Abstract
This article reviews the state of the art in processing and extracting ocean manganese nodules. It briefly reviews the mining sites where abundant rich nodules occur and also describes the metal distribution in nodules. The physicochemical characteristics of the nodules, touching upon different facets such as mineralogy, geochemistry, moisture content, and textural properties of the nodules, are discussed in detail. In the extraction aspects of the nodules, is article reviews two different extraction routes often mentioned in the literature during the last two decades, namely hydrometallurgical and biohydrometallurgical treatments. The hydrometallurgical techniques include leaching by mineral acids, ammoniacal leaching, leaching at high temperature, and pressure and leaching with reducing agents. Reduction of manganese dioxide by organics and its probable application in leaching of nodules is described in a detailed manner. The biohydrometallurgy section extensively reviews the bioreduction phenomenon for manganese dioxide and suggests possible application in leaching of nodules. Projected future developments in the field, with an eye on commercialization, are also discussed.
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