Mechanical Dewatering by Pressure

In mineral processing plant,As particles get finer the resistance against removing water increases. Gravity dewatering can no longer be used. We have to use pressure.
By creating a differential pressure Δp across a cake of solids, liquid can be removed by

Compression
“Dewatering by compression means replacing the liquid in a cake with particles”
Through blow
Dewatering by through-blow means replacing the water in a cake with air”
For vacuum filters air-through blow is used
For vertical plate pressure filters either compression or a combination of compression and air through-blow is used
For Tube Presses either compression or a combination of compression and airpurge is used. The Tube Press also enables cake washing.
Vacuum filtration is the simplest form of “through blow” dewatering. A pressure differential created by a vacuum applied to the inside of the filter drum causes air to flow through the filter cake thereby displacing the contained water. The solids are retained on a filter cloth and are carried to discharge point by the rotation of the drum.
Drum filter
1. Drum filter cloth mounted on segment grids. Internal drainage pipes.
2. Drum drive variable speed
3. Support frame
4. Tank
5. Vacuum head seal arrangement to connect rotating drum to stationary vacuum piping
6. Agitator to suspend solid particles in tank
Belt Drum Filter
The belt discharge drum filter is similar to the standard drum version except that the cloth is separated from the drum and allowed to pass over a discharge system.
This design allows cloth washing and is preferred for dewatering of slurries containing fine particles which produce a filter cake that is sticky and difficult to discharge. Three cake discharge options are available.
Top Feed Drum Filter
A top feed drum filter is designed to dewater slurries containing coarser particles.
The Top feed principle promotes segregation of the coarser particles forming a “pre coat” on the filter cloth thereby increasing filtration rate.
Vacuum Filters Vacuum Requirement
By evacuating the air inside the filters dewatering can be achieved by air “through-blow”.
Vacuum requirement is calculated as the volume of thinned air per effective
filter surface area per minute.
Thinned air volume is volume at actual reduced pressure.
Free air volume (used for sizing of compressors) is the volume at normal atmospheric pressure.
Vertical Plate Pressure Filter
The Pressure Filter model VPA is of “medium pressure” type operating in the pressure range of 6-10 bar. The machine mainly relies on the “air through blow” dewatering concept, whereby water is displaced by air as it passes through a filter cake.
Air penetration through a pore system
The driving force of this filtration method is the pressure differential across the cake. A higher pressure drop will give a faster dewatering rate and a lower residual moisture.
Pressure Filter Operation
For optional results of filter operation the pulp fed to the machine should be as high in solids as possible.
Dewatering Cycle
Start position
Step 1 – Filtration
Slurry is pumped into the filter chambers and the filtrate is expelled.
Step 2 – Compression
in which the rubber membrane in each chamber is activated and the filter cake is compressed (densely packed).
Air drying
Compressed air is forced through the filtercake driving out more liquid.
These are the dewatering steps. In cases when throughblow is not applicable and
filter is used for compression, only step 1 and 2 are used.
In addition to the above dewatering steps the complete process includes a number of so called service steps.
4. Opening cake discharge doors
5. Opening the filter, discharging the filter cakes
6. Vibrating the filter cloths (discharge control)
7. Closing the cake discharge doors
8. Rinsing the filter cloths
9. Closing the filter
Pressure Filter Product System
In a complete dewatering plant the compressed air filter is only one part of what we call the VPA system.
The VPA system consists of the following equipment:
Thickener to feed the filter with correct pulp density.
Buffer tank for deaeration and pulp density control prior to pump feeding.
Slurry pump for feeding during the filtration cycle. (3)
Valves for pulp, water and air. (4)
Rinse water system for the filter cloths. (5)
Weighing system for optimization of the operational parameters of filtration, compressed air drying, etc.
Compressor for compressed air supply. (6)
Computer based control system for operation and control of the filtration process.
Tube Press
As particles continue to get even finer the VPA system is overruled by the strong particle binding to water due to extremely powerful capillary forces. The only way to continue with mechanical dewatering is to move up to higher pressure differences across the filter cake.
This has to be done in a tube, as a conventional pressure filter cannot take up this pressure.
The tube press is a variable volume filter using a flexible membrane to apply compression to the slurry to be dewatered.
By applying a higher pressure or “driving” force to the filtration process a drierfilter cake with better handling characteristics can be produced.
The Tube press operates at pressures of up to 140 bar (2000psi) and was originally developed for dewatering of fine Kaolin slurries. It has since been applied to a variety of difficult filtration operations.


The outer casing has a flexible membrane (bladder) fastened at each end
The inner candle has a filter media around its outer surface
The candle has a series of filtrate drain holes around its circumference
The feed slurry enters the Tube Press through the feed ports
Fluid is pumped into and out of the unit through the pressure ports to create the filtration pressure
The filtrate drains away through the drain pipe
Tube Press Applications
MINERALS
Kaolin
Calcium Carbonate (including precipitated varieties)
Clays (other than Bentonitic types)
Seawater Magnesia
Steel making Sludges (BOF sludge)
Titanium Dioxide
Copper Concentrate
Tin Concentrate
Underground water at precious metal mines
CHEMICALS
Tri-Calcium Phosphate
Di-Calcium Phospate
Copper Pyro-Phosphate
Calcium Hypochlorite
EFFLUENTS
Titanium Dioxide wastes
Fluoritic muds
Spent bed slurry
OTHERS
Pharmaceuticals
Sugar refining carbonates
Pigments
Yeasts
Waxes (in oil production)
The following materials are not suited to dewatering in a Tube Press
Fibrous materials (sewage, water treatment sludges, pulp & paper, fruit)
Oily materials (oil contaminated muds, scrap effluents)
Very dilute slurries
Bentonite type clays
Rubber wastes and latex materials
Tube Press Material of Construction
Wetted Parts All metallic components of the Tube Press which come into contact with the process slurry is made from Duplex Stainless Steel.
Casing The casing and non-wetted parts are generally made from Carbon Steel.
Bladder Standard material is Natural Rubber. Other elastomers can be considered for special process applications.
Filter Cloth Selected against specific process requirements.
Tube Press System
A Tube Press plant will contain the appropriate number of Tube units according to the overall capacity required.
The units are usually supplied and installed in modules. Each module consists of a support raft to take two Tube units, complete with all local valving and service header pipework. The rafts are designed to be coupled to allow the Tube units to be configured in single or double lines.
The support steelwork, ladders, walkways, etc., will be purpose designed to suit the installation.
The service ancilliaries to operate the plant are usually supplied as independent skid mounted units and consist of the following
Slurry Pump Set
Low Pressure Filtration Pump Set
High Pressure Filtration Pump Set
Vacuum Vessel and Pump Set
Filtration Fluid Storage Tank
Oil Hydraulic Power Pack (for candle movement at discharge)
The pipework and cabling to connect these items to the raft modules will be purpose designed to suit the installation.
The plant is controlled by a PLC based system which will normally incorporate full graphics and data storage/handling for optimum plant management.
For the Tube Press to operate it requires an infrastructure of ancillary equipment to provide the necessary services. A general product system is shown below.

These services are:
Slurry feed
Filtration Pressure System
Low pressure
High pressure
Vacuum
Candle Jack Hydraulics
Oil Hydraulic Power Pack
Compressed Air
PLV Based Control System
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