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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 6 ,
N0.2
LABORATORY AND PLANT FILTER PRESSES’ B y E. J. SWEETLAND
T h a t there is great room for improvement in t h e methods of filtration commonly employed cannot be doubted. T h e plate a n d frame filter press ordinarily used comprises anywhere from 40 t o I O O filter chambers, each of which must be cleaned individually b y h a n d every time t h e filter is filled. It was t o eliminate t h e excessive amount of h a n d labor in cleaning filter presses t h a t t h e filter under discussion was primarily designed, b u t as t h e construction was developed m a n y incidental advantages of importance developed with it, as will be pointed out further on. T h e single advantage in plate a n d frame filter presses t h a t has k e p t t h e m in favor is t h e fact t h a t their peculiar construction affords a very large filter area in a comparatively small space. Filtration cannot be forced beyond a certain reasonable limit; when this limit is reached, it is necessary t o increase filter area t o increase capacity. To endeavor t o increase capacity b y increasing pressure instead of filter area is futile, for excessive pressure merely presses t h e cake t o a compact, almost impervious layer a n d t h e object is defeated at once. With this i n view, t h e modern filter must not dep a r t from a construction t h a t permits of ample filter surface. Other things being equal, t h e capacity, a n d consequently t h e value, of a filter is proportional t o t h e filter area. T h e sluicing t y p e of filter press is one example of a filter press construction t h a t eliminates h a n d labor in cleaning. In this t y p e t h e cake is disintegrated The b y a stream of water forced into each chambe;. cake t h u s liquefied flows t o t h e waste reservoir, or d u m p . T h e usefulness of this method is limited for obvious reasons, chief of which is t h e need of recovering t h e cake as d r y as possible in most processes. T h e uses of filter presses are so varied t h a t great flexibility is required. One process will demand a filter capable of handling a thick sludge containing t w e n t y or t h i r t y per cent solids, forming cakes probably four inches i n thickness, which have t o be thoroughly washed; t h e next problem m a y consist of clarifying a viscous fluid containing b u t traces of solids, so finely divided as t o make a thick cake a n impossibility. One problem calls for recovery of t h e solids only, t h e filtrate being of no value, while another rejects t h e solids as refuse after recovering t h e liquid content; still another requires recovery of both liquid a n d solid. Some plants make certain products at certain seasons only a n d m u s t use t h e same filter press for a n entirely different product a t other seasons. T o be a commercial success it would seem t h a t t h e modern filter press should lend itself t o these various require1 Presented a t t h e 6 t h Annual Meeting of t h e American Institute of Chemical Engineers, T h e Chemists’ Club, I i e w York, December 10-13, 1913.
ments a n d t h e construction be made adjustable as far as possible. As will be seen from t h e following description, t h e filter under discussion is a radical departure from t h e plate a n d frame filter press a n d is even more simple in construction. CONSTRUCTION
This filter has become known as t h e “clam-shell” t y p e due t o t h e shell being in t w o p a r t s hinged together t o open a n d close somewhat after t h e manner of t h e clam-shell bucket used on steam1shovels.
COUNTER-WEIGHTED PRESS OPEN
T h e .filter body comprises two semi-cylindrical members hinged together, one member being provided with a n inserted gasket which seals t h e joint between t h e tm-o t o form a mater-tight cylinder when closed.
X plurality of openings is provided along t h e t o p t o receive t h e stem or outlet nipple of t h e filter leaves. These outlets are spaced quite close together so t h a t t h e leaves m a y be closely spaced if desired, as is t h e case when t h i n cakes are t o be formed; b u t t h e leaf spacing m a y be increased t o a n y degree b y taking out p a r t of t h e leaves a n d stopping off t h e intermediate outlets. Variable spacing is quite essential, for i t is not always possible t o ascertain in advance exactly what spacing will be best suited t o t h e material; a n d further, there is always t h e possibility of t h e process being modified t o change t h e filtering properties of t h e material a n d make a change in leaf spacing desirable. T h e filter leaves a r e usually made of wire screen, well crimped t o provide ample drainage. T h e screen is bound at t h e edges t o stiffen t h e construction a n d protect t h e cloth. An outlet nipple with a side open-
Peb., 1914
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
ing furnishes a n exit for t h e filtrate a n d means of holding t h e leaf in t h e filter body. T h e filter cloth covers t h e entire metal construction of t h e leaf except t h e outlet nipple a n d is held on either with metal clips or b y being stitched around t h e edges. T h e leaves are held in place in t h e filter b y a cap n u t which engages t h e t o p of t h e nipple a n d draws t h e shoulder of t h e leaf tightly against t h e rubber washer which fits in a recess inside t h e filter body. T h e side outlet of t h e nipple aligns with t h e filtrate delivery fittings on t h e outside of t h e press. T h e accompanying cross-sections of t h e filter body clearly show how t h e filter leaves are held in place
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seemed t h e great problem in working o u t this improvement was t h e need of accurate hinge adjustment t o t a k e u p wear in t h e gasket. This problem like many mechanical problems proved t o be simpler t h a n was anticipated, for i t was solved b y simply making t h e hinge pins into one continuous shaft reaching from end t o e n d of t h e press, a n d turning t h e bearing which received all of t h e hinges on t h e upper member eccentric t o t h e normal axis of t h e shaft. This construction not only affords adjustment, b u t makes
COCNTER-WEIGHTED PRESSCLOSED
b y t h e nipple at t h e t o p . Each leaf has separate outlet fittings comprising shutoff cock, glass delivery t u b e , etc. These fittings allow t h e operative t o see t h e filtrate from each individual leaf as it flows through t h e glass, a n d a n y leaf t h a t might become injured or require renewal of cloth is indicated at once b y a t u r b i d filtrate in t h e glass. I n this event t h e shutoff cock governing t h e defective leaf is closed until t h e filter is opened a n d t h e leaf replaced. T o remove a leaf only t h e cap n u t has t o be removed-the outlet fittings are not disturbed. I n t h e earlier designs of this filter t h e t-wo halves of t h e shell were held in closed position b y hand-operated swing bolts. These were fairly quick a n d convenient t o operate b u t still left room for improvement. It was desired t o modify t h e construction t o a n ext e n t t h a t would reduce t h e time t o unlock a n d open t h e filter from a few minutes t o a few seconds, if possible. This was accomplished b y a very simple change in t h e construction. T h e hinges, which formerly took no strain while t h e filter was in locked position, were increased in strength a n d number a n d made t o t a k e t h e strain formerly t a k e n b y t h e swing bolts on t h e back of t h e filter body. T o eliminate a n y possibility of breakage, t h e hinges are made of steel, which can be fully depended upon for strength. W h a t
Y CROSS AND
LONGITUDINAL SECTIONS
OF SWEETLAND
FILTERPRESS
adjustment of all hinges simultaneous b y merely turning t h e hinge shaft slightly when adjustment is necessary t o draw t h e upper a n d lower halves of t h e shell closer together a s t h e gasket gradually flattens out from continued use. T h e shaft is held from turning by set screws, which are loosened when adjustment is made. An eccentric shaft with swing bolts rests in babbitted bearings along t h e front of t h e upper half. Each bolt has a n adjusting n u t a n d lock n u t for initial adjustment a n d for taking up wear. T h e swing bolts are tightened or loosened simultaneously, due t o t h e eccentricity of t h e shaft, when t h e latter is turned through a n arc of 180°, a n d means is provided for swinging t h e bolts clear of t h e lugs on t h e lower half just at t h e point when t h e y have loosened sufficiently t o disengage themselves. This arrangement makes i t possible t o open or close a filter press of moderate size in a fern- seconds. T h e lower half of t h e shell is poised with counterweights t o facilitate opening a n d closing.
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
I n large machines a n eccentric shaft is used on both front a n d back, a n d a hydraulic cylinder operates t h e two simultaneously by means of a crosshead a n d connecting rods. The lower member in this case is swung open a n d s h u t by a hydraulic cylinder a t either end which is so arranged t h a t t h e shell is opened so wide a s t o invert it a n d t h u s clean itself of a n y cake t h a t might have fallen before it was fully opened. OPERATION
In operating t h e filter t h e liquid t o be filtered is forced into t h e filter body under pressure, expelling t h e air a n d entirely submerging t h e filter leaves. T h e liquid filters through t h e cloth a n d deposits t h e suspended solids on t h e outside of t h e leaves, forming t h e “cakes.” Filtration continues until t h e cakes on adjacent leaves are separated only by a narrow space which may vary in width from l / g inch upward. When t h e cakes are formed t o t h e proper thickness t h e wash water valve is opened a n d t h e sludge valve closed simultaneously. The wash water enters
1701.
6, No. I”
testing samples of t h e washed cake from various parts of t h e press. All filters of t h e leaf t y p e wash by “direct contact” methods; t h a t is, t h e wash water is in direct contact with t h e cake; but t h e methods of washing t h a t have heretofore been practiced with leaf filters whether vacuum or pressure are indeed anything b u t direct. The usual method of forming a n d washing the cakes on filter leaves is as follows, assuming t h a t t h e cake has t o be washed first with weak liquor a n d then with water:
1st. Fill filter tank. 2nd. Deposit cake on leaves. 3rd. Withdraw excess solution. 4th. Fill filter t a n k with weak solution. j t h . Wash cakes with weak solution. 6th. Withdraw excess weak solution. 7th. Fill filter t a n k with water. 8th. Wash with water. 9th. Withdraw excess water. 10th. Discharge cakes from leaves. Here are t e n distinct operations, which are still further complicated by t h e fact t h a t t h e cakes will not cling t o t h e leaves unassisted, b u t must be held on with either compressed air or vacuum during t h e time each quantity of excess is withdrawn. I do not dispute t h a t t h e cakes can be washed well b y this method, b u t raise t h e point t h a t it is a n extremely round-about method of accomplishing a simple object. Most of t h e power consumed is not devoted t o filtration b u t t o transferring solutions, much of t h e time is not devoted t o filtering b u t t o waiting for transfers t o be made, a n d t h e operator is kept busy manipulating valves.
HYDRAULICALLY OPERATEDPRESS OPEN
t h e space between t h e cakes, forcing t h e small quantity of unfiltered sludge t o deposit its solid component upon t h e cake surface. The space between adjacent cakes is now occupied by water which, being under pressure, percolates through t h e cakes, driving t h e strong liquor ahead a n d effectively displacing i t until t h e cakes are washed t o t h e desired degree, which is indicated by frequent tests of t h e effluent.
I feel justified in enlarging somewhat upon t h e subject of washing t h e cakes, for therein lies a great difference between this a n d other leaf filters. It should be noted t h a t there are no chambers or containers t o be packed solidly full of cake. The spaces between t h e leaves are not packed solidly full, b u t a cake is deposited on t h e surface of each leaf a n d these cakes are allowed t o build up until only a narrow space remains between t h e m , a n d in this space lies t h e secret of successful automatic discharging of t h e filter, a n d efficient washing. The cakes have been formed under equal conditions, t h e sludge being evenly distributed a n d t h e pressure equal throughout t h e filter chamber; therefore, t h e cakes m u s t ’ b e of even thickness a n d density a n d must wash evenly. T h a t this actually occurs has often been demonstrated by
The delay in filtering necessitated b y t h e many transfers materially cuts down t h e d u t y of t h e filter, a n d t h e process becomes a complicated one, requiring many t a n k s a n d much accessory apparatus t o t a k e care of excess solutions. Nor is this all. The transfers of excess solutions referred t o introduce other complications. The cakes must be supported upon t h e leaves a t all times by pressure. Air pressure (whether produced by suction or otherwise) must be used during t h e transfers. If too much pressure is used, t h e cake will be cracked (due t o t h e shrinkage of partial drying) a n d t h e wash water will “short-circuit,” t h a t is, pass through t h e channels formed b y cracking. If too little pressure is used, all or p a r t of t h e cakes may fall from t h e leaves, which would preclude t h e possibility of efficient washing. Lastly, if t h e cakes are of a slimy character, the partial drying during t h e transfer packs t h e particles of slime, a n d makes it less pervious t h a n ever t o t h e passage of wash water. T h e origin of t h e many complications cited above is traced directly t o void or “ d e a d ” space in t h e filter t a n k or chamber which contains t h e filter leaves. If there were no dead space, there would be no excess solutions, a n d consequently none of t h e complications which a t t e n d them.
Feb., 1914
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
T h e “clam-shell” construction of filter body has no void spaces. It contains nothing b u t t h e filter leaves a n d manifold pipe. When t h e filter leaves are loaded with cake, t h e entire filter chamber is full except t h e feed channel for distribution along t h e b o t t o m , a n d t h e narrow space between t h e cakes for t h e entrance of wash water. There is, therefore, no excess solution t o handle. Filtration is continuous from t h e time cake-forming is commenced until washing is finished, a n d t h e process of filtering a n d washing is reduced t o t h e simplest possible form. When washing is finished, t h e cakes are partially dried with compressed air, t h e lower half of t h e filter body is swung open a n d t h e cakes are dumped into a car or conveyor underneath. T o assist t h e cakes t o dislodge from t h e leaves, t h e current is reversed b y turning s t e a m or compressed air inside of t h e leaves, which n o t only causes t h e cakes t o drop off, b u t benefits t h e cloth b y keeping t h e pores open. For t h e benefit of those who insist t h a t better washing can be obtained b y draining t h e filter body before introducing wash solutions or water, I would point o u t t h a t t h e filter construction here described offers every facility for carrying o u t this method, if desired; b u t after installing m a n y of t h e m , large a n d small a n d under widely varying conditions, a n d seeing trials made both ways, I strongly advocate t h e direct method without withdrawing solutions. If t h e material is very slow-filtering a n d t h e space between t h e cakes wide, t h e n i t is better t o introduce t h e wash water through t h e manifold pipe a n d let t h e water entering t h e t o p fill t h e chamber b y downward displacement, forcing t h e excess solution o u t of t h e drain valve a t t h e bottom. T h e water, being lighter t h a n t h e sludge, does not mix with i t , b u t floats on t o p during displacement. I n this case filtration is continuous, pressure is maintained in t h e chamber, n o t i m e is lost, a n d t h e cakes are constantly submerged. It is so frequently asked why t h e body of this filter was designed cylindrical instead of square t h a t I shall t a k e this opportunity t o point out t h e reason. Obviously t h e principle could be carried out in a filter body of square or rectanglar section. T h e reasons for cylindrical construction are: I . T h e cylindrical construction affords great strength whereas a n y shape with straight sides would require so much reinforcement t o give it t h e necessary strength as t o make it cumbersome a n d awkward. 2. T h e leaves are exposed t o view b y simply swinging t h e body open. 3. A circular leaf is conveniently supported from one point a n d is very strong a n d light. 4. If t h e chamber were square, t h e leaves would be inaccessible, or else would have t o be lifted o u t with t h e cover t o discharge, a n d this would involve mechanical difficulties. T h e circular form of leaf does not involve a s much waste in filter cloth a s might be imagined. There is t o be sure a slight waste of cloth cut from t h e corners b u t most of this has a n i m p o r t a n t use i n reinforcing
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t h e cloth a t t h e edges of t h e leaf; on t h e other h a n d , a large percentage of t h e cloth used in plate a n d frame presses is not used for filtering, b u t t o form a joint between t h e frames. If we figure i t o u t in a n y case, we find t h e percentage of cloth purchased t h a t is utilized as filter medium, is about t h e same for either shape. I n fact, t h e cost of cloth per unit of work done is much less in t h e case of t h e leaf construction, because t h e capacity per square foot is greater, a n d t h e cloth is n o t injured b y being squeezed between t h e frames. ,4s t o filtering mixtures containing coarse particles, these offer difficulty i n a n y t y p e of filter, due t o t h e coarse particles settling t o t h e bottom, making t h e b o t t o m of t h e cakes more porous t h a n t h e t o p a n d resulting in uneven washing. T h e remedy for this trouble is t o keep t h e contents of t h e filter chamber in a s t a t e of agitation. T o accomplish this, t h e feed p u m p is provided large enough t o supply t h e necessary pressure for filtration with additional capacity enough t o permit a restricted overflow t o r u n from t h e manifold pipe back t o t h e supply t a n k during filtration. New light is being thrown on t h e subject of filtration with almost every new installation, a n d problems are being successfully solved t h a t appeared a t t h e s t a r t t o be impossible of solution. T o cite a n instance: a sample of a heavy viscous liquid was submitted for test. It contained a g u m m y residue t h a t blocked t h e filter cloth from t h e s t a r t , t h e rate of flow of filtrate per square foot of cloth was b u t a few cubic centimeters per hour. a n d t h e cloth became so coated as t o defy cleaning b y a n y practical method. A s t h e solution only was required, a n d t h e residue was worthless, there seemed t o be no obiection t o adding some inert granular substance t o t h e liquid as a n aid t o filtration. A series of experiments was undertaken, adding various “filter aids ’ t o t h e fluid, a n d t h e outcome of this work was t h a t a n average r a t e of filtration of three gallons per square foot of cloth per hour was obtained a n d t h e g u m m y residual m a t t e r was sufficiently broken up t o permit t h e filter cloth t o be readily cleaned b y reversal of current. Upon completion of t h e tests, a filter was installed in t h e plant a n d has fully demonstrated t h e practicability of t h e method on a working scale. I t was found t h a t a n y one of several substances could be used as a n aid t o filtration, among which are fine sand, fuller’s e a r t h , kieselguhr, sulfate or carbonate of lime, sawdust, wood pulp, etc. I t is not argued t h a t such methods are applicable t o all cases, b u t there are unquestionably m a n y problems t h a t will find solution i n similar schemes, which make possible rates of filtration such as are usually obtainable only in sand filters, with t h e added advantage t h a t t h e leaf filter offers a n enormous filter area in a small space, combined with extraordinary ease of cleaning. Another method of filtering materials containing a g u m m y residue t h a t tends t o choke t h e cloth is t h a t of depositing a protecting layer on t h e cloth before commencing filtration. For instance, we m a y
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
first fill t h e filter body with a very t h i n slurry of carbonate of lime a n d filter this mixture for a few seconds until a film of lime, s a y l / 8 2 inch in thickness, has been deposited. This coats t h e cloth as completely and evenly a s a deposit could possibly be made. T h e lime slurry is t h e n drained off a n d filtration commenced. T h e coating of lime forms a complete film between t h e cloth a n d gummy residue, so t h e latter never even touches t h e cloth. When ready t o clean t h e cloth t h e reversal of current is effective because t h e coating of lime breaks away readily. It is frequently asked how we can tell when cakes of sufficient thickness have been formed, especially when t h e material filters a n d builds up t h e cake rapidly. This is not as difficult as would be imagined. T h e usual way is t o work a t a given pressure, say forty pounds per square inch, a n d p u m p for a certain number of minutes, determined b y practice t o form t h e required cake. Another way is t o p u m p a certain number of inches from t h e supply t a n k for each charge. As in all processes, t h e operative soon becomes expert, a n d can iudge from t h e general conduct of t h e filter when t h e charge is sufficient. T h e usual skepticism which attends t h e introduction of a n y new machine was encountered a t first, b u t so many installations are now in successful use a s t o establish fully t h e usefulness of t h e methods described. A noteworthy point is t h e fact t h a t many of t h e filters of this t y p e i n use are of much larger size a n d capacity t h a n i t is found practicable t o build filter presses. Many have been built with a capacity of 133 cubic feet of cake per charge a n d several others ranging from 80 t o 1 2 0 cubic feet per charge. One especially large one was built which dumps over z j tons of cake per charge. T h a t this apparatus is now far beyond t h e experimental stages is demonstrated by t h e fact t h a t a large beet sugar factory in California installed three large hydraulically operated machines for t h e 1913 campaign. They were installed almost a t t h e last minute before t h e campaign started and were a t once placed in service a t full capacity, a n d no difficulty was encountered, in spite of t h e fact t h a t none of t h e employees who operated t h e machines had ever seen one before. T h e time required t o open, clean a n d close one of these filters is about five minutes, which minimizes labor t o such a n extent t h a t one operator can t a k e care of several large machines, even though each machine has t o be cleaned hourly.
Vol. 6 , No.
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in a deep trench, a n d after lining this shell with firebrick a n d mineral wool and stopping t h e ends with 13-inch brick walls, it was covered with two feet’ of earth. The chamber was constructed so as t o be as
SIDE VIEW OF “COMBUSTION OF COAL”APPARATUS
nearly air-tight as possible, a n d in such manner as t o retain t o t h e greatest degree practicable a n y heat generated within its walls. A t one end a motor-driven f a n will blow in air a t a r a t e which can be accurately measured, a n d a t t h e other a stack is provided which can be opened or closed a s desired. Through t h e top of t h e chamber, a t frequent intervals, pass small pipes for withdrawing samples of air or gases a n d for inserting pyrometers for temperature measurement in the interior. The chamber will hold 6 t o 8 tons of coal when onethird full. It is expected t h a t different kinds of coal or of t h e gob or waste material from mines will be placed in t h e chamber a n d a s t u d y made of spontaneous development of heat in t h e m under various conditions. After a n active fire has been started in t h e chamber
SWEETLAND FILTERPRESSCo. 149 BROADWAY, NEW Y O R K
IMPROVISED MINE FIRES ON AN EXPERIMENTAL SCALE Received January 15, 1914
T h e Bureau of Mines has recently installed a t its Pittsburgh experiment station a n underground chamber or furnace in which t o carry on experiments relating t o mine fires a n d spontaneous combustion as occurring in mines. A section of steel t u b e cylindrical in shape, 6l/2 feet in diameter a n d 2 7 feet long, was laid on its side
CROSS SECTION OF “COMBUSTION O F COAL” APPARATUS SHOWINQ METHODOF COXSTRUCTION O F BRICKLINING
either by this means or artificially, experiments will be made on controlling t h e fire b y reducing t h e air supply or b y sealing it off entirely. T h e progressive changes in composition of t h e fire gases or of t h e air surrounding t h e coal will be followed a n d temperature measurements made, in a n endeavor t o apply t h e d a t a
