Mineral processing plants are using advanced control systems to optimize their operations. Instrumentation is a key part of it, since those instruments obtain real-time data on the processes to be used in control loops. The instruments’ data can for example be used to control valves, pumps, addition of reagents and to add dilution water to slurries to increase the stability and efficiency of the process. In this blog we will discuss the most common control variables for thickener optimization.
Struggles in thickening/dewatering
Companies can experience lots of struggles in the thickening/dewatering process. One of the most common problems is the underflow density outside the target value and the presence of solids in the overflow. If there are lower densities in a tailing thickener, it means you are losing water into the underflow and it will shorten the life of the tailing dams. If there is a higher density, you might struggle with pump blockage by moving the material from the thickener into the next process stages.
The presence of solids in the overflow are also a major problem because the quality of the product will decrease. Also there can be some problems in the downstream of the process, for example issues with operating or maintaining filters.
Other struggles in thickening/dewatering process are sliming/flare events, too much flocculant use, no insights in the throughput, no real-time measurements and rake bogging/shaft breakages.
Why process control?
In a real situation there are a lot of disturbances in the process. In the thickener, there can be many disturbances, like feed disturbances: volumetric flow, changing ore types, particle size changes and Rheology conditions. That is why it is important to have good control over the process. To be able to have good control over the process, you have to measure all the possible variables that you might have.
Traditional control strategy
There are many different control strategies possible for any part of the process. The traditional control can be the single loop (PI) controllers (controlling the underflow pumping, the speed of the underflow based on the density). The problem with single loop controllers is that excessive amounts of polymers are used to compensate for changes and in many cases an operator intervention is needed. Traditional single loop (PI) controllers are not optimal to handle the slow and complicated process dynamics.
Advanced control strategy
An advanced control strategy works better than a traditional control strategy. A Model Predictive Controllers (MPC) is usually made for each process so there is a consultant party going to the company to make a lot of different models to see which one is a good fit to the process. Those models will use all the data available and will tell you which instrumentation you need to have. This allows a real automatic control over desired setpoints (underflow density, overflow clarity). It is a standard method for control purposes with a multivariable character and complex response dynamics.
Control actions for thickeners
There are different targets to optimize thickener performance:
- An underflow density target is used to ensure optimum solids content in the tailings impoundment, and optimum water relcaim for the mill
- A bed/mud level target is used to obtain optimum loading in the thickener without overloading the drive mechanism
- Bed pressure is used as an indication of solids inventory. This helps the system to determine whether a high bed level is the result of decreased settling rate or increased solids inventory
- In some cases, a drive torque target is used as an indication of acceptable underflow rheology
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