Removal of chrome spinels from Murray Basin ilmenites by low temperature roasting

Ilmenite concentrates from the Murray Basin deposits of southeastern Australia contain discrete grains of chrome spinels as a major impurity that must be removed for the ilmenite to become a satisfactory feedstock for the sulphate route to titania pigment production. Conditions were established for a magnetising roast of the ilmenite samples, followed by a magnetic separation, that produced products with (≤0·1 wt-%Cr₂O₃. The roasting conditions were adjusted to keep the crystallite size of any rutile formed during the roast to <10 nm. Under these conditions the rutile was appreciably soluble in sulphuric acid so that the roast products had satisfactory digestion properties. The roasting conditions are a sensitive function of the mineralogy of the ilmenite grains resulting from chemical weathering in the deposits. For mildly weathered ilmenite concentrates from the southern Murray Basin, containing intimate fine grained associations of ilmenite and pseudorutile, magnetising roasts at <650°C in slightly reducing fluidising gases gave magnetic products containing, <0·1 wt-%Cr₂O₃ at >80 wt-% recovery, with high solubilities in sulphuric acid. However, similar conditions were not effective for ilmenite concentrates from the central Murray Basin that contain mixtures of unweathered Mg rich ilmenite and highly weathered ilmenite. The strongly weathered ilmenite contains high levels of intra grain chromia (0·2 wt-%Cr₂O₃) and generates high levels of rutile on roasting, making it difficult to obtain acceptable sulphate route feedstocks from this ilmenite.