Characteristics and Performance of Cerium Extraction from Cerium Hydroxide Concentrate using Tri Butyl Phosphate

This study aims to develop a liquid-liquid equilibrium model to predict the Ce distribution in the extraction system based on experimental laboratory data. Extraction feed solution (Ce hydroxide in nitric acid media) and solvent (Tri Butyl Phosphate (TBP) in kerosene diluent) with a volume ratio of 1:1 at various concentrations of feed and solvent were contacted using a mechanical shaker at 150 rpm for 15 minutes. The solution was settled for 30 minutes to separate the aqueous and organic phases. Cerium concentration in the aqueous phase was analyzed using XRF, while cerium concentration in the organic phase was calculated using a mass balance. The results showed that the extraction of Ce from the Ce-hydroxide concentrate was successfully carried out using TBP 0.92 M in kerosene diluent so that the Ce extraction efficiency of 70% was obtained. The equilibrium model that has been developed was able to represent the phenomenon of the liquid-liquid equilibrium distribution in the extraction system, which was carried out with an average relative error of 8.53%. Five stages of extraction unit need to apply to achieve 90% of extraction efficiency.

Cerium; Equilibrium; Extraction; TBP

        The availability of monazite minerals in Indonesia can be found in the Bangka Belitung and West Kalimantan regions. The volume of REE deposits reached 0.0023% of the total area of Bangka Belitung (Ministry of Energy and Mineral Resources Republic of Indonesia, 2017). The by-product of tin mining processing has so far not been utilized optimally. Research on the separation of REE from mining ore has been done a lot. Previous researchers have succeeded in developing solid-liquid extraction methods up to adsorption (Ji et al., 2022; Trinopiawan et al., 2020; Kusrini et al., 2020; 2019; 2018).

Monazite, as a by-product associated with tin mining, is a radioactive mineral because it contains uranium (U) and thorium (Th). The method of removing U and Th involves a reasonably long process to obtain REE hydroxide (REOH) (Setiawan, Anggraini, and Sunanti, 2020; Migdisov et al., 2019).

Generally, the separation of Ce from REOH is carried out by oxidation of Ce(III) to Ce(IV) and selective dissolution of trivalent REE (Bulfin et al., 2013; Ferdowsi and Yoozbashizadeh, 2017; McNeice, Kim, and Ghahreman, 2020; Pusporini, Amiliana, and Poernomo, 2020; Li et al., 2019). However, this technique results in low-purity products. Therefore, advanced separation and purification methods are still required. The Ce product resulting from the selective dissolution of REOH is a Ce-hydroxide concentrate which still contains other REE such as yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), promethium (Pm), and samarium (Sm). The separation of Ce from these elements can be done by liquid-liquid extraction. This method is easily applied with high selectivity in separating identical chemical properties of the elements (Neves et al., 2022; Dan, Ji, and Deqian, 2014).

The concept of separating Ce from other elements is carried out by forming Ce-complex in the organic phase. Various studies have been conducted to determine the effectiveness of organic solvents such as Tri Butyl Phosphate (TBP), di-(2-Ethylhexyl) 2-Ethylhexyl phosphate (DEHEHP), di-2(2-Ethylhexyl) phosphate (D2EHPA), and Cyanex923 in the extraction process of Ce from other REE (Li et al., 2018; Kuang et al., 2017; Formiga and de Morais, 2016; Cheremisina et al., 2015; Dan, Ji, and Deqian, 2014). The results showed that TBP was the most efficient solvent used in the industry to extract Ce from HNO₃ media (Dan, Ji, and Deqian, 2014).

Most organic solvents have high specific gravity and viscosity, so it is not easy to cause the solute transfer process from the aqueous phase to the organic phase. To simplify the process, the viscosity of the organic phase must be lowered by adding an organic diluent such as benzene, kerosene, and n-hexane (Hu et al., 2022; Tang et al., 2022; Shekaari, Zafarani-Moattar, and Mohammadi, 2020). Neither benzene nor n-heptane is inert. In addition, benzene and n-heptane are very volatile, toxic, and smelly, so they require special handling in terms of safety. It is different from kerosene diluent, which is inert, low volatility, and odorless.

The phenomenon of liquid-liquid equilibrium in a hybrid system involves interactions between the components. Mathematical models based on stoichiometry and non-stoichiometry to predict the distribution of neodymium (Nd) and yttrium (Y) in the extraction system with HNO₃ media and D2EHPA solvent have also been developed. The stoichiometric model was developed based on the reaction mechanism by considering the reaction coefficient. In contrast, the non-stoichiometric model is formulated by considering the phase equilibrium and chemical thermodynamics phenomenon. The non-stoichiometric model can describe the equilibrium phenomenon well (mean relative error is less than 10%) and is relatively simple (Pusporini et al., 2021).

This study aims to develop a liquid-liquid equilibrium model that can represent the distribution of Ce in the Ce extraction system from the Ce-hydroxide concentrate. The equilibrium model was developed through a thermodynamic approach based on phase and chemical equilibrium. The simulation results were then validated with laboratory research data at various concentrations of feed and solvent. The results of this study can be applied to determine the number of mixer settler stages.

2.1. Materials and Equipment

The primary material used in this study was the Ce-hydroxide concentrate leached from REOH monazite, HNO₃ solution (Merck, 65% w/w concentration), Tri Butyl Phosphate (Sigma Aldrich, 97%), and kerosene from Fischer Chemical. X-ray Fluorescence (XRF) using Ortec 7010 series. The composition of the Ce-hydroxide concentrate is shown in Table 1. Based on X-ray Fluorescence (XRF) using Ortec 7010 series analysis, the primary components in this sample were Ce, La, and Nd, with concentrations of 72.923%, 3.088%, and 6.081%, respectively. The cerium dominates the REE contents.  Monazite contains REEs with the largest element being Ce 18%, in the form of a 37.32% phosphate compound (Setiawan, Anggraini, and Sunanti, 2020). Monazite was decomposed and separated from the radioactive elements and phosphates to obtain REOH with the composition Ce 30%, La, 20%, Nd 14%, Y 1.7%, Sm 1.1% and Gd 0.22%. Separation of Ce from other REEs is by leaching REOH using dilute nitric acid. The Ce is insoluble and most of the Ce is in the solid residue, while the other REEs are mostly soluble according to the composition in table 1. A small number of other REEs are still left in the residue because the dilute nitric acid used has not dissolved all the REEs present.

2.2. Method

        The extraction process begins with a feed solution production, a dissolving Ce-hydroxide concentrate in HNO₃ (the S/L was 120 g/L). The effect of HNO₃ concentration, Ce concentration in feed solution, and solvent concentration were evaluated to investigate the effect of those parameters on the efficiencies of Ce extraction using liquid-liquid extraction. The concentration of Ce in the feed solution varied from 63.55 g/L to 86.75 g/L, analyzed using XRF, while the HNO₃ concentration ranged from 3M to 7 M. The feed and solvent solution (TBP in kerosene) with a volume ratio of (FA:FO) 1:1 was added to the Erlenmeyer. TBP concentration variation in kerosene varied from 5% to 25% (or 0.18 M to 0.92 M). The Erlenmeyer was shaken using a mechanical shaker at a speed of 150 rpm for 15 minutes. Then the mixture was allowed to stand for 30 minutes so that the aqueous and organic phases were separated. The aqueous and organic phases are separated using a separatory funnel, and then the final volume of each step is measured. The Ce concentration in the aqueous phase was analyzed using XRF. While the concentration of Ce in the organic phase was calculated using a mass balance.

2.3. Theory

The assumptions used to formulate a mathematical model for the distribution of Ce at equilibrium are 1) the organic phase and the aqueous phase are insoluble; 2) TBP solvent is only present in the organic phase; 3) no third layer is formed, and 4) the feed solution is conditioned as a dilute solution so that the Ce only interacts with TBP thus the result is Ce pure equilibrium.

Which is the concentration of free Ce in the aqueous phase (M), is the total Ce concentration in the organic phase (M) is the concentration of free Ce in the organic phase (M), is the concentration of Ce complex in the organic phase (M), is the total TBP concentration (M), is Ce phase equilibrium constant, is equilibrium constant defined in equation (5),  is the equilibrium constant defined in equation (5),  is the constant defined in equation (5), and   is the constant defined in equation (6).

The value of  are the measurement data in the study. Then the value of  is obtained from the mass balance calculation. Meanwhile, the value of  is obtained from a separate experiment, which extracts kerosene without TBP. While the value of  are evaluated by minimizing the Sum of Squared Errors (SSE) according to equation (8). Furthermore, the deviation between the data and the modeling results is expressed in the average relative error, as stated in equation (9).

3.1. Extraction System Study

Cerium dominates the REE content in the Ce-hydroxide sample, as mentioned in Table 1. Other REE elements, such as Y, La, Ce, Nd, Pr, Nd, Pm, and Sm, are still in the sample at low concentrations. This phenomenon indicated that the leaching method was not selective enough for extracted Ce; therefore, other procedures, such as liquid-liquid extraction, were needed to purify Ce.

The successful indicator of the extraction process can be seen from the extraction efficiency. The extraction efficiency is expressed as the mass percentage of solute that moves to the organic phase, as shown in equation (10).

        This research studied Ce extraction, in which the distribution in the aqueous phase and the organic phase was predicted to have reached an equilibrium condition. An equilibrium condition is achieved when there is no mass transfer from the aqueous phase to the organic phase, and instead, the amount of solute in the aqueous and organic phases remains constant. The REE extraction using neutral organophosphorus such as TBP, takes 15 minutes to reach equilibrium (Kuang et al., 2017). Thus, if the extraction time is selected for more than 15 minutes, the amount of Ce extracted into the organic phase tends to be constant, so it is ineffective.

        The stirring speed is regulated so that the contact between the feed solution and the organic solvent takes place optimally. A faster stirring speed would result in better extraction efficiency. However, it should also be noted that a quicker stirring speed could cause an emulsion to form. The emulsion makes the aqueous phase and organic phase hard to separate (Basuki et al., 2020). Therefore, a stirring speed of 150 rpm was used in this research.

3.1.1.  The effect of HNO₃ concentration

The feed solution is the aqueous phase in the extraction process which consists of a dissolved component (solute) and diluent. In this study, Ce concentrate was the solute, while HNO₃ was the diluent. Extraction of Ce on HNO₃ media using TBP solvent will follow the solvation reaction mechanism. This mechanism is based on the solvent type and the acid concentration used. The reaction steps that occur in the extraction of Ce are shown in equations (1) and (2).

Nitric acid is used as an intermediate in forming complexes between solvents and metals, as shown in equation (2). On the other hand, the acid concentration influences the extraction process. Previous studies have stated that acidic conditions can cause phosphate ester solvents such as TBP to be degraded. The mechanism of TBP degradation occurs through acid hydrolysis and dealkylation. Acid hydrolysis of TBP only occurs at low acid concentrations (<2 M), and dealkylation occurs at high acid concentrations, resulting in bond breaking (Gillens and Powell, 2013). Tri Butyl Phosphate is degraded to form Di Butyl Phosphate (DBP) and, to a lesser extent Mono Butyl Phosphate (MBP). The presence of this product affects the performance of TBP as an extraction solvent. Di Butyl Phosphate (DBP) and Mono Butyl Phosphate (MBP) react with the extracted metal to form complexes or precipitates which remain soluble in the aqueous phase of the nitrate. This phenomenon will reduce the effectiveness of extraction (Lamouroux et al., 2000).

This research has studied the effect of HNO₃ on various concentrations of 3M to 7M on the effectiveness of the extraction of Ce and other REE using 15% TBP solvent, the ratio of the volume of feed solution and the solvent is 1, the extraction time is 15 minutes, and the stirring speed is 150 rpm. The effect of variations in HNO₃ concentration on the extraction efficiency of Y, La, Ce, and Nd can be seen in Figure 1.

The extraction efficiency of Ce at 3M to 7M HNO₃ concentrations tends to be stable even though at 4M HNO₃ concentrations, the Ce extraction efficiency is at its lowest point of 46.74%. However, in these circumstances, a sizable quantity of Y and Nd were removed to the organic phase. That was 9.49% for Y and 5.73% for Nd. The extraction efficiency of Nd tends to increase along with the high concentration of HNO_3, but La in various conditions is not extracted.

Figure 1 The extraction efficiency of Y, La, Ce, and Nd at various concentrations of HNO₃ by using 15% of TBP-kerosene

Helaly et al. (2012) also conducted a similar study. Extraction was carried out at various concentrations of HNO₃, namely 4M to 10M. The results showed that at concentrations of HNO₃ 4M to 5M, the extraction efficiency of Ce would increase, but at concentrations of HNO₃ above 5M, the extraction efficiency of Ce would decrease. The increase in the concentration of HNO₃ triggers a competition against TBP to form a complex so that the extraction efficiency of Ce decreases (Helaly et al., 2012).

3.1.2.  The effect of feed concentration

In the liquid-liquid extraction process, the solute in the aqueous phase will diffuse toward the organic phase. The high feed concentration will increase the value of the distribution coefficient because it provides a significant probability for solutes to be able to diffuse into the organic phase. This diffusivity value is directly proportional to the mass transfer rate; the more significant the diffusivity value, the greater the mass transfer rate. Fick's law can explain this phenomenon.

Figure 2 The cerium extraction efficiency at various feed concentrations and 15% of TBP-kerosene

Similar results were also expressed by Helaly et al. (2011), who, in their research, extractederium from the oxidized concentrate using TBP diluted in kerosene (Helaly et al., 2012). The feed concentration is one of the variables studied, which varied from 20 g/L to 60 g/L in HNO₃ media. The Ce extraction efficiency tends to increase along with the increase in feed concentration. However, after the feed concentration reached 40g/L, the Ce extraction efficiency began to decrease.

3.1.3.  The effect of solvent concentration

Solvent concentration is one of the things that affect the extraction process. The higher the solvent concentration, the higher the viscosity. The high solvent's viscosity will reduce the diffusivity, resulting in the extraction process's difficulty. This phenomenon can be explained through the Stokes-Einstein Law.

Extraction was carried out at concentrations of TBP-kerosene from 5% to 25% at 3M HNO₃ concentration, and Ce concentration in feed was 0.6191M. The effect of TBP concentration on the extraction efficiency of REE elements, namely Y, La, Ce, and Nd, can be seen graphically in Figure 3.

Figure 3 shows that a higher TBP concentration results in higher Ce extraction efficiency. The higher TBP concentrations cause the probability to form a complex increase. This phenomenon indicates that the solvent's ability to extract the elements is also increasing. The use of TBP concentrations over 25% or 0.92 M that provide maximum extraction efficiency is also limited by viscosity. The higher solvent viscosity will decrease the mass transfer from the aqueous phase to the organic phase. As a result, efficiency is reduced, as described in the Fick and Stokes-Einstein laws, as mentioned in other experiments (Jorjani and Shahbazi, 2016; Khan et al., 2015; Helaly et al., 2012).

The results are presented in Figure 1, and Figure 3 shows that the extraction efficiency of Ce is higher than the extraction efficiency of other REEs, namely Y, La, and Nd. This phenomenon indicates that the TBP is more selective in attracting Ce, which is in line with the research results of Li et al. (2019). It can be concluded that the best conditions for Ce extraction from Ce-hydroxide concentrate are at a feed concentration of 0.62 M using 0.92 M or 25% of TBP concentration diluted in kerosene. In these conditions, the extraction efficiency of Ce has reached the highest value of 70.01%.

The research data in Figure 1, Figure 2, and Figure 3 are further developed in the model of the equilibrium approach. The deviation of each model compared to the research data is expressed in relative errors as indicated in equation (8). While the overall deviation of model data to research data is described as a relatively average error, as shown in equation (9).

Figure 3 The extraction efficiency of Y, La, Ce, and Nd at various TBP concentrations and 3M of HNO₃

3.2. The Cerium Extraction Equilibrium Model

The equilibrium model is developed by applying the simplification that chemical equilibrium is an analogy to phase equilibrium. This model can estimate the distribution of Ce in the aqueous and organic phases. This model has also been used as an approximation for other similar equilibrium systems, thereby saving the research time. In the industrial field, this model is used to design a mixer settler.

The Ce distribution equilibrium model in various feed and solvent concentrations is solved using equation (7). The value is obtained from the separate experiment without TBP. The depreciation of SSE resulted in the value of   is 1.1010.

The general equation describing the Ce distribution in this extraction system is shown in equation (11). The calculation results according to the data and model are presented in Table 2. Furthermore, the suitability between the model and data is graphically shown in Figure 4.

Table 2 The calculation result and comparison of 

Figure 4 Deviation of 

3.3. Equilibrium Model Application

       As shown in equation (11), the equilibrium model can be widely used to predict the number of stages while designing the mixer settler. The equation is developed based on the mass balance for the multistage counter current mixer settler and the Ce equilibrium (yN) equation, as mentioned in equations (12) and (13), respectively. The equations (12) and (13) are then solved simultaneously to obtain the optimum mixer settler stages.

Figure 5 Counter Current Extraction with Multistages Mixer Settler

The Ce extraction from Ce-hydroxide concentrate using TBP in a single stage can achieve extraction efficiency of up to 70%. Suppose the Ce concentration in the feed solution is 86.7540 g/L. In that case, the concentration of TBP is 20%, the volumetric ratio of feed solution to solvent is 1:1, and the extraction efficiency is desired to be 90% by entering these data and solving using equations (11) and (12) it is obtained that there are five stages of mixer settler needed. The Ce concentration in the aqueous and organic phase for each step of the mixer settler is presented in Table 3.

Table 3 The Ce concentration for each stage of the mixer settler

The Ce extraction from Ce-hydroxide concentrate leached from REOH monazite was successfully carried out using 0.92 M TBP in kerosene diluent so that the Ce extraction efficiency of 70.01% was obtained. The laboratory data is then used to develop the extraction equilibrium model to get a mathematical equation representing the Ce distribution phenomenon in the extraction system based on experimental laboratory data. The equilibrium model can describe the liquid-liquid equilibrium distribution in the extraction system, which is carried out with an average relative error of 8.53%. Five stages of mixer settler is needed to achieve 90% of separation efficiency.

     The authors gratefully acknowledge the Research and Technology Center for Accelerator for the financial support (Grant No. 080.01.1.017290/2021) and facility during this research.

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