Zone refining considerations

Figure 4.6 is interesting because it shows that for the best refining performance we need both a long zone and an impurity that is relatively insoluble in the solid (low k). Unfortunately long liquid zones can be destabilised by convection, and impurities with a low k do not come to order Commercial zone refining processes may therefore involve a large number of passes done one after the other (Fig. 4.7). This obviously adds a lot to the cost of the pure material, but the process can be speeded up considerably by using the multi-heater arrangement shown in Fig. 4.8.

Since the results of zone refining depend on the interaction of momentum, heat and mass transfer in the system, all the basic factors affecting these three processes, both molecular and convective, have to be taken into consideration. These basic factors are concentration W, Density f, viscosity /i, heat capacity Cp, temperature den-sification coefficient, thermal conductivity k, molecular dif-fusivity D, zone diameter d, zone length L, zone travel speed u, temperature difference in zone A T and acceleration g. The concentration W may affect, JJi, Cp,, k, and D as well as the properties of the P.S.Z. (mushy region). Aside from the concentration W, all... 

In the case of the solids,initial experiments showed that the conventional purification techniques such as recrystallization from solutions do not offer sufficiently pure materials. For example,PMDA samples, after several recrystallizations and vacuum sublimations appear colorless,but show high levels of ionic contaminations when analyzed for inorganic ions. To eliminate this problem,the technique of zone refining as used in semiconductor materials purifications has been used to purify the starting materials. This technique has been applied for a number of organic materials in our laboratory. PMDA for example,when subjected to a simple zone refining process ( as few as 25 zones), shows removal of impurities as high as 3% even if preceded by recrystallization and sublimation. In syntheses of polyamic acids,a variation as small as 3% in stoichiometry can cause considerable variation in the final batch-to-batch synthesis. 

The preceding discussion assumes that no convection exists in the melt, and this is rarely, if ever, the case. Next we shall consider two approaches which account for convection in the melt, a transport mechanism which is especially important in mass transfer because Dl is small and even weak convection markedly alters solute concentration profiles and may cause macrosegregation. First we shall discuss film theory which is a very simple approach that gives qualitative information and often provides considerable physical insight into the mechanisms involved. Second, we shall discuss a simplified model of zone refining. 

The speed of zoning is a factor that can have a considerable effect on the effieiency of zone refining. The correct speed is that which gives a uniform zone passage and at the same time permits impurities to diffuse away from the... 

Mechanism of pH-zone-refining CCC is shown in Figure 13A, which schematically illustrates a cross-sectional view of the separation column where three acidic compounds are eluted with a mobile phase containing ammonia and through a stationary phase containing TFA. Due to its nonlinear isotherm, the retainer (TFA) forms a sharp rear border that moves at a rate considerably lower than that of the mobile phase. Three analytes. Si, S2, and S3, competitively form solute zones behind the sharp TFA border according to their pX and hydrophobicity. Among... 

In general, materials Involved in the monolayer formation, such as spreading solvent, subphase and amphiphiles ought to be extremely purified. Use of zone refined amphiphiles has been reported to yield monolayers with considerably increased domain sizes... 

In the aquatic risk assessments, 16 out of 50 substances met the regulatory trigger values in a first-tier risk assessment. For 34 substances the trigger was not met without consideration of no-spray buffer zones and/or further risk refinement. 

Lower alkanes C2-C4 being chemically low active are mainly used as fuel material. However, in some oil extracting and oil refining zones the accumulation and desirable consumption of such hydrocarbons is technically hindered. Therefore, they are destroyed by burning on torches. In this case, not only hydrocarbon feedstock is uselessly lost, but the environment in the regions with the torches is badly damaged. Thus, the rational use of the hydrocarbon feedstock is an important problem not only from the viewpoint of economy, but also from the ecological consideration. 

The FSW resulted in considerable grain refinement in the stir zone. No evidence for the presence of alpha phase was observed in the TMAZ or stir zones of welds made on the as-received base metal, using LOM and SEM techniques. Aging of the base metal and weld samples resulted in the formation of a nearly continuous film of alpha phase along grain boundaries of the base metal and of the weld stir zone. A slight increase in hardness was observed in the stir zone of welds on as-received material and aged material. 

There was considerable scatter in the data. The stir zone exhibited increased hardness relative to the average base-metal hardness, most likely due to the refined grain size. A slight loss in hardness in the HAZ can be seen from the... 

The interfacial zone consists of adsorption, diffusion, and charge-transfer processes of oil additives and contaminants. A linear EIS [6] and NLEIS analysis [7] have led to a refinement of this model, taking into consideration the best fit of kinetic parameters to both nonlinear and fundamental-frequency experimental impedance data. The low-frequency impedance data are modeled as a combination of a CPE representing double-layer charging processes, adsorption (Z pj), diffusion (Zp, ), and charge-transfer resistance (R ) impedance segments. 

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