GB437278A - Improvements in treatment of combustion and distillation gases - Google Patents

Abstract

In the production of ammonium sulphate by treating distillation gases containing ammonia and carbon dioxide with an aqueous suspension of calcium sulphate, the calcium sulphate is continuously regenerated by separating from the used suspension a sludge containing calcium carbonate and adding this sludge to an aqueous washing medium being circulated in contact with combustion gases containing oxides of sulphur. Preferably, the operations of distillation gas washing and combustion gas washing are interconnected by exchanging calcium sulphate formed in one operation with calcium carbonate formed in the other while each of these calcium salts is still associated with some of the other left unconverted and with some of the aqueous medium in which it was suspended. Combustion gases enter a washer 2 at 1 and leave at 3, the washer being irrigated with a solution of ammonium sulphate containing calcium carbonate and calcium sulphate dihydrate suspended therein. Additional air to complete the oxidation to calcium sulphate is fed in at 6. The washing medium escapes at 5, passes to a stirred vessel 7 where it is replenished with calcium carbonate, and is delivered by a pump 8 to the inlet 4 of the scrubber. This washing medium has a temperature of about 50 DEG C. A portion of the washing medium, carrying with it calcium sulphate about equivalent to the sulphur oxides absorbed, is withdrawn at 9 and fed to a settler 10. The liquor overflowing into a launder 12 passes to the vessel 7 while calcium sulphate sludge withdrawn at 11 is delivered by a pump 13 to a distillation gas washer 14. This washer has eight bays, the upper parts thereof forming compartments A to H in which the more liquid portion of the washing medium is contacted distillation gases fed in at 15 and escaping at 16 so as to absorb ammonia and carbon dioxide therefrom, and the lower parts thereof forming receptacles a to h in which the more solid part of the medium remains to enable the reaction between ammonia, carbon dioxide and calcium sulphate to take place. The calcium sulphate sludge delivered by the pump 13 to port 21a collects in the receptacle a and the more liquid portion thereof is continuously withdrawn at 18a and sprayed into the compartment A at 19a. The sludge collecting in the receptacle a is withdrawn at 20a and fed to the next bay at 21b and thus successively through each bay finally leaving the washer at 20h while in each bay the more liquid portion of the medium circulates in contact with the distillation gases as already described. The ports such as 18a are sufficiently high in the receptacles a to h to allow sedimentation to take place below them whilst the disturbance caused by the returning circulated washing medium causes p sufficient turbulence in the receptacles above the ports such as 18a to prevent complete clarification, so that a medium containing sufficient calcium carbonate to prevent scaling is returned to the absorbing elements in the compartments A to H. The sludge leaving the washer at 20h is delivered by a pump 17 to a settler 32, the solids deposited consisting mainly of calcium <PICT:0437278/III/1> carbonate with some calcium sulphate dehydrate being withdrawn at 33 and passed to the vessel 7. Clarified ammonium sulphate liquor collecting in a launder 29, after filtration if necessary, is delivered to an evaporator 30. The evaporated liquor crystallizes, the crystals being separated from mother liquor in a centrifuge 22. The mother liquor is returned to the launder 29. The ammonia and carbon dioxide containing steam from the evaporator may be fed to a condenser 25 from which the condensate passes to a reaction vessel containing calcium sulphate whereby free ammonia and carbon dioxide react. The ammonia free liquid is fed to the combustion gas washer at 27. Any necessary further make-up water may be fed to the combustion gas washer at 28. The solution withdrawn from the launder 29 may be clarified by cooling slightly, after concentration if necessary, so as to cause some ammonium sulphate to crystallize and entrap solids in suspension. Any flue dust or grit from the combustion gases may be removed periodically or continuously by fractional sedimentation in the settler 10 or elutriation of the sludge withdrawn at 11. Specifications 374,467, 416,671, and 420,539 are referred to.