Reductive leaching of cobalt from zinc plant purification residues

The leaching kinetics of the cobalt present in zinc plant purification residues (cobalt filtercakes) was investigated in the presence of the aromatic reducing agent — phenol — without any pretreatments. As such, the effects of particle size (− 75 + 53 to − 180 + 150 μm), reaction temperature (25 to 85 °C), phenol dosage (2 to15%) and sulfuric acid concentration (0.5 to 2.0 M) on the dissolution rate of cobalt and manganese were investigated. The experimental data conformed well to the shrinking core model, and the dissolution rate of cobalt was found to be controlled by diffusion through the product layer. Due to the low percentage of cobalt (< 1%) in the filtercake, SEM and XRD studies did not show any cobalt-containing phases. It was demonstrated that sulfuric acid with phenol addition can effectively dissolve cobalt and manganese present in the filtercake. The activation energy of the reaction was calculated as 22.92 kJ/mol. The order of the reaction with respect to H2SO4 concentration, particle size, and phenol dosage was found to be 0.61, − 1.88 and 0.70, respectively. The optimum conditions for the leaching of cobalt and manganese were found to be as 2.0 M H2SO4, 85 °C, − 75 + 53 μm, and 10% phenol at the stirring speed of 600 rpm, which correspond to more than 97% cobalt and ~ 100% manganese recoveries. The rate of cobalt leaching based on shrinking core model can be expressed by a semi-empirical equation as: 1 − 2 3 X − ( 1 − X ) 2 / 3 = k 0 . [ H 2 S O 4 ] 0.61 ⋅ r 0 − 188 . [ phenol% ] 0.70 . exp ( − 22.92 R T ) ⋅ t .