High temperature dissolution of oxides in complexing media

Highlights: ► Magnetite and ferrites of Zn, Ni and Mg are effectively dissolved by NTA. ► Dissolution rate is dependent on the lability of metal–oxo bonds. ► Chromium oxide dissolution at high temperature is appreciable. ► Addition of hydrazine to NTA affects the rate of oxide dissolution. ► Of all chelants, EDTA showed maximum dissolution rate for magnetite. - Abstract: Dissolution of transition metal oxides such as magnetite (Fe3O4), mixed ferrites (NiFe2O4, ZnFe2O4, MgFe2O4), bonaccordite (Ni2FeBO5) and chromium oxide (Cr2O3) in organic complexing media was attempted at higher temperatures (80–180 °C). On increasing the temperature from 80 to 180 °C, the dissolution rate of magnetite in nitrilo triacetic acid (NTA) medium increased six folds. The trend obtained for the dissolution of other oxides was ZnFe2O4 > NiFe2O4 > MgFe2O4 > Cr2O3, which followed the same trend as the lability of their metal–oxo bonds. Other complexing agents such as ethylene diamine tetra acetic acid (EDTA), pyridine dicarboxylic acid (PDCA), citric acid and reducing agents viz., oxalic acid and ascorbic acid were also evaluated for their oxide dissolution efficiency at 160 °C. EDTA showed maximum dissolution rate of 21.4 μm/h for magnetite. Addition of oxalic acid/ascorbic acid to complexing media (NTA/EDTA) showed identical effect on the dissolution of magnetite. Addition of hydrazine, another reducing agent, to NTA decreased the rate of dissolution of magnetite by 50%.