Abstract: The invention relates to a grinding circuit for use in a flue gas desulfurization system. The circuit comprises a mill for grinding limestone in water to form a limestone particle slurry. The thus-formed slurry is pumped to a hydrocyclone classifier which separates the limestone slurry into a slurry containing oversized particles that are to be reground in the mill and a slurry containing particles suitable for product which are sent to a product tank inlet for subsequent use in the flue gas desulfurization system. In the present system, the hydrocyclone classifier is elevated above the feed inlet into the mill but below the product tank inlet and, consequently, while the oversized particles are directed by gravity from the hydrocyclone to the mill feed inlet, the particles suitable for use in the flue gas desulfurization system are pumped from the hydrocyclone outlet to the product tank inlet.