Patent GB-682393-A (GB)
Title: Improvements in the purification of industrial effluent
IPC: C02F1/72
Filed: 1949-09-23
Abstract: Toxic liquor containing thiocyanide and thiosulphate is purified by oxidizing the liquor with an oxide of nitrogen, higher than nitric oxide, or with an acid thereof, e.g. nitric acid or a mixture of nitrous acid and nitric acid, e.g. by treating the liquor with the oxidizing agent in a kettle or boiler at or near the boiling point. Examples of liquors which may be so treated are (1) crude ammoiacal liquors obtained during the manufacture of coal gas (2) such liquors as in (1) which have been deprived of the greater art of their ammonia, hydrogen sulphide and part of their phenolic constituents by a simple process of steam distillation (3) ammoniacal liquors which have previously been treated, e.g. by use of solvents for the removal of the phenolic constituents (4) ammoniacal liquors containing calcium, sodium or other alkali metal salts corresponding to the ammonia salts which normally occur in the liquors, the said alkali metal salts being produced by distillation of the liquors with fixed alkalis to release additional ammonia (5) concentrated ammoniacal solutions of the objectionable salts produced, e.g. by evaporation-processes or by ion-exchange processes (6) the drainage water from iron oxide purifiers (7) effluents from liquid purification plants, e.g. in the Seaboard process for the removal of H2S from gas mixtures (8) the circulating liquors used in the process as described in Specification 360,574 [Group III], for freeing flue gases from SO2 by means of ammonia obtained from crude ammonia liquor from coal distillation.  Liquor after treatment with butyl acetate for the removal of phenols reheated if necessary, is treated with nitric acid in a boiler into which steam is led, the ascending vapours, chiefly nitric oxide are cooled by water in the upper portion of the boiler and passed into a recovery tower where, mixed with air together with a supply of oxides of nitrogen they react with either water or cold purified or unpurified effuent introduced at the top of the tower, to reform nitric acid which contains also nitrous acid.  The gases from the tower are passed through a bed of hot fuel or through a refractory mass heated by a gas flame to destroy any hydrogen cyanide or those gases not converted into the oxidizing acid.  The hydrogen cyanide in the gases evolved on the oxidation of the liquor may be removed by passing the gases first into sodium carbonate solution to remove any nitric acid and then into caustic soda solution to absorb the hydrogen cyanide and convert it to sodium cyanide.  The hot, slightly acid treated effluent from the boiler is passed through an activated carbon filter to remove any phenols not extracted by the butyl actetate, which have been converted to nitrophenols and then through a filter containing, e.g. chalk from water softening plants, limestone or marble in granular form for neutralization of the acids, and finally the effluent is treated by biological oxidation as in town sewage practice.  Before passing it to the activated carbon filter, the oxidized liquor may be reduced by means of a reducing agent, e.g. SO2 or preferably H2S capable of reacting with nitric acid to evolve nitric oxide for subsequent recovery.