Agronomic use of aluminum thermophosphates with added slag

Abstract

Phosphate minerals are the primary source of phosphorus and serve as raw materials for most phosphate fertiliser industries worldwide; yet thermophosphate production enhances the solubility of phosphorus in the soil. In this study, thermophosphates were produced from lateritic phosphates with the addition of iron- and steelmaking slag at temperatures ranging from 100 °C to 1,000 °C, to evaluate the effect of temperature on these products and their viability as fertilisers. The mineralogical and chemical composition of the samples were characterised using X-ray diffraction and X-ray fluorescence, respectively. Crandallite is the principal aluminium phosphate found in the ore, while amorphous and iron oxide phases are found in the slag. Experiments were conducted on the agronomic efficiency of thermophosphates produced at 300 °C, 600 °C and 1,000 °C. Thermophosphate, both in its natural form and with slag at 600 °C, exhibited amorphous phases that yielded better results for plant development due to the solubility of phosphorus and other nutrients available for the crops, thereby enhancing growth and helping to regulate soil fertility. The agronomic efficiency index of thermophosphates with slag added at 600 °C outperformed natural phosphate in each of the variables, mainly due to the high levels of CaO and MgO in the slags. Aluminium thermophosphates with added slag are a viable alternative to traditional phosphate fertilisers, offering both agronomic and environmental benefits.