Membrane Distillation Crystallization Coupling Process For High Concentration Na⁺//NO₃^-,SO₄^2--H₂O Solution Treatment

The high-concentration wastewaters produced from industrial fields such as coal chemical industry and production of energetic materials are rich in nitrate and sulfate,which has a huge discharge and poses the serious damage to the environment.It is an important research direction to realize the recovery of high-quality crystals while treating high-concentration composite brine with low energy consumption.In this paper,the typical high-concentration Na⁺//NO₃^-,SO₄^2-–H₂O solution system was taken as an example to explore.First,by analyzing the phase diagram of Na⁺//NO₃^-,SO₄^2-–H₂O ternary system,the optimal operating temperature of Na⁺//NO₃^-,SO₄^2-–H₂O solution system under a specific ratio for evaporative crystallization was obtained,and the evaporation crystallization route of Na₂SO₄ crystal was determined at this optimum operating temperature.The theoretical maximum yield of Na₂SO₄ crystal was obtained by calculation,and the theoretical yield of Na NO₃ crystals when the mother liquor is cooled from 353.15 K to 313.15K is calculated.Secondly,the crystallization experiment was carried out for the Na⁺//NO₃^-,SO₄^2-–H₂O solution system under the specific ratio by the vacuum evaporation crystallization technology and the cooling crystallization technology,and the crystallization law of Na₂SO₄ crystal and Na NO₃ crystal in the actual experiment process was explored.The morphology of Na₂SO₄crystal obtained by vacuum evaporation crystallization experiment is fusiform octahedron.Due to the explosive nucleation phenomenon in this process,"hollow crystals"existed in the Na₂SO₄products,resulting in the reduction of crystal purity.The corresponding relationship between solvent evaporation and Na₂SO₄ crystal weight and yield in the vacuum evaporation crystallization process was summarized.The weight of Na₂SO₄crystals obtained by the vacuum evaporation crystallization process and the Na NO₃ crystal weight obtained by the natural cooling crystallization process were correlated,and the evaporative water requirement for Na NO₃ crystals precipitated by the mother liquor cooling crystallization process was obtained.It is proposed to use membrane distillation crystallization coupling technology to treat Na⁺//NO₃^-,SO₄^2-–H₂O solution system under a specific ratio.First,because of the advantages of low energy consumption and the capability to treat high salinity brine,the high concentration Na⁺//NO₃^-,SO₄^2-–H₂O brine is pre-concentrated and evaporated by membrane distillation technology.And in the metastable region of Na⁺//NO₃^-,SO₄^2-–H₂O solution,the nucleation energy barrier of the target salt was regulated by the interface of the microporous membrane,which effectively controled the diffusion difference and explosive nucleation of multi-salt ions in the typical evaporative crystallization process,avoiding the formation of hollow and high impurity ion crystals,improving the crystal purity and separation efficiency.At the same time,the membrane flux attenuation law in the crystallization stage of membrane distillation was explored,the three-stage theory of membrane flux attenuation was summarized.The first stage of membrane flux decay was caused by the change of the properties of the solution system,the second stage was caused by the nucleation and automatic desorption of crystals on the membrane surface,and the third stage was caused by the aggregation and the difficulty of automatic desorption of crystals on the membrane surface.Base on the three-stage theory of membrane flux attenuation,the optimal coupling interval of membrane distillation crystallization and vacuum evaporation crystallization was obtained,and determined the design and optimization idea that evaporating the solvent and inducing crystal nucleation in the advantageous separation range of membrane distillation crystallization and then making crystal growth through vacuum evaporation crystallization.This design idea can maximize the recovery of pure water and high-quality crystals,and reduce the risk of membrane surface scaling and membrane pore wetting,further improving the energy efficiency of the whole separation processThe above research provides new ideas for the development of high-concentration composite inorganic brine treatment technology and the regulation and recovery of salt crystals.