Hydrometallurgy 81 (2006) 234–240www.elsevier.com/locate/hydromet

Technical note

Reductive acid leaching of manganese dioxide with glucose:Identification of oxidation derivatives of glucose

G. Furlani ⁎, F. Pagnanelli, L. Toro

Dipartimento di Chimica, Facoltà di S.M.F.N., Università degli Studi “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy

Received 29 September 2005; received in revised form 20 December 2005; accepted 21 December 2005

Abstract

The oxidation of glucose during the reductive leaching of pure MnO2 and manganese ore in sulphuric acid at 90 °C MnO2 wasinvestigated. The aim was to identify the derivatives and the chemical pathway of glucose oxidation. Organic derivatives weremonitored by HPLC using an Aminex HPX-78H column and UV detection. Chromatographic patterns of leaching solutionsshowed that major compound formed was formic acid. Minor quantities of glycolic, glyceric and traces of gluconic acids wereidentified. Results suggest that during the leaching reaction, the carbon chain of glucose is shortened by detachment of one carbonatom as formic acid.© 2006 Elsevier B.V. All rights reserved.

Keywords: Glucose oxidation; MnO2; Reductive leaching; HPLC analysis

1. Introduction

In the leaching of manganiferous ores containingtetravalent manganese a reducing agent is necessary toreduce Mn(IV) to soluble Mn(II). Some of the reducingagents used, such as SO2, may be harmful to theenvironment. Carbohydrates are low cost and non-hazardous reducing agents that may be used either inpure form or in industrial wastes (Vegliò and Toro,1994a; Vegliò et al., 2000). Previous work about thisinnovative leaching process focused on the productionof high purity MnSO4, both in solution and in powders,by using different carbohydrates in H2SO4 medium(Beolchini et al., 2001; Vegliò et al., 2001a; Pagnanelliet al., 2004). Operating conditions have been optimized

⁎ Corresponding author.E-mail addresses: giuliana.furlani@uniroma1.it (G. Furlani),

francesca.pagnanelli@uniroma1.it (F. Pagnanelli),luigi.toro@uniroma1.it (L. Toro).

0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2005.12.008

by studying the effect of factors such as reagent kind andconcentration, grain size, temperature, stirring onleaching yield and rate. In previous studies, the stoi-chiometry was proposed as follows (Vegliò and Toro,1994b):

C6H12O6 þ 12MnO2 þ 24Hþ

¼ 6CO2 þ 12Mn2þ þ 18H2O: ð1ÞKinetic models were proposed without considering

organic intermediates (Adrover et al., 2004; Vegliò etal., 2001b).

The oxidation of glucose (and other carbohydrates)has been studied by several authors who have in-vestigated the action of various oxidants (Varela, 1987).Oxidation by MnO2 gives arabinose (lower aldose) asthe main product and poly-hydroxyacids with 6 to 3carbon atoms (Bose et al., 1959). The oxidation ofcarbohydrates with trivalent manganese leads to formicacid and the next lower aldose, which in turn can react

Table 1Leaching conditions used in different sets of tests

Set oftests

MnO2

sampleGlucoseamount

Sulphuric acid(concentrated)

Heatingand stirringT=90 °C

1 Coarsegrainsize

2×stoichiometric

2 ml Dubnoff shaker

2 Coarsegrainsize

2×stoichiometric

2 ml Electrical plate/magnetic stirrer

3 Finegrainsize

2×stoichiometric

2 ml Electrical plate/magnetic stirrer

4 Finegrainsize

3×stoichiometric

2 ml Electrical plate/magnetic stirrer

235G. Furlani et al. / Hydrometallurgy 81 (2006) 234–240

with Mn(III). Final products are therefore formic acidand formaldehyde (Bhatnagar and Fadnis, 1978). Theauthors proposed a reaction pathway via radical andcomplex ion formation (Fadnis, 1986). The oxidation ofglucose by O2 gives various products—mainly aldonicacids and mono-carboxylic-poly-hydroxyacids. Theformation of formic, acetic, glycolic and glyceric acidshas been reported (McGinnis et al., 1984; Biermann etal., 1985; Vuorinen, 1985).

The electro-oxidation of glucose has also beenstudied in various media. In acidic media the mainreaction product is gluconic acid along with glucaric,glucuronic, tartaric, glyoxylic, formic, oxalic, glycolicacids and CO2 (Kokoh et al., 1992a,b); whilst in basicmedia intermediate products may also be keto-2 or keto-5-gluconic acid, hydroxymalonic and meso-oxalic acid(Kokoh et al., 1992b).

In summary, oxidation products of glucose may bepolyhydroxyacids, aldonic and aldaric acids, loweraldoses, formic acid and CO2. In the cited works,reaction products were determined by specific chemicaltests (Bhatnagar and Fadnis, 1978; Shalaby et al., 1995),by paper chromatography (Bose et al., 1959; Fadnis,1986) or by gas-chromatography (Vuorinen, 1985). Inthis work, as in the other cited works (Isbell, 1976;Biermann et al., 1985; McGinnis et al., 1984; Kokoh etal., 1992a, 1992b) the separation and identification ofthe oxidation products have been achieved by HPLC.

The aim of this work is to identify the organicderivatives of glucose during the leaching of manganeseores. Little attention has been paid until now to car-bohydrates transformation products in the course of theleaching process. A deeper understanding of carbohy-drates degradation is needed in order to achieve a bettercomprehension and control of the leaching process. Thiswould allow a more accurate kinetic model to beobtained with further optimization of the process. It isalso important to know the composition of final leachingsolution that may need purification in any futurecommercial development. In fact the presence of organicspecies may interfere in this step by formation ofcomplexes (Pagnanelli et al., 2004). In this study, theoxidation of glucose was chosen as a model compoundduring the leaching of pure syntheticMnO2. Reaction (1)was assumed as the starting point of the present work.

2. Experimental

2.1. Reagents

Two size fractions of synthetic MnO2 were used:coarser grained (N125 μm) and finer grained (N63 μm).

Sulphuric, glucaric, oxalic, meso-oxalic, glucuronic,tartaric, gluconic, glyoxylic, glyceric, malic, glycolic,formic, acetic acids, hydroxyl–methyl–furfural (HMF)and furfural were obtained from Sigma Aldrich. Tartro-nic acid was from Alfa-Aesar. Solutions of pure com-pounds were freshly prepared before each experiment.

2.2. Glucose with sulphuric acid on heating

To investigate glucose degradation at high tempera-ture in acid solution, a solution containing 0.3 g ofglucose in 2% conc. sulphuric acid was heated at 90 °Cin a Dubnoff shaker. Chromatograms were registeredafter 1, 2, 3, 4, 24, 48, 72, 96 h.

2.3. Leaching of MnO2

In each leaching test 1 g of MnO2 and 100 ml of 2%H2SO4 conc. were placed in a glass flask and warmed atconstant temperature (90 °C). Table 1 shows theconditions chosen in each set of tests. In the first set,the reaction was carried out in a Dubnoff shaker withcontrolled agitation (200 rpm). In the other tests, heatingwas provided by an electrical hot plate with magneticstirring: with the reaction flask equipped with acondenser to prevent evaporation of water. The amountof reagents was calculated on the basis of the reaction(1). The leaching experiments were replicated at leastthree times. Samples of the leaching solution were takenat known time and analyzed.

2.4. Leaching of manganese ore

Leaching tests were similarly performed with 5 g of alow grade ore containing 20%Mn as MnO2 with 100 ml

236 G. Furlani et al. / Hydrometallurgy 81 (2006) 234–240

of 2% H2SO4 conc. and a stoichiometric amount ofglucose according to reaction (1) (i.e. 0.27 g). Theleaching was carried out for 6 h at 90 °C.

2.5. HPLC analysis

The HPLC behaviour of aldoses, acids, ketones andaldehydes on a strong cation exchange stationary phasewas already investigated (Pecina et al., 1984; Fischer etal., 1995). In the present work chromatographic se-parations were achieved with an Aminex HPX-87Hcolumn (300×7.8 mm) provided with a guard column ofthe same type. The eluent was sulphuric acid 0.01 M,flow rate 0.8 ml/min. The columns were thermostated at60 °C by a column heater. Sample loop has 20 μlvolume. Detection was performed by an UV detector at210 nm. Samples were filtered with a 0.45 μm filterbefore injection.

Chromatograms of the following solutions werecharacterised:

– Solutions of pure compounds (see Table 2)– Mixtures of pure compounds in 2% sulphuric acid– Heated solutions of 3 g/l glucose in 2% conc.sulphuric acid

– Leach liquors.

The chromatograms of solutions of single com-pounds and of their mixtures were analysed at 60 °C andat room temperature in order to verify peak correspon-

Table 2Oxidation and dehydration products of glucose according to theirelution time

Compound Elution time (min)

Sulphuric acid 6.1Glucaric acid 6.2Oxalic acid 6.5Meso-oxalic acid 6.7Glucuronic acid 7.5Tartronic acid 7.7Tartaric acid 8.2Gluconic acid 8.5Glyoxylic acid 9.3Glyceric acid 10.3Malic acid (1) 10.5Glycolic acid 11.6Formic acid 13.2Acetic acid 14.9Malic acid (2) 15.3HMF 27.3Furfural 41.2

Column Aminex HPX-78H; eluent sulphuric acid 0.01 M; flow rate0.8 ml/min; T=60 °C (malic acid shows two peaks).

dence with that of the leaching solution. Peak identifi-cation was further confirmed by adding knowncompounds to the leach liquor. Formic, glycolic andglyceric acid were quantified by a calibration plot.Quantitative analysis of formic acid in the leach liquorwas performed on suitably diluted samples.

2.6. Manganese

Manganese(II) concentration was measured by anInductively Coupled Plasma Spectrophotometer(ICP).

2.7. Glucose

Glucose was determined by a colorimetric method.Leach liquor samples were diluted 1:100 and then 1 mlof diluted sample was mixed with 5 ml of anthronereagent (380 ml concentrated H2SO4+150 ml H2O+0.75 g anthrone). The mixed solutions were heated at90 °C for 17 min, and then the absorbance wasdetermined at 625 nm.

3. Results

3.1. Pure compound identification—retention time ofsingle compounds and mixtures

The elution order of possible derivatives of glucosewas first recorded. On the basis of data known from theliterature, oxidation products of glucose may be loweraldose, poly-hydroxyacids as aldonic and aldaric acids,formic acid and CO2. As the leaching was carried out inacidic media at high temperature, dehydration of sugarmight also occur (Feather and Harris, 1997): so twoprobable dehydration products (hydroxymethyl–furfu-ral–HMF and furfural) were tested also. Table 2 reportsthe reference compounds investigated according to theirelution order.

3.2. Glucose with sulphuric acid on heating

To investigate glucose degradation at high temper-ature, a solution of glucose in sulphuric acid washeated up to 96 h. Over a short time of heating nosignificant degradation of glucose was observed andthe chromatograms of the solution show only theintense peak of sulphuric acid. Some little peaksappear after 24 h and after prolonged heating (96 h)chromatograms show four peaks, of moderate intensity,assigned to gluconic and formic acid, furfural andHMF.

Fig. 2. Chromatograms of three different leach liquors andcomparative solution: I) after 60 min complete dissolution ofMnO2 (conditions as 4th set of tests); II) after 90 min leaching ofore; S) comparative mixture (a—H2SO4 0.3 M; b—gluconic acid0.1 mM; c—glyceric acid 0.1 mM; d—glycolic acid 2 mM; e—formic acid 0.03 M).

237G. Furlani et al. / Hydrometallurgy 81 (2006) 234–240

3.3. Leaching of MnO2

3.3.1. Leaching of synthetic MnO2

In order to fittingly follow the formation of organicspecies, the operative conditions of the first set of testswere chosen with the purpose of slowing down thereaction. Leaching of MnO2 was performed usingcoarser MnO2, whilst thermostatting and stirring wasperformed by a Dubnoff shaker. Although the amountof the reducing agent was twice the stoichiometricrequired from reaction (1), only 20% of manganesewas leached after 24 h. To improve the reaction rateand get total dissolution of MnO2, the stirring speedwas improved using a magnetic stirrer giving up to60% MnO2 dissolution after 24 h. In the third set,near complete dissolution was achieved in a shortertime by leaching a finer grained MnO2. In the fourthset of experiments the amount of glucose was increasedand total dissolution was achieved after only 60 min(Fig. 1).

The chromatographic pattern was very similarbetween sets of experiments on coarser and finerMnO2. Fig. 2 shows the chromatogram (labelled as I)of the leach liquor obtained after complete dissolution offiner MnO2 and compared with a solution of purecompounds. Peak “a” is due to sulphuric acid whilstpeak “e” of formic acid is always the first appearing inthe course of reaction and the most intense. Two minorpeaks are attributed to glyceric acid (peak c) andglycolic acid (peak d). A very little peak also appearscorresponding to gluconic acid (peak b). In thechromatograms of the complete leaching of finerMnO2 a modest peak appears at a retention time higherthan that of formic acid. This peak cannot be assignedyet because its retention time does not correspond withany of those of the compounds reported in Table 1. Asthe varied leaching conditions speed up the reaction, theformic peak appears after 4 h in the first set of tests, after

Mn dissolution

0%

20%

40%

60%

80%

100%

0 5 10 15 20 25t(h)

dis

solu

tio

n

I set II set IV set

Fig. 1. Leaching of Mn(II) in different tests.

Leaching of fine grained MnO2

0,00 20 40 60 80

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

t(min)

g/l

Mn Cn(H2O)n Form

Fig. 3. Manganese, carbohydrates and formic acid concentration inliquor during leaching of fine grained MnO2 (4th set of tests).

Leaching of manganese ore

0,00 2 4 6 8

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

t(h)

g/l

Mn Cn(H2O)n Form

Fig. 4. Manganese, carbohydrates and formic acid concentration inliquor during leaching of manganese ore.

238 G. Furlani et al. / Hydrometallurgy 81 (2006) 234–240

2 h in the second and after only 5 min in the fourth. Theother peaks behave similarly.

Fig. 3 shows manganese, carbohydrates and for-mic acid concentration in the leach liquor during theleaching of finer MnO2 in the fourth set of tests.

3.3.2. Leaching of manganese oreOne set of leaching tests on a low grade manganese

ore were performed in order to compare the behaviour ofglucose with real ores. Fig. 4 shows the Mn concentra-tion in leach liquor as a function of leaching time,together with that of total carbohydrates and formicacid. At the end of the leaching test 70%Mn had leachedwhilst the concentration of total carbohydrates de-creased slightly during the first 2 h and fell to almostzero after the third hour. Formic acid concentrationremained constant after the 2nd hour of leaching andranged from 3 to 4 moles per mole of glucose initiallyadded.

A chromatogram of the leach liquor is reported inFig. 2 (labelled as II) and shows the usual intense peakof formic acid. However, two smaller peaks appear at alower retention time assignable to gluconic and glycolicacid. At the beginning of the leach, a high intensityunassigned peak appears which decreases during thecourse of the reaction.

CHO COOH COO

(CHOH)4 � (CHOH)4 CHO

CH 2OH CH2OH CH2

Glucose Gluconic acid Glycacid

traces

Fig. 5. Identified oxidation d

4. Discussion of results

Even under the most favourable conditions, thecomplete dissolution of MnO2 requires more glucosethan expected on the basis of reaction (1). One mole ofglucose reacts with 4 moles of MnO2 instead of 12expected from reaction (1). When MnO2 is completelydissolved at least 3 moles of formic acid per mole ofglucose are formed.

Chromatographic patterns were very similar underthe various leaching conditions: so the degradation ofglucose follows similar pathway whether MnO2 is con-sumed completely or not. However, under the small-scale conditions employed here, glucose may be con-sumed by the action of competing oxidants other thanmanganese dioxide, probably dissolved oxygen. Thisinhibits MnO2 reduction and its dissolution.

The presence of high concentration of formic acidsuggests sequential detachment of a single carbon atomfrom glucose giving formic acid. There is evidence ofmono-carboxylic poly-hydroxyacids such as glycericand glycolic acids: but their concentration is two ordersof magnitude lower than that of formic acid. Traces ofgluconic acid are also formed. Glycolic, glyceric andgluconic acids may accumulate in reaction mediabecause of slow degradation reaction rates. No evidenceof the other compounds reported in Table 2 was found.

As reported by various studies, detachment fromglucose of one carbon atom as formic acid leads to theformation of lower aldose (arabinose). Even if aldosesare not evidenced in the chromatograms using an UVspectrophotometer, their formation can be excluded. Infact, aldoses other than glucose are positive to theanthrone test. The rapid decrease of absorbance in thistest suggests that glucose is consumed rapidly withouttransforming into other carbohydrates.

The data collected in this study, even if not exhaustive,suggest that during the leaching reaction of pure MnO2,the carbon chain of glucose is shortened to lower aldaricacids by the detachment of formic acid via the formationof mono-carboxylic poly-hydroxyacids (see Fig. 5).

H COOH HCOOH

H CH2OH

OH

eric Glycolic Formic acid acid

prevailing product

erivatives of glucose.

239G. Furlani et al. / Hydrometallurgy 81 (2006) 234–240

During the leaching of the ore, the oxidation ofglucose seems to follow a different reaction pathway.Results from the anthrone test show that the totalconcentration of carbohydrates decreases slowly in thefirst 2 h. This could be due to the transformation ofglucose to lower aldoses (positive to the anthrone test).In this case, chain shortening would proceed viaformation of lower aldoses and formic acid. Moreover,chromatograms show an intense peak, which has yet tobe assigned, as evidence for the formation of anunknown intermediate.

The existence of different reaction pathways betweenleaching of pure compound and leaching of real orecould be due to the presence of other impurity metals inthe MnO2 mineral which reduces its activity and Eh.This can explain why the two processes require differenttime to reach the complete dissolution of MnO2.

5. Conclusions

Leaching of manganese ores can be performed usingcarbohydrates as reducing agents. Glucose was used as amodel compound. The aim of this study was to identifythe oxidation products and oxidation pathway. Leachingof MnO2 was performed in sulphuric acid under varyingreaction conditions. Data obtained suggest that glucoseis oxidized via formation of mono-carboxylic poly-hydroxyacids and formic acid. Formic acid wasidentified and quantified as the major component.Poly-hydroxyacids, especially glyceric and glycolicacid, were found in minor quantities together withtraces of gluconic acid.

Experimental data obtained from tests on naturalmanganese ore suggest that a different reaction pathwaytakes place. The oxidation of glucose during theleaching of natural ores needs further explanation andwill be the object of future investigations.

Acknowledgements

The authors would like to thank Prof. L. Campanellafor granting the use of the HPLC.

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