In past 20 years, the dry grinding circuits for the production of finished cement from cement clinker consist of two-compartment cement ball mill machines and the air separators. It is not uncommon to produce the cement in an open circuit. Advances in cement grinding technology is slow and these advances are limited to more developed countries. Approximately 95% of the feed to the cement grinding circuit are clinker and the rest of the feed are “additives” which includes grinding aids. The quality of cement is measured by the surface area or the Blane index. The unit of the Blane index is m2 /kg, and this index is determined by the Blane air permeability test. The surface area of the cement powder depends on size distribution of cement particles; smaller particles have larger surface area. If the particle size distribution is known, the Blane index can be successfully predicted (Zhang et al, 1995)
The cement clinker grinding circuit reduces the feed from 80% passing size between 10 and 20 mm to 100% passing 90 microns. The size reduction takes place in a twocompartment tube mill; the first compartment of the mill is shorter than the second compartment. The coarse clinker is ground in the first compartment where larger balls (80, 60, 50 mm) are used and the fine grinding is done in the second compartment where smaller balls (below 25 mm) are used. A diaphragm (see Figure 1) separates the two compartments and allows only particles below a certain size to pass to the second compartment. Ground material exits the mill through the discharge grate which prevents grinding balls from leaving the cement ball mill machine. A proportion of material, mostly fines, is “air-swept” out of the cement ball mill machine. The final product is the fine fraction of the air classifier and the coarse fraction returns to the mill.
In the past 20 years, high pressure grinding roller technology has been used in precrushing of clinker. Presently, many American and European cement grinding circuits have GW which increases grinding capacity and energy efficiency.
The “work horse” of the cement grinding plant is the two-compartment ball mill,commonly called the tube mill. Significant advances in model development were achieved in recent years (Benzer et al, 2001, 2003) through research on industrial scale.The breakage and transport mechanisms are better understood as well as the classification action of the diaphragm. Further advances are expected in modelling the effect of diaphragm design on classification and powder transport.
The model consist two important parameter, the breakage function (aij) that describe the material characteristic and breakage/discharge rate function (ri/di) which defines the machine characteristics and can be calculated when feed and product size distribution are known and breakage function is available. The air classifier controls the final product quality. Therefore, the air classifier has a crucial role in the circuit and a lot of attention is paid on the design and operation of the air classifier. The classification action is modeled using the efficiency curve approach (Lynch, 1997). Effect of the classifier design and operational parameters on the efficiency is complicated and work is in progress to improve the current models.
Potential benefits of using the Barmac crusher for clinker pre-crushing were studied for a cement plant. Figure left shows the proposed cement grinding circuit.
