Removal of Fe3+ from ammonium dihydrogen phosphate solution in an impact-jet hydraulic cavitation extractor

The impurity of Fe³⁺ in ammonium dihydrogen phosphate (MAP) solution has a significant influence on the morphology and quality of products. The removal of Fe³⁺ from the MAP solution by Di-(2-ethylhexyl) phosphoric acid (D2EHPA) was investigated in an impact-jet hydraulic cavitation (HC) extractor. The organic phase and aqueous phase can be highly mixed under the action of hydraulic cavitation. The extraction efficiency of 80% can be achieved when the extraction reaction was carried out for only 5 min. It was found that the extraction of Fe³⁺ with D2EHPA was an exothermic reaction, and the equilibrium equation of extraction was obtained by slope method as follows: $F{e}_{(aqu)}^{3+}+8{\left(\mathit{HD}\right)}_{2(org)}={\left(\mathit{Fe}{D}_3·13HD\right)}_{(org)}+3H_{(aqu)}^{+}$After the two-stage extraction, the extraction efficiency of up to 96.7% can be reached (only 3.4 ～ 4 ppm Fe³⁺ remained in the aqueous phase), and the MAP crystals with regular polyhedral structure, single phase nature, and high optical transmittance were obtained.