Treatment of waste pickle liquor and similar liquors

摘要

PICT:0802269/III/1 Liquors containing ferrous ions and sulphate ions and having a pH in excess of 5.5 are contacted with a gaseous mixture comprising carbon dioxide, in the presence of ammonia to precipitate ferrous carbonate, said precipitate is separated from at least a part of the liquor and ammonium sulphate recovered from the separated liquor. Preferred features are shown in the flow sheet in which spent pickle liquor is employed. This liquor is passed from tank 46 through line 29 into an upper acidic section of a "reactor-carbonator" 32 and there contacted with a gas containing ammonia and carbon dioxide which has not been absorbed in a lower section of the reactorcarbonator. A portion of the contacted liquor is recycled whilst a product is withdrawn over a weir 252 in a pump tank 150 into an "interneutralization" tank 35 in which it is treated with ammonia to provide the desired liquor having a pH greater than 5.5. It is then passed via lines 40, heat exchangers 41 and spray heads 33 into said lower "alkaline" section of the reactor-carbonator 32 into which gases containing ammonia and carbon dioxide are passed through lines 34 and 37. The liquor which collects at the bottom and includes precipitated ferrous carbonate is recycled in part through line 44, and a product is passed through heat exchanger 255 into a decanter tank 57. A slurry of ferrous carbonate is passed to a filter 70 and removed, whilst the supernatant liquor passes over weir 257 into surge tank 58 and thence into sulphiding tanks 79 into the bottom of which hydrogen sulphide is passed : a plurality of these tanks is used in sequence and when the precipitate in a given tank has settled, the liquor is passed, if desired after returning a portion to interneutraliser 35, via a polishing filter 99 to a tank 101 where iron-free sulphuric acid is added to convert excess ammonia to ammonium sulphate. The liquor is then evaporated and crystallized. The iron sulphate is withdrawn into generator 86 into which spent pickle liquor may be passed to regenerate hydrogen sulphide for reuse. To avoid oxidation it is desirable to exclude oxygen from the apparatus and this is performed by utilizing the ammonia-free, carbon dioxide-containing gases from the acid section of the carbonator-reactor which are stored in gasometer 31 having lines to the interneutralization tank 35, surge tank 58, filter 70 and pickle liquor and recycle tanks 46 and 50. A suitable ammonia-containing atmosphere is provided in the pump tank 150 by using the gas stream obtained from the suction filtration of the ferrous carbonate. The recycle tank is fed by the washings from the filter and by a portion of the liquor passed to the sulphiding tanks, and this mixed liquor is recycled to the acid section of the carbonator-reactor and to the interneutralization tank 35 to increase the volume of the liquors and thereby effect better contacting in both sections of the carbonator-reactor. Other features include (a) the use of sulphide-free, cyanide-free gases containing above 3 per cent, preferably 8-25 per cent, of carbon dioxide, not more than 4 per cent oxygen or 2 per cent sulphur dioxide, and an ammonia content similar to the carbon dioxide content; (b) the use of either ammonium bisulphate or sulphuric acid pickling liquors; (c) a preferred pH of 5.5-9.5 in the feed liquor to the alkaline section of the reactor-carbonator and of 9-9.5 in the decanter 57 : to maintain these and other factors a pH-responsive control 6 may be provided to operate the arrowed valves; (d) pressures of 3-5 atmospheres in the alkaline section of the carbonator-reactor; (e) temperatures below 60 DEG C. maintained by heat exchangers; (f) the injection of more ammonia through the lowermost lines in 32; (g) the addition of all or a part of the ammonia prior to the injection of the liquor into the "alkaline" section of the reactor-carbonator.