Hibbing Taconite is located near the center of the Mesabi Range in Northern Minnesota 100 miles from the Canadian border and 80 miles north of the Lake Superior Port Cities of Duluth and Superior.
Current owners of Hibbing Taconite are Bethlehem Steel Corporation, LTV Steel, Pickands Mather and Stelco. Inc. Pickands Mather, a subsidiary of Moore McCormack Resources, Inc., Stamford, Connecticut, also serves as manager of the operation.
The autogenous grinding flowsheet represents the latest in taconite plant design and metallurgical technology.
The flowsheet consists of one stage of crushing and one stage of grinding. Each of nine mill lines has one conveyor feed belt, an autogenous mill with an integral trommel, two slurry pumps, two stages of magnetic separation and one stage of cyclone classification. Mill concentrate flows by gravity to the concentrate thickeners prior to pelletizing.
Processing of crude ore begins in the crushing and stockpiling system. Ore is delivered to the crusher by 170 ton diesel/electric haulage trucks. The crushing plant incorporates two parallel 60″ x 109″ gyratory crushers.
Each of the mills is powered by two 6,000 horsepower wound rotor motors utilizing liquid rheostats for controlled torque starts. Each motor drives a pinion through a gearbox which in turn drives the main gear, the mills operate at 10.1 revolutions per minute or 78 percent of critical speed and are fed at approximately 420 long tons per hour. A normal charge is 35 percent of total volume.
The pulp density within the mill ranges from 65 to 70 percent. Slurry is discharged from the mill through 5/8″ slotted grates located at the outer periphery of the discharge head. Discharge lifters carry the slurry into an attached cylindrical trommel screen.
The rougher concentrate is laundered to the cyclone feed sump. A 16″ x 14″ 600 horsepower variable speed steel pump delivers the feed to a cluster of ten 15 inch diameter cyclones for classification. Coarse underflow is returned by gravity to the mill for further grinding and the tailing flows by gravity into the tailing launders.
The fine overflow fraction flows by gravity to the finisher distributor which then feeds five triple drum concurrent tank finisher magnetic separators. The primary drum is 700 gauss and the second and third drums are 500 gauss. Finisher tailing also flows by gravity into the tailing launder.
Classification control is based on the silica analysis. As the plant concentrate flows to the Agglomerator, it is sampled every seven minutes and analyzed by the on line silica analyzer located in our plant laboratory. The results are automatically relayed to a CRT located in the control room. The operator makes system adjustments to the mill circuits based on this data and trends. Water addition is controlled automatically to the cyclone feed sump to maintain a preset density. To stay within a given cyclone pressure range, the Control Room Operator may also add or subtract cyclones.
The program focused on improvements in both liner design and metallurgy. The most significant improvement in design was the reduction in the number of pieces used to line the mill. Originally outfitted with 1536 individual liners, design changes reduced this number by over 50% to 735. The reduction was accomplished primarily by replacing the original lifter bar and plate design with an integral lifter plate one-piece liner. Having fewer pieces to handle during a liner change resulted in less downtime for lining. The use of a modern hydraulic lining machine made handling of the heavier one-piece liners as easy as the original lighter lifter bars and plates.
Another improvement in mill productivity was accomplished by heating pulp in the grinding mills. Studies showed that there was a significant drop in milling rates during cold weather months. Since all process water is recovered from the reclaim area in the tailing basin, it is subject to ambient conditions.