Abstract: The purpose of the experiment is to dispose the solid chromium slag with a low toxicity.Firstly，Cr（Ⅵ） from solid chromium slag  was leached by using ultrasonic treatment.Then Cr（Ⅵ） in the leaching liquid was reduced by hydrazine hydrate.Various factors affected the reduction of Cr （Ⅵ）were discussed.Results indicated that ultrasonic treatment could leach most of the Cr（Ⅵ） from solid chromium slag in 5 h；in the alkaline range，the factors，such as amount-of-substance ratio of reactants，reaction temperature，reaction time，and pH all had obvious affection on the reduction of Cr（Ⅵ）.The optimum experimental conditions were as follows：reaction temperature was 60 ℃，pH=8.5，amount-of-substance ratio of hydrazine hydrate to Cr（Ⅵ） was 130 ∶1，and reaction time was 30 min.Under those conditions，the removal rate and final mass concentra- tion of Cr（Ⅵ） could reach 98.6% and  0.09 mg/L，respectively.

Key words: chromium slag, hydrazine hydrate, Cr（Ⅵ）, ultrasonic