Transforming Red Mud into a Strategic Resource: Selective Recovery of Scandium and Rare Earth Elements

Central Laboratory, Mitra S. K. Private Limited

Red mud, the solid residue generated during the Bayer process of alumina production, represents one of the largest industrial waste streams in the world. With more than 180 million tonnes produced annually, its disposal poses long-term environmental and economic challenges. Highly alkaline in nature and rich in iron oxides, silica, alumina, and titanium, red mud is often stored in vast impoundments, creating risks of soil contamination, groundwater pollution, and catastrophic dam failures. Yet behind these challenges lies a hidden opportunity. Red mud also contains strategically important trace metals, particularly Scandium (Sc) and Rare Earth Elements (REEs)—key materials for clean energy, aerospace, electronics, and advanced alloys. The primary difficulty has always been the selective recovery of these trace metals from complex and stable oxide matrices dominated by iron and aluminum. Recent advances in controlled ammonium sulfate roasting followed by aqueous leaching have opened a promising and scalable pathway to unlock this potential in a sustainable manner.

Controlled Ammonium Sulfate Roasting: The Breakthrough

The key innovation is the controlled roasting of red mud with ammonium sulfate followed by hot water leaching. Below 500 °C, ammonium sulfate decomposes to produce SO₃, which selectively converts scandium and rare earth oxides into water-soluble sulfates. As the temperature rises to 500–710 °C, the unstable iron and aluminum sulfates decompose back into insoluble oxides. In summary, Sc and REEs remain in soluble sulfate form, while Fe and Al stay insoluble—this thermal selectivity is the true engine behind the entire process.

Table 1. Roasting stages and dominant reactions during (NH₄)₂SO₄ processing

Stage	Temperature (°C)	Duration (min)	Objective / Dominant Reaction
Ramp1	100 → 500	30	Decomposition of (NH₄)₂SO₄; formation of SO₃ atmosphere.
Hold1	500	30	Sulfation of Sc₂O₃, RE₂O₃, Fe₂O₃, and Al₂O₃; formation of mixed sulfates.
Ramp2	500 → 710	30	Gradual desulfation of unstable Fe and Al sulfates.
Hold2	710	30	Complete desulfation of Fe₂(SO₄)₃ and Al₂(SO₄)₃; stabilization of Sc and RE sulfates.

Leaching: High Selectivity with Water

The roasted material is leached using hot deionized water at 80 °C for 2 hours with an optimised solid–liquid ratio. Under these mild conditions, scandium and rare earth sulfates dissolve readily, while iron and aluminum oxides remain in the solid residue. This enables clean separation of strategic metals without using aggressive acids, making the process both efficient and environmentally benign.

Optimizing the Ammonium Sulfate Dosage

Varying the ammonium sulfate to red mud ratio revealed a clear optimum for selective metal recovery. As the ratio increased from 0.75:1 to 1.25:1, scandium extraction rose sharply from 19 to 33 ppm, while iron dissolution remained extremely low (<0.12%). At higher sulfate addition (1.5:1), no further improvement in scandium recovery was observed, and minor aluminum co-dissolution began. Light REEs showed a slight decreasing trend at higher dosages, confirming scandium-selective enrichment. The optimum operating condition is 1.25:1, delivering maximum scandium recovery with minimal iron contamination.

Table 2. Dissolution behavior of Fe₂O₃, Al₂O₃, ΣLREEs, and Sc as a function of the (NH₄)₂SO₄ : red mud ratio, illustrating the effect of sulfate dosage on selective sulfation and subsequent leaching efficiency

Sample	(NH₄)₂SO₄ : RM Ratio	Fe₂O₃ (%)	Al₂O₃ (%)	ΣLREE (ppm)	Sc (ppm)	Key Observation
RM-3	0.75 : 1	0.023	1.32	218.5	19	Lower sulfate addition results in limited Sc activation but high selectivity; Fe and Al remain stable.
RM-4	1.0 : 1	0.049	2.68	216.5	28	Optimum onset of Sc and REE sulfation; Fe₂O₃ dissolution still negligible (<0.05%).
RM-5	1.25 : 1	0.114	3.97	212.6	33	Highest Sc extraction achieved; Fe co-dissolution remains below 0.12%, confirming selective leaching.
RM-6	1.5 : 1	0.143	4.72	207.4	32	Oversulfation leads to minor Al co-dissolution; Fe remains largely in solid residue.

Double Benefit: Simultaneous Iron Upgradation

Beyond critical metal recovery, the process delivers a second major benefit—iron enrichment of the solid residue. The Fe₂O₃ content increases from ~51% to ~72% after mild NaOH washing, transforming the residue into a potential iron-making feedstock, construction-grade pigment material, and magnetic precursor. As a result, the process generates two high-value product streams: (1) Sc–REE-rich leach solution and (2) Iron-enriched solid residue.

Conclusion: From Waste to Wealth

The controlled ammonium sulfate roasting–leaching approach marks a major breakthrough in sustainable metallurgy. It enables selective recovery of scandium and rare earth elements, suppresses iron and aluminum co-dissolution, and simultaneously delivers iron-enriched solid residues for further use. In essence, red mud is no longer just an industrial waste—it is a future-ready strategic resource.

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