Study on Solvent Extraction of Rare Earth Elements from Leaching Solution of Coal Fly Ash by P204

Abstract

Due to the increasing demand for rare earth elements (REE) resources in the market and the shortage of their direct sources, the research on REE in coal fly ash (CFA) has attracted the increasing attention of scholars because of its high content of rare earth. To extract and separate REE from the leaching solution of CFA, the method of solvent extraction after acid leaching is usually adopted. In this paper, the leaching solution of coal fly ash from Panbei, south China, with an average REE content of 478 μg/g, was taken as the research object. The extracted di-2-Ethylhexyl phosphonic acid (P204) was used to explore the solvent extraction effect and mechanism. When performed with the conditions: pH value of 2.1, oil-water ratio (O/A) of 1, extraction time of 25 min, solvent concentration of 6% and temperature of 30 ℃, the extraction rates of La, Ce, Pr, Nd, and Y were 89.16%, 94.11%, 95.56%, 96.33%, and 99.80%, respectively. It was indicated that the P204 extraction system separated REE well from the aqueous phase. The structure of the extraction complex was deduced by taking yttrium as an object of analysis, and the extraction mechanism equation was determined by using the slope method. In this extraction system, the molecular formula of the complex is YCl₂(HA₂) and the enthalpy change (△H) is +86.68 kJ/mol, which provides theoretical guidance for the extraction of REE in industrial production.

1. Introduction

Rare earth elements (REE) play an increasingly important role in the fields of ceramics, catalyst synthesis, etc., due to their unique physical and chemical properties [1,2,3]. With an ever-increasing market demand, a large number of rare earth mines have been developed, resulting in overexploitation in the past few decades. Therefore, the acquisition of new primary or secondary resources of rare earth has become a new research hotspot [4,5,6,7,8,9,10].

Coal fly ash (CFA), derived from coal combustion, is a kind of industrial waste residue with large emissions worldwide. To achieve the goal of CFA recycling, scholars have conducted research on the extraction and utilization of Al, Fe, Si, and so on [11,12,13,14,15]. As far as the recovery of rare earth elements is concerned, it was reported that the average content of REE in the global CFA is approximately 404 μg/g reported by Dai et al. [16], so CFA is sometimes regarded as an artificial rare earth deposit [17]. At present, the U.S. Department of Energy has put forward a plan to recover REE from coal and its by-products [18,19]. In Europe, many CFA samples were analyzed to evaluate the potential of REE recovery [20]. Researchers in China also explored the extraction of REE from CFA recently [20,21,22,23,24].

Among most rare earth recovery processes that have been studied so far, there are mainly two parts: leaching and purification. Firstly, REE are transferred from the solid phase to the liquid phase by means of acid addition (after calcination). Generally, acid leaching is used for REE recovery from coal-based resources [25,26,27,28,29]. Afterward, a purification process is applied for the separation of the target product and impurities. The conflict of symbiont of REE in CFA and high purity REE product results in the great significance of the purification. In addition, it is very difficult to separate the coexisting mixed rare earth elements into high-purity single rare earth elements because of the similarity of REE electronic layer structure and physicochemical properties caused by lanthanide shrinkage.

At present, the purification technologies of REE include solvent extraction, fractional precipitation, crystallization, ion exchange, redox, membrane method, etc [30,31,32,33,34,35]. Among them, solution extraction being studied for REE recovery, considering its efficiency, fundamentality, selectivity, large capacity, easy continuous operation, etc [36,37,38,39]. Although few reports on the solvent extraction of REE in the leaching solution from CFA, research on solvent extraction of REE for other source leachate provide abundant references and necessary guidance [40,41]. Di-2-Ethylhexyl phosphonic acid (P204), a phosphoric acid, is widely used in the rare earth separation industry because of its high-extraction rate, high selectivity, stable properties, and easy obtainability. In the current research on the extraction and purification of REE by P204, most of them are about scandium and heavy rare earth [42,43], while the research on light rare earth and Y is relatively few. Furthermore, the research on the specific mechanism of P204 needs to be further detailed.

In this paper, the extraction of REE (using La, Ce, Pr, Nd, and Y as proxy) with P204 from the CFA acid leachate was studied. The optimum extraction conditions were explored from the aspects of solution acidity, contact time, the volume ratio of extractant, total volume fraction of extractant, oil-water ratio, and temperature. Then, the mechanism of the process was revealed through the analysis of the reaction order by slope method and thermodynamic calculation. This research aims to offer a reference for REE recovery from CFA leachate using solvent extraction.

2. Materials and Methods

2.1. Materials

The CFA samples were collected from Panbei power plant, of which the characteristic was reported in the previous literature [44]. It mainly consisted of mullite, magnetite, quartz, and anhydrite. The leachates were obtained from the CFA by stirring and leaching for 2 h at 300 r/min with 3 M hydrochloric acid and a liquid-solid ratio (v/m) of 10 at 80 ℃. The extractant diphosphate (2-Ethylhexyl) (trade name: P204, analytical pure), was purchased from Hefei Green Technology Company of China. Sinopharm Chemical Reagent Co., Ltd. provides n-heptane, and hydrochloric acid (analytically pure). Deionized water was used for all experiments in this study.

2.2. Extraction Experiment

A series of the organic phase was prepared via mixing different proportions of P204 and kerosene as extractants. The leaching solution of coal fly ash and the organic phase were placed in the separating funnel according to a certain oil-water ratio. Then the vessel was put into a constant temperature water bath box (produced by Changzhou Jiangnan experimental instrument factory) to react for a period of time at a constant temperature to reach a separation of REE. When the reaction was complete, the raffinate (aqueous phase) was separated from the bottom of the separating funnel. The concentration of REE in the water phase was determined by an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, Agilent 7900, Santa Clara, USA), and the concentration in the organic phase was calculated by the subtraction method based on the conservation of matter. The instruments’ parameter used in this experiment are as follows: pH meter Leici, Shanghai, China), X-ray diffraction (XRD, BRUKER AXS, Bremen, German).

2.3. Experiment on Extraction Mechanism

The constant molar method and slope method were used to explore the extraction equation and the molecular formula of the extracted compound. The organic phase and the aqueous phase were mixed in a separating funnel according to a certain proportion, and put in a water bath constant temperature oscillation box to react for a period of time. When completed, the aqueous phase was separated from the bottom of the separating funnel. The concentration of rare earth ions in the aqueous phase was measured by the ICP-MS, and the concentration in the organic phase was obtained by subtraction method.

2.4. Analysis of Experimental Data

The effect of solvent extraction was evaluated based on the parameter of the distribution ratio (D) and extraction rate (E) in Equation (1) and Equation (2), respectively. The D is the ratio of the element concentration in the organic phase to that ion in the water phase. The E is the ratio of the amount of REE in the organic phase to that in the leachate.

(1)$\mathrm{D}=\frac{{\mathrm{C}}_{\mathrm{o}}}{{\mathrm{C}}_{\mathrm{a}}}$

(2)$\mathrm{E}=\frac{{\mathrm{C}}_{\mathrm{L}}{\mathrm{V}}_{\mathrm{L}}{-\mathrm{C}}_{\mathrm{a}}{\mathrm{V}}_{\mathrm{a}}}{{\mathrm{C}}_{\mathrm{L}}{\mathrm{V}}_{\mathrm{L}}}=\frac{{\mathrm{C}}_{\mathrm{o}}{\mathrm{V}}_{\mathrm{o}}}{{\mathrm{C}}_{\mathrm{L}}{\mathrm{V}}_{\mathrm{L}}}$

where ${\mathrm{C}}_{\mathrm{L}}$ is the concentration of REE in the leachate. is ${\mathrm{V}}_{\mathrm{L}}$ is the volume of the leachate. ${\mathrm{C}}_{\mathrm{o}}$ is the concentration of REE in the organic phase, ${\mathrm{V}}_{\mathrm{o}}$ is the volume of the organic phase, ${\mathrm{C}}_{\mathrm{a}}$ is the concentration of REE in the aqueous phase, and ${\mathrm{V}}_{\mathrm{a}}$ is the volume of the aqueous phase.

3. Results and Discussion

3.1. Properties of the CFA Leachate

Five rare earth elements (La, Ce, Pr, Nd, Y) were chosen as the proxy of REE in the leachate, and their contents are shown in Table 1. XRD characterization of the dried leached residue is shown in Figure 1. It can be observed that the leached residue is mainly composed of quartz and anhydrite by comparing it with the standard spectrogram. The existence of quartz was resistant to acid, and hardly dissolved during acid leaching. The majority of anhydrite also remained in the acid solution, so there is a peak (denoted as A) at 25°. The main components of magnetite and mullite were iron oxides and alumina, which easily interacted with acids, so they were leached into the solution and their peaks basically disappeared. Through XRD characterization, it can be found that the silicon and calcium elements in CFA were hardly leached while the major elements aluminum and iron were easily leached.

3.2. Exploration of Optimum Conditions

The effects of the extractant volume, the concentration of extractant P204, the pH value of the solution, the extraction time, the operating temperature, and the oil-water ratio on the experimental process were studied. The control varieties method was used. The reference conditions for setting the benchmark are as follows: a pH of 2.1, an extraction time of 25 min, temperature of 25 °C, oil-water ratio (O/A = 1:1), and the volume concentration of the extractant 6%.

3.2.1. Effect of Concentration of P204

The extractant concentration was thought as one of the most influential factors during the solvent extraction. Thus, the effect of the concentration was first investigated and the results were plotted in Figure 2. Overall, the extraction rates of REE in the leaching solution (La, Ce, Pr, Nd, and Y) are positively correlated with the volume concentration of P204, though all have a concave between 6% to 8%. The value of the extraction rate shows a large spread, from 22% to 98%. When the volume concentration of P204 is lower than 4%, the extraction rate climbs up noticeably, while the extraction rate increases slowly within the range of 4% to 6%. When the volume concentration is in the range of 8% to 10%, the extraction rate shows a slow growth trend. It was worth noting that the order of extraction rate is: La < Ce < Pr < Nd; and consisted of the rule of lanthanide shrinkage. Although the atomic number of Y is small, it is often used as a heavy rare earth element because of their similar properties. The peak of Y extraction rate appeared at the concentration of 6%, and then, there was a small decline and stabilized at about 85%. As a result, the extraction rate could increase with the increase in the concentration of P204. However, the extraction separation time also increased due to the emulsification phenomenon. The extraction rate of REE could not be significantly improved by high concentrations of P204. Thus, the best extractant volume concentration was set as 6%.

3.2.2. Effect of PH Value

To obtain the relationship between the PH value and the extraction rate, different solutions with PH values of 1.3, 1.5, 1.7, 1.9, and 2.1 were pre-made for specific experiments. The variation in extraction rates is shown in Figure 3. With the increase in PH value, the extraction rates of the REE all show an upward trend. It can be indicated that the lower the acidity is, the better the extraction results are. When the pH of the solution was 1.3, the extraction rate of La was almost zero, indicating that La was hardly extracted in this acidic environment, while the extraction rates of Ce, Pr, Nd, and Y were 19.54%, 29.46%, 33.29%, and 17.91%, respectively. The overall extraction rate was at a low level. As the pH increased from 1.3 to 1.5, the extraction rate of La, Ce, Pr, Nd, and Y sharply increased and reached 33.83%, 50.98%, 56.29%, 58.53%, and 61.87%, respectively. When the PH varied from 1.5 to 1.9, the extraction rate slowly increased. Because the extractant released H+ during the extraction process, resulting in the increase of the H+ concentration and the loss of extractant, which was not conducive to the forward reactions. As the pH continued to increase to 2.1, the extraction rates of La, Ce, Pr, Nd, and Y reached 77.53%, 86.77%, 89.49%, 90.42%, and 97.75%, respectively. As far as the reaction mechanism was concerned, the key was ion exchange, as Figure 4 showed. P204 is an acidic extractant. During the process of extraction, P204 releases hydrogen ions on ·OH, which exchange with the extracted cation, resulting in the metal cation entering the organic phase. The lower the acidity of the aqueous phase, the more favorable the reaction. The higher the acidity is, the more difficult the ion exchange will be. Under the same pH value, the extraction rate of La, Ce, Pr, and Nd increased with the increase of atomic number. The easier it is for P204 to extract metal ions, the more difficult it is to peel off. Therefore, the pH of 2.1 was used in other experiments in this paper.

3.2.3. Effect of Reaction Time

The reaction time, as a key factor affecting the yield of the extraction, was also targeted to explore the impact of the extraction rate. The results of the extraction rate in the range of 5 to 15 min are shown in Figure 5. It was found that the extraction rate grew with the extension of reaction time. Specifically, there was a clear climb from 5 min to 15 min. The plateau appeared from 15 to 25 min in the curve of Ce, Pr, Nd, and Y, and the extraction rates reached around 82%, 84%, 87%, and 81%, respectively. La reached an equilibrium state in 20 min with an extraction rate of 80.72%. At the time of the range studied, the shorter the extraction time was, the lower the extraction rate was due to the two-phase insufficient contact. On the contrary, the more full the oil-water two-phase contacting was, the more favorably the REE were extracted into the organic phase, and the higher the extraction rate was. Moreover, it facilitated the separation of the two phases while avoiding emulsification. The trend of extraction of La, Ce, Pr, Nd, and Y conformed to the decrease of the atomic radius increases, namely, La (Ⅲ) < Ce (Ⅲ) < Pr (Ⅲ) < Nd (Ⅲ) < Y (Ⅲ). The radius of yttrium ions was between holmium and erbium, its extraction rate was higher than that of the four light rare earth elements. After comprehensive consideration, 25 min was chosen as the most appropriate time for the next study.

3.2.4. Effect of O/A Ratio

In the study of the effect of the oil-water ratio on the test results, the oil-water ratio, as the independent variable, was respectively set as 0.2, 0.4, 0.6, 0.8, and 1 for discussion. The experimental results are depicted in Figure 6. It shows that the extraction rates of La, Ce, Pr, Nd, Y increased with the increase in oil-water ratio. When the oil-water ratio was 1, the extraction rates of La, Ce, Pr, Nd, and Y reached 80.65%, 88.76%, 90.47%, 91.22%, and 98.56%, respectively. The higher the oil-water ratio was, the higher the organic phase concentration was, and the more extractant dissociated in the aqueous phase. As a result, REE can be more easily extracted into the organic phase. Meanwhile, the higher the extraction rate needed the longer the phase separation time. Based on the experimental effect and cost, the oil-water ratio of 1 is more suitable.

3.2.5. Effect of Temperature

In terms of the effect of temperature on the experiment, the following temperatures (15 ℃, 20 ℃, 25 ℃, 30 ℃, and 35 ℃) were set for a comparison study, and experimental results are shown in Figure 7. The trend of extraction rate was found to goes upward followed by downward, indicating that the extraction process is endothermic. The higher temperature was more conducive to the positive progress of the reaction. For the temperature range studied in this experiment, the extraction rate of Y changed slightly, only from 93.03% to 99.80%. It can be considered that Y can be almost completely extracted within this temperature. Nonetheless, from 15 ℃ to 30 ℃, the extraction rate of La, Ce, Pr, and Nd sustained an increase. The extraction rate increased from 55.65%, 62.87%, 67.58%, and 71.99% at 15 ℃ to 89.16%, 94.11%, 95.56%, and 96.33% respectively. However, at 35 ℃, the extraction rate of several rare earth elements decreased slightly. The possible cause is that the high temperature destroys the structure and activity of extractants, resulting in a decrease in extractant concentration and extraction rate. This led to the conclusion that too of a high temperature is not conducive to the improvement of extraction rate. Therefore, a temperature of 30 ℃ was more appropriate.

3.3. Extraction Mechanism

According to the above research and analysis, as will be readily seen, the extraction rate of yttrium ion was higher than that of four light rare earth elements. Therefore, yttrium was taken as a proxy of all rare earth elements to explore the mechanism of the extraction process.

The process of rare earth ion extraction with P204 mainly includes the following steps: (1) P204 dissolution distribution equilibrium in two phases; (2) P204 dissociates in the aqueous phase; (3) Rare earth ions complexed with dissociated P204 anions in the aqueous phase; (4) The complexes formed above are dissolved in the organic phase.

P204 exists in the form of dimer in polar solvents (represented by H₂A₂ in the following). According to the extraction mechanism of acid extractant and considering its complexation with chloride ions, there exists the following equation:

(3)${\mathrm{Y}}^{3+}+\mathrm{m}\left({\mathrm{H}}_2{\mathrm{A}}_2\right)+{\mathrm{nCl}}^{-}\leftrightarrow {\mathrm{YCl}}_{\mathrm{n}}{\left({\mathrm{HA}}_2\right)}_{\mathrm{m}}+{\mathrm{rH}}^{+}$

3.3.1. The Determination of the Chemical Equation by Slope Method

The concentration of P204 and Y^{3 +} was fixed as 0.3 M and 0.01 M for the experiments to easily reveal the chemical reaction mechanism, respectively. The effect of pH on the distribution ratio of Y³⁺ in the P204 system was investigated, and the relationship between lg D and pH was drawn. As shown in Figure 8, the distribution ratio D of Y³⁺ increases with pH in the aqueous phase. The linear slope is about 1, i.e., r equals 1, which means that each Y³⁺ extracted by P204 will release an H⁺. The distribution ratio is inversely proportional to the concentration of H⁺ in the aqueous phase.

The influence of the concentration of P204 on the extraction ratio of Y³⁺ using the simulated solution with the Y³⁺ concentration of 0.01 M was investigated. The relationship between lg D and lg[H₂A₂] was drawn. It can be seen from Figure 9 that the slope is about 1, i.e., m equals 1, which demonstrates that each time a Y³⁺ is extracted, there will be an H₂A₂ involved in the reaction.

The anions in the aqueous phase have an influence on the extraction process to some extent. When REEs are in contact with anions such as chloride, nitrate, and sulfate, they complex with these anions to form various forms of species [45,46]. For example, with chloride, they present in the solution as REE³⁺, REECl²⁺, REECl₂⁺, REECl₃, and REECl₄⁻ (where REE stands for trivalent rare earth elements). In this study, only REE³⁺ is considered in the analysis, just to show the interaction between REEs and the solvent in the organic phase. Setting the experimental conditions as Y³⁺ 0.01 M and extractant concentration as 0.3 M, the effect of Cl⁻ concentration on the distribution ratio of Y³⁺ extracted was investigated, and the relationship between lg D and lg [Cl⁻] was established. It can be seen from Figure 10 that the slope is about 2. Every Y³⁺ is extracted, and there will be two Cl⁻ involved in the reaction. Therefore, it can be determined that n in the Equation (3) is equal to 2.

In summary, the behavior of P204 extracting Y³⁺ belongs to the cation exchange process, and the reaction formula can be expressed as:

(4)${\mathrm{Y}}^{3+}+{\mathrm{H}}_2{\mathrm{A}}_2+2{\mathrm{Cl}}^{-}\leftrightarrow {\mathrm{YCl}}_2\left({\mathrm{HA}}_2\right)+{\mathrm{H}}^{+}$

3.3.2. Thermodynamic Study on Extraction of Y³⁺ by P204

As the reaction formula shown in Equation (3) and the definition formula of D, the distribution ratio, the following equation can be established by extracting the equilibrium constant formula through a series of calculations:

According to Equation (4), the following equation of extraction equilibrium constant K can be obtained:

(5)$\mathrm{K}=\frac{\left[\mathrm{Y}{\left(\mathrm{Cl}\right)}_2\left({\mathrm{HA}}_2\right)\right]\left[{\mathrm{H}}^{+}\right]}{\left[{\mathrm{Y}}^{3+}\right]\left[{\mathrm{H}}_2{\mathrm{A}}_2\right]{\left[{\mathrm{Cl}}^{-}\right]}^2}$

According to the definition of partition ratio D, the concentration ratio of yttrium in the organic phase to that in the aqueous phase can be expressed as follows:

(6)$\mathrm{D}=\frac{\left[\mathrm{Y}{\left(\mathrm{Cl}\right)}_2\left({\mathrm{HA}}_2\right)\right]}{\left[{\mathrm{Y}}^{3+}\right]}$

The extraction equilibrium constant K can be expressed by the distribution ratio D by substituting Equation (6) into Equation (5) and taking the logarithm. After simplification, the following formula can be obtained.

(7)$\lg \mathrm{K}=\lg \mathrm{D}-\mathrm{pH}-2\lg {\mathrm{Cl}}^{-}-\lg \left[{\mathrm{H}}_2{\mathrm{A}}_2\right]$

Under the condition of temperature change only, the following equation can be obtained by the differentiation of the Equation (7) with T (thermodynamic temperature) (the weak influence of temperature change on pH value and the concentration of Cl⁻ and P204 were ignored:

(8)$\frac{\mathrm{d}\lg \mathrm{K}}{\mathrm{dT}}=\frac{\mathrm{d}\lg \mathrm{D}}{\mathrm{dT}}$

Combining the Van’t Hoff equation with Equation (8), the formula about D and Δ H was derived as follows:

(9)$\lg \mathrm{D}=-\frac{\Delta \mathrm{H}}{2.303\mathrm{RT}}+\mathrm{C}$

where R = 8.314 J·mol⁻¹·K⁻¹ is the gas constant. T is the absolute temperature, and C is the constant term.

Based on Equation (9), the partition ratios with different temperatures were determined under the above reference conditions. The relationship between lg D and 1000/T(K) was created in Figure 11. It can be seen from Figure 11 that the slope of the fitted curve is less than 0, indicating that the extraction process is an endothermic reaction. It was calculated after linear fitting, △H = +86.68 kJ/mol. It shows that low temperature is not conducive to the extraction of yttrium, and increasing the temperature is conducive to the occurrence of the reaction within a certain range.

4. Conclusions

In this research on the extraction of REE from the CFA of Panbei leaching solution with the extractant P204, the extraction rates of La, Ce, Pr, Nd, and Y were 89.16%, 94.11%, 95.56%, 96.33%, and 99.80%, respectively, and the optimum experimental conditions were obtained as follows: pH value of 2.1, oil-water ratio (O/A) of 1, extraction time of 25 min, solvent concentration of 6%, and temperature of 30 ℃. The results proved that P204 has a strong extraction ability for REE under optimal conditions, especially since yttrium was almost completely extracted.

By using the slope method, it can be inferred that YCl₂(HA₂) is the extraction complex of yttrium extracted by P204. The extraction mechanism can be expressed as ${\mathrm{Y}}^{3+}+{\mathrm{H}}_2{\mathrm{A}}_2+2{\mathrm{Cl}}^{-}\leftrightarrow {\mathrm{YCl}}_2\left({\mathrm{HA}}_2\right)+{\mathrm{H}}^{+}$. By studying the thermodynamic process of extraction, it was found that the extraction system is endothermic with △H = +86.68 kj/mol.

Author Contributions

Conceptualization, J.P. and X.Z.; methodology, J.P.; software, J.P. and Ji, W.J.; validation, J.P.; formal analysis, J.P. and F.Y.; investigation, W.J.; resources, W.J.; data curation, W.J.; writing—original draft preparation, X.Z. and F.Y.; writing—review and editing, J.P. and C.Z.; visualization, J.P.; supervision, J.P. and C.Z.; project administration, J.P.; funding acquisition, J.P. and C.Z. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

We appreciate the reviewers’ accurate and detailed comments.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

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Figure 1. XRD pattern of coal fly ash slag.

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Figure 2. Effect of extractant volume concentration on the extraction of REE.

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Figure 3. Effect of solution pH on the extraction of REE.

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Figure 4. Schematic of the two-phase reaction during the solvent extraction.

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Figure 5. Effect of extraction time on the extraction of REE.

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Figure 6. Effect of phase ratio on the extraction of REE.

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Figure 7. Effect of temperature on the extraction of REE.

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Figure 8. The relationship between the lg D of Y3+ and the equilibrium pH.

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Figure 9. The relationship between the lg D of Y3+ and lg [H2A2].

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Figure 10. The relationship between the lg D of Y3+ and lg [Cl−].

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Figure 11. Effect of temperature on P204 extraction of Y3+.

Table 1

Rare earth elements composition of coal fly ash leaching solution.

Element	Y	La	Ce	Pr	Nd
Content(ppm)	21.40	49.99	99.49	10.77	41.67

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