NOVELTY - A waste sludge is subjected to magnetic separation process, and magnetic materials are separated from waste sludge. The resultant waste sludge is separated into silicon dispersion liquid, and sedimentation layer. An acid is added to the separated dispersion liquid, and the metal-impurity in the dispersion liquid is dissolved. Then, the silicon particles are separated from the resultant liquid, cleaned, heat-processed, heat-melted, and solidified, to obtain silicon ingot. The silicon ingot is subjected to unidirectional-solidification process, to obtain silicon.    USE - For manufacturing silicon from waste sludge used for solar cell (claimed).    ADVANTAGE - The silicon from waste sludge is manufactured efficiently and reliably with high purity.    DETAILED DESCRIPTION - A waste sludge containing silicon particles, powdery inorganic material other than silicon particles, magnetic impurities and organic-type impurities, is subjected to magnetic separation process, and magnetic materials are separated from waste sludge by magnetic force. The resultant waste sludge is centrifuged and separated into silicon dispersion liquid in which silicon particles are dispersed, and sedimentation layer containing sedimentation substance. An acid is added to the separated silicon dispersion liquid, and the metal-impurity in the silicon dispersion liquid is dissolved, in metal-impurity dissolution process. Then, the silicon particles are separated from the metal-impurity dissolved silicon dispersion liquid and cleaned. The attached liquid containing the acid adhered to the silicon particles is removed. The cleaned silicon particles are heat-processed, and the organic compound in the silicon particles is removed. The heat-processed silicon particles are heat-melted, and solidified, to obtain silicon ingot. The silicon ingot is subjected to unidirectional-solidification process, to obtain silicon.    DESCRIPTION OF DRAWING(S) - The figure is a flowchart of manufacture of silicon from waste sludge. (Drawing includes non-English language text)