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Show 6.The Effect of Mill Conditions on the Mill Performance 6.1 Steady-state Behaviors The factors affecting the mill performance were investigated by the steady-state simulation model. Some of the results are shown below. Figure 15 shows the effect of the crushing load on the mill performance at a constant coal feed rate where -200 mesh fraction in the product and the holdup in the mill are plotted as a function of the crushing force and the cut-off size in a rotary classifier. It is noted that both the mill hold-up and the crushing force are normalized to the standard values of W.t and M.t' respectively. It is seen that the higher crushing force, the finer product and the less hold-up. Furthermore, the smaller the cut-off size, the finer the product and the higher the hold-up. ~ 90 ......, Q/Qst = 1 ~ (/) Q) E o o N 80 " I ......, 1.5 ..... (/) ,~ ~ 1.0 0.9 1.0 75 ~ m Grinding Load M/ Mst (-) 1.1 Fig.lS Effect of grinding force on the mill performance " 90--------------------------- '?f!. ~ (/) Q) E o o O/ Ost = 1 M/ Mst Constant Fineness N 80 I " I ......, 1.5 ..... (/) ,~ ~ 1.0 1.0 1.5 2.0 Classification Modulus n 2 (-) Fig.16 Effect of sharpness of classification Next, the effect of sharpness of classification on the mill performance is shown in Figure 16 where the product fineness and the relative mill hold-up are plotted against the classification modulus n2 defined by eqn.(7). For the constant value of x so=60pm, as the classification becomes sharper(i.~. the value of n increas- e es), the product becomes finer and the hold-up increases. On the other hand, the hold-up decreases if the classifier is operated so as to keep the product fineness constant by decreasing the speed of rotation as n increases. e It can be seen from these results that there are a number of ways to operate a mill to give the identical product fineness. However, a more efficient classifier and a higher crushing force will enable the product to be obtained at a lower operating cost. 6.2 Unsteady-state Behaviors The simple finite difference algorithm demonstrated for unsteady-state ball mill circuit simulation by Austin et al. 11 ) was adopted for unsteady-state simulation. The differential equations 9 |
