Clay, colloidal suspensions

In various kinds of industrial production, materials need to be treated with charged colloidal particles. In such systems, the value of the zeta-potential analyses are needed to control production. For example, in paper, adhesive, and synthetic plastics, colloidal clay can be used as filler. In oil drilling, clay colloidal suspensions are used. The zeta potential is controlled so as to avoid clogging the pumping process in the oil well. It has been found that, for instance, the viscosity of a clay suspension shows a minimum when the zeta potential is changed (with the help of pH from 1 to 7) from 15 to 35 mV. Similar observations have been reported in coal slurry viscosity. The viscosity was controlled by the zeta potential. 

Clay minerals have a permanent negative charge due to isomorphous substitutions or vacancies in their structure. This charge can vary from zero to >200cmol kg" (centimoles/kg) and must be balanced by cations (counter-ions) at or near the mineral surface (Table 5.1), which greatly affect the interfacial properties. Low counter-ion charge, low electrolyte concentration, or high dielectric constant of the solvent lead to an increase in interparticle electrostatic repulsion forces, which in turn stabilize colloidal suspensions. An opposite situation supports interparticle... 

Bodies of Water and the Chemical Sediments ,— The chemistry of the deposition of salts from sea-water has already been made the subject of special research, and van t Hoff s results in this field are already familiar. The deposition of calcium carbonate awaits a similar thorough study. Allied questions are the formation of dolomite, the deposition of various salts from inclosed bodies of water, the deposition of phosphate rocks, the precipitation of colloidal suspensions of clay and other substances, and the origin of the great deposits of sedimentary iron ore. 

Periodically, these mixtures were centrifuged and an aliquot of the clay-free supemate taken for counting analysis. Two naturally-occurring clays were selected for the experiments one was labeled kaolin (for the mineral kaolinite) while the second was referred to as attapulgite (or polygorshite). Both were obtained from the Source Clay Mineral Repository (3) as standard clays representative of each class of clay and were used as received. Stable, colloidal suspensions of each were prepared by ultrasonically dispersing weighed quantities of each clay in triple-distilled water. 

When the liquid, solution or lyophobic colloidal suspension contains asymmetric particles or when it is too concentrated, other methods must be applied to measure the viscosity. This is for instance the case with clay suspensions. In the past the viscosity of clay suspensions was measured by means of a bucket with a hole in it. The bucket was filled with clay suspension and after the stopper had been removed from the hole, the time required by the volume to drain was measured as a function of e.g. the volume and composition. Later mechanical methods were applied. One of them is based on the principle that a metal cylinder or disc, suspended from a torsion thread, is exposed to a certain resistance when you rotate it in the solution or suspension. Before the measurement the cylinder or disc is turned 360° anti-clockwise and then released. After having revolved over a certain angle, the cylinder or disc will change its direction of rotation. The rotation angle is a measure for the viscosity. 

Colloidal dispersions suspensions and aggregates Viscosity and transient electric birefringence study of clay colloidal aggregation. Physical Review E 65, 21407-21500... 

FIG. 13 The amount of NaCl needed to flocculate a clay (i.e., sodium bentonite suspension) as a function of the polyelectrolyte concentration (i.e., sodium car-boxymethylcellulose). The inserted figure is an enlargement of the initial addition of the polyelectrolyte. The lines are redrawn from An Introduction to Clay Colloid Chemistry by van Olphen [51]. 

Adsorption of materials on to the surface of colloidal oxides or clays has been a popular means of introducing electrostatic effects into photoredox processes. Adsorption of and/or bipysRu on to colloidal oxides such as AI2O3 or Si02 can result in some increase in the yield of net photoredox products, but the real benefits are modest. Improvements can be obtained with clay mineral suspensions where it has been claimedthat cyclic water cleavage can be achieved. However, turnover numbers are low, the longevity is extremely poor, and gas evolution shows a puzzling oscillatory pattern. 

Two further forces operate in any colloidal suspension or self-organised system. The first is due to Onsager and to Langmuir [54-56] who explained colloidal stability of clays and cylindrical particles in terms of purely repulsive... 

Figure 8.5. Viscosity of a 0.23-percent suspension of Na -smectite in water as a function of the amount of NaCl added to solution. (Adapted from H. van Olphen, 1977. An Introduction to Clay Colloid Chemistry. 2nd ed. New York Wiley.)...

EOD is carried out by applying an electric field between two electrodes submerged in wet clay, using a voltage imposed from an external power supply. 2 One may conduct EOD either under continuous dc or with periodic power inteiruption, when a dc power supply is used to drive the EOD. Of particular interest is combined field dewatering in which some combination of applied external pressure and EOD is used to remove water from a sludge or a comparable matrix of water trapped in colloidal suspensions. A schematic representation of EOD, with and without applied pressure, is shown in Fig. 1 (after Yoshida ). 

Similar discrepancies were noted by Blatt et al32 for colloidal suspensions such as skimmed milk, casein, polymer latexes, and clay suspensions. Actual ultrafiltration fluxes are far higher than would be predicted by the mass transfer coefficients estimated by conventional equations, with the assumption that the proper diffusion coefficients are the Stokes-Einstein diffusivities for the primary particles. Blatt concluded that either (a) the "back diffusion flux" is substantially augmented over that expected to occur by Brownian motion or (b) the transmembrane flux is not limited by the hydraulic resistance of the polarized layer. He favored the latter possibility, arguing that closely packed cakes of colloidal particles have quite high permeabilities. However, this is not a plausible hypothesis for the following reasons ... 

The petroleum industry suspension applications and problems have in common the same basic principles of colloid science that govern the nature, stability, and properties of suspensions. The widespread importance of suspensions in general and scientific interest in their formation, stability, and properties have precipitated a wealth of published literature on the subject. This chapter provides an introduction intended to complement the other chapters on suspensions in this book. A good starting point for further basic information, although focused on clays, is van Olphen s classic book, An Introduction to Clay Colloid Chemistry (I). There are several other good books on suspensions (2, 3), and most good colloid chemistry texts contain introductions to suspensions and some of their properties (4-8). 

If two electrodes (preferably of platinum) are placed in a colloidal suspension, and a potential difference is applied (e.g. 200 volts from a battery or D.C. mains), the colloidal particles migrate to one of the electrodes, which shows that they are electrically charged (Figure 17). This phenomenon is known as electrophoresis. In a colloidal suspension of clay in water, the... 

The importance of clay-water mixtures in the ceramic industry has already been referred to. The next step is to determine how our knowledge of colloidal suspensions can be applied to industrial processes. 

Dilatancy is a property that is more common in coarse non-colloidal suspensions than in clays. A suspension is said to be... 

H. van Olphen, An Introduction to Clay Colloid Chemistry, 2nd edn. Wiley, New York, 1977, Chapters 2, 3, 4, and 7 of this standard monograph form a useful adjunct to the present chapter as regards phyllosilicate suspensions. 

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