Improvements in or relating to the absorption of alkaline vapours

In a process where ammonium phosphate is formed by the absorption of ammonia from an aqueous vapour stream thereof in an aqueous solution of phosphoric acid, the vapour stream having a higher partial water vapour pressure than the aqueous acid solution, air is mixed with the vapour stream so that the resulting partial water vapour pressure is substantially not greater than that of the aqueous acid solution after absorption of ammonia from said stream and whilst the latter is in contact with the aqueous acid solution. The aqueous acid solution may be partially neutralized. The aqueous ammoniacal vapour stream may be derived from the production of an ammonium phosphate of N: P ratio >1. The added air is preferably dry but may contain ammonia in an amount less than that present in the vapour stream. In an example an aqueous ammoniacal vapour effluent at 100 DEG C. has its partial water vapour pressure reduced from 71 cm. Hg. to less than 30 cm. Hg. by the addition of dry air at 100 DEG C. The diluted vapour effluent is absorbed in phosphoric acid (51.5% P2O5, s.g.=1.63, N: P=0.1) at 15 DEG C. and 0.5 cm. Hg, to form an ammoniated phosphoric acid solution (N: P=0.4 at 100 DEG C. and 15 cm. Hg.), which is passed to a neutralizer 2 (see Figure, not shown) wherein phosphoric acid, water and ammonia are reacted to form an ammonium phosphate slurry (N: P=1.3).