Separation behavior of As, Zn and Cd trace impurities in the deep vacuum purification process of refined lead

Abstract

Environmentally friendly vacuum distillation has been employed to purify refined lead (99.996%), aiming to separate low-boiling point trace impurities of As, Zn and Cd in refined lead and to provide a foundation for the preparation of high-purity lead (5 N) and ultrapure lead (6 N). In this paper, the low-boiling point trace impurities As, Zn and Cd in refined lead are removed by low-temperature vacuum distillation (LTVD). Detailed analysis of the initial and purified lead was performed by glow discharge mass spectrometry (GDMS) to reveal that Cd and Zn were evaporated as volatiles and were removed from the refined lead in a crucible. Correspondingly, the contents of Zn and Cd impurities decreased from 4 ppmw and 1 ppmw to 0.006 ppmw and 0.01 ppmw, respectively. In contrast to the removal efficiencies of Zn and Cd by LTVD, the removal efficiency of As impurities is only 4%. To cope with this, a novel high-temperature pyrolysis-zone condensation (HTP-ZC) vacuum purification process is proposed for further As separation. The HTP-ZC results indicate that As, Cd and Zn are condensed as residues and removed in the condensation stage. Removal efficiencies of 99.96% for As, 99.98% for Zn, and approximately 100% for Cd were achieved. This approach, with huge industrialization potential, pushes the direct yield of the product to 90% and effectively removes trace impurities with low boiling points, such as As, Cd and Zn, in refined lead. The proposed HTP-ZC process has the potential to stably, cleanly and efficiently produce high-purity lead and ultrapure lead.