Slag

From Wikipedia, the free encyclopedia

For slang usage, see Slag (slang).

Slag is also an early play by David Hare.

For the Transformers character, see Slag (Transformer).

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

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. 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.

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.

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 metal 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.

[edit] Foaming slag

More recently, much attention has been focused on the foaming of slags. Slag foaming is caused primarily by the generation of bubbles of carbon monoxide, carbon dioxide, water vapour, sulphur dioxide, oxygen, and hydrogen gases within the slag, which foams much like soapy water. In a basic oxygen furnace (BOF), slag foaming is caused by the combustion of carbon in the metal itself and is a drawback to the process; the foaming slag may be violently ejected (slopping), removing metal in the slag from the furnace and creating dense, brown fumes that can cause problems in the fume removal system or cause a health and safety problem. Foaming can be controlled through the injection of stirring gas through the base of the furnace, or by introducing fine particles of metallurgical coke into the slag.

However, in an electric arc furnace (EAF), slag foaming is caused by the combustion of large coke particles deliberately introduced into the slag. Foaming is vital to modern EAF operations, as the foaming slag buries the arcs, shielding the walls and roof of the furnace from the radiant heat of the arcs and transferring more of the arc's heat to the bath, making the furnace more efficient. Foaming slag is also used in the smelting of copper, nickel, chrome and (experimentally) iron. Much research is currently underway to both further understand slag foaming, and to apply that understanding to metallurgical plants around the world.