Grinding Mill Control in Closed Circuit
The principle objective for controlling grinding mill operation is to produce a product having an acceptable and constant size
distribution at optimum cost. To achieve this objective an attempt is made to
stabilize the operation by principally controlling the process variables. The main
disturbances in a grinding circuit are;
mineral composition and mineral characteristics like
abrasiveness, hardness),
2. changes in mill operating parameters like
variation of input flow rate of material like surging of feed caused by pumps
and level of mill discharge sump.
The mill control strategy has to compensate for
these variations and minimize any disturbances to the hydrocyclone that is
usually in closed circuit. The simplest arrangement is to setup several control
loops starting from the control of water/solid ratio in the feed slurry, sump
level control, density control of pulp streams at various stages and control of
circulating load. Presently most mills use centrifugal pumps for discharging
from the sump. This helps to counter surges and other problems related to
pumping. For feed control the most likely option is to use a feed forward
control while for controlling the hopper level and mill speed and other loops
the PI or PID controller is used. The control action should be fast enough to prevent
the sump from overflowing or drying out. This can be attained by a cascade
control system. The set point of the controller is determined from the level
control loop. This type of control promotes stability.
As an example, for completely controlling a
grinding mill circuit the operation of a SAG mill is considered here as these
mills seem to be slowly displacing the normal ball mill operations.
The SAG mill main variables are:
1. solid mass transported through the mill ( solid
feed plus the circulation load),
2. the mill discharge solids, and
3. the overflow solid flow.
From the control point of view, the additional
interests are:
1. overfilling of the mill,
2. grate restrictions and,
3. power draft.
Each of these is controlled by specific controlled
inputs, i.e., feed rate, feed water and discharge water flows. The overflow
solids fraction is controlled by monitoring the ratio of total water addition (WTOT)
to the solid feed rate. The ratio being fixed by the target set point of
the overflow solid fraction.
Usually the charge volume of SAG mills occupy
between 30-40% of its internal volume at which the grinding rate is maximized.
When the charge volume is more, then the throughput suffers. The fill level is
monitored by mill weight measurement as most modern mills are invariably
mounted on load-cells.
During the operation of SAG mills, it is sometimes
observed that the sump levels fall sharply and so does the power draft. This
phenomenon is attributed to flow restrictions against the grate. When this
occurs it is necessary to control, (or in extreme circumstances), stop the
incoming feed.
The power draft is the result of the torque
produced by the mill charge density, lift angle of the charge within the mill
and fill level. The relationship between these parameters is complex and
difficult. Therefore to control mill operation by power draft alone is
difficult.
For the purpose of stabilization of the circuit,
the basis is to counteract the disturbances.
Also the set points must be held. The set points
are attributed by dynamic mass balances at each stage of the circuit.
In modern practice the structure and
instrumentation of the control systems of tubular grinding mills are designed
to operate in three levels or in some cases four levels. The control loops and
sensors for a SAG-mill and the levels of control are illustrated in Fig. 1
Fig. 1 A SAG-mill and the levels of control
Level 1
The operation at Level 1 mainly consists of
controlling the feed rate and the water inputs. In addition to this the SAG
mill revolving speed and secondary circulating load also forms ancillary loops.
There are four main control loops in Level 1 (Table
1).
Table 1. Control loop of SAG mill
The main sensors are:
1. load cell for mill weight,
2. power measurement (ammeters, voltmeters), and
3. density gauges (y-ray density gauge) for on
line, non-invasive, measurement of slurry densities.
Level 2
The function of Level 2 is to stabilize the circuit
and to provide the basis of optimizing function in Level 3. Three cascade loops
operating in level 2 controls that function in conjunction with level 1
controllers. The cascade loops are:
1. mill load feed rate (controlled by feeder
speed),
2. mill discharge, (% solids)
3. mill product, (% solids).
The set points are supplied by level 3 controllers
for all the cascade loops. The mill load and percent solids in the two streams
are calculated from signals received by sensors in the water flow stream, the
sump discharge flow rate and the density readings from density meters in the
pulp streams. The mill load cells supply the charge mass. The load cell signals
are compensated for pinion up thrust.
The set points for the mill load and the two pulp
densities are given by level 3 controls.
The points may also be set by neural method of
analysis or fuzzy logic expert systems.
To determine the set point for the optimum mill
load, a relation between load, consisting of different feed blends and
performance (the maximum achievable throughput) is established.
Similar observations are made for mill discharge
density and mill discharge flow.
For some routine standard situations ready made
supervisory work station computer programs, especially for Levels 1 and 2, are
available.
Level 3
The primary function at Level 3 is optimisation of
the SAG mill operation. That is, control of the product at optimum level. In an
integrated situation where ball mill and cyclone is in the circuit, the
optimisation must take place keeping in mind the restraints imposed by down stream
requirements. This optimisation can best be achieved by developing a software
for computer use. Usually a large database is required to cover infrequent
control actions.
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