Sustainable processing of deep-sea polymetallic nodules

Abstract

The possibility of commercialization of processing technology for sea nodules has been linked with comparisons with similar terrestrial process operations. In addition to techno-economic viability, an added focus to commercialization is the sustainability of the process route. The general context of sustainability is discussed. The importance of material flow analysis, recycle rates of the metals produced as well as the possibility of a flow sheet being developed to supply short supply critical metals are brought out in this context. Environmental management for flow sheets under development is important for sustainable operations. Cradle-to- gate environmental burdens such as greenhouse gas emissions and solid waste burden both for common metals as well as for several other metals are provided as a basis of comparison. Polymetallic nodules (PMN) have been likened to low-grade manganese ores where three approaches have been reported for processing. Two of these approaches involve initial processing similar to laterite ore processing for recovery of Ni, Cu, and Co followed by manganese recovery similar to terrestrial ferroalloy production/manganese compound precipitation. The third approach attempts manganese recovery as ferroalloy followed by recovery of Cu, Ni, and Co as practiced for terrestrial sulfide ores. The relatively high values of gross energy requirements and emissions for terrestrial Ni and Co as compared to common metals point out that flow sheet development effort needs to focus on controlling these parameters for a nodules processing operation. On the other hand, these parameters have relatively low values for manganese ferroalloys produced from terrestrial resources. Production of ferroalloys from sea nodules needs to be comparable to their production from land resources. An approach for impact analysis is evolved where a system expansion strategy is followed for Cu, Ni, and Co with a subprocess of recovering manganese bearing ferroalloy. Regarding the later process step, both the Gross Energy Requirement (GER) and Emission (150 MJ and 10 t CO2/t ferroalloy produced) far exceed the terrestrial processing values. For recovery of Ni, Cu, and Co, the results are process specific. For a roast reduction ammoniacal leach process, an energy input of 525 MJ and 40 kg CO2/kg of Ni equivalent is estimated as compared to 200 MJ and 12 kg CO2/kg of nickel equivalent for a complete hydrometallurgical process. Thus, the roast reduction ammoniacal leaching process is not sustainable for sea nodules processing for recovery of Ni, Cu, and Co because of high GER values and high specific CO2 emissions. The high pressure acid leaching route has comparable values to similar laterite processing operations. For flow sheet concepts involving manganese dissolution, recovery of manganese as electrolytic manganese dioxide is less energy intensive compared to residue smelting operation with high gross energy requirements and emissions. For nodule processing flow sheets involving use of energy chemicals (NH3, HCl, H2), appropriate reagent recycle schemes for reagents need to be conceived; else process integration with external flow sheets needs to be contemplated for enhancing sustainability. Considering resource crunch of rare earth elements with respect to terrestrial resources, the recovery of rare earth from sea nodules will enhance the sustainability of the sea bed deposits.