GB775010A - Improvements in or relating to methods and apparatus for the manufacture of ammoniumsulfate - Google Patents

Abstract

<PICT:0775010/III/1> In a cyclic process for recovering ammonia from ammonia-containing gas by circulating an aqueous sulphuric acid solution between ammonia absorption and ammonia sulphate crystallization stages, a portion of the mother liquor from the crystallization stage is subjected to vacuum evaporation and is then returned to the main circuit before the crystallization stage. The size of the crystals obtained is determined by the humidity of the ingas and the degree of vacuum in the evaporator. The humidity of the gas may be increased by the addition of ammonia still steam or steam from a non-condensing turbine circulating pump. Coke-oven gas may be treated. The system may be operated under hydrostatic or super-hydrostatic pressure. The solution from the crystallization stage may be passed directly to the absorption stage or may be mixed first with make-up acid or water. In the latter case the solution sprayed in the absorption zone may be unsaturated with respect to ammonium sulphate. Pyridine may be recovered as described in Specification 653,325 or U.S.A. Specification 2,311,134. As shown a second absorption zone 250 is employed, solution from both absorption zones being combined, a portion of the combined solution being passed via pipe 258 to the second absorption zone after the addition of make-up acid and sufficient make-up water to give an unsaturated solution, and the remainder being passed to the crystallization stage via pipe 18. A pump 26 passes solution under pressure through a closed pressure-tight crystallization bowl 14. Ammonia still steam may be used to expel pyridine from liquor withdrawn as a side stream from any part of the circuit in which case pump 26 may be a non-condensing turbine pump and steam therefrom may be added to the gas being treated. The first scrubbing liquor may contain 1-12 per cent sulphuric acid and the second up to 40 per cent sulphuric acid, and the relative volumes may be such that at least 75 per cent of the ammonia in the gas being treated is absorbed in the first absorption zone. The dew point of the out-gas is preferably equal to or less than the dew-point of the in-gas as described in Specification 773,147. Minor modifications are illustrated in Figs. 1 and 2 (not shown).