Prospective on the recovery of waste iron phosphate: Structure regulation by calcination and dissolution kinetics in acid solution

The regeneration of waste iron phosphate has not been achieved. In this study, a calcination-assisted structure regulation-solution purification process was proposed to recover waste iron phosphate based on the dissolution performance of various iron phosphate. It has been demonstrated that iron phosphate dihydrate can transform from monoclinic to hexagonal and trigonal structures at 573 K and 773 K, respectively. The removal of crystal water changes the stable structure coordinated by octahedron to the metastable structure coordinated by tetrahedral. The structure conversion leads to a change in acid dissolution performance. The dissolutions of iron phosphate dihydrate and anhydrous iron phosphates are all controlled by chemical reactions but with various apparent activation energy, 70.84 kJ/mol for iron phosphate dihydrate, 56.15 kJ/mol for tridymite FePO4, and 44.36 kJ/mol for α-quartz FePO4. The dissolution efficiency was increased from 0.23% (iron phosphate dihydrate) to 64% (tridymite FePO4) and 86% (α-quartz FePO4) at 293 K, and from 4% (iron phosphate dihydrate) to 89% (tridymite FePO4) and 88% (α-quartz FePO4) at 353 K, in 2 h. Guided by this dissolution behavior, the waste iron phosphate was calcined at 473 K, dissolved in 1 mol/L sulfuric acid, and hydrothermally treated at 393 K. Iron phosphate dihydrate that met the battery standard was regenerated. The research has provided an effective strategy to recover the waste iron phosphate.