Effect on Base-Exchange Capacity and Hydration FIGURE1. CABINET-TYPE Damn
DRYING GEL ZEOLITES M. 0.LARIAN AND CHARLES A. M A " Univeeraity of Minnesota, Minneapolis. Minn.
QT
HE importance of zeolites is based riiainly
This work is confined to the aluminosilicates, although salts of other metals can be used in place of a salt of aluminum. Aluminosilicate zeolites are preferred because of their lower cost and of their higher baseexchange capacity. A commercial gr5.8.deof water glass was used in this work for preparing the gels. It contained 29.81 per cent silica and 9.09 per cent sodium oxide, or 5 ratio of sodium oxide to silica of 1 to 3.2 (approximate!y 42O Be.). The sodium aluniinate contained 29.1 per cent sodium oxide and 36.28 per cent alumina. The aluminate was free from iron, and its solution mas clear. The gels mcre preparcd as follo~va:
upon their peculiar property of base exchange. This prorJerty was observed in the soils as early as 18% Ir).'~hornpson ( f ) and by Way (9). The latter was also able to show that the soil constituent responsible for base excliange was an aluminosilicate. Tiiese compounds, however, remained for a long time without any industrial application. IIarm (4)in 1898 was the first to propose the use oi base-exchanging compounds in sugar refining. In 1906, Gans ( 2 ) was granted a patent for the process of preparing a n artificial wolite to be used in sugar refining in place of clay which was originally suggested in Harm's patent. This process of Gans consisted in precipitating a silicic acid solution with an alkali metal aluminate and subsequently adding a chloride solution of an alkali earth. A year later he obtained B patent (5) on a fusion method of preparing artificial zeolites t o be used, in addition to sugar refining, in water softening, and in removing inanganese from water. The use of zeolites as catalyst arid as an absorbent. for vapors has also been proposed. Parallel wit,h their increased use, various processes for the ~iiimufaetureof artificial zeolites have been patented. Nevertheless, the most important processus are based upon the iirinciple of obtaining a gel when a solution of an alkali metal silicate is mixed with a solution a i a salt of aluminum. This gel has to be dried in order to make i t hard and rigid. Alt,hougIi the drying of the gel is one of the most important problems in t.!ie manufacture of zeolites, hardly any information can be found in the literature on this phase. Campbell ( 1 ) in 1907 reported that, when a gel zeolite is dried a t 100DC. instead of at mom temperature, the base-exchange equilibrium is established very slowly. The patent literature is vague, mentioning only the use of a temperature below 100°C. Since the bulk of the zeolites on the market today are of the gel type, a systematic study of the effect of air-drying upon their propertie3 was considered desirable.
A 26.5-gram portion of water glass was diluted with 190 ml. of mter, and 8.0 grama of sodium aluminate werc dimsolved in 50 ml. of water. The aluminate solution was added rapidly, while stirring, to the silicate solution >it.room temperature. The stirring was continued for 15 seconds more. The mix was then poured zt once on R gaIvnnizerl iron tray and allowed to set into B
____ Zeolites were air-dried under various conditions of temperature, humidity, and velocity. Zeolites of varying composition were also prepared and dried under identical conditions. The zeolites break up into smaller particles when they are thrown into cold water after they have been dried completely. The percentage of the fine sizes-e. 9.. those passing a 28-mesh screen-depends upon the conditions of drying. The percentage of fines is increased when drying is very rapid 196
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FEBRUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
gel. This gel was transferred to a 32-mesh, nickel-plated screen tray for drying purposes. In order t o shorten the drying period, the drying air was circulated on both sides of the tray. The setting of the mix into a gel was complete in 25 to 30 seconds from the time the aluminate solution was added. I n order to retard the setting, it would be necessary to use more dilute solutions of the reagents. With increased dilutions, however, the resulting gel is too soft to be handled on a screen tray in drying. Besides, the drying period is increased with the use of additional water, although, as reported in some patents, it may be possible in industrial practice to remove some of the water by pressing. Another way of retarding the setting is to use a proportionately smaller amount of aluminate. To obtain any great change in the rate of setting would require an appreciable decrease in the amount of aluminate to be used, and would result in a product which is more like silica gel in its properties. As a result of these considerations, and since it was not difficult to obtain thorough mixing by rapid stirring, the method given for preparing the zeolite gel was adopted for all cases where the effect of drying was studied. The uniform mixing of the constituents is indicated by the appearance of the gel. Whenever the sodium aluminate is not mixed uniformly by rapid stirring, the gel appears more opaque a t the places where the aluminate is concentrated. Even with complete mixing of the constituents, it is possible that the resulting gel may not be structurally uniform because of the short period in which the gel sets. The samples taken for study from each batch of dried gel are, however, representative, and therefore the possible case of nonuniformity of the gel will not fundamentally affect the results. Gel slabs, ‘/a inch (1.27 cm.) thick and 3 X lO3/sinches (7.6 X 26.3 cm.) in size, were prepared. Four such slabs, each divided into six approximately equal pieces, constituted a drying batch. The drying was done in a cabinet type drier (Figure I), designed and built by the Carrier Engineering Corporation. It was provided with an automatic control of temperature and humidity. A Tycos recording instrument indicated the wet- and dry-bulb temperatures. The velocity of the air was regulated, and its uniform distribution was maintained by a baffle arrangement. The air velocity was measured with a Tycos anemometer. During the process of drying, the zeolite gel undergoes certain changes in its appearance. At the beginning it is milky and opaque, but it gradually becomes transparent as water is evaporated. By the end of the constant-rate period of drying it is completely transparent. During the falling-rate period it gradually loses its transparency, and finally becomes opaque and milky, a t the point where the curve flattens out. On further drying the gel becomes chalk-white. This appearance indicates overdrying.
or when the gel is overdried. The baseexchange capacity of a gel zeolite depends upon the conditions of drying. An explanation for this behavior is given. The importance of composition in base exchange and in obtaining a rigid, nonbrittle product is indicated. The amount of the total water and also of the structural water in zeolites dried under various conditions were determined; hydration also depends to some extent upon the conditions of drying.
197
The gel pieces were dried through the transparent stage until they began t o turn milky. This offered a means of controlling the extent of drying. After drying was completed, they were “hydrated” in cold water. By this operation the dried gel pieces are broken into smaller sized particles. The proportions of the various sizes resulting from hydration depend upon the conditions under which the gel is dried. For a certain number of gels the drying rates under various conditions were determined by weighing the batch every hour (taking it out of the drier) and by determining the average length of the gel slabs at every other weighing. Assuming equally effective drying on all surfaces and assuming also equal linear shrinkage in all directions, the approximate rates of drying, as amount of water evaporated per unit area per hour, were calculated. In this way it was possible to show that from 93.5 to 96 per cent of the total water that had to be evaporated was evaporated during the constant-rate period. Thus, beginning with an average charge of 1026 grams of gel, 866 to 886 grams of water were evaporated at a constant rate, and only 38 to 59 grams of water were evaporated during the falling-rate period. TABLE I. SCREEN ANALYSISOF ZEOLITES Sample No. 1 2 3 4 5 6
7 8 9
10 11 12 13
14 15 16 17 18 34 35 36 37
-
+8
Screen + 14 Standard+Mesh 20 +28
-28
%
%
%
%
%
52.0 28.2 26.7 10.2 47.0 50.1 51.7 44.0 46.4 20.0 43.2 41.4 19.4 25.0 27.2 43.4 47.3 14.6 38.1 5.6 15.4 12.3
24.8 54.0 56.87
2.2 7.6 7.2 21.1 4 3 11.6 6.3 7.8 3.6 15.1 7.0 10.4 4.3 5.1 11.2 9.3 4.7 10.3 9.7 15.7 8.1 11.0
11.3 3.9 4.1 6.4 9.6 4.2 4.2 3.8
9.7 6.3 5.0 3.4 13.2 8.9 4.4 3.1 1.0 18.2 10.5
59.1
25.9 25.2 33.4 41.3 47.6 28.8 34.6 32.3 70.4 62.7 27.8 30.3 40.1 70.4 37.5 66.0 72.2 70.6
1.4
17.9 4.7 7.9 1.1 4.7 13.6 7.9 4.8 2.8 7.7 7.2 1.7 3.0
-
8.0
4.8 2.5 20.2 9.1 3.1 1.9 7.0
5.5
2.6 3.1
The falling-rate period represents 25 to 30 per cent of the total drying period, in spite of the small fraction of the total evaporation occurring in this period. That the gel should be dried past .the constant-rate period is necessitated by the fact that the gel becomes hard only towards the latter part of the falling-rate period. From this standpoint it would not be necessary to continue drying once the gel had become hard. If at this point drying is stopped and the gel is hydrated, then comparatively larger sized particles are obtained. This, however, is not a serious disadvantage, and the oversized particles need not be reduced by grinding (the loss due to powdering in grinding is very large) because, when the oversize particles are dried and hydrated a second time (for example, when used in a water softener), they break up again. It is surprising that all sizes which pass an 8-mesh screen are stable; i. e., they do not break up on second hydration, whereas the sizes retained by the 8-mesh screen will break up further, unless these large particles are due to overdrying. Table I shows the effect of drying conditions upon particlesize distribution. The srreen analysis was made after hydrating the dried zeolites in cold water, washing a few times with distilled water to remove most of the free alkalinity, and drying again at room conditions. The size retained by the 14mesh screen is the predominating size when the zeolite is dried a t a moderate rate. Rapid drying rates produce a pronounced case-hardening effect, one result is the production, upon hydration, of a larger percentage of fines-i. e., particles passing the 28-mesh screen-in some cases as high as 20 per cent of the total. This is a serious disadvantage in itself when the zeolite is being prepared for use in water softening because the fines will be washed out too easily.
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TABLE 11. EFFECT OF DRYINO CONDITIONS ON BASEEXCHANGE AND HYDRATION OF GEL ZEOLITES 7-1. Constant Drying Conditions- Total Dry- WetSample bulb bulb Rel. Av: drying No. temp. temp. hum. velocity time Ft. (meters)/
c.
c.
%
man.
Hr.
Exchange 2. Base- 3, Hydration at Struoof Ca$h 25' C. & Rel. turd Soln. Hum. of: Water 76% 84%
cc.
%
14.3 14.3 187 (57) 21 3310 19.3 17.8 29.0 187 3543 20.5 2 20.5 41.7 187 30 27 3725 19.2 3 23.0 55.2 187 361/4 4103 18.1 4 23.1 161/4 2915 40 20.0 14.0 187 5 20.4 191/z 3200 23.9 25.0 187 6 2O'/r 3890 19.6 26.7 35.0 187 7 187 22 4180 19.3 29.0 44.4 8 251/a 4015 18.7 31.5 5 5 . 0 187 9 21.9 121/2 2705 10 50 23.3 8.5 187 14 3190 18.1 26.0 13.8 187 11 3655 20.2 187 15 31.0 25.8 12 18.3 16l/n 3947 36.0 40.6 187 13 20'/2 4160 16.5 40.5 55.8 187 14 81/n 2760 '22.5 66 40.0 17.5 187 15 20.2 45.0 25.5 187 101/4 3391 16 123/4 3875 16.2 51.0 38.5 187 17 Room oonditiona 100 4189 17.1 18 30l/r 3590 20.2 99 (30.2) 30 17.8 29.0 19 20.5 27 3543 187 (57) 17.8 29.0 2 19.8 3426 17.8 29.0 578 (176.2) 241/4 20 99 39 4070 18.9 30 23.0 55.2 21 18.1 4103 23.0 55.2 187 36'/4 4 4055 23.0 55.2 578 32 18.5 22 22.3 19 2880 14.0 99 40 20.0 23 2915 20.0 14.0 23.1 IS7 16l/4 5 22.7 2820 24 20.0 14.0 578 121/1 3925 18.5 40 26.7 35.0 99 22'/4 25 3890 19.6 26.7 35.0 187 2031~ 7 3800 19.0 26.7 35.0 578 17 26 4078 55.0 99 28'/4 18.2 40 31.5 27 4015 18.7 31.5 55.0 187 25'/z 9 578 21 3975 19.0 31.5 55.0 28 25.8 99 3620 19.5 31.0 50 29 3655 20.2 31.0 25.8 187 12 3570 20.0 31.0 25.8 578 30 99 4100 17.3 40.5 55.8 50 31 4160 16.5 40.5 55.8 187 14 4070 18.0 55.8 578 40.5 32 3875 16.2 66 51.0 38.5 187 17 578 3800 16.9 51.0 38.5 33 Bane-exchange capaoities of seolites on the basis of 10 grams. 1
30
In order to avoid rapid drying and hence to minimize the amount of fines obtained upon hydration, it is necessary either to use lower dry-bulb temperatures or to increase the relative humidity as the dry-bulb temperature is increased. The use of a very low dry-bulb temperature, such as 30' C., has two disadvantages: (1) Drying is unnecessarily slow; and (2) the edges of the gel slabs dry into powder clusters which fall away in the form of a very fine powder, representing a net loss. Zeolites 1, 2, 3, and 4 lost during drying 2, 3, 2.5, and 1 per cent, respectively, of their b a l weight. A similar observation was made when the gel was dried at room conditions. Overdrying of the gel is harmful in all respects. When the slabs are hydrated after being overdried, the sizes retained by an 8-mesh screen and passing a 28-mesh screen predominate. The oversize can be broken up only by grinding, with the result of a greater loss due to fine powdering.
Effect of Composition of Zeolite in Drying Gels are formed within a wide range of ratios of alumina to silica with the difference that the time of setting of the gel varies with the composition. I n addition to the regular composition, a few more gels were prepared by varying the proportion of aluminate used and drying under the same conditions. As before, the amount of silicate was 26.5 grams diluted with 190 ml. of water. Thus sample 34 was prepared by using 6 grams of aluminate dissolved in 50 ml. of water: 35, 10 grams of aluminate in 75 ml. of water; 36, 12 grams of aluminate in 110 ml. of water; 37, 14 grams of aluminate in 150 ml. of water. As the amount of aluminate was increased, more water had to be used in dissolving it in order to retard
%
%
20.2 21.1 20.4 19.0 23.4 20.7 20.4 20.6 19.7 22.5 19.0 21.2 18.9 18.1 22.9 20.7 17.3 18.0 21.0 21.1 20.7 19.5 19.0 19.3 23.1 23.4 23.6 19.6 20.4 19.7 20.3 19.7 20.3 20.4 21.2 20.8 18.2 18.1 18.9 17.3 17.8
8.0 6.6 8.2 7.3 7.7 7.5 6.9 8.2 7.4 7.6 6.9 7.1
8.1
7.7 7.1 7.2 7.6 8.1 7.8 6.6 7.4 7.2 7.3 8.0 7.4 7.7 7.1 7.8 6.9 7.2 7.1 7.4
VOL. 28, NO. 2
the setting of the gel to a p p r o x i m a t e l y 30 seconds. Table I1 shows that under identical conditions the total time of drying remains almost the same, even though more water was used in preparing the gels. Apparently as the aluminate is increased to a certain proportion, e gel structure is obtained which has a lower resistance to internal diffusion of water. It was observed that, with increasing content of alumina in the gel, the edges show a tendency to break away in the form of a h e powder during drying. Thus zeolites 35, 36, and 37 lost weight because of the formation of fine powder during drying which represented 0.5, 0.8, and 1.7 per cent, respectively, of their final weight. As to the amounts of fines obtained upon hydration, there is no appreciable difference.
Effect of Drying Conditions on Base Exchange
These tests were made with the process of water softening primarily in mind. The calcium and the magnesium ions in the water are exchanged with the sodium of the zeolite. For this reason the base-exchange capacities of the different zeolites were determined in terms of the amount of calcium removed from a standard cal8.1 cium chloride solution. 7.8 7.1 The calcium chloride solution, containing the 7.2 7.6 equivalent of 0.2 gram of calcium carbonate per 7.7 liter, was prepared by using recrystallized hy7.4 drated calcium chloride. The concentration of 7.6 7.2 the solutions was determined by titrating the chloride ion with a standard silver nitrate-solution in the presence of potassium chromate as indicator. The solutions were adjusted to the required strength. The zeolites were mashed with distilled water to remove the free alkalinity. The effluent wash water was passed through a conductivity cell, and the washing was continued until the reading on a milliameter reached a low value and remained constant. The current was obtained from B dry cell. The zeolites were then allowed to come to equilibrium with atmospheric conditions in a room. Samples were weighed and kept until needed for testing. For each test about 15 grams of zeolite, of the sizes which passed a 14-mesh but were retained on the 20-mesh standard screen, were placed in tubes 20 mm. in diameter. The depth of the zeolite bed in the tubes was,about 75 mm. The bottom of the tubes was packed with glass wool, more than 20 mm. deep, in order to support the zeolite and a t the same time to make the upward flow of the water through the zeolite more uniform. The rate of flow of the calcium chloride solution through the zeolite was about half a liter per hour. Such a rate was considered slow enough to prevent channeling. The progress of each test was followed by using a soap solution. The tests were continued until the appearance of calcium ion in the effluent water. The preparation of the soap solution and the method for testing for calcium were similar to those recommended by the American Public Health Association. I n order to regenerate the zeolites, the tubes were filled with saturated sodium chloride solution, allowed to stand for half an hour, and then drained very slowly. This operation was repeated. Finally the zeolites were washed free of the excess of sodium chloride with distilled water.
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199
INDUSTRIAL AND ENGINEERING CHEMISTRY
The results are given in section 2, Tables I1 and 111. The zeolites in Table I1 were dried at various conditions of temperature and humidity; those in Table I11 were dried under the same conditions but are of various compositions. The results are expressed in terms of the volume of the standard calcium chloride solution softened on the basis of 10 grams of zeolite. They represent the average of seven values after discarding the first few, because it is only after a few regenerations that the results become uniform. This is especially true for the zeolites of high base-exchange capacity. Invariably starting with a higher value, the base-exchange capacity of these zeolites decreased gradually to a nearly constant value after the first four to six regenerations. In the case of the zeolites of very low base-exchange capacity, uniform values were obtained immediately after the first regeneration. It is possible that the irregular behavior of zeolites during the first few tests is due to the combined effect of hysteresis ( 5 ) and of adsorbed sodium hydroxide, particularly when they are freshly prepared. Thus, when the zeolites were treated, after the twelfth regeneration, with a liter of solution containing one gram of sodium hydroxide, drained, and washed with one liter of softened water, and then tested in the regular manner, higher capacities were obtained for all the zeolites except those with a very low base-exchange capacity. This difference from the average amounted to 300-500 ml. of standard calcium chloride solution. I n the next test, however, normal values were obtained. It should not be concluded from this single experiment that washing with a sodium hydroxide solution would be a desirable feature in the regeneration of zeolite, although some early patents (6)recommend the use of sodium hydroxide solution. Figure 2 represents the results obtained for zeolites 1 to 17, inclusive. The abscissas indicate the percentage relative humidity of the air, and the ordinates the volume of standard calcium chloride solution softened on the basis of 10 grams of zeolite. These, however, should not be taken as absolute values since they represent the results of a single observation only. It would have been necessary, as in dealing with other gel substances, to prepare a number of samples of zeolite under identical conditions in order to be able to obtain more representative valuefi. These curves are, therefore, useful in the sense that they represent the trend to be expected when zeolite is dried under varying conditions of temperature and humidity, and that they give the magnitude of the change of the base-exchange capacities. The curves show that for a given dry-bulb temperature the capacity of zeolite to soften hard water is increased with increasing relative humidity. Within the limits of the dry-bulb temperatures used in this work all the zeolites finally approach the same base-exchange capacity, provided they are dried with zir of sufficiently high relative humidity. The combined effect of dry-bulb temperature and relative
d
0 -I 4
A
n
50°C. 66°C.
,
2500-
IO 20 30 40 50 60 PER CENT RELATIVE HUMIDITY FIGURE2. EFFECTOF DRYINGCONDITIONS ON BABEEXCHANGE CAPACITY OF ZEOLITE
humidity in drying is expressed in terms of rates of drying. If the zeolites are compared in terms of drying rates (the total drying period taken as a measure of the drying rate), it will be observed that for very low relative humidities the rates of drying are increased with increasing dry-bulb temperature, and a t the same time the base-exchange capacities are lowered with increasing rates (compare zeolites 1, 5, 11, and 15, as well as 2, 6, 12, and 16). As a result of very rapid drying, the zeolite gel slabs show pronounced case-hardening; in other words, the capillaries at the surface are restricted and probably even closed entirely. In using these zeolites for water softening purposes by the flow method, the base exchange is mostly a t the surface, since water is not able to diffuse freely through the capillaries, and correspondingly only a smaller fraction of the available sodium is utilized. To eliminate case-hardening, it is necessary to use air with higher relative humidity, and it will be observed that zeolites of better quality are obtained. The effect of air velocity is not as pronomced as that of drybulb and wet-bulb temperatures. The three velocities used are 99, 187, and 578 feet (30.2, 57, and 176.2 meters) per minute, In some cases the tendency is more towards lower baseexchange capacity with increasing velocity. There is, however, no advantage in very slow drying, as will be seen by comparing the zeolite dried a t room conditions (sample 18) with the others. It has been mentioned that the gels were dried until opaqueness just reappeared. When the drying is continued a few hours or more past this stage-i. e., overdri’ed-a product is obtained which has a lower base-exchange capacity than the zeolite dried to opaqueness. The difference is appreciable, and, even though only a few samples were overdried, it is a general observation; the TABLE111. EFFECTOF COMPOSITION ON BASEEXCHANGE AND difference amounted to nearly 500 ml. of standard HYDRATION OF GEL ZEOLITES calcium chloride solution. Alu2. The other possibility is to stop drying before minate Baseper Ex3. the o p a q u e n e s s p o i n t is reached-that is, 26.5 -1. Constant Drying Conditionschange Hydration at Sam- G. DryWetAv. Total of 25O C. & Structo stop as soon as the gel has become hard. bulb Rel. Vedrying CaCln ReI. Hum. of: tural le Sili- bulb g o . cate temp. temp. hum. locity time G o h a 76% 84% Water This will mean a shorter drying period. The base-exchange tests indicated that, when the drying r a t e s were m o d e r a t e , there was no 0 40 26.7 34 35 187 (57) 22l/a 3785 21.1 21.8 6.9 35 7 8 40 26.7 187 208/r 3890 19.6 20.4 6,9 d i f f e r e n c e o b t a i n e d i n the quality of the 36 10 40 35 26.7 187 20 4174 19.0 20.9 8.6 zeolite. On the other hand, when the zeolites 36 12 40 26.7 35 187 201/1 4008 19.3 20.5 8.1 37 14 40 26.7 35 187 208/1 3672 20.0 21.0 8.0 were dried rapidly, the base-exchange capacity Base-exchange capacities of zeolites on the basis of 10 grama. was somewhat improved by stopping the drying before opaqueness appeared.
INDUSTRIAL AND ENGINEERING CHEMISTRY
200
Effect of Composition upon Base Exchange With increasing amount of aluminate used in precipitating the silicate, zeolites are obtained which show a gradual increase in their base-exchange capacity up to a certain composition and then begin to decrease. This means that, even though the opinion prevails that in zeolites the ratio of sodium oxide to alumina is always 1 to 1, not all of the sodium is exchanged. The advantage of partly increased base exchange ought to be compared with the increased cost of the zeolite due to the greater amount of aluminate employed. Besides cost, there is another consideration which limits the amount of aluminate to be used with a given amount of silicate. Thus, when the zeolites were dried a t room conditions after the tests were completed, zeolites 35, 36, and 37 contained a powdered fraction (passing the 28-mesh screen), representing 1.1,8.4, and 14 per cent of the total, respectively, whereas zeolites 34 and 7 had hardly any powder. This indicates a strong tendency of the zeolites to disintegrate as the aluminum content is increased. There is, therefore, an optimum ratio of silicate to aluminate which will give a strong zeolite.
Effect of Drying upon Hydration Zeolites are commonly spoken of as being hydrated aluminosilicates. This state of hydration and the capillary structure of the gel zeolites are held to be the reason for their superior base-exchange capacity compared with the natural crystalline zeolites. It is also known that complete dehydration destroys the base-exchanging power of a zeolite. The object of this part of the study was to determine the amount of water retained by the zeolite when it was brought in contact with an atmosphere of a known relative humidity. This water is composed of two parts: One is driven off when the zeolite is heated a t 100" to 110" C.,and the other is driven off only by ignition and fusion. The first may be designated as capillary water, and the latter as structural water. To determine the amount of capillary water the following procedure was used: About a half-gram sample, washed free of excess alkali as mentioned previously, was placed in a weighing bottle and kept in a desiccator maintained a t a known relative humidity until the weight of the zeolite became constant. It was then heated in an oven a t 110" C. A desired relative humidity is conveniently maintained in a desiccator by using a saturated salt solution. Sodium chloride and potassium bromide were used in this work. The relative humidities at 25" C. of their saturated salt solutions are, respectively, 76 and 84 per cent. The structural water of the zeolites was determined by igniting and fusing after they were dried a t 110" C. The results are given in section 3, Tables I1 and 111. Total hydration (structural plus capillary water, as measured under the conditions cited) diminishes for a given dry-bulb temperature of drying conditions, as the relative humidity increases. This decrease is more pronounced a t the higher dry-bulb temperatures. This difference in sorptive power may indicate a difference in gel porosity. Capillaries of maximum radius which will be filled with liquid a t a given relative humidity can be calculated by using Thornson's equation (8): r =
2Mr
-
SRT In 5
P,
VOL. 28, NO. 2
radius of capillary, cm. x 10-7 mol. weight of water y = surface tension of water at temp. of expt. S = density of water at temp. of expt. R = gas constant = 8.316 X lO'ergs/O C. T = temp., in degrees ahsolute P, = satn. vapor pressure at temp. of expt. P, = vapor pressure at given relative humidity
where r
=
M
Certain assumptions are made in the application of this equation: (1) The liquid in the capillaries wets the solid surface completely. (2) The vapor of the liquid obeys the ideal gas laws, and the density of the liquid in the capillaries is the same as that of pure liquid. (3) Surface tension is constant and is equal to the surface tension of the liquid a t a plane surface. On account of these assumptions, the equation makes it possible to compare and to determine relative porosities of gels when the difference in available pore volume is not appreciable. On this basis it is therefore possible to make the general statement that the more hydrated zeolite gels-i. e., those dried with air of lower relative humiditieshave a greater number of capillaries finer than the maximum size calculated for a given relative humidity. It is significant that the zeolites dried with air of high relative humidity are more transparent than those dried with air of very low relative humidity. Structural water represents 7 to 8 per cent of the weight of the zeolite after dehydration a t 110' C. It is not, however, affected by the conditions of drying. It was stated that the zeolites were brought to equilibrium under atmospheric conditions before samples were tested for base exchange. The atmospheric humidity was not determined, but it was probably between 40 and 50 per cent, the usual summer humidity. The hydration behavior of the various zeolites js probably the same as a t 76 and 84 per cent humidities. If the base-exchange capacities were calculated on the basis of dehydration, the same type of curves would be obtained as in Figure 1 except that the numerical values would be higher. Changing the composition of the zeolite does not appreciably affect the hydration.
Acknowledgment The writers wish to thank the Department of Entomology of the University Farm, University of Minnesota, and the Department of Chemical Engineering, University of North Dakota, for their cooperation in permitting their cabinet drier and other equipment to be used in these experiments.
Literature Cited (1) Campbell, Landw. Vers. Sta., 65,247 (1907). (2) Gans, German Patent 174,097 (Jan. 12, 1906); Chem. Zentr., 1906,11, 928. (3) Gans, German Patent 211,118 (Jan. 23, 1908). (4) Harm, German Patent 95,447 (1896); Chem.-Ztg., 1898,143. (5) Hisschemoller, Rec. trav. chim., 40, 394 (1921). (6) Staten Island Chem. Corp., Swiss Patent 74,133 (Jan. 16, 1917); French Patent 482,189 (Feb. 27, 1917); Foster, W. C., U. 9. Patent 1,249,368 (Dec. 11, 1917). (7) Thompson, J . Agr. Sci., 2, 68 (1850). (8) Thomson, Phil. Mag., [4142, 448 (1871). (9) Way, J. Agr. SOC.Engl., 11, 313 (1850); 13, 123 (1853). RECEIVED July 1, 1935. Abstracted from a thesis aubmitted by AM.0. Larian to the faculty of the Univeraity of Minnesota in partial fulfillment of the requirements for the degree of dootor of philoeophy.
