**1. Introduction**

Rare earth elements (REE) are extremely important components in high technology, which is why they are sometimes referred to as "vitamins" of the modern economy or industry [1, 2]. With the development of high technology, the demand for rare earth elements increases year on year. It is especially visible in the case of searching for clean energy sources or in the development of various types of electronic devices. Rare earth elements are important components in advanced technologies such as smartphones, computers, TVs, LEDs, hard drives, or elements used for the production of clean energy, such as magnets in wind turbines. Therefore, an increase in REE production is observed worldwide (**Figure 1**). In 2021, the extraction of rare earth elements in the world reached 280,000 tons, which was a significant increase compared to 2018, when 110,000 tons less were extracted. China is the largest producer of REE [3]. In 2021, they have produced 168,000 tones of these elements, while the United States came second in this ranking with a much lower production of 43,000 tones [4].

#### **Figure 1.**

*World mine production of REE (after Ref. [3]).*

#### **Figure 2.**

*Distribution of REE consumption worldwide in 2020, by end-use applications (after Ref. [5, 6]).*

In the natural environment, REEs do not occur as single native metals, such as gold or silver, they occur together in many ores or minerals as secondary or major constituents [1]. The most economically viable methods of extracting these elements from the ores, as well as separating them from the rest of the components, are being sought. The separation of REEs is difficult and costly due to the similar chemical properties of these elements. In addition to searching for new, more economical methods of their separation, the issue of their reuse - recycling is also very important. Each of the rare earth elements has unique chemical, optical, mechanical, electronic, or magnetic properties and can therefore be used in advanced engineering applications such as permanent magnets, luminescent materials, metallurgy, batteries, catalysts, ceramics, pigments, phosphors, nuclear industry, medicine, and nanotechnology.

The growing importance of these elements in the development of new technologies has made them sought-after raw materials worldwide. It is very important to maintain a proper supply chain for highly developed economies that want to participate in the

development of highly advanced technologies. The distribution of the use of these elements in the world, in particular, advanced applications in 2020 is presented in **Figure 2**.
