Plate and frame press

An indatable diaphragm or membrane has been used in membrane plate presses closely related to the conventional plate and frame presses. A pressure filtration period is foUowed by compression with the hydraulically operated membrane or by a hydraulically operated ram if dexible rim seals are fitted. This principle also is used in vertical presses that use either one or two endless cloth belts indexing between plates. Indatable membrane also may be used on a cylindrical filtration surface with or without a preceding pressure filtration stage. 

Plate-and-Frame Filter Presses. In the conventional plate-and-frame press (Fig. 14), a sequence of perforated square, or rectangular, plates alternating with hoUow frames is mounted on suitable supports and pressed together with hydrauHc or screw-driven rams. The plates are covered with a filter cloth which also forms the sealing gasket. The slurry is pumped iato the frames and the filtrate is drained from the plates. 

Both flush plates and recessed plates can be specified. Recessed plates obviate the need for the frames but are tougher on filter cloths due to the strain around the edges. These presses are more suitable for automation because of the difficulty of the automatic removal of residual cake from the frames in a plate-and-frame press. Recessed plates with no frames limit the chamber width to less than 32 mm to limit the strain on the cloth, whereas plate-and-frame presses allow this to be more than 40 mm if necessary. 

Filter media for plate-and-frame presses include various cloths, mats, and paper. Paper filter media usually must be provided with a backing cloth for support. 

A pressure filter is a plate-and-frame press which operates on an intermittent time cycle. Drier cakes are generally attainable from a filter press. Sludge drying beds are usually used for smaller sludge volumes, which drain and dry rapidly. Thein appHcation is usually restricted to the more arid climates. 

Recessed-plate filter press. This press is similar to the plate-and-frame press in appearance but consists only of plates (Fig. 18-113). 

Both faces of each plate are hollowed to form a chamber for cake accumulation between adjacent plates. This design has the advantage of about half as many joints as a plate-and-frame press, making a tight closure more certain. Figure 18-114 shows some of the features of one type of recessed-plate filter which has a gasket to further minimize leaks. Air can be introduced behind the moth on both sides of each plate to assist cake removal. 

Depending on the type of filter device, additional requirements may be made of the cloth. For example, in a plate-and-frame press, the sealing properties of cloths are very important. In this case, synthetic cloths are more applicable staple cloths, followed by poly filament and monofilament cloths. In leaf filters operating under vacuum and pressure, the cloth is pulled up onto rigid frames. Since the size of a cloth changes when in contact with the suspension, it should be pretreated to minimize shrinkage. 

The plate and frame press is a type of pressure filter (Figure 4.6). Filter plates are held within a frame and slurry is pumped under pressure into the spaees between sueeessive plates. As filtration proeeeds filter eake builds up and the pressure drop inereases, finally reaehing the limit of the pump. Eventually, the plate spaees beeome filled with eake and the plates are released in the frame and the eake diseharged. A wash eyele may also be introdueed to remove liquor held up in the filter eake. The operation is thus essentially bateh-wise and ean be labour intensive. 

Pressure filters or filter presses are commonly of the batch type, and are characterized by smaller floor area, high filtration rates, and lower capital cost. Dryer cakes are produced. The chemical industry uses these filters more widely than mineral processing industries, mainly because of its batch operation. The most common types of pressure filters used are the plate and frame presses. These comprise a series of vertical, alternating parallel frames and plates, with the filter cloth being held against the plate and the formation of cake occurring in the hollow frame. 

Plate and frame press (pressure operation) (Figure 10.12)... 

A plate-and-frame filter press consists of alternate solid plates and hollow frames in a sandwich arrangement. The open frames are covered by the filter medium (e.g., the filter cloth), and the slurry enters through the frames and deposits the cake on the filter medium. The operation is batch, in that the filter must be disassembled when the cake fills the frame space, then cleaned and reassembled, after which the entire process is repeated. A schematic of a plate-and-frame press is shown in Fig. 13-7. In the arrangement shown, all of the frames are in parallel and the total filter area (which appears in the equations) is... 

The rotary filter breaks down and the operation has to be carried out temporarily in a plate and frame press with frames 0.3 m square. The press takes 120 s to dismantle and 120 s to reassemble, and, in addition, 120 s is required to remove the cake from each frame. If filtration is to be carried out at the same overall rate as before, with an operating pressure difference of 75 kN/m2, what is the minimum number of frames that must be used and what is the thickness of each It may be assumed that the cakes are incompressible and the resistance of the filter media may be neglected. 

A slurry containing 100 kg of whiting/m3 of water, is filtered in a plate and frame press, which takes 900 s to dismantle, clean and re-assemble. If the filter cake is incompressible and has a voidage of 0.4, what is the optimum thickness of cake for a filtration pressure of 1000 kN/m2 The density of the whiting is 3000 kg/m3. If the cake is washed at 500 kN/m2 and the total volume of wash water employed is 25 per cent of that of the filtrate, how is the optimum thickness of cake affected The resistance of the filter medium may be neglected and the viscosity of water is 1 mN s/m2. In an experiment, a pressure of 165 kN/m2 produced a flow of water of 0.02 cm3/s though a centimetre cube of filter cake. 

A plate and frame press gave a total of 8 m3 of filtrate in 1800 s and 11.3 m3 in 3600 s when filtration was stopped. Estimate the washing time if 3 m3 of wash water is used. The resistance of the cloth may be neglected and a constant pressure is used throughout. 

A slurry is filtered in a plate and frame press containing 12 frames, each 0.3 m square and 25 mm thick. During the first 180 s, the filtration pressure is slowly raised to the final value of 400 kN/m2 and, during this period, the rate of filtration is maintained constant. After the initial period, filtration is carried out at constant pressure and the cakes are completely formed in a further 900 s. The cakes are then washed with a pressure difference of 275 kN/m2 for 600 s, using thorough washing. What is the volume of filtrate collected per cycle and how much wash water is used ... 

A sludge is filtered in a plate and frame press fitted with 25 mm frames. For the first 600 s the slurry pump runs at maximum capacity. During this period the pressure difference rises to 500 kN/m2 and a quarter of the total filtrate is obtained. The filtration takes a further 3600 s to a complete at constant pressure and 900 s is required for emptying and resetting the press. 

When an aqueous slurry is filtered in a plate and frame press, fitted with two 50 mm thick frames each 150 mm square at a pressure difference of 350 kN/m2, the frames are filled in 3600 s. The liquid in the slurry has the same density as water. 

The filter press is one of two main types, the plate and frame press and the recessed plate or chamber press. 

Figure 7.7. Plate and frame press. A-inlet passage. B-feed ports. C-filtrate outlet. D-frames. F-plates...

Figure 7.8. Optimisation of plate and frame press (data from Example 7.5)-27)...

A plate and frame press with a filtration area of 2.2 m2 is operated with a pressure drop of 413 kN/m2 and with a downtime of 21.6 ks (6 h). In a test with a small leaf filter 0.05 m2 in area,... 

When an aqueous slurry is filtered in a plate and frame press, fitted with two 50 mm thick frames each 150 mm square, operating with a pressure difference of 350 kN/m2, the frames are filled in 3600 s (1 h). How long will it take to produce the same volume of filtrate as is obtained from a single cycle when using a centrifuge with a perforated basket, 300 mm diameter and 200 mm deep The radius of the inner surface of the slurry is maintained constant at 75 mm and the speed of rotation is 65 Hz (3900 rpm). 

Cold pressing the process of separating wax from oil by first chilling (to help form wax crystals) and then filtering under pressure in a plate and frame press. 

The hltration medium may be made of perforated plates, septum of woven materials, or of granular materials such as sand. Thus, according to the medium used, hlters may also be classihed as perforated plate, woven septum, or granular filters. The hltration medium of the microstrainer menhoned above is of perforated plate. The hlter media used in plate-and-frame presses and vacuum hlters are of woven materials. These units are discussed later. 

FIGURE 7.4 (a) Sectional drawing of a plate-and-frame press (from T. Shriver and Co.) (b) an installation of a plate-and-frame press (courtesy of Xingyuan Filtration Products, China). 

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