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Crystallization of hydrates

Q: I am carrying out crystallization of hydrates by evaporation. I have a slurry at 70ºC and I want to separate the crystals from the slurry. The problem is that as soon as I start filtration, it is exposed to an atmospheric temp and agglomeration and the transformation of crystals take place. Will you please suggest a way to get rid of this? The set up is for a 200-ml batch reactor. I stop crystallization and then I start filtration by pouring the slurry into a filtration funnel. Instead of filtration, agglomeration takes place.

A: It is not clear to me whether there are different states of hydration. That is, a hydrate at 70ºC along with a higher degree of hydration at room temperature. If this is the case, the individual will want to heat the filter and make sure that the filtration occurs at operating temperature of 70ºC.

Another possibility is that post crystallization is occurring as the slurry is quickly cooled resulting in bridging and agglomeration. Again, the answer is probably to make sure that the filter is hot and remains hot and that the cake is not exposed to atmospheric temperatures.

Another consideration may be the issue of polymorphism whereby the crystals are converting to a more thermodynamically stable state with resultant change in crystal habit and the production of agglomeration. He should look at the crystals before and after via polarized light microscopy to see if it is even the same hydrate.

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