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Open AccessArticle

Leaching Process of Rare Earth Elements, Gallium and Niobium in a Coal-Bearing Strata-Hosted Rare Metal Deposit—A Case Study from the Late Permian Tuff in the Zhongliangshan Mine, Chongqing

by 1,2,*, 2 and 1
1
College of Geosciences and Survey Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
2
Chongqing Key Laboratory of Exogenic Mineralization and Mine Environment, Chongqing Institute of Geology and Mineral Resources, Chongqing 400042, China
*
Author to whom correspondence should be addressed.
Academic Editor: Houshang Alamdari
Metals 2017, 7(5), 174; https://doi.org/10.3390/met7050174
Received: 15 February 2017 / Revised: 7 May 2017 / Accepted: 11 May 2017 / Published: 15 May 2017

Abstract

The tuff, a part of coal-bearing strata, in the Zhongliangshan coal mine, Chongqing, southwestern China, hosts a rare metal deposit enriched in rare earth elements (REE), Ga and Nb. However, the extraction techniques directly related to the recovery of rare metals in coal-bearing strata have been little-studied in the literature. The purpose of this paper is to investigate the extractability of REE, Ga and Nb in the tuff in the Zhongliangshan mine using the alkaline sintering-water immersion-acid leaching (ASWIAL) method. The results show that ASWIAL can separate and extract REE, Ga and Nb effectively under the optimized conditions of calcining at 860 °C for 0.5 h with a sample to sintering agent ratio of 1:1.5, immersing at 90 °C for 2 h with 150 mL hot water dosage, and leaching using 4 mol/L HCl at 40 °C for 2 h with a liquid-solid ratio of 20:1 (mL:g). The final leaching efficiencies of REE and Ga are up to 85.81% and 93.37%, respectively, whereas the leaching efficiency of Nb is less than 1%, suggesting the high concentration of Nb in the leaching residue, which needs further extraction.
Keywords: extractability; alkaline sintering-water immersion-acid leaching; tuff; Chongqing extractability; alkaline sintering-water immersion-acid leaching; tuff; Chongqing

1. Introduction

The study of rare metal deposits hosted in coal-bearing strata is currently one of the research hotspots in the field of coal geology. The term “metalliferous coal” has been widely adopted to describe coal anomalously enriched in trace metals with potential economic and practical value for rare metal recovery [1,2,3,4,5,6,7,8,9,10,11]. From the perspective of industrial production, coal can be considered as metalliferous at the level of trace element concentrations being at least 10 times greater than the corresponding averages of world coal [1]. Many studies have shown that not only coal but some of the non-coal rock strata within or adjacent to the coal seams are also enriched in various valuable metals [8,12,13,14]. Due to the gradual depletion of many conventional rare metal ores and the difficulties in exploring new deposits, alternative rare metal sources are urgently needed to be discovered and exploited [7,8]. Furthermore, with respect to the increasing challenges of global warming and other environmental issues, the utilization of coal resources is increasingly encouraged to be economically effective and environmentally benign [2,3]. Hence, some coal-bearing strata have been regarded as promising alternative sources for rare metals recovery, to which many researchers around the world have paid great attention in recent years [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. There are already some precedents in the case of rare metal extraction from coal and coal-related materials. For example, germanium is currently being recovered mainly from three well-known Ge-bearing coal deposits, namely Lincang (Yunnan province) and Wulantuga (Inner Mongolia) of China, as well as Spetzugli of eastern Russia, which account for more than 50% of the total yearly production of Ge metal in the world [3,7,9,11]. The discovery of a super-large coal-hosted gallium and aluminum deposit in the Jungar Coalfield (Inner Mongolia), China is another example [16], which was highly recognized and considered as the third most significant and outstanding discovery for coal-hosted metal deposit production following the successful industrial extractions of uranium and germanium from coal [5,17]. Recently, a new type of Nb (Ta)-Zr (Hf)-REE-Ga polymetallic ore deposit was discovered in the late Permian coal-bearing strata of eastern Yunnan, southwestern China [14], and some other types of elemental assemblages have also been observed in coalfields in southern China [7,8,18,19].
The mechanisms of rare-metal mineralization have been discussed in detail in substantial studies [1,3,4,7,8,9,13,19], some of which have also highlighted the geologic and tectonic controls on the localization of metalliferous coal deposits [8]. However, extraction techniques directly related to the recovery of rare metals from coal deposits, which are undoubtedly the critical issue for metalliferous coal uses, have been paid less attention in the literature. At present, the possible recovery of rare metal elements from coal-bearing strata and coal combustion products (CCPs) is an exciting research area, since coal and particularly its combustion derivations may have elevated concentrations of metal elements that are comparable to or even higher than those found in conventional metal ores [5,20,21,22,23,24,25,26,27]. For this reason, the U.S. Department of Energy’s National Energy Technology Laboratory (NETL) funded 10 projects in 2015 aiming to support the lab’s research program on the recovery of REE from coal and coal byproducts [28].
Learnings from the metallurgy of conventional ores can be applied to metalliferous coal [29,30,31]. For example, scientists from the U.S. Department of Energy, inspired by the traditional hydrometallurgical method of ion exchange, carried out a series of lab-scale REE extraction tests using ammonium sulfate, an ionic liquid and a deep eutectic solvent as lixiviants, and produced a marked extraction efficiency of REE at least from the selected coal byproducts [32]. Acid leaching incorporated with acid/alkali sintering is a common metallurgical process for metal extraction [33,34], and has been a major technology applied to the recovery of metals like alumina from coal fly ash [35]. As compared with solely resorting to hydrogen fluoride(HF) digestion, which may be too hazardous for large-scale industrial use, pretreatment with an alkaline agent could effectively liberate REE and consequently bring a dramatic increase in REE extractability from Na2CO3 sintering.
We have reported an anomalous enrichment of REE, Ga and Nb in the tuff associated with the late Permian coal-bearing strata in the Zhongliangshan mine, Chongqing, southwestern China, which can be regarded as a potential economically significant coal-bearing strata hosting a polymetallic ore deposit [18]. In order to gain more insights on the modes of occurrence of these metals and further provide a reliable basis for future industrial production, this work expanded the previous findings and investigated the extractability of REE, Ga and Nb in the tuff subjected to a sequential alkaline sintering-water immersion-acid leaching process. A tentative test was conducted at first so as to primarily see the results and optimize the extraction conditions. In this work, we designed a three-step extraction experiment with a combination of alkaline sintering, water immersion and acid leaching to investigate the extractability of REE, Ga and Nb in the tuff samples examined. Subsequently, to validate the experimental procedures as well as the optimized conditions, the whole extraction process was performed again on the tuff sample under the optimum conditions. The results can be used to identify the validity of the adopted extraction strategy and gauge the accessibility of these rare metals in the tuff through chemical processes.

2. Materials and Methods

2.1. Materials

The tuff samples used in this work were collected from the bottom layer of the Longtan Formation in the late Permian coal-bearing strata in the Zhongliangshan mine, Chongqing, southwestern China. The major elements and selected rare metal concentrations of the tentative and validation experimental samples, named as T-1, Y-1, respectively, are tabulated (Table 1). According to the study by Zou et al. [18], the mineralogical composition of the tuff includes mainly kaolinite, illite, pyrite, anatase, calcite, gypsum, quartz, and traces of zircon, florencite, jarosite, and barite.

2.2. Experimental Procedure

The procedure of the alkaline sintering-water immersion-acid leaching method (ASWIAL) is illustrated in Figure 1, consisting principally of three steps. Firstly, the REE-, Ga-, and Nb-bearing minerals in the tuff sample are decomposed by reacting with anhydrous sodium carbonate, and converted into water soluble or metallic acid compounds through the process of alkaline sintering under high-temperature calcining. Secondly, the soluble components in the post-calcination material are leached out by immersing in hot water and are concentrated in the filtrate after filtration. Finally, the filtration residue after water immersion is leached using hydrochloric acid, and the acid-leaching residue is reserved. All liquid and solid products during this process were collected for later elemental determinations. Chemical reactions involved in the alkali sintering process are listed below:
Al2O3 + Na2CO3 → 2NaAlO2 + CO2
SiO2 + Na2CO3 → Na2SiO3 + CO2
TiO2 + Na2CO3 → Na2TiO3 + CO2
Ga2O3 + Na2CO3 → 2NaGaO2 + CO2

2.3. Analytical Method

Concentrations of major element oxides including SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, MnO, TiO2 and P2O5 in the tuff samples were determined by X-ray fluorescence spectrometry (PANalytical Axios pw4400, PANalytical, Almelo, The Netherlands). The contents of trace elements including REE, Ga and Nb were determined by inductively coupled plasma mass spectrometry (Thermo X series II ICP-MS, ThermoFisher Scientific, Waltham, MA, USA). The detailed experimental procedures of ICP-MS analysis have been described by Zou et al. [18]. The X-ray diffraction (XRD) analysis was conducted using a D8 advance powder diffractometer with Ni-filtered Cu-Kα radiation and a scintillation detector (Bruker Corporation, Billerica, MA, USA). The XRD pattern was recorded over a 2θ interval of 2.6°–70°, with a step size of 0.02°.

3. Results and Discussion

3.1. Tentative Experiment

3.1.1. Alkaline Sintering

The sample (T-1) used in the tentative experiment was oven-dried at 105 °C for 2 h, and ground in an agate mill (GSXX-4) at 300 r/min for 30 min. Pulverized samples were examined using a Malvern Laser Particle Sizer (MS2000 type, Malvern Instruments Ltd., Malvern, UK) to ensure that the average particle size was lower than 46 μm.
The most abundant element oxides of the tuff sample are SiO2, Al2O3 and TiO2 [18]. The mass ratio of the sample versus anhydrous sodium carbonate was determined as 1:1.5 according to the chemical reaction equation of Si, Al and Ti with anhydrous sodium carbonate. Considering the great influence of the calcination temperature on the decomposition of minerals and the liberation of rare metal elements, the samples were tentatively calcined at 740 °C, 800 °C, 860 °C and 920 °C, respectively. The resulting solids of calcination were cooled and further subjected to XRD analysis for mineralogical identification.
The tuff sample is mainly characterized by kaolinite, pyrite, anatase and jarosite in mineralogical compositions [18]. As can be deduced from Figure 2, kaolinite, pyrite and jarosite were decomposed at 740 °C, whereas the primary representative peak of anatase at approximately 25.3° 2-Theta was still obvious, thereby indicating that the anatase was not decomposed at this temperature as well as at 800 °C. As the calcination temperature increased, anatase started to decompose, which was evidenced by the disappearance of anatase peaks at 860 °C and 900 °C. Therefore, the calcination temperature 860 °C was recommended for later experiment. Since no significant difference was observed between calcination times of 0.5 h and 1 h, 0.5 h was adopted.

3.1.2. Water Immersion

Before water immersion, the sintering products were ground and stirred. The post-sintering products were immersed in hot water at 90 °C, where soluble components such as Ga- and Al-bearing compounds were able to be separated from the solid matrix into the liquid phase. The concentration of rare metals including REE, Ga and Nb in the filtrate and residue were determined by ICP-MS and the leaching efficiency was further calculated. The filtration residues after water immersion were subjected to the following acid leaching process.
Nine groups of samples were prepared in parallel for the water immersion. Accurately-weighed 10 g samples were mixed with 15 g of anhydrous sodium carbonate and then stirred carefully to reach a visually homogeneous color. The mixture was then fully calcined at 860 °C for 0.5 h. Each of the resultant nine post-calcined products were cooled to ambient temperature prior to being immersed using 150 mL ultrapure water at 90 °C for 2 h.
The water immersion extraction efficiency was calculated by the equation as follows:
β = ( 1 m 1 × ε 1 m 0 × ε 0 ) × 100 %
where β is the water immersion extraction efficiency, %; m0 is the weight of the tuff sample, g; ε0 is the weight fraction of REE, Ga, Nb or Al of the tuff sample, %; m1 is the weight of the water immersion residue, g; and ε1 is the REE, Ga, Nb or Al weight fraction of the water immersion residue, %.
As indicated by the results listed in Table 2, hot water can leach most of Ga and a portion of Al from the sample. The immersion extraction efficiency of Ga varies from 54.97% to 71.11% among the nine groups, with an average of 64.55%. The Al immersion extraction efficiency is from 39.19% to 44.53% and averages at 41.98%. Although hot water is incapable of leaching REE and Nb out, with the immersion extraction efficiencies of REE and Nb being both below than 0.03%, Ga can be separated from Si, Fe and REE, which provides a basis for the further purification of Ga.

3.1.3. Acid Leaching

The leaching experiment using hydrochloric acid as a lixiviant was carried out to extract the target metals from the water immersion residue. An orthogonal array of L9(34), which denotes four factors at three levels, was designed in consideration of the major factors relevant to the leaching process including: liquid to solid ratio, leaching temperature, leaching time, and acid concentration, which are represented by A, B, C, and D, respectively (Table 3).
The acid leaching ratio was calculated by the equation as follows:
α = ( 1 m 2 × ε 2 m 1 × ε 1 ) × 100 %
where α is the acid leaching ratio, %; m1 is the weight of the water immersion residue, g; ε1 is the weight fraction of REE, Ga or Nb of the water immersion residue, %; m2 is the weight of the acid leaching residue, g; andε2 is the REE, Ga or Nb weight fraction of the acid leaching residue, %.
The results of the range analysis are tabulated in Table 4 and Table 5. As indicated by the results, the most influential factor on the leaching efficiency of REE and Ga is the liquid to solid ratio, followed by leaching time, leaching temperature and HCl concentration.
It is clear that A1B2C1D2 is the optimum with respect to the total leaching of REE and Ga. However, the leaching efficiencies of REE and Ga in A1B1C1D1 are similar to that in A1B2C1D2. Moreover, if this experiment is applied to industrial production, the cost of the A1B1C1D1 is cheaper than the A1B2C1D2, so A1B1C1D1 offers comparatively optimum conditions for acid leaching, namely the liquid-solid ratio of 20:1 (mL:g), leaching time of 2 h, leaching temperature of 40 °C and hydrochloric concentration of 4 mol/L. The leaching efficiencies of REE and Ga reached 85.56% and 83.48%, respectively, under these conditions. However, the leaching efficiency of Nb is only 3.2% (Table 6), suggesting the significant concentration of Nb in the leaching residue, which needs to be further extracted.

3.2. Validation Experiment

To test the reproducibility of the results obtained from the tentative experiment under the optimized conditions, a duplicate tuff sample (Y-1) was subjected to the leaching procedure. The validation experiment was conducted under the optimum conditions for the ASWIAL process, listed as follows: calcining at 860 °C for 0.5 h with a sample to sintering agent ratio of 1:1.5, immersing at 90 °C for 2 h with 150 mL hot water, and leaching by 4 mol/L HCl at 40 °C for 2 h with a liquid-solid ratio of 20:1 (mL:g). It can be seen from Table 7 that the validation experiment basically produced similar results to those obtained from the tentative experiment. The water immersion efficiency and acid leaching efficiency of Ga were 67.31% and 79.71%, respectively, and the total leaching efficiency was up to 93.37%. The REE water immersion efficiency was less than 1%, and acid leaching and total leaching efficiency were both 85.81%. Nb was barely leached out during the process, but was greatly concentrated in the residue (the concentration of Nb in the acid leaching residue was up to 338 μg/g), which needs to be further extracted.

4. Conclusions

Rare earth elements, gallium and niobium enriched in the tuff of a coal-bearing strata-hosted rare metal deposit in the Zhongliangshan mine, Chongqing, China, can be effectively extracted using the method of alkaline sintering-water immersion-acid leaching (ASWIAL). According to the results of the tentative experiment, the optimum conditions for the ASWIAL process are calcining at 860 °C for 0.5 h with a sample to sintering agent ratio of 1:1.5, immersing at 90 °C for 2 h with 150 mL hot water, and leaching by 4 mol/L HCl at 40 °C for 2 h with a liquid-solid ratio of 20:1 (mL:g). The total leaching efficiencies of Ga and REE can be up to 93.37% and 85.81%, respectively. However, Nb is barely leached out through the process (<1%), which needs further extraction.

Acknowledgments

This research was supported by the National Key Basic Research and Development Program (No. 2014CB238902) and National Natural Science Foundation of China (Nos. 41502162 and 41602172). The authors wish to express their appreciation to Shifeng Dai and Lei Zhao and Lixin Zhao for revision suggestions and English polishing. The authors are grateful to the anonymous reviewers for their valuable advice and comments on the manuscript.

Author Contributions

Jianhua Zou and Heming Tian collected tuff samples in Zhongliangshan mine. Jianhua Zou was responsible for the experimental design. Heming Tian and Zhen Wang were responsible for the water leaching and acid leaching experiments. All the authors were responsible for the manuscript preparation.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Seredin, V.; Finkelman, R. Metalliferous coals: A review of the main genetic and geochemical types. Int. J. Coal Geol. 2008, 76, 253–289. [Google Scholar] [CrossRef]
  2. Seredin, V.; Dai, S. Coal deposits as potential alternative sources for lanthanides and yttrium. Int. J. Coal Geol. 2012, 94, 67–93. [Google Scholar] [CrossRef]
  3. Seredin, V.; Dai, S.; Sun, Y.; Chekryzhov, I. Coal deposits as promising sources of rare metals for alternative power and energy-efficient technologies. Appl. Geochem. 2013, 31, 1–11. [Google Scholar] [CrossRef]
  4. Seredin, V. Rare earth element-bearing coals from the Russian Far East deposits. Int. J. Coal Geol. 1996, 30, 101–129. [Google Scholar] [CrossRef]
  5. Hower, J.; Granite, E.; Mayfield, D.; Lewis, A.; Finkelman, R. Notes on Contributions to the Science of Rare Earth Element Enrichment in Coal and Coal Combustion Byproducts. Minerals 2016, 6, 32. [Google Scholar] [CrossRef]
  6. Eskenazy, G. Rare earth elements in a sampled coal from the Pirin Deposit, Bulgaria. Int. J. Coal Geol. 1987, 7, 301–314. [Google Scholar] [CrossRef]
  7. Dai, S.; Ren, D.; Chou, C.; Finkelman, R.; Seredin, V.; Zhou, Y. Geochemistry of trace elements in Chinese coals: A review of abundance, genetic types, impacts on human health, and industrial utilization. Int. J. Coal Geol. 2012, 94, 3–21. [Google Scholar] [CrossRef]
  8. Dai, S.; Chekryzhov, I.; Seredin, V.; Nechaev, V.; Graham, I.; Hower, J.; Ward, C.; Ren, D.; Wang, X. Metalliferous coal deposits in East Asia (Primorye of Russia and South China): A review of geodynamic controls and styles of mineralization. Gondwana Res. 2016, 29, 60–82. [Google Scholar] [CrossRef]
  9. Dai, S.; Wang, P.; Ward, C.; Tang, Y.; Song, X.; Jiang, J.; Hower, J.; Li, T.; Seredin, V.; Wagner, N.; et al. Elemental and mineralogical anomalies in the coal-hosted Ge ore deposit of Lincang, Yunnan, southwestern China: Key role of N2-CO2-mixed hydrothermal solutions. Int. J. Coal Geol. 2015, 152, 19–46. [Google Scholar] [CrossRef]
  10. Johnston, M.; Hower, J.; Dai, S.; Wang, P.; Xie, P.; Liu, J. Petrology and Geochemistry of the Harlan, Kellioka, and Darby Coals from the Louellen 7.5-Minute Quadrangle, Harlan County, Kentucky. Minerals 2015, 5, 894–918. [Google Scholar] [CrossRef]
  11. Dai, S.; Seredin, V.; Ward, C.; Jiang, J.; Hower, J.; Song, X.; Jiang, Y.; Wang, X.; Gornostaev, T.; Li, X.; et al. Composition and modes of occurrence of minerals and elements in coal combustion products derived from high-Ge coals. Int. J. Coal Geol. 2014, 121, 79–97. [Google Scholar] [CrossRef]
  12. Zhao, L.; Dai, S.; Graham, I.; Li, X.; Liu, H.; Song, X.; Hower, J.; Zhou, Y. Cryptic sediment-hosted critical element mineralization from eastern Yunnan Province, southwestern China: Mineralogy, geochemistry, relationship to Emeishan alkaline magmatism. Ore Geol. Rev. 2017, 80, 116–140. [Google Scholar] [CrossRef]
  13. Zhao, L.; Dai, S.; Li, X.; Liu, H.; Zhang, B. New insights into the lowest Xuanwei Formation in eastern Yunnan Province, SW China: Implications for Emeishan large igneous province felsic tuff deposition and the cause of the end-Guadalupian mass extinction. Lithos 2016, 264, 375–391. [Google Scholar] [CrossRef]
  14. Dai, S.; Zhou, Y.; Zhang, M.; Wang, X.; Wang, J.; Song, X.; Jiang, Y.; Luo, Y.; Song, Z.; Yang, Z.; et al. A new type of Nb(Ta)–Zr(Hf)–REE–Ga polymetallic deposit in the late Permian coal-bearing strata, eastern Yunnan, southwestern China: Possible economic significance and genetic implications. Int. J. Coal Geol. 2010, 83, 55–63. [Google Scholar] [CrossRef]
  15. Zhao, L.; Ward, C.; French, D.; Graham, I. Major and Trace Element Geochemistry of Coals and Intra-Seam Claystones from the Songzao Coalfield, SW China. Minerals 2015, 5, 870–893. [Google Scholar] [CrossRef]
  16. Dai, S.; Ren, D.; Li, S. Discovery of the superlarge gallium ore deposit in Jungar, Inner Mongolia, North China. Chin. Sci. Bull. 2006, 51, 2243–2252. [Google Scholar] [CrossRef]
  17. Seredin, V. From coal science to metal production and environmental protection: A new story of success. Int. J. Coal Geol. 2012, 90, 1–3. [Google Scholar] [CrossRef]
  18. Zou, J.; Tian, H.; Li, T. Geochemistry and Mineralogy of Tuff in Zhongliangshan Mine, Chongqing, Southwestern China. Minerals 2016, 6, 47. [Google Scholar] [CrossRef]
  19. Dai, S.; Liu, J.; Ward, C.; Hower, J.; French, D.; Jia, S.; Hood, M.; Garrison, T. Mineralogical and geochemical compositions of Late Permian coals and host rocks from the Guxu Coalfield, Sichuan Province, China, with emphasis on enrichment of rare metals. Int. J. Coal Geol. 2016, 166, 71–95. [Google Scholar] [CrossRef]
  20. Hower, J.; Eble, C.; Dai, S.; Belkin, H. Distribution of rare earth elements in eastern Kentucky coals: Indicators of multiple modes of enrichment? Int. J. Coal Geol. 2016, 160–161, 73–81. [Google Scholar] [CrossRef]
  21. Dai, S.; Seredin, V.; Ward, C.; Hower, J.; Xing, Y.; Zhang, W.; Song, W.; Wang, P. Enrichment of U-Se-Mo-Re-V in coals preserved within marine carbonate successions: Geochemical and mineralogical data from the Late Permian Guiding Coalfield, Guizhou, China. Miner. Deposita 2015, 50, 159–186. [Google Scholar] [CrossRef]
  22. Hower, J.; Eble, C.; O’Keefe, J.; Dai, S.; Wang, P.; Xie, P.; Liu, J.; Ward, C.; French, D. Petrology, Palynology, and Geochemistry of Gray Hawk Coal (Early Pennsylvanian, Langsettian) in Eastern Kentucky, USA. Minerals 2015, 5, 592–622. [Google Scholar] [CrossRef]
  23. Dai, S.; Zhao, L.; Hower, J.; Johnston, M.; Song, W.; Wang, P.; Zhang, S. Petrology, mineralogy, and chemistry of size-fractioned fly ash from the Jungar power plant, Inner Mongolia, China, with emphasis on the distribution of rare earth elements. Energy Fuels 2014, 28, 1502–1514. [Google Scholar] [CrossRef]
  24. Zhuang, X.; Su, S.; Xiao, M.; Li, J.; Alastuey, A.; Querol, X. Mineralogy and geochemistry of the Late Permian coals in the Huayingshan coal-bearing area, Sichuan Province, China. Int. J. Coal Geol. 2012, 94, 271–282. [Google Scholar] [CrossRef]
  25. Dai, S.; Yang, J.; Ward, C.; Hower, J.; Liu, H.; Garrison, T.; French, D.; O’Keefe, J. Geochemical and mineralogical evidence for a coal-hosted uranium deposit in the Yili Basin, Xinjiang, northwestern China. Ore Geol. Rev. 2015, 70, 1–30. [Google Scholar] [CrossRef]
  26. Blissett, R.; Smalley, N.; Rowson, N. An investigation into six coal fly ashes from the United Kingdom and Poland to evaluate rare earth element content. Fuel 2014, 119, 236–239. [Google Scholar] [CrossRef]
  27. Franus, W.; Wiatros-Motyka, M.; Wdowin, M. Coal fly ash as a resource for rare earth elements. Environ. Sci. Pollut. Res. 2015, 22, 9464–9474. [Google Scholar] [CrossRef] [PubMed]
  28. The U.S. Department of Energy. Available online: http://www.energy.gov/fe/articles/doe-selects-projects-enhance-its’s-research-recovery-rare-earth-elements-coal-and-coal (accessed on 2 December 2015).
  29. Zhang, B.; Liu, C.; Li, C.; Jiang, M. A novel approach for recovery of rare earths and niobium from Bayan Obo tailings. Miner. Eng. 2014, 65, 17–23. [Google Scholar] [CrossRef]
  30. Chi, R.; Tian, J. Chemical Metallurgy of Weathered Elution-Deposited Rare Earth Ores; Science press of China: Beijing, China, 2006; (In Chinese). ISBN 9787030178305. [Google Scholar]
  31. Li, G.; You, Z.; Sun, H.; Sun, R.; Peng, Z.; Zhang, Y.; Jiang, T. Separation of Rhenium from Lead-Rich Molybdenite Concentrate via Hydrochloric Acid Leaching Followed by Oxidative Roasting. Metals 2016, 6, 282. [Google Scholar] [CrossRef]
  32. Rozelle, P.; Khadilkar, A.; Pulati, N.; Soundarrajan, N.; Klima, M.; Mosser, M.; Miller, C.; Pisupati, S. A Study on Removal of Rare Earth Elements from U.S. Coal Byproducts by Ion Exchange. Metall. Mater. Trans. E 2016, 3E, 6–17. [Google Scholar] [CrossRef]
  33. Liu, Z.; Li, H. Metallurgical process for valuable elements recovery from red mud: A review. Hydrometallurgy 2015, 155, 29–43. [Google Scholar] [CrossRef]
  34. Li, X.; Xiao, W.; Liu, W.; Liu, G.; Peng, Z.; Zhou, Q.; Qi, T. Recovery of alumina and ferric oxide from Bayer red mud rich in iron by reduction sintering. Trans. Nonferr. Met. Soc. 2009, 19, 1342–1347. [Google Scholar] [CrossRef]
  35. Bai, G.; Teng, W.; Wang, X.; Qin, J.; Xu, P.; Li, P. Alkali desilicated coal fly ash as substitute of bauxite in lime-soda sintering process for aluminum production. Trans. Nonferr. Met. Soc. 2010, 20, 169–175. [Google Scholar] [CrossRef]
Figure 1. The schematic diagram of the alkaline sintering-water immersion-acid leaching process.
Figure 1. The schematic diagram of the alkaline sintering-water immersion-acid leaching process.
Metals 07 00174 g001
Figure 2. X-ray diffraction spectra of calcined tuff samples at different temperatures. (K: kaolinite; A: anatase; P: pyrite; J: jarosite; M: microcline; Ma: magnesiocarpholite; N: natrite; T: thermonatrite).
Figure 2. X-ray diffraction spectra of calcined tuff samples at different temperatures. (K: kaolinite; A: anatase; P: pyrite; J: jarosite; M: microcline; Ma: magnesiocarpholite; N: natrite; T: thermonatrite).
Metals 07 00174 g002
Table 1. Major elemental concentrations of experimental samples (%).
Table 1. Major elemental concentrations of experimental samples (%).
SampleSiO2Al2O3Fe2O3MgOCaONa2OK2O
T-135.6929.849.430.20.130.120.12
Y-137.5331.886.890.20.120.130.17
SD0.921.021.2700.0050.0050.025
SampleMnOTiO2P2O5REE (μg/g)Ga (μg/g)Nb (μg/g)
T-10.0232.580.062151577215
Y-10.012.970.051158586225
SD0.0090.2760.00849.56.36 7.07
Note: Rare earth elements (REE) include lanthanides and Yttrium; SD: Standard Deviation.
Table 2. Water immersion extraction efficiency (%) of elements of interest.
Table 2. Water immersion extraction efficiency (%) of elements of interest.
Sample No.AlGaNbREE
143.1363.670.030.03
242.7766.970.020.02
342.9270.650.020.03
444.5360.820.020.02
542.9271.110.010.01
639.1864.980.010.01
741.0768.850.010.01
840.4854.970.010.01
940.8358.930.010.01
Average41.9864.550.020.02
Standard Deviation1.585.190.010.01
Table 3. Acid leaching factors.
Table 3. Acid leaching factors.
LevelAB/°CC/hD/mol/L
120 mL:1 g4024
230 mL:1 g6046
340 mL:1 g8068
Table 4. L9(34) Orthogonal array design and acid leaching results of REE (%).
Table 4. L9(34) Orthogonal array design and acid leaching results of REE (%).
Sample No.FactorsLeaching Efficiency
ABCD
1111185.56
2122284.97
3133383.65
4212371.82
5223179.82
6231280.71
7313281.79
8321384.59
9332180.85
Іj254.18239.17250.86246.23
ІІj232.35249.38237.64247.47
Шj247.23245.21245.26240.06
Kj3333
Ij/Kj84.7379.7283.6282.08
ІІj/Kj77.4583.1379.2182.49
Шj/Kj82.4181.7481.7580.02
Dj7.283.404.412.47
Note: j: the column of A, B, C and D; Іj: the sum of leaching efficiency in the first level and j column; ІІj: the sum of leaching efficiency in the second level and j column; Шj: the sum of leaching efficiency in the third level and j column; Kj: the number of the same level in column j; Ij/Kj: the average of leaching efficiency in the first level and j column; ІІj/Kj: the average of leaching efficiency in the second level and j column; Шj/Kj: the average of leaching efficiency in the third level and j column; Dj: range value in the j column, Dj = max{Ij/Kj, ІІj/Kj, Шj/Kj} − min{Ij/Kj, ІІj/Kj, Шj/Kj}.
Table 5. L9(34) Orthogonal experiment design and acid leaching efficiencies of Ga (%).
Table 5. L9(34) Orthogonal experiment design and acid leaching efficiencies of Ga (%).
Sample No.FactorsLeaching Efficiency (%)
ABCD
1111183.48
2122284.62
3133380.94
4212377.11
5223178.57
6231280.42
7313282.59
8321387.12
9332177.38
Іj249.04243.18251.02239.43
ІІj236.10250.31239.11247.63
Шj247.09238.74242.10245.17
Kj3333
Ij/Kj83.0181.0683.6779.81
ІІj/Kj78.7083.4479.7082.54
Шj/Kj82.3679.5880.7081.72
Dj4.313.863.972.73
Note: the meaning of Іj, ІІj, Шj, Kj, Ij/Kj, ІІj/Kj, Шj/Kj, Dj is the same as Table 4.
Table 6. L9(34) Orthogonal experiment design and acid leaching efficiencies of Nb (%).
Table 6. L9(34) Orthogonal experiment design and acid leaching efficiencies of Nb (%).
Sample No.FactorsLeaching Efficiency (%)
ABCD
111113.2
212221.01
313332.46
421236.87
522311.36
623122.88
731323.32
8321311.02
933211.74
Ij6.6713.3917.106.30
ІІj11.1113.399.627.21
Шj16.087.087.1420.35
Kj3333
Ij/Kj2.224.465.702.10
ІІj/Kj3.704.463.212.40
Шj/Kj5.362.362.386.78
Dj3.142.103.324.68
Note: the meaning of Ij, ІІj, Шj, Kj, Ij/Kj, ІІj/Kj, Шj/Kj, Dj is the same as Table 4.
Table 7. Leaching efficiency of verifying sample (%).
Table 7. Leaching efficiency of verifying sample (%).
Experiment StepREEGaNb
Water immersion<0.467.31<1
Acid leaching85.8179.71<1
Total leaching85.8193.37<1
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