The final manufacturing stage at a cement plant is the grinding of cement clinker from the kiln, mixed with 4-5% gypsum and possible additives, into the final product-cement .
In a modern cement plant the total consumption of electrical energy is about 100 kWh/t . The cement grinding process accounts for approx. 40% if this energy consumption. Further the quality of the final cement is very dependent on the operation mode and product quality as well as consumption of electrical energy it is important that the cement grinding plant is adequately designed , and is operated properly .With the continuous development of technology, the use of the roller mill from its proven field of raw material grinding into the clinker grinding sector.
The working principle of vertical roller mills is based on two to four grinding rollers with shafts carried on hinged arms and riding on a horizontal grinding table or grinding bowl. In smaller mills the pressure of the grinding rollers was effected by helical steel springs, connecting the hinged arms (spring type roller mills), whereas in larger mills the grinding pressure of the rollers upon the mill feed is effected hydropneumatically.
The material falls onto the center of the grinding table; when it leaves the grind ing table it is entrained in the stream of air and car ried to the separator. The classified coarses are then returned directly onto the grinding bowl. Cyclones are used to collect the finished product carried out of the separator in the stream of air. Most of the air is returned to the mill, while a small portion is carried to the dust collector.
As less energy is expended in the roller mill itself than in a ball mill, less heat is produced by the grind ing process. The problems involved in dissipating heat are simpler to solve, especially since the large quantity of air required for conveying the material in the mill can also be used for cooling.
In many cases the roller mill has produced cements with low specific surfaces whose final strength values were superior to those of cements produced by ball mills.Vertical roller mill cements can have lower specific surfaces for particular final strength values than ball mill cements. Without exception, cements produced with the roller mill have a more favorable strength development than comparable cements with the same specific surface produced in ball mills. The results as a whole show this cement to be of good quality.
The net rate of wear in the roller mill is only about half of what it is in the ball mill. However, in the roller mill there remains a larger non-utilizable resi due from the parts which undergo wear, so that the overall costs in respect of wear are roughly the same for the two grinding systems.
The vertical roller mill is better to ball mill in respect of overall economy.