Ball mill
Closed circuit grinding system focuses on feed material characteristics, grinding progress in the mill, mill ventilation, classification and controls.
The main trends concerning grinding processes in the cement industry are — higher efficiency, reduction of the power consumption and system simplicity.
Matching the feed material size distribution with the most effective media top size selection as well as graded ball recharging is widely practiced.
Mill chambers sample analysis indicates progress of the grinding process along the length of the mill. In a correct operation the residue will be high initially, falling gradually as grinding progresses.
In a ball mill particles break primarily by impact or crushing and attrition. It seems however that impact breakage is predominant at coarser particle sizes whilst attrition is the main reduction mechanism at finer sizes. Experience shows that small balls are effective for grinding fine particles in the load, whereas large balls are required to deal with large particles. In addition, feed material hardness factor require high impact energy and larger media.
Other than mat grindibility and charge quantity, a good mill ventilation system ensure rapid removal of the fines which are produced even during the coarse comminution in the 1st grinding chamber. This can make decisive improvement in the size reduction results.
In general, +1 mm and +90 microns (sieve residue in %) mainly considered to judge for coarser (first chamber) and finer (second chamber) grinding performance result. Too many different sieve analysis results could be confusing to find out any solution.
At first look, this is observed there is a gradual decline in % residue retained in respective sieves — except at 4th meter on 90 microns. This is not recommended to change ball distribution pattern even if graph is found uneven/flattened for a particular sieve result within a chamber length. The charge is initially made to equilibrium where compensation for wear can be done by balls of one size only, usually the largest size in the compartment. Therefore, any change in ball size to be decided right from top size ball and individual ball share.
Under-mentioned ball distribution chart using max size ball 90 and 80 mm dia exhibits variable charge proportions respective to maximum ball size.
To define a balanced charge distribution — let us take an example of mill first chamber with 28% charge fill with quantity 40.3 Mt. where 90 dia (mm) ball is used as Max size.
From the above calculation, this is found all 90 dia ball as charged initially got converted to 80 dia after a specific quantity of material ground. And the loss in weight of 90 dia ball is almost equal to cumulative wear loss @ 0.06 Kg/Mt of grind.
Therefore it justify, the compensation charge shall be of chamber top size ball only if charge distribution is balanced consequently with the chart displayed for equilibrium charge pattern.
To obtain an useful material size reduction rate in first chamber of the mill, the +1 mm size percentage shall come down to below 5% at second chamber entry point irrespective to max feed size. If repeated sample results shows the higher % presence at this point, then the max ball dia to be selected to next higher size and distribution to be evaluated as demonstrated above.
The reduction behavior of particles size in microns within initial length of the mill is not expected to be encouraging due to bigger ball and grinding action exhibited at this section. The particular graph is expected to be more declining in the last segments of the mill.
Presence of more finer particles at mill discharge will reduce material circulating load with poor separator performance. A suitable presence of fines (here 90 microns) at mill discharge with respect to product fineness may help in to achieve better mill performance and separator efficiency.
Here cement mill samples as plotted for two different feed size characteristics and respective charge compositions, indicates efficient size reduction in first compartment. At mill discharge, residue on 90 microns sieve is found 18 and 14% respectively.
Mat circulating load factor is computed for raw-mill and cement-mill at a sep efficiency of 90 and 70% for target residue 16 and 1% respectively on 90 microns sieve. This is observed, to maintain an acceptable circulating load factor between 1.5 to 2.0, a target mill discharge residue on 90 microns sieve would be 40–50 % for raw grinding and 10–20 % for cement grinding. The above relation is displayed for raw material grinding where product residue varies with kiln requirements but for cement fineness, the residue on 90 microns normally kept 1 %.
The range of target discharge residue (in %) may vary with desired product residue as shown in above chart — which could be use as reference to select probable material discharge fineness. The mills where conventional separators are used, a little finer grinding is suggested due to shorter classifying zone available.
While analyzing both the axial graphs for finer grinding (second compartment) performance this is found cement residue at mill discharge is complying the range as marked and very satisfactory.
But same is not for raw mill, where residue at mill discharge is found less than the range as mentioned — suggests over grinding of materials in the second compartment.
Here the action may be initiated in two different ways — (1) reducing charge filling % of respective chamber or (2) lowering top size charge to next available size and keeping the distribution accordingly.
To make it simple, two sample points are found very significant to inspect mill performance — (a) second compartment entrance and (b) mill discharge end. Sample results of these two points are the main deciding factor which influence the performance and provide most useful information(s) for steps to be followed.
By analyzing the sieve residual data close to the division and outlet diaphragms, it is also possible to see if the compartments over-grind or under-grind the feed material.