The Horizontal Plate Pressure 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. 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 moistures are required. 

Slurry Inlet Filtrate Outlet Cake Outlet Air Vent Cake Scrapers Filtering Plates Scavange Plates Sight Glass Drive Shaft Hydraulic Drive To view the components move mouse pointer over the menu

Horizontal Plate Filters are very 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 Filterpresses 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.   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. Please refer to the Operational Sequence below which describes the filtration cycle in detail.

Filter sizes may vary but generally the maximum is 60 m2 area and designed for a 6 bar operating pressure.

The Plates

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 obvious 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.

There are several applications, mainly in the pharmaceutical and paint industries, where special disposable filter paper is used to cover the plates on both sides so the filtration area is doubled.

The Vessel

The vessels of 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:

• A conical bottom.
• A dished bottom.

The selection depends largely on the cake discharge arrangement as discussed later.

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 should provide 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.

Cake Discharge

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.

Horizontal Plate Filters are best selected in the following instances:

• 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.

They should be selected with care:

• When the cake does not discharge readily accessing the filter medium between the plates for washing is difficult.

• When coarse mesh screens are used the filtration step must be preceded with a precoat to retain cakes with fine particles. Precoating with a thin layer of diatomite or perlite is not a simple operation and should be avoided whenever possible.

Advantages

• The removal of the plate stack on bottom driven filters is simpler than on top driven machines since on the later the entire drive has to be removed to allow access to the stack.

• Plates with the screens mounted on the topside, as opposed to two sided plates, provide good support for the forming cake and therefore are always used on applications with thick and heavy cakes.

Disadvantages

• High headroom is required for dismantling the entire plate stack.

• The bearing of top and bottom driven filters, that supports the rotating plate stack and its sealing, is complex since it has to withstand the internal pressure and the side forces imposed by the mechanical drive. However, side loads on some machines are eliminated by the use of hydraulic motors.

• The emptying of the vessel in between cake filtration, washing and drying requires close monitoring of the pressure inside the vessel to ensure that the cake holds on to the candles.

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. 

Precoating The precoating stage 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.   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. Please refer to the section on Pressure Filters for further details on precoating and the addition of body-aid.   Heel Removal Once the filtration cycle is completed air or gas is blown into the vessel and the slurry heel that surrounds the plates is pushed and displaced downwards until it reaches the lowest part of the plate stack. At this point the remaining heel slurry is evacuated back to the feed tank by a special dip pipe that is located at the very bottom of the vessel so that the vessel is empty from slurry.     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.       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.

The major components that require attention are:

The filter tank must conform to an Unfired Pressure Vessel code, such as ASME, and checked as required by the safety regulations. The pressure relief valve that is located on the top of the tank. The bearings, retainers, o-rings, gaskets and seals where the center shaft enters the vessel. This applies to both top and bottom driven stacks with the former being susceptible to air or gas leaks and the later to slurry leaks. The large diameter caulking gasket of the dished top head cover. The ends must be cut in an angle to ensure a perfect seal. The hinged head cover locking bolts. The spacers' ring seals that press the entire plate stack together. The cleanliness of the filtrate sight glass that monitor on-line or visually enables inspection of the filtrate clarity. The interlock that disables stack spinning with a closed cake discharge outlet. The interlock that disables opening the cake discharge when the vessel is still under pressure. The maintenance hoist above the filter must pull out the entire stack vertically so that the plates will not hit the tank wall. Its exact positioning is also essential for bottom driven stacks since on many types there is no access to the bearings and gland seals from the bottom and the entire insertion is done from the top. The condition of the filter medium, cloth or mesh screen, must be done periodically to ensure that they are not damaged. The vent on top of the head must be checked for free evacuation of air. The filter must not be overfilled with cake since this causes the plates to bend.