Vacuum leaf filter

Fig. 9. Schematic diagram of the operation of a vacuum leaf filter.

Processes are classified by their rate of cake buildup in a laboratory vacuum leaf filter rapid, 0.1-10.0 cm/sec medium, 0.1-10.0cm/min slow, 0.1-lO.Ocm/hr. 

A sample of the slurry was tested, using a vacuum leaf filter of 0.05 m2 filtering surface and a vacuum equivalent to a pressure difference of 71.3 kN/m2. The volume of filtrate collected in the first 300 s was 250 cm3 and, after a further 300 s, an additional 150 cm3 was collected. It may be assumed that cake is incompressible and the cloth resistance is the same in the leaf as in the filter press. 

Figure 11.3. Laboratory test data with a vacuum leaf filter, (a) Rates of formation of dry cake and filtrate, (b) Washing efficiency, (c) Air flow rate vs. drying time, (d) Correlation of moisture content with the air rate, pressure difference AP, cake amount W Ib/sqft, drying time 6d min and viscosity of liquid Dahlstrom and Silverblatt, 1977).

Vacuum filters include the nulschc filter, enclosed agitated vacuum fillers, the vacuum leaf filter, the tipping pan filter, horizontal rotating pan filters, horizontal belt vacuum fillers, rotary vacuum drum lilters. and rotary vacuum disk filters. 

There are two types (a) the vacuum leaf filter and (b) the rotary vacuum precoat filter (Figs. 16 and 17). 

This filter has leaves, or tubes, arranged vertically in an open tank, and connected to a discharge manifold. Liquid is drawn through the filter elements by a pump coimected to the discharge manifold. Vacuum leaf filters have advantages of low cost (no pressure vessel) and ease of inspection. 

There are some liquids with solids so slimy that it is impossible to filter them on a pressure or vacuum leaf filter. Other liquids contain such a high volume of solids that the filters described previously would quickly become filled with cake, resulting in an uneconomical amount of time spent in cleaning and precoating. The rotary vacuum precoat filter was designed to meet these conditions. 

Vacuum Filters, Continuous Type.— Vacuum leaf filters which dipped into a tank or basin of the material to be filtered were the predecessors in point of time of the continuous rotating vacuum filters of today, practically the only type of continuous vacuum filter which exists today. The old leaf type of vacuum filter was intermittent in operation and one of two sets of operations were necessary with them (omitting for brevity the washing operations). In one set the leaves were stationary and after the tank or basin was filled suction was applied and a cake of the proper thickness accumulated. It was then necessary to run off the balance of the material in the tank or basin, reverse the air current and blow off and wash out the cake which had accumulated. After this was done fresh material was allowed to enter and the cycle of operations was repeated- In the other set of operations and better from a mechanical point of view the filter leaves with their accumulated cake were removed by machinery and after the cake was dropped the leaves could be returned to the tank for further solid accunaulation. 

As discussed in Chapter 9, optimisation of the batch operation of vacuum leaf filters has been discussed in the hterature [Maracek and Novotny, 1980] in hich a mathematical model... 

A slurry is tested using a vacuum leaf filter (30 kN/m vacuum) with an area of 0.05 m. In two successive 300-sec periods, filtrate volumes... 

Vacuum leaf filters are less common, although cheaper than the pressurized version, because of the lack of a pressure vessel. They consist of an open tank full of the liquid to be filtered, into which the array of vertical leaves is submerged. Vacuum is applied through the filtrate manifold, and cake builds up on the leaves until cake removal becomes due. This kind of filter is mostly used with precoat as a clarifying filter, which is not only cheaper but also more easily inspected and maintained. 

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