Carbothermic reduction

A fourth ahoy separation technique is fractional crystallization. If shica is co-reduced with alumina, nearly pure shicon and an aluminum shicon eutectic can be obtained by fractional crystallization. Tin can be removed to low levels in aluminum by fractional crystallization and a carbothermic reduction process using tin to ahoy the aluminum produced, fohowed by fractional crystallization and sodium treatment to obtain pure aluminum, has been developed (25). This method looked very promising in the laboratory, but has not been tested on an industrial scale. 

Research-grade material may be prepared by reaction of pelleted mixtures of titanium dioxide and boron at 1700°C in a vacuum furnace. Under these conditions, the oxygen is eliminated as a volatile boron oxide (17). Technical grade (purity > 98%) material may be made by the carbothermal reduction of titanium dioxide in the presence of boron or boron carbide. The endothermic reaction is carried out by heating briquettes made from a mixture of the reactants in electric furnaces at 2000°C (11,18,19). 

A stoichiometric product can be obtained by repeated grinding and reaction. Alternatively, carbothermal reduction of titanium dioxide can be used (33). The reaction is carried out in an inert atmosphere at ca 1600°C. 

Uranium and mixed uranium—plutonium nitrides have a potential use as nuclear fuels for lead cooled fast reactors (136—139). Reactors of this type have been proposed for use ia deep-sea research vehicles (136). However, similar to the oxides, ia order for these materials to be useful as fuels, the nitrides must have an appropriate size and shape, ie, spheres. Microspheres of uranium nitrides have been fabricated by internal gelation and carbothermic reduction (140,141). Another use for uranium nitrides is as a catalyst for the cracking of NH at 550°C, which results ia high yields of H2 (142). 

As previously stated, uranium carbides are used as nuclear fuel (145). Two of the typical reactors fueled by uranium and mixed metal carbides are thermionic, which are continually being developed for space power and propulsion systems, and high temperature gas-cooled reactors (83,146,147). In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO and elemental carbon to form UC (148,149). In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Carbide. Zirconium carbide [12020-14-3] nominally ZrC, is a dark gray brittle soHd. It is made typically by a carbothermic reduction of zirconium oxide in a induction-heated vacuum furnace. Alternative production methods, especially for deposition on a substrate, consist of vapor-phase reaction of a volatile zirconium haHde, usually ZrCl, with a hydrocarbon in a hydrogen atmosphere at 900—1400°C. 

Powder Preparation. There are several routes to preparing SiC powders having variable purity levels, crystal stmcture, particle size, shape, and distribution. Methods that have been examined include growth by sublimation from the vapor phase, carbothermic reduction, and crystallization from a melt. 

The carbothermic reduction can also be carried out at lower (about 1500°C—1600°C) temperature resulting in P SiC formation (72). 

Vapor—sohd reactions (13—17) are also commonly used ia the synthesis of specialty ceramic powders. Carbothermic reduction of oxides, ia which carbon (qv) black mixed with the appropriate reactant oxide is heated ia nitrogen or an iaert atmosphere, is a popular means of produciag commercial SiC, Si N, aluminum nitride [24304-00-3], AIN, and sialon, ie, siUcon aluminum oxynitride, powders. 

Many reactions among solids are important with regard to pyrometallurgical processes. While some of these reactions are true solid-solid reactions, some others occur through fluid intermediates. For instance, the carbothermic reduction of many metal oxides proceeds through the gaseous intermediates CO and C02 in the following manner ... 

The carbothermic reduction processes outlined so far apply to relatively unstable oxides of those metals which do not react with the carbon used as the reductant to form stable carbides. There are several metal oxides which are intermediate in stability. These oxides are less stable than carbon monoxide at temperatures above 1000 °C, but the metals form stable carbides. Examples are metals such as vanadium, chromium, niobium, and tantalum. Carbothermic reduction becomes complicated in such cases and was not preferred as a method of metal production earlier. However, the scenario changed when vacuum began to be used along with high temperatures for metal reduction. Carbothermic reduction under pyrovacuum conditions (high temperature and vacuum) emerged as a very useful commercial process for the production of the refractory metals, as for example, niobium and tantalum, and to a very limited extent, of vanadium. 

In order to gain an appreciation of the variation of the carbothermy in implementing it under pyrovacuum conditions (as pointed out in the preceding paragraph), the general reaction for the carbothermic reduction of a metal oxide be once again considered ... 

The last-mentioned line intersects the metal oxide line at a lower temperature than the line corresponding to the formation of carbon monoxide at 1 atm. It is, therefore, clear that the minimum temperature required for the carbothermic reduction of the metal oxide under vacuum is less than the minimum temperature for the same reaction at atmospheric pressure. Thus, by increasing the temperature and decreasing the pressure of carbon monoxide, it may be possible to reduce carbothermically virtually all the oxides. This possibility has been summarized by Kruger in the statement that at about 1750 °C and at a carbon monoxide pressure below 1CT3 atm, carbon is the most efficient reducing agent for oxides. 

When the metal can form a stable carbide, the product of the carbothermic reduction of its oxide may be a carbide instead of the metal itself. The question as to whether a carbide or the metal forms under standard conditions when the oxide is reduced by carbon is not answered by the Ellingham diagram. To obtain an answer to this question, a more detailed consideration of the thermodynamic properties of the system is necessary. 

Among the metals considered for carbothermic reduction under reduced pressure, an important example is the reduction of niobium oxides. The possibilities and the conditions for the carbothermic reduction of niobium oxides can be examined on the basis of the Pourbaix-... 

Having established the feasibility of niobium metal production by the carbothermic reduction of niobium pentoxide under temperature and pressure conditions readily attainable in the laboratory and in industry, the principles of efficient process execution may now be examined. In a high-temperature vacuum furnace operation, the quantity of gas that is to be pumped off can influence the choice of the vacuum process. When the reduction of niobium pentoxide with either carbon or niobium carbide is attempted according to the following overall equations ... 

Oxygen and carbon have substantial solid solubilities in niobium at the temperatures normally required for reduction. As the activity coefficients of both carbon and oxygen in niobium are low, their retention in the niobium metal produced by the carbothermic reduction of niobium oxide is expected. It is, however, possible (as explained later) to remove these residual impurities by extending the pyrovacuum treatment to still higher temperatures and lower pressures. 

The principles of tantalum metal formation by the carbothermic reduction of tantalum pentoxide and the technology of tantalum metal production by this method are similar to those pertaining to niobium metal production by carbothermy. 

Tantalum obtained by carbothermic reduction at 2000 °C and 10-4 torr is more than 99.8% pure. The levels of the principal impurities, carbon and oxygen, are less than 0.1% each. 

Carbothermic reduction, as a method of metal production, becomes more versatile when a metal product generated in the vapor form is acceptable. Vaporization of the metal product is considered to be an undesirable phenomenon in the carbothermic reduction of refractory metals. In the production of common metals, however, this phenomenon can be accommodated (e.g., in the process for the production of zinc). Another important example in this regard is the production of magnesium. 

The impurities that occur in the crude zinc produced by the carbothermic reduction of zinc oxide are 2-3% lead, 0.3-0.4% cadmium, and 0.05% iron. Zinc is more volatile than... 

When a metal contains both carbon and oxygen, as is invariably the case with metals prepared by carbothermic reduction under vacuum, deoxidation occurs by the following two processes at high temperatures and low pressures ... 

This process has been tried out on a pilot plant scale mainly as a means of producing pure aluminum from an impure aluminum-iron (20-45%)-silicon (2-20%) alloy, obtained by the carbothermic reduction of bauxite in an electric furnace. 

The Alcan (Aluminum Company of Canada) process is based on the direct reduction of the ore to form an aluminum alloy (by carbothermic reduction carried out in an electric arc furnace at about 2000 °C). The alloy is then reacted with aluminum trichloride at 1300 °C to form aluminum monochloride. The monochloride is next contacted with molten droplets of aluminum to form aluminum metal the trichloride is regenerated for further reaction ... 

Carborods, 4 201, 202 Carbosulfan, 2 550t Carbothermal reduction, 77 210-211 as magnesium manufacturing process, 75 342... 

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