Introduction to Rare Earth Industry Technology
·Rare earth is not a metallic element, but a collective term for 15 rare earth elements and yttrium and scandium. Therefore, the 17 rare earth elements and their various compounds have various uses, ranging from chlorides with a purity of 46% to single rare earth oxides and rare earth metals with a purity of 99.9999%. With the addition of related compounds and mixtures, there are countless rare earth products. So, rare earth technology is also diverse based on the differences of these 17 elements. However, due to the fact that rare earth elements can be divided into cerium and yttrium groups based on mineral characteristics, the mining, smelting, and separation processes of rare earth minerals are also relatively unified. Starting from the initial ore mining, the separation methods, smelting processes, extraction methods, and purification processes of rare earths will be introduced one by one.
Mineral processing of rare earths
·Mineral processing is a mechanical processing process that utilizes the differences in physical and chemical properties between various minerals that make up the ore, utilizes different beneficiation methods, processes, and equipment to enrich useful minerals in the ore, remove harmful impurities, and separate them from gangue minerals.
·In the rare earth ores mined worldwide, the content of rare earth oxides is only a few percent, and some even lower. In order to meet the production requirements of smelting, rare earth minerals are separated from gangue minerals and other useful minerals through beneficiation before smelting, in order to increase the content of rare earth oxides and obtain rare earth concentrates that can meet the requirements of rare earth metallurgy. The beneficiation of rare earth ores generally adopts flotation method, often supplemented by multiple combinations of gravity and magnetic separation to form a beneficiation process flow.
The rare earth deposit in the Baiyunebo Mine in Inner Mongolia is a carbonate rock type deposit of iron dolomite, mainly composed of accompanying rare earth minerals in iron ore (in addition to fluorocarbon cerium ore and monazite, there are also several niobium and rare earth minerals).
The extracted ore contains about 30% iron and about 5% rare earth oxides.After crushing the large ore in the mine, it is transported by train to the beneficiation plant of Baotou Iron and Steel Group Company. The task of the beneficiation plant is to increase Fe2O3 from 33% to over 55%, first grinding and grading on a conical ball mill, and then selecting a primary iron concentrate of 62-65% Fe2O3 (iron oxide) using a cylindrical magnetic separator. The tailings continue to undergo flotation and magnetic separation to obtain a secondary iron concentrate containing more than 45% Fe2O3 (iron oxide). Rare earth is enriched in flotation foam, with a grade of 10-15%. The concentrate can be selected using a shaking table to produce a coarse concentrate with a REO content of 30%. After being reprocessed by beneficiation equipment, a rare earth concentrate with a REO content of over 60% can be obtained.
Decomposition method of rare earth concentrate
·Rare earth elements in concentrates generally exist in the form of insoluble carbonates, fluorides, phosphates, oxides, or silicates. Rare earth elements must be converted into compounds soluble in water or inorganic acids through various chemical changes, and then undergo processes such as dissolution, separation, purification, concentration, or calcination to produce various mixed rare earth compounds such as mixed rare earth chlorides, which can be used as products or raw materials for separating single rare earth elements. This process is called rare earth concentrate decomposition, also known as pre-treatment.
·There are many methods for decomposing rare earth concentrates, which can be generally divided into three categories: acid method, alkali method, and chlorination decomposition. Acid decomposition can be further divided into hydrochloric acid decomposition, sulfuric acid decomposition, and hydrofluoric acid decomposition. Alkali decomposition can be further divided into sodium hydroxide decomposition, sodium hydroxide melting, or soda roasting methods. The appropriate process flow is generally selected based on the principles of concentrate type, grade characteristics, product plan, convenience for the recovery and comprehensive utilization of non rare earth elements, benefit for labor hygiene and environmental protection, and economic rationality.
·Although nearly 200 rare and dispersed element minerals have been discovered, they have not been enriched into independent deposits with industrial mining due to their rarity. So far, only rare independent germanium, selenium, and tellurium deposits have been discovered, but the scale of the deposits is not very large.
Smelting of rare earths
·There are two methods for rare earth smelting, hydrometallurgy and pyrometallurgy.
·The entire process of rare earth hydrometallurgy and metal chemical metallurgy is mostly in solution and solvent, such as the decomposition of rare earth concentrate, separation and extraction of rare earth oxides, compounds, and single rare earth metals, which use chemical separation processes such as precipitation, crystallization, oxidation-reduction, solvent extraction, and ion exchange. The most commonly used method is organic solvent extraction, which is a universal process for industrial separation of high-purity single rare earth elements. The hydrometallurgy process is complex and the product purity is high. This method has a wide range of applications in producing finished products.
The pyrometallurgical process is simple and has high productivity. Rare earth pyrometallurgy mainly includes the production of rare earth alloys by silicothermic reduction method, the production of rare earth metals or alloys by molten salt electrolysis method, and the production of rare earth alloys by metal thermal reduction method etc.
The common characteristic of pyrometallurgy is production under high temperature conditions.
Rare earth production process
·Rare earth carbonate and rare earth chloride are the two main primary products in the rare earth industry. Generally speaking, there are currently two main processes for producing these two products. One process is the concentrated sulfuric acid roasting process, and the other process is called the caustic soda process, abbreviated as the caustic soda process.
·In addition to being present in various rare earth minerals, a significant portion of rare earth elements in nature coexist with apatite and phosphate rock minerals. The total reserves of world phosphate ore are approximately 100 billion tons, with an average rare earth content of 0.5 ‰. It is estimated that the total amount of rare earth associated with phosphate ore in the world is 50 million tons. In response to the characteristics of low rare earth content and special occurrence status in mines, various recovery processes have been studied both domestically and internationally, which can be divided into wet and thermal methods. In wet methods, they can be divided into nitric acid method, hydrochloric acid method, and sulfuric acid method according to the different decomposition acids. There are various ways to recover rare earths from phosphorus chemical processes, all of which are closely related to the processing methods of phosphate ore. During the thermal production process, the rare earth recovery rate can reach 60%.
With the continuous utilization of phosphate rock resources and the shift towards the development of low-quality phosphate rock, the sulfuric acid wet process phosphoric acid process has become the mainstream method in phosphate chemical industry, and the recovery of rare earth elements in sulfuric acid wet process phosphoric acid has become a research hotspot. In the production process of sulfuric acid wet process phosphoric acid, the process of controlling the enrichment of rare earths in phosphoric acid and then using organic solvent extraction to extract rare earths has more advantages than early developed methods.
Rare earth extraction process
Sulfuric acid solubility
Cerium group (insoluble in sulfate complex salts) – lanthanum, cerium, praseodymium, neodymium, and promethium;
Terbium group (slightly soluble in sulfate complex salts) - samarium, europium, gadolinium, terbium, dysprosium, and holmium;
Yttrium group (soluble in sulfate complex salts) – yttrium, erbium, thulium, ytterbium, lutetium, and scandium.
Extraction separation
Light rare earth (P204 weak acidity extraction) – lanthanum, cerium, praseodymium, neodymium, and promethium;
Middle rare earth (P204 low acidity extraction)- samarium, europium, gadolinium, terbium, dysprosium;
Heavy rare earth elements (acidity extraction in P204) - holmium ,
Introduction to Extraction Process
In the process of separating rare earth elements, due to the extremely similar physical and chemical properties of 17 elements, as well as the abundance of accompanying impurities in rare earth elements, the extraction process is relatively complex and commonly used.
There are three types of extraction processes: step-by-step method, ion exchange, and solvent extraction.
Step-by-step method
The method of separation and purification using the difference in solubility of compounds in solvents is called the step-by-step method. From yttrium(Y) to lutetium (Lu), a single separation between all naturally occurring rare earth elements, including radium discovered by the Curie couple,
They are all separated using this method. The operating procedure of this method is relatively complex, and the single separation of all rare earth elements took over 100 years, with one separation and repeated operation reaching 20000 times. For chemical workers, their work
The strength is relatively high and the process is relatively complex. Therefore, using this method cannot produce a single rare earth in large quantities.
Ion exchange
The research work on rare earth elements has been hindered by the inability to produce a single rare earth element in large quantities through step-by-step methods. In order to analyze the rare earth elements contained in nuclear fission products and remove the rare earth elements from uranium and thorium, ion exchange chromatography (ion exchange chromatography) was successfully studied, which was then used for the separation of rare earth elements. The advantage of ion exchange method is that multiple elements can be separated in one operation. And it can also obtain high-purity products. However, the disadvantage is that it cannot be processed continuously, with a long operating cycle and high costs for resin regeneration and exchange. Therefore, this once the main method for separating large amounts of rare earths has been retired from the mainstream separation method and replaced by solvent extraction method. However, due to the outstanding characteristics of ion exchange chromatography in obtaining high-purity single rare earth products, currently, in order to produce ultra-high purity single products and separate some heavy rare earth elements, it is also necessary to use ion exchange chromatography to separate and produce a rare earth product.
Solvent extraction
The method of using organic solvents to extract and separate the extracted substance from an immiscible aqueous solution is called organic solvent liquid-liquid extraction, abbreviated as solvent extraction. It is a mass transfer process that transfers substances from one liquid phase to another. The solvent extraction method has been applied earlier in petrochemical, organic chemistry, pharmaceutical chemistry, and analytical chemistry. However, in the past forty years, due to the development of atomic energy science and technology, as well as the need for the production of ultrapure substances and rare elements, solvent extraction has made great progress in industries such as nuclear fuel industry and rare metallurgy. China has achieved a high level of research in extraction theory, the synthesis and application of new extractants, and the extraction process for rare earth element separation. Compared with separation methods such as graded precipitation, graded crystallization, and ion exchange, solvent extraction has a series of advantages such as good separation effect, large production capacity, convenience for rapid and continuous production, and easy to achieve automatic control. Therefore, it has gradually become the main method for separating large amounts of rare earths.
Rare earth purification
Production raw materials
Rare earth metals are generally divided into mixed rare earth metals and single rare earth metals. The composition of mixed rare earth metals is similar to the original rare earth composition in the ore, and a single metal is a metal separated and refined from each rare earth. It is difficult to reduce rare earth oxides (except for oxides of samarium, europium,, thulium, ytterbium) into a single metal using general metallurgical methods, due to their high heat of formation and high stability. Therefore, the commonly used raw materials for the production of rare earth metals nowadays are their chlorides and fluorides.
Molten salt electrolysis
The mass production of mixed rare earth metals in industry generally uses the molten salt electrolysis method. There are two methods of electrolysis: chloride electrolysis and oxide electrolysis. The preparation method of a single rare earth metals varies depending on the element. samarium, europium,, thulium, ytterbium are not suitable for electrolytic preparation due to their high vapor pressure, and instead are prepared using reduction distillation method. Other elements can be prepared by electrolysis or metal thermal reduction method.
Chloride electrolysis is the most common method for producing metals, especially for mixed rare earth metals. The process is simple, cost-effective, and requires minimal investment. However, the biggest drawback is the release of chlorine gas, which pollutes the environment. Oxide electrolysis does not release harmful gases, but the cost is slightly higher. Generally, high priced single rare earths such as neodymium and praseodymium are produced using oxide electrolysis.
The vacuum reduction electrolysis method can only prepare general industrial grade rare earth metals. To prepare rare earth metals with low impurities and high purity, vacuum thermal reduction method is generally used. This method can produce all single rare earth metals, butsamarium, europium,, thulium, ytterbium cannot be produced using this method. The redox potential of samarium, europium,, thulium, ytterbium and calcium only partially reduces rare earth fluoride. Generally, the preparation of these metals is based on the principles of high vapor pressure of these metals and low vapor pressure of lanthanum metals. The oxides of these four rare earths are mixed with fragments of lanthanum metals and compressed into blocks, and reduced in a vacuum furnace. Lanthanum is more active, while samarium, europium,, thulium, ytterbium are reduced to gold by lanthanum and collected on condensation, making it easy to separate from slag.
Post time: Nov-07-2023