Patent GB-812428-A (GB)
Title: Improvements in or relating to the purification of gases
Inventor: MARSH JOHN DAVID FRANCIS
IPC: B01D53/34
Filed: 1956-03-28
Abstract: A process for the simultaneous removal of hydrogen sulphide and hydrogen cyanide from fuel gas, comprises washing the gas with a weakly ammoniacal zinc sulphate solution containing sufficient ammonium sulphate to prevent the precipitation of zinc hydroxide, separating the precipitated zinc sulphide and insoluble cyanogen compounds of zinc from the resultant solution, processing the solution for the recovery of crystalline ammonium sulphate substantially free from zinc compounds, and reconverting the precipitate to zinc sulphate for re-use in the process.  Fuel gas, such as coal gas or coke-oven gas, has 80 per cent of its hydrogen sulphide content removed by washing with an ammoniacal solution in such a manner that little change is effected in the ammonia concentration in the gas.  It is then passed through washers 1, for final zinc precipitation in working up excess zinc liquor prior to the ammonium sulphate recovery, the main washers 2 where the hydrogen sulphide is substantially eliminated and the ammonia and hydrogen cyanide concentrations are reduced, the washers 3 where substantially neutral zinc sulphate solution completely removes the residual ammonia and hydrogen cyanide, and the condenser 4 where the purified gas is cooled and excess water vapour removed.  The liquor from the main washers 2 flows into a slurry separator 5 which separates the liquor into a concentrated slurry of zinc sulphide and cyanide, which is passed to the regenerator 10, and a decant liquor, most of which is returned to the washers 2 by way of mixing tank 6 and return tank 7 and a small part of which is passed through the washers 1 where, in contact with the hydrogen sulphide in the gas, the precipitation of zinc as sulphide is completed.  The zinc sulphide precipitate is separated in the continuous filter 9 from the ammonium sulphate solution and passed to the regenerator 10, the ammonium sulphate being sent to a still 11 where low-pressure steam strips off the dissolved ammonia, carbon dioxide, hydrogen sulphide and hydrogen cyanaide and sends them to the condenser 14.  The ammonium sulphate solution passes to the evaporative crystallizer 12 <PICT:0812428/III/1> where indirect steam heating drives off the excess water as steam which is mainly passed to the stripping still 11, a small part passing to the regenerator 10.  The ammonium sulphate crystals are passed as a slurry to the centrifuge 13 where they are washed with part of the aqueous condensate from the condenser 4.  The crystals are sent to a drier whilst the mother liquor and washings are returned to the still 11.  In the regenerator 10, 78 per cent sulphuric acid reacts with the zinc sulphide to produce an acidic zinc sulphate solution which is passed to the mixing tanks 6, 8, and hydrogen sulphide gases which are cooled at 15, the removed water vapour flowing to the tank 6.  The mixing tank 6 also receives condensate from condenser 14, the gases therefrom returning to the unpurified coal gas stream, and excess liquor from the tank 8 which serves washers 3.  The liquor from tank 6 is returned to tank 7 for re-use in the main washers 2.  Aqueous condensate from the condenser 4 which is not used in the centrifuge 13 or returned to the tank 8 passes to waste.  In a modification the zinc sulphate is regenerated from the wash liquor by washing the precipitate to reduce its ammonium sulphate content and then roasting in a fluidized bed with sulphur or hydrogen sulphide.