PURPOSE: To produce ferrite when mine water contg. ferrous ions and dissolved silica is treated by carrying out an aging stage at ordinary temp. after a heat treating stage.

CONSTITUTION: Alkali is added to mine water contg. ferrous ions and dissolved silica to coprecipitate part of the ferrous ions as iron hydroxide with the silica and the formed coprecipitate is separated. Alkali is further added to the resulting soln. contg. the remaining ferrous ions to recover ferrous hydroxide. This ferrous hydroxide is mixed with ferric hydroxide in 1:1W1:2 ratio. The ferric hydroxide is recovered by oxidizing mine water and adding alkali to the oxidized mine water contg. ferric ions. Alkali is added to the mixture and they are brought into a ferrite forming reaction by heating and then allowed to stand in a room. Thus, ferrite having sufficiently strong magnetizing force can be refined.