In broad terms they are incorporated in flowsheets for the following purposes:
Thickening in the downward direction to increase underflow density generally to reduce the size of downstream filters or centrifuges.
Clarification in the upward direction to reduce the amount of fines that report to the overflow to meet:
statutory requirements for effluent quality.
recycled overflow clarity for internal process requirements.
reduction in loss of product to the overflow.
Hydroseparation in the upward direction to classify and remove a desired particle size fraction or solids of a different property by controlling the overflow rate. Hydroseparators are not discussed in this section.
Classification of Particle Subsidence and Equipment Selection
The basic rule for efficient phase separation is that the suspended particles settle in a laminar environment where the upward velocity of the liquid is lower than the settling velocity of the solid matter. To meet this requirement the relative settling characteristics of solids in a liquid are defined by three basic groups:
Class #1 – Independent Particle Subsidence
Slurries with a low solids contents that settle freely without interfering with their neighboring particles leaving a clear supernatant. This class follows basically the Stokes Law before reaching the point of entry to the compression zone.
Class #2 – Intermediate Particle Subsidence
Slurries with solids that settle with several zone boundaries of varying degrees of clarity and mostly show no sharp interface. Such slurries are often flocculated to enhance the capture of fines to speed-up settling.
Class #3 – Mass Particle Subsidence
Slurries with high solids content that may settle freely at the beginning but shortly thereafter the particles enter hindrance that causes settling rate to decrease as concentration increases due to compression. The solids in such slurries show a sharp interface between the clear supernatant and the mass of settling solids.
There are also some in between classes but they are not discussed in this section.
The selection of equipment depends largely on test work for determining the class type. As a rough guideline Class #1 slurries may suit Lamella Settlers, Class #2 Clarifiers handling flocculated slurries and Class #3 Conventional Thickeners.