Slag

For other uses, see Slag (disambiguation).

A path through a slag heap in Clarkdale, Arizona, showing the striations from the now rusting corrugated sheets used to hold it back.

The Manufacture of Iron -- Carting Away the Scoriæ (slag), an 1873 wood engraving

Slag is 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 sulfides (see also matte) and metal atoms in the elemental form. While slags are generally used as a waste removal mechanism in metal smelting, they can also serve other purposes, such as assisting in smelt temperature control and minimizing re-oxidation of the final liquid metal 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. However, in many smelting processes, oxides are introduced in order to control the slag chemistry, assisting in the removal of impurities and protecting the furnace refractory lining from excessive wear. In this case, the slag is then termed synthetic. A good example is steelmaking slag: quicklime and magnesite are introduced for refractory protection, neutralising the alumina and silica separated from the metal, and assist in the removal of sulfur and phosphorous from the steel.

Ferrous and non-ferrous smelting processes produce different slags. 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 minimize iron loss and so mainly contains oxides of calcium, magnesium, and aluminium.

In some places in northern Michigan, the slag waste was dumped into the water. This combined with such minerals as dolomite and created a glass-like rock that is very attractive. These slag rocks formed green, purple and bright blue mixtures that are often used by local jewelers in jewellery.

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

Ground granulated slag is often used in concrete in combination with Portland cement as part of a blended cement. Ground granulated slag reacts with water to produce cementitious properties. Concrete containing ground granulated slag develops strength over a longer period, leading to reduced permeability and better durability properties. Since the unit volume of Portland cement will also be reduced, concrete is less vulnerable to alkali-silica and sulfate attack.

Basic slag

Basic slag is a byproduct of steelmaking by the basic version of the Bessemer process or the Linz-Donawitz process. It is largely limestone or dolomite which has absorbed phosphate from the iron ore being smelted. Because of the slowly-released phosphate content, as well as for its liming effect, it is valued as fertilizer in gardens and farms in steelmaking areas. According to the American Association of Plant Food Control Officials, basic slag must contain at least 12% total phosphoric acid (P₂O₅) or be labeled "low phosphate".

See also

• Cement
• Dross
• Fly ash
• Ground granulated blast furnace slag
• Matte (metallurgy)
• Pozzolan
• Slag heap
• Ricky Pitcher (As in 'Pitcher you...')

References

Bibliography

• Dimitrova, S.V. (1996). "Metal sorption on blast-furnace slag". Water Research. 30 (1): 228–232. doi:10.1016/0043-1354(95)00104-S.

• Roy, D.M. (1982). "Hydration, structure, and properties of blast furnace slag cements, mortars, and concrete". ACI Journal Proceedings. 79 (6).

• Fredericci, C. (2000). "Crystallization mechanism and properties of a blast furnace slag glass". Journal of Non-Crystalline Solids. 273 (1–3): 64–75. doi:10.1016/S0022-3093(00)00145-9. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)