Flexible filter media

Flexible nonmetallic materials have been widely used as filter media for many years. They are available in the form of fabrics or as preformed unwoven materials, but also in the form of perforated plates. 

Fabrics are available in differing mesh openings, and varying thread diameters. The thread diameter affects the amount of open area in a particular cloth, which in tum determines the filtration flowrate or throughput. 

The following are some examples of different types of common flexible filter media. G/ass Cloths 

Glass cloths are manufactured from glass yams. They have high thermal resistance, high corrosion resistance and high tensile strength, and are easily handled the composition and diameter of the fibers can be altered as desired. The disadvantages 

Cotton filter cloths are among the most widely used filter media. They have a limited tendency to swell in liquids and are used for the separation of neutral suspensions at temperatures up to 100°C, as well as suspensions containing acids up to 3% or alkalies with concentrations up to 10% at 15-20°C. Hydrochloric acid at 90-100°C destroys cotton fabric in about 1 hour, even at concentrations as low as 1.5%. Nitric acid has the same effect at concentrations of 2.5%, and sulfuric acid at 5%. Phosphoric acid (70%) destroys the cloth in about six days. Water and water solutions of aluminum sulfate cause cotton fabrics to undergo shrinkage. 

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The other type of common weave is called a Dutch weave. Table 1 provides a conversion chart between the two types of weave and micron ratings. Discussions that follow focus on different types of common flexible filter media. 

Of the flexible filter media described, the synthetic fabrics are perhaps the most widely relied on in industrial applications. Each filtration process must meet certain requirements in relation to flowrate, clarity of filtrate, moisture of filter cake, cake release and nonbinding characteristics. The ability of a filter fabric to help meet these criteria, and to resist chemical and physical attack depend on such characteristics as... 

Collection efficiency high medium, especially with flexible filter media high... 

The filter press has the advantage of simplicity, low capital cost, flexibility, and ability to operate at high pressure in either a cake-filter or a clarifying-filter application. Floor-space and headroom needs per unit of filter area are small, and capacity can be adjusted by adding or removing plates and frames. Filter presses are cleaned easily, and the filter medium is easily replaced. With proper operation a denser, drier cake compared with that of most other filters is obtained. 

Pressure leaf filters are used to separate much the same lands of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of hquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. 

The fibers are naturally drawn during the production process. The LS and HS fibers serve as filter mediums, catalyst carriers, and reinforcing fibers in composites. They have excellent ablation properties. Their relatively low electrical resistance has led to their use in flexible heating elements and wherever a bendable, nonme-tallic conductor is required. 

The horizontal plate filter is a pressure filter with a number of horizontal circular drainage plates in a stack in a cylindrical shell. A filter cloth or paper is placed on each plate. Filter aid may be applied if necessary. Filtration is continued until the cake capacity of the unit is reached or the filtration rate becomes too slow owing to cake resistance. Pressure drop across the filter medium is generally designed for SO lb,/in.. Cake may be washed or air-blown prior to manual removal. This type of filter is flexible and easily cleaned or sterilized, but it has a high labor requirement. 

It is true to say that any material that is porous or can be rendered porous or made into a porous structure, whether the pores are the size of a person s fist or smaller than a micrometre, can serve as a filter medium. However, as far as is reasonable, a filter medium should be strong (in tension at least), flexible, resistant to corrosion... 

The rotary table filter is an extension of the multi-pan filter design. The pans are replaced by segments of the annulus, which still rotates around in a circle so that each segment in turn filters and washes. The segments do not tip, however, to discharge the solid, which is instead scraped off the surface of the filter medium by a screw conveyor, over a flexible outer containing wall. 

The main development of the filter press from the simpler versions just described has been to include an impervious elastomeric sheet in each plate compartment (one for each plate on the feed side of the filter medium, butting up against the corresponding sheet of the next plate). These flexible sheets can be inflated pneumatically so as to press down on the drained cake, once filtration is complete, and so dewater it by compression. This type of filter press is then known as the diaphragm filter press (as shown in Figure 3.72). (Initially this sheet was referred to as a membrane, and the term membrane filter press became quite confusingly well-known. 

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