Slag is a by-product of metal smelting and is used in various industrial processes, including as a component of concrete, road materials, ballast, and fertilizer. It is produced during smelting as unwanted impurities and as a protective crust of oxides on top of the metal. After aging, it can be sold for use in other industrial processes.
Slag is a by-product of metal smelting and hundreds of tons of it are produced each year worldwide in the metal refining process and in the production of alloys. Like other industrial by-products, waste actually has many uses and is rarely wasted. It appears in concrete, aggregate road materials, as ballast, and is sometimes used as a component of phosphate fertilizer. In appearance, the slag appears as a loose collection of aggregates, with lumps of various sizes. It is also sometimes called ash, in reference to its sometimes dark and crumbly appearance.
This substance is produced during the smelting process in several ways. First, the slag represents unwanted impurities in the metals, which float to the top during the smelting process. Second, the metals begin to oxidize as they are smelted and the slag forms a protective crust of oxides on top of the metal being smelted, protecting the liquid metal underneath. When the metal is smelted to satisfaction, the slag is skimmed off the top and disposed of in a slag heap for aging. The aging material is an important part of the process, as it must be weathered and allowed to break down slightly before it can be used.
Common slag components include oxides of silicon, aluminum, and magnesium, as well as sulfur, which is always present. It also contains phosphorus, calcium, ash, flux residues such as limescale and residues of chemical reactions between the metal and the furnace lining. Other compounds found in this material depend on the type of casting. Non-ferrous smelting, used to refine copper, lead and similar metals, produces highly ferrous slag, since iron is an undesirable element. Ferrous smelting, such as that used to make steel, creates non-ferrous slag, as all of the iron is used in the smelting process.
After the slag has been allowed to age, it can be sold for use in other industrial processes. One of the most common is as a constituent of concrete and cement. Slag works very well as a loose aggregate and can be milled to produce a more uniform grain. It is also mixed with road-building materials, used as ballast on large trains and trucks, and is also applied as a phosphate fertilizer. When used as a fertilizer, the slag is ground very fine before being spread and releases nutrients slowly because it takes a long time to break down.
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