Precipitation by chemical reaction
Figure 4.13. Strong nucleation (fine particles). Mix in Y
If we mix the two reactives continuously in Y, supersaturation is maximum, as the reactives are not diluted. This is the opposite of what happens in a reactor, where the arriving reactives are diluted in the liquid present in the device in which crystals are already large in size.
Y mixing of the product produces crystals of the order of a micrometer, or even nanometers. In an agitated recipient, if we introduce both reactives immediately on discharge from the impeller, we approach the Y procedure.
Inversely, an active circulation in the recipient benefits dilution and slows nucleation. However, too energetic an agitation generates attrition.
In the volume unit, the mass balance is written as:
k: transport coefficient (m.s-1)
c: concentration (kg.m-3);
hence, after integration:
where Acois the initial supersaturation (maximum).
We observe that the decrease in supersaturation Ac is faster if the фт volume fraction of the solid is large, and if the mean harmonic dimension of the particles is small.