Studies on the dissolution of metal values in manganese nodules of Indian Ocean origin in dilute hydrochloric acid

Abstract

Dissolution behaviour of Mn, Fe, Cu, Ni, Co and Zn from manganese nodules of Indian Ocean origin in dilute hydrochloric acid has been studied. It is observed that while more than 80% of Ni, Cu and Zn can be easily leached along with 30-35% of Fe in 1.0-1.5M HCl, extractions of Mn and Co are very low (<20%). By increasing both temperature of leaching and acid concentration the extractions of Mn and Co can be improved, but only at the cost of considerable dissolution of iron. Pulp density below 15% has only a marginal effect on the extraction of all the metals. Grain size finer than 350 µm is also found to have little effect on the extraction of important metal values. Kinetic studies reveal that the leaching characteristics of different metals are different; whereas Mn and Co follow a parabolic rate law of dissolution from constant surface area, Fe and Ni follow a rate law for diffusion through a non-porous product layer. However, the initial rates of extraction of Cu and Zn are so high that for these two metals random nucleation and growth of product are found to be the rate controlling steps. Apparent activation energy values for the leaching of different metals in dilute HCl have been estimated.