| The total amount of waste salt in China’s chemical industry is large,many categories,wide sources,difficult to treat,and high environmental risks,and it is important to realize resourceful reuse of it.Based on the current situation that the waste salt produced in the domestic printing and dyeing industry has a large volume of accumulation and imperfect treatment and disposal technology,this paper focuses on the waste salt pyrolysis-crystallization coupled resourceful comprehensive utilization process and key technology based on the principles of pyrolysis and crystallization,in order to eliminate its possible environmental risks.For the waste salt of printing and dyeing industry,the process route of pyrolysis crystallization synergistic resource recovery of sodium chloride and sodium sulfate is proposed,with a view to realize the recovery of the whole component of waste salt,which provides new ideas and experimental data support to solve the bottleneck problem of waste salt disposal in printing and dyeing industry at present.Firstly,this study provides an in-depth analysis of the waste salt generation process in the printing and dyeing industry to understand the inorganic and organic components that may be present in the waste salt.Determine the physical phase and main elemental composition of the waste salt by physical analysis of relevant parameters such as water content,particle size and composition of the industrial waste salt.The results show that the inorganic composition of waste salt produced by the printing and dyeing industry is relatively single,mainly including substances such as sodium chloride and sodium sulfate,and this kind of waste salt has a good prospect of resource recovery and utilization.Secondly,In this study,the pyrolysis process parameters of waste salt were determined by thermogravimetric analysis,and the pyrolysis process of waste salt was simulated based on a tube furnace heat treatment system.The treatment effects under two different atmospheres(air,nitrogen)and different temperatures and times were compared through several sets of comparative experiments.The optimum pyrolysis process conditions for the waste salt were finally determined:451℃ at the treatment temperature and 42 min at the treatment time under nitrogen atmosphere.In addition,the results showed that pyrolysis temperatures below 500℃ were more favorable for sulfate retention,while pyrolysis temperature and time had no significant effect on chloride ion content.These results can provide data and theoretical support for the pyrolysis treatment of waste salt in the printing and dyeing industry.Finally,In this study,the heat-treated printing and dyeing waste salt was washed and filtered to recover the solid-phase product and liquid-phase product,respectively.Among them,the solid-phase product pyrolysis carbon was determined to have a carbon yield of 19.41%after performance testing and analysis,with high surface area(53.272 m2/g),high adsorption capacity,high regeneration,and low cost,etc.This waste salt by-product pyrolysis carbon can be used as an effective adsorbent for treating wastewater containing dyes.The liquid phase products are recovered separately from the sodium chloride and sodium sulphate fractions of the waste salt by a freeze crystallisation+evaporation crystallisation process.By studying the variation patterns of crystallization conditions such as waste salt filtration produced water under different crystalline species concentration,freezing time and freezing temperature on the crystal yield and purity of sodium sulphate and sodium chloride,the mechanism of crystallization optimization of sodium sulphate and sodium chloride was further explored in conjunction with the phase diagram of NaCl-Na2SO4-H2O aqueous salt system.The purity of mannite after crystallization of frozen nitrate was 74.96%,the purity of sodium chloride after evaporation crystallization was 92.17%,and the product sodium chloride reached the standard of《GB/T5462-2015 Industrial Salt》sun-dried industrial salt secondary products. |
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