A rotary milling system includes a rotatable cylinder, a discharge grate, a movable flap valve into and out of engagement with the grate, a discharge housing surrounding the grate, and a conveying pipe extending from the discharge housing. The cylinder can include grinding media for abrading a product when the cylinder is rotated. The product can be in either a dry form or within a liquid medium. Upon conclusion of the milling, the flap valve can be opened to allow the milled product to pass through the grate and into the discharge housing. A negative pressure can be applied to draw the milled product through the conveying pipe into a separator tank.