Example 7.1 Horizontal belt filter

It is proposed to use an existing horizontal belt filter to separate phosphoric acid from a slurry containing gypsum at 30% w/w. Cake formation at 50 kPa is to be followed by displacement washing and deUquoring phases at the same level of vacuum. The three phases respectively occupy 1.5 m, 4.5 m and 3 m of the 9 m total belt length. The feed suspension and belt filter characteristics are shown in Table 7.2 in addition to other operational parameters. 

Determine performance indicators for the filter cycle and identify the parameters which characterise cake composition at points along the moving belt. 

The calculations follow the procedures outlined in Section 7.2 and divide into 

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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. 

In Section 7.1 the principal features of common continuous filter cycles are described, while Section 7.2 presents the equations required to model these cycles. Section 7.3 provides detailed example calculations for the horizontal belt filter and the rotary drum filter as these are representative of typical continuous cycles. Section 7.4 shows how computer simulations can be used to examine in detail the effects of process variables on... 

Table 7.2 Characteristic parameters for the horizontal belt filter calculation (Example 7.1). ...

Hydrocyclones are also used as classifiers simply to improve the performance of other filtration or separation equipment. A good example is in applications where the cyclone separates the feed into coarse and fine particles, and the coarse material is fed onto the horizontal belt filter first, as a precoat, with fines to follow (see Figure 6.25). This will, if the cut point of the cyclone is set correctly, give good filtrate clarity and extend the usefulness of vacuum filters to finer feeds without necessarily adversely affecting the moisture content of the cake. 

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. 

The vertical recessed plate automatic press, shown schematically in Figure 15 and described previously, is another example of a horizontal belt pressure filter. Cycle times ate short, typically between 10 and 30 minutes, and the operation is fully automated. The maximum cake thickness is about 35 mm washing and dewatering (by air displacement) of cakes is possible. Apphcations include treatment of mineral slurries, sugar, sewage sludge, and fillers like talc, clay, and whiting. 

The endless-belt percolator (Wakeman, loc. cit.) is similar in principle, but the successive feed, solvent spray, drainage, and dumping stations are hnearly rather than circulany disposed. Examples are the de Smet belt extractor (uncompartmented) and the Lurgi frame belt (compartmented), the latter being a kind of linear equivalent of the Rotocel. Horizontal-belt vacuum filters, which resemble endless-belt extractors, are sometimes used for leaching. 

For those filters which allow variations in cake formation time within a fixed cycle time (such as the horizontal vacuum belt filter, for example), the advantage of pre-thickening is that for thicker feeds the cake formation time can be shortened, thus giving more time for dewatering, washing or other cake-processing operations. On the other hand, it can be shown (see Svarovsky or section 12.2.1.3) that in an optimum cycle time, at... 

The idea is not new the flat-bed pressure filter by Hydromation Engineering Co. Ltd, for example, is based on this principle. The pressure compartment consists of two halves, top and bottom. The bottom half is stationary whilst the top half can be raised to allow the belt and the cake pass out of the compartment and to be lowered onto the belt during the filtration and dewatering stages. The filter can be considered as a horizontal filter press with an indexing cloth in comparison with a conventional filter press, however, this filter has a disadvantage in that it allows only the lower face of the chamber to be used for filtration. 

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