Rate process of the reduction leaching of manganese nodules in dilute HCl in presence of pyrite - Part I. Dissolution behavior of iron and sulphur species during leaching

Abstract

An attempt has been made to study the dissolution behavior of Fe (II), Fe (III), SO42- and release of elemental sulphur (S degrees) during the reduction leaching of ground manganese nodules of the Indian Ocean origin in dilute HCl in the presence of a low grade pyrite sample. This complex leaching system proceeds basically through two consecutive redox reactions, namely, oxidation of pyrite by Fe(III) to generate Fe(II) which subsequently reduces MnO2 in the nodules to produce Mn2+ in solution. The initial Fe(III) is supplied by the rapid dissolution of iron from nodules in dilute HCl. Thus, there is initial building-up of both Fe(III) and Fe(II), reaching their maximum values after a certain period, beyond which concentrations fall due to hydrolytic precipitation of iron as acidity of the medium is depleted with the progress of manganese dissolution. The decomposition of pyrite in sufficiently acidic medium takes place predominantly through the formation of SO42- with linear kinetics up to 60 min, but at low acidity decomposition also proceeds through the formation of S degrees. Thus, Fe(II), Fe(III), Fe(II)/Fe(III) ratio, SO42-, S degrees etc. all play interrelated roles and affect the ultimate rate of leaching of MnO2 from nodules. The effect of various parameters such as [H+], temperature, [Fe(III)], pulp density etc. on the dissolution behavior of the above species have been studied and discussed. (C) 1999 Elsevier Science B.V. All rights reserved.