Soft Sensing - Advanced Control and Supervision of Mineral Processing Plants - page 199

BM

G f
f 80

G k G p

p 80

Rod Mill

k 80

sump

Section 1

Section 2

Section 3

Figure 4.21 Grinding plant for which a work index soft sensor was developed

4.4.2.3 Development of Soft Sensors for f 80 and p 80

The requirements of these two soft sensors were determined by finding the sensitiv-
ities S Wf and S Wp of W iog with respect to f 80 and p 80 . These sensitivities, defined by
the relative change of W iog with respect to the relative changes of f 80 and p 80 ,using
(4.142) are found to be

dW iog
W iog
df 80
f 80

1
√ r 80 S Wf = = − 0 . 5 (4.144)

− 1

and

dW iog
W iog
dp 80
p 80

5 √ r 80

S Wp = = 0 .

1 ,

(4.145)

√ r 80

−

where r 80 p 80 gives the size reduction performed by the grinding plant [24].

Using measurements collected from the plant these sensitivities were found to be
S Wf

= f 80 /

61.
Over a period of 46 months, the average value of f 80 was 9774 μm with a stan-
dard deviation of 379 μm, while the average of p 80 was 310 μ m with a standard
deviation of 45 μm.
If f 80 changes by three times its standard deviation around its average, the varia-
tions of W iog was found to be quite small; 1.26 %. Therefore the average value of f 80
may be used. From a soft sensor point of view the soft sensor for f 80 is simply its

= 0 . 1084, S Wp = 0 . 608, S Wp / S Wf = 5 .