Introduction
When it comes to managing tailings, producers have a variety of equipment options to help them reduce the volume of slurry material reporting to the settling pond or tailings storage facility (TSF). One of these options is a Thickener, a liquid-solid separation device that can be used alone or as part of a larger tailings management system to reduce and/or eliminate the need for waste storage facilities.
Thickeners offer several benefits. For tailings management, they are used to concentrate/thicken the solids in the waste stream to reduce the volume of slurry reporting to the TSF and/or to reduce the load on downstream processing equipment, such as a Centrifuge or Filter Press. Reducing pond size or eliminating the need for ponds or TSFs altogether can free up valuable land space and make it easier to obtain required permits.
Thickeners also recover a clear liquid portion that can be reused as wash water in the wet plant, reducing freshwater requirements. This is especially useful in areas where freshwater is in short supply or expensive to buy and adds to the overall sustainability of the site.
How Thickeners Work
Slurry that enters the Thickener is (in most cases) mixed with a chemical polymer, such as flocculant, to agglomerate the solid particles and allow them to settle faster in the tank. The settled solids are then discharged out the bottom of the Thickener tank, while the clear liquid overflows a weir at the top of the tank and is collected in a storage tank. The thickened solids can be pumped to the TSF or to the next stage of the process.
Types of Thickeners
Thickeners can come in many shapes and sizes. While they all follow the same basic working principle, there are a few design differences that allow the various Thickeners to achieve different underflow densities and water recovery rates.
The main types of Thickeners include:
High-Rate Thickeners
High-Rate Thickeners are used to achieve moderate underflow density and good overflow clarity with low chemical dosage. They also offer high liquid recovery.
High-Rate Thickeners feature a tank with a sloped floor and rotating rake mechanism to assist with compacting the mud bed.
Ultra Rakeless Thickeners
Ultra Rakeless Thickeners are aptly named because they lack the rake mechanism that other types of Thickeners use to enhance the dewatering of the mud bed and move the settled solids to the discharge zone.
In fact, Ultra Rakeless Thickeners have no moving parts at all. They consist of a tall tank and steeply sloped floor for creating a deeper mud bed, and they take up a small footprint on site.
While these simple, low-complexity devices may be most cost-effective in terms of maintenance, Ultra Rakeless Thickeners do require a higher chemical dosage and higher bed residence time to achieve the solids settling, which can result in increased operational costs.
Ultra Rakeless Thickeners discharge a moderate underflow density between that of a High-Rate Thickener and High-Density Thickener.
High-Density Thickeners
High-Density Thickeners are similar to High-Rate Thickener but include additional features to increase underflow density and liquid recovery. These features include higher side walls and a steeper floor slope to enhance the compression effects on the mud bed. They can also feature vertical pickets on the rake mechanism to create channels in the mud bed that release additional liquid.
Paste Thickeners
Paste Thickeners get their name from the consistency of the underflow density, which often resembles a paste. These types of Thickeners achieve the highest possible solids concentration by gravity alone. While not all applications using a Paste Thickener will achieve a paste-like underflow, the underflow will be such that liquid can no longer freely flow from it.
To achieve the high underflow densities for which they are known, Paste Thickeners feature a tall tank design with a steep floor slope. These elements help to compress the mud bed that forms as a result of the solids settling to the bottom of the tank. A rotating rake equipped with vertical pickets further aids in solids compaction by creating dewatering channels in the mud bed for the liquid to be released. Longer bed residence times are also typical with Paste Thickeners to allow the solids more time to compact.
In addition to the highest solids concentration, Paste Thickeners provide maximum liquid recovery.
Other types of Thickeners include:
- Bridge Thickener, which is a design where the mechanism is suspended off a bridge that spans the entire diameter of the tank
- Clarifier, a device similar to a Thickener but where the primary focus is on achieving the best possible overflow clarity rather than the underflow density
- Column Thickener, which is a design that incorporates a center pier to support the bridge
- Conventional Thickener, a design with a large diameter, low aspect ratio settling tank that generally doesn’t use chemicals
- Elevated Thickener, a design where the tank is raised off the ground, usually on steel support columns, and enables better access to the underflow pump
- On-ground Thickener, a design where the tank floor is in direct contact with the ground.
Thickeners can be more than one type. For example, a High-Rate Thickener can also be an Elevated Thickener and a Bridge Thickener.
Which Thickener is right for me?
The type of Thickener that is best for an application will depend on many factors, including the material characteristics, process parameters and site goals. Feed material should be tested to determine the type and size of Thickener required to meet the specified goals.
With the right Thickener, producers can more easily manage their tailings waste, while recovering reusable process water and improving site sustainability.
