Chemistry - Un-denaturing industrial alcohol
Solution 1:
(I'm no chemist, but here's my take on it)
Let's see: I’m an alcoholic, and I really hate having to dole out large amounts of cash to purchase your usual "drinking"-ethanol.
So I get this amazing idea to separate the ethanol by distillation (methanol has a lower boiling point than methanol, so what I'll really be doing is distilling out the methanol, leaving behind ethanol and water). Pretty smart, right (for someone who's still inebriated)? Thus invigorated, I sally forth... and return with a dozen gallons of industrial (denatured) ethanol. However, as I slowly get sober, I begin to see two major obstacles to my plan:
It’s going to be very, very difficult to maintain a constant temperature of exactly 64.7 °C (boiling point of methanol). Now, even though ethanol boils at around 78 °C, trying to maintain the denatured alcohol at under (say) 70 or 75°C isn’t really an option either. Remember, what I’m trying to do here is criminal (where I'm from at least), plus I don’t really have a Nurdrage-worthy laboratory. If say, I were to simply buy a large metal vat with a distillation apparatus clamped on the top and light a fire below, at such a large scale (a dozen gallons in my case) and with the non-professional equipment I use, it’ll be impossible to ensure that the vessel is uniformly heated at a specific temperature. So what’ll happen is, I’ll end up distilling out a mixture of ethanol and methanol, and the ethanol that remains will also have significant amounts of methanol in it (10ml of methanol's enough to make you go blind… just to give you an idea about what I mean by “significant”). Regardless, I wouldn't want to engage in something that involves producing methanol vapors (be it in a proper laboratory or otherwise)
Say I did manage to get hold of the required laboratory apparatus (I, uh, happen to know this cancer-stricken, bald, high-school Chemistry teacher who was willing to lend me them), and I were to distill the alcohol in small batches to ensure I evenly heat the solution at the desired temperature, but the whole process would be so time-consuming and labor intensive that it would probably just be better if I just went and bought "drinking"-ethanol in the first place.
(Should have mentioned this first, but since you were talking about methanol, ah well…) To begin with, I probably wouldn't know what the manufacturers actually used to denature the alcohol. They could have used methanol, pyridine, or some other foul-tasting, foul-smelling additive for all I know, or maybe even mixed all of them together. So trying to get ethanol out of that mess, is going to be a HUGE pain in the neck.
Also you might want to read up on denatured alcohol here.
I wasn't able to find anything on ethanol-methanol azeotropes in this Wiki page, so if anyone knows where I can find something on it, gimme a shout! :-)
Solution 2:
The reason criminals don't undenture alcohol is probably because it's cheap and easy to make your own alcohol at home. Plus it's legal in the US (click on G1) and most other English-speaking countries.
You can get equipment kits from anywhere between 50 and 200 USD (more expensive kits will get you tools that make things easier or improve quality and add an ingredients kits). Ingredients kits for beer, wine, cider, etc., run anywhere from 30 to 300 USD.
Now, if you wanted to go cheap, you could make kilju, a cheap homemade Finnish alcoholic beverage pronounced, basically, "kill-you".
For equipment, you'd get a food grade bucket from the hardware store (a 5 US gallon one is about 4 USD locally) and thin towel or piece of cheese cloth (about 2.50 USD locally) for a total of 6.50 USD for equipment.
For ingredients, you'd use 3 lbs of sugar plus 1 oz of raisins (for extra nutrients for the yeast and more reliable results) per US gallon. We need to leave a gallon or so of headspace, so in that 5 USG bucket, we'd make 4 USG. That's 12 lbs of sugar and 4 oz of raisins for 4 USG of water. I can get sugar for about 2.39 USD for 2 lbs, so that's 14.34 USD for the sugar and 1 USG of spring water goes about 0.89 USD here, so that's 3.56 USD for the water. A bag of 10 oz of raisins goes for about 1.99 USD. Champagne yeast (you can use baker's yeast which you can find in a grocery store but champagne yeast will give much better results and less yeasty flavors) runs about 0.50 USD which gives us a grand total of 20.39 USD for the ingredients.
Once you have that, you mix the sugar into the water in the bucket then add the raisins then sprinkle the yeast on top and cover the bucket with the cheesecloth. Once bubbling stops (about a week), syphon it out, leaving the dregs (raisins and dead yeast) behind. You'll have a little less than 4 USG of kilju with a potential alcohol content of about 16% (assuming I did my math correctly).
Everything I've described up to this point is legal in the US and most other English speaking countries. The next procedures I'm going to describe is probably illegal in most English speaking countries (except, apparently, New Zealand).
If 32 proof alcohol isn't strong enough for you, you can distill the kilju. An old fashioned easy way is freeze distillation (or jacking). Place your kilju in your freezer and when it gets slushy, pour it through a sieve and toss the solid portion. Repeat a few times until it doesn't freeze very well. What you'll have is moonshine (although most moonshiners seem to use heat distillation instead of freeze distillation).
Solution 3:
In Soviet Russia, where alcoholism was widespread and homebrewing illegal, undenaturing alcohol was a pretty common thing - a part of national culture, in a way - along with such practices as:
- drinking straight down various non-food grade products containing ethanol
- or extracting ethanol from them
- or stealing ethanol from industrial processes
- or, well, moonshining.
That being said, unlike in some other countries, denaturing was almost never done with methanol, for fear of people drinking themselves to death, which is bad for planned economy (and which they did anyway, when they could lay their hands on methanol in applications where it is genuinely needed). In fact, the authorities went to certain lengths to substitute methanol with ethanol wherever possible, considering illicit drinking a lesser evil than the alternative. Methanol was marked most prominently and guarded most heavily, and still regularly caused poisonings.
Denaturing alcohol usually was done with some additives that gave it bitter taste (or worse) and foul stench; distillation alone would not remove the latter. Undenaturing typically included oxidation of the additives by $\ce{KMnO4}$ and/or absorbing them with activated charcoal (both were dirt cheap in the drugstores), and then probably distillation as an additional step. Mind you, methanol can't be dealt with in this manner.
Moonshining included either distillation or freezing out; the latter was usually done outdoors during the winter, by pouring the raw fermented brew down a sloped U-beam, for domestic fridges in those days were ridiculously weak and barely could get anything to $0^\circ\,\rm C$. BTW, methanol can't be reliably removed this way either.
Consumption of non-food grade products included things so abhorrent I myself have troubles believing. How about stirring a certain kind of industrial glue with a power drill, so as to separate the glue proper from its solvent (which contained some ethanol)? Or putting a thick layer of shoe polish on a lump of bread, thus making alcohol to penetrate into the bread? Drinking cheap cologne and window washer looks pale and innocent in comparison.
Nowadays, with relatively cheap and available legal booze, most of these practices seem to fade into oblivion. Let them be recorded here, if only as a negative example for the generations to come.
Solution 4:
As @matt_black and @Aaron Abraham suggested, cleaning the denaturated ethanol is difficult and expensive. You don't know, what chemicals other than ethanol and methanol are in the solution and in what concentrations.
Selective absorbtion using zeolites, as @SteffX suggested, is also blank. You don't know what to filter out. Maybe using regenerative filter that will absorb ethanol and water only and then regenerate it untouched.
Much cheaper is preparation of your own ferment and distilling it in distillery or on your own. This takes time and care (collecting and fermentation) and knowledge and skill (distilling). Otherwise you will end up with poor-taste methanol-rich liquid. Hard alcohlic, thinking of de-denaturation of denaturated alcohol is unable to do it for sure.
Another way is to focus on very cheap drinks. Czech final-stage alcoholics, homeless mostly, drink Okena™, some of them, the rich ones, drink it through a bread.
Playing with the devil is quite common in black market in post-communist countries though, see: Czech methanol scandal and Pärnu methanol tragedy.
Solution 5:
The article "The Chemist's War" http://www.slate.com/articles/health_and_science/medical_examiner/2010/02/the_chemists_war.html goes through the various changes to denaturing agents used after the passage of the 18th amendment (Prohibition). I think the cost/benefit tradeoff still makes it unattractive