Salt splitting of effluents generated from recovery of metals from Polymetallic Manganese Nodules

The hydrometallurgical recovery of metals such as Co, Ni, Cu and Mn from PMN (Polymetallic Manganese Nodules) can be carried out in ammonia medium or sulphuric acid medium [1],[2]. In both these routes, effluents are generated after the recovery of the concerned metals from the leach liquor. The ammoniacal route for PMN processing utilises ammonia and sulphur dioxide during leaching, which generates ammonium sulphate effluent upon reaction. Meanwhile, in the acid route, the neutralisation of the acid after leaching leads to the formation of sodium sulphate. Despite requiring safe disposal, both effluents can also be used to recover back the respective acid and alkali values. Electrochemical salt splitting is one of the most promising and cost-effective ways to recover them from these effluents. A three-compartment cell has been used for the electrochemical splitting of the effluents. The compartments are separated by an anion and cation exchange membrane. A process for producing ammonia of 40 g/L and sulphuric acid of 10 g/L has been established through the electrochemical splitting of ammonium sulphate effluent. Similarly, up to 25% NaOH was produced from effluent containing as low as 5% Na
2
SO
4
in the electrochemical splitting cell. Various process parameters, viz. fluid flow, current density and concentration of alkali and acid, were optimised to enhance the cell's performance. Various cell designs have been tried. The processes were developed in lab-scale (36 cm
²
membrane area) and subsequently up-scaled to 1000 cm
²
cell.