Iodide decomposition process

Magnesium chloride and excess magnesium are removed by distillation at reduced pressure. Pure zirconium may be prepared by several methods that include iodide decomposition process, zone refining, and electron beam melting. Also, Zr metal may be electrorefined in a molten salt bath of potassium zirconium fluoride, K2ZrFe... 

The iodide decomposition process for any rare metal is always more expensive than other methods of production, because of the small scale... 

The demand for hafnium metal is small. Figure 9.7 shows how it can be obtained in a high state of purity as a by-product from the manufacture of zirconium. The conversion of the pure by-product solution to oxide is via sulphite as in the case of zirconium, and similarly several alternative precipitants would each be satisfactory. The Van Arkel iodide decomposition process has been shown, in its cheapest form, i.e. based upon a carbide feed. Either of the two metal-producing stages shown for zirconium are equally applicable. 

Fig. 9.7. Extraction of hafnium (zirconium by-product, oxide, carbide, iodide decomposition process).

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. 

CVD developed slowly in the next fifty years and was limited mostly to extraction and pyrometallurgy for the production of high-purity refractory metals such as tantalum, titanium, and zirconium. Several classical CVD reactionswere developedatthattimeincludingthecarbonyl cycle (the Mond process), the iodide decomposition (the de Boer-Van Arkelprocess)andthemagnesium-reduction reaction (the Kroll process). 

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. 

A small scale sulphur iodine process loop made of Pyrex glass was built and operated. The sulphuric acid section and Bunsen reaction section was operated successfully in 2006. In 2008, hydrogen iodide decomposition aided by electro-dialysis (EED) (Hong, 2007) was demonstrated to produce 3.5 litres per... 

In (92), there is a four-center exchange of a bonds. In principle, six-center trimolecular reactions involving three diatomic molecules could be in this group, providing only o bonds are permitted. Although several elementary reactions of the type (92) have been studied (Glasstone et al., 1941), there appear to be few gas-phase processes which follow this indicated mechanism. The once-classical example of a bimolecular reaction, i.e. hydrogen iodide decomposition (93), is actually a multistep... 

Rare metals have been produced in the highest state of purity by the thermal decomposition of their iodides, a process first applied to titanium by Van ArkeU> in 1925. Although the products of this process are in a compact form and are generally superior in purity to those from any other rare metal production process, it cannot unfortunately be operated on a large production scale. 

Attention was focused on the crystal bar zirconium made by the iodide route in addition to the Kroll magnesium reduction process. The iodide decomposition route gives a superior product, but at a higher cost. 

Past studies have focused on not only the process evaluation using a commercial computer code, but also the screening test of the component structural materials. Experiments to develop the catalysts for sulfur trioxide and hydrogen iodide decompositions were carried out successfully, and their manufacturing technologies were established. The experimental feasibility test of a 3.5 NL H2/h-scale SI test facility under atmospheric operation conditions has been performed in early 2008. As a result, we secured the continuous operation hydrogen production data for 6 h. 

After the primary step in a photochemical reaction, the secondary processes may be quite complicated, e.g. when atoms and free radicals are fcrnied. Consequently the quantum yield, i.e. the number of molecules which are caused to react for a single quantum of light absorbed, is only exceptionally equal to exactly unity. E.g. the quantum yield of the decomposition of methyl iodide by u.v. light is only about 10" because some of the free radicals formed re-combine. The quantum yield of the reaction of H2 -f- CI2 is 10 to 10 (and the mixture may explode) because this is a chain reaction. 

Potassium iodate [7758-05-6] KIO, mol wt 214.02, 59.30% I, forms white, odorless crystals or a crystalline powder. It has a density 3.98 g/mL and mp of 560°C with partial decomposition. Potassium iodate is rapidly formed when potassium iodide is fused with potassium chlorate, bromate, or perchlorate. The solubihty in water is 9.16 g/100 g H2O at 25°C and 32.2 g/100 g H2O at 100°C. KIO is extensively used as an oxidizing agent in analytical chemistry and as amaturing agent and dough conditioner (see Bakery processes and leavening agents). 

The reaction of higher alkyl chlorides with tin metal at 235°C is not practical because of the thermal decomposition which occurs before the products can be removed from the reaction zone. The reaction temperature necessary for the formation of dimethyl tin dichloride can be lowered considerably by the use of certain catalysts. Quaternary ammonium and phosphonium iodides allow the reaction to proceed in good yield at 150—160°C (109). An improvement in the process involves the use of amine—stannic chloride complexes or mixtures of stannic chloride and a quaternary ammonium or phosphonium compound (110). Use of these catalysts is claimed to yield dimethyl tin dichloride containing less than 0.1 wt % trimethyl tin chloride. Catalyzed direct reactions under pressure are used commercially to manufacture dimethyl tin dichloride. 

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