ACID PRESSURE LEACHING OF NICKEL-CONTAINING CHROMITE OVERBURDEN IN THE PRESENCE OF ADDITIVES

Leaching of beneficiated lateritic chromite overburden sample was carr ied out at 250-260 degrees C with 0.3-0.4 g H2SO4/g of material. It wa s observed that in laboratory scale experiments, similar to 67% nickel could be extracted in 3 hours whereas > 90% extraction in 0.25-0.3 g H2SO4/g of ore is reported in a commercial plant at Moa Bay, Cuba. To improve the recovery, additives such as (NH4)(2)SO4 and jarosite were used. A combination of these two additives in the presence of sulphuri c acid resulted in similar to 98% nickel extraction. The three variabl es, namely acid concentration, (NH4)(2)SO4 concentration and temperatu re, were chosen to optimise the extraction using statistical technique s. The present paper includes the study of interaction of leaching par ameters, characterisation of beneficiated overburden and residues. Reg ression equations have been derived for nickel, cobalt and iron dissol ution at various time intervals. The linear and interaction coefficien ts 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 reg ression equations were found to be adequate to fit the experimental da ta. The regression equations were used for predicting extractions unde r different conditions after converting to natural scale. Experimental ly obtained extractions compared well with the predicted ones. Charact erisation of original sample and leach residue by X-ray diffraction te chniques revealed that the increase in nickel extraction was due to br eaking of the crystalline goethite matrix. The presence of (NH4)(2)SO4 may result in (a) formation of jarosite with initially dissolved iron at < 210 degrees C, (b) neutralization of acid due to formation of NH 4 . HSO4, (c) decomposition of jarosite so formed, added jarosite and NH4 . HSO4 at higher temperatures which results in the release of acid , thereby breaking the goethite matrix and improving nickel extraction s.