Investigation On The Resource Utilization Of Wastewater With Sodium And Ammonium

The process of production in projects such as coal chemical industry and industrial desulfurization will produce large amounts of salty liquid wastes. In wastewater treatment stage, many areas only make strict rules on organic wastewater discharge standards, while there is no clear limit to saline wastewater emission, and it is usually discharged as clean water. With environmental protection consciousness gradually strengthen, in order to save water resources and reduce environmental pollution, most areas requires enterprises to collect and classify wastewater to maximize the recycling, and some restricted by environment even demand no wastewater discharge to achieve zero liquid discharge. However, concentrated saline wastewater processing has been the key issue of restricting ZLD program.Two practical problems for industrial production processes are studied: The first one is concentrated saline wastewater produced by coal-based olefin production process from coal chemical industry, and its main ingredients are sodium sulfate and sodium nitrate. If using evaporation crystallization method to dispose concentrated liquids, a mixture of sodium sulfate and sodium nitrate crystals will be generated largely, and if not handled properly, it may cause secondary pollution under the effect of rainwater leaching; The second is a large amount of ammonium sulfate and ammonium sulfite mixture produced by ammonia desulphurization process. This kind of liquid wastes are discharged as clean water directly, which causes not only environmental pollution but the waste of resources, so it’s necessary to separate the inorganic salts from liquid wastes by way of appropriate crystallization, and it will both eliminate environmental hazards and turn waste into treasure.The specific contents and results of this research are as follows:(1) The coupling crystallization process of evaporation and cooling is adopted to recover sodium sulfate from concentrated saline wastewater. Effects of different factors on the particle size and size distribution of crystal sodium sulfate decahydrate are investigated, and the crystallization process operating conditions are optimized ultimately: stirring speed 225 rpm; crystallization time 7h; crystallization temperature 25℃;evaporation rate 150~170m?h-1;seeds added. Through twice evaporation-cooling crystallization cycle in this condition, each about 27% of the water being evaporated, the mass ratio of sodium sulfate and sodium nitrate in concentrated saline wastewater is decreased successfully from 6.67:1 to 1.4:1.The best product purity obtained complies with national standards of sodium sulfate, and the overall yield of sodium sulfate is 82.22%. Finally, the experimental results coincide with the analysis results based on the phase diagram, and receive perfect separation effects. Through reasonable assumptions and derivation, the mathematical model of sodium sulfate decahydrate crystallization are established, using the Laplace transform of batch transient method to solve the model parameters on the basis of the above experiments. Compared with other methods, this method is simpler and less time-consuming, and the obtained experimental results are also desirable.(2) The evaporation crystallization process is adopted to separate ammonium sulfate from desulfurization liquid wastes. By studying the influence of different operating parameters on the purity of ammonium sulfate crystallization products, the optimum conditions of crystallization process are defined: ammonium sulfate concentration 2.4 mol/L; evaporating temperature 60~80 ℃; stirring speed 90~120 rpm; crystallization time no more than 3 h; seeds added. Under the optimized process conditions, the purity and the particle size distribution of products have been significantly improved.Sodium sulfate and ammonium sulfate with industrial value are successfully separated from the liquid wastes in this research, and the crystal products meet the industrial requirements. The research result can provide a theoretical base for the utilization of salty liquid wastes resources.