Study On The Effect Of Minerals On Eimission Of Heavy Metals In Sludge Incineration

Sludge incineration is a kind of sludge disposal method which reduces the amount and harm of sludge.It is more and more respected by most ofcountries.However,sludge contains a large amount of heavy metals,which is easy to cause heavy metal pollution.Therefore,it is very important to control the emission of heavy metals in the process of sludge incineration.Thermodynamic analysis shows that the heavy metals in the flue gas of sludge incineration mainly exist in the form of gaseous chloride.Moreover,the silicon and aluminium components in sludge may have retention effect on heavy metals.The adsorption effects of kaolin,limestone and montmorillonite on ZnCl₂,PbCl₂ and CdCl₂ were investigated on the two-temperature zone tubular furnace to study effect of adsorbents on gaseous heavy metals in flue gas.The results show that the order of adsorption efficiency of kaolin on heavy metal chlorides is PbCl₂>CdCl₂>ZnCl₂.The adsorption efficiency of kaolin shows a decreasing trend with temperature increasing.The order of adsorption efficiency of limestone for heavy metal chlorides is CdCl₂>PbCl₂>ZnCl_2.The adsorption efficiency of limestone increases first and then decreases with temperature increasing.The order of adsorption efficiency of montmorillonite for heavy metal chlorides is:PbCl₂>CdCl₂>ZnCl₂.The adsorption efficiency of montmorillonite decreases first and then increases with temperature increasing.Kaolin is the best adsorbent under experimental conditions by comparing the efficiency of three adsorbents.The mechanism of the effect of modification on the adsorption of heavy metals on kaolin was studied by carrying out calcination and hydrothermal modification of kaolin.The results show that the hydroxyl radicals in kaolin structure are basically removed and the specific surface area decreases after calcination.So the adsorption efficiency of heavy metal chlorides decreases at low temperature.However,the chemical activity of Si and Al in kaolin structure increases,so the adsorption efficiency increases at high temperature.The hydrothermal modification of calcined kaolin gives the hydroxyl radicals to the calcined kaolin again to restore the ability of reacting with heavy metal chloride partly and increases the specific surface area,so the adsorption efficiency of hydrothermal kaolin is enhanced at low temperature.Increasing hydrothermal time and pressure will enhance the hydrothermal effect.Finally,sludge incineration experiments were carried out in a horizontal tubular furnace to study the mechanism of the effect of calcium-based additives on heavy metals in sludge incineration process.The results show that the volatility of Zn and Pb in sludge increased first,then decreased and then increased with the temperature increasing.It is mainly due to the retention of heavy metals by silicon and aluminium in sludge and volatilization kinetics.Ca?OH?₂ can decrease the content of Zn in ash at 600?and increase the content of Zn at 700-1100?.In the hand,CaO can turn ZnCl₂ into ZnO,in the other hand,reaction between calcium and aluminosilicate lowers the probability of aluminosilicate reaction with Zn.Ca?OH?₂ can decrease the content of Pb in ash at 600-1100 C.Moreover,the more Ca?OH?₂ added,the less retention of Pb is.The main mechanism is that the reaction between calcium and aluminosilicate lowers the probability of aluminosilicate reaction with Pb.