Zirconium processing

The most well-known electrodeposition process from the molten state is that of aluminum, which is deposited from a mixture of AI2O3 in AlF3-NaF at 965 °C. Other commercial processes involving molten salts exist and are exemplified by the deposition of tantalum and zirconium. Processes for Ti deposition from TiCl3 in KCl-LiCl entectics exist. All these escape almost completely97 from the H co-deposition problem of aqueous electrodeposition. 

Putvinski et al. [395] have proposed a variant of the zirconium process [392, 393] in which a succession of three different reagents are applied and it is possible to form non-centrosymmetric films. They produced an initial surface of OH groups by reacting HS(CH2)MOH with a gold... 

Figure 7.11 Kroll zirconium process as practiced at Albany, Oregon.

For the Hanford Production Reactors, the gas system protects the moderator. In NPR, the reactor atmosphere must also protect the zirconium process tubes from hydrogen embrittlement by maintaining the oxide film on the OD of the process tube. At the present operating temperatures, hydrogen embrittlement of the zirconium tubes in KET and KW Reactors will not be a problem. 

The stable double fluoride, K TiFe, dissolved in fused sodium chloride, has been employed by Messrs. Horizons Inc., by analogy with their zirconium process. Chlorine is evolved on electrolysis, leaving an electrolyte rich in fluorides which have too high an electrical resistance for satisfactory operation over a long period. Frequent replacement of the electrolyte is therefore necessary. 

The zirconium process tubes are sufficiently thick in the N Reactor to stop an appreciable fractionof gamma radiation from sources both inside the tube and outside the tube (n y capture in the graphite). Snergy losses due to neutron Interactions in the process tubes are negligible. 

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). 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Section 14 15 Coordination polymerization of ethylene and propene has the biggest eco nomic impact of any organic chemical process Ziegler-Natta polymer ization IS carried out using catalysts derived from transition metals such as titanium and zirconium tt Bonded and ct bonded organometallic com pounds are intermediates m coordination polymerization... 

Flame-Retardant Treatments For Wool. Although wool is regarded as a naturally flame-resistant fiber, for certain appHcations, such as use in aircraft, it is necessary to meet more stringent requirements. The Zirpro process, developed for this purpose (122,123), is based on the exhaustion of negatively charged zirconium and titanium complexes on wool fiber under acidic conditions. Specific agents used for this purpose are potassium hexafluoro zirconate [16923-95-8] [16923-95-8] K ZrF, and potassium hexafluoro titanate [16919-27-0], K TiF. Various modifications of this process have been... 

Decomposition of Zircon. Zircon sand is inert and refractory. Therefore the first extractive step is to convert the zirconium and hafnium portions into active forms amenable to the subsequent processing scheme. For the production of hafnium, this is done in the United States by carbochlorination as shown in Figure 1. In the Ukraine, fluorosiUcate fusion is used. Caustic fusion is the usual starting procedure for the production of aqueous zirconium chemicals, which usually does not involve hafnium separation. Other methods of decomposing zircon such as plasma dissociation or lime fusions are used for production of some grades of zirconium oxide. 

Molten Salt Distillation. Hafnium tetrachloride is slightly more volatile than zirconium tetrachloride, but a separation process based on this volatility difference is impractical at atmospheric pressures because only soHd and vapor phases exist. The triple point for these systems is at about 2.7 MPa (400 psia) and 400°C so that separation of the Hquids by distillation would necessarily require a massive pressurized system (13). 

Fluorozirconate Crystallization. Repeated dissolution and fractional crystallization of potassium hexafluorozirconate was the method first used to separate hafnium and zirconium (15), potassium fluorohafnate solubility being higher. This process is used in the Prinieprovsky Chemical Plant in Dnieprodzerzhinsk, Ukraine, to produce hafnium-free zirconium. Hafnium-enriched (about 6%) zirconium hydrous oxide is precipitated from the first-stage mother Hquors, and redissolved in acid to feed ion-exchange columns to obtain pure hafnium (10). 

Electrolysis. Electro winning of hafnium, zirconium, and titanium has been proposed as an alternative to the KroU process. Electrolysis of an all chloride hafnium salt system is inefficient because of the stabiHty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the StabiHty of in solution and results in much better current efficiencies. Hafnium is produced by this procedure in Erance (17). 

Many of the impurities are much lower than the values shown in Table 3, but these analytical lower limits are typical and more than sufficient for all but special appHcations. Zirconium content can be from 0.01 to 4.5%, and is typically 0.5—2%, but this is a function of how far the separation process was carried, not a function of the reduction or refining processes. 

Hafnium dioxide is formed by ignition of hafnium metal, carbide, tetrachloride, sulfide, boride, nitride, or hydrous oxide. Commercial hafnium oxide, the product of the separation process for zirconium and hafnium, contains 97—99% hafnium oxide. Purer forms, up to 99.99%, are available. 

Another important class of titanates that can be produced by hydrothermal synthesis processes are those in the lead zirconate—lead titanate (PZT) family. These piezoelectric materials are widely used in manufacture of ultrasonic transducers, sensors, and minia ture actuators. The electrical properties of these materials are derived from the formation of a homogeneous soHd solution of the oxide end members. The process consists of preparing a coprecipitated titanium—zirconium hydroxide gel. The gel reacts with lead oxide in water to form crystalline PZT particles having an average size of about 1 ]lni (Eig. 3b). A process has been developed at BatteUe (Columbus, Ohio) to the pilot-scale level (5-kg/h). 

Eabrication techniques must take into account the metallurgical properties of the metals to be joined and the possibiUty of undesirable diffusion at the interface during hot forming, heat treating, and welding. Compatible alloys, ie, those that do not form intermetaUic compounds upon alloying, eg, nickel and nickel alloys (qv), copper and copper alloys (qv), and stainless steel alloys clad to steel, may be treated by the traditional techniques developed for clads produced by other processes. On the other hand, incompatible combinations, eg, titanium, zirconium, or aluminum to steel, require special techniques designed to limit the production at the interface of undesirable intermetaUics which would jeopardize bond ductihty. 

The equihbrium is reversed at high temperature. The iodide is decomposed by passing the vapor over an electrically heated wire (1300—1400°C), yielding purified sohd titanium and iodine gas which is recycled. The iodide process also appHes to the purification of zirconium, hafnium, and siUcon. 

Condensation of metal vapors followed by deposition on cooler surfaces yields metal powders as does decomposition of metal hydrides. Vacuum treatment of metal hydrides gives powders of fine particle size. Reaction of a metal haHde and molten magnesium, known as the KroU process, is used for titanium and zirconium. This results in a sponge-like product. 

In this process, catalysts, such as boric acid, molybdenum oxide, zirconium, and titanium tetrachloride or ammonium molybdate, are used to accelerate the reaction. The synthesis is either carried out in a solvent (aUphatic hydrocarbon, trichlorobenzene, quinoline, pyridine, glycols, or alcohols) at approximately 200°C or without a solvent at 300°C (51,52). 

Zirconium alkoxides are used for cross-linking and hardening of isocyanate, epoxy, siUcon, urea, melamine, and terephthalate resins in the sol-gel process as catalysts in condensation and as water repellents. Zirconium alkoxides hydroly2e in moist air, but more slowly than titanium alkoxides. 

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