Liquid–Vapour Phase Transition and Thermodynamics of Antimony–Sodium Alloys

Crude antimony containing 8–9 wt.% sodium and, in some cases, up to 25–26 wt.% sodium is obtained during the carbothermic reduction of sodium antimonate through crucible smelting. The boundaries of vapour–liquid equilibrium on the Sb–Na phase diagram were constructed to assess the possibility of using distillation processes to refine crude antimony from sodium. The boundaries of the liquid–vapour phase transitions were calculated at atmospheric pressure and under vacuum conditions (133 Pa). The presence of the incongruently evaporating compound Na3Sb makes it possible to divide the binary Sb–Na system into two quasi-binary systems: Sb–Na3Sb and Na3Sb–Na. In this study, we established that the separation of antimony and sodium (the Sb–Na3Sb system) using distillation at atmospheric pressure is technically difficult because of the high boiling temperatures of the melts in the Sb–Na3Sb system (1635–2617 °C) and the very narrow temperature range of the vapour–liquid equilibrium field under vacuum. Molten sodium and trisodium antimonide (the Na3Sb–Na system) can be separated both at atmospheric pressure and under vacuum when the sodium concentration in antimony exceeds 90 at.% Na (approximately 63 wt.%). However, the first case requires temperatures of up to 1570 °C, whereas vacuum conditions require a temperature of approximately 600 °C. Under these conditions, the vapour phase will consist of almost pure sodium, while antimony will accumulate in the still residue in the form of Na3Sb. In this paper, we also present calculated thermodynamic functions, namely, the entropies and enthalpies of mixing and evaporation of antimony–sodium melts, which will supplement the existing database of physicochemical data.