Recovery of zinc and lead from zinc extraction residue of rotary filter cake by NaOH leaching

In the hydrometallurgical zinc production process from carbonated ores, while the zinc in the ores is leached with hot sulphuric acid solution, leaching residue containing significant amount of valuable metals, especially lead and zinc, forms. This residue can be considered as a source for some valuable metals such as lead, zinc, silver, gold etc. due to its high metal content. Therefore, the proper evaluation has great importance in terms of economic and environmental.In this study, lead and zinc recovery from the zinc extraction residue containing 15.14 % Pb in the form of anglesite (PbSO4) and 7.98 % Zn in the form of franklinite (ZnFe2O4) by NaOH leaching was investigated. In order to separate the franklinite having magnetic properties from the residue and enrichment of lead content in the residue, magnetic separation was applied. But, satisfactory separation yield could not be obtained due to very small particle size of the residue. For this reason, the residue was directly used in the experiments. In the study, effects of NaOH concentration, contact time, stirring speed, liquid/solid ratio and temperature on the lead and zinc extraction from the zinc extraction residue were investigated. Leaching kinetics was also determined by applying the data obtained to the heterogen reaction equations. Optimum conditions for the maximum lead extraction was determined as NaOH concentration of 15 % (w/v); contact time of 30 min; stirring speed of 300 rpm; liquid/solid ratio of 5 and temperature of 25°C. For the maximum zinc extraction, NaOH concentration of 15 % (w/v); contact time of 180 min; stirring speed of 300 rpm; liquid/solid ratio of 5 and temperature of 85°C were optimum conditions. Under these conditions, 85.55 % Pb and 29.93 % Zn could be extracted from the residue.Leaching kinetics of the lead from the zinc extraction residue fits the shrinking core model controlled by surface chemical reaction. However, in the higher temperatures (above 70°C), three mechanisms; i) surface chemical reaction, ii) diffusion through the liquid film and iii) diffusion through the ash layer control the rate of lead leaching. The rate of the zinc leaching reaction is controlled by the diffusion through the ash layer. Activation energies for lead and zinc leaching were calculated to be 31.19 ve 22.59 kJ/mol, respectively.