This research presents a simulation for using the hydrocyclone as a thickener in the precipitation of sulfate ions from waste water as gypsum by CFD – Ansys Fluent 2020. Experimental data of the conditions: inlet flowrates(3 and 11 Lmin-1), inlet concentrations of sulfate ions (100 and 400 mg.L-1) and split ratios (0.1 and 0.9) were adopted from earlier experimental work. Simple theoretical equations were used to model velocity and density for particles precipitation in the hydrocyclone. The simulation tracked the precipitation process. An attempt was made to connect the experimental, simulated and theoretical aspects. It was concluded that Ansys Fluent 2020 was capable to describe the precipitation process. This would be helpful to identify the design and operating conditions.