Metal oxidation processing

Oxidation of methanol to formaldehyde with vanadium pentoxide catalyst was first patented in 1921 (90), followed in 1933 by a patent for an iron oxide—molybdenum oxide catalyst (91), which is stiU the choice in the 1990s. Catalysts are improved by modification with small amounts of other metal oxides (92), support on inert carriers (93), and methods of preparation (94,95) and activation (96). In 1952, the first commercial plant using an iron—molybdenum oxide catalyst was put into operation (97). It is estimated that 70% of the new formaldehyde installed capacity is the metal oxide process (98). 

The sulfur removed via these fixed-bed metal oxide processes is generally not recovered. Rather the sulfur and sorbent material both undergo disposal. Because the sorbent bed has a limited capacity and the sulfur is not recovered, the appHcation of these processes is limited to gas streams of limited volumetric rate having low concentrations of hydrogen sulfide. 

Time-Resolved Laser-Induced Incandescence (by Prof. Alfred Leipertz et al.) introduces an online characterization technique (time-resolved laser-induced incandescence, TIRE-LII) for nano-scaled particles, including measurements of particle size and size distribution, particle mass concentration and specific surface area, with emphasis on carbonaceous particles. Measurements are based on the time-resolved thermal radiation signals from nanoparticles after they have been heated by high-energetic laser pulse up to incandescence or sublimation. The technique has been applied in in situ monitoring soot formation and oxidation in combustion, diesel raw exhaust, carbon black formation, and in metal and metal oxide process control. 

Even more striking is the reduction of (bpy)2CuI(OH2)+ at a glassy-carbon electrode, which occurs at —1.04 V versus NHE. The difference (—0.88 V) is due to the Cu—Cu bond energy (85 kJ mol-1) that must be overcome in the metal-oxidation process. Reduction of (bpy)2Cu1Cl occurs at essentially the same potential as that for CuI(MeCN)4Cl (—1.01 V vs. NHE) 7... 

Sometimes the relationship between the rate of the parabolic metal-oxidation process (dnox/dt), limited by oxygen transport through a dense oxide film, is also called the Wagner equation [vi] ... 

INVENTA-FISCHER Formaldehyde Methanol Silver or metal oxide process, urea-formaldehyde (UF-85) 60 1989... 

This chapter, then, deals primarily with the directed metal oxidation process, although selected examples of stability in metal matrix composites are also discussed briefly. The focus is, of course, on the applications of phase equilibria, and more generally, thermodynamic principles that are applicable to the formation of composites in the presence of molten metals. Because these general principles are the same regardless of whether the end product is an MMC or a CMC, little generality is lost by focusing the discussion on CMCs formed by directed metal oxidation. 

Before describing in detail these specific examples, the next section provides a brief overview of the directed metal oxidation process. The information provided will allow a better understanding of the phase diagram and thermochemical applications that follow. 

In this section the preparation of ceramic composites by the directed metal oxidation process is described. First, in Section II.A, the aluminum oxide system is used as an example to explain the nature of the process, and a further example, a ZrB2 reinforced ZrC composite, is discussed in Section II.B. 

Fig. 1. A schematic illustration of CMC growth to net-shape using a directed metal oxidation process where the preform is formed by cold-pressing.

Fig. 3. A schematic illustration showing the various steps employed to form a tubular component by the directed metal oxidation process. The preform is formed by slip casting.

Typically, the directed metal oxidation process involves the simultaneous reaction of molten metal, e.g., A1 with Oz, and infiltration of the reaction product and metal into a porous preform of the desired reinforcement. The directed metal oxidation process can also form composites in the absence of a reinforcement phase, termed matrix-only growth. Although the former process is more interesting commercially because of the ability to tailor the composite properties and because the product does not show significant preferred orientation, the latter case is simpler conceptually and theoretically. Thus, the thermodynamic discussion will begin with growth in the absence of reinforcements and then cover the additional complications that arise from their presence. 

To illustrate the thermodynamic complexities that arise because of the presence of a molten metal in the directed metal oxidation process, a detailed analysis is presented both with and without the Si metal present. The former analysis is applicable to more traditional ceramic processing such as sintering or hot-pressing of SiC/Si3N4 composites, whereas the latter is applicable to the directed metal oxidation process, or any other composite process where molten Si may be present. 

This analysis must be modified for the directed metal oxidation process or other processing where a molten Si phase is present. When this phase is included in the analysis, reactions (16) and (17) must be modified, becoming ... 

A. S. Nagelberg, A. S. Fareed, and D. J. Landini, Production of ceramic matrix composites for elevated temperature applications using the DIMOX directed metal oxidation process. In Processing and Fabrication of Advanced Materials for High Temperature Applications (V. A. Ravi and T. S. Srivatsan, eds.), pp. 127-142. Metallurgical Society, Warrensdale, PA, 1992. 

W. B. Johnson, Reinforced Si3N4 matrix composites formed by the directed metal oxidation process. Ceram. Eng. Sci. Proc. 13(7-8), 573-580 (1992). 

Abstract Ionic liquids as green solvents have shown important application in extraction and separation of metals. In this chapter, the new application perspective and the important fundamental and applied studies of the extraction and separation of metals in ionic liquids which include metal oxide processing, mineral processing, electrodeposition of metals (especially reactive metals such as Al, Mg, and Ti), and extraction of metal ions are presented. 

Reactions in metal-sulphur systems have been studied, as model systems, to verify mechanisms that are proposed for the high-temperature oxidation processes. The excellent correspondence between the initial theories of Wagner and observation in the case of sulphidation established confidence in the theory, which was then applied widely to the more practically important metal-oxidation processes. 

FIGURE 19. Nicalon /Al203 components fabricated using the directed metal oxidation process. 

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