Beneficiation of rock phosphate fertilisers by mechano-milling

Abstract

Mechano-milling is a process where materials are ball-milled at high energy to induce chemical and physical reactions. This study investigated the effect of milling on the properties and agronomic effectiveness of six apatite rock phosphates (RP). The effects of milling were evaluated by X-ray diffraction, BET-N2 surface area measurements, electron microscopy and solubility in 2% citric acid. Milling increased the solubility of RPs by increasing the proportion of X-ray amorphous material and reducing the size of remaining apatite crystals. However, milling also caused agglomeration of particles, which reduced the surface area. Milling increased the unit-cell a dimension of apatite, possibly due in part to the formation of low reactive fluorapatite such as occurs during calcination. These changes induced by mechano-milling might be expected to reduce the solubility of RPs; however, solubility increased because amorphisation was dominant. The fertiliser relative effectiveness of RPs based on phosphorus content of wheat plants was increased by a factor of up to three by milling. It is concluded that beneficiation of apatitic RPs by mechano-milling may greatly improve their agronomic effectiveness and milling may provide an economically and environmentally superior option for the manufacture of phosphate fertilisers and enable the utilisation of impure RPs.