7440-30-4

grey chips

Fire risk in form of dust.

Thulium is a naturally occurring rare metal that exists is very small amounts mixed withother rare-earths. It is a bright silvery metal that is malleable and ductile and can be cuteasily with a knife. Its melting point is so high that it is difficult to force it into a meltedstate. Its vapor pressure is also high, and thus, much of the molten thulium evaporates intothe atmosphere. Its melting point is 1,545°C, its boiling point is 2,950°C, and its density is9.32g/cm³.

There are a total of 46 isotopes of thulium. One of these, Tm-169 is the onlystable isotope of thulium and accounts for the total atomic mass of the element. All theother isotopes are artificially produced and radioactive and have half-lives ranging from afew microseconds to two years.

Named for Thule, the Greek word for Scandinavia, the most northerly habitable land in ancient mythology.

Thulium is the 61st most abundant element in the Earth’s crust and is found along withother rare-earths in monazite sand, which is about 50% rare-earths by weight. Only about0.007% of this is thulium. It is also found in bastnasite ore. It ranks 16th out of the 17 rareearthsin abundance. Thulium is usually found as an oxide along with other rare-earths. Likemost rare-earths, thulium can be separated from its ore by the ion-exchange process, whereits positive ion reacts with elements with negative ions like fluorine, chlorine, or oxygen toform binary compounds (e.g., Tm₂O₂). It can also be recovered as a by-product of the nuclearfission reaction in nuclear reactors.

Thulium is near the end of the lanthanide series, where the metals tend to be heavier thanthe ones located near the beginning of the series. It is so scarce that it requires the processing ofabout 500 tons of earth to extract four kilograms of thulium. The only element that is scarceris promethium, which is not found naturally on Earth.

Thulium is recovered from xenotime, gadolinite, euxenite, samarskite, and other minerals. The first step of recovery involves opening the ores. If xenotime, (Y)PO₄ is the starting material, the mineral is heated with an excess of sulfuric acid (95%). The product mixture is treated with cold water to separate water-soluble sulfates from unreacted mineral, silica, and other insoluble residues. The solution is filtered and yttrium and the individual rare earths are separated from this solution by ion exchange. The tripositive lanthanide metal ions and yttrium are absorbed on an appropriate cation exchange column and eluted with ammonium ethylenediamine tetraacetic acid (EDTA) at pH 8.4. The cation-exchange resin is pretreated with an equimolar mixture (1 M) of copper sulfate-sulfuric acid. The various eluate fractions are collected, and are treated with oxalic acid. The metals are precipitated as oxalates. Precipitate from the thulium fraction is calcined at 800°C to convert oxalate into oxide, Tm₂O₃.
If thulium is to be recovered from gadolinite, Be₂Fe(Y)₂Si₂O₁₀, pulverized mineral is opened by digesting with hot nitric acid-hydrochloric acid mixture. Insoluble silica residues are removed by filtration. The solution now contains beryllium, iron, yttrium, and the rare earths. The solution is treated with oxalic acid to precipitate yttrium and the rare earths. The precipitate is calcined at 800°C to form rare earth oxides. The oxide mixture is dissolved in an acid from which yttrium and the rare earths are separated by the ionexchange as above. Caustic fusion may be carried out instead of acid digestion to open the ore. Under this condition silica converts to sodium silicate and is leached with water. The insoluble residue containing rare earths and yttrium is dissolved in an acid. The acid solution is fed to an ion exchange system for separating thulium from other rare earths,
Thulium metal is prepared from its oxide by reduction with lanthanum at its melting point of 1,545°C. Thulium is separated from lanthanum by sublimation in vacuum. The metal vapor is condensed into crystalline metal in purified form free from lanthanum.