GB711489A - Improvements in the purification of industrial effluents from carbonising plant and manufacture of nitrogenous fertilisers - Google Patents

Abstract

711,489. Treating toxic effluents. HOLMES & CO., Ltd., W. C., COOPER, C., PRIESTLEY, J. J., and MARSHALL, G. C. Aug. 7, 1952 [May 7, 1951], No. 10639/51. Class 111. [Also in Group XII] A toxic liquor containing thiocyanate and thiosulphate is reacted with an oxide of nitrogen higher than nitric oxide or with an acid thereof, and the resultant mixture is neutralized. An excess of the oxide of nitrogen or the acid may be used. The acid may be nitric acid. In one example of the process, gasworks retort house liquor is partially dephenolated before being passed into an oxidation unit where its temperature is raised to boiling point by steam and it is treated with a moderate excess of nitric acid. Thiosulphate and thiocyanate are decomposed, the principal product being nitric oxide which mixes with air passed into the unit, after which it is recovered by absorption in water admitted to the unit as nitric and nitrous acids which can return to the unit. The treated solution is passed to a tank where it is neutralized by a suitable alkali or, where recovery of salts is desired, by an ammonia solution, preferably liquors containing volatile compounds of ammonia produced by the gas coolers and gas scrubbers. These liquors are preferably dephenolated prior to incorporation in the process. In Fig. 3 retort house liquor 51 passes through dephenolation plant 52 into an oxidation unit 54 into which passes also at, 59, the oxidizing agent which is the product of catalytic oxidation of ammonia entering at 55. Air admitted through pipe 56 is in part reacted with ammonia in the converter 57 and in part byepassed to the unit 54. The toxic liquor falls to the bottom of the unit 54 which is heated by steam admitted at 60 and is there oxidized. Gaseous products leaving at 61 are reconverted to nitric acid by water 63 in a vessel 62 and the acid is passed back to the unit at 64. Oxidized liquor 66 passes from unit 54 into a tank 67 where it is neutralized by weak ammonia liquor 71 derived from a dephenolation unit 69 fed by mixed condenser and scrubber liquor 68. Liquor from tank 67 is passed to waste at 73 or through pipe 74 for recovery of ammonia salts. To avoid excessive dilution a balance is established between nitric acid addition and subsequent neutralization of excess by ammonia liquors. Evaporation yields a solid mixture of ammonia salts for use as fertilizer and a satisfactory effluent having the same volume as the original liquor mixture. Alternatively, Fig. 4, the oxidized liquor is received in a tank 91 which corresponds to the tank of Fig. 3. Ammonia for neutralization is derived from condenser liquor 92 and scrubber liquor 93 which pass through a dephenolation unit 94 into a heat exchanger 96 where the mixed liquor is heated by gaseous ammonia 100 driven off from an ammonia liquor still 97. The liquor passes at 98 into this still to which steam is admitted at 99. The ammonia driven off which has given up heat to incoming liquor at 96 passes into a condenser 101 forming a concentrate of ammonia whence the requisite amount for neutralization purposes is passed to the vessel 91 at 103. Some passes through pipe 82 to the converter 79. Any nitric acid which may be added to augment the total content of fixed ammonium salts enters the vessel 91 at 105. The neutralized liquor passes to an activated carbon filter 107 and thence to a steamheated evaporator 109. Crystalline salts of ammonia are removed at 111 and distillate is condensed at 113 and discharged as satisfactory effluent at 114. Specification 682,393 is referred to.