Various types of cooling methods of EAF steel slag to be used as cement replacement material

Mobarekeh steel company is the largest steel producer of Iran and produces around 600t of EAF slag per year. The slag produced in this plant normally cools in the air, but the mineralogical composition can be adjusted by appropriate cooling methods in order to fulfill the mineralogical requirements of cement production. In this work various types of cooling methods were adopted in order to achieve a slag product possessing very uniform and spherical glass granules suitable for use in Portland cement.

Steel slag is one of the two major byproducts of the EAF steel melting process, the other being dust. During melting, oxidation of phosphorus, silicon, manganese, carbon, and other materials occurs and a slag containing some of these oxidation products forms on top of the molten metal. Once cooled the molten slag solidifies to form glassy and/or crystalline minerals depending on the specific rate and method of cooling. Molten slag is collected in iron slag pots during the melting of raw materials in the electric arc furnace. The slag is then cooled by pouring it into the designated area.

TXI, the Texas-based US cement firm, developed and patented a method which allows this EAF slag to be used in the manufacture of high quality cement. Such use of slag conserves resources and reduces energy consumption as compared with traditional cement manufacturing. The chemical composition of various types of steel slag are shown in Table 1.

Table 1 - Chemical compositions of various types of slag.

There are three oxides that are mostly present in various types of slag; Al₂O₃, CaO, SiO₂. The relative concentration of these oxides in different types of slag, in addition to that of Portland Cement are shown in the ternary phase diagram (Figure 1). EAF slag is a class of basic slag and as such has a similar composition to that of Portland cement, meaning the former can be used as a replacement for the latter.

The process of slag cooling has a significant effect on its latent hydraulic activity. If it cools slowly in the air, various types of crystals form which are mostly mervinite, melilit and magnetite. Formations of any crystal from the melt will lower its internal energy and the slag will loose its hydraulic activity. Investigations made on the crystal structure and particle size distribution of the glass in slag, showed that isomorphic replacements of Al³+, Mg²+ Ca²+ cations (among others) with Si4+ will produce a charged network with particle size distribution of around 300–1000Å. This feature of EAF slag can lead to hydraulic activities comparable with those of Portland cement.^1-12

In order that a material can be considered as a suitable replacement for cement, it should have a hydraulic activity that is calculated based on its chemical composition as shown in Table 2.

Table 2 - Chemical composition of EAF slag of the Mobarekeh Steel Company.

Experimental

In order to have a homogenised composition of EAF slag, approximately 1t of various batches of air-cooled slag were mixed and 50kg of it was charged and melted in a rotary kiln. The molten slag was poured in a refractory lines pot and transferred to a tank that was designed to allow jets of air, water, and mixtures of both to cool the slag according to our requirements. The discharging process was adjusted every time to give the best results according to the particle size and mineralogical composition. The particle size distribution and shape of it were investigated with a light microscope. A selection of granules were ground into a powder and used for mineralogical investigations by X-ray diffraction (XRD).

Results and Discussions

Composition of Mobarekeh steel plant EAF slag.
In order to investigate the mineralogy of the anhydrous phases present in air-cooled EAF slag a randon sample of the batch was investigated by X-ray diffraction (Figure 2). Based on this pattern, Wustite is the major anhydrous phase while some magnetite and α-CaSiO3 are also present, which is one of the anhydrous phases present in Portland cement.

Air cooled molten EAF slag
The particle size distribution of this sample (shown in Figure 3) lies in the range between –0.3 to 2mm. The percentage of granules 0.3mm and 2mm in size is 21% and 24% respectively.

From the XRD (Figure 4) it is clear this sample exists in an essentially glassy state. However, some crystals of C₂S have also formed which is shown by a sharp peak at 2θ = 34°.

Water cooled molten EAF slag
The particle size distribution of water-cooled slag are shown in Figure 5. According to the results, the average particle size distribution is between 1mm – 2 mm, while the quantity of fine particles around 0.3 mm in size is approximately 14%. The shape of the granules was investigated by light microscopy and shown to be essentially spherical and uniform (Figure 6).

The XRD pattern of this sample is shown in Figure 7. According to this pattern, the sample is mostly amorphous but some crystals of magnetite are also present, with the most intense peak for this mineral at 2θ = 36˚.

Air and water cooled molten slag
In this experiment molten slag was cooled by a combination of water and air. The particle size distribution of this sample is shown in Figure 8. The particle size is mostly around 2mm. It seems from the results that the particle size of this sample is coarser than the samples cooled by water and air alone.

The XRD pattern of this is shown in Figure 9. From this diffraction pattern the amount of glassy phase is greater than in the samples previous sample. Moreover, crystalline C₂S has also formed which may be the result of the generally increased coarseness that can lead to crystallisation during cooling.

Water pool cooling of molten slag
In this method molten slag was dropped in a pool of water, within which a jet of water was running about 10cm under waters surface. In this way the molten slag is first cooled by water and then is crushed by the jet of water under the surface.

The particle size distribution of the resulting grains was not uniform. In addition, the shape of the grains was distinctly aspherical. Mineralogical examinations of this sample by XRD also showed that the amount of glass is low while some crystals of wustite had formed Figure 10).

Conclusions

• EAF slag of Mobarekeh steel company is a basic slag and its chemical composition is similar to the composition of Portland cement.
• Based on the chemical composition, the calculated hydraulic modulus of EAF slag of Mobarekeh steel company possesses good hydraulic activity.
• The hydraulic activity of any type of raw material is mostly dependent on its mineralogical composition and the amount of glass phase present in the solid grains.
• According to the various types of cooling methods applied to the re-melted EAF slag, more rapid cooling increased the amounts of glass phase present. This can improve the hydraulic activity of EAF slag.
• From the XRD pattern of the granules created from all the varous cooling methods no free lime is present. This can prevent latent decomposition of it in the cement and concrete.
• All cooling procedures applied to the molten slag produced good results, but cooling by the mixture of water and air gave more uniform granules and a higher amount of glassy phase. This can lead to better hydraulic activity of EAF slag making it more suitable as a replacement for Portland cement in slag cement.

References

References are available from the authors.