Leaching tungsten and rare earth elements from scheelite through H2SO4–H3PO4 mixed acid decomposition

doi:10.1016/j.mineng.2020.106526

Minerals Engineering

Volume 156, 1 September 2020, 106526

https://doi.org/10.1016/j.mineng.2020.106526 Get rights and content

Highlights

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Raising reaction temperature was beneficial to leaching of tungsten and RE.
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Properly increasing acid concentration was helpful to leaching of tungsten and RE.
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Adding dihydrate calcium sulphate can promote the tungsten and RE leaching.
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Circulating leaching can increase the concentration of tungsten and RE.
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Tungsten and RE can be leached from scheelite using the H₂SO₄-H₃PO₄ mixed acid.

Abstract

Rare earth elements (REEs) possess distinctive physical and chemical properties and are mainly extracted from several rare earth (RE) and ion-absorbing type RE ores. Traditional RE sources are unable to meet the global demand, making it necessary to explore other resources for the extraction of rare REEs. While this is possible, the recovery of REEs from the leach residue of such resources is uneconomical due to the enormous consumption of mineral acids. H₂SO₄–H₃PO₄ mixed acid decomposition is an effective method for leaching tungsten from scheelite and is evaluated in this study for its efficient extraction of tungsten and REEs from the same mineral. The scheelite used in this study contains REEs such as lanthanum, cerium, and neodymium. The properties of the REs in the leaching system are used to evaluate our method. Additionally, the effects of the reaction conditions on the leaching of tungsten and REEs are investigated. The results show that increasing the reaction temperature and H₂SO₄ concentration favors the leaching process. However, high H₂SO₄ concentration also promotes the transformation of gypsum from CaSO₄·2H₂O to CaSO₄, which affects the leaching process negatively. Further, excessive H₃PO₄ concentration increases the viscosity of the leaching solution. Increasing the liquid–solid ratio is beneficial for the dissolution of scheelite. The addition of an appropriate amount of CaSO₄·2H₂O promotes the leaching of tungsten and REEs but the addition of PEG-400 does not. In terms of the experimental conditions used in this study, the leaching efficiency of W, La, Ce, and Nd can reach approximately 100%, 96.6%, 66.6%, and 57.1%, respectively. After the circulation leaching process, the concentrations of tungsten and the REEs in the leach liquor are increased, which proves these materials can be efficiently leached from scheelite using the H₂SO₄–H₃PO₄ mixed acid.

Introduction

Rare earth elements (REEs) are listed as strategic material resources all around the world, and are widely applied in advanced manufacturing due to their distinctive physical and chemical properties (Xu, 1995, Han et al., 2010). In general, REEs are mainly extracted from several independent RE ores (such as monazite, bastnäsite, and xenotime) and ion-absorbing type RE ores (Wang et al., 1989). However, these traditional RE sources have recently failed to meet the global demand for RE metals. Therefore, it became necessary to explore other resources to extract REEs. REEs can often be found in other ores, such as phosphorites, fluorites, and tungsten ores, in the form of an isomorph during the formation of hydrothermal ore deposits (Goldschmidt, 1937, Guo, 1963, Yan et al., 2015, Ghaderi et al., 1999, Liu et al., 2007).

China has the largest tungsten reserves in the world (Li et al., 2011). Zhang (Zhang et al., 1990) assayed the REEs found in tungsten ores in South China. The RE content in porphyry tungsten deposits reaches approximately 1049.7–3034.6 ppm (RE₂O₃), and the quartz vein-type tungsten ore contains 334.9 ppm RE. The significant content of REs in tungsten ores brings an importance to the extraction of REEs from tungsten ores. Tungsten is leached from tungsten ores by the alkali autoclave process (Zhao and Li, 2008). After leaching, the REEs enter the residue, which is mainly composed of calcium hydroxide/iron and manganese oxide. The REEs are then recovered from the leach residue; however, this process becomes uneconomical due to the enormous consumption of mineral acids (Xu et al., 1997). Thus far, REEs found in tungsten ores are not recovered from the leach residue.

Zhao developed a novel method for decomposing scheelite using an acid mixture of H₂SO₄ and H₃PO₄ (Zhao and Li, 2014). In this method, tungsten is leached in the form of a soluble heteropoly acid (phosphotungstic acid). Furthermore, REEs can be leached into a solution that mainly contains H₃PO₄ and H₂SO₄ using wet-process phosphoric acid (Koopman and Witkamp, 2000, Preston et al., 1996, Wang et al., 2009, Wang et al., 2010, Wang et al., 2008a, Wang et al., 2008b). This indicated that the REEs could be leached using the H₂SO₄ and H₃PO₄ mixed acid system. The leach solution of scheelite using the H₂SO₄ and H₃PO₄ acid mixture contained a certain amount of tungsten as opposed to the leach solution formed using wet-process phosphoric acid that contained no tungsten. As established, tungsten reacts with phosphorus to form phosphotungstic acid; this reaction is conducive to the decomposition of REs existing in the form of monazite (RE phosphate). Moreover, RE ions, which are similar to transition metal ions, can combine the unsaturated heteropoly anions containing a vacant seat to form the mixed heteropoly anion (Rong et al., 1987, Chen et al., 2018). For example, lanthanide ions can combine with phosphotungstic acid to form the more complex [(LnO)₃(PW₉O₃₄)₂]¹⁵⁻, under a certain condition (Fedotov et al., 1990). Certain RE ions can even combine directly with tungstate to form RE heteropoly tungstate. Wang (Wang et al., 1982) researched the coordination reaction of cerium ion and tungstate and found that H₅(CeW₈O₂₈) heteropoly acid could be obtained. Additionally, previously research indicates that the presence of tungsten in the H₂SO₄ and H₃PO₄ acid mixture improved the leaching of REEs. In other words, the properties of REEs and tungsten can promote the leaching of REEs from scheelite, using the H₂SO₄ and H₃PO₄ mixed acid decomposition. In our previous study (Chen et al., 2019), the dissolution behavior of the associated RE(La) of scheelite in the H₂SO₄ and H₃PO₄ acid mixture was researched and the results showed that REEs could indeed dissolve into a H₂SO₄ and H₃PO₄ acid mixture that contains tungsten. The results also provided a crucial reference value for the leaching process of REEs from scheelite.

Based on previous research results, a recovery method for tungsten and REEs from natural scheelite in the presence of a H₂SO₄ and H₃PO₄ acid mixture was developed. The leaching of tungsten and REEs is examined in this paper.

Section snippets

Materials and methods

Natural scheelite was provided by China Molybdenum Co., Ltd. The contents of tungsten and REs in scheelite were 18 wt% and approximately 0.1 wt%, respectively. The chemical composition of the REs is listed in Table.1. In these REs, La₂O₃, CeO₂ and Nd₂O₃ are the main components, and account for nearly 90.15% of all the REs. The mixed acid solution was prepared by diluting concentrated H₂SO₄ (analytical reagent, 98 wt%) and concentrated H₃PO₄ (analytical reagent, 85 wt%) according to the

Effect of the reaction temperature on leaching

The experimental results are presented in Fig. 1. It can be seen that raising reaction temperature favored the leaching of tungsten. As the reaction temperature exceeded 90 °C, the leaching efficiency of tungsten increased to nearly 100% (under the set conditions, almost all the tungsten can be leached). This indicates that the H₂SO₄ and H₃PO₄ mixed acid decomposition method is an efficient route for extracting tungsten from scheelite. Additionally, the leaching efficiencies of the REEs also

Conclusion

In this study, the H₂SO₄–H₃PO₄ acid mixture was used to leach tungsten and REs from scheelite. The effects of the reaction conditions on the leaching process were investigated. The results found:

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Raising the reaction temperature was beneficial to the leaching of tungsten and REs.
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Increasing the H₂SO₄ concentration favored the leaching process of tungsten and REs as the concentration did not exceed 2.5 mol/L. An excessive H₂SO₄ concentration promotes the transformation of gypsum from CaSO₄·2H₂O to CaSO₄.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (51974372) and the Sheng-hua Yu-ying Program of Central South University. We would like to thank Editage (www.editage.cn) for English language editing.

References (32)

X.Y. Chen et al.
Extraction behaviors of rare-earths in the mixed sulfur-phosphorus acid leaching solutions of scheelite
Hydrometallurgy
(2018)
G. Das et al.
Rare earth sulfates in aqueous systems: Thermodynamic modeling of binary and multicomponent systems over wide concentration and temperature ranges
J. Chem. Thermodyn.
(2019)
M.A. Fedotov et al.
17O, 31P and 183W NMR spectra of paramagnetic complexes with the heteropolytungstate anion [Ln(PW₁₁O₃₉)2]₁₁− and their constitution in aqueous solution (Ln: Rare earth element)
Polyhedron
(1990)
C. Koopman et al.
Extraction of lanthanides from the phosphoric acid production process to gain a purified gypsum and a valuable lanthanide by-product
Hydrometallurgy
(2000)
J.T. Li et al.
Kinetics of scheelite concentrate digestion with sulfuric acid in the presence of phosphoric acid
Hydrometallurgy
(2016)
J.S. Preston et al.
The recovery of rare earth oxides from a phosphoric acid by-product
Hydrometallurgy
(1996)
C.Y. Rong et al.
Bis(undecatungstogermanate) lanthanates of potassium
Inorg. Chim. Acta
(1987)
L.S. Wang et al.
Recovery of rare earths from wet-process phosphoric acid
Hydrometallurgy
(2010)
W.L. Wang et al.
Experimental determination and modeling of gypsum and insoluble anhydrite solubility in the system CaSO₄–H₂SO₄–H₂O
Chem. Eng. Sci.
(2013)
L. Zhang et al.
Adsorption behavior of rare earth elements using polyethyleneglycol (phosphomolybdate and tungstate) heteropolyacid sorbents in nitric solution
Hydrometallurgy
(2009)

X.Y. Chen et al.

Dissolution behavior of the associated rare-earth elements in scheelite using a mixture of sulfuric and phosphoric acids

Miner. Eng.

(2019)

M. Ghaderi et al.

Rare earth element systematics in scheelite from hydrothermal gold deposits in the Kalgoorlie-Norseman region, Western Australia

Econ. Geol.

(1999)

V.M. Goldschmidt

The principles of distribution of chemical elements in minerals and rocks

J. Chem. Soc.

(1937)

C.J. Guo

Rare Earth Minerals Chemistry

(1963)

F.L. Guo et al.

Behavior of accompanying rare earth during process of decomposing scheelite by sulfuric-phosphoric mixed acid

Chin. J. Nonferr. Met.

(2018)

X.Y. Han et al.

Development situation of China’s rare earth

China Nonferr. Met.

(2010)

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In recent years, some new scheelite extraction processes have been proposed by metallurgists through technical innovation. Based on the formation of phosphotungstic acid from tungsten and phosphorus in acidic solution, a process for leaching scheelite with a mixture of phosphoric acid and sulfuric acid was proposed (Chen et al., 2020). In the leaching process, scheelite reacts with sulphur-phosphorus mixed acid to produce phosphotungstic acid and calcium sulphate (Eq. (3)), and a high leaching efficiency for tungsten is obtained.12CaWO4 + H3PO4 + 12H2SO4 = H3PW12O40 + 12CaSO4·2H2O
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Leaching tungsten and rare earth elements from scheelite through H2SO4–H3PO4 mixed acid decomposition

Highlights

Abstract

Introduction

Section snippets

Materials and methods

Effect of the reaction temperature on leaching

Conclusion

Declaration of Competing Interest

Acknowledgements

Hydrometallurgy

J. Chem. Thermodyn.

Polyhedron

Hydrometallurgy

Hydrometallurgy

Hydrometallurgy

Inorg. Chim. Acta

Hydrometallurgy

Chem. Eng. Sci.

Hydrometallurgy

Dissolution behavior of the associated rare-earth elements in scheelite using a mixture of sulfuric and phosphoric acids

Miner. Eng.

Rare earth element systematics in scheelite from hydrothermal gold deposits in the Kalgoorlie-Norseman region, Western Australia

Econ. Geol.

The principles of distribution of chemical elements in minerals and rocks

J. Chem. Soc.

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Behavior of accompanying rare earth during process of decomposing scheelite by sulfuric-phosphoric mixed acid

Chin. J. Nonferr. Met.

Development situation of China’s rare earth

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Leaching tungsten and rare earth elements from scheelite through H₂SO₄–H₃PO₄ mixed acid decomposition