Dissolution-reprecipitation processes

Dissolution/reprecipitation processes were evaluated for the recycling of poly-epsilon-caprolactam (PA6) and polyhexamethyleneadipamide (PA66). The process involved solution of the polyamide in an appropriate solvent, precipitation by the addition of a non-solvent, and recovery of the polymer by washing and drying. Dimethylsulphoxide was used as the solvent for PA6, and formic acid for PA66, and methylethylketone was used as the non-solvent for both polymers. The recycled polymers were evaluated by determination of molecular weight, crystallinity and grain size. Excellent recoveries were achieved, with no deterioration in the polymer properties. 33 refs. 

The classical dissolution/reprecipitation process could explain this phenomenon. However, under the experimental conditions used, the F catalysed redistribution process is an alternative process. It is initiated by the coordination of F at silicon, inducing a catalytic... 

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. 

In nature, ferrihydrite will slowly transform to hematite or goethite. Such a mechanism is feasible in view of the much higher solubility (lower stability) of ferrihydrite (pXg = 37 39) in comparison with that of goethite and hematite (pifg = 40-44). The transformation to goethite takes place over a wide pH range by a dissolution/reprecipitation process (see Fig. 1-2). It is slow at neutral pH (months to years), but can be greatly accelerated (days)... 

For precipitation, the synthetic process consists of addition of orthophosphate anions to an aqueous solution of the metallic salt. The precipitated metallic orthophosphate was then recovered by filtration, then washed, dried at 110°C and then calcinated at various temperature. By this method CaHP04, FeP04, Ce(FlP04)2, Zr(HP04)2 were obtained. The dissolution, reprecipitation process consists of preparing a metallic salt suspension of known solubility and in adding to it an aqueous orthophosphate anion solution in order to displace the following equilibrium ... 

Many Himalayan Th-Pb monazite age data are difficult to interpret when results from a single rock are inconsistent with a single population (see also Foster et al. 2000). Monazite growth in metapelites may be due to dissolution of existing detrital grains or allanite breakdown. The dissolution/reprecipitation process is a possible age resetting and... 

The corrosion products formed may be amorphous or crystalline, depending on the rate of crystallization and formation. It is known that slow growth or aging of amorphous phases may result in a change to the crystalline state. This process can occur through slow transformation in the solid state or through dissolution-reprecipitation processes. Such is the case in... 

An additional benefit of the dissolution-reprecipitation process is that, typically, the entrapped metal becomes nonbioavailable. For example, bioavailability of Cd, Pb, Cu, Zn, and As to vegetation in contaminated soils was observed to be significantly reduced by addition of apatite (Laperche et al., 1996). Presumably, the cations substitute for Ca sites, whereas As substitutes as an oxyanion in to the anion sites. Not only did apatite addition reduce the Pb content in sudax shoot tissue from 170 to 3 mg kg the accumulated Pb in the plant root was stabilized by formation of pyromorphite (Pb5(P04)3Cl) (Laperche et al., 1996). Consequently, phosphate soil amendmenls have received considerable attention for contaminant immobi-... 

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