INDUSTRIAL AiVD ENGINEERING CHEMISTRY
June, 1930
The time of contact of the vaporized naphtha solutions with the caustic was extremely short. The solutions were passed in a t the rate of 150 to 200 cc. per hour, which means that the vapors were in the presence of the caustic not more than 1 or 2 seconds. Yet the effect was very pronounced in some cases. Mercaptans in crude naphthas were removed very completely, so that the doctor test was negative. I n all the pure compounds tested the distillates were also sweet to doctor. The compounds tested and the results are given in Table 11. From the above results it is seen that alkyl sulfides, a t least the lower ones, are very little affected. The small differences are probably not significant except that the two heptyl sulfides are very considerably attacked. It is not clear just why there should be this difference, which apparently is not connected with structure at all but with molecular weight.
611
Aluminum chloride reacts very differently on alkyl sulfides of differing structure, but its effect does not vary with the molecular weight. Phenyl sulfide is not greatly affected, although it is attacked more than the alkyl sulfides. The olefinic allyl sulfide is thoroughly removed. Thiophene is attacked to about the same degree as diphenyl sulfide. Yet the two disulfides are very thoroughly removed, quite as thoroughly as mercaptans. At present this seems to have no practical significance, since disulfides have not been reported in any quantity in naphtha, although they are undoubtedly formed in traces in doctor sweetening, especially if air blowing or free sulfur is used. Literature Cited (1) Youtzand Perkins, IND.ENG.C H E M . , 19, 1247 (1927)
Hydration of Calcined Gypsum’ W. C. Hansen EXPERIMHNTAL L A B O R A T O R I E S , AMERICAN CYANAMID
ALCINED gypsum or plaster of Paris is produced by the following reaction:
C
+ +
CaSOd. 2H20 heat = CaS04.l/2HZ0 l1/zH20
COMPANY, AND STRUCTURAL
A study has been made to determine why soluble materials when added to calcined gypsum-water pastes either accelerate or retard the setting of the pastes. From a study of time-temperature curves obtained from the hydrations of calcined gypsum in various solutions, it appeared that the controlling factor in the setting was the rate at which precipitation took place from the paste. A study of the rate at which gypsum precipitated from carefully mixed solutions of calcium nitrate and ammonium sulfate to which had been added other salts showed that foreign salts markedly affect the rate at which gypsum precipitates from its supersaturated solution. This effect of foreign materials upon the rate of precipitation of gypsum from its supersaturated solutions appears to explain the ability of foreign materials to accelerate or retard the setting of calcined gypsum pastes.
This calcined gypsum when mixed with water rehydrates and sets into a hard mass. It is well known that almost all materials which are a t all soluble in water will, when mixed with calcined gypsum and water, either accelerate or retard the hydration of the calcined gypsum. Two theories have been offered to exulain the action of foreign haterials upon this reactio-n of hydration. Neither of these theories adequately explains this action. This investigation was undertaken in an effort to find a more complete explanation for this action of foreign materials upon the hydration of calcined gypsum. Theories on Setting of Calcined Gypsum
Lavoisier (4) probably was the first to offer an explanation for the setting of calcined gypsum. His explanation was that gypsum which had been dehydrated by heating took u p water with avidity producing a sudden and irregular crystallization. The small crystals of gypsum which precipitated, becoming confused with one another, formed a very hard mass. Marignac (6) observed that calcined gypsum with water gave solutions far more concentrated than could be obtained from gypsum-that is, the solutions from calcined gypsum were supersaturated with respect to gypsum. Le Chatelier (5) studied the setting of calcined gypsum and ctated the theory which is generally accepted today. He said: On mixing burnt plaster with water each particle soon becomes surrounded by a layer of a solution which is saturated with respect t o the hemihydrate but greatly supersaturated with respect t o the stable gypsum. Crystallization of gypsum soon begins, either spontaneously or more probably from nuclei of
’ Received .4pril 25, 1930
GYPSUMC O R P O R A T I O N ,
LINDEN,
N. J.
gypsum which have persisted unchanged t h r o u g h o u t t h e burning process. Growth of the gypsum crystals t a k i n g place from many neighboring centers, radiating growths are formed, and the interlocking of these is a t least one of the causes of the strength of the mass.
This theory has been modified to include a colloidal stage. Cavazzi ( 1 ) and others (7, 10) claim that calcined gypsum and water first form a colloidal gel or an adsorption complex from which the gypsum crystals form. Combining these two theories. one mav outline the mechanism of the hydration and setting of calcined gypsum into the following stages:
(I) Calcined gypsum plus water forms a gel or an adsorption complex in which only physical or secondary chemical forces are concerned. (2) The adsorption complex dissolves to produce a solution supersaturated with respect to gypsum. (3) Gypsum crystallizes from the supersaturated solution.
This seems to be the mechanism as Xeville sees it. Theories Explaining Accelerating and Retarding Influences of Foreign Materials upon Hydration of Calcined Gypsum
Rohland (8) studied the influence of various materials on the rate of setting of calcined gypsum and concluded that generally those materials which increased the solubility of the hemihydrate in water accelerated the setting, while those which decreased the solubility of the hemihydrate in water retarded the setting. A number of investigators haye found exceptions to Rohland’s theory. Welch (11) has summarized the literature on this subject, so it need not be repeated here. Traube and Neville claim that foreign materials added to calcined gypsum accelerate or retard the formation of the gel or adsorption complex, thus accelerating or retarding the setting. In the case of salts they claim that the action is due
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
612
principaIly to the positive ions. The negative ions have a weaker opposite effect. Neville states: The exact role of the accelerating ions is not clear, although it is evidently twofold, that is, the adsorption of the water to form the gel is accelerated by these ions; then the chemical reaction between the hemihydrate and water is catalyzed.
Experimental
The hydration of calcined gypsum is an exothermic reaction, and the rate of the reaction may be obtained by measuring the rate of evolution of this heat. Emley (2) has discussed this method in some detail. The method used in this study was as follows: Two hundred grams of calcined gypsum were stirred with 140 cc. of solution in a paper cup for I,’? minute. A thermometer was then inserted about midway into the gypsum paste, and the cup was placed in a large covered beaker. 20
b
b
2
z
\ /o
F
6 0
/O
10
$0
40 TIME IN
50 Y~uwrEs
60
70
60
Time-Temperature Curves Obtained for the Hydration of 200 Grams of Calcined Gypsum in 140 cc. of Solution 1 - 0.1 N NHaCzHs09 2 - 0.1 N (NHdzHPOa 3 - Distilled water
- 0.1 N NHiCl 5 - 0.1 N (NH4)zSOa 6 - 0.1 N NHaCzHaOz in which 10 grams of calcined gypsum had been stirred 35 minutes 7 - 0.2 N NH4CI 8 - Distilled water in which 10 grams of calcined gypsum had been stirred 35 minutes 4
Typical time-temperature curves obtained by this procedure are given in the accompanying graph. It is seen that these curves are made up of a more or less flat portion and a steep portion. If the heat changes possible in this reaction are analyzed, it is possible to see why this type of curve is obtained. (1) Calcined gypsum plus water will liberate a little heat due to the heat of wetting. Probably most of this heat is liberated by the time the mixing is completed and before the thermometer is inserted into the mass. Therefore, these curves probably do not show this heat. (2) Calcined gypsum dissolves to form a solution supersaturated with respect to gypsum. This occurs with an adsorption of heat. (3) Calcined gypsum hydrates to form gypsum. This occurs with a liberation of heat. (4) Gypsum precipitates with liberation of heat.
The heat of solution (2) is balanced by the heat of precipitation (4),so that the heat manifested is the heat of hydration (3). It is not known whether the heat of hydration is liberated when the hemihydrate dissolves or when the gypsum precipitates or during both of these changes. However, there is very little temperature rise in these mixtures until gypsum precipitates. Therefore, it would seem that during the time interval represented by the flat portion of the curve the solution is becoming supersaturated with respect to gypsum, with little or no precipitation of gypsum, and that during the time interval represented by the steep
T’ol. 22, X O . 6
portion of the curve the gypsum is precipitating and hydration is proceeding rapidly. Curve 3 represents the hydration of this calcined gypsum in distilled water. Curves 4 and 7 were obtained for the hydration in 0.1 and 0.2 N ammonium chloride solutions, and curve 5 for the hydration in 0.1 N ammonium sulfate. The solubility of gypsum in a number of different sulfate solutions has been determined by various investigators (9). They have found that ammonium sulfate and many other sulfates decrease the solubility of gypsum. Ammonium sulfate, as is seen from curve 5 , and other sulfates accelerate the hydration of calcined gypsum. This is contrary to Rohland’s theory. Likewise the solubilities of gypsum in ammonium chloride and other chlorides have been determined. These salts increase the solubility of gypsum and, as seen from curves 4 and 7 , ammonium chloride accelerates the hydration of calcined gypsum. This is in keeping with Rohland’s theory. The solubility of gypsum in water has been found to be increased by ammonium acetate. Curve 1 shows the hydration curve for calcined gypsum in 0.1 h’ ammonium acetate. This salt, which increases the solubility, retard? the hydration, which is contrary to Rohland’s theory, if we assume that these solutions have the same effect upon the solubility of the hemihydrate as they have upon the solubility of gypsum. Welch gives some values for the calcium sulfate dissolved from calcined gypsum in certain solutions which indicate that the solubility of the hemihydrate increases in about the same manner as does the solubility of gypsum in salt solution. From the results obtained with ammonium salts in this investigation and from the results given in the literature for other salts, it appears that the solubility of gypsum in the salt solution is in no way a criterion of the power of the salt to accelerate or retard the hydration of gypsum. As pointed out above, Neville believes that the accelerating effect of salts upon this hydration is due in part to the action of the positive ions in the formation of the adsorption complex between the calcined gypsum and water. From the curves given it is seen that ammonium sulfate and chloride accelerate the hydration while acetate and phosphate retard the hydration. This hydration was also studied in 0.1 &Y ammonium citrate. It was found for the citrate that the time-temperature curve remained flat for 4 hours and then rose 4 degrees in 2 hours. These results indicate that neither the solubility nor the colloidal theory explains the behavior of these salts on this hydration. Curve X represents the results obtained by adding 190 gram? of calcined gypsum to 140 cc. of distilled water with which 10 grams of calcined gypsum had been stirred for 35 minutes. It is seen that this treatment eliminated almost completely the flat portion of the curve. Curve 6 was obtained in the same manner with 0.1 N ammonium acetate. The rate of hydration of the calcined gypsum was increased markedly by this treatment. The only difference in the pastes for curves 3 and 8 was that for curve 8 the paste contamed some freshly precipitated gypsum crystals. These crystals could seri-e as nuclei t o cause the precipitation of gypsum as fast as it was formed by the dissolution of the hemihydrate. All the curves indicate that the solution of hemihydrate proceeds rapidly after the first few minutes. Therefore, one would expect that the solution of hemihydrate would proceed rapidly from the very start unless some condition existed to retard it. Comparison of curves 6 and 8 with curves 1 and 3 indicates that the hydration proceeds slowly a t first because the gypsum does not precipitate as fast as the hemihydrate can dissolve; that is, in samples 6 and 8 the reaction went rapidly from the start because the psstes
June, 1930
I-VDUSTRIAL 9,VD ELVGILVEERINGCHEMISTRY
were inoculated with a large number of newly formed gypsum crystals which caused the gypsum to precipitate probably about as fast as the hemihydrate dissolved. It appears from this that the speed a t which a calcined gypsum paste hydrates is controlled by the rate at which the supersaturated tolution can precipitate gypsum. If this is truc, then ain~noniumchloride and sulfate must increase the rate at which gypsinn will precipitate from a supersaturated solution, whereas ainmoniuni acetate, phosphate. and citrate must decrease the rate at n hich g y p w n precipitates from a super-aturated wlution. A number of experiment\ were formed in an effort to obtain data to prove or disprove the above hypothesis. It IQ difficult to get quantitative data when working with supersaturated solution
