Study On Supercritical Water Oxidation Treatment Of Tannery Sludge And The Effect Of Ultrasonic Co-removal Of Cr

Tannery sludge contains a large number of organic compounds,among which the high content of Cr(?)is the main factor to limit its complete harmless and resource utilization.The research for harmless sludge,efficient recovery and treatment technology of Cr(?)has been a hot topic for hazardous solid waste researchers.In order to keep an eye on the treatment problems of Cr(?)in tannery sludge,for this study chromium-contaning sludge was selected from different companies on the basis of morphological analysis of Cr.The effect of ultrasond-assisted supercritical water oxidation(SCWO)degradation of organic matter and removal of Cr was studied to provide a theoretical and technical basis for the complete separation of Cr from tannery sludge.This study focused on two-phase separation(solid-liquid)experiment of Cr in the ultrasonic co-oxidation process under different reaction conditions of SCWO.The optimal technological parameters of ultrasond-assisted SCWO were determined.On this basis,the morphological changes of solid Cr after oxidation and the composition of liquid components were characterized,and the ultrasonic cooperative mechanism was analyzed.The research conclusions are as follows:(1)Ca,Fe,Cr and A1 metals are present with higher contents in tannery sludge but Cr has the most significant environmental risk.In tannery sludge,Cr was in acidic and organic combination state which accounted 66.17%of the total chromium.After acid extraction,there was still 15.87%residual Cr,which was not completely harmless.(2)When the tannery sludge was treated by ultrasonic,the effects of ultrasonic on organic matter degradation and Cr migration under different moisture content of sludge and different ultrasonic conditions were investigated.The moisture content had a great influence on the ultrasonic effect.When the moisture content was 85%and 90%,the TOC value of the supernatant could increase by about 40%after ultrasound.Meanwhile,when the moisture content was less than 75%,the ultrasonic effect was ineffective.Ultrasonic time and power were positively correlated to the breaking effect of organics with 50%power ultrasound for 40 min and the TOC value of the supernatant was from 937.7 mg/L to 1467.1 mg/L.Under neutral and slightly alkaline conditions,Cr(III)still deposited in the solid phase and Cr had no significant effect in separating from the sludge.(3)When the tannery sludge was treated by SC WO,the effects of different moisture content and different SCWO conditions on Cr morphology transformation and mixing in the sludge were investigated experimentally.When the content of oxidizer was insufficient,the TOC of supernatant increased but Cr did not change.With an increase in the oxidation ratio to 1:1 under 450?,the degradation rate of TOC reached 97%and the mobility rate of Cr reached 47.7%.At 500? and 30 MPa,the degradation rate of TOC reached 99.8%.But with the decrease in the supernatant TOC from 1671.1 mg/L to 9.1 mg/L,the total Cr increased from 11.9 mg/L to 842.9 mg/L and the separation rate of Cr from the solid phase was 49.58%.Most Cr remained in the solid phase in the form of inorganic salts.Oxidation became more intense with an icrease in temperature but the reaction pressure had no significant effect on the degradation of TOC and the oxidation of chromium.(4)The optimal process parameters for ultrasonic combined supercritical water oxidation treatment of leather sludge were:ultrasonic power 750 W,ultrasonic 30 min,sludge moisture content 85%,supercritical temperature 470?,pressure 30 MPa.After the ultrasonic reaction,the sludge was oxidized under the relatively mild SCWO condition.The degradation rate of TOC was 99.8%while the separation rate of Cr from solid phase was 99.6%,all existed in the form of Cr2O72-.Solid residue chromium was harmless,and supernatant Cr(Cr2O72-)was recycled.The above studies proved that ultrasonic pretreatment plays a crucial role in cracking the combination of chromium,organic matter and inorganic matter in sludge.Cavitation effect and free radical effect can crack the structure between large molecules of organic matter and reduce the size of inorganic components.When the sludge is broken,Cr was exposed to the surface of the sludge particles,which made it easier for subsequent experiments to convert Cr(III)into Cr(VI)and provided important conditions for removing of Cr from sludge by leaching and SCWO.