Reduction-dissolution of cobalt oxide in acid media: A kinetic study

The kinetics of cobalto-cobaltic oxide (Co3O4) dissolution in sulphuric acid have been investigated. The dissolution rate is significantly enhanced by the use of Na2SO3 in aqueous media for the reduction of Co3+ to Co2+. The dissolution was studied as a function of rate of agitation, leaching time, acid concentration, temperature and Na2SO3 concentration. The reaction has a 1/2 order of dependence on acid concentration between 1.0 and 4.0 M. The rate of dissolution appears to be independent of the SO32- concentration above 0.025 M. Primary identification of the rate-controlling step has been accomplished by close fitting experimental data to the predicted reduced time plots for diffusion of the reactant species across the unreacted solid layer. The activation energy for the temperature range 303-343 K was found to be 25 kJ/mol, thereby confirming diffusion as the rate-controlling step.