(402a) Recovery of Metallic Iron from the Loaded Organic Phase After Solvent Extraction by Precipitation-Stripping with Hydrogen Gas

In hydrometallurgical processes, removal of iron from a pregnant leach solution (PLS) prior to recovery of the valuable metals is an essential, but challenging step [1], [2]. Traditionally, iron is removed from the PLS by precipitation of a ferric compound such as goethite or jarosite, but this generates large amounts of solid waste. Solvent extraction (SX) is an alternative approach for iron removal and it has the advantage that the iron can be recovered after SX in the form of a marketable iron product, typically hematite (Fe₂O₃). In this work, an iron-removal process based on the innovative use of SX was tested: Fe(III) was extracted to an organic phase comprising the carboxylic acid extractant Versatic Acid 10 (VA10), followed by direct reduction of Fe(III) in the loaded organic phase to metallic iron by hydrogen gas. Indeed, H₂ is promising in hydrometallurgical flowsheets due to its green properties and its high reactivity in reduction processes [3], [4]. This method has previously been investigated for recovery of copper, nickel and cobalt [5], and for the reduction of Fe(III) to Fe(II) [6]. Notwithstanding some promising preliminary experiments, no convincing proof for the formation of metallic iron has been given yet [7]. In these experiments, ammonia was added as a base and a H₂ pressure of 60 bar was used. No direct observation of metallic iron in the final solid product was made due to iron’s tendency to be very easily oxidized.

We studied the experimental conditions for the precipitation of metallic iron. The solids formed in the organic phase were characterized to determine their chemical composition, purity, morphology, and particle size distribution. The objective was to find the mildest-possible experimental conditions to obtain a well-filterable metallic phase. Two different bases were tested: NH_3(g) and Mg(OH)₂. Mg(OH)₂ is readily soluble in Versatic Acid 10 via an acid-base reaction, even at temperatures below 100 °C. Such base addition helped to shift the reaction equilibrium to the right for this kinetically very slow reaction in the case of iron [5]. The relevant reactions are: and , where V is the organic acidic extractant Versatic.

The experiments were carried out in 80 mL Hastelloy reactors in a multiple reactor system (Parr industry) with p(H₂) ≤ 10 bar. The organic solutions (Fe, 3–5g/L; 10-30 vol.% in the diluent Shell GTL Fluid G80) loaded into the reactor were previously obtained by the loading of Fe from an aqueous Fe₂(SO₄)₃ solutions of Fe(III) [8]. Seeding particles were added to aid the precipitation, with Ni or Fe seeds (<5 µm). The reactors were loaded with 15 to 30 mL of organic solution and with solid Mg(OH)₂ or of NH₃ gas. The stoichiometric excess of H₂ over Fe could reach 15. After 2 hours at 200 °C, the reactor was cooled and the solid formed was then filtered through a 1.6 µm filter, rinsed with ethanol, dried under vacuum at 50 °C and then stored under N₂ atmosphere, until its characterization by XRD and SEM-EDX. The residual organic solution was analyzed by ICP-OES in a pure organic 1-butanol matrix, to measure the Fe, Mg and Ni concentrations. In all experiments where Mg(OH)₂ was added, the Mg concentrations showed the total dissolution of the pre-loaded Mg(OH)₂ solid. No Ni of the Ni seeds went into solution.

The Fe precipitation yields gave higher dissolution in the presence of a base, e.g. 5% Fe precipitate without base vs 13% with Mg(OH)₂. With the use of Fe seeds with Mg(OH)₂, the precipitation yields were higher for Fe (40%) rather than with Ni seeds (13%). These observations can be explained by SEM-EDX results in the presence of Ni seeds, which show pure Fe precipitation (Fig. 1). For the first time, metallic iron particles obtained directly from a loaded organic phase have been characterized by SEM-EDX analysis, and they typically have a size between 10 and 20 µm (Fig. 1). The XRD diffractograms do not give direct indication for formation of metallic particles because the peaks of Ni metal are overlapping with those of Fe metal. However, no iron oxide peaks were observed. Upon using NH₃ as a base, a metal-rich third phase was formed, demonstrating the need for a phase modifier such as 1-decanol.

This work illustrates the differences in metal precipitation mechanisms in Versatic Acid 10 and the ability to replace NH₃ by the solid base Mg(OH)₂ to aid in the reduction of Fe in metallic form from organic SX solution. The Mg(II) in the organic phases can be replaced by another metal ion such as Fe(III) by exchange extraction upon contacting it with an iron-rich aqueous solution. MgSO₄ can be recovered from the Mg-rich aqueous solution by evaporation of water, giving a certain circularity to the process by the reuse of the organic solution.

References

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