:!: :!: Some important info from here:
http://journeytoforever.org/biodiesel_glycerin.html
Soap
The crude glycerine by-product from homemade biodiesel makes a powerful degreaser.
Remove the residual methanol first.
Letting the by-product stand in an open container for a few weeks will NOT evaporate the methanol as it's often said it will, or not much anyway.
Boil it off -- NOT over an open flame, do it in the open, don't inhale any fumes, or (better) use a simple condenser to recover the methanol for re-use.
The disadvantage of raw by-product is that it contains most of the lye catalyst used in the processing, which makes it very caustic, it can burn the skin if you don't use gloves.
Saponifying the by-product makes an even better cleaner, and it won't burn your skin -- in fact it's kind to the skin because of the high glycerine content. Glycerine is a natural product of the soapmaking process. Glycerine moisturizes the skin, but commercial soap manufacturers remove the glycerine for use in lotions and creams, which are more profitable. Handcrafted soap retains the glycerine, and hence the boom in do-it-yourself craft soapmaking, and the high prices of handmade soaps.
Saponified glycerine by-product is a great cleaner, we use it for all cleaning jobs, whether to clean old machine parts filthy with dirt and grease, or as an effective and economical dishwasher, laundry soap, or an excellent hand-cleaner. It cuts through oil, grease and dirt like a knife, and it doesn't take much of it.
There are various recipes for making by-product "soap", but they're rather vague and imprecise. Basically, remove the methanol first, then mix extra lye with water, add it to the heated by-product and mix for 10 or 15 minutes while maintaining the temperature. Then you have to cure it for a couple of weeks.
:!: The difficulty with by-product soap is in calculating how much extra lye to use. It depends how much you used in the first place.
The results also depend on which catalyst you used: sodium hydroxide (NaOH) will give a solid bar soap, potassium hydroxide (KOH) makes liquid soap. We prefer KOH as a catalyst and seldom use NaOH, and we have no experience of making bar-soap from the by-product. We'd rather have a liquid cleaner anyway.
:!: You have to stay with the catalyst you started with -- if you used NaOH in the biodiesel process you can't then use KOH to make soap from the by-product, it has to be NaOH, and vice versa: if you started with KOH, make soap with KOH.
Use the soapmaking resources listed below to find a good general method to tell you how to go about heating, mixing and curing.
We use 100 ml of water per litre of by-product, which works well with KOH to make liquid cleaner. That should be okay for NaOH bar soap too, or try using less water.
The lye quantity is more critical. It depends on the titration of the oil you used. Common recommendations are that if you used say 5-7 grams of NaOH per litre of oil in the biodiesel process, then you need another 30 to 40 grams of NaOH to saponify the by-product. We think it should be more precise than that -- we've found if you don't use enough lye the soap tends to leave an oily film, and if you use too much it's too harsh.
Here are some starting figures you could try, apply them proportionally according to the titration of your oil.
If your oil titrated at 1 ml 0.1% NaOH solution try using 22 grams of NaOH; if the titration was 2.5 ml 0.1% NaOH solution, try 30 grams of NaOH. Make a few tests first with small quantities, varying the amount of lye by 2 grams on either side until you get the best result.
For KOH, multiply by 1.52 for 92% pure KOH, by 1.56 for 90% pure KOH, or by 1.65 for 85% pure KOH.
If you separate the glycerine by-product from the impurities (see Separating glycerine/FFAs), you'll be left with about 80-90% pure glycerine. You can add it to plain liquid soap to make a high-glycerine shower-soap or shampoo. It doesn't need much -- try 10 to 20cc per 500cc of liquid soap, and add some essential oils for fragrance.