976
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 13, No. 11
F I L T R A T I O N SYMPOSIUM Papers presented before the Division of Industrial and Engineering Chemistry at t h e 62nd Meeting of the American Chemical Society, New York, N. Y., September 6 to 10, 1921.
Washing and Washing Ports in Connection with Chamber” and ‘‘Frame’’ Filter Presses
with the strength of the surrounding liquor in the manner shown by the equation
“
By Eustace A. Alliott MANLOVR, ALLIOTT & CO. LTD, 51 & 42 PARLIAMENT LONDON, ENGLAND
ST., WESTMINSTER,
Washing in the filter press is resorted to either (a) to recover a valuable liquor from the solid particles which retain it, or ( b ) to free such particles from impurities dissolved in the adhering liquor. Both requirements may of course be combined in one particular instance. I n the first case it is usually desirable to avoid dilution, and while this may not be so important in the second case, there is little doubt that methods which involve the formation of much weak liquor tend to allow some portions of material to receive comparatively little washing, and promote inefficiency. The ideal in all cases therefore is to use the minimum of wash which will give the results required, for which the primary conditions are that the cake itself should he uniform and in the proper physical condition, and that every section shall be washed a t an equal rate. It is proposed in the present paper to consider: (1) some of the physical factors which affect washing, (2) the various mechanical appliances for effecting it, and (3) some practical and experimental results. PHYSICAL FACTORS AFFECTINGWASHING The flow of wash through a filter cake certainly appears, as an experimental fact, to approximate in many cases to the rules which hold for capillary passages.l The flow is inversely as the cake thickness, and very nearly proportional to the pressure. This proportionality is very liable to be disturbed by the fact that the filter particles may become distorted or crushed under pressure, thus decreasing the area of the capillary passages, while the physical condition of the cake may change during washing. It is more difficult to show that the flow varies inversely as the fourth power of the passage diameter, and it must not be forgotten that these passages are not straight as required by the theoretical formula, but are very sinuous. Kevertheless, it is a matter of observation that the rate of flow diminishes sensibly and very rapidly with decreased size of capillary pores, and also in proportion to the viscosity. Nominally, washing consists of a displacement of mother liquor from the pores between the particles by an equal volume of wash, in which process little or no mixing of wash andliqu or takes place. Such perfection is, however, rarely obtainable, even in theory, as various disturbing factors occur, of which the following are the most important:
where S is the weight of the soluble matter adsorbed by W, the weight of adsorbent, C is the volume concentration of the surrounding wash after adsorption is complete, and K and n are constants, n being less than unity. The effect is proportional to the area of the boundary surface, and is thus of most importance in fine-structured or gelatinous precipitates, and where the cake particles are highly porous. Its general tendency is to multiply the proportion of wash required, since the amount of solubles retained decreases as a rule a good deal more slowly than the dilution of the washings. As the latter pass through the cake they displace the liquor loosely retained between the particles, but leave on the surface a t each point of the cake an amount of adsorbed solubles in equilibrium with the strength of washings a t that point. A pair of curves could therefore be drawn for the concentrations of adsorbed matter and wash for each succeeding layer, rising from zero a t the inlet face to values a t the outlet face dependent on the amount of wash and the cake thickness. wasw ‘ W X R
aK
T
c t
FEED’
T*
-
L FILTRATE
AND WASH W A T E R OUTLETS
FIG.~-SIPIIPLE WASHINGIN PLATE A N D FRAME FILTER PRESS
I n order to understand the effect of the latter factor, let us consider any thin section of the cake. It will be apparent that, the strength of the wash tailings will have fallen well below the original equilibrium concentration, i. e., the strength of the original mother liquor before the adsorbed solubles 1-Adsorption are appreciably reduced. Hence they can be further em%-Capillary diffusion ployed to remove a portion of impurity from a succeeding 3-Formation of chemical compounds 4-Colloid formation on removal of electrolytes layer, which therefore will not require so great an approADsoRPTIoN--Adsorption is the unequal distribution of the priation of additional wash as if it had been treated separately. solubles a t the boundary between the solid and liquid phases of The tailings from this can again remove a portion from a third the cake owing to surface energy effects, and in many cases layer, and so forth, each succeeding layer requiring a somea large excess of solubles is thus retained in the surface what smaller addition to the total amount of wash. It film and does not pass away with the displaced liquor. will be clear that a thick cake will require less wash than a This occurs only when the solubles in question tend to redu‘ce thin one, but the advantage, though considerable for the the surface energy, and the amount retained is in equilibrium first increases in thickness, does not increase a t the same rate 4 PI-P2 if further thickness is added; and it must be remembered 1 The flux in a capillary tube is given by F = (T),that very thick cakes are not always practicable because where PI and Pz ate the pressure heads a t either end, a is the radius of the the rate of flow is seriously diminished unless they are reasonpassage, .I is its length, and V is the viscosity of the liquid a t the particular ably permeable. The washings, however, may be graded temperature.
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 CHEMISTRY
Nov., 1921
FIG. THOROUGH WASHINGIN RECESSED PLATEPRESS
andLused in succession, beginning with a strength not too far removed from that of the original strong liquor, following this by a weaker one, and finishing with a small portion of pure wash. The introduction of weak liquor into a process may thus be reduced to minimum, even if adsorption effects are great. CAPTLLARY DIFFUSION-Absorption of mother liquor by capillary diffusion into the pores of the cake particles may take place. I n this case, the washings have to be dilute before its effectual removal begins, and a time factor, which may be considerable, also comes into play. I n the case of, say, iodine solution and charcoal, weeks may be required before an equilibrium is established. The effects are similar to, if more complex than, those of adsorption, involving an additional proportion of wash and a longer time.
977
COLLOID F O R ~ M ~ T I O X - precipitates ~ ~ ~ ~ ~ are held in the coagulated state only in the presence of suitable electrolytes, and revert to a finer structure or even disappear when these are removed. Hence, when wash enters a cake of such a nature and removes the electrolytes, the inlet face of the cake becomes highly gelatinous and impermeable. If the wash does succeed in penetrating deeper layers it meets a higher percentage of adsorbed electrolyte, and its content of electrolyte increases until enough accumulates to cause coagulation to occur again, choking the pores of the cake and rendering it still more impermeable. If washing is persisted in, a further decoagulation is obtained, and the cycle is repeated until the precipitate finally passes out of the press through the cloths. Here stage treatment with washes of graded strength has a peculiar advantage, as the entering wash always contains a little electrolyte, which is not completely withdrawn from the first layers till the bulk has been removed from the later ones, and the latter may therefore be washed well without decoagulation occurring. Thus i t is often possible to obtain purer cakes than by any other method, with reasonable consumption of wash and time. CONTROL OF QUALITY SIZE OF GRAIN-It will be clear that precipitates should not be of too fine a grain, in order that trouble due to surface action may be avoided, and that the cake pores may be sufficiently large to allow a reasonable flow. As a rough guide, 2 p may be taken as a limiting size of grain, above which washing is usually comparatively easy, and below which C
A
D
C
D
B
B
FIG.4: A-FEED
~
INLET;
B-WASH
C-AIR OUTLET (C APPEARS); D - W A S B WATER WASHINGALL THE OUTLET
WATER INLET;
SHOULD B E CLOSED AS SOON AS W A S H W A T E R OUTLET;E-OUTLET FOR FILTRATE (WHEN
COCKS E SHOULD B E CLOSED)
F ~ G3-PLATE
t
WITH AND
FEEDAND
W A S H CHANNEL IN
OUTSIDE L U G S
HAVING RUBBER JOINT RINGS
CHEMICAL CoMBIKATIox-Further, the solubles may be retained by chemical combination with the substance of the filter particles, such compounds being stable only when the concentration of the liquid phase is in excess of a certain critical amount. n'one of the solubles will then be removed till the washings become diluted to this critical concentration, which will be maintained till all the unstable compounds have came away. Such cases often occur in fine chemical manufacture and are especially to be observed where organic bases are isolated as picrates. The excess of wash required varies directly as the quantity of solubles retained] and inversely as the critical concentration.
troublesome conditions begin. In this connection it must not be overlooked that small! precipitated grains are more soluble than large ones, and it is often possible to control the quality of a precipitate by prolonged digestion in contact with mother liquor. The small grains tend to disappear, and the larger ones to increase. The user has a valuable means of control ready to his hand, which is often overlooked, but which may have more bearing on good results than the most perfect mechanical appliance. The varying degree of attention given to this point renders almost impossible any close comparison of results obtained in various works with different types of plant. I n extracting soluble matters from ores and earths fine grinding is usually advantageous, as it permits the free escape of the solubles, which should be brought into solution before the liquor is pumped to the press. It is still desirable not to form more very fine slime than can be avoided, and prolonged boiling is helpful in bringing the particles into the best condition for filtration and washing. TEMPERATURE-It must not be overlooked that increased temperatures are always desirable where permissible, as viscosity is generally very appreciably reduced by their use, permit,ting a more rapid flow, while adsorption is usually decreased. .'. I
T H E J O U R N A L OF I N D U S T R I A L A N D EA‘GIXEERING CHEMISTRY
978
fums -
WAan I N L n P U T E F I G . 5--THOROUDH
WASHING WITH
METHODS OF WASHING We now come to the question of the manner in which the wash is best applied to the filter cake. The rarious mechanical disturbances that are liable to occur will be dealt with a t the same time. SIMPLE WASIIING-The simplest method, known as “simple washing” (Fig. l),is to use the feed passage for the admission of the wash water. For the best results the cake should not be fully formed, but left incomplete with a hollow center, into which the wash passes, driving out the unfiltered liquor and itself passing through each face of the cake. Various users report that they are able to get satisfactory results even if the cake is solid, but it is clear that, in this case, there are possibilities of uneven results, as there will be very appreciable resistance to wash passing any distance through the solid cake, and it will escape more readily near to the point of admission. Where the cake is not fully formed the wash has free access to every part, and there is also an automatic tendency during filtration to equalize the resistance of all portions of the cake. While there is such a tendency, i t does not, however, necessarily follow that it can work out fully unless conditions are favorable, and as soon as the two halves of the cake touch a t any point, it ceases to exist. Unfortunately, there are disadvantages connected with this method of washing. Care has to be taken that just the right amount of liquor of correct strength is pumped into the press, and space and press capacity are of course wasted and more labor is required for emptying out than if a firm cake were to be formed. This method is therefore usually reserved for materials which form a solid cake with difficulty. It also is a useful stand-by if i t becomes necessary to wash when no specially constructed presses are available. The position of the feed passage affects this method very little,
RUBBFR
ANCS
R G . 6-MBTHODS
CLOTH SLEEVE OF
MAKING JOINTS
Vol. 13, No.11
ON
EXTERNAL FLANGBS
but a bottom feed permits the remaining strong liquor to be removed from the press before admitting the wash, which otherwise would dilute it, and the chambers can also be thoroughly drained before opening the press. This may, however, be done by compressed air, if available, while with materials tending to settle rapidly the port from a bottom feed might become silted up.
EXTERNAL LUG,.
THOROUGH WA Him-Another method is to force the wash right through the cake from one face to the other, and this may be referred to as “thorough washing” (Fig. 2 ) . I n its simplest form there is a wash-admission passage in the rim of the press, with ports connecting it to the business surface of each alternate plate. Before comnxncing to wash, the taps on the wash inlet plates are closed, while those on the intermediates are left open. The wash must now pass through the cake and cloths to the intermediate plates, where it escapes through the taps. It will be seen that in this case a fully formed cake is necessary. A serious defect in this method, which is largely in use on old pattern presses and is now becoming obsolete, is that air is likely to be trapped in the filter surface, or may leak back through the taps a t the close of filtration. This cannot always escape readily through the joints, especially as wash pressures are not always high. Thus the wash is prevented from penetrating the upper portion of the cake. One way of avoiding this difficulty is to provide an air outlet tap a t the top of each chamber. This is done in some modern presses (Fig. 3). There is, however, still a defect, which in some cases will lead to unequal washing: namely, the wash can run straight out of the taps after it has passed through the cake and does not necessarily form a wall of water behind the latter similar to that on the inlet side. This means that a t the bottom of the cake there is an unbalanced hydrostatic pressure which may lead to an undue amount of wash passing through a t this point. When it is understood that in some instances the total head desirable for washing may amount to only a very few feet of water it will be seen that this may be a serious factor. It is further to be borne in mind that although recessed plates .entail less labor than plates and frames, they are inferior to the latter for washing purposes, inasmuch as the edge of the cake is necessarily thinner than the remainder, allowing a greater flow of wash, which entails mixed washing. Further, in recessed plates, the feed channel is necessarily inside the rim, and in nearly all cases the feed is internal to the cake itself. There is no special reason why the wash should pass through this core of material in the feed passage if it is solid, while if it is soft an opportunity may offer for short-circuiting. How can all these factors be taken care of? For the best results it would appear necessary to secure the following conditions:
1-The press should be of the plate and frame type. 2-Means should be provided for air elimination. 3-The wash must leave a t the top of the press, SO that the cakes are immersed in a bath of wash eliminating any unbalanced hydrostatic head.
Presses in which all these conditions are observed m y be referred to as of the “thorough washing” type. I n a common form, the wash inlet passage is arranged inside the rim of the press a t the bottom corner and the top corners are occupied by the air and wash outlet passages, respectively. The wash first passes through connecting ports to the filter surfaces of alternate plates rising behind
T H E JOURhTAL OF IiVDUSTRIAL A N D ENGINEERING CHEiMISTRY
Nov., 1921
I
II
I1
FPS.'I--sECTION OF
I
'I
SHOWING H O W RUBBERR l N G S ARE INSERTED. MAKING JOIKTS O N E X T E R N A L FLANGES
METHOD
the cloths and driving out any air into the air passage, which is connected by ports to the wash inlet plates only. As soon as water overflows from the outlet cock of the air passage, the latter is closed, and the wash must then pass to the intermediate plates, where it rises and overflows into a wash o u t k t passage which also provides a means of escape for any trapped air Crom the intermediate plates. Fig. 4 shows this method applied to wood plates and frames. All the holes, but one, are a t the top of the plate, which avoids difficulties due to shrinkage and simplifies adjustment of the cloth. I n ai1 alternative method of construction, the holes are all in orre side piece of the wood plate, which facilitates and cheapens repairs and renewals. I n cast-iron presses, in cases where the cIoths have to be frequently changed, it is worth while to go to the additional expense of having the passage formed in external ears in the manner shown in Fig. 5 . The joints on the ears or external lugs are made by means of cloth sleeves (Fig. 6), which are provided with the necessary holes corresponding to the various passages. The main cloth covering the face of the plate is thus quite plain and may be speedily taken off and replaced, as there is no need to adjust it with regard to the various passages, as is the case in the type formerly mentioned. Further, if shrinkage of the cloth occurs, it is not of any consequence. Against these advantages there is of course the additional expense of the cloth sleeves, and the plates themselves are more costly to manufacture. It is therefore not usually worth while to instal this particular type unless cloth renewals are likely to be fairly frequent. The cuffs or sleeves will want renewing from time to time; but if care,is taken in cleaning out the ports, a sleeve may outlast several cloths. A case came to the author's notice /= WASHINGS
WASH INLET
I
PW. 8-m'AS€IlNG
FILTRATE I
P R S S S W I T H SPECIAL
TAPSFOR
WASH
_
OUTLETS
979
where the sleeves on the air and wash outlets averaged twelve months' life, as against three months for the cloths. The sleeves on the feed ports did not last quite so well on account of the additional cleaning required. Where rubber joints are permissible, they may be used instead of the cloth sleeves. One method (Figs. 6 and 7 ) is to fit rubber rings into grooves in the lugs, which are made with slightly tapered sides, while an alternative practice is to make the passages themselves rather larger, and to fit in rubber rings which project slightly on either side. Such rings are as a rule made with a slight lip on the interior a t each end, as this tends to make a better joint under pressure, but quite good joints can, as a rule, be made even with plain rings. This loose type of ring is more readily renewed, but the others are better from the point of view of ease in cleaning the passages. The rings are of course fitted to lugs which have no connecting port to the plate or frame. I n the above instances of thorough w'a sh i ng arrangements the wash leaves by an enclosed outlet. Should it be desired to control the washings from cach separate chamber, wash outlet taps may be fitted a t the top of the plates, delivering into a special light gutter (Fig. €9,which can be lifted awav when it is desired to clean out the press; but while this is perhaps the most, perfect arrangement possible from the point of view of the control of washing, morelabor and attention are involved and the enclosed outlet type is more usually adopted. It is interesting to note some FIG. D DIAGRAM SHOWIKG ARRANGEMENT on FLANGED FRAMES, PREVENTING LEAKAGE of the possibilities OF W A S H ROUNDEDGESOF C A K E of these thorough mashing presses with enclosed wash outlet. There are a dozen or more ways in which the passages can be used. For instance. it is perfectly feasible to subject the cakes to a reverse wash by using the ordinary wash outlet as inlet passage, and the filtrate taps or the air passage as outlet,; and the cloths may be cleaned by subjecting them to a reverse flow of water, air, or steam when the press is empty. The press can also be used without alteration if it is desired to take the filtrate through enclosed outlets instead of through the taps. The only requisite for these various uses is that the necessary piping connections be made to the fast head. A fruitful source of unsatisfactory washing is leakage around the edge of the cakes. The slightest shrinkage of the cake will give rise to this, but it may be of serious consequence in case? where shrinkage is not a t all apparent.
-
980
T H E J O U R N A L OF I N D U S T R I A L ANDZENGINEERING C H E M I S T R Y
Aurow
II
h&rx (MNIV*SO A S D , r e r r c r m m ?
Vazunrs)
FIG IO-COMPARATIVEWASHINGRESULTSFOR FLANGED AND FRAMES FOR SPENT OXIDE CAKE2 IN. THICK
UNFLANQED
PRACTICAL AND EXPERIMENTAL OBSERVATIOKS H. E. Williams, faced with the difficulty of dealing with a large volume of weak liquor from filter-press washing, carried out some investigations on this problem. A highly colored solution was used as the wash, and it was clearIy demonstrated that leakage was taking place a t the junction of the edges of the cake with the internal edges of the frame. Since the plane of this junction was parallel to the direction of the wash pressure, there was no possibility oi forming an effective seal. The problem was finally solved by making an internal flange or rabbet on the wash outlet side of the frame (Fig. 9). Several filter presses fitted with this special type of frame were in use for some time in the extraction of prussiates from spent oxide, and comparative figures were obtained on a works scale from chambers fitted with both the flanged and unffanged type of frame. The material was ground with weak liquor, limed and filtered a t 60 lbs. per sq. in., and containing about 34 to 36 per cent of liquor. This was washed with water a t 80 lbs. per sq. in. The pressure of the wash, which should be applied as rapidly as possible, forced the cake against the flange or rabbet on the wash outlet side of the frame, forming a very efficient seal and completely preventing any leakage round the edges of the cake. Each cake was 2 in. thick, and the retained liquor contained 9 . 2 g. of prussiate of soda per 1OOcc. I n Fig. 10 the actual displacement of solid matter is plotted against the volume of wash liquor, and demonstrates very forciblv the advantage gained" by the use of flange. The nercentFIG. DIAGRAM OF HYDROMETER ATTACH- age Of so'ub1e matter left in the cake is RISO MBNT ON WASH WATER OUTLET shown for both cases. The average strength of the washings was 6 . 6 g. of prussiate of soda per 100 cc. for the complete washing with the flanged chamber, but for the unflanged chamber the strength was only 3.2 g. per 100 cc., while less than half the soluble matter had been removed. LOCATION OF LEAKACE--MF.
'>
Vol. 13, No. 11
It was found that several years ago a patent had been taken out for a very similar device, which apparently has not received the recognition it deserved. It is surprising that it has not been generally adopted, as the slight extra labor involved in the cleaning may be very amply compensated by the increased strength of the washings. The author's own experiments confirm the fact that leakage may exist a t the cake edges even in the case of SO easily washable a cake as spent lime. I n tests on 2-in. cakes with colored washes the edges were stained through, and color issued before the cake was half penetrated in the center. It should be borne in mind that cakes which go moist when shaken in the hand are specially liable to shrinkage through any slight disturbance, and are then more prone than ever to allow wash to short-circuit a t their edges. This is due to the fact that when the particles of the cake consist of hard grains which are actually in contact, the most compact formation is not assumed in the first instance, and rearrangement is likely to occur. PESTING STRENGTH OF WASHINGS-TO obtain the best results it is well not to rely on washing for a given period, or putting a given amount of liquor through the press.
"'*YIT,
F I G . 12-WASHlNG
CARBONATE DF LIMEFILTERC A K E WTTH COLLOIDAL SILICA)
(CONTAMINATRD
Regular tests should be made of the strength of the washings, and the process should be stopped RS soon as these fall off. For this purpose it is well to have a hydrometer bowl (Fig. 11) fitted to the press head, so that readings may be taken continuously as the washings issue. It is also a convenience to fit a thermometer, and where the gravity of the washings is not a reliable guide, it is often possible to devise a simple chemical test which can be applied even by an unskilled operator. SELECTION OF CLoTHs-The l a t e and efficiency of washing is affected by the cloth. The essential point is that the cloth shall not be finer than is requisite to give a clear filtrate after a small amount of liquor has been pumped through. If this precaution is observed the cloths will clog far less readily and will not require renewal so often. If fine cloths are requisite they should be of light weight and supported on strong backings of open weave. PRESSURE APPLmD-Probably the conditions under which the cakes are made have as much influence on good washing as any other factor. If the cake is incorrectly formed the most perfect mechanical arrangement will not give satisfaction. The first point is the pressure a t which the cake is finished off, which should be high enough (except in the case of simple washing) to give a firm and complete cake.
Nov., 1921
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
WASWINGS U U U L A T E D AS PERCENTAGE VOLUME
981
OF
E O T W e R LIQUOU IN CAKE
FJG.IS-WASHING
m'EAK CACSTIC S O D A SOLUTIONS X'KOM CARBONATE O F
Excessive feed pressure, however, may render the cake unduly impermeable, especially if it is inclined to be gelatinous, while channeling of the cake and short-circuiting is very liable to occur. Further, a time element is necessary to allow for diffusion of the solubles out of the interior of the particles if-these are porous, and this is not necessarily reduced by a rapid flow. I n an instance of washing tin slimes containing wolfram, where the cake had been consolidated a t 80 Ibs. per sq. in., the best results were obtained with a wmh pressure of only 4-ft. head of water. While high pressures are requisite in some cases, washing should be carried out, in general, slowly and at the lowest practical pressure. FEEDING O F FILTEiz-often the buyer objects to paying for a suitably large pump, so that this portion of the plant is usually cut down to its finest limits, Although this minimizes pumping of the liquor through the relief valve in the later stages of the pressing operation, time is given for selective settling in the press, resulting in the larger particles being in excess a t the bottom of the cake and the finer ones a t the top. This gives rise to varying permeabilities, washing does not proceed on a parallel front, and unsatisfactory results occur. A similar effect is very likely to result through any stoppage or disturbance in the pumping, due to a broken belt, change of shift, or other reason. It is therefore essential for good washing that the cake shall be formed at maximum rapidity with a steady and regular flow. The coarser particles are then uniformly distributed among the more slimy ones and perform their proper function of keeping the whole cake more open and porous. There is no doubt that from
LIMEC A K E
(EXPERIMENTAL)
this point of view by far the more satisfactory results are obtained by feeding by means of compressed air, using a monte-jus or sludge ram, which it may occasionally be desirable to fit wit,h an agitator. The ram should be of a suitable size to charge the press in one operation, but if the volume of liquor is too great it is an advantage to have a second ready to start feeding as soon as the first is empty. As a result of the perfectly steady feed pressure, the cake itself is likely to be more open and permeable than that obtained with a reciprocating pump, even though the latter were of suitable size. The pulsations, especially of a single-acting pump, tend to disturb the particles in the cake, assist them to settle into their most compact and least permeable structure, and may also cause them to clog the cloth, so that the penetration of the wash is made difficult. If plunger pumps are used they should have an air vessel of very ample size. It should be noted, however, that a compressed air feed will give the very high rate of flow it is often possible to obtain when the press is first being started up, even a t comparatively low pressures, without the use of any relief valve or other troublesome working part, since the larger volume of the air a t lower pressures compensates for the extra duty. In one instance which came to the miter's notice, in washing a cake of rather sandy nature in which some slime was present, it proved impossible to attain a lower limit than 2 per cent of soluble matter, no matter how much time was taken or how great a quantity of wash was put through. When the small plunger pattern feed pump in use was replaced by a monte-jus fitted with a suitable agitator, no difficulty was
982
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
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 mas 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.
