The pyrometallurgical processing of lateritic nickel ores produces large amounts of FeNi slag. Its chemical composition depends on the step of the metallurgical process (smelting, refining, and desulphurization). Magnesium silicates and iron, cobalt, and nickel oxides are present in the slags. In general, cobalt is present in laterites at levels below 0.2% m/m, and a specific processing to recover this valuable metal by a pyrometallurgical route was not found in searches. Therefore, the development of routes to process this waste is fundamental to recover cobalt and nickel, strategic metals for modern technologies, and to reduce this environmental liability.
This work presents a study of the acidic leaching of FeNi slag from refining process of nickel laterites to recover cobalt and nickel. The leachants were mixtures composed of an inorganic acid (HCl 6.0 mol L-1, H2SO4 9.0 mol L-1, H3PO4 7.5 mol L-1) and H2O2 (2.5 mol L-1). The effects of time (2-4 h), temperature (25-75 °C), and solid/liquid ratio (50-150 g L-1) were tested.
The reproducibility of leaching was determined to be ± 4%. Hydrochloric acid had the best performance as leachant. The effect of temperature was not significant to Co and Ni leaching. Thus, it is possible to apply the method at 25°C, reducing energy consumption. Under the best experimental conditions (25 °C, 2 h, S/L ratio 100 g L-1), 99 wt.% of nickel and cobalt were leached in the presence of HCl + H2O2 mixtures. The mass balance showed the best yields in 3 h (> 75 % m/m). However, flame atomic absorption spectroscopy (FAAS) of the liquors indicates that Co and Ni were better leached in 2 h (≥ 99 % m/m). The leaching performance with a solid/liquid ratio of 50 g L-1 (80-84% m/m) was slightly better than at 100 g L-1 (72-75% m/m). Consequently, it is more profitable to use a less volume of acidic leach solution. X-ray powder diffraction (XRPD) analyzes indicate SiO2 and Fe3O4 (HCl) and calcium salts (H2SO4 and H3PO4) as components of the insoluble matter from leaching. The X-ray fluorescence (XRF) results indicate cobalt and nickel contents below the detection limit.