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. (C) 2013 Elsevier B.V. All rights reserved.