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Where do rare earth magnets come from?
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Where do rare earth magnets come from?

Author: Site Editor     Publish Time: 2020-12-24      Origin: Site

Where do rare earth magnets come from?

Do you know how the powerful NdFeB magnet is made? It needs to pulverize the alloy containing the necessary materials into particles, gather these particles to form, and then sinter them. Then cut to the required size to complete. This process always feels like making a cake.

High-performance aero-engines require high technical support, and the requirements for raw materials are also very high. The performance of raw materials directly determines the performance of the engine. The operation process of an aero engine is more complicated. Will produce very high temperature, so engine parts must be high temperature resistant. The difficulty of this technology is also very demanding. The creation of these alloys requires a lot of rare earth metals as raw materials. As a high-temperature resistant rare earth metal, cobalt is an important raw material for the production of heat-resistant alloys, hard alloys, anti-corrosion alloys, magnetic alloys and various cobalt salts.

The rare earth market is diversified. It is not just a product, but 15 rare earth elements, yttrium, scandium, and various compounds, ranging from the purity of 46% chloride to a single rare earth oxide and rare earth metal with a purity of 99.9999%. Has a variety of uses. Together with related compounds and mixtures, there are countless products. First, starting from the initial mining of ore, we introduce the separation methods and smelting process of rare earth one by one.

Rare earth mining

Rare earth beneficiation

Beneficiation is the use of the differences in the physical and chemical properties of the various minerals that make up the ore. Using different beneficiation methods, with the help of different beneficiation processes and different beneficiation equipment, to enrich the useful minerals in the ore, remove harmful impurities. And Mechanical processing to separate it from gangue minerals.

In the rare earth ores mined around the world, the content of rare earth oxides is only a few percent, or even lower. To meet the production requirements of smelting, the rare earth minerals are separated from gangue minerals and other useful minerals by beneficiation before smelting. To increase the content of rare earth oxides, a rare earth concentrate that can meet the requirements of rare earth metallurgy can be obtained. The beneficiation of rare earth ore generally uses flotation and is often supplemented by a combination of gravity separation and magnetic separation.

Decomposition method of rare earth concentrate

Rare earth in concentrates generally exists in the form of carbonates, fluorides, phosphates, oxides, or silicates that are difficult to dissolve in water. The rare earth must be converted into compounds soluble in water or inorganic acids through various chemical changes. After dissolving, separating, purifying, concentrating or burning, etc., various mixed rare earth compounds such as mixed rare earth chlorides are made, as products or separated For a single rare earth raw material, such a process is called rare earth concentrate decomposition or pretreatment.

There are many methods for decomposing rare earth concentrates, which can be divided into three categories in general, namely acid method, alkali method, and chlorination decomposition. Acid decomposition is divided into hydrochloric acid decomposition, sulfuric acid decomposition, and hydrofluoric acid decomposition. Alkaline decomposition is divided into sodium hydroxide decomposition or sodium hydroxide melting or soda roasting method. Generally, the appropriate process flow is selected according to the principles of the type, grade characteristics, product plan, facilitating the recovery and comprehensive utilization of non-rare earth elements, labor hygiene and environmental protection, and economic rationality.

Flow chart of rare earth mining and processing (2)

Smelting of rare earth

There are two rare earth smelting methods, hydrometallurgy, and pyrometallurgy.

In the chemical metallurgy of rare earth hydrometallurgy, most of the whole process is in solution and solvent. For example, the decomposition of rare earth concentrates, the separation and extraction of rare earth oxides, rare earth compounds, and single rare earth metals use precipitation, crystallization, redox, Chemical separation processes such as solvent extraction and ion exchange. The most common application is the organic solvent extraction method, which is a general process for industrial separation of high-purity single rare earth elements. The hydrometallurgical process is complex, the product purity is high, and the application of this method to produce finished products is broad.

The pyrometallurgical process is simple and the productivity is high. Rare earth fire smelting mainly includes the production of rare earth alloys by silicon thermal reduction, the production of rare earth metals or alloys by molten salt electrolysis, and the production of rare earth alloys by metal thermal reduction. The common feature of pyrometallurgy is production under high-temperature conditions.

The 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 a concentrated sulfuric acid roasting process, and the other is called the caustic soda process, referred to as the alkaline process.

In addition to various rare earth minerals, a considerable part of the rare earth elements in nature coexist with apatite and phosphorite minerals. The world's total reserves of phosphate rock are about 100 billion tons, and the average rare earth content is 0.5‰. It is estimated that the total amount of rare earth associated with phosphate rock in the world is 50 million tons. Given the low rare earth content in the mine and its special occurrence state, a variety of recycling processes have been studied at home and abroad, which can be divided into wet methods and thermal methods: in wet methods, depending on the decomposition acid, it can be divided into the nitric acid method, Hydrochloric acid method, sulfuric acid method. There are many kinds of rare earth recovery from the phosphorus chemical process, all of which are closely related to the processing method of phosphate rock. In the thermal production process, the rare earth recovery rate can reach 60%.

With the continuous utilization of phosphate rock resources, it is turning to the development of low-quality phosphate rock. The sulfuric acid wet-process phosphoric acid process has become the mainstream method of the phosphorus chemical industry. The recovery of rare earth in sulfuric acid wet-process phosphoric acid has become a research focus. In the production process of sulfuric acid wet-process phosphoric acid, the process of controlling the enrichment of rare earth in phosphoric acid and then using organic solvents to extract rare earth is more advantageous than the earlier developed methods.

The rare earth extraction process

Sulfuric acid solubility

  • Cerium group (hardly soluble in sulfuric acid double salt)-lanthanum, cerium, praseodymium, neodymium and promethium;

  • Terbium group (sulfuric acid double salt slightly soluble)-samarium, europium, gadolinium, terbium, dysprosium and holmium;

  • Yttrium group (sulfuric acid double salt is easy to dissolve)-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 and dysprosium;

  • Heavy rare earth (P204 acidity extraction)-holmium, yttrium, erbium, thulium, ytterbium, lutetium and scandium.

In the process of separating rare earth elements, since the physical and chemical properties of multiple elements are very similar, and there are many impurity elements associated with rare earth elements, the extraction process is more complicated. There are three commonly used extraction processes: Step method, ion exchange, and solvent extraction.

Stepwise method

The method of separating and purifying compounds based on the difference in solubility in solvents is called a stepwise method. From yttrium (Y) to lutetium (Lu), the single separation of all naturally occurring rare earth elements, including the radium discovered by the Curies, are all separated by this method. The operation procedure of this method is relatively complicated. The single separation of all rare earth elements has taken more than 100 years, and the separation and repeated operations have reached 20,000 times. For chemists, the work intensity is relatively high and the process is relatively complicated. Therefore, single rare earth cannot be produced in large quantities with this method.

Ion exchange

The research work of rare earth elements was hindered by the fact that the stepwise method could not produce single rare-earth in large quantities. In order to analyze the rare earth elements contained in nuclear fission products and remove the rare earth elements in uranium and thorium, the ion exchange chromatography was successfully studied ( Ion exchange method) and then used for the separation of rare earth elements. The advantage of the ion exchange method is that multiple elements can be separated in one operation. Moreover, high-purity products can be obtained. However, the disadvantages are that it cannot be processed continuously, the operation cycle is long, and the cost of resin regeneration and exchange is high. Therefore, the main method used to separate large amounts of rare earth has been withdrawn from the mainstream separation method and is extracted by solvents. Law replaces. However, because the ion exchange chromatography has the outstanding feature of obtaining high-purity single rare earth products, currently, in order to prepare ultra-high purity single products and the separation of some heavy rare earth elements, it is necessary to separate and prepare a rare earth product by ion-exchange chromatography. 

Solvent extraction

The method of using organic solvents to extract and separate the extract from the immiscible aqueous solution is called organic solvent liquid-liquid extraction, or solvent extraction for short. It is a method of transferring substances from a liquid phase to Another liquid phase mass transfer process. Solvent extraction has been applied earlier in petrochemical, organic chemistry, medicinal chemistry, and analytical chemistry. However, in the past 40 years, due to the development of atomic energy science and technology and the need for the production of ultra-pure substances and rare elements, solvent extraction has made great progress in the nuclear fuel industry, rare metallurgy, and other industries. China has reached a very high level in the research of extraction theory, the synthesis and application of new extractants, and the extraction process of rare earth element separation. Compared with separation methods such as fractional precipitation, fractional crystallization, and ion exchange, the solvent extraction method has a series of advantages such as good separation effect, large production capacity, convenient rapid continuous production, and easy realization of automatic control. Therefore, It gradually becomes the main method of separation rare earth.

Rare earth purification

production material

Rare earth metals are generally divided into mixed rare earth metals and single rare earth metals. The composition of the mixed rare earth metal is close to the original rare earth composition in the ore. And the single metal is the metal separated and refined from each rare-earth. Using rare earth oxides (except for samarium, europium, ytterbium, and thulium oxides) as raw materials are difficult to reduce to a single metal by general metallurgical methods, because of its large heat generation and high stability. Therefore, the commonly used raw materials for the production of rare-earth metals today are their chlorides and fluorides.

Molten salt electrolysis

The industrial mass production of mixed rare earth metals generally uses molten salt electrolysis. There are two methods of electrolysis: chloride electrolysis and oxide electrolysis. The preparation method of a single rare earth metal differs depending on the element. Samarium, europium, ytterbium, and thulium are not suitable for electrolysis preparation due to their high vapor pressure, and reductive distillation is used. Other elements can be prepared by electrolysis or metal thermal reduction.

Chloride electrolysis is the most common method to produce metals, especially the mixed rare earth metal process is simple, low cost, low investment, but the biggest disadvantage is the release of chlorine, which pollutes the environment. Oxide electrolysis does not emit harmful gases, but the cost is slightly higher. Generally, single rare earth such as neodymium and praseodymium with higher production prices are electrolyzed with oxides.

Vacuum reduction

The electrolysis method can only prepare general industrial grade rare earth metals. To prepare rare earth metals with low impurities and high purity, they are generally prepared by vacuum thermal reduction. This method can produce all single rare earth metals, but samarium, europium, ytterbium, and thulium cannot use this method. The redox potential of samarium, europium, ytterbium, thulium, and calcium only partially reduces rare earth fluoride. Generally, these metals are prepared by using the principle of the high vapor pressure of these metals and the low vapor pressure of lanthanum metal. These four kinds of rare earth oxides and lanthanum metal fragments are mixed and compacted and then reduced in a vacuum furnace. Lively, samarium, europium, ytterbium, and thulium are reduced to metals by lanthanum and collected on the condensation, which is easy to separate from the slag.

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