Purification of synthetic laterite leach solution by solvent extraction using D2EHPA

The world mineral industry is experiencing an unprecedented interest in nickel-cobalt extraction from laterite ores through acid pressure leach and SX-EW processes. The recovery of cobalt and nickel from the leach solution through direct solvent extraction is of great interest as this would result in significant capital and operating cost savings. In the direct solvent extraction approach, the separation of zinc, calcium, copper and, in particular, manganese from cobalt and nickel is highly important. A series of shakeout tests was undertaken to investigate the fundamentals of the separation of the above impurities from cobalt and nickel using di-2-ethylhexyl phosphoric acid (D2EHPA) in kerosene. D2EHPA pH-extraction isotherms from solutions each containing a single element showed that the extraction order for the seven elements of interest as a function of pH,, was Zn2+ > Ca2+ > Mn2+ > Cu2+ > Co2+ > Ni2+ > Mg2+. This confirmed that manganese would be extracted from sulfate solution ahead of cobalt and nickel. Extraction isotherms from solutions containing Zn, Ca, Mn, Cu, Co, Ni and Mg showed that the separation of zinc and calcium from the other elements was not difficult and the separation of copper and manganese from cobalt and nickel was possible. The separation of manganese from cobalt and nickel by D2EHPA in kerosene was affected by temperature and pH. At pH 3.0, better separation of manganese from cobalt and nickel was achieved at room temperature (23 degrees C). At pH 3.5, better separation of manganese from cobalt was achieved at room temperature (23 degrees C). However, better separation of manganese from nickel could be obtained at elevated temperatures (40-60 degrees C). The McCabe-Thiele diagram for the system showed that at pH 35 and 40 degrees C, two theoretical extraction stages at A/O ratio 1:1 were needed to extract 99.9% manganese from the aqueous solution and to reduce the manganese concentration from 2.0 g/L to 3 ppm. Multiple stage extraction with fresh aqueous solution showed that cobalt and nickel were crowded out by zinc and manganese. Multiple stage extraction with fresh organic solution showed that manganese and copper in the aqueous solution were eliminated. Multiple stage scrubbing of the loaded organic solution with manganese solution indicated that after one stage of contact, only about 3 ppm cobalt and nickel were present in the organic solution. (C) 2000 Elsevier Science B.V. All rights reserved.