Leaf 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. 

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

A static leaf filter is used for cleaning machine tool coolants. These are used on the suction side of a pump circulating system, with the same pump employed for withdrawal of the filtrate as for backdushing the filter elements. SoHds in this case are removed from the sump by a scraper conveyor. 

Horizontal Vessel, Vertical Leaf Filters. In a cylindrical vessel with a horizontal axis (Fig. 18), the vertical leaves can be arranged either laterally or longitudinally. The latter, less common, arrangement may be designed as the vertical vessel, vertical leaf filters but mounted horizontally. Its design is suitable for smaller duties and the leaves can be withdrawn individually through the opening end of the vessel. 

Vertical Vessel, Horizontal Leaf Filters. These filters, like all horizontal leaf filters, are advantageous where the flow is intermittent or where thorough cake washing is required. Filtration areas are limited to about 45 m. ... 

Horizontal Vessel, Horizontal Leaf Filters. These filters consist of a horizontal cylindrical vessel with an opening at one end (Fig. 19). A stack of rectangular horizontal trays is mounted inside the vessel the trays can usuaUy be withdrawn for cake discharge, either individuaUy or in the whole assembly. The latter case requires a suitable carriage. One alternative design aUows the tray assembly to be rotated through 90° so that the cake can faU off into the bottom part, designed in the shape of a hopper and fitted with a screw conveyor. 

The trays may be fitted with rims this is particularly useful for flooding the trays in washing operations. Scavenger leaves are often used. FUtration areas up to 50 m are avaUable. Like aU horizontal leaf filters, horizontal vessel, horizontal leaf filters are particularly suitable when thorough washing is needed. 

The OMD leaf filter (Stella Meta Filters) is a vertical leaf filter with a mbber diaphragm suspended between the leaves. The cake that forms on the leaves eventually reaches the diaphragm at which point pump pressure is used to inflate the diaphragm and compress the cake. The cake discharge is by vibration. 

Pressure Leaf Filters Sometimes called tank filters, they consist of flat filtering elements (leaves) supported in a pressure shell. The leaves are circ ilar, arc-sided, or rectangular, and they have filtering surfaces on both faces. The shell is a cylindrical or conical tank. Its axis may be horizontal or vertical, and the filter type is described by its shell axis orientation. 

Horizontal pre.s.sure leaf filters. In these filters the leaves may be rectangular leaves which run parallel to the axis and are of varying sizes since they form chords of the shell or they may be circular or square elements parallel to the head of the shell, and aU of the same dimension. The leaves may be supported in the sheU from an independent rack, individuaUy from the shell, or from a filtrate manifold. Horizontal filters are particiilarly suited to diy-cake discharge. 

Vertical pre.s.sure leaf filters. These filters have vertical, paraUel, rec tangular leaves mounted in an upright cylindrical pressure tank. The leaves usually are of such different widths as to aUowthem to conform to the curvature of the tank and to fill it without waste space. The leaves often rest on a filtrate manifold, the connec tion being sealed by an O ring, so that they can be lifted individuaUy from the top of the fil-... 

FIG. 18-118 Horizontal-tank pressure leaf filter designed for dry cake discharge. Sparkler Filter, Inc.)... 

Pressure leaf filters are used to separate much the same lands of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of hquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. 

Depending on the type of filter device, additional requirements may be made of the cloth. For example, in a plate-and-frame press, the sealing properties of cloths are very important. In this case, synthetic cloths are more applicable staple cloths, followed by poly filament and monofilament cloths. In leaf filters operating under vacuum and pressure, the cloth is pulled up onto rigid frames. Since the size of a cloth changes when in contact with the suspension, it should be pretreated to minimize shrinkage. 

When the eake is heavy and must be supported as opposed to a Vertieal Leaf Filter where the cake forms on a vertieal surfaee and may fall-off onee the pressure drops. 

Vertical Pressure Leaf Filters are essentially the same as Horizontal Plate Filters except for the orientation of the filter elements which are vertical rather than horizontal. They are applied for the polishing slurries with very lov solids content of 1-5% or for cake filtration with a solids concentration of 20-25%. As with the horizontal plate filter the vertical leaf filters are also well suited for handling flammable, toxic and corrosive materials since they are autoclaved and designed for hazardous environments when high pressure and safe operation are required. Likewise, they may be readily jacketed for applications whenever hot or cold temperatures are to be preserved.The largest leaf filters in horizontal vessels have a filtration area of 300 m and vertical vessels 100 m both designed for an operating pressure of 6 bar. 

Select a Vertical Leaf Filter for an application when... 

The operation of a vertical pressure leaf filter is labor intensive and requires a complex manipulation of valves so present day installations are in most cases fully automated. The operational steps are as follows ... 

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. 

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