Cake Filtration Equipment

There are three general categories of filters used. These are:

Clarifying Filters - Usually cartridge-type or bag filters designed to remove small amounts of particles from a solution. Laboratory personnel typically use these types of filters. These types of filters are also used for 0.2 micron terminal sterilization.

Cross/low Filters - These are usually membrane-type filters used for ultrafiltration. In the field of biotechnology these types of filters are used in ultrafiltration devices used in concentrating solutions, and performing buffer exchanges.

Cake Filters - These are filters that are used to remove large amounts of solids from a slurry solution. They would normally be seen in biotechnology in the primary clarification of fermentation batches and in a variety of solids removal steps seen in the production of drugs via organic synthesis. The first two categories, clarifying and crossflow filters, have been very well developed and optimized for use in biotechnology and standard wastewater treatment applications. Equipment is easily available for these applications, whether as small 0.2 micron sterilizing filter used to terminally sterilize 100 ml of product solution, or a small 500 ml crossflow filter used to concentrate a small amount of antibody solution. Many vendors of this equipment to wastewater treatment applications have their origins in the CPI (Chemical Process Industries), and have incorporated many of the scale-up and optimization properties developed in much larger units used in large scale chemical production. As a result, these two filtration unit operations are one of the most optimized and efficient used in wastewater treatment.

The third category, cake filters, although well developed in many wastewater treatment applications, are the least developed of the filtration equipment use by the Biotech Industry. In the organic synthesis laboratory sometimes very simple equipment like a funnel and filter paper is used to accomplish this operation. Some other operations used for this filtration step in the lab are more sophisticated, but many are very labor intensive and limit the capacity of the overall production process itself. As a result, there is a need for optimization of the cake filtration equipment used in biotechnology. Cake filtration equipment is available in batch and continuous modes. Following are several examples of cake filtration units:

Batch or Semi-Batch Equipment

Plate-and-Frame Filter Press

Pressure Leaf Filter

Agitated/Monoplate Nutsche Filter/Dryer Horizontal Plate Filter/Dryer Continuous Equipment Rotary Drum Filter Centrifugal Filter Horizontal Belt Filter

THE PLATE-AND-FRAME 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.

Plate And Frame Filter Equipment Process
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.

IT MAKES SENSE TO SELECT A FILTER PRESS

of When a very low moisture content is required for thermal cake drying or incineration.

rar When high fíltrate clarity is required for polishing applications.

rar When good cake release assisted by squeezing is required.

«y 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.

• 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.

• 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.

Cake disposal is relatively straightforward. 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 and de-lump the sole hard cake and, for belt conveyors, it is also recommended to increase the number of belt support rollers below the discharge chute at the point of impact.

The filterpress by itself requires little maintenance however the automation features that accompany the machine should be checked regularly and with particular attention to safety devices such as:

• The infra-red curtain that protects the operator during the closure of the plate pack should stop the hydraulic pump within 2 milliseconds.

• The switch that warns when a loss of pressure in the hydraulic plate closing system detects leakage between the plates.

• The filtrate flow meter microswitch that stops the slurry feed pump when the chambers are full.

• The microswitch that is attached to the drip trays is interlocked 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.

• The zippered 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.

Body-aid (i.e., the filter-aid) and precoating are often mentioned in connection with pressure filtration and the difference in their application is: (1) Body aid is used when the slurry is low in solids content with fine and slimy particles that are difficult to filter. To enhance filtration coarse solids with large surface area are added to the slurry and serve as a body-aid that captures and traps in its interstices

The following materials can serve as body-aid or are used to form a precoat:

Diatomaceous Earth (also called Diatomite) consisting of silicaceous skeletal remains of tiny aquatic unicellular plants.

Perlite consisting of glassy crushed and heat-expanded rock from volcanic origin.

Cellulose consisting of fibrous light weight and ashless paper like medium.

■Special groundwood is becoming popular in recent years since it is combustible and reduces the high cost of disposal. There are nowadays manufacturers that grind, wash and classify special timber to permeabilities which can suit a wide range of applications.

. ____ * * i ••"■ - _- ■- .-.-- -.1 ___|_|___________

the slow filtering particles and produce a porous cake matrix. The amount added depends on the nature of the solids and varies from ? for non-compressible and up to 5 times for gelatinous solids; (2) Precoating the plates with a 2-3 mm thick medium of a known permeability and its application requires skills since it takes-up effective cake volume, lengthens the cycle time and an over consumption can be quite costly. Precoating prior to filtration serves two main purposes:

• When the contaminants are gelatinous and sticky it forms a barrier that avoids cloth blinding. Likewise the interface between the precoat and the cloth parts 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.

THE PRESSURE LEAF FILTER

This device is similar to a plate-and-frame filter press but the whole plate assembly is also housed in a tank or pressure vessel. This design allows higher pressures to be used, and also allows the filtration operation to be done more efficiently in many applications. There are two basic configurations, namely a horizontal plate and a vertical pressure leaf filter.

Horizontal plate pressure filters were commonly applied to the fine chemical process industries such as antibiotics , pesticides or pigments when the load of impure insolubles is low and polishing is required to obtain a high product clarity. In more recent years they may be seen more and more in heavier industries such as fertilizers or precious metals when the product is the cake and efficient washing and low moistures are required. These units are well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved. These features are not possible on filter presses which require the opening of plates to the atmosphere and shifting them one by one to allow cake discharge at the end of each cycle.

Normally the filter structure consists of a stack of plates attached to a hollow shaft which are mounted inside a pressure vessel with each plate covered with a suitable filter medium. The slurry is fed under pressure into the vessel and the cake, which is retained by the filter medium, forms on the top of each plate whilst the filtrate passes through the hollow shaft further to the process. Filter sizes may vary but generally the designed for a 6 bar operating pressure. Each circular plate in the stack is constructed with radial ribs that are welded to the bottom and support a horizontal coarse mesh screen which is covered with a finer woven metal screen or filter cloth to retain the cake. The bottom of the plate slopes towards the hollow central shaft which lets the filtrate flow freely through circumferential holes and further down the shaft to the filtrate outlet. The clearance between the plates is maintained by special spacers

HORIZONTAL PLATE FILTERS ARE BEST SELECTED IN THE FOLLOWING SITUATIONS:

When minimum floor space for large filtration areas is required.

When the liquids are volatile and may not be subjected to vacuum.

When there is a risk of environmental hazard from toxic, flammable or volatile cakes specially secured discharge mechanisms may be incorporated.

When high filtrate clarity is required for polishing applications.

When handling saturated brines that require elevated temperatures the tank may be steam jacketed.

When efficient washing is required.

When the cake is heavy and must be supported as opposed to a Vertical Leaf Filter where the cake forms on a vertical surface and may fall-off once the pressure drops.

When the cake may be discharged either dry or as a thickened slurry.

with "o" rings to positively seal between the slurry that surrounds the plates and the shaft that collects the filtrate. The height of the spacers determine the clearance for cake build-up and may be replaced to meet various process conditions. One of the differences between polishing and cake filtration is the space between the plates. For polishing applications the clearances are about 20 mm as opposed to cake filtration applications where, depending on the percentage of solids and cake build-up properties, clearances may reach 100 mm. Hence, polishing filters accommodate more plates than cake filters so for the same vessel size more effective area is available with polishers. The vessels that are employed with horizontal plate filters are, as opposed to vertical leaf filters, always constructed vertically to accommodate the plates stack. All have removable dished heads but there are two options for bottom design; namely a conical bottom and a dished bottom. The selection depends largely on the cake discharge arrangement. The head of the larger vertical vessels is often pivoted so that it is swung away to allow the upwards removal of the plates stack. The layout ususally provides sufficient headroom for raising the stack over the vessel and additional floor space next to the filter for stack maintenance and replacement of damaged plates. It is good practice to design a special rig that will support the removed stack. The vessels at their bottoms are fitted with highly secured cake discharge openings to ensure safe sealing of the tank under pressure. The concept of cake filtration, as opposed to polishing, was enabled by substantial improvements in the cake discharge mechanisms since such filters are operating on a short cycle time. There are two types of cake discharge mechanisms and both use centrifugal force to throw the cake against the cylindrical wall which then falls to the bottom of the tank:

• The rotating disc stack.

• The vibrating disc stack.

The rotating type may be driven from either the top or the bottom whilst the vibrating type is always driven from the top. The removal of the tank head cover from top driven filters is generally more complex than those driven from the bottom. On the other hand bottom driven filters are more susceptible to slurry leaks. The position of the cake outlet depends on the construction of the tank bottom. There are two types available:

• With a conical bottom and a central outlet.

• With a dished bottom and a side outlet.

Tanks with conical bottoms discharge cakes by gravity and those with dished bottoms have a spade that rakes and conveys the cake towards the outlet. Hence, the conical types require more headroom as compared to the dished type having the same filtration area. Conical tanks also have often an additional scavenging plate at the lower part of the cone to filter the residual slurry heel that remains below the main plates. The slurry heel that remains at the very bottom of the tank is removed through a special dip pipe to avoid discharging a wet cake. To facilitate better cake discharge there are designs with sloping plates. With this concept the cake, owing to the centrifugal force, flies off the plate in a horizontal trajectory without being dragged and subjected to the frictional radial shear over the surface as with conventional flat plates. The cake that accumulates on the plates may be discharged as a wet thickened sludge or as a dry cake. For wet cakes the vessel will normally have a small outlet that is fitted with a valve whilst for dry cakes the opening is large and the closure locks up electrically or hydraulically with a bayonet wedge.

The operation of a horizontal plates filter is labor intensive and requires a complex manipulation of valves so present day installations are in most cases fully automated. The operational steps are as follows:

Step 1. Precoating - The precoating stage is done only in the following cases: (a) 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; (b) When a clear filtrate is required immediately after the filtration cycle commences otherwise recirculation must be employed until a clear filtrate is obtained.

Step 2. Filtration - 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.

Step 3. Heel Removal - Once the filtration cycle is completed it is necessary to remove the slurry heel that surrounds the plates. This is done by blowing air into the tank which displaces the slurry down to the lowest plate and further to the scavenger plate if one exists. The remaining slurry at the very bottom is recirculated through a dip pipe back to the feed tank until the entire slurry has been evacuated.

Water Filtration Equipment

Step 4. Cake Drying - The air then continues to pass through the cake until the captive moisture is reduced to a minimum and the cake is in practical terms considered to be dry.

Step 5. Cake Discharge - At this point the air pressure is released, the cake outlet is opened and the plate stack is rotated to discharge the cake. The cake outlet opening must be interlocked with the motor drive since its spinning is conditional to the outlet being open. On some filters the cloth or mesh screen may backwashed with water after cake discharge to dislodge and remove any cake residue that adhered to the medium.

The horizontal plate filter requires attention on a regular basis to safety devices and automation features that accompany modern filters. The space above the filter should have a hoisting device and sufficient headroom to lift the entire disc stack and move it horizontally to a location adjacent to the filter tank. It is recommended to have a special rig that will hold the plate stack for maintenance since the bigger ones may reach a length 3 meters or more. Space must also be allocated for the cover which may be either if it is hinged or removed.

Vertical Pressure Leaf Filters are essentially the same as Horizontal Plate Filters except for the orientation of the filter elements which are vertical rather than horizontal. They are applied for the polishing slurries with very low solids content of 1-5% or for cake filtration with a solids concentration of 20-25%. As with the horizontal plate filter the vertical leaf filters are also well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved.The largest leaf filters in horizontal vessels have a filtration area of 300 m2 and vertical vessels 100 m2 both designed for an operating pressure of 6 bar.

During operation the slurry is pumped under pressure into a vessel that is fitted with a stack of vertical leaves that serve as filter elements. Each leaf has a centrally located neck at its bottom which is inserted into a manifold that collects the filtrate. The leaf is constructed with ribs on both sides to allow free flow of filtrate towards the neck and is covered with coarse mesh screens that support the finer woven metal screens or filter cloth that retain the cake. The space between the leaves may

Vertical Pressure Leaf Filter Cake

vary from 30-100 mm depending on the cake formation properties and the ability of the vacuum to hold a thick and heavy cake to the vertical leaf surface.

The two types of vessel geometries employed are vertical and horizontal. In most of the fine chemicals processes the leaves are fitted into vertical vessels whereas horizontal vessels are used in the heavier process industries such as the preparation of sulfur in phosphoric acid plants. The leaves inside horizontal tanks may be positioned either along the tank axis or perpendicular to the axis. In order to utilize the tank volume for maximum filtration area the width of the leaves is graduated so they fit to the circular contour of the tank. This also reduces the slurry heel volume that surrounds the leaves. The vessels are fitted with highly secured cake discharge openings to ensure safe sealing of the tank under pressure.

The cake that accumulates on the leaves may be discharged as a wet thickened sludge or as a dry cake. For wet cakes the vessel will normally have a small outlet that is fitted with a valve whilst for dry cakes the opening is large and the closure locks up electrically or hydraulically with a bayonet wedge. The head cover of vertical vessels is often pivoted so that it is swung away to allow the upwards removal of the leaves in the stack. It is good practice to design a special rig that will support a leaf that is removed from the vessel. Special quick opening bolts are fitted around the cover so that tightness is secured during operation but enable easy opening when access to the stack is required. Figure 10 illustrates a vertical leaf filter. An advantages of the vertical leaf filter compared to the horizontal plate filter is when cakes depart easily from the filtering medium. In such cases it is not necessary to incorporate means to assist discharge since gravity will release the cake and let it drop towards the discharge opening. For such cakes that do not discharge readily a special mechanism that vibrates the entire stack is incorporated and this will in most instances release the cake. However, with this method care must be taken so that the cake does not bridge between the two adjacent plates since

Selection Criteria

Select a Vertical Leaf Filter for an application when

Minimum floor space for large filtration areas is required.

Liquids are volatile and may not be subjected to vacuum.

There is a risk of environmental hazard from toxic, flammable or volatile cakes specially secured discharge mechanisms may be incorporated.

High filtrate clarity is required for polishing applications.

Handling saturated brines that require elevated temperatures the tank may be steam jacketed.

The cake may be discharged either dry or as a thickened slurry.

this will impair cake discharge. There are instances when the cake is disposed to ponds or repulped for further treatment. In these situations special oscillating high impact jet headers sweep the medium surface and sluice the cake through the discharge outlet. These headers also serve to wash the filtering medium and dislodge particles that clog the metal screen or cloth.

Vertical Leaf Filter
Figure 10. Vertical leaf filter machine.

The operation of a vertical pressure leaf filter is labor intensive and requires a complex manipulation of valves so present day installations are in most cases fully automated. The operational steps are as follows:

Step 1. Precoating - The precoating stage is done only in the following cases: (a) When the contaminants are gelatinous and sticky the precoat layer 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; (b) When a clear filtrate is required immediately after the filtration cycle commences otherwise recirculation must be employed until a clear filtrate is obtained.

Step 2. Filtration - Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the leaves 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 leaves 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.

Step 3. Heel Removal - Once the filtration cycle is completed it is necessary to remove the slurry heel that surrounds the leaves otherwise the cake will be wet while being discharged. For this purpose a special dip pipe at the very bottom of the tank evacuates the remaining slurry heel which is recirculated back to the feed tank.

Step 4. Cake Drying - The air then continues to pass through the cake until the captive moisture is reduced to a minimum and the cake is in practical terms considered to be dry.

Step 5. Cake Discharge - At this point the air pressure is released, the cake outlet is opened and the leaf stack is vibrated to discharge the cake. The cake outlet opening must be interlocked with a pressure sensor to avoid opening under pressure. On some filters the cloth or mesh screen may be backwashed with water after cake discharge to dislodge and remove any cake residue that adhered to the medium.

The maintenance requirements on these machines is labor intensive and mirrors those of its horizontal counterpart.

THE AGITATED, MONOPLATE, NUTSCHE FILTER/DRYER

This family of filters consist of a vertical pressure vessel with a horizontal filter plate at the bottom. The filtrate from this equipment flows out a nozzle on the bottom of the filter. These devises are usually used for slurries where large amounts of solids are being collected. Variations of this equipment include equipment with removable lower heads for easy cake removal, ability to pressure or vacuum filter, ability to wash the filter cake, an agitator to break-up and rewash the filter cake, and heating or cooling jackets for the whole vessel. The Nutsche filter is the industrial version of the well known laboratory scale Buchner Funnel with the exception that it is designed to operate under either on vacuum or pressure.

sufficient holding volume is required for fast charging and emptying of the vessel . The vessels are generally designed for an operating pressure of 2-3 bars but higher pressures may be specified if required. The vessel's cover supports the hydraulic system that controls the raising and lowering of the rotating auger and paddle arms as well as the various flanged connections and maintenance manhole. With larger units the dished head cover is bolted on its top. With this arrangement it is necessary to enter the vessel for the replacement of the filter medium. On smaller filters the head and cylinder are in one part that bolts to the filter floor featuring easy access to the filter medium. This arrangement has usually swing type fast locking bolts which are attached to the circumference of the filter floor. For applications that require preservation of temperature the vessel may be double-walled or half-pipe jacketed to provide effective heat transfer to the product. The filter floor consists of a densely perforated plate sufficiently strong to hold the cake weight and the pressure that is exerted on the cake's surface. On the larger filters additional support to the plate is given by ribs between the vessel's floor and the filter plate. There are three types of filter medium that cover the filter plate: woven metallic mesh screen, synthetic filter cloth, and sintered metallic plate. Selection depends entirely on the characteristics of the solids, the liquid and the appropriate materials of construction. Medium selection cannot be determined on a bench scale leaf test and it is only pilot testing that can establish long term effects such as medium blinding and fluctuations in the feed properties.

The unique design feature of this machine is the rake arms which are used for cake washing, smoothing and discharge of the cake. These tasks are done by paddle or paddle and auger systems: The paddle system consists of two arms with slanted blades that rotate in one direction to re-slurry the cake during washing and discharge it at end of cycle. The paddle arms are rotated in the opposite direction for cake smoothing to seal cracks prior to cake drying. In this system one hydraulic arrangement raises and lowers the paddles depending on the filtration cycle.

The auger system consists of two paddle arms that rotate in one direction to perform re-slurrying as described for the paddle system. In addition two auger arms smoothen the cake by rotating in one direction and convey the cake towards the center for discharge by rotating in the opposite direction. In this system two separate hydraulic arrangements raise and lower the paddle arms and the auger arms independent-ly depending on the filtration cycle. The cake may be conveyed for discharge towards a piped chute that is located in the center of the vessel or near

The Nutsche in its full configuration consists of four major components: •3" The vessel •s" The filter floor and cloth, woven mesh screen or sintered metal plate medium

«5* The slurry arms for cake washing and smoothing «3* The cake discharge mechanism i the outlet located on the vessel's cylinder just above the filter plate. The operational sequence of this machine is complex, requiring a combination of manipulation of valves and hydraulics. Systems range from automatic to semi-automated. The operational sequence involves the following steps:

1. Filtration - The filter is charged with slurry and pressure is applied to displace the filtrate leaving the cake retained over the filter medium. For slurries with a wide distribution of coarse fast settling solids and slow settling fines there is a risk of segregation with the finer fraction settling over the coarse fraction. When this happens the fines seal the cake and slow down the cake formation so keeping the slurry in suspension with rotating arms during filtration assists in forming a homogeneous cake.

2. Cake Washing - In the washing stage a spray ring or connections on top of the cover introduce the wash liquid over the cake. This displaces the mother solution with the wash liquid but with such in-situ washing the efficiency may be quite low if the cake forms with an uneven thickness. One of the advantages of the Nutsche Filter is the ability to smoothen the cake's surface prior to applying spray wash so that the entire bed is washed evenly. Washing efficiency may be further improved if air or gas are not allowed to enter the cake in a multi-washing system so the wash liquids always displace the solutions in a "piston" like manner. This is achieved by a special detector that monitors the surface of the cake for moisture and once air or gas start entering the bed a signal is transmitted to close the filtrate valve and reopen it once next washing commences.

3. Cake Repulping - Many processes require high washing efficiency to remove the contaminating liquid from the product and washing the cake by repulping yields the most efficient product purity. This is done by resuspending the cake with the paddle arms for thorough mixing with the wash solution. During resuspension the rotating arms are moving slowly downwards and are "shaving" the bed gradually layer after layer until the entire cake enters the slurry.

4. Pressure Drying - In the drying stage air or gas purges the cake until the captive moisture is reduced to an asymptotic level and in practical terms the cake is considered to be as dry as possible. To obtain minimum moisture the cake is smoothened by reversing the rotation of the paddle or auger arras and exerting controlled pressure on its surface with the hydraulic system. This seals cracks in the cake so that air or gas will not bypass the bed.

5. Vacuum Drying - Further reduction in cake moisture may be obtained by slowly rotating and lowering the paddle arms to scrape and delump the cake. To take advantage of the drying ability of the Nutsche Filter it is worth considering the option of heating components such as the vessel, filter floor and paddles to enhance drying.

6. Cake Discharge - Once all the stages are completed the cake discharge valve opens and the paddle arms on the smaller machines or the auger arms on the larger ones are rotated and lowered to convey the dry cake towards the center. The same procedure also applies to side discharge machines however it should be noted that in this case the cake comes out intermittently and not continuously. This may have a layout impact on the downstream facility such as the conveyor that handles the product to storage.

THE HORIZONTAL PLATE FILTER/DRYER

These filters are similar in configuration to a Nutsche filter, but instead of one filter plate there is a series of plates inside the vessel. The filtrate is pulled through the filter media in the center of each plate to a central pipe that discharges out the bottom of the devise. The bottom plate of the filter usually discharges its filtrate thought a separate nozzle. These filters are usually used with slurries where a smaller quantity of solids is to be collected.

These devices have the same options as the Nutsche filter described above. Some configurations have variations on the method of cake removal from manual to totally automated. Automatic discharge is usually via scrapers or centrifugal action. Variations of this type of filter are mounted in horizontal and vertical vessels. These machines were commonly applied to the fine chemical process industries in such applications as antibiotics , pesticides or pigments manufacturing when the load of impure insolubles is low and polishing is required to obtain a high product clarity. However, in recent years they may be seen more and more in heavier industries such as fertilizers or precious metals when the product is the cake and efficient washing and low moisture is required. They are very well suited for

Selection Criteria

Nutsche Filters are best selected when: O Minimum floor space is required

O There are several unit operations upstream and downstream filtration, such as reaction and thermal drying, are required by the process flow-scheme O There is a risk of environmental hazard from toxic, flammable or volatile cakes

O Handling saturated brines or process conditions require elevated temperatures the vessel, filter floor and paddles may be heated O Res lurry washing, being more efficient than in-situ displacement washing, is required.

O Sharp separation between the mother and wash solutions is required O The cake tends to crack smoothing avoids the wash liquid, air or gas purge from by- passing.

handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved.

The filter structure consists of a stack of plates attached to a hollow shaft which are mounted inside a pressure vessel with each plate covered with a suitable filter medium. The slurry is fed under pressure into the vessel and the cake, which is retained by the filter medium, forms on the top of each plate whilst the filtrate passes through the hollow shaft further to the process. Filter sizes may vary but generally the maximum is 60 m2 area and designed for a 6 bar operating pressure. Each circular plate in the stack is constructed with radial ribs that are welded to the bottom and support a horizontal coarse mesh screen which is covered with a finer woven metal screen or filter cloth to retain the cake. The bottom of the plate slopes towards the hollow central shaft which lets the filtrate flow freely through circumferential holes and further down the shaft to the filtrate outlet. The clearance between the plates is maintained by special spacers with "o" rings to positively seal between the slurry that surrounds the plates and the shaft that collects the filtrate. The height of the spacers determine the clearance for cake build-up and may be replaced to meet various process conditions.

One of the differences between polishing and cake filtration is the space between the plates. For polishing applications the clearances are about 20 mm as opposed to cake filtration applications where, depending on the percentage of solids and cake build-up properties, clearances may reach 100 mm. Hence, polishing filters accommodate more plates than cake filters so for the same vessel size more effective area is available with polishers.

THE ROTARY DRUM FILTER

This continuous filter is used when the solids in the slurry can be easily suspended in solution and remain there. In this filter a cylindrical drum sets horizontally half submerged in a trough holding the filtration slurry. The drum is coated with a filter media and a vacuum is pulled on the inside of it. The filtrate passes through the filter media leaving the solids on the outside of the drum. The drum is rotated continuously allowing the solids to be washed and dried before they are removed by a knife blade (doctor blade) as the drum moves past it. The cleaned filter media then is rotated through the trough to pick up more solids. An illustration of a rotary drum filter is shown in Figure 12. This machine to the bottom feed group of filtering devices and is one of the oldest filters applied to the chemical process industry. The filter consists of the following subassemblies: • A drum that is supported by a large diameter trunnion on the valve end and a bearing on the drive end and its face is divided into circumferential sectors each forming a separate vacuum cell. The internal piping that is connected to each sector passes through the trunnion and ends up with a wear plate having ports which correspond to the number of sectors. The drum deck piping is arranged so that each sector has a leading pipe to collect the filtrate on the rising side of the drum and a trailing pipe to collect the remaining filtrate from the descending side to ensure complete evacuation prior to cake discharge. However, in some instances, only leading or trailing pipes are provided and this depends on process requirements. The drum is normally driven with a variable speed drive at cycle times of between 1 rpm 10 rom.

• A valve with a bridge setting which controls the sequence of the cycle so that each sector is subjected to vacuum, blow and a dead zone. When a sector enters submergence vacuum commences and continues through washing, if required, to a point that it is cut-off and blow takes place to assist in discharging the cake. The valve has on certain filters adjustable blocks and on others a fixed bridge ring. Adjustable bridge blocks enable the optimization of form to dry ratio within the filtration cycle as well as the effective submergence of the drum when the slurry level in the tank reaches a maximum.

• A cake discharge mechanism that can be either a scraper, belt, roll and in rare cases a string discharge. Blow is applied only to filters with scraper and roll discharge mechanisms but not to filters with a belt or string discharge.

The following sketches (Figures 13 through 16) illustrate key features of the operation of this machine.

Selecting the suitable mechanism largely depends on the release characteristics of the cake from the filter media. Scraper discharge mechanisms generally tend to suit cakes that release readily whereas roller discharge mechanisms are best suited for cakes that exhibit thixotropic behavior.

Vacuum Rotary Drum Filter Sketch
Figure 12. Rotary drum filter.
Water Filtration Equipment
Figure 14. Details of belt discharge.
Figure 16. Details of string discharge.

THE HORIZONTAL BELT FILTER

This devise consists of a continuous traveling horizontal belt that looks like a conveyor belt. Slurry is loaded at one end and vacuum is pulled on the underside of the belt. The filtrate passes through the belt and the solid cake remains on top. As the belt moves the cake is washed and dried and is then discharged off the end of the belt as is wraps around under the machine. This equipment is usually used for slurries that have solids that are not easily suspendible in solution. Although there are many of these industrial-type cake filters in operation outside biotechnology, few have been specifically optimized or "scaled-down" use in the Biotech lab or pilot plant. Some manufacturers do provide small scale versions of their equipment, but typically these units are used to run pilot tests on a customer's slurry for the purpose of designing the proper large scale equipment. A photograph of an installation is shown in Figure 17 and a schematic of the operation is given in Figure 18.

Horizontal belt filters are perhaps the most commonly used vacuum filters in the chemical processing industry due to their flexibility of operation, adaptation to corrosive slurries and suitability to handle large throughputs. Applications to water treatment have been liomited, but still this is an important machine worth mentioning. The development of the horizontal belt filters for the chemical process industries was closely associated with the progress in rubber technology since they incorporate an endless and thick rubber belt of a complex design to support the cake retained by the filter cloth.

Sutro Baths History
Figure 17. Photo of a horizontal vacuum belt filter installation

The first known filters were the Landskrona and Lurgi built in the 1920's and the Giorgini which was a belt filter but with attached trays. The belts were very narrow and short, with a 30 cm wide by 4-5 meters length, and were primarily applied to the washing of phosphate rock.

Later, being top feed filters that facilitated multi-washing stages, they were applied in phosphoric acid plants to replace the chains of 3 or 4 internal feed rubber covered drum filters used for gypsum washing. As the demand for area has gone up filters were manufactured with three and four 30 cm wide belts running in parallel since the rubber manufacturers were unable to catch-up with the growth of the chemical plants. For this reason the main rivals over the years to belt filters were the Tilting Pan and Table Filters so when rubber belts were the constraint to filtration area growth these filters were in demand and vice versa. Belts 4 meter wide for 120 m2 filters weigh more than 10 tons and are manufactured in one piece from sophisticated rubber compounds are common in the industry. The filter cloth retains the cake and moves together with the belt. With some exceptions, they are made from synthetic materials such polypropylene or polyester with monofilament or multifilament yarns and with sophisticated weaves. A vacuum box below the belt that is mounted along the filter and collects the filtrate through a manifold to the receivers. The box at its topside has two lips covered with low friction synthetic strip liners that seal through intermediate wear belts between the bottom side of the

Continuous Feed

Continuous Cake Wash continue*!, cake Qi^vhjl Overflow Weir {stationary! by Scrap: r '»'sire -yr Rsiter

Continuous Feed

Continuous Cake Wash continue*!, cake Qi^vhjl Overflow Weir {stationary! by Scrap: r '»'sire -yr Rsiter

Poly Multifilament

V.oiîicf Liquor

Wash Filtrato

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Figure 18. Schematic of operation.

V.oiîicf Liquor

Wash Filtrato

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Figure 18. Schematic of operation.

belt and the surface of the strips. Since the belt is the most expensive part of the filter these endless narrow belts serve as a sacrificial component that takes the wear between the surfaces, protects the rubber belt and secures against vacuum leaks. A special mechanism allows parallel lowering or swinging of the vacuum box for

Select a Horizontal Belt Filter:

<> For solids that are fast settling and cannot be kept as a homogenous slurry in bottom or side feed filters such as Drum or Disc Filters.

<> When long drying time is required to reach asymptotic moisture in the cake. On Drum Filters, for example, the ratio of dry to form cannot normally exceed 1.5 since it is determined by its geometry and the number of circumferential compartments. <> When very short cycle times are required for fast dewatering cakes such as phosphate slurry.

<> If a clear filtrate is required right from the start it is good practice to form a thin heel that serves as filter medium over the exposed cloth. This is done by either a "cloudy port outlet" that is recirculated or, if solids are seUling fast, by allocating the first 20-30 cm to act as a "sedimentation pool" prior to entering the vacuum zone.

cleaning from fines that may have settled inside. The mechanism is designed to accurately seal between the underside of the main belt and the two narrow wear belts that move together along the slide strips attached to the top shoulders of the vacuum box. A feed box and one or more wash boxes are mounted over the filter and designed to distribute evenly the slurry and wash water across the belt. Once the belt reaches the end of the vacuum box the cake drying portion of the cycle terminates and the cloth leaves the rubber belt. The cloth continues moving, changes direction over the discharge roll and the cake drops through a chute for further handling. A deck attached to the frame and mounted underneath the belt is designed to support the heavy rubber belt and the cake load. The friction between the surfaces is reduced by injecting water for lubrication and blowing air that floats the belt or by a moving floor constructed of narrow endless belts that move together with the main rubber belt. A filtrate manifold collects the mother and wash liquids to one or more vacuum receivers. It should be kept in mind that a short path of filtrate between the vacuum box and the receivers reduces to a minimum the losses of vacuum for both the single phase flow of the mother filtrate and the two phase flow of air and wash filtrates. A pneumatic or electrical tracking mechanism controls the filter cloth from slipping sideways by guiding it to the left or to the right. There are several types of mechanisms but the following are very common: Two pairs of rolls that pinch the cloth alternatively and are positioned on both sides, and A roll is that spans across the cloth, is hinged at one end and swings forwards or backwards on the other end.

These machines are designed to meet a wide range of process requirements many of which are subjecting its components to severe and demanding conditions. Systems run at high speeds, handle thick and heavy cakes, operate at high temperatures and often in an unfriendly environment hence, they are of a sturdy design. From a maintenance standpoint, the following are areas that often require attention: (1) Cracks in the rubber belt may cause separation of the plies which are encapsulated between the rubber layers. This weakens the belt and should be repaired on site without delay; (2) The shrouds on both sides of the belt are subjected to high tension while going over the head and tail pulleys. Their duty is to contain the incoming feed and if the edges tear slurry may pour all over so inspection and their repair is essential; (3) The vacuum box is hinged and swings to one side so as to enable the periodical cleaning of its internals from settled fmes. The repositioning of the box is one of the main reasons for loss of vacuum and special care must be taken to seal the box's antifriction liners against the sacrificial wear belts and the bottom side of the main belt; (4) The endless wear belts must be inspected to ensure that they are in good condition otherwise the main belt may be damaged. Likewise, the wear belts should be checked if they seal properly between the stationary vacuum box and the moving belt; (5) The life of the belt and the main drive depend largely on the water lubrication between the surfaces of the moving and stationary parts hence, the tubes leading to those parts must be kept clean; (6) It is recommended that the alignment of the filter is inspected from time to time. This applies mainly to large filters since misalignment due to differential settling of the building foundations during the first years after start-up can cause difficulties in sealing the long and segmented vacuum box.. Also, across the filter, the thickness of the cake may taper in one direction causing uneven cake washing. The alignment across the filter is particularly important for thin cakes since a 0.5% slope on a 2 meter wide belt and a 20 mm cake reduces cake thickness on one side from 20 to 10 mm.

Select a Horizontal Belt Filter:

O When intensive cake washing is required since belt filters make it possible to apply countercurrent washing. O When cakes tend to crack under vacuum measures such as a flapper, compression blanket or pressure roll may assist in sealing the cracks thus avoiding loss of vacuum. When such measures are used it is necessary to make sure that the belt supporting system can take these extra vertical loads. O When scale formation due to flash evaporation is a problem or filtrate temperature must be maintained a vacuum box steam jacketing may be provided.

O When the cake tends to clog the cloth its continuous removal after cake discharge enables dislodging of particles by thorough washing of the cloth on both sides with high impact nozzles.

THE DISC FILTER

The Disc Filters belong to the side feed group of filtering machines. They are generally used in heavy duty applications such as the dewatering of iron ore, hematite, coal, aluminum hydrate, copper concentrate, pyrite flotation concentrates and other beneficiation processes. The filter consists of several discs, up to 15 in the larger machines, each made up from sectors which are clamped together to form the disc. The sectors are ribbed towards the neck and designed for a high capacity drainage rate. One of the main features is that the required floor space taken up by disc filters is minimal and the cost per m2 of filtration area is the lowest when compared to other vacuum filters. During operation each sector enters submergence and a cake is formed on the face of the discs. It then emerges to the drying zone, the liquid drains to a central barrel and from there through a valve to the vacuum receiver. The valve with its bridge setting controls the timing so that once the sector leaves the drying zone it moves over a separating bridge and a snap or low pressure blow is applied to discharge the cake. Scraper blades on the side of each disc guide the cake to discharge chutes which are positioned between adjacent discs and are wide enough to avoid their clogging by the falling cake. A paddle type agitator located at the bottom of the tank maintains the slurry in suspension which in most of the metallurgical applications contains solids with high specific gravity which are fast settling and abrasive.

The main features of this machine include:

• Discs and sectors which may be made in injection molded polypropylene, metal or special redwood.

• A center barrel supported by the main bearings and consisting of piped or trapezoidal filtrate passages. The sectors are attached to the barrel through "o" ring sealed connections in a number equal to the number of disc sectors.

• A valve with bridges and internal compartments for form and dry under vacuum and cake discharge under pressure with 2-2.5 bar snap or 0.2-0.25 bar constant blow. Most disc filters are fitted with one valve only however two valves are often mounted on both drive and nondrive ends with long barreled filters or when the hydraulic loadings are high.

• An agitator with paddles that are positioned between the discs and far enough not to interfere with the forming cake.

• A tank which, on its discharge side, has separated slurry compartments for the discs and discharge chutes for the blown-off cake.

• Two cake discharge blades on both sides of each disc are suspended from a frame mounted on the tank. These serve to deflect and guide the cake to the discharge chutes.

• An overflow trough that spans across the entire tank length and ensures full submergence of the sectors in the cake formation zone (an exposed sector in the 6-o-clock position will cause immediate loss of pressure).

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