Role of impurities in solvent extraction-electrowinning of copper and nickel for the processing of sea nodules

Abstract

During the leaching of roasted nodules from Indian Ocean the soluble amine species of iron, manganese and zinc along with valuable metals such as Cu, Ni and Co enter the ammoniacal solution. Even after the purification step, the metal impurities in small amounts remained in the leach solution. In metal separation using LIX 64N, iron and manganese found their way in the nickel and copper electrolytes. Depending on the feed pH of the nickel spent electrolyte, the selective nickel stripping could be achieved with minimum contamination of copper. A flow sheet based on co-extraction-selective stripping has been developed to process the ammoniacal leach liquor of Indian Ocean nodules. While running the continuous SX-EW circuit, the build-up of metallics (Fe and Mn) and organic impurities in the nickel electrolyte was observed which affected the deposition. The cut-off impurity level in electrolyte for nickel winning was determined as : 0.175 Kg/m(3) Mn, 0.010 Kg/m(3) Fe and 20 ppm organic reagent. A purification scheme for nickel bleed stream was developed to recycle the electrolyte back to SE-EW circuit. Copper being a more noble metal than nickel can usually tolerate higher level of impurities. Organic levels of less than 100 ppm in the copper electrolyte produced good sheet deposit. The results showed the possibility of producing electrolytic grade copper and nickel metals of 99.94% and 99.8% purity.