The difficulty in controlling minerals processing plants is a product of constantly changing feed characteristics, stringent product quality and maximising recovery.
A key part of successful plant control is the operation of the flotation circuit.
Flotation cells have three main control parameters: reagent dosing rate; froth depth; and, air addition rate. While many other parameters may vary such as feed rate, particle size distribution and head grade, these are the output of upstream processes and are not controlled in the flotation circuit itself.
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 the impact of changes in either dosing rate or reagent type is difficult to determine 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.
So in a situation where the ore changes and marginally less reagent could be used, the operator generally should not chase this small reduction because it is difficult and time-consuming to optimise. 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 control of concentrate grade. 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 these froth depth relationships are not linear in nature.
Once a froth depth has been established for a particular flotation duty (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.
Common symptoms of this are pump hoppers overflowing and flotation cell pulping.
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 feedback for operators.
Changes to air addition rate may be made several times in a normal shift as operators seek to optimise the 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.
Leading-edge minerals processing plants incorporate automatic process control through some form ofPID-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 automated approach has seen a significant improvement in recovery when compared to a manually monitored plant.
In figure 1, the variation between automatic control (line 1) and manual control (line 2) can easily be seen. In line 2, the operator manually sets the air addition rate – so has to regularly monitor the concentrate grade and recovery rate.
This approach is time-consuming and also not necessarily the best means of achieving the targeted setpoint. In line 1, where the circuit was completely automatic, the control system constantly monitors and responds accordingly to any variations from the setpoint goal.
Andrew Okely* works for Outokumpu Technology in Australia.