[alex00]

I think when the solution is cooling down it might forms some other new nucleations, but they are not stable.if we heat-up the solution again they will be dissolved again, and in the cooling down phase the growth possibility will be increased.

[Dex]

This simulation is mostly based on what ''Discodermolide'' and ''Fledarmus'' explained in above comments. But as ''rasool083'' said earlier, I guess in the cooling process the nucleation starts because the solution gets supersaturated, the more the solution gets cold (about 2-5 c per hour), the more the nucleation happens.
In the cooling down phase, a lot of nucleations are formed but those nucleation that are formed at the beginning of the cooling process had enough time to grow, but those that are formed at the nearly end of the process didn't grow, so they are still so small.Now by heating-up the solution a little, the last nucleations are dissolved again but the big ones stay put. Now if the cooling phase starts again, the remnant nucleations form previous cooling phase(those that had enough time to grow), strat growing again and absorb the energy. Again at the end of this process there are some small crystals that should be dissolved by heating-up a little. This process make the crystals big.
Plz correct me if I'm wrong.

[discodermolide]

To get the smaller particles back into solution.

[Dex]

I think there was another reason that made the crystals so small. When the crystals are formed in the solution , they get heavy and intend to go downward the solution at the button. Then after a while the concentration around the button near the crystals is decreased and they can't grow more. If we could make the crystals stay up in the solution not in the button, then they are more in touch with other molecules and are able to grow more.

[curiouscat]

I think there was another reason that made the crystals so small. When the crystals are formed in the solution , they get heavy and intend to go downward the solution at the button. Then after a while the concentration around the button near the crystals is decreased and they can't grow more. If we could make the crystals stay up in the solution not in the button, then they are more in touch with other molecules and are able to grow more.

Add in some gentle circulation?

[Dex]

I'm gonna try this tonight and will report back the results !!

[Dex]

Actually I had made an experiment and in that I didn't just pour the seeds inside the saturated solution, I used strings (nylon ones) and tied several seeds in different location along it.Then I hang it over the solution and didn't move it at all. Sounds like the crystals are intended to stick to these seeds rather that creating a new nucleation. So the crystals grow

[fledarmus]

...which is how rock candy is made. However, that does leave you with the issue of removing the supports from the crystals without leaving contaminants behind.

In general, I would expect that for slow controlled crystallization, the diffusion of the compound through the solvent will be at least on the same order of speed as the crystal growth, if not greater. You shouldn't see any large concentration gradients based on distance from the crystals. You are right about the surface, however - evaporation of solvent from the suface is another way of forming nucleation sites. This can be avoided by doing the recrystallization in a closed container where the atmosphere inside the container is already saturated with solvent. Soaking a gauze patch with pure solvent and attaching it the inside of the lid of your container would keep the air over the container saturated and slow the formation of crystals on the surface.

[curiouscat]

...which is how rock candy is made. However, that does leave you with the issue of removing the supports from the crystals without leaving contaminants behind.

In general, I would expect that for slow controlled crystallization, the diffusion of the compound through the solvent will be at least on the same order of speed as the crystal growth, if not greater. You shouldn't see any large concentration gradients based on distance from the crystals. You are right about the surface, however - evaporation of solvent from the suface is another way of forming nucleation sites. This can be avoided by doing the recrystallization in a closed container where the atmosphere inside the container is already saturated with solvent. Soaking a gauze patch with pure solvent and attaching it the inside of the lid of your container would keep the air over the container saturated and slow the formation of crystals on the surface.

If the freshly made crystals fall down to the bottom in a heap though he might have a problem. Solvent may have a hard time accessing these crystals.

[Dex]

Circulation didn't work out well. Do you have any suggestion ''curiouscat''?

[curiouscat]

Circulation didn't work out well. Do you have any suggestion ''curiouscat''?

What did you do to provide circulation?

[Dex]

I used a motor with 5 rpm speed and and attached a blade to it's rotor to provide circulation.

[alex00]

hey guys we know all these are in lab. what is the industrial methods really?

[discodermolide]

Similar to the lab methods only bigger.
It depends upon what you want from your large scale crystallisation.