An Integrated ammonia and acid leaching based process for recovery of metal values from Indian Ocean manganese nodules

The polymetallic manganese nodules of Indian Ocean contain 0.1-1.2% Cu, 0.26-1.1% Ni and upto 0.1% Co [1], making it a rich source of Co and Ni for the subcontinent, for which land based deposits are sparse. Over the past three decades attempts have been made by several research groups to recover these metal values using different approaches. The ammoniacal-SO
2
based process developed by CSIR-IMMT [3],[4], was first demonstrated at 500kg/day in CRDL (HZL), Udaipur [7]. Upon switching from three metal to four metal recovery process through hydrometallurgy route and studying various approaches [5],[6],[8],[9], an alternate sucrose based reductive acid leaching process was developed and demonstrated at 100L/batch by CSIR-IMMT for recovering the four major metals namely Mn, Cu, Ni & Co. Currently, CSIR-IMMT is working towards an integrated approach combining the advantageous aspects of both acid and ammonia based process routes to recover Mn, Cu, Ni, Co, focussed on reutilization of solid residues and effluents within the process to achieve an improved product portfolio, value addition and waste minimization. The major operations in the integrated process are as follows: ammoniacal-SO
2
leaching, reductive acid leaching, demanganization, iron removal, smelting, bulk sulphide precipitation, mixed sulphide dissolution, solvent extraction, electrodeposition, precipitation and electrochemical salt splitting. From the integrated process, Mn is recovered as Electrolytic manganese dioxide (EMD), MnCO3 from acid dissolved liquor and as suitable feed material for Ferro-silico-manganese production by processing leach residues. The leach liquors are purified by precipitation, solvent extraction to produce 99.9% pure Cu, Ni and Co metals by electrowinning from both ammonia and acid leaching routes. Part of iron from residue is removed as pig iron, while the ammonium sulphate effluent is used for electrochemical splitting to regenerate ammonia and acid to be reused in leaching operations. Based on the recoveries from individual pilot scale tests on ammonia and acid processes, the overall theoretical recovery of Mn from the integrated process is projected to be above 90%, leading to maximum utilization of the nodule mass. The integrated process flowsheet for nodule extraction is currently being optimized and tested at lab scale. The diversified and improved product portfolio, while utilizing all major effluents and solid residues back in the process with minimal waste generation makes the integrated process very lucrative for nodule processing in the Indian context.