Mineral processing process control modes
In mineral processing plant, the
process control is important and necessary. Processcontrol systems can be divided into two major groups:
1. Continuous control that involve monitoring and
controlling of events continuously,
2 Digital controls that involve the use of
computers and microprocessors.
Controlling, say the level of a flotation tank
which is being filled continuously and from which the pulp is withdrawn
continuously, can be done crudely by observing the rise (or fall) of level and
restoring it manually by manipulating valves and increasing or decreasing the input
flow rate to the tank. Such an on-off method would result in an unsteady
profile of level (Fig. 1). This situation is unacceptable in most mineral
processing circuits. To solve the problem instruments have been devised and
strategies developed to minimize the fluctuations in level. Automatic
controllers have therefore being devised which serve to control flow rates,
density of slurries, tank and bin levels, pump operations and almost all unit
operations like crushers, ball mills, screens, classifiers, thickeners, flotation
vessels and material handling systems.
Fig.1. Manual on-off control of flotation cell
level.
The two basic control strategies or modes of these
controllers are known as:
1. feed back control system, and
2. feed forward control system.
In the feed back control system the output from a
process is monitored continuously by a sensor. When the output changes the
sensor detects the change and sends signals to a comparator which compares the signal with the set point
for normal steady state operation. It then estimates the error or the
deviation from the mean. The error signal is passed on to the controller which
compares the signal with the true set point and sends a signal to an operating device
to reduce the error to zero. The signals are electrical, mechanical or
pneumatic devices. Fig. 2 is a typical block diagram illustrating the feed back
system.
Fig.2. Block diagram of Feed Back control system.
It can be seen that the comparator has three
functions. Its first function is to correctly receive signals of measured value
from the signal monitor. Its second function is to compare the signal with the
set point and then compute the deviation against the norm (set point) and its
third function is to activate the final controller to correct the error.
There are two process factors that make the feed
back control unsatisfactory. These are the occurrence of frequent disturbances,
often of large magnitude, and the lag time within the process between
occurrence of an event and delay in recognizing the signal. As shown later, these
disturbances and lag times can be measured and corrective steps applied.
In the feed forward set up, the input signal, say
of the feed, is monitored and controlled prior to the feed entering the
process. In so doing it is expected that the feed to the process is unaltered
and therefore the process performance remains unaffected. A block diagram of
the feed forward system (Fig. 3) illustrates the principle of its operation.
Fig. 3. Block diagram of Feed Forward system of
control.
In this set up the indicator in the input stream
indicates the deviation in the input stream characteristics, (like feed flow
rate) to the controller. The controller confines its activity to the incoming
stream (and not on the process), computes the magnitude of the error and
signals to the controller to provide appropriate action to restore the input
stream characteristics to its original level.
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