GB1487325A - Electrostatic deposition of particles - Google Patents

Abstract

1487325 Electrostatic spraying IMPERIAL CHEMICAL INDUSTRIES Ltd 13 Nov 1974 [21 Nov 1973] 53939/73 Heading B2F [Also in Divisions H1 and H2] Apparatus for electrostatically depositing particulate material, comprises a duct 10, 12, 20 forming a closed circuit for particles suspended in a non-conducting fluid, a capacitor 22 located around the duct and electrically insulated to avoid charge leakage, a needle 24 electrically connecting the capacitor with the inside of the duct for transferring charge from the particles in the duct, which are charged tribo electrically and/or by corona discharge, to the capacitor, and a tube 32 having a nozzle 34 through which a separate supply of the particulate material to be deposited is projected into the vicinity of the charged capacitor, said particulate material having been charged triboelectrically and/or by corona discharge to the same polarity as the capacitor so that said material is repelled from the capacitor to be deposited on an object 36. Alternatively, a part of the charge on the capacitor surface may be used to charge the material to be sprayed. Parts 10, 20 of the duct comprise aluminium tubes, part 12 being of an insulating material such as polymethylmethacrylate. The powder in the closed circuit duct may comprise silicon carbide. Earthed needles may project into part 12 to increase the potential obtained on the capacitor. Baffles may be provided in tubes 10, 20 to increase the turbulence. Alternatively, tubes 10, 20 may comprise resin-impregnated fabric having an inner rubber lining. In a modification, the entire duct is of non-conducting material, e.g. polyethylene, an internal coaxial earth wire or earthed needles projecting through the walls of tubes 10, 20 being provided to remove surface charge from the duct walls. The tubes 10, 20 may be twisted around each other to provide a helical flow path. To effect a corona discharge in the duct, needle or knife edge electrodes extend into the charging region of the duct being insulated from the duct walls and a voltage of 1 to 20 KV is applied between the electrodes and the duct wall. Alternatively, the electrode may comprise a conducting wire extending within and axially along a length of the duct. the wire being earthed and the inner face of the duct charged to 5 to 25 KV. The capacitor may be spheroidal or toroidal. The apparatus may be used for spraving crops, coating metals with plastics, coating rubber with talc and for the application of corrosion inhibitors such as borax to metal. Powders of solids or liquid droplets may be sprayed and/or conveyed in the duct.