The kinetics of the dissolution of malachite in acid solutions

The dissolution of screened samples of pure malachite in dilute (0.033 to 0.15 M) sulfuric acid solutions has been shown to be rapid and the kinetics of dissolution are consistent with rate-determining chemical reaction on the surface of the shrinking particles. There is no evidence of mass transport controlled dissolution even at the highest rates. The rate is inversely proportional to the particle size and increases with increasing acidity. However, the relationship between the rate and the acid concentration is not linear but is directly proportional to the proton activity as obtained from pH measurements. The effect of the addition of chloride ions on the rate is relatively small and it appears to involve a specific effect of chloride at low concentrations and the effect of chloride on the proton activity at high concentrations. The rate increases with increasing temperature and the activation energy of 53.6 kJ mor is in the range expected for a rate-determining chemical reaction. A simple fundamental rate equation has been developed that will allow prediction of the dissolution curves for any particle size at any acid concentration and temperature within the range studied. From a practical perspective, these results suggest that heap leaching of malachite in dilute acid should present no chemical problems and that there is little advantage to be gained from the addition of chloride ions to increase the rate of leaching.