Preparation Of Gasification Fine Slag Residual Carbon Based Expandable Graphite And The Flame Retardant Mechanism In Polypropylene

Gasification fine slag（GFS）is one of the main solid wastes generated in the process of coal gasification.At present,landfill and construction filler are the main treatment methods.A large number of landfills have caused serious environmental problems.In addition,the carbon content in the gasification fine slag is about 30 wt.%,resulting in resources wasting.When used as construction filler,the application of GFS sample is greatly limited due to its loose structure and low mechanical strength.The result of XRD test showed that the residual carbon in the fine slag had preliminarily possessed microcrystalline graphite structure due to the high temperature treatment in the gasifier,which provided a basis for the preparation and application of the residual carbon based microcrystalline graphite samples.In this thesis,structural advantages of the residual carbon particles were fully utilized.And the microcrystalline expandable graphite structure is further constructed based on the microcrystalline graphite structure of residual carbon particles,and the flame retardancy of expandable graphite on polypropylene was explored,which provided a new way for the resource utilization of residual carbon from gasification fine slag.The research on basic properties of residual carbon particles showed that the structural characteristics of residual carbon were suitable for preparation of graphite materials.The fixed carbon content（92.07%）of the residual carbon was close to that of the anthracite which was typically used to preparing graphite materials.And the carbon structure was arranged in a relatively orderly manner,showing a trend of transformation to graphite structure.The crystalline Si O₂,main mineral contained in the residual carbon particles,could promote the graphitization process with the mass ratio of 5%which could be obtained by changing the acid leaching time.The porosity and pore volume of the residual carbon were 48.30%and 7.88m L/g,respectively.The porous structure provided a channel for the diffusion of additive vapor in the residual carbon particles,increasing the contact area between the additive and the residual carbon particles,which was conducive to the improvement of facilitation efficiency.The effects of graphitization temperature on conventional graphitization process were studied.With the increase of temperature,the graphitization degree of the prepared graphite materials increased,the surface layer structure became more obvious,the grain diameter and stacking height increased correspondingly,and the proportion of ordered carbon structure in the graphite structure increased.The results showed that increase of graphitization temperature could provide more energy for the reaction system and increase the carbon atoms in the activated state.Thus the chemical bond breaking of side chain and the carbon atom rearrangement reaction could be promoted,which was conducive to the development and accumulation of graphite lamellar structure.The pressure in the graphite furnace was 120～130k Pa.When the graphitization temperature was greater than 400K,ΔG_T<0,the graphitization reaction was going forward.When the graphitization temperature exceed 2600 ^oC,further increasing the graphitization temperature had no obvious effect on the graphitization process.Therefore,the graphitization temperature should be at the range of 2000～2600 ^oC.The mechanism of Fe Cl₃·6H₂O and H₃BO₃ additives in the graphitization facilitation of residual carbon was analyzed.With the increase of additive amount,the graphitization degree and grain diameter of graphite materials increased at the same temperature,and the lamellar structure was better.When the mass ratio of additive was 10%,the graphitization degree was higher than 90%and the grain diameter was higher than 30nm.During the graphitization facilitation process,Fe Cl₃·6H₂O and H₃BO₃could both cooperate with the original Si O₂ in the residual carbon to produce Fe₃Si and Si C-B,respectively.These two substances can provide a framework for the arrangement of graphitized carbon.At the same time,these two substances further interact with the amorphous carbon in the residual carbon to generate Si C,which promotes the graphitization process.With the additive mass ratio of 10%,the graphite samples facilitated by Fe Cl₃·6H₂O presented higher graphitization degree,larger grain diameter and more complete crystal structure than that facilitated by H₃BO₃.This was mainly due to the different lattice spacing between Fe₃Si and Si C-B generated during graphitization facilitation process.Fe₃Si had a smaller lattice spacing,so it caould make the surrounding graphite fragments more closely connected and the crystal structure more complete.The experimental condition of oxidation intercalation of residual carbon based graphite were optimized.It was found that the particle size had little effect on the oxidation intercalation process.The appropriate condition was obtained,namely,the mass of KMn O₄ was 15g,the volume of HNO₃ was 15m L,the volume of HCl O₄ was20m L,the reaction temperature was 70^oC,and the reaction time was 80min.Under this condition,expandable graphite sample was prepared by two-step oxidation intercalation method,and the expansion volume（EV）at 900^oC was 257m L/g.The oxidation intercalation expansion process was analyzed by characterizing the properties of expandable graphite samples.With the action of oxidant,a large number of functional groups such as hydroxyl and epoxy groups were generated on the base surface of the graphite sample.The same charge was generated between the graphite sheets,and the repulsive force was constantly improved,which leaded to the increase of the distance between the sheets,providing conditions for the intercalating agent to enter the graphite layers.When the expandable graphite was expanded by heat,due to the rapid pyrolysis of graphite intercalating agent molecules,the stress between graphite layers increased sharply,and the spacing between graphite layers expanded in geometric multiples,resulting in a significant increase in the particle size of the sample,which finally appearred as worm-like particles.An expansion model suitable for carbon based expandable graphite was established.When the type of intercalating agent,the amount of intercalation,the initial pore size of expandable graphite,the radiation temperature,the particle expansion pressure and other parameters were known,the model could be used to predict the relationship between the radial expansion height and the expansion time of samples.The synergistic flame retardant effects of carbon based expandable graphite（EG）and ammonium polyphosphate（APP）on polypropylene（PP）were investigated.The initial decomposition temperatures of EG and APP were 60 ^oC and 120 ^oC,respectively.Therefore,in the PP flame retardant process,EG can form an expanded carbon layer earlier than APP,which was good for the flame retardancy.At 700 ^oC,the residual char rate of EG was 71.42%,which was significantly higher than that of APP（32.40%）,indicating that the expanded carbon layer of EG presented better thermal stability than that of APP.Thus the flame retardancy efficiency of EG was better than that of APP,as the stable existence of the expanded carbon layer could promote the thermal and oxygen insulation.Compared with the single flame retardant of EG or APP,the flame retardancy efficiency was improved by mixing the both flame retardants.When the mass ratio of EG to APP was 2:1,the total heat release（THR）and total smoke generation（TSR）of PP combustion process were reduced by 19.90%and 28.11%,respectively.At the same time,the synergistic flame retardant mechanism of the composite flame retardant system was studied.The flame retardancy of PP-EG-APP flame retardant system conformed to the"condensed phase gas phase"synergistic flame retardant mechanism.In the condensed phase,the polyphosphoric acid generated from APP decomposition promoted the charring of the flame retardant system,and the"fiber effect"of EG further improved the stability of the expanded carbon layer and enhanced the oxygen insulation performance of the flame retardant system;In the gas phase,incombustible gases such as NH₃ and N-H generated by APP decomposition diluted the concentration of oxygen and combustible gas,and could take away the heat inside the system to inhibit further combustion process.The combination of the two flame retardants was conducive to the adhesive advantages of APP expanded carbon layer,making the flat expanded carbon layer generated by EG decomposition more closely connected,and ultimately improving the thermal stability of the overall expanded carbon layer.