Glycerol is recovered form spent lye using the following technique

The soap formed in colloidal form is precipitated by adding NaCl. [Na+] increases and thus the ionic product (Qsp) of sodium stearate exceeds Ksp  leading to precipitation of soap (sodium stearate). The left over solution containing glycerol is called spent lye. Glycerol decomposes much below its boiling point. So it is recovered from spent lye using vacuum distillation.