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Optimise thickener efficiency for profitability

Once extracted from the ground, mined ore is subjected to a complex series of steps before it can be converted into its purest state, ready for use. A key step in most ore refining operations is the physical separation of process water from the useful extract or tailings. The optimisation of this process is critical for both cost and quality reasons.

Thickeners are used in many mineral refining processes, including alumina, gold, nickel, mineral sands and coal washeries. In fact, most minerals go through a solid, or ‘liquid separation in a thickener’ at some point in their processing. Most mining process operators agree that one of their major challenges is to accurately and reliably monitor the bed level and bed mass of their thickeners which together with other critical processes allows optimal thickener efficiency.

Bed level

The ‘bed level’ is the interface between the aggregated solid material and the process water. Incorrect measurements can lead to water being drawn out through the underflow, sludge spilling over in the overflow or incorrect flocculation. Wasted flocculant or reprocessing costs pose unnecessary expense. Depend-ing on the application conditions, different techniques are employed to determine a thickener’s bed level.

Hydrostatic pressure measurement essentially reports the mass of the liquid column pressing down on it. Since the height of the liquid is limited due to the constant overflow, a calculation based on the force acting downwards divided by a constant height factor provides an estimation of the bed level. As the liquid height is fixed and the density of water is known, a second calculation provides information on the total amount of solids in the thickener. This information can be used to increase or decrease the inflow rate.

The sensing element (diaphragm) of this device is very vulnerable since the slurry can cause abrasion and larger suspended matter can impact and damage the stainless steel diaphragm. The best way to overcome these problems is through the use of a highly-robust, retractable pressure transducer. Trans-mitters with ceramic sensing elements are highly resistant to abrasion – several times that of stainless steel. Retractable transmitters are specifically designed to be attached directly to the bottom of the tank. Assembly consists of an isolation ball valve and a retracting mechanism which allows the unit to be cleaned and validated without stopping the process.

In processes with slow and predictable settling behaviours, using only the hydrostatic pressure technique can be adequate. However, processes prone to disturbances from variances in chemical make-up or with varying flow rates often require additional measuring systems in order to provide reliable results.

A submerged ultrasound sludge blanket transducer (figure 1-‘C’) can be used to provide a profile of all interfaces within the thickener. A turbidity sensor (figure 1-‘A’) which is either positioned at a fixed height or attached to a motorised cable spool determines the turbidity of the process water. The ‘buoyancy based electromechanical system’ (figure 1-‘B’), uses a sensing weight which is lowered on a measuring tape into the thickener in order to detect the bed level.

Bed mass

The ‘bed mass’ is effectively the density of the settled sludge. The higher the density, the less process water is pumped out of the thickener’s underflow. The optimisation of water content in the underflow allows for maximum recycling of process water while still keeping the underflow slurry sufficiently liquid to be managed by the underflow pump. If the thickener’s underflow is destined for a tailings dam, the optimisation of the slurry density helps to reduce negative environmental impacts through leaching at the dams.

Bed mass is calculated based on the total volume of combined water and solids in the tank and the total hydrostatic pressure measured near the bottom of the tank. As the specific gravity of water is a known constant, the mass of the solid content can be calculated.


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