The recovery of phosphorus value from incineration ashes of sewage sludge using solvent extraction

A process has been developed to recover phosphorus value from the incineration ashes of sewage sludge which contains about 18 wt% P2O5. Of the value contained in the ash, 80% can be leached with 0.8 mol/l hydrochloric acid, together with several undesirable metallic ions. Iron in the leachant is at first removed by extraction using bis (2-ethylhexyl) phosphoric acid (D 2 EHPA). Ammonium solution is then added to the raffinate to precipitate phosphate salt as aluminium phosphate, mast of the metallic ions being left in the filtrate solution. The precipitate is dissolved in hydrochloric acid, from which phosphoric acid is extracted with tri n-butyl phosphate (TBP). The pregnant solution is scrubbed with phosphoric acid, which comes from the stripping stage to remove coextracted hydrochloric acid. The present extraction is successfully carried out, since the extraction of phosphoric acid by TBP is much enhanced by the presence of hydrochloric acid. On the basis of these results, a flowsheet of the recovery process of phosphoric acid is proposed.