Why can some modern copper smelters operate without auxiliary fuel?

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Molten copper pours from a large industrial furnace into rectangular molds at a copper smelting facility, with glowing metal and industrial equipment visible. — copper processing Molten copper being made into plates.

Modern copper smelters can operate without auxiliary fuel—a process known as autogenous smelting—by utilizing the heat generated from the oxidation of iron and sulfur within the concentrate.1

In newer flash or bath smelting processes, dry concentrate is combined with flux and subjected to a blast of oxygen or oxygen-enriched air. This reaction is highly exothermic, meaning it releases sufficient heat to smelt the material into liquid matte and slag without the need for the external fuel sources required by traditional reverberatory or electric-arc furnaces. Beyond eliminating the need for auxiliary fuel, these processes are more efficient at capturing sulfur dioxide for the production of sulfuric acid.1