Analysis of Rubidium Resources and Application Prospects
“Rubidium has shown its wide range of applications in pharmaceutical catalysts, electronic components, magnetohydrodynamic power generation, special glass, and even rocket ion propulsion systems. Due to the irreplaceable role of rubidium in high-tech fields, the demand is increasing day by day, and its energy reserves are becoming increasingly tight. How to use rubidium efficiently and how to improve the extraction technology of rubidium will be an important topic in the future prospects of rubidium applications.”
“Rubidium was discovered in 1861 by German scientists Robert Bunsen and Gustav Robert Kirchhoff through spectral analysis using a spectroscope while examining lithium mica minerals. Rubidium is a silver-white, waxy metal that is light and soft, with a specific gravity of only 1.532 g/cm³. It melts into a liquid state at temperatures between 102.7°F and 39.2°C and boils at 686°C. Rubidium belongs to the alkali metals, with an atomic number of 37. It is more reactive than potassium and easily emits electrons under the influence of light, making it the second most electropositive and stable alkali metal element. It reacts violently with water to produce hydrogen gas and rubidium hydroxide. It also easily reacts with oxygen to form oxides. Due to the large amount of heat released upon contact with water, the hydrogen gas produced can ignite immediately. Because it can spontaneously combust at room temperature, pure metallic rubidium is usually stored in sealed glass ampoules. In China, rubidium resources are widely distributed but are mostly found within other minerals. Currently, no independent rubidium minerals have been found in nature, so rubidium is mainly extracted from other minerals like lepidolite, potash mica, seawater, and some strata water, as well as salt lake brine. Among these, lepidolite contains the largest total reserves of rubidium. Rubidium has 45 isotopes (from rubidium-71 to rubidium-102), of which one isotope is stable. The naturally occurring rubidium-87 has weak radioactivity. Due to its unique chemical and physical properties, rubidium plays an important role in various fields.”
Characteristics of Rubidium Minerals, Mineral Morphology, and Distribution of Rubidium Resources
Rubidium is widely distributed in the Earth’s crust, with a principal value of 16, which is even higher than that of copper. Its content is approximately 0.0279%, making it a relatively abundant element in terms of resources. However, rubidium on Earth has not been concentrated to form independent minerals; instead, it is dispersed in other rock-forming minerals. Currently, it is mainly found in natural minerals such as lepidolite, cesium lepidolite, potash mica, lepidolite, seawater, and salt lake brine. Lithium mica contains about 1.5% rubidium and is an important source for industrial rubidium products, followed by potassium minerals and some potassium chlorides. In these minerals, rubidium mainly exists in the form of isomorphs with potassium.
Globally, the largest reserves of rubidium are found in countries like South Africa, the United States, Canada, Zambia, and Namibia. Data shows that just in the sediments of Baenic Lake in Canada, there are over 2,000 tons of rubidium resources. According to available information, the total proven reserves worldwide are approximately 10.77 million tons (not including rubidium in seawater, where the concentration is about 0.12 mg/L), of which over 90% is found in salt lake brine.
In China, rubidium resources are also very abundant, mainly found in lithium mica and other ores, as well as in salt lake brines. The rubidium content in lithium mica accounts for 55% of the country’s total rubidium resource reserves. In terms of the form and geographical distribution:
Rubidium resources in ore form are mainly distributed in Xinjiang, Jiangxi, and Tibet. Among these, Yichun in Jiangxi has the most abundant resources and is currently the main source of rubidium products in China.
Rubidium in brine form is widely distributed in salt lakes and geothermal waters in Sichuan, Qinghai, and Tibet. The rubidium content in the deep potassium-rich brine of marine sediments in Sichuan reaches 32.55 mg/L, which is rare in the world. China has a total salt lake area of 37,927 km². It is estimated that just in Chaka Salt Lake (5,856 km²), there are at least 62.155 million tons of Rb2O.
As of 2013, the total amount of rubidium oxide confirmed in China has reached 131,700 tons, with 37% being extractable. Currently, a new rubidium deposit with a large resource volume has also been discovered in the Meizhou area. This rubidium mineralization occurs in the contact alteration zone between medium-fine-grained biotite granite and the overlying wall rock. Tests have shown that rubidium is found in the potassium feldspar in the granite and in the mica minerals in the upper metamorphic sand shale, and it is estimated to be a coeval mineralization product. Currently, work is only being done on the upper metamorphic sand shale, and the actual controlled amount of RB2O through drilling has reached 27,300 tons. This phenomenon of mineralization occurring simultaneously in both granite and metamorphic rocks is a rare type of rubidium mineralization.
Current Status of Research on Rubidium Extraction Technology
Due to the dispersed nature of rubidium, it often coexists with other alkali metal elements such as potassium, sodium, lithium, and cesium. Their physical and chemical properties are very similar, which brings great difficulty to the separation, purification, and deep processing of rubidium, increasing the complexity of rubidium extraction and purification processes. Starting from the initial stepwise crystallization method, various rubidium processing techniques such as precipitation, ion exchange, and solvent extraction have been gradually developed and are continuously being researched and explored. Currently, the main extraction methods used in China are the traditional precipitation method and the relatively emerging solution extraction and ion exchange methods. Since the precipitation method is rarely used in the extraction of rubidium from brines, this article focuses on introducing solution extraction and ion exchange methods.
Solution Extraction Method
The use of solvent extraction for the separation and extraction of rubidium is a technology that has been extensively researched in recent years and has significant application potential. It is a separation and recovery technology that is easy to implement for continuous automated operation. It can be used for extracting rubidium from brines as well as from minerals. The key area of research focuses on the improvement of the extracting agents. Currently, crown ethers, phenolic reagents, and di-secondary amines and their derivatives are mainly used as extracting agents. Di-secondary amines and their derivatives have been rarely used in recent years due to various inherent factors.
The most commonly used phenolic reagent is 4-tert-butyl-2-(alpha-methylbenzyl)phenol (t-BAMBP). It is a weakly acidic substituted phenolic extracting agent with advantages such as good stability, low water solubility, low volatility, strong selectivity, rapid reaction, ease of back-extraction, low toxicity, and relatively low cost. It serves as a specialized extracting agent for rubidium. When using t-BAMBP to extract rubidium, attention should be paid to the diluent effect. Inert solvents with low dielectric constants should be selected as diluents, as these solvents can prevent the hydrogen-bonding-induced aggregation of t-BAMBP in the diluent, achieving a higher distribution ratio. Chinese technicians have used t-BAMBP to separate and extract rubidium from high-potassium brines in Sichuan Pingluo. They examined the effects of various factors such as the type of diluent, concentration of extracting agent, extraction phase ratio, alkalinity, extraction time, water-washing phase ratio, back-extraction agent acidity, and back-extraction time on the separation process. They obtained RbCl with a purity of 97.5% and an overall extraction rate of 92.7%.
Ion Exchange Method
The ion exchange method is the primary technique for extracting rubidium from brines and salt-making brines. This method is relatively simple in terms of process, but it offers high recovery rates and good selectivity. The selection in ion exchange mainly depends on the exchanger, so research on this method focuses on finding exchangers with better selectivity. Some new ion exchangers with good selective adsorption properties have been developed, which can be categorized into inorganic and organic exchangers.
Inorganic Exchangers
The main types of inorganic exchangers are synthetic zeolites and heteropoly acids. Zeolites are crystalline aluminosilicates that are abundant and inexpensive. Their mineral structure has a large specific surface area and pore volume, offering good adsorption and ion exchange properties. They can separate and concentrate Rb+ ions from large volumes of dilute solutions without consuming a lot of energy, a feature that has attracted attention. However, further research is needed to solve issues related to the enrichment of Rb+ in the elution solution and the purity of the product. Studies have shown that inorganic ion exchangers can significantly improve the selectivity of ion exchange. They are extremely efficient in separating rubidium from brines, with extraction rates exceeding 90%, and the exchangers can be reused.
Heteropoly acid exchangers mainly include phosphomolybdates (ammonium phosphomolybdate, zirconium phosphomolybdate), phosphotungstates (ammonium phosphotungstate, zirconium phosphotungstate), arsenomolybdates, and silicomolybdates. Among them, research on ammonium phosphomolybdate is more extensive. However, ammonium phosphomolybdate has a fine, microcrystalline structure and poor hydrodynamic properties, making it unsuitable for column operations. There are also reports on the exchange properties of other heteropoly acids, but due to their low separation factors, they do not have practical application value.
Rubidium Usage and Future Prospects
Currently, the global demand for rubidium is quite low, amounting to only a few thousand tons per year. This is because rubidium has very reactive chemical properties (metallic rubidium can react naturally at room temperature), requiring strict management and safety measures in storage and transportation. These high standards limit the use and development of rubidium in some aspects. On the other hand, the methods for extracting rubidium from minerals are limited, inefficient, and have low yields and high production costs. In some areas, cesium serves as a substitute for rubidium, and cesium is easier to obtain and has a more stable supply. These factors collectively restrict the stability and economic benefits of rubidium as a general industrial product, thereby affecting its current usage. However, due to the unique properties of rubidium and its compounds, it plays an important role in traditional fields such as electronics/power, catalysts, special glass, biochemistry, and medicine. In some high-tech areas like magnetohydrodynamic power generation, thermal ion conversion power generation, ion propulsion rockets, and laser-to-electricity conversion devices, rubidium is showing an increasingly important role. Below are examples of rubidium’s current usage in both traditional and emerging fields.
Power and Electronics
One of the main application areas of rubidium is in the power and electronics industry. Utilizing rubidium’s ease of ionization, it plays a significant role in magnetohydrodynamic power generation. The biggest difference between this technology and other power generation technologies is the use of rubidium materials for power generation. Due to the unique chemical and physical properties of rubidium, the efficiency of thermal conversion has been greatly improved. According to statistics, magnetohydrodynamic generators using rubidium materials have increased the overall thermal efficiency by 100% compared to traditional nuclear power plants.
Similarly, due to its ease of ionization, a general light source can cause it to release free electrons. Rubidium thus has excellent photoelectric properties, electrical and thermal conductivity, and photochemical activity. This has led to it being highly valued in the electronics field. It is a staple material essential for infrared technology and is also an important component in photovoltaic cells, photomultiplier tubes, and atomic clocks.
Special Glass
Another major market for rubidium resources is in the production of special glass. Adding rubidium carbonate to glass has been found to significantly reduce the glass’s electrical conductivity, thereby enhancing its stability and safety during use. This type of glass is widely used in night vision equipment and optical fibers for communication.
Medical and Other Industries
In medicine, rubidium chloride and several other rubidium salts are used as density gradient media in the ultracentrifugation separation process of DNA and RNA. Radioactive forms are used for blood flow radiotracing. Rubidium iodide is sometimes used as a substitute for potassium iodide in the treatment of enlarged thyroid glands. Some rubidium salts serve as sedatives, anti-shock agents after the use of certain medications, and in the treatment of epilepsy.
Other Applications
In addition to the aforementioned application areas, rubidium and its compounds have the following typical uses: Rubidium and its alloys formed with potassium, sodium, and cesium can be used as getters for trace gases in vacuum electron tubes and for removing residual gases in high-vacuum systems. Rubidium acts as a chemical tracer, tracing various types of manufactured goods. The decay of 87Rb to 86Sr is widely used for dating rocks and minerals (Rubidium-Strontium method).
Future Prospects
Before the invention of electrolytic aluminum, aluminum was a very expensive metal due to its low production and high cost. So much so that Napoleon used to serve his generals in silver bowls, while he himself used an aluminum bowl to signify his nobility. Similarly, today, with the continuous development and refinement of science and technology, the application areas for rubidium will continue to expand. The unique characteristics of rubidium and its compounds have shown great potential and important scientific and commercial value. For example, the demand in aerospace, national defense industries, and other fields is steadily increasing. Coupled with the growing scarcity of global energy, the demand for rubidium in the energy sector will also continue to rise.
Factors that have constrained the application of rubidium resources are being resolved one by one. For instance, the refining technology for rubidium is continuously improving and maturing. Current ion-exchange and solvent extraction methods have very high industrial application prospects, which have increased rubidium production and reduced extraction costs, thereby enhancing its economic viability and stability in industrial products. In terms of ease of use, various methods have been explored for managing and storing rubidium, such as using inert gases, liquid paraffin, and vacuum conditions, making the use and management of rubidium increasingly convenient.
It can be foreseen that the application prospects of rubidium in various fields will become increasingly broad in the future, especially in high-tech areas. For example, in military communications, bubble rubidium atomic frequency standards are currently the most widely used atomic frequency standards. Due to the advantages of rubidium resources in terms of price, stability, and accuracy, rubidium atoms have replaced traditional cesium atoms as the onboard frequency standards in satellites launched by countries around the world in recent years. The same is true for the Global Positioning System (GPS). Therefore, we have reason to believe that with the continuous deepening of rubidium ore development and the continuous improvement of mineral processing and metallurgical technology, making the extraction cost lower and lower, and driven by the demand of technological development, the social supply of rubidium will also increase significantly, and social demand will also become increasingly large.
Due to its irreplaceable roles in high-tech fields, it can be foreseen that the application prospects and usage of rubidium will continue to expand in the next few decades. The continuously increasing demand requires significant improvements in rubidium extraction technology. Nowadays, emerging technologies like solvent extraction and ion-exchange methods have become increasingly mature, leading to significant improvements in the quality and extraction rates of produced rubidium. How to further enhance the extraction rates, innovate extraction methods, and continuously expand the application fields of rubidium will be the key directions for future research.