November, 1932
I N D U S T R I ii L A N D E N G I N E E R I N G C H E hI I S T R Y
logically speak of a latent heat of transformation. Certainly there is no latent heat in the same sense as one speaks of the latent heat of a true polymorphous inversion such as the At point. However, it is possible and permissible, although not in common acceptance, to look upon the heat effect accompanying the change as a latent heat distributed over a temperature range. Such phenomena are not unknown, since quartz exhibits a somewhat similar behavior to which the term “distributed latent heat” has already been applied (18).
(2) (3) (4) (5) (6) (7)
(8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (15)
COA-CLUSIOS In conclusion one may say that with a feTv minor exceptions, such as the values for gamma iron between -100” and $100” C., the heat capacity of both forms of iron is known with reasonable accuracy from 20 O absolute to 900” C. Above this latter temperature the data are not satisfactory, and more determinations carefully made by investigators who understand the needs for a new technic and the necessity of working with carefully prepared specimens are badly needed.
(19) (20)
LITERATURE CITED
(25)
(1) Dearden, Iron Steel Inst. (London), Curnegie Schol. dfem., 17, 89 (1928).
(21) (22) (23) (24)
1235
Dewar, Proc. R o y . SOC. (London), 889, 158 (1914). Eucken and Werth, 2. anorg. allgem. (‘hem., 188, 152 (1930). Griffiths and Griffiths, Proc. Roy. SOC.(London), ASS, 549 (1913). Gunther, Ann. Physik, [41 51, 528 (1916). Klinkhardt, I b i d . , [4] 84, 167 (1927). Lewis and Randall, “Thermodynamics,” p. 72, JIcGraw-Hill, 1923. Lemis and Randall, Ibid., p. 448. Mehl and Mair, J . Am. Chem. Soc., 50, 55 (1928). hleuthen, F e r r u m , 10, 1 (1912). Kernst and Lindemann, 2. Elektrochem., 17, 820 (1911). Oberhoffer and Grosse, Stahl u. E i s e n , 47, 576 (1927). Osmond, Compt. rend., 103, 743, 1135 (1886). Ralston, Bur. Mines, Bull. 296 (1929). Roberts and Darey, Metals & Alloys, 1, 648 (1930). Rodebush and Michalek, J . Am. Chem. Soc., 47, 2117 (1925). Roth and Bertram, 2. Elektrochem., 35, 297 (1929). Sosman, “The Properties of Silica,” p. 310, Chemical Catalog, 1927. Stansfield, J . I r o n Steel I n s t . (London), 56, 169 (1899). Tammann-Mehl, “States of Aggregation,” p. 59, Van Nostrand, 1925. Umino, Science Repts. Tohoku I m p . L‘nfv., 15, 331, 597 (1926). Umino, Ibid., 18, 91 (1929). Weiss and Beck, Arch. sei. p h y s . nat., 25, 529 (1908). Weiss, Piccard, and Carrard, I b i d . , 42, 378 (1916); 43, 22, 113, 199 (1917). K i s t , Meuthen. and Durrer. 77. D. I . Forschungserbeiten, 1918, 204; Z . I n s t r u m e n t e n k . , 39, 294 (1919).
RECEIVED July 12. 1932.
Manufacture of Sal Ammoniac RUDOLFFREITAG, Schenkendorfstrasse 49, Leipzig, Germany OR the production of sal ammoniac, or ammonium
F
chloride, there have been made in past years many proposals, of which the greater number have been recorded in patents that are now out of date. The direct production of ammonium chloride from the gaseous components (hydrogen chloride and ammonia) seems to hare been prevented heretofore by difficulties which were probably due to the kind of equipment-a truly regrettable fact in view of the enormous excess of hydrochloric acid from the sulfate industry. The union of ammonia obtainable by the Haber process with hydrochloric acid from sulfate processes has been attempted. The union of the gaseous components takes place in a chamber of acidresisting stoneware, in which a water-cooled drum rotates. The gaseous hydrogen chloride and ammonia. which are added in molecular proportions, unite to form solid ammonium chloride, which is deposited on the surface of the cooled drum and is removed by a scraping device and carried away by means of a screw conveyor. In this way continuous operation can be maintained. According to one quite original patent (Y), ammonia and hydrochloric acid gas are conducted into methanol, in which both gases are soluble. The ammonium chloride formed by the reaction is insoluble in the alcohol and precipitates out. The considerable heat set free by the reaction and the intensive cooling made necessary thereby, along with the low boiling point of methanol and attendant difficulties in the matter of equipment, would seem to make improbable any utilization of this patent in industrial operations. On the other hand, proposals well known ill the literature attempt to reach ammonium chloride formation by double decomposition between ammonium salts and chlorides, chiefly those of the alkalies and alkaline earths (a, 6, 6). According to these patents the simultaneous action of sulfur dioxide and ammonia on sodium chloride solution, according to the equation:
SO2
+ 2SHa + 2NaCl + HzO = 21CHaCl + NazSOa
may be employed to obtain ammonium chloride which remains in solution, and also anhydrous sodium sulfite which separates out. Nothing is known concerning the introduction of this process in industry. Furthermore, the conversion of waste potash liquors with ammonia was suggested for producing magnesium hydroxide and ammonium chloride. The decomposition of ammonium sulfate, the most conveniently produced ammonium salt, with sodium chloride is accomplished technically on a large scale and will be described later. Mention may be made of the double decomposition of ammonium sulfate with calcium or barium chloride. The latter reaction in particular is of little value, although it permits the recovery of precipitated barium sulfate, or blanc fix6, as a valuable by-product, because the concentration of the initial salt solution must be maintained very low, so that the barium sulfate can be removed by filtration as it comes to the separator. Consequently, the dilute solution of ammonium chloride must then be concentrated by evaporation-an operation which makes the process unprofitable. LARGE-SCALE PROCESSES Two processes which are today being conducted on a large scale are important. They are the modified ammonia-soda process and the ammonium sulfate-sodium chloride process. In the ammonia-soda process, ammonia and carbon dioxide under pressure act on a solution of sodium chloride, whereby principally sodium bicarbonate and ammonium chloride are formed by double decomposition. Sodium bicarbonate, being difficultly soluble, separates out, and ammonium chloride remains in solution. The process is illustrated by the following equations:
+ ++
COz HIO = SHdHCO‘ NHIHCOs NaCl = NaHCOs “4Cl NHa
+
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INDUSTRIAL AND ENGINEERING CHEMISTRY
While it was formerly customary to treat the resultant ammonium chloride solution with caustic lime to drive off the ammonia, most factories today, on account of the expansion of nitrogen usage due chiefly to the Haber process, prefer to obtain ammonium chloride as such and to supply for each day's run the ammonia requisite for the process. Thus, the ever-present ammonia loss during volatilization with lime is avoided, and a valuable by-product is obtained. The production of sal ammoniac by double decomposition of ammonium sulfate with sodium chloride, a process known for a long time, has recently been patented in various forms (4, 9). According to the latter patent the reaction is supposed to take place when the concentration is such that the resulting solution contains less than 7 5 grams of ammonium chloride per 100 grams of water. The specifications of the former patent require that equivalent quantities of ammonium sulfate and sodium chloride be brought together a t 100" C. with only sufficient water to yield a final solution which is saturated a t the transition temperature with respect to both reaction products. The precipitated sodium sulfate is filtered off and the filtrate cooled to 30" C., a t which point ammonium chloride crystallizes out. If one operates according to the patent description, it is not possible to obtain ammonium chloride free from sodium sulfate. Neither the salt concentration nor the temperature stated in the patent is applicable in practice. In general, it may be stated that exact molecular quantities are not used in large-scale operations, but instead an excess of sodium chloride is added. Furthermore, the transition temperature is a t least 112" C. Below this temperature the solid phase is undecomposed ammonium sulfate. The temperature to which the solution is permitted to cool after the transition is especially important. In view of the fact that a considerable amount of sodium sulfate is dissolved in the final solution, and that the transition point between anhydrous sodium sulfate and Glauber's salt containing 10 molecules of water lies near 33" C. and is probably higher in the presence of large quantities of ammonium chloride, it is obvious that cooling the solution to this temperature could only cause trouble. I n practice it is cooled to 39" C., and the mother liquor is then removed from the ammonium chloride that has separated. When cooled below this temperature, Glauber's salt also separates out, rendering the ammonium chloride impure, and can be removed by washing only with difficulty, while anhydrous sodium sulfate which separates above 39" C. can, as a matter of fact, be remoI-ed in later washing quite easily. Marketable ammonium chloride must, however, be absolutely sulfate-free.
DETAILSOF PROCESS The practical application of the process on a commercial basis offers no difficulties if the foregoing conditions are fulfilled. Twenty-five hundred liters of a solution of ammonium chloride, containing 300 grams of the salt per liter, are introduced into a lead-lined oak vat (capacity about 4000 liters) provided with a vertical stirring device made of strong hoop iron and also covered with lead. The vat is provided with a wooden cover which has an opening for the addition of raw materials. A second opening, over which a wooden pipe is placed, serves as an outlet for steam developed during boiling. A lead H-shaped ice coil is fitted to the walls of the vat, and is so attached that repairs which are frequently necessary can be made quickly. The condensed mater coming from the hot coil is tested from time to time for its ammonium chloride content, so that even the slightest leak can be repaired promptly. The ammonium chloride solution is heated in the vat and then ammonium sulfate is added until the total salt content, calculated as ammonium chloride, amounts to 520 grams per liter. Thus, for 2500 liters of the liquor mentioned above,
Vol. 24, No. 11
680 kg. of ammonium sulfate and 630 kg. of sodium chloride are to be introduced simultaneously, the latter weight being the equivalent of the added sulfate plus 5 per cent. The mixture is agitated with the power stirrer for one hour. In the event of failure of the stirring device, great care must be taken to empty the vat immediately; otherwise, solidification of the crystal paste prevents the stirrer from being started again, and, besides, the solidified mass has t o be chiseled loose, an operation which is an objectionable interruption to the process and may damage the lead lining; it should be avoided as far as possible. When the stirring is finished, the paste is drained from the vat onto two vacuum filters by means of a heated cock a t one side of the bottom. Before filtration the filters are heated moderately with steam, and during the filtration steam is also applied. After releasing the vacuum, the filtered ammonium chloride solution iq allowed to flon. through a heated valve into the crystallizing pan. The ammonium sulfate on the filter, which contains a little ammonium chloride, is washed with hot water with frequent stirring until its ammonium chloride content is 2 per cent. Further washing is useless and only increases unnecessarily the quantity of water. The wash water is caught in a specially built reservoir and evaporated in a cooking vat during the night to an ammonium chloride content of 300 grams per liter. The concentrated solution is then used as initial liquor in a later run. The vacuum filters are round oak vats, having a capacity of about 2500 liters, and should be lead lined. Approximately in the center of the vat is a filter grate made of lead-covered iron bars, which is covered with filtering cloth or porous stone The material remaining on the filter is 90 to 95 per cent sodium sulfate, and can either be sold as waste sulfate, or recrystallized to yield 98 per cent sulfate and sold as Glauber's salt. The crystallizing pans are capacious iron boxes, holding 7,000 to 10,000 liters. They are not more than 90 em. deep, and are either made of acid-resisting sheets or lined with lead. Here the ammonium chloride solution cools to 39" C. in the course of 10 to 30 hours, depending upon the outside temperature. On cooling to this temperature, the mother liquor is emptied into an acid-proof reservoir below, The crystallized ammonium chloride is brought onto a small filter for draining, and, if colored yellow by iron compounds, it can be washed with a little cold water containing hydrochloric acid. For the predrying, the material is whirled in a centrifuge which is completely leaded, and brewed with cold water until a sample no longer gives the sulfate reaction with barium chloride solution. The ammonium chloride, which is about 96 per cent pure after centrifuging and mashing, is dried in a Buhler hot-air drier to 99 per cent, and is then packed in 400-kg. drums ready for shipping. The residual mother liquor which contains 280 to 300 grams of ammonium chloride per liter is used in new mixtures, after its total salt content as ammonium chloride is brought to 520 grams by the addition of the calculated quantity of ammonium sulfate. Should the liquor become highly colored with use and become contaminated with lead and iron, it is necessary to treat it with bleaching powder to oxidize the iron and to add sodium sulfate to precipitate the lead. Ferric iron can then be precipitated with ammonia, For decolorizing highly colored solution, animal charcoal has proved quite satisfactory. It should be emphasized explicitly that, in order to obtain a pure white product, it is absolutely necessary that contact of the liquid with iron parts be strictly avoided throughout the entire process, Wherever wood cannot be used, all vessels should be heavily leaded. All connecting pipes must be made of lead or clay, and all iron parts used in the operations must be painted with tar. If the foregoing specifications are rigorously observed, it ia
