Study On Pre-Treatment And Electro-Membrane Processes For Acid-Base Production From High-Salinity Wastewater In The Rare Earth Industry

During the rare earth smelting and separation process,a large amount of high-salt wastewater is generated.Sodium bicarbonate is commonly used as a precipitant in the rare earth industry to recover rare earth products,resulting in weakly acidic rare earth carbonate precipitation wastewater.In addition to sodium chloride,the wastewater also contains pollutants such as calcium and magnesium ions,fluoride,and organic matter.Direct discharge would result in water resource waste and ecological environmental pollution.In order to reduce resource waste and environmental pollution,this study explores the pretreatment process of high-salt wastewater in the rare earth industry to remove impurities such as calcium and magnesium ions,fluoride,and organic matter,so that it can meet the water quality standards specified in the"Pollutant Discharge Standards for Rare Earth Industry"(GB 26451-2011).Furthermore,the study investigates the desalination and concentration performance of electrodialysis technology on the wastewater,and utilizes bipolar membrane electrodialysis to convert sodium chloride in concentrated saltwater into sodium hydroxide and hydrochloric acid for recycling.The main research conclusions are as follows:(1)Ion exchange was employed to remove calcium and magnesium ions from the high-salt wastewater.The effectiveness of different types of resins(strong acidic cation exchange resin and weak acidic cation exchange resin)in removing calcium and magnesium ions from the rare earth carbonate precipitation wastewater was compared.The optimal resin type,dosage,and treatment time for removing calcium and magnesium ions from the wastewater were determined.The study found that the weakly acidic anion exchange resin CH-93 achieved adsorption equilibrium for calcium and magnesium ions at 30 and 60 minutes,respectively,when the resin dosage was 1.5 g/L.Furthermore,the residual concentrations of calcium and magnesium ions were below the detection limit of ICP,with removal rates close to100%.(2)Adsorption method was used to remove COD from wastewater,and the removal efficiency of different functionalized macroporous resins on COD in rare earth carbonate precipitation wastewater was compared to obtain the resin type,dosage,and treatment time suitable for COD removal from wastewater.The results showed that the type I quaternary amine functionalized macroporous resin(A-722MP)at a dosage of 2 g/L and a treatment time of 60 min could remove COD from wastewater to 19.57 mg/L with a removal efficiency of 70.49%.(3)Two adsorbents,ZF-2000 and CLP-F1,were used to remove fluoride from the wastewater containing zirconium and lanthanide cerium elements,respectively.The adsorption behavior of ZF-2000 and CLP-F1 for fluoride ions in the wastewater was investigated.The results showed that ZF-2000 could rapidly and efficiently remove fluoride ions in a p H range of 3 to 9.The maximum adsorption capacity was obtained at p H 6.5 and 25°C,with a value of 24.19 mg/g.When a dosage of 0.8 g/L ZF-2000 was used,the fluoride ion concentration in the wastewater was reduced to4.78 mg/L,meeting the water quality standards for rare earth wastewater discharge.The adsorption of fluoride ions on ZF-2000 followed the Langmuir isotherm model and the pseudo-second-order kinetic model.Tests such as XPS indicated that fluoride ion removal was achieved by forming metal fluorides and by fluoride ion substitution for hydroxyl groups on the adsorbent.(4)The study investigates the desalination/concentration of rare earth carbonate precipitation wastewater by conventional electrodialysis.The effects of membrane type(homogeneous and heterogeneous membranes),initial salt concentration,and operation mode(continuous and intermittent desalination)on the desalination and concentration of high-salt solutions are compared.The results indicate that intermittent electrodialysis using a homogeneous membrane is effective in desalination and concentration of the wastewater.When the initial Na Cl concentration is 50 g/L and the treatment time is 135 min,the concentration of the electrodialysis concentrate exceeds 150 g/L,and the resulting electrodialysis dilute water can meet the discharge requirements for the rare earth industry wastewater.(5)The influence of initial salt concentration,initial acid-base concentration,and current density on the current efficiency and energy consumption per unit of product during bipolar membrane electrodialysis for acid-base production was investigated.Results showed that the higher the initial salt concentration,the higher the current efficiency and the lower the energy consumption per unit of product;the higher the initial acid-base concentration,the lower the current efficiency but the lower the energy consumption per unit of product;and the higher the current density,the lower the current efficiency and the higher the energy consumption per unit of product.At a current density of 60 m A/cm~2,an initial acid-base concentration of0.05 mol/L,and a concentrated salt solution of 150 g/L,acid and base concentrations of approximately 3.2 mol/L and 3.3 mol/L,respectively,were obtained,with a current efficiency and energy consumption per unit of product of approximately 50%and 2.6 k Wh/kg,respectively.The ion exchange membrane was characterized,and it was found that the degree of membrane fouling was small in the bipolar membrane acid-base production system,indicating that the process can achieve long-term stable operation.