Preparation Method Of Rare Earth - Purification

All 16 rare earth elements except Pm can be purified to 6N (99.9999 percent ) purity. It is relatively complicated and difficult in chemical process to separate and extract single pure rare earth elements from the mixed rare earth compounds obtained after the decomposition of rare earth concentrates. There are two main reasons. First, the physical and chemical properties of the lanthanide elements are very similar, and most rare earth ions have a radius between two adjacent elements, which are very similar, and are stable trivalent states in aqueous solutions. Rare earth ions have a high affinity with water, and because they are protected by hydrates, their chemical properties are very similar, so separation and purification are extremely difficult. The second is that the mixed rare earth compounds obtained after the decomposition of rare earth concentrates have many accompanying 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, not only the separation of these dozen rare earth elements with extremely similar chemical properties, but also the separation between rare earth elements and accompanying impurity elements must be considered.

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. Rare earth oxides (except samarium, europium, ytterbium and thulium oxides) are difficult to be reduced to a single metal by general metallurgical methods because of their large heat of formation and high stability. Therefore, today's common raw materials for the production of rare earth metals are their chlorides and fluorides.

Molten Salt Electrolysis

Industrial mass production of mixed rare earth metals generally uses molten salt electrolysis. This method is to heat and melt rare earth compounds such as rare earth chlorides, and then perform electrolysis to precipitate rare earth metals on the cathode. There are two methods of electrolysis: chloride electrolysis and oxide electrolysis. The preparation methods of single rare earth metals vary from element to element. Samarium, europium, ytterbium, and thulium are not suitable for electrolytic preparation due to their high vapor pressure, and reduction distillation is used instead. Other elements can be prepared by electrolysis or metallothermic reduction.

Chloride electrolysis is the most common method for producing metals, especially for mixed rare earth metals with simple process, low cost and small investment, but the biggest disadvantage is the release of chlorine gas, which pollutes the environment.

Oxide electrolysis does not emit harmful gases, but the cost is slightly higher. Generally, single rare earths with higher production prices, such as neodymium and praseodymium, are electrolyzed by oxides.

Vacuum reduction

The electrolysis method can only prepare rare earth metals of general industrial grade. If you want to prepare metals with low impurities and high purity, vacuum thermal reduction is generally used to prepare them. Generally, rare earth oxides are first made into rare earth fluorides, which are reduced with metal calcium in a vacuum induction furnace to obtain crude metals, and then remelted and distilled to obtain purer metals. This method can produce all single Rare earth metals, but samarium, europium, ytterbium, and thulium cannot be used in this way. The redox potentials of samarium, europium, ytterbium, thulium and calcium only partially reduce rare earth fluorides. Generally, these metals are prepared by using the principle of high vapor pressure of these metals and low vapor pressure of lanthanum metal. The oxides of these four rare earths are mixed with lanthanum metal scraps, and then reduced in a vacuum furnace. The comparison of lanthanum Active, samarium, europium, ytterbium, thulium are reduced to metal by lanthanum and collected on the condensation, which is easily separated from the slag.