Study On Phosphorus Species Transformation In Municipal Sludge Incineration Process And Phosphorus Recovery From Incineration Ash By Leaching With Oxalic Acid

Phosphorus（P）is an indispensable element for all living organisms and human beings.Unfortunately,P is a non-renewable resource.Thus,more and more attentions have been paid to the recovery of P from secondary phosphorus resources.Municipal sludge contains considerable amounts of P.After incinerating,P can be enriched in sludge incineration ash,which is a potential secondary resource for P recovery.In this dissertation,the effects of incineration temperature on characteristics of incinerated sewage sludge ash（ISSA）were investigated.The P recovery efficiency and the leaching conditions were optimized in P recovery from ISSA via leaching process with oxalic acid solution.The effects of different sludge conditioning systems on the characteristics of ISSA and subsequent P recovery efficiency by oxalic acid leaching were compared,and the utilization of leached residue from ISSA after P recovery as building material was explored.The main contents of this dissertation are as follows:1.Effect of incineration temperature on the characteristics of incinerated sewage sludge ashThe effects of incineration temperatures（600,700,800,850 and 900 ^oC）on the properties of ISSA were investigated.Thermogravimetric analysis indicates that the organic matters in raw sludge was almost completely decomposed at 600 ^oC.The results of Standard Measurements and Testing（SMT）show that P in ISSA was almost entirely composed of inorganic phosphorus（IP）.As the incineration temperature increased from 600 ^oC to 850 ^oC,a part of non-apatite inorganic phosphorus（NAIP）was converted to apatite inorganic phosphorus（AP）,which accounted for 71.9%of inorganic phosphorus at 600 ^oC and decreased to 53.7%of inorganic phosphorus at 900 ^oC.The XRD patterns show that as the incineration temperature increased,the peak strength of FePO₄ and AlPO₄ decreased,but the peak intensity of Ca₃（PO₄）₂ increased,which is consistent with the SMT results.The SEM images indicate that the fine ISSA particles have a phenomenon of melt agglomeration and the specific surface area decreases as the incineration temperature increases.Acid leaching experiments showed that the leaching efficiency of P by H₂C₂O₄ solution was slightly higher than that by H₂SO₄ solution for ISSA obtained at different temperatures.With the increasing of the incineration temperature,the leaching concentrations of heavy metals were greatly reduced.2.Optimization of leaching phosphorus from incinerated sewage sludge ash by H₂C₂O₄ solutionThe ISSA at 850 ^oC was used as the research object,and the leaching experiments were performed with H₂C₂O₄ solution as the extractant.The concentration of oxalic acid solution,leaching time and liquid-solid ratio were selected to optimize the P leaching from ISSA by H₂C₂O₄ solution.The results illustrate that the leaching efficiency of P increased along with the oxalic acid concentration and leaching time increasing,and decreased slightly with the increased of liquid-solid ratio.On the basis of single factor experiments,the optimum conditions were optimized by Response Surface Methodology（RSM）to maximize the leaching efficiency of P.The leaching efficiency of P was up to 95.17%when the oxalic acid load was the ratio of H/P was 12.18,the leaching time was 9.12 h and the liquid-solid was60.61 mL/g.3.Effects of sludge conditioning system on phosphorus recovery from incinerated sewage sludge ashCompared the characteristics of four kinds of ISSA respectively prepared by incineration of raw sewage sludge and dewatered sludge cakes obtained by Fenton,Fenton-lime and sodium persulfate-phosphogypsum at 850°C after drying.According to the SMT and XRD results,conditioning agent had a great influence on the distribution of P and phase in ISSA.The distribution of heavy metals indicated that the form of heavy metals in ISSA from different conditioning systems is mainly in residual state.The leaching effciency of P from different ISSA with the optimal oxalic acid leaching conditions was compared.In descending order of efficiency,they were:ISSA from Fenton conditioning（100%）>ISSA from raw sewage sludge（94.41%）>ISSA from sodium persulfate-phosphogypsum conditioning（93.70%）>ISSA from Fenton-lime conditioning（83.07%）.Through ion exchange and chemical precipitation,the products of P recovery which were based on aluminum hydroxyphosphate could be obtained from the oxalic acid leaching solution.The oxalic acid leaching residue could be used as the admixture of concrete.The mechanical properties of the test block by replacing the appropriate amount of cement with the oxalic acid leaching residue were better than the traditional cement test block’s.Accoding to the above results,it is feasible to use the oxalic acid leaching residue as a suitable substitute of cement to produce the mineral admixture of concrete.