Extraction of tellurium and high purity bismuth from processing residue of zinc anode slime by sulfation roasting-leaching-electrodeposition process

Abstract Anode slime and its processing residue contains a variety of valuable elements, which are the main source of tellurium and bismuth. A systematic study on the extraction of tellurium and bismuth from processing residue of zinc anode slime and electrodeposition to obtain high purity bismuth was carried out. Sulfation roasting thermodynamics, leaching thermodynamics, and reduction potential involved in the extraction were calculated and used as theoretical guidance for the experiment. In the process of extracting tellurium and bismuth: the minimum amount of sulfuric acid and the appropriate low temperature of sulfation roasting conditions for processing residue of zinc anode slime are explained that the ratio of sulfuric acid to anode slime was 0.54:1, and the temperature was 435 °C. The leaching process of the roasting product was optimized. The optimal water leaching conditions were a liquid to solid ratio of 6:1 and a reaction time of 0.5 h at 25 °C. The optimal alkali leaching condition is 10% NaOH liquid to solid ratio of 6:1 and reaction time of 0.5 h at 25 °C. Tellurium and bismuth were separated, and tellurium was enriched from a low content of anode slime raw materials. The recovery rate of tellurium and bismuth were 84.9% and 97.1%, respectively. The reduced deposition process of bismuth ions on the cathode was characterized by cyclic voltammetry in the nitric acid. In the stable deposition state, electrodeposition overpotential of the bismuth ion on the titanium plate was 0.1068 V. Tributyl phosphate (TBP) was used as extraction solvent and nitric acid solution was used as stripping solution to purify the impurities in the basic bismuth sulfate. After extraction process, the removal rates of tellurium (Te), arsenic (As), copper (Cu), and antimony (Sb) were 96.5%, 93.3%, 99.1% and 100%, respectively. The content of other impurity elements also reduced greatly. The electrical separation process further reduced the content of impurities in the electrolyte, in which the concentration of tellurium was reduced to less than 1 ppm. A standard-compliant high purity elemental bismuth was obtained by electrodeposition from purified electrolyte.