Behavior of major and minor elements during directional crystallization of Fe-Ni-Cu-S-(Rh, Ru, Ir, Pt, Pd, Ag, Au) melt

The Cu-Fe-Ni-S system is unique in terms of the number of crystalline phases with a variety of combinations of properties, which makes it relevant for prospective material studies. The phases of this system compose typical associations of massive zonal sulfide Cu-Ni ores, and their copper-rich zones are characterized by a high content of noble metals. Therefore, this system is among the most important of those used for the geochemistry of sulfides and for the metallurgy of copper and nickel. There is insufficient quantitative information on the equilibrium distribution coefficients of macrocomponents and the behavior of impurities upon crystallization of solid solutions in the region of the solid-melting diagram corresponding to natural ores or intermediate products of metallurgical production. Therefore, the goal of the work was to obtain new data on the phase diagram of the Cu-Fe-Ni-S system and corresponding phases of noble metals (Rh, Ru, Ir, Pt, Pd, Ag, Au) during the process of fractional crystallization of the melt simulating zonal copper-rich ores of platinum-copper-nickel sulfide deposits. We conducted quasi-equilibrium directional crystallization of the melt with a composition of (at. %): Fe 29.20, Ni 5.85, Cu 17.60, S 47 with addition of 0.05% of Rh, Ru, Ir, Pt, Pd, Ag, and Au. The obtained sample was studied using optical and scanning electron microscopy, energy-dispersive spectrometry (SEM/EDS), and X-ray phase analysis. Differential thermal analysis (DTA) was used to determine the liquidus temperatures along the crystallization path. The distribution of macrocomponents along the cylindrical ingot showed that it consisted of five primary zones. Primary phases and phase associations crystallized from the melt in the following sequence: mss / mss + iss / iss / iss + bnss / bnss + pnss, where mss is monosulfide solid solution (FexNi1-x)S1±y, iss is intermediate solid solution (Cu,Fe)S1-x, bnss is bornite solid solution Cu5±xFe1±xS4±y, and pnss is pentlandite solid solution (FexNi1–x)9±yS8. This indicated a complex structure of the solid-melting diagram in the studied region. We determined the crystallization temperatures of mss and iss. A new type of secondary (phase) zoning was identified, formed as a result of subsolidus transformations of primary phases, which can be present in Cu-Ni sulfide ores. It was found that impurities can dissolve in the main sulfide phases, form individual microphases in the sulfide matrix, or be present in these microphases in the form of solid solutions. The main concentrators of Pd were pn and sug. Ir, Rh, and Ru were distributed between mss and pn, and Ag preferred bnss. Most impurities of noble metals formed inclusions as independent microphases: RuS2, Pt3Fe, Au* gold-based alloy, Pt-Fe-Au alloy, CuIr2S4, and native Ag. The results of the work showed that the behavior of macrocomponents could be described using distribution coefficients, and the behavior of microcomponents did not strictly correspond to the classical theory of fractional crystallization of multicomponent melts with impurities