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.
Highlights
Reactions at a solid–liquid interphase play a vital role in understanding a dissolution phenomenon
The present study showed that the adsorption of oxalate ion to the surface of hematite and the electron-transfer of Fe(III) to Fe(II) facilitated the formation of humboldtine
Considering the phase diagram (Lee et al, 2007) and the XRD results, excess amounts of oxalic acid are seen to favor the formation of crystalline humboldtine
Summary
Reactions at a solid–liquid interphase play a vital role in understanding a dissolution phenomenon. Sulfuric and nitric acid are commonly used leaching agents for iron oxides, but more effective dissolution can be achieved using a solution that contains complexing ligands Complexing agents such as ethylenediaminetetraacetic acid (EDTA), trisodium nitrilotriacetate (NTA), hydroxyethylethylenediaminetriacetic acid (HEDTA), pentetic acid (DTPA), and methanesulfonic acid (MSA) have been used to enhance the dissolution of iron oxides (Chang & Matijevic, 1983; Blesa et al, 1984; Wang et al, 2017). Among these ligands, oxalic acid has received a lot of attention as it can dissociate into strong complexing oxalate ions for the dissolution (Cornell & Schindler, 1987; Panias et al, 1996; Lee et al, 2007; Salmimies et al, 2016). The reaction scheme for dissolution of hematite in oxalic acid can be described via Eqs. 1–10 (Stumm & Furrer, 1987; Siffert & Sulzenberger, 1991; Panias et al, 1996)
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