Kinetics of sulfuric acid leaching of cadmium from Cd–Ni zinc plant residues

Cd–Ni filtercakes are produced continuously at the third purification step in the electrolytic production of zinc in the National Iranian Lead and Zinc Company (NILZ) in northwestern Iran. In this research, the dissolution kinetics of cadmium from Cd–Ni residues produced in NILZ plant has been investigated. Hence, the effects of temperature, sulfuric acid concentration, particle size and stirring speed on the kinetics of cadmium dissolution in sulfuric acid were studied. The dissolution kinetics at 25–55 ◦C and t ≤ 5 min was found to follow a shrinking core model, with inter-diffusion of cadmium and sulfate ions through the porous region of alloying layer (Cd5Ni, Cd2Ni1.9 and Cd10Cu3) as the rate determining step. This finding is in accordance with the apparent activation energy (Ea) of 13.363 kJ/mol and a linear relationship between the rate constant and the reciprocal of squared particle size. Arrhenius constant was calculated as 6.3942 min −1. The order of reaction with respect to sulfuric acid concentration, solid/liquid ratio and particle size were also achieved. The rate of reaction at first 5 min based on diffusion-controlled process can be expressed by a semi-empirical equation as: [1 − (2/3)X − (1 − X)2/3] = kd t = 997[H2SO4]^(1.0475)(SL)^(−0.5969)r^(−2.1819)0 exp(−13.363/(RT))t. It was determined that the dissolution rate increased with increasing sulfuric acid concentration and decreasing particle size.