Semi-silica

Fig. 9. The Na20-semi-silica vertical section of the Na20-Al203-Si02 system.

Semi-silica compositions The NazO-95% Si02/5% A1203 vertical section (Fig. 9) shows that as Na20 reacts with a semi-silicate refractory, all Si02 phases should disappear, and tridymite, the only stable form of silica, will form very slowly. Albite will form only at temperatures below 1050°C and liquid is present down to 740°C. Semi-silica refractories are not resistant to alkali attack, and may be even less alkali-resistant than high-silica refractories. 

Fig. 15. Photograph of semi-silica refractory before and after reaction with soda vapor at 1350°C/12 h.

Semi-silica Refractory Semi-siliceous Refractory... 

Semi-silica Refractory Semi-siliceous Refractory. See siliceous REFRACTORY note, however, that ASTM-C27 defines a semi-silica refractory as containing 72% Si02. A tentative ISO recommendation defines a semi-siliceous refractory as containing 85-93% SiOj. 

There is another variety of silica bricks called semi-silica bricks. The raw materials for these bricks are low-grade ganisters or artificial sand/clay mixtures. The properties are in between those of silica and firebricks. They are inexpensive with low after-shrinkage—an advantage over firebricks. Semi-silica bricks have better spalling resistance than silica bricks. Once the surface is glazed, they are not attacked by slag. They exist in a metastable state. Their structure can be visualized as a matrix of firebrick in which silica particles are embedded. 

Semi-silica bricks are cheaper than silica bricks and are used as backing for silica bricks. In this position, conditions are less severe. Semi-silica bricks are used in coke ovens, kiln roofs, and in flues. 

Today, sand bricks are used for lining furnaces such as L-D converters. These have properties similar to semi-silica bricks and are cheaper and quicker to install than normal bricks. They last as long as regular bricks and tiieir thermal conductivity are low. This reduces the heat loss from the furnace. Small induction melting furnaces with up to a three-ton capacity are lined with pure silica sand, which is rammed against a steel former. Afterward, induction heating fires it. 

The properties of firebricks depend on the amoimt of almnina present in them. Therefore, they are classified based on their alumina content. The first class is aluminous firebrick, which contains 38%-45% alumina. When the percentage is between 22 and 38, the brick is called ordinary. The third class is semi-silica firebrick. It contains 10%-22% alumina. 

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

The materials originally used as stationary phases for GPC were the xerogels of the polyacrylamide (Bio-Gel) and cross-linked dextran (Sephadex) type. However, these semi-rigid gels are unable to withstand the high pressures used in HPLC, and modern stationary phases consist of microparticles of styrene-divinylbenzene copolymers (Ultrastyragel, manufactured by Waters Associates), silica, or porous glass. 

The more useful types of chirally active bonded phases are those based on the cyclodextrins. There are a number of different types available, some of which have both dispersive or polar groups bonded close to the chirally active sites to permit mixed interactions to occur. This emphasizes the basic entropic differences between the two isomers being separated. A range of such products is available from ASTEC Inc. and a separation of the d and / isomers of scopolamine and phenylephrine are shown in figure 4. The separations were carried out on a cyclodextrin bonded phase (CYCLOBOND 1 Ac) that had been acetylated to provide semi-polar interacting groups in close proximity to the chiral centers of the cyclodextrin. The column was 25 cm long, 4.6 mm in diameter and packed with silica based spherical bonded phase particles 5pm in diameter. Most of the columns supplied by ASTEC Inc. have these dimensions and, consequently, provide a... 

This method was developed by Johnston et al. in 1991 and well described in Ref. [10], according to which the method is introduced as follows. The principle of optical interference is shown schematically in Fig. 1. A coating of transparent solid, typically silica, of known thickness, is deposited on top of the semi-reflecting layer. This solid thus permanently augments the thickness of any oil film present and is known as a "spacer layer. The destructive interference now obeys the equation ... 

The disk was coated with a 20 nm sputtered chromium semi-reflecting layer, a silica spacer layer was sputtered on top of the chromium. This spacer layer varied in thickness in the radial direction, but was approximately constant circumferentially round the disk [10]. 

Prior to the development of modern SPE formats, liquid-solid partitioning with charcoal, silica, Florisil, and/or alumina was common to aid in the removal of lipids in the determination of nonpolar pesticides, but these sorbents are less useful in the cleanup of semi-polar and polar pesticides owing to the large elution volumes needed. Applications of modern SPE are discussed in Section 3.2. 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

It is very difficult to obtain values for the intrinsic hardnesses of silicate and related types of glass. Therefore, no attempts at quantitative analyses will be made here. A semi-empirical method has been proposed by Yamane and Mackenzie (1974) based on the geometric mean of bond strength relative to silica, shear modulus, and bulk modulus. For 50 silicate glasses it yields estimates within ten percent of measured values, and for a few non-silicate glasses it is quite successful, as Figure 14.2 indicates. 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

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