Cement and Energy

,· process. The grindi ng balls gain their kinetic energy together, which is then transformed into gri nding energy. The grinding process is achieved when a sufficient amount of material comes between the grinding media and the mill shell lining. The grinding balls come in differen t sizes depending upon the task intended, and as a result they nrc activnted differentl y by the mill shell lining. The lirst chamber of a ball mill produces an accelerating and lifting eflcct by the shell l_i ning on to the grinding halls to achieve impact grinding which is known as cataracting. The second chamber of the mill creates a I ifti ng effect known as cascading. Recently there have been a great number or publications deal ing wi th aspects of energy saving by mill intern als. They include some or the following ideas: • Roll and grooved liners have a big savi ng effect in specifi c energy consumption of ball mills. This is due to the much lower activation of grinding balls and material to be ground with this type of lining compared to common lining systems. The use of this lin ing system is recommended onl y for certain conditions because in general use this system is not particularly energy saving. • The type of grinding media that is used in ball mills al so influences the specific energy con sumption of the mill. It is known that cylpebs show a higher energy consumption than the same amount of grinding media. This is due to cylpebs needing more energy to move and roll against each other than grinding media. In certain designs of grinding mills this is used purposely as cylpebs also have a higher specific surface than grinding balls of the same mass whi ch in turn aims towards a better grinding efficiency. • Smaller grinding balls can be used to produce the same effect as cylpebs which accounts for the decrease in the use of cylpebs in cement production. • Grinding with a charge of cylpebs produces much more grinding heat than when grinding wi th ball charges with temperatures differing between 10 and !5° C. • Grinding quartz sand in ball mills has shown that, at the same production rate, the use of heavi ly deformed grinding media resulted in 30 per cent higher energy consumption than by using well shaped, fully rounded grinding balls. By looking at the design of the lining plates 111 a two– chamber mi ll (Figure I ) it is possible to see that the motion and circulation of the mill shells is of great importance. The aim of the design is to achieve a constan t relati ve mot ion between the grindin g media and the material to be ground . In the second chamber the mill shell has two tasks, firstl y, grinding the media and material and secondl y, the grinding media has to be classified hy size. It is on ly when both these operations take place that energy is etTiciently used. For the mi ll shell lining to work effect ively throughout its life span it is advantageous for it to be made of a cast alloy. Cast material min imises wear and provides a better guarantee agai nst breakage. INLET FLOW OF MATERIAL DIRECTION ~ OUT .... . . LET ~:NROTA- '\.\~~:·'"'~ Compartments Figure I Design ofLiners in 1st and 2nd compartments 5