Integrative Study Assessing Space and Time Variations with Emphasis on Rare Earth Element (REE) Distribution and Their Potential on Ashes from Commercial (Colombian) Coal

Ana Cláudia Santos,Aurora Futuro,David French,Alexandra Guedes,Bruno Valentim

doi:10.3390/min12020194

Ana Cláudia Santos, Aurora Futuro + Show 3 more

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https://doi.org/10.3390/min12020194

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Abstract

The increasing demand for rare earth elements (REEs), which is associated with their economic importance and the supply risk, has motivated the research for alternative secondary sources of these elements. Coal and coal combustion ash have been pointed out as promising REE raw materials. This research seeks to understand REE fractionation, from feed coals to ashes, considering seasonal variations, and to assess the trends within the ash fractions that can be used for further beneficiation processes. Colombian commercial feed coals, combustion ashes, and their respective fractions were sampled from a Portuguese power plant and were characterized via petrographical, mineralogical, and chemical analyses. The total REE concentrations in the feed coals studied range between 6.97 and 23.15 ppm, while, in the ashes, they vary from 159.9 to 266.6 ppm. Fly ash (FA) from electrostatic precipitator (ESP) presented higher concentrations than the bottom (BA) and economizer (ECO) ashes. Furthermore, REEs and the LREE/HREE ratio increased slightly towards the back rows of the ESP. In the feed coals, the REEs are significantly correlated with ash, and they occur in micrometric phosphate minerals intermixed with clays. In the ashes, the REEs were mostly detected in micrometric particles, with P and Al-Si as the major components. The results from the fractioned samples show that the REEs were enriched in the fine (<25 µm) and nonmagnetic fractions of the ESP FA. A single trial combining sieving and magnetic separation enabled the attainment of a REE recovery of 53%, and a final enrichment factor of 1.25. Coal combustion ashes and their respective size fractions are promising REE raw materials; however, the REE oxide concentrations are below the economical cutoff of 1000 ppm.

Highlights

The rare earth elements (REEs) [1] are considered strategic materials because they play a key role in vital segments of the economy, such as defense, green energies, electronics, and the chemical industries [2,3]
The growing demand rate (3.7–8.6% per year) and their supply risk led to the classification of REEs as “critical” raw materials and has stimulated the search for alternative sources [6,7,8,9,10,11]
The samples used in the present study were collected at the pulverized coal-fired thermoelectric power plant of Pego (Abrantes, Portugal), which operates at approx. 1500 ◦ C

Summary

Introduction

The REEs [1] are considered strategic materials because they play a key role in vital segments of the economy, such as defense, green energies, electronics, and the chemical industries [2,3]. The supply of REEs in Europe is dominated by China, at 98% [5], a total of 8%. The growing demand rate (3.7–8.6% per year) and their supply risk led to the classification of REEs as “critical” raw materials and has stimulated the search for alternative sources [6,7,8,9,10,11]. According to Ketris and Yudovich [31], the average REE content (lanthanide plus Y and Sc) in world hard coals is 72.1 ppm, approx.

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