Phase Equilibrium between Ni S Melt and FeOX SiO2 or FeOX CaO Based Slag under Controlled Partial Pressures

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(1)Materials Transactions, Vol. 43, No. 9 (2002) pp. 2219 to 2227 c 2002 The Japan Institute of Metals. Phase Equilibrium between Ni–S Melt and FeO X –SiO2 or FeO X –CaO Based Slag under Controlled Partial Pressures Hector M. Henao ∗ , Mitsuhisa Hino and Kimio Itagaki Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan To provide thermodynamic data for converting the nickel matte to liquid nickel, an experimental study was conducted in the phase equilibrium between the Ni–S alloy and the FeO X –SiO2 or FeO X –CaO based slag in a magnesia crucible at 1773 K under controlled pSO2 of 10.1 kPa and pO2 in a range between 5.1 × 10−3 and 1.6 Pa by using CO–CO2 –SO2 gas mixtures. The solubility of nickel in these slags at pO2 of 1.6 Pa, above which precipitation of a FeO X –NiO solid solution was anticipated, was found to be 10.1 and 20.3 mass% for the FeO X –SiO2 and FeO X –CaO based slags, respectively. The contents of iron, sulfur and oxygen in the nickel melt equilibrated with these slags were 2, 3 and 0.5 mass% for the FeO X –SiO2 based slag and 1, 3 and 0.5 mass% for the FeO X –CaO based slag, respectively. The dissolutions of nickel and sulfur in the slags were discussed on the basis of distribution ratio and sulfide capacity, respectively. (Received May 27, 2002; Accepted July 3, 2002) Keywords: phase equilibrium, distribution ratio, activity coefficient, FeO X –SiO2 slag, FeO X –CaO slag, nickel matte, nickel converting. 1. Introduction The conventional pyrometallurgical process of nickel sulfide ores consist of two stages of smelting and converting. The concentrates are oxidized in the smelting stage to produce the Ni–Cu–Fe–S matte, which is further converted to the Ni–Cu–S matte. The converting matte is separated into the sulfides of Ni3 S2 and Cu2 S by the controlled slow cooling method. Most of the recovered Ni3 S2 is further treated by the hydrometallurgical processes to make nickel, while the rest by the pyrometallurgical processes. To convert Ni3 S2 to liquid nickel, INCO1) developed a process in which oxygen is injected in a top blown rotary converter (TBRC) for providing the conditions of high temperatures around 1923 K and efficient gas-liquid mixing required for conversion to nickel. To discuss the pyrometallurgical route for the conversion of Ni3 S2 to liquid nickel from the standpoint of thermodynamics, the equilibrium studies for the Ni–S melt and the slag or the flux are of practical importance. Although numerous studies2) have been made on the equilibrium between the iron-silicate based slag and solid nickel phases under the controlled partial pressure of oxygen, no data are hitherto reported on the equilibrium between the slag and the Ni–S melt with widely varying content of sulfur from that of Ni3 S2 to those with precipitation of solid NiO in the slag phase, under controlled partial pressures of SO2 , S2 and O2 . Hence, following a series of experimental studies3–5) for the Ni–Fe–S matte/slag equilibration, the phase equilibrium between the Ni–S melt and the iron-silicate or calcium-ferrite based slag in a magnesia crucible was investigated at 1773 K in the present study. The experiments were made under the partial pressure of SO2 controlled at 10.1 kPa and the partial pressures of O2 and S2 were also controlled in the ranges between 5.1 × 10−3 and 1.6 Pa and between 8.0 × 10−2 and 9.2 kPa, respectively, by using CO–CO2 –SO2 gas mixtures. Although the iron content in the nickel sulfide produced by the controlled slow cooling method is considerably small at ∗ Doctor. Student, Tohoku University. E-mail [email protected]. less than 1 mass%, the authors have chosen the slags containing the iron oxide in the present study for the reason that the iron oxide is possibly incorporated as a contamination or a concomitant to the SiO2 or CaO based flux. The result for the iron free slag system will be reported in a separated paper.6) 2. Experimental CO–CO2 –SO2 gas mixtures were used to control the partial pressures of SO2 , O2 and S2 . The flow rate was regulated at 2.5 × 10−6 m3 /s by using capillary flow meters. A CO gas train was equipped with soda lime and P2 O5 columns to trap CO2 gas and moisture, respectively, while CO2 gas passed through a P2 O5 column. The partial pressure of SO2 was fixed at 10.1 kPa and those of O2 and S2 were varied by changing the CO/CO2 ratio in the gas mixtures. These were calculated by using a ChemSage Ver. 4. 11 software (GTT-Technologies, Aachen, Germany). A furnace assembly consists of a silicon carbide heating element and an alumina reaction tube. Temperature of a sample was controlled at 1773 K within 2 K by a SCR controller with a Pt/Pt-Rh thermocouple. About 8 g of pre-melted slag were equilibrated with about 8 g of the total amounts of Ni3 S2 and Ni with a Ni/S molar ratio close to a predicted equilibrium composition to achieve the equilibration in a short time. The compositions of starting FeO X –SiO2 slag are FeO: 63, Fe2 O3 : 4 and SiO2 : 33 mass%, while those of the FeO X –CaO slag, FeO: 55, Fe2 O3 : 18 and CaO: 27 mass%. The sample was put in a MgO crucible of 1.5 × 10−2 m i.d and 6 × 10−2 m height. The gas mixture was introduced into the reaction chamber through an alumina tube. After equilibration, the sample was taken out of the furnace and quenched in a jet stream of argon. Specimens for the chemical analysis were prepared by cutting the alloy sample into small pieces and grinding the slag sample, followed by removing the small alloy particles included in the slag with a magnet. The slag and alloy specimens were analyzed by the inductively coupled plasma spectrometry (ICP) and by the conventional methods of chemical analysis with volumetric titration and gravity measurements..

(2) 2220. H. M. Henao, M. Hino and K. Itagaki. The total iron and Fe2+ in the slags were determined by the volumetric titration with K2 Cr2 O7 . The difference between the total iron and Fe2+ contents was taken as the Fe3+ content. The total oxygen content in the alloy specimen was determined by inert gas fusion-infrared absorptiometry (EMGA650, HORIBA Co, Ltd., Kyoto, Japan), while the total sulfur content in the slag specimen by a combustion-infrared spectrometer (EMIA-580, HORIBA Co, Ltd., Kyoto, JAPAN). Some preliminary experiments were carried out to determine the time required for the equilibration. When the FeO X – CaO based slag was melted with pure nickel, a Ni–S melt with 15 mass% or Ni3 S2 having 26.7 mass%S under pSO2 of 10.1 kPa and pO2 of 0.12 Pa, the change in the sulfur content of alloy phase with the holding time is shown in Fig. 1. It is found for the alloy with initial 15 mass%S that the sulfur content reaches the equilibrium value of 17.6 mass% at about 70 ks. For the initially pure nickel, although the sulfur content simply increases with the holding time, it does not reach the equilibrium value even at 260 ks. On the other hand, it is found for the initial Ni3 S2 melt that the sulfur content hardly changes for 86 ks. This suggests that the desulfurization of the Ni–S melt hardly occurs in the Ni–S melt-slag system under the controlled CO–CO2 –SO2 gas mixture. The present preliminary experiments have clarified that the equilibrium can be made in a restricted time of less than 100 ks by adjusting the sulfur content of starting alloy phase so that it may be a little smaller than the equilibrated one. Subsequently, in the present study, the holding time was set at 129.6 ks and the sulfur content of starting Ni–S melt was adjusted to be about 2 mass% smaller than the estimated equilibrated one. 3. Results It was found that the phase separation between the alloy and slag phases occurred in a range of PO2 between 5.1×10−3 and 1.6 Pa, accordingly, pS2 between 9.2 kPa and 8 × 10−2 Pa. A NiO–FeO X solid solution precipitated in the slag phase at higher PO2 more than 2 Pa, while the Ni–S melt and the FeO X –CaO based slag did not show any phase separation between them at higher PS2 more than 9.2 kPa. The micro-. scopic analysis showed that a thin layer with a thickness of about 1.5 × 10−3 m and 3 × 10−5 m was formed between the slag and the crucible for the FeO X –SiO2 and FeO X –CaO based slags, respectively. The electron probe micro analysis (EPMA) clarified that these layers corresponded to olivine 2(Mg, Fe)O·SiO2 for the FeO X –SiO2 based slag and magnesiowüstite (Mg, Fe)O for the FeO X –CaO based slag, as suggested from the phase diagrams reported by Muan and Osborn7) and Johnson and Muan.8) Compositions of the slag and alloy phases after equilibration are listed in Tables 1 and 2 for the FeO X –SiO2 and FeO X –CaO based slags, respectively. The summation of the analytical values for both slag and alloy are a little more or less than 100 mass% due to the error of the analysis. The partial pressures of oxygen and sulfur are expressed as dimensionless ones, defined in the cases of oxygen by pO2 = (PO2 /Pa)/(101325 Pa) and sulfur by pS2 = (PS2 /Pa)/(101325 Pa). 3.1 Alloy phase 3.1.1 Sulfur content The relationship between the sulfur content in the alloy and log pS2 or log pO2 for both slag systems are shown in Fig. 2. The limits of pO2 and pS2 , between which the experiments were carried out, are indicated with the dash-dot lines. Meyer et al.9) determined pS2 in relation to the sulfur content in the Ni–S melt by equilibrating with H2 –H2 S gas mixtures, which is shown with a broken line in Fig. 2. It is found that the present results agree fairly well with the data by Meyer et al.9) within scatter of the data even though the present system contains iron. 3.1.2 Iron content It was found that iron was introduced into the Ni–S melt from the slags. The alloy composition was plotted on the Ni– Fe–S ternary diagram in Fig. 3 because the contents of other elements including oxygen are very small. It is shown that. log p O2 -5. 30. 10. 0. 100. 200. 300. Time, t /ks Fig. 1 Change of the sulfur content in Ni–Fe–S melts with holding time at log pO2 of −5.91 and 1773 K.. 10. 0 -7. Meyer et al.. 2 3. 20. equilibrium with Ni S -FeS. 20. FeOX -CaO-MgO precipitation of NiO compound. 26.7 mass % S 15.0 0. mass%S in Alloy. 2. initial alloy composition. mass%Ni in Alloy. -7. FeOX -SiO -MgO. 30. 0. -6. -6. -5. -4 -3 -2 log p S2. -1. 0. Fig. 2 Relationships between the sulfur content in Ni–Fe–S melts and log pS2 or log pO2 in the equilibration with the slags in the MgO crucible at 1773 K..

(3) Phase Equilibrium between Ni–S Melt and FeO X –SiO2 or FeO X –CaO Based Slag under Controlled Partial Pressures. 2221. Table 1 Compositions of FeO X –SiO2 –MgO slag and nickel–sulfur alloy melted in the MgO crucible at 1773 K. Slag. Alloy. mass% No 1 2 3 4 5 6 7 8 9 10. log pS2 −1.0 −1.9 −2.7 −2.5 −3.8 −4.8 −4.9 −5.0 −5.3 −5.8. mass%. log pO2 −7.3 −7.0 −6.6 −6.6 −6.0 −5.5 −5.4 −5.3 −5.2 −5.0. aFeO Fe2+. Fe3+. FeTotal. Ni. SiO2. MgO. S. 19.6 33.4 25.4 27.4 15.4 24.1 22.6 18.4 21.5 26.3. 4.50 8.49 7.11 8.63 8.62 11.6 18.3 11.1 15.6 15.5. 31.7 41.9 32.5 36.0 24.1 35.6 40.9 29.5 37.1 41.8. 0.44 0.90 1.83 1.70 3.78 6.50 7.21 7.63 9.20 10.12. 33.0 22.8 33.9 31.0 33.3 27.9 17.1 31.6 21.8 15.0. 22.2 19.0 20.7 21.7 25.0 20.6 23.9 19.0 20.6 19.8. 1.09 0.82 0.32 0.39 0.22 0.09 0.07 0.05 0.04 0.12. 0.86 0.82 0.74 0.71 0.87 0.80 0.85 0.77 0.60 0.67. γNiO 4.70 3.16 3.07 3.74 5.33 6.59 7.57 7.94 7.59 8.79. Ni. Fe. S. O. 49.2 59.2 70.3 66.3 80.3 84.5 88.9 95.3 98.0 97.2. 24.8 16.3 10.1 10.6 4.98 3.30 2.50 2.63 2.44 2.20. 24.5 24.5 21.1 22.1 14.8 10.1 6.20 2.67 2.15 2.70. 0.99 0.77 0.66 0.65 0.61 0.62 0.58 0.57 0.56 0.52. Table 2 Compositions of FeO X –CaO–MgO slag and nickel–sulfur alloy melted in the MgO crucible at 1773 K. Slag. Alloy. mass% No 1 2 3 4 5 5 7 8 9 10 11 12. log pS2 −1.5 −1.9 −2.5 −3.2 −3.3 −3.9 −3.9 −3.9 −4.6 −5.3 −5.9 −6.1. mass%. log pO2 −7.1 −6.9 −6.6 −6.3 −6.2 −5.9 −5.9 −5.9 −5.6 −5.2 −4.9 −4.8. aFeO Fe2+. Fe3+. FeTotal. Ni. CaO. MgO. S. 31.8 31.4 22.0 21.8 20.2 16.1 18.8 17.9 20.0 14.1 12.0 10.0. 5.67 16.6 19.7 22.2 26.3 23.8 25.7 24.5 23.7 27.9 28.4 20.7. 37.4 46.7 41.7 44.1 46.6 39.9 44.5 42.4 43.7 42.0 40.4 30.7. 5.99 4.82 3.42 4.01 3.52 7.04 6.48 5.47 11.0 15.3 17.0 20.3. 28.8 29.2 28.2 31.5 27.3 23.6 24.7 15.5 24.3 21.0 21.4 23.9. 6.72 6.50 5.92 3.57 6.90 8.65 5.38 3.74 6.48 5.04 5.91 7.89. 14.7 8.30 4.21 2.20 1.63 0.56 0.80 0.72 0.25 0.11 0.03 0.08. 0.39 0.40 0.41 0.46 0.49 0.34 0.36 0.36 0.52 0.49 0.68 0.52. γNiO — — — — 3.10 2.74 2.95 2.89 3.25 4.14 5.36 4.97. Ni. Fe. S. O. 56.3 67.9 72.8 77.2 76.1 80.4 82.4 83.2 92.9 93.8 94.1 99.1. 16.6 10.5 6.97 4.96 3.78 1.70 2.02 2.13 1.28 1.10 1.05 0.97. 26.0 22.7 22.1 21.2 19.5 15.5 17.1 16.7 8.95 2.80 2.96 1.22. 0.53 0.60 0.62 0.61 0.62 0.59 0.60 0.58 0.58 0.53 0.46 0.45. 3.1.3 Oxygen content As shown in Fig. 4, the oxygen content in the Ni–S melt equilibrated with the FeO X –SiO2 based slag presents a considerably large value of about 1 mass% at log pO2 of −7.3 and decreases remarkably with increasing log pO2 up to log pO2 of −6.3, then, changes very gently in the higher region of log pO2 . While, that for the FeO X –CaO based slag increases with increasing log pO2 up to −6.3, then changes very gently in the higher range of log pO2 . It is shown that the solubility of oxygen in the Ni–S melt in the range of log pO2 above −6.3 is very similar between both slag systems.. Fig. 3 Composition of Ni–Fe–S melts equilibrated with the slags in the MgO crucible at 1773 K.. the mole fraction of iron ranges between 0.01 and 0.21 (1 and 25 mass%) and that, at a given sulfur content, the iron content in the alloy equilibrating with the FeO X –SiO2 based slag is larger than that for the FeO X –CaO based slag.. 3.2 Slag phase 3.2.1 Precipitation of solid phase Microphotographs of the solidified FeO X –SiO2 and FeO X – CaO based slags equilibrated with the Ni–Fe–S alloy at log pO2 of −5.0 and −4.7 are shown in Figs. 5(A), (B) and Figs. 5(C), (D), respectively. Glassy bulk and dendritic phases are found for both slag systems at log pO2 of −5.0, while the third phase with a nearly oval shape at log pO2 of −4.7. The EPMA analysis clarified that the glassy and dendritic phases corresponded to the FeO X –SiO2 –MgO or FeO X –CaO–MgO.

(4) 2222. H. M. Henao, M. Hino and K. Itagaki. 1.5 FeOX -SiO -MgO. mass%O in Alloy. 2. FeOX -CaO-MgO Ni/(CO-CO )gas 2. 1. 0.5. 0 -8. -7. -6 log p O2. -5. -4. Fig. 4 Relationships between the dissolution of oxygen in the alloy and log pO2 for the FeO X –SiO2 –MgO and FeO X –CaO–MgO slags melted in the MgO crucible at 1773 K.. slag and the FeO X –NiO solid solution. It is considered that the oval phase was formed in the liquid slag at the experimental temperature of 1773 K, while the dendritic phase during solidification of the slag. Hence, it is suggested that the slag is saturated with the FeO X –NiO solid solution at an oxygen partial pressure between log pO2 of −5.0 and −4.7. 3.2.2 Solubility of MgO The slag compositions are reproduced in Fig. 6 on the AB-C ternary diagram with a mole fraction scale where A corresponds to MgO, B to SiO2 or CaO and C to (FeO X + NiO). Although the solubility limit in the slag phase is considerably scattered, as shown with the estimated broken liquidus lines, it is obvious that the solubility in the FeO X –SiO2 based slag is larger than that in the the FeO X –CaO based slag. Despite the presence of NiO, the liquidus line in the FeO X –SiO2 based slag is similar to that reported by Muan and Osborn,7) which is very close to a tie line combining between FeO and MgO·SiO2 . It is reported by Johnson and Muan8) that the solubility of MgO in the FeO X –CaO–MgO slag in coexistence with the solid magnesiowüstite changes very little with the FeO X content. This is in accordance with the present result. 3.2.3 Activity of FeO Activities of FeO in the slags are given in Tables 1 and 2, which were calculated on the basis of the partial pressure of oxygen and the iron content in the Ni–Fe–S alloy from the standard free energy change10) for the reaction 1 O2 (g) = FeO(l) (1) 2 The data on the iron activity in the Ni–Fe–S ternary alloy reported by Hsieh and Chang11) were used in the calculation because the contents of other elements including oxygen are very small. 3.2.4 Fe3+ /Fe2+ ratio It is shown in Fig. 7 that the mass% ratio, (Fe3+ /Fe2+ ), increases with increasing pO2 . For the FeO X –SiO2 based slag, a linear relationship with a slope about 1/4 is observed between log(Fe3+ /Fe2+ ) and log pO2 , corresponding to the chemical reaction of FeO + 14 O2 = FeO1.5 . For the FeO X –CaO based Fe(l) +. slag, a similar relationship is observed in the range of higher log pO2 more than −6.5 but a notable deviation from the linear relationship is observed in the lower range. This deviation is considered to be ascribed to the considerably large solubility of sulfur in the FeO X –CaO based slag, which combines the ferrous ion in the range of lower pO2 , as described in 3.2.6. It is also shown that, at a given oxygen pressure, the (Fe3+ /Fe2+ ) in the FeO X –CaO based slag is larger than that in the FeO X –SiO2 based slag. The data reported by Pagador et a1.12) for the FeO X –SiO2 –MgO slag equilibrated with liquid Ni–Fe binary alloys are plotted with a broken line in Fig. 7. The present data extrapolated in the range of lower oxygen pressure agree well with their data. 3.2.5 Nickel content The nickel content in the FeO X –CaO or FeO X –SiO2 based slag equilibrated with the Ni–Fe–S alloy is plotted in Fig. 8, in relation to log pO2 . It is shown that the solubility of nickel in the FeO X –SiO2 based slag increases with increasing log pO2 from 0.5 mass% at log pO2 of −7.3 to 10 mass% at log pO2 of −5.0. While, that in the FeO X –CaO based slag decreases in a region of lower oxygen pressure from 6 mass% at log pO2 of −7.1 to 4 mass% at log pO2 of about −6.5, then, remarkably increases in the range of higher log pO2 and becomes 20 mass% at log pO2 of −4.8. It is noted that the solubility of nickel in the FeO X –CaO based slag is larger than that in the FeO X –SiO2 based slag. 3.2.6 Sulfur content The sulfur contents in the FeO X –CaO and FeO X –SiO2 based slags are shown in Fig. 9, in relation to log pS2 . It is noted that the solubility of sulfur in the FeO X –CaO based slag abruptly increases in the range of higher log pS2 and becomes 15 mass% at log pS2 of −1.5. While the change of sulfur solubility against log pS2 is very gentle for the FeO X –SiO2 based slag, increasing from 0.05 mass% at log pS2 of −5.8 to 1 mass% at log pS2 of −1.0. 4. Discussion 4.1 Dissolution of nickel in slag The dissolution of nickel and the activity coefficient of nickel oxide in the slags will be discussed on the basis of the distribution ratio of nickel between the slag and alloy phases, which is defined by eq. (2). L s/Ni Ni = (mass%Ni in slag)/{mass%Ni in alloy}. (2). When nickel is dissolved in the slag as an oxide, the distribution ratio can be analyzed thermodynamically on the basis of the following reaction to form a mono-metallic oxide. Ni(l) + v/4O2 (g) = NiOv/2 (s). (3). Where, the solid NiOv/2 was taken as a reference of the NiOv/2 activity for the lack in the thermodynamic data of the liquid NiOv/2 . The equilibrium constant of eq. (3) is given by K = aNiOv/2 /(aNi pOv/4 ). (4). 2. where aNi and aNiOv/2 are activities of Ni and NiOv/2 , respectively. By combining eqs. (2) and (4) and converting the mole fraction, N , in the activity relationship of a = N γ with the Raoultian activity coefficient of γ , into mass%, the following.

(5) Phase Equilibrium between Ni–S Melt and FeO X –SiO2 or FeO X –CaO Based Slag under Controlled Partial Pressures. 2223. Fig. 5 Microphotographies showing the structure of the slags at 1773 K just before and after the precipitation of a FeO X –NiO solid solution.. equation is obtained. log. L s/Ni Ni. = v/4 log pO2 + log[{γNi }/(γNiOv/2 )]. + log[(n T )/{n T }] + log K. (5). where, ( ) and { } denote the slag and alloy phases, respectively. n T is the total number of moles in 100 g of each phase, which is calculated on the mono-cation base. The L s/Ni Ni , which were obtained from the data on the nickel.

(6) 2224. H. M. Henao, M. Hino and K. Itagaki. 0.4. 0.6. SiO. N. O. 0.8. 0.2. 1. 0 0. 0.2. 0.4 0.6 N(FeO X + NiO). (SiO2 or CaO). 0.8. 1 (FeOX +NiO). FeOX -CaO-MgO. mass%S in Slag. Ca O. N. 2. 0.4. 2. 0.6. FeOX -SiO -MgO. N Mg. or. 20. FeOX-SiO2-MgO FeOX-CaO-MgO. 10. 10. 0 -7. 2. FeOX -CaO-MgO Pagador et al.. 3+. -6. -4 -3 -2 log p S2. -1. 0. Fig. 9 Relationships between the dissolution of sulfur in the slags and log pO2 for the FeO X –SiO2 –MgO and FeO X –CaO–MgO slags melted in the MgO crucible at 1773 K.. FeOX -SiO -MgO. 2+. 1 FeOX -SiO -MgO 2. FeOX -CaO-MgO. 1 s/Ni. 0.1. L Ni. mass%Ratio, (Fe /Fe ). Fig. 6 Compositions of the slags equilibrated with the Ni–Fe–S alloy melted in the MgO crucible at 1773 K.. 1 4. 0.1 -8. -7. -6 log p O2. -5. NiO. 0.01. 1. -4 2. Fig. 7 Relationships between the mass% ratio, (Fe3+ /Fe2+ ), and log pO2 for the FeO X –SiO2 –MgO and FeO X –CaO–MgO slags melted in the MgO crucible at 1773 K.. 0.001 -8. -7. -6 log p O2. -5. -4. Fig. 10 Relationships between the distribution ratio of nickel and log pO2 for the FeO X –SiO2 –MgO and FeO X –CaO–MgO slags melted in the MgO crucible at 1773 K.. 30 FeOX -SiO -MgO. mass%Ni in Slag. 2. FeOX -CaO-MgO. 20. 10. 0 -8. -7. -6. -5. -4. log p O2 Fig. 8 Relationships between the dissolution of nickel in the slags and log pO2 for the FeO X –SiO2 –MgO and FeO X –CaO–MgO slags melted in the MgO crucible at 1773 K.. solubility obtained in the present study, are plotted in Fig. 10 in relation to log pO2 . For the FeO X –SiO2 based slag, a nearly linear relationship is observed between log L s/Ni Ni and log pO2 with a gradient of about 1/2. This suggests that nickel dissolves in this slag as NiO, according to eq. (3) with v = 2. A linear relationship with a nearly same gradient is also found for the FeO X –CaO based slag in a range of higher oxygen pressure. However, it is to be noted that the L s/Ni Ni in the range of lower oxygen pressure decreases with increasing pO2 . This anomalous behavior is considered to be ascribed to a different mechanism of the nickel dissolution other than that of the dissolution as the oxide given by eq. (3). Owing to extremely large solubility of sulfur in this region, as shown in Fig. 9, the dissolution of nickel as a sulfide may be suggested, as is reported by Font et al.5) for the FeO X –CaO slag/Ni3 S2 –FeS matte equilibration. The linear relationship observed in Fig. 10 suggests that the sum of activity coefficient ratio and n T ratio in eq. (5) does not change appreciably for each slag. Hence, the Raoultian.

(7) Phase Equilibrium between Ni–S Melt and FeO X –SiO2 or FeO X –CaO Based Slag under Controlled Partial Pressures. 25. activity coefficient of NiO in each slag can be calculated on the basic of eq. (6). The standard free energy change and equilibrium constant of eq. (6) is given in a data book.13) The Raoultian activity coefficient of nickel in the ternary Ni–Fe–S alloy, {γNi }, is obtained from the data reported by Hsieh and Chang11) at 1673 K by extrapolating to the present experimental temperature of 1773 K with the regular solution model. The calculated γNiO are listed in Tables 1 and 2 for the FeO X –SiO2 and FeO X –CaO base slags, respectively. The γNiO presents values ranging between 3.1 and 8.8. In a general trend, the values for the FeO X –SiO2 based slag are larger than those for the FeO X –CaO based slag. This trend may be mainly ascribed to a chemical property of NiO, that is relatively neutral for the predominant slag components of basic CaO and MgO as well as acidic SiO2 , as suggested by Takeda and Yazawa.14) The experimental results for the solubility of nickel in the FeO X –CaO based slag in the range of lower oxygen pressure, where log L s/Ni Ni –log pO2 plots deviate from the linear relationship, may be analyzed thermodynamically by combining the oxidic and sulfidic dissolution of nickel, (mass%Ni)ox and (mass%Ni)s , in the slag. According to Nagamori,15) (mass%Ni)ox and (mass%Ni)s will be given by eqs. (7) and (8), respectively. 1/2. (mass%Ni)ox = AaNi pO2 /γNiO (mass%Ni)s = BaNiS (mass%S). 15. Calculated Total Oxidic Sulfidic. 10 5 0 -8. -7. -6 log p O2. -5. -4. Fig. 11 Relationships between the dissolution of nickel in the FeO X –CaO–MgO slag and log pO2 , calculated by assuming sulfidic and oxidic dissolution.. 1 FeOX -SiO -MgO 2. FeOX -CaO-MgO. 0.1. 0.01. (7) (8). where A and B are constants. aNiS is known from the data reported by Hsieh and Chang11) and the content of sulfur in the slag, (mass%S), was obtained in the present study. The present experimental data for the solubility of nickel in the FeO X –CaO based slag were analyzed by regression on the basis of eqs. (7) and (8). It was found that they could be reproduced well when A and B in eqs. (7) and (8) are 26000 and 6.4, respectively, as shown in Fig. 11. 4.2 Sulfide capacity There are two important equilibria16) for the dissolution of sulfur in slag, which are represented by the following equations: 1 1 S2 (g) + (O2− ) = O2 (g) + (S2− ) (9) 2 2 1 3 (10) S2 (g) + O2 (g) + (O2− ) = (SO2− 4 ) 2 2 and it is known that eq. (9) prevails against eq. (10) at lower oxygen pressures. On the basis of eq. (9), a sulfide capacity is defined by eq. (11). C S2− = (mass%S)( pO2 / pS2 )1/2 = K 9 aO2− /γS2−. mass%Ni in Slag. G ◦ /(J/mol) = −247750 + 92.52T /K. 20. 2-. (6). Experimental. CS. 1 Ni(l) + O2 (g) = NiO(s) 2. 2225. (11). Where K 9 is the equilibrium constant of eq. (9), and aO2− and γS2− are activity of O2− and activity coefficient of S2− in the slag, respectively. It is described by Richardson17) that C S2− for various silicate and aluminate slags with a considerably low content of sulfur containing CaO, MgO and MnO are in-. 0.001 -8. -7. -6 log p O2. -5. -4. Fig. 12 Relationships between the sulfide capacity and log pO2 for the FeO X –SiO2 –MgO and FeO X –CaO–MgO slags melted in the MgO crucible at 1773 K.. dependent of pO2 in its low pressure range and that C S2− increases with raising slag basicity (aO2− ) and with the presence of components with greater affinity for sulfur. C S2− for the present FeO X –SiO2 and FeO X –CaO based slags were calculated from the sulfur content and partial pressures of O2 and S2 determined in the present study and are shown in the Fig. 12, in relation to log pO2 . At a given pO2 , C S2− for the FeO X –SiO2 based slag is considerably larger than that for the FeO X –CaO slag. This is considered to be ascribed to the more basic property of FeO X –CaO based slag with larger aO2− . It is to be noted that C S2− for both slags increase with the oxygen pressure. This suggests that (aO2− /γS2− ) in eq. (11) is dependent on pO2 , provided that eq. (9) is applicable to the dissolution of sulfur in the present slag. It is shown in Tables 1 and 2 that the slag compositions vary a little or more with the oxygen pressure. This variation may be ascribed to the change in the (aO2− /γS2− ) against pO2 . It is noted that the (Fe3+ /Fe2+ ) ratio and the nickel content present systematic changes of increasing with the oxygen.

(8) 2226. H. M. Henao, M. Hino and K. Itagaki. pressure. This may suggest that these quantities have the effect on the (aO2− /γS2− ). It is possible to make another interpretation for the change of C S2− against pO2 . It is reported by Sano18) that C S2− for calcium ferrite and calcium-alumino-silicate melts present critical oxygen pressures between log pO2 of about −4 and −5, above which the dissolution of sulfur as sulfate, as given by eq. (10), is prevailing, and that the C S2− shows a noticeable increase around the critical pressures. The oxygen pressures in the present study are also considerably high at log pO2 between about −7 and −5. Hence, there might be a possibility that these oxygen pressures are to be in the transitional region between the sulfide and sulfate dissolution of sulfur in the slags. 4.3 Oxygen in alloy It is generally known in the metal/gas equilibration that the oxygen content in the metal increases with increasing oxygen pressure up to the precipitation of metal oxide. As an example, the solubility of oxygen19, 20) in liquid nickel at 1773 K is shown with a broken line in Fig. 4. However, as shown in Fig. 4, the oxygen content in the Ni–S based alloy equilibrated with the FeO X –SiO2 and FeO X –CaO based slags present an anomalous behavior against pO2 . The oxygen content for the FeO X –SiO2 based slag decreases sharply with increasing pO2 up to log pO2 of about −6.3 and that for the FeO X –CaO based slag increases very slightly in this range of oxygen pressure. It is reported by Font et al.5) that the nickel mattes with iron equilibrated with the FeO X –SiO2 and Fe X –CaO based slags contain considerable amounts of oxygen and that it decreases sharply with decreasing iron content in the matte. It is noted in Tables 1 and 2 as well as in Fig. 3 that the iron content in the Ni–S based alloy is considerably large in the range of lower oxygen pressure and that it decreases sharply in the range of higher oxygen pressure. It is also found that the iron content in the range of lower oxygen pressure is much larger for the FeO X –SiO2 based slag than for the FeO X –CaO based slag. These behaviors of iron in the alloy against pO2 may be ascribed to the solubility of oxygen in the alloy shown in Fig. 4. Although the change of oxygen content against the oxygen pressure is very gentle, as shown in Fig. 4, the content tends to decrease with the oxygen pressure in the range above log pO2 of −5.5. The reason for this decrease can not be clarified. 4.4 Application to nickel smelting It was clarified in the present study that, at log pO2 of −5, that is just prior to precipitation of a FeO X –NiO solid solution, the compositions of alloys equilibrated with the FeO X – SiO2 and FeO X –CaO slags (fluxes) in a MgO crucible at 1773 K are Ni: 95.0, Fe: 2.4, S: 2.1, O: 0.5 mass% and Ni: 95.5, Fe: 1.1, S: 2.9, O: 0.5 mass% (the analytical results in Tables 1 and 2 are normalized to 100 mass%), respectively. On the other hand, the contents of nickel in the respective slags are 10.1 and 20.3 mass%. Although the nickel contents in the alloys are relatively large at more than 95 mass%, considerable amounts of iron, sulfur and oxygen are contained in the alloys as the impurities. Hence, when a process of smelting the Ni–S alloy with the FeO X –SiO2 and FeO X –CaO based slags (fluxes) is considered, a refining process for the metal. product will be indispensable. Furthermore, it is to be noted that the content of nickel in the slag is considerably large at more than 10 mass%. This means that the loss of nickel in the slag will be substantial in the smelting process. Hence, the amount of produced slag or used flux in the process must be kept small to reduce the loss of nickel in the slag or flux. The authors have found that iron-free fluxes such as Al2 O3 – CaO–MgO system are suitable for making nickel from the Ni–S alloys, as will be reported in a separated paper.6) 5. Conclusion As a fundamental study of smelting the nickel sulfide to produce liquid nickel, experimental works were made on the phase equilibrium between the Ni–S melt and the FeO X –SiO2 and FeO X –CaO based slags in a MgO crucible at 1773 K under controlled PSO2 at 10.1 kPa and PO2 in a range between 5.1 × 10−3 and 1.6 Pa. It was found that the content of sulfur in the alloy decreased with increasing oxygen pressure (decreasing sulfur pressure) while the content of nickel increased, and that a considerable amount of iron was transferred from the slag to the alloy in a range of lower oxygen pressure. It was clarified that the content of oxygen in the alloy equilibrated with the FeO X –CaO based slag changed very slightly with the oxygen pressure, while that for the FeO X –SiO2 based alloy decreased sharply with increasing oxygen pressure in the range of lower oxygen pressure due to the existence of considerable amount of iron in the alloy. The content of nickel in the FeO X –SiO2 slag was found to increase with increasing oxygen pressure and reach 10.1 mass% at log pO2 of −5.0 above which precipitation of a FeO X –NiO solid solution is anticipated. On the other hand, the content of nickel in the FeO X –CaO based slag decreased with increasing oxygen pressure in the range of lower oxygen pressure, due to the dissolution of nickel in the slag as the sulfide which is prevailing in this pressure range. In the range of higher oxygen pressure, the nickel content in the FeO X –CaO based slag increased with increasing pO2 and reached a considerably large value of 20.3 mass% at log pO2 of −4.8. The content of sulfur in the FeO X –SiO2 and FeO X –CaO based slags decreased with increasing pO2 . REFERENCES 1) P. Queneau, C. E. O’Neill, A. Illis and J. S. Warner: J. Met. July (1969) 35–45. 2) Terry: Extractive Metallurgy of Nickel, ed. by A. R. Burkin (John Wiley & Sons, 1987) pp. 13–28. 3) J. M. Font, M. Hino and K. Itagaki: Mater. Trans., JIM 39 (1998) 834– 840. 4) J. M. Font, M. Hino and K. Itagaki: Mater. Trans., JIM 40 (1999) 20–26. 5) J. M. Font, M. Hino and K. Itagaki: Metall. Mater. Trans. 31B (2000) 1231–1239. 6) H. M. Henao, M. Hino and K. Itagaki: to be submitted to Metall. Mater. Trans. 7) A. Muan and E. F. Osborn: Am. Ceram. Soc. 39 (1956) 121–140. 8) R. E. Johnson and A. Muan: Am. Ceram. Soc. 48 (1965) 359–364. 9) G. A. Meyer, J. S. Warner, Y. K. Rao and H. H. Kellogg: Metall. Trans. 6B (1975) 229–235. 10) O. Knacke, O. Kubaschewski and K. Hesselmann: Thermochemical Properties of Inorganic Substances, Second Edition, (Springer-Verlag, 1991) pp. 688..

(9) Phase Equilibrium between Ni–S Melt and FeO X –SiO2 or FeO X –CaO Based Slag under Controlled Partial Pressures 11) K. Hsieh and Y. Austin: Can. Metall. Quart. 26 (1987) 311–327. 12) R. U. Pagador, M. Hino and K. Itagaki: Metall. Rev., MMIJ. 13 (1996) 90–103. 13) O. Knacke, O. Kubaschewski and K. Hesselmann: Thermochemical Properties of Inorganic Substances, Second Edition, (Springer-Verlag, 1991) pp. 1455. 14) Y. Takeda and A. Yazawa: Proceedings of Intern. Conf. on Productivity and Technology in the Metallurgical Industries, (TMS, Köln, 1989) pp. 227–240. 15) M. Nagamori: Metall. Trans. 5 (1974) 531–538.. 2227. 16) J. Fincham and F. D. Richardson: Proc. Roy. Soc. A233 (1954) 40–62. 17) D. Richardson: Physical Chemistry of Melts in Metallurgy, Volume 2, (Academic Press, London and New York, 1974) pp. 291–296. 18) N. Sano, W. K. Lu, P. V. Riboud and M. Maeda: Advanced Physical Chemistry for Process Metallurgy. ed by N. Sano et al. (Academic Press, 1997) pp. 59. 19) E. S. Tankins, N. A. Gokcen and G. R. Belton: Trans. Metall. Soc., AIME. 230 (1964) 820–827. 20) P. Nash: Phase Diagrams of Binary Nickel Alloys, (ASM International, The Materials Information Society, 1991) pp. 230..

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