Carbon carbothermic reduction

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. 

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. 

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. 

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. 

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. 

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. 

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 ... 

Also, the metal can be obtained by nonelectrolytic reduction processes. In carbothermic process, alumina is heated with carbon in a furnace at 2000 to 2500°C. Similarly, in Subhalide process, an A1 alloy, Al-Fe-Si-, (obtained by carbothermic reduction of bauxite) is heated at 1250°C with AlCl vapor. This forms the subchloride (AlCl), the vapor of which decomposes when cooled to 800°C. 

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... 

Ke W, He X, Wang L, Ren J, Jiang C, Wan C. Preparation of Cu6Sn5-encapsulated carbon microsphere anode materials for Li-ion batteries by carbothermal reduction of oxides. J Electrochem Soc 2006 153 A1859-A1862. 

There are several reports on the preparation of SiC nanowires in the literature but fewer on the preparation of SisKi nanowires.38-39 The methods employed for the synthesis of SiC nanowires have been varied. Since both SiC and Si3N, are products of the carbothermal reduction of SI02, it should be possible to establish conditions wherein one set of specific conditions favor one over the other. We have been able to prepare SijN nanowires,40 by reacting multiwalled carbon nanotubes produced by ferrocene pyrolysis with ammonia and silica gel at 1360... 

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. 

In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO3 and elemental carbon to form UC. In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

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,... 

However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at > 1400°C (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form P-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie er a/. 1994a). 

Only recently was a new caibothermal reduction process developed in which the WC is synthesized by a rapid carbothermal reduction of tungsten oxides in a vertical graphite transport reactor (RCR entrainment process) [3.49]. Rapid heating of the WO3/C mixture driven by thermal radiation allows conversion of the mixture into a carbide precursor (WCi. c) within very short reaction times (a matter of seconds). In a second step, additional carbon is added to the carbide precursor to form a mixture, which then imdergoes a second heat treatment to convert the precursor into substantially pure WC. 

Carbothermal reduction of tungsten oxides with carbon monoxide [3.47], or gas mixtures of CO/CO2, CO/H2, CH4/H2 [3.50], C2H4/H2, and C2H4/H2 [3.51], as well as by reaction between metal oxide vapor and solid carbon [3.52] have recently attracted attention for producing high surface area tungsten carbides (up to lOOm /g), for use as catalyst (see Section 10.4), and for nanophase WC/Co composite powders (see also Section 9.2.1.4) [3.53]. 

Silicon is a member of the Group IV elements in the Periodic Table. However, little of the chemistry of silicon can be inferred from carbon, one of its closest neighbors. Although silicon is the second most abundant element in Earth s crust (approximately 26%), it does not exist in nature as a free element. Silicon must be freed from its oxides through a chemical process known as carbothermic reduction. In this reaction, sihca and a carbon source (generally wood) are heated together at extremely high temperatures to yield silicon in its elemental form. The Swedish chemist Jons Jakob Berzelius (1824) was the first to isolate silicon from its natural matrix. Sificon is widely used in the electronics and chemical industries. 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

G.W. Meng, Z. Cui, L.D. Zhang, and F. Phillipp, Growth and characterization of nanostructured P-SiC via carbothermal reduction of SiO xerogels containing carbon nanoparticles, J. Cryst. Growth., 209, 801-806 (2000). 

---