Back leaf filters

The fraction of oU removed in the full press is laden with fine meal particles, typically in the range of 5-15% by weight. These meal particles are typically separated from the oU using the traditional method of pumping the oil into a tank with 30 to 60 minutes of residence time to allow the heavier meal particles to settle and be continuously dredged from the base of the tank. After gravity separation, the oil is then pumped across a vibratory screener or through a pressure-leaf filter for final meal particle separation. Meal particles separated from the clean oU stream are saturated with oil and recycled back into the process ahead of the full press. 

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. 

Kgure 11.48 Vertical leaf filters back-flushing and sluicing discharge (Courtesy of U.S. Filter Co., Vanpipe Co. Ltd, Burslem, Stoke, U.K.)... 

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 well as for back flushing the filter elements. Solids in this case are removed from the sump by a scraper conveyor. 

Fit the test leaf with a filter cloth expected to give reasonable results and seal the back of the leaf and side of the dam with sihcone or other suitable material. 

Powder the 25 G sample and place it into a glass mason jar that has a screw top (make sure the seal on the jar top is not soluble in the solvent). Add at least 125 ml (1/2 cup) of solvent. Screw on the top. Shake well for 2 minutes. Let sit for a minimum of 24 hours (several days might yield a more complete extraction). Shake periodically (at least 5 or 6 times over the course of the extraction period). Then pour the entire contents of the jar though a fine mesh wire strainer (a tea strainer will do). Save all the solvent. With the back of a spoon squeeze the stuff in the strainer dry. Save the solvent that you squeeze out. Re-extract the filtered leaf material in an addition 125 ml fresh solvent. As... 

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 ... 

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