I N D U S T R I A L A N D ELVGINEERING CHEMISTRY
966
Test Analyses
As a test of the method, 0.0250 gram of pure arsenious oxide was dissolved in 90 cc. of concentrated hydrochloric acid, 20 grams cuprous chloride added, and the solution distilled. After about 70 cc. of the solution had distilled over a further 25 cc. of acid were added and the distillation continued until a further 25 cc. of distillate had collected. At the same time a blank was run on the reagents used. Titration gave: -Arsenic, 0.0198 0.0007
In solution In reagents
Gram0.0195 0.0007
--
-
Arsenic returned Arsenic added
0.0191 0 . 0 2 6 0 X 0.7571
0,0188 0.0189
A Bureau of Standards standard steel sample (Fo. J l c ) , with arsenic given as 0.012 per cent, yielded on titration, 0.009 per cent and 0.010 per cent. Comparative determinations made on two arsenic bearing steels made by gravimetric methods (weighed as As&&) and the above volumetric method yielded the following: Samole 1
2
Gravimetric Gram 0.256 0.254 0.268 0.268 0.199 0 . 1 9 4 0.202
Volumetric Gram 0.262 0.258 0.263 0 . 2 6 2 0.202 0.198 0.205 0.211
Vol. 17, 30. 9
From the foregoing results it is evident that the volumetric method as outlined will give results that are quite as accurate gs those obtained by the more laborious and timeconsuming gravimetric method. With steels carrying up to 0.25 per cent arsenic an accuracy to within 0.01 per cent is possible. Acknowledgment
The author expresses his thanks to W. T. Hall, associate professor of analytical chemistry, Massachusetts Institute of Technology, for criticism and heip during the course of the investigations. Bibliography 1-Bureau of Standards Method for Arsenic, private communication 2-Lord and Demorest, “Metallurgical Analyses,” 1992. 3-American Rolling Mill Co., “Research and Methods of Analyses of Iron and Steel.” 4-Lord and Demorest, “MetaHurgical Analyses,’’ 1918. &-Hal and Williams, ”The Examination of Iron, Steel, and Brass.” I 7 0 f f Steel Insl. (London), 1896, p. 100. -tend, 7-Mazzetti and Agnostini, Ibid., 1923, 11, p. 485 (abs.). 8-Mellor, “Quantitative Inorganic Analyses.” &Ramberg and Sjostrom, Commission on Arsenical Poisoeing in Sweden; Cox, Analyst, PO, 3 (1925). 10-Bangs, see Reference 9.
Device for Maintaining a Slow, Constant Flow of Liquid‘ By Charles Van Brunt GENERALELECTRIC Co.. SCHENECTADY. N. Y.
E Y E R Y chemist has experienced the difficulty of adjusting and maintaining a slow but steady flow of liquid when the head is such that a cock, valve, or clamp must be “cracked” for the purpose. Sooner or later the flow slackens, and ultimately may stop altogether. Presumably, this is due to solid impurities in the liquid, which gradually accumulate behind the minute opening of the valve. Capillary adsorption may also play a part12but it is believed that the first-mentioned cause is the principal one, since an effect due to adsorption would reach an equilibrium. Further, the flow can always be reestablished by opening the valve slightly for a moment only. The desired throttling of flow can better be attained by using an elongated capillary of such bore as readily to pass any solid matter in the liquid. Ostwald has recommended two or more screw clamps in series on rubber tubing, which is an application of the same principle. But the length of capillary required is often inconvenient, and the nature of the suspended matter frequently makes such a remedy quite impracticable. The writer has adopted a modification of the long capillary idea with such success as to justify the belief that others will find it of service. A reference to the device was made in a recent article,3 but its efficiency in the trying circumstances under which it was there used justifies more specific mention. The problem was that of continuously draining off, from the bottom of a vessel, a t a rate as low as 5 cc. per minute, and under a 30-cm. head, a suspension of gelatinous or gummy carbonaceous and soapy matter more or less emulsified with 1 2
8
Received June 8, 1925. Wilson and Barnard, THIS JOURNAL, 14, 683 (1922). Van Brunt and Miller, Ibid., 17, 421 (1925), see Figure 4.
oil in a dilute solution of silicate of soda and sodium soaps. This mixture contains the various solid impurities removed from used automobile crank-case oil in the process described. It is inhomogeneous and “lumpy’’ to a degree which causes immediate clogging of any cracked valve or any tube of less than 3 or 4 mm. bore. Such a tube would not only have to be of inordinate length to produce the desired rate of flow under the head used, but it must have a sharp downward slope throughout to avoid stoppage by sedimentation. The problem was solved by cutting a helical groove about 0.5 mm. deep around an 8-mm. steel rod. When this “screw” is inserted into a close-fitting housing it forms a comparatively fine capillary of any desired length in a very compact form. Clogging is prevented by rotating the rod in its housing, which serves as a bearing. The r. p. m. may be as low as desired. A small windmill run by a jet of compressed air is a convenient source of power. The direction of rotation should be such that any solid particles are carried by the screwing action in the direction of flow. So far as the writer’s experience has gone this device is absolutely nonclogging. Not a single stoppage, or anything more than a momentary slowing up, has occurred in several months of use. Evidently, the only thing that could cause stoppage would be a mass too large to enter the capillary and too tough to be disintegrated by the movement, and this would have to be caught and held in the continually moving opening of the groove. Any particle entering the capillary must eventually work through unless it sticks in the groove so tightly that friction against the walls of the housing cannot move it. This has so far never occurred. It should be understood that the helical pump or screw conveyor action of this device does not appreciably affect the
September, 1925
IiVD USTRIAL AiTD EhTGINEERISG CHEMISTRY
flow of the liquid phase unless this is of a higher range of Tiscosity than is met with in the application now being considered. Fundamentally, it is simply a polygonal capillary, one of the walls of which is in motion with respect to the others. The liquid-solid system, however, may, and does, a t times become temporarily pasty through sedimentation. The device then functions as a conveyor, and the rate of flow will be dependent on the r. p. m. But apart from this, the flow is steady for days or weeks a t a time. Eight or ten turns of the groove, or screw thread, about
967
the 8-mm. shaft gives the stated rate of flow in the specific case described. Exact adjustment is easily secured by moving the shaft in or out of the housing. Ordinary machine screws are entirely satisfactory, except that wear is rapid owing to the sharp edge of the thread. A light cut on the lathe will take off this edge. Of course, a metallic device is affected by the question of chemical attack. But there seems no reason why hard rubber, or one of the synthetic resins, or even graphite, bhould not be used.
The Hydration of Anhydrite' By Marie Farnsworth hTONMET.4LLIC
~ I I N ~ R AEXPERIMENT LS S T A T I O N , BVREAU OF
MINES, N E W
B R L - N ~ W IN C KJ .,9
A'
According to the rebearches ,THOUGH anhydrite Anhydrite (CaS04)as a natural mineral or as a chemof van? Hoff3 solutions of (CaS04) exists in naical by-product finds little use, but the hydrated form, ture in fairly large calcium sulfate, when evapgypsum (CaSO4.2H20), has many important uses. quantities, so far there has orated in open containers, Therefore, methods of hydrating anhydrite have been been very little outlet for this and therefore under atmossought. mineral. It occurs in most pheric pressure, deposit gypTwo methods are studied. It is found that anhydrite gypsum mines to a certain exsum or anhydrite according cannot be hydrated by a pressure as high as 19 atmostent and in some to a very to the temperature reached at pheres. When the material is heated in a closed tube large extent; it also occurs as saturation. Up to 66" C. with an excess of water at temperatures varying from gypsum separates, above that a by-product in the manufac100" to 210" C. no hydration takes place. Gypsum limit, anhydrite; however, if ture of certain chemicals. As heated under the same conditions changes into plaster time advances its utilization the solution contains other of Paris at 110" C. and into anhydrite at 160" C. The salts the boundary temperawill become of increasing imsecond method which was successful was by means of ture for gypsum-anhydrite portance, for most of the evivery fine grinding. The material, ground in the Predence points to an increase in will be lowered. In the presmier colloid mill or in a pebble mill, wet or dry, varied the proportion of anhydrite ence of sodium chloride, anin average particle size from 7 to 18 microns. The time in gypsum mines as greater hydrite begins to form a t 30" required for complete hydration of the coarsest sample depth is attained. The chief C . ,and the gypsum deposited was 18 weeks, whereas with the finest sample the time uses of gypsum are as plaster below that temperature will, was reduced to 3 weeks. The gypsum so formed can of Paris, gypsum boards or in contact with a solution satbe used as a retarder in Portland cement and it makes other calcined products, as a urated with sodium chloride, an excellent plaster of Paris. retarder in Portland cement, change into anhydrite. In and as land daster, the the evaDoration of sea water first two utilizing a large percentage of the gypsum mined. t'he crystallizing point of anhydrite is 25O-C. Thus it is reasonAnhydrite cannot be used for any calcined products; it is able to suppose that the beds deposited from sea water came sold to a certain extent, mixed with gypsum, as a retarder down as anhydrite. I n examining these beds, it is found that$ for cement, but opinion is divided as to whether or not it is the upper portions are gypsum and the lower portions ansuitable for this use. There is a small outlet for it in the form hydrite. The logical conclusion is that originally the entire of land plaster, but this industry is not large. bed was.anhydrite but that by hydration the upper portions The desirability of developing some process whereby an- have been transformed into gypsum. The fact that gypsum hydrite might be hydrated to gypsum and thereby rendered is the stable form up to 60" C. and anhydrite above that has available for use in calcined products led the Bureau of been established by the solubility curves of Melcher.4 He Mines to undertake a study of the problem a t its Nonmetallic determined the solubilities of gypsum and anhydrite a t difMinerals Experiment Station. Owing to lack of fundamental ferent temperatures and found that the anhydrite curve cut data the experimental work involved a study of the physical the gypsum one a t 60" C. Although it is possible to form and chemical properties of the calcium sulfates. anhydrite at that temperature, it is not a practical method as dehydration is extremely slow. Relation of Anhydrite to Gypsum in Nature
Early Experiments on Hydration
-4nhydrite in contact with water a t ordinary temperature and pressure hydrates slowly to form gypsum. I n nature this process is extremely slow, and laboratory experiments were conducted to determine the best methods of accelerating the hydration. Once hydrated, the material could be used as ordinary gypsum, and especially it could be utilized in the large field of calcined products.
Definite proof has been given that anhydrite will change into gypsum by simply standing with water. Gill5 found that both anhydrite and dead-burned gypsum had set, after six years' standing with water, the dead-burned gypsum more completely than the anhydrite. The latter was approxi-
Received April 20, 1925. Published by permission of the Director, U.S. Bureau of Mines. In coiiperation with Rutgers University, New Brunswick, N . J .
"Untersuchungen iiber die Bildingsverhaltnisse Salzablagerungen," Leipzig, 1911. 4 J . A m . Chem. Soc., 31, 50 (1910). 6 J. A m . Cernm. Sor., 1, 65 (1918). J
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der oreanischen
