NOVELTY - Silicon sludge containing silicon powder (c) having silica film containing metal impurity, and hydrofluoric acid washing liquid (b) are injected into a washing tank. The oxygen concentration of silicon powder is measured. The total weight of silica film formed in the powder in the washing tank is calculated from the measured value. The minimum amount of hydrofluoric acid for removing all silica film is computed. Washing liquid containing hydrofluoric acid more than the calculated value is supplied to the washing tank and silicon sludge is washed.    USE - Washing of silicon sludge produced during outer peripheral grinding and slicing of silicon wafers used in solar cells.    ADVANTAGE - The method efficiently washes silicon sludge. Stable high purification of silicon powder is attained. Uniform high purification of silicon powder covering between batches is realized. Since the usage amount of hydrofluoric acid washing liquid is made into minimum quantity, washing cost is reduced. The purified silicon powder provides silicon type solar cells having excellent photoelectric conversion efficiency.    DETAILED DESCRIPTION - Silicon sludge produced in a silicon process and containing silicon powder in which a silica film containing metal impurity is formed in the surface layer, and hydrofluoric acid washing liquid are injected into a batch type washing tank. The silica-containing film is dissolved by the hydrofluoric acid component in the washing liquid, and the metal impurity is removed from the silicon powder. The oxygen concentration of silicon powder is measured. The total weight of silica film formed in the powder in the washing tank is calculated from the measured value. The minimum amount of hydrofluoric acid which can remove all the silica film formed in the powder is computed. Washing liquid containing hydrofluoric acid more than the calculated value is supplied to the washing tank and silicon sludge is washed.    DESCRIPTION OF DRAWING(S) - The drawing shows an expanded sectional view explaining the solution state of natural oxide film containing metal impurity formed in surface of silicon powder. (Drawing includes non-English language text)    Hydrofluoric acid washing liquid (b)    Silicon powder (c)