Behaviour of lead and selenium during sequential sodium carbonate and nitric acid leaching of slime generated by the sulphuric acid plant of the Balkhash copper smelter

Abstract

This paper examines the process of selenium precipitation from the slime generated by the sulphuric acid plant of the Balkhash Copper Smelter. Contrary to similar tailings generated by non-ferrous metals industry, such slime has a high concentration of selenium (12.5–4.6 wt. %). By utilizing it in the production process, one can raise the output of selenium by more than 30% compared with the amount of selenium recovered by the smelter from copper anode slimes. The slime that was used in the experiments had been separated from the slurry produced as a result of scrubbing of metallurgical gases generated by copper smelting and matte converting processes. The slime was rinsed with water to remove sulphuric acid from it and then dried at 105 oC. With the help of new-generation analytical equipment, the slime was analyzed for qualitative, quantitative and material compositions. 15 elements with significantly varying concentrations were detected. The main components include lead (51.17 wt. %) and selenium (12.49 wt. %), which are present in the form of lead sulphates and elemental selenium. Other lead and mercury compounds are also present as disseminations – i.e. different salts of selenium acids. Considering the forms in which lead and selenium are mainly found in the slime, lead was initially converted into carbonates by sodium carbonate leaching, and then nitric acid was used to make lead transfer from the cake into the solution. The authors looked at the effect produced by the sodium carbonate concentration (50–200 g/dm3), solid-to-liquid ratio (1:(2–6)), process duration (20–120 min) and temperature (20–70 oC) at constant slime feed and stirrer RPM on the behaviour of lead and selenium. Optimum conditions were identified in which both elements almost completely remain in the carbonate cake, and only 0.019% Pb and 0.17% Se get extracted into the solution. Different quantities of three carbonate-containing compounds of lead and seven selenium compounds were found in the cake. When the carbonate cake is leached with nitric acid, almost all of the lead transfer to the solution, while selenium remains in the cake as an elementary substance and mercury selenides. As compared with the slime, the concentration of selenium in the cake increased by 5.6 times. This research was funded by the Science Board of the Ministry of Science and Education of the Republic of Kazakhstan under the following programme: Development and Implementation of Innovative Technology Ensuring a Higher Recovery of Non-Ferrous, Noble, Rare and Rare Earth Metals, as well as Fulfillment of Production Targets by Industrial Companies of the Republic of Kazakhstan Set for 2018–2020.