Separation of REs from Ca and Mg Ions by Ammonium Bicarbonate Precipitation and the Influence of Fe and Al Ions

Abstract

The presence of impurities such as Ca, Mg, and Al during the precipitation of rare earths (REs) using ammonium bicarbonate directly affects product purity. It is necessary to optimize precipitation methods and conditions to improve the separation efficiency between REs and impurities. In this study, RE (La and Ce) ions were precipitated using ammonium bicarbonate solution, and the separation efficiency of REs from Al, Fe, Ca, and Mg ions was investigated with or without the addition of triammonium citrate (TAC). The results showed that as long as the precipitation yield of REs was controlled below 94%, Ca and Mg ions would not enter the precipitation in the absence of other impurities, and the purity of the obtained rare earth oxides (RE₂O₃) was close to 100%. The presence of Al and Fe impurities would reduce the separation efficiency of REs from Ca and Mg. Therefore, Al and Fe must be separated before the precipitation of REs. First, Fe was completely precipitated by controlling the pH value to 4.12. Then, by filtering out the isolation and adjusting the pH value to 4.6, approximately 84% of Al³⁺ was precipitated, with a loss of REs of about 6%. Finally, the pH value was increased to 6.43, and REs were completely precipitated, yielding rare earth carbonate. The RE₂O₃ purity of its calcination product was 97.8% with Al and Mg contents of 1.05% and 0.21%, respectively, and no Ca or Fe was detected. This indicated that Mg can enter the product by co-precipitation with Al. To address this, a small amount of TAC was added during the pre-removal of Fe and Al to facilitate the complete removal of Al. By controlling the precipitation yield of REs at 94%, the purity of the final RE₂O₃ reached 99.6% with an Al content of 0.09%. Furthermore, using a continuous precipitation crystallization method, RE₂O₃ purity can be achieved at 99.8% with an Al content of 0.06%.

1. Introduction

The application of rare earths (REs) in fields such as electronics, information technology, new energy, and environmental protection is continuously expanding, driving the ongoing advancement of rare earth separation theory and technology, thereby achieving cost reduction, quality improvement, and efficiency enhancement [1,2,3,4,5]. The primary methods for efficiently separating and purifying REs from impurity ions include solvent extraction, ion exchange, and precipitation [6,7,8,9]. Among these methods, precipitation separation plays a crucial role in the production of high-purity REs and is the main method for separating RE³⁺ from solutions containing Fe³⁺, Al³⁺, Mg²⁺, Ca²⁺, and their coexisting anions [10,11]. During the early stages of rare earth industrialization, the oxalic acid precipitation method was the most widely adopted approach, primarily due to its excellent separation selectivity, large precipitation crystal particles, and simple solid–liquid separation process [1,12,13]. However, oxalic acid is costly, and the subsequent treatment of acidic wastewater is also cumbersome [1,14]. Replacing oxalic acid with ammonium bicarbonate (ABC) for rare earth precipitation can reduce production costs and facilitate subsequent wastewater treatment as well as material recovery so as to achieve green and clean production [1,15,16,17,18]. Therefore, extensive research has been conducted on the mechanism and precipitation separation process of REs by ammonium bicarbonate, and a series of precipitation separation methods have been proposed for different REs solutions and target product requirements [19,20,21,22]. Among these studies, the achievements of Nanchang University have been effectively applied in industrial production [1,17].

In recent years, the supply of rare earth mineral raw materials has been constrained, and their quality has been declining. Meanwhile, to reduce ammonia nitrogen pollution, Ca, Mg, and Na have been used as substitutes for ammonia, resulting in relatively high levels of impurity ions in mineral and separation products. Therefore, current precipitation methods cannot meet product quality requirements [23,24,25,26]. In fact, these impurities can be separated using extraction methods. However, adding additional separation steps significantly increases separation costs, particularly for high-abundance lanthanum, cerium, and yttrium products [27]. Therefore, it is the most realistic and effective technical solution to optimize operating conditions as well as process flows and add a small amount of auxiliary reagents in precipitation separation to improve the separation efficiency of RE elements from impurity ions [1,15,16,28,29].

By controlling precipitation conditions, it is possible to separate rare earth elements from Ca and Mg using ammonium bicarbonate [1]. In previous studies conducted by our research group [30], it was found that the TAC-assisted ammonium bicarbonate precipitation method can remove most of the Al and all of the Fe from La/Ce solutions, with RE losses of only 5.6%. By using citric acid chelation to eliminate Fe interference, RE₂O₃ with a purity of 99.59% can be obtained [30]. In the present study, the effects of Al and Fe impurities on the separation of REs from Ca and Mg were further investigated, and it was found that the co-precipitation behavior of Ca and Mg is related to the coexistence of high-valent ions such as Fe³⁺, Al³⁺ etc. To obtain qualified rare earth carbonate products, it is necessary to completely remove these high-valent (Fe and Al) impurity ions first. The mutual influence patterns between low-valent (Ca and Mg) and high-valent (Fe and Al) impurity ions during the precipitation of RECl₃ using ammonium bicarbonate were investigated. Based on these patterns, methods and conditions for precipitating and separating REs from solutions containing impurities such as Al, Fe, Ca, and Mg were proposed, and qualified rare earth carbonate products were prepared, which provided theoretical and technology support for the precipitation separation of REs from impurities.

2. Materials and Methods

2.1. Materials and Reagents

The actual RECl₃ solution used was provided by Jiangxi Jin Century Rare Earth New Materials Co., Ltd. (Nanchang, China), with the following main components in

Table 1. The elements and contents of the actual feed solution.

Elements	Mg	Al	Ca	Fe	Co	Ni	Zn	Mn	Na
Content (g/L)	1.30	2.31	2.02	0.23	0.01	0.01	0.02	0.01	1.23
Elements	Y	La	Ce	Pr	Nd	Gd	Other REs		K
Content (g/L)	0.06	19.81	52.81	0.18	0.06	0.01	0.01		0.68

(actual feed solution, AFS). At the same time, we prepare a simulated feed solution (SFS) without Al and Fe by mixing NaCl, KCl, MgCl₂, CaCl₂, LaCl₃, and CeCl₃ according to the same concentration as that of AFS. The concentrations of each ion are as follows: Na (1.23 g/L), K (0.68 g/L), Mg (1.30 g/L), Ca (2.02 g/L), and REs (72.93 g/L). These solutions were used in subsequent experiments. The chemical reagents used in this study, such as ammonium bicarbonate (ABC) and triammonium citrate (TAC), are all analytical-grade reagents purchased from Xilong Technology Co., Ltd. (Shantou, China).

2.2. Experimental Process

(1) Pretreatment: In this study, for rare earth chloride solutions containing Ca, Mg, Al, and Fe impurities, a 2% ammonium bicarbonate solution was used to adjust the solution to a specific pH value before filtration. The precipitation yield was measured, and the filtrate was collected for subsequent precipitation reactions.

(2) Stepwise precipitation: In the stepwise precipitation process, a certain volume of the aforementioned rare earth chloride solution was placed in a beaker. A 10% ABC solution was gradually added under stirring. A pH-3c pH meter (Anhui Leqi Instrument Co., Ltd., Chuzhou, China) was used to monitor the pH value. After stabilization (5–10 min), the pH value and the volume of the 10% ABC solution consumed are recorded.

(3) Continuous precipitation: In a continuous crystallization apparatus, a certain volume of deionized water and seed crystals (rare earth carbonate precipitates) are added, and the feed percentages of the rare earth chloride solution and 10% ABC solution are controlled to stabilize the reaction pH within a certain range. The rare earth carbonate product is prepared through simultaneous feeding and discharge. The apparatus schematic diagram is shown in Figure 1.

Finally, it should be noted that all reactions were carried out at room temperature unless otherwise specified.

Finally, the precipitates obtained from stepwise precipitation and continuous precipitation were washed 2–3 times with deionized water, thoroughly dried, and calcined at 950 °C (muffle furnace, Nanjing Boyuntong Instrument Co., Ltd., Nanjing, China) for 1 h to obtain the RE₂O₃ product.

2.3. Analytic Methods

The concentrations of impurity ions (Ca, Mg, Al, Fe) and rare earth elements (REs: La, Ce, Y, Gd, Pr, Nd, etc.) were determined using an inductively coupled plasma mass spectrometer (ICP-MS, Thermo Fisher, Waltham, MA, USA) and EDTA titration, respectively. The EDTA titration method involves the following steps: Approximately 0.5 g of ascorbic acid, 5 mL of 10% sulfonyl salicylic acid, and 10 mL of 20% hexamethylenetetramine solution (Ph = 5.5) were added to the test solution in the measurement volume. The solution was then titrated using a 0.003843 mol/L EDTA solution with dimethyl orange as the indicator. The titration endpoint is identified by a color change from purple-red to bright yellow. Each sample is analyzed three times, with the relative standard deviation of this method being less than ±2%, and the detection limit is 100 ppb. In comparison, the detection limit of the ICP method is 10 ppb, with a standard deviation of less than ±20%. The corresponding XRD patterns were obtained using an X-ray diffractometer (Bruker D8ADVANCE, Karlsruhe, Germany, Cu target, λ = 0.15406 nm) at a scanning speed of 5°/min.

2.4. Chemical Reactions and Formulas

(1) The reaction equations and the corresponding K_sp involved are as follows: [30].

Fe³⁺ + 3OH⁻ = Fe(OH)₃ ↓ log_Ksp = −37.40

(1)

Al³⁺ + 3OH⁻ = Al(OH)₃ ↓ log_Ksp = −32.89

(2)

La³⁺ + 3OH⁻ = La(OH)₃ ↓ log_Ksp = −19.00

(3)

Ce³⁺ + 3OH⁻ = Ce(OH)₃ ↓ log_Ksp = −19.82

(4)

The corresponding pH values of theoretically complete precipitation for the above four reactions are 3.2, 4.7, 8.0, and 7.4, respectively.

(2) The relevant calculation formula is as follows:

Precipitation yield (%) = (C₀ − C₁)/C₀ × 100%

(5)

Purity or content (%) = C_M/C_N × 100%

(6)

C₀ is the initial concentration of each ion at the start of the reaction, C₁ is the concentration of that ion after the reaction; C_N is the sum of the total concentrations of all ions in the products, and C_M is the concentration of one of those ions. All concentrations submitted here are mass concentrations.

3. Results and Discussion

3.1. Method and Conditions for Separating REs from Ca and Mg by Ammonium Bicarbonate Precipitation

Using 150 mL of simulated feed solution, the precipitation yield of REs and solution pH were determined through stepwise precipitation by controlling the addition of 10% ABC. The results are shown in Figure 2a. As the ABC dosage increased, the solution pH also increased, and the precipitation yield of REs gradually increased. A good linear relationship existed between V_10%ABC and precipitation yield. Additionally, under different dosage conditions, the effect of precipitation yield of REs on the purity of REs and the Ca and Mg content in the RE₂O₃ product was investigated, as shown in Figure 2b. It was found that when the pH value exceeds 5.6, Ca and Mg gradually enter the product, affecting the purity of RE₂O₃. When the precipitation yield of REs is around 94%, the product purity is high. The effect of aging time on the purity of Res, as well as the Ca and Mg content in RE₂O₃ product, was shown in Figure 2c. When the aging time is 2 h, the purity of REs in the product is the highest, and neither Ca nor Mg enters the product.

3.2. The Effect of Al and Fe on the Precipitation Separation of REs from Ca and Mg by Ammonium Bicarbonate

As previously reported [30], the precipitation yield of Al and Fe significantly increased as the pH value rose from 3 to 4.4. At a pH value of 4.12, Fe was completely precipitated, while the precipitation yield of Al was only 60%, and did not significantly improve until the pH reached 4.4. At a pH value of 4.9, the precipitation yield of Al was only 75%, while the precipitation yield of REs reached 10%.

In the actual RECl₃ solution, the pH was adjusted to 4.12. After Fe³⁺ was completely precipitated by ABC and filtered out, a 10% ABC solution was further added to 100 mL of this RECl₃ solution gradually to adjust the pH from 4.4 to 5.2. Figure 3a showed the relationship between the precipitation yields of REs, Al, Ca, Mg, and pH of the solutions. At a pH value of 4.6, the precipitation yield of Al was 84%, and the precipitation yield of REs was 6%. As the pH increased, the precipitation yields of Al and RE also increased, and remained below 90% and 10%, respectively. However, the precipitation yields of Ca and Mg were approximately 40% and 20%, which were very different from those of the RECl₃ solutions containing only Ca and Mg impurities. These results indicated that the presence of small amounts of Al can cause the co-precipitation of Ca and Mg, increasing the impurity content in the final product.

Therefore, it is recommended to filter the aluminum slag at a pH of 4.6, then precipitate REs from the filtrate with ABC until the pH reaches 6.1. The ion concentrations of Mg²⁺, Ca²⁺, and RE³⁺ in the supernatant were determined to be 0.28, 0.65, and 0.067 g/L. Subsequently, the pH was further increased to 6.43, 6.68, 6.78, and 6.92, respectively. The mixture solution was left to stand at room temperature for 2 h before being filtered under vacuum. The ion concentrations in the supernatant were measured, and the precipitation yield was calculated. The precipitate was washed multiple times, dried, and calcined at 950 °C to obtain RE oxides. Their content of REs and impurities was measured. The results were shown in Figure 3b. When the pH exceeded 6, the solution contained only a small amount of Al, and the REs were almost completely precipitated. Ca began to precipitate at a pH value of 6.68, while Mg showed no significant change. This is because the solubility product constant (K_sp) of calcium carbonate(logK_sp = 8.52) is much smaller than that of magnesium carbonate(logK_sp = 4.62) [31]. The results in Figure 3c showed that as the pH increases, the Ca content in the precipitate increased with increasing Al content, while the purity of REs decreased. At a pH value of 6.43, the product purity was the highest, reaching 97.8%. The Al and Mg contents were 1.05% and 0.21%, respectively, while the Ca and Fe contents were both below the detection limit. This indicated that the presence of Al affected the precipitation of Ca and Mg, particularly Mg due to its coprecipitation with Al.

The effects of aging time and temperature on the purity of RE₂O₃ after removing Fe and Al from the precipitated REs were investigated at pH = 6.43. The results in Figure 4 indicate that an appropriate aging time is beneficial for improving product purity. This is because the impurity ions adsorbed during rapid precipitation can be released through the dissolution and recrystallization of precipitate particles during aging. However, prolonging the aging time leads to dissolved impurities being re-adsorbed onto the surface of RE carbonates through electrostatic interactions, thereby affecting the purity of RE₂O₃ [32]. Meanwhile, moderately increasing the temperature to 40–60 °C can improve product purity. At low temperatures, the diffusion coefficients of impurities are weak, so that Mg²⁺ and Al³⁺ are difficult to release. Conversely, high temperature promotes the re-precipitation of soluble impurities and surface complexation, ultimately reducing product purity [32]. Therefore, the highest RE₂O₃ purity is obtained by aging at 50 °C for 6 h, with a RE purity of 98.5%, only 0.9% Al and 0.13% Mg, and no Ca or Fe was detected.

3.3. Purity of RE₂O₃ Obtained Using Ammonium Bicarbonate Stepwise Precipitation from a Solution of RECl₃ Containing Ca, Mg, Al, and Fe

To investigate the effect of coexisting ions (Al, Fe, Ca, and Mg) on product purity, an actual solution of RECl₃ containing Ca, Mg, Al, and Fe was first precipitated under a set pH condition, and the precipitate was filtered out. 150 mL of the purified solution was taken for the precipitation of REs at different pH values adjusted by the addition of the ABC solution. The precipitation yields of REs were measured with three parallel replicates. The results shown in Figure 5a show a good linear relationship between the precipitation yield of REs and V_10%ABC.

Meanwhile, the purity and impurity content in the products obtained at different precipitation yields were determined. The results in Figure 5b indicate that the Mg content in the RE₂O₃ product increases almost simultaneously with the precipitation yield of REs. When the precipitation yield of REs is between 40% and 75%, the Mg content in the product is approximately 0.06. When the precipitation yield of REs is around 88%, the Mg content is approximately 0.16%. This demonstrates that Mg co-precipitates into the product due to the presence of Al. Interestingly, when the precipitation yield of REs is ≤40% or ≥90%, Ca also undergoes co-precipitation. When the precipitation yield of REs is between 60% and 88%, the product contains almost no Ca. However, when an excess of ABC is used, Ca enters the precipitate in the form of calcium carbonate. Therefore, controlling the precipitation yield of REs is crucial for separating Mg and Ca from REs. Additionally, aging time also has a significant impact, as shown in Figure 5c. When the precipitation yield of REs is set to 60% and the aging time is controlled at 2 h, the purity of RE₂O₃ is the highest, with a RE purity of 98.8%, 0.73% Al, 0.06% Mg, and no Ca or Fe detected. This represents an improvement in purity compared to the precipitation product in Section 3.2.

3.4. Triammonium Citrate-Assisted Ammonium Bicarbonate Precipitation Separation of REs from Ca, Mg, Al, and Fe

The above results demonstrate that the separation of REs from Fe and Ca is highly effective, but the separation of REs from Al and Mg is still incomplete. Therefore, based on the selective coordination ability of TAC toward trivalent ions, TAC chelating agents were introduced to enhance Al removal efficiency and improve Mg removal efficiency. According to the previous results, adding a small amount of TAC can enhance aluminum removal efficiency, and when the molar ratio of TAC to Fe is 1:1.87, the separation efficiency reaches its optimal level [30]. TAC not only lowers the precipitation pH of Al and Fe but also reduces the precipitation yield of REs, minimizing the co-precipitation of Ca, Mg, and REs, thereby improving the separation selectivity between REs and Al, Fe, Ca, and Mg, resulting in high-purity RE₂O₃ products with low impurity content. However, the TAC dosage ratio and precipitation pH control range play a critical role.

Figure 6a shows the relationship among the addition amount, pH, and the precipitation yield of REs when precipitating rare earths with a 10% ABC solution. As ABC is added, the solution pH and rare earth content gradually increase, and there is a linear relationship between V_10%ABC and the precipitation yields of REs. Therefore, the precipitation yield of REs can be controlled by adjusting the V_10%ABC addition amount. The results of product purity and impurity content at different precipitation yields of REs are shown in Figure 6b. When the precipitation yield of REs is around 94% (pH = 5.4) or below, Ca and Mg are almost not incorporated into the precipitate, and the contents of REs and Al in RE₂O₃ remain relatively stable. However, when the precipitation yield of REs exceeds 99%, Ca and Mg in the solution are incorporated into the precipitate, leading to a decrease in product purity. Considering the impact on production efficiency and product purity, the optimal precipitation yield of REs is around 94%. The effect of aging time on product purity is shown in Figure 6c. When the aging time is 2 h, the purity is optimal. The REs content reaches 99.6%, with only 0.09% Al, while Ca, Mg, and Fe were not detected.

3.5. Triammonium Citrate Coordination Assistance Continuous Precipitation for Preparing High Purity of RE₂O₃

The above results demonstrate that TAC plays a crucial role in the precipitation separation of REs from Ca, Mg, Fe, and Al using an ABC precipitation method. The dosage of the precipitant (10% ABC) and the pH range are also critical factors, both of which can be controlled through a continuous precipitation process. In a continuous precipitation technology, the role of seed crystals can also be fully utilized to accelerate the precipitation crystallization rate of rare-earth carbonates. This method employs simultaneous feeding of solution and precipitant while discharging rare earth carbonate precipitates; the requirements are only the control of the feed ratio and solution pH. For this purpose, the feed solution after impurity removal filtration was mixed with the 10% ABC solution for continuous precipitation, and the effects with and without added TAC after impurity removal were compared. Based on the determined relationship between pH and precipitation yield of REs, the pH was maintained at approximately 5.4–5.6, at which point the precipitation yield of REs was approximately 94%. Only the volume ratio of the precipitant (10% ABC) to the feed solution needed to be controlled at approximately 55:100. Figure 7 shows the purity data of RE₂O₃ obtained by calcining rare earth carbonates produced with and without TAC addition and under different aging times at 950 °C for 1 h. Figure 7a shows that without adding TAC, the purity of RE₂O₃ reaches 98.9%, with Al content of 0.41%, Ca content of 0.08%, Mg content of 0.16%, and Fe undetected. In contrast, Figure 7b shows that with TAC added, the purity of RE₂O₃ reached 99.8%, with only 0.06% Al; Fe, Ca, and Mg were undetected. It can be seen that adding TAC during the impurity removal process effectively reduces the content of Ca, Mg, Al, and Fe in RE₂O₃, improves product purity, and yields better results in continuous precipitation crystallization.

The XRD analysis of the rare earth oxide products obtained from calcination is shown in Figure 8. The rare earth elements in the feed solution used in this study primarily consist of La and Ce, with a ratio of approximately 1:2.7 between the two elements. Due to the larger ionic radius of La³⁺ (1.16 Å) compared to that of Ce³⁺ (0.97 Å), a systematic shift in the peak positions to lower diffraction angles and lattice expansion occurred in the final product after calcination, as compared to the standard cards [33]. Moreover, no impurity phases, such as aluminum oxide, were observed in the XRD pattern, which may be attributed to the extremely low content of impurity ions in the product, making them undetectable; this is consistent with the measured data.

4. Conclusions

Using a 10% ABC precipitant to separate REs from a solution containing only Ca and Mg impurities, as long as the precipitation yield of REs is kept below 94% (pH value below 6), neither Ca nor Mg will enter the precipitate, enabling effective separation from the REs, with product purity approaching 100%.

For RECl₃ solutions containing Al, Fe, Ca, and Mg, the presence of Fe and Al causes Ca and Mg to precipitate prematurely, thereby affecting product purity. Therefore, the pH is adjusted to 4.12, and the precipitated Fe is filtered out. Then, the pH is adjusted to 4.6, and 84% of the precipitated Al is filtered out. Finally, at a pH value of 6.43, 100% of the REs are precipitated, but the product still contains a small amount of Al and Mg. This indicates that the presence of a small amount of Al can still cause Mg to coprecipitate, thereby affecting product purity. When the precipitation yield of REs is controlled between 60% and 88%, no Ca enters the precipitation, and only a small amount of Mg is carried into the precipitation by Al coprecipitation. At this point, the rare earth purity reaches 98.8%, with Mg and Al contents of 0.06% and 0.73%, respectively.

By introducing TAC during the removal of Fe and Al, the stepwise precipitation method enables effective separation of REs from Ca and Mg. When the precipitation yield of REs is around 94%, the purity of RE₂O₃ can reach 99.6% with 0.09% Al. Meanwhile, the continuous precipitation crystallization method can better separate REs from impurity ions, achieving a purity of 99.8% for RE₂O₃ with 0.06% Al and no detectable Ca, Mg, or Fe.