Abstract: A process for treating optionally oxidized sulphidic material containing one or more heavy metals, by aerobic microbial oxidation of the optionally oxidized sulphidic material in an aqueous suspension using a sulphur oxidizing micro-organism to convert optionally oxidized sulphide into soluble sulphate at a pH sufficient to maintain most of the heavy metals present in a soluble form. A preferred sulphidic material is sulphidic waste, preferably sulphidic slag. The formed sulphate is environmentally less harmful than the sulphide, while the heavy metals compounds are suitable for recovery of the metals using known methods.