At present, the content of rare earth oxides in rare earth ores mined in China and other countries in the world is only a few percent, or even lower. In order to meet the production requirements of smelting, rare earth minerals are separated from gangue minerals and other useful minerals by beneficiation before smelting, so as to increase the content of rare earth oxides and obtain rare earth concentrates that can meet the requirements of rare earth metallurgy. Mineral processing of rare earth ores generally adopts flotation, often supplemented by gravity separation and magnetic separation, forming a variety of combined mineral processing flows.
Bayanobo rare earth deposit in Inner Mongolia is a carbonate deposit in ankerite. Rare earth minerals (besides bastnaesite and monazite, there are several niobium and rare earth minerals) coexist with the main iron ore. The extracted ore contains about 30% iron and about 5% rare earth oxides. After the large pieces of ore in the mine are broken, they are transported to the concentrator of Baotou Steel Group by train. The task of the concentrator is to increase Fe2O3 from 33% to over 55%, firstly grind and classify it in a cone ball mill, and then select the primary iron concentrate with Fe2O3 (iron oxide) of 62-65% by a cylinder magnetic separator. Its tailings continue flotation and magnetic separation to obtain secondary iron concentrate containing more than 45% Fe2O3 (iron oxide). Rare earth is enriched in flotation foam with a grade of 10 ~ 15%. The rough concentrate with 30% reo content can be selected by shaking table, and the rare earth concentrate with more than 60% REO content can be obtained by reprocessing with mineral processing equipment. 1, chromogenic liquid absorbs 15ml filtrate, add 7ml of 5% oxalic acid and 3ml of chlorophosphonazo in 50ml conical flask, and shake well to obtain chromogenic liquid.
2. After the reference solution is operated with the chromogenic solution, add 1-2 drops of sodium hypophosphite solution (two drops is enough), use it as a reference solution (blank solution) after fading, pour it into a 2cm colorimeter with the wavelength of 660nm, and measure its absorbance and content. You can do it in the second aisle. Note: The color developing liquid is black ink. Rare earths in rare earth concentrates usually exist in the form of water-insoluble carbonates, fluorides, phosphates, oxides or silicates. Rare earth must be converted into compounds soluble in water or inorganic acids through various chemical changes, and then mixed rare earth compounds such as mixed rare earth chloride are made into products or raw materials for separating single rare earth by dissolution, separation, purification, concentration or combustion. This process is called rare earth concentrate decomposition, also known as pretreatment.
There are many methods to decompose rare earth concentrate, which can be generally divided into three categories, namely acid method, alkali method and chlorination decomposition method. Acid decomposition is divided into hydrochloric acid decomposition, sulfuric acid decomposition and hydrofluoric acid decomposition. Alkali decomposition is divided into sodium hydroxide decomposition, sodium hydroxide melting or soda roasting. Generally, the appropriate technological process is selected according to the principles of concentrate type, grade characteristics, product scheme, being beneficial to the recovery and comprehensive utilization of non-rare earth elements, being beneficial to labor hygiene and environmental protection, and being economical and reasonable.
Although nearly 200 rare and dispersed elemental minerals have been discovered, there are only rare independent germanium, selenium and tellurium deposits, and the scale of the deposits is not large.
Sulfuric acid dissolution
Cerium group (insoluble in sulfate double salt)-lanthanum, cerium, praseodymium, neodymium and promethium;
Terbium family (slightly soluble in sulfate double salt)-samarium, europium, gadolinium, terbium, dysprosium, holmium;
Yttrium group (soluble in bissulfate)-europium, erbium, thulium, ytterbium, lutetium and scandium. There are two methods of rare earth smelting, namely, hydrometallurgy and pyrometallurgy.
Hydrometallurgy belongs to chemical metallurgy, and the whole process is mostly carried out in solution and solvent. For example, the decomposition of rare earth concentrate, the separation and extraction of rare earth oxides, rare earth compounds and single rare earth metal are all chemical separation processes such as precipitation, crystallization, redox, solvent extraction and ion exchange. Organic solvent extraction is widely used now, which is a general process for industrial separation of high-purity single rare earth elements. Hydrometallurgical process is complex and the product purity is high, so the finished products produced by this method are widely used.
Pyrometallurgy has simple process and high productivity. Rare earth pyrometallurgy mainly includes preparation of rare earth alloy by silicothermic reduction, preparation of rare earth metal or alloy by molten salt electrolysis and preparation of rare earth alloy by metal thermal reduction. The same characteristic of pyrometallurgy is that it is produced at high temperature.
Step by step method
From yttrium (Y) discovered in 1794 to lutetium (Lu) discovered in 1905, all natural rare earth elements were separated in this way, and so was radium discovered by Curie and his wife. Step-by-step method is to separate and purify compounds by using the difference of the difficulty (solubility) of dissolving compounds in solvents. The operation procedure of this method is as follows: firstly, the compounds containing two rare earth elements are dissolved in a suitable solvent, and then heated and concentrated, and some elemental compounds in the solution are precipitated (crystallized or precipitated). In the sediment, rare earth elements with small solubility are enriched, and rare earth elements with large solubility are also enriched in the solution. Because the solubility difference between rare earth elements is very small, it is very difficult to separate these two rare earth elements through repeated operations. The single separation of all rare earth elements took 100 years, and repeated operations reached 20,000 times. For chemists, the degree of hardship can be imagined. Therefore, a single rare earth cannot be produced in large quantities by this method.
ion exchange
Because a single rare earth can't be produced in large quantities by step-by-step method, the research work of rare earth elements is also hindered. After World War II, the American atomic bomb development plan, the so-called Manhattan Project, promoted the development of rare earth separation technology. Because the properties of rare earth elements are similar to radioactive elements such as uranium and thorium, rare earth elements are used as substitutes to promote the research of atomic energy as soon as possible. Moreover, in order to analyze the rare earth elements contained in atomic fission products and remove the rare earth elements from uranium and thorium, ion exchange chromatography (ion exchange method) was successfully studied, and then it was used for the separation of rare earth elements.
The principle of ion exchange chromatography is: firstly, the cation exchange resin is filled in the column, then the mixed rare earth to be separated is adsorbed at the end of the column entrance, and then the eluent flows through the column from top to bottom. The rare earth forming the complex will leave the ion exchange resin and flow down with the eluent. During the flowing process, the rare earth complex is decomposed and then adsorbed on the resin. In this way, the rare earth ions flow to the outlet end of the column with the eluent, and are adsorbed and separated from the resin at the same time. Due to the different stability of the complex formed by rare earth ions and complexing agents, various rare earth ions move downwards at different speeds, and the rare earth with high affinity flows downwards faster and finally reaches the outlet end first.
The advantage of ion exchange method is that many elements can be separated in one operation. But also high-purity products can be obtained. The disadvantages of this method are that it can not be treated continuously, the one-time operation cycle is long, and the cost of resin regeneration and exchange is high. Therefore, this once main method for separating a large number of rare earths has been retired from the mainstream separation method and replaced by solvent extraction. However, due to the outstanding characteristics of obtaining high-purity single rare earth products by ion exchange chromatography, in order to prepare ultra-high-purity single products and separate 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 extracting and separating the extracted substance from its immiscible aqueous solution with organic solvent is called organic solvent liquid-liquid extraction, which is a mass transfer process of transferring the substance from one liquid phase to another.
Solvent extraction was used in petrochemical industry, organic chemistry, medicinal chemistry and analytical chemistry earlier. However, in recent forty years, due to the development of atomic energy science and technology and the need to produce ultra-pure substances and rare elements, solvent extraction has made great progress in nuclear fuel industry, rare metallurgy and other industries. China has reached a high level in the research of extraction theory, the synthesis and application of new extractant, and the extraction process of rare earth element separation.
Compared with separation methods such as fractional precipitation, fractional crystallization and ion exchange, solvent extraction has a series of advantages, such as good separation effect, large production capacity, convenient rapid continuous production and easy automatic control, so it has gradually become the main method for separating a large number of rare earths.
The separation equipment of solvent extraction method includes mixing clarifier, centrifugal extractor and so on. The extractants used to purify rare earth include cationic extractants such as P204 rare earth extractant represented by acid phosphate and P507 rare earth extractant, anion exchange liquid N 1923 represented by amine, and solvent extractant represented by neutral phosphate such as TBP and P350. These extractants have high viscosity and specific gravity and are not easy to separate from water. Usually diluted with kerosene and other solvents before use.
Light rare earth (P204 weak acid extraction) -La, Ce, Pr, Nd, HM;
Medium rare earth (P204 low acidity extraction)-samarium, europium, gadolinium, terbium, dysprosium;
Heavy rare earth elements (acidic extraction in P204)-holmium, europium, erbium, thulium, ytterbium, lutetium and scandium. 16 rare earth elements except Pm can be purified to the purity of 6N(99.9999%). It is complicated and difficult to separate and extract a single pure rare earth element from the mixed rare earth compound obtained by decomposition of rare earth concentrate. There are two main reasons for this. First, the physical and chemical properties of lanthanides are very similar. Most rare earth ions have very close radii between two adjacent elements, and they are all stable trivalent in aqueous solution. Rare earth ions have great affinity with water. Due to the protection of hydrates, their chemical properties are very similar, so it is extremely difficult to separate and purify them. Second, the mixed rare earth compounds obtained after the decomposition of rare earth concentrate contain many associated impurity elements (such as uranium, thorium, niobium, tantalum, titanium, zirconium, iron, calcium, silicon, fluorine, phosphorus, etc.). Therefore, in the process of separating rare earth elements, we should not only consider the separation between these ten rare earth elements with extremely similar chemical properties, but also consider the separation between the associated impurity elements of rare earth elements.
raw material
Rare earth metals are generally divided into mixed rare earth metals and single rare earth metals. The composition of mixed rare earth metals is close to the original rare earth composition in the ore, and a single metal is a metal separated and refined from each rare earth. Rare earth oxides (except oxides of samarium, europium, ytterbium and thulium) have high heat of formation and high stability, so it is difficult to reduce them to a single metal by general metallurgical methods. Therefore, the common raw materials for producing rare earth metals today are their chlorides and fluorides.
Molten salt electrolysis
Molten salt electrolysis is generally used for large-scale production of mixed rare earth metals in industry. In this method, a rare earth compound such as rare earth chloride is heated and melted, and then electrolyzed to precipitate a rare earth metal on a cathode. There are two kinds of electrolysis methods: chloride electrolysis and oxide electrolysis. The preparation method of single rare earth metal varies with different elements. Samarium, europium, ytterbium and thulium have high vapor pressures and are not suitable for electrolytic preparation, so the reduction distillation method is adopted. Other elements can be prepared by electrolysis or metal thermal reduction.
Chloride electrolysis is the most commonly used method to produce metals, especially mixed rare earth metals, with simple process, low cost and small investment, but the biggest disadvantage is that chlorine gas is released and the environment is polluted.
Oxide electrolysis does not emit harmful gases, but the cost is slightly higher. Generally, the single rare earth with higher production price, such as neodymium and praseodymium, is electrolyzed by oxides.
Vacuum reduction
Electrolysis can only prepare general industrial grade rare earth metals. If the metal with low impurity and high purity is to be prepared, it is generally prepared by vacuum thermal reduction. Generally, rare earth oxides are first made into rare earth fluorides, and then the crude metal is obtained by reduction with metal calcium in a vacuum induction furnace, and then the purified metal is obtained by remelting and distillation. All single rare earth metals can be produced by this method, but samarium, europium, ytterbium and thulium cannot be produced by this method. The redox potentials of samarium, europium, ytterbium, thulium and calcium only partially reduce rare earth fluoride. Generally speaking, these metals are made by using the principle that the vapor pressure of these metals is high and the vapor pressure of lanthanum metal is low, mixing and briquetting the oxides of these four rare earths with fragments of lanthanum metal, and reducing them in a vacuum furnace. Lanthanum is relatively active, and samarium, europium, ytterbium and thulium are reduced to metals by lanthanum, which are collected during condensation and easily separated from slag.