Why McLanahan Thickeners?
With McLanahan Thickeners, producers can recover up to 90% of their water for reuse to create a more sustainable operation. They are ideal for sites where water is scarce or expensive. McLanahan Thickeners also reduce the amount of material reporting to waste. This allows producers to reduce the size of their settling ponds or tailings storage facilities, which can be expensive and dangerous to maintain, cause permitting issues and potentially cover mineable reserves.
Most McLanahan Thickeners (but not all) incorporate a low profile rake design with or without dewatering pickets, and the rake drive is powered either by a hydraulic power pack or direct coupled electro-mechanical gearbox. A modern feedwell design based on computational fluid dynamics (CFD) modeling ensures minimal flocculant usage. Optional sensors for monitoring bed level, underflow density and overflow turbidity ensure optimal operation.
Types of Thickeners
McLanahan offers a range of thickener types, including High-Rate, Ultra Rakeless, High-Density and Paste, that are tailored to an individual application. They can be used as stand-alone solutions or in conjunction with other equipment.
High-Rate Thickeners are used by the aggregate and mining industries, as well as environmental contractors, to recover approximately 85% of water for reuse. This high-level of water recovery keeps water consumption at sustainable levels, as well as drastically reduces the slurry volume reporting to waste ponds. They use minimal amounts of flocculant to quickly settle the particles suspended in the slurry, making High-Rate Thickeners environmentally friendly. High-Rate Thickeners can be supplied with flat-bottom, on-ground tanks or elevated tanks, which feature short suction lines that reduce the risk of plugging.
Ultra Rakeless Thickeners (ULT) feature a deep mud bed and a steep floor slope to assist in compressing the mud without the use of rakes. This allows ULTs to be a more economical option. Another benefit of the ULT is that while it offers high water recovery rates, the underflow is not as dense as that from a Paste Thickener and can therefore be pumped with a centrifugal pump.
High-Density Thickeners discharge an underflow mud with a yield stress between 50-100 Pa., which can be pumped with a centrifugal slurry pump. They are commonly characterised by having higher side walls to provide enhanced compression effects, a steeper floor slope to assist with moving the solids to the discharge area and a low-profile rake structure combined with pickets to enhance bed dewatering. High-Density Thickeners are available with on-ground, flat-bottom tanks or elevated tanks, which allow for shorter suction lines and reduced pumping requirements.
Paste Thickeners achieve the highest possible solids concentration by gravity alone, providing maximum water recovery as high as 90% and discharging a mud with a yield stress greater than 150 Pa. Because of high percentage of solids in the mud, producers can greatly reduce the size of their settling ponds or tailings storage facilities. Paste Thickeners feature tanks with high side walls and steep floor slopes to aid with the compression effects of the mud. A rake and picket system powered by a rugged drive helps create dewatering channels in the mud bed, allowing further compaction of the mud.
Frequently Asked Questions
What is the difference between a Thickener and a Clarifier?
Both Thickeners and Clarifiers separate solids from liquid, but they are used for different reasons. Clarifiers focus on the clarity of the overflow, while Thickeners focus on the density of the underflow. Read Thickener vs. Clarifier: What's the Difference to learn more.
What does a Thickener do?
Thickeners dewater slurries containing fine particles ranging from 250 to 45 microns (60-400 mesh) in size, recovering reusable process water and reducing the volume of material reporting to the settling pond or tailings storage facility.
How do I monitor and control the thickening process taking place inside the Thickener?
Thickeners should be viewed as a continuous settling and thickening process. When the Thickener is in a steady state, the solids are being withdrawn at the same rate as which they are being fed. Therefore, a principle of mass in equal to the mass out should be observed, and instruments for measuring both the feed, as well as the underflow density and flow is recommended. Since feed characteristics, such as particle size distribution and composition may vary, additional controls for measuring and controlling include flocculant dosage, rake torque, bed level, bed pressure and overflow clarity. The sophistication of the controls package needs to be evaluated compared to the importance of the equipment output and the degree of feed variation anticipated.
How do I properly size a Thickener?
To properly size a Thickener, the input and output requirements need to be accurately defined. Second, the settling characteristics need to be established. Ideally, this is done by testing the representative samples in a lab; however, if samples are not available, experienced personnel can make an assessment based on the input specifications (solids specific gravity, density and particle size distribution) correlated against a database of successful applications.
What types of testing are necessary to select the correct Thickener?
To determine the correct Thickener for an application, the following laboratory tests should be undertaken:
- Feed characterization
- Settling flux to establish the optimum solids for maximum rate
- Static cylinder testing at optimum solids to establish a free-settling rate
- Dynamic cylinder testing to confirm an acceptable flux rate
- Extended deep tube testing to determine mud residence times
- Yield stress (rheology) curve to confirm required rake torque
- Indicative flocculant and/or coagulant dose
Subsequent to laboratory testing, a pilot plant test can be used to demonstrate viability at a larger scale; however, McLanahan can provide a process warranty on the basis of laboratory test work alone.
Why and when is a lifting device used in a Thickener?
A lifting device is a mechanical attachment to the drive head that lifts the Thickener's entire rake mechanism out of the mud bed. While not always necessary, a rake lift is commonly used to provide some insurance against bogging of a Thickener. Should feed or flocculation conditions change and process control tools are either insufficient or inoperative, the rake lift is programmed to activate once a pre-set drive torque is measured. The drive torque is assumed to have increased due to an increase in the thickness of the mud. By lifting the rake mechanism out the thick mud, the operator can buy some time to address the change in conditions prior to the rake drive stalling. Watch How A Thickener Works to see how the rake lifts when torque conditions are high.
What is the role of the feedwell in a Thickener?
The role of the feedwell in a Thickener is to mix the slurry feed and flocculant under the right shear conditions to promote the fastest free settling. It is designed to translate horizontal feed momentum into a gentle vertical motion. Achieving the appropriate velocity and shear conditions are critical to minimise the polymer consumption. The feedwell may incorporate auto or forced dilution of the slurry feed to obtain the optimum solids level as identified in the testing procedure. Watch How A Thickener Works to see how the feedwell mixes and distributes the slurry particles.
How much does a Thickener cost?
Thickener costs vary due to the wide range of duties and styles. Please consult your local McLanahan representative to identify the most cost-effective option for your application.