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

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.

Insights into the rare earth element potential of coal combustion by-products from western Canada

“…Geochemical data from previous CCB studies 20,21,23,24,26,27,30,34,39–63 was compiled to compare REE contents of western Canadian CCBs with other localities and to identify elemental indicators of REE enrichment using unsupervised machine learning. Where available, major oxide, trace element, REE, coal rank, country, and sedimentary basin information was compiled for FA and BA samples from around the world.…”

“…[20][21][22][23] Among these, coal combustion by-products are emerging as a promising source of REE based on several factors including: (i) availability as a low-cost waste product with strong incentives for reuse due to environmental liabilities of ash storage; (ii) a reduced requirement for excavation or beneciation thereby eliminating the most energy intensive processes involved in REE mining; and (iii) the elimination of radioactive tailings. 23,24 During the combustion process, REE are immobile and can be enriched up to 30 times in the ashes relative to the source coal, 25,26 with the geochemical composition of the coal, combustion conditions in the boiler, carbon content, and particle size controlling the REE abundance. 27 Several studies have investigated REE abundances in CCBs from a number of countries including: Canada, 28 the United States, 23,25,29,30 Brazil, 31 China, 32,33 the United Kingdom, 21 South Africa, 34 and Poland.…”

Physicochemical Properties of Fe-Bearing Phases from Commercial Colombian Coal Ash

“…The coal combustion ashes were provided by a Portuguese pulverized-coal thermoelectric power plant that burns low-sulfur Colombian commercial coals [49]. Proximate analysis, gross calorific value, and mineralogy of feed coals from the corresponding sampling campaigns (S1 and S4) previously published in [49] are summarized in Table 1.…”

“…The coal combustion ashes were provided by a Portuguese pulverized-coal thermoelectric power plant that burns low-sulfur Colombian commercial coals [49]. Proximate analysis, gross calorific value, and mineralogy of feed coals from the corresponding sampling campaigns (S1 and S4) previously published in [49] are summarized in Table 1. SX-C, coal sample (C) corresponding to the X sampling campaign; M, moisture; A, ash yield; V, volatile matter; daf, dry-ash-free basis; GCV, gross calorific value; n.d.-not determined.…”

The Discrepancy between Coal Ash from Muffle, Circulating Fluidized Bed (CFB), and Pulverized Coal (PC) Furnaces, with a Focus on the Recovery of Iron and Rare Earth Elements

“…Coal ash, as an artificial metallic mineral deposit, is a promising resource of Al, Fe, Li, Ga, and so on [ 13 , 14 ]. Among various metals, rare earth elements recovery from coal ash is impressive due to the “Rare earth Program” by the Department of Energy, the U.S.…”