TeUurium

Tellurium is not an essential element, and teUurium compounds are in general more toxic than their selenium counterparts. MetaUic teUurium is known to have a teratogenic effect in rats, though no studies have been done on the toxicity of teUurium donor compounds (35). 

Halogen fluorides react with sulfur, selenium, teUurium, phosphoms, sHicon, and boron at room temperature to form the corresponding fluorides. Slight warming may be needed to initiate the reactions (4) which, once started, proceed rapidly to completion accompanied by heat and light. The lack of protective film formation aHows complete reaction. 

Cold-roUed alloys of lead with 0.06 wt % teUurium often attain ultimate tensile strengths of 25—30 MPa (3625—5350 psi). High mechanical strength, excellent creep resistance, and low levels of alloying elements have made lead—teUurium aUoys the primary material for nuclear shielding for smaU reactors such as those aboard submarines. The aUoy is self-supporting and does not generate secondary radiation. 

Wrought or extmded lead—teUurium (0.035—0.10 wt %) aUoys produce extremely fine grains. The binary aUoy is, however, susceptible to recrysta11i2ation. The addition of copper or sUver reduces grain growth and retains the fine grain si2e. Because teUurium is a poison for sealed lead—acid batteries, the teUurium content of lead and lead aUoys used for such purposes is usuaUy restricted to less than 1 ppm. 

Dilution with water reverses the reaction, and heating the solution Hberates sulfur dioxide. Upon being added to a solution of teUurides, teUurium forms colored polyteUurides. Unlike selenium, teUurium is not soluble in aqueous sodium sulfite. This difference offers a method of separating the two elements. Like selenium, teUurium is soluble in hot alkaline solutions except for ammonium hydroxide solutions. Cooling reverses the reaction. Because teUurium forms solutions of anions, Te , and cations, Te" ", teUurium films can be deposited on inert electrodes of either sign. 

Although this procedure yields tellurium as the same compound found in the original feedstock, the copper teUuride is recovered in a comparatively pure state which is readily amenable to processing to commercial elemental tellurium or tellurium dioxide. The upgraded copper teUuride is leached with caustic soda and air to produce a sodium teUurite solution. The sodium teUurite solution can be used as the feed for the production of commercial grade teUurium metal or teUurium dioxide. 

If the final product desired is teUurium metal, excess free caustic soda is required in the sodium teUurite solution. The solution is electrolyzed in a ceU using stainless steel anodes to produce teUurium metal (20). This technology is used at the CCR Division of Noranda MetaUurgy Inc., Canada, and at Pacific Rate Metals Industties Inc., the Philippines. Typical electrolysis conditions ate given in Table 2. 

Alternatively, if teUurium dioxide is the product desired, the sodium teUurite solution can be neutralized in a controUed fashion with sulfuric acid. As the pH is lowered, precipitates containing impurities such as lead and sUica that form ate filtered off. At pH 5.6 the solubUity of teUurous acid teaches a minimum and essentiaUy aU of the teUurium precipitates (>98%). After filtration and drying, commercial teUurium dioxide is obtained. A diagram for the process of deteUurizing of slimes and recovering teUurium products is shown in Figure 1. 

Metal teUurides for semiconductors are made by direct melting, melting with excess teUurium and volatilizing the excess under reduced pressure, passing teUurium vapor in an inert gas carrier over a heated metal, and high temperature reduction of oxy compounds with hydrogen or ammonia. 

There are no official U.S. specifications for teUurium and producers pubUsh thek own standards. TypicaUy the producer specifies the weight and shape of the pieces, a screen analysis of powders, and a maximum content of certain impurities. 

Comprehensive accounts of the analytical chemistry of teUurium have been pubUshed (5,26—30). The analytical methods for the determination of teUurium are to a considerable extent influenced by the element s resemblance, in many of its properties and in its limited terrestrial abundance, to selenium. 

Although gravimetric methods have been used traditionally for the determination of large amounts of tellurium, more accurate and convenient volumetric methods are favored. The oxidation of teUurium(IV) by ceric sulfate in hot sulfuric acid solution in the presence of chromic ion as catalyst affords a convenient volumetric method for the determination of tellurium (32). Selenium(IV) does not interfere if the sulfuric acid is less than 2 N in concentration. Excess ceric sulfate is added, the excess being titrated with ferrous ammonium sulfate using o-phenanthroline ferrous—sulfate as indicator. The ceric sulfate method is best appHed in tellurium-rich materials such as refined tellurium or tellurium compounds. 

The oxidation of teUurium(IV) by permanganate as an analytical method has been studied in some detail (26). The sample is dissolved in 1 1 nitric-sulfuric acid mixture addition of potassium bisulfate and repeated fuming with sulfuric acid volatilises the selenium. The tellurite is dissolved in 10 vol % sulfuric acid, followed by threefold dilution with water and titration with potassium permanganate ... 

Elemental tellurium and the stable teUurides of heavy nonferrous metals are relatively inert and do not represent a significant health hazard (43—47). Other, more reactive teUurides, including soluble and volatile teUurium compounds such as hydrogen teUuride [7783-09-7] teUurium hexafluoride [7783-80-4] and alkyl teUurides, should be handled with caution. Some of these materials can enter the body by absorption through the skin or by inhalation and ingestion of dust or fumes. No serious consequences or deaths have been reported in workers exposed to teUurium and its compounds in industry (48). 

The unusual physical complaints and findings in workers overexposed to teUurium include somnolence, anorexia, nausea, perspiration, a metallic taste in the mouth and garlic-like odor on the breath (48). The unpleasant odor, attributed to the formation of dimethyl teUuride, has not been associated with any adverse health symptoms. Tellurium compounds and metaboUc products have been identified in exhaled breath, sweat, urine, and feces. Elimination is relatively slow and continuous exposure may result in some accumulation. No definite pathological effects have been observed beyond the physical complaints outlined. Unlike selenium, teUurium has not been proved to be an essential biological trace element. 

The threshold limit value (TLV) set by the American Conference of Industrial Hygienists (ACGIH) for teUurium and its compounds is 0.1 mg/m which is about ten times the amount which has been known to produce the adverse garUc odor (45,50). The ACGIH TLV for teUurium hexafluoride is 0.1 mg/m or 0.02 ppm of air. Likewise, the U.S. Occupational Safety and Health Administration (OSHA) has estabUshed its permissible exposure limit (PEL) for teUurium and its compounds at 0.1 mg/m the PEL for teUurium hexafluoride is 0.2 mg/m or 0.02 ppm of air (50). 

Tellurium forms inorganic compounds very similar to those of sulfur and selenium. The most important teUurium compounds are the teUurides, haUdes, oxides, and oxyacids (5). Techniques and methods of preparation are given in the Uterature (51,52). The chemical relations of teUurium compounds are iUustrated in Figure 2 (53). 

Tellurium Sulfide. In the hquid state, teUurium is completely miscible with sulfur. The Te—S phase diagram shows a eutectic at 105—110°C when the sulfur content is 98—99 atom % (94—98 wt %). TeUurium—sulfur aUoys have semiconductor properties (see Semiconductors). Bands attributed to teUurium sulfide [16608-21 -2] TeS, molecules have been observed. 

Tellurium Selenides. TeUurium selenides or selenium teUurides are unknown. The molten elements are miscible in aU proportions. The mixtures are not simple soUd solutions but have a complex stmcture. Like the sulfides, the selenides exhibit semiconductor properties. 

Carbon Sulfotelluride. Carbon sulfoteUuride [10340-06-4] CSTe, exists as a yeUow-red Hquid having a garlic-like odor. It is decomposed by light, even at —50°C, to carbon disulfide, carbon, and teUurium. 

Carbonyl Telluride. Littie is known about carbonyl teUuride [65312-92-7] COTe. It is formed in poor yield by passing carbon monoxide over teUurium at a high temperature. It is less stable than the selenide. 

Tellurium Nitride. TeUurium nitride [12164-01 -0] Te N is an unstable, citron-yeUow soHd that detonates easUy when heated or stmck, but it can be kept under dry chloroform. It is said to explode on contact with water, possibly because of the heat of wetting. 

Tellurium Tetrafluoride. TeUurium tetrafluoride [15192-26-4] TeF, forms white, hygroscopic needles melting at 129.6°C. It decomposes at 194°C to TeFg and is readily hydrolyzed. Tellurium tetrafluoride attacks glass, sUica, and copper at 200°C, but it does not attack platinum below 300°C. 

Tellurium Decafluoride. TeUurium decafluoride [53214-07-6] Te2F2Q, is a stable, volatile, colorless Hquid, melting at —33.7°C and boiling at... 

Tellurium Hexafluoride. TeUurium hexafluoride, TeF melts at —38° C and subHmes at —39° C, forming a colorless gas. It hydrolyzes slowly to orthoteUuric acid and is reduced by teUurium to TeF. ... 

Tellurium Dichloride. TeUurium dichloride [10025-71-5] TeCl2, is a black hygroscopic soHd, melting at 208°C to a black Hquid it boUs at... 

C to a bright red vapor. The soHd is stable when pure. It disproportionates in organic solvents and is decomposed by acids and alkaHes. It is hydrolyzed to H2Te02, Te, and HCl, and decomposed by HCl to Te and H2TeClg [17112-43-5]. Air oxidizes teUurium dichloride to Te02 and HCl. 

Basic teUurium nitrate, Te02, is made by dissolving Te in HNO3. Thermal decomposition begins at 190°C and is complete at 300°C. 

Basic teUurium sulfate 2 Te02 S03 [12068-84-8], 2 Te02 Se03 selenate and teUurate, 2 Te02 Te03, are known. The sulfate is made by the slow evaporation of a Te02 solution in H2SO4. It is stable up to 440—500°C and is hydrolyzed slowly by cold, and rapidly by hot water. 

Copper—lead—teUurium aUoys have high wear resistance in sliding contacts. In copper—2inc aUoys, the benefits of teUurium decrease with increasing 2inc content and almost disappear when the 2inc content exceeds 35%. 

Lead Alloys. A teUurium—lead aUoy containing 0.02—0.1% teUurium, with or without antimony, was introduced in 1934 (81) as teUurium lead or Teledium. This aUoy has higher recrysta11i2ation temperatures and corrosion resistance and takes a significantly longer time to soften at 25°C after cold work. 

Adding teUurium to lead and to lead aUoyed with sUver and arsenic improves the creep strength and the charging capacity of storage battery electrodes (see Batteries). These aUoys have also been suggested for use as insoluble anodes in electrowinning. 

Other Metals. Tellurium has been added to copper-base, lead-base, and tin-base bearing aUoys. In babbit-type aUoys, teUurium controls the stmcture and improves uniformity and fatigue resistance by restraining the tendency to segregation (see Bearing Materials). 

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