Study On The Recovery Method Of Chromium From Tannery Sludge By Ultrasonic Acid Leaching And Oxidation

The Cr content of tannery sludge produced by tannery chromium-containing wastewater after physico-chemical precipitation treatment is usually 40-80 g/kg,and the organic component content is about 50%-70%.Tannery sludge has a large annual output and high environmental risks,so its treatment and disposal is one of the key issues related to the sustainable development of the tannery industry.At present,the chemical extraction treatment of tannery sludge has the problem of incomplete Cr removal and the subsequent recycling of Cr is difficult.This thesis aims at the technical problems of incomplete Cr removal and difficult recovery of Cr.Ultrasonic assisted chemical extraction is used to extract Cr in tannery sludge.The leaching effect of three different inorganic acids on Cr is evaluated.The best chemical extraction agent,and the best experimental conditions were determined by orthogonal test.The effectiveness of different ultrasonic methods,ultrasonic power and ultrasonic time on the chemical extraction was evaluated by the leaching rate of difficult-to-extract form Cr.Potassium dichromate(K2Cr2O7)+hydrogen peroxide(H2O2)was used as oxidant to remove dissolved organic matter(DOM)in the Cr-containing extracts,and the removal effect of DOM was characterized by the TOC removal rate.The orthogonal test was used to determine the optimal input of oxidant.Otherwise,the conditions for the occurrence of Fenton-like reactions in the liquid phase system of Cr and H2O2 were subsequently investigated.It is expected to provide an engineering basis for the treatment and disposal of tannery sludge to achieve its harmlessness,reduction and resource utilization.The main findings are as follows:(1)The three inorganic acids HCl,H2SO4,and HNO3 were used as chemical extractants to treat tannery sludge.Under the experimental conditions of solid-liquid ratio of 1:30,H2SO4 concentration of 0.5 mol/L and reaction time of 3 h,the leaching rate of Cr reached 85.42%.By adding 0.1 mol/L citric acid,it could exert its dual effects of acid leaching and chelation,and increased the Cr leaching rate to 88.82%.(2)The probe-type ultrasonic assisted chemical leaching method had a significantly better effect on leaching difficult-to-extract form of Cr than the tank-type ultrasonic,and increasing the ultrasonic power could effectively increase the Cr extraction rate.Under low ultrasonic power,increasing the ultrasonic time would increase the Cr leaching rate more obviously.Under high ultrasonic power,the effect of the ultrasonic time on the Cr leaching rate would be weakened.With the ultrasonic power of 750 W and ultrasonic time of 120 min,the Cr leaching rate reached 95.62%,and the sludge reduction was 47.09%.(3)Under the conditions of 50?,reaction time 2 h,K2Cr2O7 solid powder dosage of 0.4%(mass fraction)and 30%H2O2 dosage of 15%(volume fraction),the removal rate of TOC in the extraction solution reached 78.02%.After oxidation,all Cr existed in the form of Cr(?),which met the needs of leather tanning process.In the liquid system,Cr(?)and Cr(?)reacted with H2O2 to generate hydroxyl radicals(·OH)via Fenton-like reaction usually under the conditions of neutral or weak alkaline with pH>5.In the subsequent pilot scale experiments of acid extraction and oxidation,the Cr removal rate of tannery sludge reached 79.1%,and the TOC removal rate of extraction solution reached 73%.The final Cr solution met the recycling requirements of leather tanning process.