Controversy surrounds thallium production in Japan and

Thallium is the last element of group IIIA in the Periodic Table (group 13, according to the new IUPAC rules). This group also includes boron, aluminum, gallium and indium. Thallium is a soft, silvery metal that rapidly tarnishes as a result of the formation of a black oxide layer. It resembles lead in terms of its physical properties but is closer to zinc with respect to chemistry and geochemistry.

There are no thallium-ores and thallium minerals are exceedingly rare. They are found only in Wyoming, Sweden, Kazakhstan (in the Ural Mountains), and particularly in the so-called Lengenbach dolomite of the Swiss Binntal. So far, at least 10 thallium minerals have been identified in Switzerland, several of which are unique to the location. The dolomite is quarried in the summer by a consortium of universities strictly for scientific purposes. (This is actually the only active mine in all of Switzerland.) The world production of thallium is of the order of 16 metric tons per year, the major producers being Belgium and several Japanese lead-zinc smelters. Total Japanese production is estimated at more than 9 tons. Owing to thallium’s toxicity (1 gram will easily kill an adult man), the European industry is literally scared to death of the element. Requests for information are almost never acknowledged, let alone answered. Trade companies that list the metal in their catalogue generally deny they have it in stock and refuse to so much as talk about it, for fear that Greenpeace activists will climb their stacks to protest involvement with poisonous substances.

A more rational attitude toward thallium is found in Japan, but not across the board. Mitsubishi Materials, which imports thallium residues from German zinc smelters, flatly denies it is in the thallium business, even though its production is known to be around 3 tons per year. Likewise, the Mitsui Kinzoku plant in Tokyo is positively off-limits for visitors. On the other hand, Sumitomo Metal Mining, another large thallium-producer in Japan, will bend over backwards to show you its old stock of thallium-rich residue which was used up in 1992. Nowadays, it is processing flue-dust from its Nikko zinc smelter, which yields 500-600 kg of thallium per year.

Each of the three Japanese companies that are not afraid to talk thallium uses a different process, although they all start with the dust from their sintering machine, where lead-zinc concentrate is roasted prior to reduction in the Imperial Smelting furnace. It is assumed that thallium chloride is evaporated during the sintering process, and is entrained with zinc, lead and cadmium oxide dust in the Cottrell precipitator and dissolved in sulphuric acid, yielding a sulfate solution.

Sumitomo Metal Mining in Kakogawa uses an ion exchange process at its Harima Works to recover nearly all of the thallium and half of the cadmium in the sulfate liquor. Both metals are leached off the cationic resin with a sodium chloride solution, with soda ash added to precipitate cadmium carbonate. Thallium remains in the solution, where it is oxidized to the trivalent state with potassium permanganate. It can now be precipitated as thallium hydroxide T1(OH)3. The latter is filtered, washed and dissolved in sulphuric acid. Sodium sulfite reduces the trivalent thallium back to the monovalent state. The remaining lead is precipitated out as sulfate; soda ash precipitates both zinc and cadmium carbonate.

Zinc powder is then added to the solution to cement out a thallium-sponge. The latter is melted under a layer of sodium hydroxide, which picks up impurities and prevents the oxidation of thallium. The raw metal is again dissolved in sulfuric acid and hydrogen peroxide, so as to precipitate selenium; soda ash is then added for precipitating still more zinc and cadmium carbonate. Finally, thallium is cemented out on zinc plates, from which the shiny thallium-sponge is scraped off periodically. It is melted into 1-kg ingots with a purity of 99.99%.

Mitsui Mining and Smelting (MMS) treats its Cottrell-dust sulfate liquor with lime and soda ash in order to precipitate zinc hydroxide, camium carbonate and calcium fluoride. The heavy metals in the solution are precipitated as sulphides, including thallium. They are filtered off and redissolved in sulphuric acid, into which sulphur dioxide is bubbled. The solution is neutralized with sodium hydroxide in order to precipitate iron and lead hydroxides, and thallium is cemented out with zinc powder, with the resulting sponge containing 20% zinc. It is thoroughly washed in water and dissolved in hot nitric acid. The solution is neutralized with soda ash to precipitate zinc carbonate. Thallium nitrates crystallize out of the solution upon cooling; it is dried at 50C and sold directly, as MMS does not produce thallium metal.

Surprisingly, Nippon Mining & Metals Co. produces thallium by an electrochemical process at its large Saganoseki copper smelter near Oita on Kyushu Island. This is where the Cottrell-dust from its Nikko zinc smelter is leached with sulphuric acid. The lead sulfate precipitates while floculating out the thallium and is added to the nearly-thallium-free lead sulfate which is obtained as a byproduct of the copper smelter. This material is then reduced to crude lead with coke and is electrolytically refined to cathode-lead in a potassium fluorosilicate solution. Thallium accumulates in the electrolyte; as soon as a certain concentration is reached, 6 cubic metres are bled off the tankhouse in order to plate out the thallium electrolytically on stainless steel cathodes. The raw thallium sponge is then cast into anodes, which are electrolytically refined to 99.99% purity. Thallium sulfate is still legal as a rodent poison in Japan, but the nitrate is the major thallium product. It is used for manufacturing special grades of optical glass with an even higher index of refraction than lead-glass. This glass is needed for the compact optics and character-readers of copiers and fax machines. A minor outlet is the inert lead-silver-thallium anodes used in continuous, electrolytic, zinc-plating plants.

— The author is an expert on the nature and uses of thallium.