The dissolution of iron in the hydrochloric acid leach of an ilmenite concentrate

Hydrochloric acid leach processes for upgrading ilmenite to synthetic rutile are being considered for use with the Australian ilmenite concentrates that cannot be treated by the commonly used Becher process. These acid leach processes consist of a pre-leach thermal treatment, a hydrochloric acid leach that extracts iron from the concentrate, and a regeneration step that forms hydrochloric acid from the ferrous chloride produced in the leach. The acid leach processes differ in their thermal pre-treatments, but common to them all is a leach at temperatures near the boiling point of the acid. The present investigation of the acid leach step, applied to a Western Australian ilmenite concentrate, clarifies the chemical processes that occur in the leach and, through a designed experiment, identifies factors that affect the rate of extraction of iron. The four controlled variables studied were: the concentrations of ethanol, hydrochloric acid, and ferrous chloride in the leach liquor, and the quantity of oxygen injected into the liquor. The extraction of iron was shown to occur in two stages: an initially rapid dissolution of iron, decomposing the 45% Fe2O3 ilmenite into an impure rutile containing 0.42% to 0.65% Fe2O3; and a slower dissolution of this iron out of the rutile. This initially rapid rate of iron extraction is altered by process liquor conditions. Thus, it is reduced by ethanol addition, and increased in more concentrated acid. It is also increased by ferrous chloride addition, up to an optimum concentration (determined by the acid concentration), but decreased by its further addition. (C) 1999 Elsevier Science B.V. All rights reserved.