Magnesia compaction

Fig. 3.26 Comparison plots for compacts of silica and magnesia. In each case the adsorption of nitrogen at 78 K on the compact is plotted against that on the uncompacted powder, (a) and (b), comparison plot and adsorption isotherm for silica powder compacted at 130 ton in (c) and (d), comparison plot and adsorption isotherm for precipitated magnesia compacted at 10 ton in. Note that the upward sweep of the comparison plot commences at a relative pressure below the inception of the loop.

The evidence obtained in compaction experiments is of particular interest in the present context. Figure 3.22 shows the results obtained by Avery and Ramsay for the isotherms of nitrogen on compacts of silica powder. The hysteresis loop moved progressively to the left as the compacting pressure increased, but the lower closure point did not fall below a relative pressure of 0-40. Similar results were obtained in the compaction of zirconia powder both by Avery and Ramsay (cf. Fig. 4.5), and by Gregg and Langford, where the lower closure point moved down to 0-42-0-45p° but not below. With a mesoporous magnesia (prepared by thermal decomposition of the hydrated carbonate) the position of the closure point... 

When alumina is combined with the silica, forming a natural clay, a much more compact and fusible compound is formed with the lime than when the silica is alone. Indeed, it has been observed as a general principle, that tire point of fusion is materially affected by the relation and number of bases the whole materials contain thus, a more liquid scoria is obtsined by the addition of a limestone containing magnesia than with a pure limestone. But experience is against the use of a magnesieu limestone, because it deteriorates the iron produced, while the purity of the metal iB the primary consideration. That which contains much silica should also be used sparingly, as silica combines with the iron and injures its quality, -The purest limestones are the most suitable for flux. Common marble is nearly a pnre carbonate of lime but is too rare and expensive to be used as a flux. 

Preparaiim of the Bom Ash,—The hones usually employed in this manufacture arc those of oxen, of sheep, or of horses, The bones of sheep are preferred, as the ash which they yield is less compact, and more easily attacked by an acid. They contain, besides sabphosphate of lime, carbonate of lime, and a small per cottage of other salts, as phosphate of magnesia, chloride of sodium, fluoride of calcium, et cetera also a large proportion of animoi matter, estimated at from thirty-three to fifty per cent., according to the kind of bone, and the age of the animal. The following analysis of the bones of the ox is by Berzelius —... 

The results with magnesia led us to a planned series of experiments with doped aluminas. Nickel was evaporated in vacuo onto the surface of grains of undoped or doped alumina or, alternatively, onto compact nickel. These preparations were then used as catalysts for the donor model reaction of formic acid dehydrogenation as above. Table II shows the results. 

Crypto-crystalline (compact) magnesites with a low content of iron oxides. They are used in the manufacture of caustic magnesia (i.e. low-fired active MgO for the chemical industry, for the manufacture of oxychloride cements, etc.). When calcined at suitably high temperatures, they serve as an excellent raw material for the manufacture of refractories. 

Slurry compaction is a critical factor in the magnesia precipitation process. Compaction directly affects filtration solids, which in turn affects plant production rates. Slurry compaction can be influenced by a number of factors in the precipitation process however, it is primarily dependent upon seed density in the reactor as well as reactant concentrations. 

Detailed laboratory and field studies have shown that Paraho retorted shale can be compacted easily is not subject to dusting, erosion, or auto-ignition and can be handled to create structures of very low permeability. A basis for these beneficial properties can be found, in part, by an examination of the chemical and physical properties of the retorted shale. For example, it is believed that the strengths achieved are caused by the hydration of magnesia, reactions of the gypsum minerals, and the compaction of the silty-gravel mix. 

Figure 3. Selected exemplary magnesia-chromite RADEX VFG (a and b), RADEX 0X6 COMPACT (c and d) and alumina-chromia RESISTAL RK30SR (e and f) bricks after the rotary kiln test at 1500°-1550°C (left side a, c, e) and 1650°C (right side b, d, f). Slag coating (C). Crack formation (arrows). 

The immediate brick hot face was covered with a 1-2 mm thin reaction zone. Within this area the magnesia component is completely dissolved, only some chromite relics are still visible. Below the reaction zone a deep reaching infiltration and corrosion of the brick microstructure can be observed. The lowest infiltration of the brick microstructure was observed for RADEX 0X6 COMPACT and the highest for RADEX FM5 (see also Table I). 

From the different investigated magnesia-chromite bricks the RADEX FGLI shows the best result and the lowest solubility rate. But the difference of the various brands with exception of RADEX 0X6 COMPACT is low. 

The aim of the work is to fabricate alumina-based ceramics with high wear resistance. It is achieved by the usage of weakly aggregated nanopowders and by the apphcation of magnetic-pulsed compaction as well as by the addition of titania, magnesia and zirconia. 

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