Air-cooled basic oxygen steel slag

The production of Basic Oxygen Steel Slag (BOS) is unavoidable during the steel making process, with approximately 150kg being produced for every tonne of steel. Clearly, therefore, it makes sense to utilise BOS where possible, not only to avoid the creation of unsightly slag heaps, but also to reduce the need to extract finite primary minerals. The introduction in April 2002 of taxation on primary aggregates sold for construction purposes does not apply to by-products such as BOS.

Basic oxygen steel (BOS) slag is a by-product from the further refining of liquid iron to produce steel by removal of alloy elements through oxidation and in combination with lime flux. Approximately one million tonnes is produced annually in UK shared between the three 'Corus' (previously British Steel) operations at Redcar, Scunthorpe and Port Talbot. Annual BOS production is usually fully utilised in construction materials, mainly as roadstone aggregate but also in agriculture as a liming agent, in rockwool insulation manufacture and in cement. BOS is a good example of a by-product from one industry providing raw materials for another. BOS production is controlled within specific parameters during the steel making process.

It is processed into construction aggregates broadly in a similar manner to Blastfurnace Slag- crushing and screening. Particular control is applied over the way in which it is naturally 'weathered' prior to use as roadstone aggregate to counter its inherent volume instability. It must be stated however that the mechanism's causing expansion in BOS slag are different to those that affect BFS and are not fully understood, although the hydration of 'free' CaO and also MgO in the presence of water is considered to have a significant contribution.

Basic Oxygen Steel Slag Production

The BOS furnace operates on a batch basis and is initially charged with scrap metal and lime flux. After the scrap has been charged, the molten iron (transported from the blastfurnace in torpedo cars and transferred into the steel making plant in ladles) is added into the 'tilted' furnace vessel which is then up-righted ready for the 'blow' with an oxygen lance. The blow lasts about 12 minutes but during this time the formation of the slag is critical to the formation of good quality steel. At the start of the blow a slag high in FeO and MnO is formed which quickly dissolves the lime. During the blow any silicon in the iron is oxidised to the oxide and enters the slag. Towards the end of the blow the FeO and MnO decrease. When the temperature and chemical analysis are judged to be correct, the lance is retracted. The vessel is tilted in the opposite direction with molten steel then poured from below the slag into a teeming ladle waiting on the floor of the plant. Once the steel pouring is complete the vessel is once more tilted in the original direction and the slag poured into the waiting ladles below. The slag ladles are then transported on 'kress carriers' across the steel works to open air pits where the slag is poured at a temperature of approximately 1600 C in layers enabling it to cool to ambient temperature prior to being excavated ready for processing. Water is sprayed onto the BOS slag to facilitate cooling. Approximately 150kg BOS slag is produced per tonne of steel.

Basic Oxygen Steel Slag Chemistry and Mineralogy

BOS Slag consists primarily of calcium silicates together with oxides and compounds of iron, manganese, alumina and other trace elements. The chemical composition of each batch of BOS slag varies dependent on the type of steel being produced (i.e. high, ordinary or low sulfur steel) which is largely influenced by the flux used (i.e. lime or dolomitic lime). A typical chemical analysis on processed BOS slag aggregate is: 42-44% CaO, 27-31%
Fe₂O₃, 10-12% SiO₂, 5-6% MgO, 3-4% Mn₃O₄, 1-2% P₂O₅, 1-2% Al₂O₃, 0.5% TiO₂. BOS slag is usually dicalcium silicate (bredigite) (2CaO.SiO₂), containing other elements up to 1-2% (e.g. Mn, Fe, Mg), tricalcium silicate (3CaO.SiO₂), free lime (CaO), wustite (FeO), calcium ferrite (also dicalcium ferrite and calcium aluminoferrite), and minor amounts of periclase (MgO) and magnesiowustite (solid solution of FeO and MgO). Usually around 1% free metal in fine globules is present. The proportions of each phase vary mainly with the CaO and SiO₂ proportions of the slag. Tricalcium silicate only appears at high lime/silica ratios (>>3.0). Wustite and ferrites are major phases in iron-rich BOS slags.

Basic Oxygen Steel Slag Processing

The cooled BOS slag is excavated from theopen air pits and hauled to a screening and magnetic demetalling operation to remove iron which is then fed back into the steel works for recycling. The residual slag is then subjected to a period of natural weathering typically as <125mm material in managed stockpiles. This is designed and controlled to ensure any unreacted lime is sufficiently hydrated prior to sale as roadstone aggregate.

Weathered BOS slag is then available, if required, for processing into aggregate single sizes, which are then subjected to further natural weathering in discreet stockpiles. The slag's volumetric expansion is tested after typically 3 - 6 months in accordance with the European Standard test referred to in EN1744-1 and only released for sale when expansion test data complies with internal control limits to ensure fitness for purpose.

This practice ensures any further exposed lime resulting from the crushing process is hydrated prior to sale. Any batch to batch variation in BOS slag chemistry is homogenised by the crushing and screening operation resulting in aggregates with variation in mechanical properties comparable to that which would be expected of natural aggregates.

Utilisation of BOS Slag

Air cooled BOS slag primarily provides quality controlled aggregates for use in construction. It can be used in most applications that would otherwise require the use of natural aggregates.

The principal applications include aggregates for use in asphalts and surface dressings in accordance with BS EN 13043 and aggregates for use in unbound mixtures, BS EN 13242, covering sub-bases, cappings and fills.

BOS aggregates can be blended with blastfurnace slag aggregates to produce self-hardening bases. BOS slag is particularly well suited to use in asphalt surfacings because of its high abrasion resistance and aggregate shape which contributes to surface texture which is a key requirement for providing skidding resistance.

A significant application for 'unweathered' BOS slag is as an agricultural liming agent where it neutralises the soil and provides valuable basic trace elements essential for animal well being.

A relatively new market application for BOS fines ( 0/2mm ) is in brownfield land remediation, where CaO and other beneficial trace elements are released slowly to maintain soil pH ideal for growing grasses and trees in soft landscaping.

Cement production also provides a suitable end use for BOS slag with its inherent iron and alumina providing a valuable contribution to the cement chemistry.

BOS slag acts as a source of lime, silica and alumina in mineral wool production for thermal insulation.

BOS slag has been extensively used as protective armour stone for both river, sea and coastal erosion schemes because of its high density and has played a major role in land reclamation projects (e.g. Seal Sands on Teesside).

The hard wearing properties of steel slag make it suitable as railway track ballast.

For more information contact John Bullock

Please note this website is maintained to provide information and guidance on UK issues, products and applications of those products.