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

Apatitic rock phosphates (RP) are commonly calcined to remove impurities an d to increase phosphorus (P) concentration but calcination decreases the ag ronomic effectiveness of RPs used for direct application to soils. This stu dy investigated the effect of calcination on 6 apatite RPs (Christmas Islan d A-ore, Egypt, Morocco, North Carolina, Queensland, and Sechura). RPs were uncalcined (25 degreesC) and calcined at 500 degreesC, 900 degreesC, and 1 100 degreesC. They were evaluated by X-ray diffraction (XRD) and BET-N-2 su rface area technique. P dissolution in 2% citric acid with a 128 h extracti on time was measured. Chemical results were compared with those from a plan t growth experiment, where wheat was fertilised with the calcined RP produc ts. 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 Ang strom to approximately 9.38 Angstrom, indicating that the carbonate contain ing apatite RPs altered to less-soluble fluorapatite. Apatite average cryst al size (coherently diffracting zone) more than doubled and BET-N-2 specifi c surface area decreased by 95%, due to crystal growth and sintering. Conse quently, the extent of dissolution in 2% citric acid and agronomic effectiv eness decreased substantially. Calcination at 500 degreesC and 900 degreesC produced similar but smaller changes in mineral properties. It is conclude d that beneficiation of apatitic RP by calcination will adversely affect th e agronomic effectiveness of RP used for direct application to soils.