Thickeners and clarifiers are both used to separate solids and liquid, but how do they differ?
Simply put, thickeners focus on the settled solids, and clarifiers focus on the clear overflow liquid.
A clarifier will typically treat a low-density slurry of less than five per cent w/w solids with a focus on producing a clear liquid with suspended solids of less than 100 parts per million (ppm).
To achieve these high levels of clarity, the up-current flow, or rise rate, needs to be low to prevent entrainment of fine particles.
Sizing a clarifier
A starting point for sizing clarifiers for most sand and mineral processing applications is 1gpm/ft or 2.2m3/hr/m2 (volumetric overflow divided by settling area, often abbreviated to m/hr). While this provides a guideline, testing is highly recommended to confirm the required rise rate to meet the target clarity.
Testing comprises a matrix of different throughput rates, chemical dosages and shear conditions.
For any given flow, the tank area or diameter required is determined by the nominated rise rate, the chemical dosage regime and the specific design of the equipment.
How clarifiers work
The clarification process uses gravity but is often enhanced with chemical treatment. To achieve high levels of clarity, both a flocculant and a coagulant may be required.
The coagulant can be added initially to neutralise surface charge, and the flocculant can be added afterward to pull all the particles together.
A complication may arise for very low-density feed slurries (less than one per cent solids) where the particles are too dispersed to be easily collected by the flocculant or coagulant.
In these cases, an improved solids-contact mechanism may be required where careful control of shear conditions is required to ensure sufficient energy to create large and faster settling colloids but not high enough shear to break apart the loosely bound flocs.
One commonly employed technique is known as high density sludge, or HDS, where a portion of the underflow is recycled to a pre-contact tank to seed the incoming feed.
Another technique is to settle the solids on steeply inclined laminar plates. In this example, the solids fall faster by sliding down the plate surface rather than in a free settling mode.
In principle, this is a great idea, but the requirements to build a plate stack that can structurally manage a high solids loading restricts this nifty idea to smaller, lightweight solids applications.
Should the process specification require a suspended solids level, perhaps discharge to public waterways, a sedimentation process alone may not be a cost-effective solution.
Combination with a security filter may be an option. Think of your swimming pool filter; with the clarifier overflow passing through a medium, such as sand or another more exotic material. Though as with the swimming pool filter, there is a complication of having to periodically backwash to clean the media.
A thickener, in contrast, is designed to do just that — thicken.
While a part of this process by definition releases the liquid, the quality of the overflow is secondary. Suspended solids clarities of up to 1,000ppm can be tolerated in some cases, though a typical specification is less than 300ppm.
Like clarifiers, thickeners can use flocculants to clump smaller particles together, allowing them to settle faster than they would on their own without the use of the flocculating agent.
A rake mechanism in the thickener helps guide the settling solids to the centre of the tank for discharge via centrifugal underflow pump.
Depending on the type of thickener, the underflow will range in density from a free-flowing mud to a paste.
Thickeners are characterised by having higher side walls, steeper floor slopes and a more robust mechanism.
How to size a thickener
To get the highest underflow densities, features such as tank geometry and rake design will be more critical.
The underflow density, to a degree, is proportional to the mud residence time (how long the particles remain the tank) and the compression effects of the column of settled muc so driven by the side-wall height.
Clarifiers or thickener?
It can be tempting to try to do both the thickening and clarifying functions to maximum benefit in one go. While technically this is feasible, you’d need a really large diameter (for low rise rate and good clarity), yet tall tank with higher side walls and a robust drive and rake mechanism.
As you would reasonably expect, this will come at a substantial cost premium, and it may be difficult to convince the money men it is a sound proposition.
The trick is to figure out what is really important for your application, thickening or clarifying and design the equipment to suit.