Environmental Engineering Reference
In-Depth Information
PI controller. Table 5.1 lists control and controlled variables and their corresponding
controllers.
Tabl e 5. 1 Control and controlled variables and their corresponding controller
Control var.
Controlled var. Controller
1. Pump speed
Sump box level
PI
2. Ore feedrate
Water addition
P
to the rod mill
3. Water addition
Product fineness
GPC
to the sump box
4. Ore feedrate
Circulating load
GPC
Process model parameters are identified by using a set of identification data and
a least-squares method. Figure 5.11 shows controlled and control variables through
several changes in set-points and ore hardness.
As a result of using a tight level control around the sump box, the speed pump
propagates flow disturbances from the sump box towards the battery of hydrocy-
clones, producing undesired transient dynamics in the grinding circuit. These are
emphasized when an increment in ore hardness occurs, as seen in Figure 5.11 at
410 min.
In this case, the grinding process is represented by a 3
×
2 MIMO system, in
which the following control variables are considered:
1. water addition to the sump box ( u 1 );
2. fresh ore feedrate to the rod mill ( u 2 );
3. pump speed ( u 3 ).
The outputs considered are:
1. product fineness ( y 1 );
2. sump box level ( y 2 ).
Under this formulation, several operation strategies can be designed by changing
weight parameters in the cost function to be optimized.
The control objective for a typical operating strategy used in grinding plants max-
imizes fresh ore feedrate to the rod mill and circulating load indirectly, considering
constraints in product size, sump box level, fresh ore feedrate, speed pump, as well
as the slew rate of control variables.
The control of circulating load is important, as high recirculation at a fixed prod-
uct size means lower energy consumption [20]. However, circulating load is a non-
measured variable most of the time. Therefore, sump box level and speed pump are
taken as a good indication of the circulating load. A high level indicates maximum
circulating load and a low level, minimum load [20]. Therefore, tonnage is increased
to a point where sump box level constraints are met.
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