| The coal gasification industry has rapidly developed in recently years due to abundant coal reserves,clean energy production and the increased energy demand in China.Coal gasification process has the characteristics of high pollution and high water consumption,and it is mostly distributed in areas with poor environmental capacity.In order to alleviate the contradiction between environmental protection and rapid development,the Ministry of Environmental Protection has put forward strict near zero liquide discharge requirements for coal gasification wastewater treatment.The coal gasification brine(CGB)is the reverse osmosis concentrated at the end of coal chemical wastewater treatment.Due to the characteristics of complex water quality,high pollutant concentration and difficult treatment,it has become a bottleneck for the near zero discharge of coal chemical wastewater treatment.This paper studied the separation efficiency of CGB,solute rejection mechanism and influencing factors,the membrane fouling mechanism during separation process by nanofiltration.Based on above research,the disc tube nanofiltration membrane(DTNF)was applied to resouce the salt from CGB in the pilot scale experiment,and further analyzed the industrial salt quality,environmental benefits,economic benefits,investment costs and operating costs.The pilot scale experiment provides the theories basis and application reference for the application of nanofiltration to realize the utilization and near zero discharge of CGB.Nanofiltration separation mechanism and influence factors of monovalent salt,multivalent salt and organic matter were discussed by comparing the rejection ratio as a function of pressure,p H value and concentration of solution.The experimental result shows that the rejection of sulfate and the chemical oxygen demand of NF270 were 90.52% and 74.64%,respectively,which were lower than 3.1% and 12.75% of the salt mixture.The coexistence conditions of high concentration of salt and organic matter in CGB have a significant impact on the rejection.The organics attached to the nanofiltration membrane surface shields the negative charge and weakens the electrostatic repulsion,thereby reducing the sulfate rejection.The membrane pore swelling phenomenon induced by the high concentration salt of solution reduces the steric hindrance effect,thereby reducing the organics rejection.Chloride rejection was negative in the separation process of nanonfiltration.The reason is that sulfate,as a multivalent common ion of CGB,was preferentially rejected by the electrostatic repulsion effect.In order to balance the electrically neutral conditions of solution on both sides of the nanofiltration,the transport of chloride through the nanofiltration into the permeate was promoted.The chloride concentration in permeate of nanofiltration is higher than that of CGB,resulting in negative rejection.The experiment of separate neutral and positively charged organics by nanofiltration is conducted to investigate the rejection efficiency of trapical refractory organic(TRCs)in CGB.The accurancy of the DSPM&DE(Donnan steric pore model & dielectric exclusion model)model for predicting the rejection ratio of TRCs was optimized and evaluated.The rejection of NF270 membrane for phenol and indole were 61.05% and 85.62%,which were lower than the model predicted re jection of 63.27% and 86.46%.The molecular weight of undecane is larger than that of phenol and indole.The experiment rejection rate and model predicted rejection rate of undecane are 97.83% and 97.09%,respectively.The rejection of 5 neutral TRCs of nanofiltration showed that nanofiltration can effectively reject TRCs with a molecular weight smaller than molecular weight cut off of the membrane.Comparing the TRCs rejection of the three nanofiltrations,the NF270 membrane has the smallest pore size and the highest organics rejection,so it can be concluded that the TRCs rejection is inversely proportional to the membrane pore size.By comparing the rejection of TRCs with different physical and chemical properties,it is analyzed that the molecular size,molecular structure,and hydrophilicity of neutral TRCs are the reasons for the small deviations between the model predicted rejection and the experimental rejection.The predicted rejection of positively charged TRCs of DSPM&DE model is slightly higher than its experimental rejection,and the experimental rejection of positively charged TRCs is lower than the experimental rejection of neutral TRCs with the same molecular weight.Based on the above research,the accuracy of the DSPM&DE in predicting the TRCs of CGB was improved by correctting the partition effect and dielectric effect.Membrane fouling mechanism and influence factors is analyzed through the investigate the change of membrane flux,water quality and the membrane characterization.The flux decreased to 57.97% at 20 h,and the TOC(Total organic carbon),silica and TDS(Total dissolved solids)rejection decreased from 84.41%,73.26% and 71.55% to 69.37%,50.99% and 53.64%,respectively.The chloride rejection increased from-9.82% to-4.23% due to the weakening the Donnan effect.Reducing the concentration of amorphous silica and increasing the p H increase membrane flux,the rejecion of organics,TDS and amorphous silica increased.The characteristic of fouled membrane indicated that the porous br occoli-like composite fouling layer is is mainly composed of silica colloid and also includes organics and salt.Conclusion,it could be considered that the organic firstly attached to the nanofiltration membrane surface.The silica concentration near to m embrane surface increased and aggregated to form the membrane fouling layer,the cation in CGB and silica colloid built a bonding through absorption and bridging effect lead to serious membrane fouling.This paper further investigates the chemical clean so lution according to the membrane fouling type.The best membrane flux recovery has observed after caustic clean.The 1.0% Na OH solution and 1.0% EDTA solution rinsing nanofiltration membrane 1.0 h in the circle mode,the membrane flux recovered to 93% and 99% of the initial membrane flux,respectively,which both higher than that of the citric acid clean.The pilot-scale experiment of separation and resource utilization of industrial salt from CGB by the thermal-membrane coupling technology was conducted to explore the efficiency of industrial salt resource utilization.The pilot-scale experiment applied the integrated technology of "passivation complexation pretreatment-DTNF-catalytic oxidation-evaporation crystallization" to realize the industry salt preparation.The DTNF significantly increases the mass ratio of sodium chloride to total salt and sodium chloride to sodium sulfate in the CGB from 0.64:1 and 2:1 to 0.91:1 and 27:1,respectively.The average evaporation of the MVR evaporator was 492.12 kg/h,the feed flow was between 0.8 and 1.3 m3/h,and the difference of temperature and pressure between the inlet and outlet of the compressor were 0.2 ? and 1.5 k Pa.The output of sodium choride was 33 kg/h and the quality meet the meets the national dry salt superior grade standard.Finally,the investment cost and operating cost were analyzed based on the pilot-scale experiment.The pilotscale experiment results show that this is a very promising technology for realizing the resource utilization of industrial salt from CGB. |
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