Beneficiation of apatite rock phosphates by calcination: effects on chemical properties and fertiliser effectiveness

Apatitic rock phosphates (RP) are commonly calcined to remove impurities and to increase phosphorus (P) concentration but calcination decreases the agronomic effectiveness of RPs used for direct application to soils. This study investigated the effect of calcination on 6 apatite RPs (Christmas Island A-ore, Egypt, Morocco, North Carolina, Queensland, and Sechura). RPs were uncalcined (25 degreesC) and calcined at 500 degreesC, 900 degreesC, and 1100 degreesC. They were evaluated by X-ray diffraction (XRD) and BET-N-2 surface area technique. P dissolution in 2% citric acid with a 128 h extraction time was measured. Chemical results were compared with those from a plant growth experiment, where wheat was fertilised with the calcined RP products. Calcination at 1100 degreesC reduced the agronomic effectiveness of apatite RPs by about 90%, by altering the crystal properties and the particle size of the RPs. Unit-cell a dimension increased from values of 9.324-9.375 Angstrom to approximately 9.38 Angstrom, indicating that the carbonate containing apatite RPs altered to less-soluble fluorapatite. Apatite average crystal size (coherently diffracting zone) more than doubled and BET-N-2 specific surface area decreased by 95%, due to crystal growth and sintering. Consequently, the extent of dissolution in 2% citric acid and agronomic effectiveness decreased substantially. Calcination at 500 degreesC and 900 degreesC produced similar but smaller changes in mineral properties. It is concluded that beneficiation of apatitic RP by calcination will adversely affect the agronomic effectiveness of RP used for direct application to soils.