Clarification and thickening

The liquid bulk flow limits the upward flow of small particles from the internal side and has a significant influence on the separating effect. Hydroclones are applied successfully for classification, clarification and thickening of suspensions containing particles from 5 to 150 tm in size. 

Svarovsky L (2000), Gravity clarification and thickening, In Svarovsky L (ed), Solid-Liquid Separation, Butterworth-Heinemann, Oxford. 

Separation involving sedimentation is dependent upon settling velocity, which requires a difference in density between solid particles and the suspending liquid. Gravitational sedimentation operations are divided into clarification and thickening. Clarification involves dilute suspensions and frequently has the objective of liquid recovery. Thickening refers to solid recovery by forming more concentrated slurries. Particle size, liquid and particle densities, and liquid viscosity are important factors in sedimentation processes. 

Typical uses Clarification and thickening to produce a solids sludge. 

If the primary purpose is to produce the solids in a highly concentrated slurry then the process is called thickening and the equipment is known as the gravity thickener. The feed to a thickener is usually more concentrated than to a clarifier, the primary purpose of which is to clarify the feed. A correctly designed and operated thickener, however, can accomplish both clarification and thickening in one stage. 

If hydrocyclones are to be used to produce thick underflows, the total mass recovery of the feed solids has to be sacrificed because throttling the underflow orifice inevitably leads to some loss of the solids to the overflow. A hydrocyclone as a single unit cannot therefore be used for both clarification and thickening at the same time. Typically the underflow concentrations that can be achieved with hydrocyclones are up to 50% by volume or more. In this they compare favourably with gravity thickeners and hydrocyclone systems are often used to replace the much larger and more expensive gravity thickeners. 

The lamella plate and inclined tube principles have been used both in clarification and thickening since the early 1960s. Examples of appUcatioDS include the treatment of coal, gas scmbber effluents, fly ash and leach solutions, or in battery manufacture. In coal concentration the typical overflow rates are from 1.7 to 2.9 m/hour. 

Figure 6.19 A three-stage arrangement for clarification and thickening of dilute feeds...

Fouthly, there are some major revisions or total rewrites such as Introduction to Solid-Liquid Separation, Gravity Clarification and Thickening, Vacuum Filtration, Centrifugal Filtration and Countercurrent Washing. 

Sedimentation is the separation of suspended solid particles from a fluid stream by the action of a body force on the settling behavior of the particle. The body force may be either gravitational or centrifugal force. This section covers gravity sedimentation, represented by clarification and thickening. The equipment used for these two operations are called clarifiers and thickeners, respectively. Centrifugal sedimentation will be discussed in the next section. 

From a unit operation standpoint, clarification and thickening are essentially based on the same design principles, and each combines features of the other. The key objective of a clarification operation is to remove small quantities of suspended particulates from the liquid stream to produce a clarified effluent or overflow stream. In thickening operation, the goal is to concentrate the dilute suspensions for their further treatment in filters or centrifuges. Key features of these two types of operation are presented in Fig. 8. 

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