Study On "Zero-discharge" MVR Evaporation Crystallization Process And Purification Of Slag Salt In High-Salt Wastewater Of Photovoltaic Industry

The treatment of high-salt wastewater,which is common in industry,is a difficult and painful point in the wastewater treatment industry.Direct discharge without treatment will inevitably result in great environmental pollution.Zero discharge of wastewater is the trend of environmental protection.In some traditional chemical industries,the“zero-discharge”treatment of high-salt wastewater has gradually achieved,but the wastewater treatment process is relatively backward for emerging industries such as photovoltaic industry.In this thesis,a technology involving MVR evaporation crystallization was deigned to treat high-salt wastewater by evaporate and purify slag salt for secondary utilization achieving zero-discharge of salt wastewater.The main contents and results were as follows:1.A process route of“pretreatment+membrane treatment+MVR evaporation crystallization”was designed to treat high-salt wastewater based on the research idea of zero-discharge.The pretreatment unit and membrane system unit effectively reduced the hardness,COD,SS and other indicators in the wastewater,and the salt content of wastewater was concentrated to 12%.The evaporation system adopted the double-effect evaporation scheme of“falling film evaporator+MVR forced circulation evaporator”.The vacuum of the main evaporator was 0.055 MPa.The salt content of the wastewater reached 25%by one-effect falling film evaporator,and then evaporated and crystallized through a two-effect forced circulation evaporator.A high-speed Roots blower with a temperature rise of 18? was selected.The secondary steam at 85? produced in the evaporator was lifted to a high grade saturated steam of 103? and re-engage in the system cycle.A pilot-scale scheme of evaporation system with a throughput of 0.5 t/h was designed,and the process parameters of the equipment were calculated.Meanwhile a complete process flow chart?PID diagram?with control points was drawn.Finally,the system only produced slag salt by accompanying reuse of the water produced by each channel,therefore achieving zero-discharge of wastewater.2.The separation and purification of the evaporated slag salt was carried out by the anti-solvent crystallization method.The mixed salt solution with different ratios and concentrations was prepared.The methanol solvent with better dissolution and purification effect was selected as the experimental dissolution agent.The optimized dissolution time and temperature were 2 h and 25?,respectively.The dissolution and purification process were investigated with different solute ratios and different initial concentrations?5%,10%,12%,15%,20%of salt content?at different solvent ratios.The experimental results showed that the solvent could exert better effect when the salt content of solution was above 10%.If the solvent ratio of the solution was selected to be 4:6 and 5:5,the salt could be precipitated in a large amount and high purity.3.The phase diagram method was used to purify the slag salt.The mixed salt solution containing Na₂SO₄ was separately prepared.The experiment was carried out at100? by analyzing the dissolution characteristics of Na2SO4.Five mixed salt solutions of m _Na2SO4:m _NaCl=5:1?4:1?3:1?2:1?1:1 were prepared as mixed salt test groups.Six different concentrations of COD?100 mg/L?200 mg/L?300 mg/L?400 mg/L?500mg/L?600 mg/L?were set up on the basis of mixed salt test groups as simulated wastewater test group,COD was simulated by adding anhydrous glucose.The experimental data of the pure solution group showed that the products with a nearly 100%of purity were obtained at all ratios,and the average single yield reached 50%.The simulated wastewater group showed that the salt yield of Na₂SO₄ was 35%-80%,the purity was above 93%,and the effect of COD on the purity of salt was less than 7%.The purities of the all products reached the standard of industrial anhydrous sodium sulfate class III qualified products?w?92.0%?,and some even reached the first class of industrial class I?w?99.0%?.This experimental data can provide a basic reference for the purification of slag salt in the industry,which helps to find suitable processing conditions quickly and accurately.