Sciencemadness Discussion Board

How to make Silicon

gnitseretni - 16-3-2010 at 03:45

One can make silicon from SiO2 by reducing it with magnesium. But as you know the devil is in the details and this is where I need some help. How much of each do I need?

Google was a little helpful though. I think, from what I've read, I'll want a slight excess of magnesium. That way instead of silicon and some leftover SiO2 I'll end up with silicon and Mg silicide. After the reaction has subsided, drowning it with HCl will turn the Mg silicide into silane and what I'll have left is pure silicon. Or that's if I understood it right.

But I couldn't find anything about how much of Mg and SiO2 I needed, so por favor, help please ;)
Also, my sand is pretty fine. I have 200 and 325 mesh silica. Will my magnesium have to be just as fine or is scraping some shavings of a magnesium bar just fine?

woelen - 16-3-2010 at 04:18

You don't have experience with stoichiometric calculations?

The reaction is: 2Mg + SiO2 --> 2MgO + Si

From this you should be able to compute how much Mg reacts with how much SiO2 (molar weights). As you say, use a slight excess of Mg and then perform the reaction. Afterwards crunch the material and add to dilute HCl. Keep in mind though that the reaction between Mg and SiO2 does not start easily, but once it is going, a lot of heat is produced. Do the reaction in a closed vessel or a test tube, otherwise any Si formed in the reaction will burn in the surrounding air. I did this experiment, not for making Si, but formaking Mg2Si, but the procedure I used might be useful for you as well:

http://woelen.homescience.net/science/chem/exps/silane/index...

You need finely powdered sand and finely powdered Mg. Shavings from a big piece of Mg are too coarse, you really need a fine powder.


[Edited on 16-3-10 by woelen]

aonomus - 16-3-2010 at 05:56

IIRC, the bog standard method for producing semiconductor silicon is to take clean sand (mostly SiO2) mixed in with excess carbon, heat strongly without oxygen to reduce the silicon and produce CO2, CO. You get a mixture of Si metal, carbon, and other contaminants. Following the reduction, repeated zone refining is used to purify the silicon...

Now if *you* wanted to make Si metal, you could probably reduce with carbon (outdoors cause of the CO!), then melt it all down (if you can even reach 1400degC) and skim off the crap from the top.

gnitseretni - 16-3-2010 at 06:42

Woelen: I have no background in chemistry at all. All I know about chemistry I learned on forums like this one.

So let's see... you said I need 2 Mg and one SiO2. Mg is 24 g/mol and SiO2 is 60 g/mol. So I'd need 48g Mg and 60g SiO2. Correct? Plus a little extra Mg to make sure all SiO2 gets reacted.

If I were to use stoichiometric amounts, I'd get Si + MgO. But because I'll use a slight excess of Mg, the excess Mg will react with some of the Si that formed and so what I'll get is Si + MgO + Mg2Si.

Wen the reaction has subsides, I break the lumps up into a powder and add it to dilute HCl, which will dissolve the MgO and react with the Mg2Si to make silane. So filtering this solution should leave (fairly pure?) Si on the filter paper, if I'm not mistaken.

woelen - 16-3-2010 at 07:13

Very good, you understand the concept of stoichiometry. Indeed, 48 grams of Mg for 60 grams of SiO2. I would say, try first with a little bit more than 4 grams of Mg and exactly 5 grams of SiO2 (same ratio as 48 : 60) to get a feeling of the reaction.

If the sand and Mg are sufficiently fine and well-mixed, then I think that you will get fairly pure silicon, after dissolving the MgO and Mg2Si. In practice don't expect much better than 95% purity, because we are talking about a solid-solid reaction and these always tend to have unreacted parts in the reaction mix. After dissolving the result of the reaction I expect that even when a slight excess of Mg is used, that still some SiO2 remains unreacted and so I expect you to get silicon with a few percents of leftover SiO2 after the washing with dilute HCl.

Try to have your powders as fine as possible!! The finer the better the purity of the end product.

gnitseretni - 16-3-2010 at 07:56

Thanks for your help Woelen.

watson.fawkes - 16-3-2010 at 12:19

Quote: Originally posted by aonomus  
IIRC, the bog standard method for producing semiconductor silicon is to take clean sand (mostly SiO2) mixed in with excess carbon, heat strongly without oxygen to reduce the silicon and produce CO2, CO. You get a mixture of Si metal, carbon, and other contaminants. Following the reduction, repeated zone refining is used to purify the silicon...
Simple reduction will get you to metallurgical-grade silicon. For semiconductor-grade, you then make and distill silane (and/or a substituted chloro-silane). After that, you pull a boule and use zone refining.

aonomus - 16-3-2010 at 12:34

Ah, I was taught a somewhat stripped down version for my separations & chromatography course then.