For the extraction of metal values from poly-metallic nodule several alternative techniques have been used. Out of these, the most promising ones are based on selective reduction of metal oxides / hydroxides in aqueous slurry. The present work on metallurgical process development in India, Japan, Korea, as well as the earlier work in France, Russia, and USA are based on such techniques. The main advantages of reductive leaching technique are:
use of nodule in the form of a slurry, and therefore removal of naturally occurring moisture is not necessary,
use of chemical reagent selectively for Cu, Ni, Co, and Mn, and
use of moderate temperature during leaching, and therefore case of reactor design and process control.
The aqueous reduction processes use different types of reductants e.g. Fe(II), Mn(II), SO2, CO, thiosulphate, glucose, and carbon in acidic as well as in ammoniacal media. Some of these reductants, such as CO, operate through a Cu(I).. Cu(II) couple to reduce the oxides, while reductants like Fe(II), and Mn(II) can operate directly, or through Cu(I) - Cu(II) couple. For almost a decade, the authors and their colleagues are involved in studying different aspects reductive leaching of polymetallic nodule, and currently involved in pilot testing of a SO2 based process. The paper gives a brief over-view of the processes using aqueous reduction technique. It particularly compares different processes developed using aqueous reduction technique.
The occurrence of poly-metallic manganese nodules (PMN) was first reported by the HMS Challenger Expedition during 1872–1876. Since then the composition and the origin of nodules were largely studied as a scientific curiosity till early 1960, when their economic potential was recognised. There is now little doubt that PMN forms an important source for Ni, Co, Cu and possibly other elements like Mn.