Effect of milling on metal leaching: Induction of galvanic effect in a secondary lead smelter matte by prolonged milling

Abstract

The effect of milling on the leaching of Pb (and other metals like Fe, Cu, Ni and Zn) from a secondary lead smelter matte was studied. The effects of milling time, milling environment (dry or wet milling) and leaching reagent (Fe(III)-HNO3 or citrate solution) on the leaching efficiency of metals were investigated. A 1–2mm ferromagnetic fraction of the matte, containing 54% Fe, 5.8% Pb, 24% S and minor levels of Cu (1.2%), Ni (0.2%) and Zn (0.3%), was used. Pb leaching in a Fe(III)-HNO3 solution was enhanced from 53% (no milling) to 85% after 5min of dry milling. Prolonged milling resulted in an agglomeration of particles causing a galvanic effect (between FeS and Pb), which led to electrochemical precipitation of Pb2+ cations from the solution as PbS. Because of this electrochemical precipitation, almost no Pb2+ was detected in the Fe(III)-HNO3 solution after 10min of dry milling. In order to avoid/reduce agglomeration and to obtain a finer particle size and higher leaching efficiencies, wet milling was carried out. The highest Pb leaching was obtained after 20min of wet milling (88% in the Fe(III)-HNO3 and 94% in the citrate solution). Citrate solution was more selective towards Pb leaching, with <2% Fe leaching from the matrix material compared to 20–24% in the Fe(III)-HNO3 solution. The optimised milling and leaching steps were combined to attain a process for the selective recovery of valuable metals from secondary lead smelter matte.