Recovery of silica fume from blast-fumace slag, a by-product of iron manufacture, was attempted through the acid treatment at room temperature. The waste slag contains quite valuable pure silica with only trace metallic elements such as TiO2 and CaO. The present lab-scale experiments revealed that highly pure silica foam could be obtained through a simple process of an effective reaction between the slag and acid. Its rapid exothermic reaction enables to reduce the reactor volume per unit production. The reaction rate was associated with the absolute amount of acid rather than the reaction time or temperature. It was also found that the reaction efficiency would depend primarily on acid concentration.