Alumina precipitation

This reaction is used, also, to a very small extent in calico-printing. A hyposulphite of alumina is formed in the cold by adding a solution of chloride of aluminium to a sol ution of hyposulphite of soda. The result is a solution of chloride of sodium and hyposulphite of alumina. This Is thickened iu the regular manner, and printed on the fabrio. The piece is then exposed to a temperature of 212° by means of steam, when the salt is decomposed, alumina being deposited on or in the cloth. Tin s alumina so deposited dyes np with madder precisely like the alumina precipitated in the ordinary manner from the acetate. 

The Si02/Al203 ratios vary from 84 100 (Ross and Kerr, 1934, report a low value of 74 100) to 197 100. Most values are considerably lower than the theoretical value 200 100 of the kaolinite minerals. It appears that when silica and alumina precipitate, alumina is usually in excess of that required for a stable 1 1 structure. 

Vapor-phase hydrogenation results and experimental evidence of this type lead to the conclusion that catalysts on basic supports are suitable for nonsplitting prehydrogenation-type reactions and that acidic supports are best used for splitting catalysts. Activated alumina was found to be the best support because of rapid reduction of tar acids. Especially, alumina precipitated from aluminum salts at constant pH was satisfactory and produced catalysts that could be formed into pellets of high mechanical strength. 

According to this concept, the stabilizer function of alumina reduces to paracrystal-line lattice defects an analogous effect is to be expected with Cr203, Sc203, etc. [262], [296]. Another theory is based on the observation that during reduction part of the alumina precipitates with other promoters into the surface of the iron crystallite in a molecularly dispersed distribution [258], [297] or in small islands [254]. This patchy monolayer of alumina acts like a spacer between iron atoms of neighboring crystallites and prevents sintering by means of a skin effect [263]. (See also [101], [110], [253].)... 

No satisfactory method has been found for removing the 3-4% of A-fraction presumably remaining in the Pentasol non-precipitated B-traction. Bates, French and Rundle have suggested that this may be removed by repeated treatment with cotton. Attempts to apply this purification have not been successful. Various grades of cotton, cellulose pulp, charcoal, activated alumina, precipitated aluminum hydroxide, bentonite and fuller s earth have been tested, without any significant improvement in the purity of the B-fraction. It is possible that Bundle s cotton treatment may introduce traces of lipid material (fatty acids or sterols) which mask the iodine adsorption. 

Preparations took place in a stirred glass reactor equipped with sensors (pH electrode, thermocouples and torquemeter on stirring shaft) which allow to control silica gelation and alumina precipitation. 

The silica hydrogel is a compliant material whose network building-up is not completed when alumina is added. So, alumina precipitation can easily modify the silica gel structure and induce drastic texture modifications. 

The X-ray powder diffraction patterns are shown in Fig. 2. The X-ray diffraction pattern of alumina precipitated from gel shows two broad maxima at 20=46 and 67° characteristic of Y-AI2O3. Introduction of small amounts of titania markedly reduces the intensity of these peaks. Accordingly, the sample with 15% appears to be virtually amorphous to X-rays. This result is in agreement with the data of Ramirez [15]. Amorphisation of AI2O3 by Ti02 also explains the increase in Sbet (see above). 

Alumina is precipitated, as hydrate, from its solutions by potash, soda, ammonia, and their carbonates but the precipitate readily dissolves in an excess of the two first. Hydrosulphuret of ammonia, which precipitates none of the previously-described metallic oxides, causes in solutions of alumina a precipitate, which, however, is not a sulphuret, but the hydrate of alumina precipitated by the ammonia of the test. 

To further process the washed and/or (spray-) dried alumina or silica-alumina precipitates, a dough is prepared by mixing the precipitate with water. 

All operating parameters governing the alumina precipitation must be controlled to avoid variations in its effects on the hydrogel texture. 

From the results presented above, it can be assumed that the textural changes observed are for the main pa caused by silica netwodc modifications (alumina precipitation is assumed to lead to the same effects in each case). Figures la,b and 2a,b higlight the textural effects of the drying step (constriction of the gel network) and of alumina precipitation (formation of larges pores as demonstrate by curves 1). 

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