Sodium silica formers

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

The usual extraction procedure is to roast the crushed ore, or vanadium residue, with NaCl or Na2C03 at 850°C. This produces sodium vanadate, NaV03, which is leached out with water. Acidification with sulfuric acid to pH 2-3 precipitates red cake , a polyvanadate which, on fusing at 700°C, gives a black, technical grade vanadium pentoxide. Reduction is then necessary to obtain the metal, but, since about 80% of vanadium produced is used as an additive to steel, it is usual to effect the reduction in an electric furnace in the presence of iron or iron ore to produce ferrovanadium, which can then be used without further refinement. Carbon was formerly used as the reductant, but it is difficult to avoid the formation of an intractable carbide, and so it has been superseded by aluminium or, more commonly, ferrosilicon (p. 330) in which case lime is also added to remove the silica as a slag of calcium silicate. If pure vanadium metal is required it can... 

Formerly derived from the natural mineral lapis lazuli, ultramarine blue pigments have, for more than a century, been manufactured synthetically. The materials used in the manufacture of ultramarines are china clay (a hydrated aluminosilicate), sodium carbonate, silica, sulfur and a carbonaceous reducing material such as coal tar pitch. For the manufacture of the blue pigments, the blend of ingredients is heated to a temperature of 750 800 °C over a period of 50-100 h, and the reaction... 

Several silaceous materials have been used successfully to induce flow. Among those quoted in the literature are pyrogenic silica in concentrations as low as 0.25% and hydrated sodium silioaluminate in concentrations of around 0.75%. The former has the additional property of being able to scavenge moisture, which might otherwise contribute to restricted flow characteristics. 

Increased Surface Area of Walls. If glass columns are used the surface area may be increased by treatment with hot ammonium or sodium hydroxides. If aluminum columns are used a similar treatment can be employed. In the former case a layer of porous silica is formed, in the latter a layer of porous alumina. 

Silica sol which is a stable sol of monodispersed particles is made by the decomposition of sodium silicate at low concentration under controlled pH and surface treatment and then evaporation of water. A stable floccular aggregates of silica particles are made by the peptization of dilute hydrogels under controlled pH. The former material is used as fiber and paper sizing, binder and the latter is used for water treatment. 

Lhommet and co-workers synthesized (- )-436 by the route shown in Scheme 66 [422). Wittig-Homer reaction between the 2,5-rrans -disubstituted pyrrolidine aldehyde 509, made from (S)-pyroglutamic acid [423), and the protected keto-phosphonate 510 introduced all the skeletal carbon atoms of the target. Simultaneous hydrogenation and //-deprotection of enone 511 gave the aminoketone 512, which spontaneously formed the bicyclic enamine 513 in 98% yield when exposed to trifluoroacetic acid—apparently the first time that such an intermediate has actually been isolated en route to indolizidines. Reduction with sodium cyanoborohydride in acidic medium acid produced a diastereomeric mixture (92 8) of (- )-436 and (+ )-437. The former was isolated in 84% yield after chromatography on silica gel. The overall yield of this 15-step sequence was 8% based on (S)-pyroglutamic acid. 

Because the only variable changed in this dissolution study was the type of alkali metal hydroxide, differences in dissolution rate must be attributed to differences in adsorption behavior of the alkali metal cations. The affinity for alkali metal cations to adsorb on silica is reported (8) to increase in a continuous way from Cs+ to Li+, so the discontinuous behavior of dissolution rate cannot simply be related to the adsorption behavior of the alkali metal cations. We ascribe the differences in dissolution rate to a promoting effect of the cations in the transport of hydroxyl anions toward the surface of the silica gel. Because differences in hydration properties of the cations contribute to differences in water bonding to the alkali metal cations, differences in local transport phenomena and water structure can be expected, especially when the silica surface is largely covered by cations. Lithium and sodium cations are known as water structure formers and thus have a large tendency to construct a coherent network of water molecules in which water molecules closest to the central cation are very strongly bonded slow exchange (compared to normal water diffusion) will... 

This deionized colloidal silica is not a strong binder. The removal of the sodium hydroxide, which appears to act as a binder catalyst, and/or the presence of the alumina on the surface, weakens the overall bond strength developed. However, this acidic version of colloidal silica does form stable mixtures with several water soluble polymers, the most important of which is polyvinyl alcohol (PVA). PVA is itself a good film former and imparts this property of much improved film formation to colloidal silica even when present as only about 3-5% of the total solids. The mixture of PVA and acid stabilized colloidal... 

Figure 9.6 would generally be accepted as the more conventional route. The purification stages are based upon sodium hydroxide breakdown and TBP purification, now that satisfactory methods are available for the removal of residual silica from the former, which would otherwise cause... 

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