Operational experience with wear protection for high-pressure grinding rolls

High-pressure grinding rolls have been used in the cement industry for more than 10 years. There are now a total of 32 units in use in Germany for grinding cement raze material, granulated blastfurnace slag and cement. This article reports on the operational experience with wear protection for the grinding rolls in 21 units. As expected, the electrical power consumption is significantly lower than with ball mills but in many cases the durability of the wear protection on the grinding rollers has proved inadequate, which has sometimes greatly restricted the availability. There are now three types of wear protection in use - hardfacing, cast armouring in the form of a tyre or segments, and so-called studded armouring. Service lives of 6000 to over 16 000 h can be achieved with the welded hardfacing wear protection. Short service lives tend to occur where there are unsuitable combinations of material between the base material, buffer layer and hard layer and changing hardness gradients. Chill cast tyres can be an advantage because of the high compressive strength. One particularly positive development has turned out to be the use of tyres made of alloyed spheroidal graphite iron which have so far reached service lives of more than 10 000 h without any problems. Chill cast segments have proved less successful because of their low load dispersal. The stressing of the roller surfaces by the bed of material has to be taken into account when evaluating the durability of the wear protection. The effective stress, composed of the compressive and shearing stresses, has its maximum below the working surface. Individual forces of unknown size can act on the working surfaces of the grinding rollers as a result of individual oversize particles in the feed material, and can stress the wear protection beyond the limit of its fatigue strength.