[Dex]

I'd like to recrystallize a salt. So I heat the water to 50-60 degree centigrade and then try to dissolve the salt into water slowly until it gets saturated. Then I chill it to ambient temperature and then to about 10 degree. This way it gets supersaturated and I can have the salt crystals. But the problem is that the crystals are so small and I want them to be much bigger. So any of you genius chemists can give me a word of advise or recommendation?
tnx in advance

[curiouscat]

Add some seed crystals?

What's your apparatus? What's your scale? What salt?

[Dex]

Actually I've add some seeds too, but still the crystals are not big enough. The scale is about a couple of Kg. I wanna produce these two salt: KCl and C4H3KO8·2H2O and for none of these salts I've not yet been able to get big crystals.They are so small. Should I add some more water and make the solution thin?

[alex00]

I think the seed primary size is important.try with bigger seed.
what is your idea ??

[discodermolide]

You need to cool it slowly so the crystals get a chance to grow. By slow I mean slow, about 2 - 5°C per hour.

[fledarmus]

If you want large crystals, you want slow crystal growth on very few nucleation sites. The thing that helps you here is the tiny energy *Ignore me, I am impatient* required to form a nucleation site. This means that the solution will need to be somewhat colder to form a nucleation site than it would be to grow crystals if there was a nucleation site already present. Ideally, you would keep the temperature and the concentration in that sweet spot between cold and concentrated enough for crystals to grow, but not so cold or concentrated to form new nucleation sites.

To start, you want to be sure there are no nucleation sites present. Make sure there is no dust or scratches in your glassware and that all of your solid is thoroughly dissolved - this generally requires a few minutes of boiling, not just raising it to boil and then cooling it, and filtration to remove any insoluble impurities that might create a nucleation site. Then allow the hot, clear solution to cool slowly - Discodermolide's 2-5°C per hour is a good place to start, certainly no faster than that if your crystals are too small now. Before your solution gets supersaturated, add just a very few seed crystals - 3 or 4 tiny perfect crystals is a good start. Use a magnifying glass to pick out single crystals. If those seeds all dissolve, wait another hour (another 2-5°C) and add a few more seeds. Once the seeds remain in place without dissolving in, cool very, very slowly to grow the largest crystals.

If that doesn't give you large enough crystals, the other option is to start with an unsaturated solution and allow it to evaporate slowly at room temperature. Again, add just a few seed crystals every hour or so (or every day, depending on the evaporation rate of the solvent) until you start seeing crystal formation. This essentially raises the concentration of the solution rather than lowering the temperature to get crystallization.

The trick with both methods is to grow the crystals s-l-o-w-l-y. If you try to speed up the process, you will get smaller crystals.

[Dex]

Thanks dear discodermolide and fledarmus. You are life savers. 
I shall try what you suggested to see what happens. Then I'll post the results here to keep you posted in the progress of the process. But just one more thing: I used to think there is special temperature for every unique salt that in that temperature the nucleation happens. From what you chemists said, It seems I'm wrong. Right? Sounds like it doesn't matter anymore.
Since I'm not chemist let me review the process to see if i got it right  .The solution should be saturated at a high temperature and be filtered. Then it should be warmed up a little (about 2-3 degree), so the solution is not saturated anymore. Next I should add a couple of crystals as seeds before saturation happens. Finally the solution must be got cold very slowly to the environment temperature. Did I get the process right?

[discodermolide]

More or less.
Crystallisation is an equilibrium process, especially at the start. The small crystals are constantly coming out of and going back into solution. Small crystals = rapid cooling, large crystals = larger crystals. It can be a very time-consuming process. I had one that lasted 5 days, principally because the compound was quite low melting. We were heating up, cooling a bit heating up a bit cooling again and so on. The inly good news was that once it crystallised it never stopped. This actually caused us problems, because the seeds in the air made it crystallise every time, even in the plant, where it blocked up all the tubes and pipes!

[alex00]

I tried it and I saw the very fast nucleation on the surface of solution.
why the surface have high speed nucleation?
is possible for air pollution or due to surface temperature
Thanks

[discodermolide]

This is presumably because you are getting solvent evaporation causing local concentration changes resulting in nucleation.
Try it in a closed system, that is once you have dissolved all your solid cover the top up to prevent evaporation.

[alex00]

W0o0ow. it was really interesting point. I will try it and inform the result.

[Dex]

But I guess covering the top of the system will might result in increasing the pressure of the system and might effect the process.Am I wrong? But still this method is worth trying. So I'm gonna give it a shot and see the changes.

[discodermolide]

At 50-60°C it should mot make any difference.

[Dex]

Dear discodermolide
Based on the process you mentioned above, I simulated the cooling process as below

[curiouscat]

What's the purpose of the small heating-up phases?