Leaching behaviour of light rare earth elements from lignite-derived ash

Abstract

The growing demand for rare earth elements (REEs) has driven research into secondary sources such as coal ash. However, recovery from these materials remains challenging, particularly due to the high thermal treatment temperatures typically applied to parent coals. This study explores the potential of lignite, used in fluidized bed combustion (FBC) boilers, as a secondary source of REEs under lower thermal treatment conditions. Lignite coal and its derived ash, obtained at various laboratory-controlled temperatures below 1000 oC, were systematically characterised. The results revealed 8 times enrichment of REEs content from the raw coal to the derived ash, although the total rare earth element (TREE) concentration remained below 1000 ppm. Thermal treatment reduced particle size and induced mineralogical transformations consistent with the conversion of kaolinite to amorphous metakaolinite. To better replicate FBC conditions, the duration of thermal treatment was subsequently reduced based on observed combustion behaviour. Sequential leaching experiments demonstrated that, in raw lignite, that most light rare earth elements (LREEs) were associated with monosulfides and carbonates. In contrast, the derived ash exhibited primarily acid-soluble LREEs, with a fraction complex-bound and a substantial portion rendered insoluble following thermal treatment. The derived ash was subjected to leaching tests using a mineral acid (HCl) and an organic acid (tartaric acid). Screening results indicated that HCl achieved a 50% recovery of light rare earth elements (LREEs) compared to 36% for tartaric acid. The influence of key operational parameters, including liquid-to-solid ratio, lixiviant concentration, leaching duration, and temperature, was systematically evaluated. Among these, the liquid-to-solid ratio was identified as the most influential factor, although the range tested was narrower than in comparable studies. Kinetic experiments involving leaching duration and temperature revealed that the overall process is diffusion controlled. Model fitting suggested multiple rate-controlling mechanisms: product-layer diffusion dominated during the initial 5 minutes, followed by film-layer diffusion control over the remaining leaching period. The experimental results suggest that the use of lignite coal as a secondary source of REEs may be limited under the conditions investigated. Although the material exhibited a promising distribution of elements, the total rare earth element (TREE) content remained low even after enrichment. Leaching experiments at low thermal treatment temperatures achieved a maximum LREE recovery of 77%, but this required relatively high HCl concentrations (3 M) and a high liquidto-solid ratio (100:1), which may constrain the process's practical applicability. Finally, a preliminary design of a 35 t/h coal-fired power plant was presented on the basis of the experimental results. A mass balance over the leaching section of the design resulted in a PLS TREE mass flow rate of 28 t/yr. To achieve this amount, high amounts of HCl would be required. Therefore, further separation steps would be required to recycle large amounts of the lixiviant back to the leaching section.