High-temperature dissolution of nickel chromium ferrites by oxalic acid and nitrilotriacetic acid

Abstract

A study of the dissolution of a number of spinel-type oxides containing iron(III) ions by oxalic acid and nitrilotriacetic acid (NTA) is reported. Increasing the chromium content of oxides of general composition Ni_0.6Cr_xFe_2.4–xO₄(x= 0.3–1.5) brought about a marked reduction in dissolution rate, and it is suggested that this arises by a mechanism involving a change from kink-site attack to ledge-site attack as the former become ‘blocked’ by less reactive chromium(III) ions. The Ni_0.6Cr_0.6Fe_1.8O₄ oxide was investigated in more detail. The concentration dependences determined suggest that both oxalic acid and NTA are adsorbed at surface sites prior to dissolution. NTA brings about dissolution simply by ‘complexing’ attack, but with oxalic acid it was not possible to distinguish directly between complexing and reductive attack. The divalent cation also plays a role, for in the dissolution of a number of ferrites, AFe₂O₄(A = Co, Fe, Mn, Ni), in oxalic acid appreciable differences in dissolution rate were found, the order of reactivity being Fe > Mn > Co > Ni. The reasons for this are discussed.