Study On Crystallization Refining Of Crude Bismuth

The actual refining process generally exists some problems, such as complicated process, low automation degree and serious environmental pollution. It is necessary to develop new processes of removing impurities or to improve actual process for crude bismuth, in order to solve the above-described shortcomings. In this paper, the removal of impurities and improvement of desilverizing by adding zinc for crude bismuth were investigated.A certain temperature gradient was established along the axial direction of crude bismuth melt, impurities were continuously crystallized, and enriched at the end of the bismuth ingot. The results show that, a certain temperature gradient from283℃to355℃is established along the axial direction of crude bismuth melt, copper and zinc are enriched at the top of the ingot, copper content can be increased to1.53%from0.6%, copper content at the bottom can reduce to0.17%where the copper removal rate is71.67%; while zinc content at the top of the ingot can be enriched to0.04%from0.01%, zinc content at the bottom can reduce to0.0004%where the zinc removal rate is96%. A certain temperature gradient from297℃to479℃is established along the axial direction of Bi-Zn melt with14.5%of zinc content, composition of melt was changed with the temperature gradient established, zinc is enriched at the top of melt where the zinc content is up to91.8%, and the content of zinc at the bottom can reduce to5.8%.The enrichment condition of lead and silver was investigated by directional crystallization in two different ways of melt elevated and descended. The results show that, lead and silver are enriched at the bottom when the bismuth melt is elevated, lead is enriched at the bottom, lead content can be increased to11.2%from2.12%, while silver content can be increased to0.55%from0.087%; otherwise, as the bismuth melt is descended, silver is enriched at the top, and silver content can be increased to0.12%from0.06%.In addition, the process of desilverizing was investigated by the means of low silver-bismuth melt passing through the liquid zinc zone, the effects of liquid zinc height, melt temperature and reaction time on desilverization was studied. The results show that the silver content in crude bismuth reduces from0.06%to0.001%and the removal rate of silver reaches98.01%under the conditions of liquid zinc height of12cm, melt temperature of500℃, reaction time of120min, under the abovementioned conditions, the silver content in crude bismuth reduces from0.18%to0.0033%and the removal rate of silver reaches93.45%; under the conditions of liquid zinc height of12cm, melt temperature of570℃, reaction time of120min, the silver content in crude bismuth reduces from0.18%to0.001%and the removal rate of silver reaches98.53%.