Study on the thermal field material of FZ-Si crystal waste graphite purified by ultrasonic enhanced acid leaching

2.1 Analysis of raw materials

The WG used in this study was sourced from a monocrystalline silicon plant in Yunnan, China. Tables 1 and 2 present the industrial analysis along with the elemental composition and content. Fig. 1 illustrates the phase analysis. The figure shows that the primary components consisted of graphite and silicon carbide. Considering that the other impurities were present in low quantities, the WG was subjected to calcination at 900 °C in an air atmosphere. X-ray diffraction (XRD) analysis of the resulting ash identified impurities such as silicon dioxide, iron oxide, and silicon carbide. For all experiments, deionized water and various reagents, including hydrochloric acid, nitric acid, hydrofluoric acid, and sulfuric acid, were used. These acids were commercially sourced and of analytical grade.

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Fig. 1

XRD analysis of raw materials (a: XRD analysis of WG, b: XRD analysis of ash).

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2.2 Experimental procedure

Both the conventional leaching experiment and the ultrasonically enhanced leaching experiment were conducted using the DF-101ST magnetic stirring water bath. The detailed procedure for the conventional leaching experiment is as follows: First, the WG sample was pulverized to a specific particle size. Then, a predetermined concentration of acid was placed in a beaker and heated to the desired temperature using the water bath. The WG sample was then added to the beaker to initiate the reaction. After a specific reaction time, the liquid and solid components were separated using an extraction filter, to yield the acid-leaching solution and the acid-leaching residue. The optimal acid type, concentration, reaction time, temperature, and liquid-to-solid ratio were determined through the evaluation of the fixed carbon content of the acid-leaching residue. Ultrasonic emitters (GBS-SCL10A, Guobiao, China) were used during the ultrasonic experiments. The experimental procedure for ultrasonically enhanced acid-leaching was similar to that of conventional acid-leaching, except that the optimal conditions identified in the conventional experiments were adjusted to account for ultrasonic treatment. Subsequently, the resulting acid-leaching residue was oven-dried at 80 °C for 24 h, after which the fixed carbon content was determined. The content of impurity elements in the acid-leaching solution was calculated using the following formula.

The mass of impurity in the filtrate was calculated using Eq. (1): $$m_i=c_i\times \frac{v}{m_0}$$ where c_i(g/ml) is the concentration of impurity i in the filtrate; v(ml) is the volume of the filtrate, m₀(g) is the mass of the raw materials used in each process, and m_i (g) is the mass of impurity i in the filtrate.

The impurity removal efficiency (RE) was calculated using Eq. (2): $$R{E}_i=\frac{m_{\mathit{ri}}}{m_{\mathit{ti}}}$$ where $m_{\mathit{ri}}$ (g) is the mass of impurity i removed during the alkali-acid method process, and $m_{\mathit{ti}}$ (g) represents the mass of impurity i in the WG powder.

The layer spacing d<0 0 2> was measured via XRD. Using Franklin’s formula (Eq. (3)), we can determine the degree of graphitization. The XRD method allows for direct measurement of the graphite layer spacing d002, which can then be used to calculate the degree of graphitization. This method is both simple and efficient. $$d<002>=\sqrt{\frac{2}{3}{\times d}^2}$$

The Mastersizer (Winner 2008 A, MA, USA) was used to measure the particle size distribution of the material following ultrasonic treatment. The concentration of dissolved impurity ions in the supernatant after ultrasonic-assisted flotation was determined via inductively coupled plasma atomic emission spectrometry (ICP-AES, OPTIMA8000, PerkinElmer, Waltham, MA, USA). X-ray photoelectron spectroscopy (PHI5000 Versaprobe III, Tokyo, Japan) was employed for structural analysis and examination of chemical bonding. The morphology of the samples was characterized via scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS, Merlin Compact, Germany).

3.1 Analysis of conventional acid-leaching experiment

Four types of acids—hydrochloric acid, nitric acid, sulfuric acid, and hydrofluoric acid—were used as leaching agents to remove impurities from WG. Through the variation of the acid concentration, leaching time, leaching temperature, and liquid-to-solid ratio, the relationship between the fixed carbon content in the leaching residue and the leaching system was established.

A moderate increase in the liquid-to-solid ratio is beneficial for enhancing the leaching effect. The optimal leaching effect is achieved at a liquid-to-solid ratio of 6. At this ratio, the leaching rates of WG with hydrofluoric acid, hydrochloric acid, sulfuric acid, and nitric acid all peak, resulting in the maximum carbon content of the leached graphite rising from approximately 94.5 % to 98.23 %. This phenomenon occurs because at a certain concentration of the leaching agent, an appropriate increase in the liquid-to-solid ratio reduces the viscosity of the system and the concentration of valuable metal ions in the leaching solution. This enhances the mass transfer rate between the solid and liquid phases, thereby improving the leaching rate. However, as the liquid-to-solid ratio continues to rise above 6, the fixed carbon content of the WG gradually levels off and may even slightly decrease. This observation indicates that an appropriate increase in the liquid-to-solid ratio benefits the mass transfer rate of the system. However, a very high liquid-to-solid ratio does not significantly enhance the leaching effect and instead increases economic costs and environmental burdens. Therefore, a preferred liquid-to-solid ratio of 6 is recommended. Additionally, as shown in Fig. 2(b), as the leaching temperature rises from room temperature to 70 °C, the fixed carbon content of the WG correspondingly increases. When the leaching temperature reaches 70 °C, the fixed carbon content of WG is at its highest, reaching 98.32 %. However, with a further increase in temperature, the leaching effectiveness of hydrofluoric acid, hydrochloric acid, and nitric acid significantly declines, and the leaching rate decreases rather than increases. This trend is attributable to high temperatures causing the volatilization of the acids, which reduces their concentration and subsequently weakens their leaching capacity. Because sulfuric acid does not have strong volatility, higher temperatures do not significantly reduce its leaching effectiveness. Therefore, a leaching temperature of 70 °C is preferred. Fig. 2(c) illustrates the correlation between different acid concentrations and the fixed carbon content of WG. As shown in Fig. 2(d), WG is sensitive to increased concentrations of all acids except sulfuric acid. As the acidity increases from 1.5 mol/L to 6 mol/L, the fixed carbon content and WG increase significantly, with the optimal leaching effect achieved at an acid concentration of 8 mol/L. Beyond this point, the fixed carbon content no longer increases with further increases in acid concentration. This trend is attributable to high acid concentrations causing an imbalance in ion concentrations in the solution, which affects the progress of the leaching reaction. In some cases, high acid concentrations may promote the formation of solid product layers, which can hinder further acid-leaching of the WG, thereby reducing the leaching rate. Regarding the relationship between leaching time and WG, the fixed carbon content reaches its maximum after 120 min of leaching. Beyond this point, the fixed carbon content does not increase significantly, indicating that all the leachable impurity phases have been extracted within the 120-minute timeframe. Between 60 and 120 min, while the leaching effect improves, the enhancement is not substantial. Therefore, considering energy consumption and production costs, a pickling time of 60 min is preferred. According to the findings from the leaching experiments, optimal conditions were established to maximize the leaching effect. These conditions include a liquid-to-solid ratio of 6, a leaching temperature of 70 °C, an acid concentration of 8 mol/L, and a leaching time of 60 min. Under these conditions, the fixed carbon content of WG increases significantly from ∼94 % to ∼98.5 %.

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Fig. 2

Effect of pickling under different conditions (a: Liquid-solid ratio b: Temperature c: Acid concentration d: Time).

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As shown in Fig. 3, XRD analysis, both before and after acid-leaching, does not indicate any phase changes resulting from the leaching process, mainly because the impurity phase content is below the detection limit of XRD, making it difficult to accurately detect the removal of these phases. Consequently, similar ash treatment methods are not applicable to the raw materials being studied. The measurement of $d_{002}$ has proven to be a widely used parameter for assessing the graphitization of carbon materials (Ingaki, 2013). In graphite, the $d_{002}$ value corresponds to the disordered layer structure, which is significantly larger than that of a complete graphite crystal (Li et al., 2017). The hexagonal layers in this disordered structure form a two-dimensional lattice, with an average $d_{002}$ value of 0.344. The degree of graphitization in graphite is directly proportional to the degree of deviation; a smaller deviation value indicates a closer approach to complete crystallization (Gogotsi et al., 2000). Compared with perfectly crystallized graphite, graphite with a smaller deviation value can effectively regulate neutrons. The spacing between adjacent layers, denoted as $d_{002}$ , is determined by weak van der Waals interactions (Li et al., 2017). Progressive heat treatment of graphite can reduce this spacing (Li et al., 2017; Howe et al., 2003). Using the Bragg equation, the crystal plane spacings of the acid-leaching slag after HF, HNO₃, H₂SO₄, and HCl leaching were measured as 0.3364, 0.3361, 0.3361, and 0.3367, respectively. Compared with WG, the sample’s layer spacing increases by approximately 0.0003 nm after acid-leaching. This suggests that under the action of the acid, impurities are removed while the layer spacing expands. This expansion may be due to reactions between the acid and functional groups within the graphite layers, which result in the removal of interlayer materials or structural expansion. High acid concentrations and increased leaching temperatures can lead to the degradation of the graphite layer structure, potentially reducing the layer spacing or causing the stripping of graphite layers. Additionally, the acid-leaching process may produce by-products that can accumulate between the graphite layers, further affecting the layer spacing. However, the current analysis indicates that the existing process does not adversely affect the relatively regular structure of the original graphite. This finding suggests that after acid-leaching and purification, WG still holds potential as a raw material for applications in the graphite thermal field or other graphite devices (Yu et al., 2021a, 2021b).

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Fig. 3

XRD pattern of acid-leaching residue and WG.

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According to the comprehensive leaching results, hydrofluoric acid proves to be the most effective for purifying WG, regardless of adjustments made in the leaching system. This efficacy is attributed to the ability of hydrofluoric acid to effectively remove SiO₂ from the WG, a result that other acids cannot achieve. Additionally, among the acids tested, hydrochloric acid demonstrates a superior purification effect for WG compared with sulfuric acid and nitric acid. This effect can be attributed to calcium (Ca) being the predominant metal impurity in WG, alongside iron (Fe). When sulfuric acid is used as a leaching medium, it chemically reacts with Ca, forming insoluble CaSO₄, which accumulates in the acid-leaching residue and reduces purification efficacy to some extent. The WG originates from the single crystal furnaces used in the production of Czochralski (CZ) monocrystalline silicon. To ensure high purity and prevent the introduction of impurities during the crystal-pulling process, WG is utilized as a thermal field device. The metal oxide impurities found in WG originate from the graphite thermal field itself, rather than being introduced during the crystal pulling process. The raw materials used in the production of graphite thermal fields consist of natural graphite combined with petroleum coke or needle coke carbon materials. During the preparation process, high-temperature graphitization is essential for removing the majority of impurities. However, to achieve the higher strength and density requirements for a graphite thermal field, the material must undergo multiple cycles of roasting and impregnation prior to graphitization. Through this repeated roasting and impregnation process, the density of the graphite thermal field can be increased to the desired levels. Additionally, some impurity phases become trapped within the structure during this process, making it difficult for them to volatilize as a gas during high-temperature graphitization. An analysis of the Ellingham diagram provides the following insights (see Fig. 4):

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Fig. 4

Ellingham diagram.

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According to the Ellingham diagram, the reduction of iron oxides is expected to occur at low temperatures, even though metallic iron is not detected in the XRD analysis. However, iron (Fe) likely remains present until temperatures reach 2000 °C (Frossard et al., 2000). Considering that the melting point of aluminum (Al) is slightly higher than the reduction temperature, Al may exist in a liquid state. Ultimately, at its boiling point of 2470 °C (Frossard et al., 2000), Al can be removed through high-temperature methods.

Other significant impurities include alkali metals such as potassium (K), calcium (Ca), and sodium (Na). According to the Ellingham diagram, these impurities have boiling points well below their reduction temperatures, indicating that they are removed directly in the gaseous state during reduction. Additionally, owing to the low melting points of K₂O and Na₂O (Herman, 1989; Frossard et al., 2000; Wang et al., 2021a, 2021b), it is expected that these compounds will also be eliminated. Although the results shown in the Ellingham diagram suggest the effective removal of calcium (Ca) during the graphitization process, a considerable amount of Ca is still detected in the raw material testing. This observation is attributable to the aforementioned high density, which hinders the volatilization of impurity elements, including Al (Liang et al., 2021).

3.2 Cause analysis of retained impurities

The acid-leaching purification experiment reveals variations in the effectiveness of different acids in purifying WG. Additionally, the acids exhibit differing capacities for removing impurity phases from the WG. This discrepancy is attributable to the varying binding affinities of metal ions when introduced into the system (Fig. 5).

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Fig. 5

Potential pH diagram of Fe, Al and Ca.

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Considering that Fe and Ca are the primary metal impurities in WG and that hydrofluoric acid (HF) and hydrochloric acid (HCl) demonstrate superior purification effects, we will focus our discussion on the behavior of Fe and Ca when exposed to Cl⁻ and F⁻ ions.

According to the analysis of the potential–pH diagram, the use of hydrofluoric acid as a leaching medium significantly expands the dominant region for Fe. This expansion is due to the formation of fluorine-containing complex ions, specifically FeF⁺ and FeF²⁺, which can enter the solution simultaneously. However, the introduction of F⁻ ions into the Ca-F-H₂O system does not create a favorable region. Additionally, the addition of F⁻ to a solution containing Ca leads to the formation of insoluble CaF₂, thereby reducing the advantageous region for soluble calcium. Consequently, the use of hydrofluoric acid to remove Ca from WG is not recommended. F⁻ ions have a strong complexation ability, allowing aluminum ions to readily react with them to form several fluorine complex ions, including AlF²⁺, AlF²⁺, AlF⁴⁻, AlF₅²⁻, and AlF₆³⁻. Consequently, F⁻ addition to the Al-H₂O system facilitates the formation of complexes between aluminum ions and F⁻ ions, thereby expanding the dominant region for aluminum and enhancing its dissolution in the liquid phase. When hydrochloric acid is used as the leaching medium, the dominant region for ionic Fe in the aqueous solution is significantly expanded. However, as the leaching process continues, the consumption of H⁺ ions reduces the solution’s acidity, causing the pH to gradually rise. This shift results in the conversion of soluble Fe into a secondary precipitated phase. To prevent this, it is essential to control the pH and maintain the acidity of the solution. Similarly, the addition of Cl⁻ ions expands the dominant region for Ca²⁺, promoting the presence of Ca in ionic form and facilitating its entry into the solution. In summary, the presence of metal oxide impurities in an aqueous solution is unstable under acidic conditions, making acid-leaching effective for removing these impurities from WG. Conversely, the addition of F⁻ significantly expands the dissolution region for Fe and Al in the aqueous solution, while the dissolution region for Ca is markedly reduced. This suggests that using hydrofluoric acid as the leaching medium effectively removes Fe and Al impurities from WG but is less effective in eliminating Ca impurities. Therefore, to achieve maximum removal of metal oxide impurities through the acid-leaching process, it is essential to employ a mixed acid system that includes both hydrochloric acid and hydrofluoric acid.

As shown in Fig. 6, similar conclusions can be drawn from the SEM–EDS analysis of the four types of acid-leaching residues. Notably, CaF₂ is present in the residue from hydrofluoric acid (HF) leaching, indicating a strong binding interaction between Ca²⁺ and F⁻ during the acid-leaching process. This finding is consistent with both theoretical analysis and experimental results. Additionally, the XPS analysis of the HF leaching residue supports this evidence. Examination of the Ca element across the four acid-leaching residues reveals that in addition to the HF residue, only CaO characteristic peaks are present in the other three samples. In contrast, the characteristic peak for CaF₂ is exclusively found in the HF leaching residue. The leaching residue from nitric acid (HNO₃) contains various impurity elements, predominantly oxygen. This is due to the reaction between nitric acid and metal impurities, which involves oxidation followed by subsequent chemical reactions. Nitric acid has limited effectiveness in removing metal impurities from WG. Consequently, certain impurities do not transition into the liquid phase during the acid-leaching process and instead remain in the leaching residue. In contrast, the impurity compositions of the residues from hydrochloric acid and sulfuric acid are relatively similar, consisting of unremoved iron oxides and silicon carbide. Calcium sulfate is expected to form through partial reactions in the sulfuric acid-leaching residue, which cannot be eliminated. Although the SEM analysis reveals no distinct phase, further discussion on this aspect is beyond the scope of this paper. Overall, aside from minor remnants of impurity particles, the majority of the acid-leaching residue shows a low level of residual impurities. Additionally, the graphite sheets display a clean and orderly overlapping structure, indicating that the purification process effectively removes impurities without damaging the lamellar arrangement of the graphite. This finding aligns with the XRD results regarding the degree of graphitization following purification.

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Fig. 6

SEM-EDS diagram of different acid leaching residues (1: HF 2: HCl 3: HNO₃ 4: H₂SO₄).

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As shown in Fig. 7, the XPS spectra reveal peaks at 284.6, 285.1, 286.9, 287.7, and 288.9 eV, corresponding to sp²-bonded carbons, sp³-bonded carbons, C–O, C=O, and O=C–OH functional groups, respectively (Monk et al., 1993; Hassan et al., 2021).. Furthermore, a comparison of the XPS spectra of Si in the four types of acid-leaching residues reveals that apart from the HF acid-leaching residue, distinct characteristic peaks of SiO₂ are present in the other three residues. This indicates that HF effectively removes SiO₂ from the WG, while the other acids lack the same capacity (Monk et al., 1993). This finding further reinforces the earlier explanation for the higher fixed carbon content in the HF leaching residue. In addition to SiO₂, SiC is also present in the acid-leaching residue. While HF effectively removes Fe and Al, it is less efficient at eliminating Ca. Therefore, a mixture of HF and HCl is employed as the acid-leaching medium for the purification experiment. According to previous research, concentrations of 3  mol/L for both hydrochloric acid and hydrofluoric acid were selected. The effects of leaching time, leaching temperature, and acid concentration on the leaching efficiency of each element were analyzed. As shown in Fig. 8(a), Fe is nearly completely leached at a temperature of 40 °C, with its leaching rate showing little variation beyond this point. In contrast, the leaching rate of Ca continues to increase after 40 °C and stabilizes at 70 °C. As depicted in Fig. 8(b), at a leaching time of 120 min, Fe is almost entirely leached, while the leaching rate of Ca continues to rise with extended leaching time, although within a limited range. When the acid concentration is held at 3 mol/L, Fe is effectively leached to completion, and its leaching rate does not increase with higher acid concentrations. However, the leaching rate of Ca continues to increase in proportion to the acid concentration, as shown in Fig. 8(c). This phenomenon occurs because although Ca ions tend to combine with F ions to form the less soluble CaF₂ compound—hindering their dissolution in the liquid phase—increasing the acidity improves leaching rates at pH levels below 0. This allows calcium to still enter the solution as ions (Kirkby and Pilbeam, 1984). Both theoretical analysis and experimental results indicate that using mixed acids for leaching WG enhances the removal efficiency of impurity elements to some extent. However, the complete removal of metal oxides has not yet been achieved, indicating that some impurities remain trapped in the WG. The reasons for these retained impurities have been analyzed. In addition to the differing effectiveness of various acids in removing specific impurity phases, the presence of a graphite coating on the impurity or its embedding within the graphite matrix may hinder its reaction with the leaching acid, resulting in incomplete removal. To achieve complete impurity removal (Habbache et al., 2009; Sádaba et al., 2015; Yu et al., 2020), we implemented an ultrasonically enhanced acid-leaching process. Considering that the removal mechanisms for each metal oxide during the acid-leaching process are generally similar, the leaching behavior of Fe was used as a benchmark to evaluate the efficacy of ultrasonically enhanced acid-leaching (Yu et al., 2021a, 2021b). The ultrasonic-assisted leaching process generates localized high temperatures when cavitation bubbles collapse. Several factors influence cavitation, including the surface tension coefficient, temperature, and liquid viscosity (Khan et al., 2024). Lower values of surface tension and viscosity coefficients are conducive to cavitation. However, as temperature increases, both the viscosity and surface tension of the liquid also rise, leading to a higher cavitation threshold. Temperature is crucial in the formation of cavitation bubbles. The thermal effect of ultrasound raises the temperature of the solution, increasing the solubility of soluble impurities in WG. Consequently, higher temperatures enhance the dissolution of these substances. Additionally, temperature variations significantly impact the chemical reaction process, with increased temperatures promoting the reaction between Fe₂O₃ and acid. As the temperature rises, the leaching rate of Fe gradually improves. Under the influence of ultrasonic enhancement, the leaching rate of Fe shows a noticeable increase, as illustrated in Fig. 8(d). Specifically, the rate increases from 85.11 % to 85.37 % within the temperature range of 40 °C to 50 °C. However, beyond 50 °C, the degree of improvement starts to reduce. At 70 °C, the effects of ultrasonically enhanced leaching become nearly indistinguishable from those of conventional leaching (Tzanakis et al., 2016). As the Fig. 8(e) shows, the leaching rate of Fe gradually increases with extended leaching time. When the time reaches 120 min, the leaching rate of Fe reaches 92.69 % and stabilizes; beyond this point, it remains relatively constant. However, the leaching rate of Fe still experiences a notable increase under the influence of ultrasound. This indicates that Fe present on the WG surface can be fully leached within 120 min. Nonetheless, some Fe impurity phases are embedded or coated within the graphite, limiting their exposure and reaction with the leaching acid, even in the presence of ultrasound (Tang et al., 2009). Ultrasonic enhancement has been shown to significantly reduce reaction time and increase the leaching rate of impurities. This improvement is due to the instantaneous high temperature and pressure generated when cavitation bubbles collapse under ultrasonic conditions. Ultrasonic cavitation induces surface effects, energy effects, turbulence effects, and perturbation effects within the solid–liquid system. Mechanical effects, such as microjets, shockwaves, and acoustic flow, create macroscopic turbulence in the liquid, leading to high-speed collisions between solid particles in the leaching system. Compared with the thermal convection velocity of the solution, the acoustic flow velocity is significantly faster (Bu et al., 2022; Bao et al., 2023). This acoustic flow effect accelerates circulation and induces vigorous vibrations within the solution. These intense oscillations and localized high temperatures can disintegrate solid reactants, resulting in increased dispersion. This disruption enhances eddy diffusion and affects, strips, and erodes both liquid–liquid and liquid–solid interfaces. The perturbation effect refers to the fluctuations induced by ultrasonic action on solid particles. This subtle disturbance within micropores enhances the diffusion rate of materials, a result that is challenging to achieve through mechanical agitation alone. During the leaching process, the combined effects of acoustic cavitation and high-speed acoustic flow facilitate a faster and more thorough separation of inorganic impurity solid particles from the carbon in WG under ultrasonic conditions. As shown in the Fig. 8f the leaching rate of Fe gradually increases with rising acidity levels. When the acidity reaches 3.5 mol/L, the leaching rate of Fe peaks at 98.76 %. Beyond this concentration, there are no significant further improvements in the leaching rate. However, under the influence of ultrasound, the leaching rate of Fe continues to show stable enhancement, with an overall improvement of approximately 4.6 % owing to the ultrasound field strengthening effect. This enhancement is attributed to the influence of ultrasound on the structure and properties of the medium, which can lead to potential changes or even destruction. According to ultrasonic crushing theory, the generation of numerous cavitation bubbles induced by high-energy ultrasound can result in grain refinement. Furthermore, the mechanical and cavitation effects of ultrasound effectively enhance particle diffusion rates in the solution (Maleki, 2018)and facilitate Brownian motion (Šarc et al., 2017). Consequently, the overall mass transfer in the reaction system is enhanced, improving the kinetic conditions for the reaction. In summary, the application of ultrasound has shown significant effectiveness in enhancing the acid-leaching process for impurity removal from WG. It not only improves mass transfer and interface reactions within the system but also disrupts the internal structure of the graphite, exposing more impurities and facilitating their subsequent removal.

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Fig. 7

XPS analysis of acid leaching residue with different acids.

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Fig. 8

Leaching rates of Fe and Ca in mixed acid leaching and Fe under ultrasonic enhancement under different conditions (a: Temperature b: Time c: Acid concentration d: Temperature (Ultrasonic enhancement) e: Time (Ultrasonic enhancement) f: Acid concentration (Ultrasonic enhancement)).

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Ultrasonically enhanced acid-leaching not only reduces the impurity content of WG but also leads to a reduction in particle size. As shown in Fig. 9, panel (a) displays the particle size distribution of the leaching residue, while panel (b) illustrates the distribution prior to leaching. It is important to note that reducing particle size during the purification process is highly beneficial for its use as a raw material in the thermal regeneration of graphite. This is because isostatically pressed graphite may experience embryo cracking due to condensation reactions that occur during the roasting process. To prevent this issue, multiple dipping processes are typically employed to enhance bulk density. However, when fine-grained graphite is used as a raw material alongside petroleum coke for isostatic graphite production, it effectively fills the voids in the petroleum coke. This leads to a higher bulk density of the raw material before the kneading process, thereby reducing the need for excessive asphalt glue during kneading. Consequently, the quality decline during the roasting process is mitigated. Particle size analysis of the samples before and after ultrasonic acid-leaching shows a significant reduction in particle size after treatment compared with the raw material. This supports the conclusion that the ultrasonic acid-leaching process effectively reduces the particle size of graphite, as illustrated in the accompanying figure.

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Fig. 9

Particle size analysis of WG and ultrasonic mixed acid leaching slag (a: Leaching residue b: WG).

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Moreover, a comparison of the particle size analysis between samples subjected to conventional acid-leaching and those treated with ultrasonic acid-leaching reveals that the latter results in a significant reduction in particle size. This indicates that ultrasonic acid-leaching accelerates the reaction rate between the acid and impurities, enhances overall reaction efficiency, facilitates faster separation of impurities, and induces partial disruption of the original particle structure, thereby releasing trapped impurities.

As shown in Fig. 10, this conclusion is further supported through a comparison of the SEM images taken before and after ultrasonically enhanced acid-leaching. The comparison reveals that the samples treated with ultrasound exhibit more voids and smaller particles. In contrast, the samples before ultrasonic treatment display a more regular and distinct bulk structure. Similar insights can be gained from an analysis of the XRD results before and after ultrasonic treatment. The XRD patterns indicate that the main phase remains unchanged, suggesting that ultrasonic mixed acid-leaching does not effectively remove SiC impurities from the material. However, the layer spacing of graphite samples after ultrasonic treatment has significantly increased, resulting in a certain reduction in the graphitization degree. This increase in layer spacing has advantages when used as a negative electrode material for lithium-ion batteries, as it facilitates the faster removal of Li⁺ ions, thereby enhancing electrochemical performance.

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Fig. 10

XRD and SEM-EDS of ultrasonic enhanced acid leaching residue.

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