PURPOSE: Waste water containing ferric ions is neutralized and the resultant reaction mixture is combined with a solution containing a specific amount of ferrous ions and an excessive amount of calcium carbonate and oxidized to form ferromagnetic precipitate whereby a ferromagnetic material is recovered from waste water containing ferric ions.

CONSTITUTION: CaCO3 slurry is added to waste water containing ferric ions such as ferric sulfate to adjust the pH to 3.8, thus forming precipitate or solution containing ferric ions. The product is combined with a solution of ferrous sulfate so that the ratio of Fe3+/Fe2+ becomes less than 2. Then, CaCO3 slurry is added until the pH reaches the saturated value, then air is blown into the mixture at 65°C to effect oxidative treatment. Ferromagnetic precipitate is formed and recovered by magnetic separation and utilized as a starting material of wave absorber an vibration damper.