Abstract: Based on properties of wastewater produced in the process of producing iron oxide red with high Fe²⁺ content and low pH，a new process of producing iron oxide red crystal seed using the sludge produced from the treatment of acid ferrous wastewater in red iron oxide plant was developed.Results showed that pH of effluent could be adjusted to 6~9，removal efficiency of Fe²⁺ reached 99.5%，and chroma was less than 40 times，while adding caustic soda solution（mass fraction of 30%）2.8 L per ton of wastewater.The effluent quality could meet the secondary standard of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant（GB 18918—2002）.Mixed sludge of Fe（OH）₂ and Fe（OH）₃ could be used to produce iron oxide red crystal seed and to further produce iron oxide red.Fe2O3 content of the final product reached 96.8%，which could meet the standard of Iron Oxide Pigments（GB/T 1863—2008）.

Key words: wastewater treatment, iron oxide red, crystal seed, iron sludge