Introduction

Aluminum, the giant non-ferrous metal produced globally, has recently entered a relatively stable growth trend in global primary aluminum production. Carbon-rich solid wastes (spent carbon anode and spent cathode carbon) are inevitably generated during the primary aluminum production process and electrolysis cell repair, which contain high levels of soluble fluoride and cyanide and are toxic (Mao et al. 2021b; Zhu et al. 2022a). Various methods of harmless disposal and utilization have been proposed in recent years, but large-scale industrial applications have yet to be formed, and the vast majority of them still focus on storage and landfills (Chen et al. 2022a; Dong et al. 2023). Contact of carbon-rich solid wastes with rainwater causes dramatic reactions and releases toxic gases and liquids. Meanwhile, the formation of fluorine-containing liquids would pollute the surrounding water quality and endanger aquatic organisms; landfills are also at risk of soil and groundwater contamination (Ali et al. 2016; Bibi et al. 2017). Improper disposal will result in significant harm to ecosystems, plants, animals, and humans (Fig. 1) (Wang et al. 2023). Therefore, large-scale disposal and utilization methods are urgently needed.

Fig. 1
figure 1

Hazards, disposal, and utilization of carbon-rich solid wastes. Depicts the hazards of cyanide and fluoride to the environment, plants, animals, and humans, and the applications after fluoride extraction and cyanide decomposition. The applications mainly include safe landfills, the synthesis of fluorine products, and recycled carbon applied in different fields depending on the purity difference. Multi-industry solid waste synergies are a current research trend. SCC and SCA: spent cathode carbon and spent carbon anode; nNaF + xAlF3: Na3AlF6: cryolite, Na5Al3F14: chiolite, NaF: sodium fluoride, AlF3: aluminum fluoride

Full size image

Spent carbon anode and spent cathode carbon have many similarities in composition and treatment methods, and the main idea is to separate the valuable components from carbon materials to achieve detoxification and resource recovery (Fig. 1). However, the current reviews still discuss spent cathode carbon and spent carbon anode independently, as well as the advantages and disadvantages of each method in a conventional manner with a focus on harmlessness. Thus, we present a unified review of carbon-rich solid wastes from aluminum electrolysis, describing the detoxification and recycling mechanisms in detail and discussing the classification of recycled products from a new perspective. The review is expected to provide new inspiration for the application of large-scale disposal of carbon-rich solid wastes from aluminum electrolysis.

Carbon-rich solid waste sources and hazards

The prebaked anode material used in aluminum electrolysis cells is carbon. During electrolysis, the anode drives current into the cell for the electrochemical reaction (Amrani et al. 2021). Due to the washing and erosion of carbon anodes by the high-temperature aluminum solutions and molten electrolytes during electrolysis, as well as selective oxidation and uneven combustion, some carbon particles on the surface are dislodged into the cell, forming a spent carbon anode (Hou et al. 2020; Zhao et al. 2021c). When excess carbon particles collect on the surface of the electrolyte, the resistivity of the electrolytic cell increases, thereby reducing current efficiency by decreasing pole distance and increasing power consumption (Yang et al. 2020c). Furthermore, the dissolution rate of alumina decreases, and the precipitation at the bottom of the furnace increases, causing cell conditions to deteriorate. Notably, the long-term operation of electrolysis cells in an overheated state will hasten anode oxidation and cathode breakage, shortening cell life (Hankel et al. 2022). Therefore, periodic salvage is necessary to ensure the regular operation of aluminum electrolysis. Spent carbon anode is an unavoidable hazardous solid waste continuously emitted from the aluminum electrolysis industry. The main components include carbon, calcium fluoride (CaF2), magnesium fluoride (MgF2), aluminum fluoride (AlF3), aluminum oxide (Al2O3), and cryolite-type compounds such as cryolite (Na3AlF6), simmonsite (LiNa2AlF6), chiolite (Na5Al3F14), elpasolite (K2NaAlF6); the carbon content is 14–42% and the fluoride content is above 60% (Li et al. 2021b, 2024; Mao and Zhang 2021b). Spent carbon anode can be reduced using high-quality prebaked carbon blocks and maintaining appropriate electrolyte levels (Amara et al. 2021).

Spent cathode carbon comes from the spent pot liner, which arises from the replacement of the electrolysis cells. Due to the prolonged contact of the carbon blocks on the bottom and sides of the electrolysis cells with the high-temperature aluminum liquid, which results in constant penetration of sodium and electrolyte, the cells are eroded and require regular maintenance and replacement (Li et al. 2022a). The average service life of electrolysis cells is 4–8 years, depending mainly on the cells' material, design, and operation (Nunez. 2020; Tropenauer et al. 2019; Zhao et al. 2021a). Spent pot lining can be divided into two spent carbon materials (about 55%) and refractory materials (about 45%). The spent carbon materials include spent cathode carbon and side carbon blocks with a carbon content of 40–70%, fluoride content of 10–20%, and cyanide content of 0.1–0.2% (Holywell et al. 2013; Liu et al. 2020a; Nemmour et al. 2023; Silveira et al. 2002; Yu et al. 2021). Spent refractory materials mainly include impermeable materials and insulation bricks, whose chemical composition is primarily aluminosilicate and a few electrolytes (Brial et al. 2021; Ospina et al. 2017).

Figure 2c indicates the composition range of spent cathode carbon and spent carbon anode. The carbon content in spent cathode carbon fluctuates widely due to the incomplete separation of refractory and spent carbon materials during spent pot lining cutting. The main difference in spent carbon anode is caused by the salvage method. However, in general, the carbon content dominates in spent cathode carbon, and the fluoride content dominates in spent carbon anode. Further analysis has shown that soluble sodium fluoride and cryolite are the main contributors of fluoride in spent cathode carbon. In contrast, various cryolite compounds are dominant in spent carbon anode. According to statistics, each ton of primary aluminum produced will generate 5–15 kg of spent carbon anode and 20–30 kg of spent pot lining (Flores et al. 2019; Miksa et al. 2003; Yang et al. 2020c; Zhao et al. 2021c). In 2022, the global primary aluminum production will be 68.5 million metric tons, and China's production will be 40.4 million metric tons (Fig. 2a and b) (IAI 2023). Globally, 700,000 metric tons of spent carbon anode and 900,000 metric tons of spent cathode carbon are expected to be produced. Carbon-rich solid wastes from aluminum electrolysis are dangerous, resulting in widespread and potentially latent pollution if not properly treated (Andrade-Vieira et al. 2019; Castro et al. 2018; Palmieri et al. 2016). Figure 3 represents the current treatment methods regarding spent carbon anode and spent cathode carbon, namely, multiple hazardous waste co-disposal, hydrometallurgy, pyrometallurgy, and landfill (Dongyue et al. 2021; Han et al. 2023b; Hui et al. 2023; Yang et al. 2022b; Yuan et al. 2018a). Due to these solid waste materials containing many valuable resources, achieving safe disposal and resource utilization have become urgent environmental and aluminum industry concerns.

Fig. 2
figure 2

a Global primary aluminum production in different regions from 2013 to 2022; b global total primary aluminum percentage in different regions from 2013 to 2022; c spent cathode carbon and spent carbon anode composition range. The range of compositions varies significantly due to the raw materials, electrolysis processes, and scale of spent cathode carbon and spent carbon anodes used. The spent cathode carbon is also limited by the location of the cuts associated with the splitting process of the refractory. CN−: cyanide ion

Full size image
Fig. 3
figure 3

Treatment methods and recycled products for carbon-rich solid wastes from aluminum electrolysis. Current disposal could be divided into four categories: safe landfills have rarely been applied, multiple solid waste synergies are still in the exploratory stage, and conventional hydrometallurgy and pyrometallurgy methods. From the point of view of recovered products, the progression is divided into separation recovery to obtain mixtures of multiple fluorides and separation synthesis of single fluorides. SCA/SCC: spent carbon anode and spent cathode carbon; Na3AlF6: cryolite; AlFx(OH)3−x·nH2O: aluminum hydroxy fluoride hydrate; AlF3: aluminum fluoride; CaF2: calcium fluoride; Na2SO4: sodium sulfate; NaCl: sodium chloride; Na2CO3: sodium carbonate; NaF: sodium fluoride

Full size image

Detoxification and recovery

Separation recovery

Carbon and electrolyte components are separated to achieve resource recovery and detoxification. Currently, the focus is on roasting, vacuum distillation, high-temperature calcination, and flotation methods. Recovered electrolytes are of high purity but contain mixed products, such as cryolite, sodium fluoride, and calcium fluoride mixtures. The purity and graphitization of recovered carbon under different disposal methods are as follows: high-temperature calcination is better than vacuum distillation, followed by roasting and flotation. The results of the separation recovery part of the study are shown in Table 1.

Table 1 Separation recovery by thermal treatment of carbon-rich solid wastes
Full size table

Thermal behavior

Li et al. (2020b) analyzed the thermal behavior of fluoride and cyanide in spent cathode carbon using thermogravimetric and differential scanning calorimetry combined mass spectrometry analysis (TG/DSC-MS); fluoride can only volatilize at temperatures beyond the respective melting points of the various materials and is almost unaffected by the combustion of carbon in spent cathode carbon particles. Cyanide can decompose effectively at high temperatures in an argon–oxygen (Ar–O2) atmosphere, with 75% converting to nitrogen (N2) and 25% converting to nitric oxide (NO). Lu et al. (2020) used numerical simulations with electrothermal coupling to verify that the effective removal temperature of fluoride from spent cathode carbon is at or above 1700 °C with removal rates of 93% or more. For cyanide, Su et al. (2022) proposed synergistic removal using ferric oxide (Fe2O3) as a microwave heating sensitizer, achieving a maximum removal rate of 97%. At standard atmospheric pressure, most fluorides evaporate at temperatures above 1300 °C (cryolite, 1660 °C), reducing the volatilization temperature of the electrolyte by lowering the vacuum pressure (Xie et al. 2020a; Zhao et al. 2021a). Volatilization of fluoride and decomposition of cyanide at high temperatures are harmless processes for charcoal-rich solid wastes.

Roasting

Combustible materials such as carbon and hydrogen in carbon-rich solid wastes are fully combusted at a specific temperature, and the unburned material is the electrolyte. Adding dispersants and catalysts can prevent molten electrolyte adhesion and accelerate carbon gasification (Cai et al. 2023; Dong et al. 2023). An advantage of roasting is that the electrolyte has high purity but requires highly purified dispersants and catalysts, resulting in high costs; furthermore, the roasting process produces carbon dioxide (CO2) and fluorine gas (Sun et al. 2019). In particular, the fluidized bed roasting method has higher efficiency and combustion intensity than the traditional method. However, sub-cryolite in the raw material will inhibit the carbon–oxygen combustion reaction, resulting in a low carbon removal rate (Li et al. 2021a; Mao and Zhang 2021b).

The raw material's roasting temperature and particle size mainly influence the results. The higher the temperature, the smaller the particle size and the higher the weight loss. Theoretically, at 500–800 °C, carbon is oxidized to carbon dioxide (Chen et al. 2023b). In practice, due to insufficient contact between carbon and oxygen, when the temperature is higher than 800 °C, the electrolyte in the spent carbon anode starts to decompose and volatilize, and some carbon particles are wrapped by the melted electrolyte, which hinders contact between the carbon and oxygen and slows the oxidation kinetics resulting in poor separation, but elevating the temperature (1500 °C) can avoid insufficient combustion (Zhu et al. 2022a). A reasonable combustion aid and dispersant can reduce fluorinated gas emissions, while recovering high purity electrolytes at low temperatures.

High-temperature calcination

Based on the differences in the physicochemical properties of fluoride, cyanide, and carbon material, electrolytes and carbon are effectively separated by volatilizing fluoride and decomposing cyanide at high temperatures. The carbon material does not combust in an oxygen-insulated or inert gas environment (Li et al. 2021a). High temperatures result in high purity and a high degree of graphitization of the carbon material. However, waste gas pollution and high costs are concerns (Han et al. 2023a). Since, cyanide does not decompose completely during high-temperature calcination (without oxygen), Zhao et al. (2021a) achieved complete decomposition of cyanide with controlled oxygen (5%).

Increasing temperature and decreasing particle size can significantly increase the fluoride removal rate. The temperature is positively correlated with the defluorination rate and graphitization, and ultrahigh-temperature calcination at 2400–2600 °C results in the purity of recovered carbon reaching more than 98% (Yang et al. 2021, 2019a). Microwave assistance can melt the internal fluoride from the cracks, resulting in 95% fluoride removal at 1500 °C for 2 h, exploiting the significant difference between carbon and fluoride for wave absorption (Zhu et al. 2022b). The temperature trend of high-temperature calcination is gradually shifting towards very high temperatures. With the help of microwaves, the recovered carbon has a high degree of graphitization, and the process reduces energy consumption.

Vacuum distillation

Separation via vacuum distillation is achieved due to the different vapor pressures of electrolytes and carbon. Electrolytes are recovered by volatile condensation, while carbon remains at the bottom (Xin et al. 2022). The advantage is the high purity of the resulting electrolytes. The disadvantage is the low purity of the resulting carbon, and the residual carbon slag still contains more fluorinated salts and oxides with low vapor pressures (Wang et al. 2018; Xie et al. 2020b). Lower pressures reduce the temperature needed for fluoride volatilization. Temperature and vacuum pressure are decisive factors affecting the detoxification and purity of the recovered carbon. For example, from a temperature of 700 to 1700 °C and vacuum pressure of 60–3000 Pa, the purity of carbon increased from 67 to 98% (Li et al. 2019a; Xie et al. 2020b). Xie et al. (2020a) proposed a combined temperature-vacuum controlled process, showing that the volatilization of fluoride is a primary reaction with an apparent activation energy of 15.08 kJ mol−1. The volatilization process rate is mainly controlled by the migration of fluoride from the interior of the spent carbon anode to the liquid phase boundary layer; the removal of fluoride can be promoted by increasing the temperature and retention time. Fluoride volatilization can also be achieved by changes in vacuum pressure at appropriate temperature ranges, which will place higher demands on the vacuum equipment.

Flotation

The flotation method is physical separation based on the hydrophobic difference between carbon and electrolytes (Xia et al. 2019). The flotation agent and spent carbon anode are stirred in a flotation machine, and air is introduced to induce bubble formation. The carbon rises to the surface of the pulp with bubbles to form a froth layer, while the electrolyte is discharged from the bottom stream (Xu et al. 2019). As shown in Table 2, under the flotation method, the highest purity of recovered charcoal and electrolyte was 89 and 93%, respectively. Factors affecting the flotation effect are raw material size, slurry concentration, agitation speed, flotation chemicals, aeration, and the number of flotation stages (Angelopoulos et al. 2021; Li et al. 2021b, 2020a). Due to the well-developed pores and large specific surface area of the spent carbon anode surface, both large and small particle sizes can adversely affect flotation (He et al. 2023; Li et al. 2021b). The ideal particle size range is 200 mesh (70–90%) (Yang et al. 2019b). The slurry concentration can cause changes in flotation time. Increasing the number of flotation stages will increase the purity of the recovered carbon, but the recovery will decrease.

Table 2 Separation recovery by flotation of carbon-rich solid wastes
Full size table

As the contents of ultrafine carbon particles, cyanide, and fluoride will gradually increase in the circulating water of the flotation process, the circulating water is detoxified by adding calcium hypochlorite with fluoride ions to oxidize the cyanide and solidify the fluoride. The flotation method has the advantages of decreased production cost, simple operation, and increased resource utilization rate. However, carbon-rich wastes from aluminum electrolytes are composed of multiple components, and spent carbon anode is affected by process technology, salvage means, and carbon anode quality. Thus, spent carbon anode composition is complex and can fluctuate, and a low carbon content will result in poorer separation. Compared with spent carbon anode, spent cathode carbon has more complex electrolyte penetration in the carbon matrix with the increase in service life, combined with particles reaching the micron level, inevitably affecting the flotation effect (Lossius et al. 2000). Flotation also suffers from disadvantages such as long process flow, many stages, high consumption of chemicals, and high fluoride content in wastewater, which requires second-stage treatment and increases recovery costs.

Separation synthesis

Compared with separation recovery, separation synthesis mainly involves chemical dissolution and synthesizing a single product. The mechanism of fluoride decomposition and synthesis under chemical leaching (Fig. 4), as in alkali fusion in pyrometallurgy, converts insoluble fluoride into soluble fluoride. Water washing then converts the soluble fluoride to an ionic form. The cryolite (Na3AlF6), aluminum hydroxy fluoride hydrate [AlFx(OH)3−x·nH2O], sodium fluoride (NaF), and calcium fluoride (CaF2) synthesis products. The byproducts are categorized divided into the sulfuric acid system-sodium sulfate, hydrochloric acid system-sodium chloride, nitric acid system-sodium nitrate, alkaline system-sodium carbonate, and other byproducts according to different disposal systems. Separation synthesis has the advantages of high separation efficiency and recovery. Notably, preparing high purity fluoride products has considerable economic benefits and development prospects. However, disadvantages such as high reagent consumption, high production costs, and complex processes must be solved (Li et al. 2021a). Tables 3 and 4 summarize the main findings of separation synthesis.

Fig. 4
figure 4

Mechanism of detoxification and synthesis under chemical leaching. Fluoride in carbon-rich materials is typically represented by cryolite or sodium fluoride. Water washing only removes soluble sodium fluoride (NaF) from spent cathode carbon. Acids and alkalis can dissolve insoluble fluorides and resynthesize cryolite (Na3AlF6). Complex leaching replenishes the fluoro-aluminum ratio in the initial solution, and the synthesized fluoride is calcined or fluorinated to produce aluminum fluoride (AlF3). SCC/SCA: spent cathode carbon and spent carbon anode; CaF2: calcium fluoride; AlFx(OH)3−x·nH2O: aluminum hydroxy fluoride hydrate

Full size image
Table 3 Separation synthesis by chemical leaching of carbon-rich solid wastes
Full size table
Table 4 Separation synthesis by combined disposal of carbon-rich solid wastes by hydrometallurgy and pyrometallurgy
Full size table

Water washing

The water washing method mainly targets soluble sodium fluoride (NaF) in spent cathode carbon (Zhi et al. 2020). The solubility of sodium fluoride in water is approximately linear, with a concentration-temperature gradient of 3 mmol/L per degree Celsius (Reynolds et al. 2017). A high liquid–solid ratio and multiple washes are often used to achieve 99% sodium fluoride removal (Zhi et al. 2020). Removal of 100% is difficult to achieve even for soluble fluoride, as small amounts of sodium fluoride remain embedded between the carbon layers (Chen et al. 2022b). The water-washing solution can be recovered by adding calcium chloride hexahydrate (CaCl2·6H2O) to precipitate fluoride ions or by the evaporative crystallization of sodium fluoride (Zhi et al. 2020). However, most insoluble fluoride in carbon-rich solid wastes must be dissolved with chemical reagents. Lu et al. (2008) verified the leaching rate of fluoride by different chemical leaching methods. The results showed that acid leaching was more effective than alkaline leaching, followed by ultrasonication-assisted aqueous washing and aqueous washing with a constant-temperature oscillator, where the acid leaching rate was approximately 79%. Generally, water washing is seldom used alone to treat hazardous waste and is often used in the pretreatment of carbon-rich solid wastes or later to achieve a neutral leaching residue.

Acid leaching

Various acids can treat carbon-rich solid wastes from aluminum electrolysis, such as hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and perchloric acid (HClO4) (Amira et al. 2021; Huang et al. 2022; Li et al. 2022d; Lisbona et al. 2013; Yuan et al. 2018a). Since, spent cathode carbon and spent carbon anode contain calcium compounds, sulfuric acid leaching generates insoluble calcium sulfate, affecting the purity of the recovered carbon; thus, hydrochloric acid leaching is often used. However, hydrochloric acid is highly corrosive to equipment. Acid leaching at atmospheric pressure, usually with a mild leaching temperature and low acid concentration, reduces hydrogen fluoride gas generation. Undersealed conditions, hydrogen fluoride generated at a high-temperature and pressure will further dissolve impurities such as silica and aluminosilicate, increasing the purity of recovered carbon to 97% (Yang et al. 2022a). Ultrasonic assistance significantly reduces leaching time and improves the purity of recovered carbon (Bu et al. 2024). Adding an oxidizing agent during leaching can widen graphite layer spacing (0.338–0.844 nm), facilitating the dissolution of internal impurities (Chen et al. 2022b). The formation of hydrofluoric acid is unavoidable under the acid-leaching method and is minimized by lowering the acidity and intensity of the reaction.

Alkali leaching

Alkali leaching can remove the vast majority of fluoride in carbon-rich solid waste. The influence of the alkali leaching process on the leaching rate is alkali concentration over temperature, followed by liquid–solid ratio and time (Yuan et al. 2018b, c). The higher the alkali concentration is, the higher the increase in the viscosity of the solution, which is unfavorable to the diffusion medium. Before alkali leaching, there is usually a water washing process to remove the original sodium fluoride and prevent the sodium fluoride generated after alkali leaching from inhibiting the dissolution of cryolite (Ghenai et al. 2019). Tropenauer et al. (2021) used a kinetic model to show that at lower alkali concentrations of 0.1 mol to 0.3 mol, the resistance to transport of the internal resistance inside the solid particles is 19 times higher than that of external resistance in the liquid film around the particles and controls the entire process.

In contrast to conventional stirring, ultrasonic assistance can increase the purity of recovered carbon powder to 95%, which is an increase of 2% (Xiao et al. 2018a). Alkaline leaching can also be combined with flotation to increase carbon purity to 84% in the alkaline leaching stage, followed by flotation to recover carbon powder at a purity of 95% (Liu et al. 2012). Alkaline leaching under high-pressure conditions can convert the insoluble impurities silicon dioxide (SiO2) and aluminum oxide (Al2O3) into soluble sodium silicate (Na2SiO3) and sodium aluminate (NaAlO2), increasing the leaching rate of fluoride (Kondrat’ev et al. 2016). Alkaline leaching significantly improves the separation of carbon materials and electrolytes.

Acid–alkali leaching

Due to the presence of multiple fluorides in the spent carbon anode and spent cathode carbon, one-step detoxification via single acid or alkali leaching is challenging to complete; for example, acid leaching cannot dissolve chiolite (Na5Al3F14) (Ma et al. 2023). In contrast, alkali leaching cannot dissolve aluminosilicates, calcium fluoride (CaF2), and diaoyudaoite (NaAl11O17) (Shi et al. 2012). Many studies have combined acid and alkali leaching to improve the purity of recovered carbon (more than 95%) (Dzikunu et al. 2023; Li et al. 2019b; Zhou et al. 2023). The two-step leaching in Table 3 recovered carbon with a purity of up to 98%. However, the process is relatively complicated, and the volume of the leach solution expands severely. The resulting leachate with both properties has been subjected to various pH levels to prepare cryolite. Furthermore, many studies have performed detoxification in the alkaline-acid sequence, mainly considering the transformation of fluoride into hydrofluoric acid under acid conditions.

Complexation leaching

Complexation leaching is currently used mainly under acidic aluminum ion (Al3+) and iron ion (Fe3+) conditions, which converts fluorine in the solid to a complex state under the action of complex ions and increases the leaching rate of fluorine without releasing hydrogen fluoride gas. The aluminum ion complexation method is used to form highly stable complex ions aluminum monofluoride (AlF2+) and aluminum difluoride (AlF2+) during the leaching process; the filtrate is recovered as fluorine to prepare aluminum hydroxy fluoride hydrate [AlFx(OH)3−x·nH2O] products. Lisbona et al. (2013) treated spent cathode carbon with 0.5 mol nitric acid (HNO3) and 0.36 mol aluminum nitrate nonahydrate [Al(NO3)3·9H2O] solutions to extract 96% of fluoride. The filtrate underwent pH adjustment to precipitate aluminum hydroxide fluoride hydrate (AlF2OH·1.4H2O), converted to industrial-grade aluminum fluoride by fluorination.

Iron and aluminum ions have similar abilities to bind to fluorine and form fluorine-iron complex ion (FeFi)3−i complexes in the leaching process, which are utilized to purify fluorinated wastewater (Kanrar et al. 2016). Li et al. (2016) established the reaction kinetics of the complex leaching process and showed that the leaching process was consistent with the unreacted contraction nucleation model. Moreover, the leaching control link of insoluble electrolytes was internal diffusion, and increasing the reaction temperature and time improved the leaching rate. Nie et al. (2020) used acidic iron-containing wastewater to achieve an 89% extraction of fluorine in the complexed state ferric monofluoride (FeF)2+ at a leaching temperature of 80 °C for 30 min with a liquid–solid ratio of 10 mL g−1, 0.48 mol hydrogen ions (H+), and 0.2 mol iron ions (Fe3+).

Using ultrasonic assistance in the chemical leaching process can significantly reduce the leaching time and simultaneously improve the purity of the recovered carbon powder. However, there are complex processes and high processing costs. Multi-step leaching can significantly increase the purity of the recovered carbon. However, the process has the disadvantages of complex processes and many chemical reagents. Importantly, chemical leaching produces solutions that need to be re-recovered for fluoride, and the concentration of the valent components in the solution is a critical factor in recovery.

Alkali fusion

Alkali fusion is generally a solid–solid high-temperature reaction; first, the raw material is mixed with an alkali, using a ball mill or solution for homogeneous mixing, and then put under high-temperature conditions; the intention is to convert insoluble fluoride into easily soluble substances such as sodium fluoride (NaF) and sodium aluminate (NaAlO2). After alkali fusion, the alkali fusion residue must be washed to remove soluble impurities and improve the purity of the recovered carbon (Wang et al. 2016). The washing stage usually uses water and acidic media (Yang et al. 2020c; Yao et al. 2020; Yuan et al. 2022a). Water transfers soluble substances (NaF and NaAlO2) into the solution, and further acid washing can be used after the water wash to remove additional impurities such as magnesium oxide (MgO), ferric oxide (Fe2O3), ferric hydroxide [(Fe(OH)3], and aluminum hydroxide [Al(OH)3], thus increasing the purity of the recovered carbon (Ge et al. 2010; Wang et al. 2016). The method has the advantage of a high purity of recovered carbon (Liu et al. 2020b). However, there are disadvantages to high alkali consumption, high economic cost, the need for a washing stage after alkali fusion, the complexity of the process, and the need for secondary resource recovery of the washing liquid.

The alkali fusion temperature is generally 450–1000 °C. Notably, the recovered carbon powder shows a unique microstructure with a slightly swollen ink layer, insufficient graphitization, defects, and a low amount of graphene (Yang et al. 2020b). The alkali fusion agent is often mixed with sodium hydroxide (NaOH), sodium carbonate (Na2CO3), or both (Yao et al. 2021b; Yuan et al. 2022a). Moreover, mixing the alkali fusion agent, which is often mechanically assisted, destroys the spent cathode carbon crystal structure, physically separating a large amount of fluoride embedded in the inner structure, increasing the specific surface area of the raw material, and accelerating the reaction process. The carbon content and fluoride leaching rate of the recovered graphite carbon under optimal conditions increase from 89 and 76% to 94 and 95%, respectively, with 99% cyanide decomposition and 97% fluoride dissolution (Yao et al. 2021a). Alkali fusion combines pyrometallurgical and hydrometallurgical processes with high purity of recovered carbon and complete fluoride extraction.

Single and collaborative disposal utilization strategies

Single disposal strategy

Single disposal mainly involves adding calcium-containing reagents to solidify fluoride and achieve detoxification. Alternatively, direct application is available in industries such as steelmaking and cement due to the high carbon content of spent cathode carbon. Wang et al. (2020) utilized the two-step decomposition of dolomite at high-temperatures. Calcium carbonate (CaCO3) generated by one-step decomposition reacts with sodium fluoride (NaF) to produce calcium fluoride to realize fluorine fixation. Cyanide is wholly oxidized and decomposed at high-temperatures, and carbon is burned at high-temperatures to provide heat for the reaction, thus realizing an effective detoxification treatment of spent cathode carbon.

Spent cathode carbon has been used in the cement and steel industries to replace coal fuel (Ghenai et al. 2019). In the cement industry, due to high calorific value, spent cathode carbon saves fuel input, improves the combustibility of raw materials, and controls sulfur evaporation; the fluoride in spent cathode carbon lowers the melting point of clinker before being converted to calcium fluoride during cement generation to prevent environmental pollution and achieve comprehensive utilization (Peys et al. 2021). Ghenai et al. (2019) compared treated spent cathode carbon (water-washed, alkali-leached, and acid-leached) with the combustion of conventional coal fuel in a cement factory and showed that the treated spent cathode carbon had lower temperatures and furnace-mouth nitric oxide and carbon dioxide emissions; these results indicated that treated spent cathode carbon is capable of replacing coal fuel. Chen et al. (2023a) used treated spent cathode carbon with sodium hypochlorite (NaClO) to remove cyanide (CN−) and fluorine, and the excellent mechanical properties of the highly graphitic carbon obtained enhanced the mechanical interlocking ability of the cement and slightly improved the mechanical properties of cement mortar.

Alternative fuels in the iron-making process lower the melting point and viscosity of slag (Zhao et al. 2018). Spent cathode carbon has physical properties similar to coke, such as fixed carbon and ash; thus, spent cathode carbon can be used as an alternative to coke or as an electric arc furnace flux (Flores et al. 2019; Mambakkam et al. 2019). Yu et al. (2019) experimentally verified that spent cathode carbon was more effective than graphite at reducing chromite and lowered the temperature to 240 °C. Furthermore, spent cathode carbon effectively upgraded ferrochrome alloy to chromium (Cr) 53% and iron (Fe) 29% concentrate at 1500 °C, recovering 68% chromium and iron. Flores et al. (2019) performed a comparative parametric analysis of spent cathode carbon with carbon-containing materials commonly used in the steelmaking industry, such as metallurgical coke, coking coal, pulverized coal injection, and coke breeze. Showed a high stability and hardness factor, compression strength, and high-temperature strength, indicating that spent cathode carbon was more vigorous and less reactive than ordinary metallurgical coke and had the potential to be a partial replacement for metallurgical coke in a blast furnace. However, the stabilized cyanide in spent cathode carbon decomposed in the blast furnace, so leaching or other means should first improve the potential of spent cathode carbon for use. In summary, the single disposal strategy has only been marginally studied in the steel and cement industries. Untreated carbon-rich hazardous wastes are added directly, which is difficult to accept. After detoxification and fluoridation, the resulting carbon materials are applied to these industries with low product value.

Combined disposal with other waste

Co-combustion

Spent cathode carbon contains high carbon and calorific value characteristics and has large oxygen-containing groups to promote combustion. However, the high ash content makes spent cathode carbon insufficient for combustion alone but can be treated in combination with other combustible materials (Chen et al. 2021; Zacco et al. 2014; Zhang et al. 2023c). In addition, waste cathodes produce large amounts of hydrogen fluoride gas and dust particles during combustion (Zhang et al. 2023d, 2024). Post-combustion soot is usually treated with limewater (Zhou et al. 2020). The addition of calcium silicate (CaSiO3) during combustion converts sodium fluoride (NaF), cryolite (Na3AlF6), and calcium fluoride (CaF2) into stable cuspidine (Ca4Si2O7F2) for solidifying fluorine and reducing fluorine-containing particles in soot (Han et al. 2023a).

Varying the addition ratio from 5 to 15% results in the best combustion efficiency when blended with coal or lean coal (Chen et al. 2021; Deng et al. 2022; Zhang et al. 2021). Chen et al. (2021) showed the lowest apparent activation energy with a 10% spent cathode carbon addition. Calcium oxide (CaO) in the pulverized coal solidified sodium fluoride to form calcium fluoride, and silicon dioxide and aluminum oxide captured free sodium ion (Na+) to form aluminosilicates (NaAlSiO4 and NaAlSi2O6) to achieve detoxification. Furthermore, lowering the co-combustion temperature below 900 °C significantly reduced the emission of fluorinated gases. Chen et al. (2022c) added food waste shells (oyster, clam, and egg shells) containing calcium to the spent cathode carbon combustion process for co-disposal. The 50% oyster shell fixed approximately 98% of the fluorine content in the spent cathode carbon mixed ash, which was better than the 50% calcium oxide (CaO) combination. Additionally, 10% silicon dioxide and 40% clam or egg shells increased fluorine fixation to 98% and 99%, respectively. The fluoride ion was converted to complex fluorine-silicon-calcium (F–Si–Ca) compounds instead of calcium fluoride. The calcium and silicon compounds facilitated the simultaneous stabilization of fluorine and sodium and the formation of sodium calcium fluoride silicate (NaCa2SiO4F), and the leached fluorine concentration of the ash was less than 100 mg L–1. When co-combusted with other combustible materials, the addition ratio is low, and fluoride will be solidified during the combustion process, resulting in a waste of fluorine resources.

Copper slag

Copper slag is a byproduct of the copper smelting process, with a copper content of 2–8%, and has obvious recovery value (Priya et al. 2020). With the use of spent cathode carbon as a reducing agent with copper slag for co-disposal at high temperatures (1300–1450 °C), the fluorine fixation rate reached 56%, and the recoveries of copper and iron were up to 96 and 47%, respectively (Li et al. 2023). With additional calcium oxide, the unstable fluoride in the slag existed as calcium fluoride, avoiding the volatilization and dissolution of fluoride, and the introduction of calcium fluoride and sodium fluoride decreased the aggregation of silicate structures, thus improving the properties of the slag, reducing the viscosity of the copper slag, and facilitating the separation of copper from the slag (Zhang et al. 2023a).

Li et al. (2022b) achieved 98% curing efficiency for fluorine at a melting temperature of 1300 °C, a holding time of 60 min, and a nitrogen (N2) flow rate of 40 mL min−1 with optimal doses of 20% and 6% calcium oxide and sodium fluoride, respectively. Zhao et al. (2021b) showed the highest recovery of copper and cobalt (99%) under the following conditions: temperature of 1400 °C, holding time of 2 h, 12% spent cathode carbon, and 10% calcium oxide. When calcium oxide was added in excess, the fluorine-containing phase in the slag transformed into cuspidine (Ca4Si2O7F2), and the soluble fluorine leaching amount in the slag was less than 100 mg L−1 (Mao et al. 2021a; Zhang et al. 2023b). Kuang et al. (2023) used spent carbon anode in collaboration with converter copper slag, and the lead silicate (PbSiO3) and zinc oxide (ZnO) in the converter copper slag were reduced to lead (Pb) and zinc (Zn), respectively. The solidification rate of fluorine reached 96%, with 82% copper (Cu), 56% lead (Pb), and 37% zinc (Zn) recovered under the following conditions: 8% spent carbon anode addition, temperature 1250 °C, and 1 h holding time. In conclusion, the co-disposal of copper slag can enhance the recovery of copper and iron to improve the nature of the slag, which has obvious advantages; however, whether the fluoride in the slag is wasted should be considered.

Electroplating sludge disposal

Electroplating sludge is a product of electroplating wastewater after acid–alkali neutralization, flocculation, and precipitation, and the heavy metal content is severely exceeded (Mao et al. 2018). In the glass curing of electroplating sludge, additives such as crushed chips, limestone, and dolomite are replaced by the addition of copper slag and spent cathode carbon. Meanwhile, spent carbon anode can be added to replace traditional reducing agents to improve the recovery of metals. Combining these three hazardous wastes can significantly reduce disposal costs, resulting in environmental and economic benefits. Xiao et al. (2022) jointly disposed of three hazardous wastes, namely, spent cathode carbon, copper slag, and electroplating sludge. During oxidation roasting, the sodium fluoride in spent cathode carbon oxidized the chromic oxide (Cr2O3) in the electroplating sludge to sodium chromate (Na2CrO4) and free fluoride ion combined with calcium oxide (CaO) to form calcium fluoride (CaF2). Under optimal conditions, 97% of chromium was recovered, 90% of fluorine was solidified to form iron-cobalt alloys, and the cobalt in the copper refining slag was also effectively recovered. Yu et al. (2021) mixed a 100% copper slag and 19% spent cathode carbon addition with electroplating sludge at a temperature of 1450 °C, a holding time of 1.5 h, and a nitrogen (N2) flow rate of 300 mL min−1 to achieve recoveries of 76%, 98% and 99% for chromium, nickel, and copper, respectively. The recovered alloy contained 75.2% iron, 8.7% copper, 9.4% nickel, and 4.7% chromium for the production of stainless steel. Disposing of carbon-rich wastes through co-disposal instead of using a traditional reducing agent to recycle hazardous wastes with excessive heavy metals could lead to waste treatment with economic benefits; thus, large-scale co-disposal has high potential.

Red mud

Red mud comes from industrial solid waste discharged from alumina production, and the main components are silicon dioxide (SiO2), aluminum oxide (Al2O3), calcium oxide (CaO), and ferric oxide (Fe2O3). During reduction roasting, the reduction sequence with increasing temperature is ferric oxide (Fe2O3), ferrosoferric oxide (Fe3O4), ferrous oxide (FeO), iron (Fe), and ferrous oxide (FeO) quickly reacts with silicon dioxide (SiO2) and aluminum oxide (Al2O3) to form aluminum spinel (FeO·Al2O3) and iron olivine (2FeO·SiO2) (Chen et al. 2014b). Xie et al. (2020c) prioritized pretreatment (alkali fusion-water leaching) to transfer silicon dioxide and aluminum oxide into the liquid phase. The recovery of iron reached 89% under the following conditions: a nitrogen (N2) atmosphere, spent cathode carbon addition ratio of 7%, temperature of 900 °C, and holding time of 4 h. and the formation of stable cuspidine (Ca4Si2F2O7) from the fluorine in the residue.

Coprocessing with red mud and reduction to recover iron can be used directly in electric furnace steelmaking. For example, Li et al. (2022c) adjusted the alkalinity to 1 when mixing spent cathode carbon and red mud at a temperature of 1350 °C and holding time of 0.5 h; the iron recovery was 92%, and the recovered product was used for steelmaking. In contrast, Zhang et al. (2020) exploited the calcium-containing nature of red mud to cure fluorine at 850 °C, increasing the red mud addition to 30% and verifying the positive thermal effect of silicon dioxide and aluminum oxide on the conversion of sodium fluoride to calcium fluoride. The combined disposal of solid waste within the aluminum industry has obvious regional advantages; however, more theoretical and primary research is needed to achieve large-scale applications.

Coal gangue synergy

Silicon carbide is a single-crystal fiber with high thermal conductivity, high-temperature stability, high strength, and chemical resistance that is widely used in abrasives, high-temperature thermally conductive materials, and semiconductor materials (Katoh et al. 2014). Silicon carbide is mainly prepared using petroleum coke or coal coke as the carbon source and silicon dioxide by high-temperature carbothermal reduction (Wang et al. 2022a). Coal gangue is the most generated solid waste in the coal production industry and consists mainly of silicon dioxide and aluminum oxide (Al2O3) (Zhou et al. 2020). Using spent cathode carbon and small amounts of silicon dioxide (purified by pretreatment) as a carbon source combined with coal gangue to prepare silicon carbide is theoretically, economically, and environmentally feasible. Zhang et al. (2022) obtained silicon carbide by mixing spent cathode carbon with coal gangue for acid leaching to get a residue containing carbon and silicon oxide mixture at 1600 °C for 10 h. Xiao et al. (2018b) prepared a median particle size 18.15-μm of silicon carbide powder by using hydrothermal acid leaching with a leaching residue (C, Si) at 1600 °C for 5 h. Liao et al. (2022) prepared fibrous silicon carbide at a temperature of 1550 °C, C/Si ratio of 3:1, and a holding time of 3 h. The utilization of carbon-rich solid wastes and gangue for the synthesis of silicon carbide has high-value regarding product development. However, the required temperatures are high, and the total economic benefits are limited.

Utilization of recycled carbon

Utilization strategies vary depending on the purity of the recovered carbon. Amira and Chaouki. (2021) used carbon powder recovered by alkali leaching at a gasification temperature of 900 °C, an equivalence ratio of 0.3, and an air oxidant to produce alternative syngas fuels. Yao et al. (2020) combined recovered carbon with some untreated raw materials at a low mixing ratio (2%), and the performance was close to that of carbon anodes prepared from petroleum coke materials. Recovered carbon is activated for further preparation of activated carbon (Heidarinejad et al. 2020). Since, the carbon component of the spent carbon anode is mainly petroleum coke, a carbon-based material with a small volume change during charging and discharging. However, the formation of pore defects shortens the lithium-ion transport distance (Liu et al. 2020b; Zhou et al. 2023). Xie et al. (2021) synthesized sodium-metal anode bodies with sodium fluoride interfaces using spent cathode carbon, which regulated sodium ion transport and inhibited dendrite growth with excellent ionic conductivity and a high shear modulus.

Many studies apply recovered carbon to battery materials. Huang et al. (2022) used a hydrothermal method to verify that a purified spent cathode carbon material possessed graphite-specific charging and discharging characteristics despite insufficient graphitization: a first discharge capacity of 97 mAh g−1 and capacity retention of 95% after 100 cycles at 1C (C in the battery: charge/discharge-rate). Furthermore, changing the treatment temperature significantly improved the performance of the harmful electrode material. Yang et al. (2020c) exhibited a reversible capacity of 267 mAh g−1 (310 cycles at 1C) by alkali fusion (450 °C), with better cycling stability than graphite. Increasing the calcination temperature to 1600 °C resulted in a carbon purity of 97% that led to a reversible capacity of 235 mAh g−1 (500 cycles at 1C) (Yang et al. 2020a). Further increasing the temperature to 2600 °C resulted in a reversible capacity of 426 mAh g−1 (114 cycles at 1C) and remained at 263 mAh g−1 (300 cycles) (Yang et al. 2019a). The carbon purity reached 99.9% at 2800 °C, and the first capacity was 359 mAh g−1 at 0.1C (Zhao et al. 2021c). Thus, high temperatures further improved graphitization increased the amount of graphene, and improved the performance of negative electrode materials. The recovered high purity carbon was synthesized with gangue at 1600 °C to form a silicon carbide powder with a recovery of 76% and a specific surface area of 4378 cm2 g−1 or by a mechanically assisted preparation of Si/C composites cycled for 100 cycles at a current density of 120 mAh g−1, and the discharge specific capacity was 382 mAh g−1 (Li et al. 2021a; Yuan et al. 2018a). After a high-temperature purification treatment, the electrochemical performance exceeded that of graphite anode materials. However, the complicated process and high-cost limit applications. The purity of the recovered carbon depends on the detoxification method, and high purity carbon implies a complex technological treatment process.

Recovery and synthesis of fluorides

The recovered fluoride from two primary sources (Fig. 5). The first primary source was separation recovery; the method usually results in a high purity electrolyte, and the electrolyte can be directly returned to the aluminum electrolytic cells but can only be used in the start-up phase of the aluminum electrolytic (Li et al. 2019a; Wang et al. 2022b). During normal aluminum electrolysis production, mixed electrolyte products are difficult to use. The second primary source consisted of fluoride products prepared from solution in separation synthesis, such as the synthesis of cryolite (Na3AlF6), aluminum hydroxy fluoride hydrate [AlFx(OH)3−x·nH2O], calcium fluoride (CaF2), and sodium fluoride (NaF). These products have obvious economic benefits and high applicability.

Fig. 5
figure 5

Recovered fluoride and byproducts using different methods. The electrolytes obtained by physical property separation are mixtures of fluorides: NaF, AlF3, Na3AlF6, Na5Al3F14, KF, and CaF2. Single fluorides are obtained by adjusting the pH of the fluoride-containing solution. The byproducts are determined according to the solution system and are recovered by evaporation or cooling crystallization. Na3AlF6: cryolite; AlFx(OH)3−x·nH2O: aluminum hydroxy fluoride hydrate; AlF3: aluminum fluoride; CaF2: calcium fluoride; Na2SO4: sodium sulfate; Na2CO3: sodium carbonate; NaF: sodium fluoride; Na5Al3F14: chiolite

Full size image

The synthesis of cryolite is generally prepared with the use of acid or alkaline solutions containing fluorine and aluminum with a pH of 9, temperature of 70–80 °C, precipitation time of 2–4 h, and F/Al ratio of 6.00 to obtain cryolite products with purities up to 96% (Chen et al. 2014a; Shi et al. 2012; Yao et al. 2020; Zhang et al. 2022; Zhao et al. 2021c). Regarding the high impurity content of the solution, the pH was first adjusted to greater than 13 to remove non-aluminum impurities (like calcium) before being adjusted again to prepare cryolite; however, the silicon content of the product is high (Yang et al. 2022a). The pH can be adjusted using internal alkaline reagents, such as carbon dioxide gas produced by the alkali fusion process and the alkaline leach solution in two-step acid–alkali leaching (Yao et al. 2021b). Aluminum hydroxyfluoride hydrate [AlFx(OH)3−x·nH2O] was prepared in an acidic system with an F/Al mole ratio of 1.60–1.62, pH less than or equal to 5.0, and temperature of 50–90 °C. Too high of a pH and the F/Al mole ratio can lead to the coprecipitation of sodium fluoroaluminate (Chen et al. 2014a; Lisbona et al. 2012b). The prepared aluminum hydroxy fluoride hydrate has a small particle size and is dominated by agglomerates. Mainly, because conventional preparation is dominated by nucleation, resulting in limited particle growth and slow hydrolysis techniques, or by using heterogeneous nucleation to meet industrial applications (Lisbona et al. 2008). The synthesized aluminum hydroxy fluoride hydrate can be mixed with aluminum hydroxide and injected into a fluidized bed to prepare aluminum fluoride by hydrogen fluoric gas or calcined alone to prepare aluminum fluoride and aluminum oxide mixtures (Lisbona et al. 2012a; Ma et al. 2023).

For solutions containing fluorine or low aluminum, calcium fluoride is recovered by adding calcium chloride bleaching powder to decompose cyanide (Xiao et al. 2018a). Depending on the system solution, the byproducts vary, such as the sulfuric acid system-preparation of sodium sulfate by freezing crystallization, hydrochloric acid system-evaporative crystallization of sodium chloride containing small amounts of aluminum, and improving the purity of sodium chloride solution by coordinating the conditions for cryolite precipitation (Yang et al. 2023). Regarding the carbonic acid system, sodium carbonate and sodium fluoride were separated by a 0.45-μm membrane, and sodium fluoride was obtained by evaporation crystallization (Yao et al. 2020, 2021b). According to the concentration of components, the fluorine-containing solution can be flexibly prepared to synthesize fluoride products, solve the solution disposal problem, and achieve high-value utilization.

Evaluation and challenges of detoxification-recovery methods

Comparing the advantages and disadvantages of current treatment methods relative to each other (Table 5), simple processes and low costs mean low recycling efficiencies (flotation, washing). Under hydrometallurgical disposal, large amounts of filtrate and complex synthesis of high-value products will be produced, but with the potential advantage of a closed-loop recycling process. High-temperature disposal produces highly pure graphite carbon, but the resulting gases cause secondary environmental problems. Vacuum distillation is expensive, and the market value of the resulting highly pure electrolyte is insignificant. Current co-disposal methods have the potential for large-scale disposal and are a trend for future development; however, co-disposal is still in the experimental research stage, and corresponding research on waste-to-waste treatment still needs to be developed and improved.

Table 5 Advantages and disadvantages of disposal methods for carbon-rich solid wastes
Full size table

Spent carbon anode and spent cathode carbon come from fluoride's constant encapsulation and penetration in high-temperature electrolysis systems, which is difficult to extract and separate using physical properties completely. Separation recovery methods based on physical properties are confronted with a complex combined state of multiple fluoride and carbon materials with low purity and limited markets for recovered products. Separation synthesis is based on chemical methods, and fluoride-containing systems have high equipment requirements. The resulting solution requires secondary disposal, and there are complex process technologies at the fluoride recovery and synthesis stage. The current closed-loop system only applies to the laboratory stage. Therefore, large-scale applications are still to be explored. Collaborative disposal of solid waste in multiple industries is a future research trend, and making full use of the inherent properties of different solid wastes is the difficulty of synergy. Harmlessness of solid waste is a prerequisite, and large-scale application is the evaluation indicator for disposal methods.

Conclusion

Spent carbon anode and spent cathode carbon from aluminum electrolysis are unavoidably high-fluorine materials; therefore, without detoxification, these wastes can cause significant harm to the environment, plants, and animals. Furthermore, carbon materials and fluoride have objective utilization values. The review concludes from a recycled product perspective and the co-disposal of multiple solid wastes: First, separation recovery is relatively simple, but the recovered electrolyte resources are mostly mixtures with limited applicability. The high-temperature-recovered carbonaceous materials with high purity and high graphitization have high economic value and can be applied to battery cathode materials; however, these methods need more feasibility studies before industrial application. Second, separation synthesis is relatively complex. Hydrometallurgy synthesizes high purity fluoride products and separates high purity carbon-based materials with significant application value; moreover, the filtrate is comprehensively utilized. However, the recycled products are still far from being of commercial grade in terms of synthesis methods. Third, co-processing includes co-firing with energy materials and multi-stage co-leaching synthesis. Carbon-rich solid wastes are used as oxidizing or reducing agents to recover metals and solidify soluble fluorine when disposed of with other solid wastes. However, there is no complete disposal route, the solid waste disposal volume is small, and disposal is difficult due to regional transportation and policy restrictions. The future for solid waste is not a single disposal method but the full use of various solid waste properties to collaboratively achieve waste treatment that results in detoxified solid waste and high-value products. Performed in the same industry chain or at the same plant, the co-disposal of a variety of solid wastes can increase the economic benefits of the enterprise and the environmental friendliness and sustainable development of the industry.