Received: 16 June 2023 Revised: 21 September 2023 Accepted: 21 September 2023 DOI: 10.1111/jace.19523 RESEARCH ARTICLE Experimental and thermodynamic modeling study of phase equilibria in the PbO–NiO–SiO2 system Hamed Abdeyazdan Pyrometallurgy Innovation Centre (PYROSEARCH), School of Chemical Engineering, The University of Queensland, Brisbane, Queensland, Australia Correspondence Hamed Abdeyazdan, Pyrometallurgy Innovation Centre (PYROSEARCH), School of Chemical Engineering, Room 342, 1016 Banksia Bldg, Long Pocket Campus, 80 Meiers Rd, Indooroopilly, The University of Queensland, Brisbane, QLD 4072, Australia. Email: h.abdeyazdan@uq.edu.au Funding information Australian Research Council Linkage program, Grant/Award Number: LP180100028) Maksym Shevchenko Evgueni Jak Abstract An integrated experimental and thermodynamic modeling investigation of the phase equilibria in the PbO–NiO–SiO2 system in air and also in equilibrium with liquid metal has been undertaken to better characterize the chemical reactions taking place in the Ni-containing Pb processing slags. New experimental phase equilibria data at 720◦ C–1740◦ C were obtained for this system using hightemperature equilibration of synthetic mixtures with predetermined compositions in sealed silica ampoules or in Au/Pt–Ir foils, a rapid quenching technique, and electron probe x-ray microanalysis of the equilibrated phase compositions. Phase equilibria and liquidus isotherms in the quartz/tridymite/cristobalite (SiO2 ), olivine (Ni2 SiO4 ), monoxide (NiO), Ni-barysilite (Pb8 NiSi6 O21 ), massicot (PbO), and di-lead silicate (Pb2 SiO4 ) primary phase fields were revealed and the extent of the high-SiO2 two-liquid immiscibility gap in equilibrium with cristobalite was determined. New experimental data were used in the development of a thermodynamic database describing this ternary system. Also, modeling revision of the NiO–SiO2 binary system was conducted, resulting in a smaller miscibility gap in ternary systems that was closer to the experimental results. KEYWORDS lead oxide, liquidus, Ni recycling, nickel oxide, PbO–NiO–SiO2 , phase equilibria, silica, slag 1 INTRODUCTION Characterizing phase equilibria and chemical reactions taking place in Ni-containing Pb processing slags is important for optimizing recycling of Ni metal. Through the pyrometallurgical processes, Pb-smelting reactors are potentially considered for recycling a range of valuable minor elements including Ni metal,1 as both Pb and Ni represent major elements in the battery design sector and may mutually contaminate when the recycled batteries of different kinds are dealt with simultaneously. There is however a lack of accurate phase equilibria information for the oxide subsystems containing Ni oxide. Phase equilibria studies of the minor elements in nonferrous metallurgy are broadly focused on copper smelting and converting processes. There are a range of investigations for distribution of Ni2–6 and other minor elements, for example, Au, Ag, Co, Bi, Sn, Sb, and so forth7–16 between slag-matte/metal in the Cu–Fe–O–S–Si system. For the Pb–Fe–O–S–Si system however, there are only limited investigations available for distribution of Au and Ag,17 as well as Sn and Sb.18 No information was found for distribution of Ni in the Pb-processing slag-matte/metal equilibrium in the literature. Liquidus studies of the minor elements-containing ternary and quaternary systems are limited to a few investigations on the metal systems This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2023 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society. J Am Ceram Soc. 2024;107:1383–1407. wileyonlinelibrary.com/journal/jace 1383 1384 ABDEYAZDAN et al. containing Cu and Ni19–21 and PbO-based oxide systems containing SnO/SnO2 .22 While PbO–NiO–SiO2 is the key oxide subsystem to be investigated for better characterizing the Ni recycling in Pb smelters, there is no phase equilibria information for this system in the literature. Importantly, the NiO–SiO2 binary system has been reviewed by Prostakova23 in 2013. In this study, new phase equilibria data at 720◦ C–1740◦ C were obtained for the PbO–NiO–SiO2 system in air and also in equilibrium with liquid metal. The results were used for optimization of the parameters in a thermodynamic database that describes the broader, more complex system. 2 EXPERIMENTAL METHODOLOGY The experimental technique and apparatus used in this study have been detailed in previous publications.24–26 The initial chemical mixtures were prepared by blending selected proportions of high-purity powders of PbO, NiO, and Ni metal (99.9 wt.% purity), and SiO2 (99.9 wt.% purity, pre-dried at 400◦ C for 1 h before mixing), supplied by Alfa Aesar. The Pb3 O4 powder was prepared by oxidation of PbO powder in an MgO crucible in air for 24 h at 450◦ C. To reduce the extent of lead oxide vaporization, a master slag of Pb4 Si6 O16 composition was prepared by mixing appropriate ratio of Pb3 O4 and dried SiO2 powders, and heating initially for 2 h at 600◦ C and then a further 2 h at 900◦ C in Pt crucible in air. Excess oxygen from Pb3 O4 was released to atmosphere during heating, so that the final master slag contained only Pb2+ ; direct use of PbO reagent was avoided to protect the Pt substrate from accidental destruction due to local reducing conditions, observed in a previous study by the authors.22 The composition of the initial mixtures is given in Table 1. The mixtures were pelletized and divided into 0.2–0.3 g samples for the experiments. The initial compositions were selected to ensure that the liquid slag was in equilibrium with at least one crystalline phase, and the crystalline solid phase(s) fraction in the final sample at the equilibration temperature was preferably approximately 10 vol% and not greater than 50 vol% to promote the retention of the liquid as an amorphous phase during quenching. The substrates used for equilibration were (i) vacuumsealed silica ampoules (99.9% purity, 5–10 cm long, 1.3 cm outer diameter, supplied by Lianyungang Guoyi Quartz Products Co., Ltd.) for high-SiO2 mixtures that were in equilibrium with tridymite and cristobalite at the target temperature; and (ii) Au and Pt–Ir foils for low-SiO2 mixtures, which were in equilibrium with olivine, barysilite, massicot, or monoxide at the experimental temperature. TA B L E 1 The composition of the initial mixtures in wt%. Mixture name Pb3 O4 NiO Pb4 Si6 O16 SiO2 Ni PN1 90.9 9.1 – – – PNS1 – 34.8 48.7 16.5 – PNS2 63.1 9.9 27.0 – – PNS3 36.0 9.9 54.1 – – PNS4 – – 80.4 10.7 8.9 PNS5 – 9.4 47.2 27.6 15.7 PNS5a – 11.0 36.2 37.0 15.7 PNS6 – 15.5 84.5 – – PNS7 – 16.4 35.1 33.6 14.9 PNS7a – 18.7 26.1 40.3 14.9 PNS8 – 26.9 19.2 38.5 15.4 PNS9 85.9 3.0 11.1 – – PNS10 15.2 5.1 79.8 – – PNS11 53.6 2.6 43.8 – – PNS12 66.4 2.3 31.3 – – PNS13 34.0 2.9 63.1 – – PNS14 – – 91.8 4.8 3.4 PNS15 – 27.6 31.3 26.1 14.9 PNS16 – 23.1 36.6 25.4 14.9 PNS16a – 23.8 38.0 24.1 14.1 PNS17 – 13.4 24.4 46.5 15.7 PNS18 – 6.4 32.0 45.6 16.0 PNS20 51.1 0.5 48.4 – – NiSi0 – 53.8 – 46.2 – NiSi1 – 70.0 – 30.0 – NiSi2 – 41.1 – 58.9 – Au foils were used for low-temperature (T < 1000◦ C) and Pt–Ir foils were used for high-temperature (T > 1000◦ C) experiments. The experiments were conducted in a high-temperature vertical tube resistance PYROX furnace using lanthanum chromite heating elements under air or argon atmosphere. The samples were suspended in the center of the uniform hot zone of the furnace on a Kanthal (Fe–Cr–Al alloy) wire (0.7 or 1 mm diameter), with 15–20 cm platinum wire added to the Kanthal wire for T > 1450◦ C to avoid failure at high temperatures. Where there was no risk of excessive evaporation of lead oxide (i.e., sealed ampoules or low temperatures), the samples were first pre-melted at 20◦ C– 50◦ C above the target temperature for 5 min to support the formation of a homogeneous liquid. Otherwise, when the rate of lead oxide evaporation was high (i.e., low-SiO2 slags in the monoxide primary phase field), preheatings were avoided, and the equilibration times minimized to 0.2–0.5 h. Substrate Mixture name Pre-melt Atmosphere T, ◦ C Au foil PNS12 Air 760 Au foil Au foil 3 4 PNS13 PNS11 PNS11 Air Air Air Au foil PNS9 Air Au foil Au foil Au foil PtIr foil 6 7 8 9 PNS6 PNS10 PNS10 PNS10 Air Air Air Air Liquid slag + monoxide (NiO) + olivine (Ni2 SiO4 ) 5 Liquid slag + monoxide (NiO) + massicot (PbO) Au foil 2 1150 – 950 950 830 800 760 850 Liquid slag + monoxide (NiO) + barysilite (Pb8 NiSi6 O21 ) 1 – – – – – 700 720 – 700 Pre-cool T, ◦ C 1100 1000 900 800 800 750 750 735 720 Final equilibration T, ◦ C 5 5 17 72 5 168 17 72 17 Time, h 41.8 ± 0.23 0.07 ± 0.04 0.01 ± 0.01 51.7 ± 0.28 0.00 ± 0.00 33.5 ± 0.10 Olivine 0.01 ± 0.01 33.7 ± 0.07 Olivine Monoxide 0.19 ± 0.11 Slag 44.7 ± 0.24 52.0 ± 0.24 Olivine 0.22 ± 0.06 0.57 ± 0.22 0.01 ± 0.01 0.51 ± 0.16 33.9 ± 0.16 Monoxide Monoxide 45.6 ± 0.23 Slag 0.04 ± 0.03 52.3 ± 0.25 Olivine 33.6 ± 0.28 Monoxide Slag 46.1 ± 0.22 0.52 ± 0.13 52.7 ± 0.20 0.50 ± 0.23 Slag 100.0 ± 0.02 0.00 ± 0.00 Massicot Barysilite 0.15 ± 0.01 39.9 ± 0.12 Monoxide 85.4 ± 0.31 53.5 ± 0.44 0.94 Slag 14.1 ± 0.33 0.73 51.9 ± 0.37 Barysilite 0.11 ± 0.09 47.1 ± 0.44 0.00 39.9 ± 0.50 Monoxide Monoxide 0.15 53.5 ± 0.52 33.5 ± 0.31 Slag 53.5 ± 0.23 65.9 ± 0.28 40.0 ± 0.15 Barysilite Slag 67.6 ± 0.16 0.26 ± 0.03 31.9 ± 0.16 0.14 ± 0.02 Monoxide 66.4 ± 0.24 33.6 ± 0.24 Di-lead silicate Slag 73.5 ± 0.22 0.85 ± 0.62 26.1 ± 0.21 0.31 ± 0.18 PbO/Pb Monoxide SiO2 /Si Slag Phase Composition, mol.% Experimental conditions and measured compositions of the condensed phases for PbO–NiO–SiO2 system in air/argon atmosphere. Liquid slag + monoxide (NiO) + di-lead silicate (Pb2 SiO4 ) No. TA B L E 2 (Continues) 66.5 ± 0.10 99.9 ± 0.04 5.5 ± 0.35 66.3 ± 0.07 99.6 ± 0.16 3.3 ± 0.14 66.1 ± 0.16 98.9 ± 0.38 2.1 ± 0.11 66.3 ± 0.26 99.0 ± 0.36 1.3 ± 0.11 0.01 ± 0.02 99.7 ± 0.10 0.44 ± 0.14 6.6 ± 0.17 98.3 1.0 ± 0.17 6.6 ± 0.03 99.9 0.56 ± 0.12 6.5 ± 0.12 99.6 ± 0.06 0.45 ± 0.07 0.01 ± 0.01 98.8 ± 0.79 0.39 ± 0.10 NiO/Ni ABDEYAZDAN et al. 1385 PtIr foil 11 PNS1 PNS6 Mixture name Air Air – – Pre-melt Atmosphere T, ◦ C – – Pre-cool T, ◦ C 1450 1200 Final equilibration T, ◦ C SiO2 ampoule SiO2 ampoule SiO2 ampoule SiO2 ampoule PNS16a PNS4 PNS14 PNS14 Au foil Au foil Au foil 16 17 18 PNS11 PNS9 PNS11 Liquid slag + monoxide (NiO) 15 14 13 12 Air Air Air Air ampoule Argon ampoule Argon ampoule Argon 920 – 850 1550 1150 950 950 – – – 1480 – – – 900 900 800 1530 1100 900 800 Liquid slag + olivine (Ni2 SiO4 ) + quartz/tridymite/cristobalite (SiO2 ) + Pb–Ni metal PtIr foil Substrate (Continued) 10 No. TA B L E 2 5 2 17 17 5 72 168 2 3 Time, h 0.03 ± 0.02 0.00 ± 0.00 Monoxide 0.03 98.5 ± 0.53 37.9 ± 0.21 100.0 0.35 ± 0.28 60.5 ± 0.20 Quartz Pb metal 34.0 ± 0.09 0.04 ± 0.02 0.04 75.5 ± 5.8 0.05 ± 0.03 12.1 ± 0.08 0.01 ± 0.00 0.01 1.0 ± 0.27 99.9 0.77 ± 1.4 0.03 ± 0.03 60.6 ± 0.26 33.2 ± 0.09 99.8 0.01 ± 0.03 Tridymite Pb metal Ni metal Olivine Cristobalite Ni metal 0.40 ± 0.19 63.4 ± 0.15 0.12 ± 0.03 0.03 ± 0.03 Monoxide Monoxide 35.3 ± 0.13 Slag 0.13 ± 0.10 87.4 ± 0.30 11.7 ± 0.25 Monoxide Slag 62.9 ± 0.26 0.26 ± 0.02 36.3 ± 0.26 0.03 ± 0.04 Slag Slag Slag 61.8 ± 0.10 95.8 ± 0.62 33.9 ± 0.20 0.01 ± 0.01 100 ± 0.01 0.25 ± 0.18 Tridymite Pb metal Olivine 0.04 33.9 Olivine Slag 39.4 ± 0.23 0.03 ± 0.02 59.7 ± 0.22 33.9 ± 0.27 Olivine Slag 22.2 ± 0.16 0.02 ± 0.03 58.3 ± 0.26 33.2 ± 0.23 0.01 ± 0.02 33.4 ± 0.22 Olivine Olivine 0.05 ± 0.03 0.00 ± 0.00 Monoxide Slag 38.1 ± 0.29 53.8 ± 0.27 Composition, mol.% PbO/Pb SiO2 /Si Slag Phase (Continues) 99.9 ± 0.04 1.4 ± 0.12 99.5 ± 0.22 0.87 ± 0.15 99.7 ± 0.05 0.83 ± 0.11 99.0 ± 0.28 0.24* 66.8 ± 0.10 27.4 ± 0.27 99.9 ± 0.05 23.7 ± 5.5 0.05* 66.0 ± 0.20 4.1 ± 0.08 4.0 ± 0.77 0.00 66.1 1.5 ± 0.09 1.1 ± 0.34 0.00 66.1 ± 0.28 0.85 ± 0.09 100.0 ± 0.02 66.8 ± 0.25 19.4 ± 0.17 66.6 ± 0.21 100.0 ± 0.03 8.1 ± 0.14 NiO/Ni 1386 ABDEYAZDAN et al. Au foil Au foil Au foil PrIr foil PtIr foil PtIr foil PtIr foil PrIr foil PtIr foil PtIr foil PtIr foil PtIr foil PtIr foil PtIr foil PtIr foil PtIr foil 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 Substrate (Continued) 19 No. TA B L E 2 PNS2 PNS2 PNS3 PNS3 PNS3 PNS3 PNS3 PNS2 PN1 PNS3 PNS2 PNS2 PN1 PNS3 PNS2 PN1 Mixture name Air Air Air Air Air Air Air Air Air Air Air Air Air Air Air Air – – – – – – – – – – – – – – – – Pre-melt Atmosphere T, ◦ C – – – – – – – – – – – – – – – – Pre-cool T, ◦ C 1300 1300 1300 1300 1200 1200 1200 1200 1200 1100 1100 1100 1100 1000 1000 1000 Final equilibration T, ◦ C 0.25 0.25 1 1 0.5 0.5 0.5 0.25 0.2 1 0.5 0.5 0.4 3 2 1 Time, h 2.7 ± 0.22 99.7 ± 0.16 97.3 ± 0.24 99.7 ± 0.05 0.22 ± 0.05 0.03 ± 0.03 62.6 ± 0.46 0.03 ± 0.03 29.6 ± 0.41 0.00 ± 0.00 Monoxide Monoxide 0.00 ± 0.00 68.8 ± 0.41 Slag 0.05 ± 0.03 23.7 ± 0.20 Monoxide 0.00 ± 0.00 49.8 ± 0.56 Slag 0.05 ± 0.03 40.2 ± 0.48 Monoxide 0.00 ± 0.00 46.8 ± 0.55 Slag 0.09 ± 0.08 42.8 ± 0.54 Monoxide 0.02 ± 0.05 42.3 ± 0.04 Slag 0.09 ± 0.08 50.0 ± 0.09 Monoxide 0.02 ± 0.05 44.6 ± 0.15 Slag 0.09 ± 0.08 47.9 ± 0.32 Monoxide 0.02 ± 0.05 46.7 ± 0.08 45.8 ± 0.16 Slag 0.05 ± 0.03 0.02 ± 0.04 Monoxide Slag 68.6 ± 0.90 26.5 ± 0.84 Slag – 4.5 ± 0.25 95.5 ± 0.23 Monoxide 99.9 ± 0.12 0.08 ± 0.04 – 0.04 ± 0.08 Monoxide Slag (Continues) 100.0 ± 0.03 7.7 ± 0.25 100.0 ± 0.03 7.5 ± 0.30 99.9 ± 0.03 9.9 ± 0.16 99.9 ± 0.03 10.4 ± 0.08 99.9 ± 0.14 7.7 ± 0.05 99.9 ± 0.14 7.5 ± 0.17 99.9 ± 0.14 7.5 ± 0.09 99.9 ± 0.05 4.8 ± 0.15 4.4 ± 0.12 99.7 ± 0.06 0.18 ± 0.03 54.0 ± 0.32 Monoxide 3.1 ± 0.09 41.6 ± 0.27 Slag 3.1 ± 0.12 99.7 ± 0.06 0.09 ± 0.06 69.5 ± 0.23 27.4 ± 0.22 Monoxide 2.8 ± 0.11 Slag 71.2 ± 0.28 0.18 ± 0.03 25.7 ± 0.33 0.09 ± 0.06 Slag – 0.32 ± 0.11 – Monoxide 0.18 ± 0.10 Monoxide Slag 99.6 ± 0.23 56.0 ± 0.59 41.2 ± 0.55 0.18 ± 0.14 Slag 1.9 ± 0.13 99.9 ± 0.10 70.5 ± 0.34 0.09 ± 0.10 27.6 ± 0.33 0.04 ± 0.03 1.7 ± 0.22 99.8 ± 0.06 98.3 ± 0.24 0.09 ± 0.04 Monoxide – NiO/Ni Slag – Monoxide Composition, mol.% PbO/Pb SiO2 /Si Slag Phase ABDEYAZDAN et al. 1387 closed Ir wire closed PtIr foil closed PtIr foil NiSi1 PNS1 PNS3 PNS3 PNS3 PNS6 Mixture name Air Air Air Air Air Air PtIr foil PNS1 Air – – – 45 44 43 ampoule SiO2 ampoule SiO2 ampoule SiO2 PNS5 PNS5 PNS4 Argon ampoule Argon ampoule Argon ampoule 1550 1510 1180 – – – 1540 1500 1150 1450 1400 2 2 4 2 6 38.8 ± 0.11 0.02 ± 0.02 36.3 ± 0.26 0.02 ± 0.02 32.5 ± 0.26 0.02 ± 0.02 18.0 ± 0.15 0.03 ± 0.02 45.4 ± 0.10 0.04 ± 0.07 47.5 ± 0.25 0.04 ± 0.07 51.5 ± 0.23 0.04 ± 0.07 53.4 ± 0.15 0.03 ± 0.04 Monoxide 71.8 ± 1.9 0.06 ± 0.02 18.4 ± 0.09 0.29 ± 0.64 0.01 ± 0.02 65.6 ± 0.16 Pb metal Ni metal 2.0 ± 0.45 16.7 ± 0.14 0.02 ± 0.01 1.9 ± 0.29 68.9 ± 0.22 99.9 ± 0.07 0.02 ± 0.03 Cristobalite Ni metal Ni metal Slag 0.02 ± 0.03 0.00 99.8 Cristobalite Slag 0.07 32.7 ± 0.19 99.9 Tridymite 62.1 ± 0.19 99.7 Tridymite Slag 33.2 ± 0.23 Olivine 0.01 19.5 ± 0.10 0.02 ± 0.03 62.9 ± 0.23 Slag 0.01 ± 0.01 0.02 ± 0.02 33.2 ± 0.1 99.8 ± 0.13 22.5 ± 0.16 64.0 ± 0.16 – Monoxide Slag – 39.3 ± 0.21 0.03 ± 0.03 Slag Monoxide Slag Monoxide Slag Monoxide Slag Olivine 1450 0.03 ± 0.02 0.00 ± 0.00 Monoxide Slag 34.5 ± 0.60 53.8 ± 0.65 Composition, mol.% PbO/Pb SiO2 /Si Slag Phase Tridymite Air 0.5 0.33 0.33 0.33 0.33 2 Time, h ampoule PNS1 1700 1550 1400 1400 1400 1300 Final equilibration T, ◦ C + hole SiO2 – – – – – – Pre-cool T, ◦ C Liquid slag + tridymite/cristobalite (SiO2 ) + Pb–Ni metal (optional) 42 41 1710 – – – – – Pre-melt Atmosphere T, ◦ C Liquid slag + olivine (Ni2 SiO4 ) + tridymite (SiO2 ) 40 39 38 PtIr foil closed PtIr foil 36 37 PtIr foil Substrate (Continued) 35 No. TA B L E 2 (Continues) 98.1 ± 0.29 0.12* 14.3 ± 0.19 98.0 ± 0.45 0.19 16.0 ± 0.16 99.9 ± 0.03 27.9 ± 1.6 0.07* 5.2 ± 0.11 0.27* 66.8 ± 0.25 17.6 ± 0.14 0.16* 66.8 ± 0.1 13.5 ± 0.18 99.97 ± 0.03 60.7 ± 0.21 99.9 ± 0.03 28.6 ± 0.17 99.9 ± 0.08 16.0 ± 0.11 99.9 ± 0.08 16.2 ± 0.13 99.9 ± 0.08 15.8 ± 0.10 100.0 ± 0.02 11.7 ± 0.13 NiO/Ni 1388 ABDEYAZDAN et al. ampoule SiO2 ampoule SiO2 ampoule SiO2 ampoule SiO2 ampoule SiO2 ampoule SiO2 ampoule SiO2 Substrate (Continued) NiSi0 PNS17 PNS18 PNS5a PNS8 PNS7 PNS7 Mixture name Air ampoule Argon ampoule Argon ampoule Argon ampoule Argon ampoule Argon Argon 1682 1645 1620 1615 1610 1615 1560 Pre-melt Atmosphere T, ◦ C – – – – – – – Pre-cool T, ◦ C 55 54 53 Ir wire ampoule SiO2 ampoule SiO2 NiSi1 PNS15 PNS16 Air ampoule Argon ampoule Argon 1670 1590 1570 – 1520 1500 Liquid slag + monoxide (NiO) + cristobalite (SiO2 ) + Pb–Ni metal (optional) 52 51 50 49 48 47 46 No. TA B L E 2 1655 1560 1540 1680 1630 1600 1600 1600 1600 1550 Final equilibration T, ◦ C 0.8 4 4 0.4 3 3 3 2 2 2 Time, h 0.59 ± 0.27 10.9 ± 0.18 0.02 ± 0.00 0.41 ± 0.10 7.2 ± 0.11 0.00 ± 0.00 59.7 ± 0.24 99.6 ± 0.03 0.03 ± 0.03 50.6 ± 0.44 Ni metal Cristobalite Ni metal 0.42 ± 0.06 13.0 ± 0.16 0.01 ± 0.01 0.79 ± 0.09 11.3 ± 0.16 0.01 ± 0.01 1.0 ± 0.26 0.00 ± 0.00 66.3 ± 0.45 99.7 ± 0.03 0.03 ± 0.04 79.3 ± 0.21 99.9 ± 0.04 0.05 ± 0.05 Ni metal Cristobalite Ni metal Cristobalite Ni metal 12.2 ± 0.12 0.01 ± 0.01 0.04 ± 0.03 1.0 ± 0.05 99.5 ± 0.04 0.00 ± 0.00 Cristobalite Ni metal – 0.01 ± 0.01 99.0 Monoxide Cristobalite – – 44.0 ± 0.31 Slag Slag 54.9 ± 0.25 1.3 ± 0.12 0.00 ± 0.00 Ni metal 0.00 ± 0.00 0.01 ± 0.02 99.9 ± 0.02 Cristobalite Monoxide 13.4 ± 0.13 0.01 ± 0.01 57.5 ± 0.19 – 0.00 ± 0.00 99.0 Monoxide Slag Cristobalite – 0.11 ± 0.06 0.04 ± 0.05 Ni metal Slag 45.1 ± 0.17 10.0 ± 0.54 0.01 ± 0.01 55.6 ± 0.97 99.2 ± 0.01 Slag Cristobalite Slag Slag 0.11 99.3 Cristobalite Slag Slag 0.03 13.9 ± 0.48 99.6 55.8 ± 0.58 Composition, mol.% PbO/Pb SiO2 /Si Cristobalite Slag Phase (Continues) 1.0* 99.99 ± 0.01 56.0 ± 0.31 99.0 ± 0.05 0.50* 100.0 ± 0.01 32.9 ± 0.29 98.7 ± 0.12 0.06* 100.0 ± 0.01 29.2 ± 0.19 1.0* 54.9 ± 0.17 99.8 ± 0.08 0.81* 34.4 ± 1.0 99.0 ± 0.26 0.13* 9.4 ± 0.13 99.2 ± 0.09 0.30* 20.6 ± 0.39 99.6 ± 0.06 0.41* 42.2 ± 0.41 99.6 ± 0.11 0.42* 29.3 ± 0.27 99.4 ± 0.27 0.35* 30.3 ± 0.61 NiO/Ni ABDEYAZDAN et al. 1389 Substrate (Continued) Mixture name Ir wire ampoule SiO2 ampoule SiO2 Au foil PNS20 NiSi2 NiSi0 PNS8 Argon Air Air Air ampoule 780 – 1705 650 – – – Pre-cool T, ◦ C 730 1740 1703 1660 Final equilibration T, ◦ C 120 1 0.4 2 Time, h 6.3 ± 0.06 0.12 ± 0.10 2.0 ± 0.21 51.9 ± 0.38 0.9* 1.3 ± 0.16 53.2 ± 0.22 99.6 ± 0.10 0.51* 3.3 ± 0.17 38.7 ± 0.72 NiO/Ni Zn ). Real solubility of NiO in 53.4 ± 0.32 40.3 ± 0.32 Barysilite 62 58.0 ± 0.21 41.9 ± 0.21 – Slag2 Slag – 48.1 ± 0.38 98.0 ± 0.21 Slag1 61 99.1 Cristobalite – – Slag 1 – 0.02 ± 0.02 Ni metal 98.7 ± 0.16 0.40 ± 0.11 99.5 Cristobalite 46.8 ± 0.22 0.01 95.7 ± 0.21 Slag 2 6.3 ± 0.19 0.99 ± 0.05 55.0 ± 0.68 Composition, mol.% PbO/Pb SiO2 /Si Slag 2 Slag 1 Phase *NiO in quartz/tridymite/cristobalite is an artefact of secondary x-ray fluorescence, similar to other elements with high energy of characteristic lines (e.g., Fe, Cu quartz/tridymite/cristobalite is expected to be <0.1%. 59 Slag + barysilite 58 Slag 1 + slag 2 57 56 1675 Pre-melt Atmosphere T, ◦ C Slag 1 + slag 2 + cristobalite (SiO2 ) + Pb–Ni metal No. TA B L E 2 1390 ABDEYAZDAN et al. ABDEYAZDAN et al. 1391 (A) (B) (C) (D) (E) (F) F I G U R E 1 Back-scattered electron micrographs of typical phase assemblages in the PbO–NiO–SiO2 system illustrating: (A) liquid slag, monoxide (NiO), and barysilite (Pb8 NiSi6 O21 ) equilibrium at 735◦ C; (B) liquid slag, monoxide, and olivine (Ni2 SiO4 ) equilibrium at 800◦ C; (C) liquid slag, monoxide, and massicot (PbO) equilibrium at 800◦ C; (D) liquid slag, tridymite (SiO2 ), olivine, and Pb metal equilibrium at 900◦ C; (E) liquid slag, tridymite, Ni metal, and Pb metal equilibrium at 1150◦ C; and (F) liquid slag 1, liquid slag 2, cristobalite (SiO2 ), and Ni metal equilibrium at 1660◦ C. The labels indicate the experiment number, sample name, and experimental temperature as given in Table 2. Following equilibration, the samples were rapidly quenched in calcium chloride brine at −20◦ C, washed with water and ethanol, dried, and mounted in epoxy resin. Polished cross-sections were prepared using conventional metallographic techniques. The samples were examined using optical microscopy and then carbon-coated. The compositions of the phases were measured using electron probe x-ray microanalysis (EPMA) (JEOL 8200 L EPMA; Japan Electron Optics Ltd.). The EPMA was operated with a probe current of 20 nA and acceleration voltage of 15 kV. The Duncumb–Philibert atomic number, absorption, and fluorescence correction (ZAF correction) was applied. 1392 ABDEYAZDAN et al. F I G U R E 2 Phase diagram of the NiO–SiO2 system with new experimental results at 1655◦ C–1740◦ C and lines estimated using the thermodynamic database, compared to previous model.23 (A) (B) (D) (C) F I G U R E 3 Calculated excess functions: (A) enthalpy, (B) entropy, (C) Gibbs energy, (D) heat capacity of mixing of the hypothetical supercooled single liquid in the NiO–SiO2 system at 1700◦ C relative to pure supercooled liquid components, compared to previous model.23 Wollastonite (CaSiO3 ) and Ni metal (supplied by Charles M. Taylor Co.) and PbO–SiOO2 K456 glass (71.4 wt.% PbO, supplied by NIST) standards were used for Si, Ni, and Pb calibration of the EPMA, respectively. The equilibrated samples were checked for Fe and Al impurities from the Kanthal wires; measurements in the areas containing these elements were rejected. Only the concentrations of metal cations were measured with EPMA. According to literature and authors’ previous studies,27–29 all cations in this system (Pb2+ , Ni2+ , Si4+ ) were expected to have the same valences in slag regardless of conditions being oxidizing (air) or reducing (equilibrium with metal). Therefore, the results from both conditions can be presented on the same liquidus phase diagram. The presence of Pb–Ni ABDEYAZDAN et al. TA B L E 3 1393 Thermodynamic parameters of the PbO–NiO–SiO2 system optimized in the present study (J/mol). Excess parameters in liquid slag Excess interaction parameters Value, J/mol Model 0 Δ𝑔Pb 2+ ,Ni2+ 12 970.4 +17.3636 T MQM (site) 9,0,1 𝑞Si 4+ ,Pb2+ ,Ni2+ 3,0,1 𝑞Si 4+ ,Pb2+ ,Ni2+ 3,0,2 𝑞Si 4+ ,Pb2+ ,Ni2+ 1,0,1 𝑞Si 4+ ,Pb2+ ,Ni2+ 0,0,1 𝑞Si 4+ ,Pb2+ ,Ni2+ 0 Δ𝑔Si 4+ ,Ni2+ 3,0 𝑞Si 4+ ,Ni2+ 7,0 𝑞Si 4+ ,Ni2+ 0,1 𝑞Si 4+ ,Ni2+ 9,0 𝑞Si 4+ ,Ni2+ 0 Δ𝑔Si 4+ ,Pb2+ 0,1 𝑞Si4+ ,Pb2+ 0,2 𝑞Si 4+ ,Pb2+ 1,0 𝑞Si 4+ ,Pb2+ 2,0 𝑞Si 4+ ,Pb2+ 3,0 𝑞Si 4+ ,Pb2+ 5,0 𝑞Si4+ ,Pb2+ 7,0 𝑞Si 4+ ,Pb2+ 9,0 𝑞Si4+ ,Pb2+ 11,0 𝑞Si 4+ ,Pb2+ 13,0 𝑞Si 4+ ,Pb2+ 15,0 𝑞Si 4+ ,Pb2+ 12 970.4 MQM (pair) −5230 −12.552 T MQM (pair) −12 970.4 MQM (pair) −101 252.8 +62.76 T MQM (pair) −6694.4 +1.6736 T MQM (pair) −3305.36 +7.9496 T MQM (site) 73 638.4 MQM (site) 299 867.28 –121.336 T MQM (site) 7322 MQM (site) 10 627.36 Bragg–Williams −19 939.4 −0.901 T MQM (pair) −7860.1 MQM (pair) 6599.9 MQM (pair −5966.4 MQM (pair) 17 714.5 MQM (pair) 20 138.4 MQM (pair) −1186.9 MQM (pair) −2448.9 MQM (pair) 6461.7 MQM (pair) −1925.8 MQM (pair) −2081.9 MQM (pair) 12 627.2 MQM (pair) Excess parameters in liquid metal Excess interaction parameters Value, J/mol Model 0 Δ𝑔Ni,O −299 574.4 −2.9288 T MQM (pair) 1,0 𝑞Ni,O 8,0 𝑞Ni,O 0,1 𝑞Ni,O 0,2 𝑞Ni,O 0 𝐿Pb,Si 0 Δ𝑔Pb,O 1,0 𝑞Pb,O 3,0 𝑞Pb,O 0,1 𝑞Pb,O 0,0,1 𝑞Pb,O,Ni 0 𝐿Ni,Pb 1 𝐿Ni,Pb 2 𝐿Ni,Pb 3 𝐿Ni,Pb 0 Δ𝑔Ni,Si 0 Δ𝑔Si,O 1,0 𝑞Si,O 23 430.4 +1.6736 T MQM (pair) 33 890.4 −10.46 T MQM (pair) 20 920 MQM (pair) 92 048 −16.736 T MQM (pair) 17 154.4 Bragg–Williams −336 937.52 +80.835 T −8.24248 Tln(T) MQM (pair) 175 728 −75.312 T MQM (pair) −105 855.2 +54.392 T MQM (pair) 41 840 MQM (pair) 29 288 MQM (pair) 14 470.553 −3.104133 T Bragg–Williams 2685.23 −0.25543 T Bragg–Williams −8680.7633 +5.87333 T Bragg–Williams −580.57 Bragg–Williams −44 936.16 +6.94544 T MQM (pair) −342 878.8 +17.9912 T MQM (pair) 41 840 MQM (pair) (Continues) 1394 TA B L E 3 ABDEYAZDAN et al. (Continued) Excess parameters in solid (FCC) metal Excess interaction parameters Value, J/mol Model 0 𝐿Ni,Si −238 488 +50.208 T Bragg–Williams 1 𝐿Ni,Si 0 𝐿Ni,Pb −28 451.2 Bragg–Williams 19 691.39 +36.05 T Bragg–Williams Endmember in liquid slag Endmember ΔH𝟎 298 , J/mol S𝟎 298 , J/mol K Temperature range, K Cp (T), J/mol K SiO2 −906 244.805 43.730 0–50 0.0003T3 −8.03274×10−6 T4 +6.14328×10−8 T5 50–298.15 −1.58728+0.130883T+0.000801T2 −4.05645×10−6 T3 +5.54804×10−9 T4 298.15–3500 −209.89−0.155843T+9.22876×10−6 T2 −1 384 359.64T−2 −2886.8039T−1 +11.663394T0.5 +2130.928475T−0.5 3500–4000 82.8079 0–50 0.0161T2 +7.15644×10−5 T3 −1.08544×10−5 T4 +1.16012×10−7 T5 50–400 −5.90711+0.618113T−0.003429T2 +9.98691×10-6 T3 −1.04244×10-8 T4 400–4000 64.9984 0–50 1.92258×10−5 T3 +9.62836×10−8 T4 50–298.15 −6.43526+0.150407T+0.000996T2 −4.91334×10−6 T3 +6.04796×10−9 T4 298.15–600 −142.44945+0.64610016T+3 531 493T−2 −0.000511168T2 600–4000 71 PbO NiO −200 875.5642 −202 570.045 75.4 42.78 Endmember in liquid metal Endmember ΔH0 298 , J/mol S0 298 , J/mol K Temperature range, K Cp (T), J/mol K Ni 18 253.76059 42.41963 0–50 0.007T+3.48508×10−5 T3 −3.87456×10−7 T4 +5.32392×10−9 T5 50–298.15 −11.3231+0.375198T−0.001541T−2 +2.95524×10−6 T3 −2.13808×10−9 T4 298.15–1728 22.096+0.0096814T+1.60574×10−19 T6 1728–5000 43.1 0–298.15 0.171588T−0.000273T−2 298.15–600.65 24.5242+0.007318T+1.4637×10−6 T−2 +2.52605×10−17 T6 600.65–1200 32.4914−0.003092T 1200–5000 18.9641+0.005766T−5.88864×10−7 T2 +5 393 509.8T−2 Pb O Si 4672.912535 130 747.228 50 200.00062 72.53827 32.3 48.61229 5000 33.2876 0–298.15 0.000433T2 −7.75504×10−7 T3 298.15–2990 24.5+0.001968T+7.76287×10−7 T2 −645 034.4T−2 2990–5000 36.2 0–1685 22.8092+0.003871T−352 992.8T−2 1685–5000 27.196 (Continues) ABDEYAZDAN et al. TA B L E 3 1395 (Continued) Endmember in liquid slag Temperature range, K ΔH𝟎 298 , J/mol S𝟎 298 , J/mol K Endmember ΔH0 298 , J/mol S0 298 , J/mol K Temperature range, K Cp (T), J/mol K Ni 1839.06979 33.02261 0–50 0.007T+3.48508×10−5 T3 −3.87456×10−7 T4 +5.32392×10−9 T5 50–298.15 −11.3231+0.375198T−0.001541T2 +2.95524×10−6 T3 −2.13808×10−9 T4 298.15–1728 22.096+0.009681T 1728–3000 43.1−1.01479×1033 T−10 3000–5000 43.0828 0–50 0.041198T2 −0.000780T3 −2.72169×10−6 T4 +1.04927×10−7 T5 50–298.15 25.1899+8.49961×10−5 T+1.94497×10−5 T2 −10009.3586T−2 298.15–600.65 24.5242+0.007318T+1.4637×10−6 T2 600.65–1200 32.4914−0.003092−7.2508×1027 T−10 1200–5000 18.9641+0.005766T-5.88864×10−7 T2 +5 393 509.8T−2 −7.2508×1027 T−10 5000 33.2876 24.5+0.001968T+7.76287×10−7 T2 −645 034.4T−2 Endmember Cp (T), J/mol K Endmember in solid (FCC) metal (D = 0.52, TC = 633, p = .28) Pb 0 64.78505 O 129 747.228 29.2 0–2000 2000–5000 31.3807 Si 50 999.99806 40.61963 0–1687 22.8318+0.003826T+2.1312×10−8 T2 −353 334T−2 1687–3600 27.196+3.78 332×1032 T−10 3600–5000 27.197 0 0 Compound ΔH 298 , J/mol S 298 , J/mol K Temperature range, K Cp (T), J/mol K SiO2_ Quartz(l) −910 700 41.46 0–50 0.000122017T3 −1.51256×10−6 T4 50–298.15 −3.5035+0.16874T+0.00048566T2 −2.96634×10−6 T3 +4.20792×10−9 T4 298.15–373 80.012−3 546 684T−2 −240.276T−0.5 +491 568 369T−3 373–848.02 80.012+0.00844T−3 546 684T−2 −4.52127×10−5 T2 +6.0550446×10−8 T3 −240.276T−0.5 +491 568 369T−3 848.02–3000 80.012−3 546 684T−2 −240.276T−0.5 +491 568 369T−3 0–50 0.000122017T3 −1.51256×10−6 T4 50–298.15 −3.5035+0.16874T+0.00048566T2 −2.96634×10−6 T3 +4.20792×10−9 T4 298.15–3000 80.012−3 546 684T−2 −240.276T−0.5 +491568369T−3 0–50 0.0003T3 −8.03274×10−6 T4 +6.14328×10−8 T5 50–298.15 −1.587+0.13088T+0.000801248T2 −4.05645×10−6 T3 +5.54804×10−9 T4 298.15–3000 75.3727−5 958 095T−2 +958 246 123T−3 SiO2_ Quartz(h) SiO2_ Tridymite(h) −908 627 −907 045 44.2068 45.5237 (Continues) 1396 TA B L E 3 ABDEYAZDAN et al. (Continued) Endmember in liquid slag Endmember ΔH𝟎 298 , J/mol S𝟎 298 , J/mol K Temperature range, K Cp (T), J/mol K SiO2_ Cristobalite(h) −906 377 46.0288 0–50 0.0003T3 −8.03274×10−6 T4 +6.14328×10−8 T5 50–298.15 −1.587+0.13088T+0.000801248T2 −4.05645×10−6 T3 +5.54804×10−9 T4 298.15–3000 83.5136−2 455 360T−2 −374.693T−0.5 +280 072 194T−3 0–50 1.91456×10−5 T3 +9.62837×10−8 T4 50–298.15 −6.4443 +0.15074T+0.0009913T2 −4.97177×10−6 T3 +6.00761×10−9 T4 298.15–2230 514.27+0.39759T+5 035 500T−2 −3.4982×10−5 T2 −68 525T−1 −24.001T−0.5 0–50 0.28711T−0.000778629T2 +2.5234×10−5 T3 50–298.15 −13.19+0.57007T+0.000197912T2 −1.95592×10−6 T3 298.15–3000 214.997−4 944 530T−2 −1030.75T−0.5 +623 705 000T−3 0–50 0.0155T2 +8.727016×10−5 T3 −1.153202×10−5 T4 +1.223595×10−7 T5 50–298.15 5.401+0.16529T+0.00144058T2 −1.352167×10−5 T3 +4.12124×10−8 T4 −4.425483×10−11 T5 298.15–1159 47.639+0.012255T−45 546 016T−3 −65.75326T−0.5 1159–3000 64.998−120 000T−2 −7 826 793 210T−3 0–50 0.0160264T2 +2.434455×10−5 T3 −8.428563×10−6 T4 +8.541893×10−8 T5 50–298.15 5.028+0.25756T−0.000225791T2 −2.078854×10−6 T3 +7.11424×10−9 T4 −7.259215×10−12 T5 298.15–1159 47.639+0.012255T−45 546 016T−3 −65.75326T−0.5 1159–3000 64.998−120 000T−2 −7 826 793 210T−3 0–50 0.001109T3 −2.82901×10−5 T4 +2×10−7 T5 50–298.15 −1.9176+0.591015T−0.00141064T2 +1.66112×10−6 T3 −3.6643×10−10 T4 298.15–1200 −35.227−0.13966T+9.6807T0.5 NiO −237 393 42.6585 (D = 0.933, TN = 523, p = .28) Ni2 SiO4 −1 394 775 128.3 (D = 0.75, TN = 29.15, p = .28) PbO_litharge_(red)(l) PbO_massicot_ (yellow) (h) PbSiO3 Pb2 SiO4 Pb3 Si2 O7 _Pb_ barysilite −219 268 −218 062 −1 149 128 −1 369 422 −2 518 645 67.14 68.699 110.688 184 294.588 1200–3000 132.5303 0–50 0.0025585T3 −7.21063×10−5 T4 +5.6×10−7 T5 50–298.15 6.297+0.846782T−0.00187134T2 +1.52262×10−6 T3 +5.46982×10−10 T4 298.15–1200 −24.98−0.17976T+12.475T0.5 0–50 0.0036675T3 −0.0001004T4 +7.6×10−7 T5 50–298.15 4.3796+1.437796T−0.003282T2 +3.18374×10−6 T3 +1.80552×10−10 T4 298.15–1200 −60.207−0.31942T+22.1557T0.5 (Continues) ABDEYAZDAN et al. TA B L E 3 1397 (Continued) Endmember in liquid slag Endmember ΔH𝟎 298 , J/mol S𝟎 298 , J/mol K Temperature range, K Pb5 SiO7 −2 007 200 408.72 0–50 0.0480792T2 +0.00263153T3 −9.7392×10−5 T4 +8.16257×10−7 T5 50–298.15 21.381+1.61947T−0.00254871T2 −4.713942×10−6 T3 +2.18897×10−8 T4 −2.17776×10−11 T5 298.15–1400 76.9129−0.1478776T+13.98408T0.5 0–50 0.080132T2 +0.00779722T3 −0.000258462T4 +2.107095×10−6 T5 50–298.15 44.0312+3.82816T−0.006743T2 −5.82641×10−6 T3 +3.72121×10−8 T4 −3.62961×10−11 T5 298.15–1300 57.142−0.57953T+43.733316T0.5 0–50 0.00957222T3 −0.000257277T4 +1.92×10−6 T5 50–298.15 −1.5202+4.20836T−0.0083939T2 +4.71795×10−6 T3 +5.635854×10−9 T4 298.15–1200 323.402−0.52057T+5 035 500T−2 −3.4982×10−5 T2 +39.6718T0.5 −68 525T−1 Pb11 Si3 O17 Pb8 NiSi6 O21 −5 204 405 −7 587 000 895.5 854 Cp (T), J/mol K Note: MQM (site): modified quasichemical model expressed as polynomial in terms of site fractions; MQM (pair): modified quasichemical model expressed as polynomial in pair fractions. T A B L E 4 The invariant reactions in the NiO–SiO2 , PbO–NiO, and PbO–NiO–SiO2 systems calculated by FactSage using the database developed in the present study. Reaction L2 = L1 + SiO2 (cristobalite), monotectic Tmodel , ◦ C 1703 mol.%, liquid PbO NiO SiO2 0 1.7 98.3 0 53.3 46.7 L = NiO (monoxide) + SiO2 (cristobalite) 1655 0 55.5 44.5 NiO (monoxide)+ SiO2 (cristobalite) = Ni2 SiO4 (olivine) 1551 11.9 32.3 55.8 L = NiO (monoxide) + PbO (massicot) 883 99.20 0.80 0 L + NiO (monoxide) = Pb8 NiSi6 O21 (barysilite), saddle point 783 56.3 0.90 42.8 L + NiO (monoxide) = Pb8 NiSi6 O21 (barysilite) + Ni2 SiO4 (olivine) 767 50.6 0.99 48.4 L = Pb8 NiSi6 O21 (barysilite) + Ni2 SiO4 (olivine) + PbSiO3 758 49.4 0.91 49.7 L + Pb8 NiSi6 O21 (barysilite) = NiO (monoxide) + Pb2 SiO4 738 67.5 0.50 32.0 L + PbO (massicot) = Pb5 SiO7 733 79.1 0 20.9 L + PbO (massicot) = NiO (monoxide) + Pb5 SiO7 731 78.7 0.36 20.9 L = SiO2 (quartz) + PbSiO3 728 77.1 0.36 22.6 L + Pb5 SiO7 = Pb11 Si3 O17 726 77.3 0 22.7 L = SiO2 (quartz) + Ni2 SiO4 (olivine) + PbSiO3 725 40.6 0.62 58.8 L + Pb5 SiO7 = Pb11 Si3 O17 + NiO (monoxide) 724 77.1 0.36 22.6 L = PbSiO3 + Pb2 SiO4 720 59.5 0 40.5 L = Pb8 NiSi6 O21 (barysilite) + PbSiO3 + Pb2 SiO4 720 59.5 0.08 40.5 L = Pb2 SiO4 + Pb11 Si3 O17 719 74.1 0 25.9 L = NiO (monoxide) + Pb2 SiO4 + Pb11 Si3 O17 718 73.8 0.37 25.9 1398 ABDEYAZDAN et al. F I G U R E 4 Calculated (A) enthalpy and (B) entropy of melting of the NiO monoxide to liquid, and (C) heat capacity of solid and supercooled liquid NiO, compared to literature.55–60 (A) (B) (C) metal (liquid or solid) would provide additional information on thermodynamic behavior of the system (slag-metal distributions). However, the Pb-rich liquid metal is corrosive to the available metallic substrates (Au or Pt–Ir), and therefore this option was only used for experiments in SiO2 ampoules (quartz/tridymite/cristobalite primary phase fields). Achievement of equilibrium in the samples was ensured by using the four-point test approach26,30 : (i) variation of equilibration time, (ii) assessment of the compositional homogeneity of phases by EPMA, (iii) approaching the final equilibrium point from different starting conditions, and (iv) consideration of reactions specific to this system that may affect the achievement of equilibrium or ABDEYAZDAN et al. 1399 to expected from the initial bulk mixtures were accepted. Also, formation of certain solids (e.g., olivine Ni2 SiO4 from monoxide NiO + cristobalite SiO2 , or any solids at low temperatures from viscous slags) could be problematic due to small thermodynamic driving force, therefore equilibration was followed from the pre-annealing of sample at the temperature higher or lower than the target equilibration temperature, to exclude potential metastability. (A) (B) F I G U R E 5 Calculated (A) enthalpy and (B) entropy of melting of the Ni2 SiO4 olivine to liquid (metastable relative to monoxide and cristobalite), compared to literature for the Ni2 SiO4 ,49,54 Mn2 SiO4 ,50 Fe2 SiO4 ,51,52 Co2 SiO4 ,49 and Mg2 SiO4 53 olivine–liquid transitions. 3 The experimental study was conducted parallel with a thermodynamic database development describing the system. The database was used in conjunction with the FactSage 7.2 thermodynamic package31 to assist the selection of sample compositions used in the experimental study. As new experimental data were obtained, the thermodynamic parameters were progressively optimized and incorporated in the revised database. The slag phase was described using a modified Quasichemical formalism32–35 that combines the Gibbs energy parameters of three types: (i) the Bragg–Williams model, a polynomial in terms of pure component concentrations for the random mixing configurational entropy; (ii) the Quasichemical model, which assumes the formation of A–A, B–B, A–B pairs with the Gibbs free energies that are a polynomial function of the overall composition; and (iii) the Quasichemical model, which assumes the formation of pairs with the Gibbs free energy that are polynomial functions of pair fractions. The binary parameters were extrapolated into the corresponding ternary system using a geometric formalism with the Toop (asymmetrical) approach,36,37 where the acidic component SiO2 was placed at the top apex of the Toop model triangle, while the basic PbO and NiO were placed at its base. 4 4.1 F I G U R E 6 Calculated heat capacity of Ni2 SiO4 olivine and supercooled liquid, compared to literature for liquid Ni2 SiO4 ,54 Mn2 SiO4 ,50 and Fe2 SiO4 .51 reduce the accuracy. Among the latter reactions, the one of primary concern was evaporation of lead oxide. Several separate areas were tested in each sample in open substrate, and if any areas were found to contain unreasonably reduced PbO concentration, they were rejected, while those with gradient-free slag and PbO concentrations close THERMODYNAMIC MODELING RESULTS AND DISCUSSION Experimental results Figure 1 shows micrographs of the quenched PbO– NiO–SiO2 samples at selected temperatures and bulk compositions illustrating the phase assemblages observed on equilibration. Examples of microstructures are presented for liquid slag–monoxide–barysilite equilibrium at 735◦ C in Figure 1A liquid slag–monoxide–olivine at 800◦ C in Figure 1B, liquid slag–monoxide–massicot at 800◦ C in Figure 1C and liquid slag–tridymite–Ni metal–Pb metal equilibrium at 1150◦ C in Figure 1E. Note that, Ni oxide has a very steep liquidus at low temperature and 1400 ABDEYAZDAN et al. F I G U R E 7 Liquidus projection of the PbO–NiO–SiO2 system according to the new experimental results at 720◦ C–1703◦ C in air or in equilibrium with liquid metal (as given in Table 2), and estimated with the thermodynamic database, where; PS = PbSiO3 , P8NS6 = Pb8 NiSi6 O21 , P2S = Pb2 SiO4 , P11S3 = Pb11 Si3 O17 , P5S = Pb5 SiO7 , and N2S = Ni2 SiO4 . The experimental points shown with crossline markers indicate the liquid slag composition at the target temperature. the viscosity of slag is quite high. Therefore, NiO has low chance to recrystalize into bigger regular-shaped particles and predominantly stays as conglomerate of the original mixture particles (Figure 1A–C). So, in the case of barysilite, it formed after NiO particles were already present, and so that is why NiO particles are included in large barysilite particles. The experimental results for the ternary PbO–NiO–SiO2 liquidus are given in Table 2. 4.2 Thermodynamic modeling results The updated thermodynamic parameters of quartz, tridymite, and cristobalite (SiO2 ), monoxide (NiO), olivine (Ni2 SiO4 ), litharge and massicot (PbO), lead silicates (PbSiO3 , Pb2 SiO4 , Pb11 Si3 O17 , Pb5 SiO7 ), and the parameters of the binary PbO–SiOO2 liquids were taken from the parallel studies at PyroSearch,38 ensuring physically reasonable description of these phases down to 0 K and improved description of heat capacities of liquid slag endmembers. The NiO–SiO2 binary system published before23 was reviewed in the present study as the models of binaries have changed significantly since 2013. Therefore, new optimization was required, supported particularly by experiments at higher temperatures (1655◦ C–1740◦ C) to reach previously unstudied phase assemblages—two immiscible liquids and monoxide primary phase field. The melting point of NiO is increased from 1955◦ C to 1990◦ C in the new version of the model. Both values ABDEYAZDAN et al. 1401 F I G U R E 8 Details of the high-PbO region of the liquidus projection of the PbO–NiO–SiO2 system according to the new experimental results at 720◦ C–1703◦ C in air or in equilibrium with liquid metal (as given in Table 2) and estimated with the thermodynamic database. The phase boundaries and liquidus isotherms are estimated using the thermodynamic database, where; PS = PbSiO3 , P8NS6 = Pb8 NiSi6 O21 , P2S = Pb2 SiO4 , P11S3 = Pb11 Si3 O17 , and P5S = Pb5 SiO7 . The experimental points shown with crossline markers give the liquid slag composition at the target temperature. appear in the literature.39–42 There is uncertainty due to the probable mechanism of NiO melting with decomposition to Ni–NiO metal + O2 gas, which may result into underestimated “true” congruent melting point of NiO. The updated database calculation shows that if O2 loss is considered, then NiO melts with decomposition to Ni–O metal + O2 at 1970◦ C under 1 atm O2 and at 1947◦ C in air (0.21 atm O2 ). Therefore, no contradiction to literature is introduced by the increased Tmelt (NiO), as the observed melting points with O2 loss might vary depending on the atmosphere. The phase equilibria in the multicomponent slags (particularly the slope of liquidus of NiO when combined with SiO2 , CaO, MgO, Al2 O3 , Fe2 O3 ) should be the main criteria to selecting the “true” congruent Tmelt (NiO), and the new value (1990◦ C) improves the description of those phase equilibria (to be published elsewhere). The new optimization and review of this system resulted in improved binary and ternary miscibility gap description. In particular, (a) the monoxide (NiO) liquidus at 1700◦ C has improved; (b) the monoxide-cristobalite eutectic has increased from 1650◦ C to 1655◦ C, in agreement with the new experiments; (c) the monotectic temperature has increased from 1675◦ C to 1703◦ C and the composition of the high-SiO2 liquid changed to more SiO2 -rich, also supported by the experiments of the present study (Figure 2). The enthalpy of mixing of the NiO–SiO2 liquid is modeled to be positive, responsible for the miscibility gap (Figure 3). The difference from the previous model23 is more smooth shape of thermodynamic functions of mixing, resulting in a miscibility gap that is disappearing more readily on addition of the third component (PbO). 1402 ABDEYAZDAN et al. F I G U R E 9 An isothermal section of the calculated phase diagram using the database developed in this study along with the experimental results at 1100◦ C, where N2S = Ni2 SiO4 . The experimental points shown with crossline markers indicate the liquid slag composition at the target temperature. The thermodynamic properties of solid and supercooled liquid NiO were revised to make them more consistent with the Third Law and avoiding the Kauzmann Paradox43,44 (Figure 4). The heat capacity of supercooled liquid NiO was increased from 67 to 71 J/mol K above 600 K. The value of 67 J/mol K is believed to be mistakenly assigned by45 similar to molten FeO (the only chemically similar oxide with low melting point for which actual calorimetric data exist). However, that value by Coughlin et al.46 was actually 68.2 J/mol K, and not for FeO but for Fe0.947 O, which gives over 70 J/mol K when normalized to two atoms instead of 1.947 per formula. In addition, partial heat capacities of all divalent oxides measured in slags by Stebbins et al.47 and Lange and Navrotsky48 were all above 80 J/mol K, which gives more probability in favor of higher values for supercooled liquid NiO. There is limited information in literature on thermodynamic functions of melting for the Ni2 SiO4 olivine. Sugawara and Akaogi49 measured the enthalpies of cooling of liquids along the CaMg0.5 AlSi2 O7 –Ni2 SiO4 composition line to room temperature by drop calorimetry, and extrapolated that to the enthalpy of metastable melting of pure Ni2 SiO4 to be 221 kJ/mol, and the entropy of melting 115 J/mol K. This value seems to be overestimated, as it poorly compares to the entropies of melting of all other related olivines—Mn2 SiO4 ,50 Fe2 SiO4 ,51,52 Co2 SiO4 ,49 and Mg2 SiO4 .53 The present thermodynamic model fits qualitatively all these values (Figure 5). Its difference from the previous model23 is Third-Law consistency—tendency ABDEYAZDAN et al. 1403 (A) (B) F I G U R E 1 0 Phase diagram of the NiO–PbO system with new experimental results at 1000◦ C–1200◦ C and lines estimated using the thermodynamic database, (A) phase diagram at 800◦ C–2000◦ C, and (B) a closer view at 850◦ C–1250◦ C. F I G U R E 1 1 Composition of Ni–Pb liquid metal in equilibrium with quartz/tridymite/cristobalite, liquid slag, and olivine or solid (fcc) Ni metal: experimental results at 800◦ C–1660◦ C and lines calculated with the new thermodynamic database. The labels legend is as given in Figure 7. of the entropies of both solid and supercooled liquid to near-zero values at absolute zero temperature. This is achieved by a consistently higher heat capacity of the supercooled liquid compared to solid (Figure 6), which is more in agreement with available literature for liquid Ni2 SiO4 54 (estimated), and chemically related Mn2 SiO4 50 and Fe2 SiO4 51 (directly measured). The parameters for the PbO–NiO binary, PbO–NiO– SiO2 ternary liquid were introduced for the first time based on the experimental results of the present study. Several ternary parameters with high powers on SiO2 were needed to describe the shape of the miscibility gap correctly. These were needed presumably due to lack of binary parameters with powers higher than 15, which did not allow sufficiently accurate description of the binary systems NiO– 1,0,1 0,0,1 SiO2 and PbO–SiOO2 . The 𝑞Si 4+ ,Pb2+ ,Ni2+ and 𝑞Si4+ ,Pb2+ ,Ni2+ parameters work in the central part of the diagram and describe the monoxide, olivine, and tridymite liquidus. Note that their values have been significantly reduced compared to our earlier attempts before reoptimization of the NiO–SiO2 binary and NiO melting point. Also, the Pb8 NiSi6 O21 solid phase was added to the database as a stoichiometric compound. The thermodynamic parameters are given in Table 3. The PbO–NiO–SiO2 phase diagrams calculated using the database developed in this study along with the experimental results are shown in Figures 7 and 8. Also, an isothermal section of the calculated phase diagram with the experimental points at 1100◦ C is given in Figure 9. In these figures, the experimental points indicate the liquid slag composition at the target temperature. As can be seen in Figures 7 and 8, the phase equilibria and liquidus isotherms in the quartz/tridymite/cristobalite (SiO2 ), olivine (Ni2 SiO4 ), monoxide (NiO), Ni-barysilite (Pb8 NiSi6 O21 ), massicot (PbO), and di-lead silicate (Pb2 SiO4 ) primary phase fields have been measured, and the extent of the high-SiO2 two-liquid immiscibility gap in equilibrium with cristobalite has been determined. The invariant reactions of the system calculated by FactSage using the database developed in the present study is given in Table 4. The NiO–PbO phase diagram calculated using the database developed in this study and along with the present experimental results is shown in Figure 10. Only the low-temperature part of it has been studied (due to high volatility of PbO and low viscosity that prevents successful quenching above 1200◦ C). For metallurgical processes, it is important to consider the slag–metal equilibria. In this system, the PbO–NiO– SiO2 slag coexists with Pb–Ni–O metal of variable composition: the Pb/(Pb+Ni) ratio can change over the whole range from 0% to 100%, while O is limited to less than 1 mol.% in metal. Another feature of this system is presence of miscibility gap in the liquid Pb–Ni binary system, 1404 ABDEYAZDAN et al. (A) (C) (B) F I G U R E 1 2 Phase diagram of the Ni–Pb system in equilibrium with quartz/tridymite/cristobalite, liquid slag or olivine (A), a closer view to the eutectic point at high-Pb side of the phase diagram (B), and solubility of oxygen in the liquid metal (C), estimated using the thermodynamic database. which is only slightly affected by the presence of minor oxygen. Therefore, Figures 11 and 12 show the dependence of metal composition and phase assemblage (one liquid, two immiscible liquids, liquid + solid, etc.) on the slag composition and temperature projected on various axes. The measured mol.% Pb/(Pb+Ni) ratios in metal are presented as a function of mol.% PbO/(PbO+NiO) in slag for a series of quartz/tridymite/cristobalite isotherms (800◦ C– 1600◦ C) in Figure 11. At low temperatures (<1163◦ C), the highest %Ni that can be dissolved in liquid Pb-rich metal is limited by olivine (Ni2 SiO4 ) liquidus that limits %NiO in slag. At 1163◦ C–1350◦ C, solid Ni (fcc) forms on addition of NiO to PbO–SiOO2 slag before olivine liquidus is reached. A monotectic is reached in the Ni–Pb system at 1350◦ C, with formation of a second Ni-rich liquid metal (Figure 12). 5 CONCLUSIONS This study presents the first systematic study of liquidus in the PbO–NiO–SiO2 system and its Ni-containing sub- systems, as well as compositions of the metal (Ni–Pb liquid or Ni (fcc) solid) in equilibrium with some of the slags. Volatilization of PbO was a major issue limiting the temperatures that could be studied to only 1200◦ C in the PbO-rich corner, but the low-PbO compositions could be studied up to 1740◦ C. The experimental phase equilibria and liquidus data were measured in the quartz/tridymite/cristobalite, olivine, monoxide, Nibarysilite, massicot, and di-lead silicate primary phase fields and the extent of the high-SiO2 two-liquid immiscibility gap in equilibrium with cristobalite was determined. Further, only one ternary phase, Pb8 NiSi6 O21 barysilite, was found in the system. Through the thermodynamic modeling of the system, the parameters for the PbO–NiO binary and PbO–NiO–SiO2 ternary liquid were introduced for the first time based on the present results. Also, the NiO–SiO2 binary system was reviewed, which lead to improved binary and ternary miscibility gap description, and the Pb8 NiSi6 O21 solid phase was added to the database as a stoichiometric compound. The results of the study are merged into the multicomponent (Pb–Zn–Cu–Fe–Ca– Si–O–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni–Co–Na) mul- ABDEYAZDAN et al. 1405 tiphase thermodynamic database, and will allow accurate predictions of Ni impurities in PbO–SiOO2 -rich slags and Pb impurities in NiO-rich slags, both of which are expected to become more common in the industry due to increased proportion of low-grade feeds and recycling (particularly of batteries). AC K N OW L E D G M E N T S The authors would like to thank Australian Research Council Linkage program (LP180100028), and consortium of lead producers: Aurubis, Boliden, Kazzinc Ltd., Glencore, Nyrstar, Peñoles, and Umicore for the financial support for this study. The authors are very grateful to Prof. Peter Hayes for initiating and contributing in this research program. 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Ceram Int. 2021;47(15):20974–91. 1407 How to cite this article: Abdeyazdan H, Shevchenko M, Jak E. Experimental and thermodynamic modeling study of phase equilibria in the PbO–NiO–SiO2 system. J Am Ceram Soc. 2024;107:1383–1407. https://doi.org/10.1111/jace.19523