Pressure leaf filters are designed for final discharge of solids in either a dry or wet state, under totally enclosed conditions with fully automatic operation.
Each type of pressure leaf filter features a pressure vessel in which are located one or more filter elements or leaves of circular of rectangular construction.The filter media may be in the form of a synthetic fiber or other fabrics, or metallic mesh. Support and intermediate drainage members are in coarse mesh with all components held together by edge binding. Leaf outlets are connected individually to an outlet manifold which passes through the wall of pressure vessel.
Working Process:
The material to be filtered is fed into the vessel under pressure, and separation takes place with the solids being deposited on the leaf surface, and the liquid passing through the drainage system and out of the filter. Cycle times are determined by pressure, cake capacity or batch quantity. Where particularly fine solids must be removed, a layer of precoat material may be deposited on the leaves prior to filtration, using diatomaceous earth, Perlite or other suitable precoat materials.
Cake washing, for the recovery of mother liquor or for removal of solubles, may be carried out before discharging of the solids as a slurry or a dry cake.
Pressure leaf filters are supplied in a wide range of size and materials of construction. One typical design is the "verti-jet" unit with a vertical leaf filter, as shown in the figure below with rectangular leaves mounted individually but connected to a common outlet manifold.
For sluice cleaning either a stationary or oscillating jet system using high efficiency spray nozzles is fitted so as to give complete cake removal. For recovery for dry solids, vibration of the leaves allows automatic discharge of the solids though a bottom discharge port provided with a quick opening door.
In the "Auto jet" design, circular leaves are mounted a horizontal shaft which serves as a filtrate outlet manifold. The leaves are rotated during the cleaning cycle although, in addition, extra low speed continuous rotation during operation ensures uniform cake build up in difficult applications. The leaves are of metallic or plastics construction covered with fabric or wire cloth for direct or precoat operation, and rotation of the leaves during cleaning promotes fast efficient sluice discharge with minimum power consumption. As an alternative, the leaves may be rotated over knife blades which remove the cake in a dry state. Units of this type are used for handling foodstuffs and also for the processing of minerals and effluents.
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It is assumed that there is a well-defined boundary between the filter cake and the filter cloth. The initial stages in the build-up of the filter cake are important, however, because these may have a large effect on the flow resistance and may seriously affect the useful life of the cloth.
The blocking of the pores of the filter medium by particle is a complex phenomenon, partly because of the complicated nature of the surface structure of the usual types of filter media , and partly because the lines of movement of the particles are not well defined. At the start of filtration , the manner in which the cake forms will lie between two extremes-the penetration of the pores by particles and the shielding of the entry to the pores by the particles forming bridges. HEERTJES considered a number of idealised cases in which suspensions of specified pore size distributions were filtered on a cloth with a regular pore distribution. First, it was assumed that an individual particle was capable on its own of blocking a single pore, then, as filtration proceeded, successive pores would be blocked. So that the apparent value of the specific resistance of the filter cake would depend on the amount of solids deposited.
The pore and particle size distributions might, however, be such that more than one article could enter a particular pore. In this case, the resistance of the pore increases in stages as successive particles are trapped until the pore is completely blocked. In practice, however, it is much more likely that many of the pores will never become completely blocked and a cake of relatively low resistance will form over the entry to the partially blocked pore.
One of the most important variables affecting the tendency for blocking is the concentration of particles. The greater the concentration, the smaller will be the average distance between the particles, and the smaller will be the tendency for the particle to be drawn in to the streamlines directed towards the open pores. Instead, the particles in the concentrated suspension tend to distribute themselves fairly evenly over the filter surface and form bridges. As a result, suspensions of high concentration generally give rise to cakes of lower resistance than those formed from dilute suspensions.
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