Method and device for producing polyphosphoric acid

Method for producing polyphosphoric acid from pure solutions of orthophosphoric acid, comprising:

• a) burning a fuel in a burner in the presence of combustion air in a combustion chamber having an upper part and a lower part, wherein the fuel is burned in the upper part of the combustion chamber to form a flame and combustion gases,
• b) spraying a spray fluid comprising pure orthophosphoric acid, towards the said burner into the said flame in the upper part of the combustion chamber, while simultaneously polymerizing-condensing the said pure orthophosphoric acid to form a polyphosphoric acid in the form of an acid mist accompanied by formation of gases which mix with the combustion gases to form a mixture causing sudden lowering of the temperature of the combustion gases,
• c) separating the said polyphosphoric acid mist from the said mixture to form a gas mixture,
• d) collecting the said polyphosphoric acid thus formed at the bottom of the said combustion chamber, in a vessel and
• e) outputting the said gas mixture via the lower part of the said combustion chamber that is separate from the said collecting of polyphosphoric acid.