Soluble Anhydrite as a Desiccating Agent TYPICAL (;RAN~;LE SIZES OF
TEE
N1:w DESICCANT
I. Preparation and Gerrcral CXiaracteristics \V. A . i l a ~ ~ AND o ~JAMES ~ i R. WITHROW,Ohio Slhtt? I n i v e r s i t y , Cultiinbus, Ohio
F
OR some years it lias been knon-n that, when cal-
mental i a c t m . l'licsa discrepancies >%-erep o i n t e d out by Davk (,5) iuliose important paper cium sulfate dihydrate, C:tSOI.2€120,or the fiali-hydrate, on the subject was one of tlie first t u a p p c a r in E n g l i s h . C~SOC'/~H~O, is h e a t e d , t h e Davis p l o t t e d tlie d a t a pubproperties of tlie anliydrous salt obtained vary vith tlie temperalished by 1,eChatelier aiid obtained a curve slioming transiture and d u r a t i o n of heating. Tliir filct was first observed by tiun points a t 128' and 163' C. Potilitzin (17)in tliestudyof t h e Ife also noted tlie property of soluble anhydriteof takingmoissolubility of anhydrous calcium siilfat.e, a n d was verified by ture from tlie ziir to r e i w t to the form of the half-hydrate. Lacroix (11) ivlio distinguislied crg~tnllo~apliically between the Glasenapp (8)studied micros c o p i c a l l y tlie products ubnnhgdrous salt obtained from the d i l i y d r a t e Ijy long heating at tained a t various tempcra.turee and noted tlrst those obt.ained lGOC. and the rintural anliyabove 400" C. dissolred m o r e ilrite. Cloca (4)reitfirmed the slovly a n d r e h y d r a t e d more i.oncliwion reached by L a c r o i x slowly or not at all. Wilder Iwing his decision on his ob(2//), coinnicriting 011 tlie disetwation that,,unlike the natural c o r d a n t s t a t e m e n t s in the a n h y i l r i t c or the product obtained at high temperat.ures, tlie liberature, stat,ed: "Davis (5) in 1907 said that in spit.e of the salt obtained a t 145" readilv recomijinerl rvitii water. Van't work of such well-known cliemists as Lavoisier, M a r i g n a c , Hoff (9)also recognized this Le Cliatelier. van't Hoff. and rlist,inction and, because of the grcat,er solubility of the material produced a t low tempera- others, tlic confusimi tliut exists on t,he subject is without tires, and to distingiiish it from the natural anhydrite, parallel in inorgaiiic clieniistry. In 1916 Kcane (10) was c:iIled this form of tlie anliydrous salt "soluble anhydrite." obliged to reiterate this statement, alt,liougli in tlie meantime Many workers have studied the dehydration of calcium important articles 011 t,he subject had been piiblidied by wlfatc and endeavored to define the conditions under xhich Desch, Blake, Hursli, Rohland, and Glasenapp." More each of the several transformations takes place. Thus recently Cliassevent (S),using the conductivity method, has , a ~ o i ~ i(el rg ) ,working with gypsum, arrived a t a good under- sliown that nnliydruus calcium sulfate prepared from the distanding of the loss and recovery of wat,er iii the preparation hydrate by heating below 313" C. hydrates instantaneoi~sly and setting of plaster of Paris. Le Chatelier (is) pointed on contact with water to form supersaturated solntions of tlie out the existence of two hydrates and explained the calcination haii-hydrate. of tlie dihydrate and the identity of plaster of Pais BS the Tbus, while Lavoisier (18) implied that l i s enpbanat,ion lialf-hydrate practically as t.hese facts are understood at pres- left nothing t,o be desired, the whole matter of tlie phase ent,. Cloez 0,failing to distinguish between tlie ~:liemically transformations of calcium sulfate and the properties of its combined and hygroscopic moisture taken up by the anliy- deliydration products has remained a suhjcct of controversy drous ponder, failed to recognize tlie lialf-hydrate as a distinct to the present time. As far as it has been possible to deterphase, Van't Hoff (9)and co-workers did considerable irork mine, no use has ever been made of t.lie affinity of soluble on vapor pressure equilibria and transition temperatures and anliydrite for water, aiid only varying and general statements arrived at conclusions wliicli were not i n accord >\-it11esperi- are found in tlie literature wliicli aould sugpst. the possibility
lrldirsfrial applicalioa.s of calciuni sulfale IIUWIirrdoJore been confined largely lo its use in lite form of the dihydrate u,s a n ingredient i a fertilirers; as a retarder in Portland cement; as a filler i n papers, fabrics, etc.; in the form ofthe half-hydrale (plaster of Paris) in the m r i o m plasfers, architectural and art molding, and niollpliag; arid in cerarnic and metal casting. This series of lwo papers brings to the cheniical and other industries a property of calcium sulJate not hitherto recognized and describes this compound in fornis not heretofore available. The new product is an eflcienf, versatile, inexpensive, regenerafive desiccaiil and absorbent, rzulral, inert, and irlsol~zblein organic liquids, prepared in fhe forni of powder or granules of any desired size, arid well suited in ils physical properties for use in lhe drying of industrial gases, air-coriditioaing, the drying of solids, and in the corriplele and rapid drying of the alcohol.? and rnosf other organic 1iquid.s.
_"
653
654
INDUSTRIAL AND ENGIKEERING CHEMISTRY
of such use. Soluble anhydrite has been known to occur in small amounts as a contamination in plaster of Paris, formed by partial overcalcination. By its gradual reversion to the half-hydrate, it causes changes in the properties of the product. It has therefore been considered only as an elusive
Yol. 25, No. 6
Soluble anhydrite, when exhausted as a desiccant by conversion to the half-hydrate, may be regenerated by reheating exactly as TThen originally prepared, and the material may be re-used and regenerated repeatedly without any apparent alteration in its physical properties or its affinity for water.
PHYEJCAL AKD CHEMICAL PROPERTIES
FIGURE1. TEMPER.4TURE GRADIEXT DECOMPOSITION OF GYPSUM (5)
FOR
and undesirable constituent of other products and not as an available material of value on its own account'. PREPAR.4TION
I n this work the material used was a snow-white massive crystalline form of natural gypsum containing approximately 99.8 per cent of CaS04.2H20. The mineral was air-dried and crushed to inch (0.95 cm.) and finer. By the use of a series of screens, quantities of uniformly sized granules were obtained. The fraction passing l/lo inch (0.25 cm.) was ground to powder. The sized granules were dehydrated to 1 inch depth in clean iron or by spreading in layers of aluminum pans and heating in an electric oven a t 230" to 250" C. for 2 to 3 hours, the larger granules requiring the longer time. The powdered material may be dehydrated in the same way, but in this work the process was hastened by heating portions of about 250 grams in an &inch (20.3-cm.) porcelain casserole over a Bunsen burner while stirring with a stout thermometer. Figure 1 represents the original data of Le Chatelier as plotted by Davis ( 5 ) . Figure 2 shows the time-temperature relation during dehydration as observed in the preparation of typical batches used in the course of this work. The preparation of soluble anhydrite for use as a desiccant involves merely the heating of either the dihydrate or the halfhydrate to practically complete dehydration a t temperatures safely below 300" to 313" C. If heated much above these temperatures or a t these temperatures for any considerable time, the product recombines with water too slowly to be effective as a desiccant. Starting with the dihydrate, the reaction CaS0~2H~ e0CaS02/2Hz0 1'/~HZO (1)
+
proceeds actively to the right, at and above 120" C.; the second transformation CaS0g1/2H20 e CaSOa
+ '/zHzO
(2)
goes forward rapidly a t and above 170" C. I n the present work the heating was carried to 230-250" C., as these temperatures proved sufficient for complete dehydration and presented no danger of dead-burning. The dehydrated material, either powdered or granular, was transferred while hot to a desiccator containing a previously prepared portion of the same material, or other efficient desiccant, or was placed directly in strong glass containers and tightly sealed. The cooled material was ready for use, and, in handling, every precaution was taken to avoid contact with moist air.
The pure dihydrate, CaSO4.2H2O,contains 20.92 per cent of water of crystallization. Dehydration involves no appreciable change in volume, and the resistance offered by the dehydrated granules to crushing and abrasion is only slightly less than that of the mineral dihydrate. This is contrary to the general impression that dehydration reduces all crystalline hydrates to powder. Thus hlellor (15) in discussing water of crystallization states: "Crystals of gypsumCaS04 2HnO-form white chalky powder when the water is driven off." Foster (7') explains: "When a hydrate is heated, it gives up water. This may be shown by heating a crystal of gypsum or of blue vitriol in a clean, dry test tube. Water deposits upon the colder part of the tube, and a powder-the anhydrous salt-is left in the test tube." This is only one of the many unfortunate errors which persist in the literature about gypsum. When leaves of selenite 0.5 mm. or less in thickness are dehydrated, they become opaque but retain their form and size and much of their physical strength. Metal castings are frequently made in plaster molds which have been completely dehydrated. The granules of soluble anhydrite prepared as described above cannot be crushed in the fingers, they withstand repeated handling in large masses with only slight dusting, and they may even be washed with water or other liquid xithout disintegration. There is no
FIGURE2. TIME-TEMPERATURE RELATION FOR DEHYDR.4TI03 OF GYPSUM ON LABOR.4TORY SC.4LE (250 grams powder agitated)
shrinkage or expansion of the granules during the absorption of water in use or in the subsequent regeneration. They remain snow-white unless definitely contaminated. Since there is no shrinkage on dehydration, it is clear that the dehydrated granule must be of a highly porous structure. Accepting the values 2.32 and 2.96 as the densities of the dihydrate and anhydrite, respectively, calculation shows the dehydrated granule to contain approximately 62 per cent by volume of solid and 38 per cent pore space. To fill them completely, reforming the dihydrate, these capillaries would require 2 molecules of water, or 26.45 per cent by weight of the anhydrous solid. I n its action as an efficient desiccant, however, soluble anhydrite returns only to the h a l f - h y d r a t e the reversal of Equation 2-and in this reversion only onefourth of the capillaries may be assumed to be refilled with water. These refilled capillaries are distributed uniformly throughout the body of the granule. All the capillaries, however, are of such small diameter as to offer an extremely
656
INDUSTRIAL
AND E N G I N E E R I N G CHERTISTRY
of nearly complete reversion to the half-hydrate-tLat is, to the point of saturation considered as a desiccant. The straight-line character of these curves shows that there is no clogging of the surface of the desiccant, and that the rate of absorption of water vapor by soluble anhydrite depends
Vol. 25, No. 6
similar to those in the first run except for the fact that the samples over the two forms of soluble anhydrite dried more rapidly than those over phosphorus pentoxide. The results of these three experiments in drying sawdust are summarized in Table 11. Experiments similar to those on moist sawdust were made on the drying of brown sugar and ground coffee, these materials being selected because of their low moisture contents and very low rates of drying. I n these experiments, e q u a l w e i g h t s (100 grams) of the same five desiccants were used. The results obtained in the drying of brown sugar are shown in Table 111. The plotted data froni both these experiments gave drying curves for all desiccants which are practically identical.
THEDRYING OF AIR
DESICC.4.VTS AT ROOMTEMPERATURE either upon the rate of diffusion of water vapor or upon the rate a t which moisture escapes as vapor from the moist body. COhfPARISON WITH OTHER DESICCANTS The relative effectiveness of powdered and granular soluble anhydrite, phosphorus pentoxide, concentrated sulfuric acid, and a standard grade of granular calcium chloride for drying very moist substances was determined as follows: Porcelain dishes were placed in the upper compartments of five 6-inch (15.2-em.) Scheibler desiccators containing, respeetively: phorphorus pentoxide, 100 grams; sulfuric acid, 183 grams; calcium chloride, 100 grams; powdered soluble anhydrite, 200 grams; and granular soluble anhydrite, 200 grams. The larger amounts of soluble anhydrite were taken since its total water-absorbing capacity is lower than that of any of the other desiccants. Approximately 2-gram samples of a uniform. fine-grained, moist sawdust were weighed in duplicate into 30 x 50 mm. glass-covered weighing bottles and were placed in the desiccators supported over the desiccants on n-ire gauzes. Taking all precautions t o give the samples uniform treatment in every respect, the bottles were covered and weighed at intervals t,o constant weight. The results are shown in the form of drying curves in Figure 4. The samples dried most rapidly over phosphorus pentoxide followed by those over powdered and granular soluble anhydrite, sulfuric acid, and calcium chloride in the order named.
Any desiccant t o be of general usefulness must be adapted in its physical characteristics and in the avidity with which it absorbs water in the vapor phase to be used in drying columns and tubes for the drying of air and other gases. In the following two experiments, three ordinary glass-stoppered U-tubes containing 0.32-0.25 cm. granular soluble anhydrite were connected in series-the second tube to act as a detector and the third as a guard. The results shown in Table IT' were obtained by drawing saturated air through the tubes by an aspirator; those in Table V were obtained by forcing air through by displacement with water. The columns headed Gain in Weight show successive absorptions by the same sample without regeneration, except as noted in Table V. The experiments show the total water-absorbing capacity of the granules and also that the drying efficiency is maintained until saturation is reached.
AIR DRYIKG AT HIGHV E L O C I T I E S -4further study of Table IV shows that a column of granules 1.3 X 15 em., containing 26 grams, completely dried 70 liters of air saturated with water a t 25" C. passing a t the average rate of 19.5 liters per hour, and for individual runs a t the rates of 20 and 25 liters per hour. In another experiment a column 2 X 220 em., containing 660 grams of 6-1/8 inch (0.42-0.32 em.) granules, completely dried saturated air a t 25" C. a t the rate of 46 liters per hour (10 liters in 13minutes), and allowed to pass unabsorbed only 0.6 per cent of the total moisture present in saturated air flowing a t the rate of 54 liters per hour (10 liters in 11 minutes). TABLE11. DATA ON DRYINGSAWDUST OVER DESICCANTS -1--3--2DESIC- MOISTURB DEBICMOISTURED E W - MOISTURE EXPERIMEIT C A N T REMOVED CANT REMOVED C A N T REXOYED Grams % Grams 70 Grams % 50 70.09 69.36 70.32 100 P201 100 69.76 69.14 183 70.00 183 HzSOI 183 68.99 125 69.63 69.63 100 CaCh 100 250 70.16 69.09 69.63 200 G.s. anhydrite 200 70.35 69.56 250 69.98 200 P.s. anhydrite 200
SGGAR OTER DESICTABLE111. RATESOF DRYISGOF BROWN CANTS AT ROOM TEMPERATURE Desiccant
FIGURE5. GAININ WEIGHTOF SOLUBLE ANHYDRITE STORED Sugar, grams OVER CONCENTRATED SULFURIC ACID A T ROOMTEMPERATURE TIME All samples reached constant weight within 60 hours except those over calcium chloride, which continued to lose weight for 120 to 144 hours. I n a second run of this experiment, all samples, including those over calcium chloride, dried to constant m-eight in 48 to 72 hours; but in a third run in which the weights of the desiccants were changed, the results were
Days 0 1 0
314 21 28 35 42
P2Os
HzSOI
2 OiS2 2.0342 -REMOVABLE
CaClz
G
2.0334
2 0318 CONTENT--
.\IIOISTURE
9. anP a anhydrite hydrite 2.0264
%
%
%
%
%
2.19 0.68 0.54 0.50 0.26 0.14 0.11 0.06
2.23 0.68 0.53 0.47 0.27 0.13
2.16 0.66 0.52 0.46 0.26 0.13 0.11 0.05 0.00 0.00
2.13 0.68
2.li 0.67 0.54 0.48 0.27 0.14 0.11 0.06 0.00
0.00 0.00
0.1'2
0.03 0.02 0.00
0.64
0.49 0.25 0.14 0.12 0.05 0.00 0.00
0.00
I,iirr* 10
in
10 in 10
0.2260
n.22sn 0.234'' 11. "190 0.1980 0.1'364 0 . 1836
35
a1
:in
S?
10 10
32
10
24 24
ID
Gram
MznulcJ 50
si
!1.1910
0.25i0
' 7 0
1
of dry air Imt d y iiiiii:li water as corresponded to the formation of tlie l i d iydrnte Triien heated a t 100" C. in a ciirrent of ordinary undried air. Table VI preients da,t.n obtained as i l l tire precrdin: esperiments, except that U-tube 1 v a s iicateil in a paraffin ibat,lr for 30 minutes just preceding and during encli rim at a q'ecific temperature; U-t,ulie 2 acted as a detectiir at room temperature. Consecutive experiments were rim UII tire same sample.
0.87 I .71 ?.8i
3.54 4.31 a .oh:
5.77
6.50 7.411b
U.lS0U ~ I M L ~ Clrl~Srr1.s l t OF SELEINITF:, 2 M\I. 'rliICK Cryatols a i riylit rendered o p s w e by dehydrstion.
After only ,short -
