Fast precipitation

Fig. 6. Phase diagram showing the composition pathway traveled by the casting solution during precipitation by cooling. Point A represents the initial temperature and composition of the casting solution. The cloud point is the point of fast precipitation. In the two-phase region tie lines linking the...

Currently proposed licensing regulations for geologic nuclear waste repositories require a performance assessment involving long-term predictive capabilities. Previous work (J- 5) has shown the importance of solubility controls for modeling maximum actinide concentrations in repository groundwaters. However, until reliable data are available on the actinide solid phases that may be present or that may precipitate in the environment, the solubility of solid phases such as hydrous oxides that have fast precipitation kinetics can be used to initially set maximum solution concentration limits. 

M 54] [P 48] Separation-layer micro mixers with concentric multi-layered outlets can be operated in a droplet-forming mode [53] If fast precipitating solutions are contacted in this way with a solvent layer for initial separation, the part of the droplet close to the tube outlets remains transparent, which demonstrates that a tri-layered system still exists with the two reacting solutions not being intermixed, as evidenced by calcium carbonate formation in aqueous solutions as described in [39,136], At the droplet end cap the layers collide and circulation flow sets in. As a result, mixing is achieved and precipitation occurs. The circulation patterns are visualized by the particle trajectories. 

M 54] [P 50] For the concentric separation-layer mixer detailed investigations on the robustness of the reactor concept were undertaken [53], The operation time until plugging was monitored for three well-known fast-precipitating reactions from inorganic and organic chemistry. 

Fouling prevention under fast precipitating conditions... 

M 54] [P 48] The ability of the separation-layer concept to prevent fouling even under severe conditions was demonstrated for two fast precipitating reactions, bisquaternization of 4,4 -bipyridyl and amide formation from acetyl chloride [39], Stable operation for a few hours, and in selected cases longer, was achieved. 

As pH levels increase above 9.3 and in the presence of carbonate, barium carbonate becomes the dominant species (Bodek et al. 1988 Singer 1974). Barium carbonate also exhibits fast precipitation kinetics and very low solubility and in alkaline environments limits the soluble barium concentration (Faust and Aly 1981 Hem 1959 Rai et al. 1984 Singer 1974). Barium forms salts of low solubility with arsenate, chromate, fluoride, oxalate, and phosphate ions (Bodek et al. 1988 EPA 1983a Kunesh 1978). The chloride, hydroxide, and nitrate of barium are water-soluble (Bodek et al. 1988 EPA 1983a Kirkpatrick 1978) and are frequently detected in aqueous environments (Rai et al. 

This method typically results in disordered materials as the fast precipitation does not allow for ordered crystal growth. The researchers foimd that this method produced amorphous catalysts (as shown by electron diffraction) that had very high specific activities relative to those of the traditionally produced materials. 

The dispersity of hydroxide precipitates can be governed by changing precipitation conditions. Particle size increases in the following row of salts used for precipitation sulphates - chlorides - nitrates, and with passing from fast precipitation from concentrated solutions to slow precipitation from diluted ones. 

Hydrothermal routes Under ambient conditions, the low reaction temperature and fast precipitation rate have deleterious effect on the crystallization and optical performance of rare earth vanadate nanomaterials. Referring to traditional solid-state reactions, bulk YV04 Eu phosphors require a calcinations temperature above 1300 K, but it is too high for the preparation of nanomaterials. Alternatively, hydrothermal routes could provide the adequate energy for solution phase reactions, which have been widely described in preparation of ceramic powders. The high pressure and temperature largely promote the dissolution-reprecipitation process, so as to decrease the lattice defects of NCs. With fine modulation, this method is also efficient to produce nano-sized crystals. 

Mixed metal alkoxide systems are also of interest as a means of creating additional hybrid systems. However, recognition of the large differences in their hydrolysis and condensation rates is crucial. For example, if titanium isopropoxide is made to react under the same conditions as might be used for TEOS, hydrolysis and condensation rapidly occur and lead to particulate rather than network formation of Ti02- Cocondensation with TEOS under these conditions does not occur because of the fast precipitation of the titanium dioxide species. Indeed, of the general metal alkoxides, those based on silicon tend to be more easily controlled because of their slower hydrolysis... 

Although supersamration for crystallization in the pharmaceutical industry is usually measured as a difference or ratio of absolute solution concentrations, the solubility product is most often used for very fast precipitation, which is more common with inorganics. 

The increase in size of water consumption and decrease of water quality due to man s impact make the problem of water purification and treatment from different contaminants, such as suspended and colloidal disperse particles, more acute. The efficiency of dispersion precipitation can be greatly improved with the use of flocculants - high-molecular compounds that have the ability to adsorb themselves on disperse particles and form fast precipitating aggregates [1], Both natural and synthetical water soluble polymers can be used as flocculants. The most commonly used and the most active of these polymers are polyacrylamide (PAA) and some of its derivatives. 

Micromrxers provide very fast mixing, down to milliseconds or even below [9, 26]. The concentration profiles are uniform and there is good agreement between experiment and theory, which may make process development more predictable. Results at the laboratory scale may be maintained throughout the scale-out steps, since the fluid dynamics are not changed or at least shift by analogy in a known manner. This is particularly valid for very fast precipitations such as with Yellow 12 manufacture. Here, the seed formation and agglomeration process is impacted... 

The same compound has a crystalline yellow modification of known structure in which the molecules are packed so that Au atoms are positioned far from each other. The RDF of the polycrystalline powder of this phase is shown in Fig. 9b. It contains no strong peaks near 3 A typical of Au-Au bonds. Comparison of two curves confirms the above interpretation of the strong peaks in the lower curve experimentally. This is a curious example in which the metal-metal interactions occur in a kinetically preferable metastable amorphous phase under conditions of fast precipitation of the substance from a solution, whereas a stable crystalline phase with no metal-metal contacts is formed by slow precipitation. ... 

The formation of macrovoids in these asymmetric polyaniline hollow fibers is undesirable as they weaken the mechanical strength and may lead to defects in the selective layer, which make them nonviable for many separations applications. The formation of macrovoids in hollow fiber membranes is generally caused by fast precipitation kinetics of the polymer solution in the coagulation bath. The formation of macrovoids when spinning polyaniline hollow fibers was foimd to be highly dependent on... 

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