New apparatus and methods of grinding applicable in particular to the titanium dioxide

摘要

1, 040, 798. Wet grinding; continuous mixing appliances. BRITISH TITAN PRODUCTS CO. Ltd. Aug. 2, 1965 [Aug. 22,1964], No. 3444/64. Headings B1C and B2A. Apparatus for milling a particulate solid in a liquid comprises a closed vessel 1 having inlet and outlet orifices 7, 8 and a shaft 6 carrying a rotatable impeller 10, the angle between the plane of the impeller and the longitudinal axis of the shaft 6 being an angle other than a right angle. The angle is preferably in the range 70 degrees to 89¢ degrees. The impeller is provided with holes 13 which are preferably at 45 degrees to the faces of the impeller. The impeller and vessel may be made from, or coated with, polyurethane rubber, polyethylene or polypropylene. The inlet orifice may, alternatively, be in the periphery of the vessel. The surface of the impeller and the interior of the vessel may be serrated. The distance between the surface of the impeller and the interior wall of the vessel may be in the range 0. 25 to 6 inches. The impeller may have a flat central portion the plane of which is at right angles to the longitudinal axis of the impeller shaft and an outer portion the plane of which forms an angle other than a right angle with the longitudinal axis of the impeller shaft. There may be more than one impeller. The apparatus may comprise a plurality of mills the outlet of one being connected to the inlet of another by means of a conduit and means are provided for introducing into the conduit a diluting liquid or a chemical e.g. a dispersing or flocculating agent and means are provided for cooling the conduit. The particular solid may be milled in the presence of a particulate grinding medium e.g. zircon or silica sand, which has a particle size in the range 25 to 3000 microns. The peripheral speed of the impeller is preferably in the range 10 to 120 ft/sec. The particles are introduced into the vessel in the form of a slurry preferably at a concentration in the range 250 to 2000 grams/litre. The dispersing agent may be present in the slurry in a concentration in the range 0.05% to 2.5%. The amount of particulate milling medium introduced into the vessel may be 0. 5 to 3 times that of the volume of the slurry of liquid and particulate solid to be milled. The particulate solid to be milled may consist of titanium dioxide particles which have been leached. The average retention time of the particulate solid to be ground is preferably in the range 10 to 50 seconds.