Method and apparatus to recover energy from waste by means of combustion of such waste in industrial furnaces, particularly of the rotary type, that has an external calciner, feeding such waste into the tertiary air duct of the calciner and burning the waste inside the duct itself, to efficiently make use of the energy produced by such combustion, as a contribution of the total cost of fuel necessary to operate the industrial furnace. The method and apparatus permit the elimination of hazardous wastes that are efficiently disassociated when burned sufficiently to produce effluents that are typically non-toxic, producing greatly reduced or even no atmospheric pollution nor environmental damage.