GB417085A - Improvements relating to grindstones and processes of treating same - Google Patents

Abstract

417,085. Treating sandstone grindstones. McGILL, W. H., 21, Oxford Street, Winchester, Massachusetts, U.S.A. Nov. 11, 1933, No. 31510. [Class 60.] Grindstones of natural sandstone, such as are used for making wood pulp, are impregnated through at least the service volume with a waterproof binder which retains its binding property at the operating temperature of the stone, disintegration of the stone during grinding by moisture and heat being therefore avoided. In one impregnation process, the stone 2 is baked to drive out moisture and the ends of the central bore are closed by plates 4 and gaskets 8. The stone is then placed in a pressure tank 3, the central bore communicating with the external atmosphere by a tube 15 connected to a vent 13 in one of the plates 4. The end faces of the stone are preferably given a sizing coat 10 of a material impervious to the binding medium. The liquid binding medium is introduced into the tank and air under pressure is admitted through a pipe 20. A vacuum may be created in the tank before the liquid is introduced. The liquid is forced by the pressure radially inwards through the stone from the periphery to the central bore. In a typical case an initial pressure of twenty-five pounds to the square inch is applied and is increased by ten pounds at the end of half an hour and to fifty pounds at the end of an hour, this final pressure being maintained until liquid is delivered through the pipe 15. Preferably a portion of the liquid is then withdrawn from the stone by emptying the tank and maintaining the air pressure, the liquid being forced out into the central bore and withdrawn through the pipe 15. This partly empties the pores in the stone, but leaves their walls covered with the solution ; it also improves the distribution of the binder in the stone. The stone is then allowed to drain, and is finally treated in accordance with the nature of the binding medium. For example, the medium may comprise a phenol-formaldehyde resin dissolved in alcohol with a small percentage of plasticizers, in which case the stone is baked to drive off the alcohol and convert the resin to its insoluble form. Other binders which may be used are high melting point waxes, such as carnauba, candellila and montan, or silicate of soda. The impregnation may be confined to the outer parts of the stone by reducing the time of treatment and then ejecting part of the medium by forcing air into the bore of the stone. The direction in which the medium is introduced may also be reversed.