Why do relatively coarse calcareous phosphate particles perform better in a static-bed calciner?

The calcination process is a direct and clean process for upgrading of phosphate ores with high-carbonate contents. In this study, calcination experiments of calcareous phosphate were conducted. Different size fractions were used to evaluate their performance while heating in a static-bed furnace at different calcination times and temperatures. The results showed that the calcined product of coarse particles gives a higher P2O5%. A phosphate concentrate with a grade as high as 34% P2O5 was obtained. Although the larger particle size is higher in grade, the finer particles showed a higher conversion. This behavior was correlated to the change in bed-structure in terms of the particle porosity, surface area and bed permeability due to exposure to high temperatures. The fast fall in both particle porosity and bed permeability in the case of fine particles leads to the capturing of the generated CO2, as a calcination product, inside the individual particle or the entire bed till this gas forms an appreciable pressure gradient that finds its way through the cracks that appear only in fine particle bed. Calcination of calcareous phosphate ores in a static-bed furnace is particle-size dependant. Not only as a particle size but also as its effect on bed-structure, i.e. particle porosity, surface area and bed permeability. The fast fall in particle porosity and bed permeability leads to the capturing of the generated CO2, reaction reversibility as well as bed cracking. [Display omitted] ► Calcareous phosphate calcination in static-bed furnace is particle size dependant. ► The particle size has its own effect beside its effect on the bed permeability. ► The calcination of larger sizes seems better than finer ones. ► Phosphate concentrate with 34% P2O5 was obtained in case of coarse particles. ► Fines sintering leads to the capturing of generated CO2 and reaction reversibility.