Thickeners


The separation of solids from a liquid by gravity sedimentation has traces to the early days of civilization. The normal practice at those times was to use jars or pits mainly for the clarification of extracted liquids such as wine or olive oil from contaminating insoluble matter. These batch processes required four separate steps:

  • Filling the vessel with slurry.
  • Leaving the slurry for a predetermined time until the solid matter has settled to the bottom of the vessel.
  • Decanting the clarified supernatant from the upper part of the vessel.
  • Removing the settled underflow that has accumulated at the bottom of the vessel.

This cycle, depending on solid and liquid properties that effect settling rate, may require long detention times so often several vessels are incorporated in the layout to operate in sequential steps.

The method of operating on a batch process is still practiced in small flow industries but its shortcomings are obvious so once the plants grew larger the need for continuous operation became inevitable. The trend in this direction started at the late 19th century when heavy duty applications such as iron ore taconites, hematite, coal, aluminum hydrate, copper pyrite, phosphates and other beneficiation processes have grown rapidly. The high time for thickeners was in the 60's when the metallurgical industries were booming and sizes of up to 150 m diameter were constructed. Such jumbo thickeners, when centrally driven, require for most demanding applications extra heavy duty drive heads some of which reach a continuous operating torque of 3.300.000 Nm.

 

150 meter Column Type Thickener handling phosphate slimes
150m dia Thickener

The various types of thickeners may be grouped as follows:

  Thickener Types

Thickeners are a major component in a plant layout and their selection may be critical for several reasons:

Thickeners Layout

In broad terms they are incorporated in flowsheets for the following purposes:

 

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.