632 



UNITED STATES MINERAL RESOURCES 



linings in aluminum smelters is improved by the 

 addition of thallium. Thallium is also used to modify 

 the pore size of molecular sieves by partial cation 

 exchange. Thallium has extensive electronic appli- 

 cations, being used in the construction of photoelec- 

 tric cells and exposure meters to detect infrared 

 radiation, particularly in military communication 

 systems. It is used as an integral part of scintilla- 

 tion spectrometers, in thallium-activated sodium 

 iodide crystals, where it reduces background noise 

 in the detection of gamma radiation and in measur- 

 ing high-energy electrons for X-rays. The perform- 

 ance of rectifiers is also improved by utilization of 

 a selenium-thallium alloy in the cathodes. Thallium 

 is used in electrical resistors and superconductors, 

 in tungsten filaments and radiation sources in 

 X-ray fluorescence, and in the production of illumi- 

 nated phosphors. The inclusion of thallium in mer- 

 cury vapor lamps has increased their service life 

 and almost doubled their illumination output. It has 

 also been tested in lamps producing radiation in the 

 suntan range. The refractive index of optical glass 

 is increased by the inclusion of thallium. It is also 

 used with other metals and as an oxide to glaze semi- 

 conductors, capacitors, printed circuits, and other 

 electronic devices to protect them from atmospheric 

 deterioration and contamination. As a catalyst in 

 electroplating, thallium improves the adherence and 

 uniformity of gold coatings on semiconductor de- 

 vices. Technological advances have been made in 

 the use of thallium in alloys for low-temperature 

 themiometry. Thallium has also been utilized in 

 explosives as TIN3, in medicine to treat ringwoi-m, 

 and in agriculture. In experiments conducted in 

 Russia, Kolotova and Kharitonova (1970) found 

 germination and leaf growth were promoted in seeds 

 soaked with 0.001 percent TLSO, solution, resulting 

 in a 13-percent increase in corn yield and a 43- 

 percent increase in carrot yield. A solution of 0.05 

 percent is toxic. 



Current application and use for thallium are wide 

 but limited; U.S. domestic production can exceed 

 demand, and immediate reserves are substantial. 

 Thus, thallium is not a high-priority material and 

 no government-sponsored stockpiles exist. To some 

 extent, current use and demand for thallium are 

 limited by the need for additional comprehensive 

 research into its complete chemical and physical 

 properties and into its utilization potential. Lee 

 (1971) expects its future industrial use to be one 

 of a reaction intermediate in various syntheses, uti- 

 lizing to advantage the oxidizing power of T1+' 

 and the stability of many Tl^' derivatives. The 

 highly toxic nature of thalKum salts is a deterrent 



to its use, and adequate substitutes are available 

 for some of its present applications, particularly in 

 rodenticides and insecticides and in electronics. 

 Hence, the national and world economic importance 

 of thallium is relatively low. Possibly, however, new 

 technological advances in the use of thallium and 

 its compounds will change this situation. Further- 

 more, the variable abundance of thallium in specific 

 minerals may prove a valuable index to the economic 

 potential of mineralized areas for other commodities. 



World production of zinc ores containing thallium 

 is adequate for the current demand. It is doubtful, 

 however, that any substantial future or sudden need 

 for thallium could be met by increased processing of 

 residues and flue dusts from the smelters and (or) 

 by improving the recovery factor. This fact implies 

 that alternative sources of thallium must be estab- 

 lished soon. 



Present estimates of the world consumption of 

 thallium are as much as 15 short tons annually, of 

 which the U.S. requirements are given as 3.25 tons. 

 The United States now refines enough thallium to 

 meet domestic demand, although an estimated 50 

 percent is obtained from zinc ores imported from 

 Canada. U.S. imports of zinc ores containing thallium 

 are mostly from Canada ; lesser amounts are from 

 Mexico, Peru, Australia, and other countries. The 

 current shutdown of domestic smelters, however in 

 conjunction with expanded smelting capacity in 

 Canada, Mexico, Australia, and Japan, may soon 

 change the sufficiency of domestically refined 

 thallium metal to complete dependence on imported 

 sources. 



EXPLOITATION 



The first recorded use of thallium salts was in 

 medicine in 1896. In 1919 thallium salts were used 

 in the manufacture of photosensitive cells. In 1920 

 they were used as a rat poison in Germany, and in 

 1925 the principal worldwide use of thallium salts 

 became that of a rodent poison and insecticide in 

 the form of thallium sulfate, an extremely toxic, 

 odorless, and tasteless chemical. This practice was 

 curtailed in the United States by a 1965 Federal 

 ruling, because of the extreme toxicity of thallium 

 salts to humans. Lee (1971) stated that thallium 

 sulfate is a cumulative poison which may be ab- 

 sorbed by the skin. A human lethal dose is about 

 1.75 grams, death occurring in a few days through 

 respiratory failure after convulsions and delirium. 



The sparsity of characteristic thallium minerals 

 and its highly dispersed nature have prevented 

 thallium from being found in concentrations of suffi- 

 cient magnitude to be mined as the principal ore 



