Why McLanahan Thickeners?
McLanahan offers a range of thickener types and scope of supply that is tailored to an individual application. Most (but not all) incorporate a low profile rake design with or without dewatering pickets. A modern feedwell design based on CFD modelling ensures minimal flocculant usage, and the rake drive is powered either by a hydraulic power pack or direct coupled electro-mechanical gearbox.
While sand and aggregate producers have traditionally favoured an on-ground tank with steel side walls, industrial water treatment concrete tanks and mineral customers have favoured elevated steel tanks. Site location and logistics costs may also impact the appropriate selection. McLanahan design tools assist customers with choosing the best outcome for each application. McLanahan Thickeners can be used as stand-alone solutions or used in conjunction with other equipment, such as Hydrocyclones, Dewatering Screens, Pumps and Filter Presses.
How Thickeners Work
Feed to the Thickener enters from the feed tank to minimise air entrapment. The slurry is then introduced at a controlled velocity into the Thickener feedwell, where a flocculating agent is added from a polymer make-down system and is distributed evenly into the Thickener settling zone.
Flocculated solids settle and, along with gravity, are assisted to the centre of the Thickener by the rake mechanism. From the centre, the settled solids are pumped out via a McLanahan rubber lined centrifugal underflow Pump. Depending on the application and the style of Thickener in use, the underflow will have a consistency ranging anywhere from a free-flowing mud to a paste. Recovered water or liquor flows over the peripheral weir and reports to a launder for transport away from the operation. Clarified water overflows the peripheral weir of the Thickener.
Popular Applications for Thickeners
While the most common application of Thickeners are in mining and aggregate tailings, they are also found in mineral concentrate dewatering, hydrometallurgical intermediate dewatering, and counter current decantation washing processes. Clarifiers find use largely in water treatment applications.
Benefits of McLanahan Thickeners
- Modern feedwell design with proprietary auto-dilution where required
- Bolted and welded tank options
- Elevated tank with discharge drum for shear thinning requirement
- Low profile semi-truss rake arm design
- Planetary gearbox directly connected to torque tube
- Enhanced telemetry and controls
- Automatic lift function
Frequently Asked Questions
What is the difference between a Clarifier and a Thickener?
The difference is whether the design is focused on the overflow clarity or the underflow density.
What does a Thickener do?
Thickeners dewater suspensions containing fine particles ranging from 250 to 45 microns (60-400 mesh) in size, enabling a reduction in volume and raid reuse of process water.
How do I monitor and control the thickening process?
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. All these should be considered. 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, first, the input and output requirements need to be accurately defined. Second, the settling characteristics need to be established. Ideally, this is done by laboratory testing the representative samples; however, if samples are not available, experienced personnel can make an assessment based on the input specifications (solids specific gravity, density and PSD) correlated against a database of successful applications.
What types of testing are necessary to select the correct Thickener?
The following laboratory test should be undertaken
- Feed characterisation
- 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.
What is a lifting device, and why is it used?
A lifting device is a mechanical attachment to the drive head that lifts the 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.
What is a feedwell?
The feedwell mixes feed and floculant under the right shear conditions to promote the fastest free settling. It is designed to translate horizontal feed momentum into a vertical quiescent motion. Achieving the appropriate velocity and shear conditions are critical to minimise the polymer consumption. It may incorporate feed dilution to the optimum solids level identified in the testing procedure.
How much does a Thickener cost?
With a wide range of duties and styles, Thickener costs can range from $200,000 to $10 million. Please consult your local McLanahan representative to identify the most cost-effective option for your application.