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Filter presses consist of a series of either vertically or horizontally designed chambers arranged in parallel produced by either stacking flush plates of belts with distance frames separating them, or stacking a series of recessed plates, each plate or belt carrying a filter cloth or other suitable filter medium. The stack of plates is generally compressed by a handwheel, positive electromechanical or electrohydraulic pressure, the filter cloth serving as a gasket between individual chambers.Filter presses of this design have proved very successful for dewatering applications in such diverse industries as concentrate processing, industrial minerals, metallurgical processing, chemicals processing, food and pharmaceutical industries and in waste processing and effluent treatment.
TYPICAL OPERATION
In operation, when the process of filtration is started, all the chambers in the system close simultaneously.Slurry is then pumped into all chambers and cakes are formed as filtrate begins to flow through. High pressure water is then pumped behind the diaphragms in the chambers causing the cake to be squeezed further. If washing is required the diaphragms can be drained of high pressure water and wash water pumped in on top of the cake. High pressure water then fills all the diaphragms again squeezing the wash water through the cake that lies flat in the chambers. Compressed air is then blown through cakes in all chambers at the same time for air drying. Pumping pressure progressively builds up to compensate for the loss of flow rate until eventually the cake is fully formed and filtrate flow is negligible. The press is then ready to discharge, which is performed by separating the plates and allowing the cake to drop out. If necessary, cake removal may be initiated by mechanical scrapers. The press is then re-closed ready for the next cycle. For drainage the filter plates may be formed with vertical grooves, corrugations, pyramid surfaces, and other configurations. In some cases vertical grooves are used in conjunction with a wire mat or perforated metal screen, or a wire mat employed without any grooves at all. A pyramidal surface is generally to be preferred, particularly as this provides the most uniform distribution of wash water over the cake should it be desirable to wash the cake.
TYPES
VERTICAL FILTER PRESSESThese presses are generally automatic units with few continuously moving parts. They are designed to save space and because the cake lays flat in the chambers instead of standing upright as in frame and chamber type filter presses, the system can take advantage of the law of gravity. FRAME AND CHAMBER In the frame and chamber units, the product to be filtered is fed into the press under pressure, the filtrate passing through the filter media and out of the press while solids are retained in the form of a cake on the filter media, each chamber of the press performing as s separate unit. Initially, the cloth acts as the filter, but as solids are collected and build up on the surface, these themselves gradually assume the function of the primary filter media. Filtering then continues, until the cake has built up to an optimum thickness. HORIZONTAL CHAMBER PRESS In this type of press, the depth of chamber formed is equal to twice the depth of the recess on individual plates, this recess depth being varied to provide a range of cake thickness. The object is to obtain as thick a cake as possible without exceeding economic limitations - a typical maximum being about 38 mm (1-1/2"). In special circumstances where very thin cakes are called for, cake thickness may be 9.5 mm (3/8") or even less. FRAME FILTER PRESSES Plate and frame filter presses, one having passageways in the internal corners of the plates and frames, and the other having passageways in external lugs. The latter enables the filter cloths to be simply cut to length and draped in position without the necessity of punching holes in them. Joints in the lugs are made by separate cloths, cuff or lipped rubber collars or gaskets. Plate and frame presses offer scope for variation, since it is readily possible to adjust the spacing, and therefore the chamber depth. With either type, the frames are locked and compressed in stacked configuration by mechanical closure in a longitudinal direction, usually by pressure obtained by a thrust block and rotating control screw. One end of the bed being fixed and the other movable. The fixed end is normally called the head and movable end the tightening end.
PLATESPlate sizes range from as small as 100m2 plate area (4 x 4) up to 1.2m2 (over 4 ft2). A square shape is virtually universal on modern filter presses, although many of the earlier designs employed circular plates. For the retention of large quantities of solids, or incorporation of pre-coat material (eg. Kieselguhr), plate filters may be equipped with wide, hollow frames. Some presses may contain both types of filtering. For dealing with materials that need to be handled at a controlled temperature, the frames of the press may be formed with a jacket through which cooling or heating water can be circulated to maintain a specific temperature during filtration and if necessary washing. These are normally referred to as jacketed presses. Recessed plates normally have the feed inlet placed centrally, but this may be varied according to the material being treated and its ability to cake. Thus, for materials which do not form a solid cake, a bottom corner inlet may be used to enable liquid to be drained out before opening the filter. Alternatively, for materials which cake readily the inlet hole may be located near the top of the plate. Where it is not desirable for the filtrate to come into contact with air, the eyes may be joined to form a continuous passage. With any of these variations, the filter cloths which are hung over the top of the plates have holes punched in them corresponding to the feed hole position in the plate.
PLATE MATERIALSOriginally, cast iron was commonly employed for the frames, spacers, carrier membranes, and head, this material being relatively inexpensive and strong, as well as being resistant to a wide range of chemicals, particularly alkalis. Wooden presses were also favored, as a low-cost solution, where metallic contact with the filtrate was to be avoided. Modern filter press plates and frames are more usually made of stainless steel, or aluminum alloy with a synthetic resin coating. Cast iron, where employed, may be coated for compatibility with the product being handled, or rubber covered for accommodating acid solutions. Bronze construction may be favored for use with salt brines, and gunmetal for handling brewery products. Rather more limited use is made of plastic materials.
FILTER CLOTHSA wide variety of filter cloths may be used although the most common woven materials are now nylon and polypropylene where the smooth surface gives easy cake removal, helped by the good flexibility of such cloths. Cloths woven from monofilament or multifilament varns also discharge cakes more easily than those woven from staple yarns. Weave also affects the tendency for the cake to key or clog the cloth, so that satisfactory performance can only be established on empirical lines. Much of the economy of operation may depend on continued usage of a relatively expensive filter cloth, and with proper attention to cleaning so that the individual cloths are not damaged. Paper is not excluded and can prove attractive for certain applications because of its lower cost. It is more likely that paper filters will only be used once and then discarded, whilst cloths can be washed and used over and over again, provided they are not damaged in removing the cake. Some plain cloths may be discarded after a single use when handling materials which tend to adhere to and block the cloth and are difficult to remove by washing. Perforated metal screens may be used in frame filter presses where additional strength and rigidity are required, and plastics can provide low- cost screens which are easy to clean and sterilize.
PRESSURE FEEDPressure feed of the liquor to be handled can be by:
Much depends on the product being handled, as each product will have a typical filtration curve, with rapid filtering materials having a steep slope and more sludge-like materials a shallow slope. Rapid filtering materials need high pumping rates, which may be best provided by a centrifugal pump, or even gravity flow with a sufficient head. With longer filtering cycles, positive flow pumps are employed. Reciprocating (plunger) pumps are widely favored, modern types developed expressly for filter press feeds having the plunger directly coupled to (and operated by) a hydraulic cylinder. Such pumps incorporating a relief valve in the hydraulic circuit can follow closely the shape of the filtration curve. A pulsation damper may be a desirable addition to eliminate pressure pulses generated by the reciprocating motion. Diaphragm pumps are also widely used for filter press feeds and again, are usually of hydraulically operated type with two diaphragm chambers for continuous discharge. A pulsation damper is virtually essential with such pumps. The flexible rotor pump is also attractive as a low cost alternative, but for optimum performance needs to be associated with a pressure relief valve, pressure vessel and variable speed drive.
OPERATING PRESSUREOperating pressures required are not high, but generally range between the equivalent of 3 and 15 m (10 and 50 ft.) head of liquor. Filter presses are normally designed for working at pressures of up to 7 bar (100 lb/m2) Special types may be produced for very much higher working pressure, for example, up to 70 bar (1000 lb/m2). Wooden presses are not normally suitable for working pressure exceeding 5.6 bar (80 lb/m2) Even lower ratings may be adopted with rubber or plastic covered presses.
APPLICATION INDUSTRIESIn general, filter presses represent a low initial and operating costs for batch and process filtering and require a minimum of maintenance. Possible applications cover a wide variety of industries, some of which are summarized:
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Brewery industry - bronze or coated presses