Filter Cake Release

Adequate cake release is a fundamental pre-requisite in efficient pressing operations, in maintaining a low down-time, t/m. the overall batch time. The overall productivity is given by the quotient (Vf/U) where is the filtrate produced per cycle and the cycle time tc = tf+tw+ U tf and U are the fillin filtration time and wash time, respectively. 

An understanding of the failure of the release mechanism follows consideration of the balance between the forces causing adhesion of the cake to the medium and the discharge forces. Runq)f [1977] and Shubert [1977] conclude that whilst a great many studies have been published on particle adhesion, both theoretical and practical information has limited generalised applcation. The adhesion of particles dispersed in liquids is mainly the result of electrostatic and van der Waals interactions chemical bonding also plays in important role. 

Chemical bonding is characterised by direct interaction of atoms and molecules. The surface forces involved are, therefore, short range compared with other forces of attraction. Chemical bonds are so substance ecific that it is virtually impossible to generahse an outcome which is so sensitive to the state, type and structure of the surface layers on the particles. 

The adhesion force created by liquid bridges has received attention the ratio of adhesion force F to sur ce t idon y is related to particle size, particle separation and volume of liquor in F ure 4.8. 

When a liquid bridge is extended by increasing the distance between the soMs, the bridge becomes unstable and breaks at a particular separation. On the other hand, these studies show how the liquid bridge force changes with drying. 

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The critical in 5)ortance of filter cloth selection in filter press systems has been stressed in the literature [Regan, 1977], As discussed above, methods of filter cloth selection have been based on selection factors [Purchas, 1967, 1981]. These factors include textile fibre type, yam type, fabric geometry and weave pattern. Other factors include cloth shrinkage and stretching, filter cake release, cloth sur ce characteristics and cleaning. These added factors are considered below. 

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

The filtrate that is back-pulsed from the dome first expands the filter socks thereby cracking the cake. The movement of the socks and the subsequent liquid flow through them serves to dislodge the filter cake solids from the GORE-TEX membrane surface. Because of the expanded PTFE material of the membrane, the cake releases from... 

To traisfer the last of the solid/liquid from its beaker or conical flask into the funnel use a little of the filtrate in the receiving flask. Release the vacuum by opening the tap on the trap or pull dff the vacuum tubing, but do not turn off the tap on the water pump (there is a possibility of suck-back (see 9 above)). Dismantle the apparatus, pour a little of the ttitrate into the beaker of corii-cal flask, reassemble the apparatus and continue the filtration. Repeat until all the material has been filtered. Use the filtrate to wash down any of the solid sticking to the sides of the funnel onto the filter cake - it will not dry quickly on the sides of the funnel. 

Vacuum diatomaceous earth filters are a variation of this technology that offer the advantages of visibility during backwashing and of not requiring pressure vessels. Their primary disadvantage is that they run an increased risk of the release of gases in the filter cake that shorten filter runs. 

Secondary waste production is moderate. The aqueous scrubbers would produce no liquid effluents but would produce up to 500 lb per day of salts as a filter cake. The rocket motors contain lead, and the salts resulting from rocket motor processing could be hazardous for that reason. The scrap metal can be released for unrestricted use. 

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