Suppliers of Slag
Slag is a glassy by-product which is produced when smelting ores to separate the metals from the minerals in which they are contained. The actual slag chemistry can vary acording to the requirements to remove other elements from the metallic ores in order to improve the purity of the metal itself. Different slags can demonstrate significantly different properties depending on the chemistry of the ore, the process technologies and the contaminants to be removed.
In the case of the steel industry, during the production of pig iron, a high proportion of the oxide impurities present can be separated from the metal by including prescribed amounts of suitable fluxes in the Blast Furnace burden to form a fluid slag. With most iron ores, the “fluxable” oxide impurities are acidic in character (SiO2 and Al2O3) and are therefore neutralised by the use of basic fluxes such as limestone (CaCO3) and dolomite (CaCO3MgCO3). For optimum performance of the blast furnace, steel producers normally aim for a basicity ratio of 1.1 to 1.2 which is defined as:
wt%( CaO + MgO)
Basicity ratio = ------------------------------
wt%( SiO2 + Al2O3)
Typical slag compositions thus tend to be in the range of 35-45% Ca0; 5-15% MgO; 30-40% SiO2 and 5-15% Al2O3 with 1-2% S and 0-1% alkali oxides.
When the slag is tapped from the Blast Furnace, it can either be air cooled or water cooled. It is the rapid cooling associated with the use of water which creates a granular glassy material which gives blast furnace slag its cementitious properties. A rule of thumb is that some 300kg of blast furnace slag is produced per tonne of iron although there can be marked variations.
For use in the cement industry, granulated blast furnace slag (gbfs) can be inter-ground with the cement clinker or ground and stored separately – known as ground granulated blast furnace slag (ggbfs). Slag which has been ground separately can then be blended with cement at the cement plants or sold directly. Slag cement blends can contain in excess of 70% slag for certain specialised applications though the normal range is between 20% and 50% slag. From a technical perspective, slag is normally associated with improving chemical resistance and durability of concrete as well as lowering the heat of hydration in mass concrete pours.