Arkel

It was originally separated from zirconium by repeated recrystallization of the double ammonium or potassium fluorides by von Hevesey and Jantzen. Metallic hafnium was first prepared by van Arkel and deBoer by passing the vapor of the tetraiodide over a heated tungsten filament. Almost all hafnium metal now produced is made by reducing the tetrachloride with magnesium or with sodium (Kroll Process). 

Refining. KroU-process hafnium sponge and electrowon hafnium do not meet the performance requirements for the two principal uses of hafnium metal. Eurther purification is accompHshed by the van Arkel-de Boer, ie, iodide bar, process (18) and by electron beam melting. 

The iodide or van Arkel-de Boer process is a volatilization process involving transfer of an involatile metal as its volatile compound. It is used for the purification of titanium. The reaction of iodine gas with impure titanium metal at 175°C yields gaseous titanium iodide and leaves the impurities in the sohd residue. 

Titanium tetraiodide can be prepared by direct combination of the elements at 150—200°C it can be made by reaction of gaseous hydrogen iodide with a solution of titanium tetrachloride in a suitable solvent and it can be purified by vacuum sublimation at 200°C. In the van Arkel method for the preparation of pure titanium metal, the sublimed tetraiodide is decomposed on a tungsten or titanium filament held at ca 1300°C (152). There are frequent hterature references to its use as a catalyst, eg, for the production of ethylene glycol from acetylene (153). 

Refining of Vanadium. In addition to the purification methods described above, vanadium can be purified by any of three methods iodide refining (van Arkel-deBoer process), electrolytic refining in a fused salt, and electrotransport. 

High purity zirconium was first produced by van Arkel and de Boer in 1925. They vaporized zirconium tetraiodide [13986-26-0] into a bulb containing a hot tungsten filament which caused the tetraiodide to dissociate, depositing zirconium on the filament. 

Zirconium tetraiodide is the least thermally stable zirconium tetrahaUde. At 1400°C, it disproportionates to Zr metal and iodine vapor. This behavior is utilized in the van Arkel-de Boer process to refine zirconium. As with the tetrachloride and tetrabromide, the tetraiodide forms additional adducts with gaseous ammonia which, upon heating, decompose through several steps ending with zirconium nitride. 

Reduction. Reduction of haUdes usiag hydrogen—hydrocarbon mixtures is sometimes done ia the presence of a graphite carrier or usiag metals possessing high melting points, ie, the van Arkel gas deposition method (4). If a plasma gun is employed, finely powdered (< 1 //m) carbides are obtained... 

Arkel refining a sample of tire impure metal, for example zirconium, is heated to a temperature around 550 K in contact with low pressure iodine gas in a sealed system which has a heated mngsten filament in the centre. The filament temperature is normally about 1700K. At the source the iodides of zirconium and some of the impurities are formed and drese diffuse across the intervening space, where tire total pressure is maintained at 10 auiios, and are decomposed on the filament. The iodine then remrns to form fresh iodide at the source, and the transport continues. 

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... 

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... 

H. Boer and P. van Arkel, Automatic PNA (paraffin-naphthene-aromatic) analyzer for (heavy) naphtha , Chromatographia 4 300-308 (1971). 

In the chemical process industry molybdenum has found use as washers and bolts to patch glass-lined vessels used in sulphuric acid and acid environments where nascent hydrogen is produced. Molybdenum thermocouples and valves have also been used in sulphuric acid applications, and molybdenum alloys have been used as reactor linings in plant used for the production of n-butyl chloride by reactions involving hydrochloric and sulphuric acids at temperatures in excess of 170°C. Miscellaneous applications where molybdenum has been used include the liquid phase Zircex hydrochlorination process, the Van Arkel Iodide process for zirconium production and the Metal Hydrides process for the production of super-pure thorium from thorium iodide. 

In the iodide refining process described above, several conditions are implicit. Van Arkel has listed them as (i) the metals form volatile iodides (ii) the melting points of the metals are higher than the dissociation temperatures of the corresponding iodides (iii) the volatile iodides are formed at manageable temperatures (iv) the iodides easily decompose at elevated temperatures and (v) the vapor pressures of the metals are very low at the decomposition temperatures of the iodides. 

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