[Procyan]

Freezing a solution of acetic acid in water effectively concentrates the acid.  especially if done slowly.  Could someone recommend a text or explain what physical process is at play?  The regular arrangement of crystalline water molecules must exclude the solute molecules,  I would like to model the process mathematically.

[Procyan]

I'm trying to work out a gentle concentration method.  If I freeze slowly, the concentration effect is quite powerful...but slow.  If I freeze quickly, then the solute becomes entrapped in water ice. 

Rather than trial and error approach, I would like to understand and model the process to optimize.

Any thoughts or suggestion appreciated as this is all new to me.  My background is biochemistry.

[Arkcon]

Zone crystallization is a common way to purify silicon rods for use in solar cells, for example.  Basically, crystals that form slowly, are pure, and impurities are left behind.

I can only guess -- you have some vinegar, and want to make some glacial acetic acid at home.  'Kay, that sounds a little dangerous, but I'll let it slide.  You want the theoretical, or industrial procedure for such a process-- that doesn't exist, no one purifies acetic acid this way.

But if you want to formulate a mathematical theory, I'd work it from the zone crystallization of silicon, or germanium.

[Borek]

Zone crystallization is a common way to purify silicon rods for use in solar cells, for example.

AFAIK this technology is no longer used - too expensive. Now they convert everything to trichlorosilane and use distillation in so called Siemens process.

[Arkcon]

Zone crystallization is a common way to purify silicon rods for use in solar cells, for example.

AFAIK this technology is no longer used - too expensive. Now they convert everything to trichlorosilane and use distillation in so called Siemens process.

Really, so freeze distillation has even been replaced for silicon.  Heh.  Funny how that works out.

[DrCMS]

Zone crystallization is a common way to purify silicon rods for use in solar cells, for example.

AFAIK this technology is no longer used - too expensive. Now they convert everything to trichlorosilane and use distillation in so called Siemens process.

No they do not.

In the Siemens process they react already high purity silicon and trichlorosilane together to  deposit polycrystalline silicon.

2HSiCl₃ → Si + 2HCl + SiCl₄

The trichlosilane along with other chlorosilanes are made in the Müller Rochow process by reacting silicon with hydrogen chloride.  The different chlorosilanes are then separated by fractional distillation.

2Si +6HCl → 2HSiCl₃ + 2H₂ (Also makes traces of H₂SiCl₂ and SiCl₄)

The high purity polycrystalline silicon made in the Siemens process can be processed to make monocystalline silicon by Czochralski process, Bridgeman technique or Float-zone silicon.  This last one is essentially zone crystallisation

[Borek]

The trichlosilane along with other chlorosilanes are made in the Müller Rochow process by reacting silicon with hydrogen chloride.  The different chlorosilanes are then separated by fractional distillation.

So basically I was right that distillation is used for the purification, its just not called Siemens process? I thought Siemens process starts with trichlorosilane making and ends with deposition of silicone, at least that's how it was described in some journal for electronics engineers in an article that I was translating to Polish last year.

[DrCMS]

So basically I was right that distillation is used for the purification, its just not called Siemens process?

Yes

I thought Siemens process starts with trichlorosilane making and ends with deposition of silicone, at least that's how it was described in some journal for electronics engineers in an article that I was translating to Polish last year.

Yes that's it.  The trichlorosilane come from the Müller Rochow process.  I met both of them in Poland about 16 years ago.  Muller was working in Germany and Rochow in the US and they both independantly came up with the same process at the same time.

[Procyan]

In my effort to be concise, I was cryptic...

I am trying to concentrate some natural dye molecules as gently as possible before freeze drying (or immediate use).  I'm using a dilute acetic acid extract (or sometimes alkaline).  Recently, after a particularly frosty night, I found that slow freezing concentrates the solutes. Eureka!

The aim is to develop "green" technology for improved methods.  I'm trying to do something completely different, mostly to help some local purists who are spinning their own yarn and dying them with onion skins etc....

My fantasy is to find a "magic" temperature at which the molecule of interest remains on the liquid phase while the acetic acid becomes sequestered into the water ice matrix.  I would say some words about activity coefficients and chemical potential but I'm sure it would all be stupid.  I would ask, however, during crystal formation, do all "contaminates" behave the same or are some excluded more effectively than others?  I would like to model this process before experimenting.  I wish to see what the limits are as the dye becomes more concentrated for example.  The mix is constantly changing.  Mix??? Is mixing important?!  

To recapitulate, I'm wondering if there is some fundamental body of work or someone on the forum familiar with freeze concentration, even if it has become a bit passe.  In fact anyone with some dyeing experience interested in "Green", natural dyes would be most helpful.  

Clear as Guiness, right?