Treatment Of Rare Earth Industrial Wastewater And Theirmass Transfer Properties By Reverse Osmosis Membrane

A large amount of high salinity(NH4Cl/NaCl)rare earth industrial wastewater(REIW)was produced in rare earth hydrometallurgy with ammonia or sodium hydroxide as saponifier.Direct discharge will not only waste a lot of water resources,but also cause damage to water environment,which seriously restricts the green and sustainable development of rare earth industry.In this regard,the performance of reverse osmosis(RO)membrane in the treatment of simulated REIW(NH4Cl/NaCl solution)was investigated.Then,the water and salt flux transport equations were validated by the solution-diffusion(SD)model.Combined film theory and electrolyte theory,the mass transfer and separation process of water and salt in RO membrane were studied.Further,the treatment efficiency of pre-treatment technologies of coagulation-sedimentation and ultrafiltration,and membrane fouling characteristics were also also studied.The results can provide a theoretical basis for the application of membrane separation technology in the treatment of REIW.The results are as follows.(1)Separation performance of RO membrane treating NH4Cl or NaCl solutions was clarified.The desalination efficiency of RO was investigated in the treatment of NH4Cl or NaCl solutions from 0.1 to 40 g/L under different operating pressures with a commercial SG membrane.When the operating pressure was 30 bar,the rejection of NH4Cl ranged from 77.87%to 99.69%and the permeate flux was between 6.6901 and 35.7806 L m-2 h-1 in the studied range of feed concentrations.While the rejection of NaCl ranged from 86.98%to 98.46%and the permeate flux was between 5.6885 and 42.9572 L m-2 h-1.The hydrated ion radius of Na+(3.58 A)was larger than that of NH4+(3.31 A),which led to make it more difficult for the sodium ions to penetrate the membrane.While the value of hydration energy of Na+(-454 kJ/mol)was higher than that of NH4+(-331 kJ/mol).Therefor,the apparent rejection(R)and permeate flux(J)for NaCl were higher than that of NH4Cl at the same mass salt concentrations.(2)The water flux and salt flux model for RO membrane treatment of REIW rwas validated by SD model.It was found that there was a linear relationship between water flux(JN)and the net driving force(?p-??)for both NH4Cl and NaCl solutions,which conforms to the water flux equation of the SD model.The water permeability coefficient of the NH4Cl solution(1.29 L m-2 h-1 bar-1)and NaCl solution(1.35 Lm-2 h-1 bar-1)were determined by linear fitting of water flux vs.net driving force,as Jw=A(?p-??).However,the salt transport did not show a linear relationship with concentration difference as proposed in classical solution-diffusion theories.Thus,a power function equation as Js=?(Cf,s-Cp,s)? was proposed to explain the relationship between salt flux and salt concentrations.The salt flux equations for NH4Cl and NaCl solutions was Js=1.08(Cf,s-Cp,s)0.97 and Js,=0.63(Cf,s-Cp,s)1.05,respectively.All the predicted transport coefficients agreed well with the experimental data,and the models were also validated by real REIW.Therefore,the proposed equations of water and salt transport are appropriate for the prediction of the membrane performance when dealing with real REIW.(3)The effect of concentration polarization(CP)and ion activity on the mass transfer of salt and water molecules in RO membrane were revealed.According to the film theory,the salt solution concentration(Cfm)on the membrane surface was 1.02?1.21 times higher than that in feed solution(Cf)due to the concentration polarization(CP)effect,and the Bcp/B of NaCl and NH4Cl solutions was 82.8?98.9%and 83.3?96.7%,respectively.While according to the film theory and electrolyte theory,the effect of CP and ion activity were found to substantially affect the effective local salt transport coefficients.When the concentration of salt solutions was lower than 1 mg/L,the combined result of the two effects was negligible.While the concentration of salt solutions was higher than 5 g/L,the influence of the two effects was significant:the ratio Ba/B of NaCl and NH4Cl were 1.12?1.42 and 1.14?1.49,respectively,indicating the ffect of ion activity played a dominant role.The effective local water transport coefficient(Afm)in different concentrations of NaCl and NH4Cl solution was almost equal to the apparent water mass transfer coefficient(Af),indicating that the effect of CP and ion activity have less effect on the mass transfer of water molecules in RO membrane.(4)The effect of organic matter on the separation performance of RO membrane in the treatment of REIW was clarified,and the characteristics of RO membrane fouling were revealed.The effect of the concentration of organic matter(extractability of P507)in simulated REIW(NH4Cl and P507 mixed solution)on the performance of RO was studied.It was found that the lower concentration of P507 had less effect on the rejection for RO desalination.The RO membrane was used to treat the mixed solution of NH4Cl and P507,and the salt crystallization and organic substances coexisted in fouled RO membrane.The rate of membrane fouling increased with the increase of concentration of P507.As for the treatment of real REIW by RO membrane,when the operating pressure ranged from 35 to 40 bar,the average removal efficiency of ammonia nitrogen and COD was 73.97%and 68.33%,respectively,and the removal efficiency of Pb,Cu and Zn was all above 90%.Because of the high salinity of REIW,the permeate flux decreased rapidly with the increase of time,resulting in serious membrane fouling.The fouling components of membrane was dominated by inorganic matter such as salt crystallization.Acid cleaning and EDTA cleaning had better removal effect on inorganic contaminants for RO membrane,while alkali cleaning had better removal effect on organic contaminants.(5)The operation parameters of REIW pre-treatment process were optimized.When the concentration of Polymeric Ferric Sulfate(PFS)was 6 mg/L,the COD,turbidity,Zn,Cu and Pb removal efficiency was 43.1%,69.2%,72.7%,62.7%and 60.0%,respectively.The pre-treatment technology of ultrafiltration was also studied.When the operating pressure was ranged from 1 to 5 bar,the average removal efficiency of COD and turbidity was 26.5%and 77.7%,respectively.There were mainly inorganic matters on the fouled ultrafiltration membrane surface,which dominated by the cake-layers formation and pore blocking formation.The ultrafiltration membrane fouling resistances of the membrane itself resistance(Rm)?cake layer resistance(Rc)?pore block fouling resistance(Rf)and parization layer resistance(Rp)was 41.56%,29.38%,18.33%and 10.83%,respectively.The coagulation-sedimentation combined ultrafiltration pre-treatment real REIW,the removal efficiency of COD,turbidity,Zn,Cu,and Pb was 72.9%,93.5%,82.0%,74.5%,and 68.0%,respectively.However,the removal efficiency of ammonia nitrogen was very low.The pretreated REIW had relatively few polutants,which can be further treated by RO membrane.