Fine-grained coal recovery improved

The low NPSHr value of two compact rotary lobe pumps ensures precise and reliable decantation of water from a vacuum tank

Until August 2017, two centrifugal pumps were used to decant a fine coal screen vacuum tank in an open-pit coal mine of a company in Australia. Since their high NPSHr value reduced the capacity of the plant, its management decided to replace both units with more efficient models.

Look behind the scenes
In Australian open-pit mining, fine coal dust is extracted from water-coal dust mixture with a temperature of 25 °C. "For this purpose, the mixture is first fed to horizontal belt filters," says Steve Weir, Business Development Manager at NETZSCH Australia Pty Ltd. Until August 2017, two centrifugal pumps were used for decanting the excess water.

However, these pumps were not ideal. For example, they had a high NPSHr value, which meant that the intended flow rates could not be met with precision. "Cavitations, or gas-filled bubbles, frequently develop in centrifugal pumps. They not only reduce the efficiency of the system, but make accurate dosing impossible", says Weir. "This, in turn, limited the throughput of the system." The management, therefore, decided to replace the pumps with other models, which were expected to discharge process water with more precision, reliability and efficiency.

Challenge accepted
The essential requirement for the new units was their suitability for pumping media containing solid matter. They also needed to feature flow rate of 240 m³/h, pressure of 3 bar, and NPSHr of 1.8 m. Finally, the management selected two rotary lobe pumps of the NETZSCH TORNADO® T1 series in size XLB-6/2. These pumps not only meet the requirements of the open-pit operator in terms of flow rate, pressure, delivery rate and suction; they also have exceptionally convenient dimensions. The key advantage of the two TORNADO® T1 models over the previous system is that the NETZSCH models are self-priming positive-displacement pumps, which guarantee continuous delivery.

Excellently solved
Since August 2017, the two TORNADO® T1 pumps have been employed to decant excess water from the vacuum tank, and to convey it to the clarifying or thickening tank. Water is recovered there, and then reused in the coal treatment plant. The two VSD-driven XLB-6/2 units ensure stable and lower liquid level in the vacuum receiver, which can thus be operated with a larger vacuum chamber. The vacuum for draining coal fines is thus more reliable. "Higher vacuum and more uniform vacuum level lead to more efficient recovery of coal fines," explains Weir.

In view of this result, the mine operator is very positive about the new rotary lobe pumps. "We are very satisfied with the pump operation and the current performance of the plant," says a representative of the company. "This system has already paid for itself in the first three weeks of operation". The company operates several coal mines in Australia, and plans to integrate NETZSCH pumps into the decanting system into at least one further mine. They also plan to purchase a free-shaft TORNADO® T1 pump as a replacement pump.



NETZSCH: Two TORNADO® T1 rotary lobe pumps series in size XLB-6/2

Medium: Water-coal dust mixture

Rotations speed: 218 U/min

Throughput volume: 240 m³/h

Temperature: 25°C

Pressure: 3 bar

Viscosity: 5 mPas

NPSHr: 1,8m