Selective Nickel Leaching from Nickel and Cobalt Mixed Sulfide Using

Selective Nickel Leaching from Nickel and Cobalt Mixed Sulﬁde Using Sulfuric Acid+1

Hiroshi Kobayashi+2, Hirofumi Shoji, Satoshi Asano and Masaki Imamura

NIIHAMA RESEARCH LABORATORIES, SUMITOMO METAL MINING CO., LTD., Niihama 792-0002, Japan

Sumitomo Metal Mining Co., Ltd. (SMM) has gradually increased the mixed sulfide (MS: mixture of nickel and cobalt sulfides) production from nickel laterite ore as raw material for the SMM’s unique Matte Chlorine Leach and Electro-winning (MCLE) process over the past decade. This process has significant cost advantages because it is capable of selectively and effectively leaching nickel from MS; however, chlorine leaching requires expensive corrosion-resistant facilities. A new process that could be operated using lower-cost facilities has therefore been desired. To meet its development needs, this study evaluated a process for nickel-selective leaching from MS, which is similar to the existing process for refining of ZnS. The process uses sulfuric acid, which does not require high-cost facilities. However, selective nickel leaching from MS, which was the process development goal, could not be achieved. This result is quite different from the case of selective Zn leaching from ZnS. The mechanism of nickel leaching from MS using sulfuric acid was identified. It was shown that nickel-selective leaching using sulfuric acid is difficult because of the formation of elemental sulfur and NiS2 precipitates on the MS surface that interfere with the leaching reaction. [doi:10.2320/matertrans.M2018080]

(Received March 7, 2018; Accepted June 15, 2018; Published July 23, 2018)

Keywords: mixed sulﬁde, sulfuric acid, iron(III) sulfate, nickel selective leaching, NiS2

1. Introduction neutralization of sulfate ions from selective nickel leaching from MS. We examined the possibility of applying a proven Nickel matte and mixed sulfide (MS, i.e., a mixture of selective zinc leaching process2,3,4) from sphalerite (name of nickel and cobalt sulfide) are made by rough refining from ore where the main component is ZnS) by sulfuric acid. nickel sulfide ore or nickel oxide ore. These intermediate We then examined selective nickel leaching from MS using products are used as raw materials for commercial nickel sulfuric acid. As shown in the leaching eq. (1), zinc is production. In particular, production from MS has become leached selectively from sphalerite by sulfuric acid and increasingly popular because MS is made by hydro- sulfide ions are simultaneously oxidized, mainly up to metallurgical processes, which are lower in cost than elemental sulfur, by potential control. pyrometallurgical processes used to produce nickel matte. 0 ZnS þ H2SO4 þ 1=2O2 ¼ ZnSO4 þ H2O þ S ð1Þ As a raw material, MS is used for various kinds of oxidative nickel leaching processes, including: 1) chlorine leaching by However, the reaction in eq. (1) is very slow because of chlorine gas; and 2) pressure leaching by oxygen gas. the reactivity of the redox pair (O2/H2O). To promote this (1) Chlorine leaching is a well-known selective nickel reaction it is necessary to substitute the redox pair (O2/H2O) leaching process with high efficiency, which has been with another pair, which has higher reactivity.4) We define the widely used for nickel cathode production.1) However, substitutable redox pair as an oxidation medium. for safe and environmentally friendly production, As an oxidation medium, the redox pair (Fe3+/Fe2+)is facilities require expensive corrosion-resistant materials generally used for reaction promotion because Fe ions are for handling chlorine gas. These conditions place easily removed from sulfate solution. Equation (2) and unfavorable restrictions on the location of facilities eq. (3) show the break down reaction of selective zinc from the point of material supply. leaching from sphalerite in sulfate solutions. (2) Pressure leaching is an energy efficient method, which 0 ZnS þ Fe2ðSO4Þ3 ¼ ZnSO4 þ 2FeSO4 þ S ð2Þ involves the use of oxidative heating of MS by air blowing and does not require expensive corrosion- 2FeSO4 þ H2SO4 þ 1=2O2 ¼ Fe2ðSO4Þ3 þ H2O ð3Þ resistant equipment. However, for complete oxidation Some of the sphalerite reacts with sulfuric acid as shown in of sulfur up to the sulfate ions, this method features eq. (4), as a result, zinc is leached to yield hydrogen sulfide. high operation costs for sulfate ions neutralization, ZnS þ H2SO4 ¼ ZnSO4 þ H2S ð4Þ particularly for applications to rechargeable nickel battery or nickel powder production processes. On the sulfur side, sulfate ions are yielded from sphalerite On the basis of these issues, the development of new and elemental sulfur as shown in eqs. (5), (6), and (7). refining processes, which have lower costs and lower ZnS þ 2O2 ¼ ZnSO4 ð5Þ environmental impact, is highly desired. Hence, we 0 S þ H2O þ 3=2O2 ¼ H2SO4 ð6Þ investigated processes that could reduce the costs of excess 0 S þ 4H2O þ 3Fe2ðSO4Þ3 ¼ 6FeSO4 þ 4H2SO4 ð7Þ fi +1This Paper was Originally Published in Japanese in J. Japan Inst. Met. When sulfate ions are generated from sul de or elemental Mater. 81 (2017) 320326. sulfur in the leaching reactions shown as eqs. (5), (6), and +2Corresponding author, E-mail: [email protected] (7), the leaching cost of these reactions increases because Selective Nickel Leaching from Nickel and Cobalt Mixed Sulfide Using Sulfuric Acid 1459

MS Table 1 Compounds’ ratio of MS and Sphalerite (ZnS).

Medium Air䚸O2 䠄Fe3+䠅 Electrolytic effluent 䠄Including Sulfuric acid䠅 Electro- Autoclave winning

Residue Impurity Leach-soln. removal Ni electrolyte S0 E-Ni0

Fig. 1 Concept of nickel electro-winning process from nickel sulfate

solution. Particle Distribution (μm) Sample D10 D50 D90 MS 1.5 7.4 24.5 Sphalerite large amounts of oxidant is necessary for leaching and 1.0 8.2 21.5 (ZnS) neutralizing of sulfate ions. Thus, it is industrially very important to control these reactions to achieve the lowest ratio. If nickel is selectively leached from the MS according to leaching reaction shown in eqs. (8), (9), and (3), which are the same as zinc in the eqs. (1), (2), and (3), it will be possible to produce nickel cathode material from MS cheaply, as shown in Fig. 1. 0 NiS þ H2SO4 þ 1=2O2 ¼ NiSO4 þ H2O þ S ð8Þ Fig. 2 Particle size distribution curve of MS and Sphalerite (ZnS) samples. 0 NiS þ Fe2ðSO4Þ3 ¼ NiSO4 þ 2FeSO4 þ S ð9Þ 2FeSO4 þ H2SO4 þ 1=2O2 ¼ Fe2ðSO4Þ3 þ H2O (3) Furthermore, eqs. (6), (7), and (10) for MS leaching 2. Samples and Experimental Procedure reactions proceed in the same manner as those for sphalerite. The promotion of these reactions also requires large amounts 2.1 Sulfide samples of oxidant and neutralizing reagents, necessitating their MS produced by Coral Bay Nickel Corporation6) was used control. for the sulfuric acid leaching tests. The MS was ground in a planetary ball mill to a D90 value of less than 20 µm before NiS þ 2O2 ¼ NiSO4 ð10Þ use. D90 is the diameter at which the cumulative distribution As for sphalerite, hydrogen sulfide is generated in the MS accounts for 90% in the particle size distribution. High purity leaching reaction, as shown in eq. (11). sphalerite produced from Osarizawa mine in Akita prefecture was also used for comparison. The sphalerite was ground by NiS þ H2SO4 ¼ NiSO4 þ H2S ð11Þ hand to a D90 value of less than 20 µm. The compositions of In this report, we defined the sulfate ion ratio yielded in the MS and sphalerite are shown in Table 1. These compositions MS leaching versus sulfur content in the MS source as a were determined by inductively coupled plasma mass sulfur oxidation ratio in the leaching reaction. We also spectrometry (ICP-MS) analysis and x-ray diffraction defined the reactions, in which nickel is selectively leached (XRD) measurements. Furthermore, Fig. 2 shows the particle and sulfide ions are oxidized up to elemental sulfur, as shown size distributions of the two samples. in eqs. (1), (2), and (3), as selective nickel leaching reactions. Iron(III) sulfate heptahydrate (reagent grade 1) and For example, if the nickel leaching rate from MS or the nickel(II) sulfate hexahydrate (special grade reagent) zinc leaching rate from sphalerite is higher than the sulfur produced by Wako Pure Chemical Industries Ltd. were used. oxidation ratio, nickel or zinc is selectively leached versus These reagents were used as the oxidation medium and as a sulfur. Thus, we define the value of the nickel leaching rate synthetic solution source for leaching, respectively. from MS or the zinc leaching rate from sphalerite divided by the sulfur oxidation ratio as an index of selective leaching. 2.2 Leaching reaction and its evaluation An index of selective leaching greater than 1 indicates higher 2.2.1 Comparative tests of sulfide leaching selectivity of the leaching reaction. We define reactions with First, we performed tests to simulate the reactions shown higher index values as reactions with high selectivity. To in eqs. (2) and (8) to confirm the difference between the develop the above nickel cathode production process, we leaching behavior of MS and sphalerite. As a starting aimed to clarify the nickel leaching mechanism of MS in solution, iron(III) sulfate was dissolved so that its sulfuric acid leaching. Incidentally, in studies of selective concentration in the aqueous solution was 0.26 kmol/m3. nickel leaching from nickel sulfide by sulfuric acid, there has This iron(III) sulfate concentration was determined to be been a report on electrolytic oxidation of a nickel sulfide 1.5% of the dissolved solution as an oxidation medium, anode5); however, there have been no reports on control of considering its removal from the leaching solution after the the leaching parameters. reaction. The starting solution was poured in a hastelloy 1460 H. Kobayashi, H. Shoji, S. Asano and M. Imamura

Agitator

N2 Gas ← ← O2 N2 +H2SGas N Gas Baffle plate 2 Leak valve Gas flow control valve

Agitator

Internal cylinder

H2SO4 aq. Autoclave CuSO4 aq. Oil bath Fig. 3 Schematic diagram of an autoclave for sulfuric acid leaching.

Fig. 4 Schematic diagram of a sulfuric acid leaching reactor with H2S absorber. autoclave, as shown in Fig. 3 together with MS or sphalerite for the reaction. The autoclave was produced by Nitto Kouatsu Co., Ltd. (Volume: 3.4 L, Inductive stirrer: IST-12, and heated to 105°C and maintained at that temperature Stirring blade: a two-stage, 6-Flat Turbine and 4-Paddle after replacing air in the autoclave with nitrogen. To promote Peller). oxidation, oxygen was injected into the autoclave with a mass After the chamber was filled with samples and air was flow controller and the oxygen partial pressure was replaced with nitrogen, the autoclave was heated and maintained at an excess of 0.15 to 0.5 MPa against water maintained at 105°C, which is below the melting point of vapor pressure. The reaction time was set to be a maximum the elemental sulfur formed by leaching. The sulfide slurry of 10.8 ks after reaching 105°C, as for the testing of 2.2.1. concentration was varied between 0.1 and 0.6 kmol/m3. The The heating time from normal temperature to 105°C was reaction time was set to be a maximum of 10.8 ks after 2.7 ks (0.75 h) and the cooing time was 1.2 ks (0.33 h). The reaching 105°C. The heating time from normal temperature mass flow controller used for maintaining oxygen partial to 105°C was 2.7 ks and the cooling time was 1.2 ks. The pressure was a Brooks Mass Flow Controller 5850i Series stirring speed was 500 rpm. made by IWT Japan Co., Ltd. The oxygen flow rate of the 2.2.2 Sulfuric acid leaching tests under a low over- controller was fixed at 0.5 L/min. Under these operating pressure of oxygen parameters the pressure variation could be limited. After Using the autoclave in Fig. 3, we performed sulfuric acid cooling, the sample was filtrated and the residue and solution leaching under a low over-pressure of oxygen. The leaching were both analyzed. reaction involves complete oxidative leaching of sulfide 2.2.3 Sulfuric acid leaching tests under an excess oxygen over-pressure, as shown in eq. (6). To confirm the reactivity of the sulfide and sulfuric acid, Hence, we investigated the selective nickel leaching behavior the material balance of sulfur was examined in the leaching in MS sulfuric acid leaching under a low over-pressure of reaction, based on the sulfides with adjusted particle sizes oxygen. Under these conditions the reaction in eq. (6) could and various sulfuric acid concentrations. Figure 4 shows a be suppressed. The starting solution contained iron(III) schematic diagram of the sulfuric acid leaching reactor with sulfate as the oxidation medium, and nickel(II) sulfate an H2S absorber. A 1-L separable flask was used as the dissolved in sulfuric acid solution. The reason for the use reactor and was heated to 80°C and maintained at that of a synthetic nickel sulfate solution at the start of the nickel temperature. The reactor was filled with nitrogen and H2S gas selective leaching behavior investigation is that a nickel(II) generated by the leaching reaction was absorbed in a copper sulfate solution containing sulfuric acid is yielded as an sulfate solution, forming copper sulfide. The weight of the electrolyte from the nickel electro-winning process. We obtained copper sulfide was measured and the H2S generated aimed to confirm the feasibility of the current process, hence, was calculated. From the H2S weight and leaching weight of we intended to confirm that the decrease in nickel content sulfide, the sulfur balance was calculated. The sulfuric acid by electrowinning could be compensated by selective nickel concentration was 0.2 kmol/m3 and the concentration of the extraction from MS. sulfide slurry was 0.09 kmol/m3. The reaction temperature Iron(III) sulfate is an oxidation medium that promotes was controlled at 80°C, and reaction time was 7.2 ks (2 h). leaching. Thus, to confirm the oxidation, two iron(III) sulfate 2.2.4 Evaluation concentrations of 0.04 and 0.26 kmol/m3 were selected. The The leaching slurry after the reaction was filtrated with nickel(II) sulfate concentration was 0.85 kmol/m3 and the No. 5C quantitative filters papers made by Toyo Roshi sulfuric acid concentration was set to be in the range of 0.5 Kaisha Ltd., and the residue and solution were separated. 0.92 kmol/m3. The solution was chemically analyzed. The residue was These nickel(II) sulfate and sulfuric acid concentrations completely washed with pure water and dried for 12 h in a were set based on the electrowinning conditions of the vacuum oven AVO-310N made by AS ONE corporation. The Norilsk Harjavalta refinery7) in Finland. The synthetic components of the residue were quantitatively analyzed, and solution was enclosed in the autoclave of Fig. 3 with sulfide, the leaching rate of the elements were determined from the Selective Nickel Leaching from Nickel and Cobalt Mixed Sulfide Using Sulfuric Acid 1461 analysis data. For these chemical analyses, an ICP-AES apparatus ARCOS FHS12 made by Seiko Instruments Inc. was used. The crystal structure of the outermost surface was confirmed with an X-ray photoelectron spectrometer (XPS) Versa Probe II made by ULVAC-PHI, Inc. A focused monochrome Al K¡ beam (100 µm ¤, 25 W) was used as the excitation sources for photoelectrons. The vacuum was 7.3 © 10¹7 Pa and the analysis area was set to be 500 µm © 500 µm to enable observations of the average surface. The C1s line of adventitious carbon was used as an energy reference, and the reference binding energy value of C1s was 284.6 eV.

3. Results and Discussion Fig. 6 Effect of Iron (III) sulfate concentration on MS sulfuric acid 3.1 Comparative test of sulfide leaching leaching tests. Sulfate ion/total S in MS vs extraction ratio. Differences in the leaching behavior between the oxidizing agent iron(III) sulfate and sphalerite and MS, based on eqs. (2) and (9), are shown in Fig. 5. Comparing the zinc and shows the MS leaching test results for a range of iron(III) nickel leaching ratio with the sulfur oxidation ratio (OS:X sulfate concentrations. In this test, by decreasing the axis), over 90% of zinc was leached from sphalerite when the concentration of iron(III) sulfate (i.e., the oxidizing agent) sulfur oxidation ratio was 10%. This result indicates that zinc from 0.26 to 0.04 kmol/m3, the oxidation reaction of sulfur is selectively leached against sulfur oxidation as has been by the oxidant (as shown in eq. (7)) should be suppressed previously reported.2,3,4) However, only 40% of nickel was and the promotion of selective nickel leaching from MS was leached from MS when the sulfur oxidation ratio was 10%, expected. However, we observed the opposite effect to our which indicates that the selective leaching rate of nickel is expectations. Selective nickel leaching from MS was slightly less than half as high as that of zinc. On the basis of these promoted with a high iron(III) sulfate concentration. We results, the reaction mechanisms of sphalerite and MS considered that the nickel leaching reaction shown in eq. (9) leaching are likely to be different. In the case of MS was not promoted because of the lack of an oxidizing agent. leaching, the selective leaching index, which is the gradient Thus, the sulfur oxidation ratio and nickel leaching ratio from of the line shown in Fig. 5, became a small value. This result MS showed a linear relationship with a high correlation suggests that there was a change in the mechanism of MS coefficient, regardless of the concentration of iron(III) sulfate. leaching. This result indicates that the reaction shown in eq. (6) was not promoted by the conditions tested, even if the oxygen 3.2 Sulfide leaching test with slight pressure partial pressure was increased. This result agrees with a Owing to the differences between the zinc leaching rate previous report4) that the direct leaching reaction of sulfide from sphalerite and the nickel leaching rate from MS, the and oxygen is slower than the leaching reaction with an leaching test of MS with a lower selective leaching rate oxidative medium. was performed with a synthesized electrolyte solution of Figure 7 shows MS leaching test results as the sulfuric nickel(II) sulfate and sulfuric acid, and iron(III) sulfate. acid concentration was changed. Under these conditions, For the MS leaching test conditions, the iron(III) increasing the sulfuric acid concentration from 0.50 to sulfate concentration and sulfuric acid concentration in the 0.92 kmol/m3 was expected to result in iron(III) sulfate synthesized electrolyte solution were changed. Figure 6

Fig. 5 Reactivity comparison of MS and Sphalerite (ZnS) in Iron(III) Fig. 7 Effect of sulfuric acid concentration on MS sulfuric acid leaching sulfate leaching tests. Sulfate ion/total S in sulfides vs extraction ratio. tests. Sulfate ion/total S in MS vs extraction ratio. 1462 H. Kobayashi, H. Shoji, S. Asano and M. Imamura generation shown in eq. (3) and nickel leaching from MS content, which is oxidized according to eq. (10), can be shown in eq. (9). However, even when the sulfuric acid calculated from the sulfuric acid increase in the reaction. concentration was doubled, the improvement of nickel In eq. (8), equimolar amounts of sulfur with leached nickel selective leaching from MS was slight. This result indicates are generated. The generation of sulfur, based on eqs. (11) that through the reaction mechanism change over the course and (10) was confirmed; hence, the sulfur content generated of the reaction, the sulfide leaching behavior showed in eq. (8) can be calculated by subtracting the sulfur content differences from the expected behavior, even at high sulfuric generated in eqs. (11) and (10) from the total sulfur content acid conditions. calculated based on the total amount of leached nickel. As shown in Fig. 6, Fig. 7, although the nickel leaching The generation of zinc and other elements can be ratio from MS increased to over 95%, the index of a selective calculated in the same way. In the case of sphalerite leaching, leaching remained low; hence, selective nickel leaching from as shown in Table 2, the breakdown of the sulfur balance in MS could not be achieved. sphalerite is calculated to arise from 65% hydrogen sulfide The results of the previous tests are summarized as generation, 22% as fixed residue, and 13% as sulfate ions follows. from oxidization. However, In the case of MS leaching, the (1) Under conditions of an intermediate iron(III) sulfate breakdown of the sulfur balance in MS is different from that concentration or sulfuric acid concentration increase, of sphalerite leaching. Namely, hydrogen sulfide generation selective nickel leaching from the MS was improved; was calculated to be 5%, and 87% was fixed as residue, however, the effect was less than expected. This result is and 8% formed sulfuric acid ions through oxidization. The quite different from the case of selective zinc leaching difference in the reactivity represented by hydrogen sulfide from sphalerite. gas generation is thought to contribute to this difference of (2) Hence, selective nickel leaching from MS, which was the selective leaching behavior. The morphology of the the process development goal, could not be achieved. outermost surfaces of both samples and their leaching Our findings suggest that leaching of nickel from MS has residues were examined by XPS analysis, which can identify different inhibitory factors to those of selective leaching from the surface composition of the samples. sphalerite. There has been a report8) that zinc selective The XPS results are shown in Fig. 8 and Fig. 9(a)(b). leaching from sphalerite by sulfuric acid is accompanied by Figure 8 shows the Zn LMM spectra of sphalerite and its considerable hydrogen sulfide generation. We hypothesized leaching residue. Figure 9(a) and (b) respectively show the that substances formed on the reaction surface in the nickel Ni 2p3/2 and S 2p spectra of MS, together with those of their leaching from MS were different from those yielded from leaching residues. The XPS database of the National Institute zinc leaching of sphalerite. Thus, we performed sulfuric acid of Standards and Technology (NIST)9) was used to identify leaching of MS and sphalerite to confirm our hypothesis. compounds present on the surfaces of the samples. The results in Fig. 8 indicate that zinc sulfide leached to the 3.3 Sulfuric acid leaching of sulfides outmost surfaces of the raw sphalerite and the presence of its The reactions of accelerated acid leaching can be estimated residues was confirmed. In the case of MS leaching, Fig. 9(a) from the sulfur balance of the MS and sphalerite leaching and (b) were compared to confirm the outmost surface reactions with sulfuric acid. Table 2 shows the sulfur balance components. It is thought that nickel oxide, nickel sulfate, of MS and sphalerite leaching results. The sulfur balance and nickel sulfide exist on the surfaces of the raw MS. Nickel was calculated by the following procedure. The reactions sulfide, nickel oxide and nickel disulfide are thought to exist contributing to leaching are eqs. (11), (10), and (8). on the surfaces of its residue. On the basis of the results from According to eq. (11), nickel is leached from MS by sulfuric Fig. 9(a), peaks at 860 to 863 eV in Fig. 9(a) are satellite acid and equal molar amounts of nickel sulfate and hydrogen peaks reflecting charge transfer. The results shown in sulfide are generated as a result. The generated hydrogen Fig. 9(b) suggest that nickel sulfate and nickel sulfide were sulfide reacts with copper sulfate in solution and precipitates present on the outmost surface of the raw MS and nickel as solid copper sulfide. The sulfur content, which is generated as hydrogen sulfide in the reaction, can be confirmed by calculation of the copper sulfide weight. According to eq. (10), sulfuric acid ions, which contain an equimolar amount of leached nickel, are generated and their concentration increases in the solution. The amounts of sulfuric acid ions show no increase in eqs. (11) and (8); hence, the sulfur

Table 2 Sulfur mass balance of MS and sphalerite (ZnS) in sulfuric acid leaching.

Fig. 8 XPS measurement result of Sphalerite (ZnS) and sulfuric acid leach residue. Selective Nickel Leaching from Nickel and Cobalt Mixed Sulﬁde Using Sulfuric Acid 1463

(a) (b)

Fig. 9 XPS measurement result of MS and sulfuric acid leach residue. (a) Qualitative spectrum of nickel, (b) Qualitative spectrum of sulfur. 21.

disulfide and nickel sulfide were present on the outmost H SO 㻔㼍㻕 2 4 2+ surface of its residue. Sulfur generated in leaching was likely Zn Fe2+ vaporized inside the measurement chamber, which was H2S evacuated to high vacuum. The peaks at 168 eV in Fig. 9(b) Sphalerite 䠄ZnS䠅 Fe3+ S are assigned to sulfate peaks of compounds formed in MS formation, such as sodium sulfate. By comparing Fig. 9(a) and (b), we confirmed that nickel sulfide, nickel oxide, and Fe3+ Fe3+ Ni2++SO 2- 㻔㼎㻕 2+ 4 nickel sulfate were present on the outmost surface of the raw Ni Fe2+ Fe2+ MS and that in addition to nickel sulfide and nickel oxide, fi MS MS MS nickel disul de was also present on the outmost surface of (NiS) (NiS) (NiS) the MS leaching residue. Among these compounds, nickel S NiS2 H2S sulfate is thought to be an adherence compound in MS Ni2+ H2SO4 production or generated by oxidation in storage. Nickel oxide is thought to be formed by oxidation during sample drying Fig. 10 Leaching reaction model of (a) Sphalerite (ZnS) and (b) MS with because it is not generated by MS production or by sulfuric iron(III) sulfate in sulfuric acid solution. acid leaching. The sulfur balance in the sulfuric acid leaching and the surface analysis result by XPS, of both sphalerite and solution according to eq. (4) and leaves the residue surface MS confirmed the following points: rapidly owing to the solution flow. The hydrogen sulfide is (1) In the case of sphalerite leaching, zinc leaching with oxidized by iron(III) sulfate and fixed as sulfur precipitate. generation of hydrogen sulfide is the main reaction and The absence of an inactive phase at the surface of the the outmost surface of the leaching residue is sphalerite, leaching reaction residue enables selective leaching of zinc. i.e., the same composition as that of the raw material. Conversely, in the case of MS leaching, little hydrogen Hence, the leaching mechanism does not result in any sulfide is generated by sulfuric acid and the sulfur layer change and there is no obstacle layer formed on the formation on the MS leaching residue surface mainly surface of the leaching residue in the reaction. proceeds simultaneously with nickel leaching by oxidation, (2) However, in the case of MS leaching, the ratio of nickel as shown in eq. (9). As shown in eq. (12), when nickel leaching that generates hydrogen sulfide is as low as sulfide surrounds MS and a slightly soluble NiS2 layer is 5%. Nickel disulfide is formed on the surface of the generated, selective nickel leaching from MS is suppressed. residue as the reaction proceeds. S þ NiS ¼ NiS2 ð12Þ The nickel disulfide on the outmost surface of the MS leaching residue is metastable and known to be a slightly The reason for the difference in the leaching reaction soluble compound generated in sulfuric acid solution,10,11) the mechanism is discussed thermodynamically. Calculating the formation of this compound on the MS leaching residue is standard free energy change of the zinc selective reaching considered to be the main factor of preventing selective (eq. (1)) from sphalerite and nickel selective leaching from nickel leaching from MS. This assessment is also consistent MS (eq. (8)) at 25°C,12,13) the calculated values are ¹181 and with the amounts of sulfur fixed in the MS leaching residue ¹197 kJ/mol, respectively. The difference of this value is and the balance of sulfur in the reaction. so small, that it is impossible to explain the difference in the extraction rate between zinc and nickel. As mentioned 3.4 Leaching mechanism above, the standard free energy change of hydrogen sulfide The reaction mechanisms of sphalerite and MS leaching by generation from sphalerite (eq. (4)) and the MS leaching sulfuric acid solution containing iron(III) sulfate are shown in (eq. (11)) at 25°C were calculated to be 29 and 13 kJ/mol. Fig. 10(a) and (b), respectively. In the case of sphalerite Although both standard free energies are positive, it is leaching, hydrogen sulfide was generated in the acidic empirically known that hydrogen sulfide generation from 1464 H. Kobayashi, H. Shoji, S. Asano and M. Imamura

sphalerite (eq. (4)) is accelerated as the sulfuric acid (3) We assumed that the nickel leaching mechanism from concentration increases. Thus, selective nickel leaching from MS changes as the reaction proceeds. Initially, nickel is MS likely proceeds because of the small difference of their selectively leached from MS and forms a sulfur layer on standard free energy change. However, the standard free the MS leaching residue. This sulfur then reacts with energy change at 25°C for NiS2 formation on the surface of the surrounding nickel sulfide and forms a NiS2 layer the MS leaching residue (eq. (12)), as confirmed by XPS, on the MS leaching residue surface. Subsequently, the was calculated to be ¹67 kJ/mol. Thus, we found that nickel leaching reaction proceeds by partial or complete NiS2 formation proceeds more easily than hydrogen sulfide oxidation of the NiS2 layer and new nickel sulfide generation in equilibrium MS leaching by sulfuric acid. surfaces are exposed; however, the formation of NiS2 Hence, it is thermodynamically assumed that the formation of is also promoted. Because of the change in the nickel NiS2 layer on the MS leaching residue decreases the selective leaching mechanism, selective nickel leaching from MS nickel leaching from MS. For the case of sphalerite leaching, is not promoted above a certain selective leaching a pyrite type ZnS2 layer is not formed on its residue. index. Once the NiS2 layer forms on the surface of the MS leaching residue surface, partial oxidation, as shown in Acknowledgments eq. (13), or complete oxidation as shown in eq. (14), of NiS2 is necessary to promote nickel leaching from MS. We would like to express our appreciation to Professor Hiroyuki Fukuyama and Associate Professor Makoto NiS2 þ 3=2O2 þ H2O ¼ NiS þ H2SO4 ð13Þ Ohtsuka of Institute of Multidisciplinary Research for NiS þ 7=2O þ H O ¼ NiSO þ H SO ð14Þ 2 2 2 4 2 4 Advanced Materials (IMRAM) Tohoku University for their The MS leaching residue surface reactions shown in advice in reporting this study. eqs. (13) and (14) are promoted by oxygen and new nickel sulfide surfaces become exposed; however, because the REFERENCES formation of NiS2 is promoted at the same time, selective nickel leaching from MS does not proceed above a certain 1) S. Makino, N. Kemori, N. Matsumoto and S. Matsumoto: Proc. selective leaching index. This proposed leaching mechanism The NICKEL-COBALT 97 International Symposium, (27th Annual is consistent with the fact that the selective leaching indexes, Hydrometallurgical Meeting of CIM, Sudbury, Ontario, 1997) vol. 1, pp. 123135. with the gradients shown in Fig. 6 and Fig. 7, show a 2) K. Arai: J. Japan Inst. Metals 33 (1969) 965970. constant and low value according to the reaction conditions. 3) K. Arai: J. Japan Inst. Metals 33 (1969) 14001405. 4) M.J. Collins, E.J. McConaghy, R.F. Stauffer, G.J. Desroches and B.D. 4. Conclusion Krysa: JOM 46 (1994) 5158. 5) T. Kato and T. Oki: J. Japan Inst. Metals 38 (1974) 663668. 6) N. Tsuchida, Y. Ozaki, O. Nakai and H. Kobayashi: Proc. International It is well known that selective leaching of zinc from Laterite Nickel Symposium, (TMS Annual Meeting, Charlotte, NC, sphalerite by sulfuric acid is possible and is performed 2004) pp. 151160. commercially. The applicability of this process to selective 7) J.R. Boldt, Jr. and P. Queneau: The Winning of Nickel, (D. Van nickel leaching from MS was verified by elucidating its Nostrand Company, INC., Princeton, New Jersey, 1967) pp. 370374. reaction mechanism. 8) Y. Kunieda: Ph.D Thesis, Nagoya University, (1981). 9) NIST XPS DATABASE. National Institute of Standards and The following conclusions on the selective nickel leaching Technology Material Measurement Laboratory. https://srdata.nist.gov/ mechanism from MS can be drawn: xps/Default.aspx, (cited 2017-02-22). (1) Nickel is leached from MS by sulfuric acid; however, 10) D.M. Muir and E. Ho: Miner. Process. Extr. Metall. 115 (2006) 5765. it is not possible to leach nickel with a high selective 11) M.C. Jha, J.R. Carlberg and G.A. Meyer: Hydrometallurgy 9 (1983) leaching index. 349 369. 12) A. Yazawa and M. Eguchi: Shitsusikiseiren to Haisuisyori, (Kyoritsu (2) A slightly soluble NiS2 layer likely forms through the Shuppan Co., Ltd., Tokyo, 1975) pp. 1213. reaction of sulfur on the surface of the MS leaching 13) I. Barin: Thermochemical Data of Pure Substances, (VCH Verlags residue. This layer prevents nickel leaching with a high Gesellschaft, weinheim, 1989). selective leaching index.