COMMINUTION TECHNOLOGY AND ENERGY CONSUMPTION

Abstract

The ball mill has been used for over 100 years for grinding raw meal, coal and cement, and has proved to be universally serviceable and reliable in spite of the relatively high energy consumption. However, the limitations of ball mills have become apparent with the construction of large production units. Even if the energy costs were not a prime concern it would only be possible to construct a ball mill with very high output with great difficulty and at high cost. Vertical roller mills displaced ball mills some time ago in new plants for producing raw meal and pulverized fuel. With appropriate adaptations the vertical roller mill can also be used for difficult materials and high levels of fineness as well as for the majority of types of petroleum coke. The energy consumption of older mills can also be optimized, e.g. by incorporating a high-performance classifier for raw meal production. The ball mill has now been completely replaced by vertical roller mills and high-pressure grinding rolls for grinding granulated blastfurnace slags. This affects both the entire fineness spectrum from 3 000 to over 6 000 cm2/g Blaine and the specific energy consumption, which under some circumstances is only half that of a ball mill depending on the fineness of the finished product. The wear costs can now also be lower than those of a ball mill. Roller mills cannot yet completely replace ball mills for cement grinding, but they already cover the ever expanding sector of blended cements. Good results are also being achieved with the use of high-pressure grinding rolls in conjunction with cascade separators followed by final grinding in a ball mill. The manufacturers of large gear units are now able to produce drive ratings greater than 10 000 kW. The progress in wear technology indicates that the service lives or wearing parts in roller mills can be increased still further so that operating costs can be lowered still further by increasing the intervals between maintenance and maximizing availability. The arrival of the latest instrumentation and control technology as well as expert systems has made grinding plants substantially easier to operate. It is now possible for every grinding plant to have fully automatic start-up and shut-down, and to operate at optimum capacity and energy utilization close to the quality targets.3