PURPOSE: To obtain silica sand for casting of high quality from sand contg. a large amount of impurities by using an anion capturing agent consisting essentially of a dispersant, a foaming agent and a fatty acid and adapting a floatation method, at the time of removing the impurities of raw material sand.

CONSTITUTION: Raw ore F of sea bottom sand or the like is sieved and raw sand of about 14 meshes is obtained. This is passed through then subjected to attrition treatment 6, after which the sand is passed through a cyclone 7 and a classifier, whereby it is removed of slime and is made into floatation raw sand. This raw sand is charged into a condition tank 9 contg. a dispersant, a foaming agent, a capturing agent, etc., where it is subjected to condition impartation, thence it is supplied to a floatation machine 10. An oleic acid, a linolic acid, etc. are contained in this floatation agent, and the raw sand is held controlled at pH7.0W8.5. Floating sand of impurities such as ore, sea shells and others is sent to a fine floatation machine 11 to undergone fine floatation. The sunken grit is sent to spiral classifier 14 and after it is dehydrated, it is stored as a product in a stockyard 15. The sunken grit of the fine floatation machine 10 is returned to the floatation machine 10.