Thulium

Thulium is the rarest of the naturally occurring lanthanides and the second rarest among the rare earth metals, making it correspondingly expensive. Its abundance in the Earth's crust is estimated at about 0,5 ppm.

The main host minerals for thulium are monazite and xenotime.

Xenotime, which is particularly rich in heavy rare earth elements, contains a somewhat higher concentration of thulium than bastnäsite and monazite.

Thulium is obtained through solvent extraction. For this process, the rare earth oxide (REO) mixture is dissolved in hydrochloric acid, and the solution is pumped through a series of mixer-settler units. At each stage, an organic solvent is added that selectively binds a specific rare earth element.

Due to slight differences in the acidity of the ions, the various elements move through the cascade at different rates.

Thulium, being one of the heavier lanthanides, accumulates in the later stages of the extraction cascade.

The result of this step is highly pure thulium(III) oxide (Tm₂O₃), also known as “thulia.”

To obtain metallic thulium, the oxide is usually reduced metallothermically using lanthanum or cerium.

The metal produced by reduction is often not pure enough for high-tech applications. The most important purification method is vacuum distillation.

Compared to other rare earth elements, the demand for thulium is limited and its applications are confined to niche areas.

The most important use of thulium is in portable X-ray devices. When thulium-169 is irradiated, it produces thulium-170. This isotope serves as a highly efficient, portable, and safe source of soft gamma radiation, similar to that used in X-ray equipment. Since these radiation sources require no external power supply or complex cooling, they are ideal for portable X-ray devices used in non-destructive testing (e.g., weld inspection) at hard-to-reach locations such as pipelines or construction sites, as well as for medical applications in remote areas without electricity.

Another application of thulium is in high-power lasers. Thulium-doped solid-state lasers (e.g., using YAG or YLF crystals), employed in medical surgery and LIDAR (Light Detection and Ranging) systems, are extremely powerful and efficient.

Thulium is also used as a dopant in specialized phosphors and has niche applications in materials research.

Furthermore, thulium’s excellent performance in high-temperature superconductors makes it indispensable for the development of advanced electronic devices. It contributes to the production of highly efficient lasers, especially those operating in the infrared spectrum, which are crucial for precision surgical instruments and industrial applications.