Engineering software developer and pump specialist TAS Online has secured contracts with three coal washing plants to assess the efficiency of their pumps, using portable instrumentation and their locally designed pump analysis software.

TAS Online business development director John Schulkins states that pump system assessments provide an independent analysis of pumping systems and identify areas where operational cost savings can be made.

“We have used our analytical software as a tool in conducting the initial assessments of pumping systems, with manual input of data derived from portable instrumentation. We have found coal washing plants to be a particularly rich source of potential energy savings,” he says.

Schulkins adds that many coal washing plants were built many years ago and are not energy efficient.

“These plants were built when electricity was cheap, and overall design was about limiting capital expenditure and operational downtime, rather than focusing on energy efficiency and the life-cycle cost of pumping.

“Electricity now consumes up to 90% of life-cycle cost. Over the last five years, the cost of power has gone through the roof. It has forced mining houses to consider the energy efficiency of their pumping systems,” he says.

TAS Online’s PumpMonitor software provides users with the information required to increase the efficiency of their systems for substantial cost savings, improved reliability and better throughput for almost all pump-dependent processes.

Coal Washing and Correct Medium Pumps

TAS Online technical director Greg Adcock states that during the coal washing process, coal needs to be separated from the unwanted (gangue) minerals mixed in with it during a dense medium separation process.

“The product from the mine is mixed into a magnetite-based slurry, with a specific gravity (SG) of around 2.0. The coal, with a SG of 1.6, floats on the slurry, while the gangue materials with SGs of 2.5 or more sink.

“The lumpy coal is separated by passing the slurry through drums. The fines are classified using a cyclone, where the coal-rich slurry is extracted from the top, while the gangue is extracted from the bottom.

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Coal washing plants consist of processing modules. Each coal washing module has a correct medium pump, which moves magnetite slurry through the classifying system. The resulting slurry is washed by passing it over screens and spraying it with water, thus creating product and dilute medium or dilute magnetite slurry.

“The now-diluted slurry falls into a sump or tank from which it is pumped – by a dilute medium pump – to a magnetic separator. This system recovers the valuable magnetite for remixing and circulating through the correct medium circuit,” he says.

At the start of the correct medium circuit, the dense medium is pumped into a mixing box where the mined coal is added. To maintain high-production levels, imperative in all coal plants, plant operators need to avoid underpumping at all costs.

“Any overpumped correct medium slurry simply overflows back down into the correct medium tank, but if you underpump, the production rate drops, as there is not enough force on the cyclone and the whole process is directly affected,” Adcock explains.

He further states that correct medium pumps are usually fixed-speed pumps that are continuously overpumping.

“During an assessment, we are able to quantify the levels of overpumping, which are as high as 50% in some cases. We can then determine the business case for rightsizing or for using a variable speed drive (VSD) to ensure that overpumping is reduced.

“By using VSDs you can control the flow rate very accurately so that you never underpump but you never overpump unneces- sarily either,” he states.

Adcock notes that, during a the assessment of a pump system at a local coal washing plant, the correct medium pump was overpumping by, on average, 30%. “The value of the energy wasted as a result was calculated to be about R120 000,” he adds.

“There are several other spin-off benefits. Smaller pumps can be used for the lower flow, maintenance is reduced because of the reduced speed, the pump speed can be reduced when the system is in recirculation mode and the tank level can be much more accurately controlled,” he adds.

Dilute Medium System Design

Adcock states that, on a typical dilute medium circuit, the dilute slurry is pumped to a distribution box, which can be 40 m high. From there, it is gravity-fed into a magnetic separator and then through a settler. At some operations, the dilute slurry is fed into another magnetic separator.

Following this, the relatively clean water for washing then drops down into a tank at the bottom. “This vertical system has been found to be very convenient because, once pumped up, the product can be easily distributed to any location from within the plant.’
“However, it is not efficient in terms of pumping. We are proposing a change in the process. If the water passing through the screen is taken directly to a magnetic separator below the screen, clean water falls into a tank at the bottom. This water can then be pumped back up to the washing screens,” Adcock suggests.

“Not only would the pumps pump water using 35% less energy, but the lift height can also be halved, resulting in energy savings,” he says.

Tailings and Tailings Return Pump Assessments

Adcock notes that, while lumpy gangue materials from most processing plants are conveyed to the tailings dams, the fines are first thickened and then pumped to the dam, which is usually a few kilometres away.

“Through a pump systems assessment we found that the thickener underflow slurry was pumped by a fixed speed pump to the slimes dam that was 4.2 km away.

“We found that the tailings pump had an extremely steep systems curve. The pump was operating too far to the left of the curve and, therefore, at a low efficiency of 41%. It was also oversized for the duty.

“The only way this could be determined was by taking measurements during an assess- ment. Once the actual requirements of the system had been established, calculations were made that resulted in sensible decisions,” Adcock says.

He adds that the pump flow was overspecified by nearly 100%.

“By rightsizing the pump with a 10 ME, the operating flow point was shifted to within 88% of flow, resulting in a power draw reduction from 88 kW to 71 kW, resulting in a 20% energy reduction.

“Also, we found that a pump underneath the thickener was pumping to the tailings pump 50 m away, which resulted in large frictional losses and the steep systems curve.

“By placing the second pump closer to the dam, the frictional losses were redistributed between the two pumps and the head seen by the tailings pump was reduced,” he explains.

This resulted in further energy cost savings and the tailings pump being shifted closer to the tailings dam.

Tailings Return Water

In another case study, TAS Online found that six pumps were used to return water from a tailings dam to a plant. This configuration comprised three sets of two series pumps that operated in parallel.

“During the assessment, the operational functioning and performance of the pumps was investigated, individually and in different configurations of series and parallel. When pump one operated on its own, it produced 360 ℓ/m at an efficiency of 72%. When pump three, in a parallel branch, was switched on to supplement pump one, the head went up. This was because of the additional frictional losses, which dropped the flow and reduced the overall efficiency of pump three to 60%, which was acceptable,” Adcock explains.

He adds that when pump one was coupled in series with pump two, the head shared between the two pumps and that seen by pump one dropped considerably to 32 m.

“This pushed the pump’s operating point off the end of the curve because there was insufficient pressure acting against it, the pump cavitated and generated significant turbulence, which had maintenance implications. This indicated that these two pumps can’t be operated in series. You may get the flow you need but to the detriment of maintenance costs,” he states.

The assessment thus concluded that the best efficiency and energy consumption rate and the lowest maintenance costs are achieved when two pumps are run in parallel.

Performing pump system assessments reveals pump and pumping system performance issues that are often hidden in traditionally built plants.

“While pump users are usually aware that their systems are not running perfectly, assessments can be used to quantify inefficiencies that, when addressed, can result in signifi- cantly reduced energy consumption and maintenance costs, improved plant performance and an increase in throughput,” he concludes.