Rate process of the reduction leaching of manganese nodules in dilute HCl in presence of pyrite - Part II: Leaching behavior of manganese

Abstract

The formal kinetics of the dissolution of MnO2 from two different manganese nodule samples of Indian ocean origin in dilute HCl and in the presence of a low grade pyrite: have been studied under various experimental parameters such as acid concentration, temperature of leaching, concentrations Fe(II) and Fe(III), pyrite content, particle size, liquid/solid ratio etc. While the kinetics of Mn dissolution from sample 1, containing about 10% Mn, are described by two-dimensional diffusion, the rate of dissolution from sample 2 (similar to 20% Mn) is described by one-dimensional diffusion or parabolic law in accordance with the geometry of the particles as revealed from scanning electron microscopic (SEM) studies. It has been observed that the reduction leaching of Mn in the presence of pyrite is 0.25 and 1.0 order with respect to acid concentration for samples I and 2 respectively. For sample 2, the reaction is first order with respect to the peak concentrations of both Fe(II) and Fe(III) and second order with respect to FeS2 concentration. The apparent activation energy values for sample 1 (30.6 kJ/mol) is much lower than that for sample 2 (70.8 kJ/mol) suggesting that mechanism of this complex leaching reaction depends upon Fe/Mn ratio in the nodule sample. The effect of particle size on the rate of leaching also indicates that this complex leaching process involves more than one mechanistic model. (C) 1999 Elsevier Science B.V. All rights reserved.