Patent GB-941184-A (GB)
Title: Improvements relating to the treatment of surfaces
IPC: B24C9/00
Filed: 1959-12-22
Abstract: 941,184. Grit blasting. W. E. BALLARD. Dec. 22, 1960 [Dec. 22, 1959], No. 43497/59. Heading B3D. [Also in Divisions B8 and H1] A grit blasting process is characterized in that the abrasive particles are made of a magnetic material and are recovered after impingement on the surface undergoing treatment by subjecting them to a magnetic field which causes or enables them to be moved to a collecting station, the particles being subjected to demagnetizing treatment to remove or reduce residual magnetism to an extent such that they can thereafter be employed in a grit blasting machine without adhering to each other to an extent which prejudices proper operation of the machine. A workpiece 10 disposed vertically is positioned at a treatment station into and out of which it is moved prior to and after grit blasting to clean or roughen its surface. The treatment station is surrounded by lower, upper and rear barrier walls 11, 12, 13, or vertically movable front barrier wall 14 and end walls having openings through which workpieces can be fed into and out of the treatment station. A stream of abrasive particles is projected by compressed air from a nozzle 16; several nozzles may be used. The abrasive particles are magnetic, e.g. chilled iron grit or steel particles, and spent particles accumulating on the floor 11 are removed by a collecting member 18 comprising a magnetic field structure adapted to be electrically energized and deenergized so that the collecting member can be magnetized and de-magnetized. The magnetic field structure comprises one or more pole faces which may be provided with a thin layer of non-magnetic material. The field structure and winding may be associated with a D.C. source and an A.C. source, the winding receiving the different forms of current alternatively to provide, respectively, for establishment of the magnetic flux and de-magnetization of the field structure. Separate windings may be provided for connection to the D.C. and A.C. sources, or a single winding may be used. The A.C. source may have means for reducing the supply to zero. The collecting member 18 may be supported by the forks 20 of a fork lift truck 19 or be carried by a light gantry crane. The spent abrasive particles are conveyed by the collecting member 18 to a collecting station comprising a hopper 22 delivering to a jigging conveyer 24 from which the abrasive particles pass through a de-magnetizing coil 30 into a hopper 32 and thence to a storage station from which they are returned ultimately to the grit blasting machine for further use. An alternating magnetic field is produced in the coil 30, this field being gradually reduced to zero either by movement of the abrasive particles out of the field or by gradual reduction of the field strength. When grit blasting an internal surface of a hollow article a magnetic field is set up in the interior of the article to cause movement of the particles on to a collecting member which is then withdrawn from the interior of the articles. The collecting member may be disposed stationarily within the article (but withdrawable therefrom) or adjacent the opening which affords access to the interior, the magnetic field being strong enough to cause particles from any part of the interior of the article to be attracted on to the collecting member; alternatively, the collecting member is capable of being traversed from place to place within the interior of the article where the size of the latter is larger than can conveniently be simultaneously energized by the magnetic field of the requisite strength. Demagnetization may be effected by heating the particles, with subsequent cooling by quenching. The process may be used for treating the interior faces of ships' tanks.