The permeability of a phosphoric acid hemihydrate crystal bed as well as the crystal yield and the crystal purity were studied in order to find a suitable range for the operation of a hydraulic wash column. The key parameters were the width of the crystals as a measure of crystal size, the supersaturation, and the stirring speed. The results show that a crystal bed which contains larger crystals gives a higher permeability. A higher supersaturation leads to a higher crystal yield but more impurity inclusions in crystals. A higher stirring speed causes more impurity inclusions in crystals, as well.