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Air-cooled basic oxygen steel slag (BOS)

What is Air-cooled basic oxygen steel slag (BOS)?

Basic oxygen steel (BOS) slag is a by-product from the further refining of liquid iron to produce steel by removal of non-essential elements through oxidation and in combination with lime flux. BOS slag is produced by the basic oxygen steelmaking (BOS) process – also referred to as the basic oxygen furnace (BOF) process, as air cooled steel slag (ASS) or as LD-converter slag.

Production and Processing

BOS slag is formed during the conversion of hot liquid iron from the blast furnace into steel in a basic oxygen furnace.

In this process the hot liquid iron is treated by blowing oxygen through it to remove carbon and other elements that have a high affinity to oxygen.

Lime and dolomite are added (as fluxes) which combine with silicates and oxides to form liquid slag. Small amounts of scrap iron/steel are also added to control the temperature of the exothermal reactions. The slag formed in turn absorbs the oxidized impurities (Si, P, S and Mn) from the molten iron.

The BOS slag is tipped into pits and air cooled with the assistance of water. Once cooled the slag is excavated and 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 BOS 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.

Typical Chemical Properties


% Value









Al2O3 3.2

Typical Physical Properties



Relative Density (SSD)


Water Absorption %


Magnesium Sulfate Soundness


Los Angeles


Aggregate Abrasion Value 1.8

Polished Stone Value



Applications to include:

  • Granular materials
  • General fills and capping
  • Pipe bedding
  • Aggregates for asphalt
  • Aggregates for surfacing dressing
  • Hydraulic Bound Mixtures
  • Agricultural
  • Filter media
  • Land remediation