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.

Performance indicator	Comparison Line 1 vs Line 2	Confidence Level (F-test)
Average grade error as % of target (Aug 2000)	0.03% vs 9.99%	100%
Standard deviation of daily concentrate grade (% of target)	6.63% vs 14.68%	100%
Overall Cu recovery improvement (May 2000)	2.34%	81%
Overall Au recovery improvement (May 2000)	1.78%	54%
Rougher 1 Cu recovery improvement (Aug 2000)	5.28%	99%
Rougher 1 Au recovery improvement (Aug 2000)	5.14%	95%
Average frother reduction (May 2000)	7.10%	60%

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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