Extraction of rare earths from ion-adsorption type rare earth ore by indigenous microbial community

Abstract

Indigenous microbial communities were employed after subculture in stirred and column bioleaching experiments involving ion-adsorption type rare earth ore. The microbial eukaryotic communities exhibited dramatically varying diversity and structure across culture compositions. Compared with Czapek and sucrose medium, the community cultured in a nutrient broth (NB) medium had a higher diversity, and it was mainly composed of Zygosaccharomyces, Ustilago, Kodamaea, Malassezia, and Aspergillus. These microorganisms secrete organic acids, such as citric acid, malic acid, gluconic acid, and itaconic acid, which provide effective coordination electrons through hydroxyl and carboxyl groups. Stirred bioleaching experiments were conducted to investigate the effect of community, inoculum dosage, liquid–solid ratio, and time on the leaching efficiency. Stirred bioleaching resulted in a concentration limitation phenomenon. When the inoculum dosage of the community cultured in NB medium was 70vol%, the liquid–solid ratio was 5.0 mL·g−1, and the time was 60 min, the upward trend of rare earths leaching rate has become very small. Specifically, the leaching rates of detectable La, Ce, and Y were approximately 92.49%, 92.42%, and 94.39%, respectively. The leaching efficiency and the three influencing factors all conformed to the Poly5 polynomial function, with variances above 0.99. Column bioleaching experiments were performed at a scale of 1 kg. The self-propelled low-pH environment increased the leaching efficiency, which resulted in a leaching rate of 98.88% for rare earths after 117 h. X-ray diffraction and scanning electron microscopy revealed that the samples mainly comprised quartz, kaolinite, orthoclase, muscovite, and zeolite, which were predominantly present in the form of lumps, flakes, rods, and small particles. After bioleaching, the wave intensity of quartz, kaolinite, orthoclase, and muscovite increased, and that of zeolite decreased considerably. A diminution in the number of fine particles indicated the dissolution of small quantities of clay minerals. Ultimately, the differentiated bioleaching mechanism of various forms of rare earths was discussed based on experimental phenomena.

1. Introduction

Ion-adsorption type rare earth ore (IAREO) represents a type of rare earth deposit first discovered in China in 1969. This ore is rich in heavy rare earth elements (HREEs), which are scarce in the world [1‒2]. Rare earths are desorbed through cation exchange when they come into contact with an electrolyte solution in the in-situ leaching process. Then, they migrate with the leaching agent and are collected [3‒5]. The reaction equation for ammonium sulfate is shown in Eq. (1). Based on this cation exchange principle, the mining of IAREO mainly undergoes sodium chloride pool leaching, ammonium sulfate heap leaching, and ammonium sulfate in-situ leaching processes [6‒8]. However, ammonium sulfate migrates and diffuses through seepage and surface runoff; mining areas feature ammonia–nitrogen pollution of soil, surface water, and groundwater [9‒12]. Therefore, a new generation of ammonium-free leaching agents, such as magnesium sulfate and aluminum sulfate, has been proposed and gradually applied based on soil calcium and magnesium imbalance, reduced impurity leaching, and improved leaching efficiency [13‒17]. Furthermore, novel leaching agents, such as compound leaching agents, organic leaching agents, plant leaching agents, surfactants, bioleaching agents, and other new leaching agents, are constantly being proposed [18‒22]. In addition, enhancement techniques such as electric field driving have been used to improve rare earth leaching efficiency. A direct current field causes the directional movement of rare earth ions and improves the permeability of leaching systems. An electric field of 6 V·cm−1 shortened the leaching time by 30 min and increased the flow rate of the leach solution by 26.98% [23].