Patent GB-625890-A (GB)
Title: Improvements in or relating to the conversion of cyanamide or dicyandiamine to othercompounds
IPC: C01C3/16
Filed: 1945-06-08
Abstract: In a pressure vessel for use in the conversion of cyanamide or dicyandiamide to other organic nitrogen compounds by heating in the presence of liquid ammonia, all the internal surfaces in contact with the reaction mixture consist of a high chrome steel containing at least 8 (e.g. 8-20) per cent of chromium, and preferably 0.04-0.2 per cent of carbon, with or without other minor materials such as small amounts of sulphur, selenium, manganese and silicon.  The steel may also contain 8-12 per cent of nickel provided there is also present a small proportion (e.g. 0.4-5 per cent) of a stabilizer consisting of one or more of the metals molybdenum, niobium and titanium.  In examples there are employed (1) an autoclave made of a high chrome steel containing 17.5-20 per cent chromium, 8-10 per cent nickel and a maximum of 0.2 per cent carbon, 0.6 per cent molybdenum and 0.07 per cent sulphur or selenium, and (2) an autoclave having an inner liner and internal heating coils and agitator composed of an alloy steel containing 18.37 per cent chromium, 10.34 per cent nickel, 0.44 per cent manganese, 0.42 per cent silicon, 0.05 per cent carbon and 0.77 per cent niobium.  Specification 568,015, [Group IV], is referred to.ALSO:Cyanamide or dicyandiamide is converted to other organic nitrogen compounds, which do not contain substantially more iron than the starting materials, by heating at 120-200 DEG  C. in the presence of liquid ammonia in a closed ferrous pressure vessel of which all the internal surfaces in contact with the reaction mixture consist of a high chrome steel containing at least 8 (e.g. 8-20) per cent of chromium, the reagents employed having an iron content sufficiently low to ensure that the reaction mixture contains less than 150 parts per million of iron throughout the reaction.  The high chrome steel preferably contains 0.04-0.2 per cent of carbon, with or without other minor materials such as small amounts of sulphur, selenium, manganese and silicon, and may contain 8-12 per cent of nickel provided there is also present a small proportion (e.g. 0.4-5 per cent) of a stabilizer consisting of one or more of the metals molybdenum, niobium and titanium.  As an additional precaution against increase in the iron content of the reaction mixture by corrosion of the pressure vessel, the reaction mixture may contain small amount (of the order of 25-1000 parts per million) of an inhibitor consisting of phosphoric acid, a soluble phosphate (e.g. ammonium phosphate) or a soluble dichromate (e.g. sodium, potassium or ammonium dichromate).  The product of the reaction is melamine or, if an ammonium salt is also added to the reaction mixture, a granidine salt.  In examples: (1) dicyandiamide and ammonium nitrate, containing respectively 5 and 12 parts per million of iron, are heated with liquid ammonium to 160 DEG  C. in an autoclave made of a high chrome steel containing 17.5-20 per cent chromium, 8-10 per cent nickel and a maximum of 0.2 per cent carbon, 0.6 per cent molybdenum and 0.07 per cent sulphur or selenium, producing granidine nitrate; the same autoclave may be used for producing melamine by heating dicyandiamide with liquid ammonia and methanol to 180-190 DEG  C.; (2) the first process of (1) is carried out in an autoclave having an inner liner and internal heating coils and agitator composed of an alloy steel containing 18.37 per cent chromium, 10.34 per cent nickel, 0.44 per cent manganese, 0.42 per cent silicon, 0.05 per cent carbon and 0.77 per cent niobium.  Specification 568,015 is referred to.  The Specification as open to inspection under Sect. 91 comprises also the above process without limitation as to the reaction temperature or the iron content of the reagents.  This subject-matter does not appear in the Specification as accepted.ALSO:Alloys for the construction of chemical apparatus consist of high chrome steels containing at least 8 (e.g. 8-20) per cent of chromium, and preferably 0.04-0.2 per cent of carbon, with or without other minor materials such as small amounts (up to about 2 per cent) of sulphur, selenium, manganese and silicon.  The steels may also contain 8-12 per cent of nickel provided there is also present a small proportion (e.g. 0.4-5 per cent) of a stabilizer consisting of one or more of the metals molybdenum, niobium and titanium.  Examples are given, and reference is also made to the following unsuitable alloys not conforming to the foregoing composition: (a) 0.08 per cent carbon, 18 per cent chromium, 8 per cent nickel, 0.75 per cent silicon and 0.75 per cent manganese and (b) 0.25 per cent carbon, 25 per cent chromium, 20 per cent nickel, 2 per cent silicon and 1.2 per cent manganese, the balance in each case being iron, no stabilizer being present in spite of the presence of nickel.  Specification 568,015, (Group IV), is referred to.