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Thickener Optimisation in mineral processing

Increasing thickener profitability through integrated online monitoring

Integrated online monitoring simplifies the convoluted process of monitoring data, which is central to precise process control in thickeners, in key measuring points. This results in reduction in costs (mainly of energy and flocculants), reduction in waste volumes sent to tailings dam, as well as increase in safety. Ultimately, improved recovery translates into high profitability.

‘Process optimisation’ has become a buzzword in contemporary mineral refining processes, as mine operators are continuously seeking ways of increasing ore recovery as costly effectively, safely and environmentally friendly as possible. Process optimisation is universally regarded as one the established ways of improving physical separation of process water from useful extract or tailings. With reference to thickeners, it has been demonstrated that process optimisation can be attained through accurate measurement of key parameters which enable precise thickener process control. Ultimately, through process control the following objectives can be met: a continuous and adequate feed, that the overflow is as clean as possible, and that the thickened product has the density required at the time of discharge.

However, for process control to be effective, a number of critical process parameters, which enable the optimisation of thickener efficiency, have to be accurately and reliably monitored. In particular, the following measurements are critical: Rake torque & lift, Bed level, bed mass, Underflow density, underflow rate, Overflow Clarity and Dry solids feed rate.

Integrated online monitoring system
However, the biggest challenge for mine operators is to have all data monitored and controlled to gain an efficient thickening process in plant conditions that are dirty. Hence, suitable monitoring equipment has to be selected and installed. There has been growing evidence in the effectiveness of Endress + Hauser’s online integrated system enabling mine operators achieve precise process control in thickeners.
By examining the common shortcomings or challenges experienced with conventional measurement techniques, one would appreciate better the relevance of online integrated online monitoring as an alternative.
Common challenges with conventional measurement techniques
Wrong measurement of the parameters using conventional techniques can be very costly. Commonly, this surfaces through problems encountered in bed level and bed mass.

In the ‘bed level’, which is the interface between the aggregated solid material and process water, incorrect measurements can cause three scenarios – water being drawn out through the underflow, sludge spilling over in the overflow, in addition to incorrect flocculation.  These events result in unnecessary expense due to wasted flocculent or reprocessing costs.

Lower mass of the bed mass or density of the settled sludge can result in more process water being pumped out of the thickener’s underflow.

Key aspects of the parameters

Key aspects of the six parameters to be measured for process control to be optimised have to be identified.

  • Hydraulic rake drives

In hydraulic rake drives, through measure hydraulic pressure, common process problems like insufficient underflow pump rate or blocked discharge outlets can be monitored.

  • Bed level

The “bed level”’ is the interface between the aggregated solid material and the process water, which indicates the position of the settled bed. Incorrect measurements in the bed level can lead to water being drawn out through the underflow, sludge spilling over in the overflow or incorrect flocculation. There is unnecessary expense involved in all cases due to wasted flocculent or reprocessing costs. This necessitates monitoring data from all critical measuring points at a concentrator.

  • Bed mass

Bed mass is the density of the settled sludge. The rule of the thumb is: The higher the density, the less process water is pumped out of the thickener’s underflow.

Thus, through the optimisation of water content in the underflow, recycling of process water can be maximised, while ensuring that the underflow slurry is sufficiently liquid to be managed by the underflow pump.

In cases where the underflow is to be sent to a tailing dam, negative environmental impact through leaching is reduced.

That is why, it is important to report the inventory of settled solids inside the thickener.

  • Underflow density

Monitoring underflow density is vital to control the final percentage of solids. Underflow density is central to maintaining operational stability and it prevents tailings from blocking in the thickener discharge outlet.

  • Underflow rate 

Another aspect that should be monitored is the dry solids mass flow rate of the total tailings slurry is destined for disposal. Furthermore, constant outflow density and volumetric flow measurements can be fed back to the variable speed drive controlling the outflow pump. The data gathered can be used to optimise pump speeds, reduce wasted energy and improve efficiency. Thus, the combination of density and volumetric flow rate provides an integrated mass flow rate which is useful for accounting of mineral recovery as well as waste.

  • Overflow Clarity

To ensure clarity of the recovered process water, accurate volumetric measurement of the thickener’s inflow is vital.Essentially, the in-feed flow rate can be controlled based on high clarity of the overflow. So, the clearer the outflow, the faster the inflow feed rate which leads to optimised thickener throughput.When this is combined with the mass flow measurement of the flocculent, precise ratio control of the flocculent dosing is achieved.

  • Dry solids feed rate

Through accurate measurement of flocculent mass flow in thickeners, precise dosing pump control is realised.  From the measurement, concentrations of overflow solids can be obtained, which allow water to be reused or comply with government regulations if the overflow is to be discharged. For instance, in South Africa, recycled water containing 200 mg/L to 1% solids is generally acceptable. In the end, together with other parameters, measurement reduces flocculant usage, which optimises process costs.

Thus, the combination of density and volumetric flow rate provides an integrated mass flow rate which is useful for accounting of mineral recovery as well as waste.

Endress+Hauser’s integrated online integrated as an alternative

Endress+Hauser’s integrated online package has proved to optimise concentrator efficiency in demanding conditions.  The instrumentation for level, pressure and flow comprises:

  • Prosonic T FMU30 ultrasonic level sensor measures and controls the rake height
  • Cerabar M monitors the rake torque
  • Promass F 200 or Dosimass coriolis flow meter doses flocculant in relation to bed mass
  • Cerabar M PMC51 with retractable cleaning assembly monitors bed mass
  • Promag 55S and Gammapilot M measures the underflow, slurry density and solids concentration

These instruments allow the measurement of the parameters in the following ways:

  • Rake torque & lift:   generates either a shutdown or the rake lift to lower the torque.
  • Bed Height:  reports the position of the settled bed. This feedback is analyzed and sent back to alter the flocculant gram/tonne solid feed set point.
  • Bed Mass: Reporting the inventory of settled solids inside the thickener is done with a pressure transmitter that senses the rising bed mass.
  • Underflow Density: To control the final percentage of solids, a non- invasive nucleonic density meter is mounted along the underflow outlet pipe work.
  • Underflow rate: A volumetric in-line flow meter can interact with a density meter and the flow meter presents a dry solids mass flow rate of the total tailings slurry volume that goes to disposal.
  •  For quality assurance and final confirmation of the water clarity, a turbidity sensor monitors the solid particle carry over.
  •   a pair of nucleonic density and inline flow meter can be installed. This predicts the flocculant dosing based on what dry solids feed is expected to enter as well as reacting to what happens with the bed level and bed mass.

Enormous benefits

Mine operators that have used Endress+Hauser’s  integrated online monitoring package bear testament have confirmed benefiting in the following ways:

  • Underflow density increase from 55% to 60 -66%
  • Improved compliance with environmental regulations
  • Better water utilisation
  • Reduced cyanide to tailings
  • CTD beach angles increased from zero to ~2
  • Significantly enhanced tailings storage capacity
  • Reduced risk of tank overflow
  • Cost reduction in flocculent dosing
  • Precise measurements of slurry underflow rates
  • Accurate online calculation of solids content
  • Better water utilisation     

Accurate measurement, increased profitability

From the foregoing, accurate measurement of all of these important parameters using an online integrated monitoring allows precise thickener process control which results in significant increases in thickener efficiency. These efficiency gains translate directly into overall refining process cost reductions and increased profitability.

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