process flow-sheet for gold and antimony recovery from stibnite

.Hydrometallurgy 57 2000 187199www.elsevier.nlrlocaterhydromet

Process flow-sheet for gold and antimony recovery from stibniteS. Ubaldini a,), F. Veglio b, P. Fornari a, C. Abbruzzese a`

a Consiglio Nazionale delle Ricerche, Institute of Mineral Processing, Via Bolognola 7, 00138 Rome, Italyb Uniersity of Genoa, Department of Chemical and Process Engineering, Faculty of Engineering, ia Opera Pia 15, 16145 Genoa, Italy

Received 10 February 2000; received in revised form 20 April 2000; accepted 26 April 2000

Abstract

Recovery of gold from refractory ores requires a pretreatment to liberate the gold particles from the host mineral. In . . .particular, in the case of stibnite Sb S , the antimony Sb forms stable compounds with sodium cyanide NaCN during2 3

the cyanidation process; as a consequence, cyanide consumption increases. Pretreatment is usually an oxidation step. As analternative, chemical leaching can be applied to liberate the gold particles from the sulfur matrix.

The aim of the present investigation was to ascertain at laboratory scale the best conditions for alkaline leaching of y1 .a refractory gold-bearing Sb S 13.25% Sb S ; 30 g t Au coming from South America. The solutions were constituted2 3 2 3

. .by sodium sulfide Na S and sodium hydroxide NaOH . Main parameters studied were: Na S concentration, NaOH2 2concentration, pulp density and temperature.

After leaching, antimony has been recovered by electrodeposition, in order to increase the economical convenience of thesubsequent gold extraction. Antimony recovery has been about 70% Sb for suitable conditions of leaching and electrowin-ning. Metallic antimony with high purity was obtained.

After the study of the leaching parameters, the influence of the pretreatment on the cyanidation process has beenevaluated. It was revealed that the chemical pretreatment improves the gold extraction yield and favours a low consumption

.of reagents: after cyanidation low recovery has been obtained without pretreatment about 30% Au , while a high gold .recovery was achieved in the case of the pretreated samples about 75% Au , considering also the subsequent steps of carbon

concentrationpurification and electrowinning.In conclusion, experimental results have shown the technical feasibility of the alkaline leaching pretreatment prior to the

conventional cyanidation; moreover, a complete process flow-sheet with low environmental impact, considering technicaland economical factors, is proposed. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Stibnite; Gold; Antimony; Alkaline pretreatment; Factorial experiments; Cyanidation; Refractory ores

1. Introduction

Generally, extraction and recovery of antimonyare realised by pyrometallurgy, which involves theevolution of toxic gases into the atmosphere; severalhydrometallurgical methods have been proposed

) Corresponding author. Tel.: q39-6-880-4463. .E-mail address: [email protected] S. Ubaldini .

w x13 . Different leaching agents can be used for .treatment of stibnite Sb S ore: mixture of hydro-2 3

chloric and tartaric acids, mixture of nitric and tar-w xtaric acid and hot concentrated sulfuric acid 4 .

Sb S can be dissolved also by alkaline solutions,2 3 .which consists of sodium sulfide Na S and sodium2

. w xhydroxide NaOH 2,5 . The development of alter-native methods could prove to be one of the factorswhich can help extractive metallurgy industry the

0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. .PII: S0304-386X 00 00107-9

( )S. Ubaldini et al.rHydrometallurgy 57 2000 187199188

most. In recent years, various studies have beenw xpublished on this topic 69 . Of the numerous anti-

.mony minerals, Sb S , tetrahedrite Cu SbS and2 3 3 3lead ores are the major sources as far as commercialextraction is concerned.

Decomposition of Sb S yields compounds such2 3as thioantimonites, which react with oxygen of thecyanide solution to form antimonites, during the

w xconventional cyanidation process 6 , while Sb reacts .with sodium cyanide NaCN forming stable com-

pounds; this fact generally involves large consump-tion of cyanide. Moreover, if gold is finely locked ina sulfide matrix, the gold leaching agent cannot

w xreach particles easily 10 so basic treatment can beutilised to liberate gold particles from the lattice.

In the present work, alkaline leaching pretreat-ment is proposed as an alternative to roasting andpressure leaching to break down the sulfides so that

w xthe gold is liberated 7,11,12 . This type of pretreat-ment offers several advantages among which standout: a reduction in the consumption of energy andreagents, lower operating costs, possibility of treat-ing low grade resources. Moreover, the process hasin general lower, environmental impact with respect

w xto conventional pyrometallurgical methods 10,13 .During the experimental work, the main stoichio-

metric conditions for leaching of the Sb S and the2 3electrolytic recovery of the antimony were studied:leaching solutions were submitted to electrodeposi-tion for metallic antimony recovery. Solid residuesof the alkaline process have been treated by cyanida-tion, to allow evaluation of the effect of the leachingof antimony on the gold recovery process. Antimonywas recovered as metal from alkaline solution bycathodic electrodeposion: several industrial applica-

w xtions are possible 14,15 .The main objective of this experimental work

conducted at laboratory scale was to develop aninnovative process, involving an alkaline chemicalpretreatment and a conventional treatment for goldextraction from refractory Sb S ores, using cyanide2 3

w xas lixiviant 6,7,13 used in a standard procedurecyanidation, gold adsorptionrdesorption and elec-

. w xtrowinning 1619 .

1.1. Theoretical basis

Dissolution of the Sb S in alkaline solution in-2 3volves the forming of various soluble species, such

as antimonites, thioantimonites, oxothioantimonites,sulfites, thiosulfates, etc. Sb S reacts with Na S;2 3 2

w xthe main reaction is the following 2,11,20 :Sb S q3Na S2Na SbS 1 .2 3 2 3 3

Generally, NaOH is added to prevent hydrolysisw xof Na S 2,5 :2

Na SqH ONaHSqNaOH 2 .2 2NaHSqH OH SqNaOH 3 .2 2

The overall equation is:Na Sq2H OH Sq2NaOH 4 .2 2 2

NaOH can also solubilize the Sb S , producing2 3w xalkaline oxothioantimonites and thioantimonites 2,9 :

Sb S q2NaOHNaSbOSqNaSbS qH O 5 .2 3 2 2In this way it is also possible to treat other

compounds of the antimony, in particular Cu SbS ,3 3w xaccording to the following chemical reaction 21 :

Na Sq2Cu SbS 3Cu Sq2NaSbS 6 .2 3 3 2 2After the leaching step in which the Sb dissolu-

tion takes place, this element can be recovered byelectrowinning to obtain metallic Sb. The main reac-tions involved in the electrodeposition process of the

w xantimony are the following 22 :Cathodic reaction:

4Naqq4SbSyq12eys4Sbq4Naqq8S2y2Esy0.85 V 7 .

Anodic reaction:12Naqq12OHys12Naqq6H Oq3O q12ey2 2Esy0.40V 8 .

The chemical reaction for the gold dissolution bycyanidation and gold recovery by electrowinning are

wwell-documented and reported elsewhere 14,16,x19,23 .

2. Experimental

2.1. Ore characterisation

This investigation was conducted on a refractorygold-bearing ore, a Sb S ore sample coming from2 3

.Antofagasta mine Bolivia . Representative samples

( )S. Ubaldini et al.rHydrometallurgy 57 2000 187199 189

Table 1Chemical analysis of the ore sample

.Components wrw %SiO 56.152Sb S 13.252 3FeS 11.512Al O 6.382 3TiO 2.082MgO 2.01K O 0.362Pb 0.38MnO 0.06CaO 0.95Sn 0.08Cr 0.05As 0.10Cu 0.10Zn 0.05

y1Au 30 g ty1Ag 4 g t

L.O.I. 6.49

L.O.I.sLoss on ignition.

were prepared and ground to y74 mm for character-isation and experimental tests. Mineralogical charac-

.terisation by X-ray Diffraction XRD showed the . .presence of quartz SiO , Sb S and pyrite FeS ,2 2 3 2

in decreasing order of abundance.Qualitative analysis conducted by X-ray Fluores-

.cence Spectrometry XRF demonstrates the pres-ence of the following elements: Si, Sb, Fe, Al, Ti,Mg, K, Pb, Mn, Ca, Sn, Cu, As, Zn: quantitativechemical analysis was performed by Inductively

.Coupled Plasma Spectrometry ICP : gold content of30 g ty1 and silver content of 4 g ty1 were ascer-

.tained Table 1 . Pure Sb S analytical grade was2 3also employed in the preliminary factorial experi-ments.

2.2. Chemical reagents

For chemical dissolution of the mineral, NaOHand Na SP9H O were utilized. Cyanidation tests2 2were carried out using dilute solution of NaCN using

. .calcium hydroxide Ca OH to adjust the pH at 11.2For adsorption of the complex Au-NaCN, activatedcoconut carbon was used; the size was between 0.5

w xand 2 mm 16 . Desorption was conducted by 15% . .vrv of ethanol and 1% wrv of NaOH solution.

All the reagents were of analytical reagent grade.De-ionized water was used in all the experiments.

2.3. Antimony recoery by alkaline leaching andelectrowinning

Two kinds of leaching tests were carried out inw xthe present work 2,3,5,20,21 . A first series of leach-

ing tests was carried out by using pure Sb S in a2 3w xtwo-level full factorial experiment design 24 . The

main goal of these was to evaluate the effect of themain process parameters on the antimony dissolu-tion. The four investigated independent parametersare indicated in Table 2, the dependent variable

.being the antimony extraction % in the solution at .the end of the leaching test 1 h . A systematic

w xANOVA by Yates method 24 was used in theinterpretation of the experimental results obtainedfrom the factorial design at two levels. This permit-ted assessment of the main effects of factors and theinteractions among them. Although the Sb extractionyield after 1 h of leaching has been considered in theANOVA, several samples were also collected duringthe leaching process for chemical analysis.

The second series of leaching tests was carriedout using the gold-bearing ore. In that case, samples

.of 200 g particle size-74 mm were dressed. Ex-periments were carried out in a 1 L glass reactormechanically stirred at 500 rpm. Total time of treat-

.ments was 1 h see Table 3 . Temperature wasregulated by thermostatic bath. The main goal ofthese tests was to compare the results with thoseobtained for Sb S and to supply a real liquor leach2 3solution for following electrowinning tests.

In both cases, quantitative chemical analyses ofthe liquid solutions were conducted by atomic ad-

Table 2Experimental conditions for alkaline leaching tests: stirring rate200 rpmFactor Factor Levels .code y1 0 q1

.A Na S concentration % wrv 1.0 2.5 4.02 .B NaOH concentration % wrv 1.0 2.5 4.0

.C Pulp density % wrv 0.5 1.25 2.0D Temperature 8C 30 40 50


Table 3Selected experimental factors for alkaline leaching testsFactors Levels

.Weight of the samples g 200 .Ore concentration % wrv 10

.Sb S content % wrv 0.72 3 .Na SP9H O % wrv 0.72 2

.NaOH % wrv 0.7 .Temperature 8C 50

.Time min 60 .Particle size mm 74

y1 .Stirring rate rev min 500

sorption technique after filtrationrcentrifugation pro-cedures.

At the end of the second series of leaching tests,solidrliquid separation was carried out by automaticMillipore filtration using 1.2 mm membrane. Repre-sentative solid samples were analysed by XRD.

After the liquor leach recovery, the antimony wasrecovered from leaching solutions by electrodeposi-tion, whereas solid residues were submitted tocyanidation for gold recovery.

Electrowinning tests were conducted under mag-netic stirring conditions in a glass electrolytic cell ofabout 200 mL, connected to a thermostatic waterjacket. The cell was equipped with three electrodes:

.a steel cathode working electrode , a steel anode .counter electrode and a saturated calomel electrode . w xSCE as reference electrode 25,26 . The cell wasconnected to the potentiostatgalvanostat apparatus

.mod. 555B Amel, Italy , equipped with an instru-ment system to automatically control process param-

eters programmable function generator mod. 568,interface mod. 560rA, integrator mod. 721, differen-

.tial electrometer mod. 631 .Metallic antimony was recovered from the leach-

ing solution on the cathode. The cathode usefulsurface was 60 cm2, the external surface being madeinert by using insulating plastic material. Average ofinitial antimony concentration was of 20 g Ly1,while 180 mL was the solution volume utilised aselectrolytic bath. Tests at various levels of current

2 .density 100, 125, 150, 175 Arm were carried out,selecting time, temperature, pH and stirring condi-

. w xtions see Table 4 4,25 . During the laboratorytests, samples of solution were withdrawn to verifythe trend of the process vs. time. Antimony deposit

was detected gravimetrically and previous dissolu-tion process was also analysed to evaluate puritywith respect to the minor elements, using a PerkinElmer ICP Spectrophotometer. Quality of the metal-lic deposit was evaluated by XRD technique.

2.4. Cyanidation of the pretreated residue for goldrecoery

After alkaline leaching, the pretreated solid residueobtained was filtered, washed, dried, weighed andsubmitted to cyanidation. This was conducted at258C in a hemispherical 3 L glass reactor, utilising

w xstandard parameters 23 . 1 kg of solid sample was .charged in the glass reactor to give 50% wrv pulp

.density, selecting a pH 11 adjusted by Ca OH ,2while stirring rate was fixed at 300 rpm. NaCN

y1 w xconcentration was 25 g L 23 . Samples ofcyanide-leached solution were taken periodically forgold determination by atomic adsorption spectro-

.photometer AAS . Five experimental tests were car-w xried out to evaluate the experimental error 27 .

2.5. Concentration, purification and precipitation of(the gold adsorption r desorption r electrowinning

)tests

At the end of the cyanidation test, the pulp wasfiltered: this was submitted to gold analysis by AAS,after chemical dissolution. The leached Au was con-centrated and purified by adsorption onto activated

w xcarbon 16 and subsequent desorption processw x18,28 . This experimental phase was carried out in a

.fixed bed glass column 50 mL volume , with two .stages adsorption and desorption : the liquor leach

and successively the stripping solution passedthrough the fixed bed bottom via a peristaltic pump,in thermostatic conditions.

Table 4 .Parameters for electrowinning tests Sb recovery

Factors Levels .Bath temperature 8C 50

.Cell voltage V 2.23.0 .Current intensity mA 6001,050

pH 12.8 .Time h 6

.Stirring rate rpm 200


Table 5Parameters for gold adsorption onto activated carbonParameters Levels

.Temperature 8C 25Flow rate of stripping 5

y1 .solution mL miny1 .Cyanide concentration g L 25

pH 11 .Time min 30

.Active coconut carbon 10 g was used as adsor-bent material, with a particle size distribution in therange 0.52 mm. After the desorption stage, un-loaded carbon was regenerated by chemical and ther-mal treatments; then it was dried at room tempera-ture before being weighed for subsequent adsorption

w xtests 28 . Table 5 summarizes the investigated pa-rameters applied to gold adsorption onto activated

w xcarbon 16 . After adsorption, gold concentration was4.8 mg Aurg carbon. Table 6 shows the experimen-tal conditions for gold desorption from activated

w xcarbon 17 .After gold desorption, samples of carbon and

solutions were analysed for gold determination, andthe enriched gold solution was stored for the subse-quent electrowinning process. The electrolysis testswere carried out treating 180 mL of gold cyanide

.solution 94 mgrL Au , obtained after the desorp-w xtion stage 14,19 .

On the basis of the previous study, the auriferoussolution was submitted to the electrowinning processusing an electrolytic glass cell of 200 mL capacity,previously described for the metallic antimony re-

Table 6Parameters for gold desorption from activated carbonParameters Levels

.Temperature 8C 70Flow rate of stripping 5

y1 .solution mL minCyanide concentration 25

y1 .g LEthanol concentration 15 .% vrvpH 11

.Time h 2

Table 7 .Parameters for electrowinning tests Au recovery

Factors Levels .Bath temperature 8C 40

.Cell voltage V 2.52.8 .Current intensity mA 80

pH 11 .Time min 40

.Stirring rate rpm 200

w xcovery 25 . Metallic gold was recovered onto a steelcathode, while platinum wire was used as the anode.Table 7 reports the main parameters of electrowin-ning tests.

3. Results and discussion

3.1. Alkaline leaching of the synthetic Sb S and2 3Sb S ore2 3

Before performing the overall leaching study onthe ore, preliminary replicated leaching tests werecarried out using synthetic Sb S and ore under2 3study, in order to evaluate the reproducibility of the

.experimental results data not reported here . Twomain conclusions were obtained from the analysis ofthese tests: the Sb dissolution is very fast in the

investigated experimental conditions T s 808C;Na SP9H O 20 grL; NaOH 20 grL; ore concentra-2 2

.tion 100 grL and reproducible Sb extraction yieldswere obtained in the five replicated tests for eachemployed mineral: 85% of Sb extraction yield wasobtained in the first 5 min of leaching and it reaches

its maximum value after 1 h of treatment about.90% . Similar results were obtained using pure syn-

thetic Sb S under the same experimental conditions2 3the Sb S mineral concentration was selected to2 3give similar content of Sb such as in the investigated

.ore . In this case, Sb extraction values were largerwith respect to the ore leaching: a difference ofabout 10% in terms of Sb extraction yield was found.This last result may be explained by the presence ofother minor chemical reactions in the ore, due to thepresence of other mineralogical forms that consumethe chemical reagents employed in the leaching tests.


Table 8Treatments of the 24 full factorial experiment and antimonyextraction yield in the leaching tests

.No. run Treatment A B C D Time min

5 15 30 60 .1 1 y1 y1 y1 y1 59.9 81.6 85.0 84.1

2 A 1 y1 y1 y1 82.3 96.7 99.9 100.03 b y1 1 y1 y1 79.3 83.6 83.0 100.04 ab 1 1 y1 y1 85.7 83.0 93.9 97.65 c y1 y1 1 y1 34.6 44.7 50.4 47.56 ac 1 y1 1 y1 66.3 78.6 80.2 86.77 bc y1 1 1 y1 74.9 85.1 85.7 80.68 abc 1 1 1 y1 89.3 89.6 93.7 91.59 d y1 y1 y1 1 71.3 84.0 80.9 89.410 ad 1 y1 y1 1 89.2 91.9 92.5 93.311 bd y1 1 y1 1 94.4 89.1 95.0 99.712 abd 1 1 y1 1 92.6 85.7 93.6 97.913 cd y1 y1 1 1 44.4 53.1 54.8 56.514 acd 1 y1 1 1 79.5 88.7 89.1 90.015 bcd y1 1 1 1 95.6 97.9 97.1 94.116 abcd 1 1 1 1 79.2 89.2 90.8 91.5R1 central point 0 0 0 0 80.4 91.2 95.6 98.0R2 central point 0 0 0 0 80.4 93.6 88.2 100.0R3 central point 0 0 0 0 81.9 93.0 92.2 99.5R4 central point 0 0 0 0 73.5 94.6 93.1 97.5R5 central point 0 0 0 0 79.0 93.0 92.3 98.8

After these preliminary tests, a two-level 24 full .factorial experiment was planned see Table 2 in

order to evaluate the influence of the main process

parameters in the chemical dissolution of Sb startingfrom the pure synthetic Sb S .2 3

Table 8 indicates the investigated treatments andthe related experimental results obtained in the 16tests, plus five replicated leaching tests in the centralpoint of the experimental design. This last experi-mental condition was performed to evaluate the ex-perimental error variance to be used in the related

w xANOVA 24 : this value was estimated as 10.6%with 14 degrees of freedom by using also the repli-cated tests obtained in the preliminary leaching testsw x24 . The evaluation of the main and interactioneffects coupled with the relative statistical F-tests fortheir significance are reported in Table 9. Fig. 1shows the Sb extraction yield calculated by factorialdesign model including only the significant effects

w xfound in Table 9 24 . The analysis of these resultspermits verification of the good assumption of thenormal distribution of the experimental error that is,in general, assumed before performing the F-tests in

w xthe ANOVA analysis 24 .From the analysis of the results, it was observed

that all the factors examined had a significant effecton the antimony dissolution. In particular, as ex-

pected, the Na S and NaOH concentrations factors2.A and B have a positive effect on the Sb extraction

yield, the mineral concentration has a negative effect .factor C and, finally, although the temperature

Table 9 . .Analysis of variance ANOVA of the Sb extraction yield SbEY after 1 h of leaching by Yates method

. . . . . . .Run no. SbEY % I II III IV Effects % MS F Significance %

1 84.09 184.1 381.7 688.0 1406.82 100.00 197.6 306.3 718.8 102.8 A 12.85 660.9 62.23 100.03 100.00 134.2 380.3 63.6 99.1 B 12.39 614.3 57.85 100.04 97.61 172.1 338.5 39.2 y94.7 AB y11.84 561.0 52.82 100.05 47.50 182.7 13.5 51.5 y117.3 C y14.66 859.3 80.92 100.06 86.66 197.6 50.1 47.7 71.8 AC 8.98 322.3 30.35 99.997 80.60 152.9 2.0 y46.6 42.3 BC 5.29 111.8 10.53 99.418 91.51 185.6 37.3 y48.2 y46.7 ABC y5.84 136.6 12.86 99.709 89.44 15.9 13.5 y75.4 30.8 D 3.86 59.5 5.60 96.7110 93.28 y2.4 37.9 y41.8 y24.4 AD y3.04 37.1 3.49 91.7311 99.73 39.2 14.9 36.5 y3.8 BD y0.48 0.9 0.09 22.5412 97.88 10.9 32.8 35.3 y1.6 ABD y0.20 0.2 0.02 9.7313 56.50 3.8 y18.3 24.4 33.6 CD 4.20 70.5 6.64 97.8114 96.37 y1.9 y28.3 17.9 y1.3 ACD y0.16 0.1 0.01 7.6615 94.12 39.9 y5.7 y10.0 y6.5 BCD y0.82 2.7 0.25 37.6916 91.51 y2.6 y42.5 y36.8 y26.8 ABCD y3.35 44.98 4.24 94.13


Fig. 1. Sb extraction yield calculated vs. Sb extraction yield experimental: factorial model including only the significant main and interaction .effects see Table 9 .

exerts a positive influence in the Sb dissolution, itseffect is relatively of minor importance. This lastexperimental observation confirms the relative lowimportance of the temperature in the leaching pro-

w xcess found also in other works 5,20 : this fact mayindicate that the chemical reaction is not the kineticlimiting step of the process and then only masstransfer kinetics need to be considered in the kinetic

w xmodelling description of the process 29 . In ourcase, the kinetics were very fast and at this stage, itwas considered important just to study the effect of

Fig. 2. Two-way table to focalize the significant first order .interaction AB .

the main process parameters on the final Sb extrac-tion yield by using empirical models.

Some first-order interactions were found to besignificant in the ANOVA. In particular, Figs. 2 and3 highlight the structure of the interaction AB andAC as examples. These two interactions were se-lected because they are the largest values among thefirst order interactions. The negative interaction ABindicates that the positive effect of the Na S de-2

Fig. 3. Two-way table to focalize the significant first order .interaction AC .


Table 10Antimony recovery, from the ore, at the best experimental condi-

.tions %SbPretreatment Electrodeposition Whole process recovery

70 98.3 68.6

creases, increasing the NaOH content in the leachsolution and probably highlights that only the NaOHcontributes to the Sb dissolution as reported by Eq. .5 .

The positive interaction AC and similarly for the.interaction BC indicates that the negative effect of

the mineral concentration decreases, increasing theNa S concentration and similarly the same results2

are observed for the interaction BC regarding the.NaOH content in the leach solution . These results

may be due to stoichiometry requirement of theoverall process. Most experiments provided morethan the stoichiometric needs for complete Sb disso-lution: the Na S was always in excess with respect2

.to the stoichiometry reported in the Eq. 1 with theexception of the tests No. 5 and 7 where NaOH wasalways in excess with respect to the chemical reac-

.tion 5 in all the investigated treatments. From the .analysis of the chemical reaction 1 , a stoichiomet-

ric Na S concentration of about 1.5% should be2necessary for a mineral concentration of 2%. In fact,the experimental tests carried out with greater stoi-chiometric conditions always permitted reaching veryextensive Sb dissolution. Moreover, as reportedabove in the analysis of the two-interactions and as

.observed by Eq. 5 , the NaOH concentration seemsto have not only to hinder the Na S hydrolysis, but it2is also involved in the chemical dissolution of Sb S .2 3

From the analysis of the results obtained in thefactorial design, some preliminary leaching tests wereplanned to obtain Sb dissolution and to perform apretreatment for the following cyanidation process ofthe leach solid residues. At the same time, the liquor

leach of the Sb S leaching was used after filtra-2 3.tion as suitable solution for metallic antimony re-

covery by electrowining. Table 3 shows the selectedexperimental conditions investigated for the oreleaching by Na S and NaOH although several other2tests, changing mainly the stirring conditions, have

.been carried out data not reported here : as observed

w xin the literature review 5,20 , the effect of thestirring conditions on the kinetic dissolution processhas been observed, confirming that mass transfer inthe fluid external film may be considered the kinetic

w xlimiting step 29 . Regarding the other selected ex-perimental conditions, the Na S and NaOH were2selected by using similar reagent excesses as testedin the factorial experiment, the temperature was setat 508C taking into consideration its positive effecton the Sb extraction yield. It is clear that optimisedconditions can be evaluated when a cost function isintroduced in process analysis. Since our main goalwas to evaluate this leaching process also as apretreatment process for gold recovery in the follow-ing cyanidation process, only the Sb extraction re-sults have been considered after 1 h of leaching inthe selected experimental conditions. The maximumSb extraction yield was obtained after 5060 min ofleaching in the selected experimental conditions: asolution containing 70% of the initial Sb was at-

.tained Table 10 . The Sb extraction yield that shouldbe obtained by using the factorial model is about80%. As observed in the preliminary leaching tests,there is a systematic difference of 10% in terms ofSb extraction yield when we compare the Sb S2 3leaching of pure synthetic Sb S and the real ore.2 3

The Sb dissolution was also confirmed by XRDsemi-quantitative analysis of the solid residue.

Fig. 4. Concentration decay curves of antimony by electrowin-ning.

()

S.Ubaldini

etal.r

Hydrom

etallurgy57

2000187

199195

Fig. 5. X-ray diffraction pattern of metallic antimony deposit obtained at 150-Army2 current intensity.


3.2. Sb recoery by electrowinning

After this phase of the study, experimental tests ofelectrodeposition were carried out, with the aim toproduce metallic antimony. The amount of antimony

.electrodeposited was measured Fig. 4 .During the electrolysis, Sb deposits on the cath-

ode, while at the anode O forms. This fact deter-2mines the production of Na S that can be recycled to2the alkaline chemical leaching stage. In this manner,it should be possible to reduce the costs for thereagent consumption.

Best results have been obtained at current density,150 Arm2. After 6 h electrolysis, high recovery of

.antimony about 98.3% Sb in metallic form was .obtained Table 10 . Antimony deposit was adherent

to the cathode being of good morphology, excellentgrade of quality and purity XRD curves reported in

.Fig. 5 . An average Faradic efficiency of 53% wasobtained under these experimental conditions.

3.3. Cyanidation of the pretreated residue for goldrecoery

A test was carried out to evaluate the effect of thechemical pretreatment on the subsequent cyanidation

w xfor gold recovery. By statistical analysis 27 , anexperimental error of about 3% was determined.Bench scale chemical experiments permitted goldrecoveries higher than those obtained from directcyanidation; in fact, about 60% Au in solution wasobtained after 12 h of cyanidation, while about 80%Au was solubilised after 24 h from pretreated sam-ples, obtaining about 24 mg Ly1 Au in solution; datawere confirmed from gold content of solid residue y1 .about 6 g t Au . In the case of the un-pretreatedsamples, gold recovery decreases to about 30% Au .Table 11 . The results permit us to affirm thatpretreatment influences positively the gold extraction

Table 11 .Gold recovery by cyanidation %Au

Time Without With .h pre-treatment pre-treatment

1 5.2 41.112 15.3 60.224 32.1 80.1

Table 12 .Gold recoveries %Au obtained in each section of the process

Pretreatment- Gold Gold Totalcyanidation purification precipitation gold

.concentration electrowinning recoveryadsorptionr

.desorption

80.1 97.0 96.1 74.7

yield; they are very good, because the kinetics of thechemical process have not been optimised. More-over, pretreatment allows a decrease from 2 to 0.4 kgty1 of lime consumption, while cyanide consumptiondecreases from 14 to 8 kg ty1. This fact can beattributable to the alkaline environment of the pre-treatment; in fact, when pretreatment occurs in acid

w xenvironment 6,19,23 , reagent consumption in-creases during cyanidation.

3.4. Concentration, purification and precipitation of(the Au adsorption r desorption r electrowinning

)tests

The results obtained were the following: for thepurificationconcentration cycle adsorptionrdesorp-

. .tion phase 97% Au was recovered Table 12 . Anexperimental error of about 3.5% has been deter-

w xmined 27 . Very good gold concentration and purifi-cation by application of these consolidated technolo-gies, economical and environmental friendly, hasbeen obtained: gold was concentrated from about 24 . y1 leached solution to about 94 mg L Au in a

.volume of 600 mL .The possibility of re-using vegetal coconut carbon

in subsequent cycles has been determined: the un-loaded carbon was utilised five times after chemicalregeneration and thermal treatment.

More than 96% Au was recovered after 30 min ofelectrowinning from concentrated and purified solu-tions. Final concentration of gold in the barren solu-tions, after 40 min of electrolysis, resulted in -1mg Ly1 Au. Purified solutions can be recycled tocolumns in the adsorptionrdesorption stage. Thecurrent efficiency was about 6%, due to the highdilution of the solution and to the concurrent para-

sitic reactions at the electrodes. Gold deposit adher-.ent to the cathode was of good characteristics.


3.5. Preliminary flow-sheet of the integrated process

Considering the experimental results, it is possibleto hypothesise a preliminary flow-sheet for an inte-

grated hydro- and electrometallurgical process Fig..6 .

The input stream is constituted from a Sb S ore2 3with 30 g ty1 Au and 9.5% Sb: with alkaline chemi-cal attack about 70% Sb passes in solution. Theliquor leach obtained after filtration is submitted to

an electrowinning process in which about 98% Sb isrecovered at the cathode in metallic form. Final Sbdeposited at the cathode is about 69% of the initialcontent. This result can be optimised by changing theleaching process conditions and taking into consider-ation economic and environmental constraints. Theresidual solution of the process can be recycled atthe level of the chemical alkaline leaching stage.

Solid residue with 1.9% Sb is then submitted tothe circuit of gold recovery cyanidation, purifica-

Fig. 6. Schematic flow-sheet of the overall process: leaching and electrometallurgical treatment for antimony and gold recovery from arefractory Sb S .2 3


.tion, concentration and precipitation of the gold .The product attained is a waste with 6 g ty1 Au and1.9% Sb content, respectively, while about 75% Auis recovered at the cathode. Tables 10 and 12 sum-marize gold and antimony recoveries obtained afterthe main phases of the complete process. Residualsolution from electrowinning can be recycled tocolumns for gold concentration and gold purification.As in the case of the Sb pretreatment process, thispermits us to reduce the reagent consumption and itshould be important from both economical and envi-ronment points of view. A section is provided forcarbon regeneration; there is the possibility to re-usecoconut carbon in subsequent cycles as well-demon-strated in this experimental work and elsewherew x16,28 .

Although the technical feasibility of the processhas been demonstrated, critical leach parameters foruse in flow-sheet development and plant designshould be well-defined in continuous leach testing atlaboratory and pilot-plant scale.

4. Conclusions

This preliminary study demonstrates the technicalfeasibility, on laboratory scale, of the integratedprocess to treat a refractory auriferous Sb S from2 3South America.

Main experimental results of this study show that,approximately, 70% Sb and 75% Au contained in theinitial samples can be recovered by electrowinning.Moreover, the process demonstrated by technolo-gies with relative low environmental impact per-mits economic gold extraction. After 6 h electrolysis,cathodic deposition of about 98% Sb was obtained,at 150-A my2 current density with current efficiencyof 53%. Antimony deposit was adherent to the cath-ode, being of good morphology, excellent qualityand purity.

The chemical alkaline pretreatment prior to theconventional leaching causes increase in gold recov-ery. Preliminary factorial experiments were carriedout in order to evaluate the influence of the mainprocess parameters in the leaching process. Only30% Au extraction was attained after cyanidation ofthe un-pretreated ore whereas about 80% Au recov-ery was realised by cyanidation after the basic pre-

treatment. Gold recovery after the complete cycle oftreatment pretreatment, cyanidation, gold concentra-

.tion, purification and precipitation reached about75% Au.

The implementation of the gold purificationgoldelectrodeposition cycle on an industrial scale couldimprove the efficiency and the economy of the pro-cess.

Currently underway is a phase of the study con-cerning the optimisation of the parameters of thealkaline chemical treatment process, with the aim toincrease the recoveries of gold and antimony; after afirst scale-up of the process, an economic balancewill be determined. Following successful completionof the investigation, there are many possibilities fortechnology transfer.

Acknowledgements

The authors are grateful to Mr. Roberto Massidda,Mr. Marcello Centofanti and Ms. Emanuela Tem-pesta for their helpful collaboration in the experi-mental work; moreover, the authors thank Mr. Gi-ampaolo Marruzzo for XRD analysis.

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