company insight

Aggressive gold sludge: special pump technology delivers long service life with minimal addition of water

Global demand for gold and silver remains as high as ever. Precious metals are not only sought after by investors, but are indispensable materials in modern industry. However, mining these resources places machines under extreme stress.

The sludge of crushed, gold-bearing stone is extremely abrasive and can destroy pumping systems within a few days. Adding water does make it possible to convey the product, but also increases the effort required in the gold concentration process. NEMO® progressing cavity pumps from NETZSCH Pumpen & Systeme GmbH are increasingly used to save on maintenance and process costs, as they are capable of withstanding the valuable yet problematic medium thanks to smooth pumping technology and exceptionally robust components.

Both silver and gold are normally only available as small particles and therefore have to be mined with the surrounding stone. The precious metal can then be separated from the crushed material matrix using a variety of different methods. However, the mixture must first be condensed and concentrated, so that the water content of the stone-gold sludge should ideally be no more than 40 percent. In the past, centrifugal pumps were typically used in the mining industry for this conveying step, although centrifugal pumps are in fact designed for fluids and therefore require a medium that is at least 50 percent water. Additional water was injected to operate these pumps, although the water made subsequent extraction more difficult. .

As the pumps are speed-controlled, it was possible to determine the ideal speed using a frequency inverter and thereby minimise wear and tear without the risk of medium being deposited in the pump.

More than half of the country’s coal mines are managed by pro-Russian separatist militia.

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Material damage caused by abrasive stone in the gold sludge

In search of alternatives, a rotary lobe pump was installed on a trial basis. However, the pump housing was completely destroyed within just two days. The high abrasiveness of the gold sludge was to blame. The stone particles in the sludge behave like sandpaper, particularly at high speeds, and wear away all surfaces extremely quickly – until the system fails completely. This effect can only be reduced by particularly smooth pumping at an economically viable volume, as further tests with a NEMO^® progressing cavity pump demonstrated.

A progressing cavity pump operates with a combination of rotor and stator. The helix rotor rotates within a stator jacket fitted to the rotor with a reversed helix to form conveying chambers with the same shape between the two components. The medium is conveyed in these chambers from the suction to the discharge side without pulsation, shear forces or changes in pressure. As even viscous substrates and substrates containing a large proportion of solid matter can be reliably conveyed in this way, only a limited amount of water needs to be added to the gold sludge. It was therefore possible to reduce the water content of the stone-gold sludge from 50% to nearly 40%.

Robust materials, low speed and smooth pumping reduce wear and tear

To achieve a system with a service life as long as possible, despite heavy wear, the designers at NETZSCH selected tungsten carbide for the rotor. Tungsten carbide, with a scratch hardness of 9.5 on the Mohs scale, is almost as hard as diamond and is therefore scarcely affected by the grinding of the crushed stone. SBE, a nitrile material, was used for the stator. It is similarly extremely wear resistant, but also soft enough to prevent the points of contact with the rotor from wearing out too quickly.

An additional advantage of this pump technology is the availability of speed control. The rotor speed can be adjusted with complete flexibility via a variable frequency in-verter, so that it is possible to determine the perfect speed. The aim was to find a bal-ance between as low a surface speed as possible to reduce abrasion and a sufficient throughput to prevent the medium from settling. The 20 hp motor was shifted down, first to 189 rpm, then finally to 130 rpm. At that speed, the system conveys a total of 10 m³/h at 9 bar pressure.

Conveying chambers form between rotor and stator in the NEMO^® pump and convey the medium smoothly and at stable pressure. Combined with robust materials, it is therefore possible to reduce abrasion.

Increased life cycle despite lower proportion of water

The progressing cavity pumps are also low power consumers and are easy to clean and maintain, which in turn reduces operating and maintenance costs. However, it is the durability of the pumps which is decisive when it comes to conveying gold. In the first test, the pump ran for three months without damage to the stator or rotor. The mine operator has since installed a further three Nemo® pumps with an average service life of approximately 3.5 months. The service life exceeds the lifetime of earlier centrifugal pumps, which were completely worn out much quicker, despite more addition of water.