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Production and separation of rare earth

The date of: 2018-09-13
viewed: 3

The rare earth market is a diversified market with 15 rare earth elements, yttrium, scandium and other compounds from the chloride with 46% of purity and the single rare earth oxide with the 99.9999% of purity and the rare earth metals, which all have various usages, instead of some individual product. If the relevant chemical compounds are added, the products are numerous.   Therefore, we will introduce the separating method and smelting process of the rear earth one by one with the initiating from the exploitation of the ores.


1. Rare earth concentration


Mineral concentration refers to mechanical process to concentrate the useful minerals in the ores and eliminate the hazardous impurities by taking advantages of the differences of the physicochemical properties of all kinds of minerals of ores with different mineral concentration methods, different technologies and different appliances to separate the minerals from the gangue minerals.


Currently, among the rare earth ores exploited in China and other countries in the world, the content of the rare earth oxide is only several percents or even much less.  To meet the demands of the smelting and production, the mineral concentration before smelting can separate the rare earth minerals from the gangue minerals and other useful minerals to improve the contents of the rare earth oxide to get the rare earth concentrates that can satisfy the requirements of rare earth smelting.  The common method of the mineral concentration of rare earth ores is the flotation method assisted with the process with the combination of gravity concentration and magnetic concentration or some other combinations.


The rare earth ore deposit on the mine of Bayan Obo in Inner Mongolia is the carbonatite deposit of ankerite.  The iron mine is always associated with the rare earth minerals (also some other rare earth minerals contained niobium, except for bastnaesite and monazite).  The ores exploited out contains about 30% of Fe, and 5% of rare earth oxide.  The large ores are firstly broken down on the miners, and then transported to the mineral concentration plant of Baotou Iron & Steel (Group) Co. Ltd by the railway.  The task of the mineral concentration plant is to improve the content of Fe203 from 33% to 55% or above; grind and make classification on the conical mills; use the drum magnetic separators to select out the iron concentrates contained 45% of Fe203 in the first concentration; and then take gravity concentration and magnetic concentration of the tailings to get the iron concentrates with 45% of Fe in the second concentration.  The rare earths are enriched in the flotation froth with the grade of 10% to 15%. The rough concentrates with 30% of REO can be selected out from the concentrates with the tables.  After the re-treatment by the mineral concentration equipment, the rare earth concentrates with 60% or above of REO will be gained.


2. Smelting methods of rare earth


There are two methods for rare earth smelting, hydrometallurgy and thermo-metallurgy.  The hydrometallurgy method belongs to the method of metal chemical metallurgy.  The whole process is conducted in the solution or solvent.  For example, the decomposition of the rare earth concentrates, and the separation and extraction of rare earth oxides, rare earth compounds, individual rare earth metals all adopt the chemical separation technologies, such as precipitation, crystallization, redox, solvent extraction and iron exchange, etc.  The most common-used method is the organic solvent extraction method, which is the universal method to separate the individual rare earth elements with high purity in industry.  The process of the hydrometallurgy method is complicated; however, the purity of the products is higher.  Therefore, this method is broadly used in producing the products.


The thermo-metallurgy technology is very simple and the productivity is higher. The rare earth thermo-metallurgy mainly include the silicothermic process to produce rare earth alloy, fused salt electrolysis process to produce rare earth metals or alloy, and metal thermal reduction to produce rare earth alloy, etc.  The common feature of thermo-metallurgy is producing the substances under high temperature.


1. Decomposition of rare earth concentrates


The rare earths in the rare earth concentrates generally present in the form of carbonate, fluoride, phosphate, oxide and silicate, etc., which are difficult to be soluble in water.  The rare earths shall be transformed into the compounds soluble in water or inorganic acid though various chemical changes.  Then these compounds are made into mixed rare earth compounds, such as mixed rare earth chlorides though dissolution, separation, purification, concentration, calcinations and other procedures.


The methods used are selected based on the types of the concentrates, grades, products plan and the principle of being easy for the recycling and comprehensive utilization of non-rare earth elements, helpful to protect the labor sanity and environment and the economic feasibility.


There are many methods to decompose the rare earth concentrates. Generally speaking, these methods can be classified into three categories: acid, alkaline and chlorination decomposition.  The acid decomposition includes hydrochloric acid decomposition, sulfuric acid decomposition and hydrofluoric acid decomposition.  The alkaline decomposition consists of sodium hydrate decomposition or sodium hydrate melting and soda calcinations, etc.


Production of rare earth carbonate and rare earth chloride:


These are the main two primary products in the rare earth industry.  Generally speaking, there are mainly two technologies to produce these two products.


One technology is calcinations with concentrated sulfuric acid, which refers to the calcinations of the rare earth concentrate mixed with the sulfuric acid in the rotary kiln.  When the mineral water after calcinations infuses, the soluble rare earth sulfate would come into to the water solvent, which is called lixivium.  Then adding ammonium hydrogen carbonate into the lixivium, the rare earth will be precipitated in the form of carbonate.  After filtering, the rare earth carbonate is acquired.


The other technology is called alkaline decomposition.  The procedures of this method are as follows: mixing the 60% of rare earth concentrates with the dense alkali; after the melting reaction under high temperature, the rare earth is decomposed and the rare earth becomes rare earth hydrate; eliminate the sodium salts and excessive alkalis on the alkali-cake with water; dissolve the rare earth hydrate washed with water in the hydrochloric acid; after the rare earth dissolved into  rare earth chloride solution, adjust the acidity and remove the impurities.  As a result, the rare earth hydrate solution after filtering will become the solid rare earth chloride after concentrate and crystallization.


2. Separation of rare earth elements


At present, the purity of all the rare earth elements except for Pm can be improved to 6N (99.9999%) separating and extracting the individual pure rare earth element from the mixed rare earth compounds gained from the decomposition of the rare earth concentrates is very difficult from the perspective of the chemical technology because of two reasons: on one hand, the physical and chemical properties of the lanthanum elements are extremely similar.  The iron radius of most rare earth is between two elements and very close.  In addition, in the water solvent, they are all in stable tri-valence.  The affinity of rare earth ions with water is large, and due to the protection of the hydrates, the chemical properties are similar.  Therefore, the separation and purification is extremely difficult.  On the other hand, there are many impurity element associated with the mixed rare earth compounds gained from the decomposition of the rare earth concentrates (such as uranium, thorium, niobium, tantalum, titanium, zirconium, iron, calcium, silica, fluorine and phosphorus, etc.).  Therefore, in the process of the rare earth element separation, not only the separation of the rare earth elements with extremely similar chemical properties, the separation of the impurity element associated with the rare earth elements also shall be taken into consideration.      Currently, the separation methods adopted in rare earth production include: (1) fractional steps methods (crystallization fractionation, fractional precipitation and redox process); (2) iron exchange method; (3) solvent extraction method.


(1) Method of fractional steps


The individual separation of all the natural rare earth elements is conducted with the method of fractional steps, from the yttrium discovered in 1794 to lutetium discovered in 1905, including the radium discovered by Madame Curie.  The method of fractional steps takes advantages of the difference of the solubility in the solvents to separate and purify the rare earth elements.  The operational procedures are as follows: dissolve the compound of two rare earth elements in the proper solvents; some element compounds will be separated out (crystallization or precipitation) after heating and concentration.  In the substance separated out, the rare earth elements with small solubility will be concentrated, and the rare earth elements with large solubility will be concentrated in the solvents.  Due to the small difference of the solubility of the rare earth elements, only with several times of operation of the above procedures can these two rare earth elements be separated.  Therefore, it is very tough. The individual separation of all the rare earth elements takes more than 100 years and the procedures have been operated for 20,000 times.  The toughness for the chemists is imaginable.  Therefore, this method cannot be used to produce large amount of individual rare earth.


(2) Ion exchange method


The method of fractional steps cannot be applied in the production of a large amount of individual rare earths.  The research of the rare earth elements is impeded.  After the Second World War, the plan of atomic bomb research and development, that is, the Manhattan project, promoted the development of the rare earth separation technology.  Due to the similarities of the properties of rare earth elements and the radioactive elements, such as uranium and thorium, to accelerate the research upon the atomic energy, the rare earths are taken into use as the replacements.  Moreover, to analyze the rare earth elements contained in the products of the nuclear fission, and eliminate the rare earth elements in uranium and thorium, the iron exchange method is developed out successfully for the separation of rare earth elements.


The principle of the iron exchange method is: firstly, filled the cation exchange resins into the column; secondly, absorb the mixed rare earth needed to be separated on mouth of the column and then pour the eluent on the column and make the eluent flow from the top to the bottom.  Then the rare earth complexes will be separated from the ion exchange resins and flow to the bottom with the eluent.  In this process, the rare earth complexes will be decomposed and absorbed on the resins again.  In this way, the rare earth irons are absorbed on the resins, separated from the resins, and flows to the mouth of the column with the eluent.  Due to the different of the stability of the rare earth irons and the rare earth complexes, the flowing speed of the rare earth irons is different. The speed of the rare earth with large affinity flows faster.  As a result, it reaches to the mouth of the column firstly.


The advantage of the iron exchange method is that several elements can be separated out with once operation and the products got are with high purity.  The weakness of the method is that this method cannot be used continuously and once operation may takes long time.  In addition, the cost of the regeneration and exchange of resins are high.  Therefore, as the main method to separate the larger amount of rare earth in the past, this usage of this method decreases gradually and is replaced by the solvent extraction method.  However, due to that using this method can produce the individual rare earth products with high purity, this method is still used to produce the individual rare earth products and separate the important rare earth elements at rpesent.


(3) Solvent extraction method


The solvent extraction method refers to extract and separate the extracts from the aqueous solution with the organic solvent, which is immiscible with the aqueous solution.  It is a process to transfer the substance from one liquid phase to another liquid phase.


The solvent extraction method is applied early in petrochemical industry, organic chemistry, medicinal chemistry and analytical chemistry.  In the past 40 years, with the development of the atomic science and technology and due to the demand of the production of the super-pure substances and rare earth, the solvent extraction method has gained great development in the fields of nuclear fuel industry and rare metallurgical industry.  The research of extraction theory, compound and application of the new-type extraction agents and the extraction technology of rare earth elements separation in our country all reach the very high level.


Compared with the separation methods of fractional precipitation, fractional crystallization and ion exchange, the extraction process of the solvent extraction method has many advantages, such as good effect of separation, larger production capabilities, production rapidly and continuously and realization of the automatic control easily, etc.  Therefore, this method becomes the main method to separate large amount of rare earth.


In the solvent extraction method, the facilities used for separation include mixer settler and centrifugal extractor, etc.  The extraction agents used to purify the rare earths include: cation extraction agent with the acidic phosphate as the representative, such as P204 and P507; the anion exchange liquid with amine as the representative, such as N1923, and the extraction agent with the neutral phosphate, such as TBP and P350 as the representatives.  The viscosity and weight of these extraction agents are higher and it is difficult to separate them from water. In addition, they are always adopted after the dilution in the solvent, such as kerosene.


The extraction process can be divided into three main stages: extraction, washing and back-extraction.


3. Manufacturing of rare earth metals


The production of rare earth metals is also called thermo-metallurgy production of rare earth.  The rare earth metals can be divided into mixed rare earth metals and individual rare earth metals.  The composition of the mixed rare earth metals is close to the original rare earth in ores.  The individual metal is manufactured and separated from the rare earth. With the rare earth oxides (except for the oxides of samarium, europium, ytterbium and thulium) as the raw materials, it is difficult to reduce into individual metals with the common metallurgy process, due to the large heat of formation and high stability.  Therefore, the currently common used raw materials to produce rare earth metals are the chlorides and oxides of the rare earth elements.


(1) Fused salt electrolysis process


In the industry, the large volume of the mixed rare earth metals is produced with the fused salt electrolysis process.  In this method, the rare earth compounds, such as rare earth chlorides, are melted by heated, electrolyzed.  Then the rare earth metals are separated out on the negative electrode.  The electrolysis can be divided into two methods, chloride electrolysis and oxide electrolysis.  For the individual rare earth metals, the methods to manufacture are different based on the different elements. Due to the high vapor pressure, the samarium, europium, ytterbium and thulium are not proper to be manufactured with the electrolysis process and shall be manufactured with the reduction and distillation method.  Other elements can be prepared by the means of electrolysis and metal thermo-reduction.


Chloride electrolysis is the most common method used to produce metals, especially the mixed rare earth metals with simple process, low costs and small investment.  The largest weakness is that it may release chlorine and pollute the environment.


With the oxide electrolysis, there is no hazardous gas released, and the cost is higher.  The general individual rare earths with higher prices, such as neodymium and praseodymium, etc., are produced with oxide electrolysis.


(2) Vacuum-thermal reduction method


The electrolytic method just can be used to manufacture common industrial rare earth methods. For example, the metals with lower impurities and high purity are manufactured with the vacuum thermal reduction methods generally: firstly, produce the rare earth fluorides with rare earth oxides; then use the cerium metals to make reduction in the vacuum induction furnace to produce the raw metals; finally, get the purer metals through re-smelting, and distillation.  This method can be used to produce all the single rare earth metals; while this method cannot be used to manufacture samarium, europium, ytterbium and thulium.


The redox potential of samarium, europium, ytterbium, thulium and calcium just can make the rare earth fluoride reduction partly.  In general, these metals are manufactured based on the principles of high vapor pressure of metals and low vapor pressure of lanthanum metals: mixing the chippings of four rare earth oxides and lanthanum metals and pressing into blocks; realizing the reduction in the vacuum induction furnace. Due to the high activity of lanthanum, after the reduction into metals by lanthanum, the samarium, europium, ytterbium and thulium are collected on the condenser and are easier to separate from the slag.


3. Method of the classification of rare earth products


At present, there is no uniform method to classify and uniform name. The boundaries are not defined.  About the names of the rare earth, the mineral products and primary products (or raw products) are called upper-stream products; the deeply processing products (or individual products, or high-purity products) are called middle stream products; the applied materials and applied products (or apparatus) are called downstream products.  There are many kinds of rare earth products. Based on the depth of processing, the rare earth precuts can be divided into smelting products and applied products. The former refers to rare earth concentrates produced in the rare earth mines and smelting enterprises, single and mixed rare earth oxide, metals and the alloy and single and mixed rare earth salts, etc., with the total of more than 300 kinds and more than 500 specifications.  The latter refers to all the finished products contained rare earth, such as rare earth permanent magnets, rare earth phosphor, rare earth polishing powder, rare-earth trace fertilizer, rare earth laser crystals and rare earth hydrogen storage materials, etc.


From the rare earth raw materials to the final products, there are several stages. For the stage closer to the final products, the technological content is much higher and the added value is much larger.  To produce the final products, all the steps from raw materials, materials, apparatus to products have key technologies.  Much closer to the final products, the technological content is much higher and the added value is much larger.  Therefore, the development of the rare earth application products and the products with high added value is the hope of the Chinese rare earth in future.

 

 

 

 


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