Ageing of dispersions

Dispersants are used so infrequently in some locations that stockpiles are sometimes in place for as long as 20 years before use. In Great Britain, the government studied the aging of dispersants by testing effectiveness [JJO]. The laboratory performed both long-term tests and short-term accelerated tests in various containers and at 20 and 30 °C. The tests did not show dispersant deterioration as indicated by the effectiveness values. 

Waite, Arthur Edward. The holy Kabbalah a study in the secret tradition in Israel as unfolded by sons of the doctrine for the benefit and consolation of the Elect dispersed through the lands and ages of the greater exile. London Williams Norgate Ltd, 1929. xxvi, 638p. 

A manufacturer observed that the compound produced by one twin-screw extruder exhibited much better dispersion than that produced in a similar extruder making the same compound. The only difference between the two extruders was the age of the screws and barrel. What could be causing the observed difference ... 

Fig. 2. Li abundance in main-sequence and turnoff stars in the open cluster IC 4651 aged of 1.5-1.7 Gyr vs Teff from photometry (open points and triangles for actual determination and upper limits respectively). Black points show rotating model predictions (CT99 and Palacios et al. 2003) at 1.5 Gyr for an initial rotation velocity of 110 km.sec-1 and for various stellar masses. For 1.5Mq models for an initial rotation of 50 and 150 km.sec-1 are also shown (black triangle and square). This provides an estimate of the expected dispersion for stars inside the Li dip. See Pasquini et al. (2004) for more details...

The lifetime of the molecular cloud is considered to be a time line running from cloud formation, star evolution and finally dispersion in a period that is several tci. The chemistry of the TMC and, to a good approximation, all molecular clouds must then be propagated over a timescale of at most 20 million years. The model must then investigate the chemistry as a function of the age of the cloud, opening the possibility of early-time chemistry and hence species present in the cloud being diagnostic of the age of the cloud. The model should then expect to produce an estimated lifetime and the appropriate column densities for the known species in the cloud. For TMC-1 the species list and concentrations are shown in Table 5.4. 

Figure 1.1.19 illustrates such hematite particles coated with chromium hydroxide in which the latter was produced by the hydrolysis of a chrome alum solution in the presence of a-Fe203 cores on aging the dispersion at 85°C. For greater efficiency the chromium salt solution was either preheated or preheated with KOH (but avoiding precipitation of chromium hydroxide) (144). 

Aging MnS04/urea solutions yielded directly crystalline cubic MnC03 (rhodo-chrosite) particles (Fig. 7.1.3) (27). Interestingly, Hamada et al. obtained the same kind of dispersions by reacting solutions of MnS04 and NHjHC03 (29). 

Of course i will depend on the average distance between the dispersed particles, on the size of these particles, on the rate of diffusion through the continuous phase, and on the state of turbulence. In a continuous stirred tank reactor l will be the same throughout the tank, while for a piston flow reactor c will change with the average age of the concerning point. 

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