Reducing carbothermic

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. 

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. 

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-... 

Other markets for char include iron, steel, and sili-con/ferro-silicon industries. Char can be used as a reducing agent in direct reduction of iron. Ferro-silicon and metallurgical-grade silicon metal are produced carbothermally in electric furnaces. Silica is mixed with coke, either iron ore or scrap steel (in the case of ferro-silicon), and sawdust or charcoal in order to form a charge. The charge is then processed by the furnace to create the desired product. Char can be substituted for the coke as a source of reducing carbon for this process. Some plants in Norway are known to have used coal-char in the production of silicon-based metal products as late as mid-1990.5 The use of char in this industry is not practiced due to lack of char supply. 

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%. 

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. 

Silica is reduced via a carbothermic process to silicon, which is converted to a variety of chlorosilanes. The major monomer, dimethyldichlorosilane, is produced in well over a billion pounds per year by several basic producers... 

There have been sporadic attempts to produce aluminum by carbothermic reduction [3, 4]. In this approach, akin to the way iron oxides are reduced to iron in the iron blast furnace, the consumption of electrical energy is avoided or at least reduced. There have also been investigations of the production of aluminum by electrolysis of aluminum compounds other than the oxide (e.g. [5]). Some of these alternative electrolytic technologies have even reached a commercial scale [6] but the only method for aluminum production in industrial use today appears to be electrolysis in Hall-Heroult cells. Consequently, the present paper is confined to these cells. The literature on these cells is large. A recent search of the web of science with the subject Hall cell and similar subjects revealed 79 titles aluminum electrolysis yielded 109 publications. This number excludes papers published in the annual Light Metals volume of the Minerals Metals and Materials Society (TMS). Light Metals contains approximately forty papers each year on Hall cells. Consequently, the authors have made no attempt at a comprehensive examination of the literature on these topics. Rather we have included... 

Although the reducing agent—carbon—is very cheap, and carbon reduction was the early basis of tungsten powder production, so far none of the numerous carbothermic procedures has been established in the production of pure tungsten. 

Tungsten is reduced in preference to iron so that ferrotungsten with high W content can be produced even if the ore is high in iron. The WO3 contained in these compounds can be reduced either carbothermically in electric arc furnaces or metallothermically by silicon and/or aluminum. The carbothermic or silico-carbothermic method is preferred for cost reasons and, moreover, the tolerance level for impurities like As and Sn in the raw materials is higher. 

Carbothermic-silicothermic production. The process is carried out in three successive stages in an electric arc furnace. Sixty percent of the oxygen in WO3 are reduced by carbon and 40% by silicon added as 75% FeSi. 

As FeOCl was not carbothermally reduced, it is suggested that FeOCl activates and transfers ojqrgen, just like CuCl. Interestingly, the application of FeOCl leads to the firrmation of surfiice oxygen complexes, whereas catalytic soot oxidation by Fe203 does not [19]. This is another indication that chlorine diemically affects the catalytic soot oxidation activity of metal oxides and that the previous scheme also holds for FeOCl. 

A third technique employs monovalent aluminum. By bringing vapors of aluminum fluoride or aluminum chloride into contact with carbothermically reduced aluminum ahoy at 1000—1400°C, the foIlowiQg reaction occurs... 

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. 

Reduce the oxide with carbon at high temperature (carbothermal reduction) and subsequently react it with the nonmetal. 

Besides oxides, no-oxides have also been synthesized by using solid-state reaction method. For example, phase-pure and fine y-AlON powders have been synthesized by using a combinational method of carbothermal reduction and solid-state reaction [69]. The combined method addressed the disadvantages of the respective individual method. Sucrose was used as the reducer instead of carbon... 

Carbon dioxide is a relatively new threat to most metal-producing industries. Production of silicon is — and will be for many years to come — dependent on the carbothermic reduction process. Bioreduction materials such as charcoal, paper, saw dusts and other carbon-containing biomass wastes are now considered as reduction alternatives to coal and coke. Since the 1990 the West European silicon industry has reduced the CO2 released/Mt silicon produced by nearly 20 %. The reduction is mainly a result of more efficient processes, higher silicon yields and use of charcoal and woodchips in the furnace charge. Further improvements will depend on the development of biomass reduction materials and a higher use of these materials (Table 3). 

In addition, various synthesis approaches such as hydrothermal synthesis, microwave synthesis, carbothermal reduction method, and high-temperature quick-melting method also have been developed to reduce the production cost, as shown in Table 9.3. 

Nanocrystalline B4C was synthesized by an inexpensive carbothermal reduction method using carbon black and B2O3 as precursor a full conversion was achieved at 1350 °C. The average particle size of the synthesized B4C powder was 260 nm, but this was reduced to 70 nm after separation of the small particle fraction from the larger particles by sedimentation. The most likely reaction was the reduction of B2O3 vapor at the surfaces of the carbon particles after its vapor transport from the liquid B2O3 [140]. 

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