Impurities precipitation processes

The mathematical model of point defect dynamics can be adequately used on the basis of the physical model in which the impurity precipitation process occurs before the formation of microvoids or dislocation loops (V.I. Talanin I.E. Talanin, 2010b). The model of point defect dynamics can be considered as component of the diffusion model for formation grown-in microdefects. 

The calculations of the formation of microvoids and dislocation loops (A-microdefects) demonstrated that the above assumptions do not lead to substantial differences from the results of the previous calculations in terms of the model of point defect dynamics. This circumstance indicates that the mathematical model of point defect dynamics can be adequately used on the basis of the physical model in which the impurity precipitation process occurs before the formation of microvoids or interstitial dislocation loops. Moreover, the significant result of the calculations is the confirmation of the coagulation mechanism of the formation of microvoids and the deformation mechanism of the formation of interstitial dislocation loops. Therefore the model of the dynamics of point defects can be considered as component part of the diffusion model for formation grown-in microdefects. 

Synthetic hydroxyapatite prepared by mixing stoichiometric amounts of aqueous solutions of calcium nitrate and ammonium phosphate was used in this study. The pH of the boiling suspension was maintained at about 10 by flowing a mixture of NH and throughout the precipitation process. The precipitate was repeatedly washed until the conductivity of the supernatant liquid was observed to be constant. The washed sample was freeze-dried and analyzed. An elemental analysis of the batch preparation showed the Ca/P molar to be ratio 1.64, with the predominant impurity being 2... 

Once mycelia have been separated via continuous filtration from exhausted production media, citric acid may be recovered by using three different methods, such as direct crystallization upon concentration of the filtered liquor, precipitation as calcium citrate tetrahydrate, or liquid extraction. Since molasses are extremely rich in impurities, direct crystallization cannot be applied unless very refined raw materials, such as sucrose syrups or crystals, are used. The precipitation process (that is based on subsequent addition of sulfuric acid and lime to clarified fermentation broths) is used by the great majority of world citric acid manufacturers, including Archer Daniels Midland Co. (ADM) in the United States. Liquid extraction with mixtures of trilaurylamine, n-octanol, and Cio or Cn isoparaffin was used by Pfizer Inc. in Europe and Bayer Co. (formerly Haarmann Reimer Co., subsidiary of Miles) in the Dayton (OH, USA) and Eikhart (IN, USA) plants only (Moresi and Parente, 1999), even if such plants might have been shut down in 1998. 

With the system that employed these raw materials, numerous advantages of fluid mixtures were established, but a need to improve the economics of the procedure was obvious. Pure electric-furnace acid is much more expensive than wet-process acid, so ways were sought to use the cheaper but impure wet-process acids. Direct ammoniation of wet-process acid causes precipitation of numerous impurities as an intolerable, gelatinous sludge. Also, plant-food solubilities are undesirably limited in the orthophosphate system, so the concentration (grade) of the product made with electric-furnace orthophosphoric acid was limited to 8-24-0. A series of developments by TVA led the way to solving both of these problems. 

Used engine oils can be reused. Sometimes, all that is needed is removal of particles by filtration or centrifugation. In others, it may be necessary to remove volatile acids and water by heating, followed by treatment with sulfuric acid, then lime, and, sometimes, bleaching clay. A final distillation under vacuum completes the rerefining.63 Another system uses propane at ambient temperature in a continuous process.64 The additives and impurities precipitate and settle out, after which the propane is flashed off and the... 

When process solutions of Pu(III) are available, Pu(III) oxalate precipitation may be desirable if impurity levels are too high for the PuFa-precipitation process described later. 

Direct calcination of Pu(N03)4 involves no chemical separations that could remove impurities, so a highly pure plutonium nitrate feed solution is required. The plutonium dioxide product can be hydrofluorinated to PUF4, or it can be used as a feed for the formation of PUCI3. Direct calcination has received less industrial-scale application than the precipitation processes described above [C2]. 

The process is attractive in its simplicity. However, in the subsequent metallothermic reduction the Cap2 diluent absorbs a portion of the heat of reaction otherwise needed for slag melting. Also, there is less decontamination from impurities than in the case of the other precipitation processes described earlier. 

This reaction mechanism which proceeds without any intermediate reactions involving other species generally favors a compact deposit and closely obeys Faraday s law, see Eq.(5.1), making a precise control over the deposition rate and thickness possible. Furthermore, the metal deposit is much more stable than the corresponding metal oxides that might be hydrated and readily redissolve. Since the metal electroplating is not considered to be a precipitation process, as in the case of various metal oxide depositions, the inclusion of impurities from the electrolyte are less likely. 

The first step in brine purification is chemical treatment to remove certain impurities. The elements of hardness (calcium and magnesium) must be removed, along with iron and heavy metals. This is done by precipitation, adding a source of carbonate ion to remove calcium and a source of hydroxide ion to remove the other metals. Sulfate ion also can be removed by precipitation, using either calcium or barium ion. Precipitation processes cannot usefully be considered in isolation. The nature of the solids formed in this way determines how they will behave in the later processes that are designed for their physical removal. The subsections that follow therefore describe in a general way the flow behavior and settling rates of precipitated particles. Later sections of the chapter cover the details of sedimentation and filtration of the solids. 

The properties of precipitates formed from electrolyte solutions depend on the relative rates of the precipitation processes described in Section II. These in turn depend on a number of experimental factors such as the degree of supersaturation, reactant concentration ratio, temperature, ionic strength, and the presence of impurities or additives in the crystallizing solution. It is therefore essential to design reproducible experimental procedures by which to control these factors. A simple way to control the initial supersaturation and reactant concentration ratio is to rapidly mix equal volumes of known concentrations of the cationic and anionic components of an ionic precipitate (for instance, a solution of calcium chloride and sodium oxalate to obtain calcium oxalate). Knowing the initial concentra-... 

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