Thermodynamic Evaluation of Thorium Speciation in Sulfate Systems Using Pourbaix Diagrams for Optimized Water Leaching of Baked Lanthanide Concentrate

Nurul Aniyyah Mohamad Sobri; Noorlisa Harun; Siti Zubaidah Sulaiman; Ruwaida Abdul Rasid

doi:10.22146/ijc.112317

Thermodynamic Evaluation of Thorium Speciation in Sulfate Systems Using Pourbaix Diagrams for Optimized Water Leaching of Baked Lanthanide Concentrate

https://doi.org/10.22146/ijc.112317

Nurul Aniyyah Mohamad Sobri⁽¹⁾, Noorlisa Harun^(2*), Siti Zubaidah Sulaiman⁽³⁾, Ruwaida Abdul Rasid⁽⁴⁾

(1) Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang Al-Sultan Abdullah, Lbh Persiaran Tun Khalil Yaakob, Kuantan 26300, Pahang, Malaysia; Faculty of Engineering Technology, Universiti College TATI, Teluk Kalong, 24000 Kemaman, Terengganu, Malaysia
(2) Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang Al-Sultan Abdullah, Lbh Persiaran Tun Khalil Yaakob, Kuantan 26300, Pahang, Malaysia
(3) Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang Al-Sultan Abdullah, Lbh Persiaran Tun Khalil Yaakob, Kuantan 26300, Pahang, Malaysia
(4) Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang Al-Sultan Abdullah, Lbh Persiaran Tun Khalil Yaakob, Kuantan 26300, Pahang, Malaysia
(*) Corresponding Author

Abstract

Thorium is increasingly recognized as a viable alternative to uranium for next-generation nuclear fuel, driving interest in recovery from monazite minerals. This study examines the thermodynamic stability of thorium in aqueous sulfate systems during the water leaching stage of baked lanthanide concentrate obtained through the cracking process. Pourbaix diagrams were generated to model thorium speciation under varying concentrations of thorium, phosphate, sulfate, and temperature. The introduction of sulfate in the Th–PO₄–SO₄–H₂O system significantly expanded the stability region of soluble Th(SO₄)₂, over a broader pH range compared to systems without sulfate. At 40 °C, in the presence of 0.001 M thorium and 0.2 M phosphate, Th(SO₄)₂ remained stable within the acidic pH range of 0 to 2.28 at 2.0 M sulfate, with the stability range widening at higher sulfate levels. These findings highlight the importance of the cracking process; using concentrated H₂SO₄ produces sulfate-rich baked concentrate that enhances thorium solubility during water leaching. However, elevated temperatures reduce the stability of Th(SO₄)₂ under acidic conditions, promoting the formation of insoluble species such as Th(HPO₄)₂·4H₂O and ThO₂. Thus, maintaining strong acidity, ambient temperature, and sufficient sulfate levels is essential to improve thorium solubility during water leaching.

Keywords

lanthanide concentrate; thorium; water leaching; Pourbaix diagram; thermodynamic stability

Full Text:

Full Text PDF

References

[1] Coherent Market Insights, 2025, Thorium Reactor Market Size and Share Analysis- Growth Trends and Forecasts (2025-2032), https://www.coherentmarketinsights.com/industry-reports/thorium-reactor-market, accessed on 2 July 2025.

[2] Winoto, G., Mulyono, P., Suyanti, S., and Prasetya, A., 2025, Preliminary study on leaching capacity of rare earth elements from coal fly ash by using citric acid solution, Indones. J. Chem., 25 (4), 1028–1037.

[3] Jyothi, R.K., De Melo, L.G.T.C., Santos, R.M., and Yoon, H.S., 2023, An overview of thorium as a prospective natural resource for future energy, Front. Energy Res., 11, 1132611.

[4] Kemp, D., and Cilliers, A.C., 2014, Fluorination of rare earth, thorium and uranium oxides and phosphates from monazite: A theoretical approach, Adv. Mater. Res., 1019, 439–445.

[5] da F. Iwahara, L.K., de Oliveira, M.S., and de Alencar, M.A.V., 2019, Evaluation of the effect of three constituent metals of monazita on the radiosensibility of human osteoblasts, J. Environ. Radioact., 208-209, 106011.

[6] Orabi, A.H., Mohamed, B.T., Ismaiel, D.A., and Elyan, S.S., 2021, Sequential separation and selective extraction of uranium and thorium from monazite sulfate leach liquor using dipropylamine extractant, Miner. Eng., 172, 107151.

[7] Panda, N.K., Rajagopalan, V., and Ravi, G.S., 2003, Rare-earth-element geochemistry of placer monazites from Kalingapatnam Coast, Srikakulam District, Andhra Pradesh, J. Geol. Soc. India, 62 (4), 429–438.

[8] Che Hak, R.C., Januri, Z., Mustapha, I., and Mahat, S., 2022, Extraction of thorium oxide (ThO₂) from Malaysian monazite through alkali digestion: Physical and chemical characterization using X-ray analysis, Key Eng. Mater., 908, 509–514.

[9] Korbel, C., Foucaud, Y., and Vanderbruggen, A., 2026, “Mineral Processing Applied to the Treatment of Primary and Secondary Ores” in Sustainable Processes in the Circular Economy, Eds. Chagnes, A., Elsevier, Amsterdam, Netherlands, 83–122.

[10] Gioiello, S., Cazzaniga, A., Bouzahzah, H., Pirard, E., and Santoro, L., 2025, Re-use of Montorfano granite waste (Verbania, Piedmont, Italy) for REEs-recovery along the feldspar industrial chain. Results from pre-concentration tests, Miner. Eng., 228, 109309.

[11] Belova, V.V., 2017, Development of solvent extraction methods for recovering rare earth metals, Theor. Found. Chem. Eng., 51 (4), 599–609.

[12] Demol, J., Ho, E., Soldenhoff, K., Karatchevtseva, I., and Senanayake, G., 2022, Beneficial effect of iron oxide/hydroxide minerals on sulfuric acid baking and leaching of monazite, Hydrometallurgy, 211, 105864.

[13] Supriadi, H., Trisnawati, I., Handini, T., Susilowati, S.R., Hadi Sujoto, V.S., Mulyono, P., and Petrus, H.B.M., 2023, Kinetics study of yttrium leaching from zircon tailings using sulfuric acid, Indones. J. Chem., 23 (2), 489–498.

[14] Demol, J., Ho, E., Soldenhoff, K., and Senanayake, G., 2024, The impact of apatite on sulfuric acid baking of a monazite concentrate and the benefit of goethite addition on rare earth leaching, Hydrometallurgy, 226, 106296.

[15] Demol, J., Ho, E., and Senanayake, G., 2018, Sulfuric acid baking and leaching of rare earth elements, thorium and phosphate from a monazite concentrate: Effect of bake temperature from 200 to 800 °C, Hydrometallurgy, 179, 254–267.

[16] Aziman, E.S., and Ismail, A.F., 2020, Frontier looking of rare-earth processed residue as sustainable thorium resources: An Insight into chemical composition and separation of thorium, Prog. Nucl. Energy, 128, 103471.

[17] Yemel’Yanov, V.S., and Yevstyukhin, A.I., 1969, The Metallurgy of Nuclear Fuel, Pergamon Press, Oxford, UK.

[18] Qi, D., 2018, “Extraction of Rare Earths From RE Concentrates” in Hydrometallurgy of Rare Earths, Elsevier, Amsterdam, Netherlands, 1–185.

[19] Borai, E.H., Abd El-Ghany, M.S., Ahmed, I.M., Hamed, M.M., El-Din, A.M.S., and Aly, H.F., 2016, Modified acidic leaching for selective separation of thorium, phosphate and rare earth concentrates from Egyptian crude monazite, Int. J. Miner. Process., 149, 34–41.

[20] Hou, S., Huang, W., Liu, Y., Zhang, B., and Liu, C., 2025, Effect of phosphoric acid on leaching of monazite during low-temperature sulfuric acid cyclic leaching process, J. Rare Earths, 43 (4), 784–793.

[21] Echeverry-Vargas, L., Rojas-Reyes, N.R., and Ocampo-Carmona, L.M., 2023, Recovery of light rare earth elements, cerium, lanthanum, and neodymium from alluvial gold mining waste from the Bagre-Nechí mining district in Colombia using acid leaching, oxalate precipitation and calcination, Hydrometallurgy, 216, 106009.

[22] Sadri, F., Nazari, A.M., and Ghahreman, A., 2017, A review on the cracking, baking and leaching processes of rare earth element concentrates, J. Rare Earths, 35 (8), 739–752.

[23] Al-Areqi, W.M., Ab. Majid, A., and Sarmani, S., 2014, Digestion study of water leach purification (WLP) residue for possibility of thorium extraction, Malays. J. Anal. Sci., 18 (1), 221–225.

[24] Al-Areqi, W.M., Ab. Majid, A., and Sarmani, S., 2014, Separation of thorium (IV) from lanthanide concentrate (LC) and water leach purification (WLP) residue, AIP Conf. Proc., 1614 (1), 482–485.

[25] Pedeferri, P., 2018, “Pourbaix Diagrams” in Corrosion Science and Engineering, Springer International Publishing, Cham, Switzerland, 57–72.

[26] Gao, Y., Xu, L., Zhang, M., Zhang, Q., Yang, Z., Yang, J., Xu, Z., Lv, Y., and Wang, Y., 2021, Ultra-selective ion sieve for thorium recovery from rare earth elements using oxygen-rich microporous carbon adsorption, J. Hazard. Mater., 417, 126115.

[27] Patel, A.M., Nørskov, J.K., Persson, K.A., and Montoya, J.H., 2019, Efficient Pourbaix diagrams of many-element compounds, Phys. Chem. Chem. Phys., 21 (45), 25323–25327.

[28] Gulyaev, A., Volkova, E., Zavyalov, A.M., Aksenov, V., and Kondrutieva, O., 2019, Method for the evaluation of thermodynamically stable forms of iron existence using software, 2019 IEEE Conference of Russian Young Researchers in Electrical and Electronic Engineering (ElConRus), Saint Petersburg and Moscow, Russia, 28-31 January 2019, 2308–2310.

[29] Ibrahim, W.H.W., Yahya, F.N., and Rasid, R.A., 2023, Investigate the behaviour of different temperature in the digestion of monazite concentrate through Pourbaix diagram, AIP Conf. Proc., 2792 (2), 040001.

[30] Barragan, J.A., Alemán Castro, J.R., Peregrina-Lucano, A.A., Sánchez-Amaya, M., Rivero, E.P., and Larios-Durán, E.R., 2021, Leaching of metals from e-waste: From its thermodynamic analysis and design to its implementation and optimization, ACS Omega, 6 (18), 12063–12071.

[31] Lin, P., Yang, X., Werner, J.M., and Honaker, R.Q., 2021, Application of Eh-pH diagrams on acid leaching systems for the recovery of REEs from bastnaesite, monazite and xenotime, Metals, 11 (5), 734.

[32] Nasuha Yahya, F., Nadirah Mat Suli, L., Hanisah Wan Ibrahim, W., and Abdul Rasid, R., 2019, Thermodynamic evaluation of the aqueous stability of rare earth elements in sulfuric acid leaching of monazite through Pourbaix diagram, Mater. Today: Proc., 19, 1647–1656.

[33] Ahmat, F., Mohd Yunus, M.Y., Abd Aziz, B., and Hisyam, A., 2018, Application of thermodynamic modelling and experimental investigation of leaching yttrium from liquid crystal display, J. Chem. Eng. Ind. Biotechnol., 3 (1), 17–25.

[34] Kim, E., and Osseo-Asare, K., 2012, Aqueous stability of thorium and rare earth metals in monazite hydrometallurgy: Eh–pH diagrams for the systems Th–, Ce–, La–, Nd– (PO₄)–(SO₄)–H₂O at 25 °C, Hydrometallurgy, 113-114, 67–78.

[35] Shuai, G., Zhao, L., Wang, L., Long, Z., and Cui, D., 2017, Aqueous stability of rare earth and thorium elements during hydrochloric acid leaching of roasted bastnaesite, J. Rare Earths, 35 (12), 1255–1260.

[36] Huang, H.H., 2016, The Eh-pH diagram and its advances, Metals, 6 (1), 23.

[37] Haung, 1989, Construction of Eh–pH and other stability diagrams of uranium in a multicomponent system with a microcomputer—II. Distribution diagrams, Can. Metall. Q., 28 (3), 235–239.

[38] Haung, H.H., and Cuentas, L., 1989, Construction of Eh–pH and other stability diagrams of uranium in a multicomponent system with a microcomputer—I. Domains of predominance diagrams, Can. Metall. Q., 28 (3), 225–234.

[39] Legemza, J., Fröhlichová, M., Findorák, R., and Džupková, M., 2019, Modelling of mass and thermal balance and simulation of iron sintering process with biomass, Metals, 9 (9), 1010.

[40] Mohamad Sobri, N.A., Harun, N., and Mohd Yunus, M.Y., 2025, Enhancing rare earth elements stability in ion adsorption clays during ion-exchange leaching with salt solutions: A thermodynamic approach through Pourbaix diagrams, J. Teknol., 88, 41–51.

[41] Pourbaix, M., 1974, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, Texas, US.

[42] The ThX Project, 2024, 13^th Technical Team Meeting (TTM 13) with Lynas Rare Earths, Lynas Rare Earths General Meeting 2024, Unpublished Confidential Document.

[43] She, H.D., Fan, H.R., Chen, B., Yang, K.F., Li, X.C., and Wang, Q.W., 2025, The occurrence and distribution of thorium in the giant Bayan Obo REE-Nb-Fe polymetallic deposit, China, Ore Geol. Rev., 186, 106883.

[44] Udayakumar, S., Baharun, N., Rezan, S.A., Ismail, A.F., and Mohamed Takip, K., 2021, Economic evaluation of thorium oxide production from monazite using alkaline fusion method, Nucl. Eng. Technol., 53 (7), 2418–2425.

[45] Udayakumar, S., Baharun, N., Rezan, S.A., Ismail, A.F., and Takip, K.M., 2021, Kinetic modelling of alkaline leaching of thorium hydroxide (Th(OH)₄) from monazite, AIP Conf. Proc., 2423 (1), 020027.

[46] Zhai, B., Tian, Q., Li, N., Yan, M., and Henderson, M.J., 2022, SAXS study of the formation and structure of polynuclear thorium(IV) colloids and thorium dioxide nanoparticles, J. Synchrotron Radiat., 29 (2), 281–287.

[47] Magini, M., Cabrini, A., Scibona, G., Johansson, G., and Sandström, M., 1976, On the structure of highly hydrolyzed thorium salt solutions, Acta Chem. Scand., 30, 437–447.

[48] Shahr El-Din, A.M., Borai, E.H., and Abd El-Ghany, M.S., 2018, Selective separation of thorium from rare earth elements liquor during the alkaline processing of Egyptian monazite concentrate, Main Group Chem., 17, 79–88.

[49] Azaroual, M., Kervevan, C., Lassin, A., André, L., Amalhay, M., Khamar, L., and Guendouzi, M.E., 2012, Thermo-kinetic and physico-chemical modeling of processes generating scaling problems in phosphoric acid and fertilizers production industries, Procedia Eng., 46, 68–75.

[50] Lassis, M., Mizane, A., Dadda, N., and Rehamnia, R., 2015, Dissolution of Djebel Onk phosphate ore using sulfuric acid, Environ. Nanotechnol., Monit. Manage., 4, 12–16.

[51] Liang, H., Zhang, P., Jin, Z., and DePaoli, D., 2017, Rare-earth leaching from Florida phosphate rock in wet-process phosphoric acid production, Miner. Metall. Process., 34 (3), 146–153.

[52] Schorr, M., and Valdez, B., 2016, The phosphoric acid industry: Equipment, materials, and corrosion, Corros. Rev., 34 (1-2), 85–102.

[53] Ballester, A., Blázquez, M.L., González, F., and Muñoz, J.A., 2007, “Catalytic Role of Silver and Other Ions on The Mechanism of Chemical and Biological Leaching” in Microbial Processing of Metal Sulfides, Eds. Donati, E.R., and Sand, W., Springer Netherlands, Dordrecht, Netherlands, 77–101.

[54] Elyayaoui, A., Boulhassa, S., and Guillaumont, R., 1990, Complexes of cadmium with phosphoric acid, J. Radioanal. Nucl. Chem., 142 (2), 403–415.

[55] Mashifana, T., Sebothoma, J., Falayi, T., and Ntuli, F., 2019, “Recovery of Thorium from Phosphogypsum by Acid Leaching” in Beneficiation of Phosphates: Sustainability, Critical Materials, Smart Processes, Society for Mining, Metallurgy & Exploration, Englewood, CO, US, 107–114.

[56] Chen, S., Bo, T., Zhang, Y., Wang, Y., Li, X., Zhang, S., and Liu, Y., 2025, Biomimetic phosphatization of nano zero-valent iron for thorium removal and waste remediation from rare earth leachates, Environ. Res., 285, 122390.

[57] Erintina, M.D., Md Yunus, A.I., Mohamed, A.Z., Paulus, W., and Ismail, S., 2024, Leaching of rare earth elements from Malaysian monazite: Effect of dephosphorization, IOP Conf. Ser.: Earth Environ. Sci., 1422 (1), 01210.

[58] Berry, L., Agarwal, V., Galvin, J., and Safarzadeh, M.S., 2018, Decomposition of monazite concentrate in sulphuric acid, Can. Metall. Q., 57 (4), 422–433.

[59] Jeon, J.H., Cueva Sola, A.B., Park, G.B., Lee, J.Y., Kim, H., and Jyothi, R.K., 2022, Development of a recovery process of thorium and possible separation of rare earths from Korean monazite, Geosystem Eng., 25 (5-6), 239–245.

[60] Di Bernardo, P., Endrizzi, F., Melchior, A., Zhang, Z., Zanonato, P.L., and Rao, L., 2018, Complexation of Th(IV) with sulfate in aqueous solution at 10–70 °C, J. Chem. Thermodyn., 116, 273–278.

[61] Kotowicz, M., and Jasińska, M., 2021, An improved model for interpretation of micromixing experiment with iodide–iodate method and sulphuric acid, Chem. Eng. Res. Des., 165, 270–279.

[62] Wang, P., Liu, X., Zeng, G., Ma, J., and Xia, F., 2022, Effects of temperature on the leaching behavior of Pb from cement stabilization/solidification-treated contaminated soil, Separations, 9 (12), 402.

DOI: https://doi.org/10.22146/ijc.112317