Belts filters

The main features in which the Radford process differs from the batch operation are in thermal dehydration and compounding. Water-wet nitrocellulose on a continuous vacuum belt filter is vacuum-dried followed by hot air transfusion (80°C) to reduce the moisture to less than 2%. After cooling, alcohol is sprayed on the nitrocellulose to a concentration of 15—20%. The alcohol-wet nitrocellulose is then transferred from a surge feeder to a compounder by a continuous weigh-belt along with the other ingredients of the composition, which are also weighed and added automatically. 

Fractional crystallization may be accompHshed on a batch, continuous, or semicontkiuous basis. Oil is chilled continuously while passkig through the unit and is then passed over a continuous belt filter which separates soHd fat from the Hquid oil. The process gives poorer separation compared to solvent fractionation because oils are viscous at crystallization temperatures and are entrained to a significant extent ki the soHd fraction. The Hquid fraction, however, is relatively free of saturated material. 

An example of the concentration effect on the specific cake resistance is available (12) that reports results of some experiments with a laboratory horizontal vacuum belt filter. In spite of operational difficulties in keeping conditions constant, the effect of feed concentration on specific cake resistance is so strong that it swamps all other effects. 

Horizontal belt filters are well suited to either fast or slowly draining soHds, especially where washing requirements are critical. Multistage countercurrent washing can be effectively carried out due to the sharp separation of filtrates available. Horizontal belt vacuum filters are classified according to the method employed to support the filter medium. 

Horizontal or vertical vessel filters, especially those with vertical rotating elements, have undergone rapid development with the aim of making truly continuous pressure filters, particularly but not exclusively for the filtration of fine coal. There are basically three categories of continuous pressure filters available, ie, disk filters, dmm filters, and belt filters including both hydrauHc and compression varieties. 

A horizontal belt filter has been used in place of the small dmm filter in filtration studies (17). The entire filter was placed in a large pressure vessel with no moving parts passing through the filter shell. There is no commercial filter based on this principle the utilization of the space inside the pressure vessel would be poor and the filtration areas limited. 

Belt filter presses are made up to a width of 2.5 m and produce a final soHds concentration of the discharge sludge in the range of 35 to 60%. 

Eor evaluation of flocculants for pressure belt filters, both laboratory-scale filters and filter simulators are available (52,53) in many cases from the manufacturers of the full-scale equipment. The former can be mn either batchwise or continuously the simulators require less substrate and are mn batchwise. The observed parameters include cake moisture, free drainage, release of the cake from the filter cloth, filter blinding, and retention of the flocculated material during appHcation of pressure. 

A belt filter press consists of a gravity drainage belt, foUowed by a series of roUer presses which squeeze out water. 

Expression Dewaterings of Fibrous Materials. Fibrous materials are frequently dewatered in belt-filter, screw, disk, and roU presses and in batch pot and cage presses. Table 1 Hsts appHcations of screw, roU, and pot presses. Screw and high pressure belt presses are continuous and have replaced batch pot and cage presses in most appHcations. Traditionally, however, batch presses have been used for squee2ing cocoa butter from cocoa beans, which require pressures up to 41 MPa (6000 psi) (39). A description of many types of batch presses is included in Reference 40. 

Comparisons are available on the relative performance and costs for dewatering municipal sludges (2). The relative performance of different filters and conditioners on waste sludges is shown in Table 3. The same sludge was treated on two belt-filter presses, two different centrifuges, and rotary vacuum filter (75). In another study, a variable chamber filter press, fixed-volume filter press, continuous belt-filter press, and rotary vacuum filter were compared for performance, capacity, and capital and operating costs (69). 

Property Belt-filter Standard press Vacuum-a ssisted Filter Plates press Cloth Rotary vacuum filter Centrifuge... 

Other apphcations include dewatering extremely fine (0.1 llm) laterite leach tailings (91). These pelletizing processes should be compared in flocculant consumption and operating and capital costs with belt-filter presses. 

Example 2 Sizing a Drum Belt Filter with Washing. 18-86... 

On a horizontal belt filter, the equivalent sections extend across the filter in narrow strips. Therefore, changes in vacuum do occur rapidly and may be considered as happening at a particular point along the length of the filter. 

The examples which follow show how data from the correlations justpresented and a knowledge of thephysical characteristics of a particular filter are used to determine a filtration cycle and, subsequently, the size of the filter itself. The three examples which follow involve a disk, a drum belt, and a horizontal belt filter. 

Example 2 Sizing a Drum Belt Filter with Washing Equipment physical factors, selected from Table 18-9 Maximum effective submergence = 30% max. apparent suhm. = 35% max. arc for washing = 29% portion of cycle under vacuum = 75%. 

Horizontal Belt Filter Since the total cycle of a horizontal belt filter occurs on a single, long horizontal surface, there is no restriction with respect to the relative portions of the cycle. Otherwise, scale-up procedures are similar. 

Additional equipment is sometimes integrated vvkth horizontal-belt filters to further devv ater the cake through expression. The addition of such equipment shouldn t be confused with expression equipment that utilizes filter medium belts. Belt type expression equipment is described later in the Expression subsection. 

Figure 16, Basic design features of a belt filter.

Agitated/Monoplate Nutsche Filter/Dryer Horizontal Plate Filter/Dryer Continuous Equipment Rotary Drum Filter Centrifugal Filter Horizontal Belt Filter... 

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