A novel method of iron oxalate production through the valorization of red mud

Bauxite residues known as “Red Mud” (RM) are the principal waste of caustic digestion of bauxite from the Bayer Process, whose costs of disposal are expensive and cover 5 % of the total costs of extraction and processing for aluminum production. Nevertheless, this material can be considered an important source of high-value elements, such as rare earths (REEs) and metals, Fe, Ti, Al and others. In this work, the focus has been on the recovery of iron in the form of the compound Fe(II) oxalate. Four types of acids have been used (HCl, H2SO4, H3PO4, H2C2O4) for iron extraction from RM coming from Montenegro. Hydrochloric acid shows a higher iron extraction capacity, reaching an iron extraction yield in solution of 22.6 %. Sulfuric and phosphoric acid, instead, interacted with RM leading to the formation of sulfonate and phosphate species, inhibiting the leaching ability of individual species. Oxalic acid showed the least amount of iron ions extracted but formed a stable ionic complex in solution, Fe2(C2O4)3∙2 H2O. Starting from this complex it was possible to recover the corresponding salt by a reduction and precipitation process. Through a pre-treatment with HCl and a subsequent treatment with oxalic acid, it was possible to obtain a better yield of iron oxalate. Starting from the laboratory scale, a CHEMCAD plant was conceptualized with a yield higher than 16 % per pass (repeatable 3 times with a global iron yield>50 %) and obtaining iron(II) oxalate dihydrate with purity up to 96%wt. In a holistic view of the problem, the proposed process can operate in parallel with other procedures proposed in the literature for the recovery of other valuable substances from red mud.