Precoat leaves

All the various clarification treatments (fining, centrifugation and filtration through precoats) leave wines brilliant, with a turbidity rating of 1 or lower. Sheet or membrane filtration just before bottling results in values between 0.10 and... 

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. 

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... 

However, it is not always possible to run a pilot-plant test in order to determine the depth of cut. A well-accepted alternative approach makes use of the more sophisticated test leaf illustrated in Fig. 18-97. This test leaf is designed so that the cake and precoat are extruded axially out the open end of the leaf. The top of the retaining wall on this end of the leaf is a machined surface which serves as a support for a... 

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

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. 

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. 

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. 

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. 

Several leaf tests should be performed for repeatability. Data collected will permit scaleup to plant scale operations. Significant data will be pounds of dry cake per square foot per hour, gallons of filtrate per square foot per hour, filtrate clarity, wash ratios, (pounds of solids/gallon of wash), residual moistures, filtermedia selection, knife advance time, precoat thickness, solids penetration into precoat, and submergence level should also be evaluated. For the optimization equation, refer to Peters and Timmerhaus, and Tiller and Crump. 

Precoat 0.68 kg/m filter. Solid flux for precoal 0.06 to 0.18 kg/s m. Filtrate flux through precoat = 0.1 to 1.1 L/s-m. Leaf, vacuum cycle precoat Alter 0.6 h to 20 h wash-clean. Cake formation rate 1.3 mm/s solids flux 0.009 to 0.02 kg/s m Alter area. Cake thickness <10 cm. Area per unit... 

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. 

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