Study on Influence Factors of Leaching of Rare Earth Elements from Coal Fly Ash

Abstract

Rare earth elements (REEs) are very important strategic resources, but the traditional high-quality rare earth resources are becoming increasingly scarce. Coal fly ash (CFA) in some areas in the world has a high REE content, which can be a potential supplement to the rare earth resources. CFA samples collected from the Panbei Power Plant in Guizhou Province of China, in which the REE content is 489 ppm, was used as the research object in this study. Taking three REEs (lanthanum (La), cerium (Ce), and neodymium (Nd)) with the highest content in this CFA sample as the representative and hydrochloric acid as the leaching reagent, the effects of various factors on the leaching efficiencies of REEs from CFA were investigated systematically. Further, two unreacted core shrinkage models (chemically controlled model and diffusion controlled model) were used to study the leaching kinetics of REEs from CFA. The results showed that, except for the small influence of the stirring speed, the temperature, acid concentration, liquid−solid ratio, and reaction time all have a great influence on the leaching efficiencies of REEs. Leaching efficiencies of 71.9, 66.0, and 61.9% for La, Ce, and Nd, respectively, were obtained under the conditions of a stirring speed of 200 rpm, temperature of 60 °C, HCl concentration of 3 M, liquid−solid ratio of 10 (v/m), and leaching time of 120 min. The leaching mechanism of REEs from CFA tends to be more controlled by chemical reactions. In addition, La is more easily leached from CFA than Ce and Nd. This study can provide theoretical guidance for optimizing the leaching conditions of REEs from CFA at low temperature and atmospheric pressure, thereby providing theoretical support for the early industrial development and utilization of rare earth resources in CFA.

A Comprehensive Review of Rare Earth Elements Recovery from Coal-Related Materials

“…Many studies have been published in recent years focused on the chemical extraction of REEs from coal combustion ashes [26,53,76,88,89,[91][92][93][94][95][96][97][98][99][100][101]. As shown in Table 4, acid leaching has been extensively exploited to extract REEs from coal combustion ash, and often the ash materials must be chemically and/or thermally treated prior to acid leaching to achieve better extraction performance.…”

“…Nearly 100% recovery was obtained from the samples using 12 M HCl at 85 • C, and a considerable amount of REEs (71%) were extracted from a fly ash sample of the same source even under much weaker acidity (1 M HCl). Cao et al conducted a parametric study to optimize the leaching recovery of REEs from a fly ash sample that was collected from a power plant located in Guizhou, China [91]. It was found that 71.9% of La, 66.0% of Ce, and 61.9% of Nd were leached using 3 M HCl at 60 • C. However, due to the fact that most of the REEs are encapsulated in the amorphous structures of fly ash generated by combusting pulverized coal under high temperature (~1400 • C), relatively low recoveries were usually achieved using acid leaching alone.…”

“…Sulfuric acid is expected to provide the worst efficiency since the large amount of calcium present in coal combustion ash will complex with sulfate to form gypsum, and some REEs will be incorporated into the gypsum structure and lost to the precipitate [107]. Leaching recovery of REEs is sensitive to liquid/solid ratio, acid concentration, temperature, and leaching duration [91,97,100]. Leaching recoveries of La, Ce, and Nd from a coal fly ash were nearly doubled when increasing the liquid/solid ratio from 5/1 to 20/1 or prolonging the reaction time from 30 to 180 min [91].…”

“…Leaching recovery of REEs is sensitive to liquid/solid ratio, acid concentration, temperature, and leaching duration [91,97,100]. Leaching recoveries of La, Ce, and Nd from a coal fly ash were nearly doubled when increasing the liquid/solid ratio from 5/1 to 20/1 or prolonging the reaction time from 30 to 180 min [91].…”

Ultrasound‐Assisted Leaching of Vanadium and Yttrium from Coal Ash: Optimization, Kinetic and Thermodynamic Study

“…are some studies which use coal ash to recover metals by different methods. Cao et al studied the recovery of REEs and found recoveries of the three metals La, Ce, and Nd of 71.9 %, 66 %, and 61.9 %, respectively [21]. Tuan et al used acid leaching to recover rare elements such as Y, Nd, and Dy from coal ash.…”