Elsevier

Journal of Rare Earths

Volume 40, Issue 2, February 2022, Pages 318-327
Journal of Rare Earths

Leaching recovery of rare earth elements from the calcination product of a coal coarse refuse using organic acids

https://doi.org/10.1016/j.jre.2020.11.021Get rights and content

Highlights

  • HREE and Sc recovery was improved by using citric acid and dl-malic acid.

  • Complexing ability of organic acids contributed to the recovery of total REEs.

  • Malonic, oxalic and DL-tartaric acids were inferior to other organic acids and HCl.

  • Leaching mechanisms with organic acid as the lixiviant were revealed.

Abstract

Due to the increasing criticality of rare earth elements (REEs), it has become essential to recover REEs from alternative resources. In this study, systematic REEs leaching tests were performed on the calcination product of a coal coarse refuse using hydrochloric acid and different types of organic acid as lixiviants. Experimental results show that the recovery of REEs, especially heavy REEs (HREEs) and scandium (Sc), is improved by using selected organic acids. Citric acid and dl-malic acid afford the best leaching performances; whereas, malonic acid, oxalic acid, and dl-tartaric acid are inferior to hydrochloric acid. Results of zeta potential measurements and solution chemical equilibrium calculations show that malonic acid is more likely adsorbed on the surface of the calcined material compared with citric acid and dl-malic acid. The adsorption may reduce the effective concentration of malonic species in solution and/or increase the amount of REEs adsorbed on the surface, thereby impairing the leaching recovery. Compared with light REEs (LREEs), a stronger adsorption of the HREEs on the surface is observed from electro-kinetic test results. This finding explains why organic acids impose a more positive impact on the leaching recovery of HREEs. By complexing with the HREEs, organic acids can keep the metal ions in solution and improve the leaching recovery. The adsorption of Sc3+ on the surface is the lowest compared with other REEs. Therefore, rather than complexing, the organic anionic species likely play a function of solubilizing Sc from the solid, which is similar to that of hydrogen ions.

Graphical abstract

Different types of organic acids were used as lixiviants to leach rare earths from the calcination product of a coal coarse refuse. At the same pH, citric acid and dl-malic acid were found to provide the highest recoveries of heavy rare earths and scandium.

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Introduction

Rare earth elements (REEs) are a group of 15 elements referred to as the lanthanide series in the periodic table of elements. Scandium (Sc) and yttrium (Y) are normally included in this categorization because of their similar properties and occurrence in the same ore bodies as the lanthanides.1 REEs are generally classified into two sub-groups according to their atomic weights and locations in the periodical table,2,3 namely light rare earth elements (LREEs) and heavy rare earth elements (HREEs). Promethium (Pm) is produced by radioactive decay and considered as the only element listed in the periodical table without stable isotopes.4,5 The global production of REEs was 210000 metric tons of rare earth oxide (REO) in 2019.6 Fig. 1 shows REEs production by different countries. REEs have been coded as critical materials by several international institutions and governments due to their supply risks and importance to the clean energy industry, advanced military applications, and many commodity items in high-tech industries.3,7, 8, 9 The criticality of REEs has led to increasing interests and studies in the recovery from alternative resources, such as waste electrical and electronic equipment, permanent magnets, and lamp phosphors.3,10,11

In recent years, many studies have been reported focusing on the recovery of REEs from coal-based materials.12, 13, 14, 15, 16, 17, 18, 19, 20 As reported by Luttrell et al.,21 more than 80% of REEs associated with run-of-mine coals are reported to refuse streams after coal preparation. Therefore, coal refuse is more suitable to be used as a feedstock for REEs recovery compared with other coal-based materials. Physical and hydrometallurgical approaches usually provided low recoveries of REEs from coal refuse.19 Therefore, calcination treatment prior to acid leaching has been used with encouraging results being obtained.22,23 For example, REEs recovery from a coal refuse was improved from around 20% to 90% by two hours of calcination at 600 °C using 1.2 mol/L HCl as the lixiviant.22 However, this acid concentration level is too high to make the overall recovery and purification process economically viable.12 The contradiction is that REEs recovery was reduced when using HCl solutions of a lower concentration, such as 6 × 10−3 mol/L.22 One potential solution is using organic acids to replace inorganic acids.

Organic acids, especially low molecular weight organic acids, have been applied for leaching REEs from both conventional rare earth ores and secondary resources.24, 25, 26, 27 Acetic acid, malonic acid, citric acid, tartaric acid, succinic acid, and malic acid have been used to improve the leaching recovery of REEs from an ion-adsorption type rare earth clay.28 It was found that without pH adjustment, the extraction of REEs can be improved from around 9.6 to 10.7 mmol/kg by adding 1 × 10−3 mol/L of the organic acids. The enhancement of metal leaching in the presence of organic acids is likely due to three mechanisms: (1) protons dissociated from organic acid molecules increase the acidity of leaching systems; (2) organic species affect the saturation state of leach solutions with respect to mineral solids; (3) organic species change the speciation of metal ions in solution.29

In this study, acid leaching tests were performed on the calcination product of a coal coarse refuse material using different types of organic acids. Hydrochloric acid was also utilized for comparison purposes. The advantages of the organic acids over HCl were evaluated based on leaching test results. Interaction mechanisms between the organic acids and the solid material were investigated through electro-kinetic tests and solution chemical equilibrium calculations. A comprehensive understanding of REEs recovery from calcined coal refuse using organic acids was achieved.

Section snippets

Materials

The sample used in this study was collected from the feed of a coal preparation plant, which processed coals originating from the Baker (Western Kentucky No. 13) seam located in western Kentucky, USA. The sample was dry-screened to obtain the plus 1 cm fraction, which was then density-fractionated using a dense medium of 2.2 specific gravity (SG). The medium was prepared by mixing fine-grained magnetite with tap water. The 2.2 SG sink fraction was collected and rinsed several times to remove

Sample characterization

Ash content of the coarse refuse sample was 86.49%. After calcination at 600 °C for 2 h, the associated organic matter was completely removed. As shown in Table 2, the content of total REEs in the calcined material was 364 mg/kg, which is over five times higher than the average content of World coals (68.5 mg/kg).34 Mineralogical changes of the coarse refuse before and after calcination were characterized by XRD analysis. As shown in Fig. 3, quartz was the most dominant mineral in both the raw

Conclusions

Acid leaching tests were performed on the calcination product of the coarse refuse fraction of a run-of-mine coal originating from the Baker seam located in western Kentucky, USA. Different types of organic acids, including succinic acid, malonic acid, acetic acid, citric acid, oxalic acid, dl-malic acid, dl-tartaric acid, ascorbic acid, and maleic acid, were used as the lixiviants. Moreover, hydrochloric acid was also used for comparison purposes. The impact of hydrochloric acid concentration

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