Air-cooled blast furnace slag (ABS)

Iron production is via a continuous batch process with the iron and slag produced in the blastfurnace at a temperature around 1500ºC. The furnace is fed repeatedly with materials in a sequence of a 'coke dump', followed by a 'ferrous dump'. The 'ferrous dump' typically consists of a pre-blend of mainly sinter, but also graded lump iron ore and iron ore pellets, also known as acid pellets. Sinter typically consists of 10% lime derived from calcined dolomitic limestone and crushed fine ores which consist of 60% Fe (90% Fe₂O₃), silica and alumina depending on the source. Approximately 10-20% of sinter is recycled materials from the iron and steel works. The acid pellets consist of metal iron ore balls, approximately 1cm in diameter which have been fired with bentonite as a binder. The 'ferrous dump' pre-blend, also known as the 'burden', is introduced into the top of the blastfurnace via a hopper with an airlock to prevent gas escape. The burden uniformity controls the BFS composition.

The combined oxides form blastfurnace slag, which has a lower density than molten iron and hence floats as a separate layer on top of the iron. The molten slag and iron collect at the 'hearth' at the bottom of the furnace. The iron and slag are 'tapped' (removed) together at regular intervals, held in a trough to allow separation, then the slag is allowed to run into open air pits where it cools in layers. The cooling is facilitated by water spraying which causes the slag to crystallise and crack, aiding excavation from the pit. The cooling conditions affect gas liberation (i.e. escaping gases become trapped in the slag), which in turn affects the slag porosity and density.

Typical Physical Properties

Applications

• Granular and General Fills
• Capping
• Pipe bedding
• Aggregates for Asphalt
• Aggregates for surfacing dressing
• Hydraulic Bound Mixes
• Aggregates for concrete
• Filter media
• Agricultural