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.

Drum Vacuum Filters

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

• Iron Oxide

• 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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Sinonine technology team