In the previous studies, it has been found that there is a significant influence of the solids on the liquid phase mixing time at high solid concentrations. This is due to the appearance of a clear liquid layer above the cloud solid layer at a certain specific power input. The state of solids suspension and the mixing time affect the purity, productivity and selectivity of the reaction and the size distribution and morphology of the precipitation and crystallisation. Consequently, more information in these conditions is necessary in order to operate properly the reactor handling particle suspensions. In this study, the effect of D/T ratio (the ratio of impeller to tank diameter) on solid suspension and mixing time has been investigated in a stirred tank equipped with Chemineer HE-3 impeller, which is typically used for solid-liquid mixing at high solid concentrations. The maximum mixing time for the largest D/T employed in this study is the longest despite almost the same specific power input and height of interface. However, to use the large D/T has led a high degree of homogeneous suspension and fast mixing time with a small increase in power input. On the contrary, large power input is necessary to disappear the cloud layer when the small D/T is used.