Backfilling of mine voids using paste can help to reduce the quantity of tailings which must be stored in surface impounds. (Photo: Weir Minerals)

E&MJ looks at the evolving requirements and trends in pumping for paste backfill applications

By Carly Leonida, European Editor

The practice of backfilling underground mine voids using paste — a mixture of thickened tailings and/or waste rock combined with cementitious material — has seen a fresh wave of interest over the past decade. Studies are on the increase, as are enquiries for new equipment. Although the concept is not new (the first paste backfill applications were undertaken in the 1970s and 80s) a combination of heightened environmental, social and capital concerns surrounding new mines today mean that a greater number of operators are considering its use as part of greenfield designs.

Peter Peschken, Key Application Manager for Mining and Ash at Putzmeister, explained: “Paste backfilling can help to protect the surrounding environment, as a large portion of tailings are transported and placed underground, and the amount of material placed above ground in tailings storage facilities is reduced, as are the risks associated with dam management.”

As the immobilized tailings do not come into contact with rain, wind and groundwater, this means that, with the right monitoring systems in place, some that contain hazardous materials can be stored safely underground. Meanwhile, pastes containing non-hazardous tailings can be used to stabilize underground voids as part of the mining method, enhancing safety and enabling greater productivity through a higher recovery rate; ore-rich pillars and supports can be extracted without compromising mine safety, and the footprint of the mine significantly reduced.

“Backfilling mine voids provides good mechanical control of excavations which allows an increase in cut-off grades and also improves safety which, again has a direct impact on productivity,” Peschken added. “The number of planned and realized paste backfill projects have increased a lot during the past ten years, and I’m certain we’ll see more going forward as operators look for alternative uses and storage methods for tailings.”

Which Pump?

Paste backfill can be placed using gravity or through pumping — both options have advantages and disadvantages, and the choice is usually dependent upon the operational design.

Erik Vlot, Weir Minerals’ Global Manager for Tailings and Backfill, explained that hydraulic-driven piston pumps are typically required for pumping backfill paste, because the slurry is extremely viscous and has a high-density.

“Compared to gravity paste backfill systems, pumping allows for more flexibility and makes it possible to reach remote areas of the mine in a single pumping stage,” he told E&MJ.

Paste pumps are usually installed in three locations depending upon the site and operational requirements. Firstly, a paste pump is often required to transport the slurry from the processing plant over long horizontal distances to the mineshaft. Secondly, when the processing plant is close to the shaft and the horizontal distance that needs to be covered is less, a pump is also located near the shaft. In this instance, the paste is pumped directly into the mine shaft and the longer mine reticulation system requires additional pressure to fill the remote cavities.

The third option is to install a hydraulic-driven piston pump underground to transport the slurry that’s coming from the surface via a gravity backfill system. When the gravity head isn’t sufficient to ensure the slurry is discharged to the full length of the mine, hydraulic driven piston pumps are installed as underground booster stations.

“Hydraulic piston pumps are most commonly used for pumping paste backfill, but there are a couple of different types depending on the specific application,” Volt explained. “For instance, the GEHO DHC hydraulic piston pump with cone valve can handle high-density slurries with solids up to 90% — these pumps are renowned for their reliability in paste backfill applications.

“GEHO DH series hydraulic-driven piston pumps are driven by a hydraulic power unit (HPU), which generates the reciprocating movement of the pistons. When a piston moves backwards (suction stroke), the cylinder space in front of the slurry piston is filled with slurry. When the piston moves in the opposite direction (discharge stroke), the slurry is discharged from the cylinder into the main slurry line.”

Getting the Recipe Right

In aggregate paste backfill applications, coarse material — like fractured rock, for instance — is added to the slurry. As a result, less cement or binder needs to be added to achieve the same compressibility and strength. This is a significant boon given the price of cement today, and also the increased carbon footprint associated with its usage.

Peschken said: “With very fine-grained tailings, quite a lot of cement needs to be added to achieve the correct material strength once hardened. By co-mingling the tailings with stones or aggregates, either from the mine waste stream or a nearby quarrying operation, the amount of cement required can sometimes be reduced. Of course, the mixture must be designed and evaluated by a reputable engineering firm to find the most efficient and economical approach, but it is an option for many mines.”

Volt said that GEHO DHT hydraulic piston pumps with a transfer tube can handle coarser material and, as a result, are favored by Weir customers in aggregate paste backfill applications. The company has recently installed some of these pumps with a unique
pulsation-free system that includes a third chamber.

“In a hydraulic-driven piston pump with cone valves, pulsation-free pumping is achieved by creating a discharge flow that is continuous, despite the reciprocating motion of the pistons,” Vlot explained. “In a two-cylinder pump, this can be achieved if both pistons are driven independently of each other. Using this concept, the pump is divided into two single-cylinder pumps each driven by its own hydraulic system. The technology behind the pulsation-free system comprises pre-compression of the piston chamber that has completed its suction stroke up to the level of the discharge pressure before the discharging piston chamber has reached its end position. A programmable logic controller (PLC) controls the respective piston speed profiles, so that the total net discharge flow is constant, resulting in a constant discharge pressure.”

He added that this isn’t possible in a hydraulic-driven piston pump with a transfer tube, because during the switchover from suction to discharge, the slurry discharge flow stops. During this time, the swing tube transfers from one cylinder to the other, and the hydraulic flow stops. When the flow drops abruptly, pressure spikes occur in the discharge slurry pipeline causing high fluctuating loads and hydraulic hammering.

Weir Minerals has developed an innovative solution to this; during the discharge stroke of one of the main cylinders, an additional flow above the nominal slurry flow is generated. This additional flow is sent to a third cylinder and absorbed. The hydraulic oil flow from this third cylinder is then transferred back to a hydraulic pump, which is mounted on the same motor shaft as the other hydraulic pumps and used for controlling the slurry cylinders. So, the additional energy required to move the third cylinder is fed back to the same motor shaft and the only energy losses are friction and efficiency. During the swing tube switchover, the third cylinder will discharge the accumulated volume to the slurry pipeline, and thus continue the nominal pump discharge flow.

“In the past decade, pulsation-free solutions have become standard,” said Vlot. “Traditional piston pumps didn’t have pulsation-free controls. This meant that when one cylinder was discharged, it stopped, and then the discharge of the other cylinder started. As a result, flow and pressure variations were commonly high and the pumps generated a lot of noise and hydraulic hammering.”

Putzmeister recently installed a HSP 25200 HP paste pump at the Sindesar Khurd mine in India. (Photo: Putzmeister)

Bigger Plants, Bigger Pumps

Vlot added that there’s also a trend towards increasing pressure requirements for paste pumps. Initially, pressures up to 100 bar were typical, whereas now it’s possible to see pressures up to 150 bar.

“There have also been increases in flow rates of paste backfill systems, which can now be as high as 230 m3/h,” he noted.

Newcrest’s Brucejack gold mine in British Columbia, Canada, operates a paste backfill system to increase the recovery rate of the mine and reduce the amount of tailings stored on the surface. Brucejack operates three GEHO DHC hydraulic-driven piston pumps, which are dual single acting and rated for a continuous duty of 145 m3/h at 125 bar pressure and are equipped with a pulsation-free system.

“The paste is distributed to different zones in the underground mine,” said Vlot. “Two pumps are located in the mill at surface level, while the third pump is located underground. The third pump functions as a booster pump. Pumping pressures depend on pump flow, pumping distance, height difference and slurry properties. The main wear parts are the valves and seats, the piston (cup) and the cylinder. The lifetime of these parts depends on the pump operating hours, speed, pressure and slurry consistency.”

Peschken also noted the significant increase in the average size of paste plants over the past decade and, hence, an increase in flow requirements for pumps.

“The first paste pumps used in mine backfill applications had delivery capacities of maybe 30, 40 or 50m3/h,” he explained. “But now, capacities as high as 200-250 m3/h are common. As the size of the plant increases, it’s important to size the pumps correctly to achieve reliable operation. For example, with double piston pumps, the most important thing is the diameter of the delivery cylinder. By increasing the diameter of the delivery cylinders, we reduce the speed of the pistons, which increases the lifetime of the main wear parts.

“Pressure peak dampening systems, like the Putzmeister Constant Flow (PCF), are also requested more and more often. The smooth flow and more consistent pressures these create increases the lifetime of pipelines, fastenings, welds and flanges. It seems like a small thing but, when mines have pipelines that are several kilometers long, it can add years to the wear life of certain components. These systems also reduce the amount of hydraulic hammering during paste transport, particularly with less viscous pastes, which creates a less noisy environment for personnel working underground and makes them feel safer too.”

Today with large, slow running double piston pumps, like the KOS 25.200 or HSP 25.200 from Putzmeister, which have delivery cylinder diameters of 560 mm, up to 400 m3/h of backfill material can be pumped.

In 2021, Putzmeister delivered a new HSP 25.200 high-performance (HP) pump with a PCF control system to Hindustan Zinc’s SK mine in Rajasthan, India. The pump, which is driven by a HA 2XXL 2×500 HPU with PCF and 1,000 kW of installed power has now completed commissioning and is entering production. Putzmeister also recently began production at its facility in Germany of two HSP 25.200 HPs with PCF control for Hindustan Copper’s Malanjkhand mine in Madhya Pradesh, India.

Managing Wear and Tear

There are many aspects of paste pumps that make their service and maintenance challenging, particularly the HPU. The controls on this type of pump are more complicated than those of a piston diaphragm pump and a high level of expertise is required to service them correctly. Ultimately, these tasks need to be carried out by specialist service engineers.

“To reduce wear on pumps with seat valves or S-transfer tubes, the most important things are to reduce the number of switchovers and to reduce the piston speed,” said Peschken. “For reliable operation, it’s recommended to keep switchovers below 10 per minute.” 

A reduction in piston speed can only be achieved by increasing the delivery cylinder diameter, hence Putzmeister offers pumps with delivery cylinder diameters up to 560 mm.

“Depending upon the slurry properties and the operating conditions, there are also wear parts that need to be monitored and maintained,” said Vlot. “Most notably, the cylinder liner, the piston seals and, depending on the model, either the valves and seats, or the transfer tube nozzle and spectacle plate.”

Weir Minerals recently entered into strategic cooperation with its suppliers to rationalize its portfolio of GEHO hydraulic-driven piston pumps and standardize many of the components.

“There’s a growing demand for paste backfill solutions, so it’s important that we’re in a position to service the market with shorter lead times
and in a way that’s price competitive,” Vlot explained. “Weir Minerals is also looking closely at the standard design of its HPU to make pump forecasts more predictable.”