- Email: [email protected]
Fibrous materials for the filtration of liquids
Fibrous materials for the filtration of liquids J.H. RA ISTRICK
The principles of liquid filtration are introduced and are used to explain why fibrous materials are much used by filter media manufacturers. The discussion of fibrous filter media is divided into the two principle manufacturing processes: weaving, which predominantly employs organic fibres; and wet-laying which frequently employs inorganic fibres. Wet laid filter media are discussed in the greater detail because, for toxicological reasons, there is a need for replacement materials.
FIL TRA TION MECHANISMS
Filtration can occur on the surface of the filter medium or within the depth of the medium. When a liquid containing suspended particles is passed through a surface filter, those particles which are larger than the pore size are retained. A surface fdter is used where the particles form a permeable layer, and particularly where recovery of the particles is necessary. The layer of fdtered particles, which is known as the filter cake, can be removed from a surface filter, thus enabling it to be re-used. Surface fdters are invariably thin and papers, gauzes, and cloths are the most commonly encountered forms of this type of filter, A depth filter can take the form of a 1 mm thick paper or a 1 m thick bed of sand. Depth filters are used for the fdtration of gelatinous materials or in other situations where the particles would quickly form an impermeable layer on a surface filter. Usually, the filtered particles can not be removed from a depth filter and so depth filters are used once only. A particle is retained within a depth filter by a sieving mechanism if the liquid bearing the particle flows through an orifice which is smaller than the particle. Particles can also be retained within a depth filter by adhesion to the fabric of the filter or to other particles which have previously adhered to that fabric,
In the filtration of small particles ( < 5/am) the dominant transport mechanisms are Brownian diffusion, and interception. Brownian diffusion dominates for sub-micron particles whereas interception is more important for particles larger than a micron. The interception mechanism is a consequence of the finite size of the particle. If a spherical particle, which has a radius of r, follows a streamline which approaches to within a distance of r of the fabric of the falter medium, then the particle will encounter the filter medium. Gravitational effects dominate the transport of particles larger than about 5/am. The exact size at which particles show significant gravitational effects depends on the difference between the density of the suspending medium and the density of the particles. The short range forces which are made use of in the Derjaguin, Landau, Verwey, Overbeek theory (DLVO theory) to predict colloid stability, 3'4 also govern the adhesion of particles to the filter medium. The DLVO theory defines the conditions under which adhesion will or will not occur by quantifying the attractive and repulsive forces. Attractive Van der Waals' forces always exist between surfaces separated by a short distance, but adhesion is often prevented if strong repulsive electrostatic forces exist. Electrostatic forces are a consequence of the charge which a surface in contact with a liquid usually possesses as a consequence of the adsorption of ions upon the surface or the desorption of ions from the surface. Surfaces may become either positively or negatively charged and two surfaces will repel each other if they both have charge of the same sign and they will attract each other if they have charge of opposite sign.
Filtration by adhesion is a two step process. These are transport of the particle to the surface of the filter medium and adhesion of the particle to the filter medium. 1'2 Transport mechanisms, which are discussed below, cause particles to encounter parts of the filter medium. An encounter is defined as being the close approach of a particle to the filter medium. Short range attractive and repulsive forces govern the likelihood of an encounter
PHYSICAL PROPERTIES OF FILTER MEDIA
leading to particle adhesion.
Permeability and uniformity are important properties of
Dr Raistriek is with the Mond Division of ICI Ltd, Runeorn, Cheshire, England
filter media. The permeability govems the rate at which liquid can be passed through the filter. Pressure is required
0010-4361/79/040206-03 $02.00 © IPC Business Press Limited 1979
206
COMPOSITES. OCTOBER 1979
to force liquid through a Filter and this causes compression of the Filter medium. A highly compressible filter medium might have an acceptable resistance to flow when subjected to a low pressure but a high resistance to flow when subjected to a high pressure, Permeabilities of filter media are obtained by the experimental determination of the parameters given below and substitution of the results obtained in the Darcy equation:
P _ QN
L
A~b
where P = pressure drop, L = thickness of the Filter medium, Q = rate of flow, A = area of Filter, N = viscosity of the liquid, ~b= permeability. This equation, which assumes that the applied pressure causes no change in the geometry of the Filter medium, shows that the flow rate is directly proportional to the pressure drop and inversely proportional to the liquid viscosity and the thickness of the Filter medium, Calculation of O gives a quantitative comparison of the permeabilities of filter media. The Darcy equation does not, however, relate permeability to either any fundamental property of the material which makes up the Filter medium or to the way in which the Filter is made. This is done using the Kozeny equation:
4-
E 3V2 kS2(1 _ E)2
where E = porosity or fractional void volume of the Filter medium, V = specific volume of the fibres, k = Kozeny constant (dimensionless - this constant has a value of 5.0 for fibrous Filter media), S = surface area per unit mass of fibres. The Kozeny equation can be used to quantify the extent to which permeability is influenced by compression and changes in fibre.diameter, Lack of uniformity in a Filter medium causes inefficiency no matter which Filtration mechanism applies. Surface Filters, and depth Filters which operate by the straining mechanism, initially let through particles up to the size of the largest pores. The liquid being Filtered flows through large pores preferentially so a high proportion of the particles pass through the large pores of a non-uniform filter. In those depth Filters which rely predominantly upon particle adhesion rather than straining, non-uniformity in the form of channels enables particles to pass through without encouritering a surface to which they can adhere, FIBROUS F I L T E R MEDIA
There are several reasons why fibrous materials are particularly suitable for the manufacture of Filter media, the prime one of these being the ease with which fibrous materials can be fabricated into sheets by wet-laying (ie paper or board making) or into cloths by weaving. Wet-laid sheets have the advantage that the fibres pack loosely to give a medium with high porosity and hence high permeability. Both wet-laid sheets and cloths are strong and so the pressure required to force liquids through them does not cause rupture. They are also flexible and this facilitates mounting in Filtration hardware. Lastly they have good structural integrity and so a significant release of fragments of the Filter medium into the liquid being filtered rarely occurs,
COMPOSITES. OCTOBER 1979
WET-LAID F I L T E R MEDIA
Wet-laying involves dispersing the individual fibres into an aqueous suspension and then Filtering this suspension to give a paper or board which is compressed and dried. Filter papers made from wood pulp cellulose are widely used for laboratory Filtration. These papers are thin and so they act mainly as surface Filters rather than as depth Filters. Large, strong, resin bonded cellulose papers are widely used for industrial Filtrations either on their own, or as liners in conjunction with woven Filter cloths. Cellulose fibres are relatively coarse (approximately 30/am diameter) but mechanical sheafing of an aqueous slurry of cellulose fibres causes the teasing out of fine fibrils from the surface of the 30/a fibres to give them a hairy appearance. This process is known as fibrillation. Cellulosic papers with a wide range of permeabillties and retentivities are made by varying the degree of fibrillation. Unlike cellulose, glass fibres can not be fibrillated but they have the advantage that they can be made with diameters down to about 0.05/am. Fine glass can be wet-laid and the Filter media thus formed have very high permeabilities compared with those made using cellulose. This is because the void volume within glass fibre papers is much greater than that within cellulose papers, s Glass fibre papers are more prone to damage than cellulosic papers, especially damage caused by folding. Wetting with water causes cellulosic papers to become considerably weaker than when dry but glass fibre papers are little affected. Wetting also changes the other physical properties of cellulosic papers because the fibres swell. Both glass and cellulose papers are available either binder free or impregnated with bonding agents such as melamine resin and neoprene. Glass fibre papers are thicker than most cellulose papers so they are more often used as depth Filters. In some operations, thick cellulose sheets are used for depth Filtration. In particular thick sheets containing cellulose and chrysotile asbestos fibres are used for the removal of very fine particles from potable liquids. 6 These materials are 2-6 mm thick and they are called filter sheets. The cellulose acts as a binder and its primary role is to give the Filter sheet strength and form. The asbestos fibres, which are much finer than the cellulose fibres, are primarily responsible for particle removal. Asbestos fibres on their own would form a thin fragile paper which would have a low permeability and would retain particles by surface filtration. Glass fibres are sometimes put into asbestos containing filter sheets as they increase the permeability to a greater degree than cellulose fibres. 7 Asbestos, like cellulose, fibrillates when subjected to shearing forces. Filter sheets with a wide range of properties are made by varying the cellulose: asbestos ratio and the degree to which the asbestos is fibrillated. Most Filter sheets fall into three general classifications which are: sterilizing; fine clarifying; and coarse clarifying. As the name implies, sterilizing Filter sheets are capable of removing all bacteria so as to give a sterile Filtrate. It has been reported that one mechanism by which Filter sheets remove fine particles from aqueous liquids is by adhesion of the particles to the asbestos fibres.S This adhesion occurs because the surface of asbestos becomes positively charged when in contact with water. The particles are negatively charged so this causes a strong electrostatic attraction between the particles and the asbestos.
207
Asbestos/cellulose filter sheets were once extensively used in the pharmaceutical industry but they are now less popular, Concern has been expressed over the possible toxicological effects of the small amount of asbestos which is shed into the fdtrate. 9 This concern has also had an impact upon the potable liquid industry. Legislation by the American FDA organization has restricted the use of filter sheets containing asbestos as well as those containing fme glass fibres. The search for an asbestos substitute has embraced many fibrous and non-fibrous materials. Amongst the materials
test the filter medium is immersed to a standard depth in a liquid of known density and surface tension. The minimum air pressure required to force a bubble through the filter medium is then measured and from this pressure the maximum pore diameter is calculated. The distribution of pore sizes within a filter medium is determined using the mercury porosimetry technique in which a measured pressure, externally applied, causes the progressive penetration of mercury into smaller and smaller pores within the ftlter medium.
which have been tried are the synthetic refractory fibres which are marketed under the 'Safffl' trade name. l° Long term screening tests on animals have shown that these fibres have no demonstrable toxic effects and they are made of aluminium oxide which has surface charge with the same sign as that of asbestos,
In some instances, results obtained from these tests relate directly to the sizes of particles which will be passed by surface filters. Their significance for depth f'dters is more limited, and is generally restricted to predicting the relative
Wet-laid f'flter media are usually made in the form of papers
A variety of methods are used to determine the abilities of filters to retain particles. Precipitates of barium sulphate, lead sulphate and ferric hydroxide have been used. 14'15 Whatman filter media are tested using natural chalk whiting,
or boards, but sometimes other shapes are made. In particular the porous former upon which the fibres are deposited may be cylindrical, in which case the filter medium will have a tubular shape. Tubular filter elements have the advantage of greater compactness over sheet forms. Such tubular elements are invariably depth Filters. Compact, high surface area surface filters are usually made in the form of a tube of pleated paper. These compact filter elements are particularly used in motor vehiclesJ ~ W O V E N F I L T E R MEDIA
Filter cloths are usually used in situation where cellulosic papers would be too weak. As fdter cloths are thin, surface filtration is the predominant mechanism of fdtration, Unlike most cellulosic filter papers, filter cloths are invariably re-used and so the removal of the fdter cake is a necessary part of the process. Cake release and other important properties such as permeability, cloth blinding (ie loss of permeability due to ingrained particles), Filtration efficiency and moisture content of the Filter cake depend upon the chemical nature of the cloth, the type of weave and the type of yarn. 12'13 The chief characteristics of the most widely used materials, from which filter cloths are made, can be listed zs follows: •
efficiencies of depth filters made from similar materials.
which has particles larger than 1/am, for their coarse filter media, and sub-micron polystyrene spheres for their fine Filter media, s The British Pharmaceutical Codex test for sterilizing Filters involves passing through the filter medium a volume of broth which contains Serratia Marcescens bacteria. After ftltration the broth is maintained under conditions suitable for bacterial growth. Any bacterial growth indicates failure of the test. Reduction of the bacterial content of the broth by at least ten orders of magnitude is necessary if this test is to be passed. A filter medium which can pass this test is known as an 'absolute' falter for this bacterium. For most filtration tests the analytical procedures which are used to determine the particle concentration in the filtrate are not sufficiently accurate for 'absolute' ratings to be determined. It is therefore common to quote particle retention efficiencies in terms of the particle size for which there will be a 2% penetration through a clean f'dter. REFERENCES
1
Cotton filter cloths have good mechanical strength,
good solids retention and they are cheap. Cake release and chemical resistance are not good, however, alld
2
although cotton is still widely used it is being replaced by synthetics, • Nylon has high abrasion resistance, great flexural strength and good cake release properties. It has good alkali resistance but poor acid resistance.
5
Polypropylene is the most widely used of the synthetic
6
•
materials. It has good resistance to chemical attack, good cake release properties and it is quite cheap. The major disadvantage of this material is the deterioration of physical properties that starts to occur at about 80°C. • Polyester has good resistance to acids and alkalis and
it Can be used at higher temperatures than polypropylene. Its major disadvantage is its comparatively high price.
3 4
7 8 9 10
TESTING OF FILTER MEDIA
p 614
The tests which are commonly done on falter media fall
11
into two categories. The first is the determination of some structural characteristics and the second is the direct determination of particle retention'
12 13
Mercury porosimetry and the bubble point test are commonly used to obtain structural information, s In the bubble point
14 15
208
Hetzig, J.P., Leelerc, D.M. and Le Golf, P. 'Flow of suspensions through porous media - application to deep f'dtration', Industrial and Engineering Chemistry 62 No 5 (1970) p 8 O'Melia,C.R. and Stumm, W. 'Theory of water filtration', J Amer Water Wks Assoc 59 (1967) p 1393 Verwey, EJ.W. and Overbeek, J.Th.G. Theory of the Stability o f Lyophobic Colloids (Elsevier, 1948) Ives, K.L and Gregory, J. 'Surface forces in filtration', Proc Soc for Water Treatment and Examination 15 (1966) p 93 Meakin,J.C. and Pratt, M.C.Manual of Laboratory Filtration (W and R Balston Ltd, Maidstone, 1972) Osgood,G. 'Filter sheets and sheet filtration', Filtration and Separation 4 No 4 (1967) p 327 Jones, ILK. and Hess, R.W. US Patent 3 796 659 (March 1974) Assigned to Johns-ManvilleCorporation Wnek,W. 'Eleetrokinetic and chemical aspects of water filtration' Filtration and Separation 11 No 63 (1974) p 237 $haekleton, R. 'Recent advances in beveragefiltration', Filtration and Separation 14 No 6 (1977) p 632 Raistriek,J.H. 'Saffil fibres - new media for high performance liquid filtration', Filtration and Separation 13 No 6 (1976) Foehtman, E.G. 'Cartridge filtration: technology and economies', Filtration and Separation 10 No 3 (1973) p 289 Puzehas,D.B. Industrial Filtration of Liquids (Leonard Hill, London, 1967) p 61 Wright,J. 'Filtration - media selection is the key to success', Australian Chemical Engineering 15 (1974) p 5 ASTMD981-56 (geapproved 1965) DIN53138 (1962)
COMPOSITES . OCTOBER
1979
The principles of liquid filtration are introduced and are used to explain why fibrous materials are much used by filter media manufacturers. The discussion of fibrous filter media is divided into the two principle manufacturing processes: weaving, which predominantly employs organic fibres; and wet-laying which frequently employs inorganic fibres. Wet laid filter media are discussed in the greater detail because, for toxicological reasons, there is a need for replacement materials.
FIL TRA TION MECHANISMS
Filtration can occur on the surface of the filter medium or within the depth of the medium. When a liquid containing suspended particles is passed through a surface filter, those particles which are larger than the pore size are retained. A surface fdter is used where the particles form a permeable layer, and particularly where recovery of the particles is necessary. The layer of fdtered particles, which is known as the filter cake, can be removed from a surface filter, thus enabling it to be re-used. Surface fdters are invariably thin and papers, gauzes, and cloths are the most commonly encountered forms of this type of filter, A depth filter can take the form of a 1 mm thick paper or a 1 m thick bed of sand. Depth filters are used for the fdtration of gelatinous materials or in other situations where the particles would quickly form an impermeable layer on a surface filter. Usually, the filtered particles can not be removed from a depth filter and so depth filters are used once only. A particle is retained within a depth filter by a sieving mechanism if the liquid bearing the particle flows through an orifice which is smaller than the particle. Particles can also be retained within a depth filter by adhesion to the fabric of the filter or to other particles which have previously adhered to that fabric,
In the filtration of small particles ( < 5/am) the dominant transport mechanisms are Brownian diffusion, and interception. Brownian diffusion dominates for sub-micron particles whereas interception is more important for particles larger than a micron. The interception mechanism is a consequence of the finite size of the particle. If a spherical particle, which has a radius of r, follows a streamline which approaches to within a distance of r of the fabric of the falter medium, then the particle will encounter the filter medium. Gravitational effects dominate the transport of particles larger than about 5/am. The exact size at which particles show significant gravitational effects depends on the difference between the density of the suspending medium and the density of the particles. The short range forces which are made use of in the Derjaguin, Landau, Verwey, Overbeek theory (DLVO theory) to predict colloid stability, 3'4 also govern the adhesion of particles to the filter medium. The DLVO theory defines the conditions under which adhesion will or will not occur by quantifying the attractive and repulsive forces. Attractive Van der Waals' forces always exist between surfaces separated by a short distance, but adhesion is often prevented if strong repulsive electrostatic forces exist. Electrostatic forces are a consequence of the charge which a surface in contact with a liquid usually possesses as a consequence of the adsorption of ions upon the surface or the desorption of ions from the surface. Surfaces may become either positively or negatively charged and two surfaces will repel each other if they both have charge of the same sign and they will attract each other if they have charge of opposite sign.
Filtration by adhesion is a two step process. These are transport of the particle to the surface of the filter medium and adhesion of the particle to the filter medium. 1'2 Transport mechanisms, which are discussed below, cause particles to encounter parts of the filter medium. An encounter is defined as being the close approach of a particle to the filter medium. Short range attractive and repulsive forces govern the likelihood of an encounter
PHYSICAL PROPERTIES OF FILTER MEDIA
leading to particle adhesion.
Permeability and uniformity are important properties of
Dr Raistriek is with the Mond Division of ICI Ltd, Runeorn, Cheshire, England
filter media. The permeability govems the rate at which liquid can be passed through the filter. Pressure is required
0010-4361/79/040206-03 $02.00 © IPC Business Press Limited 1979
206
COMPOSITES. OCTOBER 1979
to force liquid through a Filter and this causes compression of the Filter medium. A highly compressible filter medium might have an acceptable resistance to flow when subjected to a low pressure but a high resistance to flow when subjected to a high pressure, Permeabilities of filter media are obtained by the experimental determination of the parameters given below and substitution of the results obtained in the Darcy equation:
P _ QN
L
A~b
where P = pressure drop, L = thickness of the Filter medium, Q = rate of flow, A = area of Filter, N = viscosity of the liquid, ~b= permeability. This equation, which assumes that the applied pressure causes no change in the geometry of the Filter medium, shows that the flow rate is directly proportional to the pressure drop and inversely proportional to the liquid viscosity and the thickness of the Filter medium, Calculation of O gives a quantitative comparison of the permeabilities of filter media. The Darcy equation does not, however, relate permeability to either any fundamental property of the material which makes up the Filter medium or to the way in which the Filter is made. This is done using the Kozeny equation:
4-
E 3V2 kS2(1 _ E)2
where E = porosity or fractional void volume of the Filter medium, V = specific volume of the fibres, k = Kozeny constant (dimensionless - this constant has a value of 5.0 for fibrous Filter media), S = surface area per unit mass of fibres. The Kozeny equation can be used to quantify the extent to which permeability is influenced by compression and changes in fibre.diameter, Lack of uniformity in a Filter medium causes inefficiency no matter which Filtration mechanism applies. Surface Filters, and depth Filters which operate by the straining mechanism, initially let through particles up to the size of the largest pores. The liquid being Filtered flows through large pores preferentially so a high proportion of the particles pass through the large pores of a non-uniform filter. In those depth Filters which rely predominantly upon particle adhesion rather than straining, non-uniformity in the form of channels enables particles to pass through without encouritering a surface to which they can adhere, FIBROUS F I L T E R MEDIA
There are several reasons why fibrous materials are particularly suitable for the manufacture of Filter media, the prime one of these being the ease with which fibrous materials can be fabricated into sheets by wet-laying (ie paper or board making) or into cloths by weaving. Wet-laid sheets have the advantage that the fibres pack loosely to give a medium with high porosity and hence high permeability. Both wet-laid sheets and cloths are strong and so the pressure required to force liquids through them does not cause rupture. They are also flexible and this facilitates mounting in Filtration hardware. Lastly they have good structural integrity and so a significant release of fragments of the Filter medium into the liquid being filtered rarely occurs,
COMPOSITES. OCTOBER 1979
WET-LAID F I L T E R MEDIA
Wet-laying involves dispersing the individual fibres into an aqueous suspension and then Filtering this suspension to give a paper or board which is compressed and dried. Filter papers made from wood pulp cellulose are widely used for laboratory Filtration. These papers are thin and so they act mainly as surface Filters rather than as depth Filters. Large, strong, resin bonded cellulose papers are widely used for industrial Filtrations either on their own, or as liners in conjunction with woven Filter cloths. Cellulose fibres are relatively coarse (approximately 30/am diameter) but mechanical sheafing of an aqueous slurry of cellulose fibres causes the teasing out of fine fibrils from the surface of the 30/a fibres to give them a hairy appearance. This process is known as fibrillation. Cellulosic papers with a wide range of permeabillties and retentivities are made by varying the degree of fibrillation. Unlike cellulose, glass fibres can not be fibrillated but they have the advantage that they can be made with diameters down to about 0.05/am. Fine glass can be wet-laid and the Filter media thus formed have very high permeabilities compared with those made using cellulose. This is because the void volume within glass fibre papers is much greater than that within cellulose papers, s Glass fibre papers are more prone to damage than cellulosic papers, especially damage caused by folding. Wetting with water causes cellulosic papers to become considerably weaker than when dry but glass fibre papers are little affected. Wetting also changes the other physical properties of cellulosic papers because the fibres swell. Both glass and cellulose papers are available either binder free or impregnated with bonding agents such as melamine resin and neoprene. Glass fibre papers are thicker than most cellulose papers so they are more often used as depth Filters. In some operations, thick cellulose sheets are used for depth Filtration. In particular thick sheets containing cellulose and chrysotile asbestos fibres are used for the removal of very fine particles from potable liquids. 6 These materials are 2-6 mm thick and they are called filter sheets. The cellulose acts as a binder and its primary role is to give the Filter sheet strength and form. The asbestos fibres, which are much finer than the cellulose fibres, are primarily responsible for particle removal. Asbestos fibres on their own would form a thin fragile paper which would have a low permeability and would retain particles by surface filtration. Glass fibres are sometimes put into asbestos containing filter sheets as they increase the permeability to a greater degree than cellulose fibres. 7 Asbestos, like cellulose, fibrillates when subjected to shearing forces. Filter sheets with a wide range of properties are made by varying the cellulose: asbestos ratio and the degree to which the asbestos is fibrillated. Most Filter sheets fall into three general classifications which are: sterilizing; fine clarifying; and coarse clarifying. As the name implies, sterilizing Filter sheets are capable of removing all bacteria so as to give a sterile Filtrate. It has been reported that one mechanism by which Filter sheets remove fine particles from aqueous liquids is by adhesion of the particles to the asbestos fibres.S This adhesion occurs because the surface of asbestos becomes positively charged when in contact with water. The particles are negatively charged so this causes a strong electrostatic attraction between the particles and the asbestos.
207
Asbestos/cellulose filter sheets were once extensively used in the pharmaceutical industry but they are now less popular, Concern has been expressed over the possible toxicological effects of the small amount of asbestos which is shed into the fdtrate. 9 This concern has also had an impact upon the potable liquid industry. Legislation by the American FDA organization has restricted the use of filter sheets containing asbestos as well as those containing fme glass fibres. The search for an asbestos substitute has embraced many fibrous and non-fibrous materials. Amongst the materials
test the filter medium is immersed to a standard depth in a liquid of known density and surface tension. The minimum air pressure required to force a bubble through the filter medium is then measured and from this pressure the maximum pore diameter is calculated. The distribution of pore sizes within a filter medium is determined using the mercury porosimetry technique in which a measured pressure, externally applied, causes the progressive penetration of mercury into smaller and smaller pores within the ftlter medium.
which have been tried are the synthetic refractory fibres which are marketed under the 'Safffl' trade name. l° Long term screening tests on animals have shown that these fibres have no demonstrable toxic effects and they are made of aluminium oxide which has surface charge with the same sign as that of asbestos,
In some instances, results obtained from these tests relate directly to the sizes of particles which will be passed by surface filters. Their significance for depth f'dters is more limited, and is generally restricted to predicting the relative
Wet-laid f'flter media are usually made in the form of papers
A variety of methods are used to determine the abilities of filters to retain particles. Precipitates of barium sulphate, lead sulphate and ferric hydroxide have been used. 14'15 Whatman filter media are tested using natural chalk whiting,
or boards, but sometimes other shapes are made. In particular the porous former upon which the fibres are deposited may be cylindrical, in which case the filter medium will have a tubular shape. Tubular filter elements have the advantage of greater compactness over sheet forms. Such tubular elements are invariably depth Filters. Compact, high surface area surface filters are usually made in the form of a tube of pleated paper. These compact filter elements are particularly used in motor vehiclesJ ~ W O V E N F I L T E R MEDIA
Filter cloths are usually used in situation where cellulosic papers would be too weak. As fdter cloths are thin, surface filtration is the predominant mechanism of fdtration, Unlike most cellulosic filter papers, filter cloths are invariably re-used and so the removal of the fdter cake is a necessary part of the process. Cake release and other important properties such as permeability, cloth blinding (ie loss of permeability due to ingrained particles), Filtration efficiency and moisture content of the Filter cake depend upon the chemical nature of the cloth, the type of weave and the type of yarn. 12'13 The chief characteristics of the most widely used materials, from which filter cloths are made, can be listed zs follows: •
efficiencies of depth filters made from similar materials.
which has particles larger than 1/am, for their coarse filter media, and sub-micron polystyrene spheres for their fine Filter media, s The British Pharmaceutical Codex test for sterilizing Filters involves passing through the filter medium a volume of broth which contains Serratia Marcescens bacteria. After ftltration the broth is maintained under conditions suitable for bacterial growth. Any bacterial growth indicates failure of the test. Reduction of the bacterial content of the broth by at least ten orders of magnitude is necessary if this test is to be passed. A filter medium which can pass this test is known as an 'absolute' falter for this bacterium. For most filtration tests the analytical procedures which are used to determine the particle concentration in the filtrate are not sufficiently accurate for 'absolute' ratings to be determined. It is therefore common to quote particle retention efficiencies in terms of the particle size for which there will be a 2% penetration through a clean f'dter. REFERENCES
1
Cotton filter cloths have good mechanical strength,
good solids retention and they are cheap. Cake release and chemical resistance are not good, however, alld
2
although cotton is still widely used it is being replaced by synthetics, • Nylon has high abrasion resistance, great flexural strength and good cake release properties. It has good alkali resistance but poor acid resistance.
5
Polypropylene is the most widely used of the synthetic
6
•
materials. It has good resistance to chemical attack, good cake release properties and it is quite cheap. The major disadvantage of this material is the deterioration of physical properties that starts to occur at about 80°C. • Polyester has good resistance to acids and alkalis and
it Can be used at higher temperatures than polypropylene. Its major disadvantage is its comparatively high price.
3 4
7 8 9 10
TESTING OF FILTER MEDIA
p 614
The tests which are commonly done on falter media fall
11
into two categories. The first is the determination of some structural characteristics and the second is the direct determination of particle retention'
12 13
Mercury porosimetry and the bubble point test are commonly used to obtain structural information, s In the bubble point
14 15
208
Hetzig, J.P., Leelerc, D.M. and Le Golf, P. 'Flow of suspensions through porous media - application to deep f'dtration', Industrial and Engineering Chemistry 62 No 5 (1970) p 8 O'Melia,C.R. and Stumm, W. 'Theory of water filtration', J Amer Water Wks Assoc 59 (1967) p 1393 Verwey, EJ.W. and Overbeek, J.Th.G. Theory of the Stability o f Lyophobic Colloids (Elsevier, 1948) Ives, K.L and Gregory, J. 'Surface forces in filtration', Proc Soc for Water Treatment and Examination 15 (1966) p 93 Meakin,J.C. and Pratt, M.C.Manual of Laboratory Filtration (W and R Balston Ltd, Maidstone, 1972) Osgood,G. 'Filter sheets and sheet filtration', Filtration and Separation 4 No 4 (1967) p 327 Jones, ILK. and Hess, R.W. US Patent 3 796 659 (March 1974) Assigned to Johns-ManvilleCorporation Wnek,W. 'Eleetrokinetic and chemical aspects of water filtration' Filtration and Separation 11 No 63 (1974) p 237 $haekleton, R. 'Recent advances in beveragefiltration', Filtration and Separation 14 No 6 (1977) p 632 Raistriek,J.H. 'Saffil fibres - new media for high performance liquid filtration', Filtration and Separation 13 No 6 (1976) Foehtman, E.G. 'Cartridge filtration: technology and economies', Filtration and Separation 10 No 3 (1973) p 289 Puzehas,D.B. Industrial Filtration of Liquids (Leonard Hill, London, 1967) p 61 Wright,J. 'Filtration - media selection is the key to success', Australian Chemical Engineering 15 (1974) p 5 ASTMD981-56 (geapproved 1965) DIN53138 (1962)
COMPOSITES . OCTOBER
1979