The purpose of this article is to understand the meaning of Thickeners, its origination, purpose, operation, types of thickeners and factors determining the efficiency of a thickener.
What is a Thickener?
In very simple words, a thickener is a machine that separates liquid from solids. It is defined as a method of continuous dewatering of a dilute pulp, wherein a regular discharge of a thick pulp takes place which is of uniform density, concurrently with an overflow of clarified solution. The solids particle size range in thickeners is generally from 0.5 mm to a few microns. Thickeners are often rated on lbs. of solids per square foot of surface area.
Briefly, its functioning can be explained with Scraper blades or “rakes” that are used in a thickener, which rotate slowly over the bottom of the tank, and slope down towards the centre and move the material settled on the bottom to a central opening or discharge. Thickeners are generally used in metallurgical field with the purpose of thickening prior to agitation, filtration, in the countercurrent washing of cyanide slime, and more thickening ahead of floatation, for dewatering tailing to recover the water for reuse in the mill and for thickening concentrates.
Fundamentally, thickeners and clarifiers (which look visually like thickeners) are both used to settle solids which results in the separation of liquids and solids. The difference however is that thickeners are used to concentrate solids, while clarifiers are used to purify liquids.
Historically, thickeners were introduced to the mining industry in 1905. They are widely used in various industries including mining, coal, chemical, industrial, environmental (wastewater management), municipal and paper industry.
Working of a Thickener
Let us understand in detail about how thickeners work. To start with, it works on the principle of Gravity sedimentation and the most common construction of a thickener would be of iron or steel.
The continuous thickener consists of a cylindrical tank. Pulp is fed into the centre of the tank via a feed-well placed up to 1 m below the surface of the suspension. The clarified liquid overflows a trough, while the solids which settle at the bottom of the tank are withdrawn as a thickened pulp from an outlet at the centre. One or more rotating radial arms are there within the tank, from each of which are suspended a series of blades, shaped so as to rake the settled solids towards the central outlet.
With modern thickeners, there is a functionality of these arms rising automatically if the torque exceeds a certain value, thus preventing any damage which can result due to overloading. The blades also help in concentration of the settled particles by simple setting which enable a thicker underflow. In this way, solids move downwards, and then inwards towards the thickened underflow outlet and the liquid moves upwards and radially outwards.
Types of thickeners
The thickeners can be classified into two types depending on location of rake driving mechanism. They are known as either Centrally driven or Peripherally driven.
Further, under centrally driven type there are two types depending on method of supporting the drive mechanism and the raking arms.
- a) Column type: A kind of thickener where a central steel or concrete column takes vertically the reaction to the weight of the mechanism and horizontally the torque load.
- b) Bridge type: Here, a structure spans across the tank and is subjected vertically to the weight of the mechanism plus any solids that accumulate within the arms truss and horizontally to the twin forces imposed by the density of the raked underflow.
Generally, and for larger diameters the former type is used and for tanks up to 25-30 meter diameter the later type is preferred.
What are high-capacity thickeners and how they work?
In the 1980’s, machines known as “high capacity” or “high rate” thickeners were introduced by various manufacturers. These machines are characterized by a reduction in unit area requirement for conventional installations.
Here, the feed enters through a hollow drive shaft where flocculent (used to help thickening) is added and is rapidly dispersed by staged mechanical mixing. Further, this staged mixing action helps to improve and enhance thickening. This feed is then added into a blanket of slurry where the feed solids are further flocculated by contacting previously flocculated material. Since there is a direct contact between rising fluid and settling solids, which is common to most thickeners, it is averted with slurry blanket injection. There are radially mounted inclined plates which are partially submerged in the slurry blanket; the settling solids in the slurry blanket slide downwards along the inclined plates, producing faster and more effective thickening than vertical descent. Level sensor are used to automate the height of the slurry blanket.
Factors determining efficiency of Thickener
Several types of thickeners have been developed and classified according to the arrangement of feed and discharge in the various compartments.
For effective thickener control, key considerations are bed mass inventory, underflow density and flow, bed level, overflow clarity and flocculent dosing. All these parameters and measurements of these are not easy and one must factor in accuracy and reliability to select and install the proper equipment. Some of thickening equipment from well-known and respected brand names such as Eimco, FLSmidth, Dorr-Oliver, Phoenix, Lamella and Westech are popularly used.
Choosing a right thickener for your process and industry shall determine efficiency in its output and we strive to help you find one here!
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