Фізико-хімічні властивості залізо(II) сульфат гептагідрату як чинники для вибору режиму процесу сушіння в апараті киплячого шару

The object of the study is iron(II) sulfate heptahydrate – the main solid waste product of titanium dioxide sulfate production, the accumulation of which poses a significant environmental threat. The problematic stage of its processing is the technological stage of dehydration in a fluidized bed to a monohydrate form, for which it is important to select an acceptable hydrodynamic regime and drying regime. Experimental studies included microscopic, sieve, pycnometric and titrimetric methods of analysis. The average equivalent particle diameter was found to be 0.50 mm, with a shape factor of 0.75. The bulk density of the material is 911 kg/m3, and the true density is 1888 kg/m3. The free moisture content was found to be 2.2%, and the crystallisation moisture content was 38.7%, which corresponds to the heptahydrate form of FeSO4 · 7H2O. Chemical analysis showed that the mass fraction of FeSO4 in the samples ranges from 48.8% to 51.7%, and the Fe2+ content is 18%. Free sulphuric acid is present in an amount of 0.3–1.3%. Granulometric analysis revealed significant polydispersity of the material, in particular the presence of agglomerates and fine fractions in samples No. 1–3 of the closed storage composition of iron(II) sulfate heptahydrate. For sample No. 4, which was characterised by the most uniform particle distribution, the minimum fluidization velocity of the largest particle fraction (0.7 m/s) and the fluidization velocity (0.97 m/s) for the equivalent particle diameter of the material were calculated. It was found that particles with a diameter of less than 0.207 mm will be carried out of the boiling layer, which requires additional measures to reduce material losses. The heat transfer coefficient for particles of intermediate fractions (0.315–1.6 mm) is 77.79–349.17 W/(m2 · K), which ensures efficient heat exchange during the drying process. Based on the data obtained, the choice of a horizontal sectioned fluidized bed apparatus is justified. The proposed design provides for the division of the process into independent zones with individual control and regulation of the drying agent parameters (temperature and flow rate). This makes it possible to obtain a stable hydrodynamic regime for polydisperse materials and reduce the influence of mixing on the driving force of the process. The results obtained allow predicting the behaviour of the material in the fluidized bed apparatus and calculating the fluidization and drying regimes.