| With the social development and the progress of science and technology,liquid crystal display(LCD)has become the absolute main displays due to its excellent display performance,and correspondingly the quantity of discarded LCDs increases year by year.As the key component of LCD,the treatment of LCD panel has become the main focus and difficulty after LCD discarded.Focusing on the potential environmental risks of heavy metals contained in waste LCD panel,this study analyzed the content of the main metals(Cr,As,Al,In,Ni,Cu,Zn,Fe,Cd,Sn,Pb)contained in LCD panels and their potential ecological environmental risks,as well as their migration and transformation behavior during the thermal treatment(pyrolysis and incineration).The distribution of migrated metals in solid residue,oil and fly ash were analyzed,and the influence mechanism of organic materials on the migration and transformation behavior of heavy metals was also analyzed with the aim to provide theoretical basis for heavy metal pollution control during the thermal treatment process of waste LCD panel.The concentration,existing forms and leaching toxicity of metals in the panels from three typical LCD display devices: desktop computer display,notebook computer and liquid crystal television was analyzed.The results show that the concentration of Al,As and Fe in the mixed samples of the three typical LCD panels was the highest in this research,with the concentrations of 29568.38 mg/kg,1823.17 mg/kg and 2010.13 mg/kg in turn.Followed by Zn,Cr,Sn,Ni,In,Cu,Cd,the concentration of which decreased in turn and ranged from378.96 mg/kg to 23.10 mg/kg,Pb was not detected.Analysis of metal existing forms showed that As,Al,Cd,Sn,Cr,Ni,Fe and Zn in LCD panel are mainly in the form of residue,with low ecological risk level.In and Cu are mainly in reducible state,and Cu also exists a certain amount of oxidizable state,with high ecological risk level.In addition,the toxic leaching concentrations of As,Ni and Zn exceed the limits of toxicological index and general chemical index of groundwater quality standard III,Al and Fe exceed the limits of general chemical index of groundwater quality standard V,respectively.Attentions should be paid on the treatment and disposal of waste LCD panels.The research on the heavy metals migration in the thermal treatment process of waste LCD panels showed that the treatment temperature,reaction time,gas flow rate and organics content influenced the migration rate of metals both in nitrogen and air atmosphere.The treatment temperature has the most significant influence on the migration behavior of various metals in the thermal treatment process of LCD panels,followed by the reaction time,gas flow rate.Also the high organics content is conducive to the migration of most metals.In the thermal treatment process,it was found that the metals of Cr,In,Ni,Cu,Fe and Sn in LCD panel exhibit obvious migration under various experimental conditions.(1)The maximum migration rate of Cr,In,Fe reached 70%,78% and 48% respectively under the conditions of temperature 900℃,gas flow rate 2.0L/min and reaction time 60 min in nitrogen atmosphere,and the maximum migration rate of Ni,Cu and Sn reached 62%,49% and 44% respectively under the conditions of temperature 600℃,gas flow rate 2.0 L/min and reaction time 60 min.The content of Cr,In,Ni,Cu and Fe in the oil products were higher than those in the gas phase.(2)The maximum migration rate of Cr,Ni,Cu,Fe and Sn was 72%,72%,53%,64%and 47% respectively under the conditions of temperature 600℃,gas flow rate 0.5L/min and reaction time 60 min in air atmosphere.The maximum migration rate of In was 40% under the conditions of temperature 600℃,gas flow rate 2.0 L/min and reaction time 60 min.The migrated metals Cr,In,Ni,Cu and Sn mainly concentrated in fly ash.Experimental research on the transformation of metal existing forms during thermal treatment showed that thermal treatment has no significant effect on the existing forms of As,Cd,Sn,Cr,Ni,Zn and Fe,whose main existing form were residue state both before and after thermal treatment.To some extent,the existing forms of Cr,Ni,Zn and Fe changed under the thermal treatment.With the increase of reaction temperature,they mainly showed the transformation from residue state to acid soluble state,reducible state and oxidizable state,and In,Cu showed the most significant transformation.The ecological risk assessment of solid residue showed that:(1)Pyrolysis has no effect on the ecological risk level of As,Cd and Sn,which are all in low ecological risk level.However,high temperature treatment(≥700℃)will significantly increase the ecological risks of Cr,Ni,Zn,Fe,In and Cu,with In and Cu being the most obvious.(2)Incineration exhibit little effect on the ecological risk levels of most metals such as Cr,As,Ni,Cd,Fe,Sn and Zn,but obvious inhibitory effect on the ecological risk of Cu.The leaching toxicity of heavy metals in solid residue showed that the leaching concentration of each metal is lower than the leaching concentration limit of toxic and harmful substances in China,showing no obvious leaching toxicity.However,compared with the standard limit of groundwater,the leaching concentration of As and Ni after pyrolysis is higher than the ground water pollution limit,exhibiting high pollution level when the thermal treatment temperature exceeds 700℃.The environmental pollution level of Cr,As,Ni and Fe in the solid residue under the high temperature condition(temperature ≥700℃)of incineration is extremely high,which requires much attention.The influence analysis of the panel organics and its mechanism on the migration and transformation of heavy metals showed that the presence of polarizer exihbits promotion effect on metal migration,especially under incineration conditions.The promotion effect mainly comes from the volatile/gas products generated by polarizer decomposition,and the solid products of decomposition have inhibition effect on the metals migration such as Cr,In,Ni,Cu,Fe,etc. |
