Liquid thickening mechanism

Different theories exist about the thickening mechanism of fumed silicas. One of the first was the so-caUed chicken wire structure. That means the silica particles interact with each other via their silanol groups and form a three dimensional structure, which reduces the mobility of the hquid molecules. Under mechanical impact like shearing or shaking the structure is destroyed and the viscosity of the system decreases. After the end of the mechanical impact, the three dimensional network re-establishes itself and the viscosity increases again as a function of time. This mechanism may be valid in simple nonpolar liquids. In liquid mixtures or polymer solutions it is much more complicated and the adsorption pattern on the fumed silica surface seems to play an important role [76]. 

In a 5-I. round-bottom flask, fitted with an efficient mechanical stirrer and surrounded by an ice-salt mixture, is placed a solution of 121 g. (0.5 mole) of pure dry finely-powdered benzoylperoxide (m.p. 104°) (Note 1) in about 1.51. of dry toluene. The temperature should be below —5°. A solution of sodium ethylate, prepared by dissolving 23 g. (1 mole) of sodium in 500 cc. of absolute alcohol, and cooled to o°, is introduced from a separatory funnel while the mixture is vigorously stirred. This operation should not require more than eight or ten minutes. The liquid whitens and thickens considerably on account of the formation of sodium benzoylperoxide. 

Other uses of thickening agents include pharmaceutical preparations, paper production, and oil well drilling fluids. This latter use is necessary because oil is obtained from rock that is porous. In order to remove the oil without altering the mechanical properties of the porous rock, viscous liquids ( drilling fluids ) are pumped into the rock to replace the oil. Among the substances that can be used for this purpose are thickened aqueous solutions of polymers such as poly(acrylic acid) or poly(acrylonitrile). 

The mechanical strength and performance of liquid incendiaries can be improved by the addition of suitable thickening and gelling agents like rubbers, acrylic... 

The basic mechanism of foam degradation in porous medium is film coalescence. It depends on film thickness and capillary pressure. In the process of advancement the film thickness changes considerably thickens in the narrow parts (pore throats) and thins in the wider parts (pore bodies). Visual observations of such a stretching-squeezing mechanism are reported by Huh et al. [178]. Therefore, the film thickness would depend on the liquid/gas ratio, the rate of movement and the ratio of pore-body to pore-throat. When the critical capillary (disjoining) pressure is reached, the film will rupture. 

To obtain a formula for sizing a rotary-drum filter, the mechanism of liquid flow through a porous medium must be considered. As the filter drum rotates through the slurry tank, a porous solid deposits on the surface of the dmm, increasing the resistance to liquid flow. The surface of the filter cake is at atmospheric pressure. If it is assumed that the pressure downstream of the filter medium is constant (created by a vacmun punp), then the pressme drop across the filter cake and medium is constant. As the filter cake thickens, the liquid flow rate decreases because of the increasing resistance to flow. 

The separation of solids from liquids often involves at the same time the separation of solids from solids. In the so-called classifiers much used in Western ore mills the separation effected is watery sands from watery fines or slimes. The water content in the sands is of course very much less than in the slimes, the latter containing the bulk of the water fed to these devices. To reduce the water content in the slimes following their passage out of the classifiers they are often passed through thickening devices and the thick discharge from these is subjected to mechanical filter treatment and drying for further moisture reduction. 

The main factors to bind liquids and/or to influence the consistency of the dough are wheat glutens, pentosanes and mechanically damaged starch (through the grinding process of the com) or hydrocolloids (for example thickeners). 

FIGURE 6.7 Diagram of a single deck mechanical thickener for producing a clear supernatant and a solids enriched slurry from a pulp suspension in water. Slow-moving rakes transport sedimented solids toward thickened pulp outlet, and an exit valve for this stream allows control of the solid/liquid ratio. 

---