Cement manufacturing is one of the most energy intensive activities, which typically consumes around 100-110 kwh/t of cement produced. Dr Pandey elaborates on how energy can be saved during the comminution and pyro-processing of cement.
Comminution/grinding and pyro-processing are the two major operations in the cement process with energy consumption share of around 60 and 40 per cent respectively. Comminution of bulk solids, especially in the micron size class, is the least energy efficient operation among these two, having efficiencies of not more than 8-9 per cent. Rest of the energy is largely consumed unproductively in friction, noise, heat, etc. Therefore, any improvement in the practice of comminution would be a desirable step in improving process economics because of its role in deciding cost of breakage of raw materials/intermediates in the final product economics. Clinker grinding in its most advanced state today is dominated by any of the three types of milling systems namely close circuit ball mills, roll press systems and vertical roller mills. Let us look at some other grinding technologies, which are likely to become relevant in cement manufacturing in near future.
The basic reason behind large energy consumption in size reduction is that the particle must be stressed heavily before any substantial breakage occurs. This stress is mostly stored as an elastic energy in cracks/flaws in the particle and is lost when it fractures. As particles become smaller (typically around few hundred microns), the probability of flaws in them decreases and their strength increases. Hence, for efficient grinding of very small particles, higher applied forces and relatively high probabilities for successful application of the stress on the particles are required. Such high probabilities do not exist in the conventional ball mills as they use comparatively large grinding media. As particle size decreases, grinding media size also has to decrease for efficient grinding to take place. Further, as grinding media size decreases, media velocity has to increase to generate sufficient energy for particle breakage.
Future technology for cement milling:
These pre-conditions of efficient grinding are satisfied by stirred media mills, which can achieve very high media velocities and energy densities (300 kW/m3 of mill volume) by stirring the media at higher rates (~25 m/s tip speed). As a consequence, these mills offer significant reductions in specific energy consumption compared to conventional tumbling mills. As a result, they are likely to get increased attention in cement manufacturing due to the need of reducing energy consumption for grinding. Additionally, inter-particle comminution using high pressure compression technique is one more way of effectively reducing energy wastage. Therefore, any machine working on principle of roll press with fewer problems related to wear, maintenance, etc. is also likely to be more useful in cement grinding. There are a number of manufacturers, who supply these types of energy efficient milling systems. Following are some of the major suppliers of the technology.
Stirred Media milling:
Tower mills made by Kutoba Tower Mill Corporation, Japan are low speed vertical stirred media mills, which commonly use steel balls as grinding media. Vertimill is yet another variant of the class being manufactured by Metso minerals, Australia. The Tower and Vertimill consist of an internal screw flight agitator driven by a motor, a stationary vertical grinding cylinder, a settling classifier and a pebble port which is used to remove the grinding media. The agitators used are ´double start´ helical screw configuration wound around a central shaft. ´Double start´ refers to two screw flights being wrapped around the same shaft to increase the screw density. The ball movement and resultant grinding action within the mill are the direct result of the rotating and lifting action generated by this agitator. According to reported literature, there are over 220 Vertimill (having capacities in the range of 50-160 tph) and more than 250 tower mill installations worldwide.
Yet another variant of stirred media mills is MaxxMill made by Maschinenfabrik Gustav Eirich, Germany. Its main components are the rotating grinding chamber, one or more agitator(s), and the stationary material deflector with integrated feed pipe. Each agitator is positioned eccentrically relative to the centre of the grinding chamber. Depending on the comminution task, the agitator may rotate in the same or opposite direction as the chamber. The grinding chamber is filled with grinding media of 2-10 mm up to a volume of max. 90 per cent (bulk volume). The MaxxMill operates continuously in dry grinding processes. Together with an air stream acting as a carrier, the material to be ground is fed through the eccentrically positioned feed pipe, entering the machine at the chamber bottom. The rotation of the vessel causes the material to be drawn into the grinding chamber where it is mixed with the grinding media. In the chamber, the grinding effect is produced by the energy input via the agitator, the variable compression of the grinding ball filling in the agitator area, the compressive stress close to the bottom, and the generation of shearing forces in front of the material deflector. The comminuted product is extracted continuously from the top of the ball layer by means of an extraction pipe with a large cross section installed in the upper part of the machine, where the grinding media, due to the gravitational force, are prevented from being extracted.
Eccentric vibration mills:
In this class of energy efficient comminution, utilising vibration mechanism (which is being developed by Siebtechnik GmbH, Germany in collaboration with TU Clausthal, Germany), the mill provides elliptical, circular and linear vibrations unlike conventional vibration mills with only circular vibrations. The major technological advance of this system consists of amplitudes of vibration of up to 20 mm (normally 12 mm maximum), leading to a high degree of intensification of the impact forces among the grinding media balls or rods part from increased throughputs.
Inter-particle high pressure comminution:
In this class of grinding Horomill (Horizontal roller mill), made by FCB, France and Fratelli Buzzi S.p.A, Italy, is expected to contribute significantly in coming years. It is based on the technique of compressed bed comminution at much lower applied pressures as compared to roll press systems. The system is reported to have energy saving potential of 40-50 per cent and 20-25 per cent as compared to the ball mills and vertical roller mills respectively. In Horomills, a cylindrical shell with horizontal axis is rotated above the critical speed by means of a unit gear, pinion, reducer and engine. A roller laid out in the shell ensures the grinding. The material to be ground, centrifuged, undergoes a multiple and controlled compression between the roller and the grinding track of the shell. The system uses proven components of the ball mill, by the use of a shell supported on hydrodynamic shoes and of a girth gear drive on one hand and components close to the principle of the roller press, such as rollers and bearings but operating with moderate pressures (four to five times less than the one used with roll presses), on the other hand. Horomills with capacities up to 280tph are reportedly installed in various cement manufacturing locations worldwide.