The Reduction Process

The reduction of the industrial catalyst has been extensively studied, [89-92]. 

The reduced catalyst mainly consists of metallic Fe while the promoters remain in their oxidic state. The reduction process serves two purposes, firstly the surface of metallic iron is the active structure, and secondly the removal of the oxygen makes the material porous and increases the surface area by a large amount. 

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The complete reduction of zinc oxide is favoured by a small value of K. i.e. when log,u A, > log,o X,. Figure 3.5 shows plots of logio Xi, and logio X2 against 1/T where the two graphs intersect log)o X for the reduction process is zero and hence X = 1. 

Although the reduction process is not always a reversible one, oxidation and reduction potential values can be sometimes related to the Hiickel energies of the highest and lowest filled molecular orbital of the dye (108). 

Redox reactions, such as that shown in equation 6.22, can be divided into separate half-reactions that individually describe the oxidation and the reduction processes. 

Equivalent mechanical behavior can be achieved by either time (or frequency) or temperature manipulation. As noted in Sec. 3.2, results measured at different temperatures can be reduced to a common temperature to describe response over a wide range of times. We shall consider data reduced to a common temperature in this chapter and discuss the reduction process in Chap. 4. 

The gangue content of DRI is typically comprised of oxides such as Si02, AI2O2, CaO, MgO, Ti02, K2O, Na20, MnO, etc, and is dictated by the chemistry of the iron ore used. The phosphoms in DRI is normally in the form of P2 5- Sulfur content in the DRI depends on the sulfur level in the ore and reductant, and the amount of sulfur released or absorbed by the DRI during the reduction process. 

Refining Processes. AH the reduction processes yield an impure metal containing some of the minor elements present in the concentrate, eg, cadmium in 2inc, or some elements introduced during the smelting process, eg, carbon in pig iron. These impurities must be removed from the cmde metal in order to meet specifications for use. Refining operations may be classified according to the kind of phases involved in the process, ie, separation of a vapor from a Hquid or soHd, separation of a soHd from a Hquid, or transfer between two Hquid phases. In addition, they may be characterized by whether or not they involve oxidation—reduction reactions. 

Reduction of uranium tetrafluoride by magnesium metal has been described in detail (40,53). It is often referred to as the Ames process, since it was demonstrated at the Ames Laboratory in early 1942. The reaction is very exothermic and the reduction process is carried out in a sealed bomb due to... 

Preparation of Dispersion. The reduction process is a two-phase reaction between soluble reducing agent and insoluble dye particles, and therefore the rate of reduction is influenced by the particle size distribution of the dye dispersion. The smaller the particle size the greater the surface area and hence the more rapid the reduction process. However, if the particles are too small, migration will occur in continuous dyeing. It is therefore extremely important to control the size and range of particle size and this is a closely guarded piece of dyestuff manufacturers know-how. 

Since the reforaiing of CH4 produces 1 mole of CO for each 2 moles of H2, the dominant heat effect in the reduction process is the endothermic reduction by hydrogen. However, since the reforming process is canied out with ah as the source of oxygen, the heat content of the nitrogen component is a drermal reservoir for die overall reduction process. 

The term Birch reduction was originally applied to the reduction of aromatic compounds by alkali metals and an alcohol in ammonia. In recent years many chemists have used the term to include all metal-ammonia reductions, whether an alcoholic proton source is present or not. The author prefers to use the term Birch reduction to designate any reduction carried out in ammonia with a metal and a proton donor as or more acidic than an alcohol, since Birch customarily used such a proton donor in his extensive pioneering work. The term metal-ammonia reduction is best reserved for reductions in which ammonia is the only proton donor present. This distinction in terminology emphasizes the importance of the acidity of the proton donor in the reduction process. 

Next consider the energetics of reduction. Calculate AH for the first step in the reduction process using free reagents (BHT and AlHT). Energies for formaldehyde, and for the two intermediate adducts are provided at left. Which reduction is thermodynamically more favorable Are these results consistent with the predictions made using atomic charges ... 

For the Birch reduction of mono-substituted aromatic substrates the substituents generally influence the course of the reduction process. Electron-donating substituents (e.g. alkyl or alkoxyl groups) lead to products with the substituent located at a double bond carbon center. The reduction of methoxybenzene (anisole) 7 yields 1-methoxycyclohexa-1,4-diene 8 ... 

The scheme of the interaction mechanism (Equation 88) testifies to an electro-affinity of MeFe" ions. In addition, MeFe" ions have a lower negative charge, smaller size and higher mobility compared to MeF6X(n+1> ions. The above arguments lead to the assumption that the reduction to metal form of niobium or tantalum from melts, both by electrolysis [368] and by alkali metals, most probably occurs due to interaction with MeF6 ions. The kinetics of the reduction processes are defined by flowing equilibriums between hexa-and heptacoordinated complexes. 

Attempts to adopt the process of direct reduction of K2TaF7 by sodium for the production of capacitor-grade powders are continuing in the direction of improving both the reduction process and the chemical leaching efficiency. Purushotham et al. [585] analyzed the main characteristics of the powder produced by direct reduction of the diluted melt. It was reported that the powder contained higher levels of impurities, but the authors nevertheless believe that the impurity levels can be reduced to acceptable limits. 

It is, therefore, required that all initial compounds be dried properly prior to performing the reduction. This procedure is not at all trivial and refers, first of all, to the diluent salts, and especially to potassium fluoride, KF, which is characterized by a strong hygroscopic property and a tendency to form stable crystal hydrates. The problem of contamination due to hydrolytic processes can usually be resolved in two manners. The first is to apply another tantalum-containing complex fluoride compound that does not undergo hydrolysis. The second involves the adjustment of the reduction process parameters and use of some additives that will "collect" the oxygen present, in the form of water, hydroxyl groups or other compounds. 

According to the method developed by Izumi [593], magnesium chloride is added to the reactor as a diluent, along with K2TaF7 and NaCl, prior to the reduction process. The powder obtained by the above method is washed and treated thermally at 1200°C in vacuum. The final product has a specific capacitance of 12,000 p.C/g, contains 1800 ppm of oxygen, and its Mg content is as low as 20 ppm. 

The oxygen level in primary tantalum powder can be also increased by adjustment of the reduction process parameters [594] or by controlled stepwise additions of sodium to the reactor [595]. 

The concentration of K2TaF7 in the initial melt is the main parameter controlling the particle size and surface area of the reduced primary powder [598]. Typically, the increased concentration of K2TaF7 leads to the formation of coarse tantalum powder. According to Yoon et al. [599], the diluent prevents a strong increase in the temperature of the melt that is caused due to the exothermic effect of the reduction process. Based on the investigation of the reduction process in a K2TaF7 - KC1 - KF system, it was shown that increased amounts of diluent lead to a decrease in particle size of the obtained tantalum powder. 

The uniformity of tantalum powder is also a veiy important parameter of capacitor-grade tantalum powder. The loss of powder uniformity can initiate during the regular reduction process due to varying conditions at the beginning and end of the reduction process. At the end of the process, the concentration of tantalum in the melt is very low, while the sodium content increases. Based on the complex structure model of melts, it should be noted that the desired particle size of the powder is formed at the veiy beginning of the process, while the very fine fraction forms at the end of the process, independent of the initial content of the melt. The use of special equipment enables to perform a continuous reduction process with simultaneous loading of K TaFy and sodium, which can influence the improved uniformity of the primary powder [592,603,604],... 

Figure 21. Angular movement of the fee end of a bilayer during the flow of a cathodic current using the conducting polymer as cathode. A platinum sheet (left side of the picture) is used as anode. The reference electrode is observed at the bottom, a to e Movement during the reduction process e to a Movement under flow of an anodic current. The movement is stopped at any intermediate point (a, b, c, d, or e) by stopping the current flow, and this position is maintained for a long time without polarization.

Azoxy compounds can be obtained from nitro compounds with certain reducing agents, notably sodium arsenite, sodium ethoxide, NaTeH, NaBH4—PhTeTePh, and glucose. The most probable mechanism with most reagents is that one molecule of nitro compound is reduced to a nitroso compound and another to a hydroxylamine 119-42), and these combine (12-51). The combination step is rapid compared to the reduction process. Nitroso compounds can be reduced to azoxy compounds with triethyl phosphite or triphenylphosphine or with an alkaline aqueous solution of an alcohol. ... 

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