Kroll processes

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). 

Table 3. Analysis of Kroll Process, Electrowon, and Refined Hafnium, ppm...

Magnesium-Reduction (Kroll) Process. In the 1990s, nearly all sponge is produced by the magnesium reduction process (Fig. 4). 

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... 

The ease of oxidation of magnesium is important in the commercial manufacture of titanium metal. Titanium, when quite pure, shows great promise as a structural metal, but the economics of production have thus far inhibited its use. One of the processes currently used, the Kroll process, involves the reduction of liquid titanium tetrachloride with molten metallic magnesium ... 

Kinetics, chemical, 124 Knudsen cell, 63 Kroll process, 368 Krypton, 91 atomic volume, 410 boiling point, 307 heat of vaporization, 105... 

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) ... 

The process, now known as the Kroll process, was originally developed in 1940 by Wilhelm F. Kroll for the production of titanium. It is based on the reaction ... 

The last stages, sometimes called the Kroll process, are shown in Figure 5. These represent the most expensive operations. Replacement of the Kroll process by a plasma-based route (Figure 6) has been studied and the technical feasibility demonstrated ( 6). 

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

Figure 6. Suggested scheme for replacing the Kroll process by plasma techniques.

Betteron—Kroll process, 4 5 14 755. See also Kroll-Betterton debismuthizing 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. 

Two important reduction reactions of magnesium that are of commercial interest are the production of titanium by Kroll process and obtaining uranium from its fluoride ... 

The production of titanium always encounters difficulties because of a tendency to react with oxygen, nitrogen and moisture at elevated temperatures. Most high purity elemental titanium can he produced by the Kroll process from titanium tetrachloride. The tetrachloride is reduced with magnesium in a mild steel vessel at about 800° C under an inert atmosphere of helium or argon. The net reaction is as follows ... 

The reaction is highly exothermic providing heat needed to maintain high temperature required for reaction. The Kroll process is applied commercially to produce elemental titanium. 

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... 

Crude elemental silicon can be obtained by reduction of silica sand with coke in the electric furnace (reaction 17.33) and may be adequate for making ferrosilicon alloys (Section 16.7.5) or silicones (Section 3.5). The high purity silicon used for electronic chips can be made from silica via silicon tetrachloride, which, like TiCU, is a volatile liquid (bp 57 °C) susceptible to hydrolysis but readily purifiable by fractional distillation. Indeed, the procedure for silicon resembles the Kroll process for titanium, except that an argon atmosphere is not necessary ... 

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). 

Refining. Kroll-process hafnium sponge and electrowon hafnium do not meet the performance requirements for the two principal uses of hafnium metal. Further purification is accomplished by the van Arkel-de Boer, ie, iodide bar, process (18) and by electron beam melting. 

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