Solid carbothermic reduction

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 ... 

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. 

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%. 

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... 

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). 

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]. 

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. 

While gaseous SiO is the most abundant silicon oxide in the universe, solid SiO does not exist naturally on earth. It was first prepared by Potter in 1905 by reduction of Si02 with carbon, silicon or SiC. Today, several tons of solid SiO are produced industrially each year by comproportionation of silica and silicon at low pressure (10 -10 " mbar) and high temperatures (1250-1400 °C). The gaseous SiO formed under these conditions is then condensed at colder surfaces. However, SiO is always present whenever silica or silicates are reduced at high temperatures, such as in the carbothermal reduction of Si02 (production of SiC) or in blast furnaces. The local structure at the interface between Si02 films on elemental Si, or the nature of nanocrystalline SiO particles,is also related to that of solid SiO. 

The mechanism of this carbothermal reduction has been intensively investigated [170, 171] and a scheme of consecutive partial reactions has been proposed revealing that the reaction rate is controlled by diffusion and gas phase processes. Therefore, it can be concluded that the reaction rate may become the higher the finer the distribution of the solid educts is. That is, if we are able to start from a colloidal dispersed mixture of sUica and carbon, it is to be expected that the reaction temperature will drop as well as that finer SiC particles can be obtained. 

Carbothermic Plasma-Chemical Reduction of Uranium Oxide (U3O8). Analyze the stoichiometry of the carbothermic reduction of U3O8 (7-24). Explain why the ratio of molar fractions of CO2 and CO in products of the process is not fixed. Find out the relation between the molar fractions of CO2 and CO in the products as a function of initial composition of the solid mixture U3O8-C. Explain why the carbothermic reduction process of UO2 (7-23) assumes only CO in products, while that of U3O8 (7-24) expects formation of CO and CO2. 

S.3.2.2.3 Vapor-Solid Reaction At present the principal commercial method for SiC whisker production is the carbothermic reduction of low cost silica sources at temperatures of 1500-1700°C. The reaction for the formation of VS-cubic (3SiC whiskers occurs in two steps ... 

Yuan XY, Liu XJ, Zhang F, Wang SW (2010) Synthesis of y-AlON powders by a combinational method of carbothermal reduction and solid-state reaction. J Am Ceram Soc 93 22-24... 

A 2H wurtzite-type structure is formed by the carbothermal reduction of fine Si02 and AI2O3 with a carbon source under a nitrogen atmosphere at 1600 °C [see Eq. (20)]. Kinetically favored is the carbothermal reduction of a -SiAlONs or a -SiAlON-pre-cursor mixtures (3 Si3N4 + 3 AlN + CaO) at 1800 °C [see Eq. (21)]. Because the diffusion coefficients in covalent solids are extremely small, a solid solution was thought unlikely to be obtained by heating and annealing of the powdered solid components. 

Barker, J., Saidi, M.Y., and Swoyer, J.L. (2003) Lithium iron(II) phospho-olivines prepared by a novel carbothermal reduction method. Electrochem. Solid State Lett., 6 (3), A53-A55. 

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. 

Pan M, Lin X, Zhou Z (2011) Electrochemical performance of LiFePO C doped with F synthesized by carbothermal reduction method using NH4F as dopant. J Solid State Electrochem 16 1615-1621... 

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... 

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