Capture Of Semi-volatile Heavy Metals By Modified Kaolin During Waste Incineration

The amount of municipal solid waste is greatly increased in China with economic development and the quality of living life improved,and the waste incineration has been developed rapidly with the advantages of small space occupation,large disposal capacity and considerable waste volume reduction.The semi-volatile heavy metal emissions to the atmosphere during incineration can cause environmental pollution and serious harm to human health.Therefore,in this paper,the volatilization behaviors and emission control strategies of lead and cadmium as two typical semi-volatile heavy metals are studied.In this study,the influence and mechanism of different forms of sulfur compounds on the dynamic volatilization of lead and cadmium have been studied in a horizontal tube furnace.Sulfur compounds did not influence the volatilization of lead and cadmium below 700?.Adding S can inhibit the volatilization of lead above 700?,as lead was mainly fixed in the bottom ash in the form of PbS and PbSO₄.However,sulfur promoted the volatilization of cadmium above 700?,and only a small amount of cadmium was fixed in the bottom ash in the form of CdS.At 700-800°C,cadmium oxide was reduced to elemental cadmium in the reducing atmosphere to promote the volatilization of cadmium.Sodium sulfate does not affect the volatilization of lead and cadmium.Using natural minerals to capture heavy metal is a promising technology for heavy metal emission control.To further understand the mechanism of capturing Pb and Cd by kaolin and promote practical application of this technology,the effect of Pb and Cd on the Pb-Cd-Kaolin reaction system was explored in a self-designed two-stage fixed-bed reactor.The results showed that the capture effect of the stream of PbCl₂ by kaolin,which already captured CdCl₂,was promoted and the capture effect of the CdCl₂ steam using kaolin,which already captured PbCl₂,also was improved.The adsorption of PbCl₂ and CdCl₂ by kaolin mainly was chemical adsorption.The kaolin with the adsorption of Pb or Cd promoted the eutectic melting of the kaolin surface,leading to more inside active sites of the kaolin exposed to adsorb more Pb and Cd.The adsorption capacity of kaolin to PbCl₂ is much higher than that of CdCl₂.In order to further improve the capture ability of kaolin for Pb and Cd,PbCl₂ and CdCl₂ steam were adsorbed using Na⁺modified kaolin in the fixed bed reactor.It was found that the capture efficiencies of 10 g kaolin modified by 0.19375g NaOH for PbCl₂and CdCl₂ were 4%and 47%higher than that of kaolin,respectively.Both the capture efficiencies of the modified kaolin for of PbCl₂ and CdCl₂ were more than 65%.This modified kaolin can efficiently capture two kinds of heavy metal chlorides at the same time.the above conclusions still can be obtained if the mole ratio of lead to aluminum or the mole ratio of cadmium to aluminum is used to evaluate the adsorption effect of Al₂O₃ and S_iO₂ in the kaolin on lead and cadmium.The modified kaolin did not change its adsorption capacity for CdCl₂,but it can adsorb more PbCl₂,and its adsorption was still chemical adsorption.The capture ability of kaolin to PbCl₂ and CdCl₂ can be improved by using the kaolin modified by Na⁺in leachate due to large amounts of Na⁺in leachate.The adsorption efficiency of 10 g kaolin modified by 21.25ml leachate 2?the waste liquid obtained from the leachate treated by primary precipitation,anaerobic digestion and aerobic digestion?was increased by 36%and 53%,respectively,and the adsorption efficiency of PbCl₂ and CdCl₂ reached 97.04%and 63.88%,respectively.The capture effect of kaolin modified by this leachate for PbCl₂ and CdCl₂ was better than that modified by Na⁺.It can be seen that the change process by observing SEM of adsorption production with the amounts of heavy metals captured by kaolin or modified kaolin increasing was divided into four steps:?1?the layered structure was broken into small fragments from sites with the lower strength;?2?the small fragments melted into needle or rod;?3?the needle or rod melts into sphere;?4?the spheres merges into a whole dense plate.Kaolin is completely dehydroxyized into metakaolin when the temperature gradually reaches 800?.The adsorptions of typical heavy metals?Pb and Cd?and typical alkali metals?Na and K?by meta-kaolinite were investigated by quantum chemical calculation.All the adsorptions on Al surface were chemical except for the Cd atom adsorption.The adsorptions of metal atoms were due to the attraction from the raised O atoms in Al surface.The adsorptions of metal oxide and hydroxide were due to the attraction from the Al atoms to the O atoms.The adsorptions of metal chloride were due to the attraction from the Al atoms to the Cl atoms.Although Pb,Cd,and their compounds did not adsorb on Si surface,Na,K,and their compounds did.The Cl atom in NaCl and KCl inhibited adsorption on Si surface;however,it was in favor of adsorption on Al surface.The strong adsorptions of NaOH and KOH were due to their decomposition and the products of OH.