Ball mills and vertical roller mill use different forces to achieve the necessary size reduction of clinker and gypsum plus other materials.
In ball milling, the size of the material is progressively reduced through the impact and attrition forces on the feed material as the rotation of the mill causes the tumbling action of the grinding media. The most common configuration for a new ball mill is a two chamber mill, with the first chamber generally containing media from around 90 mm to 50 mm, and the second chamber from 40 mm down to 10 mm media. This impact and attrition continues. through the length of the mill and the material becomes progressively finer. The cement materials have a long residence time within the mill and, as the process is relatively random in nature, a wide range of particle sizes will be created. However, there is little control over the exact degree of grinding within the mill itself apart from the various proportions of the different sized media in the mill, which is not easy to change on a day-to-day basis. The process is also inefficient – energy is supplied to turn the mill shell and create the tumbling action of the media within. This energy is then transferred into grinding energy but due to the interaction between the grinding media, a significant proportion of the energy that is input into the mill is transformed into noise and heat.
By contrast, the principle of the vertical mill is to apply forces to a thin material bed, which lies between the mill table and the rollers. The high pressure forces, supplied by a hydraulic system on the rollers, cause the individual particles within the bed to fracture. This type of grinding is much more efficient than the ball mill system, as much more of the input energy is transformed into grinding the material and less noise and heat is generated. The material has a much shorter retention time between the table and roller and therefore requires a number of passes between the roller and table before the final product size is as required. The repeat passes between the roller and the table is easily achieved due to the fact that the vertical mill has an internal high efficiency separator. Material that has passed between the table and the roller is transported in the gas stream, which passes around the edge of the table, to the separator – with the fines passing out of the mill and the coarse particles being returned to the table. Due to the repeated grinding of the material, the internal circulating load (the ratio of material in the mill compared to fresh feed) is much higher in a vertical mill – sometimes up to 700 – 800%.
The method of high pressure grinding in the VRM allows more energy to be transferred to the material and therefore reduces power consumption. Furthermore, less energy is wasted in the creation of noise and heat as occurs with a ball mill. The positioning of the separator within the mill also results in a more consistent product with less over and under-grinding, which can be a problem within the ball mill circuit. A good rule of thumb is that the kWh/t for the motor power alone is around 50% less in a vertical mill compared to a ball mill. However, the fan power for the VRM system is higher due to the higher recirculating load and the location of the separator within the mill body. Overall, when the power consumption for the two circuits are compared, power savings between 25 and 40% have been achieved, with the higher power consumption savings being achieved at the higher product surface area cements.