Different applications of waste generated in reduction roasting – ammonia leaching of manganese nodules.

CSIR-NML, Jamshedpur has been engaged in technology development to recover valuable metals (Cu, Ni & Co) from manganese nodules (MN). Extensive R&D work led to development of a process based on reduction roast – ammonia leaching – solvent extraction (SX) – electrowining (EW) route. This process generates large amount of waste/residue (70% of the MN weight) after selective leaching of Cu, Co and Ni, which may be considered hazardous, if untreated, due to its fineness and heavy metal contents. Characterisation studies have shown that this leaching residue waste contains oxides/oxyhydroxides of Fe, Mn, Al and Si with a high porosity and surface area, which are suitable for catalytic activity and adsorption of various aqueous pollutants. In addition, appreciable amount of manganese content of the manganese nodule leached waste may be recovered for the improvement of the economics of MN processing. Keeping these in view, studies for utilisation of waste leach residue have been carried out in three different ways: i) The use of leaching residue as catalyst has been successfully assessed for decolourisation of methylene blue (MB). The surface complex between wMNR and MB at a rate limiting step followed by electron transfer from MB to active metal centre of wMNR and release of products has been found to be operative during the decolourisation process. ii) The potential of water washed leaching residue as adsorbent has been evaluated for removal of anionic pollutants phosphate (PO43-), selenite (SeO32-) and cationic pollutants like Cu2+ and Cd2+, showing a maximum phosphate uptake of 9.88 mgP g-1 and selenite uptake of 15.17 mg Se g-1 for wMNR calcined at 400 C. The monolayer adsorption capacities (Qo) has been found to be temperature dependent showing 26.95 mg g-1 and 40.32 mg g-1 at 303 and 323 K, respectively for Cu2+ ion while the values are 32.23 and 38.14 mg g-1 at 303 and 323 K, respectively for Cd2+ adsorption. iii) In the third application of leached residue, a value added material like silicomanganese alloy has been produced by smelting of MN leaching waste in a 50 kVA electric arc furnace, with manganese recovery of about 77%