IL46037A - Method and apparatus for the electrostatic deposition of particulate material - 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. [GB1487325A]

Claims (3)

1. *- 1. Apparatus for electrostatically depositing particulate material and comprising in combination a duct defining a closed loop path for a flowing medium; means for causing a flow through the duct of a medium comprising particulate material suspended in a non-conducting fluid vehicle, the arrangement being such that electrical charges are induced triboelectrically on the particulate material carried by the vehicle along a length of the duct; capacitor means located outside the duct and electrically insulated so as to avoid charge leakage; electrical discharge means electrically connected to said capacitor means and located in but electrically insulated from the duct for transferring electric charge from the charged particulate material o said capacitor means, whereby to build up a charge on an outside surface thereof; and means separate from said duct for suspending a separate supply of particulate material in a gas flow, electrically charging said material with a charge of the same polarity as that induced on the outside surface of the capacitor means and projecting the flow of charged particulate material adjacent said capacitor surface.

2. Apparatus for electrostatically depositing particulate material and comprising in combination a duct defining a closed loop path for a flowing medium; means for causing a flow through the duct of a medium comprising particulate material suspended in a non-conducting fluid vehicle; means for inducing an electrical charge on the particulate material carried by the vehicle through the duct; capacitor means located outside the duct and electrically insulated so as to avoid charge leakage; electrical discharge means electrically connected to said capacitor means and located in but electrically insulated from the duct remote from said charging means for transferring electric charge from the charged particulate material to said capacitor means, whereby to build up a charge on an outside surface thereof; and means separate from said duct for suspending a separate supply of particulate material in a gas flow, electrically charging said material with a charge of the same polarity as that induced on the outside surface of the capacitor means and projecting the flow of charged particulate material adjacent said capacitor surface.

3. Apparatus as claimed in claim 2 in which the electrical charge inducing means includes at least one electrode extending into the duct and electrically insulated from the adjacent duct wall, and means for establishing a corona discharge between said electrode and the adjacent duct wall. it. Apparatus as claimed in claim 1 in which one part of the duct wall is of electrically conductive material or other material capable of dissipating electrical charge built up on said part of the duct wall and another part of the duct wall is of electrically non-conductive material, and the electrical discharge means extends through said non-conductive part. £. Apparatus as claimed in claim k in which a part of the length of the duct is formed of electrically non-conductive material and the capacitor means is located adjacent that length. 6. Apparatus as claimed in claim $ in which the duct includes two tubes of electrically conducting material connected together by a tube of electrically non-conductive material. 7. Apparatus as claimed in claim 6 in which the two tubes of electrically conducting material extend parallel to each other and in close proximity to each other and the tube of electrically non-conductive material comprises a U-bend connecting the adjacent ends of said tubes of electrically conducting material together, said capacitor means being in the form of a toroid fitting over the two arms of the U-bend. 8. Apparatus as claimed in claim 6 or claim 7 in which the two tubes of electrically conductive material are twisted about one another in helical fashion. 9. An apparatus as claimed in any of claims k to 8 including a number of earthed needles projecting into the tube through said non-conductive part of the duct wall upstream and/or downstream of said electrical discharge means. 10. Apparatus as claimed in any of claims h to 9 in which at least the inner face of said first mentioned part of the duct wall has a soft surface. 11. Apparatus as claimed in claim 10 in which said first mentioned duct wall part comprises a duct length formed of resin impregnated fabric the inner surface of which is lined with rubber. 12. Apparatus as claimed in claim 1 or claim 2 in which the inner face of the duct for substantially the entire length thereof is of soft electrically non-conductive material, and means are provided remote from the electrical discharge means for removing surface charges from the wall of the duct. 13» Apparatus as claimed in claim 12 in which said charge removing means comprise an earthing wire extending axially of and within the duct and/or earthed corona discharge electrodes protruding into the duct at spaced intervals. 1 . Apparatus as claimed in any of claims 1 to 13 wherein the electrical discharge means comprise corona discharge means extending into the duct. 15>. Apparatus as claimed in any of claims 1 to 13 wherein the electrical discharge means comprise a length of duct the inner face of which is formed of electrically conducting material which is electrically insulated from the remainder of the duct. 16. An apparatus as claimed in any of claims 1 to 1^ in which the capacitor is toroidal and surrounds a part of the length of the duct. 17. An apparatus as claimed in any of claims 1 to 16 including means for increasing the turbulence of the flow of the medium in the duct. 18. Apparatus as claimed in claim 17 in which said turbulence increasing means comprise baffles located in the flow path. 19. Apparatus as claimed in any of claims 1 to 18 in which the means for electrically charging the particulate material to be 20. Apparatus as claimed in any of claims 1 to 18 in which the means for electrically charging the particulate material to be projected adjacent said capacitor surface includes corona discharge means. 21. An apparatus as claimed in any of claims 1 to 20 including a reservoir for the separate supply of particulate -material . 22. A method of electrostatically depositing particulate material on to an earthed surface, said method comprising in the vicinity of the earthed surface directing a flow of gas carrying charged particulate material adjacent to a capacitor surface which is charged to ε. high voltage of the same polarity as the charge on the particulate material by transferring to said capacitor electrical charges developed on a separate particulate material which is being transported by a non-conducting fluid medium round a closed circuit path which includes a region where the particles are charged . remote from where the charge is transferred to the capacitor, 23. A method as claimed in claim 22 in which the separate particulate dielectric material is charged triboeletrically and/or by corona discharge. 2k* A method as claimed in claim 22 or claim 23 in which the non-conducting fluid medium is air and the separate particulate dielectric material comprises a powdered solid. 2$. A method as claimed in any of claims 22 to 2i; for electrostatic, coating of products with particulate material. 26. A method as claimed in any of claims 22 to 2k for applying particulate material to vegetable growth. PARTNERS