Kinetics of metal oxide dissolution. Oxidative dissolution of chromium from mixed nickel–iron–chromium oxides by permanganate

Abstract

Mixed metal oxides of general formula Ni_x Fe_y Cr_z O₄(x+y+z= 3) react with acidic permanganate solution, releasing chromium as Cr^VI. Little nickel and virtually no iron dissolve. Reaction is extremely slow when z= 1.0, but when z= 1.5 the kinetics of chromium release can be studied. The reaction follows the Crank–Ginstling and Brounshtein equation, implying that the rate-determining step is diffusion through a solid product layer between an unreacted oxide core and the solid–liquid interface. Two oxides have been studied, x= 0.6, y= 0.9 and x= 1.0, y= 0.5. Both show rate saturation with increasing permanganate concentration, consistent with Langmuirian adsorption at the interface, and a strong but less than first-order dependence on acid, from which it is concluded that the rate-limiting diffusion may involve the HMnO₄ molecule. Reaction is strongly inhibited by the presence in solution of trivalent cations of Al, Fe, In, Tl and La.