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E.,"Casein arid It8 Industrial Ap~iiiication." P. 275. Chemical Catalog. 1927. (1) Faies. H. :\., "hlorgmie Quantitativs Arial~aia." ~~srhsrilIJha (6) van Slyke, L. L..m d Baaworth. A. W., J . Bioi. Cf'e88t., 14, ?03 1%2, 194,Century, 1926. (1915). (2) Msim, E. B.. Blatherwick, N. R., and Caw, C. A.. J . B i d . Cham.. 37, 1 (1919). Rsornviu February 15, IWdz. (3) Shew, R. W.,J. IND.Ewe. Casu.. 12,118M (19201
LITEKATURE CITED
(4) Sutermeister.
Dehydration of Zeolite Gels by Freezing Zeolite gels have 6een dehydrated by freeiiug, yielding a material of higher 6ase-exchange value than similar oren-dried gels. A dynamic method of testing zeolites for capacity is described. YNTHETIC seuliter are becoming more important in the water-softening field because of their high capacity and proven ability to soften water satisfactorily. There have been many patents issued covering the manufacture of tliese uroducts, all rif which vary in detail but not in certain principles. hrtificial zeolites are: often rnade by rxiixiug a solution of sodium silicate with solutions of one or more aluminum salts which react to form a whole-volume gel. Tbe two most widely used aluminum salts are aluminum sulfate and sodium alumiuate. One or both may be incorporated into the reaction mixture. I n order that a bard suitable product may be obtained, the water must be removed. This may be done by hreaking up the gel, ator-pressing, and tlien drying, or by drying alone. The drying reqirires a large amount of heat, especially when the gel is not filter-pressed. After drying, the residue must be rednced to the proper size. This is accomplished by immersing the chunk6 in water, which causes the material to crack up or decrepitate into small particles. The decrepitated material is washed, dried, and screened before being packed.
S
FKEEZING i'rroceaa During the last few years a large amount of work has been done in this laboratory on the subject of zeolites, and in particular the dehydration of these gels by freezing. There is some precedent for using the freezing process. Oakes (2) has recommended freezing gelatinous precipitates and filtering the solid granules obtained, as a method of avoiding tedious filtrations and washings in quantitative analysis. The materids frozen by Oakes contained a relativelv small amount
ing with it a l a r g e
amount of the free alkali and other soluble salts, leaving a deposit of suitably formed aeolit,e particles. In these experiments the raw materials used were comiiiercial sodium aluminate in solution and commercial sodium silicate. The almrinate solrrtion was diluted with Soft water to a strength of 0.20 mole of alumina per liter. Stiff wholevolume gels were made by mixing equal volumes of prepared sodium aluminate solut,ian and sodium silicate solution. The concentration of t l w silicate solution was such that the resulting gel would liave tlie desired mole ratio of silica to alumina. This pel was liroken up, filter-pressed in a plate and frame press, frozen, thawed, washed, drained, dried, and tcsted for capacity or aater-softening ability. hlErHlJl) OF
EhEEZINQ
The gel or filter cake to be frozen was placed irr %gallon ice cream containers wliicli were irnmemd within about 10 em. of the top in the brine tank of a refrigeration system. The mean temperature of the brine was -6.7" C. After 48 hours the cans wereremoved from the tank, washed with fresh water, and then inverted on the draining board. Soon after being inverted, the frozen mass would slide out. I n a few hours the ice would melt, leaving behind moist zeolite particles.
METEODo+ TESTING
In order to obtain comparable results, the material was screened on Tyler standard screens. Commercial zeolites vary in size from 10 mesh (2.W mm.) to 50 mesh (0.297 mm.). All zeolites tested were of the 28-35 meslr (0.589-0.417 mm.) size. The testing apparatim consisted of a battery of thirty miniature water softcners. Each softener was made of a piece of glass t u b i n g a p proximately 1.6 em. in diameter, 38 cm. long, and fitted with a one-hole r u b b e r stoppcr at each end. The glass tube was placed vertically in a rack. The upper s t o p p e r was connectedbyasmallglass tube and a r u b b e r tube to a pet cock on the common header. Over t h e bottom FIGURE1. MINIATURE U'APSRSOSTENEM
I N D U S T R I A L A N D E N G I N E E R I NG C H E M I S T R Y
794
stopper a disk of fine monel metal screen was placed. The disk fitted snugly against the walls of the tube. A small glass tube and about 50 cm. of rubber tubing were joined to the lower stopper. Before testing a zeolite, the screened material was mshed free of phenolphthalein alkalinity. The required amount was then placed in a softener so that a 20-cm. bed was obtained. At the start of each run, the material was backwashed to insure a uniform bed, after which 16 cc. of fine-grained salt were introduced into the top of the softener. The salt was slowly dissolved by passing water downward through the softener a t such a rate that 20 minutes of salt contact were obtained with the zeolite. When soap tests indicated that the water issuing from below was soft, the water was run into a container. The exact amount of "zero" water obtained was caught and measured. The flow of water downward during the softening process was adjusted to a rate such that a t the end of 4 hours the run would be complete. A run was considered complete when soap tests showed 17 p. p. m. of hardness. At all times during the backwashing, salting, and softening periods the bed of zeolite was kept below the water level in the tube. By measuring the volume of zeolite, the amount of water softened, and the hardness of the water, it was possible to calculate the softening capacity of the material tested. The laboratory water varies but little from a hardness of 376 p. p. m. The capacity was calculated as grams of calcium carbonate equivalent of hardness removed per liter of zeolite. The method of testing a zeolite for capacity, as outlined, is a dynamic test but is very satisfactory for comparative purposes if all tests are run exactly alike. Such a test does not yield the absolute value of the exchange but is more valuable than an ultimate test, because it gives a better indication of what may be expected of a zeolite when put into a water softener. The amount of salt used in regeneration, the rate a t which the water is run through the softener, the size of zeolite, time of salt contact, depth of the bed, hardness of the water, composition of the water, and perhaps the diameter of the tube-all seem to affect the results. This being the case, it is necessary to arbitrarily select the conditions of testing and to adhere to them. As a check for comparison with results obtained in practice, some commercial zeolites of 28-35 mesh size were screened out on Tyler screens and tested as described. The average of ten runs for Doucil was 34.6, and for DeCalso, 34.4 grams of calcium carbonate equivalent per liter of zeolite. Two samples of each of the commercial zeolites were found to check within 5 per cent. Fine Doucil has been rated a t approximately 27.5 grams per liter in commercial installations.
EFFECTOF FREEZISG The filter cake from several gels was divided into two portions. One portion was dried a t 55" C., and the other portion was frozen a t -6.7" C. It was found that freezing these gels resulted in a material having a higher capacity than when dried in the ordinary manner. Some typical examples are given in Table I. The values are the averages of ten runs. FILTER CAKE TABLEI. EFFECTOF FREEZINQ ZEOLITE
CAPACITY, CaC03 EQUIV. PER LITERO F ZEOLITE Frozen Grams
A B
C
56.0 55.0
57.8
Not frozen Grams
48.0 45.6 53.8
The compositions of the gels of zeolites A , B , and C were quite different as regards total alkalinity and silica-alumina ratio. It is not intended that the values given for Doucil and DeCalso should be compared with those given in Table I, as the materials made in the laboratory had a much lower
Vol. 21, No. 7
silica-alumina ratio than these proved commercial products. 1-111 of the testing was done a t the same time and under the same conditions. EFFECTOF FILTERIXG o x PARTICLE SIZE I n order to be a satisfactory zeolite, it is essential that the resulting material be of correct size, hard, and durable. The effect on particle size of filtering the gel before freezing iY shown in Table 11. T.4BLE
11. EFFECT OF FILTERING ON SIZE
SCREE+
Size Opening Mesh Mrn 10 1.651 14 1.168 20 0.833 28 0.589 35 0,417 48 0.295 65 0,208 100 0.147 Passing 100 mesh Total Uniformity coe5cient (3) Effective size, mm.
-AMOUNT Press cake 4,satm.
RETAINEDFilter cake Biichner funnel
Gel not filtered
?6
%
%
1.1 7.3 18.3 21.6 20.3 11.6 11.4 2.6
0.0 0.0
0.0
1.4 8.8 2.5,s 27.0 2 5 . ;3 4.7 7.0 100.0
0.1 0.6 7.3 22.4 46.0 10.8 12.8 100.0
5.5 100.0
2.93
0.230
2.14 0.186
0.0
2.18 0.128
The values given in Table I1 include all of the fines and dust passing a 48-mesh (0.295-mm.) screen, which amounted to about 19.1 of the total weight obtained from the press cake. Methods of reducing this loss are being studied. DISCUSSION OF RESULTS These experiments have been conducted on gels having a low ratio of silica to alumina. Colburn and Smith (1) have come to the conclusion that highest capacity is obtained with a ratio of 2.5 moles of silica to 1 mole of alumina. Since then, most of the work has been confined to gels having this composition. It should be pointed out that the range investigated by Colburn and Smith was rather narrow, 3.5 being the highest value recorded. With this type of gel the experiments show that a material of the correct size may be obtained by freezing under the proper conditions. It seems that two factors affect the sizethe amount of water in the material frozen and the rate of freezing. Qualitative observations indicate that rapid freezing is conducive to the formation of small particles. The results indicate that freezing has a beneficial effect on capacity. Whether or not this increase is secured with gels having a higher silica-alumina ratio or having a different degree of alkalinity, is not known. However, there are some indications that high alkalinity increases the beneficial effect of freezing as regards capacity. These points are under investigation. So far, freezing has not yielded a product having the desired hardness, unless the surface moisture is entirely removed by drying. The amount of drying to accomplish this is small. Exposure of a few hours to the open air was sufficient in the laboratory. If this method were put into practice, a drier would probably be required. As it is desirable to secure a harder product, the amount of dehydration obtained by freezing a t lower temperatures is being investigated. Many details must be worked out before a definite announcement of the commercial feasibility of this process can be made. LITERATURE CITED (1) Colburn, F. H . , and Smith, F..Iowa State College, unpublished undergraduate thesis, p. 19 (1930). (2) Oakes, B. J.. dissertation submitted to the faculty of Columbia University, New York, p. 5 (1924). (3) Tyler Company, Cleveland, W. S. Catalog 53, p. 38 (1927). RECEIVED February 19,1932.
