What is the need of a thickener?
With few exceptions, most mineral-separation processes involve the use of substantial quantities of water and the final concentrate has to be separated from a pulp in which the water-solids ratio may be high. This is a method of dewatering or solid liquid separation. By this method up to 80% of the water can be separated. The water separated can be recycled in the system thereby reducing the need of fresh water in mineral beneficiation process. Also the relatively dry concentrate coming out of this process is easy for shipment and handling.
How a thickener works?
Thickener works in the principle of Gravity sedimentation. This is the most widely applied dewatering technique in mineral processing, and it is a relatively cheap, high capacity process, which involves very low shear forces, thus providing good conditions for flocculation of fine particles. The thickener is used to increase the concentration of the suspension by sedimentation, accompanied by the formation of a clear liquid.
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, in order to cause as little disturbance as possible. The clarified liquid overflows a peripheral launder, while the solids which settle over the entire bottom of the tank are withdrawn as a thickened pulp from an outlet at the centre. Within the tank are one or more rotating radial arms, from each of which are suspended a series of blades, shaped so as to rake the settled solids towards the central outlet. On most modern thickeners these arms rise automatically if the torque exceeds a certain value, thus preventing damage due to overloading. The blades also assist the compaction of the settled particles and produce a thicker underflow than can be achieved by simple settling. The solids in the thickener move continuously downwards, and then inwards towards the thickened underflow outlet, while the liquid moves upwards and radially outwards. In general, there is no region of constant composition in the thickener. Thus solids and liquid getting separated effectively from the suspension fed into the tank.
What are the types of thickeners?
The thickeners can be basically grouped in to two types depending on location of rake driving mechanism
- Centrally driven and
- Peripherally driven.
Under centrally driven type there are two types depending on method of supporting the drive mechanism and the raking arms
- Bridge type and
- Column type.
What is drive head?
The drive head, together with the raking arms, are the heart of the thickener mechanism since the entire operation of the thickener depends on their ability to convey continuously the dense underflows from the periphery of the tank to the center. They are generally of a robust design to meet the most difficult duties. The drive unit in majority of thickeners are mechanical type; however hydraulics units are also being introduced recently. The mechanical gear box can be of worm gear type for small thickeners and for larger units it is generally of spur gear type. In case of column type thickener the gear box is designed in such way that there is central free space for mounting supports for walkway.
What is a lifting device and why?
The lifting device is the element that raises and lowers the raking arms during operation so that the blades follow the interface of the settled solids by monitoring the torque. This ensures that the torque is maintained within set limits so that the arms are raised when the torque increases or lowered when the torque decreases. This is a safety feature required to run the thickener in varied load conditions. The bridge type and column type thickeners are provided with lifting devices and peripherally driven thickeners cannot be lifted; hence they cannot be used for storage.
What is feed well and why?
The feed well it the entry point for the incoming fluids into the thickener. Its purpose is to dampen the turbulence of the incoming fluid so that there is minimum interference with the settling solids. The best way of allowing the fluids to enter the well is a tangential path so that the centrifugal swirl generated will evenly distribute the feed below the liquid separation level.
What are high-capacity thickeners and how they work?
In recent years, machines known as “high capacity” or “high rate” thickeners have been introduced by various manufacturers. The machines are typified by a reduction in unit area requirement from conventional installations.
The feed enters via a hollow drive shaft where flocculant is added and is rapidly dispersed by staged mechanical mixing. This staged mixing action is said to improve thickening since it makes most effective use of the flocculant. The flocculated feed leaves the mixing chambers and is injected into a blanket of slurry where the feed solids are further flocculated by contacting previously flocculated material. Direct contact between rising fluid and settling solids, which is common to most thickeners, is averted with slurry blanket injection. Radially mounted inclined plates 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. The height of the slurry blanket is automated through the use of a level sensor.