In this work it is shown how the methods used in chemical engineering for the analysis and simulation of chemical processes may be applied to the actual phosphoric acid plant. Attention has been focused on the dehydrate process for which the necessary experimental data and plant operation data are available. It has been shown that the knowledge of some fundamental parameters is sufficient in order to obtain a reasonable description of the process. Effects of the recycle acid from washing filter cake and the slurry recycle ratio were examined. The operation range of the amount of recycle acid, which controls the solid/ liquid ratio in the reaction slurry, could be determined based on the solubility product of calcium and sulfate ions.