Particle thickeners

Detailed studies ofmicrostmctural transitions when colloidal crystals are sheared can be found in Chen (1992, 1994). Above a critical volume fraction, concentrated suspensions of charged particles thicken in a discontinuous manner. In suspensions of latex particles that have a deep van der Waals minimum, the thickening is irreversible, while in silica dispersions that have a shallow minima, the thickening is reversible. There have been a number of studies which try to relate the observed thickening to underlying microstructure (Maranzano, 2002 Bender, 1996 Chow, 1995b). 

To obtain a suitable coating viscosity, and to prevent settling of filler particles, thickeners are often used. These are often low-cost natural products based on cellulose or casein. These are readily metabolized by many bacteria and fungi therefore, a biocide should be included. Biocides in powder form such as Captan (R.T. Vanderbilt) or Zinc Omadine (Akcros) can be dispersed with fillers and pigments. A useful starting point level is 1 percent by weight of the natural polymer to be protected. Alternatively, a biocide water dispersion, such as Nuocide 404-D (40.4%, International Specialty Products), can be used. 

L. Svarovsky and A. J. Walker, "The Effect of Eeed Thickening on the Performance of a Horizontal Vacuum Belt Eilter," 1st World Congress Particle Technolog)/, Part IV, 331 Event of EFCE, Apr. 16—18, 1986, NMAmbH, Ni>mburg, Germany, 1986. 

In addition to the normal problems of completely dissolving particles of water-thickening polymers, xanthan gum contains insoluble residues which decrease polymer injectivity. Various methods of reducing insolubles content and improving xanthan solution injectivity are available (80—87). None appears economically viable. Oxygen scavengers (88) and bactericides (77,89) are commonly used to stabili2e injected polyacrylamide and xanthan gum solutions (90—102). 

In the post-dispersion process, the soHd phenoHc resin is added to a mixture of water, cosolvent, and dispersant at high shear mixing, possibly with heating. The cosolvent, frequently an alcohol or glycol ether, and heat soften the resin and permit small particles to form. On cooling, the resin particles, stabilized by dispersant and perhaps thickener, harden and resist settling and agglomeration. Both resole and novolak resins have been made by this process (25). 

For products intended to remain stable dispersions for an extended period, a particle size of 2 p.m or less is desirable. A thickening agent is usuaUy added after the reaction has been completed and the mixture is cooled in order to prevent settling and agglomeration. Examples of thickeners are guar gum, xanthan gum, and hydroxyethylceUulose. The final products are generaUy between 40 and 50% soUds, with a viscosity of 1500 5000 mPa-s(=cP). 

Aqueous dispersions are alternatives to solutions of Hquid and soHd resins. They are usuaUy offered in 50% soHds and may contain thickeners and cosolvents as stabilizers and to promote coalescence. Both heat-reactive (resole) and nonheat-reactive (novolak) systems exist that contain unsubstituted or substituted phenols or mixtures. A related technology produces large, stable particles that can be isolated as discrete particles (44). In aqueous dispersion, the resin stmcture is designed to produce a hydrophobic polymer, which is stabilized in water by an interfacial agent. 

Fiber Slurry Pipelines. Pipelines to carry suspensions of wood, paper, sludge, etc, have found commercial acceptance. Most of them are less than 15 km long but have diameters of up to 500 mm. These slurries are often concentrated and display viscous plastic properties, although particle sizes may vary special pumps are used. One such hydrotransport system carries a cellulose slurry by pipeline from the plant to a paper plant near Heidenau, Germany. The 250-mm dia pipeline carries 60 t/d over the 3-km distance to thickeners. In Sweden, a 3.7-km, 500-mm dia pipeline moves cellulose by... 

Sedimentation is also used for other purposes. For example, relative motion of particles and Hquid iacreases the mass-transfer coefficient. This motion is particularly useful ia solvent extraction ia immiscible Hquid—Hquid systems (see Extraction, liquid-liquid). An important commercial use of sedimentation is ia continuous countercurrent washing, where a series of continuous thickeners is used ia a countercurrent mode ia conjunction with reslurrying to remove mother liquor or to wash soluble substances from the soHds. Most appHcations of sedimentation are, however, ia straight sohd—Hquid separation. 

A typical thickener has three operating layers clarification, zone-settling, and compression. Frequently, the feed is contained in the zone-settling layer which theoretically eliminates the need for the clarification zone because the particles would not escape through the interface. In practice, however, the clarification zone provides a buffer for fluctuations in the feed and the sludge levels. 

A area of particle projected in direction of motion plan area of settling tank or thickener... 

Extraction of Bertrandite. Bertrandite-containing tuff from the Spor Mountain deposits is wet milled to provide a thixotropic, pumpable slurry of below 840 p.m (—20 mesh) particles. This slurry is leached with sulfuric acid at temperatures near the boiling point. The resulting beryUium sulfate [13510-49-1] solution is separated from unreacted soflds by countercurrent decantation thickener operations. The solution contains 0.4—0.7 g/L Be, 4.7 g/L Al, 3—5 g/L Mg, and 1.5 g/L Fe, plus minor impurities including uranium [7440-61-1/, rare earths, zirconium [7440-67-7] titanium [7440-32-6] and zinc [7440-66-6]. Water conservation practices are essential in semiarid Utah, so the wash water introduced in the countercurrent decantation separation of beryUium solutions from soflds is utilized in the wet milling operation. 

Compaction, Compression, and Expression. Compaction is a newer term for compression and is used to describe the movement of particles relative to one another within a device until the matrix of particles gains enough strength to resist further consoHdation (16). Compaction occurs in a plate and frame filter both while the chamber is filling and at the end of the cycle when the chamber is nearly full and the pressure rises steeply. Compactibihty (or compressibiUty) describes the reduction in volume of the particle matrix. Compaction also takes place in the bed of a thickener as the sohds continuously deposit on the top of the bed and a thickened slurry is withdrawn from the bottom. 

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