Van Arkel-de Boer method

The nitrides can be prepared by heating a metal powder in an N2 or NH3 atmosphere to temperatures above 1100°C. The carbides form upon heating mixtures of the metal powders with carbon to temperatures of about 2200 °C. Both the nitrides and carbides can also be made by chemical transport reactions by the van Arkel-de Boer method if the metal deposition takes place in an atmosphere of N2 or a hydrocarbon. Their remarkable properties are ... 

Very highly pure titanium metal can be prepared in small amounts by decomposition of pure titanium tetraiodide, (Tih) vapor on a hot wire under low pressure (Van Arkel-de Boer method). 

Extremely pure Ti can be made on a small scale by the van Arkel-de Boer method (also used for other metals) in which pure Til4 vapor is decomposed on a hot wire at low pressure. 

Reduction to elemental titanium was first commercialized in the 1950s. A successful laboratory method is reduction of the dioxide with excess calcium hydride in a molybdenum boat. The reaction is carried out at 900 °C in a vacuum or an atmosphere of hydrogen (equation 1). Extremely pure titanium can be prepared on a laboratory scale by the van Arkel-de Boer method in this method, pure TiLi is vaporized and decomposed on a hot wire in a vacuum. 

Zirconium, too, is produced commercially by the Kroll process, but the van Arkel-de Boer process is also useful when it is especially important to remove all oxygen and nitrogen. In this latter method the crude zirconium is heated in an evacuated vessel with a little iodine, to a temperature of about 200° C when Zrl4 volatilizes. A tungsten or zirconium filament is simultaneously electrically heated to about 1300°C. This decomposes the Zrl4 and pure zirconium is deposited on the filament. As the deposit grows the current is steadily increased so as to maintain the temperatures. The method is applicable to many metals by judicious adjustment of the temperatures. Zirconium has a high corrosion resistance and in certain chemical plants is preferred to alternatives such as stainless... 

The method of choice for the preparation of Pa metal is a somewhat modified van Arkel-De Boer process, which uses protactinium carbide (Section II,C) as the starting material. The carbide and iodine are heated to form protactinium iodide, which is thermally dissociated on a hot filament 12-15). An elegant variation is to replace the filament with an inductively heated W or Pa sphere 109). A photograph of a 1.4-g sample of Pa metal deposited on a radiofrequency-heated W sphere is shown in Fig. 6. From the analytical data presented in Table V, the impurities present before and after application of this modified iodide transport process (Sections II,D and III,C) can be compared. 

A general method for preparation of all An metals is by reduction of AnF3 or AnF4 with vapors of Li, Mg, Ca, or Ba at 1100 to 1400°C the chlorides or oxides are sometimes used. There are some special methods such as the preparation of Th or Pa from their tetraiodides by the van Arkel-de Boer process, or the following reaction for the relatively volatile americium ... 

The highest purity (0.03% C and 0.006% N) is attained via the elegant recovery process of van Arkel and de Boer (method V below). This is based on the thermal decomposition of titanium iodide at 1100-1500 °C. 

We speak of ionic bond and atomic bond (also frequently called covalent bond) and not of heteropolar in contrast to homopolar (or homeo-) bond, since the last term has only a meaning in characterizing a method of approximating the real wave functions, followed by Kossel (van Arkel and de Boer) and by Heitler and London. With unequal atoms even the homopolar approximation leads to a bond which possesses an electric moment on account of the difference in electronegativity, and thus is (hetero) polar. 

Ingots are made in a water-cooled copper crucible, by melting the metal with an arc struck between it and a compressed titanium sponge cathode, the operation being conducted in an atmosphere of argon. Very pure titanium can be made by the method of Van Arkel and De Boer in which Til4 vapour is decomposed on a hot wire. 

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... 

Other special methods for growing metal single crystals are recrystallization with alternate mechanical deformation and annealing (systematically and successfully used till now with Al, Mg, and Fe) as well as a crystallization process in which a volatile metal compound decomposes thermally on a high-temperature filament and the metal deposits on it. This procedure may be used also for the preparation of some metals and very pure simple metal compounds. Some of these cannot be obtained as pure by any other method (Ti, Zr, Hf, Nb, Ta, etc.) (Koref Van Arkel Agte Burgers De Boer 0.2]). 

In 1925 van Arkel and de Boer [19.2] published their first report about their famous method of producing metals in pure form by thermal decomposition of the metal iodide. In the zirconium case, impure zirconium metal is placed in an evacuated container and treated at 300°C with iodine vapor to form Zrl,. This tetraiodide is diffused to an electrically heated filament of tungsten or zirconium wire. At 1300-1400°C the zirconium iodide is dissociated to iodine gas and very ductile zirconium metal. From 1925 to 1945 the iodide process was the method used for obtaining pure, ductile zirconium metal. 

Zirconium, atomic number 40 and atomic weight 91.22, was identified by the German chemist, Klaproth, in 1789. However, the metal itself was not isolated imtil 1824, when Berzelius produced a brittle, impure metal powder by the reduction of potassium fluorozirconate with potassium. One himdred years later, van Arkel and de Boer developed the iodide decomposition process to make a pure, ductile metal in Einhoven, Holland. The "iodide crystal bar" process continues to be used today as a method of purifying titanium, zirconium, and hafnium, even though it is slow and expensive. 

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