Plate and Frame Filter Press: the mechanism of this filter is surface filtration. The slurry enters the frame under pressure and flows through the filter medium. The filtrate is collected on the plates and sent to the outlet. Various frames and plates are used so that surface area increases and, consequently, large volumes of slurry can be processed simultaneously with or without washing.

Construction for Plate and Frame Filter Press

The construction of a plate and frame filter press is shown in the Figure given below. The filter press is made of two types of units, plates, and frames.

Frame and Press Filter

The components of this filter are as follows.
(i) Frame: It maintains the slurry reservoir and has inlet (eye) for slurry.

(ii) Filter medium: It is for solid retention.
(iii) Plate: This equipment along with section-supports the filter medium, receives the filtrate and has outlet (eye).
(iv) Assembly of plate and frame filter press.

Working for Plate and Frame Filter Press

The working of the frame and plate process can be described in two steps, namely filtration and washing of the cake (if desirable). The working of a plate and frame press is shown in Figure below. Slurry enters the frame from the feed channel and passes through the filter medium on to the surface of the plate. The solids form a filter cake and remain in the frame. The thickness of the cake is usually half the frame thickness as filtration occurs on each side of the frame. Thus, two filter cakes are formed, which meet eventually in the centre of the frame. The optimum thickness of filter cake for any slurry depends on the solid content in the slurry and the resistance of the filter cake. The filtrate drains between the projections on the surface of the plate and escapes from the outlet. With the time resistance of the cake increases and the filtration rate decreases. It is advisable to stop the process at a certain point rather than continuing at very low flow rates. On completion of filtration the press is emptied and the cycle is restarted.

Washing

Generally washing of the filter cake is necessary and for that the ordinary plate and frame press is not suitable. The cake built-up at the centre in the frame brings flow to a standstill. Thus, water washing of cake using the same channels used for the filtrate is inefficient. A modification of the plate and frame press is another option. In modified form an additional channel is included for washing, Figure below (d). In half the wash plate there is a connection from the wash water channel to the surface of the plate.

Filtration Operation with Wash Water Facility

The steps of the filtration are as follows:
(i) Slurry enters the plates and filtration proceeds in the ordinary way until the frames are filled with cake.
(ii) To wash the filter cake, the outlets of the washing plates are closed.
(iii) Wash water is pumped into the washing channel. The water enters through the inlets on to the surface of the washing plates.
(iv) Water passes through the filter cloth and enters frame which contains the cake. The water washes the cake, passes through the filter cloth and enters the plate down the surface.

Finally washed water escapes through the outlet of that plate.

Thus, provision of special washing plates makes it possible for the wash-water to flow over the entire surface of washing plate that makes the uniform resistance of cake to the flow and the entire cake is washed efficiently. The water wash is efficient only when the frames are full with filter cake. If the cake does not fill the frame completely the wash water causes the cake to break on the washing plate side of the frame and the washing will be less effective. It is essential to allow the frames become completely filled with the cake. This helps in emptying the frames as well as in washing the cake correctly.

Advantages for Plate and Frame Filter Press

(i) Simple in operation, convenient in maintenance, and safe in operation with multiple safety devices. Variety of materials such as cast iron (for handling common substances), bronze (for smaller units), stainless steel (to avoid contamination), hard rubber or plastics (to avoid metal) and wood (for lightness) can be used.
(ii) It provides a large filtering area in a relatively small floor space.
(iii) It is versatile equipment whose capacity can be varied according to the thickness and the number of frames used. Surface area can be increased by employing chambers up to 60.
(iv) This press has sturdy construction that makes use of pressure difference of 2000 kilopascals to be used.
(v) Cake washing is very efficient.
(vi) Operation and maintenance is low as it has no moving parts and the filter cloths can be easily renewable.
(vii) The presence of all external joints makes plate to be isolated in case of if any leaks and the contamination of the filtrate can be avoided.
(viii) It produces dry cake in the form of slab.

Applications

(i) Filter presses are used in a huge variety of different applications, from dewatering of slurries to purification. At the same time, filter press technology is widely established for ultrafine dewatering as well as filtrate recovery.
(ii) Solid-liquid separation in alcohol, chemical, metallurgy, pharmaceutical, light industry, coal mining, food-stuff, textile, environmental protection, energy source and other industries.