Types Of Filter Media To Choose From

There are many filter media from which to choose from; however, the optimum type depends on the properties of the suspension and specific process conditions. Filter media may be classified into several groups, however the two most common classes are the surface-type and depth-media-type.

Surface-type filter media are distinguished by the fact that the solid particles of suspension on separation are mostly retained on the medium's surface. That is, particles do not penetrate into the pores. Common examples of this type of media are filter paper, filter cloths, and wire mesh.

Figure 1. Pressure filtration

Depth-type filter media are largely used for wastewater clarification purposes. They are characterized by the fact that the solid particles penetrate into the pores where they are retained. The pores of such media are considerably larger than the particles of suspension. The suspension's concentration is generally not high enough to promote particle bridging inside the pores. Particles are retained on the walls of the pores by adsorption, settling and sticking. In general, depth-type filter media cannot retain all suspended particles, and their retention capacity is typically between 90 and 99 percent. Sand and filter aids, as examples, fall into this category. Some filter media may act as either surface-type or depth-type, depending on the pore size and suspension properties (e.g., particle size, solids concentration and suspension viscosity).

It is also common practice to classify filter media by their materials of construction. Examples are cotton, wool, linen, glass fiber, porosmooth surface caused by carrying the warp (or the weft) on the fabric surface over many weft (or warp) yarns. Intersections between warp and weft are kept to a minimum, just sufficient to hold the fabric firmly together and still provide a smooth fabric surface. The percentage of open area in a textile filter indicates the proportion of total fabric area that is open, and can be determined by the following relationship:

(mesh opening + thread diameter)2

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.

Table 1. Weave Conversion Chart.

Dutch Weave

Square Weave

Micron Rating

12x64

60

270-290

14x88

70

220-240

14x100

80

210-230

21x110

120

115-125

20x200

140

100-110

30x150

150

85-100

20x250

180

80-90

40x200

200

70-75

50x250

250

58-63

60x300

300

45-53

80x700

325

40-45

80x700

400

35-38

200x600

450

28-32

165x800

500

24-26

165x1400

635

16-18

200x1400

-

12-14

325x2300

-

8-9

Reverse Dutch Weave

Square Weave

Micron Rating

48x10

40

400

72x15

60

300

132x14

80

200

152x30

120

120

236x33

150

90

338x37

200

Glass cloths are manufactured from glass yarns. 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 of glass cloth are the lack of flexibility of individual fibers, causing splits and fractures, and its low resistance to abrasion. However, backing glass cloth with a lead plate, rubber mats or other rigid materials provides for longevity. Backing with cotton or rubber provides about 50% greater life than in cases where no backing is used.

COTTON CLOTHS

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. Woven cotton filter cloths comprise ducks, twills, chain weaves, canton flannel and unbleached muslins. Cotton duck is a fabric weave that is a plain cloth with equal-thickness threads and texture in the "over one and under one" of the warp and woof. The twill weave is over two and under two with the next filling splitting the warp strands and giving a diagonal rib at 45° if the number of warp and filling threads are equal. Canton flannel is a twill weave in which one surface has been brushed up to give a nap finish. A muslin cloth is a very thin duck weave, which is unbleached for filtering. In chain weave one filling goes over two warp threads and under two, the next reversing this; the third is a true twill sequence, and the next repeats the cycle. A duct may be preferable to a twill of higher porosity, because the hard surface of the duck permits freer cake discharge. Under high increasing pressure a strong, durable cloth (duck) is required, since the first resistance is small as compared with that during cake building. Certain types of filters, such as drum filters, cannot stand uneven shrinkage and, in some cases, cloths must be pre-shrunk to ensure fitting during the life of the cloth. Nitro-filter (nitrated cotton cloth) cloths are about the same thickness and texture as ordinary cotton filtration cloths, but are distinguished by a harder surface. It is claimed that the cake is easily detached and that clogging is rare. Their tensile strength is 70-80% of that of the specially manufactured cotton cloths from which they are prepared. They are resistant to the corrosive action of sulfuric, nitric, mixed nitration and hydrochloric acids. They are recommended for filtering sulfuric acid solutions to 40% and at temperatures as high as 90°C, with the advantage of removing finely divided amorphous particles, which would quickly clog most ceramic media. Nitro-filter cloths are composed of cellulose nitrate, which is an ester of cellulose. Any chemical compound that will saponify the ester will destroy the cloth. Caustic soda or potash in strengths of 2% at 70° C or over; alkali sulfides, polysulfides and sulfohydrates; or mixtures of ethyl alcohol and ether, ethyl, amyl and butyl acetates, pyridine, ferrous sulfates, and other reducing agents are detrimental to the cloth. Cellulose nitrate is inflammable and explosive when dry, but when soaked in water it is considered entirely safe if reasonable care is taken in handling. For this reason it is colored red and packed in special containers. Users are cautioned to keep the cloths wet and to handle them carefully.

Proper selection of the filter media is often the most important consideration for assuring efficient suspension separation. A good filter medium should have the following general characteristics:

• The ability to retain a wide size distribution of solid particles from the suspension,

• Offer minimum hydraulic resistance to the filtrate flow,

• Allow easy discharge of cake,

• High resistance to chemical attack,

• Resist swelling when in contact with filtrate and washing liquid,

• Display good heat-resistance within the temperature ranges of filtration,

• Have sufficient strength to withstand filtering pressure and mechanical wear,

• Capable of avoiding wedging of particles into its pores.

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