Separation of components of Zn–Pb alloys by enhanced gravity segregation

Abstract

A method has been developed, and tested on a laboratory scale, to purify Zn obtained by the electrothermic process when Pb is the main impurity. It is based on thermal cycling the alloy around the melting temperature, inducing the phenomenon of gravity segregation during each thermal cycle. Results are reported on the effect of the number of cycles, cycle shape, and Pb concentration on the efficiency of the purification process. The separate contributions of the solidification and melting stages of the cycles are analysed. Based on the experimental results and on metallographic observations of the specimens, three mechanisms are proposed to explain qualitatively the segregation phenomena observed: (i) sedimentation of Pb-rich liquid droplets in the immiscible liquids of the monotectic Zn–Pb phase diagram; (ii) convective liquid motion; and (iii) sedimentation of Pb-rich globules during the heating part of the cycles. It was found that the last mechanism has the highest separation efficiency for the alloy under study.