The high-compression roller mills used in cement plants in the 1980s were similar in principle to the double-roll compactors built in the 1850s and 1950s, although apparently the only compactor used as a high-compression mill at that time was a secondhand unit installed by the Fuller Company in its laboratory as a test unit for grinding clinker. The written discussions of high-compression mills describe both comminution and agglomeration, although there is a different emphasis on each mechanism according to the purpose of the machine. High-compression roller mills became known as high-pressure grinding rolls or HPGRs.
When used for comminution, the main application for high-pressure rolls to date has been for clinker grinding in the cement industry. Cement clinker contains lumps up to 50 mm, and finished cement must be less than 90 pm. The ball mills traditionally used for producing finished cement are energy intensive. Other mills such as ring-roller mills, hammer mills, and dry-grinding rod mills were used without success as primary mills; the dry rod mills were the most efficient, but the difficulties associated with frequent rod charging made them unacceptable (Rowland and Nealy 1969). The question was: Could a mill be devised that could be used in a grinding circuit in which less energy would be used per ton of finished cement?
During the 1970s, Klaus Schonert studied the breakage of rocks and other solids when subjected to very high pressure. In 1982, he was awarded a U. S. patent on a high — pressure roll crusher for the fine and very fine comminution of brittle solids (Schonert 1982), and a few years later he published several papers in which industrial uses of HPGRs were discussed (Schonert 1988). These rolls soon became widely used in the cement industry because of their low power consumption. We believe that Schonert’s patent was the most important one associated with dry grinding issued in the 20th century. At our invitation, Schonert wrote a brief review of progress from laboratory tests to the issue of the patent in 2002 (see sidebar). Several hundred HPGRs are now in use around the world.
On the matter of feed size, Schonert wrote that
Maximum feed sizes that can be handled by the high pressure rolls can be equated to approximately 3.5 times the resultant gap (between the rolls) for a particular material. Thus for the larger diameter rolls material with a maximum feed size of around 80 mm can easily be accepted. …the high pressure rolls can produce in one pass a rather large amount of fines, that is 20% of -32 pm material in the case of cement clinker. (Schonert 1988)
By 2000, high-pressure rolls were being used in many cement plants, and the energy savings in the circuits containing rolls and compartment mills were reported to be up to 50%, as compared with conventional ball mills. High-pressure rolls have also been used in ore concentrators, but the results have not always been satisfactory. They have been successful in diamond concentrators, because they appear to promote diamond liberation and the ore is relatively soft. With copper ores, however, the same wear problems that beset double-roll crushers for many years recurred, although new materials increased the wearing lives of the roller surfaces. It is likely that advances in materials technology and new designs for roller surfaces will continue to reduce the problems.