Study On The Mechanism Of Impurity Removal And Quality Improvement Of Waste Tire Pyrolysis Char Using Molten Salt Thermal Treatment

The annual yield of waste tires in China is huge,and the disposal is urgent.Pyrolysis can convert waste tires into gas-liquid-solid three-state products with great application potential.However,the high content of impurities and poor activity of the pyrolysis char seriously hinder its high-value application.Therefore,the removal of impurities and the quality improvement of the char is of great significance.In view of the problems of impurity removal efficiency and environmental pollution in the traditional impurity removal technology,the present study proposed a molten salt thermal treatment method which could effectively improve the physical and chemical properties of the waste tire pyrolysis char while removing the impurities.Based on the impurity characteristics of waste tire pyrolysis char and its reactivity with different molten salts,a molten salt system（Na OH-Na₂CO₃）suitable for the removal of impurities was selected.The influence factors and transformation mechanisms of impurity removal were examined.Furthermore,the effect of molten salt thermal treatment on the physical and chemical properties of the waste tire pyrolysis char was explored.The main conclusions are shown below:The main impurity components in waste tire pyrolysis char were ash and sulfur.The ash content was as high as 17-23%,and the main components were Zn,Si,Ca,etc,which existed in the form of Zn S,Si O₂,Ca CO₃.But the relative content varied significantly with the addition of inorganic components such as Zn O,Si O₂,and Ca CO₃ in the tire.In addition,the sulfur content was as high as 2-3%,and its existence form was basically the same in different pyrolysis chars,mainly in the form of inorganic sulfide and aliphatic sulfur.The product distribution of typical impurity component reacted in different molten salts was calculated by HSC Chemistry 6.0 software.It was found that Na OH-Na₂CO₃showed the best reactivity with typical impurity components such as Zn S,Si O₂,and Ca CO₃among the four molten salts（Li₂CO₃-Na₂CO₃-K₂CO₃,Na Cl-Ca Cl₂,Na NO₃-KNO₃-Na NO₂,Na OH-Na₂CO₃）in the range of 200-600 ^oC.Therefore,Na OH-Na₂CO₃ was expected to be the suitable molten salt system for removing impurities from the pyrolysis char.Molten salt thermal treatment had a strong universality in the removal of impurity components for different waste tire pyrolysis chars,and the ash removal rate was between70-90%.Temperature significantly affected the removal efficiency of the ashes.In the range of 350-450 ^oC,as the temperature increased,the removal efficiency of the ashes firstly increased and then decreased.Within the scope of the experiment,the ash removal efficiency was significantly improved as the increase of reaction time and the ratio of molten salt/char,but this improvement became less obvious after the ratio of molten salt/char exceeded 10:1.The removal rate of calcium by molten salt was generally higher than 95%,because Ca CO₃was fully dissolved in Na OH-Na₂CO₃ molten salt.In addition,Zn S reacted with molten salt to form soluble alkali metal sulfides and sodium zincate,resulting in the silicon removal rate of more than 90%.The removal of silicon was obviously affected by temperature and ash composition.When the temperature rose to 400 ^oC,Si O₂ could react with molten salt to form Na₂Si O₃,which greatly promoted the removal of silicon.However,as the temperature increased to 450 ^oC,the formation of Na₂Si₂O₅ would inhibit the removal of silicon.In addition,when zinc was present in large quantities,the formation of Zn₂Si O₄ also hindered the removal of silicon.After molten salt thermal treatment,the physical and chemical properties of waste tire pyrolysis char were significantly improved.Molten salt promoted the decomposition of carbonaceous deposits on the surface of the pyrolytic char,increasing the graphite carbon content on the surface of the pyrolytic char from 24.41%to 70.90%,which exceeded the carbon black N550 level.At the same time,the hydroxyl content on the surface of the molten salt treated char increased significantly.This was conducive to the combination with rubber molecules,thereby improving the reinforcing properties of rubber.Further more,the inorganic sulfides in the pyrolysis char basically disappeared,and the highly reactive aliphatic sulfur was converted into stable sulfone sulfur,which avoided the interference with the vulcanization process.In addition,the particle size of waste tire pyrolysis char was significantly reduced after molten salt thermal treatment.The average particle size was reduced from 36.63μm to 19.08μm,reaching the carbon black N550 level.And the specific surface area was increased from 49 m²/g to 73 m²/g.Furthermore,the surface roughness and structure degree of molten salt treated char increased significantly.