Tantalum chloride, anhydrous

Terbium has been isolated only in recent years with the development of ion-exchange techniques for separating the rare-earth elements. As with other rare earths, it can be produced by reducing the anhydrous chloride or fluoride with calcium metal in a tantalum crucible. Calcium and tantalum impurities can be removed by vacuum remelting. Other methods of isolation are possible. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

Another solvent extraction scheme uses the mixed anhydrous chlorides from a chlorination process as the feed (28). The chlorides, which are mostly of niobium, tantalum, and iron, are dissolved in an organic phase and are extracted with 12 Ai hydrochloric acid. The best separation occurs from a mixture of MIBK and diisobutyl ketone (DIBK). The tantalum transfers to the hydrochloric acid leaving the niobium and iron, the DIBK enhancing the separation factor in the organic phase. Niobium and iron are stripped with hot 14—20 wt % H2SO4 which is boiled to precipitate niobic acid, leaving the iron in solution. 

Another method of purifying niobium is by distillation of the anhydrous mixed chlorides (29). Niobium and tantalum pentachlorides boil within about 15°C of one another which makes control of the process difficult. Additionally, process materials must withstand the corrosion effects of the chloride. The system must be kept meticulously anhydrous and air-free to avoid plugging resulting from the formation of niobium oxide trichloride, NbOQ. Distillation has been used commercially in the past. 

Undoubtedly, the best method for the production of pure anhydrous lanthanide trihalides involves direct reaction of the elements. However, suitable reaction vessels, of molybdenum, tungsten, or tantalum, have to be employed silica containers result in oxohalides (27). Trichlorides have been produced by reacting metal with chlorine (28), methyl chloride (28), or hydrogen chloride (28-31). Of the tribromides, only that of scandium has been prepared by direct reaction with bromine (32). The triiodides have been prepared by reacting the metal with iodine (27, 29, 31, 33-41) or with ammonium iodide (42). 

From these observations it was concluded that the major products of the reduction of niobium(V) chloride with anhydrous pyridine were tetrachlorodi-(pyridine)niobium(IV) and l-(4-pyridyl)pyridinium dichloride. Oxidation-reduction titrations indicated that this reduction accounted for approximately 70% of the reaction products. In view of the rapid reaction of tantalum(V) halides with pyridine to form 1 to 1 adducts, it was assumed that the remaining 30% of niobium (V) which was not reduced was present in the reaction mixture as pentachloro(pyridine)niobium(V). On this basis the following over-all reaction is proposed ... 

Spedding (1952) has made La and Ce in kg quantities, and Pr and Nd in smaller amounts, by reducing the anhydrous chloride in a refractory-lined crucible with calcium the reactions are initiated with a little iodine. Gadolinium has been made similarly in a crucible of tantalum. It is noteworthy that Sm, Eu and Yb, which exhibit a reasonably stable +2 state, are reduced only as far as the dichlorides. Their removal is thus facilitated. 

CARBONYL CHLORIDE (75-44-5) Poisonous gas. Decomposes slowly with water, producing hydrochloric acid and carbon monoxide. Reacts violently with strong oxidizers, anhydrous ammonia, isopropanol, chemically active metals, silicon tetrahydride, sodium. Forms shock-sensitive material with potassium. Incompatible with rert-butylazidoformate, sodium azide. Attacks most metals in moist conditions however, it may not affect monel, tantalum, or glass-lined equipment. Note Sodium hydroxide or anhydrous ammonia has been used to neutralize this gas. 

Tantalum (V) chloride (30 g.) is added to 50-60 g. of anhydrous HF contained in a reactor similar to that described for the preparation of TiF (p. 250). The reflux condenser is charged with freezing mixture and the reaction mixture is boiled for several hours until the evolution of HCl ceases. The excess HF is then distilled off through a descending condenser. The reflux condenser is then replaced by a distillation head, and the TaFs is distilled off into a platinum crucible. It is stored in sealed copper or Iron containers. The yield is 65%, based on TaClg. 

More care has to be taken for the preparation of binary chlorides and bromides, RCI3 and RBr3. The anhydrous salts are moisture-sensitive and they are obtained as (hepta- or hexa-) hydrates upon crystallization from hydrochloric or hydrobromic acid solutions. Simple dehydration of the hydrates in an HCl or HBr gas flow, respectively, appears to be possible (Seifert et al. 1985) but the less skfllliil chemist produces halide products more or less contaminated with oxyhahde, ROX. Sublimation of the raw product in an all-tantalum apparatus at higher temperatures (650 C to 950 C) and low pressures (10 bar) produces pure trihalides (with the exception of the rare-earth elements europium and 3dterbium where mixed-valent (+2/+3) or even dihalides are obtained under these conditions). 

Piping, valves, and other equipment used in direct contact with anhydrous hydrogen chloride should be of stainless steel or of cast or mild steel. Carbon steel may be employed in some components, but only if their temperature is controlled to remain below about 265 F (129°C). In the presence of moisture, however, hydrogen chloride will corrode most metals other than silver, platinum, or tantalum. 

Atkinson, Steigman, and Hiskey have found distillation of anhydrous chlorides to be an effective method for separating tin from niobium and tantalums... 

Tantalum is resistant to anhydrous hydrogen chloride gas to at least 250°C. 

Other inorganic acids, such as sulfamic, methylsulfuric, or hydrobromic, do not corrode tantalum, nor do the anhydrous acid gases, hydrogen sulfide, phosphorous chlorides, SO2, SOCI2, and chlorine oxides. ... 

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