Wet process phosphoric acid (WPA) manufacturing is mainly via the dihydrate process. Phosphogypsum is the by-product. The size distribution of phosphogypsum is a major factor for the economics of the process as it greatly affects filtration efficiency of the acid. The goal of this study is to enhance slurry filtration by adding treated perlite during the phosphate rock digestion stage. This study was carried out using a semi-continuous laboratory scale unit simulating the dihydrate process conditions. Results showed that there was an optimum dose which depended on the type of perlite and it was 86 for raw perlite, 17.2 for calcinated perlite and 8.6 g/kg P2O5 for the activated perlite. The addition of activated perlite ensured high filtration and washing flows and high P2O5 yields through reducing the P2O5 losses especially the unattacked form which consequently improved plant productivity. Perlite proved to be a very effective additive for the growth of gypsum crystals it altered the morphology of the gypsum particles. Additional results indicated that the activated perlite led to decreasing the amount of fluorine (18.8%), aluminum (18.9%), iron (8.2%) and soluble organic matter (16.6%). However, it involved an undesirable increase of magnesium (14.6%) and silica (16.4%) contents. (C) 2013 Elsevier B.V. All rights reserved.