The U.S. Bureau of Mines, in cooperation with Cleveland-Cliffs, Inc. (Hibbing, Minnesota), investigated ways of enhancing the quality (compressive strength, after-tumble, and reducibility) of domestic acid and fluxed magnetite pellets by modifying the oxygen content during the preheat and induration periods of the firing operation. The rate of magnetite oxidation increased directly with the gas oxygen content during the preheat period. With at least 30 pct o2 and a preheat rate of 200 deg c/min, most of the magnetite was oxidized during the preheat period. At lower o2 levels or higher preheat rates, more coalescing of magnetite particles occurred in the pellet core, resulting in shrinking and cracking of the magnetite in the pellet core. Cracked and weak pellets resulted. With laboratory tube and minipot furnace tests, oxygen enrichment during the preheat period improved the pellet properties more in the simulated grate-kiln tests than in the simulated straight-grate tests. The longer induration period with the grate-kiln test resulted in more sintering of the residual magnetite and its reaction with silicon compounds. When flux was present in the pellets, calcium silicates and calcium and magnesium ferrites were formed. More calcium ferrite was formed when the magnetite was oxidized early and less iron ended up in the fayalitic calcium silicate slag. Both acid and fluxed pellets had higher compressive strengths, after-tumble values, and reduction rates when the magnetite was oxidized early during the firing period.
Proceeds 75th Steelmaking, 51st Ironmaking, & 10th Process Tech. Div. Braun-brumfield, Mi