A mixing-precipitation model (based on engulfment, the E-model by Baldyga and Rohani, 1987) is applied to a single-feed semi-batch precipitation proccss to describe the mixing effects on the final particle-size distribution (PSD) and particle morphology. The model describes mixing on two scales: the microscale (at which the flow directly affects nucleation and growth phenomena in the mixed-precipitation (MP) zone) and It macroscale (which controls the environment for the MP-zone). Application of the model is limited to 1 much less than Sc < 4,000 and slow feed addition rates. When the model was applied to experimental results of BaSO4 precipitation, satisfactory agreement between predicted and observed results was obtained Experiments were carried out in a single-feed 18-L Rushton reactor, whereby the effects of various operating conditions such as feed rate, intensity of mixing, mean initial reactant concentration and reactant volume ratio on barium sulfate precipitation process were investigated.