Preparation of Slow-Release Fertilizer from Fly Ash and Its Slow-Release and Metal Immobilization Properties

Abstract

In this study, SRPF with metastable KAlSiO4 as the main slow-release substance was prepared by the sintering method using fly ash and K2CO3 as raw materials, and an orthogonal experiment was conducted to optimize the raw material ratio and preparation parameters of SRPF. The optimum parameters for preparing SRPF are as follows: the potassium carbonate content is 15%; the sintering temperature is 1100 °C; heat preservation time is 60 min; cooling mode is furnace cooling, and the particle size of raw materials is not less than 150 μm. Initial leaching rates in water and 2% citric acid solution were 4.64% and 47.07%, respectively, and cumulative leaching rates at 28 days were 11.17% and 85.86%, respectively, showing that the SRPF prepared from fly ash and K2CO3 meets the standard GB/T 23348-2009 of China. A soil column leaching test was carried out to study the slow-release behavior of SRPF in soil. When the leaching medium is water, the 70-day cumulative leaching rate of SRPF in soil is about 4%, while when the leaching medium is citric acid, depending on the soil type, the 70-day cumulative leaching rate of SRPF can reach 21.2% and 43.5%. The results of the soil column leaching test showed that the total potassium content in the soil was negatively correlated with the slow-release rate of SRPF. Finally, the immobilization effect and mechanism of SRPF on lead ion immobilization was studied, and it was found that SRPF still had a considerable effect on lead ions immobilization. The BET results showed that, compared with fly ash, the BET surface area of SRPF was reduced by 48.3%, the total pore volume was reduced by 16.0%, and the average pore diameter had a small change. The decrease in total pore volume was mainly concentrated in the micropore volume and mesopore volume, which are reduced by 50% and 20% respectively, while the macropore volume hardly changes. In summary, fly ash can be used to prepare SRPF with a good release effect and similar heavy metal ions immobilization ability.