High alloy and stainless steel slags are by-products of the manufacture of these steels by either the electric arc furnace process (EAF) or the argon oxygen decarburisation (AOD) process.
An EAF produces steel by the melting of recycled steel scrap, using heat generated by an arc, created by a large electric current. Lime added (as flux) combine with silicates and oxides to form liquid slag. The EAF slag has a lower density than steel and therefore floats on top of the molten steel. Once the steel has been tapped into a ladle, the slag is either poured from the furnace onto the floor where it is cooled and removed for further processing, or into slag pots where specially designed vehicles are used to transport the slag to purpose-built lagoons, where it is tipped and cooled prior to onward processing.
The AOD process is a secondary steel making process for the refinement and manufacture of different grades of stainless steels in a ladle furnace. This process produces a slag that is then further treated to prevent dusting of the aggregate.
Both these slags are removed and tipped into pits to are air cool with the assistance of water. Once cooled the excavated slags are processed through a metal recovery facility where entrapped steel is removed. Following this the slag is treated in a similar fashion to natural aggregate being crushed and screened into the required size fractions and stockpiled.
Due to the presence of free lime (calcium oxide) and depending on the final end use of the EAF slag, the stockpiles may be weathered for a designated time period in controlled areas. The weathered slag is then tested to ensure the expansion properties of the material as identified in BS EN 1744-1 meet the requirements of its end use.
Compound
Typical value
High Alloy
AOD
Fe2O3
27
1
CaO
30
53
SiO2
15
26
MgO
8
6
Al2O3
2
Property
Relative Density (SSD)
3.40
2.98
Water Absorption
1.8
1.75
Magnesium Sulfate Soundness
1.0
Polished Stone Value
63