Ceramics Research Centre
 

Mineralogy applied to steel slag
Steel slag is a voluminous by-product of steel making (~90 kg slag per tonne steel) and forms when pig iron from the blast furnace is refined to steel. Potentially, steel slag can substitute for quarry stone in many applications (road construction) provided well-defined physical properties can be guaranteed. Using steel slag is environmentally friendly and contributes to a sustainable society. Dutch environmental regulations dictate the properties for reuse of slag as sand/gravel substitute (BRL 9310)
In the steel plant it is beneficial to use the slag, as a protective coating on the inside of the converter vessel after the steel is refined.
The environmental function as well as the coating function of the slag have specific but different requirements. In order to meet both, we use an integrated approach to slag research at CRC, combining multiple mineralogical measurement techniques on a microscale, with thermo chemical modelling of phase equilibria, and with true scale modelling of actual process steps and eventual plant tests. In this way we try to reach an optimum for all functions of the steel slag during and after production.
 
Slag research
In our research we have determined the crystallization sequence of slag to find out the successive appearance of solid phases. Steel slag looks like a volcanic rock (left) and consists of a mixture of oxide compounds, chemically dominated by CaO (~50 wt%), FeOx (15-35 wt%), SiO2 (~ 15 wt%) with subordinate amounts of MgO (~7 wt%), MnO (~ 5 wt%), Al2O3, P2O5 and TiO2 (all around 1-2 wt%).
The phase assemblage of the solid product is well known and consists primarily of the five compounds: lime (CaO); di- and tri- calcium silicate (dark phases in micrograph, C2S, C3S); magnesio-wuestite (Mg,FeO) (white, RO) and srebrodolskite (Ca2(Fe,Al,Ti)2O5 (bluish phase, C2F), as shown in the reflected light microscopy image here.
It is very difficult to quench partly solidified slag to a glass and crystals. Therefore in-situ techniques, suitable for measurement at high temperature, such as DTA and high-T-XRD are most useful to study the crystallization sequence. The heat flux and crystallization sequence during solidification of a steel slag melt were studied to determine the controlling parameters for slag freesing onto the inner-wall of the converter vessel. In the DTA cooling curve (below left) the peaks indicate the crystallization and associated heat effect of each new compound in the slag, which can be identified in the HT-XRD patterns (below right).
 
DTA HT-XRD F*A*C*T modelling
 
A crystallization sequence and the associated heat flux can also be predicted using phase equilibria model calculations in F.A.C.T. As shown above, the same compounds are found to crystallize in the slag as identified in the DTA and HT-XRD experiments.
 
Knowing the crystallization sequence we can introduce cooling steps in the production process to give the slag the right properties. The data are used for Finite Element Model calculations on heat flow and slag freesing onto the converter wall during routine operation in our steel plant. But the same data serve for FEM models to optimise the cooling of slag for recycling purposes. In this way we can produce a tough, wear-resistant rock material, an excellent substitute for quarry stone.
 
The slag, in the photograph on the right, that splashes onto the test area floor might well end up in the asphalt of a road in the future.