Migration,Transformation And Adsorption Of Heavy Metals In Two-Step Thermal Treatment Process Of Sludge

The large amount of sewage sludge is rich in heavy metals and organic pollutants,which poses a serious threat to the environment.At the same time,because it is rich in phosphorus,it can be used as an important secondary phosphorus resource to be recycled.In view of these problems,this paper innovatively proposed a highly efficient and clean recovery of phosphorus from sludge by two-step thermal treatment,which consists of low-temperature incineration and high-temperature calcination.The co-deposition behavior and strengthening mechanism of phosphorus and heavy metals,the decoupling mechanism and removal kinetics of heavy metal chlorination volatilization,and the development and modification of non-carbon-based high-temperature adsorbents were studied in the two-step thermal treatment process of sludge phosphorus resource clean and efficient recovery.This study can obtain new knowledge of heavy metal migration and transformation and adsorption removal in sludge thermal treatment process,and provide theoretical support for sludge harmless and resource recovery.Firstly,thermodynamic simulation and incineration experiments were used to reveal the migration,transformation and co-deposition mechanism of heavy metals and phosphorus during sludge incineration at 650-950℃,and the effects of Ca O additives on them were analyzed.The results showed that low temperature incineration can realize the co-deposition of heavy metals and phosphorus in ash residue and realize the directional conversion of phosphorus to apatite phosphorus（AP）.Adding Ca O was beneficial to improve the co-deposition and directional conversion of phosphorus and heavy metals in ash residue.When the addition amount was 2%,the enrichment rate of phosphorus was the highest,which can reach91.34%.Secondly,a technology adopting chlorine additive to remove the heavy metals was developed and aimed to the problem of high content of heavy metals in phosphorus-enriched incineration slag.Thermodynamic study found that chlorine additive can significantly reduce the semi-volatilization temperature of heavy metals Cu,Pb and Zn in the calcination process of cinder.Calcination experiment also showed that the addition of Na Cl,Mg Cl₂ and PVC significantly increased the volatilization rate of heavy metals,and the volatilization rate of Cu and Zn with the highest content could reach more than 90%.Meanwhile,the heavy metals in calcined ash mainly exist in stable residue state,and the proportion of heavy metals can reach more than 93%.Then,this paper comprehensively considers the content,quantity,toxicity and stability of various heavy metals in sludge,establishes an evaluation model of comprehensive control efficiency of various heavy metals,and evaluates the pollution control efficiency of various heavy metals in the new method of phosphorus resource recovery by two-step thermal treatment of sludge.The evaluation results show that the comprehensive control efficiency of heavy metals in incineration process is up to 78%,and the comprehensive control efficiency of heavy metals in calcination process is up to 67%.Meanwhile,it is found that the key to improve the comprehensive control efficiency of heavy metals is the pollution control of heavy metals in calcination flue gas.Finally,this paper introduced the new porous material-activated boron nitride（BN）into the field of high-temperature flue gas heavy metal adsorption for the first time,and prepares phosphorus-doped boron nitride（P-BN）and sulfur-doped boron nitride through thermal diffusion doping of phosphorus and sulfur（S-BN）.The results showed that the specific surface area of P-BN was the largest under the conditions of phosphorus doping mole ratio of 0.5,activation temperature of 1150℃and activation time of 4h,which can reach 837.08 m²/g.The results of heavy metal adsorption experiments confirmed that the adsorption capacity of BN,S-BN and P-BN to heavy metals at high temperature（above 200℃）was 3-16 times that of the conventional heavy metal adsorbent activated carbon.Moreover,BN and P-BN showed good adsorption selectivity to the heavy metal Zn with the largest content in the sludge,and the adsorption selectivity index was up to 6.9.