Study On The Mechanism Of The Effects Of Calcium And Magnesium Additives On Phosphorus Forms In High Iron/aluminum Sludge Incineration As

The improvement in both yield and quality of modern agriculture can be attributed to proper fertilization.Supplementing the soil with essential elements such as nitrogen,phosphorus,and potassium on top of adequate organic fertilization is the key to ensuring normal plant growth.Phosphorus fertilizer mainly comes from phosphate rock,but it is facing the challenge of decreasing reserves and declining quality.In Japan and Europe where phosphorus resources are extremely scarce,recovering phosphorus from sludge is a feasible technology for secondary utilization of phosphorus resources.However,the form and content of phosphorus in ash from sludge incineration are crucial factors determining its value for secondary utilization.The different application methods of iron/aluminum coagulants in sewage enhanced phosphorus removal process can affect the initial form of phosphorus.Phosphorus in different forms has different migration patterns under the action of additives.However,currently,there is insufficient research on the changes in phosphorus forms and applicability during the mixing and burning process of calcium and magnesium additives with different characteristic sludges.In this study,we investigated the migration and transformation of phosphorus enrichment and forms during the co-incineration of high-speed rail/aluminum sludge with calcium and magnesium additives,and discussed the reasons for the decrease in total phosphorus at high temperatures.In addition,we clarified the mechanism of phosphorus enrichment and conversion in sludge through model compound experiments,providing a scientific basis for the harmless and resourceful utilization of sludge.The main conclusions of this study are as follows:（1）The primary inorganic phosphorus in the dried sludge is low,with a relatively small proportion of inorganic phosphorus being present in apatite.This indicates that there is a limited amount of biologically available phosphorus in the original sludge.As the temperature increases,non-apatite inorganic phosphorus gradually converts to apatite inorganic phosphorus,but temperature has little effect on increasing the AP content.Furthermore,when high-iron sludge is incinerated at1050℃,the TP and IP content of the SMT test samples significantly decrease due to the large amount of Fe PO₄ in the high-iron sludge ash reacting with silicates to form low-temperature eutectics,leading to sample sintering and blockage,which in turn causes the extraction rate of the SMT method to decrease.However,after adding Ca Cl₂,Fe PO₄ reacts to form Fe₂O₃ with a higher melting temperature,which reduces the degree of sintering at 1050℃and slows down the reduction of the extraction rate caused by melting at high temperatures.If iron coagulants are used to capture phosphorus in wastewater management,special attention may need to be paid to incineration temperature and the use of additives to achieve the overall economic potential of recycling.（2）The additives can promote the transformation of phosphorus forms.However,the AP content in the mixed ash sample containing the same proportion of Ca Cl₂ and Ca CO₃ added at the same temperature varies greatly,and the AP content in the sample with added Ca Cl₂ is higher.The trend of changes in different characteristic sludges is consistent,indicating that Ca Cl₂ is more conducive to the transformation of NAIP to AP.In addition,under the action of the two additives,the extraction of TP in high-iron/aluminum sludge also follows the trend of phosphorus form changes.This is because the addition of a high-dose carbonate salt raises the p H of the extraction solution,resulting in a decrease in the efficiency of phosphorus extraction.Moreover,in the comparison with the effect of Mg Cl₂,it was found that Ca Cl₂ is more conducive to the extraction and bioavailability of phosphorus.In practical production,if iron/aluminum coagulants are used for phosphorus removal,considering the bioavailability and extraction rate of phosphorus in the later stage of treatment,it is recommended to use Ca Cl₂ as the additive.（3）The inorganic phosphorus model compound further demonstrated that compared with Ca CO₃ and Mg Cl₂,Ca Cl₂ is more conducive to the transformation of Fe PO₄,and Fe PO₄ and Ca Cl₂ have strong reactivity at lower temperatures.However,there are differences in the changes of Al PO₄ and each additive in high-aluminum sludge.In the model compound Al PO₄,the three additives are beneficial to the transformation of phosphorus forms at 750-1050℃,as shown by both the SMT extraction and mixed ash crystal mineral diffraction.Therefore,in the process of sewage phosphorus removal,if iron coagulants are used for phosphorus removal,it is recommended to use Ca Cl₂ as an additive.If aluminum coagulants are used alone for phosphorus removal in the phosphorus removal process,attention should be paid to the selection of incineration temperature,and the recommended incineration temperature is above 750℃.