Chemical variability in chrome spinel grains from magnetically fractionated ilmenite concentrates: implications for processing

Detrital chrome spinels are contaminant grains within ilmenite rich heavy mineral concentrates found along the southeast coastline of Africa and in the Murray Basin region of southeastern Australia. Separation is often difficult because of overlaps of physical properties between the chrome spinels and ilmenite. The presence of even minor levels of chromia in the concentrates will downgrade their market value as potential feedstocks for the production of titania pigment. In the present paper, the chemistry and distribution of chrome spinels in magnetically fractionated ilmenite concentrates from both regions are characterised using electron microprobe imaging and analysis techniques to establish the chemical and mineralogical factors that impact on their separation behaviour. Results show that the South African sample contains two chemically and magnetically distinct chrome spinel groups. The first group is dominated by a strong trend that reflects the mutual substitution between Al³⁺ and Cr³⁺ while the second group is characterised by compositions containing abundant Fe₃O₄ magnetite component. The presence of these two spinel groups means that physical separation from the ilmenite is relatively straightforward using simple roasting and magnetic separation procedures. In comparison, the Murray Basin chrome spinels exhibit broad chemical variation that results in no clear separation based on magnetic response. The broad spread in composition and magnetic susceptibility of the chrome spinels is responsible for the previously observed difficulties in developing successful separation methods.