Bismuth Dissolution from Smelter Flue Dust Residues

Effective bismuth removal from flue dust residues has been accomplished for flue dusts from both zinc and copper smelters. Selective dissolution of bismuth, with respect to silver and lead, can be achieved by the proper manipulation of temperature, sulfuric acid addition and sodium chloride addition. Optimum levels of these variables were determined using a response surface technique to maximize the objective function: Y = %Bi/(l + %Ag). In the case of the flue dust residue from a zinc smelter, which contains bismuth as an impurity (0.05%),86% bismuth removal can be realized with less than 1% combined silver and lead dissolution. Similarly, in the case offlue dustfrom a copper smelter, which contains bismuth as a primary constituent (3.3%),93% bismuth removal was obtained with little dissolution of silver and lead values. The processing of residues derived from various opera­ tions in the mining and metallurgical industry for byproduct recovery is becoming more prevalent. A principal target for byproduct recovery is flue dust produced by conventional smelting operations. Flue dust from zinc smelters has been processed for many years for the recovery of cadmium. i ,2 The cadmium recovery technique which consists of a selective sulfation roast, followed by a water leach and zinc dust cementation, produces a lead cake residue (see chemical analysis in Table 1), which consists primarily oflead sulfate. The high silver content, of approximately 100 per oz ton, makes lead cake a potentially valuable product. However, bismuth, even at levels of 0.05%, frequently limits the mar­ ketability of this material as lead smelter feed. In this and other systems, the selective removal of bismuth from such residues is of considerable interest. The intent of this investigation was to determine the condi­ tions under which selective bismuth removal from flue dust residues can be achieved. A hydrometallurgical approach for bismuth removal from lead cake was selected for detailed study.",4,5 In this particular case the objective was to selec­ tively dissolve the bismuth and minimize the extent of lead and silver dissolution. The low solubility of lead sulfate in sulfuric acid solution and the high stability of bismuth chloride complexes suggested that an acidic chloride leach might be effective for this purpose. Statistically designed, bench scale leaching experiments were initiated to test this hypothesis for lead cake processing. In addition, a cursory examination was made of the applicability of this technique for the treatment of copper smelter flue dust.