Slag

For 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. 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 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 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.

Also, it is often used in concrete as a replacement of cement by the concrete producers due to the increasing demands of Portland cement. Slag has a pozzolanic reaction which allows the increase of concrete strength. Slag has proven to produce very good transport properties in concrete allowing an increased durability.

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, 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. It 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 deliberate introduction and combustion of large coke particles into the slag and 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, as a result of this and of long-arc operation, 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.

Slag

Foaming slag

See also