Dissolution of iron oxides by oxalic acid

The iron content of industrial minerals can be reduced by physical and chemical processing. Chemical processing is very efficient to achieve a high degree of iron removal at minimum operating cost. Both inorganic acids and organic acids have been used for clay refining. However, due to environmental pollution and contamination of products with the SO42- and Cl-, inorganic acids should be avoided as much as possible. This research investigated the use of oxalic acid to dissolve iron oxides and the dissolution characteristics of natural iron oxides.The dissolution of iron oxides in oxalic acid was found to be very slow at temperatures ranging from 25℃ to 60℃, but increased rapidly at a temperature above 90oC with increasing oxalic acid concentration, whereas the pH caused the reaction rate to decrease at pH&gt2.5 and improved the rate from pH 1 to pH 2.5. The iron oxides such as goethite (α-FeOOH), lepidocrocite (γ-FeOOH) and iron hydroxide (Fe(OH)3) can be dissolved faster at the presence of magnetite which exhibits an induction period at the initial stage and showed the bell-shaped curves for the dissolution.In titration tests, however, the increase of temperature causes an increase in solubility of the oxalate complexes, resulting in an increased stability of ionized species in solution. During the addition of NaOH, NaHC2O4⋅H2O was precipitated without forming Na2C2O4⋅H2O, but it was re-dissolved at pH&gt4.0. On the other hand with NH4OH, NH4HC2O4⋅H2O and (NH4) 2C2O4⋅H2O co-precipitated at pH 0.93, but also re-dissolved at over pH 2.03. The reaction temperature was found not to affect the removal of iron from the ferric oxalate complex solution using lime. Iron is removed as iron hydroxide and calcium oxalate is then precipitated during the iron removal step. The formation of Fe(OH)3 in the solution was affected by the dissociation of Ca(OH)2.The thermodynamics of sodium, ammonium and iron oxalate complexes were investigated and the standard free energy, ΔGo was calculated using thermodynamic data and solubility products.The dissolution of pure hematite by oxalate was found to follow a shrinking core model of which the kinetic step of the reaction is the controlled mechanism