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For the British slang usage, see Slag (slang).

For the Transformers character, see Slag (Transformer).

Slags are the by-product of smelting ore to purify metals. They can be considered to be a mixture of metal oxides however they can contain metal sulphides and metal atoms in the elemental form. Slags are generally used as a waste removal mechanism in metal smelting however they can also serve other purposes such as assisting in smelt temperature control and to minimize re-oxidation of the final bullion product before casting.

In nature, the ores of metals such as iron, copper, lead, aluminium, and other metals are found in impure states, often oxidized and mixed in with silicates of other metals.

During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. The collection of compounds that is removed is the slag.

Different smelting processes produce different slags. In general they can be classifed as ferrous or non-ferrous. The smelting of copper and lead in non-ferrous smelting, for instance, is designed to remove the iron and silica that often occurs with those ores and separates it as an iron silicate based slag. Slag from steel mills in ferrous smelting on the other hand is designed to minimise iron loss and so mainly contains calcium, magnesium, and aluminium.

Slag has many commercial uses, and is rarely thrown away. It is often reprocessed to separate any other metals than it may contain. The remnants of this recovery can be used in cement, railroad track ballast, and as fertilizer. It has been used as a road metal and as a cheap and durable means of roughening sloping faces of sea walls in order to progressively arrest the movement of waves.

See also

• Dross

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