2,546,551 United States Patent Office
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Patented Mar. 27, 1951 2,546,551 UNITED STATES PATENT OFFICE 2,546,551 METHOD OF MAKING ALKALIMETAL CYANATES Louis L. Lento, Jr., Springdale, and David W. Jayne, Jr., Old Greenwich, Conn., assignors to American Cyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application January 25, 1950, Se rial No. 40,544. In France May 23, 1949 10 Claims. (CI. 23-75) 1. 2 The present invention relates to the prepara zinc, bismuth, etc., are unsuitable on account of tion of alkali metal cyanates with a very low con their low melting points. Silver was found to be tent of cyanides and other impurities traceable unsuitable because the metal quickly recrystal to the material of the reaction vessel used in said lized and permitted the charge to seep through preparation. the pot. Platinum was found to be inadequate It is an object of the invention to prepare because it not only catalyzed the formation of alkali metal cyanates containing not more than cyanides but also dissolved quickly into the 0.1% metal cyanide and with no more than traces charge. A copper pot was tried but it was found of Silica, or similar contaminants due to the that a film of copper oxide formed and then con character of the reaction vessel. It is a further tinuously dissolved into the charge. A graphite object of the invention to prepare alkali metal pot was found to be unsuitable because the charge Cyanates that may be handled without the forma leaked through the interstices of the graphite tion of dangerous amounts of hydrogen cyanide. particles, solidified therein, and cracked the con A still further object of the invention is to pre tainer. Gold was ruled out because of its ex pare a potassium cyanate in unusually high yield cessive cost for a commercial operation. Various from dicyandianide and potassium carbonate. ceramic pots were tried, including such dense Another object is to prepare a low-cyanide potas ceramics as zirconia, but it was found that these sium cyanate in a semi-permanent reaction ves dissolved into the charge with varying degrees sel. It is a still further object of the invention of rapidity, necessitating rapid and costly re to react nitrogenous organic compound with an 20 placement even when the dissolved matter was alkali metal carbonate in an aluminum container. acceptable from a commercial standpoint. Additional objects will appear from the discus Magnesium was known to be oxidized in the sion hereinafter. presence of potassium cyanate with almost ex The alkali metal cyanates are valuable weed plosive violence, and hence could not be used as a killers, potassium cyanate being particularly ef 25 container. - fective. However, its use as such in the past has The surprising discovery was then made that suffered from the disadvantage that the potas aluminum, despite its known corrodibility in the sium cyanate of commerce contained appreciable presence of fused alkalies, could be used, and was quantities of cyanides, which hydrolyzed over a in fact the dinly pot found that could be used period of time owing to the presence of atmos 30 commercially. - - pheric moisture, or hydrolyzed quickly when The following examples illustraté without lim placed in aqueous solution, in either event evolv iting the invention. ing dangerous quantities of hydrogen cyanide. Eacample 1 In the prior commercial manufacture of potas sium cyanate, using dicyandiamide or the like, 3. 5 An aluminum pot of 400 lbs. melt capacity with potassium cyanide in the amount of 1 or 2% was a vertical divider was used so that the reactants always formed because the reaction was con could be fed to and the reaction take place sub ducted in an iron pot. It is believed that iron stantially in one portion of the pot, and the catalyzes the reduction of cyanates to cyanides, product could flow into the adjacent portion and and thus appreciable contamination with cy 40 from there be drawn off. The pot was heated anides was always encountered. It is believed with external electrical resistance strip heaters, that this catalysis works indirectly as follows. but any other suitable heating means may of Iron, nickel, and all the metals of group 8 of the course be used. A heel of 200 lbs. of potassium periodic table catalyzed the decomposition of cyanate Was fused in the bottom of the pot, and annonia to nitrogen and hydrogen. It is be 45 the temperature was brought up to 400°-450° C. lieved that the hydrogen so-formed the reacts A mixture of 3000 lbs. commercial grade potas With the cyanate to form a cyanide and Water. sium carbonate (about 99% pure) and 1300 lbs. An iron pot also has the disadvantage of a high of commercial grade dicyandiamide (about 99% rate of corrosion, resulting not only in low potlife pure) was then run into the reaction portion of but discoloration of the product. 50 the pot, and permitted to fuse. The melt formed Consequently, Workers in this field attempted was sufficient to fill both sides of the pot, and as to use pots made of metals other than those of soon as the melt ceased bubbling, denoting sub group 8 of the periodic table. Since the reaction stantial completion of the reaction additional mix must be conducted at 340°-550° C. for efficient was added to the reaction portion of the pot, yields, obviously, metals such as tin, cadmium, 55 thereby forcing out material from the exit side, : . 2,546,55i 3 which was collected as pigs and allowed to cool. best yields of cyanate, other factors being equal. Thereafter, additional mix was added at a rate While it is preferred to use dicyandiamide as such that the melt being discharged bubbled very the source of cyanate nitrogen a Wide variety of little. This was equivalent to adding mix at the other organic nitrogen compounds added as Such rate of about 179 lbs. per hour. The addition is or produced in situ, as from ammonia, and carbon Conveniently carried out continuously by means dioxide, may be used to make low-cyanide cy of a hopper feeding a vibrating trough, a number anates free from discoloration in an aluminum of which mechanisms are standard, and easily reactor, and Without corroding the reaction ves adjustable to provide the feed rate desired. Sel. Among Such organic nitrogen compounds Among the gaseous by-products of the reaction O are urea, biuret, melamine, ammeline, amnelide, are about 350 lbs. of ammonia, and 450 lbs. of car melam, melem, melon, guanidine carbonate, bi bon dioxide, Which may be separated and re guanide, and the like. covered for use, or the mixture may be dis Solved in Water for use as a fertilizer. Eacample 3 5 Eacample 2 Using apparatus and procedure analogous to that of the preceding examples, 394 lb. of urea The foregoing example may be compared With and 362 lb. of potassium carbonate were reacted a Second example in which a Smaller pot was at 450° C. for 1 hr., to give 422 lb. of 92% pure potassium cyanate of low cyanide content, com used, as follows. parable to that of the preceding examples. Nitrogen recovery based on urea, Was 36.5%, about Example 1 Example 2 half that obtainable from dicyandiamide. If desired, sodium carbonate may be substituted Pot capacity, lbs. of melt------- 400 49 Pottemperature, C.----- 440-450 375 25 for Some of the potassium carbonate in the Feed, Ibs. K2CO3/lbs. dicyan 2.30 2.19 charge. It is preferred, however, When using Feed rate, lbs.fhr--------------- 179 20 Product recovery rate, lbs.fhr------------- 46 16 Sodium carbonate in the charge, to keep the Residence time, hrs--------- 2.74 3.06 potassium carbonate to Sodium carbonate mol Libs. productibS. feed----- 0.817 0.800 Per cent KCNO in product- 94-96 97.64 ratio at about 1/3 or higher, as a lower ratio Per cent KCN in product---- 0.01-0.05 <0.003 requires dangerously high pottemperatures. The 30 thus-prepared mixed potassium-Sodium cyanates The effect of low reaction temperature on are very low in cyanide content and are valuable cyanide content is shown clearly in Example 2. Weed killers. Here the feed rate was fast enough to give a re While the examples herein have been described action temperature of only 375 C., resulting in 5 on a continuous basis, they may obviously be extremely low cyanide formation. The relatively conducted on a batch basis by simply emptying high residence time was neutralized by the low the pots completely after the evolution of the re temperature, So that the overall result was low action gases ceases, and starting anew. The con cyanide formation. tinuous embodiment of the process is obviously In general, to keep cyanide formation low it is the preferred one. desirable to use as short a residence time and as This invention is a continuation-in-part of our low a temperature as possible, within known lim application, Serial No. 90,044, filed April 27, 1949, its. In one experiment on a laboratory scale, a now abandoned. residence time of only 30 minutes was found pos While the invention has been described with sible: Ordinarily, it will be considerably longer. particular reference to specific embodiments, it The temperature may vary from the fusion point is to be understood that it is not to be limited - of-the-charge, about 340°C. up to about 550° C., thereto but is to be construed broadly and re at which temperature the aluminum pot becomes stricted solely by the scope of the appended dangerously Weakened.