Balling Circuit Modifications to Improve Pellet Sizing


Erik Lacksonen, US Steel

United States Steel’s Minntac facility produces flux pellets on the Iron Range in northern Minnesota.  Minntac is the largest pellet producer in North America and operates five grate-kiln lines in its pelletizing plant fed by 24 balling drums.   As a continuous improvement initiative, balling circuit modifications were investigated with the goal of improving the percentage of on-size pellets (%-1/2 in. +3/8 in.).  The parameters that were studied included roll screen gapping, roll screen speed, and drum speed.  Extensive testing of different roll screen seed and product gap combinations was conducted until the gapping which provided the best green ball % on-size without significantly impacting recirculating load stability was discovered.  The resulting seed and product gap adjustment increased the green ball % on-size by 3.5%.  Doubling the roll screen speed from approximately 140 rpm to 280 rpm further improved the % on-size by an additional 8.8%.  Changes to the balling drum speed did not produce any statistically significant improvements to green ball % on-size.  The plant is currently undergoing conversion to the new roll screen gapping and increased roll screen speed.  The potential impact of improved permeability in the furnace is also being studied as the conversion of each grate kiln lines is complete.
Keywords: United States Steel, Minntac, Iron Ore, Grate, Kiln, Balling Drums, Quality, Sizing, Roll Screens, Screening