The trade in minor metals occupies itself with the movement of by-product elements from their place of origin to factory gate. As various as end-uses are, so is the diversity of substances needed.
Of the variety of these materials, none has more contrasting properties than Thallium. The most toxic of all elements; once consumed, it may never be expended from the body but migrates to the skeleton where it accumulates for life. 800 milligrams, according to John Emsley in his excellent book ‘The Elements of Murder’ (publ. Oxford 2005), is enough to kill a human.
Thallium only arises in commercial quantities when actively separated from Zinc/Lead concentrates. But usually it is not recovered at all and remains in a polymetallic residue which is disposed of. Only in a few parts of the world, such as Kazakhstan, is Thallium separated, refined and cast into ingots. Here, two main grades are produced, Tl-0 and Tl-1, as previously defined by Soviet state standards; the former being pyramid ingots of higher purity Tl 99.99% minimum. Due to its toxicity and tendency to oxidize, each ingot was wrapped in paraffin-impregnated paper, sealed in with an inch-thick layer of wax and then placed in wooden crates. [But nowadays due to restrictions on the import of goods into the EU in wooden packaging, the ingots have to be safely removed and repacked under argon into steel drums according to IMDG Code 6.1/3288/I.]
Today, due to the unique way in which Thallium diffracts light in glass within digital camera lenses and fibre optic repeaters, it has found entirely new and hi-tech applications. With the growth in demand for delivery of 3G and 4G data, fibre optics’ popularity has led to the roll-out of cable networks across the globe, akin to the first laying of copper cable in the 19th century. It all leads to one thing – a startling increase in the commercial demand of this element; which just goes to show that elements are not ‘bad’ or ‘good’ – it is rather how man uses them.
Thallium was discovered in 1861 by William Crookes in London, England and was isolated by C. A. Lamy in Paris, France. The origin of the word Thallium comes the Greek word for ‘green shoot’, because the metal produces a green line when analysed spectroscopically. It is a soft, silvery-white metal, that can be cut with a knife and tarnishes readily in moist air to create a bluish-grey tinge (similar to lead). It also reacts with steam to form TlOH and is easily attacked by acids and especially so by HNO3. A heavy layer of oxide builds up on thallium if left in air, and in the presence of water thallium hydroxide is formed. To preserve the metal it may be kept under water.
When combined with sulphur or selenium and arsenic, thallium has been used in the production of high-density glasses that have low melting points in the range of 125 and 150 °C. These glasses have room temperature properties that are similar to ordinary glasses and are durable, insoluble in water and have unique refractive indexes.
Although the metal is reasonably abundant in the Earth’s crust at a concentration estimated to be about 0.7 mg/kg, it exists mostly in association with potassium minerals in clays, soils, and granites and so is not generally considered to be commercially recoverable from those forms. As far as we know the only commercial route for the recovery of thallium is the trace amounts found in lead and zinc ores but it is also present in some copper and other sulphide ores.
There is ongoing research with thallium to develop high-temperature superconducting materials (wires) for applications such as magnetic resonance imaging, storage of magnetic energy, magnetic propulsion, and electric power generation and transmission, as well as filters in wireless communication.
The odourless and tasteless thallium sulphate was widely used in the past as a rat poison and ant killer. In the United States and many other countries this use is no longer allowed due to safety concerns.
Among the growing uses for thallium are the semiconductor and laser industry, in fibre (optical) glass, in scintillographic imaging, in superconductivity, digital cameras, and thermometers to measure low temperatures.
Relative atomic mass