Separation and Purification of Fluorine from Fluorine Slag and Alkali Leaching Conversion Mechanism

Abstract

This work proposes a novel and direct alkaline leaching process for extracting and recovering fluorine from fluorine slag, designed to overcome the current limitations in research on fluorine slag and the unclear leaching mechanisms. 80.40% of fluorine can be leached from the fluorine slag to the solution under optimal leaching conditions (NaOH 3.0 mol/L, temperature 80 °C, time 2 h, liquid-to-solid ratio 18 mL/g, and speed 800 r/min). The leaching kinetics of fluorine from fluorine slag in a concentrated NaOH solution was analyzed using multiple models, and key kinetic parameters, including activation energy and reaction order, were determined. The factors limiting fluorine leaching were identified through density functional theory calculations. Additionally, the economic feasibility of the process was evaluated, providing important insights into its potential for industrial application. This research aims to establish a foundation for optimizing processes and conducting mechanistic studies on the recovery of fluorine resources by leaching fluorine slag.

1. Highlights

• 1.The efficient conversion of fluorine form fluorine slag was achieved using a simple alkali leaching process.
• 2.The mechanism of inhibited fluorine conversion rates under conventional leaching processes was revealed.
• 3.The concentration of sodium hydroxide and the temperature are key factors influencing the leaching process.
• 4.The purity of NaF recovered from fluorine slag reached 98.02%.

2. Introduction

Fluorine is an indispensable element in the cement and glass industry, aluminum smelting industry, iron and steel smelting industry, and fluorine chemical industry. Fluorine is primarily sourced from fluorite ore, and China’s fluorite ore reserves rank second in the world. , In 2021, China produced 5.4 million tons of fluorite, representing about 62.8% of the world’s total production of 8.6 million tons. However, as a nonrenewable resource, the availability of fluorite ore is limited. With the gradual consumption of fluorite, the shortage of resources is a challenge for the industry, and there is expected to be a shortfall of 600,000–800,000 tons of global fluorite mineral resources in 2026. Finding alternative fluorite mines to provide fluorine resources is urgent and necessary.

Fluorine slag is a solid waste produced during manufacturing and industrial processes such as smelting, semiconductors, electroplating, glass, and ceramics, and its chemical composition is primarily composed of CaF₂, MgF₂, CaSO₄, and NaMgF₆, etc. , It is regarded as an emerging pollutant that poses a significant risk to plant, animal, and human health and is rich in fluorine resources. − Therefore, it is essential to safely and correctly dispose of fluorine slag and recover fluorine resources to avoid harming the environment and human body , and provide another source of fluorine resources.

Currently, most fluoride slag is exposed in the open air without any treatment, and the rest are landfilled after solidification/stabilization. , These treatments occupy a large amount of land resources, and there is a risk of secondary pollution, such as introducing fluoride into groundwater with rainfall and contaminating soil and drinking water. − Some scholars use chemical methods to treat fluorine slag, which can be divided into pyrometallurgy and hydrometallurgy. Yang performed high-temperature roasting on the fluoride slag, and the fluoride could be removed by roasting at 1600 °C for 1 h. Zhu’s study showed that particle size, roasting temperature, and time had a significant effect on the fluoride removal effect, with an average particle size of 10 mm and a fluoride removal rate of 88% by roasting at 1499 °C for 3.33 h. However, CaF₂ remained in the slag. Dong used H₂SO₄ as additive and fluorine slag for coroasting under the optimal conditions (m(H₂SO₄)/m(fluorine slag) 1.4:1, 360 °C, and 2 h), the fluorine removal rate was 99.4%. Lisbona washed the fluoride slag in two stages by water washing and Al₂(NO₃)₃·9H₂O solution, and the total fluoride leaching rate was 76∼86% after 24 h of reaction. Huo utilized response surface methodology to optimize acid leaching parameters, achieving a fluoride removal rate of 18.54% at 2.0 mol/L H₂SO₄ concentration, a liquid-to-solid ratio (L/S) of 25 mL/g, and a temperature of 60 °C. The pyrometallurgical method removes or converts fluorine by roasting the fluorine slag, which requires a large amount of energy, and the volatile fluorine resources are difficult to collect for secondary utilization. , Hydrometallurgical treatment of fluorine slag has the problems of being long-consuming and having a low-leaching efficiency. Therefore, finding a low-cost and high-leaching method to recover fluorine from fluorine slag is needed.

In this work, fluoride slag was leached with an alkali solution and the effects of various leaching conditions on fluoride leaching efficiency were systematically investigated. The leaching kinetics were studied at different temperatures, NaOH concentrations, L/S, and stirring speeds, developing leaching kinetic equations and exploring the reaction mechanism. Additionally, density functional theory (DFT) calculations were employed to elucidate the mechanism underlying the inhibited fluorine conversion rates observed in conventional leaching processes. Finally, the leachate was purified to obtain the NaF product, and an economic assessment of the process was conducted. This work developed a simple leaching technique to recover fluorine resources from fluorine slag, which is expected to provide an experimental foundation and theoretical support for the further efficient leaching of fluorine slag.

3. Experimental Section

3.1. Materials and Characterization

Fluorine slag was supplied by Jiangxi Ruida New Energy Technology Co., Ltd. (Jiangxi Province, China). After drying, the fluorine slag was ground through a 200-mesh sieve, and its detailed chemical composition is shown in Table S2. XRD and SEM images are presented in Figure . In this experiment, all chemical reagents (NaOH, MgF₂, N₂₃₅, etc.) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). After being treated by a water purification system (Advantage A10, Millipore, Burlington, MA, The United States), deionized water was obtained. The fluorine in fluorine slag was measured by alkali melting roasting and deionized water dilution according to Chinese industrial standard (HJ 999-2018); Supporting Information (SI) for specific steps.

1.

Characteristics of raw fluorine slag: (a) XRD; (b) SEM image.

3.2. Experimental Procedures

In this study, the leaching test of solid waste containing fluorine was carried out with sodium hydroxide solution as a solvent. For each run, 30,000 g of fluorine slag was mixed with a certain volume of NaOH solution, and the leaching experiment began when the constant temperature water bath reached a predetermined temperature. After leaching, the fluoride concentration in the filtrate was measured by filtration, and the filter residue was dried for subsequent microscopic observation and analysis. The specific parameters of the leaching test are shown in Table S3. The corresponding leaching efficiency for fluoride can be determined by eq :

$$\eta =\frac{C\times V}{m}\times 100\%$$ 1

where η is the F leaching efficiency (%), C is the concentration of F^– in the leaching solution (g/L), V is the volume of the leaching solution (L), and m is the F content in fluorine slag.

Kinetic experiments were carried out under optimized reaction conditions based on single-factor experiments. The change of the fluorine leaching rate with time under different temperatures, NaOH concentrations, L/S, and speeds was studied. Appropriate data were selected from kinetic experiments to fit kinetic equations, and the reaction’s control type and apparent activation energy were calculated.

3.3. Theoretical Calculations

The first-principles, energetic and electronic structure calculations, were carried out within the framework of the DFT using projector augmented wave (PAW) pseudopotentials as implemented in the Vienna ab initio simulation package (VASP). − We used the generalized gradient approximation (GGA) formulated by Perdew, Burke, and Ernzerhof (PBE) as an exchange-correlation function. A kinetic energy cutoff of 400 eV for the plane wave basis set was generated. The Brillouin zone was sampled with Γ-centered 5 × 5 × 5 k points. The energy and force convergence criteria were set to 10^–5 eV and 0.01 eV/Å.

3.4. Characterizations

The fluoride ion content in the solution was tested using ion chromatography (ICS-2100, DIONEX, The United States). X-ray fluorescence spectroscopy (XRF, UltimaIV, Rigaku, Tokyo, Japan) determined the elemental composition of the reaction residue. X-ray diffraction (XRD, Axios MAX, PANalytical B.V., Almelo, Netherlands) was used to analyze the phase composition and crystal structure under a scanning speed of 1°/min and the range of 2θ = 5–85°. The morphology and elemental composition of the reaction residue were analyzed by scanning electron microscopy (SEM; SU-8020, 175 HITACHI; Japan) and energy-dispersive spectroscopy (EDS, EX-230**BU, Japan).

4. Results and Discussion

4.1. Leaching Characteristics of Fluorine Slag

The effects of NaOH concentration, temperature, time, L/S, and speed on the leaching efficiency of fluoride slag were investigated. The relationship between the reactivity and physicochemical properties of fluoride slag was revealed by combining the phase changes before and after leaching.

4.1.1. Effect of NaOH Concentration on Fluoride Leaching

A single-factor experiment with different NaOH concentrations was performed at 30 °C, 1.5 h, 16 mL/g, and 400 r/min. It can be seen from Figure a that the increase in NaOH concentration promotes the leaching efficiency. The leaching efficiency was 21.98% at a NaOH concentration of 1.5 mol/L. As the NaOH concentration increased, the efficiency significantly improved, reaching 42.42% when the NaOH concentration was 3.0 mol/L. At this time, the efficiency reached a plateau, and there was no significant increase with further increase of NaOH concentration. According to Figure a, the characteristic peak of MgF₂ gradually weakened with the increase of NaOH concentration, while Ca­(OH)₂, Mg­(OH)₂, and MgAl­(OH)₁₄·XH₂O gradually increased. Furthermore, considering the production practice and economic benefits, the optimum NaOH concentration was 3.0 mol/L.

2.

Effect of factors on fluorine slag leaching efficiency: (a) NaOH concentrations; (b) temperatures; (c) time; (d) L/S; (e) speed; and (f) leaching efficiency of other elements under optimal conditions.

3.

XRD pattern of reaction residue under different conditions: (a) NaOH concentrations; (b) temperatures; (c) time; (d) L/S; (e) speed; and (f) the ΔG variation trend with temperature.

4.1.2. Effect of Temperature on Fluoride Leaching

To study the effect and optimal value of temperatures on the leaching efficiency of fluoride slag, the experiments were performed with temperatures ranging from 30 to 90 °C. NaOH concentration, leaching time, L/S, and speed were kept constant at 3 mol/L, 1.5 h, 16 mL/g, and 400r/min, respectively. The results are shown in Figure b, which indicates that higher temperatures were more conducive to the reaction. However, it can also be observed that when the temperature exceeds 80 °C, the leaching efficiency of F remains unchanged. The characteristic peaks of MgF₂ are still present in the leaching residue after the reaction at 80 °C (Figure b). Hence, the optimal temperature was determined to be 80 °C.

4.1.3. Effect of Time on Fluoride Leaching

Prolonging reaction time is beneficial to the reaction. Therefore, the leaching experiments were carried out for durations between 0.5 and 2.5 h under the NaOH concentration of 3.0 mol/L, temperature of 80 °C, L/S of 16 mL/g, and speed of 400 r/min. As shown in Figure c, the experimental results revealed that the leaching efficiency improved with the time increased. For example, the leaching efficiency reached 49.18% after 0.5 h and increased to 66.49% after 2.0 h. With the extension of time, the leaching efficiency of fluorine slag tends to be stable and gradually stabilizes at about 66%. XRD results (Figure c) also showed a similar trend, with the characteristic peaks of MgF₂ weakening as time increased. Thus, the optimum time was 2.0 h.

4.1.4. Effect of L/S on Fluoride Leaching

To investigate the effect and optimal value of L/S, the experiments were performed with L/S ranging from 12 to 20 mL/g. NaOH concentration, temperature, time, and speed were kept constant at 3 mol/L, 80 °C, 2.0 h, and 400 r/min, respectively. As shown in Figure d, the leaching efficiency of fluoride were 45.90 and 69.74%, as ratios of L/S were 12 and 18 mL/g, separately. It was attributed to the increase in L/S, which promotes contact between the fluorine slag and the solution, thereby improving the leaching efficiency. However, characteristic peaks of MgF₂ still exist at an L/S of 20 mL/g (Figure d). Considering both leaching efficiency and XRD results, a L/S of 18 mL/g was chosen for subsequent experiments.

4.1.5. Effect of Speed on Fluoride Leaching

The effect of speed on fluoride leaching was investigated (NaOH concentration of 3 mol/L, temperature of 80 °C, leaching time of 2 h, and L/S of 18 mL/g). Figure e shows that the fluoride leaching efficiency is only 69.90% with 200 rpm stirring, which achieves 80.40% under 800 rpm, and the upward trend is still maintained with continual speed. Continuing to increase the stirring speed, the leaching efficiency of fluorine remained stable. As seen in Figure e, the intensity of the characteristic peaks of MgF₂ weakens with increasing stirring speed, but they remain present. Therefore, the optimum mixing speed is 800 r/min.

In summary, the optimal alkaline leaching conditions were a NaOH concentration of 3.0 mol/L, a temperature of 80 °C, a leaching time of 2 h, an L/S of 18 mL/g, and a speed of 800 r/min. Under these conditions, fluoride slag leaching achieved 80.40%. Additionally, Figure f shows that the leaching efficiency of Mg, Al, and Ca were 4.75, 3.77, and 3.12%, respectively, under the optimal conditions. According to the XRD analysis of the leaching residue under different conditions, the possible reactions in the leaching process are as follows: when the fluorine slag is leached with the NaOH solution, MgF₂ in the slag can react with the NaOH solution to form Mg­(OH)₂ and NaF, and NaF is soluble in water and exists in the leach solution. NaF crystals with higher purity could be obtained by further solution follow-up. Meanwhile, other phases in the fluorine residue, such as CaSO₄ and Al₂(SO₄)₃, will also react with the NaOH solution to form Ca­(OH)₂ and Al­(OH)₃. The generated Al­(OH)₃ further combines with Mg­(OH)₂ to form MgAl­(OH)₁₄. The decomposition of fluorine slag during NaOH leaching, described by eqs eq –, was computed for its Gibbs-free energy (ΔG) change using HSC (6.0) software, as illustrated in Figure f. The ΔG of eq – is negative in the range of 0∼100 °C and does not change significantly with increasing temperature, which suggests that the reaction of NaOH with the phases of the fluorine slag can occur spontaneously:

$${\mathrm{MgF}}_2+\mathrm{NaOH}\to \mathrm{Mg}{(\mathrm{OH})}_2+\mathrm{NaF}$$ 2

$${\mathrm{CaSO}}_4+\mathrm{NaOH}\to \mathrm{Ca}{(\mathrm{OH})}_2+{\mathrm{Na}}_2{\mathrm{SO}}_4$$ 3

$${\mathrm{Al}}_2{({\mathrm{SO}}_4)}_3+\mathrm{NaOH}\to \mathrm{Al}{(\mathrm{OH})}_3+{\mathrm{Na}}_2{\mathrm{SO}}_4$$ 4

$$\mathrm{Al}{(\mathrm{OH})}_3+\mathrm{NaOH}\to {\mathrm{NaAlO}}_2+{\mathrm{H}}_2\mathrm{O}$$ 5

$${\mathrm{Al}}_2{({\mathrm{SO}}_4)}_3+\mathrm{NaOH}\to {\mathrm{NaAlO}}_2+{\mathrm{Na}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}$$ 6

$$\mathrm{Mg}{(\mathrm{OH})}_2+\mathrm{Al}{(\mathrm{OH})}_3+{\mathrm{H}}_2\mathrm{O}\to \mathrm{MgAl}{(\mathrm{OH})}_{14}·{\mathrm{XH}}_2\mathrm{O}$$ 7

4.2. Leaching Kinetics

Pure MgF₂ was used in leaching kinetic analysis to investigate the leaching mechanism and reaction control of fluoride slag under the above conditions.

The unreacted core model, widely utilized for describing the dissolution, leaching, and chemical reactions among solid particles in multiphase reaction systems, is applied extensively. Therefore, this work employs an unreacted core model to describe the leaching process of MgF₂. The model describes the dissolution of solid particles in three steps: (1) reactants diffuse from the solution to the surface of the particles; (2) reactants diffuse through the product shell to the surface of the unreacted core; and (3) the reaction occurs on the surface of the unreacted core. It can be divided into three situations: (1) dissolution control, (2) solution diffusion control, and (3) product layer diffusion control. , The equations for these three control modes are as follows:

$$\mathrm{External}\mathrm{diffusion}-\mathrm{controlled}\mathrm{step}:X=K_{1}t$$ 8

$$\mathrm{Chemical}\mathrm{reaction}-\mathrm{controlled}\mathrm{step}:1-(1-X{)}^{1/3}=K_{2}t$$ 9

$$\mathrm{Internal}\mathrm{diffusion}-\mathrm{controlled}\mathrm{step}:1-3(1-X{)}^{2/3}+2(1-X)=K_{3}t$$ 10

where X is the leaching efficiency of fluorine; k ₁, k ₂, and k ₃ are the reaction rate apparent constants (min^–1); and t is the reaction time (min), which are calculated from eq –, respectively.

At the same time, the leaching kinetics of MgF₂ was described by the Avrami model. The Avrami model (eq ) was initially used to describe the dynamics of crystal growth in the solution and assumed that crystal growth occurred randomly and uniformly during nucleation, with the same growth rate in all directions. − Based on the above assumptions, solid leaching dissolution is the reverse process of crystal growth by crystallization in the solution. Therefore, the Avrami model is also suitable for the solid leaching process. Some researchers have applied the Avrami model to describe the leaching kinetics of waste V₂O₅/TiO₂ catalyst, laterite, sodalite, and other minerals in this paper. The Avrami model will also be introduced to describe the dynamics of magnesium fluoride, and its equation is as follows:

$$-\ln (1-x)=K_4{t}^n$$ 11

where X is the leaching efficiency of fluorine at reaction time t (min); k ₄ is the rate constant of a chemical reaction, and n is the reaction procedure parameters.

The Avrami model also contains both chemical reaction and diffusion control and is related to the parameter n. When n > 1 indicates chemical controlled, 0.5 < n < 1 suggests that both diffusion and chemical controlled, while n < 0.5 indicates diffusion control.

4.2.1. Leaching Kinetics with Different Temperatures

The kinetic experiments were carried out at a NaOH concentration of 3.0 mol/L, L/S of 18 mL/g, speed of 800 rpm, and temperatures of 30, 60, 70, 80, and 90 °C, with reaction times of 5, 10, 15, 20, 30, 60, and 120 min, respectively.

As shown in Figure a, the leaching efficiency changes over time at different temperatures. With the extension of time, the leaching efficiency increases, and the higher the temperature of the same leaching time, the greater the leaching efficiency. Figure b–e and Tables S4–S5 show the results of the mechanical fitting of the leaching rate action under different temperature and time conditions, and we can see that the three equations are poorly fitted (<0.95), and the Avrami model is the best-fitted, where the R ² > 0.95. Furthermore, it can be seen from Table S5 that the characteristic parameter n of the kinetic equation for NaOH leaching of MgF₂ is 0.5570 (the average value of n at different experimental temperatures) >0.5, indicating that both chemical and diffusion reactions control the leaching process.

4.

(a) Fluorine leaching efficiency with different temperatures; (b) X; (c) 1–(1–X)^1/3; (d) 1–3­(1–X)^2/3 + 2­(1–X); (e) ln­(−ln­(1–x)) variation with time at different temperatures; (f) Arrhenius plot.

The rate constant K of a chemical reaction is determined by the temperature and apparent activation energy (Ea), as described by the Arrhenius eq (eq ). Therefore, utilizing the equation, a curve is constructed by plotting lnk against 1/T to determine the activation energy necessary for leaching fluorine slag:

$$\mathrm{lnk}=-\frac{\mathrm{Ea}}{\mathrm{RT}}+\mathrm{lnA}$$ 12

where k is the rate constant of the reaction, A is the constant, Ea is the apparent active energy (kJ/mol), and R is the universal gas constant (8.3145 J mol^–1 k^–1).

Generally, an apparent activation energy <12 kJ/mol indicates diffusion control. At 12–42 kJ/mol suggests controlled by both chemical reactions and diffusion, while values >42 kJ/mol indicate controlled by chemical reactions. Figure f shows that the Ea for the leaching of MgF₂ in a NaOH solution is 23.25 kJ/mol, further suggesting that the leaching process is influenced by both chemical reactions and diffusion. The research suggests that kinetic equations provide a better understanding of the leaching process. − Therefore, further fitting was performed under different NaOH concentrations, L/S, and speed.

4.2.2. Leaching Kinetics with Different NaOH Concentrations

Kinetic experiments were conducted at NaOH concentrations of 1.5, 2.0, 2.5, 3.0, and 3.5 mol/L, with a temperature of 80 °C, a L/S of 18 mL/g, and a speed of 800 r/min.

It can be seen from Figure S1­(a) that the leaching rate increases with the increase in NaOH concentration and reaction time. Similarly, four different kinetic models were fitted to the leaching data (Figure S1­(b-d) and Figure a). It can be seen from Table S6 that the Avrami model had the best fitting effect (R ² > 0.98). Furthermore, the constant value of the apparent rate was calculated according to the slope of Figure b, and lnK was used to plot ln­[C_NaOH], which showed that the empirical reaction order of the NaOH concentration was 0.8229.

5.

ln­(−ln­(1–x)) variation with time at different (a) NaOH concentrations, (c) L/S, (e) speed; Arrhenius plot of (b) NaOH concentrations, (d) L/S, and (f) speed.

4.2.3. Leaching Kinetics with Different L/S

Kinetic experiments were conducted at L/S of 12, 14, 16, 18, and 20 mL/g under the conditions of 3.0 mol/L NaOH, 80 °C, and 800 r/min.

From Figure S2­(a), it can be seen that the increase in the L/S contributes to the increase in the leaching rate. A kinetic model was fitted to the leaching rates at different L/S, and it can be seen from Table S7 that all equations have a poor linear fit except for the Avrami model (R ² > 0.99) (Figure S2­(b–d) and Figure c). The constant apparent rate was determined from the slope of Figure d, and natural logarithms of k were plotted against the natural logarithm of L/S, revealing an empirical reaction order of 0.6444 with respect to L/S.

4.2.4. Leaching Kinetics with Different Speed

Under the conditions of NaOH concentration of 3.0 mol/L, temperature of 80 °C, and L/S of 18 mL/g, kinetic experiments were conducted at speeds of 200, 400, 600, 800, and 1000 r/min. Figure S3­(a) shows that the rotational speed increases the leaching efficiency. Similarly, kinetic fitting of the leaching rates at different stirring speeds over various time intervals indicates that the Avrami model achieved the best fit, with a correlation coefficient greater than 0.98, as shown in Table S8 (Figure S3­(b-d) and Figure e). According to the slope of Figure f, the empirical response order of the speed is 0.3777.

The control factors of the leaching experiment included NaOH concentration, temperature, L/S, and speed. To complete the kinetic model, the apparent rate constant is related to these factors as follows:

$$K_4=K_0({\mathrm{C}}_{\mathrm{NaOH}}{)}^{\mathrm{a}}(\mathrm{L}/\mathrm{S}{)}^{\mathrm{b}}(\mathrm{rpm}{)}^{\mathrm{c}}\mathrm{e}-\frac{\mathrm{Ea}}{\mathrm{RT}}$$ 13

Substituting eq into eq yields eq

$$-\ln (1-X)=K_0({\mathrm{C}}_{\mathrm{NaOH}}{)}^a(\mathrm{L}/\mathrm{S}{)}^b(\mathrm{rpm}{)}^{c}e-\frac{\mathrm{Ea}}{\mathrm{RT}}{t}^n$$ 14

where K ₀ is the leaching efficiency of fluorine at the reaction time t (min); Ea is the apparent active energy (kJ/mol); R is the universal gas constant (8.3145 J mol^–1k ^–1); T is temperature; t is leaching time; and a, b, and c are the empirical reaction order of NaOH concentration, L/S, and speed, respectively.

According to the above results, the kinetic equation of NaOH leaching MgF₂ can be obtained as follows:

$$-\ln (1-X)=K_0({\mathrm{C}}_{\mathrm{NaOH}}{)}^{0.82}(\mathrm{L}/\mathrm{S}{)}^{0.64}(\mathrm{rpm}{)}^{0.37}{e}^{-\frac{\mathrm{23,250}}{\mathrm{RT}}}{t}^{0.56}$$ 15

Based on eq , it can be inferred that under different alkali leaching parameters such as temperature, NaOH concentration, L/S, and speed, the leaching efficiency of MgF₂ varies to various extents. Among these parameters, NaOH concentration and temperature exhibit the most significant influence on the alkali leaching process, followed by L/S and speed. Therefore, according to eq , alkali leaching can be controlled under certain conditions by adjusting the NaOH concentration, reducing the temperature and L/S appropriately, and optimizing the process parameters during the leaching process.

4.3. Alkali Leaching Mechanism Analysis

From the XRD results in Figure a, it can be seen that the F element in the fluorine slag primarily exists in the form of the MgF₂ phase. To better and more thoroughly investigate the alkaline leaching conversion mechanism of fluorine-containing components in fluorine slag, repeated alkaline leaching experiments were conducted by using pure MgF₂ under optimal process conditions. The leaching efficiencies from multiple trials are shown in Figure S4­(a), with an average efficiency of 80.52%, which is close to the optimal leaching efficiency of fluorine slag (80.40%). The XRD patterns of the leaching slag revealed characteristic peaks of Mg­(OH)₂, along with the presence of MgF₂ characteristic peaks (Figure S4­(b)), confirming that MgF₂ has not been wholly converted to NaF.

To further understand the factors limiting the leaching efficiency, we evaluated the Gibbs-free energy difference through theoretical calculations to comprehend the preference for forming Mg­(OH)₂ during the growth process involving MgF₂ and NaOH. We choose two representative sets of chemical potentials for evaluating the corresponding Gibbs-free energy, corresponding to Mg­(OH)₂-rich conditions and Mg­(OH)₂-poor conditions, respectively (rich indicates a higher potential for generating more Mg­(OH)₂, while poor represents the opposite). In considering the Mg­(OH)₂-rich and Mg­(OH)₂-poor conditions, we adjust the chemical potentials of the reactants and products to reflect different environmental conditions. The chemical potential at pressure p can be easily obtained by using eq . For each condition, the Gibbs-free energy difference of the reaction is calculated as follows eq –:

$$\mu (T,P)=\mu (T,P^0)+KT\ln (\frac{P}{p^0})$$ 16

For the Mg­(OH)₂ -rich condition:

$$\mathrm{\Delta }{G}_{\mathrm{rich}}=[G_{\mathrm{Mg}(\mathrm{OH})2}+\mathrm{\Delta }{\mu }_{\mathrm{rich}}+2G_{\mathrm{NaF}}]-[G_{\mathrm{MgF}2}+2G_{\mathrm{NaOH}}]$$ 17

For the Mg­(OH)₂ -poor condition:

$$\mathrm{\Delta }{G}_{\mathrm{poor}}=[G_{\mathrm{Mg}(\mathrm{OH})2}+\mathrm{\Delta }{\mu }_{\mathrm{poor}}+2G_{\mathrm{NaF}}]-[G_{\mathrm{MgF}2}+2G_{\mathrm{NaOH}}]$$ 18

The calculation results, shown in Table , provide the species Gibbs-free energies under our experimental conditions. We determined the chemical potential for the Mg­(OH)₂-rich condition to be −27.10 eV. Subsequently, we considered the Mg­(OH)₂-poor condition with a chemical potential of −25.79 eV. The Gibbs-free energy differences for the chemical reaction under these conditions were calculated to be −0.94 and 0.37 eV, respectively.

1. Gibbs-Free Energies of the Species.

	G (eV)				Delta G
	MgF2	NaOH	Mg(OH)2	NaF
Mg(OH)2-rich	–16.14	–13.75	–27.10	–8.74	–0.94
Mg(OH)2-mid	–16.14	–13.75	–26.16	–8.74	0.00
Mg(OH)2-poor	–16.14	–13.75	–25.79	–8.74	0.37

Based on the above analysis, the possible leaching mechanism of MgF₂ in fluorine slag is shown in Figure . Initially, the energies of Mg­(OH)₂-rich condition are low, the Gibbs-free energy of the whole reaction is <0, and the reaction can occur spontaneously. As the chemical reaction progresses and the amount of the Mg­(OH)₂ product increases, the chemical potential of Mg­(OH)₂ gradually decreases. This results in a positive Gibbs-free energy difference for the chemical reaction, causing the response to cease. The leaching efficiency did not change significantly with the increase in NaOH concentration, reaction time, and temperature. The calculated results align well with the experimental findings.

6.

A possible leaching mechanism of MgF₂ in fluorine slag.

4.4. Procedures for NaF Product

In previous research, high-purity NaF products were obtained after the fluorinated solution under the best purifying condition (extraction: O/A of 2, 5 min, 25 °C, four extraction cycles; stripping: NaOH concentration of 0.8 mol/L, O/A of 1:1, 3 min, 20 °C). According to this condition, the leaching solution was purified, and the crystal phase of the product was identified by XRD analysis (Figure a), which showed that the crystal phase of the purified product was consistent with NaF (JCPDS No. 01-1184). The SEM image of NaF is demonstrated in Figure b, which displays a regular block-like structure. Furthermore, the purity of the product after evaporation and crystallization is 98.02% according to standard GB/T1264-1997, which complies with the quality requirements for NaF chemical reagents (Table S10).

7.

(a) XRD pattern of product NaF; (b) SEM of product NaF.

4.5. Economic Accounting

To assess the practicality of the alkali leaching process for fluorine slag proposed in this research, an economic evaluation was conducted, and a detailed analysis is provided in SI. In the economic assessment, the total input costs, including chemical reagents, water, and energy consumption, are highlighted in blue. At the same time, the output benefits from the evaporative crystallization products are indicated in red (Figure ). Table S11 shows that when 1.0 kg of fluorine slag is treated using the process proposed in this work, the cost is 54.30 ¥, resulting in a benefit of 77.02 ¥ and an economic profit of 22.72 ¥. In contrast, the mechanical ball milling method for processing the same amount of fluorine slag yields a benefit of 21.5 ¥. Consequently, compared to the mechanical ball milling technologies, the proposed method exhibited higher economic benefits. This suggests that the alkali leaching for F recovery and purification from the fluorine slag has high profits and good environmental benefits.

8.

Cost, energy, and water consumption of this process.

It is important to note that these data are derived from small-scale laboratory experiments and can provide only a preliminary commercial feasibility analysis. Therefore, this economic assessment may not fully reflect the realities of large-scale commercial recovery operations.

5. Conclusions

This work investigated the technological parameters and kinetics of leaching fluorine slag with a NaOH solution. Fluorine leaching efficiency can be achieved by 80.40% under optimal conditions. The kinetic study showed that the leaching process was most consistent with the Avrami model and controlled by diffusion and chemical reaction with an apparent activation energy of 23.25 kJ/mol, and the leaching kinetic equation was as follows: −ln­(1–X)=K₀(C_NaOH)^0.82(L/S)^0.64(rpm)^0.37 $e^{-\frac{\mathrm{23,250}}{\mathrm{RT}}}$ t ^0.56. With the increase of chemical potential energy of Mg­(OH)₂ during the reaction, the Gibbs-free energy of >0 limited the leaching of fluorine slag. The purity of NaF product in the final recovered leachate can reach 98.02% after purification. Recovering fluorine from fluorine slag in the form of NaF not only significantly enhances its economic value but also contributes to environmental sustainability by reducing hazardous waste discharge. Future research should focus on improving the leaching efficiency of fluorine slag by optimizing leaching methods and reagent selection, facilitating the rapid application of the findings to industrial production.

Glossary

Abbreviations

DFT

Density functional theory

L/S

Liquid-to-solid ratio

PAW

Projector augmented wave

VASP

Vienna ab initio simulation package

GGA

Generalized gradient approximation

PBE

Perdew, Burke, and Ernzerhof

Ea

Apparent activation energy

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c01584.

• Detailed description of the methods for determining the total fluorine content, purification methods of fluorine-containing solutions, detailed data of the leaching process, economic analysis data (PDF)

X.Z. and D.P. conceived and designed the study and wrote the manuscript. H.Z., M.C., and J.S. developed the mechanochemical method and carried out experimental studies. P.M., F.Y., and H.Y. carried out the mechanism analysis and some data tests such as XRF, XRD, SEM, and DFT. All authors were involved in analyzing the experimental data.

The authors declare no competing financial interest.