Outokumpu Technology’s Andrew Okely considers control
of minerals processing plants, a difficult but essential part
of any successful operation.

The difficulty in process plants comes from the constantly
changing feed characteristics, stringent product quality requirements
and the economic need to maximize the recovery of a finite resource.
A key part of successful plant control is the operation of the
flotation circuit.

Flotation cells have three main control parameters
(1) reagent dosing rate (2) froth depth and (3) air addition
rate. Many other parameters may vary such as feed rate, particle
size distribution and head grade, however these are the output
of upstream processes and are not controlled in the flotation
circuit itself.

Each of the flotation cell control parameters are discussed
below.

Reagent dosing rate

The selection of reagent type and dosing rate is critical
to successful processing of a given ore. It offers a coarse
control mechanism as it is difficult to determine the impact
of changes in either dosing rate or reagent type unless significant
change in flotation performance is observed. In a relatively
stable operation, the addition rate of reagents does not vary
greatly. The operator seeks to ensure that a slight excess of
reagent is available for the flotation process. Too much reagent,
however, results in wastage and economic loss whilst too little
results in either reduced grade or recovery and again economic
loss. Thus, in a situation where the ore changes and marginally
less reagent could be used, the operator generally should not
chase this small reduction as it is difficult and time-consuming
to optimize. The exception to this is where the ore change is
expected to last for a long time.

Once the correct level and type of reagent is established,
the next step in control is correctly setting the pulp level
and thus froth depth.

Froth depth

Froth depth is fundamentally used to provide concentrate
grade control. This occurs in two ways – firstly, the depth
determines the residence time in the froth phase and thus the
time available for froth drainage.

Generally the greater the froth depth, the more drainage
of entrained gangue (waste) and the richer the concentrate grade.
There is a limit to the froth depth that a given flotation situation
will support. If the froth gets too deep it begins to collapse
on itself. The depth at which collapse begins is determined
by the structure of the froth. Froth structure is driven by
factors such as reagent type, reagent dose rate and the quality/level
of mineral in the ore. Froth depth also plays
a role in the recovery rate of the concentrate from the cell.
As the froth gets deeper, the rate of froth removal reduces
at a constant air addition rate. It is important to note that
froth depth relationships are not linear in nature.

Once a froth depth has been established for a particular
flotation duty (i.e rougher, cleaner etc), changes are generally
small and infrequent. A flotation circuit where the slurry level
is subjected to large or frequent changes is usually going to
be in a constant state of flux as the changes in one cell will
impact other cells in the circuit. Pump hoppers overflowing
and flotation cell pulping are common symptoms of this.

Air addition

Air addition rate offers the finest control of flotation
cells. Small changes in concentrate recovery rate and grade
can be achieved via changes in air addition rate. The impacts
of changes in air addition rate are observed quickly in the
plant providing a good source of operator feedback. Changes
to air addition rate may be made several times in a normal shift
as operators seek to optimise concentrator performance. As air
addition represents a fine control method, changes should be
small and one needs to wait several minutes before these results
can be seen. Sudden large changes in air addition rate can create
issues with level control as the pulp in the flotation cell
will experience a rapid expansion and may overflow the cell
launders. The ability to make regular changes to air addition
rate in a convenient manner has led to automatic air control
being the norm in modern concentrators. Changes in the concentrate
grade that result from changes in air addition rate can be observed
rapidly by utilising an on stream analysis system.

The next level – automated control

Leading-edge minerals processing plants incorporate
automatic process control through some form of PID-driven system.
In the case of flotation plants, the ideal system uses the three
parameters discussed above to control a single parameter such
as froth speed. Instruments such as FrothMaster use vision technologies
to measure the speed of the froth over the lip. The desired
froth speed can then be determined by monitoring the concentrate
grade via an on stream analysis system. This type of control
system automates the minute-to-minute running of the flotation
circuit, which is driven by the desired concentrate grade. In
plant trials, this approach has seen a significant improvement
in recovery when compared to a manually monitored plant.

In
figure 1, above, the variation between automatic control (Line
1) and manual control (Line 2) can easily be seen. In Line 2,
the air addition rate is manually set – so has to regularly
monitor concentrate grade and recovery rate. This is time-consuming
and is also not necessarily the best means of achieving the
targeted set point. In Line 1, where the circuit was completely
automatic, the control system constantly monitors and responds
accordingly to any variations from the set point goal. In this
particular trial, fully automated control brought real dollar
benefits – increasing overall recoveries, substantially
reducing deviations from targets and reducing use of reagents
in the cell.

Conclusion

Being able to successfully manage the different control
parameters in a flotation circuit is a critical exercise in
minerals recovery. Whilst the air addition rate offers the finest
means of control, other parameters such as reagent type, reagent
dosage rate and froth depth are also important controls for
an operator to understand. It is also vital for an operator
to understand the cause and effect relationships of these controls.
Automated technologies such as on-stream analysers, along with
newer developments such as FrothMaster or froth imaging systems,
take this control to the next level. Not only do these automated
systems monitor and analyse froth characteristics highly efficiently,
but they can also optimise recovery, reduce reagent use and
free up operator-time.

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