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Vol. 17, s o . 9
with an average particle size of about 8 microns. The that of ordinary plaster of Paris and gypsum. Sample G was material was uniform in size. This sample hydrated very very rapid setting, owing to traces of acid left from the chemrapidly and complete hydration %'as attained in about 4 ical process. This acid would have to be removed before the weeks. At first the rate of hydration was about the same as material could be used commercially. The tensile strength that of Sample D,but i t maintained a much more rapid rate could not be tested as the set was broken up in mixing. until complete hydration as there were comparatively few of Table I1 the larger sized particles present. Watercarrnng All of these samples, with the exception of Sample C, were capacity agitated during each week day, but Sample C was only agiDer 100 Unretarded Temperature -grama setting tated once a week when a sample was removed to test for time TENSILE STRENGTH calcination plaster Sample '.C. Grams HrO Minutes Kg./sq. cm. Lb./sq. i n . moisture content. A comparison of the rates of hydration c 150 82.5 19 18.20 259.3 shows that agitation cuts down the time required for hydraD 160 78.0 9 16.29 232.0 E 150 87.5 10 18.43 262.5 tion by about 20 per cent. If the sample were agitated conE 200 87.5 15 21.17 301.5 tinuously rather than during the day only, the rate of hydraG 150 Uncertain ADurox. B but approxition would be increased even more. mating those above Sample F consisted of anhydrite obtained as a by-product in the manufacture of hydrofluoric acid from fluorspar by the Practical Application action of sulfuric acid. The material as obtained was very well crystallized, with a maximum particle size of about 70 The hydration process described herein may be the first microns and an average of about 9 microns. Its rate of hy- stage in progress toward a wider utilization of anhydrite. It dration was found to be exceedingly slow; although it had has been shown that anhydrite can be hydrated by a very the same average particle size as Sample D,it had taken up simple process within a reasonable period of time, and that only 1 per cent moisture after one week, whereas Sample D the product can be calcined to give plaster products comparhad taken up 10 per cent. The sample was then ground wet ing favorably with those manufactured from natural gypsum. for a short time in a pebble mill, after which the well-defined Immediately the question arises-Can the process be used crystals were found to have been broken down, the maximum commercially or is the cost prohibitive? particle size decreased to about 30 microns, and the average This first step which demonstrates the easy possibility of particle size to about 7 microns. This material is designated hydration is important to an extent that can be determined Sample G. Its rate of hydration was then more rapid than only by subsequent developments. It is highly desirable any of the other samples tested, complete hydration being that commercial scale tests be made to determine the actual attained in about three weeks. This increase is, of course, cost of hydration. A suggested process is tube mill grinding, out of all proportion to the decrease in particle size. Evifollowed by agitation in tanks, partial dewatering, and calcidently, the crystalline form is a factor as well as the particle nation in rotary kilns in slurry form similar to the wet process size, the abrasion or roughening of the crystal faces causing a of cement manufacture. Anhydrite is easily ground, no classimarked increase in the rate of hydration. Some evidence fication by size is requisite, and complete hydration is unwas obtained in this test that the hydration is a t least to some necessary as the standard specifications for commercial gypextent a process of solution, and reprecipitation for the hy- sum require a minimum of only 65 per cent CaS04.2Hz0. drated substance was again well crystallized, with an average The degree of hydration would probably be governed by the particle size of 17 to 18 microns, therefore much coarser strength requirements of the resulting plaster. The process grained than before hydration. would seem to be particularly advantageous for dealing with The hydration rate of these seven samples is shown in by-product anhydrite which is already in a fine-grained form, Figures 1, 2, and 3, the various curves being marked A , B, etc., and which, because of its purity, would form a high-grade to correspond with the samples. Figure 4 shows the change calcined product commanding a comparatively high price. in rate of hydration in relation to particle size, average rate of hydration being plotted against average particle size. A tabulation of the data used in plotting this curve is given in Surface Tension and Foaming Table I. ~
Table I Time of Average particle hydration size Weeks Microns 3 7
A
Editor of Industrial and Engineering Chemistry:
S
4
Per cent hydrated in 3 weeks 100 75
9 10 12 18
9 11 13 18
27.27 23.07 14.44
33.33
These results indicate that the hydration of anhydrite is greatly facilitated by fine grinding and the abrasion of crystal surfaces, and that the rate of hydration is further increased by agitation of the fine particles in water. When Samples C, D,E, and G were completely hydrated or practically so, they were filtered off, dried a t 45" C., and then calcined to plaster of Paris. Each was then tested for watercarrying capacity, setting time, and tensile strength according to the standard methods of the A.S.T.M. From the data in Table I1 it is seen that the hydrated material makes plaster of Paris well above the specifications of the A.S.T.M. which demands a tensile strength of 200 pounds per square inch, The material is excellent in appearance. As a retarder in Portland cement its action is fully equal to
We note in Millard and Mattson's recent article [THIS 17, 685 (1925)] a quotation from a n article by me [THIS JOURNAL, 15, 1239 (1923) ] which, although correctly quoted, represents a possibly incorrect statement which should be explained. There is a very close connection between salts in solution and foaming; there is, also, a close connection between the ease with which bubbles pass the film on such solutions, between the liquid and the gas, or air, above. In our efforts t o explain foaming troubles we have unfortunately confused surface tension with this characteristic of the surface film. As a result, our statement as published should have been reversed, as an increasing surface tension condition no doubt better explains the condition which will be followed with a decrease of so-called foaming, and a more rapid breaking of steam bubbles. However, although changing surface tension may not be the proper explanation of foaming, I am convinced that in the surface film and the factors relating t o it is hidden the explanation of what causes a water to foam, and I do not consider the conclusion of the article responsible for this letter sufficiently positive one way or another to be considered in any way final. D. K. FRENCH JOURNAL,
DEARBORN CHEMICAL CHICAGO, ILL. July 8, 1925
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