Effects Of Chlorine On The Migration Of Heavy Metals During The Preparation Of Cement Clinker Utilizing Sewage Sludge

Sludge is a solid precipitate produced by the process of water and sewage treatment.Sewage sludge produced through Anaerobic-Anoxic-Oxic technology in the sewage treatment plant usually contains heavy metals,toxic organic compounds,a large number of pathogenic microorganisms and other harmful components.It will be directly harmful to the environment without appropriate treatment.On the one hand,the main chemical compositions of sewage sludge are SiO₂,CaO,Al₂O₃,Fe₂O₃ etc.,which could replace part of raw materials.On the other hand,attributed to advantages,the use of cement kilns for disposing heavy-metal-containing sewage sludge is promising.Cyclic utilization of solid waste is an important way of sustainable development.However,sewage sludge usually contains Cu,Ni,Pb and Zn etc.Those trace toxic heavy metal components not only affect the cement clinker production but also are harmful to the environment.In addition,reaction of chlorine and heavy metals will be generated volatile heavy metal chloride in cement kiln,and the volatile characteristics of these heavy metals may be different at different temperatures.Therefore,it is very important to study the volatilization and solidification behavior of heavy metals during the co-combustion of sewage sludge in cement kiln,which can provide a guideline for appropriately handling solid wastes containing heavy metals on the basis of security.This paper characterized the physical and chemical properties of sewage sludge.Then,effect of sludge on the sintering of cement clinker was studied,and the leaching of heavy metals in cement was tested to prove the environmental safety of the clinker.Finally,the effects of the addition and type of chlorides and temperature on the volatilization behavior of heavy metals?Cu,Ni,Pb and Zn?were studied.The specific conclusions are as follows:?1?The 12 kinds of sludge in Guangzhou city were characterized,and the content and existing partitioning of the heavy metal were mainly discussed.The main chemical compositions of sewage sludge are SiO₂,CaO,Al₂O₃,Fe₂O₃ etc.The content of Cu?71.55-2670.98 mg/kg?,Cr?53.66-2406.55mg/kg?and Zn?118.92-1985.64 mg/kg?are most,but the content of Cd,Pb and Ni are not more than 100 mg/kg,which are organio-bound and residual fraction,and the leaching toxicity meets national standards.?2?The addition and type of chlorides and temperature are the important factors that affect the volatilization of heavy metals:?The addition of chlorides has strongly positive effect on the volatilization of Cu,Ni,and Pb.When the addition of CaCl₂ increased from 0% to 1.2%,the volatilization ratio of Cu and Ni increased approximately 30%.However,chlorides did not exert a significant effect on the volatilization of Zn.?AlCl₃·6H₂O,FeCl₃·6H₂O,and CaCl₂ were typically more effective for the removal of heavy metal removals compared with NaCl.Maximum volatilization ratio of heavy metals at 1450?C were 60.8% for Cu?AlCl₃·6H₂O?,63.6% for Ni?FeCl₃·6H₂O?,83.0% for Pb?FeCl₃·6H₂O?,and 67.5% for Zn?FeCl₃·6H₂O?.?The higher the temperature,the easier formation of volatile heavy metal chlorides,resulting in dramatically increase in the volatilization of Cu,Ni and Pb.More than 44.8% of Pb was volatilized at 950?C and increased to 74.68% at 1050?C in the present of CaCl₂.?3?The mechanism of heavy metal volatilization by chlorine is summarized.The ability of chloride to release free radicals at different temperatures is different.Temperature affects the release ability of chlorine radical and the binding ability of heavy metal and chlorine.?4?The sludge?0-25%?was used as an alternative material to prepare clinker: The appropriate amount of sludge?3%¹6%?has a certain effect on the burnability of raw material.With the increase of the addition of sewage sludge?0⁹%?,the size of alite in the clinker was increased.When the sludge content was 9%,the maximum compressive strength of cement paste of 28 days reached the value of 102 MPa,but the compressive strength of cement paste would decrease with the increase of the content of sludge.However,increase of the addition of sludge was not conducive to the formation of the alite.When the content of sludge was 25%,the leaching values of heavy metals of the cement paste curing 3,7 and 28 days was less than the minimum values of ground water type ??GB/T 14848-93?.The cement clinker has a good solidification effect on the heavy metals.