Dissolution of iron oxide using oxalic acid

Iron oxide is the main contaminant of clay and silicate minerals used during the production of high quality ceramics. Its content has to be removed to generally less than 0.1% for achieving the required whiteness of 90% ISO or higher for clay and silicate materials. Oxalate has been used to dissolve iron oxide from various sources. The dissolution is affected by oxalate concentration, solution pH and temperature. The mineral phase is also critical in determining the reaction rate. Hematite is slow to dissolve whereas iron hydroxide and hydroxyoxides such as goethite and lepidocrosite can be easily dissolved. As the dissolution requires a pH controlled in the region 2.5–3.0 for maximum reaction rate, it is essential to create a hydroxide-oxalate mixture for use in the leaching process. The characteristics of NaOH-, KOH- and NH4OH-oxalic acid mixtures were also determined in this study. Due to the precipitation of salts such as Na2C2O4(s) and NaHC2O4(s) the NaOH-oxalic acid could act as pH buffer for the leaching. Such precipitation also reduces the concentration of the free bioxalate, HC2O4− required for the dissolution of iron oxide. KOH behaves the same as NaOH whereas NH4OH precipitates the less stable salt NH4HC2O4(s) which easily re-dissolves forming soluble oxalate species. Ammonium hydroxide is therefore the most suitable reagent that can be used for pH control during the leaching of iron oxide using oxalate. Using STABCAL, several Eh–pH and stability diagrams were developed to explain the dissolution process.