Basic slag is a by-product of the steel industry. In the steelmaking process the impurities in the crude iron are oxidised and together with lime form the slag. This slag which contains the phosphorus in the form of silicophosphates is ground to a fine powder. Traditionally ball or tube mills are used, but recently in this country a new Aerofall mill has been installed, in which the slag is mainly self-grinding.
Current advances in the technology of basic slag production include methods of granulation to avoid the dust problem associated with the normal powder product, magnetic beneficiation to yield an enriched product, and the use of additives to improve the physical characteristics. They also include the production of mixed fertilisers from basic slag, notably potassic slags.
Whereas basic slag supplies only about 10% of the world usage of phosphatic fertilisers, in W. Europe the proportion is nearer 25%.