Study of the leaching of goethite and hematite.

The direct dissolution of various samples of hematite and goethite in hydrochloric, sulphuric and perchloric acids using a particulate (-65, + 150 mesh) feed has been investigated. The reductive dissolution of hematite samples using acidified sulphur dioxide solutions was also investigated. Goethite and hematite leached by a common mechanism with comparable rates. The various shapes of leaching curves obtained for different hematite samples can be correlated with their grain size to particle size ratio. An increasing rate of dissolution was found for a large number of grains per particle, and an approximately constant rate was observed for particles containing relatively few grains per particle. The effect of acid concentration on the rate of dissolution for different acids has led to the modification of a previously proposed chemical mechanism dependent on the complexing power of the anion of the acid with iron. The increase in dissolution rate for a weak complexer was proportional to the acid concentration e.g. HCIO₄, and for a strong complexer, if was proportional to the acid concentration squared e.g. HCl. The rate of dissolution in the presence of sulphur dioxide was extremely rapid compared with the acid dissolution.