By Tom Vulcan
Thallium is not only a highly effective depilatory, it is also an extraordinarily nasty poison. While its use as a hair remover and as a rat and bug poison is now severely restricted, there have still been some high-profile instances of its recent use against people. In one instance, it was little children who died.
Back in the summer of '07, a student at the China University of Mining and Technology, in Xuzhou City, Jiangsu province, poisoned three of his classmates by lacing their drinks with thallium nitrate. They all survived.
Sadly, there was no such happy ending for the 10 members of two families who were poisoned by thallium-laced cake in Baghdad, Iraq, on Jan. 22, 2008. In this incident, two children (ages 2 and 11) and two adults died. The surviving six adults were severely poisoned.
The poisoned cake, a "gift" from a disgruntled former employee, was intended for the board members of an Iraqi Air Force football club. However, only two of them were left at the club when the cake arrived belatedly to a board meeting. Taking their slices home, lamentably, it was these members' families who became the unwitting victims of the assassination attempt.
Arsenic and cadmium poisoning may be unpleasant, but thallium poisoning is particularly nasty. Among other trying symptoms, including hair loss and "burning feet syndrome," victims can suffer from "hallucinations, lethargy, delirium, convulsions and coma."
At atomic number 81, thallium, a blue-white, malleable and soft heavy metal, sits between mercury and lead in the periodic table. Named after the Greek for "green shoot," the metal produces wonderful green lines when analyzed spectroscopically.
As a naturally occurring element, thallium is ubiquitous, with an average content in the Earth's crust estimated at anywhere between 0.5-0.7 parts per million. However, the metal is usually only found in low concentrations, such as the sulfide ores of heavy metals such as zinc, copper, iron and lead, and in minerals of caesium, potassium and rubidium.
There are few thallium-only deposits, one such being the thallium-only deposit discovered in 2005 at Xiangquan, in the northeastern margin of eastern China's Yangtze block. And the discovery of a thallium-rich murunskite deposit on the Russian Kola Peninsula was announced in 2006. Other important thallium mineral deposits include the Norilsk-Talnakh deposit in Siberia and another at Allchar in Macedonia.
Commercially, thallium is most often recovered from flue dusts and residues, following the processing of these heavy metals, although it can sometimes be found together with sulfides of antimony, arsenic and silver. Occasionally, it is recovered as a by-product in the production of sulfuric acid.
Although the metal can be found in higher concentrations (sometimes up to 60 percent) in such minerals as lorandite (TlAsS2) and crookesite ((Cu,Ag,Tl)2Se), the metal is rarely recovered commercially from these. Thallium is also found in coal, but as of yet, nobody has discovered a commercially viable way of recovering it.
According to the U.S. Geological Survey [USGS], no thallium has been recovered domestically in the U.S. since 1981, although "flue dust and residues from base-metal smelters, from which thallium metal and compounds may be recovered, are exported to Canada, France, the United Kingdom, and other countries." Consequently, the country is 100 percent dependent on imports of the metal. Previously, companies recovering the metal in the U.S. included both ASARCO and Umicore out of its operation in Hoboken, N.J. Even the big Canadian mining concern, Teck, does not itself recover the metal.
Information on thallium production is extraordinarily difficult to ferret out. However, it likely remains safe to say that the world's largest thallium-producing countries are Kazakhstan and China—with Kazzinc in Kazakhstan arguably being the world's biggest producer. Over the last five years or so, according to the USGS, the U.S. has imported thallium and thallium compounds primarily from Russia, but also from The Netherlands, Germany and Belgium. (However, whether the thallium was originally mined in these last four countries is another matter altogether—most likely not.)
That said, however, overall global production of thallium is miniscule. For 2007 and 2008, the USGS estimates mine production at just 10 tonnes each year.
Applications Of Thallium
Like other poisonous metals such as cadmium and arsenic, thallium still has some important and interesting applications, although in recent years, these have shifted away from bodily use to highly specialized, high-tech applications.
In the past, thallium was used not only as an insecticide and rodenticide (in Europe against rats and in the U.S. against ground squirrels), but also in the treatment in human ailments such as malaria, ringworm of the scalp, tuberculosis, typhus and venereal diseases.
Today, however, thallium's two biggest uses are in the fiber optic industry and in glass lenses. In fiber optics, thallium acts as a dopant in the lenses, prisms and windows of repeaters, in order to speed signals on their way. And, as thallium nitrate, the metal is frequently used in the manufacture of the glass lenses for digital cameras. Not surprisingly, therefore, three of the largest consumers of thallium are now China, Japan and South Korea.
Other common uses of thallium are in:
- Acousto-optic and laser equipment
- Catalysts for organic reactions; e.g., the oxidation of hydrocarbons and olefins
- High-temperature superconductors [HTS]
- Mercury lamps
- Photocells sensitive to infrared light
- Scintillometers (for detecting gamma radiation)
- Special high-resistance, low-melting-point glasses: often together with selenium
- Thermometers for low temperatures (as low as -59°C); alloyed with mercury it is a eutectic
Of thallium's many applications, perhaps the ones with the most promise are its use in radioisotopes, and its nascent use in HTS materials.
In medicine, the radioactive isotope of the metal thallium-201 is used for the diagnosis of melanoma and in scintigraphy of heart, liver, thyroid and testes. Although at first glance radioisotopes may not appear to be big business, an article published in Nuclear Engineering International magazine in July 2008 noted that the estimated worldwide value of radioisotope production of the three main radioisotopes (cobalt-60, molybdenum-99 and thallium-201) accounted for some $360 million out of $460 million for all such radioisotopes. (Thallium-201 itself accounted for 12 percent of the overall market.)
In August of last year, examining the then-prevailing shortage (particularly in Canada) of certain medical isotopes, CBC ran a story on the possibility of alternatives, and thallium-201 featured prominently as one such substitute, particularly for technetium-based nuclides for cardiac evaluation.
The thallium-201 radioisotope has the singular advantage that it does not need to be produced in a nuclear reactor. It can quite easily be produced in a cyclotron (often owned by hospitals) in a process that starts by bombarding a thallium (or lead or bismuth) target with protons.
Superconducting materials conduct electricity with little or no resistance. Apart from the potential use of such materials in applications such as magnetic propulsion and the storage of magnetic energy, being able to distribute energy along power lines without energy loss through resistance is one of the holy grails of efficient power transmission. Enter the high-temperature superconductor, or HTS. ("High" in this context is a relative term, since it merely means not super-cooled.)
Since 1986, when the superconductivity of lanthanum-barium-copper oxide at 35ºK (-396.67ºF) was discovered by Bednorz and Müller, research has continued in the development of HTS materials, particularly in places like Oak Ridge National Laboratory, as part of the U.S. Department of Energy's national effort on HTS' electrical power applications.
In 1993, researchers discovered the superconductivity of thallium-doped mercury-barium-calcium-copper oxide at the even higher temperature of 138ºK (-211.27ºF), a temperature higher even than that for the compound thallium-barium-calcium-copper oxide.
HTS materials are now used in electronic filters for cell phones and in a few superconductor power cables (manufactured by the likes of American Superconductor). But problems with their use still abound, not least of which being that these brittle ceramics are notoriously difficult to make into long wires.
Robert F. Service's words in Science in November 2006 still ring true today: "Cost remains the biggest challenge for HTS devices, particularly for HTS wire that's priced three to five times higher than its copper equivalent. With the production of 2G wire now being scaled up, companies say they hope to close that gap by the end of the decade. But if the last 20 years have offered any lesson for entrepreneurs, it's that HTS hype inevitably yields to a more sober assessment of what the new science can deliver to the marketplace."
While thallium has yet to be used in the widespread manufacture of HTS power cables, there could still be potential for the metal in this context, even if its toxicity remains a stumbling block.
One further area of potential use for thallium is in newly developed and more effective thermoelectric materials. In July 2008, researchers at Ohio State University announced that they had "invented a new material [thallium-doped lead telluride] that will make cars even more efficient, by converting heat wasted through engine exhaust into electricity." Since experts have argued that as much as 60 percent of the energy produced by a typical gasoline engine is lost through waste heat, "much of which escapes in engine exhaust," being able to convert this heat into electricity can only be a good thing.
If the hopes of the research team are converted into reality, then there could be great potential for thallium use in thermoelectric materials applications, not only in automobiles, but also in such items as heat pumps and power generators.
Even if you were so inclined, thallium is one metal for which a physical holding would be ill-advised for anybody other than a serious professional. Because of its extreme toxicity, the metal and its compounds need to be handled and stored with extreme care.
Anthony Lipmann in a full hazmat suit off to inspect a holding of thallium
Source: Lipmann Walton & Co Ltd
That's because after exposure to air, the surface of the metal oxidizes and any contact with skin becomes extremely dangerous.
Thallium ingot covered in oxide
Source: Lipmann Walton & Co Ltd
Since thallium is produced in such small amounts, and the current average price is around $40-60 per kilo, the value of the whole global market in the metal appears rather small. (Of course, any estimate of the market's value based on the metal alone does not include the value of the market in its main compounds, most of which have significant value added.)
In addition, since the thallium is produced almost without exception as a by-product, no pure plays in the metal exist. Furthermore, for those companies that do produce it (for example, Kazzinc, 69 percent of which is owned and operated by subsidiaries of Glencore), it will be just a tiny part of their overall business.
However tiny the market for the metal may be, at least one junior explorer has its eye on thallium's potential profits: Cascadero Copper (Bloomberg Ticker - CCD:CN). In a press release from the end of November last year, the company made much of its discovery of a "Potential Large Deposit of Thallium an Electronic [sic] Metal."
For those interested in maintaining a watching brief on thallium, perhaps the most constructive course of action would be to remain up-to-date with any developments made in the areas of both thermoelectric and HTS materials, where the metal is used as a dopant.
Since it is more than likely the metal will continue to be used in both camera lenses and fiber optics, investors should watch its use in these areas, too. If demand for the metal should increase dramatically, then we may see either companies looking specifically for "thallium-only"-type deposits, or its recovery from previously unexploited existing thallium resources.
And then, of course, there is still the radioisotope market to keep in mind, especially if there are further shortages.
As with a number of other toxic but useful metals—like cadmium and mercury—the benefits derived from a decrease in thallium's use will always be subject to the law of diminishing returns. Since so many of the toxic metals are recovered from anthropogenic sources, more often than not as by-products, decreasing their use just because they are toxic in no way addresses the fact that we will continue producing the fly ash, residues, flue dusts, etc., that contain them. If these metals are not going to be recovered, used and recycled responsibly, then it is questionable as to whether it is any more responsible for them to end up in landfills, or left around for future generations to deal with.
In this context, the opening paragraph of the "Conclusions and Recommendations" section of the INCHEM International Programme on Chemical Safety report on thallium may be particularly relevant:
"The currently limited industrial uses of thallium are unlikely to pose a threat to the general environment. At industrial facilities such as metal mining and smelting operations and cement plants using pyrite, which can release significant amounts of thallium, the concentration of thallium in industrial raw materials as well as stack gases and waste water should be monitored and, if necessary, controlled. Waste materials containing water-soluble thallium compounds should be sealed and marked to avoid leaching and pollution by dust."
Perhaps one of the greatest investment opportunities out there for thallium and other toxic elements lies in remediation. But that is another story altogether.
Kazakh Thallium Metal in Boxes
Source: Lipmann Walton & Co Ltd