Study On The Effect Of Corn Straw On The Transformation Of Phosphorus Form In Sludge Ash

Phosphorus plays a vital role in the growth of plants and animals and is essential in cell structures and metabolism.Phosphorous is a depletable resource while the consumption of phosphorous fertilizer increases with growing population size.Phosphorus recycled from incinerated sludge ash can be complements to phosphatic fertilizer in districts suffering phosphorus resource shortages（e.g.Germany,Japan and Sweden）.The apatite inorganic phosphorus（AP）content in incinerated sludge ash is a key factor influencing the recoverability and bioavailability.Biomass straw is rich in calcium minerals and can be used as an additive to be blended with sludge to increase the AP content.But the current research has little discussion on the bioavailability of various Ca-Mg-P minerals produced by adding biomass,and the biomass itself has a slagging problem due to the high content of alkali metals,which still remains after blending with sludge.It will seriously affect the stable operation of the equipment.Water leaching pretreatment can alleviate the slagging problem of biomass itself,but the effect of biomass after pretreatment（it has been removed most of soluble components）on the phosphorus specification of incinerated sludge ash is still unclear.In this study,aiming at the transformation of phosphorus species during sludge incineration,the effect of co-combustion of corn straw/washed corn straw and sludge on phosphorus enrichment and the transformation in incineration ash was investigated,By comparing the phosphorus species of the two and exploiting the model compound experiments,the effects of soluble and insoluble alkali metals in corn straws on the phosphorus specification in incineration ash were discussed.The results provide a scientific basis for the recovery of phosphorus resources in incineration ash.The main conclusions of this paper are as follows.（1）The content of apatite inorganic phosphorus in the incineration ash of sludge or corn stover alone is low,and apatite inorganic phosphorus determines the bioavailability of phosphorus,so the phosphorus bioavailability in the incinerated ash of sludge or corn straw alone is poor.With the increase of the incineration temperature,the conversion of non-apatite inorganic phosphorus（NAIP）in the pure sludge incineration ash to AP was promoted.More Ca₂P₂O₇ and Ca P₂O₆ are generated in the incineration ash at higher temperature.Ca P₂O₆ is insoluble in citric acid,so although the AP content increases,the bioavailability did not change much with temperature.（2）The increase of Ca/P promoted the conversion of NAIP to AP in the incineration ash of the blended samples,and the bioavailability increased accordingly.In the blended samples,Al PO₄ and Fe PO₄ were converted into apatite inorganic phosphorus,not only Ca₂P₂O₇ but also Ca₇Mg₂P₆O₂₄appeared.This is the result of the reaction of magnesium-containing minerals in the mixed-burned corn straw,and the effect of magnesium-containing minerals may be similar to that of calcium-based additives,which is beneficial to the conversion of NAIP to AP.Comparing the crystal mineral composition of pure sludge and incineration ash of mixed samples at different temperatures,it can be seen that Ca₇Mg₂P₆O₂₄ can be formed when temperature is high（>950℃）and calcium and magnesium is enough.（3）With the increase of temperature,the content of NAIP in the incinerated ash of sludge mixed with water-washed corn straw decreased gradually,and the content of AP gradually increased,but the content of TP and IP decreased significantly at 1050℃.The total phosphorus and inorganic phosphorus contents of the original sludge also decreased significantly at 1050℃,and the total phosphorus and inorganic phosphorus contents changed little in mixed samples of corn straw and sludge,which indicated that the phosphorus fixation effect of the raw corn straw was stronger than that of the water-washed corn straw.The soluble alkali metal minerals in the original corn straw are mainly KNO₃,K₂SO₄ and KCl O₄.These minerals decomposed at low temperature.It is speculated that the decomposition of these minerals at low temperature enables them to react with phosphorus-containing minerals at lower temperatures,thereby fixing phosphorus to incinerated ash.（4）The conversion of NAIP to AP was promoted with the increasing ratio of washed corn straws.But the conversion of NAIP to AP was incomplete in SCw_2.5-1050.Comparing the apatite inorganic phosphorus in the samples mixed with the same proportion of water-washed corn straw and corn straw at the same temperature,it can be seen that the AP content in the samples mixed with corn straw is higher,and the difference is more obvious with the increase of temperature.This indicated that the soluble alkali metal minerals in corn straw not only had the effect of fixing phosphorus in the incineration ash,but also contributed greatly to the conversion of NAIP to AP.（5）The total phosphorus and inorganic phosphorus in the original sludge decreased significantly at 1050℃,and the decrease of total phosphorus and inorganic phosphorus content decreased after mixing with potassium chloride,and the total phosphorus and inorganic phosphorus of the samples mixed with sufficient potassium chloride no longer decreased,indicating that Potassium chloride has the effect of fixing phosphorus.The content of total phosphorus and inorganic phosphorus in the incinerated ash with different proportions of CaO sludge decreased significantly at1050°C,indicating that the calcium oxide additive did not have an obvious effect on phosphorus fixation.At the same temperature,the content of non-apatite inorganic phosphorus gradually decreased with the increase of the proportion of CaO blending,and the content of apatite inorganic phosphorus increased.However,the phosphorus-fixing effect of this additive is not obvious.There is no obvious phosphorus loss in the incineration ash of sludge mixed with potassium chloride and calcium oxide composite additives at 1050°C,and the content of total phosphorus and inorganic phosphorus does not change much at 750°C and 1050°C,indicating that the combined addictive can effectively fix phosphorus.With the increase of the additive blending ratio,the content of non-apatite inorganic phosphorus decreases,and the content of apatite inorganic phosphorus increases,indicating that the combined additive has the ability to fix phosphorus and promote the conversion of non-apatite inorganic phosphorus to apatite inorganic phosphorus.（6）Comparing the AP content in the incineration ash of the samples mixed with corn straw and the samples added with the corresponding proportion of combined additives,the AP content in the sludge incineration ash mixed with corn stalks is higher.Because the potassium-containing minerals and calcium-containing minerals in the corn straws include but not limited to potassium chloride and calcium oxide,other alkali metal and alkaline earth metal minerals may be more reactive.