Investigation of Mercury Reduction in Gold Stripping Process ...

University of Nevada, Reno

Investigation of Mercury Reduction in Gold Stripping Process

at Elevated Temperature

A thesis submitted in partial fulfillment of the

Requirement for the degree of Master of Science in

Metallurgical Engineering

By

Irawan Pramudya

Thom Seal, Ph.D, PE/Thesis Advisor

August, 2015

We recommend that the thesis

prepared under our supervision by

IRAWAN PRAMUDYA

Entitled

Investigation of Mercury Reduction in Gold Stripping Process at Elevated

Temperature

be accepted in partial fulfillment of the

requirement for the degree of

MASTER OF SCIENCE

Thom Seal, Ph.D., PE, Advisor

Carl Nesbitt, Ph.D., Committee Member

Dhanesh Chandra, Ph.D., Graduate School Representative

David W. Zeh, Ph.D., Dean, Graduate School

August, 2015

THE GRADUATE SCHOOL

University of Nevada, Reno

i

ABSTRACT

Mercury is present in many gold ores. By processing these ores, there is a

potential of emitting mercury to the environment. Carbon regeneration kiln stacks have

been observed as one of the primary source of mercury emission into the atmosphere.

Before it is recycled back into the carbon in leach (CIL) or carbon in columns (CIC),

carbon used in the gold extraction process needs to be reactivated thermally. Emission of

mercury can be minimized by keeping the mercury left in the carbon low before it goes to

the carbon regeneration kiln stacks.

The objective of this study is establishing the optimum elution conditions of

mercury cyanide from loaded carbon (which includes the eluent, concentration,

temperature and elution time) with respect to gold stripping. Several methods such as

acid washing (UNR-100, HCl or Ethanol/UNR-100) were investigated prior to the

stripping process. Furthermore, conventional pressurized Zadra and modified Zadra were

also studied with regards to mercury concentration in the solution and vapor state as well

as maximizing the gold stripping from industrial loaded carbon. 7% UNR-100 acid

washing of loaded carbon at 80oC was able to wash out approximately 90% of mercury

while maintaining the gold adsorption on the carbon (selective washing). The addition of

alcohol in the UNR-100 acid washing solution was able to enhance mercury washing

from 90% to 97%. Furthermore, mercury stripping using conventional pressurized

(cyanide-alkaline) Zadra was best performed at 80oC (minimal amount of mercury

reduced and volatilized) whereas using the same process only 40% of gold was stripped,

which makes this process not viable. When alcohol was added to the stripping solution, at

ii

80oC, 95% of gold was detected in the solution while keeping the reduction and

volatilization of mercury low.

The outcome of this study provides a better understanding of mercury behavior

during the acid washing and stripping processes so that the risk of mercury exposure and

contamination can be minimized while maximizing the gold overall recovery.

iii

Executive Summary

Experiments of UNR-100 (acetic acid) used for the acid washing on gold loaded

activated carbon were performed to determine the optimum condition of removing

mercury prior to the gold stripping process. Based on the experiments, 90% of mercury

was removed by UNR-100 at 80oC. When alcohol was added into UNR-100 solution,

97% of mercury was removed from the carbon at the same temperature.

Both UNR-100 or HCl solutions have a comparable result in removing calcium

and magnesium salts from the loaded carbon during the acid wash step. However, UNR-

100 is superior in removing mercury compared to HCl. With respect to gold, UNR-100 at

80oC removed very little gold from carbon as compared with industrial standard HCl acid

washing, 0.25% versus 0.96%. At 80oC, only minimum amount of mercury was reduced

and volatilized.

Based on this study, mercury removal from the gold loaded carbon prior to the

downstream process (gold stripping, electrowinning and carbon regeneration) minimizes

the mercury emissions, thus reducing workers exposure and environment impacts.

iv

ACKNOWLEDGEMENTS

I would like to extend my gratitude towards my Master’s thesis advisor Dr. Thom

Seal for his encouragement and guidance in my research and preparation of this thesis.

His advice, support, and help throughout finishing this task provided me motivation. I

respect his diligence and his devotion towards work that has motivated me to develop my

knowledge and experience.

I am thankful to Barrick Gold Corporation, Newmont Mining Corporation, and

Goldcorp, for financially supporting this project and giving me a chance to add

knowledge in this area.

I would like to thank Mr. Kangnian Zhong for his advice, help, and guidance, as

well as giving me a chance to work together with him in developing this project. I also

thank my friends (too many to list here but you know who you are!) for providing support

and friendship that I needed.

In addition, I would like to give my deepest gratitude to all of my committee

members, Dr. Thom Seal, Dr. Carl Nesbitt, Dr. Dhanesh Chandra for their valuable

thoughts and suggestions for this thesis.

I especially thank to my dad, mom, brother, sister and my little nephew. My hard

working parents have sacrificed their lives for us and provided unconditional love and

care. I would not have made it this far without them. My brother and sister have been my

best friend and thank them for all advice and support. I hope I can be a good role model

for my sister, so she will pursue her education as high as possible.

v

I dedicate this thesis to

my parents, brother, sister and Lionel K. Irawan

for their constant support and unconditional love.

I love you all dearly!

“Sometimes life is about risking everything for a dream no one can see but you” –

anonymous

vi

Table of Contents

Abstract i

Executive summary ii

Acknowledgement iv

Table of Contents vi

List of Tables ix

List of Figures xii

Chapter I - Introduction and Literature Review 1

1.1 Characteristic of mercury

1.2 The recovery of mercury from mineral extraction residues using

Hydrometallurgical techniques 2

1.3 Mineralogy and aqueous chemistry and toxicity of trace toxic elements 5

1.4 Deportment of trace toxic elements in gold ore processing by cyanidation 9

1.5 Minimizing mercury pollution in gold ore processing by UNR-group 13

1.6 Carbon elution/ stripping 18

1.6.1 Atmospheric Zadra stripping 20

1.6.2 Pressurized Zadra stripping 23

1.6.3 Anglo American Research Laboratory (AARL) stripping 25

1.6.4 Alcohol stripping 29

1.6.5 Micron stripping 35

1.7 Acid washing carbon 37

1.8 Acid washing followed by pressure Zadra by UNR-group 39

vii

Chapter II – Experimental 46

2.1 Materials 46

2.1.1 Industrial loaded carbon supplied by Nevada Gold Mines 46

2.1.2 Cyanide alkaline solution for the stripping process 47

2.1.3 Acetic acid for the acid washing solution 48

2.1.4 Other reagents 48

2.1.5 Equipment – autoclave 49

2.2 Experimental procedures 50

2.2.1 Synthetically loading carbon 50

2.2.2 Optimization of acid washing using UNR-100 in an autoclave 50

2.2.3 Zadra stripping in an autoclave 52

2.2.4 Acid washing using UNR-100 in a column 54

2.2.5 UNR technology on digestion of mercury from the carbon 55

2.2.6 Acid washing using mixture of UNR-100 and ethanol 56

2.3 Characterizations 56

2.3.1 Jerome 431-X Mercury Vapor Analyzer 57

2.3.2 Atomic Absorption Spectroscopy – Varian 55B 59

Chapter III – Results and discussion 62

3.1 Optimization of acid washing using UNR-100 on synthetically loaded carbon

in an autoclave 62

viii

3.2 Comparing acid washing on synthetically Au-Hg loaded carbon using HCl

and UNR-100 85

3.3 Column acid washing followed by stripping using Zadra on industrial gold

loaded carbon 87

3.4 Stripping using Zadra of industrial gold loaded carbon 101

3.5 Alcohol alkaline stripping of industrial gold loaded carbon 112

3.6 Acid washing using mixture of UNR-100 and alcohol of industrial gold

loaded carbon in an autoclave 114

3.7 Washing using alcohol on industrial gold loaded carbon 117

Chapter IV – Conclusion and future work 122

4.1 Conclusion 122

4.2 Future work 124

References 126

Appendix 137

ix

List of Tables

Table 1.1 Physical properties of mercury 2

Table 1.2 Summary the estimated NPI reported emissions (2008-2009) from

metal ore mining and basic non-ferrous metal manufacturing in

Australia in tonnes/year 6

Table 1.3 Carbon elution process selection 20

Table 1.4 General operating schedule for a pressurized Zadra stripping cycle 25

Table 1.5 Operating schedule for an AARL stripping 29

Table 1.6 Acid washing in gold mining plants 39

Table 1.7 Au, Hg, Ca and Mg content in the loaded carbon (mg/g) 40

Table 2.1 Parr Instrument Autoclave (Model A1120HC) 49

Table 2.2 Jerome 431-X specifications 57

Table 3.1 Synthetically Hg loaded carbon 62

Table 3.2 Acid washing using 7% UNR-100 at T = 70oC 63





Table 3.7 Acid washing using 0% UNR-100 (water only) at Toptimum = 80oC 71

Table 3.8 Acid washing using 3% UNR-100 at Toptimum = 80oC 72

Table 3.9 Acid washing using 6% UNR-100 at Toptimum = 80oC 73

Table 3.10 Synthetically Au-Hg loaded carbon 77

x

Table 3.11 Au in the solution during 3% UNR-100 acid washing at T = 80oC 78




Table 3.15 Acid washing using 5% HCl at 80oC 86

Table 3.16 Acid washing in column on industrial loaded carbon at 80oC and

5% UNR-100 88

Table 3.17 Au and Hg stripping using Zadra at 80oC on UNR-100 washed

loaded carbon 90

Table 3.18 The mass balance of Au and Hg for the whole process (UNR-100

acid washing and Zadra) 92


loaded carbon 93


loaded carbon 95


loaded carbon 97

Table 3.22 Au and Hg stripping using Zadra at 80oC on industrial loaded carbon 102




Table 3.26 Au and Hg stripping using alcohol alkaline solution at 80oC on

industrial loaded carbon 113

xi

Table 3.27 UNR-100/alcohol washing on industrial loaded carbon at 80oC 116

Table 3.28 Alcohol washing on industrial loaded carbon at 80oC 118

Table A-1 Mercury vapor at various temperatures 137

Table A-2 Concentration of saturated mercury vapor at different temperatures 138

xii

List of Figures

Figure 1.1 Cinnabar – red brick mineral (HgS) 8

Figure 1.2 Mercury minerals associated with gold or silver mineral 9

Figure 1.3 Stripping of mercury by cyanide/alkaline solution as a function of

temperature in 24 hours 17

Figure 1.4 Stripping of mercury using different concentration of NaCN and

NaOH at 80oC 17

Figure 1.5 Stripping of mercury from loaded carbon using various acids at 80oC 18

Figure 1.6 Atmospheric Zadra process schematic 21

Figure 1.7 Temperature versus gold stripping rate 22

Figure 1.8 Effect of electrowinning on gold stripping (after 20 hours) 23

Figure 1.9 Pressurized Zadra process schematic 24

Figure 1.10 The effect of temperature on the elution carbon 26

Figure 1.11 AARL striping schematic 27

Figure 1.12 Various kind of alcohol in the stripping process of gold 30

Figure 1.13 Alcohol effect on gold stripping process 31

Figure 1.14 Ethanol concentration versus gold desorption from carbon 32

Figure 1.15 Effect of temperature in alcohol stripping process 33

Figure 1.16 Optimization of NaOH concentration 33

Figure 1.17 Optimization of NaCN concentration 34

Figure 1.18 Micro stripping diagram 37

Figure 1.19 Acid washed column 40

xiii

Figure 1.20 Acid washed comparison between UNR-100 and HCl at 10%

concentration 41

Figure 1.21 Comparison of 5% and 10% UNR-100 wash at 85oC. 41

Figure 1.22 Ca-Mg removal using 5% UNR-100 solution 42

Figure 1.23 Ca-Mg removal using 5% HCl solution 43

Figure 1.24 Stripping of HCl washed loaded carbon at 140oC 44

Figure 1.25 Stripping of UNR-100 washed loaded carbon at 140oC 44

Figure 2.1 Activated carbon from coconut shell 46

Figure 2.2 SEM image of activated carbon microporous (0.4 < 1 <1 nm) 47

Figure 2.3 Sodium cyanide and equilibrium distribution diagram for cyanide

as a function of pH 48

Figure 2.4 Sodium hydroxide (NaOH) 48

Figure 2.5 Acetic acid – CH3COOH 48

Figure 2.6 Stirring hotplate - KIA C-MAG HS7 and stirrer bar 51

Figure 2.7 Jerome 431-X mercury vapor analyzer 51

Figure 2.8 Zadra stripping experimental setup 53

Figure 2.9 Muffle/retort furnace for burning the carbon 54

Figure 2.10 Experimental set up of acid washing in a column 55

Figure 2.11 Mercury digested as a function of digestion stage 56

Figure 2.12 Principle operation of Jerome 431-X when sample solenoid bypass

closes, and when sample solenoid bypass opens 58

Figure 2.13 Hollow cathode lamp and Varian 55B - AAS 60

Figure 2.14 Optical system and detector 61

xiv

Figure 3.1 Acid washing using 7% UNR-100 at T = 70oC 64





Figure 3.6 Acid washing using 7% UNR-100 at various different temperatures 69

Figure 3.7 Percent of Hg in the solution within 4 hours 7% UNR-100 acid washing

and Hg in the vapor within 0.5 hour acid washing 70

Figure 3.8 Acid washing using 0% UNR-100 (water only) at Topt = 80oC 71

Figure 3.9 Acid washing using 3% UNR-100 at Topt = 80oC 72

Figure 3.10 Acid washing using 6% UNR-100 at Topt = 80oC 74

Figure 3.11 Acid washing using various concentration of UNR-100 at Topt = 80oC 75

Figure 3.12 Four hours acid washing using various concentration of UNR-100 at

Topt = 80oC 75

Figure 3.13 Acid washing using various concentration of UNR-100 at T = 70oC 76

Figure 3.14 Comparison of acid washing using various concentration of UNR-100

at T = 70oC and 80oC 77

Figure 3.15 Acid washing on synthetically Au-Hg loaded carbon using 3%

UNR-100 at 80oC 79


UNR-100 at 80oC 80


UNR-100 at 80oC 82

xv


UNR-100 at 80oC 83

Figure 3.19 Percent Hg in the solution at 80oC and various concentration of

UNR-100 acid washing 84

Figure 3.20 Percent Au in the solution at 80oC and various concentration of

UNR-100 acid washing 84

Figure 3.21 Percent Au and Hg in the solution at 80oC and various concentration

of UNR-100 acid washing 85

Figure 3.22 Comparison of acid washing using 5% HCl and 7% UNR-100 at 80oC 87

Figure 3.23 Column acid washing set up 87

Figure 3.24 Hg and Au in the solution when acid washing using 5% UNR-100

was performed in a column at 80oC 89

Figure 3.25 Au and Hg presence in the solution – Zadra at 80oC 91

Figure 3.26 Hg(0) detected in the vapor – Zadra at 80oC 91





Figure 3.31 Au and Hg presence in the solution –Zadra at 140oC 97

Figure 3.32 Hg(0) detected in the vapor –Zadra at 140oC 98

Figure 3.33 Hg in the solution – Zadra after prewashed (UNR-100 acid

washing) industrial loaded carbon at various different temperatures 99

xvi

Figure 3.34 Au in the solution – Zadra after prewashed (UNR-100 acid

washing) industrial loaded carbon at various different temperatures 100









Figure 3.43 Summary of stripped Hg at various different stripping temperatures 110

Figure 3.44 Summary of stripped Au at various different stripping temperatures 110

Figure 3.45 Summary of reduced mercury detected by Jerome vapor analyzer at

various different stripping temperatures 111

Figure 3.46 Au and Hg presence in the solution – alcohol alkaline solution at 80oC 113

Figure 3.47 Hg(0) detected in the vapor - alcohol alkaline solution at 80oC 114

Figure 3.48 Hg and Au in the solution – UNR-100/alcohol washing on industrial

loaded carbon at 80oC 116

Figure 3.49 Hg(0) detected in the vapor – UNR-100/alcohol washing on industrial

loaded carbon at 80oC 117

Figure 3.50 Hg and Au in the solution – alcohol washing on industrial loaded carbon

at 80oC 119

xvii

Figure 3.51 Hg(0) detected in the vapor – alcohol washing on industrial loaded carbon

at 80oC 119

Figure 3.52 Gold ore processing flow diagram 121

Figure A.1 Jerome 431-X display and actual curves 139

1

CHAPTER 1

INTRODUCTION

Mercury is contained in many gold ores. By processing these ores – by the

conventional method of cyanide leaching, there is a potential of emitting mercury to the

environment. Carbon regeneration kiln stacks have been observed as one of the primary

source of mercury emission into the atmosphere. Before it is recycled back into the

process, the carbon in leach (CIL) or carbon in columns (CIC), carbon used in the gold

extraction process needs to be reactivated thermally [1,2]. Emission of mercury can be

minimized by removing the mercury from the carbon before it goes to the carbon

regeneration kiln stacks. The objective of this study is establishing the optimum elution

conditions of mercury cyanide from loaded carbon (which includes the eluent,

concentration, temperature and elution time) with respect to gold stripping off carbon. By

removing the mercury prior to gold stripping, the impact of mercury to the downstream

process is reduced. Thus, the pollution due to heavy metals, i.e. mercury particularly can

be minimized.

1.1. Characteristic of Mercury

Mercury is a silvery white liquid with an exceptionally low melting point of

234.321K (-38.829oC). In the periodic table, mercury is grouped in IIB related to zinc and

cadmium. Mercury has the atomic number of 80 and atomic mass of 200.59 [3-5 32-34].

2

Physical properties of mercury are summarized in Table 1-1 below [6,7].

Table 1-1. Physical properties of mercury [6,7].

Phase Liquid

Melting point 234.321 K (-38.829oC)

Boiling point 629.88 K (356.73oC)

Density 13.534 g/cm3 (at 0oC; 101,325 kPa)

Heat of vaporization 59.11 kJ/mol

Molar heat capacity 27.98/mol.K

Vapor Pressure 2 atm at 398oC

1.2. The recovery of mercury from mineral extraction residues using

hydrometallurgical techniques

The selective leaching of mercury from some concentrates and solid residues can

be achieved by the formation of a range of soluble mercury (II) species obtained by

complexation with certain reagents such as chloride, bromide, EDTA etc. Unit processes

involved in the mercury removal are precipitation, ion exchange, solvent extraction,

adsorption, membrane diffusion, cementation, electrolysis, and biological methods [5-8] 34-

38]. Precipitation of an insoluble mercury (I) or mercury (II) compound is the most

common process used [8]. HydroMet Corporation Limited, Australia has been involved in

developing some commercial processes for the recovery and removal of mercury from

mineral concentrates and residues. The final product of this process has been a high grade

mercury concentrate sold on the international market [8].

3

Selective Leaching of Sulfide Concentrates

Sulfide concentrates that contain mercury (in some cases as cinnabar) cause

problems and result in commercial penalties [8]. HydroMet has discovered that an acidic

(HCl) chlorobromide leach with a hypochlorite-bromine oxidant was exceptionally

effective for mercury leaching in the case of one certain Australian complex sulfide

concentrate [8]. This process has been commercialized in which mercury (II) sulfite

precipitated is recovered [37]. Cinnabar can be leached using the conventional process i.e

hypochlorite leaching followed by either carbon adsorption or zinc cementation to yield a

Zn-Hg amalgam [8].

Recovery of Mercury from Gold Cyanide Solutions

Precious metal bearing minerals often contain cyanide soluble mercury which is

eventually deposited on the carbon, cell cathodes and in the tailing pond. Several kind of

ion exchange resins such as Duolite A-7, Ionac A-305, and Schering TN02327 have been

studied for this mercury recovery purpose [9].

Selective Precipitation of Mercury Sulfide

Thioacetamide and thiocarbamate have been used to selectively precipitate

mercury to produce sulfide from a copper-mercury solution obtained by acid leaching.

Precipitation of mercury sulfide from solution by thioacetamide works best at pH of

about 2, so that it can be separated by filtration before significant copper is also

precipitated [9].

Treatment of Mercury Sulfur Residues

Trace levels of mercury is often found in sulfuric acid which is commonly

produced from SO2 gas generated by roasting of sulfide concentrates. The mercury

4

contaminated gas is usually scrubbed by weak acid and mist Cottrells. The mercury

contained in this washed liquor can be then cemented with aluminum metal pellets to

yield a solid residue which is primarily elemental mercury [9, 10].

Sulfatization process is another technique of removing mercury from roaster gases

which is closely related to the Outkumpu zinc plant at Kokkola in Finland in 1970 [9].

Roaster gasses are contacted with a recirculating 85-90% sulfuric acid in a sulfatizing

tower and then are scrubbed with a weak acid to remove chloride which is in the form of

HCl gas and HgCl2 gas. When selenium bearing concentrates are treated this way, the

sulfatizing product is washed to remove the soluble salts leaving a complex mercury-

selenium precipitate (HgSO4.xHg (0, S, Se, Te)) [9, 10]. Then, the mercury can be

recovered as Hg2SO3 and selenium as the element [9,10].

The roasting gas is scrubbed with an acidified of a mercuric chloride complex

(Hg2+ - chloride) dissolved in water at 30-40oC. Calomel (Hg2Cl2) is formed as follow:

Hg(v) + Hg2+ (aq) 2Hg(s)

or in another form as:

Hg + HgCl2 Hg2Cl2

The precipitate is then separated from the solution by decantation. The calomel is then chlorinated to

oxidize and convert the mercurous ion (Hg+) back to the soluble mercuric ion (Hg2+) as

shown below:

Hg2Cl2 (s) + Cl2 (g) = 2 HgCl2 (aq)

5

The chlorinated solution becomes the electrolyte for the electrowinning reactions

(courtesy: Thom Seal Ph.D).

Ethylene Leaching

Ethylene gas is being investigated as a reagent to form a strong complex with

mercury (II) which is operated at 4 atm. After the leaching process, pressure is released

and HgO can be precipitated [9].

1. 3. Mineralogy and aqueous chemistry and toxicity of trace toxic elements

Trace elements deportment during mineral processing is an area that concerns the

mining industry. Metal extraction processes incline to accumulate trace elements to

degrees that may cause hazards to both human health and the environment [10]. The role

of the minor trace elements in gold cyanidation; specifically lead, chromium, cobalt,

cadmium, mercury, arsenic, antimony, bismuth, selenium and tellurium; including their

mineralogy, aqueous solution chemistry, and toxicology is discussed in this section [10].

These trace elements have been studied in mineral processing as impurities in

base metals and concentrate products. Nevertheless, recent studies have been focused at

improving the safety of employees and meeting environmental regulations [12]. National

Pollutant Inventory (NPI), an Australian public database that displays information about

diffuse sources and emissions of 90 different substances to air, land and water reported

by industrial facilities, listed the trace elements that need to be measured and reported

annually for all mine and mineral processing facilities [13]. Table 1-2 summarizes the

estimated NPI reported emissions (2008-2009) from metal ore mining and basic non-

ferrous metal manufacturing in Australia in tonnes/year [9, 12].

6

Table 1-2. Summary the estimated NPI reported emissions (2008-2009) from metal ore

mining and basic non-ferrous metal manufacturing in Australia in tonnes/year [12, 14]

Pb Cd Hg As Sb Se

Metal ore mining

Air

150 4.7 0.7 44 2.3 0.9

Water

7.8 1.3 < 0.1 5.5 1.3 2.1

Land

1.6 < 0.1 < 0.1 4.9 0.2 0.3

Total

159 6 0.7 54 3.8 3.3

Basic non-ferrous metal manufacture

Air

260 10 9.9 47 8.4 0.8

Water

4.1 0.8 < 0.1 1.2 0.3 0.5

Land

< 0.1 < 0.1 < 0.1 0.4 < 0.1 < 0.1

Total 264 10.8 9.9 49 8.7 1.3

Bismuth and tellurium are not included as they are not required to be included in the

report under current legislation. Several other countries also assemble similar data which

is available publicly on the Internet, for instance Toxics Release Inventory – USA, and

National Pollution Release Inventory - Canada [15, 16].

Some emissions from gold mining operations contain arsenic which is largely

quick to disappear (dust containing arsenic minerals – unless dissolved like acid rock

drainage) and mercury (approximately 0.7 t/year), which is primarily from ore roasting

7

and carbon regeneration [10]. Lead and cadmium are also known for their toxicity and are

classified as a human carcinogen, however, little is known of their deportment in the gold

processing solutions. A more recent issue is mercury as a trace element in roasting of the

ore, oxidative pressure leaching and gold cyanidation [17, 18].

Another known carcinogen, arsenic, has been extensively investigated in gold

processing [19]. Mainly, the investigations were on precipitation and disposal of

solubilized arsenical wastes and several commercial processes from roasting, pressure

leaching or bacterial leaching of gold ores and concentrates [20]. Long term stability of

these treated wastes continues, as do investigations that focused at further minimizing

health and environmental risks.

Because of their lower concentration in gold ores, little is known about antimony

and bismuth. Furthermore, bismuth has a lower toxicity compared to antimony which is

comparable to arsenic in terms of its hazard on humans [20].

Selenium is an indispensable nutrient in trace amount, but unfortunately it is very

toxic at higher concentrations and ranked as the third most toxic trace element after

mercury and lead [19, 20]. Tellerium is believed to be less toxic, although it has to be

handled with care. Currently, little information is available relating the deportment of

selenium or tellurium in gold processing plants. Consequently, the chemical behavior of

antimony, selenium, bismuth and tellurium in gold processing needs to be studied so that

health and environment risks assessment can be investigated in more detail [20].

Mineralogy and aqueous chemistry of mercury

In this particular study, mercury will be reviewed in more detail related to the

mineralogy and aqueous chemistry in gold processing systems. Commonly, lead,

8

cadmium and mercury are grouped in environmental studies on account of their

toxicities, in spite of the significantly different lead solution chemistry [20-22].

Cadmium and mercury are grouped in IIB of the periodic table with a main

valence states of +II; however, mercury also forms elemental mercury (Hg0) and several

mercuruous (Hg+) compounds such as calomel (Hg2Cl2). While the highest concentration

of mercury ores contain up to 2.5%wt of Hg, the lowest one contains at least 0.1% wt of

Hg. Mercury is found frequently as the brick red mineral cinnabar (HgS) shown in

Figure 1-1 [23].

Figure 1-1. Cinnabar – red brick mineral (HgS) [7, 23].

Native mercury is rare, but and some of the other mercury minerals linked with gold or

silver ore are tiemannite (HgSe), corderoite (Hg3S2Cl2), coloradoite (HgTe), and gold or

silver mercury amalgam shown in Figure 1-2 [24-27].

9

Figure 1-2. Mercury minerals associated with gold or silver mineral [24-27].

Generally, mercury minerals have lower solubility in cyanide solution than gold.

In cyanidation solutions, mercury minerals dissolve to form a number of stable

complexes such as Hg(CN)3- and Hg(CN)4

2- which is included in the total cyanide

analysis [28].

1. 4. Deportment of trace toxic elements in gold ore processing by cyanidation [28]

It is known that pure gold dissolved at slower pace in cyanide solution than gold

containing small amount of other metals [29]. The presence of other metals such as lead,

mercury, thallium or bismuth lessen the passivation effect on the surface of gold,

therefore speeding up the rate of gold leaching [30]. These metals are unfavorable, when

10

leached, because they compete for the adsorption sites and consume the stripping

reagents [28]. Additionally, they may also report to the final product and therefore need to

be eliminated during refining, but also to the tailings from where they may need to be

stabilized [28].

These toxic trace metals need to be isolated with the environment from gold

cyanidation after roasting of ores or concentrates, leaching, and also during reactivation

of carbon. Amalgamation is still used in developing countries by small gold miners to

recover gold. This process has released thousands of tons of mercury into the atmosphere

during separation of the gold from the mercury [28, 31].

In this segment, the deportment of trace elements especially mercury during gold

processing is reviewed, including leaching, carbon adsorption and stripping, cyanide

detoxification and tailings storage [28].

Behavior of Mercury during Cyanidation

As mentioned previously, trace amounts of soluble mercury in the cyanidation

process can elevate the rate of gold leaching [30, 32]. However, when amalgams are

created, the presence of mercury can actually decelerate the cyanidation [33]. The

extraction of mercury along with gold is about 10-40% which is highly depending on the

concentration of cyanide used, the mineralogy and liberation of the mercury [34] with the

leaching rate appearing to be slower than for copper and gold [35]. Further study by Oraby

et. al reported that mercuric chloride dissolved completely within 24 hours in 2.5 g/L

NaCN solution in an open air, whereas cinnabar (HgS) and elemental mercury only 10%

and 1% respectively under the same condition [36].

11

Adsorption onto Activated Carbon

The use of activated carbon to eliminate heavy metals from natural water and

wastewater has been well studied and documented [37]. However, the behavior of trace

metals during gold adsorption is less understood [28]. Based on a study conducted by

McDougall, Hg(CN)2 strongly adsorbs onto activated carbon and competes with gold for

adsorption sites whereas Hg(CN)42- species does not [38-40]. Fleming reported that at

higher pH (>9) and higher cyanide concentration the adsorption of mercury onto

activated carbon can be minimized due to the formation of highly charged mercury

complexes [41].

Stripping from Activated Carbon

Unlike gold and copper, at atmospheric temperature with the Zadra stripping

process, mercury is inadequately stripped from activated carbon [34, 35]. However, pressure

Zadra stripping was preferentially able to desorb mercury [42, 43]. Bunney et al. showed

that stripping of mercury was effectively stripped at temperatures below 100oC in strong

caustic cyanide solution whereas gold was best stripped at temperatures above 100oC [43].

Carbon Regeneration, Gold Electrowinning and Smelting

During carbon regeneration and gold smelting, lead, cadmium, and mercury can

be emitted to the atmosphere. Since lead and its compounds are not volatile, they can be

emitted to the atmosphere as fine particles. On the other hand, due to their volatility,

cadmium and mercury are emitted as gases to the atmosphere [44].

In processing plants using Merrill-Crowe to remove gold and silver, mercury co-

precipitates, mercury co-precipitates with zinc [45]. Furthermore, in gold electrowinning,

mercury (more electropositive) in the solution is reduced to its elemental state [35]. In both

12

cases, retorting at 600-700oC is effectively used to remove the elemental mercury from

gold [35, 46]. From this process, the isolated mercury can be captured as liquid metal and

sold for recycling [56]. Sheya et. al from US Bureau of Mines pointed out that mercury

from gold containing cyanide solution can be selectively electrowon [47].

The emission of mercury from carbon regeneration by scrubbing technologies has

resulted in initiating better mercury management during gold processing [48, 49]. Hence,

installation of a mercury capture system and this on-going research of removing mercury

prior to carbon regeneration and smelting were conducted [50-52].

Tailings

The complex forms of cyanide, such as cadmium and mercury cyano complexes,

are more resilient and release cyanide ions slowly as they decompose compared to free

cyanide [53]. Pore water from a tailings deposit containing mainly cinnabar was

investigated by Shaw et al. bolster the finding by Zagury et al that the weakly-complex

metal cyanide species were found in the fresh tailings, and only the strong-complex metal

cyanide remained in the aged tailings (presumably ferro- and ferricyanides) [54, 55]. These

two investigations designated that the more stable mercury cyanide complexes continue

to exist and could be recycled in the process or be contained from the environment [28].

Fixation and Encapsulation of Mercury Process

In general, two processes, fixation and encapsulation of stabilizing certain wastes

containing significant amount of trace metals have been examined. By definition, fixation

is precipitating the trace elements in the waste as insoluble compound or adsorption onto

suitable adsorbents [28]. On the other hand, encapsulation, sometimes referred as

solidification, is enclosing the trace metals in an insoluble matrix for long term disposal

13

in an environmentally responsible manner such as cement solidification, sulfur polymer

cements (SPC), chemically bonded phosphate ceramics (CBPCs), carbon and silicon

based polymers and calcined dolomite binders. [28, 56, 57].

The main methods of mercury fixation are adsorption on activated carbon and

precipitation with sulfide, sodium dimethyl dithiocarbamate (NaDTC) followed by

flocculation and dissolved air floatation, sodium polymeric thiocarbonate, or 1,3-

benzenediamidoethanethiol (BDET) as investigated by Tourro and Lipps, Tessele at al.,

Buckham, and Matlock et al. respectively [52, 58-60]. Based on those studies, it has been

claimed that the reagent used in fixation can remove most heavy metals from the solution

(including arsenic and selenium), without interfering with gold leaching or the adsorption

process [58, 59].

When there is no cyanide, mercury (II) may also be eliminated from solution by

adsorption onto finely divide minerals such as magnetite, vermiculite or zeolite [61], ion

exchange resins [62], synthetic crandallite [63], and iron oxyhydroxides for both inorganic

and methyl mercury [64]. Additionally, mercury binding compound, such as 2-mercapto-

benzothiazole, have been used as adsorbents for heavy metals removal from industrial

waste water [65].

1. 5. Minimizing mercury pollution in gold ore processing by UNR-group [66,1]

As mentioned previously, the precious metal industry is one of the biggest source

of heavy metal pollution [2]. The principle or primary emissions from gold mining

operations are arsenic and mercury, which are primarily from ore roasting and carbon

regeneration. However, little is known of their deportment in the gold processing

solutions. Lead and cadmium are also known as having toxicity and classified as human

14

carcinogen, respectively. A more recent issue is mercury as a trace natural mineral which

is affected by roasting of the ore, oxidative pressure leaching and gold cyanidation.

Significant attention has been applied on capturing and recycling the mercury emission.

The recycled mercury plays an important role in the mercury production [67]. The

deportment of mercury from the gold stripping process is deficiently studied and

understood [68]. Fuerstenau et. all has investigated the mass balance of mercury in loaded

carbon during the stripping process at 130-140oC in an autoclave for 8-14 hours [1]. Based

on his study, elemental mercury, Hg(0), was detected. Hence, during the stripping at high

temperature, some of Hg(II) was reduced to form Hg(0) [1].

Marsh, J. E. et al. described that Hg(CN)2 might be reduced in an aqueous

solution at 230oC as follow [69]:

HgC2N2 + 4H2O = Hg + CO + (NH4)2CO3

Based on the reaction above, Hg(II) in Hg(CN)2 is reduced to Hg(0) and then due to high

temperature of stripping, it is then volatized from the solution. Furthermore, Galvin J. P.

and Pritchard, H. O. reported that the thermal decomposition of mercury cyanide vapor is

in the range of 147-237oC [70].

Hg(CN)2(g) Hg(g) + C2N2(g)

When OH-, CN-, and Cl- ions, which have the ability to form Hg2(aq)2+ complexes,

are presented in the aqueous solution, the Hg2(aq)2+ ion can disproportionate to Hg(0) with

an equilibrium constant shown below [71]:

Hg2(aq)2+ Hg(aq)

2+ + Hg(aq)0

Kdisprop = [Hg2+ ][Hg(aq)

0 ]

[Hg22+]

= 5.5x10 -9 M

15

Solubility of metallic Hg in water at various temperatures is 3.2x10 -7 M at 20oC,

1.22x10-6 M at 78.7oC and 5.11x10-6 M at 134.8oC, they are 0.0161, 0.0612 and 0.256

mg/250 mL H2O respectively. The values analyzed of Hg(II) should include dissolved

Hg(0) [71, 72].

Aqueous Hg(CN)2 solution was investigated by using pulse technology by Fujita

et. al [73]. Based on the study, HgCN and Hg2(CN)2 were detected in the solution.

Absorption of the electron energy in water releases some free radicals and molecular

products as shown below [73]:

H2O 𝑒𝑎𝑞− , H, OH, H2, H2O2, etc.

Hydrated electron could produce mercurious species – HgCN shown below [73]:

𝑒𝑎𝑞− + Hg(CN)2 Hg(I)

Whereas the H radical in the solution could act as a reducing agent for Hg(CN)2:

2H + Hg(CN)2 Hg(I) + 2HCN

Furthermore, cyanide solution could be hydrolyzed at high temperature as shown

in the reaction below [74, 75]:

CN- + 2H2O NH3 +HCO2−

The formation of CN- ultimately leads to formation of ammonia, hydrogen and carbon

dioxide. The presence of hydrogen as a reductant reduces the Hg(CN)2 to Hg2(CN)2 and

increases the formation of Hg(0) significantly [74, 75].

Additionally, M. D. Adams reported that the NaCN used in the stripping process

is not a stable compound. At high temperature, NaCN is easily oxidized by oxygen and

hydrolyzed [76, 77]. The mechanism of reaction is shown as follow [76, 77]:

16

Hydrogen is a reducing agent [78], and the presence of hydrogen helps the

reduction of Hg(II) into Hg(0). Thus, several processes were investigated in this study to

remove as much as Hg(II) possible prior the stripping. Thus, when stripping is performed

at 110-140oC, a minimum amount of Hg(0) will be produced.

Fuerstenau et al. investigated mercury stripping and washing from loaded carbon

to minimize mercury pollution in gold ore processing [79]. Based on this study, it is also

concluded that the elution of mercury is best at lower than 100oC which supports the

prior study done by Jeffrey et al in 2009 [80]. When the temperature was increased, no

mercury was found in the stripping solution nor in the carbon [79]. It indicates that the

mercury was volatilized, in the form of its elemental state.

Mercury elution/stripping was also investigated as a function of time and

temperature as shown in Figure 1-3 below. 3.33 grams of loaded carbon (4.7 mg/g Au

and 4.1 mg/g Hg) was stripped in 500 ml solution contained 2.5% NaOH and 2.5%

NaCN for 24 hours in an autoclave. Based the graph, the stripping of mercury is best

conducted at 80-90oC with less than 5% Hg left in the carbon.

17

Figure 1-3. Cyanide stripping of mercury by cyanide/alkaline solution as a function of

temperature in 24 hours [79].

Different concentration of NaCN and NaOH was also studied on the stripping of

mercury as shown in Figure 1-4 below. It shows that 2.5% of NaCN and 2.5% NaOH

gives the better result (Figure 1-3).

Figure 1-4. Cyanide stripping of mercury using different concentration of NaCN and

NaOH at 80oC [79].

18

Furthermore, the effects of different acids were also studied on the mercury

stripping from the carbon as shown in Figure 1-5 [79]. Based on the graph, acetic acid was

able to strip mercury better at lower concentration.

Figure 1-5 Stripping of mercury from loaded carbon using various acids at 80oC [79].

1.6. Carbon elution/ stripping

An efficient elution process is crucial in order to achieve a good gold recovery.

Many methods are used commercially for recovering gold and silver from loaded

activated carbon [81]. The main processes are [81]:

Atmospheric Zadra Stripping

Pressurized Zadra Stripping

Anglo American Research Laboratory (AARL)

Alcohol Stripping

Micron Elution Procedure

19

The complete recovery of chemically desorbing gold from the carbon (stripping)

allows the carbon to be recycled for further use. Each method mentioned has advantages

and disadvantages which should be assessed and considered when determining which

process to use [81]. Table 1-3 summarizes the carbon elution processes that have been

used in the gold industry [82]. From a “system approach” point of view, Randol prefers the

Anglo Elution or AARL Strip process. The main reason why Anglo elution is favored is

it utilizes seven bed volume of fresh water to elute the carbon, which results in the

removal of chemicals such as acid and/or hydroxide from the carbon pores before the

thermal regeneration process [82].

20

Table 1-3. Carbon elution process selection [82].

1.6.1 Atmospheric Zadra Stripping[83]

Atmospheric pressure Zadra was the first commercial process developed

successfully for stripping gold from carbon by J. B Zadra et. al. at the U. S. Bureau

of Mines (USBM) in the early 1950’s and applied for the first time at Golden

Cycle Gold Corporation’s Carlton Mill at Victor, Colorado in 1951 [81, 84]. This

method is still used due to its simplicity.

21

This process consists of 1% sodium hydroxide and 0.1% sodium cyanide

solution upflow trough a fixed bed of a loaded carbon at a flow rate of about 2 bed

volumes per hour at about 200oF (93.3oC).

Adsorbed gold on the activated carbon as a sodium or calcium/gold

cyanide ion pair is desorbed from the carbon via reversed adsorption kinetics [85].

Gold is recovered from pregnant solution by electrowinning. The gold free

solution is then reheated and recycled back to the stripping for reuse as illustrated

in Figure 1-6 below [81].

Figure 1-6. Atmospheric Zadra process schematic [81].

The elution time for this process usually takes approximately 48 – 72

hours. Generally the gold content in carbon is minimized from 150 oz Au/ton of

carbon to under 3 oz Au/ton of carbon [81]. The tanks are made of mild steel and

manual control are used which are very simple, thus it provides benefit in system

22

design and operation. The primary disadvantage of Zadra process is its low rate of

desorption; thus, it requires larger carbon inventories and equipment compared

with the alternative techniques [81]. Elution temperature is the most important

parameter in this process; that is, keeping the temperature as close to the boiling

point of the solution is important. Figure 1-7 shows the effect of temperature and

bed volumes to the gold desorption from the carbon [82]. Reduced boiling points of

the solution is unavoidable at mines located at high elevation, and this condition

will reduce the desorption rate quiet significantly compared with mines located

near sea level [81].

Figure 1-7. Temperature versus gold stripping rate [81].

M. J. Nichol observed that stripping efficiency also depends on the

efficiency of the electrowinning cells. Recycled eluent that contains high level of

23

gold reduces the stripping rate of gold from carbon as shown by Figure 1-8 below

[86].

Figure 1-8. Effect of electrowinning on gold stripping (after 20 hours) [86].

1.6.2. Pressurized Zadra Stripping [83]

Stripping at higher temperature could increase the stripping rate of gold

from carbon was disclosed by the USBM through its subsequent research on the

conventional Zadra process [87]. In order to have higher temperature than boiling

point of solution, the process must be performed at a pressure higher than the

vapor pressure of the solution [81]. The effect of temperature on the stripping rate

is shown by Figure 1-7 mentioned before [81].

Practically, a solution containing about 1% sodium hydroxide and 0.1%

sodium cyanide at about 280oF (138oC) and 65 psig is passed through a pressure

Electrowinning efficiency (η)

Ca

rbo

n l

oad

ing (

oz/

ton

)

24

vessel filled with loaded carbon at a flow rate of 2.0 bed volume per hour. Elution

time for this process is usually in the range of 10 to 14 hours [81]. Barren strip

solution is typically pumped through a heat exchanger and a solution heater. The

solution is then circulated up through the bed of carbon and overflows near the

top of the stripping vessel. The solution is then cooled by exchanging heat with

barren solution and flows to the pregnant solution holding tank through a back

pressure control valve. The pregnant solution flows to the electrowinning cells

and gold is recovered by electrolysis in these cells. Barren solution is then

recycled. The mechanism of the pressurized Zadra process is illustrated in Figure

1-9 [81].

Figure 1-9. Pressurized Zadra process schematic [81].

25

The upper limit temperature of the pressurized Zadra is 356oF (180oC)

based on the USBM study. At temperatures higher than 356oF (180oC), cyanide

was decomposed and metallic gold precipitated in the carbon. However, in

practice, 300oF (149oC) is the upper limit used for maximum stripping efficiency

[2]. The stripping columns are generally designed with ratio of height to diameter

of 4 to 1 and upflow rate of 1 to 4 bed volumes per hour. This flow rate does not

significantly affect the stripping efficiency [81]. Table 1-4 summarizes the

operating schedule of pressurized Zadra process.

Table 1-4. General operating schedule for a pressurized Zadra stripping cycle [81].

Process Solution Time

Load column Transfer water 60 minutes

Elution 0.1% NaCN, 1% NaOH 480 minutes

Carbon cooling Fresh water 60 minutes

Unload column Transfer water 30 minutes

TOTAL 11 hours

1.6.3. Anglo American Research Laboratory (AARL) Stripping [81, 88]

President Brand Gold Mine, South Africa used the AARL technique for

the first time on a large scale in 1980 [88]. J. R. Davidson et al. studied the elution

that involved pretreatment of loaded carbon with sodium cyanide and sodium

hydroxide followed by elution with water. Three different sources of loaded

carbon were observed. Based on his observation, elution temperature was the

26

most important parameter, and column operation at 110oC (Pgauge = 50 – 100 kPa)

was recommended [88]. Figure 1-10 shows the effect of temperature on the gold

elution process from SCII coconut-shell carbon (minus 2380µm plus 1000µm)

loaded from Golden Dumps Ltd [88]. The sample contained 1604 g/t of gold and

182 g/t silver [88]. Since that time, the application of AARL stripping has become

the standard procedure in South Africa and Australia.

Figure 1-10. The effect of temperature on the elution carbon [88].

Generally, a series of procedures started with an acid wash and water wash

to remove the residual acid is involved. The prewashed carbon is then soaked for

about 30 minutes in a solution of 3% sodium cyanide and 1% sodium hydroxide.

Fresh water heated to 230oF (110oC) is then pumped through pressurized stripping

vessel to produce the pregnant eluent. Gold is recovered from the pregnant

27

solution by electrowinning and the barren solution is discarded or recycled back

to the process. The schematic of the AARL stripping is illustrated in Figure 1-11

below.

Figure 1-11. AARL striping schematic [81].

Acid washing has been shown to generally increase the efficiency of

AARL stripping. The most common acid used in this process is hydrochloric acid

with concentration around 3%. However, research at the University of Nevada,

Reno has shown acid washing using acetic acid, which also works well [89]. The

advantages of acid washing in the elution vessel include:

28

Eliminating a carbon transfer which reduces gold losses from fine carbon

breakage of loaded carbon.

Conservation of heat if hot acid washing is employed, and

Reduction in stripping cycle time by eliminating carbon transfer [81]

On the other hand, acid washing in a separate vessel has advantages which

include:

Less potential for mixing acid and cyanide in the event of operator error or

equipment malfunction, and

Less rigorous requirements for materials of construction in the stripping

circuit since acid proof equipment is not required [81]

The carbon is washed and rinsed with fresh water after acid washing. The

objective of this procedure is to prevent acid and chlorides from entering the strip

circuit [81].

The next step is presoaking which is started by preheating the loaded

carbon with hot water, followed by soaking the carbon with hot 3% by weight

sodium cyanide and 1% by weight of sodium hydroxide for about 30 minutes at

90oC. Based on John L. Fast’s study, the concentration of sodium cyanide is

critical, elution efficiency is reduced when less than 3% by weight of sodium

cyanide is used for soaking, while higher concentration does not affect the

stripping efficiency. The duration of soaking has been reported that it does not

significantly affect the stripping efficiency [81].

The final step is elution/stripping which is generally performed using

about 6 bed volumes of good quality water at a rate of about 2 bed volumes/hour.

29

This water quality and water temperature has a notable effect on stripping

efficiency. Operation at 236oF (113oC) requires operating pressures of 10 to 15

psig to restrain flashing steam in the system [81]. Table 1-5 summarizes the

common operating schedule for an AARL stripping [81].

Table 1-5. Operating schedule for an AARL stripping [81]

Process Solution Time

Load column Carbon 90 minutes

Acid wash 3% HCl 20 minutes

Water rinse Potable water 90 minutes

Pre heat Potable water 30 minutes

Pre soak 3% NaCN, 1% NaOH 30 minutes

Elution Potable water 180 minutes

Cooling Potable water 30 minutes

Carbon transfer Transfer water 30 minutes

TOTAL 7 hours 50 min

1.6.4. Alcohol stripping [81, 90]

Adding alcohol to the stripping solution can increase the stripping rate.

Based on research at the USBM [85]. In 1973, Benzaria patented a stripping

process of constituents from activated carbon using hot alkaline alcohol solutions

[91]. It was followed by Fischer in 1976 which found that using caustic-cyanide

solution containing 75% volume of alcohol (methanol or ethanol), loaded carbon

30

from pure sodium cyanide solution was readily desorbed at room temperature [23].

This techniques, however, it is not effective to desorb gold and silver from loaded

carbon generated at commercial mine [90]. The most commonly used alcohols in

the stripping process are ethanol and methanol which gives almost the same

stripping efficiency. Although, isopropanol works as well, the efficiency is lower

than the alcohols mentioned previously [81]. Figure 1-12 shows the effect of

different kind of alcohols that can be used in the stripping process. Due to the

health risk and economical reason, ethanol is preferred over methanol.

Figure 1-12. Various kind of alcohol in the stripping process of gold [90].

Heinen et al. conducted research to study the effect of alcohols on the

desorption of gold and silver from activated carbon. 1% weight of sodium

hydroxide and 0.1% weight of sodium cyanide strip solution at 93oC was used

31

with and without 20% volume of ethanol. The result of this study is displayed in

Figure 1-13 below [90]. Based on the graph, addition of ethanol significantly

increases the stripping of gold from the carbon.

Figure 1-13. Alcohol effect on gold stripping process [90].

The concentration ethanol on the stripping process was also investigated.

It was found that the optimum condition was peaked at 20% volume of ethanol.

Higher ethanol content does not increase the stripping or desorption of the gold

from carbon. Figure 1-14 explains the effect of ethanol concentration on the

stripping or desorption gold [90].

32

Figure 1-14. Ethanol concentration versus gold desorption from carbon [90].

Heinen et al. also reported the optimum temperature and concentration of

sodium hydroxide in the alcohol stripping process as illustrated in Figure 1-15

and 1-16 which is 80oC and 1% by weight NaOH [90]. Nearly all gold was

stripped from carbon within 5 to 6 hours of elution process [90]. The concentration

of sodium cyanide has no significant influence on the gold stripping process as

shown in Figure 1-17 below [90]. As shown in Figure 1-17, the gold stripping

efficiency is only slightly affected by concentration of sodium cyanide at the

range of to 0 to 0.5 % weight [90].

33

Figure 1-15. Effect of temperature in alcohol stripping process [90].

Figure 1-16. Optimization of NaOH concentration [90].

34

Figure 1-17. Optimization of NaCN concentration [90].

The main disadvantage of the alcohol stripping is the potential for fires.

Fires have been reported at several plants that utilize alcohol stripping, especially

in electrowinning section since it has the potential of generating sparks [90].

Another hydrocarbon frequently used in the gold stripping process to

enhance the speed of atmospheric Zadra process is ethylene or propylene glycol

with commonly used stripping time of 24 to 36 hours [92]. The advantage of using

glycol is it is not flammable, however; glycol is costly and its stripping rate is

lower than the alcohols. Typically, 20 to 25 % weight of ethylene or propylene

glycol and 2 % weight of sodium hydroxide are required. Adding sodium cyanide

is not necessary in this process. Approximately 20 to 40 gallons of glycol is

consumed per ton of loaded carbon stripped. The eluent is heated to about 190oF

35

(88oC) and pumped through the stripping vessel at a flow rate about 2 bed

volumes per hour. Then the gold is recovered by electrowinning [81, 92].

1.6.5. Micron stripping [81, 93]

The newest stripping procedure used in the gold industry is micron

stripping which was developed at Micron Research Australia [93]. The process

requires pretreatment of loaded carbon with a caustic cyanide solution followed

by alcohol mixture elution [81, 93]. The micron stripping takes advantage of the

alcohol stripping (enhanced stripping rate) but enclose the alcohol to the closed

stripping unit in order to reduce or minimize the potential of fire dangers as the

pregnant eluent in this process does not contain alcohol anymore [81]. The elution

unit is arranged like a packed bed distillation tower with a heater on the base and

condenser on top. The micron stripping process is composed of the following

steps [81]:

1) Presoaking

The loaded carbon is first soaked in a 1 to 2% sodium hydroxide and 5 to

10% sodium cyanide at room temperature. If the carbon contains high

concentrations of gold, silver or copper, the sodium cyanide concentration

may be increased up to 20%. Once presoaking is finished, the solution is

then drained.

2) Desorption or stripping

About 0.5 bed volume alcohol, is then added. Heat is then applied on the

base of the packed bed tower or vessel. Organic vapor rises through the

36

carbon bed and is condensed. The condensate is pumped back to the top

and sprayed on the carbon.

3) Alcohol recovery

Once the stripping process is completed, alcohol recovery starts by

evaporating the alcohol from the pregnant solution. Once the solution

reaches the boiling point of water, the evaporation is then discontinued

and the draining of pregnant solution commences.

Following these three steps, the eluent from this micron stripping contains

negligible alcohol, with high concentration (about 2 to 3 times higher than those

in the loaded carbon) of recovered metals [81]. The illustration of the micron

stripping process is shown in Figure 1-18 below [81].

37

Figure 1-18. Micro stripping diagram [81].

1.7. Acid Washing of Carbon

Acid washed is often discussed in carbon elution or stripping and usually it is

carried out at different points in the carbon elution depending on operating management

and preferences [94]. Generally, AARL elution requires acid washing followed by caustic

cyanide solution before the elution itself. There are two reasons reported on the

advantages of acid washing loaded carbon before the elution. First, acid washing opens

38

the carbon pores thus intensifying the elution process [94]. Second, acid washing removes

calcium carbonate and other substances from the carbon pores [95]. The previous study by

Adams et. al. has shown that acid treatment results in formation of an Au CN- type

species shown below [96, 97].

Adams also observed that following treatment of loaded carbon in caustic

solution, in the absence of cyanide, results in reduction of some of the gold cyanide into

gold metal [94, 96].

Due to this metallization of gold in activated carbon results after acid washing followed

by cyanide free elution, it is critical to neutralize the acid treatment before the free

cyanide elution processes [94-96].

Some studies have suggested acid washing after cyanide elution process, due to

decomposition of cyanide to ammonia and carbonate which can cause scaling on the

carbon [94]. Table 1-6 summarizes some of acid washing used in gold extraction plants

[94].

39

Table 1-6. Acid washing in gold mining plants [94].

Operation Acid Wash

Barrick Mercur, UT 3% HNO3 prior to elution to remove thallium

Big Springs, Elko, NV 5% HCl rinse before modified AARL elution

Getchell, Winnemucca, NV 30 min nitric acid soak ahead of pressure Zadra

Morro Do Ouro, Paracatu,

Brazil

HCl acid wash ahead of micron elution

Mount Gibson, W. Australia

3.3% HCl acid wash 15 minutes at ambient temperature before

AARL

Newmont Gold, Carlin, NV

(A) 3% HCl (ambient) ahead of AARL

(B) 3% HCl (ambient) ahead of Zadra process

Waihi Gold Mine, Waihi, NZ 3% HCl 20 minutes (ambient) before AARL

Summitville, CO

Whenever CaCO3 levels exceed 5% of carbon weight, carbon

is acid washed in HCl (pH = 1) for 4 hours. This reduces Ca

levels to 0.5% CaCO3 before elution

1.8. Acid washing followed by Pressure Zadra by UNR-group [96]

UNR has been investigating the behavior of mercury in the stripping solution and in

the vapor phase for several years. Loaded carbon in these experiments was provided by a

Nevada gold mine. The content of gold, mercury, calcium and magnesium is tabulated in

Table 1-7. The mercury in the solution was characterized using AAS Varian 55B and the

mercury in the vapor was analyzed using a Jerome 431-X Mercury Vapor Analyzer [97].

40

Table 1-7. Au, Hg, Ca and Mg content in the loaded carbon (mg/g)

Product

Content on loaded carbon (mg/g)

Hg Au Ca+Mg

Loaded carbon before acid washing 0.313 5.39 11.45

Prior to using Zadra stripping, loaded carbon was acid washed using either 10%

UNR-100 and 10% HCl in a glass column (15 x 300 mm) equipped with a controlled

heating tape as shown in Figure 1-19 below. Washed carbon was then dried in a vacuum

oven.

Figure 1-19. Acid washed column [96].

The effect of mercury stripping from the carbon using acid washed (UNR-100 and

HCl) is shown in Figure 1-20. Based on the graph, UNR-100 provides a significantly

higher stripping of mercury than HCl at the same temperature and concentration. UNR-

100 is even more effective at 90oC, as it was shown to strip about 70% of mercury in 8

hours washing. Different concentration of UNR-100 was also studied. 5% UNR-100 was

41

also used to wash the carbon. It was then compared with 10% UNR-100 at 85oC shown in

Figure 1-21. Based on this graph, 5% UNR-100 shows its superiority compared to 10%

UNR-100 at 85oC.

Figure 1-20. Acid washed comparison between UNR-100 and HCl at 10% concentration [96].

Figure 1-21. Comparison of 5% and 10% UNR-100 wash at 85oC [96].

42

Based on further investigation, the UNR-100 and HCl were also very selective

which means it stripped only mercury. 5% UNR-100 and 5% HCl only stripped 0.15%

and 1.5% of Au respectively. Thus, this acid wash provides a favorable technique to

separate the gold and mercury before gold stripping process using cyanide solution.

Acid wash has also offered a significant role in removing the calcium and

manganese in the carbon. Figure 1-22 and 1-23 show the Ca-Mg removal along with Hg

removal when 5% of UNR-100 and 5% HCl were used to wash the loaded carbon

respectively.

Figure 1-22. Ca-Mg removal using 5% UNR-100 solution [96].

43

Figure 1-23. Ca-Mg removal using 5% HCl solution [96].

Based on the results shown in Figure 1-22 and 1-23, the removal of Ca-Mg was

optimum at 6-8 bed volumes using acid wash (UNR-100 and HCl). HCl removed Ca-Mg

about 10% better than UNR-100 whereas UNR-100 showed its superiority on removing

Hg. At about 16 bed volumes, UNR-100 removed approximately 55% of Hg whereas

HCl removed less than 1%.

The conclusion of this acid wash is that HCl is favorable to remove Ca-Mg

whereas UNR-100 is favorable to remove Hg from loaded carbon. Additionally, neither

HCl and UNR-100 significantly removes Au from the carbon.

The washed loaded carbon was then cyanide stripped using Zadra in an autoclave

at 140oC. 2 grams of HCl washed loaded carbon was stripped in 300 ml solution

contained 1% NaOH and 1% NaCN. The loading was 0.313 mg/g of Hg and 5.39 mg/g of

Au. Figure 1-24 shows the stripping of 5% HCl washed loaded carbon at 140oC.

44

Figure 1-24. Cyanide stripping of HCl washed loaded carbon at 140oC [96].

2 grams of HCl washed loaded carbon was stripped in 300 ml solution contained

1% NaOH and 1% NaCN. The loading was 0.15 mg/g of Hg and 5.39 mg/g of Au.

Figure 1-25 shows the stripping of UNR-100 washed loaded carbon at the same

temperature.

Figure 1-25. Cyanide stripping of UNR-100 washed loaded carbon at 140oC [96].

45

Based on the two figures above, at high temperature, a lot of Hg(0) was produced

during the stripping process. The behavior of reduction of Hg on both stripping are

similar. Hg(II) reduction to Hg(0) is inevitable at 140oC in an extended period of time (>1

hour). There are two possible ways to avoid this reduction and possible emission. First,

removing Hg(II) as much as possible from loaded carbon prior to pressure Zadra

stripping and/or decreasing the elution time at the beginning of elution in which most of

the adsorbed Hg(II) is already desorbed. Secondly, replacing the stripped solution with

new solution to the autoclave for the rest Au elution. Hg(II) in then removed from the

stripped solution before the stripped solution enters the electrowinning process [98, 99].

46

CHAPTER 2

EXPERIMENTAL

2.1 Materials

2.1.1 Industrial loaded carbon supplied by Nevada Gold Mines

Activated carbon (Figure 2-1) is commonly used and plays an important roles in

precious metals processing, particularly in gold recovery in heap leaching circuits, carbon

in leach and carbon in pulp operations. It has a very high porosity (Figure 2-2), thus

yields a very high surface area of more than 500 m2/grams. Activated carbon is produced

from carbonaceous materials such as coconut shell, nutshells, peat, wood, etc. via

physical reactivation (using hot gases) or chemical reactivation prior to carbonization

(impregnation with certain chemicals such as acid, strong base or salt) [100-102]. For this

study, a sample of loaded activated carbon was provided by a Nevada gold mine, which

contained 4.4-4.7 mg/g of Au and 1.5-1.7 mg/g of Hg.

Figure 2-1. Activated carbon from coconut shell [103].

47

Figure 2-2. SEM image of activated carbon microporous (0.4 < 1 <1 nm) [104].

2.1.2 Cyanide alkaline solution for stripping process

Sodium cyanide (NaCN) and sodium hydroxide (NaOH) were used as a stripping

solution. NaCN is a highly toxicity, white, water soluble, inorganic compound shown in

Figure 2-3 (a). Due to its high reactivity with metals, this compound is commonly used

in gold mining industry [105]. Toxic gas of hydrogen cyanide (HCN) can be formed when

NaCN is mixed with an acid as shown in Figure 2-3 (b) [106].

Figure 2-3 (a) Sodium cyanide and (b) Equilibrium distribution diagram for cyanide as a

function of pH [105-106].

48

NaOH, also known as caustic soda, is a white solid, highly metallic caustic base

and alkali salt. It is soluble in most organic solvent such as ethanol, methanol and in

water (Figure 2-4) [107]. In the mining industry, NaOH is commonly used to adjust the pH

to the desired condition.

Figure 2-4. Sodium hydroxide (NaOH).

2.1.3 Acetic acid for acid washing solution

Acetic acid or the UNR 100, (IUPAC: ethanoic acid) is a colorless, pungent,

water soluble liquid that has chemical formula CH3COOH (molecular weight of 60.05)

[108]. The liquid has a density of 1.049 g/mL, boiling point of 118oC. Besides water, it is

also miscible with alcohol, glycerol, ether and carbon tethachloride. It is a very stable

aqueous solution at room temperature, if well sealed [109].

Figure 2-5. Acetic acid – CH3COOH.

2.1.4 Other reagents

Mercury and gold standards for atomic absorption spectroscopy (AAS), mercury

cyanide – Hg(CN)2, ethanol – CH3CH2OH, nitric acid (HNO3), hydrochloric acid (HCl),

49

potassium permanganate (KMnO4), sodium persulfate (Na2S2O8), tin chloride (SnCl2).

All reagents were all commercially available (Sigma Aldrich) and used without further

purification unless explained.

2.1.5 Equipment – Autoclave

Some of the acid washing and stripping processes were conducted in a Parr

Instrument Autoclave (Model A1120HC). The autoclave is made of T316 stainless steel

that has a capacity of 1900 ml and maximum pressure of 3600 psi. Table 2.1 summarizes

the specification of the autoclave.

Table 2.1. Parr Instrument Autoclave (Model A1120HC)

Vessel

Model Parr Instrument A1120HC

Style Moveable

Material of construction T316 Stainless Steel

Fe(%) 65

Ni (%) 12

Cr (%) 17

Mo (%) 1.2

Mn (%) 2

Si (%) 1

Inside Diameter (inch) 4

High Pressure inside diameter (inch) 3.75

Inside depth (inch) 10.5

Weight of vessel (pounds) 6

Reactor dimensions

Width (inch) 11.5 inch

Height 6.4

Weight (pounds) 11

Reactor Mounting Bench Top

Closure Split-Ring (6 Cap Screws)

Valve Connections 1/8" Male NPT

50

2.2 Experimental procedures

2.2.1. Synthetically loading carbon

Fresh activated carbon was dried in an oven at 80oC for 24 hours to

eliminate moisture adsorbed during the storing. Approximately 3% by weight

moisture was removed. Hg(CN)2 and AuCN purchased from Sigma Aldrich (99%

purity) were used to load the carbon.

25 grams of dried carbon was placed into an Erlenmeyer flask. Then, 200 ml

solution contains 1% NaOH, 0.1% NaCN, and desired amount of AuCN (144.5 mg)

and Hg(CN)2 (32.2 mg/g) were poured into the Erlenmeyer. The Erlenmeyer was

sealed and placed in a gyratory shaker for 24 hours at room temperature (23oC). The

speed of the shaker used was 40%.

Once finished, the carbon was isolated from the solution by filtration. The

filtrate was collected for analysis so the exact amount of Hg and Au no adsorbed

can be determined. Furthermore, the carbon was then washed using distilled water

until the water reached pH < 9 (about 30 times washing).

By following this procedure, approximately 4.91 mg/g Au and 0.94 mg/g

Hg were loaded onto the carbon. This carbon was then used for optimization of acid

washing using UNR-100 in an autoclave.

2.2.2 Optimization of acid washing using UNR-100 in an autoclave

2.5 grams of synthetically loaded carbon was used to study the mercury

washing using various concentrations of UNR-100 solution. UNR-100 solution was

heated up by the digital stirring hotplate (KIA C-MAG HS7) and a stirrer bar as

51

shown in Figure 2-6. When the temperature was 80oC, the carbon was added and

the autoclave was quickly sealed and to start the washing time.

Figure 2-6. Stirring hotplate - KIA C-MAG HS7 and stirrer bar.

7-8 ml samples of the solution was removed at 0.5, 1, 2, 3, and 4 hours and

then analyzed using AAS. The vapor collected inside the autoclave was also taken

by a needle syringe to measure the mercury content by Jerome 431-X mercury

vapor analyzer (Figure 2-7).

Figure 2-7. Jerome 431-X mercury vapor analyzer.

After 4 hours, the residue carbon was removed and washed using tap water

and then digested by UNR technology (see 2.2-5 below). This process will

52

determine how much mercury left in the carbon (tail) after washing using UNR-

100.

2.2.3. Zadra stripping in an autoclave

Zadra stripping of precious metals at various temperatures was conducted to

determine the optimum condition for removal of gold from the carbon while

maintaining a low reduction and volatilization of mercury. The stripping was

conducted in an autoclave with 250 ml solution containing 1% by weight NaCN and

1% by weight NaOH. If alcohol alkaline solution was used, then 250 ml stripping

solution contains 1% NaCN, 1% NaOH and 20% ethanol was used. The autoclave

was then heated up to the desired temperature (Figure 2-8). In this process, there

was no pressure added. However; due to elevated temperature, a pressure was read

on pressure gauge due to the heated solution. At 89, 100, 120, 140oC, pressure of 0,

9, 20, and 43 psig were read, respectively. The same sampling procedure was used

as the previous experiment, the vapor collected in the autoclave was also sampled

using a syringe needle at 0.5, 1, 2, 3, and 4 hours, measured by the Jerome 431-X

mercury vapor analyzer. Additionally stripped solutions were also taken

simultaneously as the vapor and analyzed by AAS for both mercury and gold.

53

Figure 2-8. Zadra stripping experimental setup.

After 4 hours, the stripped carbon was washed repeatedly using tap water

and then digested by UNR technology and analyzed to determine the remaining

mercury and gold on the carbon. Finally, the digested carbon was combusted in a

muffle/retort furnace (Figure 2-9), the ash was then dissolved by 50 ml aqua regia

to measure the gold remained in the carbon after all of these processes (at this point,

it was assumed no mercury was left on the carbon).

54

Figure 2-9. Muffle/retort furnace for burning the carbon.

The result from all procedures were used to calculate a mass balance of gold

and mercury throughout the stripping process. When a good balance was obtained,

the stripping kinetic of gold and mercury was then reported.

2.2.4. Acid washing using UNR-100 in a column

Acid washing of loaded carbon using UNR-100 in a column was also

conducted to mimic an industrial-scale process. The column used was made from

stainless steel, ID = 25 mm (0.984 inches) and H = 300 mm (11.81 inches). The

washing temperature was held at 80oC (26.67oF). Approximately 72 grams of wet

loaded carbon was place in a stainless steel cartridge, 24 × 200 mm (0.944 × 7.87

inches), the carbon had ± 85 cm3 (5.187 in3) of bed volume contained. The column

was heated using electrical heating tape and controlled by a thermostat.

1.204 L of UNR-100 solution was pumped from the bottom of the column,

collected, and recycled. The experimental set up is shown in Figure 2-10. The

55

flowrate of washing solution was controlled by Pulsatron Electronic, 1 L/h pump.

The washing was done in 6 hours. Each sample was taken at 0.5, 1, 2, 3, 4, and 6

hour intervals of washing and analyzed using AAS.

Figure 2-10. Experimental set up of acid washing in a column.

After 6 hours, the carbon was washed with water and then dried in an oven.

A weight of 48.6 grams was obtained. The dried washed carbon was then stripped

with Zadra in an autoclave as described previously.

2.2.5. UNR technology on digestion of mercury from the carbon

Obtaining a metallurgical balance is very crucial. Thus, a digestion of

mercury from the carbon was developed by UNR group. There are 2 steps used in

this digestion process, oxidants and lixiviant. The oxidants used are 5% KMNO4

and 5% Na2S2O8 whereas the lixiviant is aqua reqia (HCl : HNO3 = 3 : 1).

56

A known weight of dried carbon (0.5 – 2 grams) was digested in 10 ml of

aqua regia and 5% Na2S2O8 for about 25 minutes at its boiling point. Then, 5 ml of

5% KMnO4 was added and heated for about 25 minutes. The pregnant solution was

then poured into 200 ml volumetric flask. This procedure is repeated 10 times. Once

all pregnant solution was collected, the solution was then analyzed.

Fuerstenau et al. reported that, digesting carbon ten times using this

technique was able to remove all of mercury from the carbon as shown in Figure 2-

11 below [79]. As seen from the graph, 5 times digestion is already able to desorb

approximately 90% of the adsorbed mercury from carbon.

Figure 2-11. Mercury digested as a function of digestion stage [79].

2.2.6. Acid washing using mixtures of UNR-100 and ethanol

A similar procedure as 2.2-2 was used for this experiment. The only

difference was the addition of 20% of ethanol was added into the UNR-100

solution. The addition of ethanol was expected to increase the removal of mercury

from the carbon and keep the gold on the carbon.

57

2.2 Characterizations

There are two characterization techniques used in this study, i.e. Mercury Vapor

Analyzer and Atomic Absorption Spectroscopy.

2.3.1. Jerome 431-X Mercury Vapor Analyzer

Jerome 431-X is designed for simple, effortless and precise analysis of

mercury vapor. Its sensitivity is in the range of 0.001 to 0.999 mg/m3 Hg [110-112].

Table 2-2 summarizes the specifications of this instrument.

Table 2-2. Jerome 431-X specifications [112].

Specifications

Resolution 0.001 mg/m3

Detection range 0.001-0.999 mg/m3

Precision 5% relative standard deviation at 0.100 mg/m3

Accuracy ± 5% at 0.100 mg/m3

Response time

13 seconds in sample mode

4 seconds in survey mode

Flow rate 750 cc/min

The principle of operation of Jerome 431-X is based on the unique quality

that mercury will change resistance of a gold film [110, 111]. The sensor of this

instrument consists of two thin gold films, a reference and a sensor which is

configured in a Wheatstone Bridge Circuit [111]. The reference film is protected so

that it does not contact mercury. The sensor film is exposed to mercury resulting in

58

resistance changes measured by the circuit. This configuration is able to detect very

small changes in electrical resistance [111]. The concentration of mercury is then

computed by microprocessor prior to displaying the results [111].

When “sample mode” is activated, air is drawn by the pump through a

scrubber and blown into the system [110]. After 2 seconds, the sample solenoid

bypass opens and closes off the scrubber filter from the flow system. The sample air

passes through a filter which removes any acidic gasses that may interfere the

sensor’s response to mercury. The air from the filter is drawn over the gold film

sensor which adsorbs the mercury vapor. Nine seconds after it starts, the sample

solenoid bypass closes and the remainder of the sample is drawn through the

scrubber filter and the flow system [110]. The mercury concentration is then

calculated and displayed in milligrams per cubic meter (mg/m3) [110]. The schematic

of this process is explain by Figure 2-12 [111].

Figure 2-12. Principle operation of Jerome 431-X (a) when sample solenoid bypass

closes, and (b) when sample solenoid bypass opens [111].

When the sensor becomes saturated (500 nanograms of mercury), it needs to

be regenerated. The regeneration is accomplished by a 10 minute heating cycle. The

solenoid bypass closes during the sensor regeneration cycle, causing air to pass

59

through the scrubber filter, providing clean air for the purpose of regeneration [110,

112].

2.3.2. Atomic Absorption Spectroscopy – Varian 55B

Atomic absorption spectroscopy (AAS) is one of the most common

analytical technique to measure element concentration in aqueous the sample. It is

able to measure as low as several part per billion of a gram in a sample [113]. The

main principle of this technique is detection of change in light wavelength

specifically absorbed by an element.

AAS has been used in various application such as [113-114]:

Clinical analysis. Metal analysis in biological fluids such as blood and urine.

Environmental analysis. Tracking various elements in rivers, seawater,

drinking water, etc.

Pharmaceuticals. Monitoring quantitatively of a catalyst in the final

pharmaceutical products.

Industrial analysis. Analyzing the amount of metals in rocks for mining

industry, examining the major elements of raw materials.

The working principle of AAS is that each atom of different elements

absorbs specific wavelength of light. For instance with lead, a lamp containing lead

emits light from excited lead atoms that create the exact mix of wavelengths that are

absorbed by lead atoms presented in the sample. Furthermore, the sample first must

be atomized – converted into ground state free atoms in the vapor state – and then a

beam electromagnetic radiation is emitted from the excited lead atoms that passes

60

through the vaporized sample. Some of the radiation is absorbed by the lead atoms

in the sample. The higher the number of atoms in the vapor, the more of the

radiation is absorbed [113].

Construction of calibration curves is required by running a number of

known lead concentrations under the same condition [113-115]. Finally, by comparing

the amount of sample absorbed with the calibration curve, the lead concentration in

the sample can be calculated [113-115].

The frequent source of light is a “hollow cathode lamp” shown in Figure 2-

13 (a), it contains a tungsten anode and a cylindrical hollow cathode made of

specific element to be detected [113]. The AAS instrumentation used in this project is

shown in Figure 2-8 (b).

Figure 2-13. (a) Hollow cathode lamp and (b) Varian 55B - AAS.

Selecting the specific wavelength is done using a monochromator. This

allows users to detect the desired element in the presence of other elements. The

selected light is led onto a detector (commonly a photomultiplier tube). This creates

an electrical signal corresponding to the light intensity (Figure 2-14) [103, 116].

Optical

window

Cathode

Anode

(a) (b)

61

Figure 2-14. Optical system and detector [113].

From here then the results of the experiments are reported and discussed in the

next chapter.

62

CHAPTER 3

RESULTS AND DISCUSSION

3.1. Optimization of acid washing using UNR-100 on synthetically loaded carbon in

an autoclave.

The UNR-100 or acetic acid, used for removing Hg was optimized in this study.

Activated carbon was synthetically loaded with Hg only and both Au-Hg as described in

previous chapter. Table 3-1 summarizes the synthetically Hg loaded carbon obtained.

Table 3-1. Synthetically Hg loaded carbon.

Weight of

carbon

(g)

Vol. of

solution

( mL)

Initial Hg Wt (mg) Final Hg in

solution

(mg)

Adsorbed

Hg (mg)

Adsorption of

Hg (mg/g) Hg(CN)2 Hg

25.0 200 32.1 25.5 0.6~1.2 24.9 0.97~1.0

The effect of temperature on acid washing using 7% UNR-100 as pin point was

investigated. The UNR-100 washing was conducted in an autoclave at various

temperatures (70, 80 85, 90, and 100oC) which was heated by a stirring hotplate. 250 ml

of washing solution 7% UNR-100 was used to wash the 2.5 grams of synthetically loaded

carbon.

The Hg in the vapor and solution were analyzed using Jerome 431-X Mercury

Vapor Analyzer and AAS Varian 55B, respectively. The washed carbon (tail) was

63

digested using the UNR Hg digestion procedure to analyze the remaining Hg in the

carbon.

Table 3-2 summarizes the acid washing result at T = 70oC. Within 4 hours of acid

wash, approximately 79.4% of the Hg was removed from the carbon into the solution,

whereas only insignificant amount of Hg vapor was detected as shown in Figure 3-1

respectively.

Table 3-2. Acid washing using 7% UNR-100 at T = 70oC.

T (oC)

Hg loaded

(mg/g)

Elution Hg(II) in solution Hg(0) in autoclave

time (hour) mg % mg %

0 0 0 0 0

0.5 1.107 45.6 0.0021 0.085

70 0.97 1 1.53 63.1 0.0015 0.06

2 1.813 74.7 0.0015 0.06

3 1.9 78.4 0.0016 0.068

4 1.925 79.4 0.0012 0.05

64

Figure 3-1. Acid washing using 7% UNR-100 at T = 70oC.

7% UNR-100 acid washing at T = 80oC was able to wash approximately 90% of

Hg from the carbon within 4 hours and only small amounts of Hg vapor were detected.

Table 3-3 and Figure 3-2 summarize this phenomenon.


T (oC)

Hg loaded

(mg/g)

Time Hg(II) in solution Hg(0) in autoclave

(hour) mg % mg %

0 0 0 0 0

0.5 1.555 64.1 0.0201 0.83

80 0.97 1 1.849 76.2 0.0077 0.32

2 2.002 82.6 0.0023 0.1

3 2.133 88 0.00089 0.04

4 2.168 89.4 0.00067 0.03

65


UNR-100 acid washing at T = 85oC was able to wash approximately 90% of Hg

from the carbon within 4 hours and only a small amount of Hg vapor was detected.

Comparing this result with UNR-100 acid washing at 80oC, both of these temperature

conditions are comparable in removing the Hg from the carbon. Table 3-4 and Figure 3-

3 summarize the 7% UNR-100 acid washing at 85oC performance.

66


T (oC)

Hg loaded

(mg/g)


(hour) mg % mg %

0 0 0 0 0

0.5 1.616 67.3 0.0124 0.52

85 0.96 1 1.817 75.7 0.0082 0.34

2 2.027 84.5 0.0043 0.18

3 2.08 86.7 0.0021 0.087

4 2.151 89.6 0.0011 0.045


When the acid washing temperature was increased to 90oC, the effectiveness of

Hg removal slightly declined as shown in Table 3-5 and Figure 3.4 below. Within 4

hours, only approximately 85% of Hg was detected in the solution, and the Hg in the

vapor was four times higher than the 85oC acid washing.

67


T (oC)

Hg loaded

(mg/g)


(hour) mg % mg %

0 0 0 0 0

0.5 1.719 70.9 0.0623 2.6

90 0.97 1 1.862 76.8 0.0432 1.8

2 1.967 81.1 0.0205 0.85

3 2.004 82.6 0.0083 0.34

4 2.059 84.9 0.0039 0.16


At even higher temperature, T = 100oC, the Hg concentration in the solution was

even lower and more Hg was detected in the vapor as shown in Table 3-6 and Figure 3-5

below. Within 4 hours of acid washing, approximately 40% of Hg was in the solution and

more than 2% was detected in the vapor, which was more than ten times detected at 90oC.

68


T (oC)

Hg loaded

(mg/g)


(hour) Mg % mg %

0 0 0 0 0

0.5 1.229 50.7 0.6118 25.2

100 0.97 1 1.219 50.3 0.3925 16.2

2 1.254 51.7 0.1192 4.9

3 1.061 43.7 0.076 3.1

4 1.011 41.7 0.0564 2.3


Based on these result, it can be concluded that acid washing at 80-85oC is the

optimum condition to remove about 90% of the Hg from the carbon with minimum

amount of Hg vapor emitted. Figure 3-6 summarizes the effect of temperature in the

removal of Hg from the carbon into the solution.

69

Figure 3-6. Acid washing using 7% UNR-100 at various different temperatures.

Figure 3.7 shows the summary of Hg in the solution and vapor at various

different temperatures. Based on the graph, the higher the temperature, the more Hg

vapor was detected. At 100oC, approximately 25% of Hg was detected in the vapor only

within 30 minutes of acid washing. At 80oC, approximately 90% of Hg was detected in

the solution, and insignificant amount of Hg was found in the vapor detected at this range

of temperature.

70

Figure 3-7. Percent of Hg in the solution within 4 hours 7% UNR-100 acid washing and

Hg in the vapor within 0.5 hour acid washing.

The concentration of UNR-100 was also optimized at Toptimum = 80oC in regards to

Hg removal from the carbon. Table 3-7 and Figure 3-8 show the Hg removal when only

water was used to wash the synthetically loaded carbon. Using just water to wash the

synthetically loaded carbon at 80oC, approximately 40% of Hg was detected in the

solution and only 0.2% in the vapor.

71

Table 3-7. Acid washing using 0% UNR-100 (water only) at Toptimum = 80oC.

UNR-100

(%)

Acid washing

time (hour)

Hg(II) in Solution Hg(0) in autoclave

Mg % mg %

0 0 0 0 0

0.5 0.66 27.2 0.00132 0.054

0 1 0.748 30.9 0.00108 0.045

2 0.875 36.1 0.0028 0.115

3 0.942 38.8 0.0044 0.181

4 0.933 38.5 0.005 0.206

Figure 3-8. Acid washing using 0% UNR-100 (water only) at Topt = 80oC.

When 2.5% of UNR-100 was able to wash out approximately 62% of the Hg and

minimal amounts of Hg in vapor were detected. Furthermore, 3% UNR-100 acid washing

72

at 80oC was able to remove 66% of Hg from the carbon into the solution and low amount

of Hg in vapor was exposed as shown in Table 3-8 and Figure 3-9 below.

Table 3-8. Acid washing using 3% UNR-100 at Toptimum = 80oC.

UNR-100

(%)

Acid washing

time (hour)


Mg % mg %

0 0 0 0 0

0.5 1.183 48.8 0.0032 0.13

3 1 1.364 56.2 0.0023 0.09

2 1.475 60.8 0.0026 0.1

3 1.529 63.1 0.0028 0.11

4 1.6 66 0.0021 0.09

Figure 3-9. Acid washing using 3% UNR-100 at Topt = 80oC.

73

When the concentration of UNR-100 was increased to 4% and 5%, the Hg in the

solution were 78% and 84%, respectively. Both of these conditions released small

amounts of Hg in the vapor -- less than 1%. At 6% UNR-100 solution, approximately

88% of Hg was measured in the solution and minimal amount of Hg in the vapor was

detected as shown in Table 3-9 and Figure 3-10.

Table 3-9. Acid washing using 6% UNR-100 at Toptimum = 80oC.

UNR-100

(%)

Acid washing

time (hour)


Mg % mg %

0 0 0 0 0

0.5 1.705 70.3 0.00104 0.043

6 1 1.794 74 0.00074 0.031

2 1.953 80.5 0.00028 0.012

3 2.067 85.2 0.00018 0.007

4 2.142 88.3 0.00021 0.009

74

Figure 3-10. Acid washing using 6% UNR-100 at Topt = 80oC.

At 7% UNR-100, approximately 90% of the Hg from carbon was removed (as

obtained from the previous work when temperature was optimized) and low Hg in the

vapor was detected. When the UNR-100 was increased to 8%, 8.5% and 10%, the Hg

concentration in the solution started declining – 84, 87 and 73% respectively. Thus, from

the experiments, 7% UNR-100 works best for washing the Hg from the carbon. Figure 3-

11 summarizes all different concentration of UNR-100 on the acid washing. Hg in the

solution at 4 hours acid washing and various concentrations of UNR-100 is displayed in

Figure 3-12.

75

Figure 3-11. Acid washing using various concentration of UNR-100 at Topt = 80oC.

Figure 3-12. Four hours acid washing using various concentration of UNR-100 at Topt =

80oC.

76

At T = 70oC, various concentrations of UNR-100 were also investigated as shown

in Figure 3-13 and compared to T = 80oC shown in Figure 3-14. From this comparison,

it showed a consistent trend of optimum UNR-100 concentration on removing Hg from

carbon. Both conditions showed that at 7% concentration of UNR-100, a maximum

amount of Hg was washed out from the carbon into the solution. At 70oC and 7% UNR-

100, approximately 80% of Hg in the solution was found which 10% lower than acid

washing at the same condition at 80oC.

Figure 3-13. Acid washing using various concentration of UNR-100 at T = 70oC.

77

Figure 3-14. Comparison of acid washing using various concentration of UNR-100 at T

= 70oC and 80oC.

Further study on the Au behavior in the acid washing process was also explored in

this work. The ideal acid washing outcome is achieved when Hg is removed during this

process and the Au remains on the carbon.

Synthetically Au-Hg loaded carbon is recapitulated in Table 3-10 below. This

carbon was then used to study the Au behavior in the acid washing using UNR-100

solution.

Table 3-10. Synthetically Au-Hg loaded carbon.

Initial Au in the

Solution (mg)

Initial Hg in the

Solution (mg)

Remained in the

Solution (mg)

Adsorbed

(mg)

Adsorption

(mg/g)

AuCN Au Hg(CN)2 Hg Au Hg Au Hg Au Hg

144.5 127.7 32.2 25.6 4.77 2 122.9 23.6 4.91 0.94

78

When 3% UNR-100 was used for washing the synthetically Au-Hg loaded

carbon, as much as 76% of Hg and only 0.11% of Au were measured in the solution

within 4 hours acid washing process as shown in Table 3-11 and Figure 3-15. From this

result, the selectivity of acid washing to remove only Hg is satisfied since low amounts of

Au were found in the solution. The same trend of the selectivity of acid washing on Hg

will be supported in the next discussion.

Table 3-11. Au and Hg in the solution during 3% UNR-100 acid washing at T = 80oC.

Loaded UNR-100 Acid washing

time (hour)

Hg in Solution Au in Solution

Au+Hg Conc. (%) mg % mg %

0 0 0 0 0

Au 4.91 0.5 1.255 53.2 0.024 0.2

mg/g 3 1 1.547 65.6 0.028 0.23

2 1.725 73.1 0.035 0.29

Hg 0.94 3 1.777 75.3 0.017 0.14

mg/g 4 1.794 76 0.014 0.11

79

Figure 3-15. Acid washing on synthetically Au-Hg loaded carbon using 3% UNR-100 at

80oC.

At 5% UNR-100, approximately 77% of Hg was removed from the syhtnethically

Au-Hg loaded carbon and 0.27% of Au in the solution was detected. Au and Hg in the

solution were tabulated in Table 3-12 and shown in Figure 3-16. The Hg detected in the

vapor remained very low under this condition (only 0.015%). Thus, another strong proof

supporting the previous work that the UNR-100 acid washing technique is very selective

on removing Hg with minimal Au removed from the carbon.

80



80oC.

Table 3-13 and Figure 3-17 summarizes the Au and Hg detection in the solution

when 7% UNR-100 was used to wash the synthetically Au-Hg loaded carbon at 80oC. At

this condition, approximately 83% of Hg was removed from the carbon and only 0.26%


time (h)


Au+Hg Conc. (%) mg % mg %

0 0 0 0 0

Au 4.91 0.5 1.508 63.9 0.055 0.45

mg/g 5 1 1.493 63.3 0.065 0.53

2 1.722 73 0.06 0.49

Hg 0.94 3 1.761 74.6 0.058 0.47

mg/g 4 1.827 77.4 0.034 0.27

81

of Au was detected in the solution. 7% of UNR-100 at 80oC shows the optimum

condition obtained for removing the Hg from the loaded carbon and keeping most of the

Au in the carbon since at 9% UNR-100, less than 80% of Hg and 0.39% of Au were

detected in the solution as tabulated in Table 3-14 and Figure 3-18.



time (h)


Au+Hg Conc. (%) Mg % mg %

0 0 0 0 0

Au 4.91 0.5 1.528 64.7 0.041 0.34

mg/g 7 1 1.761 74.6 0.035 0.29

2 1.905 80.7 0.032 0.26

Hg 0.94 3 1.96 83.1 0.032 0.26

mg/g 4 1.95 82.6 0.031 0.25

82


80oC.



time (h)


Au+Hg Conc. (%) Mg % mg %

0 0 0 0 0

Au 4.91 0.5 1.345 57 0.084 0.68

mg/g 9 1 1.564 66.3 0.062 0.51

2 1.709 72.4 0.047 0.38

Hg 0.94 3 1.812 76.8 0.046 0.38

mg/g 4 1.84 78 0.048 0.39

83


80oC

Figure 3-19, 3-20, and 3-21 summarize the different concentrations of UNR-100

acid washing at 80oC on the removal of Hg and Au, respectively, from the synthetically

Au-Hg loaded carbon. From the graph, it can be inferred that 7% UNR-100 provides the

best removal of Hg from the carbon. Furthermore, it is also clear that acid washing

using UNR-100 is very selective on removing most of the Hg with minimal amount of

Au washed out from the carbon. Thus, using this process, the loss of Au can be

minimized.

84

Figure 3-19. Percent Hg in the solution at 80oC and various concentration of UNR-100

acid washing.

Figure 3-20. Percent Au in the solution at 80oC and various concentration of UNR-100

acid washing.

85

Figure 3-21. Percent Au and Hg in the solution at 80oC and various concentration of

UNR-100 acid washing.

3.2. Comparing acid washing on synthetically Au-Hg loaded carbon using HCl and

UNR-100

HCl, which is known to be beneficial to wash the loaded carbon due to its ability

to remove Ca-Mg from the pores of the carbon, was also explored and studied. The HCl

acid washing was conducted in the autoclave, which was heated by a stirring hotplate.

The total washing solution used was 250 ml with 5% HCl, whereas 2.5 grams of

synthetically Au-Hg loaded carbon was used. The Au-Hg loading during washing is

summarized in Table 3-10. The Hg in the vapor and solution were analyzed using Jerome

431-X Mercury Vapor Analyzer and AAS Varian 55B, respectively.

The behavior of Au and Hg on the HCl washing was compared to UNR-100 acid

washing. The result of this experiment is tabulated in Table 3-15 and Figure 3-22 which

illustrates its performance comapared with 7% UNR-100 performance.

86

Based on the graph, 5% HCl was only able to remove less than 3% of Hg and

almost 1% of Au from the carbon. Furthermore, at 80oC HCl acid washing temperature,

only 0.21% of Hg was detected in the vapor compared to 2.3% with 7% UNR-100.

Table 3-15. Acid washing using 5% HCl at 80oC.

Loaded HCl

(%)

Acid washing

time (hour)

Hg in sol. Au in sol. Hg(0) in autoclave

Carbon Mg % mg % Mg %

0 0 0 0 0 0 0

Au 4.7 0.5 0.155 4.1 0.072 0.58 0.00419 0.11

mg/g 5 1 0.208 5.5 0.074 0.6 0.00811 0.22

2 0.178 4.7 0.091 0.74 0.0117 0.313

Hg 1.5 3 0.165 3.1 0.112 0.91 0.0067 0.177

mg/g 4 0.098 2.61 0.118 0.96 0.0079 0.21

On the other hand, as shown in Table 3-13, 7% UNR-100 at the same condition

was able to wash out approximately 83% of Hg and 0.25% Au from the carbon. Thus, 7%

UNR-100 by far works better on removing Hg from the carbon and less Au in the

solution obtained (approximately four times less than 5% HCl) as shown in Figure 3-22

below.

87

Figure 3-22. Comparison of acid washing using 5% HCl and 7% UNR-100 at 80oC.

3. 3. Column acid washing followed by stripping using Zadra on industrial loaded

carbon

Since the best condition of UNR-100 acid washing in autoclave had been

determined to be 7% UNR-100 at 80oC (pH of 2.25 fresh solution and 2.5 washed

solution), then a semi pilot scale of acid washing was conducted using a column shown in

Figure 3-23. The detail of the column dimension, set up and procedure have been

described in previous chapter in detail.

Figure 3-23. Column acid washing set up.

88

72 Grams of wet industrial loaded carbon (48.6 grams of dried carbon) was acid

washed using 1.2 L of 5% UNR-100 in the column. The solution was recycled during the

process. The initial carbon loading was determined to have 1.5 mg/g Hg and 4.78 mg/g

Au. The flowrate of washing solution was 1.20 L/h pump, while the bed volume of 14.11

was determined. The result of this acid washing is tabulated in Table 3-16.

Table 3-16. Acid washing in column on industrial loaded carbon at 80oC and 5% UNR-

100.

Washing

time (hour)

Hg(II) in solution Hg(II) in

solution (%)

Au in solution Au in

solution (%) mg mg/g mg mg/g

0.5 9.03 0.19 12.4 0.4 0.0082 0.17

1 13.36 0.28 18.3 0.4 0.0082 0.17

2 18.65 0.38 25.6 0.4 0.0082 0.17

4 20.6 0.42 28.3 0.36 0.0074 0.16

6 23.25 0.48 31.9 0.36 0.0074 0.16

As shown in Figure 3-24, within 6 hours of acid washing, approximately 32% of

Hg was removed from the carbon and only 0.16% of Au was detected in the solution. The

Hg in the solution of this process is actually lower than the acid washing in the autoclave

at the same condition. The reason behind this siginificant difference is the bed volume

used and contact time of the acid solution with the loaded carbon.

89

Figure 3-24. Hg and Au in the solution when acid washing using 5% UNR-100 was

performed in a column at 80oC.

The UNR-100 acid washed loaded carbon was then stripped using Zadra process

in an autoclave at various temperatures. The washed loaded carbon ‘head’ concentration

was 1.02 mg/g of Hg and 4.77 mg/g of Au. A 250 ml of stripping solution (1% of NaCN

and 1% of NaOH) and 2.5 grams of washed loaded carbon were used. The Hg and Au in

the solution were measured by AAS Varian 55B and the Hg in vapor was analyzed by

Jerome 431-X Mercury Vapor Analyzer. No pressure was added during the process, the

pressure built in the autoclave was due to the solution heated at 80, 100, 120 and 140oC

(0, 9, 20, and 43 psig, respectively).

Table 3-17 and Figure 3-25 show the presence of Hg and Au (during the Zadra

stripping at 80oC) in the solution which were about 90% and 36% respectively. At this

temperature, small amounts of Hg vapor were detected (less than 1%) as shown in Figure

3-26.

90

Table 3-17. Au and Hg stripping using Zadra at 80oC on UNR-100 washed loaded

carbon.

Stripping

time (hour)

Hg(II) in solution Au(I) eluted

Mg mg/g % Mg mg/g %

0 0 0 0 0 0 0

0.5 1.55 0.62 60.78 3.31 1.32 27.6

1 2.02 0.81 79.22 3.39 1.36 28.5

2 2.07 0.83 81.18 3.77 1.51 31.6

3 2.26 0.9 88.63 4.07 1.63 34.1

4 2.35 0.94 92.16 4.27 1.71 35.8

Some of Hg vapor was also condensed on the surface of the autoclave. Hg vapor

condensed was volatilized by heating up the autoclave at 250oC and taking the sample

until the Hg vapor reading approached zero. Approximately 0.01 mg/g (1%) of Hg was

detected from this process. Thus, the total Hg reduced into its initial state was

approximately 1.5%.

91

Figure 3-25. Au and Hg presence in the solution – Zadra at 80oC after UNR-100 acid

washing.

Figure 3-26. Hg(0) detected in the vapor – Zadra at 80oC after UNR-100 acid washing.

The Hg and Au remaining on the carbon after stripping was also analyzed. The

stripped carbon was digested by Hg digestion developed by the UNR group described

previously and then analyzed using AAS for both Hg and Au. The digested carbon was

burnt in a furnace at 750oC for about 2 hours. Finally, it was dissolved in aqua regia

92

solution before analyzing its Au content by AAS. From this procedure, the mass balance

of Au and Hg was implemented as tabulated in Table 3-18. The total Au and Hg from the

mass balance is very close to the actual ‘head’ concentration of Au and Hg. Thus, the

vapor measurement using Jerome 431-X and AAS Varian 55B are relatively accurate.

Table 3-18. The mass balance of Au and Hg for the whole process (UNR-100 acid

washing and Zadra process).

Au (%) Hg (%) Au (mg) Hg (mg)

UNR-washed 0.2 31.8 0.36 23.25

Stripped 35.7 62.4 4.27 2.35

Digested 11.5 4.8 1.375 0.1825

Ashed 52.6 N/A 6.275 N/A

Vapor N/A 1 N/A 0.036

Total 100 100

When Zadra stripping was performed at 100oC for 4 hours, approximately 74% of

the Hg and 63% of the Au were stripped into solution as tabulated in Table 3-19. Figure

3-27 show the kinetic of Hg and Au stripping at this condition. Thus, the rate of stripping

of Au is approximately 20% higher than Au stripping at 80oC. The Hg in the vapor was

detected to be about 1.7% within 4 hours of stripping at 100oC (which was about four

times higher than the stripping at 80oC).

93


carbon.

Stripping

time (hour)


Mg mg/g % mg mg/g %

0 0 0 0 0 0 0

0.5 1.12 0.45 43.92 4.04 1.61 38.2

1 1.41 0.56 55.29 5.02 2.01 47.7

2 1.61 0.64 63.14 5.8 2.32 55.1

3 1.76 0.7 69.02 6.47 2.59 61.5

4 1.88 0.75 73.73 6.68 2.67 63.4


washing.

94


As mentioned previously, some of Hg vapor was also condensed on the surface of

the autoclave. Hg vapor condensed was re-volatilized by heating the autoclave to 250oC

and taking the sample until the Hg vapor reading was undetectable. Approximately 0.048

mg/g (4.7%) of Hg was detected from this process. Thus, the total Hg reduced into its

ground state was approximately 6.5%. The residue carbon was also digested, burned in a

furnace, and analyzed to get the mass balance of Hg and Au.

Zadra stripping was also performed at 120oC for 4 hours, approximately 37% of

Hg and 85% of Au were stripped into solution as tabulated in Table 3-20. Figure 3-29

show the kinetics of Hg and Au stripping under this condition. Thus, the rate of stripping

of Au is even higher than Au stripping at 100oC.

95


carbon.

Stripping

time (hour)


mg mg/g % mg mg/g %

0 0 0 0 0 0 0

0.5 1.47 0.588 57.65 6.78 2.71 62

1 1.69 0.68 66.27 8.34 3.34 76.4

2 1.68 0.67 65.88 8.88 3.55 81.2

3 1.46 0.584 57.25 9.06 3.62 82.8

4 0.96 0.383 37.64 9.25 3.7 84.7


washing.

96

Based on Figure 3-29, the concentration of Hg in the solution is getting lower as

a function of stripping time. This phenomenon happened because as the stripping time

increased the amount of reduced Hg going into vapor also increased as shown in Figure

3-30. Approximately 27% of Hg in the vapor was detected within 4 hours of stripping at

120oC.


At higher temperature, more Hg was vaporized, thus more condensed Hg was

obtained. When the autoclave was cleaned from condensed Hg, approximately 0.12 mg/g

of Hg vapor was detected (11.37%) on the autoclave at 250oC. The stripped carbon was

digested and then bruned to analyze the ‘tail’ Hg and Au concentration and to obtain

mass balance of the metals. About 10.5% of Hg remained on the stripped carbon.

Finally, the Zadra was conducted at 140oC. At even higher temperature, there was

only a small quantity of Hg detected in the solution (2.4%) within 4 hours stripping

whereas approximately 83% of Au found in the solution shown in Table 3-21 and Figure

97

3-31. As expected, the Hg vapor detected in the vapor was about 54% in 4 hours of

stripping, as illustrated in Figure 3-32.


carbon.

Stripping

time (hour)


mg mg/g % mg mg/g %

0 0 0 0 0 0 0

0.5 1.25 0.5 49.02 4.74 1.9 44.3

1 0.98 0.39 38.43 8.89 3.56 83

2 0.067 0.027 2.623 9.05 3.62 84.4

3 0.061 0.024 2.392 8.71 3.48 81.1

4 0.061 0.024 2.392 8.93 3.57 83.2


washing.

98


Based on Figure 3-22, the longer the stripping the lower the quantity of Hg in the

vapor was detected (this because, the longer the stripping the higher the Hg vapor

produced and thus the higher the condensed Hg generated). When the autoclave was

cleaned from the condensed Hg at 250oC, there was approximately 0.5 mg/g (49%) of Hg

detected. The stripped carbon was digested and burned in a furnace to analyze the

residual Hg and Au in the carbon after this process, so a mass balance could be generated.

99

Figure 3-33. Hg in the solution – Zadra after prewashed (UNR-100 acid washing)

industrial loaded carbon at various different temperatures.

Based on the graph, the stripping rate of Hg can be best conducted at lower

temperature Figure 3-33. On the other hand, stripping of Au can be best performed at

higher temperature, 140oC, as shown in Figure 3-34. However, at higher temperature, the

higher Hg vapor generated which can increase exposure to the environment as well as

employees. The ideal condition for this stripping would be high stripping rate of Au with

low reduction of Hg into its vapor state.

100

Figure 3-34. Au in the solution – Zadra after prewashed (UNR-100 acid washing)

industrial loaded carbon at various different temperatures.

Based on these results, it can be concluded that Hg(CN)2 can be reduced into its

elemental state, Hg(0), in NaCN and NaOH solution. Based on a reported study from S.

Fujita group, Hg(CN)2 can be reduced just in deaerated water and by CO2 by pulse

radiolysis technology. Furthermore, the NaCN used in the stripping process is not a stable

compound. As stated in Chapter 1, at high temperature, NaCN is easily oxidized by

oxygen and hydrolyzed. The mechanism of reaction is shown as follow:

Hydrogen is a reducing agent, and the presence of hydrogen helps the reduction of

Hg(II) to Hg(0). The reduction of Hg is even higher at higher temperature. At 80oC

stripping, less than 2% of Hg was reduced into Hg(0) whereas almost all Hg was reduced

101

at 140oC. Based on this result, the reduction of Hg is enhanced significantly by increasing

the temperature. Thus, any Au stripping process on carbon with a high Hg content at

temperature higher than 100oC will result in emission of Hg vapor, and consequently

jeopardize the environment and workers at the site.

3. 4. Stripping using Zadra on industrial loaded carbon

Zadra stripping without prior acid washed on industrial loaded carbon was

conducted to investigate the behavior of Hg stripping. The stripping was performed in a

2L autoclave and 2.5 grams of industrial carbon was used. 250 ml of stripping solution

that contained 1% of NaCN and 1% NaOH was utilized. Each stripping of the loaded

carbon was done at various temperatures. The head concentration on the carbon was 1.68

mg/g Hg and 4.4 mg/g Au.

Table 3-22 and Figure 3-35 show the percent stripped mercury at T = 80oC as a

function of time. Within 4 hours, approximately 93% of Hg and about 42% of Au were

measured in the solution. The trend is similar as the previous stripping experiments

showed that at 80oC low concentrations of Au and relatively high concentrations of Hg

(>90%) were stripped into the solution.

102

Table 3-22. Au and Hg stripping using Zadra at 80oC on industrial loaded carbon.

Stripping

time (hour)

Hg(II) in solution Au(I) in solution

mg mg/g % mg mg/g %

0 0 0 0 0 0 0

0.5 2.344 0.9376 55.81 2.77 1.11 25.21

1 2.782 1.1128 66.24 2.84 1.14 25.82

2 3.198 1.2792 76.14 3.86 1.54 35.07

3 3.676 1.4704 87.52 4.28 1.71 38.88

4 3.928 1.5712 93.52 4.64 1.86 42.21

Figure 3-35. Au and Hg presence in the solution – Zadra at 80oC.

At relatively low temperature, there was only a minimal amount of Hg vapor

produced as shown in Figure 3-36, which has the same behavior as Zadra on UNR-100

prewashed loaded carbon. The Jerome 431X mercury vapor analyzer detected

approximately 0.2% of Hg vapor at this temperature. It was also found that

approximately 0.06 mg/g (3.8%) of Hg vapor was condensed on the surface of the

103

autoclave. Thus, approximately 4% of Hg was reduced into Hg(0) during this process. By

analyzing the residual carbon (after elution), only 3% of the Hg is left on the carbon after

4 hours stripping at 80oC.

Figure 3-36. Hg(0) detected in the vapor - Pressurized Zadra at 80oC.

Zadra on industrial loaded carbon was also conducted at 100oC. From Table 3-23

and Figure 3-37, it can be seen that the Hg in the solution decreased between 3 and 4

hours stripping as a function of time. This phenomenon happened because the longer that

stripping at higher temperature, the more Hg in the solution was reduced as a vapor (see

the vapor of Hg produced below). Approximately, 65% of Au was detected in the

solution within 4 hours of stripping.

104


Stripping

time (hour)


mg mg/g % Mg mg/g %

0 0 0 0 0 0 0

0.5 3.381 1.35 80.50 4.95 1.98 45.03

1 3.809 1.52 90.69 6.20 2.48 56.35

2 3.814 1.53 90.81 7.22 2.89 65.66

3 3.855 1.54 91.79 7.47 2.99 67.94

4 2.653 1.06 63.17 7.23 2.89 65.72


Figure 3-38 shows the behavior of Hg vapor on stripping at 100oC, even though it

seems low Hg vapor was detected, it was found that much of Hg vapor was condensed on

the surface of the autoclave. When the autoclave was cleaned by heating it to 250oC,

105

there was approximately 0.57 mg/g (34% of Hg) detected. Thus, at 100oC stripping, there

is a significant amount of Hg vapor produced. The higher the Hg vapor produced, the

higher the risk of Hg exposure. The residual carbon Hg content (after elution) was also

analyzed by digesting it using UNR developed technology to measure the ‘tail’ Hg

concentration in the carbon. It was found that only 2% of the Hg was left in the carbon

after 4 hours stripping at 100oC.

Figure 3-38. Hg(0) detected in the vapor – Zadra at 100oC.

Based on this result, stripping at 100oC provides a better Au stripping rate than

stripping at 80oC. Approximately 20% more Au found in the solution at 100oC compared

to 80oC stripping. However, the higher Au stripping comes with cost of higher Hg vapor

produced. Thus, this condition is not the ideal stripping process. The ideal condition

aimed for this project is high Au stripping and low Hg vapor produced or high Hg

stripping thus there will be less Hg released during the carbon regeneration.

When the stripping temperature was increased to 120oC, the Hg in the solution

increased at some point and then it decreased as a function of stripping time as shown in

106

Table 3-24. Whereas for Au, the stripping rate is higher as a function of stripping time.

Furthermore, the concentration of Au in this process found in the solution was even

higher compared to 100oC stripping. Within 4 hours stripping more than 80% of the Au

was stripped from the carbon as shown in Figure 3-39.


Stripping

time (hour)


mg mg/g % mg mg/g %

0 0 0 0 0 0 0

0.5 1.931 0.77 45.35 5.24 2.10 47.63

1 2.903 1.16 68.17 7.68 3.07 69.83

2 3.248 1.30 76.28 8.88 3.55 80.72

3 2.273 0.91 53.38 9.11 3.64 82.81

4 1.089 0.44 25.59 8.95 3.58 81.38


107

At 120oC stripping, there was considerably more Hg vapor produced. Figure 3-40

shows the Hg vapor detected by the Hg vapor analyzer. Within 4 hours of stripping,

approximately 22% of Hg vapor was detected. The stripped carbon was digested by using

the procedure described in previous chapter. It was found that only about 4% of Hg

remained on the carbon. From the mass balance, approximately 1.3 mg/g (75%) of

mercury was reduced and condensed on the autoclave walls. Thus, at 120oC, higher Au

was stripped with even higher Hg released as vapor. The total Hg vapor produced was

more than 95% which is not an environmentally friendly result.


Lastly, Zadra at 140oC was also performed. At this temperature, almost all of the

Hg vapor was reduced from its vapor within 4 hours of stripping as seen in Table 3-25

and Figure 4-41 below. On the other hand, more than 80% of the Au was detected in the

solution within 4 hours stripping at 140oC. Thus, the stripping at 140oC strips as much Au

as stripping at 120oC. However, both of the stripping processes release almost all Hg as

vapor.

108


Stripping

time (hour)


Mg mg/g % mg mg/g %

0 0 0 0 0 0 0

0.5 4.036 1.61 96.10 5.54 2.22 50.36

1 4.001 1.60 95.26 8.06 3.22 73.24

2 0.064 0.03 1.52 9.15 3.66 83.21

3 0.003 0.00 0.07 9.08 3.63 82.56

4 0.0012 0.00 0.03 9.09 3.64 82.64


Figure 3-42 shows the rate of Hg vapor emission. As can be seen, it shows that

within 4 hours of stripping at 140oC, there was not much Hg vapor detected. This

happened because almost all of the Hg vapor was condensed on the autoclave early in the

process. The digestion of carbon yielded only approximately 3% of Hg remaining on the

109

stripped carbon. Thus, there was more than 95% of the Hg vapor condensed on the inner

surface of the autoclave.


Figure 3-43 summarizes the stripping rate of Hg using Zadra at various

temperatures. Based on the graph, the stripping of Hg was best performed at low

temperature (80oC). Furthermore, at 80oC, an insignificant amount of mercury was

reduced. At higher temperature, the solubility of Hg(0) in the solution was less which can

be explained using Le Chatelier’s principle. It states that if placed under stress, an

equilibrium system shifts in a way that relieves the stress. In this research, the “stress” is

the addition of heat [116]. Thus, the higher the stripping temperature, the higher the Hg

vapor produced.

110

Figure 3-43. Summary of stripped Hg at various different stripping temperatures.

On the other hand, Figure 3-44 shows the stripping rate of Au using Zadra at

different temperatures. The graph tells that the striping of Au was best conducted at

higher temperatures, 120oC and 140oC. However, the drawback when stripping

performed at higher temperature is the high emission of Hg vapor. This high emission of

Hg is one of the issues of stripping at higher temperature because the more Hg vapor

produced, the higher risk of exposure of worker and environment from the Hg toxicity.

Figure 3-44. Summary of stripped Au at various different stripping temperatures.

111

Furthermore, the Hg vapor detected by Jerome 431X is provided in Figure 3-45

below. At 80oC, it shows low Hg vapor detected and the condensed Hg on the inner

surface of the autoclave was low as well. Thus, a small amount of Hg was reduced at low

temperature. However at 100oC, even though Figure 3-45 shows a low Hg vapor

detected but when the autoclave was cleaned, about 34% of the Hg was obtained. At

120oC, more Hg vapor was detected from Figure 4-45, and when the autoclave was

heated at 250oC to measure the condensed Hg, as much as 75% of the Hg vapor was

recovered. Eventually, at 140oC, Figure 4-45 shows that there was minimal amount of

Hg vapor detected, but it was found that more than 95% of Hg vapor was recovered

when the autoclave was heated up to 250oC to remove the condensed Hg.

Figure 3-45. Summary of reduced mercury detected by Jerome vapor analyzer at various

different stripping temperatures.

Because we want to get a high stripping of Au with minimal amount of reduced

Hg/maximal stripping Hg so there is minimal Hg released during the carbon regeneration,

then another stripping process was investigated to achieve this goal.

112

3.5. Alcohol alkaline stripping on industrial loaded carbon

Based on the previous study by H. J. Heinen et. al, alcohol alkaline solution

increases the stripping rate of gold. Within 6 hours of stripping using alcohol alkaline

solution, almost all of Au was desorbed/stripped from the carbon. For that reason, alcohol

alkaline solution was investigated in regards to Hg behavior.

2.5 grams of industrial loaded carbon with 4.26 mg of mercury (1.7 mg Hg/gram

of C) and approximately 11 mg of gold (4.4 mg Au/gram of C) was used in this study.

The industrial loaded carbon was stripped in the stainless steel autoclave. 250 ml of

solution contained 20% ethanol, 1% NaCN and 1% NaOH was used to strip the carbon at

80oC.

Table 3-26 and Figure 3-46 summarize the Au and Hg detected in the solution

within 4 hours of stripping using alcohol alkaline solution. About 87% of the Hg and

95% of the Au were detected in the solution. Thus, the addition of alcohol increases the

stripping rate of Au significantly. As we recall form previous work, conventional Zadra

at 80oC was able to strip only about 42% of the Au which was about 40% lower than

alcohol alkaline stripping. Another advantage of this modified Zadra stripping is it can

also strip 87% of Hg with only a small amount of Hg left on the carbon.

113

Table 3-26. Au and Hg stripping using alcohol alkaline solution at 80oC on industrial

loaded carbon.

Stripping

time (hour)


mg mg/g % Mg mg/g %

0 0 0 0 0 0 0

0.5 3.466 1.386 81.397 6.743 2.697 61.415

1 3.478 1.391 81.671 7.975 3.190 72.637

2.5 3.363 1.345 78.970 10.372 4.149 94.466

3 3.639 1.456 85.457 9.946 3.978 90.585

4 3.683 1.473 86.499 10.432 4.173 95.011

Figure 3-46. Au and Hg presence in the solution – alcohol alkaline solution at 80oC.

The reduction of Hg was also investigated and it showed that at relatively low

temperature, insignificant amount of Hg vapor was detected as shown in Figure 3-47

below. The cleaning of the autoclave from the condensed alcohol recovered

114

approximately 0.0731 mg/g of Hg (4.3%). The digestion of stripped carbon was

performed, and approximately 4% of Hg was found left in the carbon. Thus, the total

reduced Hg was approximately 9% which was 5% higher than conventional Zadra

stripping.

Figure 3-47. Hg(0) detected in the vapor - alcohol alkaline solution at 80oC.

Since the stripping efficiency of Hg and Au at this temperature was already high,

high temperature alcohol alkaline stripping experiments was not investigated.

3.6. Acid washing using mixture of UNR-100 and alcohol on industrial loaded

carbon in an autoclave

Based on previous work, it was known that 7% UNR-100 was selectively

effective to remove Hg selectively at 80oC washing in an autoclave. The process

mentioned was able to remove approximately 83% of the Hg and little Au was stripped in

the solution (< 1%). On other hand, alcohol alkaline stripping was able to strip 86% of

Hg and 95% of Au. Based on these results, a mixture of UNR-100 and alcohol was used

115

to wash the industrial carbon in the autoclave. The expectation of this process is for

higher Hg to be removed while leaving the Au in the carbon for further stripping process.

2.5 grams of industrial loaded carbon containing 4.26 mg of mercury (1.7 g

Hg/gram of C) and 11 mg of gold (4.4 mg Au/gram of C). The carbon was washed in the

stainless steel autoclave using 250 ml of solution containing 7% acetic acid and 20% of

ethanol at 80oC.

Based on Table 3-27 and the graph below (Figure 3-48), a mixture of acetic acid

and ethanol solution showed that it was able to remove up to 97% of the Hg

(approximately 10% higher than just regular UNR-100 washing) and only about 1.4%

of the total gold (about 1% higher than regular UNR-100 washing and 0.4% higher than

HCl washing). Therefore, by using UNR-100 and alcohol solution, almost all of the Hg

was washed from the carbon. The only disadvantage of this process is higher Au found

in the solution, although it was still relatively low (1.4% of Au in the solution).

Compared to HCl acid washing, the gold concentration in the solution detected using

UNR-100/alcohol washing is 0.5% higher. However, UNR-100/alcohol washing is

superior in regards to mercury washing.

116

Table 3-27. UNR-100/alcohol washing on industrial loaded carbon at 80oC.

Stripping

time (hour)


mg mg/g % mg mg/g %

0 0 0 0 0 0.000 0.000

0.5 2.233 0.893 52.445 0.208 0.083 2.110

1 2.965 1.186 69.628 0.175 0.070 1.773

2 3.126 1.250 73.405 0.162 0.065 1.649

3 3.925 1.570 92.179 0.144 0.058 1.466

4 4.172 1.669 97.977 0.149 0.060 1.514

Figure 3-48. Hg and Au in the solution – UNR-100/alcohol washing on industrial loaded

carbon at 80oC.

The Hg vapor was also studied from this washing, as shown in Figure 3-49.

Within 4 hours of stripping, less than 2% of Hg vapor was detected.

117

Figure 3-49. Hg(0) detected in the vapor – UNR-100/alcohol washing on industrial

loaded carbon at 80oC.

Further optimization of this acid/alcohol washing needs to be conducted in order

to lower the Au levels found in the solution. This washing technique offers a promising

result, so it can be used to separate the Hg from the carbon prior to the Au stripping

process. Additionally, once the UNR 100 acid washing and Au stripping process is

conducted, the loaded carbon will contain low amounts of Hg, thus this process can

minimize the reduction of Hg as well as the contamination of stripped solution with Hg.

In the long term, it will significantly reduce the environmental pollution and employee

exposure of Hg.

3.7. Washing using alcohol on industrial loaded carbon in an autoclave

Since the mixture of UNR-100 and ethanol was able to wash out 97% Hg from

the carbon and only little Au, the industrial loaded carbon was washed using only

ethanol. 2.5 grams of industrial loaded carbon has 4.26 mg of mercury (1.7 g Hg/gram of

C) and 11 mg of gold (4.4 mg Au/gram of C). The carbon was washed in the stainless

118

steel autoclave. 250 ml of solution contains 20% of ethanol was used to wash the carbon

at 80oC.

The results of the experiment summarizes in Table 3-28. As shown, 20% ethanol

solution was able to wash approximately 50% of the Hg within 4 hours in the autoclave,

as reiterated in Figure 3-50. It means 20% ethanol washed out 40% less Hg compared to

mixture of UNR-100 and ethanol washing. On the other hand, the alcohol solution

removed about 5% of Au from the carbon. This result is not promising since significant

amount of Au were stripped, which means more Au could be lost to tailings.

Table 3-28. Alcohol washing on industrial loaded carbon at 80oC.

Stripping

time (hour)


mg mg/g % Mg mg/g %

0 0 0 0 0 0 0

0.5 1.726 0.69 41.10 0.357 0.143 3.247

1 1.457 0.58 34.69 0.136 0.054 1.234

2 2.212 0.88 52.67 0.266 0.106 2.416

3 2.337 0.93 55.64 0.372 0.149 3.384

4 2.138 0.86 50.90 0.507 0.203 4.606

119

Figure 3-50. Hg and Au in the solution –alcohol washing on industrial loaded carbon at 80oC.

Based on Figure 3.51, the alcohol washing solution did not produce significant

amount of Hg vapor. Based on the graph, Hg vapor detected by Jerome 431-X Mercury

Vapor Analyzer was only less than 0.3%.

Figure 3-51. Hg(0) detected in the vapor –alcohol washing on industrial loaded carbon at 80oC.

120

Objective of the study

In gold ore processing, activated carbon is often used in carbon in column (CIC)

or carbon in leach (CIL) to adsorb the gold. Once adsorbed, the gold has to be strip from

the carbon by a process called stripping. However, prior to the stripping process, acid

washing is often performed to increase the stripping efficiency.

Once the gold is stripped, the pregnant solution (solution that is rich of gold) is

fed into electrowinning to obtain the gold while the carbon has to be regenerated in a kiln

prior recycled back to CIL or CIC. On the other hand, the product of the electrowinning

often contains mercury has to be removed by retorting. Removal of mercury is crucial

prior to stripping so that not only a minimal amount of mercury is left in the carbon prior

to be sent to the carbon regeneration kiln, but also a minimal amount of mercury is

stripped in the solution to be electrowon as summarized in Figure 3-52. By doing the

acid washing that selectively removes mercury from the carbon prior to the stripping

process, worker exposure to mercury in workplace can be minimized and this study

provides scientific evidence of the volatilization of mercury when process temperatures

exceed 100oC.

121

Figure 3-52. Gold ore processing flow diagram [119].

122

CHAPTER 4

CONCLUSION AND FUTURE WORK

4.1 Conclusion

UNR-100 acid washing of loaded carbon to remove mercury prior to the stripping

process is best conducted at 80oC. At temperatures higher than 100oC, significant amount

of mercury is reduced and volatilized into its elemental state, vaporized and then

condensed. 7% UNR-100 was optimized and acid washing in an autoclave at 80oC was

the best condition found, and the experiment was able to remove 90% of the mercury

from the carbon. Furthermore, the UNR-100 acid washing is very selective, it only

removes mercury and leaves the gold on the carbon. Thus, acid washing prior to gold

stripping process is a promising technique to minimize the mercury reduction in a

conventional Zadra which is typically conducted at >100oC.

Washing of loaded carbon with just water at 80oC was able to remove

approximately 40% of the mercury with only 0.2% of mercury reporting in the vapor.

UNR-100 acid washing in an autoclave offers better mercury removal (45% higher) as

compared with UNR-100 acid washing using a column. The bed volume and contact

volume of washing solution to the carbon play an important role here.

By comparing HCl and UNR-100 acid washing, UNR-100 is superior in washing

the mercury and keeping the gold on the carbon (less than 0.12% of gold was detected

in the solution). HCl washing was also able to strip almost 1% of the gold and 3% of the

mercury.

123

When 20% of alcohol is added into the UNR-100 solution, it is able to remove 97%

of mercury and 1.5% of gold. The addition of alcohol in UNR-100 acid washing not

only increased the removal of mercury from the carbon (almost all of the mercury was

in the solution within 4 hours of alcohol/UNR-100 washing at 80oC in an autoclave)

but also slightly increase the gold desorption during acid washing by 1.3% compared to

UNR-100 only washing. Furthermore, in a conventional Zadra stripping process,

mercury stripping was best conducted at 80oC, so that the reduction of mercury can be

minimized. However, stripping at this temperature is not able to strip most of gold from

the carbon (only 42% of gold can be stripped). The gold stripping is best performed at

140oC using conventional Zadra process, where 90% gold can be stripped. However, at

this temperature, almost all of the mercury on the carbon will be reduced and vaporized.

Thus, a new technique to improve the stripping of gold efficiency and minimize the

reduction of mercury is achieved by adding alcohol in the cyanide alkaline solution. By

adding 20% alcohol into the cyanide/alkaline solution, gold stripping can be increased

significantly (95% of gold in the solution detected) at 80oC stripping. Additionally, by

stripping at 80oC stripping, 87% of mercury can also be stripped from the carbon into the

solution with only about 1.5% of the mercury being reduced. Hence, a very small amount

of mercury is left in the carbon, thus minimizing the mercury pollution during the carbon

regeneration process. In conclusion, acid washing with UNR 100 at 80oC, removes up

to 90% of the Hg, nearly the same Ca & Mg salts as compared with the industry

standard HCl acid washing and 74% less gold (0.25% Au v 0.96%).

124

4.2 Future work

Since the mixture of alcohol/UNR-100 is able to selectively strip mercury, this type

of acid washing needs to be optimized. Different concentrations of alcohol, washing

temperature and time need to be performed so that the higher mercury removal can be

achieved while minimizing the desorption of gold.

Besides removing mercury, acid washing is also beneficial in removing the calcium

and magnesium (and their salts) contained in the carbon. The calcium and magnesium

balance for the washing industrial carbon by HCl, UNR-100, and mixture of UNR-

100/alcohol needs to be determined. By comparing different solutions, optimization of

calcium and magnesium removal can be determined.

Furthermore, investigating the feasibility of the acid solution and/or mixture of

acid-alcohol solution to be recycled. Thus, it will increase the economic visibility of the

processes. Moreover, determining a mercury precipitation technique from the washing

solution will be a great approach to reach the objective of this project, i.e. minimizing

mercury pollution in gold ore processing.

Experimentation with washing of loaded carbon with water at 80oC showed a Hg

removal of 40%. Further experiments are recommended to follow the water wash with

UNR 100 and UNR/alcohol combinations to optimize the Hg removal while minimizing

the gold removal. Additionally, optimization of column washing (bed volume, washing

time, temperature) is recommended to achieve comparable result as washing in an

autoclave.

Further study on identifying the Hg species in the vapor during the stripping process

is also recommended. Based on the results of this study, there is tendency that the

125

mercury detected by Jerome 431-X is not only from Hg(0), it can be other species. This

identification can be done by connecting the vapor outlet of the autoclave to the gas

chromatography mass spectroscopy (GC-MS). From GC-MS, molecular weight of the

species can be determined.

Acid washing might offer benefits in washing the precipitate formed by Merril

Crowe. Thus, acid washing before or after precipitation should be investigated as well.

Eventually, economic analysis on this project by tracking the reagents used will be

a good study to determine whether the processes are economical.

126

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APPENDIX

Calculating the weight of mercury from Jerome 431-X reading

1 ml of saturated Hg vapor at known temperature is injected into Jerome 431-X.

The sample is processed and the reading displayed. For instance, at 20oC, mercury vapor

saturated concentration is 13.2 mg/m3, the instrument will show the concentration of

0.129 to 0.174 (which is the mixed concentration of mercury vapor and air) with average

of 0.151 mg/m3. Table A-1 displays the readout of mercury vapor at various

temperatures. Table A-2 shows the saturated mercury vapor at various temperatures [111].

Table A-1. Mercury vapor at various temperatures [111].

Temperature (oC) Digital meter response

16 0.091 to 0.123

17 0.100 to 0.135

18 0.108 to 0.146

19 0.118 to 0.159

20 0.129 to 0.174

21 0.138 to 0.187

22 0.151 to 0.204

23 0.164 to 0.222

24 0.177 to 0.240

138

Table A-2. Concentration of saturated mercury vapor at different temperatures.

Temperature (oC) Hg concentration (mg/m3)

0 2.180

10 5.880

16 10.082

17 10.972

18 11.935

19 12.974

20 14.094

21 15.303

22 16.606

23 18.009

24 19.520

30 31.287

40 65.875

50 132.212

By combining the information from both tables, a relationship between the

reading of Jerome 431-X and an actual mercury concentration can be determined. Curve

1 and 2 shown in Figure A-1 are the display reading by the Jerome 431-X whereas curve

3 is the actual Hg concentration at various temperatures [118].

139

Figure A-1. Jerome 431-X display and actual curves.

Example:

Known parameters from experiment:

Tautoclave = 139.5oC

Pressure = 43.0 psi

Vsample = 0.25 ml

Vvapor in the autoclave = Vautoclave – Vsolution = 2 – 0.25 = 1.75 L

Display on Jerome 431-X = 0.210 mg/m3

Finding the actual weight of mercury

From Figure A-2, when the reading shown 0.210 mg/m3 (from left y-ordinate), if the

horizontal line is taken to the right, then 19.5 mg/m3 of actual mercury is obtained

(from right y-ordinate).

1

2

3

140

Since the instrument is based on 1 ml of sample, thus if only 0.25 ml sample taken,

then the value of reading should be multiplied by 4. Thus, the actual mercury

concentration is 4 × 19.5 = 78 mg/m3.

The pressure factor = 43+14.7

14.7= 3.93

Vapor volume = 1.75× 10-3 m3.

Thus, weight of Hg = 78 × 1.75× 10-3 × 3.93 = 0.54 mg of Hg

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