Chemical Engineering Journal 264 (2015) 425–436

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Mineral carbonation of red gypsum via pH-swing process: Effect of CO2

pressure on the efficiency and products characteristics

http://dx.doi.org/10.1016/j.cej.2014.11.1251385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: asadullah@salam.uitm.edu.my (M. Asadullah).

Amin Azdarpour a,b, Mohammad Asadullah b,⇑, Erfan Mohammadian b, Radzuan Junin a, Hossein Hamidi b,Muhammad Manan a, Ahmad Rafizan Mohamad Daud b

a Department of Petroleum Engineering, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia 81310, UTM, Johor, Malaysiab Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia

h i g h l i g h t s

� Red gypsum can be used as a raw material for sequestering CO2 by mineralization.� Ca extraction as the primary step of pH swing process affects the carbonation efficiency.� A direct relationship between CO2 pressure and carbonation efficiency is observed.� High purity calcium carbonate (CaCO3) is produced.

a r t i c l e i n f o

Article history:Received 22 September 2014Received in revised form 24 November 2014Accepted 25 November 2014Available online 1 December 2014

Keywords:Carbon capture and storageMineral carbonationpH swingRed gypsumCalcium carbonate

a b s t r a c t

The objective of this study was to investigate the feasibility of indirect carbonation of red gypsumthrough pH swing process. The effect of CO2 pressure as one of the major controlling parameters in car-bonation process was investigated in details. The dissolution of red gypsum was performed using 2 MH2SO4 at 70 �C for 60 min reaction time. The pH was regulated using NH4OH, which also precipitatedout the impurities from the solution. Finally, the calcium rich solution was poured in an autoclave reactorand CO2 was injected into the reactor for carbonate precipitation. The experimental results showed thatthe Fe, the main impurity can be effectively separated prior to carbonation stage (92–95%). At low pH theFe was separated while at high pH Ca was precipitated as carbonate. The carbonation efficiency wasdirectly affected by the CO2 pressure and the maximum efficiency (100%) was achieved when 8 barCO2 pressure was used. In addition, carbonation experiments resulted in CaCO3 production in the formof calcite, aragonite, and vaterite with 98% purity.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Human beings have had an unprecedented role in controllingthe global environment over the past century. Civilization, mod-ernization and industrialization of countries have been changingthe life style environmental conditions on a day-by-day basis.Undoubtedly, the energy plays a vital role to maintain that lifestyle. CO2 is the result of the largest single anthropogenic productof the modern era due to excessive fossil fuel burning for energy.Due to the high cost of other energy sources, societal pressureand the established infrastructure of countries, fossil fuels willremain as the world’s primary energy sources for the near future.

Fossil fuels are the main energy source in the world because oftheir availability and affordability in providing energy. Humanbeings are very dependent on fossil fuels, and availability of theseenergy sources provides prosperity and security for them. Accord-ing to the Emission Database for Global Atmospheric Research,excessive fossil fuel consumption has increased the global CO2

emission to 33.4 billion tonnes in 2011, which is 48% more thanthat of two decades ago. Over the past century, atmospheric CO2

level has been increased more than 39%, from 280 ppm duringpre-industrial time to the record high level of 400 ppm in May2013 with a corresponding increasing of global surface tempera-ture of about 0.8 �C [1–11].

Capturing the generated CO2 from the atmosphere and thenlong-term storage of it is called carbon capture and storage(CCS). The basic steps of CCS methods consist of CO2 capture,transportation to the sites, and finally CO2 storage. The CCS

426 A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436

methods are the best viable option for reducing atmospheric CO2

concentration as a long-term technology to minimize the impactsof CO2 on climate change. In fact, CCS can contribute up to15–55% of the cumulative global climate change mitigation effortby 2100 [12–15]. The CO2 mineralization, or mineral carbonation,is an artificial rock weathering and was first proposed by Seifritzin 1990, whereas natural rock weathering is a geological time-scaleprocess [16–19]. The energy state of mineral carbonate is60–180 kJ/mol lower than that of CO2 (400 kJ/mol). Therefore,mineral carbonation provides a permanent and leakage-free CO2

disposal, as the carbonate produced is environmentally benignand stable [20,21].

Mineral carbonation has the potential to sequester billions oftonnes of CO2; however, there are several technical challenges thatneed to be addressed carefully for a widespread application of thistechnology. The effective development for the application of theproduct produced by mineral carbonation could help make thistechnology economically viable and facilitate its deployment.Based on literature, direct mineral carbonation suffers from ther-modynamic limitations, which results in low overall carbonationefficiency [22–25]. The extraction of reactive ions from mineralsin the first step and then carbonation in the following steps, as aresult of chemical reactions with either gaseous CO2 or aqueousCO2, is called indirect carbonation. One of the major advantagesof indirect carbonation is that the impurities, such as silica andFe, can be removed prior to the carbonation stage; therefore, highpurity carbonates can be produced. Moreover, carbonation rate issignificantly increased through indirect carbonation process.Indirect carbonation can be accomplished through different tech-nologies and process routes, such as indirect multistage gas–solidcarbonation, the pH swing process, HCl extraction, the molten saltprocess, other acid extractions, bioleaching, ammonia extractionand caustic extraction [11,26–28].

Ilmenite, which contains approximately 43–65% titaniumdioxide (TiO2), is widely used as the main raw material fortitanium dioxide manufacturing. TiO2 is extracted from ilmenitethrough a stepwise process. The first stage is digestion of the orewith H2SO4. Then, the resulted liquor is clarified with H2SO4 forremoval of undissolved solids from the solution. In the next step,the clarified liquor is hydrolyzed with steam to produce andprecipitate hydrated titanium. After that, the precipitated titaniumdioxide is separated from the liquor by vacuum filters. Finally, theimpurities are washed out from the precipitated TiO2 by washingwith water; a weak acid is also produced during this washing step.Red gypsum (RG) can be produced as a by-product in the last stage.The produced weak acid in the final stage is neutralized with limeor limestone, which generates a by-product named RG. In the nextstep, the RG is filtered and separated from water, and the producedwater is recycled in the process [29,30].

RG contains significant amounts of calcium (Ca) and iron (Fe),which can be used effectively for CO2 fixation. Azdarpour et al.[21] concluded that calcium and iron carbonates could be pro-duced through direct carbonation of red gypsum, however thelow product purity and carbonation efficiency are still big chal-lenges. In another study by Azdarpour et al. [31] the feasibility ofRG dissolution by different solvents has been investigated. Theyconcluded that H2SO4 is a good candidate for RG dissolution, whichresults in 100% Ca extraction in a shorter time step. Therefore, inthis study the feasibility of indirect carbonation of RG through apH swing process has been investigated. H2SO4 is used in the firststep to extract Ca from RG and then NH4OH is used to increase thesolution pH and facilitates carbonate precipitation in the secondstep. In addition, the effect of CO2 pressure as one of the major con-trolling parameter on carbonation efficiency, CaCO3 purity, andoverall pH swing efficiency process has been identified.

2. Materials and methods

2.1. Materials

Fresh RG samples used in this study were obtained from landfillin Kemaman, Terengganu, Malaysia. The samples were dried in anoven overnight at 45 �C to avoid any loss of their hydrated water.Ammonium hydroxide (NH4OH) and sulfuric acid (H2SO4) werepurchased from Rankem for the pH swing carbonation experi-ments. Analytical grade CO2 (purity > 99.9%) was purchased fromMalaysian Oxygen.

Fig. 1(a) represents the schematic diagram and real dissolutionexperimental setup used in the present study. RG dissolutionexperiments were performed in a 500 mL three-necked glass reac-tor vessel (2) as shown in Fig 1(a). The reactor was first placed on aheating mantle (1) equipped with a temperature controller whereall solutions were well mixed using a mechanical stirrer (4). A tapwater cooled condenser (3) was used to prevent solution vaporfrom escaping. A pH meter was used to monitor the solution pHafter each experiment.

Fig. 1(b) represents the schematic diagram of the HPHT reactorused for pH swing carbonation experiments. A 100 mL high pres-sure and high temperature (HPHT) autoclave reactor was usedfor the carbonation experiments. The autoclave reactor is madeof stainless steel 316 (SS316) which is very suitable for high pres-sure and high temperature experiments. The maximum operatingpressure and temperature of the autoclave reactor were 200 barand 450 �C, respectively. A temperature controller was used to con-trol the tube furnace temperature. CO2 gas was injected into thereactor from a CO2 cylinder using 1/8 inch stainless steel tubing.The direct carbonation of RG and also carbonation experimentsthrough the pH swing method have been carried out using thisHPHT reactor.

2.2. Methods

The physical and chemical properties of RG were determinedusing various analytical instruments, including XRD, XRF, andMASTERSIZER 2000. For accurate measurement of particle size,two samples were analyzed with MASTERSIZER 2000. In addition,the final products were collected after finishing the experiments,dried overnight at 105 �C, and then analyzed with TGA, XRD, andSEM. The standard procedure of each instrument was followedcarefully during the analysis of samples. Moreover, a sample of1 mL from filtrates was collected and then acidified with 2 mL of70 wt% HNO3 and then diluted to 100 mL with deionizedwater and analysed by ICP-OES for concentration of Ca and Femeasurements.

2.3. Experimental procedure

In the first step of the pH swing experiments, 2 M H2SO4 wasused in all experiments. A batch of 200 mL of 2 M H2SO4 was pre-pared and poured into the reactor. A temperature controller wasused for adjusting the reaction temperature at 70 �C. A batch of10 g of 100–212 lm RG was prepared and added into the reactorafter stabilizing the reaction temperature at 70 �C. The stirring rateof 1000 rpm was applied using a mechanical stirrer and the reac-tion was continued until 60 min. Then the reaction was stoppedand cooled and the solution was removed from the reactor and fil-tered immediately. The solution, which is named as filtrate 1 (F1),was separated from the solid part. The solid part was named asproduct 1 (P1). The solutions were sent to ICP-OES for measuring

Fig. 1. Schematic diagram of RG dissolution set up and carbonation set up.

A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436 427

of the concentrations of Ca and Fe. This procedure was repeated forall pH swing carbonation experiments.

After measuring the concentration of Ca/Fe in solution, theextraction efficiency of each experiment was calculated using Eq.(1) [8,32,33]. In this equation, the MX-extracted is the mass of themetal in the leachate obtained after the extraction, Mt is the totalmass of the material used in the extraction experiment, CXO isthe metal oxide content of the material determined by the XRFanalysis, MwX is the molecular weight of the metal and MWXO isthe molecular weight of the metal oxide. In this equation, X standsfor either Ca or Fe ions extraction efficiency. The results of thisstudy were used to find the most suitable solvents and also todetermine the optimum solvent concentration for futureexperiments.

X extraction efficiency ð%Þ ¼ MX�extracted ðgÞMt ðgÞ � CXO ð%Þ

100 � MwX ðg=molÞMwXO ðg=molÞ

ð1Þ

After RG dissolution, the impurities were removed from the Carich solutions. In this step, 100 mL of F1 sample was taken andpoured into a beaker. Then, NH4OH was added to the F1 samplesto increase the solution pH to approximately 9. The NH4OH wasadded dropwise and very slowly into the F1 samples to ensure iso-lated precipitation of Fe oxide without any Ca precipitation.Approximately 90–110 mL of NH4OH was required to increasethe solution pH from acidic condition to approximately 9. A pHmeter was used at all times during the addition of the ammoniasolution to the F1 sample to monitor the solution’s pH. After reach-ing the desired pH of 9, NH4OH addition was stopped, the sampleswere filtered and the solutions were separated from the solids. Theresulting solid and filtrate were labeled as product 2 (P2) and fil-trate 2 (F2). The F2 samples were sent to ICP-OES for Ca and Fe con-centration measurement. This procedure was applied for allcarbonation experiments. The F2 samples are considered as thecarbonation feed, which will be introduced into the carbonationstage in the third step.

In this study, the pH swing process consists of three main steps.The first step is the preparation of the Ca ion solution from RGusing H2SO4. The second step is NH4OH addition to the Ca-richsolutions to increase the solution pH and impurity removal(mainly Fe). The third step is the carbonation reaction of the Casolution with CO2 at the desired reaction conditions. Fig. 2 repre-sents the overall pH swing carbonation process. After preparingthe initial Ca solution and the removal of impurities, the solutionswere ready for the carbonation experiments. Two sets of control

experiments were defined in this study prior to investigate theeffect of CO2 pressure. The control experiments are aqueous solu-tion volume optimization and reaction time optimization. Afterthat, the effect of CO2 pressure on overall pH swing carbonationwas investigated.

For the first set of control experiment, initially 20 mL of solutionwith a known concentration of Ca and Fe was poured into the car-bonation reactor. Then, the reactor was closed, and CO2 with 10 barpressure was injected into the reactor at ambient temperature. Thecarbonation reaction was stopped after 30 min reaction time. Next,the reactor was depressurized, and the solutions were filteredimmediately. After filtration, the solid part, which was supposedto be carbonate products, was named as product 3 (P3), and the fil-trate was named as filtrate 3 (F3). The F3 samples were sent to ICP-OES to measure the concentration of unreacted Ca and Fe. This pro-cedure was repeated for 30–80 mL Ca ion solutions at constant10 bar CO2 pressure and ambient temperature.

After determining the concentration of Ca and Fe using ICP-OES,the Ca/Fe carbonation efficiency and Ca/Fe removal efficiency werecalculated. The Ca removal efficiency was calculated using Ca con-centration in the leachate from the extraction and carbonationtests according to Eq. (2) where MCa-extracted is the mass (g) of Cain the leachate obtained prior to the carbonation experiment (F2)and MCa-carbonation is the mass (g) of Ca in the leachate obtainedafter CO2 injection (F3). The carbonation efficiency, which repre-sents the percentage of Ca removed by the precipitation of CaCO3

per 1 g of RG sample, was determined using Ca concentrationsbefore and after CO2 injection. The carbonation efficiency was cal-culated using Eq. (3) where MCa-extracted is the mass (g) of Ca in theleachate obtained after the extraction test (F1), MCa-carbonation is themass (g) of Ca in the leachate obtained after CO2 injection (F3), Mt

is the total mass (g) of the material used in the extraction test, andCxO is the calcium and iron oxide content of the material deter-mined by XRF analysis, MWX is the molecular weight of Ca or Fe,and MWXO is the molecular weight of CaO or Fe2O3 [8,34].

Removal efficiency ð%Þ¼MX�extracted ðgÞ�MX�carbonation ðgÞMX�extracted ðgÞ

�100 ð2Þ

Carbonation efficiency ð%Þ¼MX�extracted ðgÞ�MX�carbonation ðgÞMt ðgÞCXO ð%Þ

100 �MwX ðg=molÞ

MwXO ðg=molÞ

�100 ð3Þ

Fig. 2. Overall pH swing carbonation process.

Table 1Chemical compositions of red gypsum from XRF.

Compound Concentration (wt%)

Al2O3 0.7SiO2 1.9SO3 31.6CaO 32.2TiO2 3.4V2O5 0.22Cr2O3 0.032MnO 0.41Fe2O3 28.99CuO 0.063ZnO 0.04SrO 0.03ZrO2 0.065RuO2 0.39Eu2O3 0.26Re2O7 0.001HgO 0.034

0 10 20 30 40 50 60 70

Inte

nsity

, (a.

u)

2 (degree)

CS- CaSO42H2OCS

CS CS CS CS CSCS

CS

CS

428 A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436

The second set of control experiments was designed to optimizethe carbonation time. In this study, 50 mL of aqueous solution wasused consistently in all experiments. After loading the reactor withaqueous solution, the reactor was closed, and CO2 with 10 barpressure was injected into the reactor at constant ambient temper-ature. The carbonation experiments were continued until5–90 min. After depressurizing the reactor, the solids and solutionswere separated by filtration. The filtrates were sent to ICP-OES tomeasure the concentration of unreacted Ca and Fe, and, finally,Ca/Fe carbonation efficiency and Ca/Fe removal efficiency were cal-culated for each experiment using Eqs. (2) and (3), respectively.

For investigating the effects of CO2 pressure on overall pH swingcarbonation, initially, 50 mL of aqueous solution with a knownconcentration of Ca and Fe was prepared and poured into the reac-tor. Then, the reactor was pressurized with CO2 at the desired pres-sure (1–70 bar) at ambient temperature. After giving 30 minreaction time, the experiments were stopped, the reactor wasdepressurized and the solutions were filtered. The solid part wascharacterized using TGA, SEM, and XRD, while the concentrationof Ca and Fe was determined using ICP-OES.

The TGA results were used to determine the weight loss of CO2

and calculate the carbonates purity. Weight loss of the sample at200–450 �C and also 600–850 �C is assigned to weight loss of CO2

from FeCO3 and CaCO3, respectively [35,36]. The CaCO3 and FeCO3

purity measurements were performed using Eqs. (4) and (5),respectively. Alternately, the XRD and SEM results were used forphase and morphology determination and to confirm the presenceof CaCO3 as the main product in the final solid residue. The ICP-OESresult was used to calculate the amount of reacted Ca and Fe, Ca/Fecarbonation efficiency and Ca/Fe removal efficiency using theequations explained earlier.

PCaCO3 ¼DW ð%Þ �MwCaCO3

MwCO2

� 100 ð4Þ

Fig. 3. X-ray diffraction of bulk red gypsum.

PFeCO3 ¼

DW ð%Þ �MwFeCO3

MwCO2

� 100 ð5Þ

3. Results and discussions

3.1. RG characterization

The XRF result shows that the red gypsum is clearly dominatedby CaO (32.2%) along with other ingredients such as Fe2O3 (28.99%)and TiO2 (3.4%) as well as some minor elements such as Al, Ti, Mn,and etc. The complete XRF results of red gypsum is presented inTable 1. The XRD analysis of RG showed that the RG is mainly com-posed of gypsum mineral (CaSO4�2H2O). It shows four major peaksat 11.68�, 20.79�, 23.44�, and 29.16�, and some minor peaks at35.41�, 40.64�, 47.84�, 50.34�, and 51.18� which are assigned to

CaSO4�2H2O in RG as shown in Fig. 3. In addition, the absence ofFe2O3 peaks as the other major constituents present in RG couldbe attributed due to the possibility that the iron can be in amor-phous hydroxides because of the high concentration of Fe2O3

obtained by XRF [30]. The particle size analyzer results showedthat the average particle size of the RG is in the range of 0.02–2000 lm, with a mean (d50) particle size of 56.03 lm and a specificsurface area of 7.01 m2/g as shown in Fig. 4 [21].

3.2. Ca ion solution preparation and pH regulation

The pH swing process in this research produce three filtratesand three solids. Filtrates (F) 1 and 2 are the filtrates prior to

Fig. 4. Particle size distribution for the two bulk red gypsum sample used.

Table 2Summary from ICP-OES analyses of filtrates produced in the carbonationexperiments.

Set Carbonation condition Filtrate 3concentration(mg/L)

Volume ofaqueoussolution(mL)

Pressure(bar)

Temperature(�C)

Reactiontime(min)

Calcium Iron

1 20 10 25 30 6749 <LD30 10 25 30 5629 <LD40 10 25 30 2784 <LD50 10 25 30 <LD <LD60 10 25 30 1458 <LD70 10 25 30 2729 <LD80 10 25 30 4651 <LD

2 50 10 25 5 7780 12150 10 25 10 5080 13850 10 25 15 4031 8950 10 25 20 1317 <LD50 10 25 30 <LD <LD50 10 25 60 <LD <LD50 10 25 90 <LD <LD

3 50 1 25 30 5285 <LD50 2 25 30 4582 <LD50 4 25 30 3137 <LD50 6 25 30 1495 <LD50 8 25 30 <LD <LD50 10 25 30 <LD <LD50 15 25 30 <LD <LD50 30 25 30 <LD <LD50 50 25 30 <LD <LD50 70 25 30 <LD <LD

A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436 429

carbonation stage and filtrate 3 is the filtrate collected after per-forming the carbonation experiment. The concentration of Ca andFe ions were identified using ICP-OES during the three stages ofpH swing process. Table 2 represents the summary of Ca and Feconcentration in F3 samples in this research. As it is shown in thistable, three sets of data are provided, which refer to the three set ofcarbonation experiments conducted in this study. The first set ofexperiments includes aqueous volume optimization in whichreaction temperature, CO2 pressure and reaction time were usedconstantly and aqueous volume inside the reactor was varied.The second set of experiments includes carbonation time optimiza-tion where different reaction time was given to experiments andaqueous volume, reaction temperature and CO2 pressure wereused constantly in all experiments. In the third set of experimentsthe effect of CO2 pressure on pH swing process was investigatedwhere CO2 with variable pressure was injected to the reactor andaqueous volume, reaction time and reaction temperature were

used constantly in all experiments. The concentration of calciumand iron in F3 was determined by ICP-OES and was used for car-bonation and removal efficiency calculations.

Initially, 2 M H2SO4 was used to extract Ca/Fe from RG. H2SO4

was able to extract a significant fraction of Ca and Fe from RG, aswas expected. The maximum Ca extraction efficiency of 100% fromRG was achieved in all dissolution experiments. On the other hand,Fe extraction efficiency from RG was ranged between 59.53% and68.13%. The concentration of Fe in F1 samples was about3232 mg/L with 217–219 mg/L variation. These results are quitesimilar to the data presented in the literature in which metal ionswere extracted from serpentine and steelmaking slags usingNH4HSO4, H2SO4, HCl, HNO3, HCOOH and CH3COOH [37,38].

After this stage, the solution pH was increased from very acidicconditions to more basic conditions using NH4OH. The pH regula-tion step was designed and aimed to remove impurities, mainlyFe, from the Ca-rich solution. The pH regulation results showedthat nearly 92–95% of available Fe in solutions successfully sepa-rated from Ca solutions. In the second stage, after pH adjustmentand impurity removal, the concentration of Fe reduced to about211 mg/L with 22–23 mg/L variation. This resulted in 95% Feseparation from the Ca rich solution prior to carbonation stage.On the other hand, some of Ca ions also precipitated along withFe ions, which resulted in Ca concentration of 11,796 mg/L with137–208 mg/L variation.

Fig. 5(a) shows the F1 solution after filtration. The solution haslight color, which is attributed to the presence of Fe+2 as descriedby Casas et al. [39]. Fig. 5(b) shows that a slight addition of NH4OHto the solutions results in the precipitation of Fe ions. It also clearlyshows that the precipitated Fe ions are separated from the solu-tion. By increasing the amount of NH4OH in the solution, a higherconcentration of Fe was precipitated. The color of the solutionchanges to a red color due to the precipitation of Fe ions, whichis shown in Fig. 5(c). After performing the pH adjustment andincreasing the solution pH, the solution color changes to a reddishcolor, which is due to the presence of Fe ions in the solution, asshown in Fig. 5(c). Fig. 5(d) represents the collected solid after fil-tration. This solid, which is considered to be composed of impuri-ties, mainly consists of Fe. The filtered solution, which is rich in Caions, was used for the carbonation stage for CaCO3 production.

3.3. Carbonation experiments

3.3.1. Aqueous volume optimizationVariable volumes of aqueous solution were subjected to the car-

bonation stage at ambient temperature and 10 bar CO2 pressure.The concentration of reacted Ca was measured for each experimentafter giving 30 min of reaction time, and finally Ca carbonation effi-ciency was calculated for each experiment. Fig. 6 shows the calcu-lated data of Ca carbonation efficiency. Fe carbonation efficiency isnot included in this section because the concentration of Fe is neg-ligible compared to Ca concentration. Moreover, Fe carbonationefficiency is not the focus of the present study.

Experimental results showed that the minimum Ca carbonationefficiency was achieved using 20 mL aqueous solution. The mini-mum Ca carbonation efficiency of 41.36% was achieved in thisexperiment. However, further increasing the volume of aqueoussolution resulted in increasing the Ca carbonation efficiency. Themaximum Ca carbonation efficiency of 100% was achieved byincreasing the aqueous solution volume to 50 mL. All of the Capresent in the solution reacted with CO2 during this experiment.Further increasing the volume to 80 mL resulted in decreasingthe Ca carbonation efficiency to about 59.59%.

The amount of available CO2 inside the reactor was significantlylower when the higher volume of aqueous solution was used. Inaddition, the mass of CO2 in the gas phase inside the reactor was

(b) (a)

Fe precipitates in solution Ca solution

Fe precipitates after filtration

(d)(c)

NH4OH

Solution after filtration

pH meter

Fig. 5. Fe removal from Ca rich solutions.

0

20

40

60

80

100

120

20 30 40 50 60 70 80

Ca

carb

onat

ion

effi

cien

cy (

%)

Aqueous Volume (mL)

Fig. 6. Ca carbonation efficiency versus variable aqueous volume.

430 A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436

higher when the lower volume of aqueous solution was used. Theconjunction of these two facts limited the reaction of Ca ions andCO2 at very high and low aqueous volumes. Huijgen et al. [40,41]stated that, there is always a balance between the Ca ions andCO2 during carbonation reactions. Below or above this point, theCa conversion changes significantly due to an inadequate amountof dissolved CO2 and Ca ions. A higher CO2 volume, in principle,increases the carbonation degree because of an increase of theCaCO3 precipitation rate. However, above a specific CO2 volumewhere all Ca ions have already reacted with CO2, the carbonationdegree becomes independent of the CO2 volume because the (bi)carbonate activity in the solution is no longer rate limiting, andthere are no more Ca ions in solution. Therefore, further increaseof the CO2 volume has a direct reduction effect on the carbonationefficiency.

0

20

40

60

80

100

120

0 20 40 60 80 100

Ca c

arbo

natio

n effi

cien

cy (%

)

Reaction time (min)

Fig. 7. Ca carbonation efficiency versus carbonation time.

3.3.2. Reaction time optimizationFig. 7 shows the Ca carbonation efficiency with variable carbon-

ation reaction time. Apparently, for this leachate, calcium carbon-ation proceeds very fast in the first 10–20 min. The carbonationefficiency is at its lowest value of 32.40% after 5 min of reaction.Then, by increasing the reaction time to 15 min, the efficiencyincreased to 64.98%, and, by further increasing the reaction timeto 30 min, the efficiency increased to 100%. It was expected thatfurther increasing the reaction time to 60 and 90 min would keepthe carbonation efficiency at its maximum value. The favorablecondition is to have a complete reaction between the Ca ions pres-ent and CO2 to result in high carbonation efficiency. For 30 min car-bonation time, the reaction was completed, and all the present Caion in solution successfully reacted with CO2. The concentration ofCa in F3 is below the detection limit value (LD), implying that allthe Ca ions are consumed.

He et al. [33]; Lee et al. [36]; and Santos et al. [42] stated thatincreasing the reaction time has a positive effect on the carbon-ation rate, and prolonged reaction time proportionally increasesthe carbonation conversion. By increasing the reaction time, thereactants have sufficient time to be reacted with each other. There-fore, more Ca ions reacted with the gaseous CO2, consequently theoverall conversion of Ca ion was increased. Experimental results

showed that the Ca conversion rate into carbonate is affected byreaction time; however, an optimum temperature was observedin this study. The conclusion from this section is that 30 min reac-tion time could be the optimum reaction time. Experimentalresults showed that the carbonation efficiency at 30 min and90 min does not differ significantly, since the maximum Ca carbon-ation efficiency of 100% was achieved at 30 min and onward.

A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436 431

3.3.3. Effects of CO2 pressureThe effects of CO2 pressure on carbonation efficiency, removal

efficiency, and product purity were investigated as one of the mainobjectives of this study. The carbonation time and aqueous volumewere optimized in previous experiments. In this section, the CO2

pressure was varied from 1 bar to a maximum of 70 bar at a reac-tion temperature of 25 �C.

3.3.3.1. Ca/Fe removal efficiency. The Ca/Fe removal efficiency wascalculated with respect to Ca/Fe concentrations in F2 and F3 solu-tions. Table 3 represents the Ca/Fe removal efficiency with respectto the pressure changes. The minimum Ca removal efficiency of51.58% was achieved at 1 bar CO2 pressure, however, furtherincreasing the CO2 pressure to 6 bar resulted in Ca removal effi-ciency of 86.13%. Finally, the maximum Ca removal efficiency of100% was achieved at 8 bar CO2 pressure and onward. Additionally,as is shown in Table 3, all Fe in solution reacted with CO2 duringthe carbonation experiments. This resulted in 100% Fe removalefficiency for all experiments from 1 to 70 bar pressure.

Calcium reactivity is slightly higher than the Fe, which leads tothe expectation to have higher Ca removal compared to the Fe. Asshown in Table 3, the Fe removal efficiency is always higher thanthe Ca removal efficiency in the range of 1–6 bar pressure. The rea-son for having this phenomenon is because the Fe concentrationinside the carbonation reactor is much lower than the Ca concen-tration. Therefore, Fe could easily react with CO2 inside the reactor,which resulted in 100% Fe removal efficiency. In addition, even at1 bar CO2 pressure, a significant fraction of CO2 reacted with Carather than Fe, which can be clearly observed from the concentra-tion of reacted Ca and Fe.

3.3.3.2. Ca/Fe carbonation efficiency. The carbonation efficiency ofCa and Fe at different CO2 pressures was calculated using the con-centration of Ca and Fe in the F1 and F3 solutions. Fig. 8 shows thecarbonation efficiency with respect to the variable CO2 pressure. Asshown in this figure, the Ca carbonation efficiency is at its lowestvalue of 54.08% at atmospheric pressure. Increasing the CO2 pres-sure to 6 bar increases the Ca carbonation efficiency to 87.01%. ACa carbonation efficiency of 100% was achieved at 8 bar CO2 pres-sure. Further increasing the CO2 pressure from 8 to 70 bar resultedin full consumption of all Ca in solution during the carbonationreaction. The Ca carbonation efficiency was found to be in directrelationship with CO2 pressure in which increasing the CO2 pres-sure increased Ca carbonation efficiency significantly. On the otherhand, Fe carbonation efficiency exhibited different behavior thanthe Ca. The Fe carbonation efficiency ranged between 59.45% and68.08% during the carbonation reaction with 1–70 bar CO2 pres-sure. As was explained earlier, all Fe in solution reacted with CO2

during the carbonation experiments, however, relatively poor Fecarbonation efficiency was achieved.

The Ca/Fe carbonation results suggest that, the carbonation effi-ciency is affected by the CO2 pressure and also Ca/Fe extraction

Table 3Ca/Fe removal efficiency with respect to variable CO2 pressure.

Pressure (bar) Ca removal efficiency (%) Fe removal efficiency (%)

1 51.58 1002 58.07 1004 71.22 1006 86.13 1008 100 10010 100 10015 100 10030 100 10050 100 10070 100 100

efficiency. The effect of CO2 pressure on carbonation efficiency isvery clear with Ca carbonation efficiency where the carbonationefficiency increased significantly with increasing CO2 pressure. Aswas explained earlier, the maximum Ca extraction efficiency fromRG was achieved using 2 M H2SO4 in these experiments. Thisresulted in the availability of all Ca ions present in RG to be in reac-tion with CO2. On the other hand, the carbonation experimentsresulted in complete consumption of all Fe ions in solution duringcarbonation reactions. However, the reason for having relativelylow Fe carbonation efficiency was due to the fact that the Feextraction efficiency from RG was relatively low (maximum of68.13%). As the focus of this research, the effect of CO2

pressure on Ca carbonation efficiency will be exploited in detailssubsequently.

An observation from the carbonation efficiency data is that Cacarbonation efficiency is in direct relationship with the concentra-tion of reacted Ca with CO2. Increasing the amount of reacted Caincreases the carbonation efficiency significantly in which the con-sumption of all available Ca results in 100% carbonation efficiency.This behavior is well shown in Fig. 8 for Ca. Speaking of Ca carbon-ation efficiency for this set of experiments shows that, at atmo-spheric pressure, the amount of reacted Ca is 5629 mg/L. Theconcentration of Ca in F1 is 11,509 mg/L, which results in a carbon-ation efficiency of 54.08%. Increasing CO2 pressure to 2 bar con-sumes 6346 mg/L of Ca out of 11,509 mg/L Ca in F1, whichresults in a carbonation efficiency of approximately 60.19% in thisexperiment. Similarly, increasing the CO2 pressure to a higher pres-sure consumes more Ca in the reaction until all the available Caions have reacted with CO2. At any pressure above 8 bar, allavailable Ca has reacted with CO2, which resulted in 100% Cacarbonation efficiency.

The experimental results show that reaction rate is increasedwith increasing CO2 pressure. Lower carbonation rate is achievedat low CO2 pressure while increasing CO2 pressure results in max-imum carbonation rate. This shows that kinetics of reaction arechemically controlled during carbonation reaction. For the carbon-ation process, an optimal temperature, pressure, and pH exist dueto thermodynamic constraints on the stability of the carbonates.Carbonate precipitation during indirect aqueous mineral carbon-ation route is controlled by the concentration of CO3

2� ions in thesolution and also concentration of Ca2+. Increasing CO2 pressure,decreasing temperature, and increasing pH to around 9, promotethe dissolution of gaseous CO2 to form HCO3

� [43]. Wang et al.[44] stated that carbonation efficiency can be improved by increas-ing either the Ca2+ or CO3

2� ion concentration. In the present study,the concentration of Ca ion in solution is almost constant for allexperiments; therefore, the CO3

2� concentration needs to beimproved. At a certain constant temperature, the concentrationof CO3

2� will change only as the atmospheric CO2 partial pressureor pH changes. Therefore, it is feasible to increase the CO3

2� concen-tration by increasing the CO2 partial pressure or by increasing thesolution pH. This statement is in good agreement with resulteddata in the present study in which carbonation efficiency wasimproved significantly by increasing CO2 pressure.

Increasing carbonation efficiency with increasing CO2 pressureis due to increasing CO2 solubility with increasing CO2 pressure,as stated by Santos et al. [42]. They concluded that increased CO2

pressure results in greater CO2 solubility due to the formationand dissociation of carbonic acid. Increasing dissociation of car-bonic acid will result in increasing the concentration of carbonateions in solution. As a result, more carbonates will react with Caions. Therefore, higher carbonation efficiency would result inhigher CO2 pressure rather than lower CO2 pressure. This state-ment is in good agreement with He et al. [33], who stated that uti-lizing NH4HCO3 instead of CO2 increases the carbonation efficiencysignificantly because more carbonate ions are present during the

0

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Ca carbonation Fe carbonation Reacted Ca

Fig. 8. Ca/Fe carbonation efficiency and reacted Ca with respect to CO2 pressure.

432 A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436

carbonation reaction with NH4HCO3. Jo et al. [8] concluded thatCO2 dissolution provides carbonate ions. The presence of Ca+2 insolution and, subsequently, the reaction of this Ca+2 ion with theproduced carbonate ions results in the precipitation of Ca carbon-ate. Hence, increasing the CO2 pressure will facilitate the genera-tion of carbonate ions for carbonation reactions. Therefore,increasing the CO2 pressure plays a positive role in enhancingthe conversion rate of Ca ions to carbonates. In addition, the Caconcentrations in the filtered samples after the termination ofthe carbonation tests also confirmed the progress of the carbon-ation reaction during CO2 injection.

3.3.3.3. CaCO3 purity. The carbonates purity at different CO2 pres-sures was calculated using TGA results. As was expected, both cal-cium carbonate and iron carbonate were produced duringcarbonation reactions. The CaCO3 purity, which is the main focusof this research, was at its lowest value of 89.03% at 1 bar CO2 pres-sure. By further increasing the CO2 pressure to 2 bar, the purity alsoincreased to 94.48%. The carbonation efficiency experimentsshowed that the complete reaction between CO2 and Ca ionsoccurred at 8 bar CO2 pressure. At this condition, the purity of cal-cium carbonate is approximately 98.07%. It remained high withfurther increasing of CO2 pressure even until 70 bar.

Fig. 9 represents the relationship between CaCO3 purity andreacted Fe concentration. As is shown in this figure, CaCO3 purityhas an inverse relationship with reacted Fe concentration. Calciumcarbonate purity increases when the concentration of reacted Fedecreases. Considering experiments at 50 and 70 bar shows that,although a higher concentration of calcium was reacted with CO2

at 70 bar the purity was slightly lower. The concentration ofreacted calcium with CO2 was 10,701 mg/L and 10,857 mg/L, whilethe purity is approximately 98.81% and 98.66% at 50 and 70 barCO2, respectively. The lower purity of CaCO3 at 70 bar CO2 pressureis because only 163 mg/L Fe reacted with CO2 at 50 bar, which islower than the concentration of reacted Fe with CO2 (183 mg/L)at 70 bar. The same behavior is well observed for other pressuredata as well. For instance, the concentration of Fe at 30 and50 bar CO2 pressure was 239 and 163 mg/L, respectively. As isshown, the concentration of Fe at 30 bar is higher than the concen-tration at 50 bar pressure. The final purity of CaCO3 at 30 bar pres-sure is 98.25%, which is slightly lower than the CaCO3 purity of98.81% at 50 bar pressure.

In this study, CaCO3 purity of 89.03% was achieved at atmo-spheric pressure, which is relatively lower than that of the CaCO3

purity reported in the literature. It is believed that CaCO3 purityhas a direct relationship with impurity removal prior to the car-bonation step, as was stated by Teir et al. [45] and Wang and Maro-to-Valer [38]. In the present study, however, the completeseparation of Fe ions is never achieved even under the best condi-tions, approximately 162–258 mg/L Fe was introduced to the car-bonation stage along with calcium. Under lower pressure, thecarbonation efficiency is significantly lower. For instance, the car-bonation efficiency is 54.08% at atmospheric pressure, while itincreases to the 100% at 8 bar CO2 pressure. At lower CO2 pressure,the presence of Fe ions as impurities is clearer than at higher CO2

pressures. In addition, insufficient CO32� ions are present under

lower CO2 pressures in the reaction. The ideal reaction in the car-bonation process is the reaction that occurs between CO2 and Caion in the solution. However, in the presence of Fe ions, the avail-able CO2 also reacts with Fe ions. Therefore, the reduction in theamount of reacted Ca reduces the purity of CaCO3. The combinationof these facts results in having lower CaCO3 purity in lower CO2

pressure compared to higher CO2 pressure. As was explained, thepresence of Fe ions as impurities reduces the CaCO3 purity moreobviously at lower CO2 pressure where carbonation efficiency issignificantly low.

3.3.3.4. Characterization of carbonates. Fig. 10 shows the XRD pat-tern of the carbonated products of the experiments at 50 bar CO2

pressure. In this figure, the spectra (a), (b), and (c) represent pureCaCO3, pure siderite, and reaction product at 50 bar pressure,respectively. The representative peaks at 29.32�, 35.88�, 39.33�and 47.07� in spectrum (a) are assigned to calcite polymorphousof CaCO3, while peaks at 23.01�, 43.06�, 48.42�, 56.48�, and57.35� are the representative peaks of vaterite polymorphous ofCaCO3 [45–48]. The representative peaks at 15.55�, 18.54�,18.95�, 23.03�, 24.9�, 32.05�, 34.39�, 42.91�, 48.22�, 50.42�, and53.19� in spectrum (b) are assigned to siderite [35,49,50].

In spectrums (c) the significant representative peaks of CaCO3

and siderite, as mentioned above, are present, showing that car-bonation reactions took place successfully. In addition, the repre-sentative peaks at 26.24�, 45.80�, and 48.62� are also detected,which show the presence of aragonite in the final products[46,51,52]. It is stated in the literature that setting the reaction

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O3

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ty (%

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CaCO3 purity

Reacted Fe

Fig. 9. CaCO3 purity and Fe concentration relationship.

Fig. 10. XRD patterns of (a) pure CaCO3; (b) pure siderite and (c) carbonation product at 50 bar.

A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436 433

temperature below 80 �C results in precipitation of vaterite, arago-nite, and calcite. Increasing the reaction temperature to roughly170 �C favors the precipitation of only vaterite and calcite, and, attemperatures above 170 �C, only vaterite precipitates [46,47]. TheXRD results prove the existence of aragonite, calcite and vateritein the final products, which is in agreement with the statementsin the literature.

Fig. 11(a–d) represents the SEM analysis of the final products ofthe carbonation experiments. In these figures, the representativeA, C, and V are assigned to aragonite, calcite and vaterite, respectively.Although different morphologies have been reported for differentpolymorphs of CaCO3, the most common structures of calcite, ara-gonite and vaterite are rhomboidal, needle/rod and spherical,respectively [47]. As is shown in these figures, products with cubicforms, which are assigned to calcite, are predominant. In addition,particles with spherical and needle shape are widely spread out,which confirms the presence of aragonite and vaterite in the finalproduct. Another observation from these figures is that calcite,

aragonite and vaterite with variable sizes are present. The SEMresults confirm the presence of calcite, aragonite and vaterite asthe product of the carbonation experiments. The SEM results arein good agreement with the published data in the literature[53,54].

Fig. 12 represents the TGA curves of carbonation product as wellas the TGA curves of pure CaCO3 and pure FeCO3. The major weightloss of pure FeCO3 occurs in 200–450 �C, while it is in 600–850 �Cfor pure CaCO3 as indicated by the DTA curve. The weight loss isdue to the evolution of CO2 from the decomposition of FeCO3 andCaCO3, respectively. On the other hand, for the final product, oneminor weight loss was observed within the FeCO3 decompositiontemperature range and one major weight loss was observed withinthe CaCO3 decomposition temperature range. It is indeed a clearindication of the formation of CaCO3 as a major product and FeCO3

as a minor product in the carbonation reaction. In addition, themajor DTA curve for the product is slightly shifted toward left sidefrom the DTA curve of pure CaCO3, indicating a slight faster rate of

(d) (c)

(b)(a)

Fig. 11. SEM analysis of carbonate products.

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Product (TGA) Pure CaCO3 (TGA) Pure FeCO3 (TGA)

-- Product (DTA) -- Pure CaCO3 (DTA) -- Pure FeCO3 (DTA)

Fig. 12. TGA curves of carbonation product, pure CaCO3 and pure FeCO3.

434 A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436

evolution of CO2 from the product than that of the pure CaCO3. Thismay be due to the effects of the impurities of the product or thedifferent polymorphism of CaCO3.

3.3.3.5. Optimum pressure. The optimum pressure was determinedby balancing carbonation efficiency and CaCO3 purity. The reason

for excluding Fe carbonation efficiency data was that the concen-tration of Fe ions during the carbonation experiments was negligi-ble as compared to Ca. Therefore, the optimum CO2 pressure couldbe considered based on the carbonation efficiency and CaCO3 pur-ity. A relatively wide range of CO2 pressure (1–70 bar) was utilizedin this study. Fig. 13 shows the relationship between CaCO3 purity

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%)

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CaCO3 purity Ca carboantion efficiency

Fig. 13. Ca carbonation efficiency and CaCO3 purity with respect to variable CO2 pressure.

A. Azdarpour et al. / Chemical Engineering Journal 264 (2015) 425–436 435

and carbonation efficiency with respect to CO2 pressure. As isshown in this figure, CaCO3 purity is relatively high (89%) even atatmospheric pressure, and, in contrast, carbonation efficiencyincreases from very lower value of 54% to maximum value of100% with increasing pressure. Therefore, careful management ofCO2 pressure to result in maximum carbonation efficiency withmaximum CaCO3 purity is of great importance. A carbonation effi-ciency of 100% is achieved at 8 bar pressure, and it remains at 100%at any pressure above 8 bar. Moreover, CaCO3 with purity of 98% isachieved at 8 bar pressure, and further increases of CO2 pressureremains the CaCO3 purity above 98%. Therefore, it could be con-cluded that 8 bar is the optimum CO2 pressure for this set of exper-iments, which results in maximum carbonation efficiency withmaximum CaCO3 purity.

4. Conclusions

The experimental results showed that increasing CO2 pressureis favorable in improving the carbonation efficiency. The minimumvalue of Ca carbonation efficiency was the result of using atmo-spheric pressure, while increasing the CO2 pressure to 8 barresulted in the complete reaction between Ca ions and CO2. Theincrease in CO2 concentration in the aqueous phase obviouslyaccelerates the carbonation reaction. The complete reactionrequires the availability of a balance between Ca and CO2 in solu-tion. On the other hand, the Fe carbonation efficiency exhibited dif-ferent behavior as Ca. The Fe carbonation efficiency was in therange of 59–68%, which was due to inability of 2 M H2SO4 toextract 100% Fe from RG. From the experimental data, it could beconcluded that experiments with variable CO2 pressure are chem-ical kinetically controlled because the reaction rate is increasedwith pressure. The purity of CaCO3 was significantly affected bythe presence of Fe ions as an impurity in the solution. In otherwords, the primary reason for lowering the CaCO3 purity was thepresence of Fe ions during the carbonation reaction. A maximumCaCO3 purity of 98% was achieved during these experiments.

Acknowledgments

This research is financially supported by the Ministry of Educa-tion, Malaysia under the project No. Q.130000.2542.06H82 andUniversiti Teknologi Malaysia (UTM); and also the Ministry of Edu-cation, Malaysia under the project No. 600-RMI/FRGS/5/3(98/2013)

and 600-RMI/RAGS5/3(79/2013); and the Research ManagementInstitute, Universiti Teknologi Mara.

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