2010 Jun: Issue 3 
Achieving stable pulp levels throughout a flotation circuit can be a challenging task, but an essential one for maximising metal recovery and stabilising concentrate grade. What is not essential is spending precious capital funds on expensive “black-box” or expert systems. If your aim is stability then look no further than your Distributed Control System (DCS) or Programmable Logic Controller (PLC).
Some of the main reasons why flotation level control can be so problematic include:
• Sticky tail valves on flotation banks and cells.
• Jerky pulp level measurements in flotation banks and cells.
• Incorrect tuning of flotation level controllers.
• Incorrect tuning of transfer sump level controllers.
• Not using all process measurements.
When faced with these issues, busy plant staff often just spend the money on new expert systems. While this will probably result in stable flotation levels such expenditure and added complexity is unnecessary.
Most plants have invested extensively in PLCs or DCSs that have a sufficiently rich library of control blocks to build effective flotation level control strategies. Most modern PLC or DCS systems can be used to implement a highly effective flotation level control strategy. They can also be used to develop and implement a wide range of other advanced regulatory and optimising control strategies. Sadly these powerful and flexible control systems are an underutilised tool in the world of mineral processing.
The other advantage of using your existing PLC or DCS is that you will probably already have an existing maintenance and support infrastructure – meaning additional support contracts will not be required. With the appropriate training existing plant instrumentation people can perform the day-to-day maintenance required. In today’s fly-in, fly-out sites this can be particularly attractive.
How do I get started?: The first and most important thing is to check your field instrumentation. This means dart or pinch valves must be working freely and level measurements must be correctly installed and calibrated. Also, all sump level measurements (usually ultrasonics) should be correctly installed and calibrated. The sump level controllers should also be correctly tuned.
Flotation level control – Is it a multi-variable problem?: No. This is just hype put around by some suppliers. Effective flotation level control can be achieved with appropriate use of relatively simple feedforward control. A suitable Proportional Integral (PI) control algorithm with feedforward capability is perfectly adequate. Signal noise filtering is sometimes required and additional flow measurements can be used if available.
By effectively controlling flotation levels using their DCS/PLC, operators can expect to achieve better utilisation of plant operators and their skills; improved disturbance rejection; less variation of final concentrate grade and improved metal recoveries from 1-4%. Not to mention spending precious and unnecessary capital on a needlessly complex black-box system.
Author: Article based primarily on interviews with Andrew Thornton, mineral processing and advance process control expert and MIPAC’s Principal Process Control Engineer.
More information:
Flotation Level Control
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