Iodine vapor process

Thorium and Pa are most conveniently prepared from carbides, but at low temperatures made possible by an iodide intermediate in the iodine vapor process, based on the reaction of carbide with iodine vapor at 300 C. The actinoid iodide is transported to a hot surface (such as a W wire or sphere at 1200°C), where it decomposes and deposits the actinoid metaF". The overall reaction sequence is... 

When a metal contains impurities more volatile than the metal, such as a volatile reductant metal or its halide, heating the metal well above its melting point in vacuum serves as a useful purification step. The lighter rare earth and actinoid metals, especially those prepared by the iodine vapor process, are efficiently refined by vacuum melting . Vacuum melting is not often employed industrially because of the expenses associated with heating and corrosion of crucibles. 

Moist iodine vapor rapidly corrodes metals, including most stainless steels. The initial process is the formation of corrosion centers where small amounts of metal iodide are formed which deHquesce, and the corrosion then takes place electrochemically (41,42). Only titanium and molybdenum steels are unattacked by iodine (42,43). The corrosion of molten iodine has also been studied. 

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. 

Exercise 28-8 The vapor-phase photochemical decomposition of 2-propanone proceeds in the presence of iodine vapor, but the amount of carbon monoxide formed becomes very small, Explain how this result argues against the one-step process,... 

Iodine is one of the most striking and beautiful of all elements. As a solid, it is a heavy, grayish-black, metallic-looking material. When heated, it does not melt. Instead, it sublimes. Sublimation is the process by which a solid turns direcdy to a gas without first melting. The resulting iodine vapor has a violet color and a harsh odor. If a cold object, such as an iron bar, is placed in these vapors, iodine changes back to a solid. It forms attractive, delicate, metallic crystals. 

Fig. 4-8 Schematic of JAERI bench-scale apparatus for the iodine-sulfur process thermochemical cycle for hydrogen production (LL - liquid-liquid, VL - vapor-liquid), from [73]...

The reaction of the potentially tripodal JV,N-bis(2-mercapto-ethyl)-2-(methylthio-ethylamine with anhydrous Ni(OAc)2 yields a dimeric complex. Chemical oxidation with iodine in a 2e process yields metal oxidized products which exist as two monomeric nickel species. One of these complexes is believed to have been derived from the formation of thiyl radicals. Both the reactions of solid [CUL2] (HL = acetylacetone, trifluoroacetylacetone, benzoylacetone) with iodine vapor and those of [CUL2] solutions with iodine solutions in weakly polar solvents have been interpreted as proceeding through an oxidative mechanism. ... 

Properties Yel. cryst. m.w. 134.87 Toxicology LCLo (Inh., rat, 1 h) 580 ppm poison mod. toxic by inh. TSCA listed Precaution Flamm. when exposed to heat, flame, or oxidizers reacts with water to evolve flamm. phosphine gas ignites when heated in chlorine, bromine, or iodine vapors incandescent reaction with nitric acid Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of POx and phosphine Storage Moisture-sensitive Uses Fumigant for animal feed, processed foods... 

Experimental results (References...) and computer simulations of the chemical kinetics involved (Reference...) show that an important factor determining the effectiveness of sprays against elemental iodine vapor is the concentration of iodine in the spray solution. Experiments with fresh sprays having no dissolved iodine were observed to be quite effective in the scrubbing of elemental iodine even at a pH as low as 5 (References...). However, solutions having dissolved iodine, such as the sump solutions that recirculate after an accident, may revolatilize iodine if the solutions are acidic (References...). Chemical additives in the spray solution have no significant effect upon aerosol particle removal because this removal process is largely mechanical in nature. 

Sublimation is the process in which molecules go directly from the solid into the vapor phase. Deposition is the reverse process, that is, molecules make the transition from vapor to solid directly. Naphthalene, which is the substance used to make mothballs, has a fairly high (equilibrium) vapor pressure for a solid (1 mmHg at 53°C) thus, its pungent vapor quickly permeates an enclosed space. Iodine also sublimes. Above room temperature, the violet color of iodine vapor is easily visible in a closed container. 

The chromatographic plate may be exposed to iodine vapor by placing it in a closed chamber containing several crystals of iodine. As the iodine forms complexes with the various organic compounds, the spots become brown. Since the process is reversible and the spots fade, it is necessary to circle the spots with a pencil in order to have a permanent record of the chromatogram. 

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