Acid pressure leaching of nickel-containing chromite overburden in the presence of additives

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Leaching of beneficiated lateritic chromite overburden sample was carried out at 250–260°C with 0.3-0.4 g HZSO 4/g of material. It was observed that in laboratory scale experiments,∼ 67% nickel could be extracted in 3 hours whereas > 90% extraction in 0.25-0.3 g H 2SO 4/g of ore is reported in a commercial plant at Moa Bay, Cuba. To improve the recovery, additives such as (NH 4) 2SO 4 and jarosite were used. A combination of these two additives in the presence of sulphuric acid resulted in ∼ 98% nickel extraction. The three variables, namely acid concentration, (NH 4) 2SO 4 concentration and temperature, were chosen to optimise the extraction using statistical techniques. The present paper includes the study of interaction of leaching parameters, characterisation of beneficiated overburden and residues. Regression equations have been derived for nickel, cobalt and iron dissolution at various time intervals. The linear and interaction coefficients which were found to be significant for nickel were also significant for iron dissolution. Therefore, it is expected that in the presence of both additives, iron contamination would be more. The estimated regression equations were found to be adequate to fit the experimental data. The regression equations were used for predicting extractions under different conditions after converting to natural scale. Experimentally obtained extractions compared well with the predicted ones. Characterisation of original sample and leach residue by X-ray diffraction techniques revealed that the increase in nickel extraction was due to breaking of the crystalline goethite matrix. The presence of (NH 4) 2SO 4 may result in (a) formation of jarosite with initially dissolved iron at < 210°C, (b) neutralization of acid due to formation of NH 4 · HSO 4, (c) decomposition of jarosite so formed, added jarosite and NH4. HS04 at higher temperatures which results in the release of acid, thereby breaking the goethite matrix and improving nickel extractions.