Microwave-assisted method investigation for the selective and enhanced leaching of manganese from low-grade pyrolusite using pyrite as the reducing agent

The recovery mechanism of manganese from low-grade pyrolusite was studied through microwave and conventional leaching, respectively, and pyrite was used as the reducing agent. An improvement on the manganese leaching rate with microwave heating was noticed which may be caused by the suppressing of the formation of sulfur passivation layer by the unique dipole rotation heating mechanism of microwave energy. To confirm this hypothesis, the leaching time, the amount of reducing agent, and the concentration of sulfuric acid were studied, and the surface compositions of the leaching residues were analyzed. The results evidenced that the sulfur content on the surface of residue produced by microwave leaching was significantly reduced compared to the results produced by conventional heating, which proved the rationality of the hypothesis. The recovery of leaching assisted with microwave heating was improved compared to that assisted with conventional heating under the same experimental conditions (liquid-solid ratio: 10:1, leaching temperature: 90 ℃, M(pyrite)/M(pyrolusite): 0.2, sulfuric acid concentration: 1.2 mol/L, stirring speed: 400 rpm). The corresponding peak value of the leaching rate was 95.07 % and 75.08 %. Additionally, microwave leaching is very environmentally friendly since it significantly reduces the amount of reducing agent and sulfuric acid as well as reaction time.