Extraction of lead from sulfate cakes of copper smelter dust leaching

Autoclave technology of sulfuric acid leaching of electrostatic dust of a copper smelter allows quantitatively (92–95%) to transfer zinc and copper into solution with minimal arsenic transfer (≤3.5%); the bulk of lead and arsenic remains in the insoluble residue (cake), which has the following composition, %: 24 Fe; 11.5 As; 7.2 Pb; 1.8 Cu; 0.5 Zn. For lead extraction, it is rational to use salt leaching of the cake, which allows lead to be transferred into solution, leaving arsenic in the insoluble residue for disposal. The purpose of the work is to select the optimal parameters for the process of salt dissolution of lead. A thermodynamic analysis was performed by constructing equilibrium phase diagrams for the PbSO4 – H2O – NaCl system to identify compounds in the liquid and solid phases. The presence of soluble compounds PbCl42–, PbCl3_, PbCl2 and PbCl+ was established, the ratio of which depends on the temperature and concentration of sodium chloride. The process of spontaneous dissolution of lead sulfate occurs with the formation of PbCl42–, the appearance of other soluble lead products is associated with the decomposition of PbCl42– anions. The optimal process parameters were established: solvent concentration, g/dm3: 250 NaCl; 50 CaCl2; L:S = 10; T = 60 °C; ω = 31.4 rad/s; duration 1 h. In accordance with the shrinking core model, the data on the kinetics of lead leaching were processed using equations corresponding to external and internal diffusion kinetic modes of dissolution. The minimum values of the rate constant were found for the kinetic mode (k = 0.0007÷0.0009 s–1), and the maximum values were found for the internal diffusion mode (k = 0.0074÷0.0078 s–1). They are determined by values of the activation energy: maximum (E = 5.77 kJ/mol) for the kinetic mode and minimum (E = 1.21 kJ/mol) for the internal diffusion mode. In general, the process corresponds to the diffusion mode. The introduction of an integrated technology for dust leaching with subsequent processing of cake containing lead and arsenic by dissolution in chloride solutions allows for an increase in lead extraction by 3-5% compared to the Waelz technology, ensuring a profitability of about 60 million rubles/year and converting arsenic into a low-toxic product suitable for disposal.