FILTRATION AND CLARIFICATION Presented before the Division of Sugar Chemistry and Technology at the 102nd Meeting of the American Chemical Society, Atlantic City, N. J. (Pages 403 to 429)
Clarifying Efficiency of Diatomaceous Filter Aids A.B. CUMMINS Johns-ManvilleResearch Laboratories, Manville, N. J.
Some relations for filter aids are shown by calculations of particle size, specific surface, cake porosity, etc., and by comparison of the results obtained in the filtration of particles of known character and particle size. Filtration tests made with simple dispersed systems, such as carbon black, metallic oxides, wax emulsions, latex, etc., emphasize some of the complicated relations between the type of filter aid employed and the degree of clarification obtained.
The role of filter aids in “clarification filtrations” and some of the relations between several types of filter aids and different kinds of suspended particles are considered. Data are presented to show the correlation between the particle size and shape and other properties of a number of diatomaceous silica products, and the results obtained in the filtration of liquids containing suspensoid or emulsoid particles of different types.
F
ILTRATION may be defined as the process of separating suspended matter from liquids by forcing the liquid through a porous member which serves to retain the solid particles. The material collected on the porous member is known as the filter cake. Difficulties in filtration are generally of two types-low rate of flow and inadequate clarification. If the nature of the solids to be filtered out is such as to form an open or porous cake, there is little difficulty in the filtration from the standpoint of the rate of filtration and generally little trouble in obtaining satisfactory clarification. If, however, the solids are colloidal or gelatinous or tend to form tight nonporous cakes, then the filtration becomes more difficult. With many materials i t is hard to obtain the requisite degree of clarification, and the rate of flow may be undesirably low. Raw sugar solutions contain impurities of these types, and the satisfactory filtration of sugar liquors has always been one of the more troublesome steps in sugar refining. A filter aid is a material which serves to obtain or improve clarification or increase the filtration rate, or both. I n sugar refining it is necessary generally to employ a filter aid in order to use successfully any one of the commercial types of pressure filters. Diatomaceous silica is the most widely used type of
filter aid, and most American sugar refineries and beet sugar factories have utilized it for many years. The fundamental function of a filter aid is to provide a porous cake structure. ThiR is built up simultaneously with the deposition or retention of the suspended particles. The particles of the filter aid, therefore, must form a rigid skeleton or lattice structure which is capable of entraining the gelatinous or fine particles of the unfiltered liquid and yet leave channels in the filter cake through which the pure liquid, freed of its suspended matter, may pass at a rate and under such pressure differentials as are practical economically. The filter aid particles should be of such character as to remain suspended in the liquid prior to the filtering step and must be microscopic in size to provide cakes which will filter out the small particles of colloidal suspensions. I n addition, a filter aid must be free from objectionable impurities such as soluble salts, organic matter, coloring materials, taste- or odor-imparting bodies, gritty particles, etc. It must be also inert or unreactive in the material being filtered. Diatomaceous particles are of many different types, Rome of which are markedly more efficient than others for filtration purposes. Similarly, liquids to be filtered are greatly differ-
403
INDUSTRIAL AND ENGINEERING CHEMISTRY
404 -----PERCENTAGE
I
I
20
I
COARSER
I
40 PERCENTAGE
I FINER
THAN
I
60
I
1
80
I
I
100
THAN-
Figure 1. Particle Size Distribution of Celite Filter Aids ent and the solids to be removed are of many types. With raw sugars, important differences in the colloid contents and filterabilities are found for different sugars. As a result, different types of diatomaceous filter aids have been developed and are employed for various applications, depending upon the relations between the filter aid particles and the particles of the suspension. Filter cake structures of different size openings or interstitial channels are provided by varying the size of the diatomaceous particles, but such factors as size distribution and particle shape are of significant importance. Another paper (4)describes methods and apparatus for laboratory scale filtration tests, including both the rate of filtration and the evaluation of the degree of clarification. Differences in filtration results as influenced by the kind of sugar and the type and amount of filter aid are discussed. By the use of coarse particle-size filter aids and a suitable amount of calcium phosphate precipitate ( I ) it is possible under certain conditions to secure a satisfactory rate of filtration, a high degree of clarification, and a substantialcolor removal. Consideration of the theoretical aspects of filter aid filtrations is beyond the scope of the present paper. I n most commercial filtrations which require a filter aid, the best conditions for operation are usually established by small-scale filtration tests and subsequent study of the filtered liquid. The quality factor of the filtrate and its suitability for further treatment is frequently more important than the filtration rate. Filtrate quality is difficult to evaluate since it varies widely for different materials and is still more difficult to express in formulas which are of practical value. It is proposed later tb consider the subFigure 2. ject of filtration and the use of filter aids from a somewhat more fundamental standpoint. Work under way has been directed to a study of filter aid filtrations in the hope of developing further the mathematical theories of filtration. A number of valuable contributions have been made in this direction, but it appears that further correlation between filtration formulas and practical working conditions is desirable.
General Properties of Celite Filter Aids Celite filter aids cover a range of filtration requirements and are available in grades which differ in the character of
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filter cakes formed. They are particularly suited for a study of this kind, since Celite filter aids have all been prepared from the same type of diatomaceous raw material-i. e., marine plankton diatomite of high purity and uniformity. As will be pointed out later, the diatom forms of this earth comprise elongated, filiform, spicular, or acicular shapes, discoid forms of different sizes, fine particles of different kinds, and some accessory forms. (Products of one manufacturer only are reported here.) Celite is composed principally of a mixture of different sizes of filiform or needle-shaped diatoms, disk-shaped diatoms, and fragments of each. The filiform, or spicular, diatoms are mainly Synedra, Thallasiothrix, Thallasionema, and Kitzschia. The disk diatoms are principally Coscinodiscus, Arachnoidiscus, Xctinoptychus, ilulacodiscus, Auliscus, and Asterolampra, of which Coscinodiscus is the more prevalent discoid form. The percentage of filiform to disk shapes varies somewhat for different strata of the deposit. Some contain as high as 75 or even 85 per cent of filiform shapes, whereas other strata may run as low as 15 to 20 per cent filiform types or fragments thereof. Over two hundred varieties of diatoms have been noted as accessory forms in Celite. The residues other than those of diatoms are of silicoflagellae, radiolaria, and sponge spicules. Some of the accessory forms of diatoms are Savicula, Biddulphia, Dicladia, Xanthiopyxis, and Rouxia. The principal impurities of Celite are very fine quartz grains, clay particles, amorphous silica, traces of inorganic salts, and small amounts of organic matter. The total amount of all these impurities is low for the commercial grade crudes, averaging about 2 t o 8 per cent. The impurities are greatly reduced by processing in the manufacture of filter aids. Table I lists some of the general properties of six Celite filter aids which cover the range from fine to quite coarse particle size. Filter-Cel is prepared from natural Celite crude and contains some free and all of the combined water in the original opaline silica. Standard Super-Cel is a calcined powder of a light salmon pink color. Hyflo Super-Cel, Celite 503, Celite 535, and Celite 545 are all white powders
Photomicrographs of :Filter-Cel (/eft) and Celite 545 (right) at 200 Magnifications and are flux-calcined. The apparent densities or “loose weights” of these products do not vary greatly, but become progressively higher vith the increase in particle size. I n cake density all products are about the same, usually 17 to 22 pounds per cubic foot, depending upon the procedure in forming the cake, pressure cycle, concentration of filter aid, rate of filtration, type of liquor handled, etc. Twenty pounds per cubic foot is the value generally accepted for filter cakes in sugar filtration. A cake density of 20 pounds per cubic
”
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405
TABLEI. GENERALPROPERTIES OF CELITE FILTER AIDS Condition
Color
Density, Lb./Cu. F t . Apparent Cake
Filter-Cel
Natural
Gray
8.5
17-22
Standard Super-Cel Hyflo Super-Cel
Calcined Flux-calcined
Pink White
9.2 9.7
17-22 17-22
Celite 503
Flux-calcined
White
10.5
17-22
Celite 535
Flux-calcined
White
11 .O
17-22
Celite 545
Flux-calcined
White
11.5
17-22
Mixed forms &fragments of elongated, filiform, & discoid diatoms; many fines Same with less fines Same: with microscopic aggregate structure, few fines More microscopic aggregate structure, practically no fines Still more microscopic aggregate structure, no fines Pronounced microscopic aggregate structure, no fines
foot corresponds to a porosity of about 85 per cent, which is the relative volume of openings for liquid and entrapped impurities. Since this is approximately the same for all of the filter aids, it means that the differences in the filter cakes lie in the number and size of the openings or channels. For Filter-Cel these are numerous and of small size, whereas with Celite 545 the openings are much fewer but of larger size. The character of the particles in these different filter aids is substantially the same, but the relative percentages of the different sizes and shapes are different. Filter-Cel, for instance, has an important percentage of small diatomaceous particles and fragments referred to as fines. Standard SuperCel has less of these fines whereas the faster flow rate products have few or none. I n the flux-calcined filter aids there is a secondary or multiple structure in the nature of porous microscopic aggregates or clusters, which is peculiar to filter aids as produced by flux calcination. The fines and smaller particles have been caused to adhere to the larger particles, and the result is an increase in the particle size. Celite 545 has much larger aggregates than Hyflo.
Particle Size of Celite Filter Aids Under the heading ‘General, Particle Size Distribution” in Table I, the ranges in particle sizes which constitute about 75 per cent of the filter aids are given. These figures serve to characterize the products. There is a definite increase in the particle size from Filter-Cel, which consists predominantly of particles from 1 to 12 microns, up to Celite 545 which contains particles mostly within the range 12 to 45 microns. Particle sizes as referred to here were determined by sedimentation methods (Od6n or pipet) and are expressed in “equivalent” micron sizes; that is, 20-micron particles are all those, irrespective of shape or configuration, which settle out of liquids at a rate equal to that of a 20-micron-diameter sphere of the same specific gravity. As will be shown later, many diatomaceous particles are extremely different in form, but the equivalent method of expressing particle size is highly useful and convenient when considering filter aids. Some figures are given in Table I for the specific surfaces of the filter aids. They vary from about 30,000 sq. em. per gram for Filter-Cel to about 6000 for Celite 545. These
PARTICLE SIZEDISTRIBUTION OF TABLE 11. APPROXIMATE FILTER AIDSOBTAINED BY SEDIMENTATION METHODS --Microns FilterCel 2.5 8.0 14.0 19.0 37.5 19.0
Standard, Super-Cel 4.5 10.0 20.0 24.5 33.5 7.5
(Equivalent Sphere Sizes) Hyflo Celite Celite Super-Cel 503 535 12.0 6.0 17.5 15.5 32.0 25.5 33.5 32.0 29.0 19.5 22.0 16.0 21.5 13.5 2.5 1.5 0.5
..
Ap rox. E s t d . 8 p . Surface Sq. Crn./Gra&
1-12
30,000
Mostly 2-16 Mostly 4-20
20,000 10,000
5-30
8,000
8-38
7,000
12-45
6,000
values were calculated from particle size measurements and geometrical considerations (making certain assumptions and corrections for irregular outlines, complicated surfaces, etc.). The order of magnitude for the specific surfaces agrees satisfactorily with values obtained from adsorption and porosity test measurements. More detailed information on the particle size distribution of the six filter aids is given in Table 11. Wide differences in particle size characteristics are shown between the fine- and coarse-filtering products. Thus, Celite 545 has 76.0 per cent of particles coarser than 20 microns, whereas Filter-Cel has but 10.5 per cent. Conversely 545 has only 1.0 per cent of particles finer than 6 microns, whereas Filter-Cel has 56.5 per cent. However, all of the filter aids have a substantial percentage of particles coming within the important size range of 6 to 20 microns. Of particular interest are the differencesbetween Filter-Cel, Standard Super-Cel, and Hyflo, the three filter aids which give the high clarities required for many applications, including sugar filtration. Standard Super-Cel, for example, has only about 7.5 per cent of particles finer than 2 microns as compared with 19.0 per cent for Filter-Cel. For total particles finer than 6 microns, Standard Super-Cel has 41.0 per cent, whereas Filter-Cel has 56.5 per &at. Differences of this magnitude are highly significant. However, there are other important differences; Standard Super-Cel is a heattreated product, aDd some of the differences in filter aid characteristics between Filter-Cel and Standard Super-Cel are due to calcination effects. Other particle size relations are shown in Figure 1. Here the six filter aids show somewhat related particle size curves which do not overlap. The 2-micron size appears to be significant for the high-clarific’ation filter aids-Filter-Cel, Standard Super-Cel, and Hyflo. The 6-micron size is correspondingly significant for the coarser ‘filter aids-Celites 503, 535, and 545. Comparing the 50 per cent marks and the 10-micron sizes, the following may be noted: 50% Finer Than: Filter-Cel Standard Super-Cel Hyflo Su er Cel Celite Celite 635 Celite 545
SOP
Mesh Size >40 40-20 20-10 10-6 6-2 12
General Particle Size Distribution, Miorons
Character of Particles
-
7“ microns
11 15 20 30
Finer Than 10 Microns:
% 45 33 18 6
Microscopic Structure of Celite Filter Aids
1
Celite 545 24.0 52.0 18.5 4.5 1.0
..
These filter aids co&t principally of elongated, filiform spicular, or acicular particles, disklike particles, fragments of both types, fine particles of all the types, and light microscopic aggregates of the different shapes and sizes. All of the particles are so small as to require the microscope for their observation.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
A.
20 microns
Figure 2 shows the microscopic structures of Filter-Cel and Celite 545, which serve to demonstrate the differences. I n Filter-Cel the great number of particles are shown throughout the field. The finer particles do not show up very well a t the low primary magnification of 200 X, but the intermediate size particles (largely fragments with definite structure) are prevalent. With Celite 545 the extremely fine particles are not present, and the microscopic aggregate structure of this type of filter aid is clearly evidenced. (Details of diatomaceous structure are difficult to show in the small reproductions included here. The features referred to are clearly evident in larger prints.)
Filtration Characteristics of Celite Filter Aids The general and detailed uses of Celite filter aids in the sugar industries are well known; Filter-Cel and the Super-Cels
have been employed as standard materials for 28 and 18 years, respectively. The newer types of faster flow rate products are also of interest and will be referred to again. The practice of using filter aids varies at different refineries in the selection of the grade to be employed. Clarification requirements, which are not always the same, principally determine the choice of filter aid, but -there are also other important considerations, such as bone char practice, filter area available, types of sugar, etc., which may influence or determine the type of filter aid. It is usually recognized, however, that there is a fairly general inverse correlation between filtration rate and clarity. The clarity-flow rate relations between the different Celite filter aids are well recognized within the sugar industry; but as pointed out previously, these relations do vary considerably, depending upon the type of sugar, filter technique, amount of filter aid used, etc.
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April, 1942
407
The other fdter aids come regularly and consistently within these defining limits. The experimental examples given may be considered as typical of many laboratory evaluations on raw cane sugar. The values and ratios for both flow rate and clarity vary considerably, however, for different sugars and different test methods. The flow rate ratios, for instance, may narrow so that Hyflo is only three times as fast as Filter-Cel, or they may expand so that Celite 545 is fifty times as fast as FilterCel. The spread in clarities is also variable but generally less so than the flow rates. Except in most unusual cases, however, there are few exceptions to the general flow rateclarity relation; i. e., the faster the flow rate, the lower the clarity. I n refining practice the clarity requirements are variable, but generally it has been found that a clarity not far from that of Hyflo is the minimum requirement. Some refiners endeavor to obtain better clarity than that of Hyflo; some operate a little on the other side. Clarities like those found here for Celite 535 and 545 are not satisfactory for cane sugar refining, and these filter aids have not been used extensively for this purpose without modifications in process which may permit or favor their use. Thus, it was shown recently that these coarse-particle-size filter aids can be used in combination with calcium phosphate under certain conditions to give both satisfactory flow rates and clarities, together TABLE 111. FILTRATION CHARACTERISTICS O F CELITE FILTER with substantial decolorizing. This method of employing AIDS open-cake filter aids is described in another paper ( I ) . An (Based on laboratory filtrations. Puerto Rican raw sugar'. 0.3% filter aid; application in the filtration of afhation sirups with the regular' pressure marease) Oliver precoat type of filter was described earlier (3). Clarifioationb,
Table I11 gives the results in determining the flow rates and clarities of six Celite filter aids with a Puerto Rican raw sugar. The tests were made with the improved bomb filter press assembly described in another paper (4). Clarities were determined with the Cummins-Badollet-Mdler photometric Tyndallmeter (8). The clarities are expressed as foot-candles, which is the light intensity of the Tyndall beam produced in the filtered liquor as observed under standard conditions. Photometer wedge units of the Tyndallmeter may be employed equally well, as done in Table VI1 and Figure 4. The higher the wedge reading, the better the clarity. When expressed in foot-candles, the lower figures indicate the better clarities (4). The "clarity factor" is an expression employed to give approximate comparative ratings based on the footcandle readings. The filtration rates which are determined as the volume of filtrate obtained per unit time per unit of filtering area are expressed in terms of Filter-Cel, the flow rate of which is taken arbitrarily as 100. In the tests reported the raw sugar was melted to 60' Brix and filtered a t 80" C. (176' F.); 0.3 per cent of filter aids was employed. Pressure increase was regular throughout a 30-minute filter cycle. The rate of flow for Filter-Cel was 6.5 gallons per square foot per hour.
Filtration Rate 1005 213 534 910 1269 1830
a
Standard.
b 25-30
Ft.-Candles 8.79 10.33 15.23 21.09 25.35 27.80
Clarity Faotor 1005
Clarity-Flow Rate Relations
86 67.5 41.5 34.6 a1.6
minute filtrate.
The ratios between the flow rates found for Filter-Cel, Standard Super-Cel, and Hyflo were 1.00, 2.13, and 5.34, which are not uncommon in actual cane sugar refining practice. Higher ratios were obtained for the faster flow rate iilter aids-9.1 for Celite 503 and 18.3 for Celite 545. The clarities, on the other hand, show inverse relationships, being dehitely best for Filter-Cel and poorest for Celite 545.
The clarity-flow rate relations between filter aids of different particle size distribution and type depend upon the character and size of the suspended particles in the liquid to be filtered, the viscosity of the liquid, and other factors. The size and type of particle are of particular importance in considering the type of product which can filter out such particles. Filtration tests with dFfferent colloidal suspensions and dispersions should give helpful information on the use of filter aids and will serve to emphasize the fact that relations between filter aids which are valid for one type of liquor (raw sugar, for instance) do not hold necessarily for all other systems.
OF DISPERSED SYSTEMS EMPLOYED IN FILTRATION TESTS TABLE IV. PROPERTIES
Dispersed System Aluminum hydroxide in water Coarse suspended material from molasses in 60° Brix sucrose Colloids from molasses in 60' Brix sucrose Cane wax emulsion in 60' Brix sucrose Coarse carbon in 30° Brix sucrose Barium sulfate in water Zinc oxide in water Rubber latex emulsion in water Sulfur in water Sea moss extract Prune pulp Puerto Ricc raw sugar
Character of Particle Flocculated Amorphous, compressible Amorphous, compressible Amorphous, compressible, waxlike Microscopic aggregates Isotropic, noncompressible Rounded plates, noncompressible Rounded, compressible Colloidal
Concn. of Dispersed Material Size of Particle, Microns Remarks 5-200 0.1% Al(0R)s Readily filterable, high clarity Readily filterable, high clarity About 3, some up to 10, no 0.2% colloidal Filters like affination sirup About 1 1 .O% sugar Filters like raw or washed raw
70% less than 0.2
0.270 on
1-50
0.1% on sugar
About 1
0.1%
0.2-1.0
0.1%
About 80% less than 0.2 Less than 0.2
0.125% 0.05%
Large organic particles, Large particles, 10-200 2.0% emulsoid colloid Fibrous pulpy tiesue, col- Relatively coarse to colloidal 10.0% loidal material Suspensoid, emulsoid About 0.570 Colloidal to suspended
High clarity Poor clarity at high filtration rate Not clarified a t high flow rate Hyflo does not clarify Standard Super-Cel does not clarify Difficultly filterable; fast-flow filter aid advantageous Difficultly filterable; requires fast-flow filter aid Typical raw sugar
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Vol. 34, No. 4
TABLEV. CLARITY-FLOW RATERELATIONS OF FILTER AIDS WHENFILTERINQ DIFFERENT DISPERSIONS
Dispersed System AI(0H)s Coarse material from mo1as8es Colloids from molasses cane wax emulsion Coarse carbon Bas04 ZnO Rubber latex Sulfur Sea moss Prune pulp Raw sugar
0.25
Filtration Rate of Filter-Cel Gal./Sq. Ft./dr. 12.7
0.25 0.25 0.5 0.4 0.25 0.50 0.25
34.2 3.9 5.95 19.2 29.0 3.5 9.2
0.25
43.8
1.0
3.7 1.6 6.5
Filter Aid,
%
20% on pulp 0.3
Comparative Filtration Rates FilterCel
... ... 1
Standard 1
Hyflo 1
503
545
1.1
1.5
1 1 1 1 1
1 1.5 2.1 1.8 1.6 2.3 1.7
4.1 3.2 2.4 4.6
6.5
2.2 2.25 6.5 4.0 4.0 8.0 10.0
1
1.7
4.0
5.5
1
6.2 2.1
1 18.2 5.3
1.2 25.3 9.1
... 1
...
i:0
Clarification, Ft.-Candles FilterCel Standard Hyflo 503 0.4 0.4 0.4
..
... ...
... 7.1
i:8 24 1.0 1.9 33 9,0
1.0 6.4 43 1.6 14.3 70 9.4
53 2.7 127 393 101
...
0.3
55.0
No
1.55 52.5 18.3
3+:5 8.8
40:s 10.3
... ...
1 .o 7.6 59 3.0 284 920 NO
clarification NO
545 0.4
... ... ... ... ... ... ... ...
clarification clarification 7.8 9.2 10.3 103.3 121.3 136.1 15.2 21.1 27.8
T A B LVI. ~ FILTRATION DATAON FRACTIONS FROM LONPOC DIATOMACEOUS CRUDE WITH 60’ BRIXRAWSUGAR Fraction, Microns Complete 0-2 2-4 4-6 6-8 8-10 10-12 12-15 15-20 +20 10-20 T.
Figure No.
3;d
3B 3C 8D
3Y 3F 3G 3X ,
..
Type of Particles 5 0 7 filiform, elongated; 25% fines, 2570 disks and fragments Fin: fragments 90 filiform 107 disk fragments 8 0 7 filiform*2 0 2 disk fragments 5 0 9 filiform’ 5 0 7 disk fragments 3 0 9 filiform’ 7 0 9 disk fragments 30q filiform’ 7 0 2 disk fragments Ma& disk;, about l/a unbroken Mainly disks about unbroken 40% large diAks, 40-50% crystalline impurities Filiform, disk fragments and disks; no small partioles
Distribution,
F l o w Rate (Relative to Original)
100.0 29.6 26.9 17.1 9.6 6.6 2.4 2.4 1.9 4.5
100 20 128 244 468 626 750 1035 1190 1810’ 1070
%
...
Clarifioation Similar to: Filter-Cel Filtkkel Standard Super-Cel Hyflo
Celite 501 Celite 503 Celite 535 Celite 535 Celite 545“ Celite 535
Crystalline material removed.
Table IV includes information on the dispersions, character of dispersed particles, size of particles, and concentration of eleven different dispersions and a raw sugar; the results obtained in filtering them are given in Table V. The aluminum hydroxide dispersion consisted of relatively large flocculated aggregates. The coarse suspended material from molasses was separated originally from the molasses by centrifuging. The particles were amorphous and compressible, The “colloids” from molasses (precipitated by alcohol, later treated with water, and then dehydrated by heating) were of smaller particle size than the coarse suspended matter, but were likewise amorphous and compressible. The particles of the cane wax emulsion were small, mostly coming within the range of colloidal suspensions. These particles were compressible and to some extent deformable. The coarse carbon dispersion consisted of microscopic aggregates, covering a broad size range up to 50 microns. The barium sulfate and zinc oxide dispersions represent systems containing small rigid particles. The particles in the rubher latex emulsion were mostly less than 0.2 micron in size, and were not only compressible but appreciably deformable under pressure. The sulfur dispersion consisted of colloidal particles, practically all of which were well under 0.2 micron in size. The sea moss extract (2.0 per cent concentration) contained large organic particles and much colloidal material, which was of a highly hydrated condition (emulsoid). The prune pulp obtained by the digestion of dried prunes contained heterogeneoilsly sized particles ranging from fibrous or pulpy tissue down to colloidal material. The comparative filtration rates found (Table V) with FilterCel, Standard Super-Cel, Hyflo, Celite 503, or Celite 545 are expressed either in terms of Filter-Cel or of the lowest flow rate filter aid in the series, which is taken a s 1.0. With the aluminum hydroxide suspension nearly equal flow rates were found for all of the filter aids tested. Similarly the clarities were nearly optically perfect for each of the filter aids. This is an example of a readily clarified, easily filtered dispersion. It does not require a filter aid.
With the coarse suspended particles from molasses a fast rate of flow was obtained with Standard Super-Cel, but 2.2 times this flow rate was possible with Celite 503. I n both cases the clarities were high and equal. This is an example of easy clarification, but there is an advantage in flow rate with the coarser particle size filter aid. The colloidal material from molasses behaves differently. Here the particles are smaller and more concentrated. Relatively small but definite differences in the flow rates obtained with the different filter aids are noted, but there is also a definite difference in the clarities. These clarity differences are not nearly so great as with some of the other dispersions noted below. The filtration of the molasses colloid preparation is somewhat similar to a filtration of affination sirup. In the cane wax emulsion greater differences are observed between both the flow rate ratios and the clarity evaluations. The higher the flow rate, the poorer the clarity. The results are somewhat like those obtained with raw sugar or washed raw sugar. The coarser carbon suspension shows somewhat similar relations, but the degrees of clarification were high. With barium sulfate and zinc oxide important differences are observed. Both systems gave flow rate ratios for the different filter aids which were different in degree, but both were within the ranges found for sugar filtrations and many other commercial filtrations. The rates of flow with barium sulfate were high, whereas zinc oxide gave relatively low flow rates. The clarity differences, however, are of greater interest. Filter-Cel gave brilliant clarity for barium sulfate; greatly inferior clarity resulted with Standard Super-Cel, and poor clarity with Celite 503. Zinc oxide was partially clarified with Filter-Cel and less so with Standard Super-Cel. Unsatisfactory clarities were obtained with both Hyflo and Celite 503. With the rubber latex emulsion Filter-Cel and Standard Super-Cel gave about equal clarities. Hyflo provided only about 10 per cent as good clarification, and Celite 503 was unable to retain any of the particles.
April, 1942
I N D U S T R I A L AND E N G I N E E R I N G C H E M I S T R Y
The sulfur suspension showed pronounced differences. With Filter-Cel almost perfect optical clarity was obtained. Standard Super-Cel gave 55 foot-candles of turbidity as compared with 0.3 for Filter-Cel. The coarse-particle-size filter aids were useless in holding back the colloidal sulfur particles. With sea moss a totally different set of conditions was encountered. This is a highly viscous system consisting of large gelatinous particles and much fine colloidal material which is apparently greatly hydrated and emulsoid in character. A flow rate of 3.7 gallons per square foot per hour was obtained with Hyflo. This was increased only to 6.7 with Celite 545, but a difference of this magnitude is important for a material of this type. The clarity of the Celite 545 filtrate was measurably inferior to that obtained with Hyflo, but the difference is not so important as the increase in flow rate that is possible with the more open-cake filter aid. This is a case where a coarse-particle-size filter aid such as Celite 545 is advisable. It is more advantageous or even necessary to employ the coarse filter aid in the prune pulp filtration. Here the rate of flow possible with Filter-Cel or Standard Super-Cel was too low for economic consideration. Filtration with these aids stops completely after a short cycle. With Celite 503 and, more particularly, Celite 545 quite rapid flow rates were obtained; and the clarification with these open-cake filter aids was relatively good for a dispersion of this type, even in comparison with Filter-Cel or Standard Super-Cel. The particles in the prune pulp extract range from relatively coarse, fibrous, or pulpy tissue to truly colloidal material. The large particles tend to plug or seal the minute channels afforded by the fine-particle-size filter aids, whereas the coarse-particlesize cake lattice can retain such particles without complete stoppage of the channels. Here, the cake, as it is formed with filter aid and entrapped tissue particles, becomes capable of retaining also many of the finer particles, even down to submicroscopic sizes. It must be recognized that here, as in other filter aid filtrations, the true filter member is not a cake of diatomaceous particles alone, but is a complex mixture of entrapped particles and/or gelatinous bodies intimately built up simultaneously with the filter aid particles. The above examples serve to show some of the complicated relations between different filter aids and different dispersions. The proper choice of filter aid depends upon the type and size of suspended material to be filtered out. Flow rate ratios between different filter aids vary greatly with different systems, and the clarity varies to an even greater extent. I n most cases difficult filtrations become a compromise between flow rate and clarity. The selection of the correct filter aid for a specific requirement is the best compromise that can be made under a given set of conditions.
Effect of Particle Shape Particle shape, as well as particle size, is important in determining the kind of openings or interstitial spaces in a diatomaceous filter cake. It is difficult to separate these two factors, but a step in this direction can be made by a study of different types of diatomic particles. Up to this point the principal consideration has been mostly particle size. With the data given, however, a further study of diatomaceous materials is helpful. A relatively pure, good quality diatomaceous crude from Lompoc, Calif., was separated by water classification into various particle size ranges. This crude consisted of about 50 per cent filiform, elongated, or spicular particles, about 25 per cent fine particles, and about 25 per cent disklike forms or fragments thereof. Most of the fraction smaller than 2 microns in size had fine diatomic structure, but a minor percentage also was without evident structure.
409
The 2 4 micron fraction was nearly all filiform and consisted of broken particles of diatom forms which were originally much longer, being particularly elongated or acicular types. The 2 4 micron fraction comprised 26.9 per cent of the original crude. All of the fractions up to 12 microns in size contained substantial proportions of these same acicular or spicular particles, but they became of increasing lengths in the coarser fractions. The 6-12 micron fractions were also characterized by the fragments of discoid or saucerlike forms. From 12 microns up, unbroken disks appeared, together with larger disk fragments. The coarser discoid fractions were found in smaller proportions in the original crude than were the intermediate and fine fractions. The percentages of the coarser fractions are: 2.4 per cent within the 10-12 micron range, 2.4 per cent within the 12-15 micron range, 1.9 per cent between 15 and 12 microns, and 4.5 per cent coarser than 20 microns. Of the latter, about half or 2.25 per cent consists of crystalline particles and impurities which are separated and discarded in the manufacture of a first-quality filter aid. More complete information on the properties of the fractions and their filter aid characteristics are given in Table VI. Since these fractions consist of different shapes of diatomaceous particles (as well as different sizes), it is of interest to compare their filtration characteristics. Raw sugar solution was the liquor tested. Test conditions were the same as those in the study of the commercial filter aids. Taking the original crude as the base of 100 for the comparative flow rate evaluations, the fines are found to be of no value as such for use as a filter aid. The 2-4 micron fraction has a flow rate of 128 with clarity like Filter-Cel, and so on. A mixture of all particles from 10 to 20 microns has filtration properties almost like those of the 12-15 micron fraction. It would thus appear that particle shape is less important than particle size in determining filter aid performance. However, both the elongated spicular particles and the discoid particles and fragments found in this type of diatomite have valuable filter aid properties, as will be shown later, and for this reason the pronounced differences in particle shapes observed for the different fractions do not affect greatly the iilter aid ratings. Figure 3 shows the microscopic appearance of the different fractions included in this study. Additional tests made with a variety of different types of diatomaceous preparations serve to show the importance of particle shape. Raw sugar was employed as the test medium, The results are given in Table VII. Preparation 1 was also employed in the tests included in Table It contains thin-walled plankton disks coarser than 20 microns (sedimentation value), with diameters ranging from 50 t o 125 microns. It has a fast flow rate but only fair clarity for such a flow rate. Preparation 2 is a mixture of disk fragments and elongated spicular particles, and has an excellent clarity-flow rate relation. Preparations 3 and 4 are like 2 but larger in size. They show still faster rates of flow than 2 but have the same favorable flow rate-clarity ratio. Preparations 2, 3, and 4 were not obtained from the same crude diatomite as No. 1, and show better comparative clarification. None of these preparations are commercial filter aids. No commercial filter aids are yet available which are composed exclusively of marine disk particles, on the one hand, or exclusively of long spicular particles, on the other. The test materials have these characteristics and were prepared with care .solely to bring out principles of filter aid performance, Pre aration 5 is almost pure disk forms and fragments. The particyes are smaller than those of preparation 1 and differ from those of 2, which is a mixture of disks and thin, elongated forms. Preparation 5 has a fast flow rate and excellent clarity for this flow. Preparation 6 is a mixture of relatively large fresh water forms. These oval, elliptical, or cymbiform sha es are compact structures which are not so favorable for clari&ation. The character of. these shapes is not fully apparent from the two dimensional views shown in Figure 5E,but the cake formed by these particles is not highly favorable for a good clarity-flow rate relation.
VI.
INDUSTRIAL AND ENGINEERING CHEMISTRY
410 TABLE VII.
Vol. 34, No. 4
EFFECTOF PARTICLE SHAPEAND SIZEON FILTER AID CHARACTERISTICS OF DIATOMACEOUS MATERIALS Particle Size, Microns Sedimentation Microscopic value dimensions
Numb er from Fig. 4
Phptomicrograph No.
Description
1
3H
Miocene marine disks
>20
2
5A
Disk fragments & elongated particles
10-20
3
5B
4
5c
5
5D
Large disks & elongated particles Max. size disks 8: elongated particles Disk forms
6
5E
7
5F
iMixed f r e s h w a t e r forms Naviculoid forms
8
5u
Large naviculoid forms
9
5H
Denticular forms
10
51
Fresh water disk forms
11
5J
Melosira pillbox forms
12
5K
13
5L 2
Elongated s p i c u 1 a r forms Small elongated forms & disk fragments Filter-Cel
...
Celite 503
... .. .
Character of Diatom Forms
50-125 diam.
Thin-walled d a n k t o n disks Disks 15-60, elongated Plankton. characteris50-200 tic miht. of forms: ure Disks 30-75, elongated Life 2 but larger; pure 40-200
Disks 60-150 gated.50-406 25-75 diam.
elon-
Like 3 but larger; pure
Thin plankton disks: smaller than 1: different stratum from 2 45-160 1 o n g, 15-30 Oval, elliptical, cymbi10-30 wide form 35-100 1 o n g, 10-25 Boat-shaped, t h i n5-25 walled wide 150-340 1 o n g, 15-60 "Giant" forms; heavy Coarser than structure wide 40 22-60 long, 10-12 wide Heavy compact struc8-18 ture 25-60 diam. 10-25 Medium t o h e a v y structure 5-10 d i a m., 10-20 Heavy-walled cylinders 5-10 depth 5 wide, 200 long Uniform size very elongated; puke spicules Characteristic Celite 2-10 4-25 mixed forma Wide range of sizes; 1-20
+
1
t a
1400
a
-I
0
Literature Cited 1200 100
400
800
FILTRATION
1200 RATE
VS.
1600
2000
FILTER-CEL
Figure 4. Effectof Particle Shape and Size on Flow Rate-Clarity Relation of Diatomaceous Materials (Table VII)
(1) Cummins, A. B., IND. ENG.CHEM.,34, 398 (1942). (2) Cummins, A. B., Badollet, M. S., and Miller, M. C. (to JohnsManville Corp.), U. S. Patent 2,045,124 (June 23, 1936). (3) Cummins, A. B., and Morris, D. C., Facts About Sugar, 33, 23-7 (1938). (4) Cummins, A. B., and Weymouth, L. E., IND.ENG.CHEM.,34, 392 (1942).
April, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
a
411
A.
Preparation 2
B.
D.
Preparation 5
E. Preparation 6
F.
a.
Preparation 8
H.
I . Preparation 10
J . Preparation 11
Preparation
Preparation 9
K. Preparation 12
C.
Preparation 4
Preparation 7
L. Preparation 13
Figure 5. Photomicrographs of Diatomaceous Materials ( X 200) Described in Table VI1
