Mechanochemical activation of phlogopite to directly produce slow-release potassium fertilizer

Phlogopite, one trioctahedral mica, was activated by solvent-free ball milling to serve directly as slow-release potassium fertilizer. Before the final breakdown of the crystal structure, phase transformation of trigonal phlogopite into monoclinic by milling at 300 rpm for 120 min was observed for the first time. Different solubility of the amorphous product and monoclinic crystalline product in water or 2% citric acid was also obtained. The content of water soluble potassium (quick acting potassium) and 2.0% citric acid soluble one (slow release potassium) in the products could be easily changed by adjusting the milling speed depending on the requirement for plant growth. 100.0% K extraction in 2.0% citric acid solution from the amorphous sample and near 30% K extraction in water from the sample milled at 300 rpm was achieved. Several analytical methods such as XRD, SEM, FT-IR and TG-DTG were used to characterize the activated sample and to understand the reason for the easier dissolution of potassium. The proposed process was economical and environment-friendly, which could help to maintain a balanced supply of potassium fertilizer for sustainable development of agriculture by compensating partially current high-cost chemical fertilizer.