Minerals Engineering 24 (2011) 1703–1709

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Minerals Engineering

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Diagnostic pre-treatment procedure for simultaneous cyanide leachingof gold and silver from a refractory gold/silver ore

Mojtaba Saba a, Ali MohammadYousefi a, Fereshteh Rashchi a,⇑, Javad Moghaddam b

a School of Metallurgy and Materials Engineering, University of Tehran, P.O. Box 11155/4563, Tehran, Iranb Department of Materials Engineering, Sahand University of Technology, Tabriz, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 May 2011Accepted 22 September 2011Available online 22 October 2011

Keywords:Gold oresPrecious metal oresSulfide oresCyanidationLeaching

0892-6875/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.mineng.2011.09.013

⇑ Corresponding author.E-mail address: rashchi@ut.ac.ir (F. Rashchi).

This study investigates the optimization of simultaneous dissolution of gold and silver from a refractorygold ore through determination of pre-treatment stages. Based on the mineralogical studies (thin layerand polished section) and chemical analysis on the ore sample, a ‘‘diagnostic leaching’’ procedure wasdesigned. Results from diagnostic leaching suggest that the most effective pre-treatment agents for goldand silver are ferric chloride and sulfuric acid media, respectively. Optimum conditions for the simulta-neous dissolution of gold and silver were determined using a two factorial design technique. Pre-treatments with sulfuric acid and ferric chloride reagents increased the efficiency of the dissolution ofgold from 54.7% to 82% and silver from 37.4% to 81.6%.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction performance applied to sulfide ores are bio-oxidation, pressure

One of the most important problems facing the gold and silverindustry is that the placer and free milling gold and silver ores arealmost terminating. Hence, the use of refractory ores has been in-creased during the recent years. In general, gold refractory ores areclassified into carbon, sulfide and tellurium ores. Among the refrac-tory ores, the sulfide type is the most common (Lunt and Briggs,2005). Each of these ores, in turn, requires special flow sheet forgold and silver extraction (Gupta, 2003).

To extract gold and silver by hydrometallurgical route, cyanideleaching has been the dominant process in gold mining industry inthe past two centuries. To reduce environmental and toxicity risksassociated with the use of cyanide, many efforts have been made toreplace cyanide media with other reagents such as thiosulfate(Aylmore and Muir, 2001), thiourea (Groenewald, 1977), halide(Tran et al., 2001), oxidative chloride (Sparrow and Woodcock,1995), bisulfide (Hunter et al., 1998), ammonia (Han, 2001), thiocy-anate (Monhemius and Ball, 1995) and bacterial leaching (Sparrowand Woodcock, 1995). Cui and Zhang (2008) compared with differ-ent leaching methods for recovering precious metals. Cyanidationis still the dominant method for extracting gold and silver fromthe ores. However, cyanidation is not suitable for refractory ores;therefore, according to the ore characterization, a variety of pre-treatments are used to increase dissolution efficiency. The methodsused for increasing the efficiency of conventional cyanidation

ll rights reserved.

oxidation and roasting (Miller and Brown, 2005; Thomas, 2005;Thomas and Cole, 2005; Hammerschmidt et al., 2005). In cases,where gold sulfide ores are presented in the form of fine particlesinside the matrix of gangue minerals, ultra fine grinding to�10 lmis also applicable as a pre-treatment method.

In addition to the above-mentioned methods, diagnostic leach-ing based on oxidative leaching has also been used as pre-treat-ment for the refractory ores (Lorenzen and Van Deventer, 1992;Henley et al., 2000; Torres et al., 1999). The basis of this methodis, first, to identify the host phase(s) for the precious metals usinga series of oxidative leaching reagents; then, determining the re-quired pre-treatments to liberate the metals. The oxidizing mediado not have the ability to bring gold into the solution; therefore,cyanidation steps are also required. Most of the researches in thiscase are focused on gold (Lorenzen and Tumilty, 1992; Torres et al.,1999) and a few on silver (Celep et al., 2009; Rohde et al., 2011)and lead them (Greet and Smart, 2002).

The main problem facing the silver diagnostic leaching is thesilver affinity to form complexes with some of the oxidizing re-agents. In the presence of nitric acid as an oxidizing reagent, silvernitrate (AgNO3) is formed. The nitrogen in nitrate ion is reducedfrom 5 to 4 and nitrogen dioxide gas forms. Also black precipitatesof silver oxide (Ag2O) are produced (Cotton, 1997). In the presenceof ferric chloride AgCl precipitates is formed which is dissolvedlater in the presence of excessive chloride ions to form differentsilver chloride complexes of AgCl�2 and AgCl2�

3 (Griffith, 1990).According to the following reactions, the AgCl precipitates dissolvein the presence of excessive chloride ions to form different silverchloride complexes:

1704 M. Saba et al. / Minerals Engineering 24 (2011) 1703–1709

AgClþ Cl� ! ½AgCl2��ðaqÞ ð1Þ

½AgCl2��ðaqÞ þ Cl� ! ½AgCl3�

2�ðaqÞ ð2Þ

½AgCl3�2�ðaqÞ þ Cl� ! ½AgCl4�

3�ðaqÞ ð3Þ

However, the previous researchers have not considered thislarge amount of silver which could be as high as 1 g/L, as suggestedby Dinardo and Dutrizac (1985).

In this study, a diagnostic leaching process was designed for thesimultaneous dissolution of gold and silver from a refractory sul-fide ore. Accordingly, a suitable pre-treatment procedure forincreasing the efficiency of cyanide dissolution of gold and silverwas proposed. Two factorial design of experimental techniqueand ANOVA were used to optimize the pre-treatment procedurefor simultaneous cyanide dissolution of gold and silver.

2. Design of experiments

The leaching procedure used in the diagnostic leaching proce-dure in this study is shown in Table 1. This table was constructedbased on the pre-treatment leaching stages suggested by Lorenzen(1995). According to the mineralogical studies in this work, someof the stages suggested by Lorenzen (1995) were eliminated, as itis clear from Table 1 that cyanide washing by sodium cyanide(NaCN) was done to destroy the precipitated gold and silver. Inthe leaching pre-treatment stage, sodium cyanide, hydrochloricacid (HCl) and sulfuric acid (H2SO4) were used to leach gold, cal-cite and the labile base metal sulfides, respectively. To destroysphalerite, galena and tetrahedrite, ferric chloride (FeCl3) wasadded and nitric acid (HNO3) was the leaching reagent for the pyr-ites mineral.

Optimization of the pre-treatment stages was carried out usinga two factorial technique. Optimization parameters were consid-ered as categorical parameters to determine their value at whichmaximum dissolution of gold and silver were reached. By usingtwo factorial technique, oxidizing reagents in diagnostic leachingwere divided into two categories. The first group (Factor A) con-sisted of ferric chloride and hydrochloric acid and the second group(Factor B) consisted of sulfuric acid and nitric acid, are illustrated inTable 2.

Table 1Mineralogical analysis and selective pre-treatment leaching stages and the minerals destr

Metallic phase (10%) Non-metallic phase (90%) Pre-treatment stag

Content Content

Pyrite 65–70 Silica 60 NaCN washingGalena 15–23 Barite 25 NaCNSphalerite 3–7 Calcite 15 HClTetrahedrite <2 H2SO4

FeCl3

HNO3

Table 2Experimental design using two factorial technique.

Std Run Factor A F1 = FeCl3, �1 = HCl 1

2 1 1 �3 2 �11 3 �1 �4 4 1

3. Experiments

3.1. Materials

The sulfide ore used in this study was obtained from Sardashtarea in Northwestern of Iran. Elemental analysis of the sample ispresented in Table 3. It is shown that the ore sample containedca. 5.4 ppm Au and 325 ppm Ag. Inductively coupled plasma(ICP) model VISTA-PRO was used to measure gold and silver re-ported to the solution. The leach residue was also analyzed by fireassay and atomic absorption spectroscopy (AA) to determine massbalance for both gold and silver and calculate the metals recovery.

The ore sample was divided into two homogeneous fractions.For the mineralogical tests, one fraction was divided into six sizefractions; 100% passing coarser than 300 lm (+50 mesh), 212–300 lm (50–70 mesh), 150–212 lm (70–100 mesh), 75–150 lm(100–200 mesh), 63–75 lm (200–230 mesh) and finer than63 lm (�230 mesh). For the diagnostic leaching experiments, an-other fraction of the homogenized ore sample was ground into80% passing 37 lm (400 mesh) using a laboratory ball mill for45 min. All of the reagents used in this study were analytical gradeobtained from Merck.

3.2. Mineralogical analysis

To design a diagnostic leaching procedure efficiently, a com-plete mineralogical examination of the ore sample is required(Lorenzen, 1995). Mineralogical analysis was done using X-ray dif-fraction (XRD), polished sections and thin sections microscopy tostudy all the phases presented in the ore sample. For this purpose,first, the sample was divided into six size fractions (Section 3.1)and then, thin and polished sections were prepared from each sizefraction. Thin sections were prepared by cementing a thin slice ofthe tailing to the glass. A cover slip was cemented on top of thesample. Transmitted light microscopy was used to examine miner-als that transmitted light in the thin sections. These minerals in-cluded non-metallic minerals. A polished section of the samplewas also prepared and reflected light microscopy was used toexamine minerals that did not transmit light in a thin section,but reflected light to varying degrees when polished (Espiariet al., 2006). The XRD results are given in Fig. 3a and b. Fig. 3a sug-gests that the ore sample consisted of the major phase of quartzand the minor phases of barite, pyrite and calcite. Fig. 3b illustrates

oyed (adopted from Lorenzen, 1995).

es Minerals likely to be destroyed

Precipitated goldGoldCalcite, calcium carbonateSphalerite, labile copper sulfides, labile base metal sulfides, labile pyriteSphalerite, galena, tetrahedritePyrite, arsenopyrite, marcasitef

actor B Ag Rec (%) Au Rec (%)= H2SO4, �1 = HNO3

1 50.9 78.71 86.5 74.51 55.8 70.91 81.6 82.0

Table 3Elemental composition of the ore sample.

S Ti Na Mg K Fe Ca Al Element

5.72 <0.005 0.02 0.06 0.09 6.03 0.88 0.3 Content (%)

Ag Au Hg Cu As Sb Pb Zn Element

325 5.4 56.5 970 937 743 3160 7830 Content (ppm)

M. Saba et al. / Minerals Engineering 24 (2011) 1703–1709 1705

solid residue produced from diagnostic leaching which will be dis-cussed later.

The thin section images for two different particle sizes areshown in Fig. 1a and b. As can be seen in these figures, most ofthe non-metallic mineral content of the sample was fine andmicrocrystalline quartz, which constituted ca. 60% of the non-metallic minerals in all size fractions. Quartz minerals in the formof mosaic texture pieces with fine sericite (KAl2(Si, Al)4O10 (OH,F)2) blades and coarse quartz crystals were not observed in thesample. The amount of barite and calcite in most fractions wereindependent of the silica; however, sericite was highly conflictedwith silica.

Polished sections for different particle sizes, from fine to coarse,are shown in Fig. 2a and b, respectively. It is shown that coarse pyr-ite has a high degree of freedom even in the coarse fractions; how-ever, granulated pyrite conflicts with silica-sericite particles andrarely with barite. Other metallic parts are galena, sphalerite andvery low amounts of tetrahedrite ((Cu, Fe)12Sb4S13). Iron com-pounds such as goethite and limonite were produced by alterationof pyrite. Gold and silver do not exist in their pure or elementalform and are mainly combined with each other in an electrum

Fig. 1. Mineralogical analy

structure, i.e., gold, silver and copper alloy. However, there is a pos-sibility that some parts of silver are located in minerals such as ga-lena and/or tetrahedrite (Tumilty et al., 1987).Therefore, it can besuggested that some parts of the gold and silver in the ore are pre-sented as electrum and the rest should be presented in the form ofsolid solution and/or colloidal gold.

The thin section and polished section results of the ore sampleare illustrated in Table 1. As it is shown in this table based on thepercentage of each metallic/non-metallic phase, pyrite with 65–70% was the main metallic phase; whereas, silica with 60% wasthe main non-metallic phase in the sample.

3.3. Leaching procedure

All leaching experiments were performed in a 500 mL roundbottomed three neck flask equipped with a condenser immersedin a water bath for temperature control at ±1 �C tolerance. In a typ-ical diagnostic leaching experiment, the ore sample was mixedwith a specific volume of the oxidizing reagent to give a pre-deter-mined S/L. The mixture was agitated using a mechanical stirrer atconstant rate of 1000 rpm at pre-determined temperature. After

sis with thin section.

Fig. 2. Mineralogical analysis with polished section.

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filtration, the solution was analyzed for gold and silver and the res-idue was washed thoroughly with distilled water and used as theleaching feed for the second stage, i.e., leached with the second re-agent (Table 4). The procedure was repeated for all the studiedreagents.

Types of the leaching reagents and sequence of their addition inthe diagnostic leaching procedure are presented in Table 4. As it isshown in this table, the diagnostic leaching procedure consisted ofthree stages, cyanide leaching followed by cyanide washing andthen the acid leaching treatment. The cyanide washing stage wassuggested by Lorenzen (1995) to remove the precipitated goldand silver.

4. Results and discussion

4.1. Diagnostic leaching

The oxidizing reagents used to destroy the associated phaseswith gold and silver were ferric chloride or nitric acid. Other chem-icals given in Table 1 may also interact with the mineral phasesmentioned before. The possible reactions are as following:

CaCO3 þ 2HCl! CaCl2 þH2Oþ CO2 ð4Þ

MSþH2SO4 !MSO4 þH2SðM ¼ Zn;Cu; PbÞ ð5Þ

Cu12Sb4S13 þ 44FeCl3 ! 12Cu2þ þ 4Sb5þ þ 13S0 þ 44FeCl2 þ 22Cl2

ð6Þ

Ferric chloride also reacts with silver (reactions are given in Sec-tion 1).

2FeS2 þ 10HNO3 ! 2Fe3þ þ 2Hþ4SO2�4 þ 10NOþ 4H2O ð7Þ

Not only nitric acid reacts with pyrite, but also with the pre-sented silver (see Section 1). Therefore, the silver could be dis-solved and transported to the solution by either ferric chloride ornitric acid. Silver ions are also produced by cyanidation. A flow-sheet for the oxidation stage by either of the two oxidizing re-agents is illustrated in Fig. 4. As it is shown, the total silverleached (i.e., liberated) should be considered from the summationof the above reactions. The XRD pattern of the leaching residueafter the last leaching step is shown in Fig. 3b. As it is evident fromthis figure, the calcite and pyrite phases were completely removed.

Overall results of diagnostic leaching of the ore sample aregiven in Fig. 5. In Fig. 5a and b, the Au and Ag recovery from con-ventional cyanide leaching was compared with the recoveries ob-tained from diagnostic leaching procedure. According to Fig. 5a,the most effective leaching reagents were ferric chloride, hydro-chloric acid, sulfuric acid and nitric acid, respectively. It can be sug-gested that most of the gold content of the sample was trapped intetrahedrite and galena phases, according to Table 1. From the pre-vious studies, tetrahedrite is a common host mineral for gold(Chyssoulis and McMullen, 2005) and galena is not; however, insome cases gold might be associated with galena (Ferron, 2005;Henley et al., 2000). For silver, Fig. 5b shows that the most effectiveleaching reagents were sulfuric acid, hydrochloric acid, nitric acidand ferric chloride respectively. Based on the results in Table 1

Fig. 3. XRD patterns for (a) initial ore sample (b) solid residue from diagnostic leaching.

M. Saba et al. / Minerals Engineering 24 (2011) 1703–1709 1707

most of the silver was probably trapped in labile sulfide phases. Byusing the results of diagnostic leaching, Fig. 5 shows that ca. 98.1%of the gold (summing of the Au recoveries) and ca. 99.8% of silver(summing of the Ag recoveries) were extracted from the refractorysulfide ore.

4.2. Modeling and optimization

Analysis of variance (ANOVA) for diagnostic leaching of goldand silver is shown in Table 5. The F-value for a term is the test

Table 4Diagnostic leaching stages (conditions and parameters).

Treatment stage Reagent Leach p

Cyanide leaching NaCN 25 �C, 2Cyanide washing NaCN NaOH (0Acid leaching HCl 70 �C, 8Cyanide leaching NaCN 25 �C, 2Cyanide washing NaCN NaOH (0Acid leaching H2SO4 80 �C, 5Cyanide leaching NaCN 25 �C, 2Cyanide washing NaCN NaOH (0Acid leaching FeCl3 + HCl 95 �C, 8Cyanide leaching NaCN 25 �C, 2Cyanide washing NaCN NaOH (0Acid leaching HNO3 60 �C, 6Cyanide leaching NaCN 25 �C, 2Cyanide washing NaCN NaOH (0

for comparing the variance associated with that term with theresidual variance. It is the mean square for the term divided bythe mean square for the residual. The model F-value for gold andsilver recovery implies that the model is significant. The probabil-ity values larger than 95% (a = 0.05) have been considered signifi-cant with respect to the experimental error. The model P-valuesis very low which implies that the model is significant. There isonly 1.79% chance for gold recovery and 0.01% for silver recoverythat a ‘‘Model F-Value’’ this large could occur due to noise. The Pvalues were used as a tool to check the significance of each of

arameters Reagent concentration

4 h, L/S = 1:1, pH 10.5 (with CaO) 5 kg/t.02 g/L), L/S = 2:1 0.1 g/Lh, L/S = 2:1 %12 (V/V)4 h, L/S = 1:1, pH 10.5 (with CaO) 1 kg/t.02 g/L), L/S = 2:1 0.1 g/Lh, L/S = 2:1 %48 (V/V)4 h, L/S = 1:1, pH 10.5 (with CaO) 1 kg/t.02 g/L), L/S = 2:1 0.1 g/Lh, L/S = 2:1 100 g/L + 2 M4 h, L/S = 1:1, pH 10.5 (with CaO) 1 kg/t.02 g/L), L/S = 2:1 0.1 g/Lh, L/S = 10:1 %55 (v/v)4 h, L/S = 1:1, pH 10.5 (with CaO) 1 kg/t.02 g/L), L/S = 2:1 0.1 g/L

Fig. 4. Flowsheet for the oxidation stage by the two oxidizing reagents (ferric chloride or nitric acid).

(a)

(b)

Fig. 5. Deportment to the solution (a) gold (b) silver.

Fig. 6. Optimum condition for simultaneous dissolution rate of gold and silver.

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the coefficients, which, in turn, are necessary to understand thepattern of the mutual interactions between the test variables.Eqs. (I) and (II) are derived from the ANOVA results (Table 5) byusing empirical regression.

Au recovery in terms of codec factors:

Au ¼ 76:53þ 3:82Aþ 1:72B ðIÞ

Ag recovery in terms of codec factors:

Ag ¼ 68:70� 2:45Aþ 15:35B ðIIÞ

Table 5Analysis of variance (ANOVA).

Element Source Sum of squares Degree of fr

Au Model 70.42 2A-A 58.52 1B-B 11.90 1Residual 0.022 1Total 70.45 3

Ag Model 966.50 2A-A 24.01 1B-B 942.49 1Residual 0.000 1Total 966.50 3

These equations were obtained from analysis of variance ofeffective pre-treatments in gold and silver recovery.

Through equations derived from ANOVA, recovery of gold andsilver using the designed pre-treatment procedure were optimizedand characterized. Fig. 6 shows that using the model, combiningtwo pre-treatment stages, i.e., adding ferric chloride and sulfuricacid at the same time (Table 2), the simultaneous Au and Ag recov-eries were predicted to be 82% and 81.6%, respectively.

5. Conclusions

1. Using mineralogical analysis and a diagnostic leaching designprocedure, four oxidizing leaching reagents were used torecover 98.1% Au and 99.8% Ag.

2. Diagnostic leaching and mineralogical analysis suggested thatmost of the gold in the sample was trapped in tetrahedrite,while most of the silver was trapped in unstable sulfide phases.

eedom Mean square F-value P-value

35.21 1565.00 0.017958.52 2601.00 0.012511.90 529.00 0.02770.022

483.25 6.366E+007 <0.000124.01 6.366E+007 <0.0001942.49 6.366E+007 <0.00010.000

M. Saba et al. / Minerals Engineering 24 (2011) 1703–1709 1709

3. To dissolve gold in cyanide solution the most effective leachingreagents were ferric chloride, hydrochloric acid, sulfuric acidand nitric acid, respectively.

4. To dissolve silver in cyanide solution the most effective leachingreagents were sulfuric acid, hydrochloric acid, nitric acid andferric chloride, respectively.

5. A correlation for dissolution of gold and silver was proposedusing ANOVA.

6. The most effective pre-treatment reagents for gold and silverare ferric chloride and sulfuric acid, respectively. Pre-treat-ments with ferric chloride and sulfuric acid increased the effi-ciency of the dissolution of gold from 54.7% to 82% and silverfrom 37.4% to 81.6%.

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