Crystal bar process

Iodide compounds are not very stable, so heating them can result in an easy recapture of the metal elements in the compound. For example, the van Arkel-de-Boer process (or crystal bar process) is used to purify zirconium. This is done by passing the heated gas (Zrl ) over a white hot tungsten filament, and over time the metals form on the tungsten. 

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. 

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... 

Derivation (1) Reduction of thorium dioxide with calcium (2) fused salt electrolysis of the double fluoride ThF4 KF. The product of both processes is thorium powder, fabricated into the metal by powder metallurgy techniques. Hot surface decomposition of the iodide produces crystal bar thorium. 

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. 

Note that titanium metal produced as crystal bars by the Van Arkel-Deboer process is so negligible industrmlly in comparison with the Kroll process that it was not taken into account in this study. 

In soap bar processing free fatty acid is usually added in formulations to create so-called super-fatted soap. An acid-soap complex with a fixed stoichiometric ratio between alkaline soap and the fatty acid is formed. For example, the ratio of potassium acid soap is 1 1 while sodium soap forms acid soaps with various ratios. The fixed ratio complex exits not only in anhydrous crystalline phase but also in a hydrous liquid crystalline phase (11, 12). Oleic acid and its potassium soap form a 1 1 complex acid soap when equal molar acid and soap are mixed. Above the Krafft boundary, the acid soap in water forms a lamellar liquid crystal phase at low surfactant concentration, from a few percent, and the lamellar liquid crystal phase extends to ca 60% surfactant concentration. A hexagonal liquid crystal phase is formed after the lamellar liquid crystal phase with further increasing the surfactant concentration. This phase behavior is different from the soap and water phase behavior, in which the hexagonal liquid crystalline phase is formed first followed by the lamellar liquid crystalline phase. Below the Krafft boundary the acid soap complex forms a solid crystal and separates from water (4). 

Now, the influence of convective gas flow on the heat transfer can be estimated for Bridgman-type crystal-growth processes carried out under high gas pressure conditions such as the growth of GaAs (3-7 bar) InP (30-40 bar) and GaP ( 80 bar). Assuming a temperature difference AT between the water-cooled vessel and the outer thermal insulation of the heaters of 100 K and a distance I of more than 5 cm one obtains for a gas pressure p > 1 bar from Eq. (5.3) a Grashof number Gr > 1000 and a Nusselt number Nu 1. 

Impurities KroU process sponge Electrowon crystals Refined from KroU Electron beam ingot sponge Iodide bar... 

The manufacture of silver nitrate for the preparation of photographic emulsions requires silver of very high purity. At the Eastman Kodak Company, the principal U.S. producer of silver nitrate, 99.95% pure silver bars are dissolved in 67% nitric acid in three tanks coimected in parallel. Excess nitric acid is removed from the resulting solution, which contains 60—65% silver nitrate, and the solution is filtered. This solution is evaporated until its silver nitrate concentration is 84%. It is then cooled to prepare the first crop of crystals. The mother Hquor is purified by the addition of silver oxide and returned to the initial stages of the process. The cmde silver nitrate is centrifuged and recrystallized from hot, demineralized water. Equipment used in this process is made of ANSI 310 stainless steel (16). 

In this exercise we shall estimate the influence of transport limitations when testing an ammonia catalyst such as that described in Exercise 5.1 by estimating the effectiveness factor e. We are aware that the radius of the catalyst particles is essential so the fused and reduced catalyst is crushed into small particles. A fraction with a narrow distribution of = 0.2 mm is used for the experiment. We shall assume that the particles are ideally spherical. The effective diffusion constant is not easily accessible but we assume that it is approximately a factor of 100 lower than the free diffusion, which is in the proximity of 0.4 cm s . A test is then made with a stoichiometric mixture of N2/H2 at 4 bar under the assumption that the process is far from equilibrium and first order in nitrogen. The reaction is planned to run at 600 K, and from fundamental studies on a single crystal the TOP is roughly 0.05 per iron atom in the surface. From Exercise 5.1 we utilize that 1 g of reduced catalyst has a volume of 0.2 cm g , that the pore volume constitutes 0.1 cm g and that the total surface area, which we will assume is the pore area, is 29 m g , and that of this is the 18 m g- is the pure iron Fe(lOO) surface. Note that there is some dispute as to which are the active sites on iron (a dispute that we disregard here). 

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