Rare earth elements are known as "industrial vitamins" and have excellent magnetic, optical and electrical properties. The mainland is a big country of rare earths, with relatively rich rare earth resources, and the proven reserves of rare earths are 3.7×105 tons (converted into oxides), accounting for 67% of the world's total rare earth resources. The mainland supplies 97% of the global demand for rare earths, but, according to relevant statistics, the reserves of rare earth resources in the mainland have dropped from nearly 90% of the global total in the 70s of the 20th century to 69% in the 80s, to about 45% at the end of the 90s, and to less than 36% at present. According to the current scale of mining and export, China will change from a large resource country of rare earth ores to a poor country of rare earths. With the increasing demand for rare earths in emerging industries such as new energy vehicles, rare earth resources are becoming increasingly scarce, and the recovery of rare earths from secondary resources has become an inevitable trend.
Red mud is a solid waste produced in the production of alumina, which mainly contains elements such as Fe, Si, Ca, Al, Ti and a small amount of Sc. The common rare earth elements in red mud include lanthanides (such as lanthanum, cerium, praseodymium, neodymium, etc.) and yttrium (such as yttrium, dysprosium, holmium, etc.), and their content is mostly more than 500×10-6, and the enrichment coefficient is about 2, which has high comprehensive utilization value.
China's alumina production accounts for more than 50% of the world's total, and for every 1t of alumina produced, 1.0~2 t of red mud will be attached. The annual production of alumina incidental red mud can reach more than 100 million tons, but the comprehensive utilization rate is only 7.6%, and it is mainly used in the building materials industry, in which the rare earth elements can not be fully utilized. Therefore, the use of reasonable methods to extract and utilize rare earth elements in red mud can not only improve the comprehensive utilization rate of red mud and produce extremely high economic benefits, but also has great significance for environmental protection and the sustainable development of rare earth resources.
craft
At present, the methods of leaching rare earth elements from red mud are mainly divided into wet leaching process and combined fire-wet process. Due to the different reagents and operating processes, the wet leaching process can be divided into hydrochloric acid leaching method, sulfuric acid leaching method, nitric acid leaching method, biological leaching method, etc., and the combined process of fire method and wet process can be divided into high temperature roasting-leaching method and sulfation roasting and leaching method.
Whether it is a wet leaching process or a combined fire-wet process, the resulting rare-earth-rich leaching solution will always contain impurities such as Fe and Al, so it needs to be purified by subsequent separation processes. At present, the separation and extraction processes of rare earths in red mud leaching solution are widely used, including solvent extraction and ion exchange. Ion exchange methods are less widely used due to the limited performance and cost of exchangers. The extraction method has become the main separation and extraction process due to the advantages of mature process, simple operation and high recovery rate.
The principle of solvent extraction is to use the difference in the solubility of solutes in two immiscible (or slightly soluble) solvents to transfer the compounds to the target solvent, so as to achieve the purpose of separation. Compared with other rare earth elements, scandium has a small ionic radius and simple electronic structure, which makes scandium extracts different from other rare earth elements and suitable for solvent extraction. Since scandium extraction must remove a large number of impurities such as iron and titanium, the selection of extractants and extraction processes is the key to achieving high recovery of scandium. At present, the commonly used extractants are mainly divided into phosphoric acid extractants, carboxylic acid extractants, primary amines, etc.
equipment
Centrifugal extractor is a new, fast and efficient liquid-liquid extraction and separation equipment, which is essentially different from traditional extraction equipment such as mixing clarifier and extraction tower.
The centrifugal extractor is to use a motor to drive the drum to rotate at high speed, and two liquids with different densities and immiscible with each other complete the mixed mass transfer under the action of the shear force generated by the rotation of the drum or paddle, and quickly separate under the action of the centrifugal force generated by the high-speed rotation of the drum, which is widely used in many fields such as chemical pharmaceuticals, API extraction, hydrometallurgy, environmental protection, oil-water separation, and food.
CWL-M centrifugal extractor
Source: CNKI, Zhihu, and other public information collation
Image source: Internet
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