An Introduction to Thickener Underflow Pumping
Introduction
Thickener tanks are designed to separate slurry into sludge and water to aid the recovery of useable material, and to enable proper disposal of waste. The slurry is introduced into the settling tank in a way that minimises disturbance, resulting in a calm body of fluid to promote the settling out of solids. The clear water overflows the top of the tank, whilst the solids are pumped out the bottom as ‘thickener underflow’.
Thickener Pumping problems
Whilst this energy-efficient, gravity-driven process is simple in theory, the pumping of thickener underflow can be a very complex subject. For adequate separation, the underflow must be pumped out at a rate that maintains a consistent bed of sludge at the base of the tank. If the outflow rate is too high, it will start to draw out clean water that should be overflowing. If the underflow rate is too low, the whole tank can fill up with sludge, contaminating the overflow water.
Coupled with this need for precise control, the underflow itself presents a challenging, paste-like substance that demands a lot from the pumping system. In mine and quarry thickener tanks, the solid mineral particles and grits in the water are usually abrasive and have a wide mix of sizes and shapes. These variables can further complicate the design process.
Non-settling velocity
Whilst we want the solids to settle in the tank, we don’t want them settling out once in the underflow exit pipework. We now need to do the opposite of what was happening in the settlement tank; keep the slurry moving at a turbulent flow. Each slurry composition has its’ own settling velocity and we must keep the pipeline velocity high enough to prevent deposition (settlement) to avoid silt blockages. This is flow velocity is known as the limit settling velocity (LSV) or limit deposit velocity (LDV).
The difficulties with calculating the magnitude of underflow pumping implications are manifold:
1. The percentage and type of particle can be wide-ranging, and if for example, it’s all clay with disk-shaped particles, the settling velocity is vastly different to more angular particles, such as sand. Larger solids tend to settle out of suspension easier than fine, similarly sized particles.
2. Most thickener underflow applications have a non-negligible amount of flocculant remaining, which tends to increase the viscosity as particles continue clumping together.
3. The static measured viscosity can be different to the viscosity when in flow, as a high density of micro-particles can make the slurry anti-thixotropic, or dilatant so that they thicken up when under shear stress. An example of this is corn starch in water
4. Higher viscosity slurries tend not to flow turbulently, but rather have enormous frictional losses and stay laminar. Generally, laminar flow is not efficient for slurry containing coarse sand particles. Increasing the pipe diameter reduces the friction loss exponentially, usually resulting in a radically reduced settling velocity. This effect more than compensates for the slight reduction in velocity from using a wider pipe.
Thickener Underflow Characteristics
The thicker the underflow, the greater its shear stress. Shear stress occurs at the interface between the fluid’s relative movement against pipe surfaces, pump parts etc, or between layers of fluid moving at different velocities.
Many thickener underflow slurries in the mineral processing industry are dilatants – one of the non-Newtonian fluids. Dilatants are fluids whose viscosities increase when under shear stress, which can cause the thickened underflow to temporarily thicken up, possibly beyond the point of pumpability. We sometimes need to keep the flow rate low to prevent the velocity of these slurries from reaching their critical shear speed.
Where the underflow slurry is a Newtonian fluid, we don’t need to worry about changes to viscosity (although temperature and pressure changes can affect this). Our concern here is that the velocity does not become so low that solids settle out of the slurry, causing blockages through settled silt solidifying in the pipeline.
As a general rule of thumb, the target LSV for pumping Newtonian fluids is 2 – 3m per second, although Atlantic Pumps have gone down much lower on a very thick, paint-like homogenous slurry with no settling issues. Selection of the correct pipe diameter, and factoring in resistance from the material used, plus any valves and bends in the pipeline system, is critical in designing underflow pump systems.
To work out a suitable thickener underflow process system, we ideally start with a lab study of samples of the slurry to calculate its solids content percentage, specific gravity (SG), and suspended particle size range. The number most often used to represent the average particle size is the ‘d50’. This is taken as the mesh size that allows 50% of the solid particles through. To give a fuller picture of the size range, d20 and d80 are useful where the range of sizes is significant.
Mathematical formulas are available to estimate parameters in slurry pumping process engineering, developed by Cave, Stokes et al. These however have their limitations, as practically speaking there are so many different characteristics of slurry particles and compositions, whereas the formulas are only applicable to specific parameters. For example, Stokes's formula relates only to spherical solids in viscous fluids with a low Reynolds number. Cave’s Settling Velocity Formula
Caves formula provides an estimate of the minimum LSV for thinner, settling-type slurries with a narrow range of particle sizes and homogeneous flow.
Slurries of a d50um are usually homogeneous and considered non-settling.
Using such theories, the pump system engineer can establish a likely configuration of pump performance and pipe diameter needed for success.
The surest way to determine the necessary pump parameters for thickener underflow is to build a pilot trial piping system across a short length of pipe and measure the friction loss. If a non-Newtonian slurry is suspected, then recirculating it around the system reveals its reaction to the shear forces applied over time.
The Reynolds number of a fluid is dimensionless and therefore useful to enable the results of a smaller model to be scaled up for real-life system performance.
Thankfully, practical experience of pumping common, but often challenging thicker underflow slurries, in industries such as wastewater from quarrying, and construction/demolition material recycling, means Atlantic Pumps can offer a proven solution in a short timeframe. Where a novel application or specialist slurry is involved, our inhouse engineering team will undertake extensive research and testing to ensure the project’s success.
If you have a challenging thickener underflow issue, contact Atlantic Pumps’ technical team to see how they can help.
