Study On Concentration Of Desulfurization Wastewater From Power Plant By Electrodialysis

Currently,desulfurization method using limestone-gypsum wet flue gas has been applied in coal-fired power plant.This desulfurization process would produce a lot of high-salt desulphurization wastewater.This weakly acidic wastewater is containing a large number of suspended solids,and the salt ions,leading to high hardness,high chlorine,high corrosion and scale.If discharging the salty wastewater directly,it is easy not only to cause water mineralization and soil salinization,but also easy to cause waste of resources.The traditional desulfurization processes for wastewater mainly included unit operations such as pretreatment,evaporation,and crystallization.However,the salt content and hardness of desulfurized wastewater was high after pretreatment.After ultrafiltration and reverse osmosis,the salt content was about 6%-7%.If the wastewater was evaporated,the evaporation cost was far higher than that using conventional membrane concentration.Therefore,it was imperative to choose an efficient concentration before evaporation.In this paper,the treatment process of "pretreatment&electrodialysis advanced treatment" was used to treat desulfurization wastewater.The main research contents and experimental results were discribed as follows:(1)In this study,"softening,Fenton oxidation,coagulation and NaCIO deep oxidation"was used as the pretreatment unit.The experiments results showed that the pretreatment process had a significant effect on the removal of COD and chromaticity of wastewater.After pretreatment,the desulfurization wastewater can meet requirements of electrodialysis.Double-alkali process can be used to dispose desulfurized wastewater.When sodium carbonate was theoretical dosage and sodium hydroxide was 130%of the theoretical dosage,the removal rates of Ca2+and Mg2+reached 99.5%and 99.9%respectively.Based on the best conditions,pretreatment process on the desulfurization wastewater had reached:COD decreased from 2770 mg/L to 30 mg/L,chroma decreased from 370°to 26°,SS decreased from 80 mg/L to 5 mg/L.(2)The desulfu-ization wastewater of pretreatment was concentrated by electrodialysis.The first-stage electrodialysis experiments,the multi-step electrodialysis concentration experiments and the multi-stage electrodialysis concentration experiments were studied respectively.The limit current was set to be 21.5 A,with the optimal operating voltage of 10 V and the best circulation flow of 1 80 L/h.(3)In order to reach the salt content of 15%-18%of desulfurization wastewater,the multi-step electrodialysis and multi-stage electrodialysis were carried out,respectively.The factors of concentration of Cl-,concentration of SO42-,relative concentration ratio and energy consumption in electrodialysis were studied and both electrodialysis can achieve an electrodialysis concentration effect with a salt content of 15%.The four-stage electrodialysis was relatively simple,and it is corresponding energy consumption was also high(150 kW·h/m3),and the water migration phenomenon was obvious.Seven-step electrodialysis can seesctively produce more fresh water.However,the energy consumption of complicated process was low(140 kW.h/m3).Therefore,multi-stage electrodialysis can reduce energy consumption.(4)The multi-step electrodialysis with different feed concentration was studied.The results showed that the initial high feed concentration require fewer steps to concentrate to 15%.When the feed concentration was 4%and 6%,the desalination chamber can reach 0.3%at the end of each electrodialysis process,which indicated that fresh water can be recycled.When the feed concentration was 8%,the effluent concentration of the dilution chamber was too high to be recycled as circulating water.The analysis of total energy consumption under different feed concentration showed that the high feed concentration leading to low energy consumption.Thus,concentrations of 4%and 6%were suitable for desulfurized hydrodialysis.(5)Three typed of cleaning methods including water cleaning,acid-base cleaning and reverse electrode cleaning were compared for membrane cleaning.The results showed that reverse electrodialysis was more effective in the removal of cleaning membrane.Therefore,this experiment finally chose regular reverse electrode to clean the membrane.The experimental results showed that the process of "pretreatment&electrodialysis advanced treatment" was feasible to treat desulfurization wastewater,and it can provide informarion for the electrodialysis application in the treatment of desulfurization wastewater.