In order to offer high grade concentrate to produce alumina by Bayer, the reverse-flotation technology of bauxite was investigated. The results show that sodium hexmetaphosphate has different depression on the flotation of diaspore and kaolinite. The recoveries of diaspore decrease markedly with the increase of the concentration of sodium hexmetaphosphate with dodecyl-amine as collector. Fourier transform infrared spectra indicate that the absorbance band 875 cm⁻¹of the asymmetric stretching vibration of bridge oxygen P-O-P shifts to 880 cm⁻¹, and the 1 264 cm⁻¹of the asymmetric stretching vibration of the bridge PO₂shifts to 1 267 cm⁻¹in the diaspore’s spectra. This demonstrates that sodium hexmetaphosphate interacts through bridging PO₂groups with the Al atoms exposed on the diaspore and kaolinite surfaces to form P-O-Al bond. Adsorption measurements also testify that sodium hexmetaphosphate adsorbs easily on the surface of diaspore. The adsorption density of PO₃⁻on the surface of diaspore is about 4.7 × 10⁻⁶mol/m², while that on the kaolinite is only about 3.5 × 10⁻⁷mol/m²when pH value is 6.