November, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
T a b l e 11-Specific Gravity of Phosphoric Acid Solutions as R e p o r t e d by Different Observers Specific Gravity as Found by.. Concentration , Per cent --Authors-Knowlton and Mounce Hager HaPo, 17'/4'C. 25'/4'C. 25'14' C. 17.5'/4'C. 90 1.747 1.742 1.748 1.760 91 1.769 1.754 1.760 1.772
.
The values given in Table I for the specific gravity of phos-
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phoric acid solutions of 90 and 91 per cent concentration are compared in Table 11with the corresponding values reported by Hager and by Knowlton and Mounce. The comparison shows that the results found in the present work for the two concentrations in question are somewhat lower than those given by Hager and slightly lower although in fair agreement with the values found by Knowlton and Mounce.
Laboratory Production of Pure Magnesia Ware' By R. F. Mehl, J. L. Whitten, and D. P. Smith PRINCETON UNIVERSITY, PRINCETON, N. J.
OR the preparation of metals or alloys of such high
F
is dispersed, and by the proportions in which magnesia, shellac
are taken. degrees of purity as are needed for electrical measure- and "solvent" ) I n accordance with the findings of previous experimenters, ments, crucibles and refractory articles are desirable the( 5magnesia must, before mixing, be heated t o a temperature which consist solely of inert basic oxides, as free as possible equal to t h a t at which the ware is to be fired, if shrinking and from impurities, particularly from silica. Such wmes are cracking during firing are to be avoided. (6) The strength of the ware and its permanence under atnot commercially obtainable and must in general be made in mospheric exposure increase with the temperature of firing, a t the laboratory. least up to the highest temperature investigated, or 1800" C. Because of various considerations* the choice of refractory Process oxides for such purposes seems practically to be limited to magnesia or zirconia; and although numerous methods have With these facts established, the satisfactory manufacture been described for making crucibles of these s ~ b s t a n c e s , ~of magnesia ware is merely a matter of details, and has now none of them appears to meet the three prime requisites: been used by several workers with unvarying success. (1) that the completed articles shall consist wholly of the pure The magnesia is first heated for one hour to the "firing inert oxide; (2) that the ware shall be strong enough to be temperature" of 1800" C., and after cooling is used as soon handled without special care, and to withstand the weight as possible, being meantime preserved in air-tight containers. of molten metal without danger of collapse; and (3) that the Note-The temperature was determined by means of an optical pyromprocess of manufacture shall not demand costly equipment not eter, calibrated at the melting points of gold and of palladium. Samples generally available. In the case of magnesia ware the second of these metals certified by the Bureau of Standards were employed. Since of these requirements does not seem to have been met without the life of furnace core and winding is very short at 1800' C . , a somewhat sacrificing the first; for the preparation of zirconia ware such lower temperature is probably preferable in most instances. extremely high temperatures are needed as to preclude its This heating is carried out in an alundum tube; but since use save in exceptional cases. magnesia and alundum react somewhat a t the temperature The results of experiments conducted during several years named, that portion of the magnesia which has been in imupon the making of magnesia crucibles and tubes, and de- mediate contact Rith the alundum must be rejected, in order scription of a method finally evolved which may be depended to avoid contamination by silica. upon to produce articles fully satisfying the requirements Immediately before a crucible is to be molded, the metal is outlined are given in this paper. prepared by covering its entire inner surface, including the Requirements of Pure, Strong Magnesia Ware surface of the core, with heavy mineral oil, and spreading upon the oiled surface a double layer of waxed paper. The following essentials for making pure and strong magTo make the plastic mass for molding, 12 grams of orange nesia ware seem to have been established by the experiments: shellac of the best commercial quality are dispersed with gentle (1) The fragility of magnesia wares molded from plastic warming in 80 cc. of absolute alcohol (ethyl or methyl), and the masses prepared by t h e use of water or aqueous solutions is in liquid, while still warm, is well worked up with 120grams of the large part due to hydration, followed during drying and firing by dehydration, with accompanying shrinking and cracking. previously heated magnesia, the working being continued Hence the plastic mass should be made with nonaqueous liquids until, as a result of atmospheric oxidation, the mass has become only. a deep purplish red. The mixture is then without delay ( 2 ) The texture, density, and strength of the finished ware may be greatly modified by varying the organic adhesive used as the temporary binder of the plastic mass. (3) Of all the many adhesives tried, shellac produces by far the strongest wares, exerting an effect qualitatively different from that obtained with other adhesives. Denser wares are given by some others, but they are much inferior in strength. (4) The action of the shellac, and hence the quality of the ware, is affected by the nature of the medium in which the shellac Received June 4, 1925. Mehl, Trans. Am. Electrochem. Soc.. 1924. 3 Watts, Wssconsrn En&, 17, 64 (1913). Burgess and Aston, Trans. A m . Electrochem. Soc., 16, 369 (1909); Weintraub, Chem. Met. Eng., 10, 308 (1912); Cain, Schramm, and Cleaves, Bur. Standards, Sci. Paper 266 (1916); Yensen, Trans. A m . Electrochem. Soc., Sa, 176 (1917); Ferguson, J . A m . Ceram. SOL., 1, 439 (1918); Ru€f and Lauschke, Z. anorg. Chcm., 87, 198 (1914); Podszus, J. Sac. Chem. I n d . , 36, I, 217 (1917); Marden and Rich, Bur. Mines, Bull. 186 (1921). 1
2
Note-In certain experiments the shellac was dissolved in alcohol, the resulting solution was filtered and evaporated t o dryness, and the purified shellac thus obtained was employed. I t was thus shown that silica or other impurities which might be present in commercial shellac were not responsible for the strength of the magnesia ware. In cases in which the utmost precautions against silica are demanded, it would doubtless be well to use only purified shellac. For most work, even of a high order of refinement, the amount of silica which may enter from the shellac may he neglected, as is shown b y the analyses of many aluminium-magnesium alloys which have been made in such crucibles.
tamped into the mold in small portions; or better, the entire mass is forced in a t one operation by means of a press. The latter method obviates any danger of leaving tamping layers, which produce planes of weakness in the finished crucible. The core is now withdrawn, and the crucible, still in the mold, is given a preliminary drying a t room temperature by
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I S D C S T R I A L d-3-D E-I-GISEERISG CHE.IfISTRI*
passing through it a moderate current of air for one hour or more, after which the mold is removed. The magenta-colored crucible is next placed in an air oven and very gradually brought to approximately 150' C., a t which temperature i t is maintained for a t least 2 hours. During this baking the color becomes much lighter, and the crucible hardens to such an extent that it may thereafter be handled with little concern. The final firing is conducted in a molybdenum-wound alundum tube furnace, having a bore considerably larger than the diameter of the crucible, which is placed centrally and embedded in loose magnesia. The temperature is raised in the course of 3 or 4 hours to 1800" C., and is there maintained for one hour, after which the furnace with its contents is allowed to cool naturally to room temperature. With the furnace used by the writers, this cooling requires about 4 hours, and the crucible undergoes in consequence a thorough annealing. During the firing all the organic binder is burned out of the ware, so that the latter, when removed, consists solely of the original pure magnesia and has throughout the white color of that substance. Properties
The ware so produced is hard and fairly dense, although sufficiently porous to be readily permeable to gases. When struck it emits a "ring," and its strength is very little less than that of the ordinary fire-clay crucibles. As a rough measure of strength the following test was made: The lower half of a crucible of 25-mm. bore and 8-mm. wall, having a solid bottom 10 mm. thick, and weighing 74 grams, was suspended by a string from the ceiling, over a glass plate 7 mm. thick and about 20 cm. square lying upon the concrete floor. By burning the string the crucible was then allowed to fall from successively greater heights. From heights of 56 cm. and of 65 cm. no injury resulted to either crucible or plate. When the crucible was allowed to fall one meter, the glass was shattered into a number of pieces, the crucible remaining intact. It will therefore be apparent that the ware has ample strength for all ordinary demands. The process described above is in all its details the result of a large number of experiments, and the modification of an apparently small detail is likely to lead to the production of very inferior wares. Thus, a change in the quantity of shellac from 12 grams to 10 grams, the amount of magnesia remaining the same, produces a crucible of distinctly inferior strength; a decrease in the proportion of alcohol yields a mass not sufficiently plastic to pack well into the mold; and a n increase in alcohol makes the mass sticky and hard to remove from the mold, besides necessitating a longer cold drying; the use of 95 per cent alcohol, instead of absolute alcohol, causes so much hydration of the magnesia as to bring about shrinking and cracking; and the use of magnesia preheated a t 1200" C., instead of a t 1800' C., results in extreme shrinkage during firing, which is in most instances accompanied by cracking. Crucibles made as described do not shrink, but retain after firing the form and dimensions of the mold. Crucibles for Lower Temperatures
Crucibles of a type which is fairly satisfactory for use a t low temperatures, and which has been successfully employed in one rather protracted investigation, may be made by a process similar to that described, save that the preliminary heating of the magnesia and the final firing of the ware, instead of being conducted a t 1800" C., are carried out a t the highest temperature which can be obtained with a furnace having a winding of nickel-chromium alloy, or about 1150' C. These crucibles are inferior to the high-fired ware in strength, having much the consistency of chalk; and they are also much
Vol. 17, No. 11
more susceptible to the action of moist air, so that they must be kept in a desiccator until immediately before use, while the harder crucibles, although best preserved out of contact with the atmosphere, are far less rapidly hydrated and softened if left exposed. The low-fired ware, however, has distinct advantages in the relative economy and rapidity with which it can be made, and may be recommended for use at temperatures below about 1000" C. Action of Shellac
The peculiar efficacy of the shellac in the productioii of strong magnesia ware is undoubtedly due to a chemical reaction between the shellac and the magnesia, in which atmospheric oxygen also plays a part. This is shown by the very striking change of color during the mixing, and by the fact that this color develops most rapidly upon surfaces exposed to the air. It is also noticeable that if the mixture is packed into the crucible before the red color has developed to a considerable degree, much alcohol is extruded and a friable material is left which cannot be removed from the mold without breaking. A supposition, a t least plausible, as to the action of the shellac is as follows: The shellac and magnesia, influenced in some essential way by both the alcohol and the atmospheric oxygen, form a chemical compound, possibly a "resinate." This compound forms a crystal network which, when the organic matter is burned out, remains as an interlaced skeleton of magnesia, giving to the ware a cohesion very much greater than that obtained in articles of magnesia produced in other ways. This seems analogous to the formation of charcoal from cellulose, and affords an explanation of the marked dependence which the strength of the ware shows upon the proportions of the materials, and of the time effect immediately after mixing, to which reference has been made. Whatever the real reason, this particular mixture of shellac, absolute alcohol, and magnesia, treated in the way described, produces crucibles of absolutely pure magnesia which possess mechanical strength such as appears hitherto to have been secured only by the use of silica as a binder. Magnesia Tubes
I n addition to the crucibles already described, magnesia tubes having a bore of 8.5 mm., a wall thickness of 4 mm., and a length of 15 nim., have been molded in a n entirely similar manner. These are satisfactory in respect to mechanical strength, but are more porous than would be desirable for many purposes for which tubes are likely to be required. Since the only glazes practicable for very high temperatures are probably such as contain silica, the porosity of the tube was diminished by the following treatment: One end of the tube being stopped, the other was attached to suction and the tube was thrust into finely divided metallic aluminium (aluminium bronze), which was thus drawn into the pores of the tube. The latter wa5 then heated with access of air to a temperature sufficient to oxidize the aluminium to its oxide, which formed with expansion within the pores. Although only preliminary experiments have been made with this treatment, they indicate that a very marked diminution of porosity may thus be secured without adding to the ware anything except the inert oxide alumina, which would be unobjectionable for the purposes for which magnesia is specially required, and for which a silica glaze would be wholly inadmissible. Conclusion
Although the method described permits the preparation of excellent crucibles as pure as the starting material, magnesia of really satisfactory purity, particularly as regards silica, is at present both costly and difficult to secure.
