An Introduction to Thickener Underflow Pumping

by Atlantic Pumps Admin

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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.

From motor size and pump speed to wear life and operating costs, an imposing array of choices face a buyer intent on reaching and maintaining optimum pump performance. Here are some tips from experts.

by russell a. carter, contributing editor

Slurry pumps are essential for moving hard-to-handle, high solids-content fluids and sludge, and annual demand for these pumps reflects just one aspect of the significant space they occupy in several industry sectors. The global market for all types of slurry pumps is estimated at well more than a billion dollars each year, and although those sales represent only a single-digit portion of overall pump sales, slurry pumping costs take up a lot of space in mining&#;s collective energy budget. Process equipment supplier Metso estimated that slurry pumps account for only about 5% of centrifugal pumps &#; the most common type used for this purpose &#; installed throughout the mining industry, yet this small segment represents up to 80% of the industry&#;s total operational pumping costs.

The space they physically inhabit in a mining operation is typically harsh &#; at the bottom of a sump, a prep plant or thickener-underflow discharge point, or serving a pipeline carrying abrasive slurry. Their duty cycles range from continuous to sporadic depending on the application, often with highly variable flow rates and particle sizes. Internal wear can be severe in some applications, with as much as 2 mm of material a day disappearing from crucial component surfaces. Due to the increased probability of high wear rates from the materials being transported, pump builders add thicker, heavier components and/or internal liners, making slurry models larger and heavier than their water-pump brethren.

The wide range of pump-performance requirements encountered at thousands of mine, mill and other industrial sites requires an equally wide variety of pump types, sizes and mounting configurations. Two recent product introductions illustrate the range of available choices.

Going Big, Going Mobile

Late last year GIW Industries announced that it had developed the TBC-92 slurry pump specifically for use in oil sands operations. Named for its 92-in.-diam (234 cm) impeller, GIW claims the TBC-92 is the largest and heaviest slurry pump available in the mining industry.

At the other end of the size and portability scale, Gorman-Rupp&#;s transportable SludgeKat self-priming, positive displacement hydraulic piston pump is designed for convenient pumping of sludges and slurries from clarifying pits, wastewater treatment, mud pumping, environmental cleanup and similar applications.

The SludgeKat has 4-in. (100-mm) suction and discharge ports and is capable of flows up to 226 gpm (14.3 lps) and heads up to 390 ft (118.9 m). Depending on the product being pumped, SludgeKat can pass up to 2.4-in.-diam solids without damaging or clogging the pump. Units are equipped with Kohler Tier IV diesel engines.

Each SludgeKat comes standard with a wheel kit. The pump end frame is mounted to a 52-gallon (197-l) fuel tank base and offers a full-load run time of 25.5 hours. The pump end frame can be detached from the unit and when connected to optional 150-ft (46-m) hoses, provides increased portability around the job site.

In the space between these two very different pump solutions lies an array of conventional horizontal and vertical centrifugal models, submersibles and other types offering a wide range of performance characteristics that can be applied to specific slurry pumping requirements.

Pumps, unsurprisingly, can also fail to perform adequately if specified or installed incorrectly.

Look Beyond the Pump

The industry&#;s continuous drive to increase production from existing assets makes it important to view pump systems as one part of a much larger picture. In a recent blog post, Metso&#;s head of pump product management and marketing, Chris Wyper, outlined some important points to consider about pumps when aiming for plant-wide production increases. Among his recommendations:

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Ensure motor power availability: &#;A well-designed plant has enough power allocated to mill pumps. Pumps typically operate on variable speed drives, meaning there are many process variables affecting speed and, finally, the power draw. It is a good idea to look at SCADA data on historical power drawn to better estimate the amount of power that would be available for tonnage increases. Rather than using engineering data sheets that are somewhat oversimplified, it is beneficial to use a point cloud type plot showing flow and pump pressure as a function of time. This information makes it possible to determine the optimal size of all the pumps and cyclones for the plant.&#;

Consider gearbox cooling at higher power: &#;As pump duty is increased, it usually also increases the power transmitted through the gearbox. This means that the amount of heat increases as well: a gearbox that is sized marginally for air to air cooling may overheat with higher continuous duty. Consideration must be given to the cooling capacity of the lubrication system, particularly at higher ambient temperatures and altitudes.&#;

Ensure gland seal water pressure at higher heads: &#;The pump gland seal water system should be sized so as to be able to deliver a constant flow of gland water under all operational conditions. This applies to the pump duty, including any increase in head due to tonnage increases. It should also be checked that the gland seal water system is adequate when other demands are placed on it, such as hose downs or flushing.&#;

Take a close look at pipe sizing: &#;If you double the speed, the rate of material loss increases 16-fold and the rate of abrasive wear on the surface is approximately proportional to the fourth power of velocity. If there is a significant increase in input, it is necessary to consider whether the pipe sizing is optimal. The right size allows friction losses and wear to be minimized. Of course, if there is a large variation in flow, then minimum velocity to prevent settling should be examined.&#;

Prepare for crash stops by calculating floor sumps: &#;In the case of a plant crash-stop, prepare for the maximum inflow based on calculations on the live volume of floor sumps. This may include the mill static overflow and any dump valves to empty pipes and sumps. If sump size is increased or the mill volume changed, then the sumps may be undersized. In this case, the existing sumps can be deepened or enlarged, to deal with the volume, or then additional sumps created. Typically, mill sumps should be separated from the other sumps in the plant due to the possibility of mill balls entering the sump.&#;

Expanding Future Options

As industry-wide figures indicate, slurry pumping can serve as a prime example of purchased capital equipment where operating and maintenance (O&M) costs rapidly eclipse the initial procurement cost. A myopic view of TOC (Total Cost of Ownership) factors when selecting a pump can result in a variety of bad outcomes ranging from the need to prematurely replace an inadequate unit, to sky-high maintenance costs and production losses from unscheduled downtime. Conversely, pump OEMs and aftermarket suppliers are increasingly cognizant that their customers can&#;t always predict future events and consequently are expanding their product and services portfolios to provide affordable options when mining conditions, maintenance resources or technology changes occur over time.

Manufacturers are also looking at ways to incorporate more performance flexibility into their pump models and ease some of the concerns associated with necessary pump modifications. &#;For example, we are developing a line of pumps designed with a solid casing with replaceable all-metal, liner-like elements,&#; said Will Pierce, manager of engineering, Schurco Slurry. The metallurgy for these wear components is a novel enhancement to the proven 27%-28% chrome white iron that the industry has used for decades. We have hard rock customers that started with rubber liners 20 years ago, now they&#;re in a different ore deposit at the same mine and the material is sharper or has different abrasive characteristics and the rubber isn&#;t lasting. With the shell we&#;ve developed, they&#;re able to convert to a completely metal lined pump without major impact to the overall installation through using backward compatible adapter plates,&#; Pierce explained.

The new design also offers Shurco&#;s coal clients notable benefits: &#;Our coal customers almost always use metal-lined pumps, but the industry is very price-sensitive right now, so this new development doesn&#;t have the traditional massive ductile iron outer shell and metal liner &#; instead, it has replaceable metal wear components. There&#;s no quality compromise on the pump&#;s internal components, no change in wear or hydraulic performance. It&#;s just a lower-cost alternative.&#;

Designing for Durability

A rule of thumb when selecting a slurry pump is to look for the most robust pump, in terms of performance, wear resistance, power and maintainability, that falls within the service class rating for the type of material being pumped. Even that simple process can be complicated when special circumstances arise, such as unusually high mechanical wear experienced in a specific application, or intermittent operation rather than steady running. Pump manufacturers generally have vast knowledge of what works and what doesn&#;t under many conditions, and they incorporate the features that do work into their latest designs. For example:

FLSmidth Minerals expanded its line of Krebs millMAX slurry pumps with the introduction of the millMAX-e, which features a unique wear-ring design that the company claims solves grinding and recirculation problems within the pump by maintaining clearances between the impeller and the suction side. By maintaining the design performance without increasing the speed, the wear ring extends the life of all wet end parts and reduces power consumption.

The millMAX-e model is unlined and offers a compact, space-saving exterior design aimed at reducing capital and replacement costs as well as motor-power requirements. However, according to the company, millMAX-e&#;s power frame uses the same bearing and shaft components as the equivalent millMAX power frames and is capable of handling applications requiring high speed and power. The millMAX-e is equipped with the patented Krebs pump belt tensioning system that allows users to quickly change out v-belts without having to realign the sheaves.

Tsurumi Manufacturing&#;s entries in the mining-class slurry pump market include its GPN and GSD series, rated at motor outputs of 7.5-50 hp (5.5-75 kW) kW and 50-100 hp (37-75 kW), respectively. Both series comprise submersible three-phase, high head and high volume heavy-duty slurry pumps driven by a four-pole motor. They are equipped with high-chromium cast iron agitators that the company said assist in smooth handling of settled materials. Motors are enclosed by a water jacket that assures efficient cooling even when the motor is exposed to air. Pumps in this series incorporate seal pressure relief ports that prevent pumping pressure from affecting the shaft seal.

Finland-based Flowrox&#;s heavy-duty CF-S horizontal centrifugal pump is the first in a series of centrifugal pumps to be introduced by the company and capable of continuously pumping highly abrasive and dense slurries. The company said the new pump can provide flows from as low as 2.3 m3/h to more than 4,000 m3/h at heads exceeding 76 m. The pump&#;s split-case design is claimed to provide a good balance between efficiency and wear, and models are available with a range of liner material options. The pump is compatible with Flowrox&#;s Digital Services platform, a customized IIoT-based process data collection and analysis system.

MBH Pumps unveiled the Ni-Hard series submersible slurry pumps, designed and built to pump slurries containing abrasive solids up to 65% by weight. These heavy-duty pumps, according to the supplier, are equipped with an external agitator that breaks settled or compacted solids, while its adaptive spiral plate technology delivers higher pumping with less energy consumption.

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