Migration Behavior Of Typical Heavy Metals During The Firing Of Electroplated Sludge Into Ceramic Pellets

Electroplating sludge usually contains high concentrations of heavy metals,which has a strong potential environmental hazard and is currently classified as hazardous waste in China.Using electroplating sludge to burn ceramic pellets is a potentially effective way to utilize electroplating resources.Researchers have conducted a lot of studies on the heavy metal content and leaching risk of pottery pellets fired from electroplating sludge,as well as studies on the compliance of conventional pollutants in the flue gas of the firing process.However,the understanding of the migration and transformation mechanism of heavy metals in electroplating sludge fired ceramic pellets is very limited,which restricts the effective control of heavy metal pollution in flue gas.Therefore,in this paper,based on different raw material compositions and different additives,the distribution characteristics and migration and transformation laws of heavy metals under different working conditions are studied,in order to provide a scientific basis for the control of heavy metal pollution.The ceramic pellets were prepared by mixing electroplating sludge,municipal sludge and shale in certain proportions,and the roasting temperature was set at 400℃,700℃,900℃ and 1140℃ by using a high-temperature tube furnace.The changes of total gaseous heavy metals and phase distribution patterns were investigated for different ratios,chlorides,sulfides and oxides.The results of the study are shown as follows.First,the greater the amount of electroplating sludge incorporated,the higher the total amount of gaseous heavy metals。With the increase of temperature,the total amount of gaseous heavy metals showed a trend of rising and then decreasing,the total amount of Cd reached the maximum at 900℃,and the total amount of Cr,Cu and Ni reached the maximum at 700℃.The total amount of gaseous heavy metals increased with the increase of residence time,and no longer increased after 25 min.The XRD results showed that the heavy metals in the raw ceramic pellets mainly existed in the form of hydroxides and Ni in the form of sulfides.With the increase of temperature,hydroxides gradually decomposed into oxides,and CdO formed minerals Cd2SiO4 and K2CdSiO4 with SiO2 and Al2O3 at 1140℃,and Cr and Ni combined with Fe2O3 to form FeCr2O4 and NiFe2O4,resulting in a decreasing trend of total gaseous heavy metals.Second,Chloride addition both promoted the migration of heavy metals to the volatile phase and increased the total gaseous heavy metals,and the promotion effect was PVC>CaCl2.Both sulfides and oxides inhibited the volatilization of heavy metals,and the inhibition effect of sulfides was Na2SO4>Na2S at 400℃ and 700℃,the inhibition effect was Na2S>Na2SO4 at 900℃ and 1140℃.The inhibitory effect of sulfide addition on Ni was not obvious.The inhibition effect of oxides on heavy metals was CaO>Al2O3.Different additions have different degrees of influence on the heavy metal binding state in the burnt expanded ceramic pellets.XRD analysis showed that after the addition of chloride,Cd,Cr and Ni combined with Cl to form metal chloride and metal chlorate,which promoted the migration of metals to the volatile phase,with the increase of temperature,Cd existed in the form of silicate,and both Cr and Ni formed FeCr2O4 and NiFe2O4 with Fe2O3.After the addition of CaCl2,CuO gradually changed to Cu2O,which was easily combined with Cl,due to the low temperature dechlorination of PVC,CuO directly generated CuCl and(CuCl)3.After the addition of sulfide,heavy metals mainly exist as metal sulfide or metal sulfate,and with the increase of temperature,the metal sulfate is decomposed into metal oxide and metal peroxide by heat.The adsorption of CaO on Cd and Ni was mainly physical,while Cr and Cu were mostly chemically adsorbed.