Synchrotron Radiation XRD Investigation of the Fine Phase Transformation During Synthetic Chalcocite Acidic Ferric Sulfate Leaching

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Article Synchrotron Radiation XRD Investigation of the Fine Phase Transformation during Synthetic Chalcocite Acidic Ferric Sulfate Leaching

Chaojun Fang 1,2 , Shichao Yu 1,2, Xingxing Wang 1,2 , Hongbo Zhao 1,2 , Wenqing Qin 1,2 , Guanzhou Qiu 1,2 and Jun Wang 1,2,*

1 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China; [email protected] (C.F.); [email protected] (S.Y.); [email protected] (X.W.); [email protected] (H.Z.); [email protected] (W.Q.); [email protected] (G.Q.) 2 Key Lab of Bio-hydrometallurgy of Ministry of Education, Changsha 410083, China * Correspondence: [email protected]; Tel.: +86-731-88876557

Received: 7 August 2018; Accepted: 12 October 2018; Published: 17 October 2018

Abstract: The fine phase transformation process of chalcocite (Cu2S) leaching in acidic ferric sulfate solution was studied by leaching experiments and synchrotron radiation X-ray diffraction (SRXRD) tests. The results showed that the dissolution process of chalcocite was divided into two stages. In the first stage, Cu2S was firstly transformed to Cu5FeS4 and Cu2−xS, then the galvanic effect between Cu5FeS4 and Cu2−xS accelerated the dissolution process of Cu1.8S → Cu1.6S → CuS, and finally Cu5FeS4 was also transformed to CuS. While in the second stage, CuS was transformed to elemental sulfur, which formed the passivation layer and inhibited the leaching of chalcocite. Specifically, Cu5FeS4 was detected during the chalcocite leaching process by SRXRD for the first time. This research is helpful for revealing the detailed leaching process of chalcocite.

Keywords: synchrotron radiation; SRXRD; chalcocite; leaching; ferric ions

1. Introduction Flotation is one of the most important methods for primary enrichment of minerals and it has significantly developed in recent years [1–4]. However, hydrometallurgy, particularly bioleaching, has the advantages of being simple, low cost, and eco-friendly, which has been successfully applied in the processing of low grade minerals all over the world [5–11]. However, there are still some problems and challenges in the leaching of secondary copper sulfide minerals. The phase transformation mechanism and dissolution kinetics of chalcocite are not clear, which is considered as the main reason for low leaching efficiency and an unsatisfactory leaching rate [12–15]. Therefore, it is necessary to reveal the detailed dissolution mechanism and promote the copper leaching rate. The crystal structure of chalcocite is more complex than its simple composition. High chalcocite belongs to the P6-3/mmc space group, in which sulfur atoms are arranged in hexagonal close-packing order, whilst three varieties of copper atoms with four-fold, three-fold, and two-fold coordination are randomly packed in the interstices of sulfur atoms [16]. The structures of low chalcocite and djurleite are derived from the high chalcocite structure and both are monoclinic [17]. It was reported that the structure of low chalcocite was either disordered or belonged to the Pc space group, with four of the 96 copper atoms taking on two-fold coordination in the monoclinic unit. The structure of djurleite is in space group P2-1/n and with a unit cell containing 8 Cu31S16, it is in general similar to low chalcocite. However, there are 62 different copper atoms, where 52 are in three-fold and triangular coordination with sulfur, 9 form a distorted tetrahedral group, and 1 is in linear coordination [18]. In addition,

Minerals 2018, 8, 461; doi:10.3390/min8100461 www.mdpi.com/journal/minerals Minerals 2018, 8, 461 2 of 10 compared with structural data of other copper-rich sulﬁdes, the tetrahedral, triangular, and linear Cu-S bond of low chalcocite and djurleite are of large distortions and wide variations in bond lengths [19]. Many publications show that the dissolved process of chalcocite acidic ferric sulfate leaching can be divided into two stages, with the production of secondary covellite as an intermediate (Equations (1) and (2)) [13,20–26], whilst ferric ions were regenerated by Equation (3).

3+ 2+ 2+ Cu2S + 2Fe → Cu + 2Fe + CuS (1)

CuS + 2Fe3+ → Cu2+ + 2Fe2+ + S0 (2)

2+ + 3+ 4Fe + O2 + 4H → 4Fe + 2H2O (3) The leaching rate of the first stage of chalcocite ferric leaching is very fast compared with the second stage. However, there exists several phase transformation processes in the first stage of leaching. It is reported that chalcocite would be transformed to djurleite (Cu1.97S), digenite (Cu1.8S), anilite (Cu1.75S), geerite (Cu1.6S), spionkopite (Cu1.4S), yarrowite (Cu1.12S), and covellite (CuS) in that order [27–29]. According to the stoichiometry, when copper extraction reached 10%, chalcocite would disappear and transform to digenite, whilst all the intermediates were transformed to covellite when copper extraction achieved 50%. In essence, the second stage is the oxidative dissolution of the secondary covellite, in which the elemental sulfur and Cu2+ are produced. It is reported that the secondary covellite has more chemical activity than the primary covellite, which may be attributed to the increasing surface area formed by the first stage of chalcocite leaching [24,30]. The dissolution behavior of the sulfur element in chalcocite ferric leaching is an important output 3+ + of this mechanism. Chalcocite and covellite can be attacked by Fe ,O2, and H attributed to its acid-soluble properties. However, different from the four dissolution pathways (metal-deficient sulfides, polysulfide, element sulfur, and jarosites) of chalcopyrite [31–38], the dissolution of sulfur element in the second stage of chalcocite ferric leaching follows the polysulfide pathway and elemental sulfur is formed [24]. Synchrotron radiation (SR) has the advantages of high-brilliance and flux, wide energy spectrum, very short pulses, and high purity, and many scientific problems could be resolved by SR-based technologies. There are three synchrotron radiation facilities in China to date, Beijing Synchrotron Radiation Facility (BSRF), Shanghai Synchrotron Radiation Facility (SSRF), and the National Synchrotron Radiation Laboratory (NSRL). In the mineral processing and hydrometallurgy research field, SR-based technologies such as SRXRD, SRXPS, and XANES were mainly used to study the fine phase transformation and variation of chemical substances on the mineral surface [39–45]. In this paper, synthetic chalcocite was chosen as the research object, and SRXRD was employed to explore the fine phase transformation mechanism in chalcocite acidic ferric sulfate leaching. The results would be helpful for explaining the key chemical reactions and key intermediates in chalcocite leaching, and finally to improve copper extraction and resource utilization of secondary copper sulfide minerals.

2. Materials and Methods

2.1. Mineral The chalcocite sample used in experiments was bought from Sinopharm Chemical Reagent Co., Shanghai, China. The purchased chalcocite was ground using a laboratory mill and sieved to the size of ~0.074 mm for leaching experiments and SRXRD tests. Chalcocite was conﬁrmed by chemical composition analysis and an SRXRD test. The SRXRD test was completed in beamline 4B9A at the Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). Figure1 shows that the SRXRD peaks of the sample were ﬁtted with the peaks of chalcocite. Table1 shows that the contents of copper element and sulfur element were 78.61% and 18.95%, respectively. The results indicated that chalcocite sample used in this paper was of high purity. MineralsMinerals2018 2018, ,8 8,, 461 x FOR PEER REVIEW 33 ofof 1010

140 Ch Ch-Cu S 2 120

Ch 100 Ch

80 Ch Ch

60 Ch

Intensity/Counts 40 Ch Ch Ch Ch 20 Ch Ch Ch Ch

10 20 30 40 50 60 70 Two Theta/deg FigureFigure 1. Synchrotron 1. Synchrotron radiation radiation X-ray X-ray diffr diffractionaction (SRXRD) (SRXRD) results results of ofsynthetic synthetic chalcocite chalcocite samples samples (BSRF). (BSRF). Table 1. Chemical compositions analysis results of chalcocite (%). Table 1. Chemical compositions analysis results of chalcocite (%). Sample Cu S Fe Ca Si Sample Cu S Fe Ca Si ChalcociteChalcocite 78.61 78.6118.95 18.950.018 0.018 0.0170.0170.053 0.053

2.2.2.2. LeachingLeaching ExperimentsExperiments OneOne gramgram ofof chalcocitechalcocite withwith aa sizesize ofof ~0.074~0.074 mmmm waswas putput intointo aa 250250mL mLshake shakeflask flaskcontaining containing 100100 mLmL ofof sterilized distilled water, water, and and then then two two grams grams of of ferric ferric sulfate sulfate hydrate hydrate (Fe (Fe2(SO2(SO4)3·xH4)3·2xHO) 2wasO) wasadded added as the as source the source of ferric of ferric ions. ions. The ThepH value pH value was wasadjusted adjusted to 1.7 to by 1.7 diluting by diluting sulfuric sulfuric acid acidand andthen thenthe shake the shake flasks flasks were were placed placed into an into orbital an orbital shaker shaker at a speed at a speed of 170 of rpm 170 and rpm temperature and temperature of 45 °C. of ◦ 45TheC. shake The shakeflask was flask sealed was sealed with a with breathable a breathable sealin sealingg film to film prevent to prevent rapid rapid evaporation evaporation of water, of water, and andthe atmosphere the atmosphere inside inside the orbital the orbital shaker shaker was wasstandard standard atmospheric atmospheric air and air the and oxygen the oxygen in the in air the could air couldenter enterthe shake the shake flask. flask. InIn the the leaching leaching process, process, the the pH pH value value was was adjusted adjusted back back to 1.7to 1.7 when when it was it was above above 2.0, and2.0, waterand water lost bylost evaporation by evaporation was supplemented was supplemented periodically periodical by distilledly by water distilled regularly. water Copper regularly. ion concentrations Copper ion andconcentrations iron ion concentrations and iron ion were concentrations measured withwere anmeasured inductively with coupled an inductively plasma-atomic coupled emission plasma- spectrometeratomic emission (ICP-AES) spectrometer (PS-6, (I BairdCP-AES) Co., (PS-6, Deford, Baird MA, Co., USA). Deford, The redoxMA, USA). potentials The redox of the potentials leaching solutionof the leaching were determined solution were by a determined Pt electrode by with a Pt reference electrode to with an Ag/AgCl reference electrode to an Ag/AgCl (3.0 mol/L electrode KCl) (BPH-221,(3.0 mol/L Dalian KCl) (BPH-221, BELL Co., Dalian Dalian, BELL China), Co., whilst Dalian, the China), pH values whilst were the measured pH values with were a pHmeasured meter (PHSJ-4A,with a pH Shanghaimeter (PHSJ-4A, LEICI Co., Shanghai Shanghai, LEICI China). Co., Shanghai, China). TheThe leachingleaching experimentsexperiments werewere setset asas multiplemultiple parallelparallel samples,samples, andand thethe samplessamples werewere removedremoved accordingaccording to to the the timetime setset inin advance.advance. TheThe removedremoved samplesample waswas filteredfiltered withwith aa neutralneutral filterfilter paperpaper andand cleanedcleaned with with distilled distilled water water three three times, times, afterwards, afterwards the, the samples samples were were dried dried in in a vacuuma vacuum oven oven for for 24 24 h forh for the the tests tests of SRXRD.of SRXRD.

2.3.2.3. SynchrotronSynchrotron RadiationRadiation X-rayX-ray DiffractionDiffraction TheThe SRXRDSRXRD spectra spectra were were obtained obtained from from beamline beamline BL14B1 BL14B1 of the Shanghai of the Shanghai Synchrotron Synchrotron Radiation FacilityRadiation (SSRF). Facility The (SSRF). amount The of amount mixed of sample mixed thatsample needed that needed to be added to be added for each for testeach was test 50was mg. 50 Themg. wavelength The wavelength of incident of incident synchrotron synchrotron radiation radi X-rayation wasX-ray 0.6895 was nm,0.6895 and nm, the and SRXRD the SRXRD spectra rangedspectra ◦ ◦ fromranged 4 tofrom 38 4°. The to 38°. background The background removal removal and phase and comparison phase comparison of the spectra of the spectra were completed were completed using theusing software the software Jade 6. Jade 6.

Minerals 2018, 8, 461 4 of 10

3. ResultsMinerals and2018, 8 Discussion, x FOR PEER REVIEW 4 of 10

3.1. Leaching3. Results Behaviors and Discussion

3.1.The Leaching copper Behaviors extraction of chalcocite leaching at different leaching times is shown in Figure2a. It indicated that the leaching of chalcocite was divided into two stages according to the leaching rate. The copper extraction of chalcocite leaching at different leaching times is shown in Figure 2a. It The first 21 h of leaching can be considered as the first stage, in which copper extraction increased indicated that the leaching of chalcocite was divided into two stages according to the leaching rate. rapidlyThe asfirst a straight21 h of leaching line. The can stage be afterconsidered 21 h of as leaching the first canstage, be in regarded which copper as the secondextraction stage, increased in which copperrapidly extraction as a straight increased line. muchThe stage slower after than 21 h the of firstleaching stage. can It be was regarded reported asthat the thesecond first stage, stage in was controlledwhich copper by the diffusionextraction ofincreased oxidant much and the slower activation than th energye first stage. was It 4–25 was KJ/mol, reported whilst that the the first activation stage energywas of controlled the second by stage the diffusion was 55–105 of oxidant KJ/mol and [22 ,30the], activation which was energy considered was 4–25 as the KJ/mol, main reasonwhilst the of the differenceactivation in copper energy extraction.of the second stage was 55–105 KJ/mol [22,30], which was considered as the main reasonThe variation of the difference of iron ion in copper concentration extraction. in the first 15 h of chalcocite leaching is shown in Figure2b. It indicatedThe thatvariation the concentrationof iron ion concentration of iron ions in the had first a sudden15 h of chalcocite decrease leachi at theng firstis shown 1 h ofin Figure leaching, and2b. then It indicated it increased that slowlythe concentration until leaching of iron for ions 10 had h, anda sudden thereafter decrease remained at the first stable. 1 h of Itleaching, has been reportedand then that ferricit increased ions have slowly a stronger until leaching oxidizing for ability10 h, and than thereafter dissolved remained oxygen [stable.46], hence It has the been direct oxidantreported in chalcocite that ferric acidic ions have ferric a sulfatestronger leaching oxidizin wasg ability ferric than ions. dissolved Although oxygen iron [46], ions hence may hydrolyze the direct to formoxidant a precipitate in chalcocite due to acidic pH changes ferric sulfate during leaching leaching, was theferric amount ions. Although was small iron near ions the may pH hydrolyze value of 2.0. to form a precipitate due to pH changes during leaching, the amount was small near the pH value of Accordingly, it can be speculated that ferric ions in solution may have directly reacted with chalcocite 2.0. Accordingly, it can be speculated that ferric ions in solution may have directly reacted with and generated some iron-containing compounds in the first 1 h of leaching, and after 10 h of leaching, chalcocite and generated some iron-containing compounds in the first 1 h of leaching, and after 10 h the generatedof leaching, iron-containing the generated iron-contain compoundsing were compounds dissolved were again. dissolved again. FigureFigure2c,d 2c,d shows shows the the variations variations of of pH pH value value and and redox redox potential,potential, respectively. respectively. It Itrevealed revealed that that pH valuepH value had had a significant a significant increase, increase, whilst whilst redox redox potentialpotential had a a sharp sharp decrease decrease at at the the beginning beginning of of the leaching.the leaching. This This can can be be attribute attribute to to the the rapid rapid consumption consumption of of sulfuric sulfuric acid acid shown shown by by Equation Equation (3), (3), andand the rapidthe rapid consumption consumption of of ferric ferric ions ions as as follows followsin in EquationsEquations (1) and and (2). (2).

60 (a) 4.25 (b)

50 4.20 /L) g

40 (

/ 4.15 on on i 30 pH=1.7~2.0 4.10 3+

Fe =0.1mol/L 20 4.05 Copper extraction/% on concentrat

10 i 4.00 ron ron I 0 3.95

0 50 100 150 200 250 300 350 -20246810121416 Time/h Time/h

700 2.5 (c) (d) 650 2.4 600 2.3

550 2.2

2.1 500

pH 2.0 450

1.9 400

1.8 350 Redox potential / mV vs. Ag/AgCl mV / potential Redox 1.7 300

1.6 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Time/h Time/h FigureFigure 2. Results2. Results of of chalcocite chalcocite acidic acidic ferricferric sulfate leaching leaching (pH (pH = =1.7~2.0, 1.7~2.0, C(Fe C(Fe3+) =3+ 0.1) =mol/L): 0.1 mol/L): (a) (a) CopperCopper extraction extraction atat differentdifferent leaching times; times; (b (b) )Iron Iron ion ion concentration concentration in inthe the first ﬁrst 15 15h; ( h;c) (Variationc) Variation of pHof value;pH value; (d) ( Variationsd) Variations of redoxof redox potential. potential.

Minerals 2018, 8, 461 5 of 10

3.2. Synchrotron Radiation X-ray Diffraction Tests Minerals 2018, 8, x FOR PEER REVIEW 5 of 10 3.2.1. SRXRD Results of the First Stage of Leaching 3.2. Synchrotron Radiation X-ray Diffraction Tests To further investigate the phase transformation process of the first stage of chalcocite leaching, SRXRD3.2.1. was SRXRD used Results to analyze of the the First intermediate Stage of Leaching species during chalcocite leaching. Figure3 and Table2 show thatTo chalcocite further investigate disappeared the phase just transformation after half an hourprocess of of leaching, the first stage while of Cuchalcocite7.2S4, Cu leaching,1.8S, Cu 8S5, and CuSRXRD5FeS4 waswere used detected to analyze by SRXRD.the intermediate It indicated specie thats during chalcocite chalcocite was leaching. quickly Figure oxidized 3 and in Table the initial stage2 of show ferric that leaching chalcocite and disappeared some intermediates just after half were an hour generated. of leaching, Afterwards, while Cu7.2 noS4, newCu1.8S, phase Cu8S5 appeared, and Cu5FeS4 were detected by SRXRD. It indicated that chalcocite was quickly oxidized in the initial stage and the peak intensity of Cu7.2S4 decreased when leached for 1 h, which indicated part dissolution of ferric leaching and some intermediates were generated. Afterwards, no new phase appeared and of Cu7.2S4. the peak intensity of Cu7.2S4 decreased when leached for 1 h, which indicated part dissolution of However, CuS appeared as an intermediate when chalcocite was leached for 2 h, whilst the Cu7.2S4. peak intensityHowever, of Cu CuS7.2 Sappeared4 and Cu as5 anFeS intermediate4 experienced when a chalcocite decrease. was Then, leached the peakfor 2 h, intensity whilst the of peak Cu 7.2S4 disappearedintensity when of Cu leached7.2S4 and for Cu 4 h,5FeS and4 experienced the peak intensity a decrease. of Cu 8Then,S5 was the when peak leached intensity for of 6h. Cu Moreover,7.2S4 the peakdisappeared intensity when of Culeached5FeS4 forwas 4 increasinglyh, and the peak weaker, intensity whilst of Cu the8S5 peakwas when intensity leached of CuSfor 6 becameh. increasinglyMoreover, stronger the peak as intensity chalcocite of wasCu5FeS leached4 was increasingly for 2 h to 6 weaker, h. whilst the peak intensity of CuS Afterbecame leaching increasingly for stronger 10 and as 15 chalcocite h, Cu5FeS was4 leacheddisappeared for 2 h to and 6 h. elemental sulfur was generated, After leaching for 10 and 15 h, Cu5FeS4 disappeared and elemental sulfur was generated, which which indicated that the generated intermediate Cu5FeS4 had dissolved after 10 h of leaching, and that part ofindicated CuS had that oxidized the generated to elemental intermediate sulfur. Cu5FeS4 had dissolved after 10 h of leaching, and that part of CuS had oxidized to elemental sulfur. According to Figure3, it can be concluded that the first stage of chalcocite leaching was not According to Figure 3, it can be concluded that the first stage of chalcocite leaching was not a a simple process where Cu2+ dissolved into solution and chalcocite directly transformed to CuS. simple process where Cu2+ dissolved into solution and chalcocite directly transformed to CuS. Instead, Instead,it was it was a complex a complex and fast and process fast process in which in which several several chemical chemical reactions reactions occurred occurred simultaneously. simultaneously. The The productsproducts of of these these reactions reactions were were several several similar similarminerals minerals containing containing Cu, S, or Fe Cu,elements, S, or which Fe elements, are whichclose are to close each to other each in other diffraction in diffraction peak. In particular, peak. In particular,Cu5FeS4 was Cu detected5FeS4 forwas the detected first time for by theSRXRD, first time by SRXRD,which was which in wasaccordance in accordance with the withresults the of results Figure of2b. Figure It will2 b.be Ithelpful will be for helpful explaining for explaining the phase the phasetransformation transformation mechanism mechanism in chalcocite in chalcocite leaching. leaching.

Ch-Cu S Co 2 Bo-Cu FeS Co Co 5 4 Co Di-Cu S 7.2 4 Co Co Co Dg-Cu S Co Co 1.8 15 h S Co Co S Ge-Cu S Co 8 5 Co-CuS Co Co 0 S - S Co Co Co 10 h S Co Co Co Co Co

Co Co Bo

Co Bo Bo 6 h Co Co Bo Co Co Co

Co Co Ge Ge Bo Co

Bo Bo Co 4 h Co Bo Ge Co Co Bo Bo Ge Intensity/Counts Co Bo Di Co Di Di 2 h Co Bo

Ge Bo Bo Ge Bo Dg Dg Di Di Di Ge 1 h Bo

Ge Bo Bo Ge Dg Bo Di Di Ge 0.5 h Dg Di Bo

Ch Ch Ch Ch Ch Ch Ch Ch Ch 0 h Ch

0 5 10 15 20 25 30 35 40 Two Theta/deg

Figure 3. SRXRD results of the phase transformation process in the ﬁrst stage of chalcocite acidic ferric sulfate leaching (Shanghai Synchrotron Radiation Facility (SSRF)). Minerals 2018, 8, x FOR PEER REVIEW 6 of 10

Minerals 2018Figure, 8, 3. 461 SRXRD results of the phase transformation process in the first stage of chalcocite acidic ferric 6 of 10 sulfate leaching (Shanghai Synchrotron Radiation Facility (SSRF)).

TableTable 2. 2.The The minerals minerals detecteddetected in different leaching leaching time. time.

Time/hTime/h Cu2S Cu Cu2S 5FeSCu45FeSCu4 1.8CuS1.8S CuCu7.27.2SS44 CuCu8S85S 5 CuS CuSS S 0 √ √ 0 √ √ √ √ 0.5 0.5 √√ √√ √√ √ √ 1 1 √√ √ √√ √ √ √ 2 2 √√ √ √ √√ √ 4 4 √√ √ √ √ 6 6 √ √ √ √ 10 √ √ 15 10 √ √ 15 √ √ 3.2.2. SRXRD Results of the Second Stage of Chalcocite Leaching 3.2.2. SRXRD Results of the Second Stage of Chalcocite Leaching The results shown in Figure4 suggest that no new phase appeared in the second stage of The results shown in Figure 4 suggest that no new phase appeared in the second stage of chalcocitechalcocite leaching, leaching, where where the the peak peak intensity intensity of CuS of wasCuS increasinglywas increasingly weaker, weaker, whilst whilst the peak the intensity peak ofintensity elemental of sulfurelemental was sulfur increasingly was increasingly stronger stronger as the leaching as the leaching continued. continued. After After leaching leaching for 933for h, the933 main h, the phase main was phase elemental was elemental sulfur and sulfur only and a fewonly CuSa few existed. CuS existed. EquationEquation (2) (2) shows shows that that the the second second stagestage ofof chalcocitechalcocite leaching leaching was was the the redox redox reaction reaction between between covellitecovellite and and ferric ferric ions.ions. In In this this process, process, ferric ferric ions ions acted acted as an asoxidant an oxidant reduced reduced to ferrous to ions, ferrous whilst ions, whilstCuS CuSwas wasoxidized oxidized to elemental to elemental sulfur. sulfur. Meanwhile, Meanwhile, it should it should be noted be noted that only that onlya small a small amount amount of of elemental sulfur would would be be continuously continuously oxidized oxidized to to sulfate sulfate under under low low redox redox potential, potential, as asshown shown in in FigureFigure2d 2d [30 [30].]. Hence,Hence, it it can can be be concluded concluded that that inin thethe secondsecond stage of of chalcocite chalcocite acidic acidic ferric ferric sulfate sulfate leaching, leaching, CuSCuS was was transformed transformed to elementalto elemental sulfur sulfur and and no newno new intermediate intermediate was generated.was generated. However, However, it should it beshould noted thatbe noted the kineticsthat the kinetics of the second of the second stage ofstag chalcocitee of chalcocite leaching leaching were were much much lower lower than than the the ﬁrst stage.first Thestage. reason The mayreason be may that be the that generated the generate elementald elemental sulfur formedsulfur formed a passivation a passivation layer and layer inhibited and theinhibited dissolution the dissolution of chalcocite. of chalcocite.

Co-CuS 0 S - S

Co S Co S

Co Co S S S S Co Co S S S S S S S S S S Co Co S Co 933 h

Co Co Co S S Co Co Co S S Co Co S S Co Co Co Co 261 h

Co Co

Intensity/Counts Co Co

Co S S Co Co Co

Co S S S Co Co S Co Co Co 189 h

Co Co

Co S Co Co S Co

Co Co Co Co Co Co 117 h

0 5 10 15 20 25 30 35 40 Two Theta/ deg Figure 4. SRXRD results of the phase transformation process in the second stage of chalcocite acidic ferric sulfate leaching (SSRF). Minerals 2018, 8, 461 7 of 10

3.3. Thermodynamic Calculation of the Generation and Dissolution of Cu5FeS4 in Chalcocite Leaching Process Whether a reaction can start spontaneously depends on the variation of Gibbs free energy of the reaction. If the variation of Gibbs free energy of the reaction was negative, the reaction can be spontaneously started. On the contrary, the reaction cannot be carried out spontaneously. According to the redox properties of various chemical components in the leaching process, it can be inferred that Cu5FeS4 detected by SRXRD might be generated and dissolved, as shown in Equations (4) and (5). 3+ 2+ 2+ 4Cu2S + 4Fe → Cu5FeS4+3Cu +3Fe (4) 3+ 2+ 2+ Cu5FeS4 + 4Fe → 4CuS+Cu +5Fe (5) Some thermodynamic data are listed in Table3. The variations of Gibbs free energy can be calculated from Equation (6): Θ ∆rGm = ∆rGm + RTlnK (6) where K was the chemical reaction equilibrium constant. According to the thermodynamic data in Table3, it can be calculated that the variations of Gibbs free energy of Equations (4) and (5) were −69.62 KJ/mol and −113.63 KJ/mol under standard state, respectively. It indicated that Equations (4) and (5) can be carried out spontaneously on thermodynamics by Gibbs free energy criterion.

Table 3. Some standard thermodynamic data (298.15 K) [47,48].

Components Enthalpy (KJ/mol) Entropy (J/mol/K) Gibbs Free Energy (KJ/mol) (298.15 K)

Cu2S −79.496 120.918 −115.548 Cu5FeS4 −380.326 362.334 −488.356 CuS −48.530 66.530 −68.366 Cu2+ (aq) 64.768 −99.579 94.458 Fe3+ (aq) −48.534 −315.892 45.649 Fe2+ (aq) −89.119 −137.654 −48.078

3.4. Disscussion of the Fine Phase Transformation of Chalcocite Acidic Ferric Sulfate Leaching In the leaching process in acidic ferric sulfate solution, chalcocite was ﬁrstly reacted with ferric ions and some intermediates Cu1.8S, Cu7.2S4, Cu8S5, and Cu5FeS4 were generated according to Equations (7) and (4). Then, Cu1.8S, Cu7.2S4, and Cu8S5 were oxidized to CuS in that order, according to Equation (8). Afterwards, Cu5FeS4 was also oxidized to CuS, following Equation (5). Finally, CuS was oxidized to elemental sulfur, following Equation (2). Meanwhile, ferric ions played the role of oxidants during the entire leaching process and were regenerated in accordance with Equation (3).

3+ 2+ 2+ Cu2S + 2xFe → Cu2−xS + xCu + 2xFe , (0 < x <1) (7)

3+ 2+ 2+ Cu2−xS + 2(1 − x)Fe → CuS + (1 − x)Cu + 2(1 − x)Fe , (0 < x < 1) (8)

It should be noting that the disappearance of those intermediates was in the order of Cu1.8S → Cu7.2S4 → Cu8S5 → Cu5FeS4 → CuS, according to Figures3 and4. In addition, it has been reported by many researchers that there exists a galvanic effect between different semiconductor minerals when they are leached [49–51]. In the ﬁrst stage of chalcocite leaching, several copper sulﬁde minerals appeared; the other potentials of these minerals were different and there existed a galvanic effect between them. The other potentials of these minerals were in the following order: Cu2S < Cu1.8S < Cu1.6S < Cu5FeS4 < CuS. Hence, it can be speculated that the galvanic effect between Cu5FeS4 and Cu2−xS accelerated the dissolution of Cu2S and Cu2−xS, and when Cu2−xS was completely transformed to CuS, a galvanic effect between Cu5FeS4 and CuS accelerated the dissolution of Cu5FeS4, then all the intermediates were transformed to CuS. Minerals 2018, 8, x FOR PEER REVIEW 8 of 10 Minerals 2018, 8, 461 8 of 10 to CuS, a galvanic effect between Cu5FeS4 and CuS accelerated the dissolution of Cu5FeS4, then all the intermediates were transformed to CuS. Therefore,Therefore, itit cancan bebe concludedconcluded thatthat thethe probableprobable phasephase transformationtransformation mechanismmechanism ofof chalcocitechalcocite leachingleaching inin acidicacidic ferricferric solutionsolution isis asas shownshown in in Figure Figure5 .5.

FigureFigure 5.5. ProbableProbable finefine phasephase transformationtransformation duringduring chalcocitechalcocite acidicacidicferric ferricsulfate sulfateleaching. leaching. 4. Conclusions 4. Conclusions In summary, the acidic ferric sulfate leaching of synthetic chalcocite was divided into two stages. In summary, the acidic ferric sulfate leaching of synthetic chalcocite was divided into two stages. Chalcocite was transformed to covellite in the first stage, then covellite was transformed to elemental Chalcocite was transformed to covellite in the first stage, then covellite was transformed to elemental sulfur in the second stage. Meanwhile, the kinetic of the second stage was much lower than the sulfur in the second stage. Meanwhile, the kinetic of the second stage was much lower than the first first stage. stage. However, the fine phase transformation process of synthetic chalcocite acidic ferric sulfate leaching However, the fine phase transformation process of synthetic chalcocite acidic ferric sulfate was a complicated process involving Cu, Fe, and S elements: leaching was a complicated process involving Cu, Fe, and S elements: (1) Firstly, chalcocite reacted with Fe3+ and was quickly transformed to Cu S , Cu S, Cu S , (1) Firstly, chalcocite reacted with Fe3+ and was quickly transformed to Cu7.2S7.24, Cu4 1.8S,1.8 Cu8S5, 8and5 and Cu FeS . Cu5FeS45. 4 (2)(2) Then, the galvanic effect effect between between Cu Cu5FeS5FeS4 4andand Cu Cu2-x2-xS acceleratedS accelerated the the dissolution dissolution of Cu of Cu2−xS,2 −andxS, → → andthe phase the phase transformation transformation order order of Cu of2− CuxS was2−xS Cu was1.8S Cu →1.8 CuS1.6S Cu→ CuS.1.6S CuS. (3)(3) When Cu22−xSxS was was completely completely transformed transformed to to CuS, CuS, Cu Cu5FeS5FeS4 4waswas transformed transformed to to CuS CuS thereafter. thereafter. (4)(4) In thethe secondsecond stage,stage, thethe generatedgenerated CuSCuS waswas oxidizedoxidized toto elementalelemental sulfur,sulfur, whichwhich shouldshould bebe thethe passivation layerlayer inhibitinginhibiting thethe efficientefficient leachingleaching ofof chalcocite.chalcocite.

AuthorAuthor Contributions:Contributions:Conceptualization, Conceptualization, W.Q., W.Q., G.Q. G.Q., and and J.W.; J.W.; Data Da curation,ta curation, C.F., S.Y. C.F., and S.Y., X.W.; and Formal X.W.; analysis, Formal C.F.,analysis, H.Z. C.F., and J.W.;H.Z., Funding and J.W.; acquisition, Funding acquisition, C.F., H.Z. and C.F., J.W.; H.Z., Investigation, and J.W.; Investigation, C.F., S.Y. and C.F., X.W.; S.Y., Methodology, and X.W.; C.F.;Methodology, Project administration, C.F.; Project J.W.;administration, Resources, J.W.; J.W.; Resources, Software, C.F., J.W.; S.Y. Software, and X.W.; C.F., Writing—original S.Y., and X.W.; draft,Writing— C.F.; Writing—review & editing, H.Z., W.Q. and J.W. original draft, C.F.; Writing—review & editing, H.Z., W.Q., and J.W. Funding: This research was funded by Natural Science Foundation of Hunan Province (No. 2018JJ1041), theFunding: National This Natural research Science was funded Foundation by Natural of China Science (No. Foundation 51774332, of 51320105006, Hunan Province 51704331 (No. 2018JJ1041), and 51374248), the HunanNational Provincial Natural Science Innovation Foundation Foundation of China for (No. Postgraduate 51774332, (CX2018B067),51320105006, 51704331 CSU Special and 51374248), Scholarship Hunan for StudyProvincial Abroad Innovation (2018154), Foundation Innovation-Driven for Postgraduate Project (CX2018B067), of Central South CSU University Special Scho (2018CX019)larship for Study and National Abroad Undergraduate(2018154), Innovation-Driven Innovation and Project Entrepreneurship of Central TrainingSouth University Program (201810533348).(2018CX019) and National Undergraduate Acknowledgments:Innovation and EntrepreneurshipAuthors acknowledge Training Program the staff (201810533348). and other professors at beamline 4B9A of the Beijing Synchrotron Radiation Facility (BSRF) and beamline BL14B1 at the Shanghai Synchrotron Radiation Facility (SSRF)Acknowledgments: for their help inAuthors beamline acknowledge operation, data the collection,staff and andother direction. professors at beamline 4B9A of the Beijing Synchrotron Radiation Facility (BSRF) and beamline BL14B1 at the Shanghai Synchrotron Radiation Facility Conﬂicts of Interest: The authors declare no conﬂict of interest. (SSRF) for their help in beamline operation, data collection, and direction. ReferencesConflicts of Interest: The authors declare no conflict of interest.

1. Chen, X.M.; Peng, Y.J.; Bradshaw, D. The separation of chalcopyrite and chalcocite from pyrite in cleaner References ﬂotation after regrinding. Miner. Eng. 2014, 58, 64–72. [CrossRef] 2.1. Qin,Chen, W.Q.; X.M.; Wu, Peng, J.J.; Y.J.; Jiao, Bradshaw, F.; Zeng, D. J.M. The Mechanism separation studyof chalcopyrite on ﬂotation and separation chalcocite offrom molybdenite pyrite in cleaner from chalcociteflotation after using regrinding. thioglycollic Miner. acid Eng. as depressant. 2014, 58, 64–72.Int. J. Min. Sci. Technol. 2017, 27, 1043–1049. [CrossRef] 3.2. Gao,Qin, Y.S.;W.Q.; Gao, Wu, Z.Y.; J.J.; Sun, Jiao, W.; F.; Yin, Zeng, Z.G.; J.M. Wang, Mechanism J.J.; Hu, Y.Y. study Adsorption on flotation of a novelseparation reagent of scheme molybdenite on scheelite from andchalcocite calcite causingusing thioglycollic an effective flotationacid as depressant. separation. J.Int. Colloid J. Min. Interface Sci. Technol. Sci. 2018 2017, 512, ,27 39–46., 1043–1049. [CrossRef ] [PubMed]

Minerals 2018, 8, 461 9 of 10

4. Li, C.W.; Gao, Z.Y. Effect of grinding media on the surface property and ﬂotation behavior of scheelite particles. Powder Technol. 2017, 322, 386–392. [CrossRef] 5. Brierley, J.A.; Brierley, C.L. Present and future commercial applications in biohydrometallurgy. Hydrometallurgy 2001, 59, 233–240. [CrossRef] 6. Zhao, H.B.; Wang, J.; Hu, M.H.; Qin, W.Q.; Zhang, Y.S.; Qiu, G.Z. Synergistic bioleaching of chalcopyrite and bornite in the presence of Acidithiobacillus ferrooxidans. Bioresour. Technol. 2013, 149, 71–76. [CrossRef] [PubMed] 7. Zhao, H.B.; Wang, J.; Gan, X.W.; Zheng, X.H.; Tao, L.; Hu, M.H.; Li, Y.N.; Qin, W.Q.; Zhang, Y.S.; Qiu, G.Z. Effects of pyrite and bornite on bioleaching of two different types of chalcopyrite in the presence of Leptospirillum ferriphilum. Bioresour. Technol. 2015, 194, 28–35. [CrossRef][PubMed] 8. Lan, Z.Y.; Hu, Y.H.; Liu, J.S.; Wang, J. Solvent extraction of copper and zinc from bioleaching solutions with LIX984 and D2EHPA. J. Cent. South Univ. Technol. 2005, 12, 45–49. [CrossRef] 9. Wang, J.; Zhao, H.B.; Zhuang, T.; Qin, W.Q.; Zhu, S.; Qiu, G.Z. Bioleaching of Pb–Zn–Sn chalcopyrite concentrate in tank bioreactor and microbial community succession analysis. Trans. Nonferrous Met. Soc. China 2013, 23, 3758–3762. [CrossRef] 10. Wang, J.; Zhu, S.; Zhang, Y.S.; Zhao, H.B.; Hu, M.H.; Yang, C.R.; Qin, W.Q.; Qiu, G.Z. Bioleaching of low-grade copper sulﬁde ore by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. J. Cent. South Univ. 2014, 21, 728–734. [CrossRef] 11. Zhao, H.B.; Huang, X.T.; Hu, M.H.; Zhang, C.Y.; Zhang, Y.S.; Wang, J.; Qin, W.Q.; Qiu, G.Z. Insights into the surface transformation and electrochemical dissolution process of bornite in bioleaching. Minerals 2018, 8, 173. [CrossRef] 12. Michael, N.; Petrus, B. The anodic behavior of covellite in chloride solutions. Hydrometallurgy 2017, 172, 60–68. 13. Bolorunduro, S.A. Kinetics of Leaching of Chalcocite in Acid Ferric Sulfate Media: Chemical and Bacterial Leaching. Master’s Thesis, University of Brithsh Columbia, Vancouver, BC, Canada, 1999. 14. Brierley, C.L. A perspective on developments in biohydrometallurgy. Hydrometallurgy 2008, 94, 2–7. [CrossRef] 15. Brierley, C.L. Biohydrometallurgical prospects. Hydrometallurgy 2010, 104, 324–328. [CrossRef]

16. Buerger, M.J.; Wuensch, B.J. Distribution of atoms in high chalcocite, Cu2S. Science 1963, 141, 276–277. [CrossRef][PubMed] 17. Howard, T.; Evans, J. The crystal structure of low chalcocite and djurleite. Z. Krist. 1979, 150, 299–320.

18. Evans, J.; Howard, T. Djurleite (Cu1.94S) and Low Chalcocite (Cu2S): New Crystal Structure Studies. Science 1979, 203, 356–358. [CrossRef][PubMed] 19. Howard, T.; Evans, J. Copper coordination in low chalcocite and djurleite and other copper-rich sulfides. Am. Miner. 1981, 66, 807–818. 20. Sullivan, J.D. Chemistry of Leaching Chalcocite; US Bureau of Mines: Washington, DC, USA, 1930; p. TP 473. 21. Fisher, W.W.; Flores, F.A.; Henderson, J.A. Comparison of chalcocite dissolution in the oxygenated, aqueous sulfate and chloride systems. Miner. Eng. 1992, 7, 817–834. [CrossRef] 22. Peterson, J.; Dixon, D.G. The dynamics of chalcocite heap bioleaching. In Hydrometallurgy 2003: Fifth International Conference in Honor of Professor Lan M. Ritchie; TMS: Pittsburgh, PA, USA, 2003; pp. 351–364. 23. Ruiz, M.C.; Abarzua, E.; Padilla, R. Oxygen pressure leaching of white metal. Hydrometallurgy 2007, 86, 131–139. [CrossRef] 24. Miki, H.; Nicol, M.; Velásquez-Yévenes, L. The kinetics of dissolution of synthetic covellite, chalcocite and digenite in dilute chloride solutions at ambient temperatures. Hydrometallurgy 2011, 105, 321–327. [CrossRef] 25. Ruan, R.M.; Zou, G.; Zhong, S.P.; Wu, Z.L.; Chan, B.; Wang, D.Z. Why Zijinshan copper bioheapleaching plant works efficiently at low microbial activity—Study on leaching kinetics of copper sulfides and its implications. Miner. Eng. 2013, 48, 36–43. [CrossRef] 26. Niu, X.P.; Ruan, R.M.; Tan, Q.Y.; Jia, Y.; Sun, H.Y. Study on the second stage of chalcocite leaching in column with redox potential control and its implications. Hydrometallurgy 2015, 155, 141–152. [CrossRef] 27. Cavallotti, P.; Salvago, G. Electronic Behavior of Copper Sulfides in Aqueous Solutions. Electrochim. Metallorum 1969, 4, 181–210. 28. Moh, G.H. Blue remaining covellite and its relations to phases in the sulfur rich portion of the copper-sulfur System at low temperatures. Miner. Soc. Jpn. 1971, 1, 180–188. Minerals 2018, 8, 461 10 of 10

29. Whiteside, L.S.; Goble, R.J. Structural and compositional changes in copper sulfides during leaching and dissolution. Can. Mineral. 1986, 24, 247–258. 30. Cheng, C.Y.; Lawson, F. The kinetics of leaching chalcocite in acidic oxygenated sulphat-chloride solutions. Hydrometallurgy 1991, 27, 249–268. [CrossRef] 31. Zhao, H.B.; Wang, J.; Yang, C.R.; Hu, M.H.; Gan, X.W.; Tao, L.; Qin, W.Q.; Qiu, G.Z. Effect of redox potential on bioleaching of chalcopyrite by moderately thermophilic bacteria: An emphasis on solution compositions. Hydrometallurgy 2015, 151, 141–150. [CrossRef] 32. Dutrizac, J. Elemental sulphur formation during the ferric sulphate leaching of chalcopyrite. Can. Metall. Q. 1989, 28, 337–344. [CrossRef] 33. Dutrizac, J. Elemental sulphur formation during the ferric chloride leaching of chalcopyrite. Hydrometallurgy 1990, 23, 153–176. [CrossRef] 34. Klauber, C. A critical review of the surface chemistry of acidic ferric sulphate dissolution of chalcopyrite with regards to hindered dissolution. Int. J. Miner. Process. 2008, 86, 1–17. [CrossRef] 35. Zhao, H.B.; Hu, M.H.; Li, Y.N.; Zhu, S.; Qin, W.Q.; Qiu, G.Z.; Wang, J. Comparison of electrochemical dissolution of chalcopyrite and bornite in acid culture medium. Trans. Nonferrous Met. Soc. China 2015, 25, 303–313. [CrossRef] 36. Zhao, H.B.; Wang, J.; Gan, X.W.; Hu, M.H.; Tao, L.; Qin, W.Q.; Qiu, G.Z. Role of pyrite in sulfuric acid leaching of chalcopyrite: An elimination of polysulfide by controlling redox potential. Hydrometallurgy 2016, 164, 159–165. [CrossRef] 37. Zhao, H.B.; Wang, J.; Qin, W.Q.; Hu, M.H.; Zhu, S.; Qiu, G.Z. Electrochemical dissolution process of chalcopyrite in the presence of mesophilic microorganisms. Miner. Eng. 2015, 71, 159–169. [CrossRef] 38. Qin, W.Q.; Wang, J.; Zhang, Y.S.; Zhen, S.J.; Shang, H.; Liu, Q.; Qiu, G.Z. Electrochemical behavior of massive bornite bioleached electrodes in the presence of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus. Adv. Mater. Res. 2009, 71, 417–420. [CrossRef] 39. Liu, H.C.; Xia, J.L.; Nie, Z.Y. Comparative study of S, Fe and Cu speciation transformation during chalcopyrite bioleaching by mixed mesophiles and mixed thermophiles. Miner. Eng. 2017, 106, 22–32. [CrossRef] 40. Yang, Y.; Harmer, S.; Chen, M. Synchrotron X-ray photoelectron spectroscopic study of the chalcopyrite leached by moderate thermophiles and mesophiles. Miner. Eng. 2014, 69, 185–195. [CrossRef] 41. Acres, R.G.; Harmer, S.L.; Beattie, D.A. Synchrotron XPS studies of solution exposed chalcopyrite, bornite, and heterogeneous chalcopyrite with bornite. Int. J. Miner. Process. 2010, 94, 43–51. [CrossRef] 42. Majuste, D.; Ciminelliv, S.T.; Eng, P.J. Applications of in situ synchrotron XRD in hydrometallurgy: Literature review and investigation of chalcopyrite dissolution. Hydrometallurgy 2013, 131, 54–66. [CrossRef] 43. Fang, J.H.; Liu, Y.; He, W.L.; Qin, W.Q.; Qiu, G.Z.; Wang, J. Transformation of iron in pure culture process of extremely acidophilic microorganisms. Trans. Nonferrous Met. Soc. China 2017, 27, 1150–1155. [CrossRef] 44. Wang, X.X.; Liao, R.; Zhao, H.B.; Hong, M.X.; Wang, J. Synergetic effect of pyrite on strengthening bornite bioleaching by Leptospirillum ferriphilum. Hydrometallurgy 2017, 176, 9–16. [CrossRef] 45. Fang, C.J.; Chang, Z.Y.; Feng, Q.M.; Xiao, W.; Yu, S.C.; Qiu, G.Z.; Wang, J. The influence of backwater Al3+ on diaspore bauxite flotation. Minerals 2017, 7, 195. [CrossRef] 46. Zhao, H.B.; Wang, J.; Tao, L.; Cao, P.; Yang, C.R.; Qin, W.Q.; Qiu, G.Z. Roles of oxidants and reductants in bioleaching system of chalcopyrite at normal atmospheric pressure and 45 ◦C. Int. J. Miner. Process. 2017, 162, 81–91. [CrossRef] 47. Liang, Y.; Che, Y. Inorganic Thermodynamic Data Manual, 1st ed.; Northeastern University Press: Shenyang, China, 1993; pp. 39–441. 48. Dean, J.A. Lange’s Handbook of Chemistry, 12th ed.; MvGrew-Hill: New York, NY, USA, 1979; pp. 9–94. 49. Wu, B.; Yang, X.L.; Cai, L.L.; Yao, G.C.; Wen, J.K.; Wang, D.Z. The influence of pyrite on galvanic assisted leaching of chalcocite concentrates. Adv. Mater. Res. 2013, 825, 459–463. [CrossRef] 50. Javad Koleini, S.M.; Jafarian, M.; Abdollahy, M.; Aghazadeh, V. Galvanic leaching of chalcopyrite in atmospheric pressure and sulfate media: Kinetic and surface studies. Ind. Eng. Chem. Res. 2010, 49, 5997–6002. [CrossRef] 51. Huai, Y.Y.; Plackowski, C.; Peng, Y.J. The galvanic interaction between gold and pyrite in the presence of ferric ions. Miner. Eng. 2018, 119, 236–243. [CrossRef]

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