Furnace Operation

After thorough washing and filtering, the wet powder is ready for furnace processing. The furnace operation also alters certain properties, particularly particle size and shape, apparent density, and green strength. In a typical operation, the powder is transferred into the charge box of a mesh belt electric furnace (Ref 7). To prevent the powder from falling through the belt, a continuous sheet of high wet-strength paper is fed to the belt, and then the powder is transferred to the paper. A roller compresses the powder to improve heat transfer. As it enters the furnace, water is driven off and the paper burns, but not before the powder has sintered sufficiently to prevent it from falling through the belt.

The furnace atmosphere is produced in exothermic gas units in which natural gas and air are blended to yield an atmosphere containing 17% H2, 12% CO, 4% CO2, and the balance of nitrogen. The gas is refrigerated to lower the dew point to the range of -22 to -40 °C (-8 to -40 °F). The gas enters the furnace from the discharge end and, because it is refrigerated, aids in cooling the powder cake. The furnace operation dries the powder, alters the particle shape, reduces the oxides, and sinters the fines. The discharge temperature is sufficiently low to prevent reoxidation of the powder cake.

By varying the furnace temperature between 480 and 760 °C (900 and 1400 °F) and altering the time of exposure, considerable change can be made in the content of fines, apparent density, and dimensional characteristics. Upon completion of the furnace operation, the cake is broken and is ready for grinding.

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