About Filter Presses and "How They Work

Filter Presses and "How They Work"....
Filter presses were introduced at the turn of the century and have been around for many years mainly dewatering waste sludge. They were considered labor intensive machines hence they did not find much acceptance in the sophisticated and highly automated process industries. It was not until sometime in the 60's that this image has changed by the introduction of advanced mechanisms that were oriented towards obtaining low moisture cakes that discharge automatically and enable the washing of the cloth at the end of the filtration cycle.

The Filter press consists of a head and tail end with a follower that contains a pack of vertical rectangular plates that are supported by side or overhead beams. The head serves as a fixed end to which the feed and filtrate pipes are connected and the follower moves along the beams and presses the plates together during the filtration cycle by a hydraulic or mechanical mechanism. Each plate is dressed with filter cloth on both sides and, once pressed together, they form a series of chambers that depend on the number of plates. The plates have generally a centered feed port that passes through the entire length of the filter press so that all the chambers of the plate pack are connected together. Likewise, four corner ports connect all the plates and collect the sludge and wash filtrates in a "closed discharge" towards outlets that are located on the same side as the feed inlet. Some filter presses have plates that are fitted with cocks at their lower side so that the filtrate flows in an "open discharge" to a trough and serve as "tell tales" on the condition of the filter cloth by the clarity of the filtrate that passes through each chamber. The disadvantage of this arrangement is that it cannot be used with filtrates that are toxic, flammable or volatile.

A typical flow scheme may look like this:

Present day Filter presses, as mentioned previously, are equipped with features that enable fully automatic operation controlled by PLC's.

The main features are:

• Shuttle shifters that separate the plates one by one for cake discharge at a rate of 5-6 seconds per plate. A special design of the shifting mechanism ensures that two adjacent plates are not pulled together due to sticky cakes.
• Cloth showers with movable manifolds and high impact jets for intensive cloth washing.

Precoating and Body Feed

Often special measures are taken to ease cake discharge and enhance filtration. These measures are called: Precoating and Addition of Body Feed

Precoating the plates prior to introducing the feed is done only in the following cases:

• When the contaminants are gelatinous and sticky it forms a barrier that avoids cloth blinding. Likewise the interface between the precoat and the cloth departs readily so the cake discharges leaving a clean cloth.
• When a clear filtrate is required immediately after the filtration cycle commences otherwise recirculation must be employed until a clear filtrate is obtained.

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing.

When the solids are fine and slow to filter a body-feed is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-feed  occupies additional volume between the plates and increases the amount of cake for disposal.

An additional consideration, for those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake.

The Plates
For many years filter presses, named Plate and Frame, used flush plates with separate frames to contain the cake. These Plate and Frame filter presses had many sealing surfaces which were the main cause for leakages, this led to the introduction of Recessed Plates which cuts the number of surfaces in half and reduces the problem of leaking. The development of Recessed Plates has gone hand in hand with advances in cloth technology which enabled 3 dimensional stretching as opposed to Plate and Frame where the cloth remains in one plane.

Present recess depths are 16, 20 and 25 mm so the corresponding cake thickness is 32, 40 and 50 mm at maximum filling. Filter presses are built for operating pressures of 7, 10 and 15 bar for cake squeezing and the largest available plates are 2 by 2 meters so the hydraulic pressure system that holds the closing force of the plates is designed accordingly. Filter press plates are available in various materials of construction such as cast iron, aluminum alloys, high-density polypropylene and PVDF. The major area of development, apart from automation, was in the design of the plates since thermoplastics have enabled new structural concepts which were not possible with metallic plates.

Thier special features are:

• Lower plate weight has reduced the downtime for shuttle shifting during the cake discharge mode.
• Effective filtration area has gone up since with the largest available plates of 2 by 2 meters, having a 20 mm recess and 150 chambers, the area is about 1000 m2 with a cake capacity of 350 cu ft.
• The introduction of water, or air to a lesser extent, from the backside of flexible membranes reduces chamber volume and squeezes the cake yielding a further lowering of the moisture content. The filter press may be arranged as a mixed pack of solid and membrane plates, full solid or full membrane pack depending on the application.

  Typical membrane plates are shown in the photo to the left.

Most plates are extruded in polypropylene which withstands temperatures of 80-85C. Operating at higher temperatures will warp the plates and cause leakage that can be dangerous at such high temperatures.

Selection Criteria
Filter presses are best selected in the following instances:

• When a very low moisture content is required for thermal cake drying or incineration.
• When high filtrate clarity is required for polishing applications.
• When good cake release assisted by squeezing is required.
• When the cake is disposed as land fill for spreading with a bulldozer provided it is hard enough to carry its weight.
• When large filtration areas are required in a limited space.

There are some situations that are not suited to filter presses, such as:

• When filtering saturated brines, or other heated slurries, since the plates cool-off during cake discharge they may require preheating prior to feeding the process slurry.
• When there is a risk of environmental hazard from toxic, flammable or volatile cakes when the plates are opened for discharge at the end of each cycle.
• When efficient washing is required since with a chamber filled with cake the wash water may not reach all its volume causing an uneven displacement. This, however, should present no problem when a gap is left between the formed cakes within a chamber so that the wash water is distributed evenly over the cake and reaches its entire surface.

Operational Sequence
The basic design of the following configurations may be viewed in both figures below.

Filter press without membrane plates:

• Slurry is pumped and fills the chambers at a high flow rate and low pressure which gradually builds-up as the cake gets thicker. The bomb bay doors which are positioned below the filter press for the collection of leakage are closed.
• When pressure reaches 6-7 bars wash water is pumped through the filter cake at a predetermined wash ratio to displace any adhering solution.
• Air blowing is applied to reduce cake moisture.
• The wet core that remains in the feed port is blown back with air for 20-30 seconds to ensure that the discharged cake is completely dry.
• The drip trays opened and the filter press is ready for cake discharge.
• The hydraulic plate closing piston retracts together with the follower.
• The shuttle shifter moves the plates one by one towards the follower and the cake discharges.
• The bomb bay doors close and are ready for the next cycle.
• The shuttle shifter moves the plates back one by one towards the fixed header. When each plate parks the cloth is washed at 100 bar with a mechanism that lowers and lifts a pair of symmetrical manifolds with high impact nozzles.

Filter press with membrane plates:

• Slurry is pumped and fills the chambers at a high flow rate and low pressure which gradually builds-up as the cake gets thicker. The drip trays are closed.
• The membranes, of empty chamber type plates, are pressed back to allow cake formation.
• When pressure reaches 6-7 bars the cake is squeezed for even distribution by pumping water to the backside of the membranes.
• Wash water is pumped through the filter cake at a predetermined wash ratio to displace any adhering solution.
• Air blowing is applied to reduce cake moisture.
• More water is pumped to the backside of the membranes for final squeezing up to 15 bar to further reduce moisture.
• The wet core that remains in the feed port is blown back with air at 6-7 bars for 20-30 seconds to ensure that the discharged cake is completely dry.
• The bomb bay doors open and are ready for cake discharge.
• The hydraulic plate closing piston retracts together with the follower.
• The shuttle shifter moves the plates one by one towards the follower and the cake discharges.
• The bomb bay doors close and are ready for the next cycle.
• The shuttle shifter moves the plates back one by one towards the fixed header. When each plate parks the cloth is washed at 100 bar with a mechanism that lowers and lifts a pair of symmetrical manifolds with high impact nozzles.

Cakes may be discharged into bins that are trucked away or transported with a belt conveyor. With very large filter presses a well formed cake may weigh 200-300 Kg per chamber and when it falls into a bin or onto a belt conveyor in one solid piece the impact is very high. Hence, special measures are required to break the hard cake.

Maintenance
The filter press by itself requires little maintenance however the automation features that accompany modern filter presses should be checked regularly and with particular attention paid to safety devices such as:

• The infra-red curtain that protects the operator during the closure of the plate pack stops the hydraulic pump within 2 milliseconds.
• A switch that warns when a loss of pressure in the hydraulic plate closing system detects leakage between the plates.
• The filtrate flow meter/pump sensor that stops the slurry feed pump when the chambers are full.
• A switch that is attached to the bomb bay doors so that the doors are fully open during cake discharge.
• The pressure switch that permits squeezing of the membranes only when the plate pack is compressed with the hydraulic closing system.
• A bellows that protect the hydraulic piston against drippings should be checked for wear and tear.
• The two manifolds that wash the cloth on both sides of the plate have high impact nozzles at a pressure of 100 bar. When some nozzles of one manifold are plugged the jet impact is uneven and the plates tend to swing.
• The cloth must be checked for holes and the optional cocks on the filtrate port of each plate help in identifying damaged cloths.
• The impregnated edges that surround the cloth and seal between adjacent plates should be checked for leakage and cleaned each cycle.