Successfully separating washed fines with a hydrocyclone separator

When producing aggregate, sand and gravel for use in roadbuilding, construction or other applications, raw material is mined from a deposit of high-grade materials intermixed with other undesirable materials and detritus. 

A physical separation must occur to extract the saleable material from the undesirable byproducts. Depending on the deposit, a variety of means of physical separation are at the disposal of the material producer.

In the course of this physical separation, fine and ultra-fine material may be generated or already present and entrained with the raw material. In these instances, a washing process occurs – and, often, an additional separation of these fines may be desirable either to extract and blend these fines with saleable product while still meeting the various specifications that exist for each product, or to concentrate and discard these fines in refuse ponds or dumps.

A common way of separating these washed fines from waste or product streams is through the use of a hydrocyclone separator.

Operating principles

Cyclones, like centrifugal pumps, have a curve by which their performance is governed.

There are defined pressure drops associated across a range of volumetric flows through the cyclone, and the cyclone’s separation performance is dictated by that pressure drop.

Essentially, the pressure of the fluid entering the cyclone generates the centrifugal forces that separate the solid and liquid phases within the device, whereby the heavier particles are flung outward from the central axis, flow in a spiral motion against the conical body section of the cyclone, and dragged into the underflow of the cyclone. The lighter, more fluid portion of the flow is directed to the central axis of the cyclone, and spirals upward into the overflow.

This classification occurs in the “separation zone,” a conical section of the equipment found below a vortex finder at the top/head box section. There are a variety of design principles that go into cyclones and separators to achieve various cuts of the media. There are performance curves for cyclones and separators – somewhat like a pump curve, where pressure drop across the cyclone is defined by a variety of parameters both intrinsic to the cyclone/separator and the pump feeding it.

Centrifugal pump performance in concert with cyclone performance

Centrifugal slurry pumps are excellent solutions for a variety of applications where solids are moved via hydrotransport – cyclone separation being one.

The slurry pump should be designed to achieve the total head to the cyclone, plus the calculated or predicted pressure drop across the cyclone for the desired flow rate and cut. Making the cut and maintaining a good separation is very dependent on maintaining the specified pressure drop.

Control of the pump then becomes critical to holding this pressure. As variable frequency drives (also known as inverters, VFDs or VSDs) have gotten less expensive and more common in industry, they have been more and more frequently applied to slurry pumps for cyclone/separator feed applications.

For maintaining optimum performance at the separation point (cyclone/separator), it is recommended to control the pump speed via VFD based on feedback from a pressure transducer measuring pressure at the cyclone/separator inlet.

It is understood that steady-state is the goal of any continuous operation, but in practice it is difficult to achieve with variable product feed and other non-steady-state processes around the mill or plant. Operating a pump in this way may lead to imbalance in tank levels, as the pump speed is varied to achieve a single-pressure set point at the cyclone/separator. But this can be designed around with adequate makeup water at the pump sumps.

Product dilution may occur in the sump when the process media delivering to the sump falls below design parameters, but the unit operation – centrifugal separation of heavier and lighter media at the cyclone or separator – will be maintained at the design point.

In general, if a centrifugal pump is feeding a cyclone or separator, the control point for the system should center on the cyclone performance. Pump performance must be maintained by ensuring adequate and constant flow to the pump via the feed hopper.

Still, if level control on the feed hopper is governing pump speed, and thereby flow and head (pressure) to the cyclone, cyclonic performance will likely suffer.

If cyclone performance is suffering, first confirm that no foreign objects entered the pump, pipeline or cyclone itself. Following confirmation of mechanical integrity of all equipment in the system, reach out to the pump manufacturer with all information relating to your system, including:

• Pump model
• Speed (or sheave sizes and motor nameplate speed)
• Installed motor power
• Pressure at pump discharge and cyclone header at duty point
• Motor amp draw at duty point
• (If present) VFD operating scheme (inputs/outputs)

With this information, the pump manufacturer should be able to assist in diagnosing and resolving cyclone performance issues, which may involve additional conversations with the cyclone manufacturer, as well.

Conclusion

Steady-state and reliable cyclone/separator performance is achievable in almost any application with a heavy-duty centrifugal slurry pump. With the right data in hand, operators can get the performance they desire out of their system and hopefully ensure greater reliability of all involved equipment moving forward.

Will Pierce is vice president of engineering at Schurco Slurry.