Zirconium Kroll process

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 Kroll process for tire reduction of tire halides of refractory metals by magnesium is exemplified by the reduction of zirconium tetrachloride to produce an impure metal which is subsequently refined with the van Arkel process to produce metal of nuclear reactor grade. After the chlorination of the impure oxide in the presence of carbon... 

Refractory metals Zirconium Hafnium Titanium Kroll process, chlorination, and magnesium reduction Chlorine, chlorides, SiCli Wet scrubbers... 

Zirconium, too, is produced commercially by the Kroll process, but the van Arkel-de Boer process is also useful when it is especially important to remove all oxygen and nitrogen. In this latter method the crude zirconium is heated in an evacuated vessel with a little iodine, to a temperature of about 200° C when Zrl4 volatilizes. A tungsten or zirconium filament is simultaneously electrically heated to about 1300°C. This decomposes the Zrl4 and pure zirconium is deposited on the filament. As the deposit grows the current is steadily increased so as to maintain the temperatures. The method is applicable to many metals by judicious adjustment of the temperatures. Zirconium has a high corrosion resistance and in certain chemical plants is preferred to alternatives such as stainless... 

Though the element was discovered in 1789 it was not prepd in the pure state until 1914. It may be prepd commercially by the reaction of zirconium chloride with magnesium (the Kroll process) and other methods. The principle ore is zircon, deposits of which are found in the USA, Australia and Brazil. A number of special properties, such as exceptional resistance to corrosion and a low absorption cross section, have led to the use of Zr or alloys contg Zr, in many... 

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). 

The famous Kroll process is based on the following reaction (in the case of zirconium) ... 

Figure 5. The final stages in the production of zirconium sponge (the Kroll process).

Kroll process, 13 84-85 15 337 17 140 in titanium manufacture, 24 851-853 Kroll zirconium reduction process, 26 631 KRW gasifier, 6 797-798, 828 Krypton (Kr), 17 344 commercial, 17 368t complex salts of, 17 333-334 doubly ionized, 14 685 hydroquinone clathrate of, 14 183 in light sources, 17 371-372 from nuclear power plants, 17 362 physical properties of, 17 350 Krypton-85, 17 375, 376 Krypton compounds, 17 333-334 Krypton derivatives, 17 334 Krypton difluoride, 17 333, 336 uses for, 17 336... 

Zirconium oxide (ZrO ) is the most common compound of zirconium found in nature. It has many uses, including the production of heat-resistant fabrics and high-temperature electrodes and tools, as well as in the treatment of skin diseases. The mineral baddeleyite (known as zirconia or ZrO ) is the natural form of zirconium oxide and is used to produce metallic zirconium by the use of the Kroll process. The KroU process is used to produce titanium metal as well as zirconium. The metals, in the form of metaUic tetrachlorides, are reduced with magnesium metal and then heated to red-hot under normal pressure in the presence of a blanket of inert gas such as helium or argon. 

Zirconium metal is produced from its tetrachloride by reduction with magnesium by the Kroll process. The oxide obtained above is converted to zirconium tetrachloride by heating with carbon and chlorine. In practice, the oxide is mixed with lampblack, powdered sugar, and a little water, and pelletized. The dried pellet is then heated with chlorine in a chlorinator to produce ziro-conium tetrachloride ... 

The Kroll process involves heating molten magnesium and zirconium tetrachloride vapor in a sealed furnace in the absence of air under a helium atmosphere. The reaction forms zirconium sponge and magnesium chloride ... 

Titanium (and also zirconium and tantalum) are made industrially by the Kroll process, which involves conversion of TiC>2 to TiCl4 by coke reduction... 

Electrolysis. Electro winning of hafnium, zirconium, and titanium has been proposed as an alternative to the Kroll process. Electrolysis of an all chloride hafnium salt system is inefficient because of the stability of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stability of Hf4+ in solution and results in much better current efficiencies. Hafnium is produced by this procedure in France (17). 

Zirconium metal (mp 1855°C 15°C), like titanium, is hard and corrosion resistant, resembling stainless steel in appearance. It is made by the Kroll process (Section 17-A-l). Hafnium metal (mp 2222°C 30°C) is similar. Like titanium, these metals are fairly resistant to acids, and they are best dissolved in HF where the formation of anionic fluoro complexes is important in the stabilization of the solutions. Zirconium will burn in air at high temperatures, reacting more rapidly with nitrogen than with oxygen, to give a mixture of nitride, oxide, and oxide nitride (Zr2ON2). 

Kroll process. A widely used process for obtaining titanium metal. Titanium tetrachloride is reduced with magnesium metal at red heat and atmospheric pressure, in the presence of an inert gas blanket of helium or argon. Magnesium chloride and titanium metal are produced. The reaction is TiCl4 + 2Mg —> Ti + 2MgCl2. Essentially the same process is also used for obtaining zirconium. 

Derivation The ore is converted to a cyanonitride and is chlorinated to obtain zirconium tetrachloride. This is reduced with magnesium (Kroll process) in inert atmosphere. The metal can be prepared in a highly pure and ductile form by vapor-phase decomposition of the tetraiodide. Hafnium must be removed for uses in nuclear reactors... 

Finely divided zirconium, such as the zirconium sponge produced in the Kroll process (Sec. 8.3), reacts sufficiently rapidly with water at ordinary temperatures to impair its mechanical properties. Thus it is not feasible to use water leaching to separate zirconium sponge from the magnesium chloride by-product of the Kroll process. 

The halides of greatest practical importance are Zrp4, as a component of fluozirconates ZrClt, as the feed material for production of zirconium in the Kroll process and Zrl4, as feed material in the hot-wire process. These will be discussed later in the chapter. 

Chlorination of zircon has been the process mainly used in the United States because it produces ZrCU, which is used in the Kroll process for making zirconium metal (Sec. 8.3), and because ZrCU was the feed material for the first process developed for separating hafnium from zirconium, using thiocyanate extraction (Sec. 7.3). 

In the United States, practically all zirconium metal is now being made by the Kroll process. This process was an adaptation to zirconium of a similar process for titanium developed by W. J. Kroll. The work of Kroll and metallurgists of the Albany, Oregon, station of the Bureau of Mines culminated in a plant to produce 135,000 kg zirconium/year at the station. A similar plant was operated by the Carborundum Metals Corporation, at Akron, New York. These have been superseded by the plant of the Teledyne Wah Chang Albany Company, at Albany, Oregon, with a capacity in 1978 of 3.4 million kg/year. 

Processes for making ductile zirconium by electrolysis of KjZrFj dissolved in molten chlorides have been described by Steinberg et al. in the United States [S4, R2] and by Ogarev et al. [01] in the Soviet Union. An advantage over the Kroll process is that a coarsely crystalline product is obtained from which coproduced halides can be removed by leaching with acidified water without undue contamination of zirconium by oxygen. The washed crystals are then vacuum dried and consolidated by arc melting. 

Zircon silicate is the most important source of hafnium. Ion-exchange and solvent-extraction techniques have supplanted fractional crystallization and distillation as the preferred methods of separating hafnium from zirconium. The metal itself is prepared by magnesium reduction of hafnium tetrachloride (the Kroll process), and by the thermal decomposition of tetraiodide (de Boer-van Arkel process). The annual world production of hafnium metal was about 40 tons at the end of the 1980s (Soloveichik... 

Industrial by-products that contain zirconium are mainly zircon and zirconia, both of which are insoluble in water, largely inert, and of low toxicity. Water-soluble zirconium compounds are converted at pH 4-9.5 into insoluble zirconia. The only possible atmospheric emission of other zirconium compounds is that of chlorinated and/or hydrolyzed oxychlorides from the processing of sponge zirconium using the Kroll process (reduction of ZrCl4). Analytical data corresponding to the extent of these emissions do not exist (Smith and Carson 1978), but there is no doubt that the exposure of the general population to zirconium compounds is small. 

The most important commercial methods of production of metallic zirconium and hafnium are based on furnace reduction, either of MCI4 with magnesium or a Na-Mg mixture (the Kroll process), yielding a metal sponge, or of metal fluorides with Ca at 2000 °C to give an ingot. The... 

The reactions of metals to form stable halides are important for various reasons. The metal halides generally have low boiling points and high volatiUty. For this reason, they are used in several important processes for the production and refining of metals, such as the reactive metals titanium and zirconium. These metals are produced using the Kroll process, in which the metal oxide is converted to metal chloride or fluoride, which is then reduced to metal. This route avoids several formidable difficulties involved in the reduction of the oxides of these metals. Details of these processes can be found in extractive-metallurgy textbooks. 

The major differences between the Kroll process for titanium and zirconium are as follows ... 

Zirconium is prepared from ZrCLj by the Kroll process (chemical reduction of the chloride by hquid magnesium at 900 °C), but the electrochemical route in molten salts is considered as a good alternative. As for Ta and Nb, pure chloride melts are not appropriate for the electrowinning to yield the metal because of too complex cathode process. The addition of fluoride ion source (KF) allows to stabilize Zr ions in the form of K2ZrFg which is reduced in one step to Zr [23]. 

Production Zirconia is usually produced from zircon, ZrSi04. Zircon sands are first melted with sodium hydroxide and transferred to zirconium (IV) oxide. With addition of coal, the zirconium oxide is transferred to zirconium carbonitride in a plasma arc, which is converted again to zirconium (IV) chloride with chlorine. The chloride is reduced to zirconium with magnesium in a helium atmosphere at 800°C in the so-caUed Kroll-process ZrCLt -I- 2Mg —> Zr -1- 2 MgC. ... 

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