Precoating Operation

The first step in the use of DE filter aid is to build up a precoat of the filter aid on the filter medium. The purpose of the precoat is threefold (a) To prevent the filter septum from becoming clogged by impurities, thus prolonging septum life, (b) to give immediate high clarity, and (c) to facilitate cleaning of the septum at the end of the cycle. 

The amount of precoat should be from 10 to 15 lb of filter aid per 100 ft of filter area (4.5-6.8 kg per 9.29 m ), the higher amount being used when distribution of flow in the filter is poor, or in starting up new filters. If it is perfectly distributed, 10 lb (4.5 kg) of filter aid per 100 ft (9.29 m ) of filter area will give a precoat of approx 1.46 in. (1.6 mm) in thickness. The use of baffles or precoating at a different rate may be necessary for an even precoat at lower precoat amounts. 

Precoat slurry concentration will depend primarily on the ratio of filter area to filter and piping volume. If it is much below 0.3%, precoating may be difficult because the formation of the bridge depends partly on the crowding effect of the particles of Celite trying to get through the septum openings. 

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. 

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These tanks should be equipped with sweep-arm agitators rotating at approx 50 rpm or slow-speed, large-bladed, propeller-type agitators. Filter aid, once in suspension, is easy to keep in suspension. Tanks should have dished, coned, or slanted bottoms so that all liquid can be drained from the tank and so that in the precoat operation a minimum heel can be maintained during circulation of liquid between the precoat tank and the filter. 

The resistivity of the cake, however, very often increases with time. Under the pressure of filtration or the friction of the fluid that continues to pass through the cake, the filtered particles continue to compact and reduce the area available for flow. Section 7.S.4.2 also describes the empirical methods that are used to characterize the compressibility of the cake. It is significant that the particles produced by precipitation from the brine are often highly compressible. At the same time, they have a tendency to become lodged in the pores of the filter medium. Filtration of these particles can become very difficult. The usual solution to this problem is the use of a filter aid. The filter aid is applied to the surface of the filter medium before introducing the brine. This is the precoating operation. Filter aids are selected for their desirable characteristics, and the precoat material protects the pores from penetration by filtered solids while at the same time offering little resistance to the flow. Compression of the precipitates still occurs, and so filter aid ( admix ) is also added in small quantities to the brine to improve the characteristics of the cake. 

The operation of the RWCS is controlled from the main control room. Filter resin back-washing and precoating operations are controlled from a local panel. The cleanup system... 

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Dynamic membranes are concentration—polarization layers formed in situ from the ultrafiltration of coUoidal material analogous to a precoat in conventional filter operations. Hydrous zirconia has been thoroughly investigated other materials include bentonite, poly(acryhc acid), and films deposited from the materials to be separated (18). 

The depth of cut involved in precoat filtration is a veiy important economic factor. There is some disagreement as to the method required to accurately predic t the minimum permissible depth of cut. Some investigators maintain that the depth of cut can be evaluated only in a quah-tative manner during bench-scale tests by judging whether the process solids remain on the surface of the precoat beck This being so, they indicate that it is necessaiy to run a continuous pilot-plant test to determine the minimum permissible depth of cut. The use of a continuous pilot-plant filter is a veiy desirable approach and will provide accurate information under a variety of operating conditions. 

Continuous Precoat Filters These filters may be operated as either pressure or vacuum filters, although vacuum operation is the prevailing one. The filters are really not continuous but have an extremely long batch cycle (1 to 10 days). Apphcations are for continuous clarification of liqmds from slurries containing 50 to 5000 ppm of solids when only very thin unacceptable cakes would form on other filters and where perfect clarity is required. 

FIG. 18-126 Operating method of a vacuum precoat filter. (DotT-Oliver, Inc. )... 

The cost of the filter station includes not only the installed cost of the filter itself but also that of all the accessories dedicated to the filtration operation. Examples are feed pumps and storage facihties, precoat tanks, vacuum systems (often a major cost factor for a vacuum filter station), and compressed-air systems. The dehvered cost of the accessories plus the cost of installation of filter and accessories generally is of the same order of magnitude as the dehvered filter cost and commonly is several times as large. Installation costs, of course, must be estimated with reference to local labor costs and site-specific considerations. 

Polishing of solutions (as performed on a precoat filter operation). 

Figure 9. Precoat drum filter flow scheme for polishing operations.

Capillary HPLC (usually SEC or LCCC), coupled semi off-line with MALDI-ToFMS for oligomer and end-group characterisation, may be operated using collection modules precoated with matrix material. 

Another method, which is even more successful in preventing binding of the septum, is the use of a precoat.53 Before filtration is begun a coating of 2-6 in (5-15 cm) of diatomaceous earth or perlite filter aid is deposited on the surface of the septum. During filtration operations the scraper is set so that it slowly removes the precoat and, of course, with it the materials that would have plugged the filter. Since the precoat causes a considerable pressure drop, the rate of filtration is slowed down. Flow rates may vary from 2 to 50 gal/hr/ft2 (0.025-0.60 m/hr). The precoat material costs around 3 or 40/lb and is used at the rate of 10- 15 lb/1,000 gal of feed (1,200 to 1,800 kg/m3). 

To obtain maximum economy in operation, a precoat type vacuum filter should have ... 

Based on the results of these tests, recommended operating conditions for Zr precipitation in the pilot plant system include reaction temperatures of 20 C or 60 C, impeller tip speeds of 320 cm/s during solid dispersal/reaction vising a high efficiency axial flow impeller, addition of 0.83 g NaF/g Zr in solution, and crystallization periods of 24 hours. A filter press will provide acceptable solid/llquid separation even during process upsets without use of a precoat. A diaphragm pump functioned well during these tests. 

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