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Optimising performance of bed level thickeners

Thickeners for mining are large tanks used to separate fine suspended mineral particles (ore) from the associated liquid, producing a clear liquid stream from the top of the thickener (overflow) and a thick sludge of settled solids (underflow) from the bottom.

Flocculants are mixed with the solid — liquid suspension in the entry feedwell to the thickener. The flocculants aggregate the fine particles, which speeds up settling of the solids.

They are crucial equipment in a wide range of mineral processing applications, such as the alumina, gold and mineral sands industries — most of the world’s minerals go through a solid — liquid separation in a thickener at some point in their processing.

Separating the process water from the ‘waste ore’ (tailings) is generally the last step in the refining process, and is one of the most common thickener applications — tailings thickening.

Accurate measuring

When Endress+Hauser asked its mining customers about the major process problems they face, high on their priority list was the ability to accurately and reliably monitor the “bed level” and “bed mass” in the tailings thickening process — and in the other thickening processes, because if these could be measured, the efficiency of the thickeners could be improved.

The ‘bed level’ is the interface between the aggregated solid material and the clarified water. Successfully monitoring the bed level during the gravity separation process is a particularly delicate and high-maintenance task. Incorrect measurements may lead to water being drawn out through the underflow, sludge spilling over in the overflow, or incorrect flocculation. There is wastage and unnecessary expense involved in all cases due to wasted flocculant or reprocessing costs.

Numerous technologies are already available for bed level measurement:

• Theoretical bed level based on the calculation of the average density using a hydrostatic level sensor, and a non-contact ultrasonic level transmitter.

• Submerged ultrasound sludge blanket transducer to sense reflections from the solid bed

• Turbidity sensor attached to a motorised cable spool which records the light absorption of the sludge.

Because these technologies rely on sophisticated and finely-tuned primary sensing elements, or because of ‘faulty’ theoretical methods, their success has been limited in the demanding thickening environment.

A more robust solution is the ‘buoyancy based electromechanical system’, where a sensing weight is lowered on a measuring tape into the thickener to detect the level of the dense bed underwater. Typically the sensing weight is a lightweight hollow container filled with the bed level material such that when it is lowered, it ‘floats’ on the bed, and at that point the bed level can be measured. This more straightforward principle of operation offers a greater ease of operation, but historically often lacked mechanical integrity and was not user-friendly.

Electromechanical system

Recently Endress+Hauser released a new, heavy-duty buoyancy electromechanical system called the FMM50. It was based on the need to minimise price, installation, engineering and maintenance on the one hand, whilst also offering a system that can be set up in the workshop without the need for specially-skilled personnel.

Easy to follow instructions on set-up and ranging appears in plain English text on the local display.

The electronically controlled motor; although capable of pulling 500N, is easily adjusted for high sensitivity detection to avoid misreading on gradient sludge blankets. The use of 316 stainless steel tape instead of rope, together with four tape wipers guarantees the reliability and longevity of the design.

To overcome issues related to the use of rakes in settling tanks, the measurement cycle of the FMM50 can be controlled by full remote control so that measurement takes place in between rake rotations.

The FMM50 has been well proven in Australia, with increasing numbers of mining and sand excavation plant users praising its ease of operation and quick return on investment.

Similarly, ‘bed mass’ is effectively the density of the settled sludge. The higher the density, the less process water is pumped out to the tailings dams, reducing negative environmental impacts through leaching at the dams. Constant density is also important to control pump speeds, reduce wasted energy, and improve efficiency.

Commonly, a hydrostatic pressure sensor inserted near the base of the tank is used to measure the bed mass. 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. Problems with the pressure sensor and diaphragm can only be determined during a plant shut down at which time the tank must be drained, and the pressure transmitter is removed, checked and recalibrated, if it is not already damaged beyond repair. The hydrostatic pressure sensor is at the heart of the thickening process — a faulty sensor could therefore cause an inefficient process to run for a long period before being detected, incurring excessive water wastage, and high pumping costs.

Innovative solution

In response to customer needs, Endress+Hauser have devised an innovative solution to avoid potential diaphragm damage in applications like this. They developed the world’s first ceramic pressure sensor, the Ceraphire, which is fitted to the popular Cerabar M pressure transmitter. Ceramic exhibits an extremely high resistance to abrasion — several times that of stainless steel. The Cerabar M uses the patented Ceraphire® ceramic sensor which has a purity of 99.9% and exhibits the least porous surface of all ceramics, giving it a super smooth surface which reduces product build-up and facilitates easy cleaning.

The Cerabar M has been modified and is installed in a stainless steel retractable assembly, perfect for attaching to the bottom of the tank. The assembly consists of a stopper ball valve and a retracting mechanism allowing the unit to be extracted, cleaned and validated without shutting down the process. The insertion length of the sensor mechanism can be varied to match the thickness of the tank and to ensure that the sensor depth once inserted in the tank is optimal. The assembly is specifically designed to prevent any injury to the operators during the validation process. Due to the requirement of the extension of the sensor into the process in this style of transmitter, the use of the ceramic sensor avoids long oil filled capillaries between the sensors and transmitter as used in metal type sensors. This means a more robust design with improved long term accuracy and stability.

The combination of using the FMM50 for bed level monitoring and the Cerabar M for bed mass measurement, ensures reduced flocculant consumption, improved thickener efficiency, and lowers overall water wastage — all very important criteria of modern mining.

Endress+Hauser Australia Pty Ltd

John Immelman, Managing Director

1300 363 707

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