Carburizing of metals

Fluid-solid reactions include thermal decomposition of minerals, roasting (oxidation) of sulfide ores, reduction of metal oxides with hydrogen, nitridation of metals, and carburization of metals. Each t3 e of reaction will be discussed finm the thermodynamic point of view. Then reaction kinetics for all of the various rate determining steps in fluid-sohd reactions will be discussed for two general models shrinking core and shrinking particle. 

The amount of metal available on the catalyst surface following C03O4 reduction rather than its dispersion determines the extent of CO chemisorption, and so the presence or absence of FT activity. Dissociative chemisorption, carbide formation and coke formation appear therefore to have been dynamic processes with migration of Co° atoms occurring as progressive carburization of metal clusters occurred. Thus metal dispersion, as measured by CO absorption at room temperature, is of questionable utility in terms of a description of FT activity since the Co° atoms are static. 

Metal Dusting. Metal dusting is a phenomenon of catastrophic carburization of metals and alloys, which are exposed to carburizing atmospheres (54). This exposure can lead to disintegration of the metals into metal particles and carbon. As such, this phenomenon is a very dangerous one and can lead to leaks in plant equipment and piping. 

Processing. Tungsten carbide is made by heating a mixture of lampblack with tungsten powder in such proportions that a compound with a combined carbon of 6.25 wt % is obtained. The ratio of free-to-combined carbon is of extreme importance. Tantalum and titanium carbides are made by heating a mixture of carbon with the metal oxide. Multicarbide powders, such as M02C—WC, TaC—NbC, and TiC—TaC—WC, are made by a variety of methods, the most important of which is carburization of powder mixtures. 

Carburization by Thermal Diffusion. Carburization of chemically processed metal or metal-compound powders is carried out through sohd-state, thermal diffusion processes, either in protective gas or vacuum. Carbide soHd solutions are prepared by the same methods. Most carbides are made by these processes, using loose or compacted mixtures of carbon and metal or metal-oxide powders. HaUdes of Group 5 (VB) metals recovered from ores by chlorination are similarly carburized. 

The equation for the rate of oxidation of the transition metals at high temperatures, which form a solid solution of oxygen before the oxide appears at the surface has die same form as that derived for die carburizing of die metal, and... 

Niobium carbide may be obtained by carburization of the metal substrate according to the following reaction at low pressure and at temperatures above 2000°C ... 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Carburization was carried out in a fixed-b i stainlras strel reactor (ID = 6 mm). Typically about 0.5 g of metal sulphide was sandwiched between 2 layers of quartz wool placed centrally in a temperature-programmed furnace. iTie sample was heated at 10 Kmin" to the... 

The quadrupole doublet has an isomer shift corresponding to iron in the ferric or Fe " state. After reduction in H2 at 675 K the catalyst consists mainly of metallic iron, as evidenced by the sextet, along with some unreduced iron, which gives rise to two doublet contributions of Fe " and Fe " in the centre. The overall degree of iron reduction, as reflected by the relative area under the bcc ion sextet, is high. Fischer-Tropsch synthesis at 575 K in CO and FI2 converts the metallic iron into the Flagg carbide, Fe5C2. The unreduced iron is mainly present as Fe ". Exposure of the carburized catalyst to the air at room temperature leaves most of the carbide phase unaltered but oxidizes the ferrous to ferric iron. 

Carburization of rhenium filaments has been used to optimize Th and Pa ionization efficiency for TIMS analysis on single filaments (Esat 1995). ReC has a greater work function than Re metal, and elemental oxidation state is maintained in the reduced or metal state by the presence of carbon in the filament. Using this method and a mass spectrometer with improved ion optics, Esat (1995) was able to improve Th transmission and ionization efficiency by about a factor of 30 over conventional methods. Using more conventional mass spectrometry, Murrell et al. (personal communication) were able to improve ionization efficiency for Pa and Th by a factor of 5-10 over conventional graphite sandwich loads on Re filaments (Goldstein et al. 1989 Pickett et al. 1994). For Pa analysis, one drawback is that Pa and U ionization commonly overlap using this... 

The conversion of iron catalysts into iron carbide under Fischer-Tropsch conditions is well known and has been the subject of several studies [20-23], A fundamentally intriguing question is why the active iron Fischer-Tropsch catalyst consists of iron carbide, while cobalt, nickel and ruthenium are active as a metal. Figure 5.9 (left) shows how metallic iron particles convert to carbides in a mixture of CO and H2 at 515 K. After 0.5 and 1.1 h of reaction, the sharp six-line pattern of metallic iron is still clearly visible in addition to the complicated carbide spectra, but after 2.5 h the metallic iron has disappeared. At short reaction times, a rather broad spectral component appears - better visible in carburization experiments at lower temperatures - indicated as FexC. The eventually remaining pattern can be understood as the combination of two different carbides -Fe2.2C and %-Fe5C2. 

The first syntheses of transition metal nitrides and carbides were derived from metallurgical processes,1,3 and consisted in the nitridation or carburization of the metal or of the oxide at severe conditions, in particular at high temperatures (>1500 K). Consequently, the resulting powders generally had low specific surface areas (Sg). 

The most common failures are associated with oxidation, carburization and metal dusting, sulfidation, chlorination, and nitridation. The most common high-temperature degradation mode is oxidation, and the protection against oxidation, in general, is given by the formation of a chromium oxide scale. The presence of a small amount of aluminum or silicon in the alloy may improve the resistance against oxidation of a... 

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