Patent ID: 7922876

Claim:
A method for recovering acid from a used aqueous etching mixture containing HF, HNO 3 , H 2 SiF 6 and HNO 2 derived from purifying polycrystalline silicon, comprising progressively distilling the used aqueous etching mixture in a first step such that approximately 20 to 50 wt. % of the used aqueous etching mixture is distilled off as dilute acid in a first fraction, the dilute acid containing more than 90 wt. % of the silicon in the used aqueous etching mixture as hexafluorosilicic acid, the water contained in the used aqueous etching mixture being reduced by approximately 10-30 wt. % to form a water-depleted mixture, and distilling a second fraction from the water-depleted mixture in a second step until a residue of about 1-5 wt. % of the initial amount of used aqueous etching mixture is obtained, and disposing of the remainder of the residue.