Abstract: A method and apparatus for treating for disposal oil-contaminated clay substrates such as drill cuttings from drilling with an oil-based mud. If necessary, the substrate is pretreated with an aqueous emulsion breaker such as alkylbenzenesulfonic acid. The substrate is mixed under high shear conditions with a mineral acid such as sulfuric acid. This can be done in an agitated reactor with sequential addition of the organic and inorganic acids. The substrate is then mixed with alkaline earth such as lime in a second agitated reactor. The reactions between the acid(s), alkaline earth and substrate are exothermic and provide heat to vaporize the oil, reaction products and water. Recoverable constituents in the vapor can be condensed in a vapor collection system. The treated substrate is essentially free of oil and has a controlled water content.