Release behavior of iodine during leaching and calcination of phosphate rock.

A series of experiments of column leaching under different pHs (pH 1.8, 3.8, 6.5, and 8.5) and calcination at different temperatures (200-1100 °C) were carried out for evaluation of release behavior of iodine in phosphate rock. The modes of occurrence of iodine in the phosphate rock and its leaching and calcination residues were extracted with sequential chemical extraction. Iodine in solution and solid samples was measured with ion chromatography (IC) and pyrohydrolysis combined ion chromatography (PIC), respectively. Mineralogical compositions of phosphate rock and the leached and calcined residues were determined by XRD (X-ray diffraction) and FT-IR (Fourier infrared spectrum). The results show that iodine in phosphate rock occurred in a descending order of significance, as forms of residual, carbonate bound, ion-exchange, organic bound, Fe-Mn oxide bound, and water soluble. Under pH 1.8, 3.8, 6.5, and 8.5, the release iodine may almost reach the maximum at the leaching time of 65, 93, 90, and 165 h, with leaching rates of 5.28%, 1.24%, 0.550%, and 1.08% and the average iodine concentrations in the leachates of 2300 μg/L, 378 μg/L, 164 μg/L, and 189 μg/L, respectively. The forms of the leached iodine were mostly ion-exchange and carbonate-bound iodine under pH 1.8 and water soluble and ion-exchange iodine under pH 3.8, 6.5, and 8.5. By calcination, the total iodine was released rapidly in 200-300 °C and 700-1000 °C, and almost released completely at 1000 °C, with a leaching rate of 96.6%. The ion-exchange and organic-bound iodine were, respectively, released at 200-1000 °C and at less than 300 °C; the carbonate-bound and residual iodine were mainly released at more than 700 °C. The release iodine in phosphate rock leached by natural water and calcined at a high temperature may lead to the increase of iodine concentration of water body and atmosphere.