Time effect flocculation

Figure 1. Effect of reaction time on flocculation performance (0) 100 h and (O) 50 h at room temperature.

Steady-state shear stress-shear rate measurements are by far the most commonly used method in many industrial laboratories. Basically, the dispersion is stored at various temperatures and the yield value and plastic viscosity r are measured at various intervals of time. Any flocculation in the formulation should be accompanied by an increase in tr and r. One rapid technique for studying the effect of temperature changes on the flocculation of a formulation is to perform temperature sweep experiments, running the samples from perhaps 5 to 50 °C. Any trend in the variation of tr and r with temperature can quickly provide an indication of the temperature range at which a dispersion will remain stable, since during that temperature range cr and r will remain constant. 

Many suspensions (particularly those that are weaMy flocculated or structured to reduce sedimentation) show time effects during flow. At any given shear rate, the viscosity of the suspension continues to decrease with increasing the time of shear on stopping the shear, the viscosity recovers to its initial value. This reversible decrease of viscosity is referred to as thixotropy. 

Flocculating agents differ from other materials used in the chemical process industries in that their effect not only depends on the amount added, but also on the concentration of the solution and the point at which it is added. The process streams to which flocculants are added often vary in composition over relatively short time periods. This presents special problems in process control. 

If there is particle—particle interaction, as is the case for flocculated systems, the viscosity is higher than in the absence of flocculation. Furthermore, a flocculated dispersion is shear thinning and possibly thixotropic because the floccules break down to the individual particles when shear stress is appHed. Considered in terms of the Mooney equation, at low shear rates in a flocculated system some continuous phase is trapped between the particles in the floccules. This effectively increases the internal phase volume and hence the viscosity of the system. Under sufficiently high stress, the floccules break up, reducing the effective internal phase volume and the viscosity. If, as is commonly the case, the extent of floccule separation increases with shearing time, the system is thixotropic as well as shear thinning. 

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