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 5% 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 neutralize 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 1% 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 undeflow 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 other 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.
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 and compression effects driven by the side-wall height.
Clarifiers are thus characterized by having larger diameter, shallow tanks with a more lightweight rake and drive system compared with having higher side wall, steeper floor slopes and a more robust mechanism.
It is tempting to try to do both functions to maximum benefit in one go, and while this is technically feasible, if you were paying attention you'll understand you'll likely need a really large diameter, tall tank with higher side walls and a robust drive and rake mechanism. As you would expect, this will come at a substantial cost premium.
The trick is to figure out what is really important for your application and design the equipment to suit.