This paper presents thermodynamic study of the conversion of metal sulphides to oxides of the CuAg sulphide concentrate as a final product after mechano-chemical leaching of tetrahedrite. The conversion of sulphides to oxides is carried out by oxidation leaching in NaOH solution. The thermodynamic calculation was performed for the sulphide concentrate containing the following sulphides: CuS, CuFeS2, FeS, Sb2S3, As2S3, Bi2S3 and HgS. Based on the change of Gibbs free energy (ΔG°) and the equilibrium constant (K), conversion of metal sulphides to oxides from the qualitative assessment of the chemical reaction can occur as the result of the thermodynamic reaction abilities

J. Bocan

M. Sidorová

M. Šofranko

English

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157METALURGIJA 56 (2017) 1-2, 157-160J. BOCAN, M. SIDOROVÁ, M. ŠOFRANKOTHERMODYNAMIC STUDY OF METAL SULPHIDES CONVERSION TO OXIDES IN HYDROMETALLURGYReceived – Primljeno: 2016-05-25Accepted – Prihvaćeno: 2016-10-25Preliminary Note – Prethodno priopćenjeJ. Bocan, M. Sidorová, M. Šofranko, Technical University of Košice, BERG Faculty, Košice, SlovakiaThis paper presents thermodynamic study of the conversion of metal sulphides to oxides of the CuAg sulphide concentrate as a final product after mechano-chemical leaching of tetrahedrite. The conversion of sulphides to ox-ides is carried out by oxidation leaching in NaOH solution. The thermodynamic calculation was performed for the sulphide concentrate containing the following sulphides: CuS, CuFeS2, FeS, Sb2S3, As2S3, Bi2S3 and HgS. Based on the change of Gibbs free energy (ΔG°) and the equilibrium constant (K), conversion of metal sulphides to oxides from the qualitative assessment of the chemical reaction can occur as the result of the thermodynamic reaction abilities.Key words: hydrometallurgy, sulphide, thermodynamic analysis, leaching, conversionINTRODUCTIONThe thermodynamic study of the conversion of met-al sulphides into oxides is applied to the hydrometal-lurgical processing of the tetrahedrite concentrate. Lab-oratory investigations were performed with tetrahedrite concentrate from Mária-Rožňava mine in Slovakia hav-ing the chemical composition of 21 % Cu, 11 % Sb, 28 % S, 1 % As, 2 % Zn, 0,2 % Ag and 1 % Hg. X-ray analysis reveal following proportions of dominating phases: 41 % Tetrahedrite (Cu4Sb4S13), 12 % Chalcopy-rite (CuFeS2) and 25 % Pyrite (FeS2) [1-4]. EXPERIMENTAL PARTThe relationship between thermodynamic efficiency of the conversion of metal sulphides to oxides and ini-tial values of NaOH and O2 was calculated using the HSC Chemistry software based on the equilibrium composition of reactants and reaction products used as inputs for the calculation [5-10]. The evaluated equilib-rium compositions and thermodynamic efficiency char-acterize the quantitative part of chemical reactions. Thus, it is possible to consider quantitative chemical thermodynamics [11].The conversion of metal sulphides was also exam-ined when developing technological projects [12-13].Thermodynamic efficiency of the conversion of metal sulphide to oxide was calculated for the following chemical reactions: CuS + 2NaOH + 2O2 =  CuO + Na2SO4 + H2O (1)CuS + 2NaOH + 1,75 O2 = 0,5 Cu2O + Na2SO4 + H2O  (2)ISSN 0543-5846METABK 56(1-2) 157-160 (2017)UDC – UDK 669.011: 661.249: 539.7: 622.46=1112CuFeS2 + 8NaOH + 8,5 O2 = 2CuO + Fe2O3 + 4 Na2SO4  + 4H2O  (3)3CuFeS2+ 12NaOH + 12,5 O2 = 3CuO + Fe3O4 + 6Na2SO4 + 6H2O (4)3CuFeS2 + 12NaOH + 11,75 O2 = 1,5Cu2O + Fe3O4 + 6Na2SO4 + 6H2O   (5)2CuFeS2 + 8NaOH + 7,5 O2 = Cu2O + Fe2O3 + 4Na2SO4 +4H2O (6) 2FeS + 4NaOH + 4,5 O2 = Fe2O3 + 2Na2SO4 +2 H2O  (7)3FeS + 6NaOH + 6,5 O2 = Fe3O4 + 3Na2SO4 +3 H2O  (8)Sb2S3 + 6NaOH + 7,0 O2 = Sb2O5 + 3Na2SO4 +3 H2O  (9)Sb2S3 + 6NaOH + 6,0 O2 = Sb2O3 + 3Na2SO4 +3 H2O (10)As2S3 + 6NaOH + 7,0 O2 = As2O5 + 3Na2SO4  +3 H2O    (11)As2S3 + 6NaOH + 6,0 O2 = As2O3 + 3Na2SO4 +3 H2O (12)Bi2S3 + 6NaOH + 6,0 O2 = Bi2O3 + 3Na2SO4 +3 H2O (13)Bi2S3 + 6NaOH + 5,5 O2 = 2BiO + 3Na2SO4 +3 H2O (14)HgS + 2NaOH + 2,0 O2 =  HgO + Na2SO4 + H2O (15)S + 2NaOH + 1,5 O2 = Na2SO4 + H2O  (16)Na2SO4 + CaO + H2O = CaSO4 + 2NaOH   (17)Na2SO4 + BaO + H2O = BaSO4 + 2NaOH  (18)Na2SO4 + MgO + H2O = MgSO4 + 2NaOH (19)The values for thermodynamic probability of the chemical reaction processes (1) ÷ (15) under standard conditions depending on ΔG° and K value are given in Table 1. Thermodynamic probability of the conversion of metal sulphides to oxides under standard conditions de-creases from HgS → HgO, eq. (15) to CuFeS2 → CuO 158J. BOCAN et al.: THERMODYNAMIC STUDY OF METAL SULPHIDES CONVERSION TO OXIDES... METALURGIJA 56 (2017) 1-2, 157-160+ Fe3O4, eq. (4) in the order listed in Table 1. The con-tents of sulphide tetrahedrite concentrate (solid phase following the mechano-chemical leaching) for calcula-tion of the equilibrium composition are given in Table 2. These products are Cu, Fe sulphide concentrate where the impurities present are Sb, As, Bi and Hg.Na2SO4 is formed in the solution after the conver-sion of metal sulphides to oxides. Using CsO or BaO it is possible to salt out sodium sulphate from the solution to form CaSO4 or BaSO4 in the solid phase and, thus, to assess sulphur. The above-mentioned reactions (17), (18) are ther-modynamically feasible; the reaction with BaO in com-parison to CaO is thermodynamically more favorable and is largely shifted towards the formation of products. Obtaining sulphur with the help of MgO is not feasible (19). Sodium hydroxide that is formed in the reactions (17, 18) is recyclable to oxide leaching of metal sul-phides when converted to oxides – reactions (1) ÷ (16).Table 1  Thermodynamic probability of metal sulphide conversion to oxides depending on ΔG° and K at t = 60 °CConversion ΔG° / kJ K Eq. no.HgS  HgO - 668,9 7,8E + 104 15CuS  Cu2O - 755,0 2,5E + 118 2CuS  CuO - 807,3 3,9E + 126 1FeS  Fe3O4 - 971,5 2,2E + 152 8FeS  Fe2O3 - 1 846,7 3,7E + 289 7Bi2S3  BiO - 2 211,2 1,0E + 308 13Bi2S3  Bi2O3 -  2 321 1,0E + 308 14Sb2S3  Sb2O3 - 2 461,2 1,0E + 308 10As2S3  As2O3 - 2 466,0 1,0E + 308 12Sb2S3  Sb2O5 - 2 649,0 1,0E + 308 9As2S3  As2O5 - 2 663,8 1,0E + 308 11CuFeS2  Cu2O + Fe2O3 - 3 133,0 1,0E + 308 6CuFeS2  CuO + Fe2O3 - 3 237,0 1,0E + 308 3CuFeS2  Cu2O + Fe3O4 - 4 635,8 1,0E + 308 5CuFeS2 v CuO + Fe3O4 - 4 792,5 1,0E + 308 4Figure 1  Thermodynamic efficiency of the conversion of metal sulphide to oxide and formation of metal oxide as a reaction product compared to initial weights of NaOH and O2 : a –Cu, b – Fe, c – Sb, d – As, e – Bi, f – Hga) b)c) d)e) f )159J. BOCAN et al.: THERMODYNAMIC STUDY OF METAL SULPHIDES CONVERSION TO OXIDES...METALURGIJA 56 (2017) 1-2, 157-160Table 2 Composition of sulphide concentrate / wt. %CuS 41,62CuFeS2 14,78FeS 36,03Sb2S3 1,13As2S3 0,82Bi2S3 0,53HgS 0,05spoil 5,04Calculation of the equilibrium composition in ac-cordance with [1] was carried out under the following conditions: mconc = 100 g, t = 60 °C, p = 0,1 MPa. The initial and equilibrium values of the conversion of metal sulphide to oxide for nNaOH = nO2 = 1,6 ÷ 2,2 mol are shown in Figure 1a - 1f.RESULTS AND DISCUSSION ΔGo and K values listed in Table 1 show that from the given sulphides the conversion HgS→HgO and CuS→Cu2O is the least probable. The agents NaOH and O2 in the amount lower than stoichiometric (< 2,06 mol and < 2,19 mol for NaOH a O2) allow conversion of CuS to Cu2O and CuO (Figure 1). Stoichiometric amounts of NaOH and O2 for the conversion of metal sulphides to oxides are given in Table 3.Table 3  Stoichiometric amounts of NaOH and O2 for the conversion of metal sulphides to oxidesMetal sulphides mNaOH / g mO2 / gCuS 34,820 27,860CuFeS2 12,886 10,950FeS 32,787 29,510Sb2S3 0,798 0,745As2S3 0,799 0,746Bi2S3 0,247 0,198HgS 0,016 0,012Σg 82,353 70,022Stoichiometric amounts of NaOH and O2 were calcu-lated on assumption that CuO, Fe2O3, Sb2O5, As2O5, Bi2O3, and HgO were formed in the conversion of metal sulphides to oxides CuO, Fe2O3, Sb2O5, As2O5, Bi2O3, and HgO were formed. The total stoichiometric amount of NaOH and O2, for all the sulphides converted to oxides is 82,353 g.  The amounts of NaOH and O2, which react in relation to their initial amount, are given in Table 4.When the initial amount of NaOH is 1,6 mol, only 62,48 g of NaOH is at the disposal. Probability of ther-modynamic capacity for the conversion CuS→Cu2O given by ΔGo and K values allows concluding that all metal sulphides convert to metal oxides before HgS and CuS. Table 3 shows that 34,836 g of NaOH is required for the conversion of HgS and CuS.When this stoichiometric value is subtracted from the total stoichiometric value (82,353 – 34,836 = 47,517 g), the result expresses the amount of NaOH required for the conversion of all other mentioned sulphides.When the value (34,836) is subtracted from the amount of NaOH to be at the disposal in the original state, i.e. 1,6 mol (62,48 g), see Table 4, the result gives the value of NaOH left for CuS conversion (62,48 – 47,517 = 14,963 g). The amount 14,963 g of NaOH is sufficient for converting 42,96  % CuS only. This is in the agreement with the value of thermodynamic effi-ciency of CuS conversion to oxides for 1,6 mol of NaOH, the value of 43 % in Figure 1a. The values given in Figures 1a - 1f were calculated on the basis of equi-librium values using HSC Chemistry software [1] and a method according to [2]. All other sulphides except for HgS and CuS convert to oxides totally, practically with 100 % thermodynamic efficiency from the very begin-ning at original values nNaOH=nO2=1,6 mol. In relation to the original NaOH and O2 values the oxides with lower valence are formed. Oxides Cu2O and BiO are formed at original values from 1,6 to 2,0 mol (Figure 1a, 1e). From 1,6 to 2,1mol, oxides Sb2O3 and As2O3 are formed (Figure 1c, 1d). Very interesting are FeS and CuFeS2 conversions, when besides Fe3O4 also Fe2O3 is formed at original values from 1,6 to 2,0 mol (Figure 1b). At original NaOH and O2 values above 2,0 mol only oxides HgO, CuO, Bi2O3 and Fe2O3 are formed. Above 2,1 mol oxides Sb2O5 and As2O5 are formed.CONCLUSIONS– Qualitative evaluation of the chemical reactions of the conversion of metal sulphide to oxides can be performed on the basis of ΔGo and K values. These enable to assess the thermodynamic reaction ability order.– Equilibrium composition of metal oxides after their conversion to oxides and subsequent assessment of thermodynamic efficiency give quantitative charac-teristics of the chemical reaction processes. Hence, this allows quantitative chemical thermodynamics to be considered.– The products obtained by oxidation leaching in NaOH solution will be the input for the reduction of metal oxides of Cu (Sb, Bi, As, Hg). Following the thermodynamic process, Cu is obtained by the re-duction in solid form. The reduction process is man-aged so as to reduce oxides of Cu without Fe. Table 4  The amounts of NaOH and O2 entering reactions in relation to their respective initial amounts Original amount of NaOH, O2 / molReacted amountNaOH O2/ mol / g / mol / g1,6 1,562 62,48 1,6 51,21,7 1,675 66,99 1,7 54,41,8 1,787 71,48 1,8 57,61,9 1,897 75,88 1,9 60,82,0 1,999 79,96 2,0 64,02,1 2,059 82,35 2,1 67,22,2 2,059 82,35 2,19 70,08160J. BOCAN et al.: THERMODYNAMIC STUDY OF METAL SULPHIDES CONVERSION TO OXIDES... METALURGIJA 56 (2017) 1-2, 157-160– Conversion of metal sulphides to oxides makes it possible to produce non-ferrous metals without pol-luting the environment with sulphur and its oxides or compounds. – Sulphur can be eventually obtained in the solid phase in the form of sulphate as a marketable prod-uct. REFERENCES[1]  P. Baláž, R. Kammel, M. Kušnierová, M. Archimovičová, Mechano-chemical treatment of tetrahedrite as a new non-polluting method of metals recovery, Hydrometallurgy, London, 1994[2]  P. 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Baláž, Spôsob lúhovania medi zo sulfidických surovín, Slovak Patent, No. 280233, (02.02.2000).  [9]  F. Sekula, P. Baláž, F. Jusko, F. Molnár, Š. Jakabský, Acta Montanistica Slovaca 3 (1998) 1, 149-156. [10]  A. Roine, Outokumpu HSC Chemistry for Windows, Ver-sion 4.1: Outokumpu Research Oy, Pori, Finland 1997. [11]  W. R. Smith, R. W. Missen, Chemical reaction equilibrium analysis: Theory and algorithms, John Willey and sons, New York, 1982, p.364.[12]  F. Sekula, F. Molnár, Finalizácia produktov CuAgAuPt koncentrátu po hydrometalurgickom spracovaní tetraedri-tu, 2 etapa, Manuscript, Fakulty BERG, TU Košice, 2002 [13]  F. Sekula, and F. Molnár, Finalizácia produktov CuA-gAuPt koncentrátu po hydrometalurgickom spracovaní te-traedritu, 3 etapa, Manuscript, Fakulty BERG, TU Košice, 2003 Note:  The responsible translator for English language is Mgr. Ludmila Petriková, TU FBERG Košice, Slovakia

Thermodynamic study of metal sulphides conversion to oxides in hydrometallurgy

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