Method for processing an etching mixture which is formed during the production of highly pure silicon

A method for purifying polycrystalline silicon, comprising purifying by prepurification and main purification with an etching mixture containing HF and HNO3, wherein first and second fractions from distillation of a used aqueous etching mixture are prepared by progressively distilling a used aqueous etching mixture containing HF, HNO3, H2SiF6 and HNO2 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, the second fraction comprising an a HF and HNO3 mixture with an HNO3 content >55 wt. % and a silicon content of less than 1 wt. %, and contacting polycrystalline silicon with the second fraction in the main purification of the polycrystalline silicon.