In more recent process, atomized aluminum powder is made from high grade aluminum ingots free from iron and silicone impurities. The aluminum ingots are melted in a reverberatory furnace which consists of two parts, the molten metal reservoir and atomizing well. The molten metal is atomized with hot compressed air is heated by an electric heating element. The atomized aluminum issuing from the nozzle is conducted through an aluminum duct to a separator where the major quantity of the product is separated from air. Fine powder not separated in the first stage is further passed through a series of cyclone which remove most of the powder. Finest particles which escape the cyclones are vented into the atmosphere.
The powder from the first separator passes through a vibrating screen to remove the coarse particles which are not used for pigment manufacture. The screened powder which is usually 325 mesh in size (spherical in shape) is collected in drum and taken to the paste section. The coarse particles are graded by sieving and other methods of classification for use in various applications. Atomized powder along with stearic acid and mineral spirit is charged into a ball mill containing a definite charge of polished steel ball of varying dimension. The ball mill are often jacketed for circulation of cold water to keep the milling temperature within dieted storage tank. The milling of aluminum granules into flakes required somewhat different conditions from those used for ball mill dispersion of pigments in paints. For aluminum milling the balls are required to deliver a hammer blow or impact action on the particles. The ratio of balls to powder must be adjusted properly and the mill must be run at the correct speed to obtain adequate cascading of the balls and to avoid attrition. Usually stearic acid used as the lubricant but other fats and oils may also be used. Similarly, in place of mineral spirit, solvent naphtha or certain plasticizers may be used as the liquid medium during milling for specific applications.
After milling, the paste is washed with mineral spirit. The batch is screened to remove coarse particles and then pressure filtered. The press cake may be dried to remove excess solvent; alternatively, the solids content of the cake is adjusted by adding the liquid medium (mineral spirit, etc.) to produce aluminum paste. The solid content of commercial aluminum paste is usually 65 percent. Mineral spirit may be replaced wholly or in parts by a solvent more compatible with a particular paint medium. The foregoing process products the leafing type of aluminum pigments. These may be treated to convert them into non-leafing type directly.
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