Silica, spheroids

FIGURE 63.1 Diagrammatic view of a section of a silica molding powder showing amorphous silica spheroidal ultimate particles 1, lightly coalesced (less than 30%) at their contact points 2 to form micropore voids 3. 

As pointed out earlier (Section 3.5), certain shapes of hysteresis loops are associated with specific pore structures. Thus, type HI loops are often obtained with agglomerates or compacts of spheroidal particles of fairly uniform size and array. Some corpuscular systems (e.g. certain silica gels) tend to give H2 loops, but in these cases the distribution of pore size and shape is not well defined. Types H3 and H4 have been obtained with adsorbents having slit-shaped pores or plate-like particles (in the case of H3). The Type I isotherm character associated with H4 is, of course, indicative of microporosity. 

Perfect spheres are rare, but spheroidal particles are present in some powders produced at high temperature (e.g. pyrogenic silicas) or by the sol-gel process. The term sphericity is useful for some purposes. Sphericity has been defined in various ways, the simplest definition being the ratio of the surface area of a sphere of the same volume as a given particle to the actual surface area of that particle (Allen, 1990). 

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. 

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). 

In deep-sea sediments, opal-CT commonly occurs as platy, blade-shaped crystals that are often arranged in spheroidal rosettes a few /zm in diameter. These rosettes are known as lepispheres (Wise and Kelts, 1972). The nearly euhedral crystal habit of natural lepisphere crystals indicates that the opal-CT precipitated in situ as an authigenic mineral this conclusion is supported by results of experimental studies (Oehler, 1973) in which completely euhedral lepisphere crystals were synthesized from amorphous silica. Thus, it appears that the conversion of biogenic silica (opal-A) to opal-CT occiurs principally, if not exclusively, through a solution-reprecipitation mechanism. 

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. 

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. 

Kulak A et al (2003) Controlled assembly of nanoparticle-containing gold and silica microspheres and silica/gold nanocomposite spheroids with complex form. Chem Mater 15(2) 528-535... 

By this invention metal-modified silica-based catalysts are produced in such a way that the metal-modified, spheroidal, silica particles of uniform size become closely packed, thus having a porosity in the interstices which is of a uniform size and distribution throughout the catalyst body. A further advantage of the catalysts of this invention is that the pores, being between particles which are already closely packed, resist further collapse when the catalysts are subjected to the elevated temperatures required in catalytic reactions. 

Also as observed by electron microscopy the ultimate silica particles in the sol should be uniformly-sized and spheroidal. By uniformly-sized is meant that 75% of the total number of particles have a diameter in the range from 0.5D to 2D, where D is the number average particle diameter. The uniform size of the particles is important in obtaining uniform voids in the formed catalysts. The uniformity of the particles can be determined by methods described in the Journal of Physical Chemistry, 57 (1953), page 932. 

FIGURE 63.4 Multiplicity of spheroidal amorphous silica powder particles 7 such as obtained by spray-drying a silica sol by a process of the invention, the individual powder particles having the structure of the aggregate of Figure 63.2 and being made up of ultimate amorphous silica particles 1. 

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