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Slag is the (lighter) dross solution that floats on top of the (heavier) molten steel that needs to be separated during refining. Slag mostly originates from:

  • slag forming additives (lime and fluxing agents)
  • byproducts of steel deoxidation products (Al2O3, MnO, SiO2)
  • remaining processes of ore gangue/scrap (FeO, MnO, P2O5, Cr2O5)
  • coal ash,
  • fluxing oxides/fluorides (Al2O3, SiO2, CaF2, Fe-oxides)
  • wear of refractory (CaO, MgO)


When slag forming materials are injected into the molten steel, slag particles begin to rise (as they are lighter than the steel) and accumulate at the top surface of the melt. It is worth noting that, even nowadays, the physical measurement of the exact total basicity of a furnace slag is still not possible. Even with optical basicity calculations, the physical properties of the slag is not fully described due to the fact that, optical calculations are only useful for a completely liquid slag.


Thus, applications are mostly based on the calculation of the basicity of the slag (ratio of basic oxides to acidic oxides) in which one makes assumptions or rely on short versions of practical ratios:

(%CaO + %MgO + % Na2O) / (%SiO2 + %Al2O3 + %Cr2O3 + %FeO + %Fe2O3 + %MnO + %CaF2)

where the relative basicity/acidity difference of oxides are ignored. However, basicity ratios help to estimate the sulfur removal capacity and to estimate to reduce refractory wear due to interaction.


Slag significantly effects the quality and the properties of the steel.  Proper slag formation is a crucial part in clean steelmaking as in the old proverb: “Take care of your slag, the steel will take care of itself”.


What is the Function of a Slag?

Slag is essential: 

  • to cover and protect the molten steel against oxidizing atmosphere
  • to absorb deoxidized compounds (SiO2, Al2O3, etc.) and trapped impurities and inclusions
  • to help desulfurization process
  • to improve the quality of the steel

to protect/be compatible with the refractories