Recovery of zinc and lead from zinc extraction residue of rotary filter cake by high temperature-pressure 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 high temperature-pressure NaOH, H2SO4 and FeCl3 leaching was investigated. In the study, effects of the type and concentration of leaching agent, contact time, temperature and pressure on the lead and zinc extraction from the zinc extraction residue were investigated. Leaching kinetics related to three leaching reagents was also determined by applying the data obtained to the heterogeneous reaction equations. In order to separate of the lead from the zinc in the leaching solution, chemical precipitation tests with CO2 and Na2S were applied and the precipitates obtained were chemically and mineralogically analyzed. In addition, pollution potentials of the leaching residues obtained were examined by TCLP and EC procedure. The obtained results have showed that the leaching yields of lead and zinc from the zinc extraction residue depend on the type and concentration of the leaching reagent and partially increase with the increasing temperature and pressure. The optimum conditions determined for NaOH, H2SO4 and FeCl3 leachings are as follows. 1. In high temperature-pressure NaOH leaching, 100 % lead and 31.58 % zinc can be leached from the residue under the conditions of 110 g/l NaOH concentration, 100°C temperature and 60 min contact time. 2. In high temperature-pressure H2SO4 leaching, only 98.59 % zinc can be leached from the residue under the conditions of 2 M H2SO4 concentration, 100°C temperature and 60 min contact time. Leaching rate of lead was below 0.04 %. 3. In high temperature-pressure FeCl3 leaching, although higher temperature and longer contact time, satisfactory leaching yields both lead and zinc could not be obtained under the investigated conditions. Maximum leaching yields for lead (9.65 %) and zinc (61.84 %) were obtained in the presence of 4 stoichiometric amount of FeCl3, at 150°C for 8 hours of contact time. Leaching kinetics of the lead and zinc from the zinc extraction residue by NaOH leaching fits the shrinking core model controlled by ash film diffusion and surface chemical reaction, respectively. However, due to low lead leaching yields in the H2SO4 and FeCl3 solutions, kinetics equations were applied only zinc leaching for these solutions. It has been determined that the zinc leachings are controlled by a mixed mechanism with ash layer diffusion and chemical reaction for H2SO4 and diffusion through the ash layer for FeCl3. Activation energy value for lead leaching by NaOH was calculated to be 2.78 kJ/mol while those of the zinc leachings for NaOH, H2SO4 and FeCl3 were calculated to be 20.06, 13.02 and 13.4 kJ/mol, respectively. In the precipitation process applied to separate lead from zinc in the leaching solution, 99.76 % and 99.85 % lead by CO2 and Na2S precipitations can be separated from the leaching solution in the forms of NaPb2(CO3)2OH and PbS, respectively. TCLP test results showed that the leaching residues obtained can be disposed in a landfill. But, according to the 2003/33/EC procedure, the residues are not inert, thus, it has been determined that their disposed in a landfill can not be acceptable. Key Words: Zinc; Lead; Zinc extraction residue; Leaching; Metal recovery