Control strategies for flotation circuits

Posted on Friday, 12 February 2021 at 04:02

Mineral processing plants are increasingly 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 found in the flotation circuit.

Flotation is known as a complex process that is affected by a lot of factors. These factors may be related to the circuit design and how the flotation plant is operated. Different process variables are affecting the circuit performance, for example the particle size, slurry density, flow rates, level, pH/added reagents and air. Instrumentation is used to collect data about these process variables on different places in the circuit. An example is measuring density and flow on the feed and output of the circuit, in this way the operator can monitor the tons of ore processed per hour. Below you can find a typical piping and instrumentation diagram (P&ID) for the flotation circuit.

The instrument data helps operators to make faster decisions and analysis over the process behavior in order to achieve a high performance and efficiency of the plant. Typically, SCADA control systems are used to acquire variables and data from field devices. All collected data is sent to servers and converted into process information. The real-time information is visually available on displays at the site control room, so that process engineers and operators can take actions over their operations.

There are different ways of monitoring and controlling your process, we can distinguish three types: manual, basic and advanced process control.

Manual process control:

  • Monitoring and reporting is done by labor
  • Shifts are responsible for sampling and controlling the process
  • Decisions are made based on experiences or observations

Basic process controls:

  • Basic instrumentation and electrification (e.g. density, flow, level measurements)
  • A local or distributed control system
  • Analyzers and sampling systems (particle size distribution, ore grades)
  • Automatic reagent dosing system

Advanced process controls

  • All basic process controls
  • Froth cameras
  • Reporting and data analysis
  • Advanced process controls
  • Advanced level controls
  • Air feed optimization
  • Froth speed control and grade-recovery optimization

Benefits of advanced process control:

  • Total control of the plant through monitoring
  • Analysis and evaluation of the process performance
  • Enhanced problem solving
  • Ability to react to process changes in real-time
  • Automatic reporting and remote troubleshooting
  • No performance variation between shifts

Why is instrumentation needed to control the process?

Circuit load and mass pull

Flotation is about recovery of valuable materials. The pulp density is on the hands of the operator to make the decision on having a maximum recovery or to lose valuable materials to the tailings (waste). The mass pull rates and circulating loads are important parameters to monitor in the flotation circuit. It is well known that circuit performance (grade and recovery) is strongly affected by these parameters. By stabilising circulating load and mass pull, it is possible to ensure consistent circuit performance. Flow meters and density meters are required to get control over the mass pull parameter.

Increasing volume and slurry rising or flooding events

If the mass pull control strategy is not well implemented, this could cause flooding events. For this reason, the pulp level is a very important variable and control parameter. The volume (of slurry) in the tank could increase in minutes when the slurry density or particle sizes are changing, affecting the efficiency of the collection process. Level measurement is used to check for increases of volume in the tanks. Also, density combined with flow measurement in the feed of the flotation circuit can predict those level rising events real-time. If the pulp density is not corrected in time, the amount of (valuable) material that can possibility be “lost” is huge. By adjusting the pulp density, particles are made available to be collected by the air. The goal is to recover desired particles and keep the stability in the circuit. Typically, process operators determine a density setpoint or “target density” for the feed of the flotation circuit.

Other parameters

The reagent dosing and air feed are other parameters that need to be controlled. Reagents and air are essential parameters for the recovery process, since both are used to collect the valuable material in the flotation cell and seperate it from the tailings (gangue or unwanted material). We distinguish two kinds of reagents, depressors and collectors. The supply line of those reagents is usually monitored by Coriolis mass flow meters. At the same time, pH is usually measured and controlled in the flotation cell. Chemicals need to be dosed efficiently to create a continuous froth flow into the concentrate launder. Efficient dosing can keep the recovery process running efficiently, since it ensures that valuable minerals are not lost to the tailings stream. In addition, froth cameras can be used to monitor the concentrate flow into the launder in real-time. These cameras are usually combined with an advanced process control system.

If you want to learn more about instrumentation in the flotation circuit, then please contact us for more information.

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