Clay-Water Suspensions

For casting with a fluid slip, a considerable proportion of water must be added to the clay which itself contains some moisture for plastic mixes that have to be moulded, less water is added finally for semi-dry pressing the moisture already in the clay may be sufficient. 

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In addition to studies which focused primarily on clay-water suspensions [55-58], Weiss and Gerasimowicz [59] have investigated using 2H NMR the... 

The pronounced amplitude dependence of the complex modulus, referred to as the Payne effect, has also been observed in low viscosity media, e.g., composites of carbon black with decane and liquid paraffin [50], carbon black suspensions in ethylene vinylacetate copolymers [51], and clay/water suspensions [52, 53]. It was found that the storage modulus decreases with... 

As shown in Figure 2.17, the flow behavior of clay-water suspensions changes in the presence of soluble electrolyte ions that can be exchanged with the ions adsorbed at the surface of the day partides. Natural clays yield predominantly Ca ... 

It should be noted that a pH of exactly 7 is quite difficult to maintain for clay-water suspensions, since any addition of water to day immediately causes a drop in pH to about 5 (this occurs because protons are rapidly released from the particle edges, as described above). 

Measurements of the zeta potential of a clay-water suspension in the presence of one or more electrolytes can be performed using a U-shaped tube (Burton tube) in which the suspension is subjected to an electric field appUed between two electrodes. The electrophoretic velocity v of the electrolyte ions is related to the zeta potential. Then, the electric force <7- E (where E = electrostatic field strength) acting on the charged particles is balanced by the frictional force 77 v/d (77 = dynamic viscosity) that is ... 

Monovalent cations are good deflocculants for clay—water sHps and produce deflocculation by a cation exchange process, eg, Na" for Ca ". Low molecular weight polymer electrolytes and polyelectrolytes such as ammonium salts (see Ammonium compounds) are also good deflocculants for polar Hquids. Acids and bases can be used to control pH, surface charge, and the interparticle forces in most oxide ceramic—water suspensions. 

Thus, a linear relationship means that in suspensions of smectites with clay/water (w/w) concentrations ranging from 0.1 to 10%, the external surface of a tactoid and the number of water layers on this surface are constant in first approximation. Increasing the concentration in solids in this range increases the number of tactoids, whereas their average size remains constant. 

Even dynamic measurements have been made on mixtures of carbon black with decane and liquid paraffin [22], carbon black suspensions in ethylene vinylacetate copolymers [23], or on clay/water systems [24,25]. The corresponding results show that the storage modulus decreases with dynamic amplitude in a manner similar to that of conventional rubber (e.g., NR/carbon blacks). This demonstrates the existence and properties of physical carbon black structures in the absence of rubber. Further, these results indicate that structure effects of the filler determine the Payne-effect primarily. The elastomer seems to act merely as a dispersing medium that influences the magnitude of agglomeration and distribution of filler, but does not have visible influence on the overall characteristics of three-dimensional filler networks or filler clusters, respectively. The elastomer matrix allows the filler structure to reform after breakdown with increasing strain amplitude. 

Low, P.F., Interparticle forces in clay suspensions Flocculation, viscous flow and swelling, in CMC Workshop Lectures, Clay-Water Interface and Its Reological Implications, vol. 4, Giiven, N. and Polastro, R.M., Eds., Clay Mineral Society, Boulder, CO, 1992, p. 157. 

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. 

Yield-dilatant (n > 1) materials are rare but several cases of yield-pseudoplastics exist. For instance, data from the literature of a 20% clay in water suspension are represented by the numbers To = 7.3dyn/cm, K = 1.296dyn(sec)"/cm and n = 0.483 (Govier and Aziz, 1972, p. 40). Solutions of 0.5-5.0% carboxypolymethy-lene also exhibit this kind of behavior, but at lower concentrations the yield stress is zero. 

The potter shown in Figure 1 uses water to help sculpt the sides of a clay pot. As he dips his clay-covered fingers into a container of water, the water turns brown. The clay-water mixture appears uniform. However, if the container sits overnight, the potter will see a layer of mud on the bottom and clear water on top. The clay does not dissolve in water. The clay breaks up into small pieces that are of such low mass that, for a while, they remain suspended in the water. This type of mixture, in which the different parts spontaneously separate over time, is called a suspension. In a suspension, the particles may remain mixed with the liquid while the liquid is being stirred, but later they settle to the bottom. 

In the low shear regime, the systems display some discontinuities on the shear stress versus shear rate plane, and the discontinuity is more severe when the clay volume fraction, 0C, is about 0.12. For pure clay in water suspensions, similar behavior is also observed, whereas the discontinuity is much reduced as the oil concentration is increased. 

Suspensions of particles in water (typically less than 2 mm), produced in limestone scrubbing plants, can be classified to produce saleable products such as coarse sand, fine sand and a clay-rich suspension of fines. Classifiers may be divided into two groups — one uses gravity to separate coarse from fine particles, while the other uses centripetal forces. 

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. 

Suspensions in general, and particularly those of clay, do not behave like true liquids, and the laws governing their flow are quite different. The laws of viscous flow are dealt with elsewhere and we shall only introduce some elementary ideas here, sufficient to understand the technology of casting slips and other clay-water systems. 

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