Tellurium Solution

Stable Te sols are obtained by the reduction of telluric acid with hydrazinlum hydroxide. 

Since HgTe is a poisonous gas with an impleasant odor and, when inhaled in large quantities, greatly irritates the bronchial 

Electrolytic preparation of HgTe is generally preferred to the method of acid decomposition of tellurides (see II) because the yield is higher. 

If a very pure product Is required, the traces of inert gas are removed by repeated melting and solidification in vacuum by fractionation or sublimation in high vacuum a considerable amount of the first and last cuts is rejected. 

Hydrogen telluride is stored in the dark either as a solid at low temperature or in the vapor state in torch-sealed glass flasks. Mercury may not be used as a sealing liquid since it is attacked even by carefully dried HgTe. 

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To produce commercial (99.5%) tellurium, tellurium dioxide is dissolved in hydrochloric acid. The tellurium solution is saturated with sulfur dioxide gas to yield commercial tellurium powder, which is washed, dried, and melted. 

Tellurium solution, 1 mg/ml. Dissolve 0.10 g of tellurium in 2 ml of cone. HNO3, and evaporate the solution to dryness. Dissolve the residue in 10 ml of cone. HCl, and dilute the solution with water to 100 ml. 

Standard tellurium solution 1 mg/ml. Place in a beaker 1.0000 g of powdered tellurium. Add 50 ml of cone. HCl, and introduce cone. HNO3 in small portions with heating until the tellurium dissolves. Then add 100 ml of water, and boil the solution for 5 min. Add 20 ml of cone. HCl, and dilute the solution to volume with water in a I-litre standard flask. 

J.K. Fink, J.J. Heilberger, R. Kumar, and R.A. Blomquist. 1977. Interaction of refractories and reactor materials with sodium. Nuclear Technol., 35 656-662. W.R. Kanne. 1973. Corrosion of metals by liquid bismuth-tellurium solutions. 

Since the hydrogen-element bond energy decreases from sulphur to tellurium they are stronger acids than hydrogen sulphide in aqueous solution but are still classified as weak acids—similar change in acid strength is observed for Group Vll hydrides. 

These closely resemble the corresponding sulphides. The alkali metal selenides and tellurides are colourless solids, and are powerful reducing agents in aqueous solution, being oxidised by air to the elements selenium and tellurium respeetively (cf. the reducing power of the hydrides). 

Crystalline tellurium has a silvery-white appearance, and when pure exhibits a metallic luster. It is brittle and easily pulverized. Amorphous tellurium is found by precipitating tellurium from a solution of telluric or tellurous acid. Whether this form is truly amorphous, or made of minute crystals, is open to question. Tellurium is a p-type semiconductor, and shows greater conductivity in certain directions, depending on alignment of the atoms. 

The lead buUion, ready to be shipped to the refinery, contains in solution impurities such as silver, gold, copper, antimony, arsenic, bismuth, nickel, 2inc, cadmium, tin, tellurium, and platinum metals. 

Wet Process. The sodium arsenate and stannate slag are treated by a leach and precipitation process to produce calcium arsenate, calcium stannate, and a sodium hydroxide solution for recycle. The sodium antimonate filtercake containing selenium, tellurium, and indium is treated in a special metals refinery to recover indium and tellurium. 

The roasted pellets or extmdes are ground and leached in water. The hexavalent selenium dissolves as sodium selenate [13410-01 -0] Na2Se04. Sodium teUurate, being highly insoluble in the now very strongly alkaline solution, remains in the residue. The separation between selenium and tellurium is readily achieved, provided all tellurium is oxidized to the hexavalent state. 

Losses of selenium and tellurium from the solution are negligible, provided the reactor is equipped with a reflux condenser. The wet chlorination is easily controlled. The reaction is rapid, allowing fast turnover of the precious metals in the slimes and yielding all the selenium and tellurium in soluble form. 

Tellurium and many other impurities remain undissolved. The solution is filtered and cooled to reverse the reaction and to deposit soHd selenium. Oeselenized liquor is recycled to the dissolution step. 

The SeBr which forms is distilled from the solution leaving the interfering elements behind. The only other metallic elements that can also distill over by this procedure are arsenic, antimony, tellurium (pardy), and germanium. 

A widely used procedure for determining trace amounts of selenium involves separating selenium from solution by reduction to elemental selenium using tellurium (as a carrier) and hypophosphorous acid as reductant. The precipitated selenium, together with the carrier, are collected by filtration and the filtered soflds examined directly in the wavelength-dispersive x-ray fluorescence spectrometer (70). Numerous spectrophotometric and other methods have been pubHshed for the deterruination of trace amounts of selenium (71—88). 

Tellurium is diamagnetic below its melting point. Its intrinsic electrical resistivity at room temperature is about 0.25 ohmcm, when the current is parallel to the i -axis, and decreases with increasing temperature and pressure. The element forms a continuous range of isomorphous solutions with selenium, consisting, in the soHd state, of chains of randomly alternating Se and Te atoms. 

Most commercial tellurium is recovered from electrolytic copper refinery slimes (8—16). The tellurium content of slimes can range from a trace up to 10% (see Seleniumand selenium compounds). Most of the original processes developed for the recovery of metals of value from slimes resulted in tellurium being the last and least important metal produced. In recent years, many refineries have changed their slimes treatment processes for faster recovery of precious metals (17,18). The new processes have in common the need to remove the copper in slimes by autoclave leaching to low levels (<1%). In addition, this autoclave pretreatment dissolves a large amount of the tellurium, and the separation of the tellurium and copper from the solution which then follows places tellurium recovery at the beginning of the slimes treatment process. 

Tellurium is recovered from solution by cementation with copper at elevated (>90° C) temperature. 

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. 

Commercial Products. Tellurium dioxide [7446-07-3] Te02 (79.9% Te theoretically), is made by heating an aqueous suspension of teUurous acid. The acid is purified, if necessary, by redissolving in caustic soda solution and neutralizing with sulfuric acid. 

Tellurium diethyldithiocarbamate [20941 -65-5] [(C2H3)2NC(S)S]4Te, is made by the reaction of diethylarnine, carbon disulfide, and tellurium dioxide in an alcohoHc solution. 

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. 

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. 

Production and Economic Aspects. Thallium is obtained commercially as a by-product in the roasting of zinc, copper, and lead ores. The thallium is collected in the flue dust in the form of oxide or sulfate with other by-product metals, eg, cadmium, indium, germanium, selenium, and tellurium. The thallium content of the flue dust is low and further enrichment steps are required. If the thallium compounds present are soluble, ie, as oxides or sulfates, direct leaching with water or dilute acid separates them from the other insoluble metals. Otherwise, the thallium compound is solubilized with oxidizing roasts, by sulfatization, or by treatment with alkaU. The thallium precipitates from these solutions as thaUium(I) chloride [7791 -12-0]. Electrolysis of the thaUium(I) sulfate [7446-18-6] solution affords thallium metal in high purity (5,6). The sulfate solution must be acidified with sulfuric acid to avoid cathodic separation of zinc and anodic deposition of thaUium(III) oxide [1314-32-5]. The metal deposited on the cathode is removed, kneaded into lumps, and dried. It is then compressed into blocks, melted under hydrogen, and cast into sticks. 

Zinc smelters use x-ray fluorescence spectrometry to analyze for zinc and many other metals in concentrates, calcines, residues, and trace elements precipitated from solution, such as arsenic, antimony, selenium, tellurium, and tin. X-ray analysis is also used for quaUtative and semiquantitative analysis. Electrolytic smelters rely heavily on AAS and polarography for solutions, residues, and environmental samples. 

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