Preparation Of Sulfur-free Expanded Graphite And Its Adsorption Properties

Preparation of expandable graphite by chemical oxidation is the most mature process.However,with the strengthening of environmental protection requirement,most of the strong acids andstrong oxidants such as concentrated sulfuric acid and concentrated nitric acid used in the traditional methods,whichare replaced by the reaction system formed by introducing a single solid co-oxidant and general acids.In this paper,80 mesh natural flake graphite?FG,Flake Graphite?was used as raw material to synthesize sulfur-free expanded graphite with low temperature and high expanded volume in KMnO₄-mixed acid system and KMnO₄+K₂Cr₂O₇-mixed acid system.The latter introduced a second co-oxidant based on a single co-oxidant,which aimed to reduce the amount of KMnO₄,increase the solubility of the co-oxidants,enhance the controllability of the reaction,and avoid the occurrence of peroxidation.The experimental scheme was optimized by single factor experiment and orthogonal experiment,and the physical properties,composition and morphology of the product samples at different stages were characterized and analyzed.At the same time,the expanded graphite?EG,Expanded Graphite?,which was synthesized by KMnO₄+K₂Cr₂O₇-mixed acid system and had an expansion volume of 550 mL/g,was used as the adsorbent to explore the optimal adsorption conditions and reusability of expanded graphite for methyl orange.The main findings are as follows:?1?The optimal synthesis conditions for the preparation of expanded graphite by KMnO₄-mixed acid system are:FG?g?:mixed acid?mL?:KMnO₄?g?=1:4:0.2;optimum reaction temperature and time,drying temperature and time are 40?and 100 min,40?and 2 h,respectively.Graphite Intercalation Compounds?GICs?prepared under optimal reaction conditions had a maximum expansion volume of 588 mL/g at an optimum expansion temperature of 500°C.The SEM,XRD,FT-IR and TGA were used to characterize the products at different stages.The analysis showed that after the FG was oxidized,the edge layers of graphite were opened,and the perchlorate,phosphate and acetic anhydride were intercalated between layers,and the crystal structure changed.After GICs was expanded,the intercalation materials vaporized,and the graphite characteristic peak of EG was consistent with the crystal plane regression of FG.?2?The optimal synthesis conditions for the preparation of expanded graphite by KMnO₄+K₂Cr₂O₇-mixed acid system are as follows:the optimum ratio of mixed solid oxidant is KMnO₄?g?:K₂Cr₂O₇?g?=1:1.The introduction of K₂Cr₂O₇made the optimum reaction temperature increase slightly to 50?and reduce the reaction time to 60 min.The GICs,which was prepared under the optimal reaction conditions,had an expansion volume of 79 mL/g at an initial expansion temperature of 200?;the maximum expansion volume at an optimum expansion temperature of 500?was 665mL/g,which was greater than that of the KMnO₄-mixed acid system.XRD,SEM and FT-IR analysis showed that the EG prepared by the method had better purity,more uniform expansion and larger winding space than the EG prepared by KMnO₄-mixed acid reaction system at the same expansion temperature.?3?EG,whowas prepared by KMnO₄+K₂Cr₂O₇-mixed acid system and hadan expansion volume of 550 mL/g,was used as adsorbent to investigate its adsorption performance for 100 mg/L methyl orange solution.The equilibrium adsorption capacity under the optimum adsorption conditions was determined to be 34.4 mg/g.The adsorption process was simulated by kinetic and thermodynamic modelto obtain?G<0,?H<0.It was indicated that the adsorption process was an exothermic reaction that can be spontaneously carried out,and conformed to the quasi-secondary kinetic model and belonged to Langmuirisothermal adsorption.By SEM,XRD and FT-IR characterization of EG₁ adsorbed with methyl orange and EG₂ after calcination and desorption,it can be found that the static adsorption method can maintain the morphology and pore structure of EG well.However,the adsorption amount of EG desorbed by calcination at 600°C was reduced from 33.3mg/g to 8.5 mg/g,which was much lower than that of EG.The adsorption performance after the second use was no longer basically change.From the analysis results of FT-IR,it was found that the removal of methyl orange also caused the loss of effective reactive groups simultaneously,which resulted in a decrease in the reusability of EG.