Insights on the additive formulation for the energy-efficient production of fused calcium magnesium phosphate fertilizer from waste sludge

Abstract

The production of the fused calcium magnesium phosphate fertilizer (FCMP) from sludge is a sustainable alternative to phosphate ore route. However, the distinct compositions of sludge and extensive energy input make the process under great uncertainty for the commercialization. In this study, a new method was proposed to determine the optimal addition amount of CaO and MgO to sludge, realizing the high phosphorus (P) availability and simultaneously reducing the melting point of the system for energy saving. The effect of P content on the product structure and P availability was first investigated at different metal oxide addition levels. The modified non-bridging oxygen to tetrahedral cation equation (NBO/T) was used to establish the linear relationship between the NBO/T and the P availability. The threshold value of the modified NBO/T ≥ 2.3 guaranteed a P-availability beyond 90%. Further, FTIR and Raman spectra showed that there was a strong correlation between the modified NBO/T and the actual glass structure. Finally, on the reference value of NBO/T = 2.30, the system exhibited the minimum melting point at 1292 °C when MgO/CaO = 0.83. This method offers a powerful tool to guide the optimal amount of CaO and MgO addition to sludge ash by simple calculations.