Five Major Grinding Processes for Steel Slag

Steel slag is an industrial solid waste produced during steelmaking in the converter and electric arc furnace. It is mainly composed of calcium silicate, calcium ferrite, and other compounds. Grinding steel slag into fine powder for use as a cement additive or concrete admixture is an important method for high-value utilization. In recent years, the application of new grinding processes and equipment for steel slag has grown. Building on traditional ball mills, China has developed advanced technologies and equipment, including roller press semi-finish grinding, roller press finish grinding, vertical mills, and horizontal roller mills.

Grinding Processes for steel slag with Ball Mill

The grinding body specifications and materials of the ball mill can be adjusted based on material properties. This allows the ball mill to grind steel slag with high hardness. However, the ball mill has drawbacks, such as high energy consumption and significant grinding loss. These limitations restrict the development of ball mills in the grinding Processes for steel slag.

Combined Grinding Technology of Roller Press + Ball Mill

Compared with ball mills, roller presses offer several advantages. These include increased production, energy savings, low noise, and low steel loss. The grindability of material particles improves significantly after passing through the roller press, reducing the energy consumption of the entire grinding system.

In a combined grinding system with a roller press and ball mill, steel slag is first squeezed by the roller press. A powder selector with drying and powder selection functions is used to select micro-powder with the desired fineness. This powder enters the ball mill for further grinding into finished products. The coarse powder is returned to the roller press for re-extrusion.

After rolling, cracks appear on the surface of the steel slag particles. This helps improve the grinding efficiency of the final grinding equipment and reduces energy consumption.

It is important to note that a considerable amount of small metal iron particles are wrapped inside the steel slag during the steelmaking process. Therefore, iron removal is crucial during grinding. First, metal iron should be extracted from the steel slag for recycling. Effective iron removal reduces wear on equipment and improves grinding efficiency. Adding multiple iron removal units to the external circulation system can reduce the iron content in the steel slag powder and protect the grinding equipment.

The energy consumption of the combined roller press and ball mill grinding system is lower than that of a ball mill-only system. The unit power consumption for grinding 400m²/kg specific surface area steel slag powder is about 80 kW·h/t. However, the system still faces challenges that limit continuous production. These include low metal iron enrichment, low drying efficiency, and poor powder selection and classification efficiency. Additionally, roller press feeding control needs improvement. Despite these issues, the combined grinding technology offers more advantages than single grinding systems for steel slag.

Roller Press is the “Line Contact” Grinding Process

The high-pressure roller press has several key characteristics: long service life, high equipment operation rate, easy maintenance, and low energy consumption.

Compared with traditional ball mills, the high-pressure roller press uses two counter-rotating rollers to squeeze the material layer during grinding. The material layer consists of many particles connected together. The applied pressure causes strong mutual squeezing and crushing between particles, which significantly improves grinding efficiency.

The energy savings of the high-pressure roller press are mainly due to the closed-loop grinding system. This system allows raw materials to directly become qualified finished products. Compared to conventional ball mills, the energy-saving effect of the high-pressure roller press system exceeds 50%. The unit power consumption for grinding 400m²/kg specific surface area steel slag powder is about 50 kW·h/t.

However, the fine powder content in the material squeezed by the roller press is relatively small. As a result, the circulation load is very high, typically more than 8 times the feed amount. The amount of micro powder in the finished product is insufficient. Although the finished product meets the required specifications, the quality of the product with the same specific surface area is worse than that ground by a ball mill. Additionally, the production capacity of a single machine is still relatively low.

Vertical Mill’s “Surface Contact” Grinding Process

Compared with ball mills, vertical mills have always been known for their high grinding efficiency and low energy consumption. However, it is generally believed that steel slag has a high iron content and the product requires high fineness, so it is not easy to use vertical mills for grinding.

Horizontal Roller Mill’s “Surface Contact” Grinding Process

Horizontal roller mill, also known as drum roller mill, is based on the principle of inter-layer extrusion of materials, and uses medium pressure and multiple extrusion to crush materials. It has the advantages of stable operation, flexible operation, online adjustable output, strong controllability, low wear and power consumption. When grinding steel slag powder to 400m2/kg, the main machine power consumption is about 45kW·h/t.

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