Pumps for Tough Applications
A look at pumps that can handle acidic and abrasive fluids in mining and mineral processing
By Simon Walker, European Editor
Of course, the need to handle corrosive and abrasive fluids is by no means unique to mining, although the combination of the two does present additional challenges in terms of the materials needed for pump construction. Steels that offer long-term corrosion resistance are different from those with good wear characteristics, so manufacturers need to design and build their pumps to achieve the best of both worlds.
In addition, mine pumps must be easy to maintain. Not only do the pumps themselves have to work in often unpleasant conditions, but so do the people who have to look after them. A quick turnaround in the workshop can make the difference between normal mine operations and panic mode if water begins to rise, not to mention the longer-term effects of having to spend time and money in pumping out a flood.
This article looks at some of the pumps that are currently available for work in corrosive and abrasive mining applications.
Submersibles and Slurry Pumps
Xylem offers a range of dewatering pumps, which, the company said, are capable of handling the most challenging of mine and slurry liquids. Its Flygt 5000 series of slurry pumps is designed for use in the most abrasive applications where solids transport is the main operation and the water is merely the carrying fluid. Their compact design and submersibility makes them easy to install and ensures quiet operation, Xylem told E&MJ.
Xylem explained that it has increased the abrasion resistance of Flygt slurry pumps in three ways. Firstly, it has increased the size of the volute to slow down the speed of the slurry, so reducing wear. Secondly, the design keeps the solids flowing. When pumping slurries, the water moves faster than the solids. If the differential becomes too great, the two separate, resulting in very high wear rates. To counter this, Xylem said, the slurry impellers on Flygt pumps feature long sweptback vanes—an advanced design that ensures a homogenous mix and keeps solids moving in step with the flow of the medium.
The third improvement has involved reducing particles’ angle of impact. The long swept-back impeller vanes make the particle flow lines follow the geometry of the volute, thus reducing wear.
Xylem added that special features on these slurry pumps include the wet end and all parts subject to wear being made of hardened high-chrome cast iron. This provides maximum wear resistance for pumping liquids with a high concentration of solids. In addition, all Flygt slurry pumps can be equipped with agitators in order to keep solids in suspension.
Flygt slurry pumps are compact and therefore easy to move around. They need no superstructure, are quick to set up, and can be used in multiple locations, the company said.
Looking at pumps for handling acidic water, Xylem offers its Flygt 2700 series of corrosion-resistant pumps. These are stainless- steel submersible dewatering pumps designed to pump salt water, sludge and slurry, corrosive and abrasive liquids, and effluent with unpredictably high or low pH values—anywhere between 2 and 10. This versatility drastically reduces unplanned operational shutdowns due to pump failure, Xylem pointed out.
Meanwhile, Slimline Flygt 2700 pumps fit well into confined spaces and can easily be moved. A ring-shaped handle provides good maneuverability during installation, and serves as a sturdy stand when the pump is upside-down for service.
Many old mining areas have a legacy of acidic water; few more so, in fact, than the Witwatersrand in South Africa. The German company, Andritz Ritz, recently reported that it has commissioned two double-suction submersible motor pumps to remove 3 million liter/h of water from disused mines that lie beneath the city of Johannesburg.
With the problem having reached a critical level, Andritz Ritz received the initial order for these pumps in 2010, although it was not until earlier this year that the units were able to be commissioned. The scale of the challenge is immense, with water flows within the old “Central Basin” mining district estimated at some 60 million liters/d.
“The water never stops flowing,” said Dieter Schmitz, head of the company’s mining business development division, and who has specific experience in dewatering flooded mine tunnels. Andritz Ritz has been a supplier of dewatering pumps to the Ruhr coal-mining district in Germany for a number of years, and noted that since 2010, RWE has gradually been replacing its existing pump models with the company’s submersible motor pumps.
However, there is a major difference. Whereas the water being pumped from the old Ruhr mines is clean, beneath Johannesburg the water is not only acidic, but can reach a pH value of 2 under certain conditions.
Because of this, the pumps for the Johannesburg project had to be completely redesigned. The design is based on the company’s HDM (Heavy Duty Mining) technology, which uses the concept of a double- suction pump. The thrusts produced are offset by the counter-rotating arrangement of the impellers, so these pumps run without axial thrust—giving a properly maintained pump a service life of 10 to 15 years, Andritz Ritz claims.
Hence, the Johannesburg pumps were not off-the-shelf units, but were tailor-made for this specific application. The company told E&MJ that it took a year from the order being placed for this special construction to the first test-bed trials. Part of the new system involves the encapsulation of the submersible motors, which have a pressure differential within and outside the pump. Since the internal pressure is higher than that outside, corrosive water cannot get in to the pump and attack—or even destroy— internal components. At the same time, the water being drained is used to cool the motor by means of a heat exchanger.
“For us, this system is an improvement on our well-tried and tested HDM technology, which now needs to prove itself,” Schmitz said, adding that the technology now allows Andritz Ritz to tap into new markets as acid water is a huge problem worldwide.
The first two pumps have been running since June. Each weighing 21 mt, and measuring 15 m long with a 1 m diameter, they were installed side-by-side in March, but could only be started after completion of a water-treatment plant. Freely suspended on 430-m-long duplex steel pipes, they transport the acid mine water to the surface and then into the plant where the pH value is raised by adding lime to neutralize the acid and precipitate dissolved metals as hydroxides.
According to Andritz Ritz, the South African authorities are planning a total of three pump stations, with invitations to tender for the Eastern and the Western Basins currently in progress. The long-term goal is to force the water level in the flooded mines back from its current level of around 200 m to a depth of 1,000 m and then to keep it there, potentially opening the way for the future re-opening of the dewatered upper levels.
The company’s HDM series are multistage, double-suction submersible motor pumps that can handle heads of up to 1,500 m and flow rates of up to 6,000 m³/h. As well as featuring counter-rotation to eliminate axial thrust, this also gives a 50% lower flow velocity, so reducing the potential for wear.
The Rand’s AMD Legacy
Central Rand Gold is one of the companies benefiting from the newly commissioned dewatering pump station in Johannesburg. In June, the company reported that during the previous month, water had reached 165 m below surface in its operations, having been rising steadily for years. However, even with only one of the two submersible pumps operating, the water level had initially been stabilized and then reduced. “The reduction provides the first substantive evidence of the ability of the submersible pumps and the high-density sludge (HDS) plant to dewater the entire Central Basin, which hosts the company’s significant resource base and exploration target,” Central Rand Gold said.
CEO Johan du Toit, added, “This is the first time since 2008, when the previous pump station was flooded, that the rising water table in the Central Basin has been halted. The drop in the water table is a positive indication that our resource within the Central Basin can still be realistically dewatered and accessed. With the HDS plant now able to operate at full capacity, it is further hoped that we will see an acceleration in the drop of the water table so that the mining operations can re-access some of the mining areas which were abandoned due to the rising water table.”
While the AMD problem is being tackled first in Johannesburg, the situation is not new. Outflows began in the Western Basin, encompassing the Krugersdorp-Randfontein area, in 2002, with initial treatment consisting of adding lime to precipitate solids and neutralize the acidity. However, this led to problems with sludge build-up in the local watercourses.
In the East Rand, pumping stopped at the last deep mine in operation, Grootvlei, in 2011, and water levels began rising. Earlier this year, the South African government announced a R319 million ($29 million) budget for an integrated pumping and watertreatment project, in an effort to stem long-term impacts from the now-defunct mines around Brakpan, Springs and Nigel.
Stainless Steel for Acid Water
Grindex told E&MJ that the stainless steel pumps in its Inox family are capable of handling extremely corrosive fluids with pH values from 2 to 10. The six members in the range are also used for pumping fluids such as various types of mud, sludge and light slurries containing solids up to 80 mm in size. Inox pumps are designed for heads between 11 and 75 m, with flow rates of 13-65 liter/second.
Submersible drainage pumps in the company’s Inox range include the Minette, Major and Master models, which are complemented by the Salvador, Senior and Sandy slurry pumps. All of the cast parts used are made of acid-resistant stainless steel, and all of the pumps in the range can be equipped with zinc anodes for extra protection.
Grindex claims that its pumps have improved reliability because of the inclusion of an air valve that cools the pump if it runs dry. Inox pumps also feature a motorprotection system to protect the pump from damage resulting from power failures, while all the starting systems needed are in-built, so no external start equipment is used.
Double mechanical shaft seals with an oil compartment between them protect the motor from water ingress, while a cast stainless-steel impeller provides a longer working life when a pump is used to handle acidic water. Models used for drainage have an adjustable rubber-lined diffuser, while in their sludge-handling counterparts the pump housing is rubber-lined, providing greater wear protection.
Progressing Cavity Pumps for Abrasive Fluids
According to the French pump manufacturer, PCM, its progressive cavity and peristaltic pumps find extensive applications in the mining industry. Designed for transferring the most abrasive fluids such as pulp and tailings, their performance and efficiency are not affected by variations in viscosity, solids content, or the pressures that are typical of dewatering or filter-press feeding applications, the company added.
The company explained that this design of pump consists of a helical, stainless-steel rotor that turns inside a helical stator, molded in a resilient elastomer. The geometry is such that when the rotor is inserted into the stator, a double chain of honeycombed-shaped watertight cavities is created. When the rotor turns inside the stator, the honeycomb progresses spirally along the axis of the pump, never changing shape or volume. This transfers the product from the pump intake to the pump discharge, without any degradation.
Progressing cavity pumps offer high suction and are self-priming, while delivering a constant, non-pulsing flow. The flow rate is proportional to the running speed, and the units are both reversible and easy to maintain, PCM said. Mineral-sector applications include handling slurries and grouts, as well as in on-site explosives preparation.
In 2012, PCM introduced its EcoMoineau C progressing cavity pump, which it claims is the shortest stainless-steel model on the market. Weighing less than half its predecessor, and with a lower energy demand than other comparable pumps, the new unit has a small footprint that allows easier installation and maintenance, the company stated.
PCM also offers peristaltic pumps in which the deformation and compression of a soft elastomer hose allows fluid to be passed through the system. Both low- and high-pressure versions are available, with flow rates from 200 liter/h to 65 m3/h.
Benefits of peristaltic pumps include their inherent wear-resistance. The hose is the only part of the pump that gets worn, resulting in lower maintenance costs. In addition, there are no seals or valves to maintain, the pumps are self-priming, and can run dry without damage. They are suitable for handling both corrosive and abrasive media, and can even be used for dry solids.
Coping with Extreme Corrosion
When it comes to corrosion, few substances are as unforgiving as salt. In 2009, the U.K-based company, Verderflex, received an order from a salt producer in Spain for peristaltic pumps to handle slurries during the salt-purification process.
According to Verderflex, the raw material contains not only salt but also silica, mud and other minerals. Following decantation separation, one of its VF65 hose pumps is used to transfer highly abrasive silica slurry to a storage tank. Two more pumps—a VF40 and a VF65—are used for handling mud slurry.
Verderflex noted that its peristaltic pumps are less affected by abrasive materials since there are no seals or valves in contact with the liquid being pumped. The slurries are entirely contained within the natural rubber hose, which is the only wear part and is quick and easy to replace.
The company also reported that one of its pumps is in use in a vanadium-recovery plant in South Africa, which takes titaniumbearing magnetite as its feed. The vanadium is recovered through a roast-leach-precipitation process, with a VF40 pump, rated for flow rates of up to 7.5 m³/hr and pressures up to 16 bar, being used to handle abrasive barren slurry with a solids content of up to 60%, more than 80% sub-micron particle sizing, and a specific gravity of more than 2.
Verderflex stated that hose pumps can pump dense fluids like this while maintaining high levels of plant availability, whereas centrifugal pumps often have higher downtime and are less able to handle high-SG slurries.