Capillary suction time

The amormt of material involved in a CST test is very small, less than 50 ml per test and cleaning between tests is sinple. Hence the technique is well suited to a series of tests investigating the effect of flocculant dosage, conditioning chemicals, etc. The test can be used in conjunction with some vacumn leaf or pressure bomb tests in order to correlate CST with specific resistance. Such a correlation tends to be specific to the material and conditions investigated, and a universal correlation should not be assumed. 

In general, correlation between specific resistance and capillary suction time appears successful only for sludge-like suspensions, ie not for granular ones. 

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Experiments to determine specific resistance, based on Equation 7, have usually been carried out by some form of vacuum filtration. These methods are time-consuming and subject to error. More rapid techniques such as the measurement of capillary suction time (CST) can be used (8), although these do not give absolute values of specific resistance. Nevertheless, the CST method is very useful for rapidly obtaining comparative data on the flocculation of fairly concentrated suspensions by polymers (9). In the present work, specific resistance has been determined by an automated technique, which will be described below. 

Filtration rate, specific cake resistance, CSX (capillary suction time slurry is poured into a small cylinder placed on a filter paper [see Figure 22.62a], A cake is formed and filtrate is sucked out radially. The time required to pass between two fixed radii is termed CSX), filtrate turbidity, and cake solidosity... 

This method relies on the capillary pressure created by edal fiher paper to suck filtrate from a slurry contained in a small cylindrical reservoir placed on the surface of the paper. It is believed to be equivalent to a constant-pressure filtration. A picture of a capillary suction time device is given in Figure 2.25, together with a schematic diagram of the test cell. 

Figure 2.25a Capillary suction time test-schematic of test cell...

Figure 2.25b Capillary suction time test equipment...

Slurry or slip casting provides a relatively inexpensive way to fabricate unifonn-thickness, thin-wall, or large cross section shapes [4o, 44, 45, 46, 42 aiid 48]. For slip casting, a slurry is first poured into a porous mould. Capillary suction then draws the liquid from the slurry to fonn a higher solids content, close-packed, leather-hard cast on the inner surface of the mould. In a fixed time, a given wall thickness is fonned, after which the excess slurry is drained. 

The questions of interest to an engineer in this case are How do the initial concentration, the particle size, and the nature of the interparticle potential affect the structure of the dispersion, the structure of the final specimen, and the processing time How long does the process take What kinds of chemical additives are suitable The permeability and the capillary suction in the mold determine the rate of production of the specimens. How does one adjust the two to optimize production These questions require a basic understanding of colloid and surface science and phenomena. 

Capillary colloidal filtration occurs when the dry substrate comes into contact with a dispersion and the pore surface is wetted by the dispersion liquid. The capillary suction of the substrate which occurs, in effect drives particles to the interface. If the surface is not permeable for the particles, the particles concentrate at the substrate-dispersion boundary and a compact layer is formed. The thickness of the compact layer grows with time according to the well-known square root time law (see Section 6.3.1), until the substrate is saturated with dispersion liquid or, in case of a dispersion of small colloidal particles, a stationary state due to back-diffusion occurs. Figure 6.8 summcirises the capillary filtration mode of dip-coating. 

Kinetics of sorption into carbon materials for different oils was studied by using the so-called wicking method [31]. The system for the measurement is schematically shown in Fig. 27.11(a). The mass increase by capillary suction of oils from the bottom into carbon sorbents, either exfoliated graphite or carbonized fir fibers packed into a glass tube with a cross-sectional area of 314 mm with different densities or carbon fiber felts cut into similar cross-sectional area, was measured at room temperature as a function of time. The change in the mass of carbon sorbents due to the sorption of oils was plotted against time (sorption curve) as shown in Fig. 27.11(b). 

Slip casting is a low-pressure filtration method where capillary suction provides the driving force (of the order of 0.1-0.2 MPa) for liquid removal and formation of a cast layer at the mould surface. The casting rate is controlled by the resistance to flow by the cast layer and the mould. Usually, the mould resistance is negligible and the increase in the cast layer thickness, Z, with time, t, can be written as follows ... 

Equations (6.9) and (6.12) indicate that the consolidation rate increases with the capillary suction pressure p of the mold. If there were no other effects, an increase in p would always lead to a shorter time for a given thickness of the cast. The capillary suction pressure varies inversely as the pore radius of the mold, and it may be thought that a decrease in the pore radius would lead to an increase in the casting rate. However, the permeability of the mold K also decreases with a decrease in the pore radius of the mold, so there should be an optimum pore size to give the maximum rate of casting (39). 

Extensive laboratory tests on electrochemical realkalization have been conducted at BAM Berlin (Mietz and Isecke, 1994 Mietz et al., 1994). It has been found that the realkalized zone around the rebar increases with time and current density, i.e. proportionately to the electric charge passed (Mietz et al., 1994). On the other hand the observed realkalization from the concrete surface is independent of current density and thus only due to capillary suction of... 

The basic design is that of the Ostwald viscometer a U-tube with two reservoir bulbs separated by a capillary, as shown in Figure 24a. The Hquid is added to the viscometer, pulled into the upper reservoir by suction, and then allowed to drain by gravity back into the lower reservoir. The time that it takes for the Hquid to pass between two etched marks, one above and one below the upper reservoir, is a measure of the viscosity. In U-tube viscometers, the effective pressure head and therefore the flow time depend on the volume of Hquid in the instmment. Hence, the conditions must be the same for each measurement. 

Recently, we have developed a new version of this system, and this is shown in Figure 36. In this system, instead of the capillary being used as a blow-out pipette, the capillaries are used as wash-out pipettes. The capillaries are dispensed in a block, and the solution flows through the capillaries sequentially as they are presented to 2 press plates which apply pressure to the side of the block and maintain contact at all time. This done with 0-rings and has an effect similar to a suction cup sliding along the block. In Figure 36, it shows how 3 capillaries can be simultaneously emptied into three different channels of an autoanalyzer so that three determinations can be done simultaneously. 

The end of the capillary tube is heated until the glass is soft, then before it has time to cool it is touched on to the surface of a thin bubble of glass and a slight suction applied. This forms the window into a concave shape and draws it slightly down into the capillary, whose ends then protect it from damage. The bubble of thin glass should be thin enough to show interference colours. These windows will stand a vacuum provided atmospheric pressure is on the concave side of the window. If they are subjected to a pressure difference in the other direction, failure occurs due to the reversal of curvature. 

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