Characteristics of Treated Monazite in Different Particle Sizes to Upgrade the Rare Earth Elements Content by Using Mechanochemical and Roasting Processes

Rare earth metals (REE) constitute one of the most crucial groups of elements globally. In Indonesia, deposits of REE have been recognized within monazite minerals extracted from tin mining operations in Bangka. Nevertheless, Indonesia has not used these deposits and has instead heavily relied on imports from other countries. This study aims to recover rare earth metals, specifically Cerium (Ce), Lanthanum (La), Ytterbium (Yb), and Yttrium (Y), from monazite sand, a by-product of tin mining. The method employed in this research is Mechanochemical Decomposition, involving the reaction of monazite samples with a 33 wt% solid Sodium Hydroxide (NaOH) to convert REE-Phosphate compound into REE-Hydroxide. The process begins with sample preparation, involving mechanochemical decomposition using a ball mill, followed by roasting at 400°C, leaching with distilled water, and subsequent steps of filtration and drying at 120°C. Various initial particle sizes, denoted as +65, -65 +100, -100 +140, -140 +170, and -170 mesh, were used to assess their effect on the recovery and grade of REE. Various initial particle sizes, denoted as +65, -65 +100, -100 +140, -140 +170, and -170 mesh, were used to assess their effect on the recovery and grade of REE. The series of procedures revealed notable improvements in the REE content in monazite. The highest recovery of Ce, La, Yb, and Y elements reached 46.43%, 70.08%, 22.69%, and 23.31%, while the highest grade of those elements obtained the values of 4.116%, 1.543%, 0.263%, and 1.681%, respectively. In alignment with prior research, the recovery of Light Rare Earth Elements (LREEs) surpasses that of Heavy Rare Earth Elements (HREEs). However, a unique observation emerges: a diminishing trend in both recovery and grade as particle sizes become finer. This unexpected phenomenon warrants further investigation to elucidate the mechanisms governing rare earth metal recovery from monazite sand under varying particle sizes. 

By-product; Monazite; Particle size; REE-Oxide; Roasting

    Rare earth metals (REE) are a mineral group consisting of 17 elements, namely 15 elements of the lanthanide group (La – Lu), Scandium (Sc), and Yttrium (Y) (Van-Gosen et al., 2014). The abundance of REE in the earth's crust is significant but not concentrated at one point (Lide, 2005). This causes the extraction of REEs as the main product does not have high economic value and is currently only extracted as a by-product (Gupta and Krishnamurthy, 2005). In the world, REE production is dominated by China by 90%, with minerals in the form of monazite and bastnasite (Bünzli and Pecharsky, 2016). Meanwhile, Indonesia currently imports 100% of its REE needs from countries such as China, Japan, Malaysia, and others (Ministry of Energy and Mineral Resources, 2020). Research from the Ministry of Energy and Mineral Resources stated that Indonesia has 180,000 tons of monazite reserves and 20,000 tons of xenotime in the Bangka and Belitung Islands. These reserves must be utilized to meet domestic needs, where the REE industrial sector is projected to increase from USD 2,831 million in 2021 to USD 5,520.2 million in 2028 (Fortune Business Insight, 2021). In Indonesia, research regarding REE is carried out not only from Monazite and Xenotime but also from by-product raw materials such as Zircon tailings, as reported by Trisnawati et al. (2020) and Prameswara et al.(2023), it can also be found in coal-related by-products such as fly ash from coal combustion reported by Prihutami et al., 2021.

Trisnawati et al. (2020) reported that Y, Gd, Er, Dy, and Yb as HREEs₂-(SO₄)₃ products can be recovered through sulfuric acid leaching, while the result from Prameswara et al. (2023) showed that Dy and Yb leaching from solid REE-hydroxide using HCl solution afforded a total of 61.71% Dy and 74.55% Yb recovery. The processing of REE from monazite can be achieved through chemical treatment, employing acid and base methods. Two common approaches involve NaOH digestion or leaching with H₂SO₄ to extract REE from monazite (Habashi, 2013). In addition to hydrometallurgical processing, pyrometallurgical processes use Ca or Mg-reducing agents through Metallothermic methods. In this study, a mechanochemical decomposition process was used, namely a reaction process that utilizes mechanical forces. The mechanochemical process has advantages, such as lower energy consumption, freedom of liquids, and absence of by-products (Mateti et al., 2021). In addition to mechanochemical, a roasting process is carried out to produce REE-Oxide and complete the reaction of NaOH with monazite.

In a few studies, the researcher has used mechanochemical processes, especially inorganic material. For instance, Bonnaillie et al. used the mechanochemical method for synthesizing SnO₂ nanostructures (Legendre, Poissonnet, and Bonnaillie, 2007). The other, Tsuzuki and Mccormick, use this process for nanoparticle synthesis and mechanical alloying (Tsuzuki and McCormick, 2004). Nowadays, the process is barely used in mineral processing, such as onazite sand (Kim et al., 2009). Kim et al. add the mechanochemical using NaOH to assist the leaching process (Kim et al., 2009). The result shows the formation of REE-hydroxide from REE-phosphate. On the other hand, Kumari et al. used NaOH during roasting at 400^o, continued with leaching, and showed promising REE recovery (Kumari et al., 2015).

2.1. Sample Preparation

Monazite sand was preliminarily dried at 80°C and subsequently underwent size reduction through ball milling, with a rotational speed ranging from 67.8 to 71.1 rpm over a 30-minute duration. The milling process yielded five distinct particle size fractions, specifically +65 mesh, -65 to +100 mesh, -100 to +140 mesh, -140 to +170 mesh, and -170 mesh.

2.2. Mechanochemical with NaOH 33wt%

Each particle size fraction was subjected to a mechanochemical treatment with the incorporation of 33 wt% sodium hydroxide (NaOH) from Pro Analys Merck 106498. The purpose of adding NaOH was to react with rare earth element phosphate (REE-PO₄) and produce rare earth element hydroxides (REE-OH). The mechanochemical process was conducted within a ball mill from NOAH NQM-4, employing a rotational speed of 650 rpm for a duration of 120 minutes.

2.3. Roasting

Before the roasting step, the mechanochemically treated samples were subjected to compaction using a Krisbow Hydraulic Press, thereby increasing the contact surface area. Subsequently, the roasting procedure was conducted in a Payun Tech Muffle Furnace within alumina crucibles, sustained at a temperature of 400°C for a duration of 120 minutes. This process served the dual purpose of finalizing the NaOH and REE-PO₄ reaction and facilitating the oxidation of rare earth elements inherent in the sample.

2.4. Distilled water washing, filtration, and drying

The roasted product was subjected to leaching with distilled water at a controlled temperature of 70°C and a rotational speed of 680 rpm, maintained for a duration of 30 minutes, which was discussed in the next section as a washing process. The process was conducted by a Hot Plate Magnetic Stirrer from Thermo Scientific Cimarec SP88857105. The primary objective of this step was to facilitate the dissolution of the formed Na₃PO₄ compound. Following completion of the leaching process, the mixture was subjected to filtration, yielding a precipitate. This precipitate was subsequently isolated and subjected to drying at 120°C for 120 minutes using Memmert UN 55 Oven.

2.5. Characterization

To identify the REE content in the initial sample, the ICP-OES test was conducted at Laboratorium Teknologi Bahan Galian Nuklir National Research and Innovation Agency (BRIN). X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) analyses were conducted on dried samples as part of a comprehensive characterization step. The objective of the XRD analysis, undertaken at the Center for Materials Processing and Failure Analysis (CMPFA) at Universitas Indonesia, and the XRF is conducted at the Laboratory of the National Research and Innovation Agency (BRIN), focused on quantifying the concentration increments of key elements, specifically Cerium (Ce), Lanthanum (La), Ytterbium (Yb), and Yttrium (Y). These analyses collectively provided essential insights into the composition and elemental content of the investigated specimens, contributing to a comprehensive understanding of their properties and potential applications.

3.1. Monazite Raw Sand Characteristics

        Following the method outlined by Zhang and Noble, the initial monazite is characterized using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). (Zhang and Noble, 2020). In this study, the REEs content detected by the characterization process involved ICP-OES present in the monazite sample is shown in Table 1. Cerium exhibited the highest concentration among the other REEs, followed by several other Light Rare Earth Elements (LREEs), as summarized in Table 1.

        Before the mechanochemical and roasting processes, sample preparation was carried out on the monazite. Firstly, the monazite was subjected to a drying process to eliminate any residual moisture present (Jung, Lee, and Yoon, 2018). Subsequently, size reduction was performed to obtain monazite particles of specific sizes: +65mesh, -65mesh +100mesh, -100mesh +140mesh, -140mesh +170mesh, and -170mesh. The obtained five different particle sizes underwent further characterization through X-ray Fluorescence (XRF) analysis. This analysis served to determine the content of cerium (Ce), lanthanum (La), yttrium (Y), and ytterbium (Yb), as presented in Table 2. The XRF results also provide information about the dominant elements present in the sample, including silicon (Si), iron (Fe), and tin (Sn).

Table 1 ICP-OES results of initial monazite samples

Table 2 XRF results of the initial monazite sample

3.2. Converting Rare Earth Phosphate to Rare Earth Hydroxide

        In order to decompose phosphates in REE, monazite is milled with the addition of NaOH. The milling process took place in a planetary ball mill for 120 minutes at 680 rpm, serving two primary purposes: mixing and milling. Monazite is mixed well with NaOH during milling, while the mechanical force helps break bonds between REE and phosphates. NaOH will react with phosphate, producing REE hydroxides and Na₃PO₄ (Kim et al., 2009). The mixture changes in color from light grey and white to dark grey. Besides that, the photo shows some agglomeration in the mixture. This might be caused by NaOH's characteristic of being hygroscopic (Ahmadi and Seyedin, 2019). The reaction occurs during mechanochemical is as shown:

Besides that, NaOH also reacts with CO₂ in the atmosphere, creating Na₂CO₃ as a product (Monash Scientific Glass Blowing Services, n.d.). The reaction occurs as shown:

3.3. Roasting in atmospheric condition

        This process aims to change hydroxide from mechanochemical to oxide compound. The method includes heating samples at 400^o for 2 hours in the atmospheric condition. The reaction that occurs during roasting is as shown:

        The result shows the change in morphology and color. The pellets show expansion and shift in color from dark grey to yellow-greenish. Expansion on +65mesh and -65mesh +100mesh seen are much more significant than the rest. Lower expansion on -100mesh +140mesh, -140mesh +170mesh, and -170mesh is caused of denser pellets. Expansion might occur because of CO₂ formation during the reaction between phosphate and Na₂CO₃, as shown in equation 1 (Kumari et al., 2015). In addition, the expansion might appear because of the voids left from compaction that expand during the heating/roasting process.

        The reaction can be validated with XRD, where REE was found in the oxide form. XRD also shows Na in Na₃PO₄ and Na₂CO₃ form, which indicates equations (1) and (2) have occurred. SnO₂, SiO₂, Fe₃O₄, and FeTiO₃ are monazite impurities as a cassiterite by-product, as shown in Figure 1 (Harjanto, Virdhian, and Afrilinda, 2013).

Figure 1 XRD results after the roasting process.

3.4. Leaching with distilled water to dissolve Na₃PO₄

        Na (Na₂CO₃ and Na₃PO₄) dissolve in water. Therefore, the by-product will dissolve by agitating the sample in distilled water, leaving only REE residue (Kim et al., 2009; PubChem, n.d.). Leaching is done at 70^o for 30 minutes at 680 rpm. The results show changes in color from yellow-greenish to brown-reddish, while the morphology does not.

        XRD shows that Na is still present as Na₂CO₃ and Na₃PO₄, indicating insufficient washing time. On the other hand, the REEs and impurities compound's form doesn't change. The analysis results of the XRD pattern are shown in Figure 2. Upon completion of the washing and drying processes, an analysis of the residue revealed an increase in the concentration levels of elements Ce, La, Y, and Yb. The X-ray Fluorescence (XRF) characterization results presented in Table 3. demonstrate the observed changes in element concentrations.

Figure 2 XRD results after distilled water washing and drying

Table 3 XRF result of the product after distilled water washing and drying

3.5 Product Morphology after roasting and after the leaching process

        SEM results with 1000 x magnification of the sample after the roasting process can be seen in Figure 3. Meanwhile, the SEM results with 1000 x magnification of the sample after the washing and drying process can be seen in Figure 4. According to EDS result, in the samples after the washing process with sizes -100mesh +140mesh and -170mesh, the Ce content was 33.17% and 0.92%, respectively. Meanwhile, other samples did not show any REE content.

Figure 3 SEM results after roasting for particle size (a) +65mesh (b) -65mesh +100mesh (c) -100mesh +140mesh (d) -140mesh +170mesh (e) -170mesh

Figure 4 SEM results after washing for particle size (a) +65mesh (b) -65mesh +100mesh (c) -100mesh +140mesh (d) -140mesh +170mesh (e) -170mesh

Table 4 EDS result of the product after distilled water washing and drying

%recovery calculation was done on the final product using REE contents based on equation (4).

        The result of the %recovery calculation from Ce, La, Y, and Yb is present on a graph shown in Figure 5. Based on that picture, the coarser the initial particle size, the higher %recovery will be earned. This phenomenon can be attributed to the fact that samples with finer particle sizes tend to experience greater losses during the processing stages, resulting in higher weight loss percentages.

Figure 5 Effect of initial particle size on (a) %recovery and (b) %grade of Ce, La, Y, and Yb

Monazite contains REE, mainly LREE. Sn, Fe, and Ti present as impurities. NaOH is an effective reagent used for mechanochemical decomposition to remove phosphate from monazite by reacting and producing Na₃PO₄. Ce, La, Yb, and Y had their content increased after the procedure. The highest %grade of Ce and La achieved at particle size -65mesh +100mesh respectively by 4.116% and 1.543% each. Yb and Y have the highest %grade at particle size +65mesh, respectively, by 0.263% and 1.681% each. The highest %recovery of Ce, La, Yb, and Y was achieved at particle size +65mesh by 46.43%, 70.08%, 22.69%, and 23.31% respectively, which have the final form of CeO₂, La₂O₃, Yb₂O₃, and Y₂O₃.

    The authors express their gratitude to the Center for Materials Processing and Failure Analysis (CMPFA) Department of Metallurgical and Materials Engineering at Universitas Indonesia for their supports in conducting this research.

Ahmadi, M., Seyedin, S.H., 2019. Investigation of NaOH Properties, Production and Sale Mark in the world Modern Spent Caustic Treatment System View project Investigation of NaOH Properties, Production and Sale Mark in the world. Journal of Multidisciplinary Engineering Science and Technology (JMEST), Volume 6(10), pp. 10809–10813

Bünzli, J.-C., Pecharsky, V., 2016. Handbook on the Physics and Chemistry of Rare Earths. 1^st Edition. Amsterdam: North-Holland

Fortune Business Insight, 2021. Rare Earth Elements Market Size & Forecast Report [2021-2028], Fortunebusinessinsight.Com. Available online at: https://www.fortunebusiness insights.com/rare-earth-elements-market-102943, Accessed on October 3, 2022

Gupta, C.K., Krishnamurthy, N., 2005. Extractive Metallurgy of Rare Earths. Florida, United States of America: CRC Press

Habashi, F., 2013. Extractive Metallurgy of Rare Earths. Canadian Metallurgical Quarterly, Volume 52(3), pp. 224–233

Harjanto, S., Virdhian, S., Afrilinda, E., 2013. Characterization of Indonesia Rare Earth Minerals and Their Potential Processing Techniques Synthesis of Zeolite Catalyst from Badau Belitung Kaolin View project Logam Tanah Jarang View Project. Available online at https://www.researchgate.net/publication/265096672, Accessed on October 15, 2022

Jung, H., Lee, Y.J., Yoon, W.B., 2018. Effect of Moisture Content on The Grinding Process and Powder Properties In Food: A Review. Processes, Volume 6(6), p. 69

Kim, W., Bae, I., Chae, S., Shin, H., 2009. Mechanochemical Decomposition of Monazite to Assist the Extraction of Rare Earth Elements. Journal of Alloys and Compounds, Volume 486(1–2), pp. 610–614

Kumari, A., Panda, R., Jha, M.K., Lee, J.Y., Kumar, J.R., Kumar, V., 2015. Thermal Treatment for The Separation of Phosphate and Recovery of Rare Earth Metals (REMs) from Korean Monazite. Journal of Industrial and Engineering Chemistry, Volume 21, pp. 696–703

Legendre, F., Poissonnet, S., Bonnaillie, P., 2007. Synthesis of Nanostructured SnO2 Materials by Reactive Ball-Milling. Journal of Alloys and Compounds, Volume 434, pp. 400–404

Lide, D.R., 2005. CRC Handbook of Chemistry and Physics, Journal of Chemical Technology & Biotechnology. Available online at https://doi.org/10.1002/jctb.280500215, Accessed on October 3, 2022

Mateti, S., Mathesh, M., Liu, Z., Tao, T., Ramireddy, T., Glushenkov, A.M., Yang, W., Chen, Y.I., 2021. Mechanochemistry: A Force in Disguise and Conditional Effects Towards Chemical Reactions. Chemical Communications, Volume 57(9), pp. 1080–1092

Ministry of Energy and Mineral Resources, 2020. Booklet Tambang Tanah Jarang (Rare Earth Mining Booklet). Available online at: https://www.esdm.go.id/id/ booklet/booklet-tambang-tanah-jarang-2020, Accessed on October 14, 2022

Monash Scientific Glass Blowing Services, n.d. NaOH. Available online at: http://www.monashscientific.com.au/NaOH.htm, Accessed on May 7, 2023

Prameswara, G., Trisnawati, I., Handini, T., Poernomo, H., Mulyono, P., Prasetya, A., Petrus, H.T.M.B., 2023. Recovery of Critical Elements (Dysprosium and Ytterbium) from Alkaline Process of Indonesian Zircon Tailings: Selective Leaching and Kinetics Study. International Journal of Technology, Volume 14(4), pp. 770–779

Prihutami, P., Sediawan, W.B., Prasetya, A., Petrus, H.T.B.M., 2021. A Product Diffusion Model for the Extraction of Cerium and Yttrium from Magnetic Coal Fly Ash Using Citric Acid Solution. International Journal of Technology, Volume 13(4), pp. 921–930

PubChem, n.d., Sodium Hydroxide. Available online at: https://pubchem.ncbi.nlm.nih.gov/compound/Sodium-hydroxide, Accessed on May 11, 2023

Trisnawati, I., Prameswara, G., Mulyono, P., Prasetya, A., Petrus, H.T.B.M., 2020. Sulfuric Acid Leaching of Heavy Rare Earth Elements (HREEs) from Indonesian Zircon Tailing. International Journal of Technology, Volume 11(4), pp. 804–816

Tsuzuki, T., McCormick, P.G., 2004. Mechanochemical Synthesis of Nanoparticles. Journal of Materials Science, Volume 39, pp. 5143–5146

Van-Gosen, B.S., Phillip L.V., Keith R.L., Joseph G., Robert R., 2014. The Rare-Earth Elements — Vital to Modern Technologies and Lifestyles. USGS Mineral Resources Program, p. 4

Zhang, W., Noble, A., 2020. Mineralogy Characterization and Recovery of Rare Earth Elements from The Roof And Floor Materials of The Guxu Coalfield. Fuel, Volume 270, p. 117533