Formation of Humboldtine During the Dissolution of Hematite in Oxalic Acid – Density Functional Theory (DFT) Calculations and Experimental Verification

Abstract

Understanding the reactions taking place in the hematite-oxalic acid system is important in order to clean iron oxides from filters and to remove iron from mineral concentrates. Previous studies reported the formation of an unwanted solid phase during this process. The objective of the current work, therefore, was to visualize and rationalize the iron dissolution steps taking place in the hematite–oxalic acid reaction by combining density functional theory (DFT) calculations and experimental data. The results of DFT calculations indicated that a precipitate was formed in this reaction; XRD analysis of the solid phase after the dissolution experiment revealed the formation of humboldtine as the precipitate. The attachment of oxalate on the hematite surface and the reduction of Fe(III) to Fe(II) were key steps for humboldtine formation. Both simulations and the experimental results showed that greater oxalic acid concentrations yielded more precipitate, suggesting a simple and novel route to synthesize humboldtine, a material which is relevant to the demand for clean energy.

Anodic Activity of Hydrated and Anhydrous Iron (II) Oxalate in Li-Ion Batteries

“…Iron (II) oxalate dihydrate (Ferrous oxalate dihydrate; FeC 2 O 4 •2H 2 O; FOD) or humboldtine is a secondary mineral naturally found with lignite, pegmatite, and brown coal [1]. It can also be synthesized, for example, from hematite and oxalic acid [2]. FOD is known as one of the simplest coordination polymers (CPs) and a one-dimensional metal-organic framework (1D-MOF) [3,4].…”

One-Step Synthesis of Iron and Titanium-Based Compounds Using Black Mineral Sands and Oxalic Acid under Subcritical Water Conditions

“…According to Equation (1), hydrogen oxalate and hydrogen ions that result from the first ionization stage of oxalic acid are prone to react with the iron-bearing minerals either hematite (Equation (3), [52]) or ilmenite (Equations ( 5) and ( 6)), leading to the formation of ferrous oxalate as a precipitate. However, due to differences in chemical composition and valency of iron in the iron-bearing minerals, the dissolution rate of either hematite [53] or ilmenite could be favored [54], reducing the amount of the available hydrogen oxalate ion to further react and form more ferrous oxalate (Equation ( 6)). Then, the rate-limiting step can be associated with the dissociation of oxalic acid.…”

“…In addition, the analysis of IH SS by reflected-light microscopy can build up on the understanding of the dissolution of this particular IH SS as segregated hematite lamellae within the ilmenite host, and hematite exsolution segregated at the ilmenite grain borders have been reported in naturally occurring ilmenite ores [55], features that could lead to a specific dissolution mechanism. Conversely, the dissolution mechanism of pure iron oxides, such as hematite, in hot oxalic acid solutions at ambient pressure has been well studied [53,56].…”

Anodic Potential and Conversion Chemistry of Anhydrous Iron (II) Oxalate in Na-Ion Batteries

“…As an example, we consider the ferrous oxalate dihydrate (FOD) or the iron (II) oxalate dihydrate (FeC 2 O 4 •2H 2 O) (humboldtine). This material is promising for clean energy applications, and can also be mined or easily synthesized [18]. Its high photocatalytic activity [19,20] and proton conductivity [21] justifies its use in photocatalysis and wastewater treatment [22,23].…”