Alumina spheroids

Waldie 56, 57), in an attempt to prepare ultrafine powders from coarser materials, obtained spheroids of oxide powders in low-power RF torches. When silica powder (50-72 p.m) was injected into a 2.5-kW, 34-MHz argon plasma at 15 gm/hr, a 15% conversion to ultrafine particles (0.015-0.15 / m) and coarse spheroids were obtained. Ultrafine powders of barium oxide (50% <0.1 ju.m) and alumina spheroids were also prepared by this technique. When alumina was injected cocur-rently into a 3.5-kW, 10-MHz argon plasma 57), 48% spheroidization of a 180-250 /xm powder was obtained at a feed rate of 36 gm/hr. Waldie obtained better results by use of countercurrent particle flow similar to the technique used by Haiti 26). Up to 26% spheroidization of a 300-500 /xm powder was measured for an alumina feed rate of up to 140 gm/hr. It is evident from this work that countercurrent spheroidization can achieve not only higher yields of spheroids but also spheroidization of a larger size range of solid. 

Most catalysts consist of active components dispersed as small crystallites on a thermally stable, chemically inactive support such as alumina, ceramics, or metallic wires and screens. The supports are shaped into spheroids, cylinders, monolithic honeycombs, and metallic mesh or saddles. 

Spherical alumina can also be formed from commercial, low cost aluminum-oxides or even from aluminum-hydroxides. In the latter case energy of the plasma should provide not only the enthalpy of melting but that of dehydration and subsequent phase transformations of alumina as well. Under the aforementioned conditions particles below 45 pm have a good chance to be spherodized. Presumably the wide particle size distribution of starting gibbsite powder accounts for the less spheroidization rate of 70%. 

Several papers report [4] that liquid alumina solidifies not in the thermodynamically most stable phase of (X-AI2O3, but rather in the form of Y-AI2O3. This is attributed to the fact that the solidified phase structure is basically determined by the relative critical free enthalpies of nucleation of alternative crystal structures. Consequently, not surprising, that considerable part of spheroidized particles composed of y-AbOs and other metastable phases (such as 8, 0) of alumina (Fig. 7). The latter were formed from the y phase according to the usual route of phase transformation on cal-... 

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

Commercial production of synthetic silica-alumina catalysts for use in fluid cracking was initiated in 1942. The synthetic catalysts were first manufactured in ground form, but means were later developed for production in MS (micro-spheroidal) form. First shipments of the MS catalyst were made in 1946. The synthetic catalysts contain 10 to 25% alumina. Synthetic silica-magnesia catalyst has also been used commercially in fluid-catalyst units (19,100). Magnesia content is 25 to 35% as MgO (276). 

Fluid grades of synthetic silica-alumina catalyst are manufactured by the American Cyanamid Company, The Davison Chemical Corporation, Morton Salt Company, and National Aluminate Corporation. At first the catalyst was dried and ground to produce the desired range of particle sizes. It was later found that by using spray driers micro-spheroidal particles of the desired size distribution could be produced directly without any grinding (7,10,145). The particle-size distribution can be altered, within limits, by changing the spray-drier conditions (145). 

Since the dominant feature of packings of spheroidal particles is the constrictions between the tetrahedral cavities formed by the Alumina microspheres, a more realistic model is required, based on the random sphere packing models. Such models are obviously more complex. Conversely, they permit a more realistic representation of the pore space among the spheroidal particles. A preliminary model has been reported for sorption [20] and relative permeability Pr [21]. 

The catalyst used is a commercial catalyst known as the super-D manufactured by Crosfield Chemicals Ltd., UK. It is in the form of particulate spheroid with an average diameter of 81 microns and consists of 15-18% ion exchanged Re sodium Y-zeolites on a support silica-alumina matrix. Heat treatment of catalyst particles at 150°C for 48 hours is undertaken before cracking reaction commenced. The isopropyl benzene (cumene) has the purity higher than 99.5% which was supplied by Fissons Scientific Apparatus. 

Snamprogetti has commercialized fluidized-bed dehydrogenation (FBD) for the catalytic dehydrogenation of light paraffins using a chromia-alumina catalyst with an alkaline promoter, which is used primarily for the dehydrogenation of isobutane to isobutylene in the manufacture of MTBE. The catalyst is micro-spheroidal with an average diameter of <100 pm and... 

Sintered deposits form at the furnace exit at lower gas temperatures and in zones subject to rapid changes in direction. The deposit is composed of spheroidal particles, <40p, bound together by a molten substance. In those cases where substantial quantities of coarse pyrites are liberated from the pulverized coal, the spheroids are nearly pure FeaOa, as shown in Figure 11. The matrix contained silica, alumina, iron, and potassium, and has an initial deformation temperature of 1832°C, as determined by differential thermal analysis. The heavier pure iron spheroids deposit as a result of inertial impact. The mineral source of the molten phase is most likely illite. 

Figure 7 shows spheroids of gamma type alumina about 3 mm in diameter used as supports in automobile exhaust catalysts. 

A. J. Slavin, F. A. Londry, and J. Harrison. "A new model for the effective thermal conductivity of packed beds of solid spheroids Alumina in helium between 100 and 500°C," Int J. Heat Mass Transfer, 43, 2059-2073, 2000. 

LiOH, A1(0H)3, and triethanolamine (TEA) in ethylene glycol (EG) while distilling off product water, is a glassy thermoplastic at room temperature, and dissolves in polar solvents, e.g. ethanol. Spheroidal, particles of <100 nm diameter were formed by flame-spray pyrolysis of ethanolic solutions of the precursor. X-ray powder diffraction showed the as-formed powders to be an intermediate phase, possibly m-alumina, which transforms to p"-alumina on heating above 1200 C. 

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