Basic Research On Resourec Treatment Of Coking Desulfurization Waste Solution By Evaporation-Catalytic Converter-Salt Leaching Method

The wet oxidation desulfurization technology is widely adopted by coking enterprises due to its making full use of ammonia in coke oven gas and without external alkali source.The coking desulfurization waste solution（CDWS）that is produced in desulfurization progress is a kind of harmful pollutants.The health and life safety of workers and the surrounding masses are threated seriously by CDWS whose efflux or leakage will cause serious environmental accidents.Hower,the existing technology can not deal with it economically and effectively,how to effectively deal with CDWS is technical problem that placed in front of all coking enterprises.Resourec treating CDWS by ’evaroration-catalyltic converter-salt leaching method’ was proposed by author’s group after large number of experimental studies and theoretical analysis,in which,CDWS produced in HPF ammonia wet oxidation desulfurization progress was taken as the study object.In this paper,the basic properties of CDWS,the distribution characteristics and removal method of COD,ammonia nitrogen and sulfide in evaporative condensate of the CDWS,the extraction of ammonium thiocyanate from solid mixed salts after evaporation were researched according to the method proposed by author’s group.On the basis of above research results,the Pilot experiments on resourec treatment of CDWS by evaroration-catalyltic converter-salt leaching method were developed and established,and pilot-scale study were also conducted systematically based on the data of laboratory.The main results and conclusions obtained by the study are as follows.1 Basic properties of CDWSCDWS contains ammonium thiosulfate,ammonium thiocyanate,suspended sulfur,PDS,free ammonia etc.The organic compounds in CDWS from TYMQH contain phenolic and nitrogenous compounds,accounting for 98.79%of the total organic matter.Phenolic compounds,accounting for 87.54%of the total organic matter,including phenol,2-methyl phenol,3-methyl phenol,2;4 dimethylphenol,and 2,3-dimethylphenol.Nitrogen-containing organic compounds include 4-methylpyridine,2-methylpyridine,isoquinoline,quinoline,indole,nitriles and account for 11.54%of the total organic matter.The mixed salt in CDWS has poor thermal stability and can be almost completely decomposed at 425℃.The three main stages in the thermo-gravimetric process of mixed salt occur at 92.5℃ to 167.7℃,167.7℃ to 231.6℃,and 231.6℃ to 349.0℃.The second stage has the largest weightlessness among the three stages,accounting for 61.39%of total weightlessness.The density and boiling point of CDWS increase with increasing of salt concentration in the solution.The relationship exists between density（Y）and salt content（X）can be expressed by linear equation of Y=3.19*10^-4X+1.007.The boiling point of the solution can reach 124℃ when the concentration is increased to 450 g/1.2 Study on distribution characteristics and removal methods of COD,ammonia nitrogen and sulfide contained in evaporation condensate of coking desulfurization waste solution（1）The distribution characteristic of COD in condensate is parabolic,which can be expressed as Y=12.25X²-1019X+21722（Y is COD,mg/L;X is time,min）during the concentration of desulfurization waste solution.The reasons of high COD concentration in condensate are that the high concentration of COD appeared in early and late of the concentration.Phenolic compounds,nitrogen and alkanes accounte for 81.57%,17.38%and 1.1%of total organics in early concentration.They are the main cause of high COD in initial condensate and contribute up to 90%of it.The contribution of thiosulfate and other inorganic reducing substance which are the main causes on the high COD of finial condensate is as high as 85%.（2）Transition metal Cu shows high catalytic activity on oxidative degradation of COD in the condensate of desulfurization waste solution.The optimal preparation conditions of catalyst are that:Cu loadings of 5%,calcination temperature of 400℃,while optimal reaction conditions are that:reaction temperature of 350℃,ratio of oxygen to phenol is more than 8,space velocity is less than 2.37 ml/g.h.Under above conditions,the degradation rate of COD is more than 90%,and the longevity of catalysts is up to 1200 h.The COD content is stable varying 2800 mg/L from 1200 mg/L after treating by the catalyst,which meet the requirement of the inlet of biochemical treatment in coking plant that COD content of less than 3500 mg/L.Analysis and characterization of the catalyst by SEM,BET and TG showes that the deactivation of catalyst is caused by the running off of Cu.（3）The main resons of ammonia coming into the condensate are the volatility of free ammonia and the hydrolysis and thermal decomposition of ammonium sulfide（or ammonium hydrosulfide）,ammonium polysulfide and ammonium thiosulfate contained in desulfurization waste solution.Adjusting pH to about 5.00,use pretreatment methods such as heat or oxidation and control the evaporation temperature can lower ammonia nitrogen in condensate in the evaporation process of desulfurization waste solution.（4）The stripping rate of ammonia improves with the increases of reaction temperature,ration of gas to liquid and pH,and the order of various gases in stripping effect is that H2>CH₄>CO>N₂>O2.Kinetic studies showed that the reaction order of the stripping ammonia nitrogen is 1.The corresponding kinetic equation satisfies the following relationship dC/dt=-AeΔE/RTNmC.During stripping,the activation energy of H2、CH₄、CO、N₂ and 0₂ are respectively 46.34,42.64,40.10,34.03 and 32.24 kJ/mol.The stripping effect of coke oven gas is better than air,and the single gas stripping rate can be calculated by the above kinetic equation under the same conditions.Then the stripping rate of coke oven gas can be calculated by weighting according to the proportion of different gases.（5）The optimal conditions of MAP precipitation treatment with MgSO4·7H₂O and Na₂HP0₄·12H₂O as precipitant are that:pH=9.63,reaction time of 20min,reaction temperature of 20℃,n(Mg²⁺):n（NH4+）:n(PO₄^3-)=1:1:1.1.The removal rate of ammonia can reach 93.94%and magnesium ammonium phosphate precipitation of larger particles and more regular be got under the optimum conditions.（6）The primary reason of high concentration sulfide appeared in the condensate of desulfurization waste solution is that the hydrolysis and thermal decomposition of ammonium sulfide or ammonium hydrosulfide.Evaporating under vacuum condition after adjusting the pH to 5.0 can reduce the content of sulfide in condensate.The sulfide content is about 25mg/L after treating,which meet the requirement that sulfide content less than 75mg/L of the inlet of biochemical treatment in coking plant.The thiosulfate and sulfite have an important influence on determination of sulfide.It is suggested that the interference of S2O32-and SO32-can be eliminated by washing sedimentation twice with KOH solution of 10%.It indicates that the improved method can accurately determine the sulfide in the condensate of dusulfurization waste solution,and the standard recovery rate is more than 95%.3 The extraction of NH₄SCN from the mixed salt after evaporationAs the increase of the material liquid ratio,extraction times,extraction temperature,agitation speed and the decrease of salt particle size,the extraction rate of ammonium thiocyanate is increased.The key of achieving a high ammonium thiocyanate extraction ratio is enhancing the separation of ammonium thiocyanate leaching liquid and filter residues and increase the leaching frequency.The intermittent leaching process of ammonium thiocyanate was in accordance with the Spiro steady model.Under the same conditions,L1 is of higher extraction efficiency compared with L2.The leaching kinetics equation of L1 and L2 are ln[4s,0-4C_l,t（αρ+1）/C₍s,0]=-0.375/αRρe-97412/T+1.386 and ln[4C_s,0-4C_l,t（αρ+1）/C_s,0]=-0.456/αRρe1.6346/T+1.386 respectively.The leaching activation energies are 8.10 kJ/mol and 8.84 kJ/mol respectively.The best conditions of ammonium thiocyanate crystallization are without seed addition,air-cooling and low speed stirring（≤50r/min）,in which conditions,Ammonium thiocyanate crystal of blockbusters can be obtained and the purity is over 99%after washing.4 Pilot experiments on resourec treatment of coking desulfurization waste solution by evaroration-catalyltic converter-salt leaching methodUnder the condition of steam temperature of 126 C,steam pressure of 0.12 MPa,the steam flow of 126.75kg/h,evaporation efficiency can reach over 50%.Evaporation efficiency and evaporation amount in unit time are decreased as the increasing of the feed flow rate.There is little effect of feed temperature on the rate of evaporation under the same feed flow rate condition.Under the same conditions,a linear relationship is existed between the evaporation rate and the vacuum degree,that is Y=11.50+1085.07*X（in which,X represent for the vacuum degree,MPa;Y represent for the evaporation rate,%）.The working efficiency of intermittent feeding is higher than that of continuous feed for dryer.In order to achieve the dry products,the working condition of dryer should be above 0.084 MPa of vacuum degree and above 0.07 MPa of steam pressure.Under the above conditions,over 85 L/h of the feed rate for evaporator,the evaporator and the dryer can matched well.,while the evaporator and the dryer could not matched well when the feed rate increased to 100L/h.Under the condition of feed quantity of 80 L/h,dryer feed amount of 60 L and the catalyltic converter temperature of about 600℃,the temperature of inlet steam can reach 400 ℃,running 15 days under above conditions,the COD,NH4-N and sulfide content of the treated liquid samples can meet the standard of biochemical treatment,which is COD≤3500 mg/L,NH4-N≤300 mg/L and sulfide content≤75 mg/L,and the operation process is stable,the linkage operation of evaporator-dryer and catalytic converter was achieved.The foundation of the industrialization operation of resourec treating coking desulfurization waste solution by ’evaroration-catalyltic converter-salt leaching method’ is formed.