Precoat Filter Leaves

AUXILIARY PARTS AND EQUIPMENT 7.1. Precoat Filter Leaves 

The shape and construction of filter leaves varies widely. Basically, there are two types three-ply and five-ply. The three-ply leaf consists of a center drainage member (which may be a coarse wire mesh screen or any of a variety of expanded or punched metals) covered by a metal or cloth septum on each side. The five-ply leaf consists of a heavy backing material with a finer mesh screen or perforated metal sheet on either side covered by a cloth or metal septum. The following precautions must be made  

The main function of the septum, or screen, is to support the filter aid, which, as we have seen, actually does the filtering. A heavy dense septum is therefore not necessary except where there may be cake instability due to pressure fluctuations or other outside influences. If the cake is discharged dry and is thick or heavy, a strong septum should be used. 

Septa are made in such a great variety of materials that a discussion of all available types is beyond the scope of this chapter. In general, septa should have  

Septa may be made of metal, natural, or synthetic yams. Plain, twill, and dutch weaves are available in the metal cloths. In the natural and synthetic fiber cloths, plain, twill, chain, and satin weaves in monofilament, multifilament, and staple yams have been used. 

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FIG. 18-117 Section of precoated wire filter leaf. (Multi Metal Wire Cloth.)... 

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

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. 

Leaf vacuum cycle precoat filter 0.6-20 h wash-clean. Cake formation rate 1.3 mm/s solids flux 0.009-0.02 kg/s m filter area. Cake thickness < 10 cm. Area per unit 1.2-180 m Ap < 80 kPa. Cake volume per unit 0.1-4.5 m. Precoat liquid filtrate flux for pharmaceuticals 0.0003-0.0017 L/s m. ... 

Leaf horizontal pressure cycle precoat filter 8 h wash-clean. Availability per cycle for filtration 65 to 85 %. Cake thickness < 10 cm solids flux 0.001-0.04 kg/s m of filter area filtrate flux 0.34-1 L/s m. Area per unit < 280 m Ap 100-700 kPa. Cake volume per unit 0.2-4.5 m. ... 

Two types of precoat filters (tubular and filter leaf) have been used for amine filtration. Typical sizing criteria and performance data are summarized in Table 3-7. A review of experience with several precoat filters in DBA plants located in Canada and the United States has been presented by Scheirman (1973A, B), Combined use of filter aid and activated carbon in a precoat filter of the pressure leaf type has been reported by Dailey (1970). Such a system removes suspended solids and undesirable dissolved compounds in one operation. However, Smith and Younger (1972) report that precoat filters using both activated carbon and filteraid are not as effective as a precoat filter combined with a dedicated activated carbon filter. 

Filtration. Any type of clarification is foUowed by filtration through leaf-type vertical or horizontal pressure filters. Carbonatated Hquors, containing calcium carbonate, may require addition of diatomaceous earth as a filter precoat. Phosphatated Hquors are generally filtered with the addition of diatomaceous earth as precoat and body feed. 

Precoat Procedure Precoat filtration tests are run in exactly the same manner as bottom-feed tests except that the leaf must first be precoated with a bed of diatomaceous earth, perhte, or other shave-able inert sohds. Some trial and error is involved in selecting a grade of precoat material which will retain the filtered solids to be removed on the surface of the bed without any significant penetration. During this selection process, relatively thin precoat beds of I to 2 cm are satisfactory. After a grade has been selected, bench-scale tests should be... 

The material to be filtered is fed into the vessel under pressure, and separation takes place with the solids being deposited on the leaf surface, and the liquid passing through the drainage system and out of the filter. Cycle times are determined by pressure, cake capacity or batch quantity. Where particularly fine solids must be removed, a layer of precoat material may be deposited on the leaves prior to filtration, using diatomaceous earth, Perlite, or other suitable precoat materials. 

Filtration. Filtration in Washington wineries is performed almost exclusively with plate-and-frame filter presses. In some cases, these filters have wide frames so that they are capable of performing diatomaceous earth filtrations. There is only one stainless steel, screened, diatomaceous earth, pressure-leaf filter in use in the Washington wine industry. Two major wineries have found they can utilize a paper septum over a medium pad in their plate-and-frame filters. This allows them to precoat and body feed with diatomaceous earth as the wine is filtered. This accomplishes both a coarse and medium filtration in one movement of the wine. 

The precoating rate will depend mainly on the viscosity of the liquid used. The rate should be sufficient to keep all the filter aid in suspension but should not be fast enough to cause erosion of precoat in the filter. For water, a typical rate is from 1 to 2 gpm/ft of filter area, or 0.04 to 0.08 m /min/m of filter area. For viscous liquids, the rate may be as low as 5 gal per ft per hour (gph/ft ), or 0.02 mWm. A general rule for precoating is to precoat at that rate that gives a differential pressure of approx 2 psi (13.8 kilopascals). For water, an upward velocity of at least 4.5 ft/min (1.4 m/min) is required for proper filter aid suspension. The suspension of filter aid can be improved in the tank, or pressure leaf filter, by recirculating part of the inlet flow from the top of the filter back to the precoat tank. 

Leaves should be rigid and free from flexing as filtration pressure increases. Any movement of the leaf during filtration will form cracks in the filter cake and permit unfiltered hquid and filter aid to bypass the precoat. 

Caustic Alkali Sodium hydroxide solution from the decomposer of the amalgam process usually has a concentration of 50% and a chloride content of only 5-50 mg kg-1. It is cooled down from 80-120 °C to 40-60 °C in steel pipes, Ni-, or Incolloy coolers. Any particles of graphite from the decomposer material or traces of mercury are removed by centrifuges, candle filters, or precoated leaf filters. 

Vertical filters are not convenient for the removal of dry cake, although they can be used in this service if they have a bottom that can be retracted to permit the cake to fall into a bin or hopper below. They are adapted rather to wet-solids discharge, a process that may be assisted by leaf vibration, air or steam sparging of a filter full of water, sluicing from fixed, oscillating, or traveling nozzles, and blowback. They are made by many companies, and they enjoy their widest use for filter-aid precoat filtration. 

FIGURE 22.34 A Sparkler leaf filter with leaves in horizontal position during precoat, filtration, washing, and drying and vertical position during the cleaning cycle. 

Effect of recovery on the choice To recover liquid in the order of preference of filters. Section 5.14 and filtering centrifuges. Section 5.13 deep bed, horizontal vacuum, pressure leaf, gravity flat table cartridge, precoat drum and plate and... 

Leaf, pressure vertical cycle (for wet cake) filter 2-80 h open, dump, close 0.4-4 h cake volume/unit 0.1-2 m corresponding to 5-90 m filter area leaves on 75 mm spacing. Ap 250-400 kPa. cake buildup flux 0.001 kg/s m. Precoat 0.68 kg/m filter, solid flux for precoat 0.06-0.18 kg/s m. Filtrate flux through precoat = 0.1-1.1 L/s m. ... 

Precoat 0.75 kg/m to give a precoat thickness of 1.6 mm. Rate for precoat concentration between 0.3 and 5 % w/w and at a rate of 0.7-1.4 L/s m. This should give a Ap = 14 kPa. For leaf or rotary filters, maintain consistent pressure differential across cake once the cake is formed. 

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