Iron ore tailings reprocessing by electrostatic separation plus two-stage froth flotation

Legacy iron ore tailings represent an underused domestic resource. This study shows that combining electrostatic separation with a two-stage (direct anionic, then reverse cationic) froth flotation circuit can upgrade a low-grade tailings feed from 24.05 weight percent Fe2O3 to a final concentrate of 96.90 weight percent Fe2O3, a grade consistent with direct-reduced iron (DRI) feedstock requirements. The approach demonstrates a practical, staged pathway for recovering iron from waste while cutting the energy and cost burden that fine grinding alone would impose, offering mine operators and researchers a route to convert tailings liabilities into a metallurgically useful product and to reduce reliance on primary ore extraction. The study establishes laboratory-scale technical feasibility. Pilot-scale optimization and economic evaluation are still required. Background Global iron production is approximately 2.5 Gt/a, and iron ore beneficiation typically generates 1.5 to 3.0 t of tailings for every ton of concentrate produced. In 2018 alone, global iron ore mining generated an estimated 1.4 Gt of tailings. These legacy stockpiles, such as those at the former Republic Mine in Michigan’s Upper Peninsula, often retain meaningful iron values that were not recovered by the beneficiation technology of their era. Reprocessing this material offers a way to recover iron, reduce the footprint and long-term risk of tailings storage, and support circular-economy goals in mining, but no single beneficiation technique reliably handles the full particle size range and mineralogical complexity of a real tailings feed. Electrostatic separation is an efficient, dry, low-energy preconcentration step, but it loses effectiveness on fine (smaller than 75 µm) or poorly conductive particles. Froth flotation can address those particles but is itself constrained. Fatty-acid-based anionic collectors tend to float fine quartz along with iron oxides, while amine-based cationic collectors struggle to float coarse quartz larger than roughly 75 to 100 µm. This study integrates the two technologies in a staged flowsheet to exploit their complementary strengths. Methods Raw tailings (24.05 weight percent Fe2O3, balance mostly quartz) were oven-dried and passed through a high-tension, roll-type electrostatic separator, which split the feed into tailings, middlings and concentrate fractions based on conductivity. The middlings fraction was ground and subjected to direct anionic flotation using oleic acid to reject coarse quartz and produce an iron-enriched concentrate. Both this direct-flotation concentrate and the electrostatic-separation concentrate were then ground further (for 90 min) and treated by reverse cationic flotation using dodecylamine (with a starch depressant) to reject residual fine quartz (Fig. 1). Iron grade was tracked at each stage using inductively coupled plasma-atomic emission spectroscopy (ICP-OES) and spectrophotometric (1,10-phenanthroline) analysis, with X-ray diffraction (XRD) and scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS) used to confirm mineralogy and mineral liberation. The 90-min grind was a laboratory procedure to ensure liberation, not a proposed industrial residence time. Results and discussion Electrostatic separation alone enriched a small mass fraction (1.9 percent) to 72.2 weight percent Fe2O3 but recovered only about 5 percent of total iron, confirming its role as a preconcentration step rather than a stand-alone upgrading process. The middlings, at an intermediate grade of 50.8 weight percent Fe2O3, responded well to direct anionic flotation: 30 min of grinding (D80 = 78 µm) was identified as the optimal liberation point, producing a concentrate of 72.76±1.24 weight percent Fe2O3 at 47.2 percent iron recovery. Subsequent reverse cationic flotation of this concentrate, after acid washing to remove residual oleic acid and 90 min of additional grinding (D80 = 29 µm), raised the grade to 96.01±3.01 weight percent Fe2O3. In parallel, the electrostatic-separation concentrate, taken directly to reverse flotation without the intermediate anionic step, reached an even higher grade of 97.83±1.29 weight percent Fe2O3 at 53.4 percent recovery, confirming that a high-purity product is achievable by either route. Fig. 1 (a) Experimental flowsheet of the multistage iron oxide beneficiation process and (b) process flow diagram showing the integrated beneficiation pathway with mass balance and Fe2O3 grade at each stage. When the two final concentrates were combined and independently verified, the composite product assayed 96.90 weight percent Fe2O3 with only 2.80 weight percent SiO2 and low levels of deleterious trace elements (Table 1). XRD confirmed a marked shift from a quartz-dominated pattern in the raw feed to a hematite-dominated pattern in the final product, and SEM-EDS imaging showed progressively cleaner, more liberated iron oxide particles largely free of quartz intergroweighths through the process sequence (Figs. 2 and 3). Because electrostatic separation and bulk direct flotation reject a large share of silicate gangue before fine grinding (one of the most energy-intensive unit operations in mineral processing), only the iron-bearing, conductive fractions proceed to extended grinding and reverse flotation, improving overall process efficiency. Reagent dosages (oleic acid 0.9 kg/t, starch 0.36 kg/t, dodecylamine 0.1 kg/t) fell within ranges commonly reported for iron ore flotation. Fig. 2 XRD patterns of the raw feed, direct flotation productand final product, showing the shift from quartz-dominatedto hematite-dominated mineralogy. The mass yield of the final high-grade concentrate reflects the specific splitter settings chosen for the electrostatic stage, which prioritized purity over yield. Retuning the separator could shift the grade-recovery trade-off toward higher overall iron recovery. Likewise, the extended grinding times used here (up to 90 min) reflect laboratory-scale liberation requirements rather than optimized industrial parameters, which would typically be achieved faster with industrial-scale ball mills. Conclusion Integrating electrostatic separation with two-stage froth flotation upgraded a 24.05 weight percent Fe2O3 legacy tailings feed to a 96.90 weight percent Fe2O3 concentrate suitable, pending final industrial qualification, as DRI feedstock. The staged approach, rejecting gangue early and cheaply and then applying increasingly selective (and energy-intensive) separation only to the enriched fractions, overcomes the individual limitations of electrostatic separation and single-stage flotation, offering a technically feasible and potentially more sustainable pathway for recovering iron from legacy mine waste. Future work should pursue pilot-scale testing, techno-economic assessment and complementary approaches such as bioleaching for residual metal recovery. Funding This work was supported as part of a subcontract to Department of Energy Grant No. DE-AR0001904, “Direct Hydrogen Reduction of Iron Ore Concentrate and Net-Shaped Fabrication of Linear Cellular Alloy Steels”, a subcontract to the Georgia Institute of Technology under US DOE ARPA-E. Fig. 3 SEM images and EDS elemental maps of (a) raw feed, (b) direct-flotation product and (c) final product showing progressive liberation of iron oxide from silicate gangue. Iron is shown in blue and silicon in green. References A list of all references is available in the full paper.

Iron ore tailings reprocessing by electrostatic separation plus two-stage froth flotation

Authored by: Vincent Bailey Arohunmolase, Akintunde Stephen Samakinde, Palas Kamlakar Borkar, Daniel Amponsah-Berko and Timothy Eisele

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