Phase Equilibrium between Ni S Melt and FeO X SiO 2 or FeO X CaO Based Slag under Controlled Partial Pressures

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1 Materials Transactions, Vol. 43, No. 9 (22) pp. 229 to 2227 c 22 The Japan Institute of Metals Phase Equilibrium between Ni S Melt and SiO 2 or 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 , 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 SiO 2 or CaO based slag in a magnesia crucible at 773 K under controlled p SO2 of. kpa and p O2 in a range between 5. 3 and.6 Pa by using CO CO 2 SO 2 gas mixtures. The solubility of nickel in these slags at p O2 of.6 Pa, above which precipitation of a NiO solid solution was anticipated, was found to be. and 2.3 mass% for the SiO 2 and CaO based slags, respectively. The contents of iron, sulfur and oxygen in the nickel melt equilibrated with these slags were 2, 3 and.5 mass% for the SiO 2 based slag and, 3 and.5 mass% for the 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, 22; Accepted July 3, 22) Keywords: phase equilibrium, distribution ratio, activity coefficient, SiO 2 slag, CaO slag, nickel matte, nickel converting. 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 Ni 3 S 2 and Cu 2 S by the controlled slow cooling method. Most of the recovered Ni 3 S 2 is further treated by the hydrometallurgical processes to make nickel, while the rest by the pyrometallurgical processes. To convert Ni 3 S 2 to liquid nickel, INCO ) developed a process in which oxygen is injected in a top blown rotary converter (TBRC) for providing the conditions of high temperatures around 923 K and efficient gas-liquid mixing required for conversion to nickel. To discuss the pyrometallurgical route for the conversion of Ni 3 S 2 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 studies 2) 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 Ni 3 S 2 to those with precipitation of solid NiO in the slag phase, under controlled partial pressures of SO 2,S 2 and O 2. Hence, following a series of experimental studies 3 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 773 K in the present study. The experiments were made under the partial pressure of SO 2 controlled at. kpa and the partial pressures of O 2 and S 2 were also controlled in the ranges between 5. 3 and.6 Pa and between 8. 2 and 9.2 kpa, respectively, by using CO CO 2 SO 2 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. henaocol@iamp.tohoku.ac.jp less than 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 SiO 2 or CaO based flux. The result for the iron free slag system will be reported in a separated paper. 6) 2. Experimental CO CO 2 SO 2 gas mixtures were used to control the partial pressures of SO 2,O 2 and S 2. The flow rate was regulated at m 3 /s by using capillary flow meters. A CO gas train was equipped with soda lime and P 2 O 5 columns to trap CO 2 gas and moisture, respectively, while CO 2 gas passed through a P 2 O 5 column. The partial pressure of SO 2 was fixed at. kpa and those of O 2 and S 2 were varied by changing the CO/CO 2 ratio in the gas mixtures. These were calculated by using a ChemSage Ver. 4. 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 773 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 Ni 3 S 2 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 SiO 2 slag are : 63, Fe 2 O 3 : 4 and SiO 2 : 33 mass%, while those of the CaO slag, : 55, Fe 2 O 3 : 8 and CaO: 27 mass%. The sample was put in a MgO crucible of.5 2 m i.d and 6 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.

2 222 H. M. Henao, M. Hino and K. Itagaki The total iron and Fe 2+ in the slags were determined by the volumetric titration with K 2 Cr 2 O 7. The difference between the total iron and Fe 2+ contents was taken as the Fe 3+ content. The total oxygen content in the alloy specimen was determined by inert gas fusion-infrared absorptiometry (EMGA- 65, HORIBA Co, Ltd., Kyoto, Japan), while the total sulfur content in the slag specimen by a combustion-infrared spectrometer (EMIA-58, HORIBA Co, Ltd., Kyoto, JAPAN). Some preliminary experiments were carried out to determine the time required for the equilibration. When the CaO based slag was melted with pure nickel, a Ni S melt with 5 mass% or Ni 3 S 2 having 26.7 mass%s under p SO2 of. kpa and p O2 of.2 Pa, the change in the sulfur content of alloy phase with the holding time is shown in Fig.. It is found for the alloy with initial 5 mass%s that the sulfur content reaches the equilibrium value of 7.6 mass% at about 7 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 26 ks. On the other hand, it is found for the initial Ni 3 S 2 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 CO 2 SO 2 gas mixture. The present preliminary experiments have clarified that the equilibrium can be made in a restricted time of less than 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 29.6 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 P O2 between 5. 3 and.6 Pa, accordingly, p S2 between 9.2 kpa and 8 2 Pa. A NiO solid solution precipitated in the slag phase at higher P O2 more than 2 Pa, while the Ni S melt and the CaO based slag did not show any phase separation between them at higher P S2 more than 9.2 kpa. The micro- mass%ni in Alloy Time, t /ks initial alloy composition 26.7 mass % S 5. Fig. Change of the sulfur content in Ni Fe S melts with holding time at of 5.9 and 773 K. scopic analysis showed that a thin layer with a thickness of about.5 3 m and 3 5 m was formed between the slag and the crucible for the SiO 2 and CaO based slags, respectively. The electron probe micro analysis (EPMA) clarified that these layers corresponded to olivine 2(Mg, Fe)O SiO 2 for the SiO 2 based slag and magnesiowüstite (Mg, Fe)O for the CaO based slag, as suggested from the phase diagrams reported by Muan and Osborn 7) and Johnson and Muan. 8) Compositions of the slag and alloy phases after equilibration are listed in Tables and 2 for the SiO 2 and CaO based slags, respectively. The summation of the analytical values for both slag and alloy are a little more or less than 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 p O2 = (P O2 /Pa)/(325 Pa) and sulfur by p S2 = (P S2 /Pa)/(325 Pa). 3. Alloy phase 3.. Sulfur content The relationship between the sulfur content in the alloy and log p S2 or for both slag systems are shown in Fig. 2. The limits of p O2 and p S2, between which the experiments were carried out, are indicated with the dash-dot lines. Meyer et al. 9) determined p S2 in relation to the sulfur content in the Ni S melt by equilibrating with H 2 H 2 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 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 mass%s in Alloy 3 2 precipitation of NiO compound -5 -SiO -MgO 2 -CaO-MgO Meyer et al. log p O log p S 2 Fig. 2 Relationships between the sulfur content in Ni Fe S melts and log p S2 or in the equilibration with the slags in the MgO crucible at 773 K. -7 equilibrium with Ni 3 S 2 -FeS

3 Phase Equilibrium between Ni S Melt and SiO 2 or CaO Based Slag under Controlled Partial Pressures 222 Table Compositions of SiO 2 MgO slag and nickel sulfur alloy melted in the MgO crucible at 773 K. mass% Slag Alloy mass% No log p S2 a γ NiO Fe 2+ Fe 3+ Fe Total Ni SiO 2 MgO S Ni Fe S O Table 2 Compositions of CaO MgO slag and nickel sulfur alloy melted in the MgO crucible at 773 K. mass% Slag Alloy mass% No log p S2 a γ NiO Fe 2+ Fe 3+ Fe Total Ni CaO MgO S Ni Fe S O Oxygen content As shown in Fig. 4, the oxygen content in the Ni S melt equilibrated with the SiO 2 based slag presents a considerably large value of about mass% at of 7.3 and decreases remarkably with increasing up to of 6.3, then, changes very gently in the higher region of. While, that for the CaO based slag increases with increasing up to 6.3, then changes very gently in the higher range of. It is shown that the solubility of oxygen in the Ni S melt in the range of 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 773 K. the mole fraction of iron ranges between. and.2 ( and 25 mass%) and that, at a given sulfur content, the iron content in the alloy equilibrating with the SiO 2 based slag is larger than that for the CaO based slag. 3.2 Slag phase 3.2. Precipitation of solid phase Microphotographs of the solidified SiO 2 and CaO based slags equilibrated with the Ni Fe S alloy at of 5. 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 of 5., while the third phase with a nearly oval shape at of 4.7. The EPMA analysis clarified that the glassy and dendritic phases corresponded to the SiO 2 MgO or CaO MgO

4 2222 H. M. Henao, M. Hino and K. Itagaki mass%o in Alloy.5.5 -SiO -MgO 2 -CaO-MgO Ni/(CO-CO )gas 2 Fig. 4 Relationships between the dissolution of oxygen in the alloy and for the SiO 2 MgO and CaO MgO slags melted in the MgO crucible at 773 K. slag and the NiO solid solution. It is considered that the oval phase was formed in the liquid slag at the experimental temperature of 773 K, while the dendritic phase during solidification of the slag. Hence, it is suggested that the slag is saturated with the NiO solid solution at an oxygen partial pressure between of 5. and Solubility of MgO The slag compositions are reproduced in Fig. 6 on the A- B-C ternary diagram with a mole fraction scale where A corresponds to MgO, B to SiO 2 or CaO and C to ( + 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 SiO 2 based slag is larger than that in the the CaO based slag. Despite the presence of NiO, the liquidus line in the SiO 2 based slag is similar to that reported by Muan and Osborn, 7) which is very close to a tie line combining between and MgO SiO 2. It is reported by Johnson and Muan 8) that the solubility of MgO in the CaO MgO slag in coexistence with the solid magnesiowüstite changes very little with the content. This is in accordance with the present result Activity of Activities of in the slags are given in Tables 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 change ) for the reaction Fe(l) + 2 O 2(g) = (l) () The data on the iron activity in the Ni Fe S ternary alloy reported by Hsieh and Chang ) were used in the calculation because the contents of other elements including oxygen are very small Fe 3+ /Fe 2+ ratio It is shown in Fig. 7 that the mass% ratio, (Fe 3+ /Fe 2+ ), increases with increasing p O2. For the SiO 2 based slag, a linear relationship with a slope about /4 is observed between log(fe 3+ /Fe 2+ ) and, corresponding to the chemical reaction of + 4 O 2 =.5. For the CaO based slag, a similar relationship is observed in the range of higher 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 CaO based slag, which combines the ferrous ion in the range of lower p O2, as described in It is also shown that, at a given oxygen pressure, the (Fe 3+ /Fe 2+ ) in the CaO based slag is larger than that in the SiO 2 based slag. The data reported by Pagador et a. 2) for the SiO 2 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 Nickel content The nickel content in the CaO or SiO 2 based slag equilibrated with the Ni Fe S alloy is plotted in Fig. 8, in relation to. It is shown that the solubility of nickel in the SiO 2 based slag increases with increasing from.5 mass% at of 7.3 to mass% at of 5.. While, that in the CaO based slag decreases in a region of lower oxygen pressure from 6 mass% at of 7. to 4 mass% at of about 6.5, then, remarkably increases in the range of higher and becomes 2 mass% at of 4.8. It is noted that the solubility of nickel in the CaO based slag is larger than that in the SiO 2 based slag Sulfur content The sulfur contents in the CaO and SiO 2 based slags are shown in Fig. 9, in relation to log p S2. It is noted that the solubility of sulfur in the CaO based slag abruptly increases in the range of higher log p S2 and becomes 5 mass% at log p S2 of.5. While the change of sulfur solubility against log p S2 is very gentle for the SiO 2 based slag, increasing from.5 mass% at log p S2 of 5.8 to mass% at log p S2 of.. 4. Discussion 4. 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/4o 2 (g) = NiO v/2 (s) (3) Where, the solid NiO v/2 was taken as a reference of the NiO v/2 activity for the lack in the thermodynamic data of the liquid NiO v/2. The equilibrium constant of eq. (3) is given by K = a NiOv/2 /(a Ni p v/4 O ) (4) 2 where a Ni and a NiOv/2 are activities of Ni and NiO v/2, respectively. By combining eqs. (2) and (4) and converting the mole fraction, N, in the activity relationship of a = Nγ with the Raoultian activity coefficient of γ, into mass%, the following

5 Phase Equilibrium between Ni S Melt and SiO 2 or CaO Based Slag under Controlled Partial Pressures 2223 Fig. 5 Microphotographies showing the structure of the slags at 773 K just before and after the precipitation of a NiO solid solution. equation is obtained. log L s/ni Ni = v/4 + log[{γ Ni }/(γ NiOv/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 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 2 N SiO or N CaO.8 (SiO 2 or CaO).6.4 -SiO 2 -MgO -CaO-MgO N ( + NiO) NMgO ( +NiO) mass%s in Slag 2 -SiO 2 -MgO -CaO-MgO Fig. 6 Compositions of the slags equilibrated with the Ni Fe S alloy melted in the MgO crucible at 773 K. mass%ratio, (Fe /Fe ) SiO -MgO 2 -CaO-MgO Pagador et al.. Fig. 7 Relationships between the mass% ratio, (Fe 3+ /Fe 2+ ), and for the SiO 2 MgO and CaO MgO slags melted in the MgO crucible at 773 K. mass%ni in Slag 3 2 -SiO 2 -MgO -CaO-MgO Fig. 8 Relationships between the dissolution of nickel in the slags and for the SiO 2 MgO and CaO MgO slags melted in the MgO crucible at 773 K log p S2 Fig. 9 Relationships between the dissolution of sulfur in the slags and for the SiO 2 MgO and CaO MgO slags melted in the MgO crucible at 773 K. s/ni L Ni.. -SiO -MgO 2 -CaO-MgO NiO. Fig. Relationships between the distribution ratio of nickel and for the SiO 2 MgO and CaO MgO slags melted in the MgO crucible at 773 K. solubility obtained in the present study, are plotted in Fig. in relation to. For the SiO 2 based slag, a nearly linear relationship is observed between log L s/ni Ni and with a gradient of about /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 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 p O2. 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 CaO slag/ni 3 S 2 FeS matte equilibration. The linear relationship observed in Fig. 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 2

7 Phase Equilibrium between Ni S Melt and SiO 2 or CaO Based Slag under Controlled Partial Pressures 2225 activity coefficient of NiO in each slag can be calculated on the basic of eq. (6) Ni(l) + 2 O 2(g) = NiO(s) (6) G /(J/mol) = T/K The standard free energy change and equilibrium constant of eq. (6) is given in a data book. 3) The Raoultian activity coefficient of nickel in the ternary Ni Fe S alloy, {γ Ni }, is obtained from the data reported by Hsieh and Chang ) at 673 K by extrapolating to the present experimental temperature of 773 K with the regular solution model. The calculated γ NiO are listed in Tables and 2 for the SiO 2 and CaO base slags, respectively. The γ NiO presents values ranging between 3. and 8.8. In a general trend, the values for the SiO 2 based slag are larger than those for the 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 SiO 2, as suggested by Takeda and Yazawa. 4) The experimental results for the solubility of nickel in the CaO based slag in the range of lower oxygen pressure, where log L s/ni Ni 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, 5) (mass%ni) ox and (mass%ni) s will be given by eqs. (7) and (8), respectively. (mass%ni) ox = Aa Ni p /2 O 2 /γ NiO (7) (mass%ni) s = Ba NiS (mass%s) (8) where A and B are constants. a NiS is known from the data reported by Hsieh and Chang ) 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 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 26 and 6.4, respectively, as shown in Fig Sulfide capacity There are two important equilibria 6) for the dissolution of sulfur in slag, which are represented by the following equations: 2 S 2(g) + (O 2 ) = 2 O 2(g) + (S 2 ) (9) 2 S 2(g) O 2(g) + (O 2 ) = (SO 2 4 ) () and it is known that eq. (9) prevails against eq. () at lower oxygen pressures. On the basis of eq. (9), a sulfide capacity is defined by eq. (). C S 2 = (mass%s)(p O2 /p S2 ) /2 = K 9 a O 2 /γ S 2 () Where K 9 is the equilibrium constant of eq. (9), and a O 2 and γ S 2 are activity of O 2 and activity coefficient of S 2 in the slag, respectively. It is described by Richardson 7) that C S 2 for various silicate and aluminate slags with a considerably low content of sulfur containing CaO, MgO and MnO are in- mass%ni in Slag Experimental Calculated Total Oxidic Sulfidic Fig. Relationships between the dissolution of nickel in the CaO MgO slag and, calculated by assuming sulfidic and oxidic dissolution. C S SiO -MgO 2 -CaO-MgO. Fig. 2 Relationships between the sulfide capacity and for the SiO 2 MgO and CaO MgO slags melted in the MgO crucible at 773 K. dependent of p O2 in its low pressure range and that C S 2 increases with raising slag basicity (a O 2 ) and with the presence of components with greater affinity for sulfur. C S 2 for the present SiO 2 and CaO based slags were calculated from the sulfur content and partial pressures of O 2 and S 2 determined in the present study and are shown in the Fig. 2, in relation to. At a given p O2, C S 2 for the SiO 2 based slag is considerably larger than that for the CaO slag. This is considered to be ascribed to the more basic property of CaO based slag with larger a O 2. It is to be noted that C S 2 for both slags increase with the oxygen pressure. This suggests that (a O 2 /γ S 2 ) in eq. () is dependent on p O2, provided that eq. (9) is applicable to the dissolution of sulfur in the present slag. It is shown in Tables 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 (a O 2 /γ S 2 ) against p O2. It is noted that the (Fe 3+ /Fe 2+ ) 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 (a O 2 /γ S 2 ). It is possible to make another interpretation for the change of C S 2 against p O2. It is reported by Sano 8) that C S 2 for calcium ferrite and calcium-alumino-silicate melts present critical oxygen pressures between of about 4 and 5, above which the dissolution of sulfur as sulfate, as given by eq. (), is prevailing, and that the C S 2 shows a noticeable increase around the critical pressures. The oxygen pressures in the present study are also considerably high at 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 oxygen 9, 2) in liquid nickel at 773 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 SiO 2 and CaO based slags present an anomalous behavior against p O2. The oxygen content for the SiO 2 based slag decreases sharply with increasing p O2 up to of about 6.3 and that for the 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 SiO 2 and Fe 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 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 SiO 2 based slag than for the CaO based slag. These behaviors of iron in the alloy against p O2 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 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 of 5, that is just prior to precipitation of a NiO solid solution, the compositions of alloys equilibrated with the SiO 2 and CaO slags (fluxes) in a MgO crucible at 773 K are Ni: 95., Fe: 2.4, S: 2., O:.5 mass% and Ni: 95.5, Fe:., S: 2.9, O:.5 mass% (the analytical results in Tables and 2 are normalized to mass%), respectively. On the other hand, the contents of nickel in the respective slags are. and 2.3 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 SiO 2 and 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 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 Al 2 O 3 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 SiO 2 and CaO based slags in a MgO crucible at 773 K under controlled P SO2 at. kpa and P O2 in a range between 5. 3 and.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 CaO based slag changed very slightly with the oxygen pressure, while that for the SiO 2 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 SiO 2 slag was found to increase with increasing oxygen pressure and reach. mass% at of 5. above which precipitation of a NiO solid solution is anticipated. On the other hand, the content of nickel in the 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 CaO based slag increased with increasing p O2 and reached a considerably large value of 2.3 mass% at of 4.8. The content of sulfur in the SiO 2 and CaO based slags decreased with increasing p O2. REFERENCES ) P. Queneau, C. E. O Neill, A. Illis and J. S. Warner: J. Met. July (969) ) Terry: Extractive Metallurgy of Nickel, ed. by A. R. Burkin (John Wiley & Sons, 987) pp ) J. M. Font, M. Hino and K. Itagaki: Mater. Trans., JIM 39 (998) ) J. M. Font, M. Hino and K. Itagaki: Mater. 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