Batch Cake Filters

Solids separation Cake formation Press, vacuum, and gravity (PVG) filters Batch ... 

Other than the fiber aid, to form a highly porous and uniform filter cake. The suqiension to be filtered is then introduced onto the filter. Clarification by filtration results by the action of the filter aid trapping the suspended solids within the filter aid cake, by the techniques described in Section 6.1. Only a thin layer of cake is usually considered to be inq>ortant in this operation. If the filtration equipment is continuous by nature, e.g. a rotary vacuiuu filter, a thick filter aid cake may be enployed as the top layer contaminated with the material filtered during the clarification can be scraped off and discarded. The layers below the surface are then exposed for fiuther filtration and, therefore, economically viable rates of filtration ensue. Alternatively, if the filtration is conducted in a batch vessel thinner filter aid cakes are usually employed, with more frequent cleaning required. 

Filtration equipment will be briefly reviewed for the following categories batch cake filters, continuous cake filters, and clarifying filters. 

Rather than specifying the mass of solids per batch as a fixed value, the effects of maximum cake thickness can be investigated for a constant cloth area and variant mass of solids entering the press. While the influences of cake thickness are similar in general trend to several of the results shown in Figures 6.16-6.22, the variation of pressure in the filter during cake... 

The trend in the use of deep bed filters in water treatment is to eliminate conventional flocculators and sedimentation tanks, and to employ the filter as a flocculation reactor for direct filtration of low turbidity waters. The constraints of batch operation can be removed by using one of the available continuous filters which provide continuous backwashing of a portion of the medium. Such systems include moving bed filters, radial flow filters, or traveling backwash filters. Further development of continuous deep bed filters is likely. Besides clarification of Hquids, which is the most frequent use, deep bed filters can also be used to concentrate soflds into a much smaller volume of backwash, or even to wash the soflds by using a different Hquid for the backwash. Deep bed filtration has a much more limited use in the chemical industry than cake filtration (see Water, Industrial water treatment Water, Municipal WATERTREATiffiNT Water Water, pollution and Water, reuse). 

Optimization of Cycle Times. In batch filters, one of the important decisions is how much time is allocated to the different operations such as filtration, displacement dewatering, cake washing, and cake discharge, which may involve opening of the pressure vessel. Ah. of this has to happen within a cycle time /. which itself is not fixed, though some of the times involved may be defined, such as the cake discharge time. 

In general, pan filters are selected for freely filtering soHds and thick filter cakes. Cake washing can be introduced easily. Most appHcations are in the mining and metallurgical industries for small-scale batch filtration. 

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. 

Filtration. In many mineral processing operations, filtration follows thickening and it is used primarily to produce a soHd product that is very low in moisture. Filtration equipment can be either continuous or batch type and either constant pressure (vacuum) or constant rate. In the constant pressure type, filtration rate decreases gradually as the cake builds up, whereas in the constant rate type the pressure is increased gradually to maintain a certain filtration rate as the cake resistance builds. The size of the device is specified by the required filter surface area. 

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