Separation behavior of arsenic and lead from antimony during vacuum distillation and zone refining

When classifying impurities in commercial pure antimony (Sb, 99.8%), arsenic (As) and lead (Pb) should be brought to the forefront consideration. Due to the known difficulty of As removal through zone refining, it is meaningful to investigate its separation tendency through alternative methodologies such as vacuum distillation, promoted by the large difference of their vapor pressures. Here, a series of vacuum distillation trials with different process parameters were at first conducted with the aim of As removal. Pb, as an always accompanying impurity, seemed to be able to be significantly separated from Sb, so that its content in the refined phase could be reduced too, e.g. from 1200 ppm to less than 30 ppm. The reduction of As, however, is highly dependent on the distillation ratio of Sb and hence limited just to 450 ppm. The biggest obstacle here was the simultaneous evaporation of Sb and As when using high temperature and low pressure. In order to suppress the evaporation of As more intensely in vacuum distillation or selectively capture As in zone refining process, the additives – aluminum (Al) and zinc (Zn) – were studied and selected by using the respective phase diagrams as well as thermochemical Software FactSage and then individually added to Sb as alloying elements with the aim of intermetallic formation with As. The addition of Al led to a considerable reduction of As in vacuum distillation as well as while zone refining process. During vacuum distillation, 67% less As was obtained in the condensate in comparison to the trial without additive. Meanwhile, a huge As concentration gradient appeared in the residual Sb. During zone refining process, As concentration in the whole bar was considerably reduced from 456 ppm to below 150 ppm after only one zone pass, due to the enrichment of Al at the end of the bar in accompanying with As in form of an intermetallic compound. The addition of Zn, on the contrary, did not convince as an effective improvement in purification of Sb. In order to achieve a higher efficiency of As removal from Sb, the authors at the first priority suggest an addition of Al directly into the zone refining process. If specifically a vacuum distillation process is preferred, a multi-stage condenser, equipped with controlled temperatures, attendant with the addition of Al in the charge material, can deliver effective results as well.