Ball mills and vertical mills use different forces to achieve the necessary size reduction of clinker and gypsum plus other materials – such as pozzolan, flyash, limestone and slag – that may also be part of the feed to the mill.
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.
The wear rates for a ball mill are higher than for LM vertical roller mill, the cost of the replacement parts is lower, and the opposite applies for the LM vertical roller mill, i.e. lower wear rate but higher replacement costs.
One of the perceived difficulties with the vertical roller mill when they were first proposed as an alternative for cement milling was the particle size distribution(PSD)of the product. Due to the high circulation factor in the mill and the high separation efficiency, vertical roller mill have the tendency to produce with a steep PSD curve resulting in cement that is dissimilar to that produced in a ball mill (a steep PSD cement results in high water demand cement).
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