Investigation on Leaching Characteristics for Optimum Processing of Two Types of Rare Earth Elements Ores

Abstract

Rare earth elements (REEs) are the most important materials used in the modern high-tech industry. In 2014, 42% of the REE reserves and 86% of REE production in the world were concentrated in China. In this study, the leaching characteristics of two types of REE ores (REE ore A and REE ore B) outside China were studied. As a result, the optimum method for processing each ore was proposed. REE ore A contained zoning structured apatite as the major mineral. The grade of REEs was approximately 10% as total rare earth oxide (TREO). REEs chemically substituting for Ca in apatite, it is difficult to concentrate the REEs by physical separation, but approximately 60% of the Fe was able to be removed by physical separation. When REE ore A was leached by sulfuric acid, the leaching level of the REEs was limited by the formation of calcium sulfate. In the leaching study using hydrochloric and nitric acid as the leaching agents, more than 90% of the REEs were leached by 2.0 M of acid. By employing a two-step leaching process, 80% of the Ca was removed by 1.0 M of hydrochloric acid in stage 1 without REE loss, and more than 90% of the REEs were leached in stage 2 using 2.0 M of acid. The amount of Ca in the final leachate was reduced remarkably. After the two-step leaching, REE oxalate could be produced and then converted to REE oxide by roasting. In REE ore B, goethite was the major mineral, and the Fe content was higher than 50%. The TREO content was approximately 2-3%. There was no advantage by physical separation, thus the ore was introduced into the direct leaching process. As a result of preliminary acid leaching, 20-30% of the REEs were leached by 2.0 M of nitric acid. At elevated temperatures, the efficiency of the REE leaching was only improved to 60-70% by 2.0-3.0 M of nitric acid. To enhance the REE leaching level, physical and chemical cracking methods were introduced to leach the ore. Microwave pretreatment and wet grinding were attempted as physical cracking methods, but there was no improvement in the level of REE extraction. However, the introduction of chemical cracking methods, such as sulfuric acid baking, NaOH digestion and high pressure acid leaching (HPAL), was successful in improving the recovery of REEs significantly. In particular, 90-100% of the REEs were leached by NaOH digestion-hydrochloric acid leaching (80 ℃), with some impurities being removed during the digestion step. To check the possibility of leachate purification, precipitation and solvent extraction (SX) were attempted. By employing two-step precipitation, 99.9% of the Fe contained in the leaching solution of the NaOH-digested sample was removed in stage 1. In stage 2, the REE carbonate could be recovered. By SX, making a cleaner leaching solution using D2EHPA was attempted after the stage 1 precipitation. In this study, the detailed leaching characteristics and processing of apatite-based ore and lateritic refractory REE ore were investigated. Insufficient studies for these types of REE ores have been done, thus, this study provides information about the ores and suggests approaches for processing unknown materials.