Characterisation and acid pressure leaching of various nickel-bearing chromite overburden samples

In an earlier study it was found that additives were required for improving nickel recoveries from preconcentrated nickel-containing chromite overburden. With a view to confirming the response, five chromite overburden samples were taken for characterisation and acid pressure leaching studies. Kansa ore was used for comparison because it does not require any additives to achieve 98% dissolution of Ni. The X-ray diffraction and TG-DTA studies revealed that the goethite phase associated with laterite ore is amorphous in nature whereas the chromite overburden samples had higher fractions of crystalline goethite. In the absence of additives, nickel associated with the amorphous goethite was leached. No quantitative relationship could be established between the crystallinity and percentage of nickel extraction from various samples, although it was possible to get good recoveries by varying the temperature and amount of jarosite from all five chromite overburden samples. The acid used during leaching can be fully or partially replaced by ammoniojarosite. The ammoniojarosite can also be prepared during leaching itself by addition of the required amounts of ferric sulphate.