Non-ammonia enrichment of rare earth elements from rare earth leaching liquor in a magnesium salt system I: Precipitation by calcium oxide
Introduction
The weathered crust elution-deposited rare earth ore is a valuable strategic mineral resource in china. It has many advantages, such as low radioactivity, simple leaching process, complete rare earth partition and abundance in middle and heavy rare earth elements. Currently, rare earth elements are recovered from the weathered crust elution-deposited rare earth ore by ammonium sulfate leaching and ammonium bicarbonate enrichment processes (Xiao et al., 2015). However, the use of (NH4)2SO4 and NH4HCO3 has brought about serious ammonia-nitrogen pollution, which destroys the ecological environment (Xiao et al., 2013). Therefore, it is crucial to develop a non-ammonia extraction process for the weathered crust elution-deposited rare earth ore.
On the one hand, to overcome the problem of ammonia-nitrogen pollution introduced by (NH4)2SO4 leaching process, experts have proposed series of enhanced leaching and new non-ammonia leaching methods(Qiu et al., 2008; Tian et al., 2013; Xiao et al., 2017; Yang et al., 2018), among which magnesium salt compound leaching agent has been developed as a new kind of potential leaching agent according to the magnesium-deficient characteristics in the red soil region of South China (Lai et al., 2018). Under the same conditions, the rare earth leaching efficiency with magnesium salt compound leaching agent is equal to that with ammonium sulfate, reaching over 95%(Xiao et al., 2016). Moreover, the application of magnesium salt compound leaching agent could solve the ammonia-nitrogen pollution problem in the leaching process, reduce the consumption of CaMg fertilizer needed and finally realize environmentally-friendly leaching of the weathered crust elution-deposited rare earth ore(Xiao et al., 2016). The commercial tests with magnesium salt compound leaching agent have been developed in Changting, Fujian province and Chongzuo, Guangxi province.
On the other hand, to solve the ammonia-nitrogen pollution problem in the enrichment process of rare earth elements from the leaching liquor, non-precipitation methods and non-ammonia precipitants have been widely studied to take the place of NH4HCO3 precipitant. Non-precipitation methods(Wang et al., 2018 Huang et al., 2017 Tian et al., 2011; Liu et al., 2017) include liquid membrane extraction, ion exchange, adsorption and so on, but these methods have the disadvantages(Tian et al., 2011) of high production cost, low processing capacity, presence of organic pollution and difficulties in stable operation. Compared with non-precipitation methods, precipitation methods can be easily controlled with simple equipments and process, have high rare earth yields, are low cost and more suitable for mass production. Therefore, non-ammonia precipitants are a popular topic of current research. Chen(Chen et al., 2016) and Xu(Xu and Li, 2018) have studied the effect of sodium bicarbonate and sodium carbonate on rare earth precipitation from the leaching liquor. However, the use of these sodium salt will result in soil salinization and high cost issues. Moreover, the precipitation conditions should be strictly controlled, otherwise rare earth carbonate is not easy to crystallize(Chi and Tian, 2006). Yu(Yu, 2014) and Huang(Huang et al., 2019) have used magnesium hydroxide as a precipitant to enrich rare earth elements from the leaching liquor. However, the solubility of magnesium hydroxide is only 9.628*10−4 g/(100 g water) in 20 °C(Speigh, 2005), leading to long reaction time and high consumption of precipitant. Moreover, due to the use of excessive magnesium hydroxide and the introduction of SO3, the purity of the rare earth concentrate can only reach 83.5%(Huang et al., 2019). Compared with magnesium hydroxide, the solubility of calcium oxide is much higher, being 0.173 g/(100 g water) in 20 °C(Speigh, 2005). Therefore, the pH of calcium hydroxide solution is higher and the dosage of precipitant needed for precipitation process is less, calcium oxide will have better precipitation performance than MgO. Gao(Gao et al., 2019) has adopted calcium oxide as a precipitant to enrich rare earth elements from rare earth liquor without magnesium ions. It is determined that the optimum precipitation conditions lead to only 83.81% rare earth purity in the rare earth concentrate, and the content of SO3 in the rare earth concentrate has reached 12.30%. The introduction mechanism of SO3 in the precipitation process has not yet been studied.
Based on the research described above, the magnesium salt compound leaching agent was a appropriate choice to leaching weathered crust elution-deposited rare earth ore because of its environmental advantages. In this way, the leaching liquor in a magnesium salt system would be obtained. Then calcium oxide was used as a precipitant to enrich rare earth elements from rare earth leaching liquor in a magnesium salt system. The green extraction process of the weathered crust elution-deposited rare earth ore with Mg/Ca salt is shown in Fig. 1. As a precipitant, calcium oxide is cheap and easy to obtain, and the precipitation process is simple and easy to control. Besides, the use of calcium oxide can supply calcium ions from the magnesium salt compound leaching agent, realize the effective circulation of Mg and Ca, and finally solve the ammonia-nitrogen problem in the whole extraction process of the weathered crust elution-deposited rare earth ore. The solubility behavior of calcium sulfate in this system has been previously determined by our lab, and the results showed that calcium sulfate precipitation would not occur during the calcium oxide precipitation process(Lai et al., 2019). In this paper, the simulated rare earth leaching liquor in a magnesium salt system was the research object, the precipitation process of rare earth elements from rare earth leaching liquor in a magnesium salt system with calcium oxide was studied. The possible precipitation mechanism and the effects of precipitation conditions on the precipitation process were investigated. Moreover, the removal of sulfate from precipitation enrichment by stirring washing was explored. The research in this paper could have great significance to the green and sustainable development of the weathered crust elution-deposited rare earth ore.
Section snippets
The mixed rare earth solution
The mixed rare earth carbonates, obtained after impurity removal process and enrichment process with NH4HCO3, were supplied by Chinalco Guangxi Chongzuo Rare Earth Development Co., Ltd. (located in Chongzuo City, Guangxi Province, China). The carbonates were dissolved in dilute sulfuric acid to prepare 20.0 g/L mixed rare earth solution (measured in REO). The rare earth partition and impurity concentrations in the solution were listed in Table 1. According to the rare earth partition, the
Thermodynamic calculation
The rare earth elements in the leaching liquor of a magnesium salt system exist in the form of sulfates. Therefore, in the precipitation process with calcium oxide, rare earth hydroxides and alkaline rare earth sulfates may be generated together(Fu, 1993). The corresponding reaction equations are shown in Eqs. (5), (6).
However, the Gibbs free energy of formation values (∆fGθ) of alkaline rare earth sulfates were very scarce, so they were first estimated
Conclusion
- 1.
The Hazen model calculation showed that the Gibbs free energy values of the precipitation reaction of alkaline rare earth sulfates and rare earth hydroxides was always less than −100 kJ/mol. Thus, in the process of precipitation of rare earth elements by calcium oxide, both alkaline rare earth sulfates and rare earth hydroxides might be produced in view of thermodynamics, which was also proved by the calcium oxide addition experiments and the characterization of the precipitation enrichment.
- 2.
The
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
The authors gratefully acknowledge the Financial Supports of National Key Research and Development Project of China (Grant No. 2019YFC0605002), National Natural Science Foundation of China(Grant No. 51964018), China Postdoctoral Science Foundation (Grant No. 2018T110661), Selective Grant Project of Postdoctoral Research Program of Jiangxi Province (Grant No. 2018KY01), Science and Technology Innovation Talents Program of Ganzhou City (Grant No. [2018] 50), Double Thousand Plan in Jiangxi
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