Why are tin concentrates roasted before smelting?

In the context of tin processing, low-grade concentrates derived from complex ores are roasted prior to smelting primarily to drive off sulfur.1

This process, conducted in a reverberatory or multiple-hearth furnace at temperatures between 550 and 650 °C (1,025 and 1,200 °F), facilitates oxidizing, reducing, or chlorinating reactions depending on the specific impurities present. Following the roasting stage, the material is often leached with water or acid solutions to remove any impurities that were rendered soluble during the heat treatment. This preparation ensures that the furnace feed consists primarily of tin oxide, which is then ready for the chemical reduction process required to produce tin metal.1