Metal directional crystallization: A sustainable green separation method for the purification of bismuth from crude bismuth while enriching lead and silver

Abstract

In the traditional crude bismuth refining process, the addition of additives to form refining slag and the subsequent removal of impurities via separating the refining slag introduce new impurities while lowering the direct yield of bismuth, thereby restricting the green development of bismuth smelters. In this study, we propose a method that uses directional crystallization of metals to purify crude bismuth and enrich lead and silver. The phase diagram was analyzed using equilibrium partition coefficients, and lead and silver impurities were primarily enriched in the liquid phase during the crystallization of crude bismuth. In addition, we investigated the ground-state structures of the clusters, calculated the average binding energies, and elucidated the tendency of Ag to be enriched with Pb rather than Bi during the directional crystallization of crude bismuth from an atomic perspective. Using small-scale experiments, we determined the optimum process conditions, which were a crystallization separator speed of 4 r/min and a 10° inclination of the crystallization separator toward the horizontal plane. The guided semi-industrial experiments yielded Bi with a purity of 98.08 wt%, bismuth with a direct recovery of 92.49%, and Pb and Ag with removal rates of 95.64% and 90.53%, respectively. The entire process is simple, involves no additives, and effectively avoids the generation of refining slag.