Experimental Study On Phosphorus Recovery From Sludge-incinerated Ash

Phosphorus（P）is a macro-nutrient essential for crop growth and for phosphate-fertilizers’production.Due to the large-scale production and use of industrial fertilizers,phosphate ores on Earth have been increasingly depleted,resulting in a phosphorus crisis.Meanwhile,excessive application of industrial fertilizers introduces a large amount of phosphorus from non-point agricultural sources into the water environment,which results in water eutrophication.Therefore,it is necessary to recover phosphorus from wastewater to mitigate the phosphorus crisis and water eutrophication.Generally,phosphorus recovery from incinerated sewage sludge ash（ISSA）is consistent with the goal of ultimate sludge disposal,which has proven technology and high recovery efficiency.Wet-chemical approaches extract phosphorus from ISSA,while heavy metals are leached simultaneously.Thus,phosphorus and metals need to be separated in the subsequent process to improve the purity of the obtained P-products.For this purpose,further studies on various ionic leaching situations in the acid leaching process are carried out,so that specific P-products can be formed and recovered,and different metal ions can be effectively removed.This study firstly investigated the kinetic characteristics of phosphorus and various metal ions in the extraction with multiple acids.The results showed phosphorus and most metal ions could be eluted from ISSA at p H=1～2.The acidic leaching of phosphorus and metals reached equilibrium within 2h,followed the second-order kinetic model.The selective extraction and separation of iron in acidic leachate of ISSA were further investigated.The maximum removal efficiency of 96%for iron was achieved using tributyl phosphate as an extractant.Meanwhile,combined with aluminum in the leachate,phosphorus was recovered as the aluminum phosphate（Al-P）.The kinetic experiments showed that,at p H=4～5,aluminum phosphate could be precipitated.Finally,a phosphorus recovery system was designed based on acid extraction and selective adsorption.The results showed that the phosphorus adsorption by hydrotalcite reached equilibrium within8 h,and its saturated adsorptive capacity was about 88 mg P·g^-1.The adsorption isotherm data were well fitted with Langmuir and Freundlich equilibrium models.During the desorption process,phosphorus desorption was completed within 8 h,following the second-order kinetic model.Phosphorus was recovered in the form of calcium phosphate（Ca-P）by adding saturated Ca（OH）₂.The precipitation efficiency of phosphorus increased with the addition of saturated Ca（OH）₂,completed within 20 min.