Steel Slag Grinding: Pioneering New Grinding Solutions for Green Building Materials

Steel slag is an industrial waste from converter and electric arc furnace steelmaking, mainly composed of calcium silicate and calcium ferrite.
It accounts for about 14% of crude steel production. Steel slags contains around 50% of minerals like tricalcium silicate (C3S) and dicalcium silicate (C2S), which have hydraulic properties.
Currently, fine grinding of steel slags as cement or concrete additives is an important way to achieve high-value utilization. However, due to calcium ferrite, RO phases, and metallic iron, steel slag is difficult to grind. When grinding to 400 m²/kg or above, traditional ball mills increase energy consumption significantly. Therefore, China has been exploring more energy-efficient grinding technologies and equipment.

Grinding characteristics of steel slag

Steel slag forms at high temperatures (above 1580°C), where it absorbs impurities like FeO and MgO, forming large crystals. Petrographic analysis shows the main mineral components in steel slag are plate-like tricalcium silicate and spherical or rounded dicalcium silicate. Next are calcium ferrite and RO phases. Tricalcium silicate can reach a maximum size of 1998 μm, with MgO particles in its inclusions ranging from 142 to 271 μm. Metallic iron in steel slags appears mainly as spherical inclusions, with a typical size of 100–300 μm, and up to 3mm at maximum. Dicalcium silicate can also reach a particle size of 943 μm.

Through comparative studies of the refractory phases and cementing properties of steel slag, it was found that the refractory components in steel slag are calcium aluminate and magnesium iron solid solution. These phases have low hydration reactivity. On the other hand, tricalcium silicate (C3S) and dicalcium silicate (C2S) in steel slag have better grindability, slightly better than slags. However, their hydration reactivity is significantly lower than that of slag. C3S and C2S in steel slag have dissolved a considerable amount of foreign ions.

Therefore, to activate the hydraulic cementing properties of C3S and C2S in steel slag, the slag must be finely ground to a high fineness. This will cause distortion in the mineral structure, reduce crystallinity, and lower the bond energy of mineral crystals. As a result, the reactivity is increased, allowing for a higher incorporation rate of steel slags in cement and concrete.

Steel slag grinding technology

Grinding process with ball mill as final grinding equipment

The ball mill is a traditional device for grinding materials after simple mechanical crushing. With continuous advancements in ball mill technology, it is now capable of grinding hard substances, such as steel slags. Ball mills offer strong adaptability and a large crushing ratio. They allow simultaneous grinding and drying. The structure is simple, and maintenance is easy. Ball mills have good sealing, stable operation, and reliable performance. These features make them widely favored, especially in cement grinding. Their history is nearly as long as the cement industry itself.

The ball mill allows adjustment of the grinding media specifications and materials based on material properties. This enables it to grind hard steel slag. However, grinding steel slag powder with a specific surface area of 400 m²/kg consumes around 100 kW·h/t. Despite its advantages, the ball mill has significant drawbacks. It has an expensive configuration, severe wear, low efficiency, and high energy consumption.

For cement production, the energy consumption per ton of cement is no less than 70 kW·h, with only about 5% of this energy used to increase the material surface area. Most of the energy is converted into heat and sound energy, which is wasted. The high energy consumption and significant grinding losses limit the development of ball mills in the steel slag grinding field.

For this reason, the grinding industry focuses on improving grinding efficiency. It aims to reduce energy consumption and minimize steel wear. This drives the research and development of new grinding equipment and technologies.

The combined grinding process of roller press and ball mill

The combined grinding process of roller press and ball mill consumes less energy than using the ball mill alone. The energy consumption per ton of steel slag powder with a specific surface area of 400 m²/kg is around 80 kW·h/t.
However, the system still faces some issues limiting continuous production, such as metallic iron accumulation, low drying efficiency, poor classification efficiency, and roller press feeding control.
Nonetheless, this demonstrates that the combined grinding technology is more advantageous than single-stage grinding technology for steel slag grinding.

Conclusion

In conclusion, steel slag grinding plays a crucial role in enhancing the value of steel slag by transforming it into useful materials for cement and concrete production. Advanced grinding technologies, such as combined roller press and ball mill systems, offer significant energy savings and improved efficiency. However, challenges such as iron accumulation and low classification efficiency still exist. Continued development and optimization of grinding techniques will further unlock the potential of steel slag in various industrial applications.

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