Thulium, symbolized as Tm, is a metallic element with atomic number 69, placing it within the lanthanide series of the periodic table. Thulium is notably one of the least abundant naturally occurring lanthanides in the Earth’s crust, second only to the radioactively unstable element promethium. Obtaining pure thulium requires a multi-stage process, starting with mining specific minerals and concluding with complex chemical separation techniques. The following sections explore where this element is naturally found and the industrial methods used to extract it for technological applications.
The Mineralogical Home of Thulium
Thulium is never found in nature as a pure, isolated metal but is chemically bound and dispersed within the crystal structures of various minerals. These elements share similar chemical properties, meaning they occur together in the same geological deposits. The element is typically grouped with the heavy rare earth elements, often referred to as the yttrium group, due to its co-occurrence with yttrium, erbium, and ytterbium.
The primary host minerals for thulium are complex phosphate and silicate minerals. Monazite, a phosphate mineral containing thorium, and xenotime, a yttrium phosphate mineral, are the most commercially significant sources. Thulium is incorporated into these mineral structures by substituting for other elements, such as yttrium or cerium, which have similar ionic sizes. For instance, monazite sands, a common ore, may contain only about 0.007% thulium by weight within the rare earth oxide fraction.
The presence of thulium in such low concentrations highlights the misnomer of the “rare earth” designation. The elements are geologically common but are rarely concentrated enough to be economically viable to mine. This dispersed nature necessitates processing vast quantities of ore to yield a usable amount of the element.
Global Geographical Distribution
While thulium-bearing minerals are found across the globe, commercially viable concentrations of these ores are geographically limited. China holds the largest reserves of rare earth elements, including thulium, and maintains a dominant position in global production and refining. The Bayan Obo mine in Inner Mongolia is a major source, producing significant quantities of the world’s rare earth oxides.
Beyond China, other nations possess significant reserves that contribute to the global supply chain. Brazil and Vietnam hold some of the world’s largest estimated rare earth reserves. Australia is also a significant producer, with substantial deposits located primarily in Western Australia.
Additional reserves are located in countries such as the United States, India, and Russia. The term “rare” is misleading because the elements themselves are abundant in the crust. The distribution of commercially extractable thulium is concentrated in a few key regions that can handle the massive-scale mining and subsequent refining.
Extraction and Purification Process
The journey to obtain pure thulium begins after the ore is mined. The initial process involves crushing the raw ore and subjecting it to flotation or gravity separation techniques to concentrate the heavy rare earth minerals. This mechanical beneficiation is followed by a chemical process known as “cracking,” where the concentrated mineral is dissolved using strong acids, such as sulfuric or hydrochloric acid, to leach the rare earth metals into an aqueous solution.
Once dissolved, the thulium ions exist alongside all the other co-occurring lanthanide ions, which have nearly identical chemical properties. The industrial gold standard for separating thulium from this complex mixture is a multi-stage technique called solvent extraction (SX). This process involves repeatedly mixing the acidic aqueous solution with an organic solvent containing a specific chemical extractant.
The extractant selectively binds to one or a small group of lanthanides, allowing them to be pulled out of the aqueous phase and into the organic phase. This process requires hundreds or even thousands of mixing and settling stages in a cascade to achieve the necessary high purity, often exceeding 99%. Alternatively, ion-exchange chromatography is sometimes used, which achieves high purity separation by passing the solution through a column of resin beads that differentially adsorb the lanthanide ions.
Why Thulium is Highly Valued
The high value of thulium is a direct result of its extremely low natural concentration and the high cost associated with its complex extraction and purification. This cost is justified by the element’s unique properties, which enable specialized technological applications.
Thulium is used to create specific types of solid-state lasers, often doped into yttrium aluminum garnet (YAG) crystals, which are valued for their infrared light output in medical and military applications. A radioactive isotope, Thulium-170, is produced by bombarding the stable isotope in a nuclear reactor and serves as a compact source for portable X-ray devices. These small, lightweight X-ray units are used in medical imaging and non-destructive testing.
Furthermore, thulium is incorporated into specialized phosphors that emit a strong blue light when exposed to ultraviolet radiation, finding use in anti-counterfeiting measures for currency. The element’s specialized role in these advanced technologies, where no easy substitute exists, ensures its continued demand despite the expense and complexity of its supply chain.