An interesting instance of adopting a process to changed conditions is found in a new apparatus for coating metals with zinc film. The Metals Coating Company of America has developed the Schoop metal spraying process in which zinc wire or other metal wire is melted in a gas flame and projected under pressure onto the surface to be coated. Under present conditions in the metal market this wire has become exceedingly expensive. To fill the need for a less costly process, the apparatus has been modified to permit the employment of zinc in the less costly form of dust. It consists of a conical receptacle which has another similarly-shaped receptacle inside, with a small space between. Zinc dust is placed in the inner cone and compressed air is admitted through a hose at the top of the space between the two cones. A valve at the bottom of the inner compartment allows the zinc dust to be caught up by the air current and carried through a discharge tube to a spraying device. The zinc dust is there blown from an inner nozzle through an acetylene gas flame, the heat of which is sufficient to melt the zinc. In the melted condition the particles of zinc are projected at high velocity upon the surface to be coated by the action of the blast. Adjustable valves regulate the quantity of zinc dust allowed to pass into the air current and also the quantity of gas used to melt the metal.
The apparatus weighs about twenty pounds and uses air at a pressure of about five pounds per square inch. Acetylene gas, the heating agent, is easily available, and has been found to give good results with zinc dust. The present cost of zinc wire is about $1.25 per pound, while zinc dust, which is a by-product of the zinc smelter, may be obtained at about thirty cents per pound. The portability of the apparatus adds to its usefulness for work in the field in coating structural or bridge steel with a film of zinc to resist corrosion. Even at ordinary price levels the use of zinc dust is less expensive than wire, a feature which considerably broadens the application of this method of coating and makes possible its use on much larger surfaces than when zinc wire is used.