Rare earth production and separation

Rare earth production and separation

Rare earth market is a diversified market, it is not just a product, but 15 rare earth elements and yttrium, scandium and its various compounds from chloride 46% purity and 99.9999% of single rare earth oxides and rare earth metal, has a variety of purposes. Add the relevant compounds and mixtures, and the products are numerous. Firstly, we introduce the separation methods and smelting process of rare earth from the initial ore extraction.

Rare earth selection

Dressing is the use of various mineral composition of ore of the physical and chemical properties of differences between different processing method, using different mineral processing technology, different mineral processing equipment, the enrichment of useful minerals in the ore, removal of harmful impurities, and the machining process of the separation of the gangue mineral.

Currently mined in China and other countries in the world of rare earth ore, rare earth oxide content in only a few percent, even some lower, in order to meet the production requirements of smelting, the dressing before smelting, rare earth minerals and gangue minerals, and other useful mineral separate, in order to improve the content of rare earth oxides, can satisfy the requirements of the rare earth metallurgy ore concentrate. The ore dressing method of rare earth ore is generally adopted flotation method, and it is often supplemented by reelection and magnetic separation.

Baiyun obo mine rare earth ore deposit in Inner Mongolia, is ankerite carbonate rock type ore deposit, the main ingredient of associated rare earth minerals in ore (except bastnaesite, monazite, there are several kinds of containing niobium and rare earth minerals). The mined ore contains about 30% iron and about 5% of rare earth oxides.

At the mine

After the large ore was broken, the train was transported to baotou iron and steel corporation. The job of the concentrator is to increase Fe2O3 from 33% to more than 55%, first to grind the grade on the conical ball mill, and then select the iron concentrate of 62-65% Fe2O3 with cylinder magnetic separation machine. The tailings continued flotation and magnetic separation, and the secondary iron concentrate containing 45%Fe2O3 was obtained. Rare earth accumulation in flotation foam, the grade reaches 10 ~ 15 %. The enriched material can be used to select a crude concentrate of 30% of REO content. After the ore dressing equipment is reprocessed, the ore concentrate of more than 60% of reog can be obtained.

Rare earth smelting method

There are two kinds of rare earth smelting methods: wet metallurgy and metallurgical metallurgy.
Hydrometallurgy is a chemical metallurgy, most of the whole process in solution, the solvent, such as the decomposition of rare earth concentrate single rare earth metal, rare earth oxides, rare earth compound, the separation and extraction process is the precipitation and crystallization, REDOX, chemical separation processes such as solvent extraction, ion exchange process.

The application of organic solvent extraction method is widely used in this paper. It is a common process for industrial separation of high purity single rare earth elements. Wet metallurgical process is complex, the product purity is high, and the production of the method is wide.

The process of metallurgical metallurgy is simple and high in productivity. The metallurgical process of rare earth is mainly included in the process of the extraction of rare earth alloy by silicon thermal reduction, the metal or alloy of the rare earth, the metal heat reduction and the extraction of rare earth alloys. The common characteristic of fire metallurgy is to produce under high temperature.

1. Decomposition of rare earth concentrate

Rare earth minerals in rare earth concentrate are generally difficult to dissolve in the form of hydrocarbonate, fluoride, phosphate, oxide or silicate. Must be through a variety of chemical change to rare earth into soluble in water or inorganic acid compounds, after dissolving, separation, purification, concentration and calcination process, made all kinds of mixed rare earth compounds such as mixed rare earth chloride, as a product or separation of single rare earth materials, such a process known as decomposition, also known as rare earth concentrate before processing.

There are many methods to decompose rare earth concentrate, which can be divided into three categories: acid method, alkali method and chlorination. The decomposition of acid method is divided into hydrochloric acid decomposition, sulfuric acid decomposition and hydrofluoric acid decomposition method. The decomposition of alkali is divided into sodium hydroxide decomposition or sodium hydroxide melting or baking soda. Generally based on the characteristics of the type and grade of concentrate, product plan, facilitate the recycling and comprehensive utilization of rare earth elements, and to labor hygiene and environmental protection, economic and reasonable principles such as choosing suitable technological process.

Production of rare earth and chloride rare earth:

This is the main two primary products in the rare earth industry. Generally speaking, there are two main processes for producing these two products.

One process is the baking process of concentrated sulfuric acid, which is a mixture of rare earth concentrate and sulfuric acid in a rotary kiln. The soluble rare earth sulfates are used in aqueous solution, called leaching solution. Then, add bicarbonate of ammonium bicarbonate to the leaching solution, then the rare earth will be deposited with carbonate, and then the rare earth will be filtered.

Another process is called caustic soda process, or alkaline process. Is usually 60% of the rare earth concentrate with strong alkali stir, melting reaction at high temperatures, the rare earth concentrate is broken down, rare earth rare earth oxide into h ydrogen, to remove the alkali cakes are washed and excess sodium salt alkali, then washed the rare earth oxide and h ydrogen dissolved in hydrochloric acid, rare earth was dissolved rare earth chloride solution, adjustable acidity to remove impurities, after filtration of the rare earth chloride solution by rare earth chloride concentration crystal is made of solid.

2. The separation of rare earth elements

At present, 16 rare earth elements other than Pm can be purified to 6N (99.9999%) purity. It is complicated and difficult to extract single pure rare earth elements from the mixed rare earth compounds obtained from the decomposition of rare earth concentrate. The main reason has two, one is between the physical properties and chemical properties of lanthanides is very similar to that of most of the rare earth ion radius in between two adjacent elements, are very similar, is stable in the aqueous solution of three valence state. The high affinity of rare earth ions and water is very similar in chemical properties due to the protection of hydrate.

It is extremely difficult to separate and purify. Second, there are many impurity elements in the mixed rare earth compounds obtained after the decomposition of rare earth concentrate (such as uranium, thorium, niobium, tantalum, titanium, zirconium, iron, calcium, silicon, fluorine and phosphorus). Therefore, in the separation process of rare earth elements, not only should consider this a dozen chemical extremely similar separation between rare earth elements, but also must consider the rare earth elements associated with the separation between the impurity elements.

The separation method used in rare earth production (wet process production process) is: (1) fractional step method (fractional crystallization method, fractional precipitation method and REDOX method); (2) ion exchange method; (3) solvent extraction method.

(1) fractional step method

From 1794 found that yttrium (Y) in 1905 to found the lutetium (Lu) so far, all the single separation between the rare earth elements in nature, and Marie and Pierre Curie discovered radium, were separated by this method. The fractional step method is used to separate and purify the differences in solubility of the solubility of compounds in solvents. The operation procedure of the method is that the compound containing two kinds of rare earth elements will be dissolved in a suitable solvent, then heated and concentrated, and a part of the elements in the solution are analyzed (crystallized or precipitated).

In the precipitate, rare earth elements with less solubility are enriched, and the rare earth elements with larger solubility are also enriched in the solution. Because of the small differences in solubility between rare earth elements, it is very difficult to separate the two rare earth elements by repeated operation. The single separation of all rare earth elements took more than 100 years, and the number of repeated operations was 20,000 times, and the degree of hardship for the chemical workers was predictable. So you can't make a lot of single rare earths in this way.

(2) ion exchange method

Due to make a single rare earth can't mass production, and research of rare earth elements have also been block, after the second world war, the us atomic bomb development program known as the Manhattan project to promote the development of the rare earth separation technology, because of the rare earth elements and nature of uranium, thorium and other radioactive elements are similar, for promoting the study of atomic energy as soon as possible, will be rare earth as a substitute. Moreover, in order to analyze the nucleus fission products containing rare earth elements, and rare earth elements in the removal of uranium and thorium, the success of ion exchange chromatography, ion exchange method), and then used for the separation of rare earth elements.

The principle of ion exchange chromatography is: first, the cation exchange resin filled in the column, then stay separation of mixed rare earth that end of the adsorption at the entrance of a pillar, and then let the eluent from top to bottom through the column. A complex of rare earths is formed from the ion exchange resin and flows down with the flow. In the process of flow, the rare earth complex is decomposed and then adsorbed on the resin. In this way, the rare earth ions absorb and disengage from the resin, and then flow to the outlet end of the column with the washing liquid. Because of rare earth ions and the stability of the complex formed by complexing agent is different, so the speed of all kinds of rare earth ions move down, the affinity of rare earth flows down fast, the first to arrive at the outlet.

The advantage of ion exchange is that an operation can separate multiple elements. And you can get high purity products. The downside of this approach is not continuous processing, a long operating cycle time, and the resin regeneration, exchange, consumption of the cost is high, so this was the main method of separation of a large number of rare earths has retired from the main separation method, and replaced by solvent extraction. But as a result of ion exchange chromatography has obtained the prominent characteristics of single high purity rare earth products, at present, for preparing super high purity single rare earth products and some heavy rare earth elements separation, ion exchange chromatography separation also need making.

(3) solvent extraction method

Use organic solvent from its not phase miscibility in aqueous solution, the method is called by extraction and separation of extract liquid - liquid liquid extraction, organic solvent or solvent extraction, it is a kind of substance from a liquid transferred to another mass transfer process of the liquid phase.

Solvent extraction has been applied in petrochemical, organic chemistry, pharmaceutical chemistry and analytical chemistry. But nearly forty years, due to the development of the atomic energy science and technology, the needs of the production of the super pure substances and rare elements, solvent extraction in the nuclear fuel industry, metallurgical industry, such as rare got great development. The research of extraction t heory, synthesis and application of new extraction agents and extraction process of rare earth elements have reached a high level.

Solvent extraction method of the extraction process with fractional precipitation, fractional crystallization and ion exchange separation method, compared with good separation effect, large production capacity, to facilitate rapid continuous production, easy to realize automatic control and so on a series of advantages, thus gradually became the main method of separation of a large number of rare earths.

Solvent extraction separation equipment such as mixer settler, centrifugal extractor, extraction solvent used in the purification of rare earths are: represented by acid phosphate ester cation extractant P204, P507, represented by amine N1923 anion exchange fluid and represented by the TBP, P350 neutral phosphate ester solvent extraction agent. The viscosity and gravity of these extractants are very high, which is not easy to separate from water. It is usually diluted and reused with kerosene and other solvents.

The extraction process can be divided into three main stages: extraction, washing and r everse extraction.

3. Preparation of rare earth metals

The production of rare earth metals is also called rare earth metallurgical production. Rare earth metals are generally divided into mixed rare earth metals and single rare earth metals. The composition of the mixed rare earth metals is similar to the rare earth elements in the ore. The single metal is the metal which separates and refined from each rare earth. Rare earth oxides (except for samarium, europium, ytterbium and thulium oxide) are difficult to be reduced to single metal in general metallurgy methods because of its high heat and high stability. Therefore, the commonly used raw materials for rare earth metals are their chloride and fluoride.

(1) molten salt electrolysis

In the industrial mass production of mixed rare earth metals commonly used molten salt electrolysis. The method is to heat and melt rare earth compounds, such as rare earth chlorides, and then electrolyze them to precipitate rare earth metals on the cathode. The electrolytic method has two methods: chloride electrolysis and oxide electrolysis. The preparation method of single rare earth metals varies from element to element. Samarium, europium, ytterbium and thulium are high in vapor pressure and are not suitable for electrolytic preparation. Other elements can be prepared by electrolysis or metal heat reduction.

Chloride electrolysis is the most common method of producing metal, especially the mixed rare earth metal technology is simple, cheap and small investment, but the biggest drawback is that chlorine gas is released, polluting the environment.

Oxide electrolysis has no harmful gas emission, but the cost is a little higher, and the single rare earth such as neodymium and praseodymium, which generally produce higher prices, are electrolyzed by oxide.

(2) vacuum thermal reduction method
The electrolysis method can only prepare the rare earth metals of industrial grade, such as the metal with low impurity and high purity, which can be obtained by vacuum heat reduction method. First generally is the rare earth oxides made from rare earth fluoride, reduction in vacuum induction furnace with metal calcium, made thick metal, and then after remelting and distillation to obtain relatively pure metal, this method can produce all the single rare earth metal, but samarium, europium thulium, ytterbium, cannot use this method.

The REDOX potential of samarium, europium, ytterbium, thulium and calcium only partially restore the fluoride rare earth. General preparation of these metals, is the use of these metal high vapor pressure and the principle of lanthanum metal low vapor pressure, the four kinds of rare earth oxides and lanthanum metal debris mixed briquetting, reduction in vacuum furnace, lanthanum is lively, samarium, europium thulium, ytterbium, lanthanum reduction into a metal after collection in the condenser, and the residue is easy to separate.

Classification methods for rare earth products
There are many kinds of rare earth products. According to the depth of processing, we can divide it into product selection and application. The former refers to the rare earth mining and smelting production of rare earth concentrate, single and mixed rare earth oxides, metals and their alloys, the single and mixed rare earth salts, etc., total more than 300 varieties, more than 500 specifications. The latter refers to all products containing rare earth, such as rare earth permanent magnets, rare earth fluorescent powder, rare earth polishing powder, rare earth micro-fertilizer, rare earth laser crystal, rare earth h ydrogen storage materials, etc.

There is no uniform classification, there is no unified call, the boundary is not clear, people familiar with the name; Mineral products, primary products (or crude products) refer to upstream products; The deep processing products (or single products, high purity products) are referred to as the mid-game products; Application materials and application products (or devices) are referred to downstream products. Single rare earth oxides, rare earth metals, mixed rare earth oxides, mixed rare earth metals.