Industrial slag is also called molten slag. The melt that floats on the surface of liquid substances such as metal during the pyrometallurgical process is composed mainly of oxides (silicon dioxide, aluminum oxide, calcium oxide, magnesium oxide), and often contains sulfides and contains a small amount of metal.

The composition of industrial slag is adjusted by adding an appropriate amount of flux (lime, quartzite, fluorite, etc.). In the smelting process, by controlling the composition and properties of the slag, the products of gangue and oxidized impurities can be smoothly separated from the molten metal or sulfur, the harmful impurities in the metal are removed, and the non-metallic inclusions in the liquid metal are not directly affected. Furnace gas is polluted, and useful metal oxides are enriched; it is also a resistance heating element in electric furnace smelting. Slag plays a decisive role in ensuring the smooth progress of smelting operations, the quality of smelted products, and the metal recovery rate. For example, in steelmaking operations, there is a saying that "good slag can be used to make good steel".

According to different metallurgical processes, slag can be divided into smelting slag, refined slag, synthetic slag; according to the nature of the slag, there are alkaline slag, acid slag and neutral slag. Many slags have important uses. For example, blast furnace slag can be used as a raw material for cement; high-phosphorus slag can be used as a fertilizer; vanadium and titanium-containing slag can be used as raw materials for refining vanadium and titanium, respectively. Some slag can be used to make industrial slag cement, industrial slag brick, industrial slag glass, etc.