Flat Bottom Classifiers, like McLanahan Lites-Out™ Classifiers, remove lightweight, deleterious materials, such as lignite and organic matter, from a minus 4 mesh (5mm) sand product stream using hindered settling.
Applications
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Flat Bottom Classifiers are part of the family of hindered settling classification equipment that includes McLanahan Hydrosizers™. These units use a dense fluidized bed of near-size sand to float off lightweight materials while allowing heavier fractions to pass through the bed.
Flat Bottom Classifiers are primarily used by aggregate producers in the production of construction sands but can also be used in specialty sands production, including glass sand and frac sand when organics are present. This technology is also utilized in soils washing for contaminated sites. Typically, contaminants such as PCBs are concentrated in the organics. The Lites-Out™ is excellent for this application where the organics are separated from the sand for disposal and the sand can be reused.
In a typical Lites-Out™ system, the Flat Bottom Classifier is fed by a Hydrocyclone to provide a controlled, dense and deslimed feed. This slurry enters the top of the unit via a central feed well, while water is injected via a plenum chamber through specialized nozzles to create an upward rising current. This establishes a zone of suspended, or teetered, solids. The solids in suspension are of a size fraction at the point of separation (better described as the cut point), suspended as neither coarser nor finer than the cut desired. Fine/light particulates don’t penetrate this dense fluidized bed and are carried up by the rising water current over the weir as overflow at the top of the unit. Coarser/heavier particles are able to penetrate the teetering bed and, by gravity, pass through the water injection zone to be collected and discharged through an underflow valve.
The McLanahan Lites-Out™ maintains separation automatically through a simple PID control loop. Operators can set the water injection rate and the desired operating density. The unit requires minimal supervision, as the Lites-Out™ control monitors the classification environment and opens/closes the dart valves at the bottom of the separation chamber. The unit automatically compensates for changes in feed rate, and the only moving parts are the underflow valves.
A manual globe valve and a flow meter is supplied with every unit so teeter water can be adjusted easily. For additional cost, the teeter water volume can also be automatically set and maintained using a separate PID loop control utilizing an air-operated control valve.
Where the Lites-Out™ units are being used for organics removal such as in soils washing or lignite removal, the overflow will contain larger organics with the finer sand (equivalent weight particulates); in this case, an interstage screen is used to separate the larger organics from the finer sands.
Typically, in construction aggregates, the screen throughs will be recombined with the coarse Underflow and fed to either Screws, Hydrocyclones and/or Dewatering Screens to produce a stackable product. Screws and Hydrocyclones assist in further removal of the finest lightweight materials.
The organics are either disposed of with plant effluent, collected for use for site restoration or sent to high temperature destruction in the case of contaminated soils projects.
McLanahan’s experienced process engineers can customize organics/lignite removal systems to meet the needs of most end users. Often these systems are add-ons to existing Classifying Tank systems, required due to changes in the deposits or the market, or are stand-alone systems for construction aggregate or contaminated soil applications. With many systems operating and one-stop-shop, field proven equipment choices, no one can match McLanahan’s offering.
As part of its effort for service and support, McLanahan Corporation provides sink-float testing at our in-house laboratory to evaluate the effectiveness of a Lites-Out™ System for specific applications. Testing for specific gravity is critical to the selection of the right equipment. Most lignite is in the 1.4 – 1.8 SG range, which is most suitable for removal from the 2.6+ SG sand.
The short answer is no. It should be noted that construction sand evaluation is based on ASTM C-123 — The Standard Test Method for Lightweight particles in Aggregate. In this test ,<50 mesh (-300µm) fractions are not considered part of the test. By the nature of the process, while there is a significant reduction in lignite, the final products are rarely lignite-free. To explain, the overflow of a Lites-Out™ contains the lightweight material and the fine useable sand. This overflow stream is directed to a screen, typically a sieve bend style. It will then screen out the coarser lightweight fractions as a waste; however, the throughs of the screen containing the fine sand will also contain fine lightweight material. This slurry is usually directed to a sump to be pumped to a recovery Hydrocyclone or directed to a dewatering Screw Washer. Both the Hydrocyclone and the Screw Washer will reject most of the lightweight materials as overflow but will not eliminate the lightweight materials.
It is possible that this material may be of higher specific gravity. The operating level in the Lites-Out™ can be raised to just below the overflow weir. The set point can be adjusted higher and the teeter water flow can be increased. A combination of all the above will typically improve this contamination. Slightly reducing the sand screen may also produce the desired effect. If you continue to experience this problem, take samples of the particulates for submission to the McLanahan lab. Before shipping the samples, obtain the necessary documentation from your dealer or the factory.
It depends. Flat Bottom Classifiers/Lites-Out™ do make separations based on the set-point in a similar way to the Hydrosizer™; however, with the batch style discharge, fines can be drawn into the coarse stream from the fluidized bed. Depending on the accuracy required for the individual products, or blend of overflow and underflow, this may not be of concern. Typically, though, the device is not accurate enough for the discrete cuts in frac sand applications.
McLanahan Lites-Out™ Flat Bottom Classifiers remove lightweight organic material from sand products.
