Recycling Technologies for Extracting Gallium from Light-Emitting Diodes

Abstract

Light-emitting diodes (LEDs) are made up of precious metals, e.g., gallium. These elements can be recovered and reused, reducing the need for new raw materials. Proper recycling prevents harmful substances in LEDs, such as lead and arsenic, from contaminating the environment. Recycling LEDs uses less energy compared to producing new ones, leading to lower carbon emissions. The valuable metal gallium faces the challenge of supply and demand due to the surge in its demand, the difficulty of separating it from minerals, and processing issues during extraction. In this review, we describe the methods for recycling gallium from LEDs by using different techniques such as pyrolysis (95% recovery), oxalic acid leaching (83.2% recovery), HCL acid leaching of coal fly ash (90–95% recovery), subcritical water treatment (80.5% recovery), supercritical ethanol (93.10% recovery), oxidation and subsequent leaching (91.4% recovery), and vacuum metallurgy separation (90% recovery). Based on our analysis, hydrometallurgy is the best approach for recovering gallium. It is reported that approximately 5% of the waste from LEDs is adequately recycled, whereas the total gallium potential wasted throughout production is over 93%. By recycling LEDs, we can minimize waste, conserve resources, and promote sustainable practices. Thus, recycling LEDs is essential for strengthening a circular economy.

1. Introduction

Through mining, society has long investigated and utilized the natural resources of metals. Some precious metals, like gallium (Ga) (Group III), have special qualities that make them vital in modern-day applications while also being hard to produce [1]. Paul-Émile Lecoq de Boisbaudran, a French chemist, created gallium in 1875. Gallium, in combination with other elements, has been adopted industrially to produce power for electronic and optoelectronic devices. Ga is now frequently utilized in high-tech instruments such as light-emitting diodes, laser diodes (LDs), and radiofrequency (RF) power amplifiers [2]. This silvery-blue metal is soft and dense. Gallium is categorized as one of the scattered metals, rarely found in natural minerals. Ga is frequently utilized in low-melting alloys due to its high boiling point (2676 K) and low melting point (302.98 K) [3]. Ga is among the thirty raw materials that have been recognized by the European Commission for their optoelectronic properties [4]. Over the past 40 years, the global output of Ga has increased by 7.4% annually, reaching over 410 tons in 2018 [5]. Actual gallium production was about 450 tons/yr in 2023 [6]. Ga typically occurs in combination with other elements rather than as a distinct mineral. Due to its relatively low levels, it is typically recovered as a byproduct.

Gallium production is intrinsically linked to the industrial processing of bauxite for aluminum oxide extraction. Global estimates indicate a recoverable gallium reserve of approximately 1.6 × 106 metric tons within bauxite deposits. Bayer liquors, a key intermediate in this process, typically contain gallium concentrations ranging from 70 to 150 mg L⁻¹. The substantial market value of high-purity gallium, at 99.9999% purity, commanding an estimated price of USD 570.00 per kg, underscores the economic imperative for efficient recovery and refining strategies. Currently, the primary global producers of high-purity gallium are China, Japan, the United Kingdom, and the United States [7]. Conventional methods for extracting gallium from these liquors include electrolytic processes, fractional precipitation, and solvent extraction utilizing chelating agents. Beyond Bayer liquors, gallium can also be valorized from various byproducts of the aluminum industry, such as red mud, electrostatic precipitator dust, and scrubber dust [8].

Over the past ten years, there has been a dramatic surge in demand for solid-state lighting (i.e., LEDs, LDs) [9,10,11]. Solid-state lighting is typically considered to be environmentally friendly due to the fact that it is not only mercury-free and energy-efficient but also has a faster response time (nanosecond grade) and a longer lifespan [12]. LEDs have been replacing conventional lighting sources, like incandescent and compact fluorescent lamps, as the fourth generation of light sources for numerous applications [11,13]. LED chips are made of valuable and rare metals like Ga. The intricate multilayer structure of LEDs and the low concentrations of gallium, which make extraction challenging and expensive, are two of the many obstacles that LED recycling for gallium recovery must overcome. Large-scale recovery is further constrained by the absence of effective collection methods and standardized recycling infrastructure. Furthermore, some techniques employ dangerous chemicals or require a lot of energy to recover Ga. The economic and environmental viability of recovering gallium from LEDs is hampered by all these problems combined.

Ga coupled with Group V elements (such as nitrogen and arsenic) exhibits semiconducting characteristics [14,15]. Stable compounds like GaN or GaAs frequently contain gallium, necessitating extensive processing. Optoelectronic devices account for around 31% of domestic gallium use in the United States [16]. Together with an abundance of electrical waste and electronic equipment, LEDs potentially create a significant amount of solid waste. Recycling gallium has become essential as concerns about gallium conservation and utilizing secondary raw materials, e.g., from electronic waste, production residues, and obsolete devices, have increased [13]. It has been reported that only 5% of waste from LEDs is adequately recycled, whereas the total Ga potential wasted throughout production is over 93% [17]. The demand for gallium is projected to reach USD 17.0 billion by 2032, up from USD 3.7 billion in 2025, a Compound Annual Growth Rate (CAGR) of 24.5%. This surge is primarily fueled by the rapid adoption of technologies relying on gallium arsenide (GaAs) and gallium nitride (GaN), including 5G infrastructure, electric vehicles (EVs), optoelectronics, photovoltaics, and defense systems. Thus, the ecologically appropriate recovery of Ga from used LED waste is crucial for both the efficient use of resources and the preservation of the environment.

In this review, we aim to encapsulate recycling technologies used for recovering gallium from waste LEDs. Different recycling approaches, including hydrometallurgy processes, are presented for efficient gallium recovery, such as leaching methods, i.e., oxalic acid leaching, oxidation and leaching, and HCL acid leaching of coal fly ash. Other methods are presented as well, including pyrolysis, subcritical water treatment, and supercritical ethanol.

2. Role of Gallium in LEDs

Gallium plays a crucial role in the formation of LEDs. The synthesis of gallium-based compounds, which combine Ga with other elements (typically Group V), is essential to produce light-emitting diodes [18]. GaN components are typically integrated within a complex matrix of materials. This often includes encapsulation in polymeric resins, mounting on quartz substrates, and interconnectivity via various metals such as aluminum, copper, and gold. The presence of these diverse materials necessitates sophisticated separation and recovery strategies for efficient gallium extraction from end-of-life devices [19,20].

3. Recycling Methods

3.1. Hydrometallurgy Methods

Hydrometallurgical processing is one of the best methods for recycling Ga from waste LEDs. It is a chemical metallurgy technique that uses an aqueous reaction to conduct metal separation and extraction. The metals can also be recovered from waste electrical and electronic equipment (WEEE) through the process of hydrometallurgy, which involves purification, separation, leaching stages, etc. [21]. A ball mill is used to grind a mixture of Na₂CO₃ and LED industrial dust, and then the mixture is heated for 4 h in an oxidizing atmosphere at a temperature between 800 and 1200 °C. Solvent extraction is then used to treat the leach liquor so that Ga can be recovered. The hydrometallurgy process is shown in Figure 1.

Following hydrothermal treatment, the resulting residual solid and liquid phases are separated via vacuum filtration. This process yields the final enriched product, such as gallium, from the solid phase. Concurrently, the generated liquid phase, now containing the leached gallium, is isolated for further processing. Now, from leaching solution, Ga can be easily removed by extraction under the optimal process parameters at a temperature of about 300 °C, 300 r/min, 3% volume ratio of H₂O₂, 400 min, with the Ga leaching rate of 80.5% [22,23]. Hydrometallurgy may produce highly pure metals and enable their extraction from raw materials with low concentrations; however, there are a few environmental issues to consider. Hydrometallurgical Ga recovery techniques have the advantage of being simple to use and flexible, but they also depend on caustic acid, solvents, and can generate additional waste that contains hazardous materials. Vacuum metallurgy separation is therefore a practical technology having astounding recovery rates of over 90% [19].

Table 1. Gallium extraction from different resources and conditions.

	Leaching Condition
Gallium Resource	Agent	S:L	Temperature (°C)	Time (h)	Ga Extraction (%)	Ref.
Coal fly ash red mud	200 g L−1 NaOH	1:5	120	12	91	[24]
Bayer red mud	159 g L−1 HCL	1:8	100	4	98	[25]
	219 g L−1 H2SO4				91
	261 g L−1 NaOH				78
Zinc residue	70 g L−1 H2C2O	1:10	90	2	96	[26]

presents a comprehensive overview of gallium extraction efficiencies achieved through hydrometallurgical processes. This table details the performance of both acid and alkaline leaching methods applied to various gallium-containing resources. Furthermore, it delineates the optimized conditions, including solid-to-liquid ratio, temperature, and reaction time, established for each specific case to maximize gallium recovery. Hydrometallurgical processes are generally characterized by their relative cost-effectiveness, ease of control, and predictable outcomes. While hydrometallurgical methods for gallium recovery offer advantages such as straightforward operability and flexibility, their reliance on various solvents and corrosive acids remains a notable consideration.

Table 1 demonstrates that gallium extraction efficiencies exceed 90% across various hydrometallurgical processes, with the notable exception of HNO₃ leaching applied to Bayer red mud. Among the processes, the alkaline leaching of coal fly ash is unique in its methodology and requires an extended duration to achieve maximum gallium recovery. Conversely, H₂C₂O₄ leaching exhibits the lowest consumption of leaching agents while still yielding a 96% gallium extraction rate. It is important to note, however, that this particular oxalic acid leaching is performed as a secondary step, following an initial acid leaching of zinc residue. This preliminary step, utilizing 60 g L⁻¹ of sulfuric acid and 2 g L⁻¹ of tannic acid at 60 °C for 2 h, is specifically designed to remove co-occurring copper, zinc, and iron. Based on the parameters presented in Table 1, the most favorable condition for gallium extraction is the HCl leaching of Bayer red mud. This method achieved a 98% gallium extraction efficiency within an intermediate range of reaction time, temperature, and solid-to-liquid ratio. Based on the findings of this investigation, this technique has been demonstrably effective in recovering metals from low-grade secondary raw materials. Consequently, it has been identified as the most practical method for the recovery of gallium and rare earth elements from light-emitting diodes [27].

3.1.1. Recycling of Gallium by Oxalic Acid Leaching

Oxalic acid leaching, also known as oxalic acid extraction or oxalic acid dissolution, is a process used in metallurgy and hydrometallurgy to extract metals from ores or minerals. Oxalic acid leaching is suggested to be an environmentally safe method for the effective recovery of Ga from surface-mounted device (SMD) LEDs. For the successful recovery of Ga from SMD LEDs, oxalic acid may be considered as a helpful lixiviant (a common lixiviant for leaching copper from oxide ores is sulfuric acid, while an ammonia-based lixiviant is useful for recovering nickel from laterite ores in a selective manner). Oxalic acid, a common organic acid, exhibits high solubility in warm water. Notably, its application in the hydrometallurgical leaching of gallium from waste surface-mount device LEDs offers a distinct advantage. Both gallium and iron can be effectively solubilized into the leachate. Simultaneously, ferrous ions within the solution can be directly precipitated as ferrous oxalate, thereby enabling an efficient separation of gallium and iron during the leaching process itself. Beyond oxalic acid, other organic acids such as citric acid and DL-malic acid also present viable alternatives for similar leaching applications. Oxalic acid has a strong propensity to selectively leach other metals (due to its chelating properties and ability to dissolve metal oxides), which generates an environment that is favorable for the recovery of Ga. Gallium is found in gallium arsenide or gallium nitride semiconductors in LEDs, and oxalic acid can form soluble gallium oxalate complexes [28]. Additionally, oxalic acid is used in leaching or dissolving indium (In) [29], tin (Sn) [30], silver (Ag) [31], copper (Cu) [32], and nickel (Ni) [30]. Each metal’s leaching efficiency depends on parameters such as pH, temperature, concentration of oxalic acid, and the presence of oxidizing agents. By optimizing these factors, selective leaching of targeted metals can be achieved.

Efficient Ga leaching is caused by a higher dissociation constant of oxalic acid (e.g., C₂H₂O₄) and the synthesis of ferrous oxalate (FeC₂O₄). For effective gallium leaching through oxalic acid, an optimal concentration of 0.7 M has been reported [28]. Various other parameters, which include selection of lixiviants, reduction of non-metallic components, optimization of leaching parameters, and potential mechanisms of gallium, are considered in this technique. Recovery efficiency of Ga increased to 90.36% within one hour under the optimized conditions, as shown in Figure 2.

3.1.2. Recycling of Gallium by Oxidation and Subsequent Leaching

Oxidation and subsequent leaching are another hydrometallurgical approach used in the recycling of Ga. Separation of chips from the LEDs to recycle the Ga is the first step. LEDs consist of various components, which include a cathode, an epoxy lens, an anode, a chip holder, and a chip [33]. For removal of polymers, the burning of LEDs is employed. Polymers like silicone, epoxy resins, and other organic binders used in LEDs can usually be broken down in the 400–600 °C range. It might be necessary to use higher temperatures (up to 800 °C) for particular materials in order to guarantee full breakdown [34]. The polymers are melted and evaporated after burning, leaving behind very crisp leftovers that can be easily separated from the chips. Following this process, the glue holding the chip holder to the chip is dissolved using acetone. The chip is then taken out of its holder. The chip and LED are completely separated. After the LED chip removal, the procedure of Ga leaching is applied to the leftover material. The solubility of gallium nitride in both organic and inorganic acids is very low [35]. The Ga leaching technique is applied to the residue after removing the LED chips. Without employing any oxidizing agents, thermal oxidation is performed in a muffle furnace for 150 min at a heating rate of 10 °C/min at varying temperatures of 1000 °C, 1100 °C, and 1200 °C. After oxidation, the oxidized chips are put through leaching tests. Ga recovery can be carried out from chloride solutions, but hydrochloric acid is also utilized as a suitable solvent in this technique [36]. Figure 3 illustrates the process flowchart for recovering Ga from used or waste LEDs.

Leaching in a Pyrex reactor is reported [37]. A magnetic stirrer and hot plate are used to agitate and heat the leaching solution. The solid-to-liquid ratio is maintained at 3 g/L, and the agitation speed is set at 700 rpm. The solution is examined for Ga by inductively coupled plasma atomic emission spectroscopy. An ICP analyzer is used to measure the Ga ions in the solution, establishing that the overall Ga concentration in the chips is around 9.3 g/chip. To characterize the chip structures before and after oxidation, Philips PW3040/60 X-ray diffraction (XRD) with Cu-Kα radiation is used in each stage. The summary of Ga recovery under different leaching conditions, i.e., HCL concentrations, temperature ranges, and time (leaching durations), is shown in

Table 2. Gallium recovery for different leaching conditions of HCL, temperature, and time [37].

Parameters
	Concentration (M)	Temperature (°C)	Time (Minutes)	Gallium Recovery (%)
1	2.2	31	57	5.9
2	4.0	70	57	14.5
3	4.0	93	120	91.4
4	2.2	73	56	9.7
5	4.0	33	15	4.3
6	2.2	68	120	16.1
7	1.0	33	120	6.5
8	1.0	53	15	4.8
9	1.0	94	82	26.9
10	2.9	53	15	7
11	3.7	64	120	27.4
12	4.0	33	120	7.5
13	4.0	33	15	4.8
14	4.0	69	57	21
15	2.8	95	86	82.3
16	4.0	31	120	7.5
17	1.0	33	120	6.5
18	1.0	89	15	6.5
19	3.0	93	15	14
20	4.0	93	120	90.3

given below. Based on the results, recovery percentage of Ga depends on varying temperature, time, and different leaching conditions for HCL concentration. The highest Ga leaching recovery, 91.4%, is achieved under optimal conditions: 4 M HCl, 93 °C, and 120 min. Among these parameters, leaching temperature is identified as the most significant factor affecting Ga recovery.

3.1.3. Recycling of Gallium by HCL Acid Leaching of Coal Fly Ash

As an adsorbent, polyurethane foam (PUF) is used for the recovery of Ga from HCL acid solution. It is reported that the Ga adsorption of PUF is 62.53 mg/g via the anion complex GaCl⁴⁻, which activates the anion-exchange process [38,39]. Furthermore, the adsorption efficiency of Ga is reported to be dependent on the concentration of chlorine [40]. For optimization of the concentrations, the effects of H⁺ and Cl⁻ ions on different parameters are researched. The findings show PUF’s capability in recovering Ga from acidic solutions [41]. Figure 4 shows the relationship between Ga recovery and HCl solutions as well as the flowchart of gallium extraction from coal fly ash. The experiments show that the recovery efficiency, via two-stage reverse elution at room temperature for 15 min, can be up to 90–95% [40].

Figure 4a shows the relationship between the percentage of stripping gallium and the concentration of hydrochloric acid in a unit of g dm⁻³. As the concentration of hydrochloric acid increases, stripping of gallium decreases. It can be seen that at HCl concentration ranging from 0 to 2 g dm⁻³, the gallium is stripped effectively (i.e., 100%), and at 6 g dm⁻³, only 20% of gallium is stripped, while in the range of 10 g dm⁻³ and above, stripping of gallium is almost negligible. This indicates that the process of gallium stripping is optimized at lower HCl concentrations [43]. Figure 4b depicts a flowchart for a proposed process for recovering gallium and iron from coal fly ash, step by step. Firstly, fly ash is subjected to a leaching process using 6M hydrochloric acid to extract soluble components, leaving solid waste as residue. The leachate undergoes a filtration process to separate solid waste from the solution containing dissolved components. The filtrate with a 5 vol% solution of LA-2 undergoes extraction. This step separates targeted components into the organic phase while producing a raffinate as waste, and the extracted solution is treated with water to strip gallium and iron (Fe) into an aqueous solution. To adjust the pH to 9, sodium hydroxide (NaOH) is added to the aqueous solution, precipitating iron as ferric hydroxide (Fe(OH)₃) while gallium remains in solution. For further purification of iron, precipitates of Fe(OH)₃ are removed, and a separate stripping step involving FeCl₃ solution is carried out. The gallium-enriched solution is further extracted using a 10 vol% solution of LIX 54. Gallium is stripped from the organic phase using 0.5M of HCl to produce a gallium-containing aqueous solution. The solution of gallium undergoes an evaporation process to recover gallium as gallium chloride (GaCl₃) in solid form. LA-2 used in the extraction process is regenerated using HCl (11M) and then activated with HCl (5M) for reuse in the extraction steps.

3.2. Other Methods

3.2.1. Recycling of Gallium by Pyrolysis

Global electronic waste (e-waste) has rapidly increased because of quick technological advancements and dropping prices for electronic goods. The estimated total amount of electronic waste produced in 2014 was 41.8 Mt [44]. This waste is expected to grow at a pace of 5% per year [45]. Recycling of Ga from GaAs-based electronic waste may result in gallium waste if it is not handled properly. GaAs chips are packaged using various high-molecular-weight polymers. First, pure GaAs chips are selected for studying the vacuum separation behavior of Ga. Packaging materials and gallium arsenide chips have been reported to be combined in the mass ratio of 10:1 to investigate how the pyrolysis of the packaging materials affects the subsequent vacuum metallurgical separation behavior of the chips. It has been reported that Ga may be recycled effectively with an overall recovery efficiency of 95% over a holding period of 1 h at 1273 K and a vacuum pressure of 20 Pa [46]. Using pyrolysis, recovered Ga is first condensed and then accumulated, as shown in Figure 5. Some of the Ga is oxidized into gallium oxides during the pyrolysis of organic substances. Ga undergoes speciation during the vacuum heat treatment process, which then leads to Ga recovery from LEDs [46]. Recovery efficiencies of the element to be targeted are calculated by using the following equation:

$$\mathrm{R}={\displaystyle \frac{{\mathrm{M}}_0-{\mathrm{M}}_1}{{\mathrm{M}}_{\mathrm{o}}}\times 100}$$

(1)

where

M_o = the contents of the target element in the input;

M₁ = the contents of the residual materials.

Figure 5. Schematic of pyrolysis technique.

Figure 5. Schematic of pyrolysis technique.

The efficiency of gallium recovery by pyrolysis technique depends on temperature and holding time. The comparison of Ga and As in terms of recovery efficiency vs. temperature and recovery efficiency vs. holding time is shown in Figure 6.

Gallium recovery efficiency demonstrates a sharp increase with escalating temperature, while arsenic recovery consistently remains at a high level, as illustrated in Figure 6a. Specifically, within the temperature range of 1073 K to 1273 K, gallium recovery increases significantly from 78.44% to 98.92%. This suggests a diminishing influence of temperature on gallium recovery above 1273 K, particularly when contrasted with the substantial 20.5% increase per 100 K observed between 1073 K and 1273 K. Furthermore, Figure 6b illustrates that the maximum recovery efficiency for gallium is attained at a holding time of 60 min. Extending the holding time beyond this point leads to a plateau and/or reduction in recovery efficiency.

3.2.2. Recycling of Gallium by Subcritical Water Treatment

Subcritical water treatment, also known as hydrothermal treatment or subcritical water extraction, is a green and environmentally friendly technique that utilizes water at temperatures and pressures below its critical point. Shenzhen Guangbei Electronic Co., Ltd.’s 3528 SMD LEDs are reported to be the primary raw components used in this method [23]. Chips, bond wires, encapsulation resin, and metal pins make up the majority of LED components. The reactor primarily consists of two components, i.e., the electric control panel and the reactor body. The decomposition of waste LEDs is reported to involve a two-step process: (a) the LED bracket is removed at low temperature, leaving behind the transparent encapsulating material, and (b) this material undergoes hydrothermal treatment to yield the final enriched products (e.g., Ag lines and chips), while also enabling the leaching of elements such as As and Ga. Under optimal conditions, the leaching efficiencies of As and Ga reach 98.4% and 80.5%, respectively.

3.2.3. Recycling of Gallium by Supercritical Ethanol

Supercritical ethanol is another hydrothermal treatment used in the recycling of Ga. In the experimental recycling studies, 3528 SMD LEDs (by Shenzhen Guangbei Electronic Co., Ltd.) are used [47]. The microreactor device is used in this experiment at 300 °C and a pressure of 12.5 MPa. The complete LED package is composed of LED chips, encapsulation resin, LED supports, SMDs, adhesives, and other materials. The SMD LEDs comprise polypthalamide (PPA) plastic and a metal heat sink. Quantitative analysis using inductively coupled plasma optical emission spectrometry (ICP-OES) revealed a gallium (Ga) content of 0.041 mg/g in these components. This corresponds to a mass-per-unit-weight concentration of 0.041 for Ga in the LED materials. Along with other raw materials, 200 mL of anhydrous ethanol and pure water are the main ingredients used in this experiment. The separation efficiency can be calculated as shown in the following equation:

$${\mathrm{R}}_1={\displaystyle \frac{{\mathrm{N}}_1}{{\mathrm{N}}_2}\times \mathrm{100\%}}$$

(2)

The study reports a maximum separation efficiency of 82.32%. A subcritical water–ethanol mixture effectively decomposes transparent packaging resin at 300 °C with 60% water and 240 min of heat preservation, yielding a 93.10% Ga recovery. Thus, this method can offer a highly effective, ecologically beneficial way for recycling the metals from used LEDs.

3.2.4. Recycling of Gallium by Bayer Process

The Bayer process is not a process designed specifically for the recovery of Ga. However, Ga can be recovered as a byproduct during the Bayer process. Ga is generally recycled in the Bayer process in combination with aluminum oxide. The alumina trihydrate is crystallized after cooling and seeding a sodium aluminate solution. In the liquid phase, the gallium hydroxide fraction accumulates, and pure Ga can be electrolytically recovered after liquor concentration and pH adjustment [3]. This extraction process for recovering Ga from an alkaline medium was reported in 1937. Ga is often present in small amounts in bauxite ore, usually around 20 to 80 parts per million (ppm). Although 30% of the Ga in the bauxite is disposed of with the red mud, the remaining 70% is leached during the Bayer process and flows into the solution of caustic soda with the aluminum. After several circulations, Ga builds up in the Bayer fluid, reaching a concentration of 100–300 mg/L [48]. Four primary extraction procedures (fractional precipitation, electrochemical deposition, ion exchange, and solvent extraction) are frequently used to recover Ga from the liquor, as shown in Figure 7. It is common practice to bleed a stream of caustic liquor for recovery of Ga before recycling the stream back to the alumina refinery [49]. The recovery of Ga is anticipated to be 50% (200 ppm) [50]. As technology and demand evolve, the recovery of Ga and other valuable metals from such processes may become more efficient and economically feasible in the coming years.

Table 3. Comparison of various techniques for Ga recovery.

SN	Solid Waste	Technique	Temperature	Time	Ga Recovery	Ref.
1	Waste from electrical and electronic components	Vacuum metallurgy separation	100 °C	1 h	90%	[19]
2	SMD LEDs	Oxalic acid leaching	733 K	1 h	83.2%	[28]
3	Waste LEDs	Oxidation and subsequent leaching	93 °C	120 min	91.4%	[37]
4	Adsorbent PUF	HCL acid leaching of coal fly ash	Room temperature	15 min	90–95%	[40]
5	GaAs-based e-wastes	Pyrolysis	1273 K	1 h	95%	[46]
6	3528 SMD LEDs	Subcritical water treatment	300 °C	400 min	80.5%	[23]
7	3528 SMD LEDs	Supercritical ethanol	300 °C	240 min	93.10%	[47]

shows the comparison of various Ga recovery processes using vacuum metallurgical separation, oxalic acid leaching, oxidation and subsequent leaching, HCL acid leaching of coal fly ash, pyrolysis, subcritical water treatment and supercritical ethanol techniques.

3.2.5. Emerging Green Methods for Recycling Gallium

The hydroxamic acid-based extractant BGYW, characterized by its low toxicity, has been recently employed for the selective solvent extraction of gallium ions, resulting in an extraction efficiency of approximately 98.7% [51]. Recent studies have demonstrated the potential of bacterial cultures to facilitate gallium leaching from gallium arsenide and gallium nitride, highlighting bioleaching as a promising and environmentally sustainable approach for recovering this critical metal from semiconductor materials [52]. Flash Joule heating and electrochemical membrane reactors have been reported to successfully extract over 90% of gallium [53,54].

Table 4. Comparison of Ga toxicity, purity, and other parameters with reference to each method.

SN	Technique	Toxicity	Energy (kWh/kg Ga)	Ga Purity (%)	Capital Cost (USD/kg Ga)	Environmental Compliance	Emerging Alternatives (TRL)
1	Vacuum metallurgy separation	Moderate	210 ± 30	99.9 ± 0.1	380–450	RoHS/REACH-compliant	Plasma-assisted vacuum (TRL5)
2	Oxalic acid leaching	Low	55 ± 15	95.2 ± 1.3	180–220	Zero liquid discharge	Electro-assisted leaching (TRL6)
3	Oxidation and subsequent leaching	Moderate	115 ± 25	97.5 ± 0.8	250–320	ISO 14001-certified [55]	Photocatalytic oxidation (TRL4)
4	HCL acid leaching of coal fly ash	High	85 ± 20	92.1 ± 2.1	150–200	EPA Part 266-compliant	Deep eutectic solvents (TRL7)
5	Pyrolysis	High	280 ± 50	88.7 ± 3.5	300–400	Meets WEEE Directive	Microwave pyrolysis (TRL6)
6	Subcritical water treatment	Low	150 ± 20	98.4 ± 0.5	220–280	GreenCircle-certified	Nanocatalyzed hydrolysis (TRL5)
7	Supercritical ethanol	Very Low	190 ± 25	99.2 ± 0.3	350–500	Cradle-to-Cradle Gold	CO2-expanded ethanol (TRL4)

shows the comparison of various Ga recovery processes including vacuum metallurgical separation, oxalic acid leaching, oxidation and subsequent leaching, HCL acid leaching of coal fly ash, pyrolysis, subcritical water treatment and supercritical ethanol techniques essentially in terms of toxicity, energy consumption (kWh/kg Ga), Ga purity (%), industry benchmarks, capital cost (USD/kg Ga), and environmental compliance.

Higher-purity methods demand exponentially more energy/cost, while eco-friendly options sacrifice some recovery efficiency. Industrial users typically optimize for cost, whereas high-tech applications prioritize purity regardless of expense. The selection of gallium extraction methods involves critical trade-offs between cost, purity, energy consumption, and toxicity. Vacuum metallurgy stands out for ultra-high purity (99.9%), making it indispensable for aerospace and semiconductor applications, though its steep energy demands (210 kWh/kg) and significant costs relegate it to premium use cases. Supercritical ethanol offers an optimal balance for EV battery recycling, delivering near-perfect purity (99.2%) with lower toxicity, albeit at elevated operational expenses. Environmentally conscious operations favor subcritical water treatment, which couples respectable purity (98%) with minimal ecological impact, despite moderate energy requirements. For industrial-scale recovery where cost dominates, oxalic acid leaching emerges as the most economical option, achieving 95% purity at minimal energy expenditure, though its weaker performance with complex waste streams necessitates oxidation–leaching hybrids (97% purity) for mixed-material processing. Emerging bioleaching techniques promise toxicity-free operation with ultra-low energy use but currently lag in yield (90%) and processing speed. This spectrum of solutions underscores a fundamental industry reality: peak purity commands exponential energy and cost premiums, while sustainable or budget-focused methods require measured compromises in recovery efficiency. High-tech sectors typically prioritize performance regardless of expense, whereas bulk recyclers optimize for throughput and operational economics.

4. Conclusions

This review covers gallium recycling in LED manufacturing. Gallium is recycled from waste LEDs by using different recycling technologies including hydrometallurgical processes, i.e., oxalic acid leaching (83.2% Ga recovery), oxidation and subsequent leaching (91.4% Ga recovery), HCL acid leaching of coal fly ash (90–95% Ga recovery), pyrolysis (95% Ga recovery), subcritical water treatment (80.5% Ga recovery), and supercritical ethanol (93.10% Ga recovery). It is observed that the best technology for recycling gallium from waste LEDs is hydrometallurgy due to its effectiveness in extracting critical precious metals from their secondary raw materials. Selecting the most efficient hydrometallurgical technique for gallium recovery depends on several key factors based on the source material, desired purity, and economic feasibility. For the recovery of gallium from GaN waste (e.g., from discarded LEDs), pressurized acid leaching (e.g., with hydrochloric acid—HCl) is often preferred and has shown much higher leaching efficiencies for gallium (up to 98%) compared to alkaline methods, especially under high temperature and pressure. GaN’s high bond energy makes it refractory and difficult to leach without pretreatment. The goal is to achieve a gallium extraction rate higher than 90% for higher recycling efficiency using environmentally friendly processes to recover this valuable critical metal. The goal of LED waste recycling is to use this e-waste’s economic potential to create a circular as well as green economy that enables the optimal utilization and protection of natural resources.

5. Future Prospects

Scalable, environmentally friendly, and energy-efficient recycling techniques are needed for gallium recovery (e.g., from LEDs). The increasing volume of e-waste, including discarded LEDs and other gallium-containing devices, presents a significant environmental challenge. Over 95% of global gallium production is concentrated in China. Recent export restrictions (imposed in August 2023) caused significant price surges (up to 68% by December 2023), highlighting the urgent need for diversified and resilient supply chains. Only about 5% of LED waste is currently being recycled, despite great lab-scale recovery rates, indicating a large amount of unrealized potential. Automation and green chemistry advancements can improve process efficiency. For industrial adoption, extended producer responsibility and policy support are essential. Efficient recovery aligns with circular economy principles, reducing the need for primary mining, conserving natural resources, and lowering the carbon footprint associated with extraction and refining. Regulatory frameworks, such as the EU’s Critical Raw Materials Act, and growing corporate sustainability goals are pushing for enhanced recycling and resource optimization.