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UNITED STATES PATENT Office MANUFACTURE of THIOUREA William F

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UNITED STATES PATENT Office MANUFACTURE of THIOUREA William F Patented Jan. 29, 1946 2,393,917 UNITED STATES PATENT office MANUFACTURE OF THIOUREA William F. Lewis, Nitro, w, va, assignor to Mon santo Chemical Company, St. Louis, Mo., a cor poration of Delaware No Drawing. Application April 15, 1942, Serial No. 439,24 3 Claims. (CL 260-552) The present invention relates to the manu the amount required for reaction with the cyan facture of thiourea and more particularly to amide is present it can be used up in Subsequent the manufacture of thiourea from hydrogen operations. In practice it has been found that Sulfide and an alkaline ?earth cyanamide. approximately two-thirds of the alkali metal hy At elevated temperatures an alkaline earth droxide necessary to absorb the hydrogen sulfide cyanamide reacts rapidly with hydrogen sulfide 5 theoretically required for reaction with the cyan to produce thiourea, especially in the presence amide is sufficient to meet practical requirements. of a large excess of hydrogen sulfide. Experience These proportions are based on the total quan has shown that for economical operation the tities used in a single operation, it being preferred hydrogen sulfide must be in substantial stoichio O to saturate the aforementioned quantity of alkali metric excess of the cyanamide in the reaction earth hydroxide with hydrogen sulfide before zone through at least the greater part of the adding any of the cyanamide. Lower amounts of reaction. - absorbent can be used but with gradually dimin In accordance with the present invention opti ishing effectiveness. mum conditions for reaction are simply and eff 5 The optimum temperature for reaction of the cliently brought about by charging the reactor hydrosulfides is the same as that for free hydrogen with an aqueous suspension of an alkaline earth sulfide, about 80-95 C. Below 100° C. the trans hydroxide and absorbing hydrogen sulfide therein fer of thiourea into thiocyanate is negligible. prior to initiating any reaction with the cyan However, it is possible, in fact preferred, to absorb amide. In this manner hydrogen sulfide is stored 20 the hydrogen Sulfide at a lower temperature as in solution in the form of an alkaline earth metal for example 15 to 20. hydrosulfide which solution can be heated to the A very important advantage of the process of proper temperature for reaction without serious this invention is that it is not affected by rea loss of hydrogen Sulfide thus obviating the neces sonable amounts of impurities, notably sulfur sity for pressure equipment. In this connection 25 compounds, frequently found in hydrogen sulfide. it seems not to have been generally appreciated It is probable that the alkaline earth hydroxides that alkaline earth hydrosulfides react with a react chemically with the impurities as well as cayanamide fully as rapidly and completely as with the hydrogen sulfide, whereas of the reac free hydrogen sulfide. tion products formed only the latter reacts with After building up the initial excess of hydrogen 30 the cyanamide. On the other hand the free sulfde by absorption in an alkaline earth hy impurities as well as free hydrogen sulfide react droxide the desired excess can be maintained with the cyanamide. However, the present in by continuing the addition of hydrogen sulfide vention is not limited to any theory or explana along with the cyanamide. It may be pointed out tion of the observed facts. that alkaline earth hydroxide is formed as a 35 As a specific embodiment of the invention by-product of the reaction as appears from the which is illustrative thereof but in nowise limi equation: tative, about 1250 pounds of water and 150 pounds of hydrated lime are charged into an MNCN -- H.S + 2HO - NH--NE, -- M(OH) iron reactor of suitable capacity. Gaseous hydro 40 gen sulfide is then fed in at 30°-50° C. until Where M is an alkaline earth metal. This fact 77.6 pounds had been absorbed. The charge is permits the use of lower amounts of absorbent then heated to 80° C. and 200 pounds of crude than would otherwise be required. However, 60% calcium cyanamide added. The addition of if desired, the alkaline earth hydroxide, as for hydrogen sulfide is then continued while another example lime, can be added sufficient to absorb 45 200 pounds of calcium cyanamide are added. all of the hydrogen Sulfide Subsequently needed. Approximately 82 pounds of hydrogen sulfide The amounts can be widely varied and even a are run in during the addition of the second 200 large excess is not detrimental except that it pounds of calcium cyanamide. Preferably the increases the amount of material which must be reaction charge is filtered at this point in order subsequently filtered and handled. The alkaline SO to avoid handling excessive amounts of insolubles earth hydroxides are in general gelatinous and and the filtrate and washings returned to the slimy and hence difficult to filter so that it is reactor and heated to 80°-90° C. The hydrogen recommended prior to any filtering operation Sulfide feed is again continued, about 93 pounds that hydrogen Sulfide be added sufficient to com being added together with 400 pounds of calcium bine with all of the absorbent. If an excess over 55 cyanamide. d 2 2,898,917 The batch is again filtered and the filtrate 1. The method of making thiourea from hy and washings pumped into a glass lined neu drogen sulfide which comprises converting the tralizing tank and 60% sulfuric acid fed in hydrogen Sulfide to an aqueous solution of alka slowly until the charge is faintly acid (pH 6.6). line earth hydroSulfide, reacting the alkaline The temperature is raised to 80 C. and after a earth hydrosulfide directly with an alkaline earth short period of heating and stirring the sulfate cyanamide by gradually adding the cyanamide cake is filtered off and the clear filtrate and to {... hot aqueous Solution but below 100° C. of the washings pumped to the concentrator. Before hydrosulfide and keeping the latter in stoichio the separation of the thiourea, however, it is ad metric excess at every point throughout the re visable to run the hot concentrated Solution O action, the excess being sufficiently large to at through a filter to remove additional small tain maximum yield with the particular grade amounts of impurities which frequently separate of hydrogen Sulfide used. as the solution is concentrated. Excellent yields 2. In the process of making thiourea from hy of thiourea of high purity are obtained directly drogen sulfide and an alkaline earth cyanamide from the concentrated Solution even from tech 5 the steps of absorbing the hydrogen sulfide in nical hydrogen sulfide containing impurities of aqueous alkaline earth hydroxide sufficient to sulfur compounds. absorb at least two thirds of the total hydrogen The data set forth below show the advan sulfide theoretically required for the complete tages of the present process. The amounts of reaction with all of the alkaline earth cyanamide, lime and hydrogen sulfide as well as the yields 20 then gradually adding cyanamide while heat are based on the mols of calcium cyanamide ing below 100° C. to initiate a reaction directly available for reaction. The "technincal' hydro with the alkaline earth hydroSulfide formed by gen sulfide contains impurities of sulfur com reaction of the hydrogen sulfide and alkaline poundS. - earth hydroxide, and while there is still a large 25 excess of hydrosulfide over that theoretically re ES quired by the cyanamide already added, but - Thiourea after the regeneration of part of the alkaline CaO, mols Excess of produced earth hydroxide, gradually forming more hydro Purity theory sulfide by gradually adding further quantities 30 of hydrogen sulfide along with the cyanamide Percent Percent 50 84. so as to keep the alkaline earth hydrosulfide in 0.83------------------------------ do------- 5 91. stoichiometric excess at every point throughout Non Technical.-- 50 69. 08------------------------------ do------- 5 84. the reaction, the excess being sufficiently large to 0-4------------------------------ do------- 5 83. attain maximum yield with the particular grade 0.33------------------------------do------- 16-23 1 90. 35 of hydrogen sulfide used. : 3. In the process of making thiourea from 1 Average. hydroger sulfide and calcium cyanamide the The above data show that the absorption of the steps of absorbing the hydrogen sulfide in lime . hydrogen sulfide in lime increases the yield of water containing sufficient calcium hydroxide to thiourea even with an overall excess of only 5% 40 absorb part of the total hydrogen sulfide theo hydrogen sulfide. The results with hydrogen retically required for the complete reaction with sulfide containing impurities of sulfur com all of the calcium cyanamide, then gradually pounds are particularly striking and show that adding calcium cyanamide to the hot solution the deleterious effect of the impurities can in of calcium hydrosulfide formed by reaction of large measure be overcome by the addition of 45 the hydrogen sulfide and lime the temperature line to the reactor charge and can completely of reaction being about 80-95 C., and while be overcome by the addition of lime coupled with there is still a large excess of calcium hydro an increase in the overall excess of the hydrogen sulfide over that theoretically required by the cyanamide already added, but after the regen sulfide.While a preferred form of- the improved proc 50 eration of part of the line gradually forming esss has been set forth to illustrate the inven more hydrosulfide by the addition of more hy tion it is to be understood that numerous changes drogen sulfide along with the calcium cyan can be made without departing from the prin amide so as to keep the calcium hydrosulfide in ciples of the invention.
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