The fate of chromium impurities during acid sulphate digestion of ilmenite concentrates

Ilmenite concentrates from the Murray Basin region of southeastern Australia are contaminated with chromium impurities that must be removed for the ilmenite to become a satisfactory feedstock for the sulphate route to titania pigment production. The chromia is present primarily as discrete, compositionally variable, chrome-rich spinel grains with a smaller amount as intra-grain chromia distributed as coatings in fractures and pores of weathered ilmenite grains. Characterisation of chromia deportment through a simulated acid sulphate digestion process showed a small but non-negligible solubility of the spinels. Most spinets were resistant to dissolution with the exception of those containing high Fe(Al,Cr)(2)O-4 and magnetite (Fe3O4) components. Intra grain chromia was highly soluble. Processing to achieve low bulk chromia using a magnetising roast procedure must ensure that well crystallised rutile is not produced because of its insolubility in the sulphate process. It is also important that the roast conditions do not substantially increase the magnetite content of the spinets making them more susceptible to dissolution. This work highlights the importance of characterising all spinet composition types within ilmenite concentrates in addition to the level of intra-grain chromia associated with the ilmenite when considering the suitability of Murray Basin primary ilmenites as a feedstock to sulphate route titania pigment plants. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.