Study On The Resource Utilization Of High-salt Wastewater For Coking Desulfurization

The coking industry usually adopts wet oxidative desulfurization process（Phthalocyanine Dinucleus Sulfonation method,improved anthraquinone disulfonate sodium method,etc.）to remove sulfur element in coke oven gas.This process discharges a large amount of desulfurization wastewater with high salt content（thiocyanate,thiosulfate and a small amount of sulfate）which is very serious and difficult to manage.Since the desulfurization waste liquid contains thiocyanate ions(SCN^-1)with strong bactericidal ability,the waste liquid must be subjected to salt extraction before further treatment.Nowsaday,the high-salt wastewater is mainly treated by fractional crystallization processes,which have several common problems such as complex processes,unstable results,and low salt purity.In this work,the high sodium salt wastewater produced in the desulfurization process of the coking industry is regarded as the research target.Taking advantage of solubility differences of inorganic salts（sodium thiocyanate and sodium thiosulfate）in polar solvents/non-polar solvents,a two-system（water-ethanol system）fractionation separation method was used for extracting secondary salts in desulfurized high-salt wastewater.In addition,the organic contaminants in the wastewater after salt extraction can be degraded and removed via photocatalytic technology.This work is expected to solve the problems of the difficult degradation of coking desulfurization waste liquid,the high cost of organic removal,and the unstabled crystallization efficiency,which can not only realize the effective recycling and utilization of resources,but also realize zero discharge of organic wastewater.The main research contents and results are as following:（1）The mixed salts（1）and mixed salts（2）were obtained by evaporating and concentrating the waste liquids produced in the improved ADA and PDS desulfurization processes（hereinafter referred to as waste liquid（1）and waste liquid（2）,respectively）.The contents of sodium thiocyanate（Na SCN）and sodium thiosulfate（Na₂S₂O₃）in mixed salts were determined by titration method.The results shown:the contents of Na SCN and Na₂S₂O₃in mixed salt（1）（white）were 24%and 59%,respectively;those in mixed salt（2）（Green due to residual PDS catalyst）were 40%and 46%,respectively.The two groups of waste liquids have higher value of salt separation and purification due to the higher content of Na SCN and Na₂S₂O₃than normal waste liquids.Besides,the total organic carbon（TOC）contents in mixed salt（1）and mixed salt（2）were 2.40%and 3.04%,respectively.（2）The waste liquid（2）needed to be decolorized before separation and purification.In this work,columnar activated carbon and No.1,No.2 and No.3 activated powder carbon were selected to decolorize the waste liquid,which shown that the decolorization effect of waste liquid is highly related to the shape and amount of activated carbon.Based on the consideration of cost in practical application,the best decolorization effect was No.3activated powder carbon（Iodine value is 900）with 3 g·L^-1of optimal dosage and 30 min of decolorization time.（3）The research indicated that methanol,ethanol,ethylene glycol,toluene and other organic solvents can be blended with water to form a dual-system solution mixture.Considering the factors such as cost and safety,the ethanol-water system was finally selected in this work.When the volume fraction of ethanol increased,the solubility of Na₂S₂O₃gradually decreased until it was almost 0;the solubility of Na SCN also decreased gradually,but it was still relatively large.The optimal volume fraction of ethanol was finally selected as0.75.At this time,the recoveries of Na SCN and Na₂S₂O₃in waste liquid（1）were 90%and68%,respectively,and the recoveries of them in waste liquid（2）were 91%and 81%,respectively.In addition,the purities of Na SCN and Na₂S₂O₃products were 94-96%and95-98%,respectively.（4）After separation,the ethanol solution was reused by rotary evaporation.The wastewater,produced by the crystallization and centrifugation of the aqueous solution and contained refractory organic compounds such as biphenyl,quinoline and pyridine,were further photocatalytically treated by KCC-1@Ti O₂photocatalyst.The experimental results shown that some organic contaminants in the wastewater can be degraded by the catalyst.Besides,the morphology,structure and photocatalytic performance of KCC-1@Ti O₂were analyzed by XRD,SEM and so on and Rhodamine B（Rh B）was used to evaluate the photodegradation performance of KCC-1@Ti O₂photocatalyst.Because Rh B,a dye reagent,is often used to determine the photocatalytic activity of photocatalytic materials.The possible mechanism of photocatalytic degradation was preliminarily inferred by free radical trapping experiment.The results shown that when the mass ratio of KCC-1 to Ti O₂was 2.5:1 and the dosage was 0.2 g·L-1,degradation efficiency of Rh B was 99%after 60 min of dark reaction and 60 min of light reaction.Free radical capture experiments shown that superoxide radical was the primary active specie during Rh B degradation.During the photodegradation process,the role of KCC-1 is adsorption function,while the role of Ti O₂is photocatalytic degradation function.The synergistic reaction of the KCC-1 and Ti O₂improves the photocatalytic activity.