Reduction leaching of manganese nodules of Indian Ocean origin in dilute hydrochloric acid

Abstract

Two manganese nodule samples from the central Indian Ocean basin have been leached in dilute hydrochloric acid (0.5-2.5 M) in the presence of reductants such as pyrite, sodium sulphite and carbon with a view to extracting Mn and Co along with Ni, Cu and Zn which are otherwise easily extracted in dilute acid in the absence of any reductant. For a manganese nodule sample containing about 9.8% Mn and 11.4% Fe, pyrite has been found to be an effective reductant for extracting more than 80% of Mn and Co when leached in 1.5 M HCl at 80°-90° C for a period of 3 h, whereas with Na₂SO₃ as a reductant lower temperature (50°-60° C) and shorter period (60 min) of leaching are sufficient to achieve similar extraction values. Grain size finer than 150 μ m has only a marginal effect on the extraction of metals during leaching in the presence of pyrite. The efficacy of carbon as a reductant has been tested with a nodule sample containing 18% Mn and 9.5% Fe. It has been observed that a combination of pyrite and carbon is more effective than that of carbon alone in achieving low Fe and high Mn, Ni, Co, Cu and Zn extractions.