Gaseous reduction–smelting process for the recovery of Cu, Ni, Co and Mn from polymetallic nodules

In India, considering the strategic importance of Ni and Co, the exploration of seabed mineral resources for their extraction has gained significant attention. It has been observed that several processes developed so far for the recovery of Cu, Ni and Co from polymetallic nodules are somewhat complex and critically controlled. Since the concentration of Cu, Ni and Co together is about 2-3 wt% of polymetallic nodules, a huge amount of residue containing Mn and Fe is generated on employing the hydrometallurgical processes based on acid or ammonia leaching. Furthermore, the valuable metals such as Cu, Ni and Co in polymetallic nodules are found to be present as an integral part of iron and manganese oxides. Thus the release of these metal values by disintegrating the matrix of iron and manganese oxide lattice structure is crucial to achieve high recovery of metals and minimise generation of residue/waste. The processes developed so far for the recovery of valuable metals from polymetallic nodules are generally driven by the theory of three-metal recovery (Cu, Ni and Co). However, the recovery of Mn, if integrated, would improve the economic prospect of the total metallic output. Therefore, an effective four-metal extraction process has been developed at CSIR-IMMT, Bhubaneswar for the recovery of Cu, Ni, Co and Mn from polymetallic nodules by employing gaseous reduction-smelting route.The sea nodules were pelletized in the size range of 2-3 mm and subsequently, the nodule pellets were subjected to gaseous reduction using compressed natural gas (CNG) as the reducing agent. The reduced nodule pellets were then melted to produce an alloy containing Cu, Ni and Co, and a slag constituting oxides of Mn, Fe and Si. Both alloy and slag produced after melting of the reduced nodule pellets were characterized for phase identification, elemental composition and percentage (%) recovery of targeted metal values. Since the manganese nodules generally contain more than 25 wt% of moisture, a significant reduction in its mass was observed subsequent to gaseous reduction operation. The gaseous reduction temperature was optimized in order to maximize the recovery of Cu, Ni and Co in the alloy and to retain Mn in the slag phase. The alloy phase, which is about 6-8% of the feed material, can be processed further through hydrometallurgical route for the recovery of individual metal values. Therefore, in the present process the solution volume will be significantly less as compared to those generated by the direct acid or ammonia leaching processes.The polymetallic nodules exhibit low Mn/Fe ratio (< 3), which was increased by separating Cu, Ni, Co and a significant fraction of Fe during the production of Cu-Ni-Co-Fe alloy through gaseous reduction-melting process. As a result, the generated slag was found to be enriched with Mn and to have a higher Mn/Fe ratio (> 7). The suitability of the Mn-rich slag for the production of silicomanganese was explored. The Mn-rich slag was processed through smelting operation to recover Mn in the form of standard grade silicomanganese. Smelting of the Mn-rich slag was carried out using coke and dolomite as the reducing agent and flux material, respectively. The effects of various parameters such as amount of coke and flux addition have been studied and optimized. The role of amount of coke as well as dolomite addition was found to be critical in producing the desired grade of silicomanganese and improving the recovery of Mn from polymetallic nodules. The silicomanganese produced from the Mn-rich slag was characterized for its composition, and percentage (%) recovery of Mn as well as Fe. The present process seems to be attractive owing to the value addition by efficient recovery of Mn from polymetallic nodules having low Mn/Fe ratio.