Method for producing polyphosphoric acid and device for such a method

A process for the production of polyphosphoric acid comprising:

• (a) introducing, into a gas/acid contactor a feed stream of the contactor of a feed solution of phosphoric acid which exhibits a concentration by weight, xp0, of P2O5 present in an amount of up to and including 70%;
• (b) introducing, into the gas/acid contactor a recirculation stream of recirculated enriched phosphoric acid solution;
• (c) introducing, into the gas/acid contactor combustion gases;
• (d) bringing into contact the feed stream of the contactor, the recirculation stream and the combustion gases, in order to form,
• an enriched phosphoric acid solution comprising a concentration by weight, xp1, which is greater than xp0 and,
• contacted combustion gases;
• (e) separating the contacted combustion gases from the enriched phosphoric acid solution, then
• discharging the contacted combustion gases from the gas/acid contactor and
• taking the enriched phosphoric acid solution out of the gas/acid contactor;
• (f) forming, from said enriched phosphoric acid solution,
• the recirculation stream of recirculated enriched phosphoric acid solution, in order to introduce the recirculation stream into the gas/acid contactor as defined in stage (b) and,
• a spray stream of the enriched phosphoric acid solution, in order to introduce it into a combustion chamber;
• (g) spraying, through a burning flame in the upper part of the combustion chamber, a mixing stream of a mixing solution of phosphoric acid, having a concentration by weight, xpm, which is greater than that of the feed stream of the contactor formed by one of:
• the enriched phosphoric acid solution;
• or
• a direct feed stream of a direct aqueous feed solution of phosphoric acid at a concentration by weight, xd, of at least 20%, in order:
• to evaporate water and thus to concentrate the mixing solution,
• to polymerize the molecules of the mixing solution of phosphoric acid in order to form a polyphosphoric acid solution, and
• to form the combustion gases;
• (h) separating the polyphosphoric acid solution from the combustion gases, and
• recovering the polyphosphoric acid solution, and
• transferring the combustion gases into the gas/acid contactor as defined in stage (c).