Recovery of Metal Values from Bauxite Residue/Red Mud: A Review

Suchita Rai¹*, Prachiprava Pradhan¹, Amar Padole¹, Anupam Agnihotri¹

Jawaharlal Nehru Aluminium Research Development and Design Centre, Nagpur, Maharashtra, 440023, India.

*Corresponding Author E-mail: suchitarai@jnarddc.gov.in, suchitarai1968@gmail.com

ABSTRACT:

Red mud/bauxite residue is an industrial waste product of Bayer's process and because of its alkalinity poses a significant environmental problem. However, it is a potential source from which valuable metals such as iron, alumina, titanium, caustic soda, gallium and rare earth elements (REEs) especially scandium can be extracted. The aim of this article is to systematically review, research on metal and rare earth extraction from red mud from the year 2000 to 2024. Extraction methods by hydrometallurgy, pyrometallurgy and bioleaching have been reviewed in the paper. It is seen that in any extraction process, the different operational parameters, such as reagent concentrations, temperature and pH influence the extraction efficiency of metals and rare earths. The recovery challenges and limitations presented in this document highlights, above all, the underlying complexities. The paper also discusses the problems associated with the recovery processes, particularly because of the complex composition of the red mud.

KEYWORDS: Red Mud/ Bauxite Residue, Metal Extraction, Recovery, Rare Earth Elements, Hydrometallurgy, Pyrometallurgy, Acid Leaching, Bioleaching, Sustainability.

INTRODUCTION:

Bauxite residue/ Red mud (RM)is a highly alkaline by- product produced during the extraction of purified aluminum hydroxide from bauxite ore. Depending upon the bauxite processed and the technology employed, production of one tonne of aluminum oxide generates 1–2.25 tonnes of RM (Paramguru, Rath, and Misra 2004). The large volume of red mud produced annually poses significant environmental risks, including soil and water pollution (Li et al. 2024). However, it also presents an opportunity for metal recovery, and is an alternative source of valuable metals (Kong et al. 2022). Red mud is a promising alternative for metal extraction such as alumina, caustic soda, iron, titanium, including rare earth elements (REEs) such as scandium and gallium, making it a strong contender for metal extraction. Red mud primarily consists of 30-60% iron oxide (Fe₂O₃) and 10-20% alumina (Al₂O₃). It also contains silica (SiO₂) at 3-50%, titanium oxide (TiO₂) in trace amounts up to 25%, sodium oxide (Na₂O) at 2-10%, and calcium oxide (CaO) ranging from 2-8% (Paramguru R.K 2004).

The red mud produced by Indian alumina refineries has the chemical composition of Iron Oxide (Fe₂O₃) 40-62%, Alumina (Al₂O₃) 16-23%, Silica (SiO₂) 4-9%, Titanium Oxide (TiO₂) 4-16%, Sodium Oxide (Na₂O) 3-6.5%, Calcium Oxide (CaO) 0.5-3% (Rai et al. 2020).

Extracting valuable metals from red mud presents a solution to multiple challenges, such as lowering waste disposal expenses, preserving natural resources, and reducing environmental pollution. Moreover, the properties of rare earth elements are of great interest because of their fundamental applications in a number of advanced technological processes. Several techniques for metals extraction from red mud include carbothermal reduction, smelting reduction, acid leaching, magnetic separation, and hydrothermal techniques (Khanna et al. 2022). These methods have shown promise in recovering iron, REEs, and other valuable metals from red mud.

This review paper aims to examine the present status of metal recovery from red mud, with a focus on the technical, environmental, and economic aspects of this processes. We will examine the challenges and opportunities that are linked to metal recovery from red mud and discuss the potential applications of these technologies. This paper reviews and discusses the research conducted from the year 2000 to the present.

RECOVERY OF METAL:

Recovery of Iron from Red Mud:

Various methods have been developed by researchers for recovery of iron from Bayer red mud such as physical sorting by magnetic separation, hydrometallurgical recovery, pyrometallurgical recovery. Physical sorting method such as magnetic separation generally carried out with high iron content red mud does not involve chemical reaction. (X. F. Li et al. 2023). Iron in red mud mainly consists of two phases, namely hematite (Fe₂O₃) and goethite (FeOOH) (Klauber, Gräfe, and Power 2011).

In one of the studies, (Liu and Naidu 2014) concluded that, a combined process was much preferred over the single process method which can achieve multiple additional values from red mud. Table 1 reviews the works carried out for recovery of iron from red mud.

Table 1: Iron recovery from red mud (national and international context)

Constituents used	Experimental conditions (optimal condition)	Observations	Method used for extraction	Key findings	Country	Reference
Red Mud, sulphuric acid	RM calcined at 873 K, 6M H₂SO₄	Follows first-order kinetics	Acid leaching	97.46% Fe, 64.40% Al recovered	Turkey	(Uzun and Gülfen 2007)
Red mud	Reduction roasting, magnetic separation	Original Fe₂O₃ content in red mud: 27.93%	Magnetic separation	89.05% Fe in magnetised separated part, 97.69% metallization	China	(Liu, Yang, and Xiao 2008).
Red mud, carbon, soda ash, lime	Preheated at 800°C for 20 min and then sinter at 600°C, 20 min	Ferric oxide can completely reduce with increase in amount of carbon added	Pyrometallurgy	Alumina recovery rate: 89.71%, and Fe recovery rate: 60.67%	China	(LI et al. 2009)
Red mud, carbon, lime, calcium, calcium carbonate, magnesium carbonate	Additive to red mud ratio: 6:100, carbon powder to red mud ratio :18:100, temp. 1300°C, 110 min.	Building materials were prepared from silicate residues after successful Fe recovery	Direct reduction roasting and magnetic Separation	88.77% Fe in concentrate and 81.40% recovery ratio	China	(W. Liu, Yang, and Xiao 2009)
Red mud, sulphuric acid	383 K (110°C), H₂SO₄ leaching	Kinetic study carried out. Controlled by intra-particle diffusion	Acid leaching	maximum Fe (III) leaching from bauxite residue at 383 K	China	(Z. R. Liu et al. 2009)
Red mud, sodium carbonate, calcium fluoride	1150°C, coal-based reduction, 180 min, 3% Na₂CO₃ and 3% CaF₂ added	Na₂CO₃ and CaF₂increases reducing reaction activity of FeO	Pyrometallurgy	grade of concentrate to 89.57%. metallization increases to 91.2% grade with addition of Na₂CO₃, CaF₂.	China	(Huang et al. 2010)
Red mud	High gradient superconducting magnetic separation (HGSMS) for fine particles (<100 μm and <10 μm)	Fe-element intergrowth observed	Magnetic Separation	58% Fe₂O₃ increased to 65% oxide in <100 μm red mud, 29 to 45% Fe₂O₃ in <10 μm red mud	China	(Li et al. 2011)
Red mud, magnesium chloride, sodium acetate	Sintering at 800°C-1000°C in a resistance type vertical tube furnace, 1-4 days	Varied effects on metal extractability	Pyrometallurgy	Increased Fe and Al leachability	India	(Ghosh et al. 2011)
Red mud, graphite (99 % C_fix) Red mud of NALCO and Vedanta	Smelting in 35 kW DC extended arc plasma reactor, reduction time 15 min, reductant 11% (graphite powder), limestone, dolomite, fluorspar and CaCO₃ used as fluxes	Study of recovery of pig iron from dry red mud, Reductant concentration effect studied	Pyrometallurgy, plasma smelting	Fe recovery of 65-70% with NALCO red mud and 79% with Vedanta red mud, reduction rate follows first-order kinetics	India	(Rath et al. 2011) (Rath et al. 2013) (Jayasankar et al. 2012)
Red mud, sodium carbonate, soft coal	8% Na₂CO₃, 1050°C, 80 min, magnetic separation	overall iron recovery rate is 95.76%.	Pyrometallurgy	90.87% Fe in final product	China	(Zhu et al. 2012)
Red mud, anthracite, lime stone	Reduction at 1400°C, 30 min., molar ratio of 1.6, and basicity of 1.0.	Temperature is identified as the strongest factor influencing the metallization rate	Reduction roasting	Chemical analysis showed that the total iron content in the nuggets is higher than in pig iron	China	(Guo et al. 2013)
Red mud, lignite (30.15% C_fix)	Microwave reductive roasting using lignite, followed by wet magnetic separation	Hematite to magnetite to iron transformation	Pyrometallurgy	Fe₂O₃ % increased from original 43% to 50% with 69.3 % metallization, 35.15 wt.% total Fe in magnetic concentrate	Greece	(Samouhos et al. 2013)
Red mud, sodium sulphate and CaO added as additives, bituminous coal as reductant	9% sodium sulphate, 9.46 % lime and 16 % coal, 1423 K (1150°C), 80 min	Na₂SO₄ improved Fe grain growth	Pyrometallurgy	Original Fe₂O₃ is 36%, 90.28 % Fe, 94.87 % metallization and 92.14% of iron recovery	China	(Chun et al. 2014)
Pyrite, red mud	30 g red mud-containing 9.24% of iron (13.2% Fe₂O₃) was mixed with 0.74 g pyrite, 600°C, 30 min, N₂ atmosphere	Hematite reduced to magnetite	Pyrometallurgy	4.5 g with 36.9% magnetically separated, Efficient Fe transformation and separation	China	(Liu et al. 2014)
Red mud, iron scraps, oxalic acid, H₂SO₄, HCl, HNO₃, acetic acid, sodium acetate, potassium permanganate	Selective leach with oxalic acid, red mud/oxalic acid/H₂SO₄ is 3:3:2, 95 °C,90 min at L/S ratio: 16:1 mL/g	High Fe recovery rate	Hydrometallurgy	Leaching with oxalic acid, reduction of iron in the leached solution using iron scraps, precipitation of FeC₂O₄.2H₂O, separation of oxalic acid and iron using H₂SO₄.	China	(Yang et al. 2015)
Red mud, reducing agent	Reduction roasting, magnetic separation	Nanotechnology enhanced Fe enrichment	Pyrometallurgy	Over 70% Fe recovery	India	(Biswal 2018)
Basic oxygen furnace (BOF) slag, red mud, lignite coke, Lime	Combined carbothermic reduction, 1500°C, 1h, BOF slag, lignite coke	Low residual Fe in slag	Pyrometallurgy	High Fe extraction, dilution slags with high Al₂O₃ content with enrichment of Ti	Red mud (Greece) and BOF slag (Germany)	(Xakalashe and Friedrich 2018)
Red mud, CO, CO_2,N₂	Reduction at 540±10°C, P_CO/P_CO2=1(0.070 atm (bar) ±0.001 atm (bar), 30min	Approximately 98% magnetite could be recovered	Reduction followed by magnetic separation	~60% grade of the magnetite achieved	America	(Gostu 2018)
Red mud, laterite nickel ore	Reduction roasting followed by magnetic separation	When Ni added up to 0.6% white microstructure turn into grey	Carbothermic reduction followed my magnetic separation	Red mud grey cast iron provides better strength and corrosion resistance	China	(Zeng 2018)
Red Mud, reductants such as coal dust, sugarcane bagasse, spent pot lining	Magnetic separation, carbon-containing wastes as reducing agent	Comprehensive study on % recovery	Magnetic separation	80% recovery with hydro-cyclone and iron (Fe₂O₃) enrichment from 52% to 70% by physical separation technique	India	(Rai et al. 2019)
Red mud, Sichuan coke (C_fix:84.4%), chlorinating agent (sodium chloride)	Roasting at 1100°C for 60 min, mass ratio: 100:15:15:10 (red mud/sodium chloride/ coke/sodium sulfate)	Improved Fe recovery from red mud	Pyrometallurgy	92.78% Fe recovery rate, magnetic concentrate sorting index with T_Fe 82.13%	China	(Ding et al. 2020)
Red Mud, Zn (NO₃)₂·6H₂O, H₂O₂, Na₂CO₃, Al (NO₃)₃·9H₂O, NaOH	Zn (NO₃)₂·6H₂O to RM is 2:1, 6h dry milling, water dosage 2 mL, 250 rpm.	Created Fe₂O₃/Zn-Al layered double hydroxide (LDH)	Mechano- chemical synthesis	Successfully synthesized Fe₂O₃/Zn-Al layered double hydroxide	China	(Li et al. 2020)
Red mud, coke, sodium sulphate, potassium chloride	Ratio of Red mud: potassium chloride: coke: sodium sulfate is 100:13:10:8 and roasted at 1100°C, 65 min	Enhanced Fe extraction	Pyrometallurgy	85.43% Fe recovery with total iron grade of 79.32%	China	(Wei et al. 2021)
Red mud, lime milk	Red mud treated with 90°C for 180 min to liberate sodium. carbothermic reduction with coal at 1000-1200°C, magnetic separation	Effective extraction process	Reduction of iron and magnetic Separation	Successful Fe extraction from red mud	Russia	(Zinoveev et al. 2021)
Red mud, CaF_2,calcified slag, coking coal	1550°C, alkalinity 1.1, carbon ratio 1.1, 30 min, 3% CaF₂	High efficiency in Fe recovery. Calcified iron tailings extracted from red mud can be used for making aluminate cement.	Pyrometallurgy	90.06% Fe recovery and 93.76% mass fraction of Fe in the metal.	China	(Yang et al. 2022)
Red mud, magnesium oxide, carbon dioxide, calcium sulfate, sodium carbonate, calcium oxide	MgO mixed with red mud at 1100°C to form MgFe₂O₄. NaAlO₂ leached out. ATH precipitated. Separated. Magnetic separation carried out. MgO/(Fe₂O₃ + MgO) ratio of 14.89% at 1100°C with Ca/ (Si + Ti): 1.8	Efficient and environment friendly extraction	Magnetization sintering process	67.54% and 73.01% of recovery rate of Fe and Al	China	(Chen et al. 2023)

Various techniques are used to recover iron from Bayer red mud, including acid leaching, pyrometallurgy, hydrometallurgy and magnetic separation, with pyrometallurgy being the predominant method. Iron recovery rates are generally impressive, often achieving 70-90% or higher under optimal circumstances, highlighting the application of red mud as a significant secondary source of iron. Research on iron recovery from red mud is being pursued worldwide, with notable contributions from countries like China, India, and Turkey, reflecting a broad interest in this area.

Recovery of Alumina and Caustic Soda:

Red mud also contains valuables such as Al₂O₃ (14-17%) and caustic soda (5-6%) which can be partially recovered. Studies show that Bayer process modifies the aluminum minerals, so using weak acids most of the Al can be separately extracted from mud while the mobilization of iron is not significant (Zsolt and Lakatos 2009). Table 2 reviews the work carried out for recovery of alumina and caustic soda from red mud in India and abroad.

Table 2. Recovery of Alumina and caustic soda from red mud (national and international context)

Constituents used	Experimental Conditions	Observations	Method of Extraction	Key Findings	Country	Reference
Red mud, sodium hydroxide, lime	45% NaOH solution, CaO: red mud ratio is 0.25, 200°C, 3.5h for Al₂O₃extraction. 7% NaOH solution at 170°C for 2 hrs for Na₂O extraction	Sodium aluminate hydrate crystallized	Mild Hydro- chemical method	87.8% Al₂O₃, 96.4% Na₂O extracted	China	(Zhong, Zhang, and Zhang 2009)
Red mud, fungi (A. Niger and P. Simplicissimum)	Biological leaching with fungi	Indigenous specimen fungi most efficient	Biological leaching	2082 mg Al₂O₃/L solubilized at 15% pulp density of red mud	Iran	(Ghorbani, Oliazadeh, and Shahverdi 2009)
Red mud, CaCO₃, Na₂CO₃	Ratio of red mud: CaCO₃: Na₂CO₃ in mole ratio of 1:0.20:0.25. 1100°C, 4h; leaching with 80 g/L 1h, 105°C	Na₂CaSiO₄, Ca₂SiO₄, CaTiO₃ and Ca₂Fe₂O₅ formation enhance the alumina extraction efficiency	Hydrometallurgy	97.64% alumina extraction efficiency	India	(Shib and Meher 2016)
Red mud, NaOH, CaO, CO₂gas	Calcification-carbonation method	High recovery rate of alkali and alumina	Calcification-carbonation method	75% alumina extraction with 95.2% Na₂O recovered	China	(Zhu et al. 2016)
Red mud	The effect of cooling methods of furnace, air, water, liquid N₂studied on roasted red mud	Specific surface area increases from 1.898 to 2.177 m²·cm-³	Hydrometallurgy	Nearly 25% total sodium recovery	China	(Liu et al. 2017)
Red mud, citric acid,	Leaching with citric acid dosage 15%, liquid-to-solid ratio 7 mL/g, 100°C, 300 rpm, 120 min.	Internal diffusion controlled dealkalization process	Acid leaching	The dealkalization rate was > 95%	China	(Li, Zhu, and Tang 2017)
Red mud, lime, CO₂gas	Calcification-carbonisation process	After Al₂O₃ and Na₂O extraction, residue can be used in cement. It also meets the requirement of soil in terms of alkalinity salinity and other soil properties	calcification-carbonisation process	Alkali content reduced to less than 0.3%and 46.5% alumina extraction	China	(Wang et al. 2018)
Red mud, sodium carbonate, coal, hydrochloric acid,	Magnetic separation roasting (1050 °C, 2 h) leaching of non-magnetic phase with 6M HCl, 90°C, 2h	High purity alumina extracted	Magnetic Separation	20.66% alumina extracted	Indonesia	(Suprapto et al. 2018)
Red mud and calcified slag	Wet grinding of calcified slag	Milling condition established. Effect of wet grinding studied.	Calcification-Carbonization	The process improved carbonisation efficiency and alumina extraction.	China	(Liu et al. 2020)
Red mud, sodium hydroxide	Mechanical activation, Calcification-carbonization with phase transformation	Milling conditions of calcified slag studied. Alumina extraction ratio increased	Calcification-carbonization	Alumina extraction increased from 32.9% to 43.1%	China	(Liu, Liu, and Zhang 2019)
Red mud, H₂SO₄, waste ammonia, iron powder	Hydrothermal reaction 40% H₂SO₄, 110°C, 5h fractional precipitation	Effective separation of Fe and Al with lower leaching of Ti and Si	Fractional precipitation process	>90% Al and Fe extraction with 45% and 95% purity resp.	China	(Yu et al. 2020)

The above table outlines techniques for the extraction of alumina (Al₂O₃) and caustic soda (Na₂O) from various sources, highlighting hydrometallurgy, calcification-carbonation, and bioleaching as prevalent methods. The efficiency of extraction for both alumina and caustic soda shows significant variation, with certain techniques reporting high recovery rates, such as a study from China that achieved 96.4% alumina extraction, and 95.2 % alkali recovered. Many of these methods aim to recover alumina and caustic soda concurrently, underscoring the significance of effectively extracting both substances. Global research efforts are underway regarding the extraction of alumina and caustic soda, mainly in China.

Recovery of Titanium Dioxide:

Titanium in bauxite primarily occurs in the forms of anatase or rutile (TiO₂). The titanium compounds are predominantly insoluble, leading to their accumulation in red mud, where the titanium content ranges from trace amounts up to 25% by weight. Titanium is extracted with iron, alumina which has been explained in the earlier section. Very few works have been conducted on selective extraction of titanium from red mud. Table 3 reviews the very recent work on extraction of Titanium from red mud.

Table 3. Recovery of Titanium from red mud

Constituent used	Experimental Conditions	Observations	Method used for extraction	Key Findings	Country	Reference
Red mud, H₂SO₄	Acid leaching with 6N H₂SO₄, 60°C and solid to liquid ratio: 5%.	At optimum conditions, iron leaching was 46%, and aluminum did not exceed 37%	Acid leaching	Titanium recovery efficiency was 64.5%	Greece	(Agatzini-Leonardou et al. 2008)
Red mud, H₂SO₄, HCl	H₂SO₄ as leaching agent, 70°C, 120 min, Solid/liquid ratio: 1:50	Hydrometallurgical treatment of red mud. Interference of Fe in Ti leaching observed.	Acid leaching	H₂SO₄ leaching most effective. Maximum titanium leaching efficiency 67%.	Grecce	(Alkan, Schier, et al. 2017)
Red mud, hydrochloric acid, sulphuric acid, extractant Cyanex923, Heptane	Sulfuric acid leaching (6mol/L), 85 ◦C, L/S = 20 Organic/aqueous phase ratio: 1. Cynax concentration was 2.59 gL^-1	The maximum leach rate of Ti (IV), Fe (III) and Al (III) were 64.53%, 34.01% and 47.09% in H2SO4. The extraction efficiency was 42.07%, 2.27%, 0.49% for Ti (IV) Fe (III) and Al (III) resp. with addition of Cynax923 to the H₂SO₄ leach liquor	Acid Leaching	High extraction efficiency for titanium (IV). Exothermic process with efficiency decreasing at higher temperatures.	China	(Zou, Chen, and Li 2021)
Red mud, leaching reagent (HNO₃, H₂SO₄, HCl, EDTA and aqua regia, Cyanex 301), precipitating agents (oxalic acid, ace tic acid, sulphuric acid, tartaric acid, thiosulfate, iodide, ammonia)	HCl leaching (12 mol/L), 25ۦ°C, 24 hours, Solid/liquid ratio: 1:10, agitator speed: 150 rpm, oxalic acid, acetic acid, sulphuric acid, tartaric acid, thiosulfate, iodide and ammonia used to precipitate	The method effectively removed impurities such as iron and aluminum, enhancing titanium recovery	Acid Leaching	Maximum titanium leaching efficiency of 95% with HCl.	Italy	(Pietrantonio et al. 2021)
Red mud, smelting separation slag, additives (NaOH, Ca(OH)₂)	Roasting (1200°C, 60 min) NaOH leaching to remove alumina bearing phases followed by HCl leaching to get titanium enriched slag.	Mineral phase reconstruction process can enable a higher metal leaching rate.	Acid Leaching	Overall TiO₂recovery was 95.46%.	China	(S. Li et al. 2023)

Different studies report efficiencies from 67% up to 95.46% for titanium extraction from red mud by acid leaching. Sulfuric and hydrochloric acid have been used as leaching agents. Generally, titanium extraction efficiencies using hydrochloric acid can be as high as 95.46%, compared to with sulfuric acid. Although processes with the optimal conditions for titanium extraction depend on the specific process, they generally involve a high acid concentration, controlled temperature, and solid-liquid ratios.

Recovery of Gallium:

Gallium is trace element present in the bauxite and enters the red mud during its processing. Red mud generally consists of 0.002 %–0.008% of gallium (Ga) (Lu et al. 2018). Table 4 reviews the work carried out for recovery of gallium from red mud. Solvent extraction and electrolysis are mainly the two methods used for recovery of Gallium from red mud

Table 4. Recovery of Gallium from red mud

Constituents used	Experimental Conditions	Observations	Method used for extraction	Key Finding	Country	Reference
Red mud, HCl, NaOH	Acid leaching with HCl from red mud; 8 mol/L HCl, L/S=4.0, 5 hours, 109 °C	Leaching rate of gallium was 95.4%.	Acid Leaching	Max. extraction rate of gallium reached 98% with 50% TBP+50% kerosene	China	(Wang et al. 2012).
Red mud, oxalic acid, sulphuric acid, nitric acid, hydrochloric acid	Selective acid leaching with oxalic acid (H₂C₂O₄) (2.5 M H₂C₂O₄, 21.7 h, 80°C,10g/L slurry conc.)	Highest extraction efficiency of Ga achieved using oxalic acid leaching.	Acid Leaching	gallium extraction 80%.	Hungary	(Ujaczki et al. 2017).
Red mud, sodium hydroxide	20 % NaOH, 120 °C, 12 hours, solid/liquid ratio 1:5	Fe is not detected in leaching solution	Hydrothermal alkaline leaching method	Maximum gallium leaching efficiency of 91.4%.	China	(Xue et al. 2019)
Red mud, sulphuric acid, D2EHPADi (2-ethylhexyl) phosphoric acid	Extraction of gallium (III) and vanadium (IV) using D2EHPA; pH 1.4-1.8, 0.3 M D2EHPA	. Yield of vanadium increases from 18.1 to 95.8%, yield of gallium nearly 98.2% at pH 1-2	Hydrometallurgy	Optimized conditions for selective separation of gallium and vanadium from aluminum.	China	(Tagiyeva L.T 2021)

Acid leaching is highly effective for gallium extraction, achieving over 95% recovery under optimized conditions. Very recently, phosphoric acid has been proposed to be used for leaching instead of H₂SO₄ (Yimin Zhu 2024). Research focuses on improving the selectivity and efficiency of gallium extraction, particularly in separating it from elements like vanadium and aluminium in complex matrices such as red mud.

Recovery of Rare Earth Elements (REEs):

The REEs comprises of the fifteen metallic elements of the lanthanide series, along with yttrium and scandium (Krishnamurthy and Gupta 2004). They are further classified into light rare earth elements, LREEs, which include lanthanum to europium, and the heavy rare-earth elements, HREEs, that contain the other remaining lanthanides from gadolinium to lutetium plus yttrium (Trifonov 1963). Rare earth elements and is composition from one of the studies of Chenna Borra is given in the Table 5 (Borra et al. 2015). Table 6 show the utilization of red mud for REEs recovery in a national and international context.

Table 5: Rare earth elements composition in red mud in g/tonne

Elements	Sc	Y	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
Composition (g/tonne)	121	76	114	368	28	99	21	5	22	3	17	4	13	2	14	2

Table 6. Utilization of red mud for rare earth elements recovery in National and international context

Study	Observation	Rare earth recovered	Ref.
Acid leaching with HCl and H₂SO₄ at two leaching conditions.	80% scandium recovery and 1% titanium recovery in HCl leaching and 96.5% titanium recovery in H₂SO₄ leaching	Scandium and Titanium	(Zhang, Deng, and Xu 2005)
Biological leaching Penicillium tricolor fungi	Two-step bioleaching process at 10% (w/v) pulp density achieved maximum extraction yield	REE	(Qu and Lian 2013)
Studied possibilities to REEs from residues such as pyrometallurgical slags, bauxite residue (red mud), phosphogypsum, mine tailings and wastewater.	Author reviewed the recovery of REEs from industrial waste	REE	(Binnemans et al. 2013) (Binnemans et al. 2015)
Sulfuric acid leaching followed by liquid-liquid extraction with various organic extractants.	99.9 % Lanthanum and cerium recovery achieved	Lanthanum and Cerium	(Abhilash et al. 2014)
Leaching experiments on Greek bauxite residue	70-80% REEs could be recover using 6 N HCl leaching for 24 h at 25°C.	REE	(Borra et al. 2015)
Author reviewed various extraction method	Can be recovered by direct leaching or by smelting followed by leaching.	REE	(Borra, Blanpain, et al. 2016a)
A combined sulfation–roasting–leaching process to selectively leach the REEs while leaving iron undissolved in the residue.	Higher acid concentrations enhanced REE extraction	REE	(Borra, Mermans, et al. 2016)
Reduction smelting process using wollastonite as a flux and graphite as a reducing agent.	high-temperature leaching at 90°C yielded > 95% scandium, >70% of REEs, and about 70% titanium	Fe, REE	(Borra, Blanpain, et al. 2016b)
Used functionalized ionic liquid titanium bis(trifluoromethylsulfonyl)imide (HbetTf2N) directly on leaching	70-85% REEs extraction yield with <3% Fe extraction yeild	REE	(Davris et al. 2016)
Recovery of Titanium and scandium using combine pyrometallurgical and hydrometallurgical methods	To overcome limitations of direct leaching, a new process combining pyrometallurgical and hydrometallurgical methods was proposed	REE	(Alkan, Xakalashe, et al. 2017)
Used sequential extraction method	60% yttrium was extracted from high iron diasporic red mud	yttrium	(Gu, Wang, and Hargreaves 2018)
Study comprises of Overview of the methods to extract rare earth elements (REEs)	Author reviewed method for extraction of REEs	REEs	(Akcil et al. 2018)
Dry digestion method followed by water leaching	Multi-stage leaching, particularly with HCl is more effective than single-stage dry digestion-water leaching	REE	(Rivera et al. 2018)
Technical feasibility of simultaneous metal extraction and separation was investigated using HCl medium and various organic extractant in liquid-liquid extraction.	88.5% of La and 99.9% of Ce recovery at 95 °C, 1 M HCl, S/L 1/100, 1 hour, 200 rpm agitation	Lanthanum and Cerium	(Abhilash et al. 2019)
An acid producing bacteria Acetobacter sp. was used. Two-step processes were used with 2-10% pulp density.	Leaching ratios of Al, Lu, Y, Sc, and Th were 55%, 53%, 61%, 52% and 53% respectively with one step process at 2% pulp density	Aluminium, Lanthanum, Yttrium, Scandium, Thorium	(Qu et al. 2019)

Recovery of Scandium:

The recent advancements in the distribution characteristics of scandium in red mud, along with the techniques for extracting scandium from red mud and the associated processing conditions, have been reviewed both domestically and internationally. (Si, Deng, and Xu 2003). The authors summarized techniques for extracting scandium from red mud, outlining key technologies for recovery from leaching solutions, solvent extraction, and refining processes, ultimately suggesting a more viable approach for comprehensive red mud utilization (Wang et al. 2008). Table 7 reviews the work carried out for extraction of scandium.

Table 7. Recovery of Scandium from red mud

Constituent used	Experimental Conditions	Observations	Method of Extraction	Key Findings	Country	Reference
Red mud, Hydrochloric acid, P_507 and kerosene	HCl leaching followed by solvent extraction	Processing conditions studied	Hydrometallurgy	The scandium had 66.09% purity.	China	(Zhang, Deng, and Xu 2006)
Modified Activated Carbon, HCl, red mud	Adsorbent dosage: 6.25 g/L,308 K, Adsorption time: 40 min	Adsorption capacity to Sc increases with time, reaching equilibrium after 40 min	Adsorption	Adsorption capacity to Sc increases with time, reaching equilibrium after 40 min	China	(ZHOU et al. 2008)
Red mud, NaOH, Na₂CO_3, ZnO	Carbonisation of red mud with CO₂	Ga, Al also co-precipitate	Carbonisation.	Zinc hydroxide was the most effective co-precipitant for scandium	Russia	(Yatsenko and Pyagai 2010)
Red mud, diluted sulphuric acid, Cyanex 272, D2EHPA	Leaching with 0.5 H₂SO₄ followed by solvent extraction	DE2HPA is best extract among remaining extract used in the study	solvent ex‐ traction	99% scandium could extract	Australia	(Wang, Pranolo, and Cheng 2013)
Red mud, HCl, H₂SO₄, lignite coke, lime	High-pressure acid leaching with HCl at 120°C and H₂SO₄at 150°C	High selectivity for scandium extraction	Hydro-metallurgy	Approx. 80% scandium recovery (HCl leaching) and more than 95% extraction (H₂SO₄leaching)	Greece	(Rivera et al. 2019)
Red mud, sulphuric acid, CaF_2,solvent extractant P507	Acid leaching with CaF₂ and H₂SO₄, 90°C, solvent extraction 10% P507, pH: 0.1 for 4 min	Effective combination of acid leaching and solvent extraction	Acid leaching	over 98% scandium extraction	China	(Zhu et al. 2020)
Red mud, ammonium chloride, sulfuric acid scandium sulfate, ethanol	5-6 M H₂SO₄and 0.5 M NH₄Cl	High scandium recovery rate achieved.	Hydro-metallurgy	99% of scandium in the form of NH₄Sc (SO₄)₂	Russia	(Pasechnik et al. 2021)
Red mud, H₂SO₄, MgSO₄, NaOH, and Al(OH)_3,sodium carbonate and lime	Acid leaching with MgSO₄ at pH 2, C_MgSO4 of 24 g L⁻¹ ,80 °C, 60 min	significant extraction even at pH 4	Acid leaching	scandium extraction efficiency of over 80% at pH 2	Russia	(Shoppert et al. 2022)
Red mud, nitric acid	Diluted nitric acid leaching at pH 2, L:S ratio 10,80 °C, 90 min	Significant effect of pH	Acid leaching	71.2% scandium extraction with Fe extraction less than 3%.	Russia	(Shoppert et al. 2022)
Red mud, hydrochloric acid	Hydrochloric acid leaching at 20% in solution, 200 rpm, 80°C, 3h, S/L:1:10(g/mL), −74 µm sample particle size	leaching efficiency is controlled by chemical reaction step.	Acid leaching	Maximum scandium leaching efficiency of 83.94%	China	(Wei et al. 2022)
Red mud, sulfuric acid	Roasting at 1023 K, 60 min H₂SO₄ leaching (H₂SO₄/RM :0.9 mL/g, 323K, 200 rpm, 2h, L:S:5mL/g)	low acid consumption, high scandium leaching rate	sulfation-roasting-water leaching method	Leaching efficiency of scandium is 91.98%	China	(Ding et al. 2022)

Various acids like hydrochloric acid, sulfuric acid, and nitric acid have been commonly used for leaching of scandium from red mud. Under optimized conditions, which generally include elevated temperatures ranging from 80 to 120°C and carefully regulated pH levels, high recovery rates for scandium can be attained, often exceeding 80% and reaching as high as 99%. The experiments were carried out in several countries, including Greece, China, and Russia, with studies being carried out in India too.

CONCLUSION:

The processes for metal extraction from red mud have been several studied and have been reviewed. Red mud consists of metals which makes their recovery an important aspect both in as far as industry is concerned and in relation to the environment. Reduction roasting is the most used method for extraction of iron from red mud. In reduction roasting, the red mud is treated at very high temperatures together with a reductant that allows the iron oxides to be converted to iron. After this roasting, it is common to get rid of iron by means of magnetic separation in which only the magnetic iron is separated. In addition to reduction roasting, acid leaching is another important method of iron extraction. Using this method, red mud is treated with acids that dissolve iron. Most of the time, researchers rely on hydro metallurgical techniques to separate alumina and caustic soda from red mud. In these processes, aqueous solutions are utilized for the removal of the targeted components from red mud. One of the most effective processes is the calcification-carbonation method, in which aluminium-containing compounds are converted into more soluble forms and then extracted. For the extraction of alumina and caustic soda both hydrometallurgy and calcium carbonation have demonstrated high extraction efficiency. For titanium recovery, strong acid leaching methods have shown more leaching efficiency. For selectively recovery of gallium from red mud there is not much literature available. Acid and alkali leaching methods are being utilized by researchers to extract gallium. The exploration of gallium recovery remains an area of ongoing research, as the demand for this metal continues to grow in various high-tech applications. There is an increasing demand on extraction of rare earth elements from red mud like cerium, scandium and lanthanum due to their applications in specialised materials.

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Received on 27.01.2025 Revised on 24.02.2025

Accepted on 20.03.2025 Published on 27.03.2025

Available online from March 27, 2025

Research J. Engineering and Tech. 2025; 16(1):31-43.

DOI: 10.52711/2321-581X.2025.00004

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