GB883242A - Improvements in and relating to the production of mixtures of phosphoric and sulphuric acid - Google Patents

Abstract

Mixtures of phosphoric and sulphuric acids are produced by absorbing SO3 in a strong mixture of phosphoric and sulphuric acids containing not more than 5% wt water. SO3 may be employed per se or in admixture with other gases, e.g. N2, containing for example 7% vol. SO3, produced by the catalytic oxidation of SO2 with air. The strong mixture of phosphoric and sulphuric acid may contain no water and includes mixtures of phosphoric acid and oleum, but preferably comprises mixtures containing 86% wt H2SO4 and 13% wt H3PO4. The process may be operated batch-wise or continuously. Continuous operation may be affected for example by flowing a mixture of phosphoric and sulphuric acids containing preferably less than 2% wt water, in counter-current with SO3 in an absorption tower, and dividing the resultant SO3-enriched acid mixture, which preferably contains not more than 1% wt of water, issuing from the tower into two portions, one being removed as product and the other being diluted with phosphoric acid, e.g. wet-process phosphoric acid containing about 43% wt H3PO4, preferably to substantially the same concentration as the original phosphoric-sulphuric acid mixture feed, and returned as feed to the tower. The acid mixtures obtained by either batch or continuous process may be diluted for example to contain about 30% wt water, by addition of phosphoric acid solution, e.g. wet-process acid containing 38-45% wt H3PO4; such diluted mixtures may then be reacted with rock phosphate to produce enriched superphosphate fertilizers. Alternatively, the undiluted acid mixture product may be ammoniated to yield fertilizers containing ammonium phosphates and sulphate.