Van Arkel process

At higher pressures, tire total pressure Pi must be included in the equations, and at low pressures, such as are used in die van Arkel process, the atomic species cannot be ignored. 

The Kroll process for tire reduction of tire halides of refractory metals by magnesium is exemplified by the reduction of zirconium tetrachloride to produce an impure metal which is subsequently refined with the van Arkel process to produce metal of nuclear reactor grade. After the chlorination of the impure oxide in the presence of carbon... 

The Van Arkel process can also be used to prepare actinide metals if the starting compound reacts easily with the transporting agent (I2). The thorium and protactinium carbides react with I2 to give volatile iodides above 350°C these are unstable above 1200°C and decompose into the actinide metals and iodine. Attempts to prepare other actinides, such as U and Pu, through the process were not successful, because from Th to Pu along the actinide series, the vapour pressure of the iodide decreases and the thermal stability increases. 

Preparation. Commercially it is prepared mainly as ferro-vanadium alloy. It can be obtained by aluminothermic reduction of the oxide. Pure V may be obtained through the de Boer-Van Arkel process by iodide thermal decomposition (vanadium iodide is decomposed on an electrical heated W wire). 

Therefore, and to avoid possible reactions with the quartz wall, it was attempted to combine synthesis and crystal growth of actinide pnictides in a modified van Arkel process Actinide metal or carbide - the latter obtained by carboreduction of the oxide - are heated in the presence of the pnictogen and of the transporting agent at the... 

Table 2. S)Tithesis temperature of actinide pnictides by the Van Arkel process ...

The Van Arkel process can also be used to make metal nitrides. Because of the tendency of NH3 to decompose at the temperatures needed for nitride formation (1300-1900 K), the gaseous reactant is a mixture of H2 and N2. The nitride forms as a layer around the tungsten wire. 

Pure zirconium is obtained using the two-step Van Arkel process. In the first step, impure zirconium and iodine are heated to produce zirconium iodide (Zr ). In the second step, Zrl4 is decomposed to produce pure zirconium. 

One method of refining of metals, called the van Arkel process, involves the formation of a volatile halide by the direct action between the metal and a halogen followed by purification of the volatile halide (by fractional distillation, for example) and its subsequent decomposition to the metal and halogen at a higher temperature. The sequence of reactions involved in the van Arkel process is... 

Purification via the volatile carbonyl compounds is similar to the van Arkel process, though used widely only for nickel ... 

Zircon silicate is the most important source of hafnium. Ion-exchange and solvent-extraction techniques have supplanted fractional crystallization and distillation as the preferred methods of separating hafnium from zirconium. The metal itself is prepared by magnesium reduction of hafnium tetrachloride (the Kroll process), and by the thermal decomposition of tetraiodide (de Boer-van Arkel process). The annual world production of hafnium metal was about 40 tons at the end of the 1980s (Soloveichik... 

The specific heat of thorium monocarbide was measured between 2 and 300 K. A sintered sample was prepared from thorium obtained by the van Arkel process and reported to contain less than 200 ppm impurities. According to chemical analyses (no details given), the overall composition of the carbide sample corresponded to ThC0965N0.015O0.020 The lattice parameter was reported to be a = (5.3444 + 0.0002) A. This sample was measured over the whole temperature range 2 to 300 K. 

Since the rare metal iodides are produced most easily by direct synthesis from the appropriate metal and iodine at relatively low temperature, the Van Arkel process has usually been employed as a purification process for the preparation of small quantities of exceptionally high-grade metal. A relatively low-grade metal, e.g. made by a calcium reduction route, is converted to iodide and then decomposed again to give the pure metal. 

The reaction mass is leached with water and dilute acid, leaving thorium metal powder. Very pure thorium metal can be prepared by the van Arkel process involving the thermal decomposition of Thl4. 

The production of uranium metal usually involves the reduction of UF4 with magnesium at 700°C. The metal may be refined by molten-salt electrolysis followed by zone melting. Because of the low melting point of uranium, the van Arkel process is not as feasible as for thorium and protactinium. 

Much modern research on the metallic state requires very pure metals. Depending on the nature of the impurities, the actinide metals can be purified by volatilization of the impurities in a very high vacuum, by volatilization of the metal itself to form films of very pure metal, or by electrodeposition from molten-salt baths. Very pure metals can be obtained by the Van Arkel process, which consists of converting the crude metal to the volatile iodide by reaction with elemental iodine at an elevated temperature, and decomposing the gaseous metal iodide on a hot filament [28-30]. This process produces exceptionally pure metals, which have been used for such demanding purposes as superconductivity measurements that require metals of the highest purity. 

Th Van Arkel processing of carbide multigram Van Arkel processing of metal... 

The remelting process virtually does not change these values. Titanium with a significantly lower oxygen content can be obtained by the van Arkel-process (iodide-titanium). This type of titanium generally contains oxygen concentrations of 10 to 100 Mg/g only. 

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