Crystallization Process Optimization Of Sodium Hexamethylene-1,6-bisthiosulfate Dihydrate

Sodium hexamethylene-1,6-bisthiosulfate dihydrate (abbrs. Duralink HTS) is anew rubber additive, which can improve the elasticity performance and reduce theaging rate of rubber. Compared to the traditional additives with single sulfur linkagebond, Duralink HTS has a long chain structure, greatly improved the flex resistanceand tear resistance of rubber. Moreover, as Duralink HTS can enhance the adhesionof steel and rubber, it is getting more and more attention from the manufacturers. Atpresent, it is synthesized by the domestic routine with the purity86%and yield75%.In this work, systematic investigation on the solubility of Duralink HTS in differentsolvents was performed, the appropriate solvent system was determined and a newpurification crystallization technology of Duralink HTS was established which hasgreatly improved the product purity and yield.The ethanol-water binary solvent mixture was determined as the crystallizationsystem after the study on the solubility of Duralink HTS in solvents like water,methanol, ethanol, etc. Solubilities of Duralink HTS in the binary solvent systemwere measured using a laser-monitoring technique at temperatures ranging from278.15to343.15K. The experimental data was well correlated by the Apelblat andCNIBS/Redlich-Kister model separately, and then the dissolution enthalpy wasdetermined with van’t Hoff plots. The result indicates that the solubility of DuralinkHTS increases with increasing temperature and mole fraction of water in these twobinary mixed solvents. Additionally, the dissolution enthalpy increases withincreasing mole fraction of ethanol.According to the thermodynamic properties summarized from the aboveexperiments, solubility of Duralink HTS increases with increasing temperature whilesolubility of NaCl (by-product) hardly varies with temperature. In view of the hugedifference in dissolving property between the target product and by-product, a newtechnology of coupling crystallization was designed. The by-product is removedfrom the system through solvent-out crystallization process, and then the targetproduct(Duralink HTS) is collected by cooling crystallization. Systematicexperiments were designed to launch further study on influncing factors such asconstituents of the solvent mixture, method of solvent addition, crystal growing time,range of temperature, cooling rate and so on. The main achievement of this newtechnology is that the purity of the product has been raised from86.0%to91.4%and the yield from75.0%to90%.