A METHOD FOR LEACHING OF Nd, La, Ce RICH BASTNAESITE MINERAL IN DESIGNED IONIC SOLUTIONS WITH HIGH EFFICIENCY COMPARABLE TO CONCENTRATED ACIDS

Abstract

The invention relates to deep eutectic solvents (DES) developed through a molecular modeling approach. These solvents serve as environmentally friendly alternatives to high volumes of acids used during the leaching process of bastnaesite mineral. This is the initial step in the lengthy process of mineral processing, and these DES can replace traditional extraction methods. Moreover, the inputs used in creating these solvents have been recognized in the literature for their applications in green chemistry.

Description

[0001] DESCRIPTION

[0002] A METHOD FOR LEACHING of Nd, La, Ce RICH BASTNAESITE MINERAL IN DESIGNED IONIC SOLUTIONS WITH HIGH EFFICIENCY COMPARABLE TO CONCENTRATED ACIDS.

[0003] TECHNICAL FIELD

[0004] The invention pertains to deep eutectic solvents (DES) developed through a molecular modeling approach, utilizing alternative green-class chemicals. These solvents are intended to replace large quantities of acids used during the leaching process of bastnaesite mineral, which is the initial stage of its long processing journey. The innovative green DESs formulated can supplant traditional methods in the extraction phase from solid materials.

[0005] BACKGROUND

[0006] Rare earth elements (REE) are extracted from carbonate minerals, particularly bastnasite. This mineral is pivotal in producing REE with a purity of 98-99% in the form of pure metal, oxide, or chloride salt. The purity can be further enhanced through additional processing (Ozbayoglu et al., 1995; Kur§un et al., 2017). In Turkey, the Eski§ehir- Sivrihisar-Beylikahir region has been found to contain 21 million tons of reserves, with 1 .3 million tons of REE at a grade of 3.52%, primarily consisting of lanthanum (La), cerium (Ce), and neodymium (Nd).

[0007] The processing of bastnasite involves grinding and leaching the mineral, during which extraction occurs from the solid material. This process requires acids such as concentrated sulfuric acid, nitric acid, and hydrochloric acid, due to the challenges associated with dissolving carbonate rocks (Kul et al., 2008; Kur§un, Ozdemir et al., 2017). To address environmental concerns, the European Union's EROCON projects have focused on minimizing acid usage. Initiatives funded by the EU, including EURare and SoSRare, have been implemented in this context (Pearson and Long, 1998; Rousseau and Benaben, 2002; Endres et al., 2017).

[0008] Leaching processes typically require significant amounts of acids, which raises economic and environmental concerns (Krishnamurthy and Gupta, 2004; Kuzmin et al., 2012). Additionally, alternative methods that use high-temperature or alkaline solutions are often considered unprofitable (Chakhmouradian and Wall, 2012; Belova, 2017). Rare earth elements (REE) solutions are processed for metal recovery through intermediate extraction methods, yet these approaches also have economic and environmental drawbacks (Abbott et al., 2009; Royen and Fortkamp, 2016). In essence, many of these methods either consume substantial quantities of acid or utilize uneconomical processes, such as high-temperature energy demands, in lieu of acid usage. Furthermore, aside from methods that employ very high concentrations of hydrochloric and sulfuric acids, most leaching processes do not achieve complete efficiency (Krishnamurthy and Gupta, 2004; Kuzmin et al., 2012). The use of acid in these methods is essential because metal oxides and carbonates do not easily dissolve in molecular solvents and require strong acids for processing (Chakhmouradian and Wall, 2012; Belova, 2017). The aqueous solutions containing rare earth elements (REE) generated from this process then undergo intermediate extraction processes for metal recovery.

[0009] Researchers aim to minimize the use of volatile organic solvents and acids due to their potential to cause significant health problems. These substances can lead to irritation, damage, and rapid evaporation into the atmosphere. The long-term effects of exposure to these chemicals are concerning, as they are likely to contaminate natural resources such as water streams.

[0010] AIM OF THE INVENTION

[0011] The invention aims to use a type of low-cost environmentally friendly ionic liquids known as deep eutectic liquids (DES) instead of traditional acids used during the leaching process of bastnasite mineral. Within the scope of our invention, these green chemicals designed with molecular modeling allow the REE elements to pass into solution faster, more efficiently, and at lower cost with the catalytic effect of electrochemical techniques in DES. Given the environmental and cost challenges associated with conventional methods, regulatory organizations such as the IPCC, OSHA, and EPA have imposed restrictions on their use, promoting the adoption of greener alternatives. Projects like ERECON further encourage the development of environmentally friendly methods (Moldoveanu and Papangelakis, 2012; Lazo et al., 2017). Therefore, the exploration of alternative green chemistry applications is of significant importance.

[0012] The aims of the invention can be listed as follows;

[0013] I. Three different deep eutectic solvent (DES) solutions were prepared using environmentally friendly components, such as choline chloride and ethylene glycol, through a molecular modeling approach for the leaching process of bastnaesite minerals. These solutions were carefully designed according to specific mixing ratios to prevent mass transfer issues in industrial applications and to optimize system performance in electrochemical techniques. The suitability of these solutions for electrochemical applications was confirmed by measuring their viscosity and conductivity values.

[0014] II. The electrochemical stability ranges of the prepared DES solutions were scanned using a potentiostat with a three-electrode system. The data obtained determined the potential ranges in which each DES solution could be used without decomposition, and this information was taken as a fundamental reference for the electrochemical leaching processes to be applied in further stages.

[0015] III. Electrochemical tests were performed on specially designed electrodes containing bastnasite minerals in DES solutions using the potential ranges determined in Article II. In these tests, the bastnasite mineral was decomposed, and the ions in the mineral leached into the solution at anodic oxidation potentials, and the possible efficiency of the pulse electrocatalytic activation process was initially proven. The experiments were carried out using voltammetry and chronoamperometry techniques and the results were recorded with high magnification microscope cameras.

[0016] IV. The amounts of La, Nd, and Ce elements in the bastnaesite mineral obtained from the mine site were determined using X-ray fluorescence (XRF) analysis. This analysis was used to calculate the concentration values of the target elements that would be present if the mineral were fully dissolved in a specific volume of solution. This is a critical step for evaluating the efficiency of our technique in future leaching tests.

[0017] V. The supplied powdered bastnaesite ore was subjected to leaching through electrocatalytic activation using a two-electrode pulse system. This process involved simultaneous leaching in strong acid, base, and alkaline solutions. Finally, the efficiency of the developed pulse system in the developed solutions was determined.

[0018] LIST OF FIGURES

[0019] Figure 1 . Measurement of viscosity and electrical conductivity as a function of temperature for DES-I solution formed by mixing ChCI: EG: TA molecules in a molar ratio of 1 : 4: 1 was performed.

[0020] Figure 2. Measurement of viscosity and electrical conductivity as a function of temperature for DES-II solution formed by mixing ChCI: LA: MA molecules in a molar ratio of 1 : 2: 1 was performed.

[0021] Figure 3. Measurement of viscosity and electrical conductivity as a function of temperature for DES-I II solution formed by mixing EG: ChCI: AICL molecules in a molar ratio of 1 : 4: 0.1 was performed.

[0022] Figure 4. Voltammetric window width response study scans to investigate the decomposition of DES solution prepared with 1 molar solid crystalline form choline chloride (ChCI), 4 molar ethylene glycol (EG) in liquid form, and 1 molar solid crystalline form tartaric acid (TA) was performed at 55±5°C temperature with a scan rate of 20mV s’1using a 0.0176 cm2Pt disk working electrode, an 8 cm2Pt counter electrode, and a pseudo silver wire reference electrode.

[0023] Figure 5. A chronoamperometric test was performed to measure the stability of the solution under a stable 1.1V applied potential difference to the working electrode for 350 seconds in the DES-I solution. All conditions during the study were kept the same as those given in the cyclic voltammeter experiment.

[0024] Figure 6. Bastnasite mineral powders were placed on the working electrode ("J" shape house-made Pt electrode) in the A7 solution obtained by mixing the choline chloride, ethylene glycol, and tartaric acid (ChCI: EG: TA) in a molar ratio of 1 :4:1 , respectively, and cyclic voltammetry was performed between -0.35V and 1.4V at a scanning speed of 10mV s’1. The cyclic voltammetry was terminated after the oxidation peak formation of 1 .4V. The working temperature was 50±5°C. The counter electrode used was a Pt or iridium-coated titanium electrode (highly resistant to chemicals and high potentials) with a surface area of 20 cm2, and a pseudo silver wire was used as the reference electrode.

[0025] Figure 7 Represents the first 8 seconds of results (0-8 sec.) of the chronoamperometry test conducted at 1 ,4V for 600 seconds at a temperature of 50 ± 5°C. The chronoamperometry procedure was performed using a homemade "J" type platinum working electrode (the electroactive area is directed upwards), which has a diameter of 1.5 mm (the electroactive area is 0.0176 cm2). Bastnaesite powder was placed onto the working electrode while it was submerged in the DES-I solution. The three-electrode system utilized an inert counter electrode composed of 20 cm2platinum or iridium-coated titanium, and a pseudo silver wire served as the reference electrode.

[0026] Figure 8. Represents the results covering 0-160 sec. of the chronoamperometry conducted for 600 seconds,

[0027] Figure 9. Represents the results covering 0-600 sec. of the chronoamperometry conducted for 600 seconds.

[0028] Figure 10. To facilitate comparison of the leaching efficiencies for La, Ce, and Nd presented in Table 6, a visual bar chart is provided.

[0029] DETAILED DESCRIPTION OF THE INVENTION

[0030] The method described in this invention outlines a way to derive an equation for determining deep eutectic solvent (DES) solutions that have industrially usable viscosity ranges. This is achieved by selecting the appropriate molecular components and their correct molecular ratios. Within the scope of the invention, it has been determined that during the preparation of DES ionic solutions, the total valence indices of the individual molecules are of great importance in terms of the final DES solution to be formed. Calculating the total valence indices for individual molecules can be done using various chemical calculation programs, such as ChemOffice, Dragon, MOPAC, and Nemesis. This calculation method is based on the approaches developed by Kier and Hall, which have been detailed in their literature (Kier and Hall, 1976; Kier and Hall, 2000; Kier and Hall, 2002). Essentially, the derived equation organizes the calculated valence indices of individual molecules to form the final operational DES.

[0031] Using the method described, a mathematical relationship has been established for the mixing ratios, polar surface areas, and total valence indices of the molecules that will form the DES, and this theory and the equations to be applied are presented below. Accordingly, the application conditions of the viscosity suitability factor (VSF) equations to be applied that should be applied during the preparation of the DES are outlined as follows:

[0032] For example let's take g:0.11 , k:0.14

[0033] Viscosity Suitability Factor (VSF) = (z*f / k) / (x*e / g)

[0034] Among the g,h, and k values, small or smaller ones are written in the denominator, and the remaining ones are written in the numerator (Equation 1)

[0035] If one or two of the values If one or two of the values g, h, k >1 g, h, k £0.1 (Equation 2) (Equation 3)

[0036] For example; if g>1 For example; if g 0.1

[0037] For example; if h >1 and k >1 For example; if g £0.1 and h £0.1

[0038] VSF = [(y*d / h)+(z*f / k)] / (x*e / g) VSF = (z*f / k) / [(x‘e / g)+(y*d / h)] Applying the viscosity compatibility factor rules mentioned above, three different DES solutions were developed using the most accurate combination of VSF inputs of 6 molecules from approximately 50 molecular structures.

[0039] Table 1 Presents the molecular formulas and properties, including polar surface area, molar ratio, and total valence indices of each molecule used in the preparation of DES solutions.

[0040] The first Deep Eutectic Solvent (DES-I) was prepared by mixing choline chloride (ChCI), ethylene glycol (EG), and tartaric acid (TA) in molar ratios of 1 :4:1 in a beaker at 65 °C for 4 hours until colorless liquid obtained. Before preparing the mixture, the VSF value was checked using the viscosity compatibility factor equations outlined in the application diagram of the invention. Since the total valence index of tartaric acid (TA) is 0.0006, it was placed in the denominator, while the indices for the other components were placed in the numerator. The VSF value was calculated to be 0.0086, based on the molar ratios of x, y, and z, which correspond to 1 , 4, and 1 , respectively.

[0041] 20,23 40,46 0,1120 / + (y * 0,1000^

[0042] VUF = = 0,0086

[0043] 115,06

[0044] 0,0006

[0045] According to the general theorem, it was determined that the viscosity at room temperature increased in cases where the VSF value deviated from the value of 0.1. Although the situation for DES-I does not show high viscosity at around 55±5 °C, which is within the limits of critical and / or industrial moderate operating temperatures as shown in Figure 1 , some solutions exhibit VSF values as low as 10'5. Such low VSF values can lead to viscosities exceeding 104cP. This proposed method allows for the identification of potential issues with final solutions that may have unacceptably high viscosity before they are prepared. The viscosity of the DES-I solution was measured to be approximately 70 cP at a temperature of 55 ± 5°C, which has been established as the operating temperature. This viscosity does not hinder mass transport or the performance of filtration systems. Moreover, since these solutions will also be utilized in electrocatalytic activation applications, it is essential for the system's electrical conductivity to be at least a few mS / cm. In this regard, the conductivity of the DES-I solution was assessed as a function of temperature, revealing a value of 3.2 mS / cm at approximately 55 ± 5°C. This conductivity is deemed appropriate for facilitating the formation of ion transport redox species, which are expected to occur in the system during the electrocatalytic activation and electrochemical tests planned for the DES systems.

[0046] The second DES solution (DES-I I) was prepared by mixing ChCI, LA, and MA molecules in a beaker at 1 , 2, and 1 molar ratios, respectively, and stirring the beaker on a heating stirrer at 65 °C for 4 hours until a transparent solution was formed. Before preparing the mixture, the VSF value was checked using Equation 3 given in application diagram. The total valence indices of LA and MA are 0.0240 and 0.0027, respectively, and thus; they are placed in the denominator. ChCI is placed in the numerator, and when using equation 3, the calculation results in 0.0055 (x, n, and m are molar ratios and corresponds to 1 , 2, and 1 , respectively).

[0047] ( 20,23 \

[0048] \x* 0 1130 / VUF= 57,53 1 74.60 = °'0055n* 0,0240+ P* 0,0240

[0049] The VSF value obtained indicates that a relatively high viscosity is expected during viscosity measurements at room temperature. This is supported by the viscosity measurement of 1300 cP at room temperature, as shown in Figure 2. Additionally, it is essential to measure the viscosity and conductivity of the DES-I I solution as a function of temperature, specifically around 55±5 °C. This temperature range has been selected as the operating conditions for the bastnasite mineral leaching process. These measurements are crucial for understanding the viscosity and conductivity of the environment in which the planned electrochemical experiments will take place, as illustrated in Figure 2.

[0050] The viscosity of the DES-II solution was measured at 140 cP, while its conductivity was recorded at 1 .6 mS / cm at an operating temperature of 55 ± 5 °C. Both measurements fall within acceptable limits for physical processes, such as mass transport and filtration, as well as for electrochemical processes.

[0051] The third DES solution (DES-III) was prepared by mixing ethylene glycol (EG), choline chloride (ChCI), and aluminum chloride (AICI3) in a beaker at molar ratios of 4:1 :0.1 , respectively. This mixture was heated to 65 °C for 4 hours until a transparent solution formed. Before preparing the mixture, the VSF value was determined using Equation 3 from the application diagram. Since the total valence index of AICI3 is 0.039, this value was placed in the denominator, while the values for ChCI and EG were placed in the numerator. The calculated result was 0.3709, using the molar ratios of y, x, and v, which correspond to 1 , 4, and 0.1 , respectively.

[0052] 40,46. , , 20,23 .

[0053] (y * 0,10 ) + < * 0,1130)

[0054] VUF = = 0,3709

[0055] 189 v *

[0056] 0,039

[0057] Since the obtained VSF value is greater than 0.1 , we can conclude that low viscosity values can be expected in the room temperature range without the need for direct viscosity measurements. The viscosity and conductivity values measured as a function of temperature, as shown in Figure 3, support the approach outlined in the invention's application diagram. The room temperature viscosity of the DES-III solution was measured at approximately 32 cP. At the decided operating temperature of 55±5 °C for leaching processes, the viscosity and conductivity values were measured at 13 cP and

[0058] 13 mS.cm"1, respectively. When compared to DES-I and DES-II, it is predicted that DES- III will facilitate much easier transportation and filtration processes.

[0059] After developing the DES-I, DES-II, and DES-III solutions, the Biologic SP240 potentiostat device was utilized to determine the widths of the electrochemical windows for these solutions. This refers to the electrical potential values at which the molecular components of the DES remain stable and do not decompose. Signs of decomposition, such as gas evolution and sudden jumps in current, indicate the decomposition potential values.

[0060] Upon examining the electrochemical cyclic voltammetry graph of the DES-I solution (Figure 4), it is understood that the cationic components forming the solution initiate reduction reactions at a value of ~-0.3 V and the process of possible cationic species decomposition begins as seen from current jumps / deteriorations after a value of ~-0.6 V. In the positive region of the cyclic voltammetry graph presented in Figure 4, the electrolyte demonstrates stability up to about 1.1 V, during which no oxidation reactions occur. However, when the potential exceeds this value, oxidation reactions involving anionic or negatively charged molecular species in the electrolyte system initiate. The maximum current densities observed for the bare DES-I solution within the scanned potential range (with an electrode surface area of 0.0176 cm2) were calculated as --12.44 mA cm-2and -31.12 mA cm-2for the reduction and oxidation peak points, respectively.

[0061] For the DES-I electrolyte, a chronoamperometric test was conducted for 350 seconds at a potential of 1 .1 V, as illustrated in Figure 5, to simulate long-term dissolution tests. The purpose of this test was to determine whether the designed DES-1 solution would decompose over time when subjected to a stable current, potentially due to factors such as solution resistance. The electrochemical analysis of the DES-I solution indicated that it remained stable and is deemed suitable for the planned experiments involving electrocatalytic activation and leaching processes of bastnasite powder over an extended period.

[0062] In the analysis of the DES-I I solution, the approach was similar to that used for the DES-I solution, as previously illustrated in Figures 4 and 5. Initially, the solution was examined using cyclic voltammetry to assess its behavior at an applied voltage of 1.1 V. During the electrochemical window width scan, the potential was varied from 0 V to -0.8 V and then increased to 1.8 V. Upon reviewing the cyclic voltammetry and chronoamperometry data for DES-I I, it was observed that the cationic components of the DES-I I solution initiated reduction reactions at approximately - -0.1 V, and the reduction process of the cationic species was found to be accelerated after — 0.2 V. When examining the cyclic voltammetry data, it was observed that the electrolyte remained stable up to approximately 1.1 V, without exhibiting any oxidation reactions. However, at potentials higher than this threshold, the oxidation of anionic or negatively charged molecular species in the electrolyte system began to occur. The oxidation peak values recorded, such as 0.1 mA, were relatively small for an electroactive solution. The current densities, calculated based on the electrode surface area of 0.0176 cm2, were approximately -3.97 mA / cm2for the maximum oxidation point reached by the reduction process, and around 5.11 mA / cm2for the pure solution within the scanned potential range. To assess the electrocatalytic activation effect of the DES-II electrolyte between - 0.1 V and 1.1 V, a chronoamperometric study similar to that performed with DES-I was conducted. The results indicated that the molecular dynamics of the prepared solution remained unchanged with the continuously applied electrochemical potential difference. Therefore, this solution was found suitable for extended electrocatalytic activation tests.

[0063] Similar to the DES-I solution system previously presented in Figure 4, a potentiodynamic study using cyclic voltammetry was conducted for the developed DES- II I solution. This involved scanning the electrochemical window from 0 V to -0.8 V, and then to 1 .8 V. The cyclic voltammetry data indicated that the cationic components of the DES-I 11 solution began reduction reactions at approximately -0.7 V, and after -0.9 V, various cationic species were likely engaged in the reduction process without any noticeable current jump or decomposition. The positive region observed in the cyclic voltammetry graph indicates that the electrolyte remains stable up to approximately 1 .22 V, showing no signs of oxidation reactions. However, at potentials exceeding this value, the oxidation of anionic or negatively charged molecular species within the electrolyte system begins. The maximum current densities recorded for the bare DES-I 11 solution within the scanned potential range (with an electrode surface area of 0.0176 cm2) were approximately -14.75 mA / cm2for the reduction point and around 214.21 mA / cm2for the oxidation peaks. Similar to DES-I, a prolonged chronoamperometric analysis for DES-I 11 was performed. It was observed that the molecular dynamics of the solution remained stable despite the continuously applied electrochemical potential difference, indicating its suitability for long-term electrocatalytic activation experiments, similar to those conducted with DES-I and DES-II solutions. Let's summarize how the electrocatalytic activation system, which is the core of this invention, operates in relation to the dissolution of bastnasite mineral powder. This system fundamentally relies on equations 1 , 2, and 3, which have been previously presented in the literature. These equations pertain to the reaction rate constants of Gibbs energy, as discussed by A. Fischer (1996), G. Compton and H. W. Sanders (1996), Brett and Brett (1998), and Browne (2018). Equation 1 Equation 2

[0064] By utilizing Equations 1 and 2, Equation 3 is derived (A. Fischer, 1996; G. Compton and H. W. Sanders, 1996; Brett and Brett, 1998; Browne, 2018).

[0065] -AGQ / ko' = Z'e 'RT Equation 3

[0066] The term ko' refers to the standard rate constant, which is derived from kf' ve kb' when the potential E is equal to the standard potential Ee. This standard rate constant can vary significantly depending on the electrode material used in the electrochemical system. For instance, when using copper as an anode in a coating system that contains chlorine, the reaction is hindered by the formation of the highly insulating compound CuCL on the surface of the copper. This blockage reduces the coating efficiency to just 10%. In contrast, using a titanium (Ti) anode can enhance the coating efficiency to 90%.

[0067] After providing the theoretical background, we address the question: "How much can electrocatalytic activation accelerate a reaction?" This question is influenced by the activation energy required to break the bond in question. To determine this, we need to assess the activation energy of each step involved in the decomposition of carbonates. Let's illustrate this with an example: suppose we have a reaction that requires an activation energy of 60 kJ / mol at room temperature (298 K), with a rate constant, k, of 1 s-1. If we increase the temperature to 398 K, we can calculate the increase in the reaction rate using the Arrhenius equation, as shown in Equation 4, where R = 8.315 J / mol K (Arrhenius, 1889).

[0068] In— Equation 4 fci

[0069] When the ki value is substituted as 1 in this equation, the k2 value is calculated to be 2 on average. This indicates that the reaction rate is doubled for a system with an activation energy of 60 kJ mol'1when the temperature increases by 10°C.

[0070] Let's consider a scenario where the system is electroactive. When we apply a potential difference of -1 V, we can assume an a value of 0.5, which is typical in the Gibbs transition region. For this system, we will consider one electron transfer to silver (n=1 ) at a temperature of 25°C. Using Equation 1 to calculate the kf' / kQ' value under these conditions, the k value can be calculated 109times higher. kf'

[0071] = exp(—0.5%96487(—l) / (8.314x298) = e20= 109Kj,

[0072] This result indicates that the reaction rate can increase by two times with a 10°C rise in temperature. In contrast, applying a potential difference of just -1V can accelerate the reaction rate by a trillion times. The redox reactions of electroactive species are closely related to the widely used theory in physical chemistry known as Fermi energy or Fermi level. The electrochemical motor force, which enables direct electron transfer, can significantly enhance the achievement of various Fermi energy levels for different forms of elemental species, providing a strong driving force.

[0073] When an electromotive driving force generated by the electrochemical potential difference is applied to bastnasite minerals in contact with an electrode, it is expected to enhance the reaction through two main mechanisms. First, cationic species that can effectively break down the bastnasite mineral in the deep eutectic solvent (DES) are likely to accumulate more rapidly around the polarized double layer of the electrode. This concentration allows redox species, which typically dissolve the carbonate present in the solution, to gather closely around the bastnasite powder. Consequently, rare earth element ions released from the mineral can diffuse into the solution more swiftly due to the current transfer. Second, the bastnasite powder, once wetted and swollen in the solution, becomes partially conductive. This increase in conductivity facilitates the direct oxidation and breakdown of the mineral through its electrical properties.

[0074] To test for preliminary electrochemical catalytic activity, the system was configured with a pseudo reference electrode and a specially designed "J" shaped working electrode, which features a platinum electroactive surface oriented upwards. Bastnasite powder was placed on this surface. A large platinum counter electrode with a surface area of 8 cm2was also used. While the primary goal of this setup is to assess catalytic electrochemical activity by observing changes in current, surface color changes were also monitored using a microscope aimed at the upward-facing working electrode.

[0075] The cyclic voltammetry graph for the bastnasite mineral powder, which was placed on a working electrode in the DES-I solution, is shown in Figure 6. During the cyclic voltammetry test, a small amount of gas was released at approximately -0.3 V, and the electrode surface exhibited a darkening at around 1.3 V. This experiment aimed to investigate whether the bastnasite mineral was electroactive during chronoamperometry, where the current passing through the unit area per unit time would be measured. The chronoamperometry was conducted in the DES-I solution at a potential of 1.4 V for 600 seconds within the electroactive region. The results of this experiment are depicted in Figures 7, 8, and 9, which illustrate the data collected at durations of 8 seconds, 158 seconds, and 600 seconds, respectively.

[0076] In the chronoamperometry experiment conducted, the platinum (Pt) electrode, which was initially dark brown, exhibited a high current density of 13.56 mA.cm2at the start. This indicated the presence of electrochemically active dissolution. Around the 200th second of the chronoamperometry procedure, the bastnaesite powder on the electroactive region began to diminish. As anticipated, this reduction in electroactive species led to a decrease in the electrode's current density to 4.81 mA.cm2by the 600th second. The DES-I solution was determined to be electroactive in its interaction with bastnaesite minerals, as evidenced by the high chronoamperometric current densities recorded. Consequently, it was deemed suitable for bulk pulse electrocatalytic leaching experiments. Similar voltammetric and chronoamperometric tests were performed for DES-II solution and some gas evolution was observed on the electrode at around 0.2V. However, when around 1.4V was reached, no observable color change occurred on the electrode. Following the cyclic voltammeter test, a chronoamperometer test was applied at a potential value of 1.4V in the electroactive region, equivalent to the system presented in Figures 7, 8, and 9 for DES-I. Although it was difficult to observe the electrode surface in the chronoamperometer test due to the muddy environment that emerged from the nature of bastnasite, a color change was obvious on the electrode, and the current flow starting at around 6.36 mA / cm2 decreased to 3.13 mA / cm2 due to the decrease in electroactive species towards the end of the chronoamperometer test. As a result, bastnaesite powder was found to behave electroactive in DES-II solution and was found suitable for batch pulse electrocatalytic-based leaching process.

[0077] For the DES-I 11 solution, the same cyclic voltammetry experiment as for DES-I, shown in Figure 6, was performed. Again, no excessive gas evolution was observed at any potential value

[0078] The chronoamperometric study was conducted under DES-I conditions for DES- III, following the methodology outlined in Figures 7, 8, and 9. Unlike the solutions of DES- I and DES-II, the DES-III solution exhibited a greater current draw over time. Upon examining the chronoamperometric data, a current density of 37.59 mA cm-2was recorded during the first 10 seconds. By around 350 seconds, the current density in the electroactive region increased to 45.88 mA cm-2and stabilized at approximately 45.14 mA cm-2by the end of 600 seconds. These findings indicate that the bastnasite mineral demonstrated electroactive behavior when analyzed in the DES-III solution.

[0079] After evaluating the usability of DES-I, DES-II, and DES-III solutions through preelectrochemical tests, we proceeded with pulse electrocatalytic activated leaching experiments. This section will directly examine the leaching rate of bastnasite mineral for the prepared DES-I, DES-II, and DES-III solutions, as well as for strong acids. The goal is to assess the leaching performance, which involves the process by which rare earth element ions within the bastnaesite mineral structure dissolve into solution. As noted in the literature review, previous studies predominantly utilized high-concentration or even pure-form acids for similar analyses. To accurately assess the leaching performance in DES-I, DES-II, DES-III, and acidic solutions, it is essential first to determine the mass percentage composition of La, Ce, and Nd in the powdered bastnasite mineral obtained from the mine site. For this purpose, the bastnasite mineral designated for the leaching processes was analyzed using the Spectra I QI I XRF device, and the results are presented in Table 2.

[0080] Table 1 . XRF analysis of the bastnasite mineral sample taken from the mine site and processed into powder form.

[0081] One of the acids used to leach the lanthanum element from bastnasite mineral is pure hydrochloric acid (HCI), which is commercially available at a concentration of 12M (37%). This substance is among the riskiest chemicals to be traded and transported in bulk due to its potential to cause serious damage to organic materials and relatively rapid corrosion of inorganic materials, including hard steels. In the leaching test system, 12M HCI will serve as the control group for the leaching experiments. The amounts of solution prepared for the leaching system and the other leaching conditions are presented in Table 3. For the experiment, 20 ml of each deep eutectic solvent (DES) system was used, and 0.02 grams of bastnasite mineral was subjected to a leaching process with pulse electrocatalytic activation at 60°C. From both acid and alkaline leaching solutions, 100 ml samples of S1 , S2, S3, S4, and S5 were taken, and 0.1 gram of bastnasite mineral was mixed into each sample for 24 hours. The leaching was conducted at temperatures ranging from 30°C to 120°C, based on the evaporation rates of the acid and alkaline solutions. In all systems, the liquid-to-solid (L / S) ratio was maintained at 1000 to prevent any decrease in efficiency due to insufficient solution and to accurately assess the true leaching performance of each solution.

[0082] Table 2. The types of DES and acids used to evaluate the leaching efficiency of bastnaesite mineral into solution are presented, along with the experimental conditions (EA: Electrocatalytic Activation).

[0083] In electrochemical leaching experiments, two electrodes are used rather than three-electrode (there was a reference electrode in these tests) used in potentiodynamic tests performed with potentiostat. Therefore, the maximum potential that bastnaesite powder placed electrode can reach is expected to be higher than applied 1.8V in three- electrode tests. In cyclic voltammetry tests, while a potential difference of up to 1.8V against the Pt electrode was sufficient for the representative silver reference electrode, 3.4V was found sufficient for titanium in the two-electrode pulse system, and at positive values above this value, solution decomposition, bubble formation, and gas evolution began to be observed. After testing various frequencies and potential values, including 0.1 Hz, 0.15 Hz, 0.2 Hz, 0.25 Hz, 0.3 Hz, 0.4 Hz, 0.5 Hz, 1 Hz, 5 Hz, 10 Hz, 50 Hz, and 100 Hz for pulse electrocatalytic activation, the most suitable frequency for the electrocatalytic decomposition of carbonate powders was found to be 0.25 Hz. The primary advantage of using pulsed current is that it allows for the relaxation of the electrode double layer during decomposition, facilitating the movement of cationic and anionic species (Mandich, 2002; Abbott, Cihangir, et al., 2018). A decomposition potential of approximately 3.4 V was applied for 3 seconds, followed by a 1 -second interruption of the current. This interruption aimed to relax the ion layer of the double layer, which is known to form on the electrode surface. It was theorized that this relaxation would enable the discharged double layer to decompose more carbonate in each subsequent pulse cycle. To ensure consistent electrode contact points of the powders in DES solutions during the experiment, we regularly mixed the powders to prevent agglomeration at a single spot and to enhance their contact rate with the electrode. Specifically, the powders were mixed for 1 minute every 15 minutes. This means that over the course of a day, we applied a total of 90 minutes of mixing, calculated as follows: 24 hours multiplied by 60 minutes divided by 16 minutes equals 90 minutes. After the processes, the DES solutions were filtered twice through fast, medium, and slow filter papers to remove solid particles, ensuring that the leach solutions intended for ICP-MS analysis contained only dissolved ionic species.

[0084] The ratios of La, Ce, and Nd detected in the filtered solutions after ICP-MS analysis are presented in Table 4. To avoid inconsistencies and questions regarding the analysis results, the XRF data shown in Table 2 will be utilized. To ensure the accuracy of the data obtained, we examined the element lanthanum, which is known to be nearly fully leached from bastnaesite minerals in hydrochloric acid (HCI). The ICP-MS analysis in Table 4 indicates that lanthanum was leached into 12M HCI at a concentration of 36.586 ppm.

[0085] Tablo 3. After completing the leaching process as outlined in Table 3, the filtered samples were sent for ICP-MS analysis, and the resulting data were obtained.

[0086] It will be started by conducting a theoretical calculation, knowing from the literature that pure HCI can completely dissolve the La element in bastnaesite. The steps for this calculation are outlined below:

[0087] • As given in Table 2 as a result of XRF analysis, Lanthanum detected in Bastnasite mineral is in La20s form and is 4.1496%.

[0088] • There will be 0.1 x(4.1496 / 100) = 0.0041496 grams of La20s in 0.1 grams of Bastnasite.

[0089] • The molecular weight of La20s is 325.8092 grams.

[0090] • The weight of La element in La2Os is 138.9054 grams.

[0091] • There is (2x138.9054) / 325.8092: 85.2679% La element in 1 mole of La20s.

[0092] • In this case, there will be 0.003538 grams of La in our 0.1 gram of Bastnasite sample.

[0093] • For the case where all the La in 0.1 gram of Bastnaesite in 100 ml of HCI is leached into the solution, 0.3538 mg / ml, i.e. 35.38 ppm La is expected to be detected.

[0094] As a result of the evaluation, it was found that the value obtained was nearly identical to the theoretically calculated value, showing an insignificant difference of only 3% (Table 4: 36.58 ppm compared to the theoretical value of 35.38 ppm). This minor discrepancy can be attributed to the expected variations from the XRF and ICP-MS analyses. In other words, all of the lanthanum (La) was successfully leached from 0.1 grams of Bastnaesite mineral using 12M HCI, as anticipated. This result is valuable for confirming the proper functioning of the instruments and for re-verifying the XRF data, indicating that the obtained results are quite accurate.

[0095] Before conducting the general performance evaluation analysis of aqueous solutions with DES-I, DES-II, and DES-III, we examined the maximum ppm values for the studied systems at an L / S ratio of 1000. This assessment focused on the complete leaching of La, Ce, and Nd elements from bastnaesite mineral into each 20 ml of DES solution. The calculations were performed using the XRF data, as described in the steps above, and the results are presented in Table 5. Table 4. The theoretical values expected to be detected by ICP-MS, assuming that all of the La, Ce, and Nd elements found in the bastnaesite mineral (with XRF data presented in Table 2) are fully dissolved in DESs, were calculated in a similar way to the La / HCI case discussed in the previous paragraph (L / S:1000). Using the values presented in Table 4, the efficiency calculations were based on the results of leaching shown in Table 5, and these calculations are summarized in Table 6.

[0096] Tablo 5. Leaching efficiencies of the rare earth elements in bastnaesite were calculated. Upon examining the bar graph presented in Figure 10, it's evident that the 6M H2SO4 solution poses significant risks during transport and use. This solution requires complex environmental regulations and operational coordination to manage its risks. It can quickly cause lung and skin irritation due to its rapid evaporation and high vapor pressure while it also accelerates the corrosion of surrounding devices and materials, significantly reducing their usable lifespans. In this study, the ratios of La, Ce, and Nd leached into the 6M H2SO4 were measured at 78.56 ppm, 81.35 ppm, and 81.75 ppm, respectively. Notably, the efficiency of this system was lower than that of the DES-III ionic solution system, while both DES-I and DES-II also yielded impressive results. The DES systems operated at a temperature of 60°C and under a total mixing period of 90 minutes (averaging 1.5 hours) within a 24-hour timeframe, which is considerably shorter and cooler than the 24-hour mixing and elevated temperatures used in the acid systems. In the examination, the pure HCI solution, which is quite risky to use in large quantities and is typically preferred for laboratory-scale applications, demonstrated the highest leaching rates. Deep Eutectic Solvents (DESs) showed results that were not far behind. When excluding the pure HCI solution from the comparison, the leaching efficiency of the La element in the different solutions can be ordered as follows: DES-III > S1 > DES-II > DES- I > S5 > S4 > S3. This indicates that DES solutions are highly effective for the leaching of the La element. Similarly, when analyzing the leaching efficiency of the Ce element, the order remains the same: DES-III > S1 > DES-II > DES-I > S5 > S4 > S3, confirming that DES solutions also yield efficient results for the Ce element. In the last comparison, the examination of the leaching efficiency of the Nd element into the solution will reveal the order DES-I ll> S1 > DES-II> DES-I> S5> S4> S3. It is evident that DES solutions perform exceptionally well when it comes to Nd leaching. When considering factors such as lower temperatures, minimal mixing energy, negligible evaporation due to the low vapor pressures of ionic systems, and relatively eco-friendly chemical inputs, it is reasonable to conclude that the study of rare earth element leaching efficiency using DES solutions with pulse electrochemical activation yields very efficient leaching results.

Claims

CLAIMS1 . An ionic solution designed for efficiently leaching bastnaesite minerals, achieving performance comparable to concentrated acids characterized by comprising ChCI, EG, and TA molecules in molar ratios of 1 , 4, and 1 , respectively.

2. An ionic solution designed for efficiently leaching bastnaesite minerals, achieving performance comparable to concentrated acids characterized by comprising ChCI, LA and MA molecules in molar ratios of 1 , 2 and 1 , respectively.

3. An ionic solution designed for efficiently leaching bastnaesite minerals, achieving performance comparable to concentrated acids characterized by comprising EG, ChCI and AICI3 molecules in molar ratios of 1 , 2 and 1 , respectively.

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