Purification of rare earth

All of the 16 rare earth elements except Pm can be purified to a purity of 6N (99.9999%). Separating and extracting a single pure rare earth element from the mixed rare earth compound obtained by decomposing the rare earth concentrate is complicated and difficult in chemical process. There are two main reasons. First, the physical properties and chemical properties between the lanthanides are very similar. Most of the rare earth ions have a radius between adjacent two elements, which are very similar, and are stable trivalent states in aqueous solution. Rare earth ions have a large affinity with water. Due to the protection of hydrates, their chemical properties are very similar, and separation and purification are extremely difficult. Second, there are many impurity elements (such as uranium, thorium, strontium, strontium, titanium, zirconium, iron, calcium, silicon, fluorine, phosphorus, etc.) accompanying the mixed rare earth compound obtained after decomposition of the rare earth concentrate. Therefore, in the process of separating rare earth elements, not only the separation of these dozens of rare earth elements with very similar chemical properties but also the separation between the rare earth elements and the associated impurity elements must be considered.
production material
Rare earth metals are generally classified 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 component in the ore, and the single metal is a metal separated and refined by each rare earth. It is difficult to reduce to a single metal by a general metallurgical method using rare earth oxides (except oxides of cerium, lanthanum, cerium and lanthanum) as raw materials, because of its high heat generation and high stability. Therefore, the raw materials commonly used in the production of rare earth metals today are their chlorides and fluorides.
Molten salt electrolysis
In the industrial production of mixed rare earth metals in large quantities, molten salt electrolysis is generally used. In this method, a rare earth compound such as a rare earth chloride is heated and melted, and then electrolyzed to precipitate a rare earth metal on the cathode. There are two methods of electrolysis: chloride electrolysis and oxide electrolysis. The preparation method of a single rare earth metal varies depending on the element.钐, 铕, 镱, 铥 are high in vapor pressure and are not suitable for electrolytic preparation, but use reduced distillation. Other elements may be prepared by electrolysis or metal thermal reduction.
Chloride electrolysis is a common method for producing metals. In particular, the process of mixing rare earth metals is simple, the cost is low, and the investment is small, but the disadvantage is that chlorine gas is released and pollutes the environment.
Oxide electrolysis does not emit harmful gases, but the cost is slightly higher. Generally, single rare earths such as lanthanum and cerium, which are relatively expensive to produce, are electrolyzed with oxides.
Vacuum reduction

The electrolysis method can only prepare rare earth metals of general industrial grade. For example, a metal with low impurity and high purity is prepared, and generally obtained by vacuum thermal reduction. Generally, the rare earth oxide is firstly made into a rare earth fluoride, which is reduced by metal calcium in a vacuum induction furnace to obtain a crude metal, and then remelted and distilled to obtain a relatively pure metal. This method can produce all the single metals. Rare earth metals, but 钐, 铕, 镱, 铥 cannot be used in this way. The redox potential of lanthanum, cerium, lanthanum, cerium and calcium only partially reduces the fluorinated rare earth. These metals are generally prepared by using the high vapor pressure of these metals and the low vapor pressure of the base metals. The four rare earth oxides are mixed with the crucible metal crumbs and compacted in a vacuum furnace. Active, sputum, sputum, sputum, and sputum are reduced to metal by hydrazine and collected on condensation, which is easily separated from slag.

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