Gaseous zirconium iodides

The enthalpy of sublimation of solid zirconium tetraiodide to gaseous ZrG was given in Section V.4.3.3.1, as was the enthalpy of formation for Zrl4(er). These values are therefore used to determine the selected enthalpy of formation of Zrl4(g)  

This value is in excellent agreement with the value selected by [97VIS/COR] -(356.2 4.1) kJ-moP whereas [78CUB/LAU] calculated a value of - (355.2 6.7) kJ-mol. Both [97VIS/COR] and the present review essentially used the same measured data as [78CUB/LAU], the differences in the selected enthalpy values arising from the auxiliary data used in the calculation of the enthalpy of formation of Zrl4(cr). 

Kleinschmidt et al. [78KLE/CUB] studied the thermodynamics of the gaseous zirconium iodides at elevated temperatures using Knudsen cell mass spectrometry. The formation of Zrl3(g) over the range 1100 to 1400 K was studied using two independent reaction mechanisms  

Both second and third law values of the enthalpy of reaction were determined using two different detection techniques, electrometer analogue and ion counting for Reaction (V.57) whereas only the former was used for Reaction (V.58). For Reaction (V.57), the third law reaction data were significantly more consistent than the second law values and, as such, were selected by [78KLE/CUB]. The values selected for the enthalpy of reaction were (136.4 4.6) and (332.6 4.6) kJ-moP for Reactions (V.57) and (V.58), respectively. The enthalpy of formation calculated from the two reactions, utilising the enthalpy of formation of Zrl4(g) and the selected auxiliary enthalpy of 

The value given by [97VIS/COR] viz. -(128.4 5.0) kJ-moP is in good agreement with the selected value in the present review. 

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Geometric structure and molecular parameter data were used by [97VIS/COR] to determine the heat capacity and entropy of the gaseous zirconium iodides at... 

K. The predicted structures of the various iodides are the same as the bromides, that is Ti symmetry (tetraiodide), planar (triiodide) and linear (diiodide). From the measured data, van der Vis et al. [97VIS/COR] calculated the following heat capacity and entropy values for the gaseous zirconium iodides ... 

Botli reactions involve the formation of a vapour-uatisporting species from four gaseous reactant molecules, but whereas the tetra-iodide of zirconium is a stable molecule, the nickel teU acarbonyl has a relatively small stability. The equilibrium constatits for these reactions are derived from the following considerations ... 

The anticipated content of impurities in the refined metal may be calculated a priori by assuming thermodynamic equilibrium at both metal/gas interfaces, and using the relevant stabilities of tire gaseous iodides. Adequate thermodynamic data could provide the activities of the impurities widr that of zirconium close to unity, but tire calculation of tire impurity transport obviously requires a knowledge of activity coefficients in the original impure material, which are not sufficiently well known. 

Gaseous diffusion, with subsequent decomposition on a hot wire, has been used for the purification of zirconium , uranium , chromium, niobium and tantalum . The metal is transported as a volatile iodide, which then decomposes on the hot wire or other heated receiver. In addition to diffusion, convection is observed in large diameter tubes (> 2 cm) at several atmospheres pressure. The tube is placed in an inclined position with the hot end downward. Silicon in the form of its dihalides has been transported in this manner . 

Dihalides of uncertain purity are prepared by the disproportionation of the trihalides. Alternate routes have also been reported. Swaroop and Flengas (549) prepared ZrCU of 95-99% purity by heating the trichloride and metallic zirconium at 675°C for 30-35 hours in an evacuated quartz tube lined with platinum foil. There is also a reference to the production of liquid dihalides by the reaction of the gaseous tetrahalides with loosely packed zirconium at 700°C for the chloride and 400°C for the bromide and iodide (270). The difluoride has been prepared (357) by the reaction of atomic hydrogen on thin layers of zirconium tetrafluoride at 350°C. New data on hafnium are lacking, although Corbett (542) has concluded that hafnium diiodide does not exist. 

For zirconium production, the Van Arkel-de Boer process [1] and the Kroll process [2] are the two main processes applied in the industry. The Van Arkel-de Boer process is also known as the iodide process or the crystal bar process, developed by the Dutch chemists Van Arkel and De Boer in 1925 [1]. It is the first industrial process for the commercial production of pure ductile metallic zirconium, and is still in use for the production of small quantities of ultra-pure titanium and zirconium. The Van Arkel-de Boer process involves the use of elemental iodine and crude metal, in the form of a sponge or alloy scrap, to form a volatile metal iodide at a low temperature. At a high temperature, the metal iodide will thermally decompose into pure metal and gaseous iodine. The Kroll process is a process used to produce titanium metal [2], developed in 1945 by... 

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