Precoats of filter aid

In operation, a precoat of filter aid up to 6 in. thick is built up on the drum by pumping a slurry of filter aid from a precoat tank through the filter and either back to the precoat tank or on to the process. After the precoat is built up, unfiltered liquid is introduced into the filter. As the drum rotates, a blade running across the face of the drum above the liquid level continually advances toward the drum, peeling off solids plus a very small amount of precoat. This continues until the knife is within 1/4-3/8 in. (6.3-9.5 mm) of the drum, at which time the drum is cleaned and precoated again. 

RVPF (Rotary Vacuum Precoat Filter) A drum filter that is coated with a thick (up to 6 in.) precoat of filter aid, such as CELITE or equivalent. 

The carbon layer is formed by a procedure similar to that previously described for forming a precoat of filter aid. 

Rotary Drum Vacuum Filters. This filter can accommodate a large thickness of cake and can take a precoat of filter aid which is up to 150 mm in depth. The feed su ension is filtered onto the precoat and the filtered solids together with a thin layer of precoat is scraped off the cake sur ce by a horizontal knife situated just before the surface passes back into the feed suqiension tank, thus e qposing a fi esh surface of precoat. The scraper knife advances automatically until it reaches a mioimum distance fi-om the cloth, usually 5-10 mm. At this point the cycle is finished and the filter is cleaned of used precoat. 

In conventional application, no coating is applied to the filter medium. In the filtration of fermentation broths, however, a thin but controlled layer of a filter aid, e.g. diatomaceous earth, is applied onto the filter medium before the drum enters the suspension as a precoat. The filtration of the fermentation broth leads to a buildup of the cake over it. When the knife blade is applied at the end of one cycle, most often only a particular fraction of this precoat of filter aid is removed. The porous precoat facilitates extended treatment of the broth, which tends to create a complex compressible cake. If the filtrate contains the desired and valuable material, the cake discharged by the knife blade is waste filter aid material added as a precoat contaminates the cake and increases the volume of this waste. It also changes the nature of the waste, which will now contain, in addition to cellular debris, cells, etc., siliceous material. 

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. 

In the precoat mode, filter aids allow filtration of very fine or compressible soHds from suspensions of 5% or lower soHds concentration on a rotary dmm precoat filter. This modification of the rotary dmm vacuum filter uses an advancing knife continuously to skim off the separated soHds and the... 

In the precoat and body feed mode, filter aids allow appHcation of surface filtration to clarification of Hquids, ie, filtration of very dilute suspensions of less than 0.1% by volume, such as those normally treated by deep bed filters or centrifugal clarifiers. Filter aids are used in this mode with pressure filters. A precoat is first formed by passing a suspension of the filter aid through the filter. This is followed by filtration of the feed Hquid, which may have the filter aid mixed with it as body feed in order to improve the permeabiUty of the resulting cake. The proportion of the filter aid to be added as body feed is of the same order as the amount of contaminant soHds in the feed Hquid this limits the appHcation of such systems to low concentrations. Recovery and regeneration of filter aids from the cakes normally is not practiced except in a few very large installations where it might become economical. 

Construction is similar to that of other drum filters, except that vacuum is applied to the entire rotation. Before feeding slurry a precoat layer of filter aid or other suitable solids, 75 to 125 mm (3 to 5 in) thick, is apphed. The feed slurry is introduced and trapped in the outer surface of the precoat, where it is removed by a progressively advancing doctor knife which trims a thin layer of solids plus precoat (Fig. 18-126). The blade advances 0.05 to 0.2 mm (0.002 to 0.008 in) per revolution of the drum. When the precoat has been cut to a predefined minimum thickness, the filter is taken out of service, was ned, and freshly precoated. This turnaround time may be 1 to 3 h. 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

Paper pulp and fiber cloths are excellent materials for precoats and filter aids. Paper pulp gives a high rate of flow, is easily discharged and shows little tendency to clog. 

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing. When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal. Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake. 

It is found that, if the cloths are precoated with filter aid to a depth of 1.6 mm, the cloth resistance is reduced to 25 per cent of its former value. What will be the increase in the overall throughput of the press if the precoat can be applied in 180 s ... 

Procedures were used in this work which could be converted to industrial process steps. Coal was refluxed with a phenol-catalyst mixture at atmospheric pressure. The hot solution was filtered through a tared filter which had been precoated with filter aid. Contents of the filter were washed, first with hot phenol-methanol mixture (20/1, v/v) to remove adhering depolymerized coal, and finally, with methanol to remove phenol and catalyst. The filter cake was dried at 10 mm Hg pressure and 140-160 C and weighed. 

Filtration Equipment Good filtration equipment and proper operation are essential if the filtration process is to be effective and economical. Modem-day bleaching earths should not, except in rare cases, require the use of filter aids such as diatomaceous earth. Rather, the clay is used as the precoat. According to Veldkamp (123), the two main objectives in precoat filtration are as follows ... 

For a given liquid, clarity of filtrate is governed principally by (a) grade and amount of filter aid for body feed (b) grade and amount of filter aid for precoat (c) length of cycle and (d) filtration rate. 

In the operation of a filtration system, the filter is first precoated by circulating a mixture of filter aid and clear or filtered liquid from the precoat tank through the filter and back to the precoat tank. This is continued until all the filter aid is deposited on the filter. The body feed injection system is then started and the filter is changed over, with minimum fluctuations in pressure, from precoating to filtering. 

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. 

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. 

Hashing is the formation of bubbles at the septum-precoat interface causing a continual disruption of the precoat and resultant bleed through of filter aid. Hashing may be due to localized boiling or release of dissolved gases. 

Plate and frame filters (Fig. 9) are made in every conceivable combination of inlet and outlet positions but for good precoating and filter aid suspension, they should have a bottom inlet and a top outlet, and in the larger sizes these should be at opposite ends of the filter. It is absolutely necessary that all air be purged from the filter before precoating is completed. This may be particularly difficult if gasketed filter septa are used. The Filtration Specialist can give the user details as to how to do this. 

A preliminary economic analysis of the cell separation step indicates that membranes are two-fold more cost effective than centrifuges, and equivalent in cost to using a precoat vacuum filter for this application. Use of a precoat vacuum filter requires disposal of filter aid. This was not considered in the economic comparison, but could represent a significant incremental cost for preooat systems. 

Precoating the filter medium prevents blinding of the medium with the product and will increase clarity. Filter aid must be an inert material, however, there are only a few cases where it cannot be used. For example, waste cells removed with filter aid cannot be reused as animal feed. Filter aid can be a significant cost, and therefore, optimization of the filtration process is necessary to minimize the addition of filter aid or precoat. Another possible detriment is that filter aid may also specifically absorb enzymes. 

Precoated rotary vacuum drum filters (Fig. 6) are used by filtering a slurry of filter aid and water first, then subsequent product filtration. Difficult filtering materials, which have a tendency to blind, are removed with a doctor blade. Precoat is removed along with the slurry to expose a new filtration surface each cycle. 

Ordinary glass funnels holding folded or fluted filter paper can be employed for liquids that filter readily. If a liquid is difficult to filter, a Buechner or similar type of funnel can be used with a suction flask. With such funnels, either filter paper or cloth may be used. If filter cloth is used, it is desirable to precoat the filter with a inch layer of filter aid to prevent any carbon from coming through in the first portions of the filtrate. When using a suction filter, the vacuum should not be broken suddenly during filtration as this may loosen the carbon cake and result in a cloudy filtrate. 

Overflow from the clarifier is combined with a similar stream from leaching circuit B and passed through six 600-ft (58-m ) U.S. pressure filters in parallel, each operated at a feed rate of 300 to 500 gal/min (1.1 to 1.9 m /min). Filters are precoated with about 0.1 lb (0.05 kg) of precoat per short ton of ore, and 0.35 lb (0.17 kg) of filter aid per ton is added to the filter feed. The filter loading cycle lasts from 4 to 24 h, depending on the solids contents of clarifier overflow. Filter cake is returned to 2 or 3 thickener. Filtered leach liquor containing about 1 g UaOg/liter is the product of the leaching system and the feed to the solvent extraction system. 

A dead-ended laboratory filtration apparatus is relatively simple. It consists of a vacuum rated filter flask, large filter funnel with perforated plate, and a filter paper disk covering the perforations. The filter paper should be wetted and vacuum applied to hold the paper against the funnel while a precoat water slurry of filter aid is applied to the funnel to create a precoat layer of approx 0.5 in. thickness. Additional filter aid should be added to the fermentation broth and well mixed before pouring broth onto the funnel. Amount of filter aid added depends on the broth characteristics, but a typical starting point would be 50 g/L of broth. 

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