Scraper discharge

The scraper or knife discharge consists of a blade that removes the cake from the dmm by direct contact with the filter cake. It is normally used for granular materials with cake thickness greater than about 6 mm. In order not to damage the filter cloth, a safety distance of 1 to 3 mm between the blade and the cloth must be observed. If the residual layer is made not of filter aid but of the product, there is danger of its blocking by fine particles and by successive consoHdation by the scraper blade. 

In conventional disk filters, cake discharge is usually performed by a scraper blade, for cakes thicker than 10 mm, or sometimes by a tapered roU air blowback is often used to assist the discharge. High pressure sprays also have been used for cake discharge. 

The Gaudfrin disk filter, designed for the sugar industry and available in Prance since 1959, is also similar in design to a vacuum disk filter but it is enclosed in a pressure vessel with a removable Hd. The disks are 2.6 m in diameter, composed of 16 sectors. The cake discharge is by air blowback, assisted by scrapers if necessary, into a chute where it may be either reslurried and pumped out of the vessel or, for pasty materials, pumped away with a monopump without reslurrying. 

Cake discharge occurs at atmospheric pressure by the action of a toU or a scraper, assisted by blowback. The cloth may be washed by a spray before the cycle starts again. Filtering areas range up to 8 m and dmm diameters up to 2 meters. The necessity for large seals limits the operating pressure to less than 300 kPa, typically. Cake thickness can be from 2 to 150 mm, depending on machine size, and the speed of dmm rotation up to 2 rpm, usually from 0.3 to 1 rpm. Apphcations occur in the manufacture of pharmaceuticals, dyestuffs, edible oils, and various chemicals and minerals. 

The American version of the dynamic filter, known as the Artisan continuous filter (Fig. 30), uses such nonfiltering rotors in the form of turbine-type elements. The cylindrical vessel is divided into a series of disk-type compartments, each housing one rotor, and the stationary surfaces are covered with filter cloth. The feed is pumped in at one end of the vessel, forced to pass through the compartments in series, and discharged as a thick paste at the other end. At low rotor speeds the cake thickness is controlled by the clearance between the scraper and the filter medium on the stationary plate, while at higher speeds part of the cake is swept away and only a thin layer remains and acts as the actual medium. 

FIG. 18-44 High-intensity mixer (a) bottom scraper (h) fluidizing tool (c) horn tool (d) flush-mounted discharge valve. Henschel Mixers America, Inc.)... 

Scraper-Discharge Filter The filter medium is usually caulked into grooves in the drum grid, with cake removal facihtated by a scraper blade just prior to the resubmergence of the drum (Fig. 18-122). The scraper selves mainly as a deflector to direcl the cake, dislodged by an... 

FIG. 18-122 Schematic of a rotary-drum vacuum filter with scraper discharge, showing operating zones. (Schweitzer, Handbook of Separation Techniques for Chemical Engineers, p. 4-38. Copqtight 1979 hq McGraw-Hill, Inc. Used with permission of McGraw-Hill Inc.)... 

A cake discharge mechanism that can be either a scraper, belt, roll and in rare cases a string discharge. Blow is applied only to filters with scraper and roll discharge mechanisms but not to filters with a belt or string discharge,... 

Selecting the suitable mechanism largely depends on the release characteristics of the cake from the filter media. Scraper discharge mechanisms generally tend to suit cakes that release readily whereas roller discharge mechanisms are best suited for cakes that exhibit thixotropic behavior. 

Sludge discharge systems should allow for easy automation and variation of volumes discharged. Mechanical scraper tip speed should be less than 1 m with speed variation of 3 1. 

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