Colloidal sedimentation

Our experiments also established that the process of aging of freshly precipitated iron hydroxides does not proceed in the same way in sediments obtained from ionic and colloidal solutions of Fe. Whereas colloidal sediments remain X-ray-amorphous for a long time, sediments from ionic solutions relatively rapidly acquire a crystal structure which is most clearly manifested in alkaline, is less ordered in acid, and almost X-ray-amorphous in neutral environments. 

This should be the case for the A site and C site because they have the same energy level with respect to gravity, so RHCP would be favored in colloidal sedimentation. In order to quantitatively define the amount of the FCC phase versus the HCP phase, an overall stacking parameter, a, is used. This stacking parameter is defined such that a = 0 for the HCP, a = 1 for the FCC phase, and a RHCP structure would have an a = 0.5. This has been used to describe colloidal crystal structure and stacking fault density. ... 

In Chapters 3 and 4 we analyze mass and heat transfer in plane channels, tubes, and fluid films. We consider the mass and heat exchange between particles, drops, or bubbles and uniform or shear flows at various Peclet and Reynolds numbers. The results presented are of great importance in obtaining scientifically justified methods for a number of technological processes such as dissolution, drying, adsorption, aerosol and colloid sedimentation, heterogeneous catalytic reactions, absorption, extraction, and rectification. 

Colloids have a relatively large surface area, so they may act as adsorbents. Colloidal sediment formed in wine dne to natnral settling or treatment generally contains varions snbstances that were not involved in the colloidal floccnlation mechanisms that caused the deposit. Thus, for example, ferric phosphate deposits frequently contain calcinm. At one time, it was even snpposed that ferric-calcinm casse had occnrred. In fact, the calcinm is not involved in floccnlation as an electrolyte, bnt is rather fixed by adsorption. 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Anotlier standard metliod is to use a (high-speed) centrifuge to sediment tire colloids, replace tire supernatant and redisperse tire particles. Provided tire particles are well stabilized in tire solvent, tliis allows for a rigorous purification. Larger objects, such as particle aggregates, can be fractionated off because tliey settle first. A tliird metliod is (ultra)filtration, whereby larger impurities can be retained, particularly using membrane filters witli accurately defined pore sizes. 

In most colloidal suspensions tire particles have a tendency to sediment. At infinite dilution, spherical particles with a density difference Ap with tire solvent will move at tire Stokes velocity... 

In practice, sedimentation is an important property of colloidal suspensions. In fonnulated products, sedimentation tends to be a problem and some products are shipped in the fonn of weak gels, to prevent settling. On the other hand, in applications such as water clarification, a rapid sedimentation of impurities is desirable. 

Piazza R, Bellini T and Degiorgio V 1993 Equilibrium sedimentation profiles of screened charged colloids a test of the hard-sphere equation of state Rhys. Rev. Lett. 71 4267-70... 

Suspended matter in raw water suppHes is removed by various methods to provide a water suitable for domestic purposes and most industrial requirements. The suspended matter can consist of large soflds, settleable by gravity alone without any external aids, and nonsettleable material, often colloidal in nature. Removal is generally accompHshed by coagulation, flocculation, and sedimentation. The combination of these three processes is referred to as conventional clarification. 

Tyj)e of dryer Applicable with dry-product recirculation True and colloidal solutions emulsions. Examples inorganic salt solutions, extracts, milk, blood, waste liquors, rubber latex, etc. Pumpable suspensions. Examples pigment slurries, soap and detergents, calcium carbonate, bentonite, clay sbp, lead concentrates, etc. does not dust. Recirculation of product may prevent sticking Examples filter-press cakes, sedimentation sludges, centrifuged sobds, starch, etc. 

Figure 18-82 illustrates the relationship between solids concentration, iuterparticle cohesiveuess, and the type of sedimentation that may exist. Totally discrete particles include many mineral particles (usually greater in diameter than 20 Im), salt crystals, and similar substances that have httle tendency to cohere. Floccnleut particles generally will include those smaller than 20 [Lm (unless present in a dispersed state owing to surface charges), metal hydroxides, many chemical precipitates, and most organic substances other than true colloids. 

Modes of Operation There is a close analogy between sedimentation of particles or macromolecules in a gravitational field and their elec trophoretic movement in an electric field. Both types of separation have proved valuable not only for analysis of colloids but also for preparative work, at least in the laboratoiy. Electrophoresis is applicable also for separating mixtures of simple cations or anions in certain cases in which other separating methods are ineffectual. 

This removal may also include diffusion of soluble U(VI) from seawater into the sediment via pore water. Uranium-organic matter complexes are also prevalent in the marine environment. Organically bound uranium was found to make up to 20% of the dissolved U concentration in the open ocean." ° Uranium may also be enriched in estuarine colloids and in suspended organic matter within the surface ocean. " Scott" and Maeda and Windom" have suggested the possibility that humic acids can efficiently scavenge uranium in low salinity regions of some estuaries. Finally, sedimentary organic matter can also efficiently complex or adsorb uranium and other radionuclides. 

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