Fabrication Of Glass-ceramic And Geopolymer Using Lead-zinc Smelting Slags And The Immobilization Mechanisms Of Pb And Cd

It is difficult to dispose of lead fuming furnace slag(LFFS)and zinc volatile kiln slag(ZVKS)safely,which are produced from lead-zinc smelting.The outputs of these two kinds of slag are large.These slags are listed in the National Hazardous Wastes Catalogue of China.Immobilizing and disposing the slags in landfills is the only practicable way to dispose them.However,the cost of this method is high.Long-term monitoring of the landfills is necessary.Thus,it is necessary to find new methods to reuse these wastes.The compositions of LFFS and ZVKS are similar.The contents of Ca,Si,A1 and Fe in the slags are high,whereas the contents of toxic elements are low,such as Pb and Cd.However,the leaching concentrations of Pb and Cd exceed the threshold of toxic wastes of TCLP.There are two difficulties to reuse the slags:a)to immobilized the toxic elements,b)to reuse the nontoxic element.Glass-ceramics immobilization and geopolymer immobilization have the potential to the problems.The compositions of LFFS and ZVKS are able to meet the composition requirement for synthesis glass-ceramics and geopolymer.These materials have the potential to be developed into many practical applications.However,there is no research on comparing the two methods.Thus,the aim of this research is to develop the methods for fabricating glass-ceramics and geopolymer using LFFS and ZVKS and to compare them.In addition,the immobilization mechanisms of Pb and Cd in the two materials are also investigated systemically.LFFS and ZVKS are suitable for fabricating augite glass-ceramics.The optimized heating treatment schedule was 850? for 2h.The optimized formula of the LFFS glass-ceramics is 46 wt%LFFS,46 wt%fly ash and 8 wt%stainless steel slag.The flexural strength,acid resistance and alkali resistance of the LFFS glass-ceramics were 146.26 MPa,? 99%and>99%.These parameters can meet the standard for glass-ceramics plate for industrial application(JC/T 2097-2011),which is an industry standard of building materials of China.The flexural strength,acid resistance and alkali resistance in this standard are>70MPa,? 96%and>98%.The leaching concentrations of Pb and Cd of the LFFS glass-ceramics were 1.342 mg L-1 and 0.021 mg·L-1,which are lower than the leaching thresholds of TCLP.The leaching thresholds of Pb and Cd of TCLP are 5 mg·L-1 and 1 mg· L-1 respectively.The optimized formula of the ZVKS glass-ceramics is 36 wt%ZVKS,52 wt%fly ash and 12 wt%stainless steel slag.The flexural strength,acid resistance and alkali resistance of the ZVKS glass-ceramics were 128.31 MPa,? 99%and ?99%.These parameters can meet the standard for glass-ceramics plate for industrial application.The leaching concentrations of Pb and Cd of the ZVKS glass-ceramics were 1.087 mg·L-1 and 0.032 mg· L-1,which are lower than the leaching threshold of TCLP.LFFS and ZVKS are also suitable for fabricating fly ash-based geopolymer.The heavy metals in the slags could be effectively immobilized in the synthesized geopolymer.The 7d compressive strength of the geoplymer with LFFS or ZVKS addition could reach up to 82.81 MPa and 60.39 MPa.The immobilizaion mechanisms of Pb and Cd in glass-ceramics and geopolymer were also investigated.In order to stimulate the heavy metals in the slags,Pb or Cd was added in the glass-ceramics or geopolymer in the form of PbO,CdO,ect.We found that Pb and Cd could be immobilized in glass-ceramics not only by physical encapsulation,but also by chemical bonding.The upper limit of the Pb content,which can be immobilized in the glass-ceramics,is about 2wt%.When the Pb content in glass-ceramics is lower than 2wt%,Pb can be involved in residual glass phase and augite crystal phase simultaneously.Pb ions could substitute the Ca ions in the structure of augite to form substitutional solutes.The upper limit of the Cd content,which can be immobilized in the glass-ceramics,is about 1 wt%.When the Cd content is lower than 1 wt%,Cd2+ can also substitute the Ca2+ in the structure of augite.The immobilization mechanisms of Pb in geopolymer depend on the chemical properties of the Pb contaminations.When Pb contamination is in the form,soluble in sodium hydroxide solution,Pb can be chemically bonded into the amorphous gel of the geopolymer via Pb-O-Si or Pb-O-Al.Immobilization of these species proceeds not only by physical encapsulation,but also by chemical bonding.In contrast,the Pb compounds,inert to alkali,are trapped in the geopolymeric matrices mainly by the physical encapsulation.CdO cannot dissolve in alkaline solutions.Thus,CdO lacks the precondition to participate in the formation of geopolymeric gel.CdO is trapped in geopolymer by physical encapsulation.The immobilization efficiency of PbO in glass-ceramics is similar with that in geopolymer.The upper limit of the PbO content,which can be immobilized in the two systems,is about 2wt%.The immobilization efficiency of CdO in glass-ceramics is better than that in geopolymer.The reason is that CdO is immobilized in geopolymer only by physical encapsulation,whereas CdO is immobilized in glass-ceramics by physical encapsulation and chemical stabilization.Glass-ceramics is better for CdO immobilization than gaopolymer.