GB1057643A - Concentrated phosphoric acid - Google Patents

Abstract

The invention relates to a composition comprising concentrated phosphoric acid containing 1 to 15% metal impurities prepared by concentrating wet process orthophosphoric acid, the concentrated phosphoric acid being liquid at ambient temperatures, having a phosphorus content of 78-98 weight per cent expressed as P2O5 equivalent on an impurity-free basis and containing substantially no metaphosphoric acid, at least 70% of its P2O5 content being present as polyphosphoric acid. The boiling point of such an acid is in the range 450-750 DEG F. SO3 may be present in the phosphoric acid undergoing concentration, e.g. in the form of H2SO4, and the concentrated phosphoric acid is then the reaction product of SO3 and phosphoric acid and preferably contains 0.8 to 3.5 weight per cent of SO3. Nitric acid, optionally accompanied by H2SO4, may also be added to the phosphoric acid undergoing concentration and the composition is then the reaction product of phosphoric, nitric + sulphuric acids. The concentrated phosphoric acid may be prepared by bringing hot gases into contact with the wet process phosphoric acid (optionally containing H2SO4 and/or HNO3) to bring the acid to a temperature in the range 450-750 DEG F. and maintaining this temperature to dehydrate the acid to the above-mentioned concentration. In one embodiment, wet process phosphoric acid is supplied to an evaporator to provide a pool therein, and the acid pool is maintained at the desired temperature by gas heating, the gaseous products of combustion being directed into the pool and the volume of such combustion gas being maintained at a substantially constant level. The wet process phosphoric acid before dehydration may contain 6-9 weight per cent of SO3 or HNO3 may be added thereto to bring the nitrogen content to 0.2-0.7%.