Process for producing technical grade phosphoric acid from sewage sludge ash

A process for recovering phosphoric acid from a solid phosphorous-bearing raw material, said process comprising:

• a) a leaching step of the phosphorous-bearing solid raw material with a sulphuric acid aqueous solution having a pH between 1 and 2 and at a temperature equal to or lower than 40° C., thus obtaining:
• a.1) a first leached solution comprising, solubilized, a main fraction of the phosphorous contained in the solid raw material in the form of phosphoric acid and phosphate metallic salts;
• and
• a.2) a non-leached solid residue comprising a remaining fraction of the phosphorous contained in the solid raw material;
• b) a transforming step wherein sulfuric acid is added to the first leached solution obtained in step (a) so as the phosphate metallic salts are transformed into phosphoric acid, thus obtaining a second leached solution comprising phosphoric acid as well as metallic impurities in the form of sulphates;
• c) a first solvent extraction of the phosphoric acid from the second leached solution obtained in step b) with an organic solvent, thus obtaining a first phosphoric acid-loaded organic liquid phase and a first aqueous raffinate also containing non-extracted phosphoric acid as well as impurities;
• d) washing the first phosphoric acid-loaded organic liquid phase obtained in step c) with diluted phosphoric acid aqueous solution, thus obtaining a second phosphoric acid-loaded organic liquid phase and a second aqueous raffinate containing sulphuric acid and metallic impurities;
• e) a first stripping of phosphoric acid from the second phosphoric acid-loaded organic liquid phase obtained in step d) with water at a temperature between 5° and 70° C., thus obtaining a first aqueous solution containing phosphoric acid and an organic phase comprising the organic solvent;
• f) subjecting the first aqueous raffinate obtained in step c) to a second solvent extraction which comprises:
• adding sulfuric acid to the first aqueous raffinate to form a mixture;
• extracting the phosphoric acid from the mixture with an organic solvent, thus obtaining:
• a third phosphoric acid-loaded organic liquid phase also containing sulfuric acid, and
• a second aqueous raffinate containing impurities and non-extracted sulfuric acid;
• and
• g) a second stripping of phosphoric acid from the third phosphoric acid-loaded organic liquid phase obtained in step f) with water at a temperature between 5° and 70° C., thus obtaining a second aqueous solution containing phosphoric acid and sulfuric acid as well as an organic phase comprising the organic solvent, and at least part of the said second aqueous solution is returned to the leaching step a).