Characteristics of Electrical Resistance Alteration during In-Situ Leaching of Ion-Adsorption Type Rare Earth Ore

1. Introduction

Rare earth elements (REEs), especially for the mid and heavy REEs, are essential in high-tech industries such as renewable energy, national defense and new materials preparation [1,2,3]. Owing to their special 4f electron layer structure, they can exhibit some unique optical, magnetic, and electrical properties [4,5]. Ion-adsorption rare earth ores are rich in medium and heavy rare earth elements [6,7,8,9] and are strategic mineral resources [10,11]. The rare earth content of ionic rare earth ores is generally 0.05–0.2 wt.% REO [12]. It usually presents in four phases: water soluble phase, ion-exchanged phase, mineral phase and colloidal sediment phase [13,14,15]. The ion-exchanged phase accounts for 60-90% of the whole-phase rare earth [16,17] and the ion-exchangeable phase rare earth is adsorbed on the surface of clay minerals such as kaolin and mica in the form of hydrated cations or hydroxyl hydrated ions [18,19,20].

The surface of the clay mineral is negatively charged and has a fixed surface electrostatic potential, which attracts counterions into the electrolyte solution to form a double layer [21,22]. The ion-adsorption type rare earth ore is mainly composed of the clay minerals, which will attract rare earth ions in the natural environment, forming double elecrotic layer. The leaching process of ion-adsorbed rare earth is essentially a process of cation adsorption by the lixiviant, desorption of rare earth ions, and redevelopment of the double elecrotic layer on the clay surface [23]. The thickness of double layer is related to the valence number of ions in the leach solution, ion concentration, temperature, and pH [19,24]. An inorganic salt solution is used as the lixiviant, and rare earth elements are leached through ion-exchange [25,26,27]. Ammonium sulfate is a widely used lixiviant for ion adsorption rare earth due to its advantages of high exchangeability of ammonium ions and low cost [18,28,29]. Ammonium ions in the lixiviant are exchanged with rare earth ions, and rare earth ions enter the leaching solution [30,31]. The equation for the ion-exchange reaction that occurs during leaching is as follows (where s represents the solid phase; aq represents the liquid phase):

[Al₄Si₄O₁₀(OH)₈]_m•nRE³⁺(s)+3nNH₄⁺(aq)⇌ [Al₄Si₄O₁₀(OH)₈] _m•[NH₄⁺]_3n(s)+nRE³⁺(aq)

(1)

Clay mineral particles have an unbalanced charge on their surfaces and have a range of electrochemical properties [32], such as conductive properties of electrolyte and ion transport properties. The migration of ions and chemical reactions at the liquid-solid interface also trigger electrical phenomena during in-situ leaching. Electrochemical impedance spectroscopy has been widely applied by scholars to investigate the mechanical properties of contaminated soil remediation, the corrosion behavior of cement materials, the mechanical properties of concrete materials, the microscopic pore structure of solid waste materials, and the hydration process of cement [33,34,35,36,37]. This method can reflect the chemical reactions and microstructural changes occurring within the substance under test and reveal the interfacial reactions between the conducting phases. Soil is a three-phase medium of solids, liquids, and gases that can be structured as an electrochemical system. Ion-adsorption rare earth are clay minerals and the EIS technique is equally applicable. Physical and chemical interactions between the leaching solution and the mineral particles occur during the leaching process of rare earth ore, which can be continuously monitored using EIS technology.

In this study, electrochemical impedance test of the ammonium sulfate leaching process was carried out by combining the previous studies on the EIS theory and its application by Macdonald J R [38], Meilun Shi [39] and Chunan Cao [40], as well as by referring to the studies on the electrochemical impedance change of clay and its influencing factors by Zhao [41], A. Elmelouky [42] and others. Combined with Nyquist and Bode diagrams and equivalent circuit element parameters, the electrochemical characterization of the whole leaching process was monitored. An equivalent circuit model applicable to the leaching process was established to explore the resistance change of the leaching process of rare earth ores, to reveal the charge migration law in the pore solution and at the solid-liquid interface, and to analyze the changing law of the pore structure and the chemical reaction in the leaching process. The electrical resistance variation characteristics of ion-adsorbed rare earth ore during in-leaching process can reflect the rare earth leaching process and leaching efficiency, which can provide more ideas for the intelligent mining of rare earth ores, the optimization of leaching process and the improvement of leaching efficiency.

2. Materials and Methods

2.1. Test Materials

The raw materials were from an ionic rare earth mine in Dingnan County, Ganzhou City, Jiangxi Province, China. The water content of the rare earth ore was determined to be 11.2%. The particle size distribution of the raw ore is shown in Table 1.

Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used to detect the rare earth content of different particle sizes, and the results are shown in Table 2.

As shown in Table 1 and Table 2, the proportion of mineral particles with a particle size of less than 1.43mm is greater than 50%, indicating that the original ore has a finer particle size and a higher degree of weathering. Mineral particles below 0.15mm in size have the highest rare earth content, accounting for 53.82% of the total rare earth and smaller size mineral particles contain higher rare earth content.

2.2. Test Methods

2.2.1. Column Immersion Test

Ionic rare earth in-situ leaching simulation tests were conducted using ammonium sulfate solution as lixiviant. Some rare earth ore samples were weighed and dried at 100°C for 12h. 200 g of dried rare earth ore samples were weighed, mixed thoroughly, and added to a leaching column of Φ59 mm*90 mm. A filter paper was placed above the samples, and a test electrode was fixed into the rare earth ore samples. A funnel and a collecting cylinder were placed below the column, with filter paper inside the funnel. Ammonium sulfate was used as the lixiviant, with a concentration of 0.1 mg/mL, a flow rate of 1.5 mL/min, a solid-liquid ratio of 1:2, pH=4, and ambient temperature.

2.2.2. Electrochemical Impedance Spectroscopy Tests

Electrochemical impedance spectroscopy is a method of electrochemical measurements in which a sinusoidal potential (or current) of a certain amplitude is used as a perturbation signal, resulting in an approximately linear relationship between the perturbation and the system's response. EIS testing was performed throughout the column leach test using the admiral squidstat series of electrochemical equipment. The leach solution was collected at regular intervals. The parameters of the rare earth leaching process were set as follows: AC sinusoidal signal amplitude of 10 mV, test frequency of 10^-2–10⁶ Hz, and three-electrode method, with the working electrode being a graphite electrode, the auxiliary electrode being a platinum electrode, and the reference electrode being an Ag/AgCl electrode. The Nyquist and Bode plots obtained from the test were fitted and analyzed using Zahner Analysis software to obtain the electrochemical component parameters of the ionic rare earth during the in-situ leaching process. Figure 1 shows the test equipment of this study.

3. Equivalent circuit model

3.1. Analysis of Conductive Pathways

The ionic rare earth in-situ leaching process includes three media: solid, liquid, and gas. The internal conductive path of the ore body mainly includes the conduction of the rare earth particles, the conduction between the particles pore liquid and the conduction between the rare earth particles and the solid-liquid interface of the pore liquid [43,44]. The conductive path is shown in Figure 2.

Among the three conductive paths, the first conduction is rare earth particles. Since they contain a certain amount of water and exist in the natural electrolyte solution for a long time, they are electrically conductive. However, the conductivity of the rare earth particles is minimal and can be ignored due to the limited water content and electrolyte solution concentration. The second type is the pore liquid conductivity in the in-situ leaching process, which is mainly induced by the ion-exchange reaction. The displacement reaction between pore liquid cations and rare earth cations leads to the migration of ions in pore liquid. The third type of solid-liquid interface conductivity is mainly caused by ion-exchange reactions. The second and third conductive effects are interrelated and interdependent , resulting from the ions in the pore liquid and the surface of rare earth particles have always existed in the physical and chemical reactions in the effective reaction stage of in-situ leaching. The ions in the pore liquid enter the solid-liquid interface through the ion-exchange reaction and eventually migrate to the surface of the rare earth particles.

3.2. Equivalent Circuit Model

An equivalent electrical circuit consists of continuously connected elements such as resistance, capacitance and inductance. The method is called the equivalent circuit method when the frequency response properties of the test material are the same as the electrical circuit described above. The equivalent circuit method can test the AC impedance spectrum because different electrochemical components have different frequency response times under the same current passage conditions. Resistive elements have the fastest response to current compared to other element and the capacitor can be considered a wire when the frequency is high. No electrochemical transfer occurs because it takes time for the ions to diffuse from the beginning of their movement. Therefore, the low-frequency region is controlled by the diffusion process.

In electrochemical reactions, there are two simultaneous processes: The first is a non-faraday process involving charging and discharging of the double layer capacitance when electrode potential is changed. The second is a faraday process involving charge transfer and diffusion at a certain electrode potential, which is represented by the series connection of R_t and Z_w. These two processes are represented by a parallel circuit and there is a "dispersion effect" in the equivalent circuit fitting process. Due to the inhomogeneity of the electrode surface, the double layer capacitance is not a constant but a value that changes with frequency, and this phenomenon of capacitance change with frequency is called the dispersion effect. The dispersion effect reflects the instability of the parameters and the constant phase angle element CPE is used to represent the double layer capacitance to avoid this phenomenon. In the present study, based on the two processes of electrochemical reaction and leaching mechanism of ion-adsorption rare earth, the equivalent circuit of circuit elements was established as a quasi-Randles model based on the Randles model [45], as shown in Figure 3.

In Figure 3, Rs is the solution resistance, indicating the conductivity of the pore solution. Rt is the charge transfer resistance, reflecting the ease of charge transfer. Zw is the diffusion impedance, indicating the impedance of the reactants diffusing from the solution to the reaction surface of the electrode. CPE is the constant phase-angle element, which indicates the double elecrotic layer at the interface between the electrode and the solution, equivalent to a capacitor.

Figure 4 shows the Nyquist diagram of the equivalent circuit model. The high-frequency region shows a circular arc, which is controlled by the dynamics. The low-frequency region shows a slash at an angle of 45° to the real axis, which is controlled by material transfer (diffusion control). The curve intercept to the real axis in the high-frequency region is Rs, the diameter of the semicircle in the high-frequency region is Rt, and the intercept of the slanting line in the low-frequency region on the real axis is σ. The diameter of the impedance arc is deflected downward by an Angle φ relative to the real axis, which is called the dispersion angle. The CPE is represented by the two parameters, CPE-T and CPE-P, in which CPE-T denotes the charged amount of the double layer and the capacitance of its diffusion layer, and CPE-P is a parameter related to the diffusion angle ϕ, where ϕ is expressed as:

ϕ = π(1−CPE-P)/2

(2)

Z_w is the diffusion impedance, which can be expressed by the diffusion coefficient:

Z_w =σω₃^-1/2 (1−j)

(3)

Where σ is the diffusion coefficient, (Ω-cm²)/s^1/2, ω₃ is the angular frequency, and j is an imaginary unit.

4. Results and Analysis

4.1. Nyquist Plot Analysis

Figure 5 shows the three-dimensional diagram of the AC impedance spectrum of the whole leaching process of ion-adsorption rare earth using ammonium sulfate leaching. In order to facilitate the study and comparative analysis, the whole leaching process was divided into four stages including the wetting stage (30-50min), reaction stage (50-80min), equilibrium stage (80-250min), and top water stage (250-350min), according to the reaction time, reaction phenomenon, and impedance value change of the reaction process. Among them, the wetting stage refers to the process from the beginning of the ore body to the leaching fluid outflow. The reaction stage refers to the process from the beginning of the leaching solution to the significant change in impedance. The equilibrium stage refers to the process in which the impedance value is relatively stable. In the top water stage, the leaching solution was finished and deionized water was added to flush the ore body until the end.

As shown in Figure 6, the Nyquist curve was composed of the capacitance reactable arc in the high-frequency region (greater than 1000Hz) and the inclined straight line in the low-frequency region (less than 1Hz), which conformed to the quasi-Randles model. It was found that the capacitive reactance arc and the slope of the line changed significantly in the wetting stage, reaction stage and top water stage, while changed little in the equilibrium stage. The capacitive reactance arc radius and the capacitive reactance arc intercept to the real axis in the wetting and reaction stages decreased with the increase of reaction the increase of reaction time, indicating that the solution and charge transfer resistance in the two stages decreased. In the wetting and equilibrium stage, the slope of the straight line and the straight line intercept to the real axis decreased, indicating that the diffusion coefficient σ decreased. The diffusion coefficient was proportional to the resistance, and the resistance decreased continuously.

It can be inferred that in the wetting stage and the reaction stage, the ion concentration in the pore liquid kept increasing and the ion-exchange reaction at the solid-liquid interface kept increasing. The diffusion rate of ions in the pore liquid of the orebody was getting faster and faster and the ion concentration difference at the solid-liquid interface was getting larger and larger, which was conducive to the diffusion.

In the equilibrium stage, the impedance amplitude changed little, and the capacitive reactance arc radius was small. The capacitive reactance arc intercept to the real axis and the intercept of a line to the real axis has not changed significantly, indicating that the resistance value changed little. The ion-exchange reaction tended to be stable and the resistance at the top water stage increased significantly. The capacitive reactance arc radius and the capacitive reactance intercept to the real axis increased with time. The linear intercept to real axis increased, the diffusion coefficient σ increased and the resistance increased. The results indicated that the ion concentration decreased and the ion-exchange reaction intensity decreased significantly with the addition of deionized water.

4.2. Bode Plot Analysis

Figure 7 shows the frequency-impedance modulus plot for the leaching process of ion-adsorption rare earth.

It can be seen from Figure 7 that the impedance modulus values in the four stages of the leaching process had a consistent decreasing with the frequency. In the frequency range of 10^-1-10², the curvature of the curve was large and the impedance modulus decreased linearly. In the frequency range of 10²-10³ Hz, the curvature of the curve and the impedance modulus decreased slowly. In the frequency range of 10³-10⁶ Hz, the change amplitude of the impedance modulus was small and tended to a certain value. During the leaching process, the impedance modulus decreased continuously in the wetting and reaction stages, then dropped to the lowest value and tended to be stable in the equilibrium stage. When deionized water was added, the impedance modulus gradually increased and tended to be stable in the final stage of top water.

As shown in Figure 8, the phase angle of the leaching process varies in the interval of 0-80°. In the wetting stage, the phase angle decreased with the frequency. The phase angle decreased rapidly when the frequency was 10-10³ Hz, and then decreased slowly when the frequency was less than 10 and more than 10³ Hz. In the reaction stage, the phase angle tended to increase first, then decrease and increase again. The phase angle increased rapidly and reached the peak when the frequency was less than 10 Hz, the phase angle decreased rapidly to nearly 0° when the frequency was 10-10³ Hz, and the phase angle increased again when the frequency was greater than 10 Hz. In the equilibrium stage, the phase angle peaked at 10^-1-10 Hz, decreased rapidly within 10-10³ Hz and remained basically stable within 10³-10⁶ Hz. In the top water phase, the phase angle showed an overall decreasing trend, with a slight decrease within 0-10 Hz, followed by a rapid decrease within 10-10 kHz, and remained relatively stable within 10 k-1000 kHz. The wetting and equilibrium phases each had a peak, and the peak in each equivalent circuit corresponded to one time constant, indicating that time constants existed in both reaction phases [45].

4.3. Equivalent Circuit Parameter Analysis

The Nyquist and Bode plots were fitted using Zahner Analysis software, and each equivalent circuit element parameter was fitted to obtain a fitting error of essentially less than 5%, as shown in Table 3.

According to Table 3, the trend diagram of solution resistance and charge transfer resistance during leaching was drawn, as shown in Figure 9.

Rs is the solution resistance, which mainly reflects the conductivity of the pore solution electrolyte of the orebody. The pore solution resistance is great and pore solution is difficult to conduct electricity when the Rs is large, which relates to the pore liquid and size [46,47]. From Table 3 and Figure 9, it can be seen that the solution resistance decreased at first and then increased during the leaching process. The solution resistance was 939Ω ∙cm² at the wetting stage(30min), reached the minimum value of 135Ω ∙cm² at the equilibrium stage(90min) and reached the maximum value of 1330Ω ∙cm² at the top water stage(350min). It could be calculated that the ore sample was unsaturated with an unstable internal structure at the wetting stage. The pores inside the ore sample contained a large amount of gas, and micro and small holes dominated the pore structure. The pore connectivity between clay minerals was poor, and the pore solution had poor electrical conductivity, so the initial resistance value was considerable. From the beginning of the wetting stage to the 90th minute of the equilibrium stage, the addition of ammonium sulfate leach made the orebody transition from an "unsaturated state" to a "saturated state". The gas in the pores was gradually replaced by pore solution, and the pore solution volume, the saturation and the conductive path 2 in Figure 2 increased. The micro and small pores in the ore body evolve into medium and large pores under the action of seepage, and the pore size of the ore body increased, the seepage effect was enhanced, and the solution connectivity was improved [48,49,50,51,52]. Due to the continuous addition of ammonium sulfate leaching solution, the number of ions in the pore solution increased, and the improvement of pore connectivity led to the increase of the freely moving anions number in the solution, so the conductive capacity of the solution was enhanced.

From the 90th to the 250th minute of the equilibrium stage, the solution resistance fluctuated within 135-154 Ω∙cm2 and showed a steady increase trend. The solution resistance in the equilibrium stage was significantly lower than that in the other three stages, indicating that an effective ion-exchange reaction was happening in the equilibrium stage. At the same time, the ore body had reached saturation [53] and the number of ions in the solution was relatively stable. Nevertheless, the pore structure of the ore body changed due to the ion-exchange reaction. The macropores number and the porosity decreased [54] and the solution connectivity deteriorated, which resulted in a slight increase in solution resistance with time in this stage. In the top water stage, the solution resistance showed a significant increasing trend due to the addition of deionized water inside the ore body. The ammonium sulfate solution and residual rare earth ions in the pore space were continuously discharged. The conductivity of deionized water was less than that of ammonium sulfate solution, so the resistance increased.

Rt is the charge transfer resistance, reflecting the ability of ions in the diffusion zone to complete charge transfer on the surface of ore body particles and the speed of electrochemical reaction [55], which is mainly determined by the ion concentration gradient and the adsorption characteristics of the ore body particles surface [24,26,27]. In the ion-exchange process, the ion-exchange reaction occurring on the surface of mineral particles desorbed rare earth ions to the binding liquid layer, resulting in a sharp increase in their concentration in the binding liquid layer. The adsorption of ammonium ions to mineral particles led to a rapid decrease in their concentration in the binding liquid layer, resulting in an apparent concentration gradient between the two layers, which promoted the directional migration of the two ions [56,57]. As shown in Table 3 and Figure 9, Rt generally presented a trend of first decreasing and then increasing, from the maximum value of 8,310ω ∙cm2 in the wetting stage to the minimum value of 21Ω∙cm2 at the 80th minute of the reaction stage. Rt reached 338Ω∙cm2 at the end of the top water, and the resistance value in the equilibrium stage was close to the minimum value with a slight change.

According to the analysis above, it can be seen that at the beginning of the wetting stage, as the leaching solution entered the ore body, it took some time for the solution to penetrate into the ore body and diffused to the surface of the ore body particles to arise.

ion-exchange reaction. Therefore, the initial charge transfer was difficult, and the resistance value had an instantaneous maximum value. In the wetting and reaction stage, the number of ions in the ammonium sulfate solution increased and continuously entered the "diffusion double layer " region on the surface of the particles with the continuous penetration of the leaching solution, which led to the exchange of RE3+ and NH4+. In this process, the number of charges in the diffusion layer at the solid-liquid interface increased continuously, forming a significant concentration difference and promoting charge transfer. Meanwhile, according to the leaching reaction equation (1), nRE3+ (aq) resolved from the surface of the mineral particles and then continuously flowed out with the leaching solution, which promoted a positive reaction. The electrical resistance value showed a significant decrease [28] and reached a minimum value at the end of the reaction stage. From the 80th to the 100th minute, the resistance was minor and close to the minimum value, indicating that the ion-exchange reaction was dominant in this period. The ore body was interfered by external factors quickly, but the degree of interference was basically the same, so the resistance value changed little.

From the 100th minute of the equilibrium stage to the end of the equilibrium stage, the charge translation resistance increased in general and fluctuated up and down irregularly. During this period, the ion-exchange reaction continued, but the ion migration in the solution was hindered due to the decrease of porosity. Therefore, the number of ions in the diffusion layer at the solid-liquid interface decreased, the charge migration was inhibited and the resistance increased. As more and more rare earth ions were replaced, the amount of negative charge carried on the surface of mineral particles increased and the water film become thicker, which led to stronger retention effect on the fluid, weaker penetration effect and lower charge transfer ability. However, compared with the other three stages, the "effective leaching" mainly occurred in ion-exchange reaction, so the resistance was small. In the top water stage, the addition of deionized water led to a decrease in the concentration of ions in the solution, and the continuous occurrence of leaching reaction also reduced the content of rare earth ions in the ore body, resulting in a significant decrease in the number of ions in the diffused double electric layer. The charge transfer became difficult, and the resistance value increased continuously until the end of the top water.

5. Conclusions

This paper used electrochemical impedance spectroscopy (EIS) to study the electrochemical characterization of ionic rare earth during the in-situ leaching process, and an equivalent circuit model was established. Through the calculation of the fitted EIS parameters, the changing law of electrochemical parameters in the leaching process was obtained, and the parameters were analyzed and interpreted. The main conclusions were drawn as follows:

(1) Based on the electrochemical characterization of the in-situ leaching process of ion-adsorption rare earth, an equivalent circuit model was proposed to reflect the electrochemical changes of the leaching process. The main circuit elements of the model included a solution resistor, a charge transfer resistor, a constant phase angle element, and a diffusion impedance. The solution resistance Rs and charge transfer resistance Rt were mainly analyzed. The solution resistance reflected the number of ions in the ammonium sulfate solution and the change of pore structure of the ore body during leaching. The charge transfer resistance reflected the ion concentration gradient and chemical reaction at the liquid-solid interface during the leaching process, which were the main factors affecting the leaching and leaching rates.

(2) According to the changing law of electrochemical parameters of the leaching process, the leaching process was divided into four stages: wetting stage, reaction stage, equilibrium stage and top water stage. During the leaching process, the resistance parameters Rs and Rt values tended to decrease first and then increase. The maximum value of Rs was 1330Ω∙cm2 at the end of the top water stage, and the minimum value was135Ω∙cm2 at the beginning of the equilibrium stage. The maximum value of charge transfer resistance was 8,310 Ω∙cm2 at the beginning of the leaching stage, and the minimum value was 21Ω∙cm2 at the end of the reaction stage. Rs and Rt in the equilibrium leaching stage were close to the minimum value and remained relatively stable.

(3) The values of Rs and Rt were negatively correlated with the size of the pore structure and the chemical reaction rate of ion-exchange. The pore structure was large and the rate of chemical reaction was great when the resistance parameter value was small. At this stage, the pore solution connectivity of the ore body was good and the solid-liquid interface was easy to form a large ion concentration gradient, which was conducive to chemical reactions.