Study On The Improvement Of The Yield Of Thiourea Dioxide

Thiourea dioxide (TD), as a novel reductant, has good stability and high redox potential, and will not be spontaneous decomposed. It is very convenience and safety for transportation, storage and utilization. Meanwhile, rongalite, which is widely used in dyeing and blenching industry, is easily decomposed and self-ignited, then cause some potential safety problems. Hence, in foreign countries, TD has been widely used in various fields as the substitute of rongalite, such as printing and dyeing industry, bleaching industry, pharmaceutical industry, organic synthesis and analysis, paper making industry, polymer industry, and photosensitive material industry and so on.Nowadays, high production yield of TD can be achieved (more than 90%) in foreign countries; while in our country, its production yield is only around 80%, moreover, the stability of the product is poor and the storage time is only about 1 to 3 months. Consequently, it is very urgent to improve the production yield and the stability of the product.In this paper, the effects of operation conditions on the production yield of TD are studied by batchwise experiments, and the optimized process conditions are obtained as following: the molar ratio of peroxide and thiourea is 2.1:1; reaction time is 1h; reaction temperature is less than 8℃; the solvent is a mixture of 50% water and 50% alcohol; crystallization time is 1~1.5h; crystallization temperature is -10℃; the amount of acetic acid potassium is 2%.Under the optimized process conditions, the production yield is about 90%.During the continuous crystallization experiments, the results show that the improvement of the granularity and the storage time of TD can't be achieved because the reaction and crystallization time are also prolonged.The effects of surfactants on the production yield and the stability of TD are also studied. The experimental results showed that heat emission efficiency of the reaction system is improved, the stability and production yield of the product are also improved by the introduction of surfactant (thiourea, about 1.5 %) to the reaction system and the yield increased about 1.5%..The synthesis process of TD is a strong exothermic reaction process, so the reaction heat must be removed immediately so as to increase its yield. For the purpose of this, a kind of indissolvable refrigerant is added into the reaction system. The optimized conditions are obtained through orthogonal experiments, and the results show that the production yield of the product can not be increased by the addition of the refrigerant, but the production yield can be increased when adding the refrigerant and the surfactant at the same time.The effects of several peroxide stabilization agents on the production yield are also investigated. The experimental results show that the production yield of TD can exceed 91% by the addition of 0.1% EDTANa and 0.1% potassium dihydrogen phosphate (KDP).The circular utilization of mother liquid can increase the production yield, and is good for environment protection, but the experimental results show that if mother liquid is circled for more than three times, both the yield and the quality of the product will decline.Several magnified experiments without organic refrigerant are carried out by using stainless steel jacket and coil as refrigerant equipments. Experimental results show that the production yield of TD can easily exceed 90%.