Two-Stage SART Process: a Feasible Alternative for Gold Cyanidation Plants with High Zinc and Copper Contents

Article Two-Stage SART Process: A Feasible Alternative for Gold Cyanidation Plants with High Zinc and Copper Contents

Humberto Estay 1,* , Minghai Gim-Krumm 1,2 and Michelle Quilaqueo 1

1 Advanced Mining Technology Center (AMTC), University of Chile, Av. Tupper 2007 (AMTC Building), Santiago 8370451, Chile; [email protected] (M.G.-K.); [email protected] (M.Q.) 2 Department of Chemistry, Universidad Tecnológica Metropolitana, Las Palmeras 3360, Ñuñoa, Santiago 7800002, Chile * Correspondence: [email protected]; Tel.: +56-2-2977-1011

Received: 2 August 2018; Accepted: 5 September 2018; Published: 7 September 2018

Abstract: The SART (sulﬁdization, acidiﬁcation, recycling, and thickening) process (SP) has been successfully implemented in gold cyanidation plants to address issues associated with high cyanide-soluble copper content ores. However, this process could produce a relatively low grade precipitate, decreasing the sale price when gold plants have high zinc and copper content in their solutions. A potential option in this case would be the use of a two-stage SART process (TSSP) to produce separate zinc and copper precipitates. The additional equipment involved with this process would increase the capital cost, thereby generating concerns about the optimal range of metal contents that could justify this option. This study presents a methodology to quantify the feasible range of Cu/Zn concentrations that would justify a two-stage SART process. The study is based on a thermodynamic model and a simple economic evaluation. Results show the TSSP is preferred when the Cu/Zn ratio ranges between 0.2 and 1.5 with copper concentration higher than 500 mg/L. The TSSP appears to be a viable option to consider for gold plants having concentrations of copper and zinc higher than 200 mg/L for both metals.

Keywords: SART process; Merrill–Crowe process; cyanidation plants; cyanide recovery

1. Introduction

1.1. SART Process Description The SART (sulfidization, acidification, recycling, and thickening) process has been installed in different gold cyanidation plants worldwide to address the issues associated with high cyanide-soluble copper content ores (additional cyanide consumption in the ore, increasing the cyanide concentration in the leach tails, decreasing gold adsorption efficiency in carbon adsorption stage, contamination of dore metal, wrong measurement of free cyanide and reduction of ore reserves) [1]. The main advantage of this technology is recovering cyanide and copper as a saleable by-product, based on the following reaction [2]. 2− 2− 2− 2Cu(CN)3 + 3H2SO4 + S ↔ Cu2S(s) + 6HCN(aq) + 3SO4 (1) 2− Equation (1) is based on Cu(I) as the stable ion in cyanide solutions. Furthermore, the Cu(CN)3 is the most stable cyano-copper specie present in cyanide solutions at typical operational pH (10.5–11.0). The main unit operations included in the SART process are reactors and solid-liquid separation stages, as shown in Figure1.

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H2SO4 Treated Solution NaSH Air NaOH Flocculant

Scrubber Copper Lime sulfide Reactor Copper sulfide THK Flocculant

NaOH

Treated Neutralization Reactor Solution Wash Water Gypsum CN- Copper THK sulfide Filter Cu2S

Wash Water Gypsum Filter Gypsum

Figure 1. Schematic flow-diagram of the SART process. Figure 1. Schematic flow-diagram of the SART process. The sulfidization reaction described in Equation (1) occurs in the copper sulfide reactor under pH conditionsThe sulfidization ranging reaction between described 4 and 5. in Copper Equation precipitate (1) occurs produced in the copper in this sulfide reactor reactor is separated under frompH conditions cyanide solution ranging in between thickening 4 and (THK) 5. Copper and filtering precipitate stages. produced The treated in this cyanide reactor solution is separated must befrom neutralized cyanide solution to return in thicke into thening cyanidation (THK) and plant. filtering For stages. this purpose, The treated milk cyanide of lime solution is added must in the be neutralizationneutralized to reactorreturn tointo achieve the cyanidation pH 10.5, generating plant. For gypsum this purpose, which is milk separated of lime in sequentialis added in steps the ofneutralization thickening and reactor filtering to achieve [1–3]. pH This 10.5, conventional generating configuration gypsum which in is the separated SART flow-sheet in sequen hastial steps been appliedof thickening successfully and filtering in different [1–3]. This gold conventional plants with highconfiguration copper content in the SART in their flow cyanide-sheetsolutions, has been suchapplied as Telfer,successfully Lluvia in de different Oro, Yanacocha, gold plants Gedabek, with high Mastra, copper Maricunga, content in their and Coplercyanide [4 solutions,–10]. The such unit operationsas Telfer, Lluvia described de earlier Oro, Yanacocha, could also useGedab to recoverek, Mastra, zinc as Maricunga ZnS in gold, and cyanidation Copler [4plants–10]. withThe high unit cyanide-solubleoperations described zinc content earlier ores,could according also use to therecover reaction zinc [ 1as,11 ZnS] in gold cyanidation plants with high cyanide-soluble zinc content ores, according to the reaction [1,11] ( )2− + + 2− ↔ + + 2− ZnZnCN(CN4) +2H2H2SOSO4 +SS ZnSZnS(s) + 4HC4HCNN( (aq) )+ 2S2SOO 4 (2(2)) 2− 2− 2− When cyanidation plants4 useuse gold2 cementation 4 ↔ with s zinc, this aq is known4 as the Merrill–CroweMerrill–Crowe (MC)(MC) process. Here, the the zinczinc powderpowder isis dissolveddissolved accordingaccording toto thethe followingfollowing equationequation [[12].12].

Au(CN− ) + Zn + H O + 2CN Au + OH + 0.5H + Zn(CN) − (3) Au(CN) + Zn0 + H O + 2CN− ↔ Au0 + OH− + 0.5H + Zn(CN)2 (3) 2 − 0 2 − 0 − 2 2− 4 Dissolved zinc–cyanide2 complex 2is built-up ↔in the cyanidation plant2 reaching4 levels that require a bleedDissolved from cyanide zinc-cyanide solutions complex [12]. In is this built-up context, in the the cyanidation SART process plant applied reaching to recover levels that cyanide require and a bleedzinc could from cyanidebe implemented solutions in [12 cyanidation]. In this context, plants the such SART as process Cerro Vangu appliedardia to recover [13], La cyanide Coipa and [14], zinc El couldPeñón, be and implemented Yanacocha, in among cyanidation others plants [15]. suchHowever, as Cerro there Vanguardia are some [13gold], La mines Coipa that [14], have El Peñ highón, andcyanide Yanacocha,-soluble among copper others content [15]. ores However, and MC there process are some installed gold mines in the that plant. have high In these cyanide-soluble cases, the copperconventional content SART ores andplant MC described process in installed Figure in1 will the produce plant. In a these blended cases, precipitate the conventional of copper SART and plantzinc. describedThis fact decreases in Figure 1the will copper produce (or zinc) a blended grade precipitate in the precipitate of copper from and around zinc. This 65% fact to approximately decreases the copper35–40%(or in the zinc) case grade of copper in the, precipitatedepending fromon the around Cu/Zn 65%molar to ratio approximately presence in 35–40% the cyanide in the solu casetion. of copper,This reduction depending on the on precipitate the Cu/Zn grade molar decreases ratio presence the sales in the price, cyanide due to solution. the increase This of reduction transportation on the precipitatecosts. Furthermore, grade decreases the precipitate the sales price price, is duegenerally to the increaseapplied only of transportation for one metal costs. contained, Furthermore, losing theincomes precipitate associated price to is the generally other metal. applied Theref onlyore, for onethe blended metal contained, copper–zinc losing precipitate incomes associatedgenerated by to the otherconventional metal. Therefore, SART process the blended loses value copper–zinc in plants precipitatewith MC and generated high copper by the contents. conventional With this SART in processmind, there loses are value some in studies plants with[16–18] MC proposing and high coppera two-stages contents. SART With process, this in recovering mind, there copper are some and studieszinc separately, [16–18] proposing based on the a two-stages pH conditions SART where process, ZnS recovering (Equation copper (2)) is andformed zinc (over separately, pH 5) based [1,11]. on the pH conditions where ZnS (Equation (2)) is formed (over pH 5) [1,11].

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1.2. Two-Stage SART Process 1.2. Two-Stage SART Process The two-stage SART process includes additional stages such as the agitated reactor, thickening, The two-stage SART process includes additional stages such as the agitated reactor, thickening, and filtration for zinc, as shown in Figure 2. and ﬁltration for zinc, as shown in Figure2.

H2SO4 Treated Solution NaSH Air NaOH Flocculant

H2SO4 Zinc sulfide Scrubber Reactor NaSH pH ~ 7.5 Zinc sulfide Flocculant THK NaOH Metal free solution to Copper Neutralization sulfide Wash Reactor Copper Water Zinc pH 4.0-5.0 sulfide sulfide THK Filter ZnS NaOH

Wash Water Copper sulfide Filter

Cu2S

Figure 2. Schematic flow-diagram of the two-stage SART process. Figure 2. Sche matic flow-diagram of the two-stage SART process. The main background of ZnS precipitation from cyanide solutions is the Velardena process, which, The main background of ZnS precipitation from cyanide solutions is the Velardena process, keeping pH, treated a barren solution in order to recover cyanide and bleed zinc from solutions [19]. which, keeping pH, treated a barren solution in order to recover cyanide and bleed zinc from Furthermore, the recent studies for the two-stage SART process [16–18] have achieved zinc precipitation solutions [19]. Furthermore, the recent studies for the two-stage SART process [16–18] have achieved efficiencies over 90% at pH 7.5 with low copper precipitation (below 5%) in the first stage. The second zinc precipitation efficiencies over 90% at pH 7.5 with low copper precipitation (below 5%) in the first stage was operated as the conventional SART process, treating the overflow of the zinc sulfide thickener stage. The second stage was operated as the conventional SART process, treating the overflow of the at pH 4.5, reaching copper efficiencies higher than 90%. Additionally, the performance separation of the zinc sulfide thickener at pH 4.5, reaching copper efficiencies higher than 90%. Additionally, the two-stage SART process assessed at pilot scale was successful, obtaining overall recoveries higher than performance separation of the two-stage SART process assessed at pilot scale was successful, 90% and 99% for copper and zinc respectively, and two separated precipitates having 67% zinc grade obtaining overall recoveries higher than 90% and 99% for copper and zinc respectively, and two and 71% copper grade each [18]. Even though, these results are promising, the two-stage SART process separated precipitates having 67% zinc grade and 71% copper grade each [18]. Even though, these involves the inclusion of extra equipment, increasing the capital and operational costs. Furthermore, results are promising, the two-stage SART process involves the inclusion of extra equipment, the studies published at this moment present results using cyanide solutions containing a specific increasing the capital and operational costs. Furthermore, the studies published at this moment Cu/Zn molar ratio and fixed pH conditions. Thus, the performance effect of different Cu/Zn molar present results using cyanide solutions containing a specific Cu/Zn molar ratio and fixed pH ratio in the cyanide solution and pH of each sulfidization stage has not been performed. conditions. Thus, the performance effect of different Cu/Zn molar ratio in the cyanide solution and The aim of this study is to present a methodology which estimates the range of Cu/Zn molar pH of each sulfidization stage has not been performed. ratio in a cyanide solution where the two-stage SART is feasible, based on a thermodynamic model The aim of this study is to present a methodology which estimates the range of Cu/Zn molar and an economic evaluation. ratio in a cyanide solution where the two-stage SART is feasible, based on a thermodynamic model and2. Methods an economic evaluation.

2.2.1. Methods Estimation of Operational Parameters Using a Thermodynamic Model The speciation of metal-cyanide complexes can be estimated using the equilibrium constants 2.1. Estimation of Operational Parameters Using a Thermodynamic Model and a mass balance of each reaction involved in an aqueous solution [20]. Furthermore, sulﬁdization EquationsThe speciation (1) and (2) of canmetal be-cyanide added to complexes the metal-cyanide can be estimated complexes using equations the equilibrium system in constants order to andestablish a mass the balance speciation of each of eachreaction cyanide involved complex in an and aqueous metal-sulﬁde solution [20]. precipitates. Furthermore, Table sulfidization1 shows the Equatequilibrium ions (1) relations and (2) involved can be added in the totwo-stage the metal SART-cyanide process. complexes equations system in order to establish the speciation of each cyanide complex and metal-sulfide precipitates. Table 1 shows the equilibrium relations involved in the two-stage SART process.

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Table 1. Values of equilibrium constants for cyanide and sulﬁde species involved in the SART process at 25 ◦C.

Reaction logKi Reference Cu+ + CN− ↔ CuCN 20 [21] − − CuCN + CN ↔ Cu(CN)2 3.94 [22] − − 2− Cu(CN)2 + CN ↔ Cu(CN)3 5.3 [22] 2− − 3− Cu(CN)3 + CN ↔ Cu(CN)4 1.5 [22] 2+ − 0 Zn + 2CN ↔ Zn(CN)2 11.07 [23] 0 − − Zn(CN)2 + CN ↔ Zn(CN)3 4.98 [23] − − 2− Zn(CN)3 + CN ↔ Zn(CN)4 3.57 [23] H+ + CN− ↔ HCN 9.21 [24] + 2− 2Cu + S ↔ Cu2S 47.3 [25] Zn2+ + S2− ↔ ZnS 23.08 [25] H+ + S2− ↔ HS− 13.9 [26] + − H + HS ↔ H2S 7.02 [26]

The species distribution in cyanide solution and sulfidization stages has been determined using the Hydra/Medusa software (Version 1) (KTH Royal Institute of Sweden, Stockholm, Sweden) [27]. This model can estimate Eh-pH and speciation diagrams of different compounds and reactions. However, the thermodynamic data base (equilibrium constants, Ki) of Hydra/Medusa has been changed according to Table1 for those reactions. Therefore, the recoveries and grades for copper and zinc at different pH were predicted using this modified software. In the presence of HS− ions content, speciation curves of metal-cyanide complexes for copper and zinc, were developed. Cu/Zn molar ratios (based on copper and zinc contents in the cyanide solution) ranging between 0.025 and 40 (0.025, 0.25, 0.5, 0.67, 1.0, 1.5, 2.0, and 40) were assessed keeping a free cyanide concentration of 100 mg/L for pH over 10.5 and sulfide stoichiometric addition equivalent to 120% stoichiometric of copper and zinc (based on Equations (1) and (2)). The free cyanide concentration value is a typical concentration set in cyanide solutions by the cyanidation plants in order to ensure gold dissolution [12], while the sulfide stoichiometric addition of 120% in the SART process is based on experimental and operational experience focused on maximizing copper recovery [1]. The results of copper and zinc recoveries and grades in the precipitate are based on the following assumptions:

• The model did not include other metals—such as iron, mercury, silver, or nickel—because these elements are typically present in low contents of cyanide solutions (<10 mg/L). • The main reactions involved in sulfidization stage were Table1. • The estimation of copper and zinc recoveries did not include process inefficiencies such as re-dissolution of metals by oxidation and precipitate lost in the thickener overflow [1,11].

Results of speciation curves at different Cu/Zn molar ratio and pH were analyzed, proposing the best pH conditions to operate the conventional (one-stage) or the two-stage SART process.

2.2. Economic Comparison between Conventional and Two-Stage SART Process Options The pH conditions for two SART process options defined in the earlier section determines metal recoveries and grades in the precipitate, for each Cu/Zn molar ratio assessed. These two parameters (recovery and grade) define the incomes expected by both SART process options. On the one hand, metal recovery determines the metal sulfide production. On the other hand, the metal grade in the precipitate defines the total mass of precipitate generated determining the transport cost. In this context, the economic criteria used to estimate the precipitate value (US$/precipitate ton) are shown in Table2. Minerals 2018, 8, 392 5 of 12

Table 2. Economic parameters values to estimate precipitate sales value.

Parameter Value Unit Copper price 3.0 US$/lb Zinc price 1.25 US$/lb Precipitate discount rate 25 % Transport cost 30 US$/dry ton

Metals prices are defined according to recent prices (March–April 2018 period) by the London metals exchange (LME), while the discount rate of precipitate is an average value of typical discount fixed by refineries treating semi-processed products such as copper concentrates. Additionally, the transport cost is based on truck transport from mining to a refinery or port located at 200 km of distance. The main issue of a blended copper/zinc precipitate produced by the conventional SART process is that precipitate price is fixed according to the value of one metal (copper or zinc). Therefore, the other metal is considered as an “impurity” in the precipitate. This fact makes the two-stage SART process an interesting option to generate additional incomes based on the incomes by the two metals. Hence, this methodology defines the price of the blended precipitate generated by the conventional SART process as the price of the most valuable metal in the precipitate (metal price × mass of metal recovered). Additionally, the price defined for both precipitates generated by the two-stage SART process is based on the individual recovery of zinc and copper achieved in each stage. The economic evaluation was performed comparing the conventional and the two-stage SART processes differences. This means that the capital and operational costs have been estimated based only on the differences between two options. Thus, the cash flow included the differential costs (capital and operational) and precipitate incomes, estimating a differential net present value (NPV) of cash flow per SART plant capacity. The parameters values used to assess the economic estimation are shown in Table3.

Table 3. Parameter values of additional equipment or supplies in the two-stage SART process, used for the economic evaluation.

Parameter Value Unit Capital cost of additional equipment 36.0 kUS$/(m3/h) Additional energy consumption 0.047 kWh/m3 Energy price 150 US$/MWh Additional ﬂocculant consumption 2.0 g/m3 Flocculant price 3000 US$/ton

The additional equipment defined in the two-stage SART process are the zinc sulfide reactor, the thickener, and the filter. These equipment must also consider instrumentation, piping, electric materials, and vent ducts. The capital cost of a SART plant is very variable, mainly depending on the specific requirements of the gold plant [1]. Nevertheless, a recent study estimated the additional capital costs of the two-stage SART process [18] approximately in 36.0 kUS$/(m3/h). This value is similar to the one used and reported by other SART plants [1]. Energy consumption was estimated based on the additional thickener rake and underflow pumps, and the flocculant consumption has been taken from recent data reported [1]. Furthermore, energy and flocculant prices are based on recent quotations of mining plants operating in Chile. NPV was estimated in a period of 10 years, using 5% of discount rate. Finally, the differential cash flow was estimated for each Cu/Zn molar ratio. Consequently, the copper concentration in the cyanide solution varied from 100 mg/L to 1200 mg/L. Minerals 2018, 8, 392 6 of 12 Minerals 2018, 8, x FOR PEER REVIEW 6 o f 12

3. Results Results and and Discussion Discussion

3.13.1.. Operational Operational Conditions Conditions and Metal Metal Recoveries Recoveries

SpeciationSpeciation curves curves of of metal metal-cyanide-cyanide complexes complexes and and sulfide sulﬁde metals were performed using Hydra/Medusa software, software, as as explained explained in in S Sectionection 2.1 2.1 ResultsResults obtained obtained from from this this software show the contentcontent ofof eacheach compound compound as as function function of pH.of pH. Figure Figure3 shows 3 shows the speciation the speciation curves ofcurves a cyanide of a solutioncyanide solutioncontaining containing a Cu/Zn a molarCu/Zn ratiomolar of ratio 2.0, beforeof 2.0, before and after and being after being treated treated in the in conventional the conventional one-stage one- stageSART SART process. process.

100 CN- HCN 100 CuCN CN- 80 - Cu(CN)2 HCN 2- 80 - Cu(CN)3 Cu(CN)2 3- 2- Cu(CN)4 Cu(CN)3 60 Zn(CN) 0 Cu(CN) 3- 2 60 4 - Zn(CN)3 Zn(CN) 2- 40 4 40

20 20 Metal-cyanidespeciation, complexes % Metal-cyanidespeciation, complexes %

0 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 pH pH (a) (b)

100 100

80 80

60 60

- 40 40 2+ Cu(CN)2 Zn 2- Cu(CN)3 ZnS 3- Cu(CN)4 20 20 Cu2S Metal-cyanidespeciation, complexes % Metal-cyanidespeciation, complexes %

0 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 pH pH (c) (d)

FigureFigure 3. 3. SpeciationSpeciation curves obtained from Hydra/MedusaHydra/Medusa software software for a a cyanide cyanide solution containing aa Cu/Zn Cu/Zn molar molar ratio ratio of of 2.0. 2.0. (a)( cyanidea) cyanide solution solution fe d fed into into the theSART SART process; process; (b) treated (b) treated solution solution by one by- stageone-stage SART SART proce process ss using using 120% 120% stoichiome stoichiometric tric NaHS NaHS addition; addition; (c) copper (c) copper spe ciesspecies distribution distribution for the for trethe ated treated cyanide cyanide solution solution in the in the SART SART proce process; ss; (d ()d zinc) zinc spe species cies distribution distribution for for the the treated treated cyanide solutionsolution in in the the SART proce process. ss.

TheseThese results show the high metal metal-cyanide-cyanide complexes present present in in a a cyanide solution when pH values are are higher higher than than 10.0 10.0 (Figure (Figure 3a).3a). Thus, Thus, the the free free cyanide cyanide concentration concentration fixed ﬁxed at at100 100 mg/L mg/L——for typicalfor typical gold gold cyanida cyanidationtion conditions conditions (Ph = (Ph 10.5) = 10.5)—is—is equivalent equivalent to approximately to approximately 4% of 4% the of weak the weak acid dissolvedacid dissolved (WAD (WAD)) cyanide cyanide concentration. concentration. In return, In return, when when sulfide sulﬁde is isadded added to to the the cyanide cyanide solution, solution, ZnS is formed formed according according to to Equation (2) (2) for pH values values lower lower than than 12.0 (Figure 33dd),), sincesince zinc-cyanidezinc-cy anide complexescomplexes are dissociated to to free free cyanide. In fact, fact, this dissociation generates free cyanide. Figure 33bb 3− showsshows the increase ofof free cyanide concentration from from 4% 4% to to around around 34% 34% and and Cu(CN) Cu(CN)443− concentrationconcentration fromfrom 8%8% toto 35%. 35%. When When pH pH decreases decreases under under 5.0 5.0 copper-cyanide copper-cyanide complexes complexes are dissociatedare dissociated to form to HCNform HCNand Cu and2S accordingCu2S according to Equation to Equation (1) (Figure (1)3 c).(Figure Therefore, 3c). Therefore, the SART processthe SART can process recover theoreticallycan recover theoretically100% of cyanide 100% complexed of cyanide to complexed zinc and copper to zinc forand pH copp valueser for under pH values 5.0. Furthermore, under 5.0. Furthermore, zinc can be zinccompletely can be completely recovered forrecovered pH higher for pH than higher 2.5, andthan copper 2.5, and reaches copper recoveryreaches recovery values of values 100% of for 100% pH for pH lower than 4.0. These results are accordant with experimental and operational results of different SART process studies [1,10,11,16,28] where copper can achieve recovery values higher than

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Mineralslower 2018 than, 8 4.0., x FOR These PEER resultsREVIEW are accordant with experimental and operational results of different7 o f 12 SART process studies [1,10,11,16,28] where copper can achieve recovery values higher than 95% at 95% at pH, ranging between 4.0 and 5.0, and zinc can reach recovery values higher than 99% when pH, ranging between 4.0 and 5.0, and zinc can reach recovery values higher than 99% when pH is less pH is less than 7.5. than 7.5. Copper and zinc recoveries have been estimated for different Cu/Zn molar ratios using the same Copper and zinc recoveries have been estimated for different Cu/Zn molar ratios using the same methodology presented in Figure 3. Thus, Figure 4 shows copper and zinc recovery for the one-s t a ge methodology presented in Figure3. Thus, Figure4 shows copper and zinc recovery for the one-stage SART process. SART process.

100 100

R40 80 R2.0 80 R1.5 R1.0 R40 R0.67 R2.0 60 60 R0.5 R1.5 R0.25 R1.0 R0.025 R0.67 40 40 Zinc recovery, % recovery, Zinc R0.5 Copper recovery, % recovery, Copper R0.25 R0.025 20 20

0 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 pH pH (a) (b)

FigureFigure 4. 4. MetalsMetals recovery recovery values values in in the the one one-stage-stage SART process process for different Cu/Zn mo molarlar ratio ratio ((RR0.0250.025––RR40,40, where where RR isis Cu/Zn Cu/Zn molar molar ratio). ratio). (a ()a )Copper Copper recovery; recovery; ( (bb) )zinc zinc recovery. recovery.

FigureFigure 4 shows the Cu22SS stability stability under under pH pH values values below below 7.0, 7.0, achieving achieving recoveries recoveries higher higher than than 90%90% at at pH pH 4.0 4.0 for for each Cu/Zn Cu/Zn molar molar ratio ratio simulated. simulated. Furthermore, Furthermore, cop copperper recovery recovery increases increases when Cu/ZnCu/Zn molar molar ratio ratio has has increased, increased, due due to to the the high high equilibrium equilibrium constant constant of of Cu Cu22SS—or—or solubility solubility constant constant ——presentedpresented in TableTable1 .1. Also, Also, ZnS ZnS is is very very stable stable for for pH pH values values ranging ranging between between 2.0 2.0 to 12.0,to 12.0, except except in the in 2− thezone zone between between pH pH values values of of 8.0 8.0 to to 12.0, 12.0, where where Zn(CN) Zn(CN)4 42− becomesbecomes stable together with ZnS ZnS.. In In this this pHpH area, area, zinc zinc recovery recovery can decrease under 80%, 80%, depending on Cu/Zn molar molar ratio ratio and and free free cyanide cyanide concentration.concentration. When When these these two two parameters parameters increase, increase, zinc zinc recovery recovery decreases. decreases. These These results results allow allow to defineto define the thepH pH operational operational conditions conditions that that maximize maximize the the metals metals recovery recovery in in both both SART SART process process options.options. Therefore, Therefore, copper copper recovery recovery can can reach reach values values over over 95% 95% at at pH pH 4.0. 4.0. At At this this condition, condition, ZnS ZnS remainsremains stable stable ensuring ensuring zinc zinc recovery recovery over over 99%. 99%. In In the the case case of of the the two two-stage-stage SART SART process, process, the the first first 3− stagestage of of zinc zinc preci precipitationpitation can be operated at pH 7.5 or lower, wherewhere Zn(CN)Zn(CN)443− speciespecie is is dissociated dissociated toto ZnS ZnS and and HCN HCN in in the the presence presence of of sulfide. sulfide. These These proposed proposed conditions conditions are are similar similar to to the the ones ones found found byby experimental experimental studies studies of of the the two two-stage-stage SART SART process process [16,18]. [16,18]. Hence, Hence, the the theoretical theoretical model model can can predictpredict the the optimal optimal conditions conditions of of the the SART SART process process—conventional—conventional and and two two-stage—maximizing-stage—maximizing metal metal recoveries,recoveries, according according to to Cu/Zn Cu/Zn metal metal ratio ratio and and cyanide cyanide concentration concentration in a incyanide a cyanide solution solution of a gold of a plant.gold plant.Indeed, Indeed, this theoretical this theoretical method methodologyology can cansupport support an anexperimental experimental study study to to extrapolate extrapolate operationaloperational conditions conditions to to others others Cu/Zn Cu/Zn molar molar ratios, ratios, understanding understanding the the effect effect of of different different parameters parameters involvedinvolved in in a a SART SART process process (pH, (pH, NaHS NaHS addition, addition, cyanide, cyanide, copper copper and and zinc zinc content). content). Figure Figure 5 shows cocopperpper and and zinc zinc recoveries recoveries and and grades grades obtained obtained for for the the conventional conventional and and the the two two-stage-stage SART SART process process forfor different different Cu/Zn Cu/Zn molar molar ratios. ratios. TheThe zinc recoveryrecovery reaches reaches 100% 100% in both in both SART SART process process options. options. Instead copperInstead recovery copper overcomesrecovery overcomes90% in one-stage 90% in option one-stage for every option Cu/Zn for ev molarery Cu/Zn ratios molar simulated. ratios However, simulated. copper However, recovery copper falls recoveryup to 65% falls in up the to two-stage 65% in the option two-stage fora option Cu/Zn for molar a Cu/Zn ratio molar of 0.025. ratio of This 0.025. fact This is explainedfact is explained by the byincrease the increase of HCN of concentration HCN concentration in the solution in the which solution fed which into the fed second into stagethe second (copper stage precipitation) (copper precipipromotedtation) by thepromoted precipitation by the reactionprecipitation of zinc reaction performed of zinc in performed the first stage in the (Equation first stage (2)). (Equation The high − (2)).HCN The content high HCN in the content feed solution in the offeed the solution second stageof the allows second to stage keep allows the Cu(CN) to keep2 complex the Cu(CN) more2− complexstable at more lower stable pH. Therefore,at lower pH. the Therefore, two-stage the SART two- processstage SART must pro carefullycess must define carefully the operational define the operational pH of the second stage in order to maximize the copper recovery, particularly when Cu/Zn molar ratio is low. In terms of metal grades in the precipitate, the two-stage option allows to obtain two different high-grade precipitates for copper and zinc. Finally, the two-stage SART process

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Minerals 2018, 8, x FOR PEER REVIEW 8 of 12 pHMinerals of the 2018 second, 8, x FOR stage PEER inREVIEW order to maximize the copper recovery, particularly when Cu/Zn molar8 o f 12 ratio is low. In terms of metal grades in the precipitate, the two-stage option allows to obtain two option achieves higher metal recoveries than the conventional option, and two separated salable differentoption achieves high-grade higher precipitates metal recoveries for copper than and the zinc. conventional Finally, the option, two-stage and two SART separated process salable option high-grade precipitates. Nevertheless, the additional capital and operational costs could limit the achieveshigh-grade higher precipitates. metal recoveries Nevertheless, than the the conventional additional capital option, and and twooperational separated costs salable could high-grade limit the implementation of this technology. For this reason, a simple economic evaluation is performed to precipitates.implementation Nevertheless, of this technology. the additional For capitalthis reason, and operational a simple economic costs could evaluation limit the is implementation performed to delimit the feasible range of Cu/Zn molar ratio when the two-stage SART process can be ofdelimit this technology. the feasible For range this reason, of Cu/Zn a simple molar economic ratio evaluationwhen the istwo performed-stage SART to delimit process the feasiblecan be implemented. rangeimplemented. of Cu/Zn molar ratio when the two-stage SART process can be implemented.

100 90 100 90 100 90 100 90 90 80 90 80 90 80 90 80 80 70 80 70 80 70 80 70 70 70 60 70 60 70 60 Cu Rec, % 60 60 60 60 50 60 Cu Rec, % 50 50 Cu Rec, % 50 50 Cu Rec, % 50 50 Zn Rec, % 40 50 Zn Rec, % 40 40 Zn Rec, % 40 40 40 40 Cu Grade, % 30 40 Zn Rec, % 30

30 30 Cu Grade, % Metal grade,% Cu Grade, % 30 30 Metal grade,% 30 20 30 Cu Grade, % Metal Recovery,% 20

Metal % grade, Metal Metal Recovery,% 20 Zn Grade, % 20 % grade, Metal 20 Zn Grade, % Metal Recovery, % Recovery, Metal 20 20 Zn Grade, % % Recovery, Metal 20 10 10 10 Zn Grade, % 10 10 10 10 10 0 0 0 0 0 0 0 0 0.01 0.1 1 10 100 0.01 0.1 1 10 100 0.01 0.1 1 10 100 0.01 0.1 1 10 100 log(Cu/Zn molar ratio) log(Cu/Zn molar ratio) log(Cu/Zn molar ratio) log(Cu/Zn molar ratio)

(a) (b) (a) (b) Figure 5. Results of copper and zinc recoveries and grades in the precipitate for different Cu/Zn molar Figure 5. Results of copper andand zinczinc recoveriesrecoveries and and gradesgrades in in thethe precipitate precipitate for for different different Cu/Zn Cu/Zn molarmolar ratios. (a) One-stage SART process; (b) two-stage SART process. ratios. ((aa)) One-stageOne-stage SARTSART process;process; ( (bb)) two-stagetwo-stage SART SART process.process.

3.2.3.2. Economic Economic Results

EconomicEconomic assessment assessment assessment takes takes takes into into into account account account an an estimation an estimation estimation of of incomes, incomes, of incomes, operational, operationa operational,l, and and capital capital and capitalcosts. costs. Incosts.In thethe In firstfirst the place, place, ﬁrst place, thethe income theincome income estimationestimation estimation for for forthethe the oneone one-stage-stag-stagee SARTSART SART option option option includesincludes includes aa a comparison comparisoncomparison betweenbetween the the precipitate precipitate value value sold soldsold as as copper coppercopper or oror zinc zinczinc precipitate. precipitate.precipitate. Thus, Thus,Thus, Figure Figure 6 6 shows shows the the income income estimationestimation developed developeddeveloped for for both both SART SART options, options,options ,considering considering the the high high value value of of the the the metal metalmetal precipitate precipitate for forfor thethethe conventional conventional SART SARTSART process. process.process. In In thisIn thisthis context, context,context, when whenwhen Cu/Zn Cu/ZnCu/Zn molar molar ratiomolar is ratio lessratio than isis lessless 0.5, than thethan precipitate 0.5,0.5, thethe precipitateincomeprecipitate in one-stageincome income in in SARTone one-stage-stage process SART SART is maximizedprocess process is is maximized sellingmaximized the precipitateselling selling the the precipitate as precipitate a zinc sulﬁde as as a azinc concentrate.zinc sulfide sulfide concentrate.Theseconcentrate. results These These are highly results results dependent are are highly highly ondependent dependent metal grades on on metal metal in the grades grades precipitate in in the the (Figureprecipitate precipitate5) and (F (Figureigure the metal 5) 5) and and prices. the the metalInmetal this prices. prices. study, In theIn this this ratio study, study, of copper/zinc the the ratio ratio of of copper/zinc prices copper/zinc is 2.4 (3.0/1.25),prices prices is is 2.4 2.4 according (3.0/1.25), (3.0/1.25), to according according Table2. to to Table Table 2. 2.

4000 4000 Pp income by Cu 5000 5000 3000 PpPp income income by by Zn Cu 3000 OverallPp income Pp income by Zn Overall Pp income 4000 4000 2000 2000

3000 Pp income by Cu 3000 1000 PpPp income income by by Zn Cu 1000 OverallPp income Pp income by Zn 2000 Overall Pp income

Income, US$/ton Income,

Income, US$/ton Income, 0 2000 Income,US$/ton

Income,US$/ton 0

1000 -1000 1000 -1000

0 -2000 0 -2000 0.01 0.1 1 10 100 0.01 0.1 1 10 100 0.01 0.1 1 10 100 0.01 0.1 1 10 100 log(Cu/Zn molar ratio) log(Cu/Zn molar ratio) log(Cu/Zn molar ratio) log(Cu/Zn molar ratio) (a) (b) (a) (b) FigureFigure 6. 6. SARTSART precipitate precipitate income income (Pp (Pp:: precipitate precipitate )) for for different different Cu/Zn molar molar ratios. ratios. (a (a) )One One-stage-stage Figure 6. SART pre cipitate income (Pp: pre cipitate ) for different Cu/Zn molar ratios. (a) One-stage SARTSART process; process; ( (bb)) two two-stage-stage SART SART process. process. SART proce ss; (b) two-stage SART process. A differential NPV value is estimated taking into account the income obtained from Figure6, AA differential differential NPV NPV value value is is estimated estimated taking taking into into account account the the income income obtained obtained from from Figure Figure 6, 6, andand capital capital and and operational operational costs costs based based on on parameters parameters defined deﬁned in in Table Table 33.. Nevertheless, the NPV and capital and operational costs based on parameters defined in Table3 3. Nevertheless, the NPV value is expressed by cash ﬂow per SART plant capacity (US$/(m 3/h)). Therefore, the income value is expressed by cash flow per SART plant capacity (US$/(m /h)).3 Therefore, the income value is expressed by cash flow per SART plant capacity (US$/(m /h)). Therefore,3 the income expressedexpressed in in terms terms of of precipitate precipitate produced (US$/ton) must must be transferred to US$/(mUS$/(m3/h)/h) using using metals metals expressed in terms of precipitate produced (US$/ton) must be transferred to US$/(m3/h) using metals concentrationconcentration in in thethe feedfeed solution. solution. Hence,Hence, typical typical copper copper andand zinczinc concentrationsconcentrations in in aa cyanidecyanide solutionsolution havehave bebeenen defined.defined. Figure Figure 7 7 showsshows the the differentialdifferential NPV NPV resultsresults of of the the two two-stage-stage SART SART processprocess with with respect respect to to the the conventional conventional option. option. For For each each Cu/Zn Cu/Zn molar molar ratio, ratio, copper copper concentration concentration in in

Minerals 2018, 8, 392 9 of 12 concentration in the feed solution. Hence, typical copper and zinc concentrations in a cyanide solution Mineralshave been 2018, deﬁned.8, x FOR PEER Figure REVIEW7 shows the differential NPV results of the two-stage SART process9 with o f 12 respect to the conventional option. For each Cu/Zn molar ratio, copper concentration in the feed thecyanide feed cyanide solution solution has varied has between varied between 100 and 100 1200 and mg/L, 1200 except mg/L, for except Cu/Zn for molarCu/Zn ratio molar value ratio of value 0.025. ofHere, 0.025. instead Here, of instead copper, of zinc copper, has zinc been has considered, been considered, in order toin avoidorder ato zinc avoid concentration a zinc concentration that distances that distancesfrom typical from gold typical plant gold conditions plant (e.g.,conditions copper (e.g. concentration, copper concentration of 1200 mg/L of and 1200 Cu/Zn mg/L molar and ratioCu/Zn of molar0.025 determineratio of 0.025 a zinc determine concentration a zinc concentration of approximately of approximately 50,000 mg/L). 50,000 mg/L).

300

[Cu] = 100 mg/L 250 [Cu] = 200 mg/L /h)

3 [Cu] = 500 mg/L 200 [Cu] = 750 mg/L [Cu] = 1000 mg/L [Cu] = 1200 mg/L 150

100

50 Differential NPV, kUS$/(m NPV, Differential 0

-50 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Cu/Zn molar ratio

FigureFigure 7. 7. DiffeDifferential re ntial NPV NPV re results sults for for two two-stage-stage SART process, process, varying Cu/Zn molar molar ratio ratio and and metals metals concentration in a feed cyanide solution. concentration in a fe e d cyanide solution. Results in Figure7 show the Cu/Zn molar ratio range where the two-stage SART process can be Results in Figure 7 show the Cu/Zn molar ratio range where the two-stage SART process can be feasible to install. In this case, for copper concentration in the feed cyanide solutions over 200 mg/L, feasible to install. In this case, for copper concentration in the feed cyanide solutions over 200 mg/L, and Cu/Zn molar ratios ranging approximately between 0.1 and 1.5. When metal concentration and Cu/Zn molar ratios ranging approximately between 0.1 and 1.5. When metal concentration is is higher than 200 mg/L and Cu/Zn molar ratio is around 0.4, the differential NPV is optimum. higher than 200 mg/L and Cu/Zn molar ratio is around 0.4, the differential NPV is optimum. This This optimum differential NPV value will be modified according to metal prices, although the curve’s optimum differential NPV value will be modified according to metal prices, although the curve’s tendency will be similar. The optimum NPV value is reached when a factor defined as the optimal tendency will be similar. The optimum NPV value is reached when a factor defined as the optimal ratio (OR = copper price/zinc price × Cu/Zn molar ratio) is approximately 1.0. ratio (OR = copper price/zinc price × Cu/Zn molar ratio) is approximately 1.0. A sensitivity analysis varying the price ratio (copper price/zinc price) has been developed to A sensitivity analysis varying the price ratio (copper price/zinc price) has been developed to determine the optimum differential NPV value. In this way, Figure8 shows a slope close to 1.0 where determine the optimum differential NPV value. In this way, Figure 8 shows a slope close to 1.0 where the price ratio is graphed against the Cu/Zn molar ratio inverse when the NPV value is optimum. the price ratio is graphed against the Cu/Zn molar ratio inverse when the NPV value is optimum. This slope represents the OR parameter. Therefore, a rapid estimation of the feasible range to apply This slope represents the OR parameter. Therefore, a rapid estimation of the feasible range to apply the two-stage SART process is estimating the OR value. However, when OR value is far from 1.0, the two-stage SART process is estimating the OR value. However, when OR value is far from 1.0, the the methodology presented here—based on a thermodynamic and economic estimation—is an excellent methodology presented here—based on a thermodynamic and economic estimation—is an excellent alternative to evaluate if the two-stage SART process could be a good option in a gold plant with high alternative to evaluate if the two-stage SART process could be a good option in a gold plant with high copper and zinc content in its cyanide solutions. The methodology presented here allows to define the copper and zinc content in its cyanide solutions. The methodology presented here allows to define type of SART process that will be include in a cyanidation plant in an early stage of a project. A final the type of SART process that will be include in a cyanidation plant in an early stage of a project. A economic evaluation of the SART plant must be performed, using the economic criteria of each mining final economic evaluation of the SART plant must be performed, using the economic criteria of each company, in order to establish the economic feasibility to install it. mining company, in order to establish the economic feasibility to install it.

Minerals 2018, 8, x FOR PEER REVIEW 10 o f 12 Minerals 2018, 8, 392 10 of 12

FigureFigure 8. 8.Price Price ratio ratio (Cu (Cu price/Zn price /Zn price) price vs. ) vs 1/(Cu/Zn. 1/(Cu/Zn molar molar ratio) ratio) at at optimum optimum NPV NPV value. value . 4. Conclusions 4. Conclusions This study has developed a methodology to estimate the feasible conditions to install a This study has developed a methodology to estimate the feasible conditions to install a two- two-stage SART process in cyanidation plants having high copper and zinc contents in their solutions. stage SART process in cyanidation plants having high copper and zinc contents in their solutions. This methodology is based on a theoretical and economic estimation using a modified Hydra/Medusa This methodology is based on a theoretical and economic estimation using a modified Hydra/Medusa software. Results suggest operational pH conditions of 7.5 and 4.0 for the first and second stage software. Results suggest operational pH conditions of 7.5 and 4.0 for the first and second stage of of the two-stage SART process, respectively. These results are similar to previous experimental the two-stage SART process, respectively. These results are similar to previous experimental studies, studies, validating this theoretical methodology, and thereby contributing to explaining the behavior validating this theoretical methodology, and thereby contributing to explaining the behavior of of speciation in a cyanide solution in presence of sulfide. Furthermore, the economic results show the speciation in a cyanide solution in presence of sulfide. Furthermore, the economic results show the feasible range of application of the two-stage SART process for high zinc and copper concentration, feasible range of application of the two-stage SART process for high zinc and copper concentration, being more than 200 mg/L for both metals. Indeed, the two-stage SART plant reaches its optimum being more than 200 mg/L for both metals. Indeed, the two-stage SART plant reaches its optimum condition when the OR parameter defined in this study is close to 1.0. Summarizing, this SART option condition when the OR parameter defined in this study is close to 1.0. Summarizing, this SART option could be an excellent solution for gold cyanidation plants with Merrill–Crowe process installed in their could be an excellent solution for gold cyanidation plants with Merrill–Crowe process installed in flow-sheets which additionally contains high cyanide-soluble copper ores. their flow-sheets which additionally contains high cyanide-soluble copper ores. Author Contributions: H.E., M.G.-K. and M.Q. conceived and designed the experiments; M.G.-K. and M.Q. performedAuthor Contributions: the experiments; H.E. H.E., M.G. analyzed-K. and the M.Q. data andconceived wrote theand paper. designed the experiments; M.G.-K. and M.Q. performed the experiments; H.E. analyzed the data and wrote the paper. Funding: This research was funded by the National Commission for Scientific and Technological Research (CONICYT,Funding: Chile)This re for search financial was support funde d through by the ProjectNational Fund Commission no. FB0809 for PIA Scie CONICYT ntific and and Te the chnological FONDEF/IDeA Research Program,(CONICYT, FONDEF/CONICYT Chile) for financial grant support number through 2017+ID17I10021. Project Fund no. FB0809 PIA CONICYT and the FONDEF/IDeA Acknowledgments:Program, FONDEF/CONICYTThe authors grant appreciate number the 2017+ID17I10021 revision and support. of Mike M. Botz, his comments have improved this study. Also, the authors thank the National Commission for Scientific and Technological Research (CONICYT,Acknowledgments: Chile) for financialThe authors support appreciate through Projectthe revision Fund no.and FB0809 support PIA of CONICYT Mike M. andBotz the, his FONDEF/IDeA comments have Program,improve FONDEF/CONICYT d this study. Also, the 2017+ID17I10021. authors thank the National Commission for Scientific and Te chnological Research Conflicts(CONICYT, of Interest: Chile) forThe financial authors support declare through no conflict Project of interest. Fund no The. FB0809 funders PIAhad CONICYT no role and in the the design FONDEF/IDeA of the study;Program, in the FONDEF/CONICY collection, analyses,T or 2017+ID17I10021. interpretation of data; in the writing of the manuscript, and in the decision to publish the results. Conflicts of Interest: The authors declare no conflict of interest. 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