Smelting and refining metals means separating out impurities from the desired metal, and those impurities—along with any additives in the process—must go somewhere. Just as with any other manufacturing process, what is done with the byproducts is just as important as any other step, both for environmental and economic reasons. A manufacturer is responsible for handling any slag, dross or similar materials that arise from metal processing.
What Byproducts Are Created
The metals used in everyday life start as ores extracted from the earth. Mixed with the desired metal are carbonates, oxides, silicates, sulfides and more that must be separated out. These impurities, as well as metal oxides formed during smelting, collect as a glass-like liquid called slag. The type of slag formed depends much on the metal and the impurities being removed; for instance, non-ferrous smelting produces slag containing iron and silica, while the slag formed from steel results from fluxes added to remove impurities.
Removing slag first requires coaxing impurities to collect in this form, both through chemical additives and continuous stirring or agitation. The impurities, insoluble in molten metal, rise to the surface, where it can be poured off and stored in a slag pot for safe transport. When it cools, the slag takes on a solid aggregate form, with faster cooling resulting in smaller grains or pellets.
Making Use of Slag
Disposal of slag by transporting it to a landfill is common, but the environmental impact is severe and costly to clean up after. However, though it’s a byproduct apart from the valuable metal, it can still have use itself. Reprocessed and recycled slag can be used as an aggregate in producing cement or asphalt, sometimes with higher-quality results than other aggregates. Some forms of steel slag also demonstrate acid-neutralizing capabilities that can be put to use in treating acidic soil.