The Plateandframe Filter Press

This type of filter allows pressurized filtration of a slurry mixture to remove solids. A set of filter plates is sandwiched together in series with a configuration similar to the plates on a plate-and-frame heat exchanger. After the plates are compressed together hydraulically, a channel throughout all the plates allows the slurry to be pumped into one side of each of the plates. The filtrate goes through the filter media on the plate leaving the solid cake on the media. Filtrate on the other side of the filter plate enters a channel, joins the filtrate coming off the other plates in the filter, and leaves the equipment. When the filter plates are filled with solids, the solids can be washed and the press opened to discharge the solids off each plate. The filter press consists of a head and follower that contain in between 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 mother 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 is illustrated in Figure 8.

Often special measures are taken to ease cake discharge and enhance filtration. The measures taken include precoating and the addition of body aid. Precoating the plates prior to introducing the feed is done only in the following cases:

Filterpresses were introduced nearly 100 years ago and have been applied extensively in dewatering waste sludges. 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 1960's that this image 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.

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.

Filter Press Flowchart
Figure 8. Process flow scheme for plate-and-frame filter press.

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-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal. Likewise, for all 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.

For many years the plate-and-frame systems have used flush plates with separate frames to contain the cake. These filter presses had many sealing surfaces which were the main cause for leakages. As a result recessed plates were introduced in order to cut the number of surfaces in half and reduced the problem of drippings. The development of recessed plates has gone hand in hand with advances in cloth technology which enabled three-dimensional stretching as opposed to simple plate-and-frame where the cloth remains in one plain. Present recess depths are 16, 20 and 25 mm so the corresponding cake thicknesses are 32, 40 and 50 mm at

MAIN FEATURES OF A MODERN-DAY FILTER PRESS INCLUDE:

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.

^Shakers that subject the plate to vibrations and assist in discharging the cake.

*+Cloth showers with movable manifolds and high impact jets for intensive cloth washing.

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, and plastics such as high-density polypropylene

__(HDPE) 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. Plastic composite plates have the following unique features:

m 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 20 m3.

• 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 flush and membrane plates, full flush or full membrane pack depending on the application.

Most plates are extruded in polypropylene which withstands temperatures of 80 to 85° C. Operating at higher temperatures will warp the plates and leakage or even squirts can be dangerous at such high temperatures.

Careful consideration to selection and sizing this equipment is required when dealing with any of the following cases: (1) When filtering saturated brines since the plates cool-off during cake discharge and require preheating prior to feeding the process slurry. For such brines autoclaved filters such as Horizontal Plates, Vertical Leaf or Candle Filters are better suited as they can be steam jacketed; (2) 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. Again, the autoclaved filters are better suited; (3) When efficient washing is required since with a chamber filled with cake the wash water may not reach all its surface 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. Filterpress without membrane plates have the following operational sequence (refer to Figure 9):

• 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 which are positioned below the filterpress for the collection of drippings closed.

• When pressure reaches 6-7 bars wash water is pumped through the filter cake at a predetermined wash ratio to displace the adhering mother 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 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 drip trays close and are ready for the next cycle.

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