Dynamic Simulation and Model-based Control System Design for Comminution Circuits - Advanced Control and Supervision of Mineral Processing Plants - page 239

In Table 5.2, fines are defined as being a size smaller than 50mm, medium as a
size in the interval of mm, and coarse for size bigger than 100mm. It is

worth noting that these size distributions have been chosen solely to illustrate their
effects. However, the distribution labeled normal corresponds to a real distribution
obtained from a SAG mill. The simulation conditions were feed flowrate of 100
ton/h, water-mineral ratio controlled at a 76% of solids, speed at 9

[ 50 , 1000 ]

5rpm and no
changes to ore hardness. The simulation starts with distribution 1, at 300 minutes it
changes to distribution 2, at 340 minutes to distribution 3 and finally, at 600 minutes
to distribution 4.

.

x 10 4

1.074

1.072

1.07

1.068

1.066

1.064

1.062

1.06

1.058

100 200 300 400 500 600 700

34

32

30

28

26

24

100 200 300 400 500 600 700

Samples (k)

Figure 5.15 Level and power draw response after changes in feed size distribution

Figure 5.15 shows that under size distribution 1, the mill operates with a filling
of 25% and power draw of 10 6MW. At time 300 minutes, ore feed distribution

has fewer coarse particles. Both level and power draw start to increase, up to a
point around 320 minutes, where power draw starts to decrease but level continues
increasing. This is an undesirable operating region. At time 340 minutes, power
draw starts to increase again, due to a distribution with coarser particles (distribution
3). After reaching a peak, power draw follows the same tendency as filling level.
From time 600 minutes on, the feed does not have medium-sized particles. The mill
increases filling level and power draw up to 29

.

72MW, respectively.
These simulations show how sensitive SAG mill operation is to changes in feed
particle size distribution. This variable represents one of the main unmeasurable
disturbances affecting operation.

. 7% and 10 .