The Tilden iron deposit is an example of a Lake Superior-type BIF deposit.

Tilden is dominated by a 100 m-scale, northwest-plunging incline. The hinge line of the incline dips steeply north, plunges 30°NW, and runs down the center of the Main Pit. The hinge line of the incline is mapped locally coincident with the Keweenawan Dike. The Summit Mountain Sill, locally termed the Pillar Intrusive, defines the asymmetry and orientation of the incline. Smaller faults and folds, on a scale of one meter to 20 m, are observed in the Main Pit to follow trends of larger, regional-scale structures. These structures tend to reflect ductile deformation in the Main Pit, where folds with sheared limbs are common (Lukey et al., 2007).

The primary ore mineral at Tilden is hematite, with other minerals including martite (oxidized pseudomorph of magnetite), goethite, and siderite (iron carbonate mineral), as opposed to strictly magnetite.

At Tilden, the Negaunee IF can be divided into five distinct facies:
• Clastic Iron Formation (IFCL) Units: Varying thickness of interbedded slate with laminated chert, iron silicate, and siderite. Clastics have a lower weight recovery (wtrec) due to the presence of interbedded clastic material. They are highly oxidized in the east side of the Main Pit.
• Carbonaceous Iron Formation (IFCB) Units: Alternating thin layers of magnetite, martite (oxidized pseudomorph of magnetite), iron silicate minerals, iron carbonate mi ........

All of iron ore mining operations are open-pit mines.

Tilden Mine
The Tilden deposit is mined using conventional surface mining methods, with surface operations including:
• Overburden (glacial till) removal;
• Drilling and blasting (excluding overburden);
• Loading and haulage;
• Crushing and rail loading.

Tilden Mineral Reserves are based on ongoing annual crude ore production of 20 MLT to 22 MLT producing approximately 7.7 MLT of wet hem flux pellets for domestic consumption.

The current LOM plan has mining scheduled for 25 years and mines the known Mineral Reserve. The average stripping ratio is approximately 1.2 waste units to 1 crude ore unit (1.2 stripping ratio).

The final Tilden pit is a single pit approximately 2.5 mi along strike, up to 0.9 mi wide, and up to 1,980 ft deep.

United Taconite mine
United Taconite operations consist of an open pit truck and shovel mine where two stages of crushing occur before the ore is transported by rail, operated by CN, to the plant site.

The Thunderbird Mine North (TBN) and Thunderbird Mine South (TBS) are mined using conventional surface mining methods. The surface operations include:
• Overburden (glacial till) removal.
• Drilling and blasting (excluding overburden).
• Loading and haulage.
• Crushing and rail loading.

The Mineral Reserve is based on the ongoing annual average crude ore production of ap ........

TILDEN
Primary crushing, which is operated and maintained by the mining department, is accomplished with a 60 in. x 109 in. Allis-Chalmers gyratory crusher operated to produce a nominal -9 in. crushed product, which is conveyed to the ore storage building ahead of the grinding circuit. Primary grinding is accomplished with eleven, 27 ft-diameter x 14.5 ft-long AG mills, each driven by two synchronous motors that have a combined output of 5,720 hp. Each primary AG mill discharges to a triple-deck screen, producing a -1.5 in. x 0.5 in. product that is used as a grinding media in the pebble mills (excess diverted to the pebble crushers or recirculated back to the primary mill), a -0.5 in. x 2 mm product that is conveyed back to the primary mill, and a -2 mm product that is advanced to the secondary pebble mills. The -2 mm discharge from each AG mill feeds two, 15.5 ft-diameter x 32 ft-long pebble mills, which are operated in closed circuit with a cluster of nine, 15 in.-diameter cyclones to produce a final grind of 80% to 85% passing 25 µm. Caustic soda and slaked lime are added to the water circuit to control pH prior to desliming and flotation.

Fluxstone consisting of dolomite and calcite is received at Tilden via truck and stored in stockpiles. Material is fed from a stockpile via apron feeders and processed in two, 15.5 ft-diameter x 30 ft-long ball mills. The fluxstone slurry is added to the iron concentrate prior to filtering to ensure homogenous mixing.

UNITED TACONITE
Crude ore is blended at the Thunderbird Mine and hauled to the primary crushing station, where it is dumped by 240 ton haul trucks into the 60 in. x 89 in. primary gyratory crusher, followed by secondary crushing in three, 30 in. x 70 in. secondary gyratory crushers located directly beneath the primary crusher. The P80 4 in. product-size material is conveyed to a 20,000 LT, conical surge pile. The surge pile is covered to avoid handling difficulties during extremely cold weather. Crushed ore is reclaimed from the surge pile by apron feeders and a conveyor located in a tunnel beneath the pile and conveyed to rail car loading silos. The material is loaded into rail cars and transported by train to the Fairlane Facility, eight miles away. The average feed rate of the primary crushing station is 3,200 LT/h.

Two additional stages of crushing are provided at the Fairlane Facility. The third stage consists of five Nordberg, seven-foot shorthead crushers operating in parallel and open circuit followed by screens producing a P80 one inch (25.4 mm) product. The P100 0.5 in. (12.7 mm) screen undersize material from the third stage is combined with the screen undersize from the fourth stage to make up the final crusher product to the concentrator. Third-stage screen oversize material feeds the fourth stage of crushing, which comprises eight Nordberg, seven-foot shorthead crushers operating in parallel and in closed circuit with screens, producing the final 85% to 90% passing 0.5 in. product. The average throughput is 50,000 LT/d. Specific power consumption is 3.1 kWh/LT.

The fine crusher product is processed in five separate rod mill – ball mill grinding and magnetic separation lines to produce final magnetite concentrate with a particle size distribution of 76% to 86% passing 325 mesh.

Lines 1 and 2
Grinding lines 1 and 2 have average feed rates of 345 LT/h at 90% operational availability. The two rod mills in lines 1 and 2 are 14 ft-diameter x 20 ft EGL (equivalent grinding length), Nordberg overflow mills operated in open circuit with 2,000 hp motors. Four ball mills (14 ft diameter x 22 ft EGL) are operated in closed-circuit with 26 in.-diameter cyclones.

Lines 3, 4 and 5
The flowsheet for Lines 3, 4, and 5 is similar to lines 1 and 2, with higher average feed rates of 435 LT/h per line at 90% operational availability. The three, 15 ft-diameter x 21 ft EGL rod mills are operated in open circuit and discharge through two 4 ft x 10 ft, 1,200 Gs cobber magnetic separators per line. Cobber tailings are final tailings. The cobber concentrate is advanced to the ball mill grinding circuit, which consists of three, 17 ft-diameter x 42 ft EGL ball mills (one per line) operated in closed circuit with cyclones and screens. The ball mills discharge to twelve, 4 ft by 10 ft rougher magnetic separators (four per line). Rougher tailings are final tailings and are discarded to the tailings hydroseparators.

Fluxstone Grinding Circuit
Fluxstone, a 50%/50% mixture of limestone and dolomite, is ground using a 14 ft-diameter x 20 ft EGL Nordberg overflow ball mill when Mustang flux pellets are being produced.

NORTHSHORE
The Mine and primary and secondary crushing plant are located in Babbitt, Minnesota and the tertiary and quaternary crushing plant is located in Silver Bay, Minnesota. Mine haul trucks dump the crude ore directly into a 60 in. x 89 in. primary gyratory crusher. The primary crushed crude ore falls directly into the four, secondary 30 in. x 70 in. gyratory crushers located directly beneath the primary crusher, and is crushed to -4 in. The -4 in. material is conveyed into rail car loading bins and then loaded into trains and transported to Silver Bay, Minnesota, where the tertiary and quaternary crushing stations, the concentrator, and the pellet plant are located. Upon arriving at Silver Bay, the secondary crushed crude ore (-4 in.) is dumped from the rail cars by automated, two-car dumpers and transported by belt conveyors to the tertiary-quaternary crushing plant storage silos. The crude ore is drawn from the silos and crushed to -0.75 in. in tertiary and quaternary Nordberg 7 ft shorthead cone crushers and then passed over double-drum dry cobbers for primary magnetic separation.

HIBBING TACONITE
The crushing plant consists of two 60 in. x 109 in. Allis Chalmers gyratory crushers that crush run of mine (ROM) ore to minus 10 in., which is then conveyed to the 450,000-ton, crushed-ore stockpile, referred to as the COSP, providing up to five days of crushed ore surge capacity ahead of the concentrator. Crushed ore is reclaimed from the COSP to feed the primary grinding circuit, which consists of nine, 36 ft x 15 ft autogenous grinding (AG) mills, which grind the ore to - 3/16 in.

The screen oversize from all grinding lines is reground in two, 1,250 hp Vertimills operated in closed circuit with cyclones to produce a cyclone overflow of 90% passing 44 microns, which is then subjected to a second stage of finisher magnetic separation.

MINORCA
The primary crusher is a 54 in. x 84 in. gyratory crusher, which crushes the ROM material to P80 6 in. The crushed material is conveyed to a coarse ore stockpile. The coarse ore is reclaimed from the stockpile with vibrating feeders and transported by the conveyor system beneath the stockpile into the secondary crushing plant crusher feed bins. Secondary crushing consists of a bank of Symons 7 ft standard cone crushers. The secondary cone crusher discharge is screened on double-deck screens. The screen oversize material is conveyed to the tertiary, 7 ft Symons short-head cone crushers for fine crushing. The crusher discharge is screened. The screen oversize material feeds conveyors that recycle the material to the tertiary crusher, and the screen is product size with a P100 5/8 in. The crushed product is conveyed to the rod mill feed bin.

The rod mill discharge flows through wet cobber magnetic separators. The cobber non-magnetic tailings flow to the tailings spiral classifier and then to the tailings thickener. The cobber magnetic concentrate is pumped to three parallel ball mills, followed by eight rougher magnetic separators, and the circuit is closed by hydrocyclones.

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