Thickeners are used in continuous process applications where liquid-solid separation by sedimentation is involved. Thickeners have three basic purposes:
- Concentration, or raising the density of a mixture of solids and liquids (feed slurry)
- Clarification, which involves removing solids from the mixture.
- Hydro-separation, where a specific fraction of solids is removed from a mixture
In some applications, thickeners recover valuable solids whereas valuable liquids are recovered in others. In addition, thickeners are an economic means for of process waters. Clarifiers are extremely efficient for treatment of waters.
MIP Process Technologies thickeners consist of the following basic parts, each of which is supplied in a number of variations to best suit a particular application:
- Drive unit and optional lifting device
- Torque Tube (centre shaft)
- Rake arms and blades
- Underflow discharge arrangement
- Dual feedwell
- Overflow launders with weirs
Thickener development and general history
Sedimentation technology was originally developed in the mining industry. Prior to the development of the continuous thickener, batch gravitational settling was employed. In such batch operations, dilute feed was pumped into a tank continuously until a clear overflow was no longer obtained. The feed was discontinued and the tank left undisturbed until the solids had settled. After a suitable retention time, clear supernate liquor was decanted and the thickened sludge removed.
After using batch-settling operations for some time, the industry developed cone settlers that could be operated continuously. This was done by pumping a continuous feed stream to the settling cone, removing underflow sludge continuously from the bottom of the cone and overflow continuously from launders at the surface on the periphery of the cone. However, to maintain a uniform underflow zone in the cone, and to ensure proper removal of the solids, the angle of the cone was steep thus limiting the diameter of the unit from a practical height standpoint.
Since the capacity of gravitational settling equipment was found largely to be a function of the cross-sectional area, it was evident that application of the settling cone was limited. As larger units were developed to handle higher capacities, it was found that mechanisms were required to move the underflow or sludge to a common underflow point. This led to the development of the continuous thickener in its present day configuration in 1905.
The use of continuous thickeners rapidly expanded to the chemical, sanitary and water treatment industries.
During thickener operation, feed enters the feedwell, via a feed pipe or open launder. In order to promote settling, it is increasing to dissipate this energy. In some cases, flocculants or coagulants are added to the feed to agglomerate particles and improve settling. Feed dilution may also take place.
After passing through the feedwell, the feed enters the main portion of the tank where settling occurs. A deflector plate attached to the torque tube may be installed to assist with the slurry distribution over a greater area of the tank. Raking arms attached to the centre shaft and drive rotate to direct the settled solids to the discharge cone from which thickened slurry is pumped out of the unit. The following paragraphs will explain the main thickener parts in more detail.
The tank is constructed from a variety of materials, the most common being mild steel and concrete. Tanks may be supported on legs, giving access to the underflow pumps and valves, or supported by the ground on compacted soil support and access gained via a tunnel. MIP’s tank design provides for a scalloped bottom mild steel tank design.
The mechanism consists of the drive with an optional lifting device, torque tube, rake arms, blades and bridge. MIP drive heads consist of a multi-stage planetary gearbox, hydraulic drive unit, and an optional lifting device.
This is used to raise the rakes above the mud bed, if the solids are not being removed from the thickener quickly enough. It promotes dependable, continuous operation by providing adequate lift to raise the raking arms out of the compacted or heavy solids zone during thickener upset conditions.
Lifting devices allow rake mechanisms to rotate while the assemblies are raised or lowered. Hydraulic lifting devices have replaced manual or motorised mechanical screw devices.
This is typically a thick walled pipe or cage that transfers the rotation of the mechanism to the rakes.
The rakes convey underflow sludge to the discharge point and thicken the sludge by creating paths in the compressed zone, which allows escape of entrapped liquid upwards.
High Rate Thickeners
Perhaps the most significant design optimisation effort in thickening in the last twenty years has been in developing ways to more effectively use the horizontal cross sectional area of the thickener. The introduction of polymeric flocculants gave an impetus to development of High Rate Thickeners. The result is now common use of the term “High Rate Thickener” which has been applied in all areas of industry.
The characteristic features of a High Rate Thickener, which distinguishes it from a Conventional Thickener, are:
- Deeper feedwell
- De-aeration of the feed (either internally in the thickener or external to it)
- A means of deflecting the feed from the vertical & distributing it within the body of the thickener
- Controlled bed level
High Rate Thickening is a process whereby mineral particles are flocculated, thoroughly mixed within a feedwell and then injected uniformly into a hindered settling zone in the thickener with largely radial motion. A pre formed hindered settling zone allows horizontal distribution of incoming feed into the settling bed which, at the point of injection, is in a fluidised state. The radial motion of the feed directs the solids across the thickener, thereby utilising the entire thickener surface area for further settling. Furthermore, the injection of solids into the fluidised bed assists in the capture of fine particles, which prevents them from escaping to the overflow. The hindered zone therefore also acts as a “filter” for improving overflow clarity.
A clear interface is formed at the top of fluidised bed and this can be accurately monitored for effective thickener control.
The design of clarifiers focuses on minimizing solids escape in the overflow liquor stream and effective solids settling and removal.
MIP Design Considerations
Thickeners and clarifiers are operating in the minerals processing, chemical water treatment, industrial and effluent treatment throughout the world. In addition to this existing technology, MIP Indusries (Pty) Ltd brings customized and tailor-made process solutions.
In our design, we consider not only your process data, but also the long-term mechanical performance, maximum circuit efficiency, availability and minimized operating costs.
Some of our design considerations include:
- Precise sizing for the duty
- High torque planetary gearbox
- Accurate torque measurement including a “high torque” audible and visual alarm with “over torque trip” for mechanism protection in the event of ultra high torque conditions
- High efficiency feedwell – adjustable feed dilution system
- Ability to monitor and/or operate via Scada, if so desired
- Multiple Flocculant addition points for easy optimization
- Full span walkway for dual emergency exits and to allow easy access.
Effect of Feed Dilution
The introduction of clear supernatant into the feedwell dilutes the pulp and thus lessens the effects of hindered settling. The graph below reflects this relationship and higher settlement rates are achieved with the lower feed concentration.
In order to ensure we provide the best technical solution, each thickener is custom designed for a specific application.
Typically, the tank is modeled with finite elements and the sub-structure and the bridge with beam elements. This modeling has resulted in MIP being able to optimise designs and provide the most cost effective solutions.
Special attention is given to the design of the thickener feedwell (for high efficiency) and specially profiled rake blades for maximum transfer of viscous and heavy material at minimal torque consumption.
As part of its development initiatives, MIP Process Technologies (Pty) Ltd has committed itself to ongoing test work and research in order to improve our competitive technology and to be ahead of our competitors.