Post-forming crystallization

Silica exists in a broad variety of forms, in spite of its simple chemical formula. This diversity is particularly true for divided silicas, each form of which is characterized by a particular structure (crystalline or amorphous) and specific physicochemical surface properties. The variety results in a broad set of applications, such as chromatography, dehydration, polymer reinforcement, gelification of liquids, thermal isolation, liquid-crystal posting, fluidification of powders, and catalysts. The properties of these materials can of course be expected to be related to their surface chemistry and hence to their surface free energy and energetic homogeneity as well. This chapter examines the evolution of these different characteristics as a function not only of the nature of the silica (i.e., amorphous or crystalline), but also as a function of its mode of synthesis their evolution upon modification of the surface chemistry of the solids by chemical or heat treatment is also followed. 

Moreover, if polyoxymethylene is recrystallized from nitrobenzene and the usual chain-folded lamellae of ca. 100 A thickness is treated with TXN in the presence of BF3, more perfect extended-chain-type crystals are formed. These facts confirm the conclusion formulated -by Wunderlich, that post-crystallization leads to chain-folded crystals because of kinetic restrictions, while crystallization in the polymerizing system gives thermodynamically more stable extended-chain crystals77). The formation of the thermodynamic product in polymerization is due to the growth of crystals at equilibrium polymerization conditions. Thermodynamically less stable crystals may redissolve as a result of depropagation, and crystals may thus grow further under equilibrium conditions. This (at least partly) eliminates kinetic restric-... 

For a long time the structural classification of the mineral todorokite was uncertain, until Turner and Buseck [4] could demonstrate by HRTEM investigations that the crystal structure of that mineral consists of triple chains of edge-sharing octahedra, which form [3 x 3] tunnels by further corner-sharing. These tunnels are partially filled by Mg2+, Ca2+, Na+, K+, and water (according to the chemical analysis of natural todorokites). In 1988 Post and Bish could perform a Rietveld structure determination from XRD data taken for a sample of natural todorokite [25], This diffraction study confirmed the results of Turner and Buseck. The cations... 

The separation of solids from liquids forms an important part of almost all front-end and back-end operations in hydrometallurgy. This is due to several reasons, including removal of the gangue or unleached fraction from the leached liquor the need for clarified liquors for ion exchange, solvent extraction, precipitation or other appropriate processing and the post-precipitation or post-crystallization recovery of valuable solids. Solid-liquid separation is influenced by many factors such as the concentration of the suspended solids the particle size distribution the composition the strength and clarity of the leach liquor and the methods of precipitation used. Some important points of the common methods of solid-liquid separation have been dealt with in Chapter 2. 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

Treated flue gas contains C02, N2, 02. Since N2 and 02 form hydrate crystals at approximately the same conditions the treated flue gas is considered a C02/N2 mixture. Thus in post-combustion capture from power plants the task is to separate C02 from a C02/N2 mixture in which the C02 molar content is approximately 15-20 %. Pre-combustion capture involves separation of C02 from a mixture with H2 in which the C02 molar concentration is approximately 40 % (Klara and Srivastava, 2002 Kang and Lee, 2000 Englezos and Lee,... 

On the other hand, we believe that greater attention should be focused to the fact that metal ions will form chelated complexes with azo compounds. Hence a vigorous post treatment with acids should be attempted if the stability of the product permits. Usually this treatment should be carried out before a final crystallization. 

Fig. 55a. In the presence of detergents, e.g. SDS, micellar fibers do not rearrange to crystals, because crystallization nuclei with head-to-tail sheets cannot be formed, b Electron micrograph of a 2-month-old gluconaniide D-28-8 gel, which was kept at 60 °C in the presence of SDS (molar ratio 10 1). Micelles and double helices occur (PTA 2% post-strained, bar = lOOnm). c Electron micrograph of a gel, which was kept at 20 °C and contained more SDS (molar ratio 2.5 1). Vesicles and multiple helices are apparent (PTA 2% poststained, bar — lOOnm) [377]...

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