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experienced in washing to 0 . 2 per cent within quite a reasonable period. It should be realized that, in “through” or “thorough” washing, the wash has to pass through the whole thickness of the cake, instead of only half, as in the case of the filtrate, and for this reason, the cakes, unless readily permeable, should be thinner when washing is involved than if filtration only is requisite. EFFECT O F XATURE O F PRECIPITATE O N TIME O F WASHING-If
the particles in suspension are of a hard and nonporous character and are confined within reasonable limits of size in either direction, washing becomes an easy problem. The theoretical curve connecting concentration and amount of wash will be a horizontal line, dropping sharply to zero as soon as an amount equivalent to the volume of liquor in the cake has been forced in. The effect of adsorption, etc. is to cause this curve to drop less and less sharply the finer the grain of the cake, as the specific surface of the grains increases with decrease in grain size. Fig. 12 shows some experimental curves obtained by the author. A weak caustic solution was filtered €rom a chalk precipitate; the cakes weighed about 92 lbs. per cu. ft., contained 38 per cent of moisture, and were formed a t 80 lbs. per sq. in., using a montojus. They were washed a t 20 lbs. and 40 lbs. per sq. in., and displacement was nearly perfect. Almost theoretical results were obtained as the cake was quickly formed and of even quality throughout. Washing was complete with about 1 . 5 to 1.8 displacement volumes or, say, 0.84 to 1 cake-volume of wash. This result was due to the fact that the pressure mas not high enough to cause appreciable leakage round the edges of the cake. I n similar experiments, in which thc cake was formed and washed by a hand-pump, using 100 lbs. pressure in each case, several times the amount of wash and a longer period of time were requisite unless great care was taken.1 These results were very fairly indicative of what could be obtained on a large scale. In ordinary commercial practice a press was used making cakes 1.75 in. thick, which were formed a t 80 lbs. pressure and washed a t 15 lbs. pressure. The time required for washing with hot mater was 10 min., the rate of washing being 8.2 gal. per sq. ft. per hr., and the amount required being about 1 . 5 cake-volumes. The residual soluble matter was 0.18 per cent on the wet cake. The case, however, is immediately altered if slimy matter is present, The following figures relate to a cake consisting of precipitate of lime contaminated with colloidal silica. Here the cakes were approximately 2 in. thick, weighed 93.5 lbs. per cu. ft., and were 51.5 per cent moist. One hour was required for washing, the rate of flow being 4 to 5 gal. per sq. ft. per hr. The amount of wash was 4.25 times the cakevolume or 5 . 5 displacement volumes. By washing in stages as shown in Fig. 13, using liquors of decreasing strengths, the amount of pure water required was reduced to approximately one-third of the total wash volume. The amount of residual soluble matter was 0 4 per cent, measured on the wet cake. Mr. R. S. Denny has made very full investigations in connection with the filter pressing and washing of gold slimes.2 For many fine-grained or gelatinous precipitates much more time and wash are required. In washing chrome hydrate, 2 . 5 hrs. were necessary, for cakes 1 25 in. thick. The wash amounted to 10 cake-volumes, the cake weighing 80 lbs. per cu. ft., with a moisture content of 75 per cent. The rate of washing was 2 . 6 gal. per sq. ft. per hr. It is impossible, however, to give any general rule, as the circumstances vary very greatly for each substnnse. Similar 1 It should be noted that a11 rates of washing are calculated on the half area only of the filter press 1 “Rand Metallurgical Practire and Recent Innovations,” Proc. South A f y i c a n Assoc En&, 11 (19051.
Vol. 13, No. 11
conditions occur with many dyes and intermediates, and even longer wash periods may be necessary. CO NCE KTRATIO x-Where the liquor contains solubles dissolved a t a high temperature, care must be taken that the concentration is such that these are not precipitated in the press, owing to the cooling which may take place there. Unless such a precipitate dissolves very readily it will be very difficult to wash out, owing to the low turbulence of the wash water, and if any part is removed there will be a tendency for the cake to shrink, giving rise to irregular washing. In dealing with a certain dye intermediate, suspended in a liquor containing a proportion of sulfate of soda, satisfactory results were not obtained even after many hours’ washing. It was found that the sulfate of soda was crystallizing out, owing to a slight drop in temperature between the vat and the press. On diluting the mixture in the vat just sufficiently to prevent this, good results wereobtained in a perfectly normal wash period. It will be realized that washing in the fdter press should be used only for removing matter already in solution. It is not a substitute for good lixiviation. In conclusion it may be said that owing to the large number of factors which enter into a washing problem, it is not possible to lay down hard and fast rules and figureis, and each problem must be considered in regard to its special circumstances. Thereis no doubt, however, that in too many cases the operation is carried out without regard to principles. X precipitate is taken as it happens to come, a cake is formed somehow, the wash applied anyhow, and finally those responsible complain that their material is difficult to handle, or that there is some defect in the design of their filter press. It cannot be too clearly understood that good results depend not merely on the mechanical arrangement of the press, but almost more on the attention paid to securing a proper physical condition of the material to be treated, and delivering it in a suitable manner to the press. Every dettiil of the Operation should be correct.
Filter Cloth and Its Relation to Filtration By Alvin A. Campbell NEWARX WIRE?CLOTH Co., NEWAT~K, N. J ,
Under the heading of filter cloths we may select a few of the leading ones, woven of cotton, wool, jute, hemp, nickel, and inonel metal. Cotton duck has been used for years for practically a11 filtering media up to the present day, wherein we look for something to replace cotton; not so much with the view of giving a better separation as of an increased efficiency, figured down to output versus cost. After all, the filter cloth does not do the filtering, but acts as the retaining wall to form the cake. With this fact in mind we have but one principle to follow: that is, to combine strength, fineness, and rapidity. STRENGTH,FINENESS, AND RAPIDITY First, we require strength in the medium in order to withstand the pressure applied through the pumping of the liquids through the press. One great cause of leakage in most plate and frame presses is that the cloth, when under pressure, finds its way into the fissures or grids of the plates, and in patented types of presses into the meshes of the heavy wire cloth backings of the plates. This causes a distortion of the meshes of the vegetable and animal fibers used in the manufactbe of filter cloths, which in turn causes larger openings and a leakage. The answer to this point is change of design in the weave.
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THE JOURNAL OF INDUSTRIAL AhTD Eh'GINEERING CHEMISTRY
Secondly, we may combine fineness with rapidity. It can be readily understood that the finer the cloth, the less the rapidity. To change these conditions means change of design, not only in the weave but in the material from which the cloth is fabricated. EFFECTOF MATERIALS TO BE FILTERED I n the chemical field we find a great variety of substances to be filtered. such as those of viscous character, alkalies, acids, and sacchaxates, and in the manufacturing field we are confronted with different industrial problems. Perhaps the greatest problem, outside of the strictly chemical field, is that of the clay industry. I n considering all these problems we must watch for a chemical action on our filtering medium. While we know that cotton duck, for instance, will produce the desired results as far as filtration is concerned, we must constantly renew our cloths because of the effect of the substances passing through. We know that both alkaline and acid conditions will bring about decomposition of cotton, thereby causing a decreasing efficiency of the cloth, and finally a complete change of filtering media. It is therefore desirable to find a material from which to make filtration media, which will withstand the action of chemicals. Any of the cloths made up of the vegetable or animal threads are not entirely satisfactory from the point of view of rapidity of filtration and the filtration pressure developed by the pressure applied to the press in pumping the liquids through. When the cloth beconies wet a swelling and softening of the fiber takes place, permitting the cloth to become distorted. Larger openings and a less efficient filtration rwult, especially when additional pressure is placed upon the pumps to speed lip the process.
METALLICCLOTHS A metallic medium seems to promise most, and inonel metal seems to he the logical metal to be used. This is indieated by experimental data on the resistance of monel to various acids and salts, and it is a knoyn fact that it is alkali proof. In the use of monel metal filter cloths we must, however, guard against one thing, the electrolytic action set up when using it in connection with iron plates. While there may be other substances which will attack monel metal filter cloth, potassium permanganate is the only one which sets up a violent electrolytic action which has come to the attantion of the writer. There are, of course, other materials which cause electrolytic action, but none are on record where the value of the cloth is impaired. Monel metal cloth is about twelve times as strong as twisted cotton of the same size. With the increased strength and the nonelastic nature of a metallic wire, we find much less chance of mesh distortion, and absolutely none where the backings in the later type presses, and the grids in the plate and frame type presses, are properly constructed. This element of press construction should be carefully studied by all manufacturers of filter presses, as it is a highly important feature for efficient filtration, no matter what cloth is used on a given press. I n regard to the best construction of metallic filter cloths, we must take into consideration fineness, strength, and cost, remembering that the finer the cloth, the greater the cost. A regular square mesh could be woven giving as small an opening as 0.0027 in. with a wire diameter of 0.0023 in., where a square foot would weigh about 1 0 8 . This would, however, lack the necessary strength, along with other essentials. By experimental work it was found that a weave could be devised wherein rapid passing of liquids could be obtained without the filter cake interfering with its progress. I n
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its manufacture we have devised a vedge-shaped opening, permitting the filtrate to pass through rapidly, and in turn causing the filter cake to form against a practically solid flat backing. It has been asked why the wedge-shaped opening is used in preference to a round, square, or rectangular opening. A drop of mater will not pass through a square or roimd opening as quickly as through a rectangular opening, for the reason that in the square or round opening i t must pass through under preesure, whereas in the rectangular it will naturally elongate and pass through from gravity. With the wedge-shaped opening we have the necessary elongation with the additional element of capillary attraction, because the opening increases from zero to about 0 003 in. The metallic filter cloth known as No. 250 is equivalent in fineness to the regular 220-mesh, square-weave, wire cloth, with about forty-eight times greater strength. This cloth has been found to be the best for most processes, although it is made in different grades of fineness, for specific uses. Tf the precipitate is the product desired. metallic filter cloth can be used without treatment of any kind. I n only one industry have we experienced inuch difficulty in filtration for the saving of the precipitate. That is in tho color industry where some of the blues, yellows, and greens are in process. Metallic cloth is not generally satisfactory in this instance, because it will not clog, whereas cotton duck becomes rapidly filled, and clogs in the mesh, holding back the extremely minute particles. Metallic cloth does. however, give very good satisfaction in other color work. The other phase of filtration involves saving the filtrate. This is of course the most difficult of all in the use of any cloth. It has been found that a finely pulverized fossilized earth, added to the batch or primarily injected into the press, will quickly form a filter cake and permit rapid filtration of thc mixture. For example, a Sweetland press of eleven 8-in. disks was being used to filter liquid soap, to g k e a clear product. The disks were covered with a heavy grade of cotton duck and, while a fairly clear filtrate was obtained, it was unsatisfactory, and the cotton leaves had to be replaced about every two weeks. Metallic filter cloth was installed in this press, and with the use of fossilized earth an absolutely clear filtrate is now obtained, with the probability that the filter leaves will last for years. Microscopic examinations of a drop of this liquid soap, filtered through the cotton cloths, and evaporated on a glass slide, show star-shaped and very uneven crystals mixed with spots of foreign matter measuring from 0.00018 to 0.0075 in. The samples taken from the metallic cloth and fossilized earth filtrate show a field of star-shaped crystals of the same proportions, mixed with no foreign matter. The first large industry to use monel metal metallic filter cloth was the sugar industry. Sheets remained in use over two years, whereas the cotton changes had been very frequent. Herewith is a report from the chemist of one of the larger sugar plants: In the filter presses handling corrosive liquids ip the beet sugar industry, filter cloth made of monel metal is the most successful material that has ever been developed. One of the factors contributing to its success is the possibility of using strong hydrochloric acid to clean it. In the midd1e of the season the filter cloths are boiled out with strong acid without being removed from the press. As the press bodies are made of cast iron, this allows not only the usual acid corrosion but also a violent electrolytic action.
At the end of the season the filter cloths are reiiioved from the presses and boiled with strong acid, and also scrubbed with metallic brushes. There is no case on record where the cloths have been injured by this treatment. While many advocate the rolling of metallic filter cloths in order to get fineness, the manufacturers of wire cloth strongly
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advise against this method, for the reason that the mesh becomes distorted and weakened at the point of contact of the cross wires. Metallic filter cloth i6 being used more each year as a liner for centrifugal filters.
Industrial Filter Media By Arthur Wright FILTRATION ENGINEGRS, INC, 253 BROADWAY, NGW YORK, N. Y
Industrial filtration involves the separation of a comparatively large amount of solids from a small volume of liquid. The rate of flow of liquid through the fdter medium is low; hence woven fabrics through which only a small flow is obtainable are used most successfully. Fabrics of high resistance to flow of mater through them have for years constituted the typical filter cloth for industrial filtration. To-day cotton duck represents one limit, the dense, and unbleached muslin the other limit, open. Filter fabrics can be divided into two main classes: those used €or neutral or noncorrosive liquors, and those for corrosive liquors. The latter are mainly special media of wool, metal, asbestos, stone, etc. For noncorrosive liquors cotton is the material used almost without exception. COTTONFILTER CLOTHS TvEAvEs-coLton fdter cloth fabrics are made up in duck or plain, twill, and chain weaves. Plain weave has the square or right-angle appearance of all ducking and is woven by the filling or weft passing over one warp and under the next, known as “over one under one.” Twill has the diagonal lines so characteristic of its weave, and is made by weaving “over two and under two,” with the next filling splitting the warp members. Chain, or as it is also known, broken twill, has a herringbone appearance and is woven with one filling going over two and under two, the next reversing this order, the third being a true twill sequence, and the next repeating the above cycle again. For each weave there is considerable modification, depending on the weights of yarn used and the number per inch. Muslins and drills are trade names €or very light duck and twill weaves. The nomenclature of the various weaves should be better standardized. At present a duck is known by a number (as 00) or by the weight per unit measure (10 02.). Twill and chain weaves are designated by the number of warp and filling members per inch, as for instance No. 2232, where there are 22 warp members per inch and 32 fillings. There is ambiguity here, for the twills woven of different yarns under the same number of members must weigh differently. A combination of weight per unit and designation of the number of warp and filling members would do much to clarify this. usm OF MUSLIN-The commercial use of unbleached muslin and other comparatively frail filter cloths marks a distinct advance in the subject of filter media, and represents the application of a principle long understood but impractical until the advent of our modern filters. Filtration through iabrics should be surface filtration wherein all the particles filtered out of the liquor accumulate on the surface of the medium, as distinct from bed filtration wherein some of the solids are caught through the depth of the filter bed, as in sand or charcoal filters. A thin fabric has not sufficient depth to hold solids within it, whereas thick media Fill often hold back solids that penetrate the surface. Proof of this is furnished when the surface of a heavy twill or duck cloth will often be quite clean, while the cloth is almost impervious on account of particles lodged within the 010th.
Vol. 13, No. 11
When bag filters, gravity or suction filters, and filter presses mere the only agencies a t hand, strength of the fabric was the primal specification for all cloths. In bag filters, strength is required lest the weight of the liquor inside the bag burst it, and in tank filters unloading by shoveling out the cake requires a cloth of substantial strength. I n filter presses the strength required is not so much due to the pressure of fdtration as to the squeezing effect a t the gasket joint between the abutting plates and frames. Too milch emphasis has often been put on this point. The absolute pressure on the cloth between the plates is not excessive save where the cloth is laid in a wrinkled condition and the pressure has to be increased to stop leakage a t such places. TO correct carelessness in laying the cloths, strong fabrics were required. For a time, manufacturers of this type of filter were too much engrossed in their schemes of drainage, washing methods, accuracy of machined s~irfaces, etc., and overlooked the cutting edge of the gasket surfaces. Only a strong cloth would not be cut through by these sharp edges. A rounded edge overcomes this and eliminates the breakage a t this point. FACTORS I N SELECTION O F CLOTH-It iS obvious that the yarn used in the cloth is the determining factor in structural strength. It is also important that the cloth be dense enough to make a tight gasket joint when the press is msde up. These factors have determined for the most part the specifications of the filter cloth used in filter presses. Other factors, especially in our modern filters, affect the selection of the best filter medium. The filter cloth is fixed to the drainage member either as a sewn bag, or a wired sheet, or a clamped covering. This precludes quick changes. In consequence a cloth must have an economical life or the attendant expense of replacing the medium will make the entire filter operation excessively costly. Also, in modern filters the discharge of the cake should be without hand labor. This means automatic or semi-automatic discharging methods, the efficiency of which is largely dependent upon the filter cloth used. PROPERTIES AFFECTING DISCHARGE OF CAKE The discharge of the cake from the filter cloth can generally be accepted as a simple matter so long as the deposited cake is entirely on the surface of the cloth. Even with the most freely filtering liquors containing granular solids in suspension, some fine solids penetrate the surface and enmesh in the interior of the cloth. Automatic means of discharge are practically worthless in cleaning the cloth from these solids. Such a condition is fatal to modern type filters, and in some industries where it is alinost impossible to prevent solids penetrating the surface of the cloth, as, for instance, raw cane-sugar manufacture, plate and frame filters are still supreme. In these filters the cloth can be changed after each operation. Naturally if the cloth is open or so thin as to prevent the fine particles from collecting within the fabric, discharge of the cake from the surface cleans the cloth. SMOOTHNESS OF EuRFAcE-Experience has proved, especially in the case of sluicing discharge, that the surface of the cloth must be smooth for the best results. ii duck weave * has proved a better cloth than a twill weave of admittedly better porosity. POROSITY--11 new filter cloth held up against the light may show open pores, and yet become positively dense when wetted or in operation for a few runs. I n this case the reverse current cannot permeate well 8nd tends to belly out the cloth without lifting the cake away- from the cloth. Some operators have had but little better success in discharging when trying out open cloths. A too porous cloth lets the reverse current through too readily, so that it discharges small patches of cake and lets the incoming sir
