Impurities from surfaces

Potassium heptafluorotantalate, K2TaF7, precipitates in the form of transparent needles. The precipitated particles must not be too fine, since fine powder usually promotes co-precipitation and adsorption of some impurities from the solution. Even niobium can be adsorbed by the surface of K2TaF7 developed during precipitation, as shown by Herak et al. [535]. On the other hand, the precipitation of large K-salt crystals should not be strived for either. Laboratory and industrial experience indicates that excessively large crystals usually contain small drops of solution trapped within the crystals. This occluded solution can remain inside of the crystal until drying and will certainly lead the hydrolysis of the material. 

Methods exist to remove undesired impurities from the surface, but it is preferable in electrocatalytic experiments to reduce chances that such impurities reach the... 

A radical approach serving to remove all impurities from the electrode surface is that of slicing away a thin layer of metal right during the measurements (i.e., without withdrawing the electrode from the solution or breaking its polarizing circuit). Sometimes, bombardment of the surface with an ion beam is used for a similarly radical surface purification. 

The adsorption and accumulation of various impurities from the electrolyte or surrounding atmosphere on the catalyst surface. The rate of accumulation of impurities on the catalyst surface depends on its activity for adsorption, which often is parallel to its catalytic activity. 

The washing of filter cake is carried out to remove liquid impurities from valuable solid product or to increase recovery of valuable filtrates from the cake. Wakeman (1990) has shown that the axial dispersion flow model, as developed in Sec. 4.3.6, provides a fundamental description of cake washing. It takes into account such situations as non-uniformities in the liquid flow pattern, non-uniform porosity distributions, the initial spread of washing liquid onto the topmost surface of the filter cake and the desorption of solute from the solid surfaces. 

Native gold and its alloys, which are free from surface contaminants, are readily floatable with xanthate collectors. Very often however, gold surfaces are contaminated or covered with varieties of impurities [4], The impurities present on gold surfaces may be argentite, iron oxides, galena, arsenopyrite or copper oxides. The thickness of the layer may be of the order of 1-5 pm. Because of this, the flotation properties of native gold and its alloys vary widely. Gold covered with iron oxides or oxide copper is very difficult to float and requires special treatment to remove the contaminants. 

CARBON, DECOLORIZING. (Activated carbon, bone black.) Forms of carbon having large surface area so that it has capacity to remove colors and impurities from air, gas, or solution. 

The effects of interfacial monolayers on the extraction from drops are particularly striking. Early work showed that traces of either impurity or surface-active additives can drastically reduce extraction rates even plasticiser, in subanalytical quantities dissolved from plastic tubing by benzene, reduces the mass-transfer rate by about ten times by retarding... 

As a reason for not using h.v.t. it is often stated that its use leads to experimental results that cannot be reproduced on an industrial scale. This is untrue. A closed system, such as an all-glass vacuum line, has more in common with an industrial plant than the typical apparatus used at the laboratory bench. Furthermore, because of the considerably more favourable surface to volume ratios in a large plant, the typical concentrations of those impurities which originate from surfaces are more accurately reproduced by h.v.t. experiments than by the typical bench experiment. This is often reflected in the problems encountered during development work when bench experiments are being scaled up to pilot plant and beyond. 

Atoms in the free surface of solids (with no neighbors) have a higher free energy than those in the interior and surface energy can be estimated from the number of surface bonds (Cottrell 1971). We have discussed non-stoichiometric ceramic oxides like titania, FeO and UO2 earlier where matter is transported by the vacancy mechanism. Segregation of impurities at surfaces or interfaces is also important, with equilibrium and non-equilibrium conditions deciding the type of defect complexes that can occur. Simple oxides like MgO can have simple anion or cation vacancies when surface and Mg + are removed from the surface,... 

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