Carbothermal reactions

Several preparation methods have been published, such as solid-state reactions, carbothermal reduction reactions, sol-gel methods, and template methods. Different methods will produce materials with different performance. 

This reaction, carried out at high (700—800°C) temperatures, also converts several impurities ia the ores to volatile chlorides, thus purifyiag the MgCl2. A patent describes the carbothermal cblorination of magnesite direcdy (15). 

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. 

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. 

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 ... 

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. 

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 ... 

Actinide carbides are prepared by carbothermic reduction of the corresponding dioxides according to the reaction ... 

Production. Silicon is typically produced in a three-electrode, a-c submerged electric arc furnace by the carbothermic reduction of silicon dioxide (quartz) with carbonaceous reducing agents. The reductants consist of a mixture of coal (qv), charcoal, petroleum coke, and wood chips. Petroleum coke, if used, accounts for less than 10% of the total carbon requirements. Low ash bituminous coal, having a fixed carbon content of 55—70% and ash content of <4%, provides a majority of the required carbon. Typical carbon contribution is 65%. Charcoal, as a reductant, is highly reactive and varies in fixed carbon from 70—92%. Wood chips are added to the reductant mix to increase the raw material mix porosity, which improves the SiO (g) to solid carbon reaction. Silica is added to the furnace in the form of quartz, quartzite, or gravel. The key quartz requirements are friability and thermal stability. Depending on the desired silicon quality, the total oxide impurities in quartz may vary from 0.5—1%. 

Carbothermic Reduction. Silicon carbide is commercially produced by the electrochemical reaction of high grade silica sand (quartz) and carbon in an electric resistance furnace. The carbon is in the form of petroleum coke or anthracite coal. The overall reaction is... 

In carbothermal reactions BN is synthesized by reduction of boric acid or borates in nitrogen atmosphere at 1000-1500 °C. One example is the... 

The extraction of zinc by carbothermic reduction of zinc oxide sinter at 1,100°C can be represented by the reaction... 

Upon further heating, a second mass loss occurs in the 1473 K - 1973 K region. In this step excess oxygen is evolved as CO, due to the carbothermic reduction reaction (D). 

Chloride Process. A flow diagram for the chloride process is shown in Figure 1. The first stage in the process, carbothermal chlorination of the ore to produce titanium tetrachloride, is carried out in a fluid-bed chlorinator at ca 950°C. If mineral rutile is used as the feedstock, the dominant reaction is chlorination of titanium dioxide. 

Other Metal Nitrides Many other metal nitrides are used in ceramic formulations. These include AIN, TiN, VN, and BN. These metal nitride powders are produced by carbothermal reduction of the relevant metal oxide in a nitrogen-containing atmosphere or reaction of the relevant metal with a nitrogen-containing reducing atmosphere. These metal nitrides are used as abrasives and in hi -temperature wear applications. 

The first six reactions form mixed oxide ceramic powders. The last three reactions are carbothermal reductions to produce different metal carbides. The most famous is the Atcheson process for synthesis of SiC from Si02 and carbon, where the carbon in the mixture of reactant powders is used as a resistive electrical conductor to heat the mixture to the reaction temperature. This reaction is performed industrially in a 10-20 m long bunker fixed with two end caps that contain the source and sink for the cLc current. The reactant mixture is piled to a height of 2 m in the bunker and a current is applied. The temperature rises to the reaction temperatures, and some of the excess C reacts to CO, providing further heat. The 10-20 m bunker is covered with a blue flame for most of the reaction period. The resulting SiC is loaded into grinding mills to produce the ceramic powders and abrasives of desired size distributions. 

The carbothermic reduction of silica is believed to follow [25] a reaction mechanism given by... 

Figure 5.16 [26] shows the free energy of these two reactions and that of the carbothermic reduction as a solid-solid reaction mechanism. At a temperature greater than 1900 K the solid—solid reaction becomes... 

As another example, we look at the carbothermic reduction of tungsten oxide, which follows a reaction mechanism... 

Carbothermic Reactions Some apparently solid/solid reactions with carbon apparently take place through intermediate CO and CO2. The reduction of iron oxides has the mechanism... 

To examine the processes of cluster formation in the gas phase a Knudsen Cell has been used. A mixture of ZnO and graphite powders was positioned inside the effusion cell. The carbothermal reaction between ZnO and graphite was used to create zinc vapor. The effusion cell was heated up to a temperature above 1000°C. 

The first reliable information on the protactinium-carbon system was reported by Lorenz and ScherfF who prepared the monocarbide, PaC, by carbothermic reduction of Pa20s. The dioxide, Pa02, is first obtained at approximately 1100°C and is then converted to PaC above 1900°C. In the presence of excess carbon there is some evidence for the formation of the tetragonal dicarbide. Protactinium monocarbide is isostructural with other actinide monocarbides possessing the fee NaCl-type of structure with Oo = 5.0608 A. In contrast to ThC and UC, however, it is stable in the atmosphere and is relatively inert toward acid solutions. By measuring the CO equilibrium pressures for the reaction,... 

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