Intro.
China, the world’s largest producer of rare-earth materials needed for manufacturing battery banks and semiconductor integrated microcircuits, has banned the export of technologies for their processing and separation. This was announced by China’s Ministry of Commerce. The Ministry has added the relevant technologies to the “Catalogue of Technologies Prohibited and Restricted for Export.”
It concerns technologies of producing inorganic materials, refining, processing, and recycling rare-earth elements, as well as manufacturing integrated microcircuits.
Inclusion of these technologies in the export prohibition list aims to protect national security and the public interests of citizens of PR China.
Since China accounts for 90% of global rare-earth metal production, experts consider these measures to be another step in escalating competition with the West for control over critical minerals.
Western experts note that several countries, including the United States, Japan, and France, possess technologies of separating rare-earth elements. However, China holds the most efficient technologies of their extraction and purification, having an economic advantage.
Production cycles of rare-earth metals producing include numerous stages, each characterized by its own complex market dynamics: ore mining, extraction of rare-earth oxides, their purification, reduction of oxides to metals, incorporation of metals and alloys into components and manufacturing of final products.
Thus, development of the rare earth metals (REM) industry encompasses a wide range of issues: from ore mining and concentration to production of pure and ultra-pure individual rare-earth elements and their application in defense and non-military sectors.
Preparation of raw materials for processing
All rare-earth ores contain less than 10% extractable components and must be concentrated to 60% for further processing.
Initially, ore materials are ground to powder, then separated from other materials in the ore body by means of various standard processes, including magnetic and/or electrostatic separation and flotation.
Bastnasite ores.
For their concentration a froth flotation process is used to remove heavier products, such as barite (BaSO4) and celestite (SrSO4) through sedimentation. As a result, bastnasite and other light minerals are concentrated to a 60% of REO (Rare Earth Oxide) concentrate. The concentrate is then treated with 10% hydrochloric acid (HCl) to dissolve calcite (CaCO3). The insoluble residue, now containing 70% of REO, is roasted to oxidize Ce³⁺ to Ce⁴⁺. After cooling the material is leached with HCl, dissolving trivalent rare earth elements (lanthanum, praseodymium, neodymium, samarium, europium, and gadolinium) to produce a cerium concentrate. This concentrate is purified and sent for sale and further processing.
Europium can be easily separated from other lanthanoids by reducing europium to its divalent form, while the remaining dissolved lanthanoids are separated using solvent extraction.
Monazit and xenotime ores.
Such ores are processed in nearly the same way since they are phosphate minerals. Monazite or xenotime are separated from other minerals by means of a combination of gravitational, electromagnetic, and electrostatic methods and then split with the help of either an acid or alkaline process.
In the acid process monazite or xenotime is treated with concentrated sulfuric acid (H₂SO₄) at temperatures ranging from 150 to 200°C. Soluble sulfates and phosphates of rare-earth elements and thorium pass into the solution.
Separation of thorium from rare-earth elements is quite challenging, as the solubility of thorium and rare-earth elements varies depending on temperature and acidity. Once thorium is separated from the rare-earth elements, the REO concentrate is used for processing to obtain individual elements.
In the alkaline process finely ground monazite or xenotime is mixed with a 70% sodium hydroxide (NaOH) solution and treated for several hours in an autoclave at 140-150°C. As a result, an insoluble rare-earth phosphate R(OH)₃ is obtained along with an alkaline solution that still contains 5-10% thorium.
Two methods can be used for its removal. In one thorium hydroxide is dissolved in hydrochloric (HCl) or nitric (HNO₃) acid, and then thorium hydroxide Th(OH)₄ is selectively precipitated by adding caustic soda (NaOH) and/or ammonia water (NH₄OH).
In the other method hydrochloric acid (HCl) is added to thorium hydroxide to decrease the pH value to around 3, in order to convert the rare- earth metals into soluble chlorides (RCl₃), while the insoluble Th(OH)₄ precipitates. The thorium-free rare-earth element solution is then sold as a mixed concentrate or can be used as a starting material for obtaining individual elements.
Separation chemistry of rare-earth metals
The processes currently used for the separation of rare-earth elements were developed in the mid-20th century in several laboratories of the U.S. Atomic Energy Commission (AEC). In the 1950s it was demonstrated that individual rare earth elements could be separated with high purity (>99.99%).
This marked the beginning of the modern rare-earth element industry, where large quantities of high-purity rare-earth elements became available for the electronics industry.
The most important role in the technology of obtaining rare-earth metals (REM) is assigned to separation and concentration methods. Despite their variety, the main methods are extraction and precipitation. It should be noted that these methods are applied in a multi-stage process, and satisfactory results can be achieved by combining the methods. As industrial extractants for REM cation-exchange and neutral reagents are used.
Among neutral extractants, tributyl phosphate is the most widely used, as it extracts rare-earth metals (REM) from neutral and strongly acidic nitrate solutions. Among organic acids di-2-ethylhexylphosphoric acid is used, which has good selectivity for lanthanoids but low capacity. Aliphatic monocarboxylic acids have low selectivity when extracting REM mixtures, and they are used for group concentration operations.
The selectivity of extraction can be increased by using reagent mixtures, as well as by carefully selecting extraction conditions (pH, temperature, salting agents, diluents).
A relatively new direction in the extraction technology for obtaining certain metals is the use of binary extractants (BE) – ion pairs formed by an organic base and an organic acid. The use of BE provides a combination of properties of the original ion-exchange extractants with new possibilities for controlling the distribution process.
The main advantages of binary extractants are: the ability to predict the properties of BE, based on data about original ion-exchange extraction systems; simplicity of preparing ion pairs and availability of extractants; increased distribution and separation coefficients; simplification of re-extraction processes; high extraction and re-extraction rates.
The use of binary extractants for extracting, concentrating, and separating rare-earth metals (REM) allows for a qualitative change in technological schemes of obtaining REM, taking into account the latest achievements in the field of extraction chemistry.
Using the liquid solvent extraction method, rare-earth element producers separate mixtures into individual elements with a purity of up to 95%.
For getting higher purity, the ion exchange method is used.
Ionic exchange
The ion exchange process is much slower, but it can provide for higher purity, exceeding 99.9% (up to 99.9999999% or higher).
For optical and phosphorus-containing materials, where purity of five nines is required, the rare-earth element is first purified through solvent extraction and then further processed by means of ionic exchange to achieve the purity needed for the specific application.
In the ionic exchange process the metal ion R³⁺ in the solution exchanges with three protons on a solid ion exchanger – either natural zeolite or synthetic resin. The strength, with which the cation is held by the resin, depends on the size of the ion and its charge.
However, separation of rare-earth elements is not possible due to the insufficient selectivity of the resin. Separation is achievable by introducing a complexing agent; when the strength of the ion complex of R³⁺ with adjacent lanthanoid ions changes significantly from one rare-earth element to another, separation takes place. Two common complexing agents, used for separating rare-earth elements are ethylenediaminetetraacetate (EDTA) and hydroxyethylenediaminetriacetate (HEDTA).
In recent years numerous studies have been conducted, using interesting physicochemical approaches, aimed at creating new effective nanostructured materials, particularly magnetically sensitive sorbents with a wide range of functions and unique properties. The use of adsorbents with magnetic properties significantly simplifies the task of separating and collecting rare-earth elements. Using chemical modification and functionalization of adsorbent surfaces allows them to be adapted for use in various physical, chemical, and biological conditions.
Solvent extraction
In the solvent extraction process two immiscible or partially immiscible solvents are used, which contain dissolved rare-earth elements. The two liquids are mixed, and the dissolved substances are distributed between the two phases until equilibrium is reached, after which the two liquids are separated. The concentrations of dissolved substances in the two phases depend on their relative affinity for the two solvents.
The product (liquid), that contains the desired dissolved substance, is conventionally called the “extract,” while the remainder left in the other phase is called the “raffinate.”
The best way to influence separation of rare-earth elements is to use a continuous process in a multi-stage counterflow separator with multiple tanks or mixer-settler cells.
Selting rare-earth metals
Depending on the melting and boiling points of a specific metal and its purity, required for a particular application, there are various methods for obtaining individual rare-earth metals:
- for industrial-grade metals (purity 95-98%);
- for high purity metals (99% and higher);
- calciothermic and electrolytic processes for low-melting lanthanoids (lanthanum, cerium, praseodymium, and neodymium);
- calciothermic processes for high-melting point metals (scandium, yttrium, gadolinium, terbium, dysprosium, holmium, erbium, and lutetium);
- For high-pressure metals (samarium, europium, thulium, and ytterbium).
Calciothermic process
The calciothermic process is used for all rare-earth metals, except for the four with high vapor pressure, i.e., low boiling points (samarium, europium, thulium, and ytterbium).
The rare-earth element oxide is converted into fluoride by heating it with gaseous anhydrous hydrogen fluoride (HF), forming RF₃. The fluoride can also be obtained by first dissolving the oxide in hydrochloric acid (HCl) and then adding hydrofluoric acid (HF) to precipitate the RF₃ compound from the solution.
The fluoride powder is mixed with metallic calcium, placed in a tantalum crucible and heated to 1450 °C or higher, depending on the melting point of R. Calcium reacts with RF₃ to form calcium fluoride (CaF₂) and R.
Since these two products do not mix, CaF₂ floats on the metal. Upon cooling to room temperature, CaF₂ can be easily separated from R.
The metal is then heated in a tantalum crucible under high vacuum to a temperature above its melting point to evaporate excess calcium. At this stage R can be further purified by sublimation or distillation.
In China, industrial calciothermic reduction is often carried out in graphite crucibles. This results in significant contamination of the resulting metals with carbon, which easily dissolves in molten rare-earth metals.
Conventional oxide crucibles, such as aluminum oxide (Al₂O₃) or zirconium dioxide (ZrO₂), are unsuitable for calciothermic reduction of rare-earth metals, as molten rare-earth elements quickly reduce aluminum or zirconium from their oxides, forming the corresponding rare-earth element.
Electrolytic process.
Low-melting metals (lanthanum, cerium, praseodymium, and neodymium) can be obtained from the oxide by means of one of two electrolytic processes.
The first method is converting the oxide into a chloride (or fluoride) and then reducing the halogenide in an electrolyzer.
An electrolyte is a molten salt consisting of RCl₃ (or RF₃) and NaCl (or NaF). Lanthanides obtained through the electrolytic method are not as pure as those produced by the calciothermic process.
The second electrolytic process directly reduces the oxide in a molten salt mixture of RF₃-LiF-CaF₂. The main challenge of this process is the relatively low solubility of the oxide, making it difficult to control the oxygen solubility in the molten salt solution.
The electrolytic process is limited to rare-earth metals that melt below 1050°C, as those with significantly higher melting points react with the electrolyzer and electrodes. Consequently, the electrolyzer and electrodes require frequent replacement, and the produced rare-earth metals are heavily contaminated.
Production of samarium, europium, thulium, and ytterbium: lanthanum thermal process.
Europium and ytterbium have high vapor pressures or lower boiling points, if compared to other rare-earth elements, making it challenging to produce them using metallothermic or electrolytic processes. Samarium and thulium also have low boiling points, if compared to other lanthanide metals, as well as to scandium and yttrium.
The four metals with high vapor pressure are produced by mixing the metal oxide R2O3 (R = samarium, europium, thulium, and ytterbium) with finely shredded lanthanum metal and placing the mixture at the bottom of a tall tantalum crucible.
The mixture is heated to 1400–1600 °C, depending on the type of metal. Metal lanthanum reacts with R2O3 and forms lanthanum oxide (La2O3), while R vaporizes and is collected on a condenser in the top part of the crucible, which is approximately 500 °C colder than the reaction mixture at the bottom of the crucible.
The above-mentioned four metals can be further purified through repeated sublimation.
CONCLUSIONS
- All rare-earth ores contain less than 10% of extractable components and must be concentrated to 60% for further processing.
- Initially the ore materials are ground to powder, then subjected to separation from other materials in the ore body by means of various standard processes, including magnetic and/or electrostatic separation and flotation.
- Using the liquid solvent extraction method, rare-earth element producers separate mixtures into individual elements with a purity of up to 95%.
- The ion exchange process allows for a higher purity, exceeding 99.9% (up to 99.9999999% or higher).
- In the solvent extraction process two immiscible or partially immiscible solvents are used, which contain dissolved rare-earth elements.
- Calciothermic and electrolytic processes are used for the smelting of rare-earth metals.
