NOVELTY - Silicon sludge is regenerated by wash-processing silicon sludge produced during manufacture of silicon with hydrogen fluoride (HF) in an amount of 1.5-9 times the molar quantity of oxygen-atom molar quantity contained in silicon sludge and 0.1-2 mass% dispersing agent, melting under inert gas atmosphere or reduced pressure and vacuum atmosphere while suppressing scattering of molten silicon accompanying rapid vaporization of silicon oxide, and solidifying molten silicon. Impurity concentration is less than or equal to 0.4 ppmw boron (B), less than or equal to 0.8 ppmw phosphorus (P), and less than or equal to 10 ppmw of iron (Fe).    USE - Regeneration of silicon sludge for reproducing silicon block used in the manufacture of polycrystalline silicon used for solar cells.    ADVANTAGE - Silicon scattering phenomenon can be prevented during silicon sludge dissolution while suppressing foaming protuberance phenomenon during HF washing of silicon sludge, resulting in high yield of recovery of silicon sludge, generation amount of spent acid after use can be reduced, and silicon block can be reproduced for manufacture of polycrystalline silicon used for solar cells at a low cost.    DETAILED DESCRIPTION - Silicon sludge is regenerated by wash-processing silicon sludge produced during manufacture of silicon with hydrogen fluoride (HF) in an amount of 1.5-9 times the molar quantity of oxygen-atom molar quantity contained in silicon sludge and 0.1-2 mass% dispersing agent, melting under inert gas atmosphere or reduced pressure and vacuum atmosphere while suppressing scattering of molten silicon accompanying rapid vaporization of silicon oxide, and solidifying molten silicon. Impurity concentration is less than or equal to 0.4 ppmw boron (B), less than or equal to 0.8 ppmw phosphorus (P), less than or equal to 10 ppmw of each metallic element of iron (Fe), aluminum (Al), nickel (Ni), and copper (Cu), less than or equal to 50 ppmw carbon (C), and less than or equal to 50 ppmw oxygen (O).