Study On The Thermal Treatment Behaviors And Fluoride Detoxification Mechanisms Of Spent Potlining

With the rapid development of China electrolytic aluminum industry,it has led to a sharp increased in the yield of spent potlining（SPL）.It brings severe environmental pollution with unreasonable treatment due to its large amount of fluoride and small amount of cyanide.The incineration treatment has gradually become the mainstream treatment method of SPL due to its simple operate process,heat utilization,fluoride recovery and complete detoxification.Based on this,the objective of this work were to investigate its thermal treatment behaviors and fluoride detoxification mechanisms.The main work and research results are as follows:（1）The thermogravimetric and mass spectrometer was used to investigate the incineration characteristics of SPL under oxy-fuel atmosphere.The thermodynamic and kinetic characteristics of SPL incineration under different O₂ concentrations using varying heating rates were investigated.The results show that the decomposition of SPL was mainly between 450 and 800°C at different heating rates.The different proportions of O₂ have no significant effect on the combustion of SPL.High concentrations of CO₂ in the oxy-fuel atmosphere promoted the SPL burnout at high temperature.The thermodynamics and kinetic results indicated SPL has better combustion performance in 70%CO₂/30%O₂ atmosphere.The lowest average E_αwas estimated at 149.81 k J·mol^-1.With theαvalue of 0.25～0.70,the kinetic model of incineration at different heating rates can be described as f（α）=1/2α,G（α）=lnα.CO₂ thermodynamically promotes the carbon gasification and rendering weight loss when the temperature above 800°C,but not in the N₂ atmosphere.The gases emission during the combustion process mainly included COx,NOx,and HF.The HF emission significantly increased when the temperature above 750°C.（2）The thermogravimetric was used to investigate the incineration characteristics of SPL under air atmosphere at different heat rates.The results show that the maximum weight loss peak and burnout temperature of SPL incineration shifted to a high temperature region with the heating rate increased,thus improved the comprehensive combustion performance.Differential scanning calorimetry analysis demonstrated the SPL incineration process under air atmosphere was an exothermic reaction.The heat release decreased from 32.54 to 20.33 k J·g^-1 with the heating rates increased from 5 to 20°C·min^-1.The two peaks of the DDSC curves indicated the two-step reaction of fast incineration of fixed carbon and decomposition of char and inorganic compounds,respectively.（3）The thermogravimetric mass spectrometer and thermodynamic simulation software（Fact Sage 7.1）were used to investigate the co-combustion characteristics and fluoride conversion of SPL and textile dyeing sludge.The results shown that increased comprehensive combustion index,and decreased burnout temperature with50%SPL indicated a better combustion and char burnout,and a shorter combustion process.The synergistic effect of minerals such as sulfate promoted the conversion of sodium fluoride.The addition of calcium-based additives could convert sodium fluoride into Ca F₂ and effectively increased the fluorine in the bottom ash.The changed migration mechanisms of F^-and S caused ash compositions to consist of Na₂SO₄ and Ca F₂ for the co-combustions.The predom diagram of Na-Ca-F-S using thermal simulations showed the stable existence regions of Ca F₂ and Na₂SO₄.The main gases evoluted from the co-combustion included HF,SO₂,COS,CS₂,HCN,NH₃,NO and NO₂.（4）The tube furnace was used to simulate the incineration of spent potlining.The transformation,retention and leaching behaviors with（out）the addition of calcium-based solid waste（red mud）during the SPL incineration were investigated.The results indicated the RM addition positively affected its retention and leaching rates.Its Ca-containing compounds caused Na₃Al F₆ and Na F to turn into more Ca F₂.30%RM converted water-soluble Na F into more stable Ca F₂ than did SPL at 850°C,thus reduced the leaching rate by 45.15%.30%red mud captured HF through its Ca content and enhanced its retention rate by 66.96%.66.01%of the total fluorine was stably retained in the bottom ash,and thus,significantly reduced the toxicity of the SPL incineration products.The simulation results indicated Si O₂ and Al₂O₃ exerted a thermally positive effect on Na F turning into Ca F₂.（5）Based on the above research results,the conversion mechanism and effectiveness of pure calcium-based additives on water-soluble Na F were investigated.The distribution of fluorine in both gas and solid phase during the incineration process will be tracked.The environmental toxicity of fluorine was also assessmented.The results indicated that the characteristics of additives and combustion temperature were the key factors affecting the conversion and retention of fluoride.The two main mechanisms detected for the fluorine transformation were the substantial formations of Ca F₂ and Ca₄Si₂O₇F₂.Ca Si O₃,Ca O,Ca（OH）₂,and Ca CO₃ detoxified Na F in the bottom ash and yielded a conversion range of 54.24 to99.45%when compared to the incineration of SPL alone at 850°C with the air flow rate of 0.8 L·min^-1 and the retention time of 60min.The fluorine leaching content of the bottom ash decreased from 2493.27 mg·L^-1 to 13.71 mg×L^-1 after the SPL incineration alone and with Ca Si O₃（Ca:F=1.2:1）.The acid-alkali solutions had no effect on the content of fluorine leaching from the bottom ash when 3≤p H≤12.The fluorine leaching concentration was less than 30mg·L^-1 and meet the Chinese criterion of hazardous waste leaching.Fluorine in SPL co-incineration with caicium additives had a strong stability and more than 70%of fluorine remained in the bottom ash.The optimal incineration condition for detoxification was SPL co-incineration with Ca Si O₃（Ca:F=1.2:1）at 850°C for 60 min.The partitioning of fluorine during the SPL incineration with the addition Ca Si O₃（Ca:F=1.2:1）at850°C for 60 min was 83.37,13.90 and 2.72%in the bottom ash,fly ash,and flue gas,respectively.