| The next phase of the trip brought
us to the Hull
Rust Mine in Hibbing. This mine like all of those on the Mesabi
Range mine taconite ore, not the higher quality hematite ore.
As the high quality ore was mined some of the companies began taconite
plants that would take lower quality taconite ore containing magnetite
and process it into pellets before shipping it to the steel
mills.
Taconite contains about 20% iron compared to the hematite ore which
might
contain as much as 60% iron. The taconite is crushed and the iron
containing magnetite is separated from the waste by the use of an
electromagnet.
The magnetite is processed into uniform pellets for shipment to the
steel
plants. Because of the uniformity of the pellets taconite mining
will probably continue even if more high quality ore is available
because
it is more economical to use. The United
Steelworkers Union has some nice photos and descriptions of the
process
on their mining
page. According to a sign at the overlook, more than 1.2 billion tons of ore have been removed from the Hull Rust mine since shipping began in 1895. The mine is more than three miles long a mile wide and up to 535 feet deep. |
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| The mine overlook is at the south
edge of
the mine in an area that used to be the City of Hibbing. When
quality
ore was found below the city, the whole city was actually moved south
by
the mining company to make room for the growing mine. The road to
the overlook still has side streets and building foundations of where
the
city once stood. The photos above look east where the bedding of the ore sediments can easily be seen sloping gently to the south. The photo at the right is looking west. Active mining operations occur in both parts. According to one of the employees at the overlook center there is enough taconite left in the mine to continue for 150 more years at the current rate of production. The quota for this year is about 8 million ton of taconite pellets. The Hull Rust is the largest iron mine in the world. The bottom right photo is looking north. At the horizon is the Laurentian Divide. It is at the junction of small continental plates that now divides three drainage systems. It is the only divide of its kind in the United States. A raindrop fall at this spot could flow north to the Hudson Bay, east to Lake Superior and the Atlantic Ocean, or south by way of the Mississippi to the Gulf of Mexico. The site is not accessible to the public because of mining operations but lies at the northwest end of the Hull Rust mine about two miles northwest of Hibbing. |
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This is a photo of one of the "small"
trucks that
are used in the mining operation, it can haul 170 ton of rock or
ore.
The trucks that are currently used in the mining operation have a 240
ton
capacity. To mine the taconite a drill is used to make holes 16 inches in diameter, 55 feet deep, 6 feet apart, 120 holes in a pattern. The holes are filled with ammonium nitrate (nitrate fertilizer) and fuel oil to within 6 feet from the top. Crushed taconite is put in the last 6 feet to plug the hole and send the shock wave down into the rock. The blasts are delayed so that a large concussion does not occur that would break windows or foundations of buildings nearby. Each Wednesday at about 11 am the blasting occurs that breaks up the rock so it can be loaded onto the trucks. The photo at the bottom left shows a close-up of a drilling rig (as close as possible without working in the mine. The photo at the bottom right has arrows pointing to three pieces of equipment. The left arrow is pointing to a red and white shovel that loads taconite, the center arrow is pointing to a drilling rig, and the right arrow is pointing to a black and yellow shovel that loads overburden. |
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| This set of photos
all look
west from the overlook site. In the photo below it is again easy
to see the bedding of the ore material. If mining in this area
were
to continue more overburden would have to be removed and the mine would
get closer to Hibbing. The photo at the bottom right shows a closer view of the one above it. Since the mining operation follows the bedding layers it slopes gently down to the south. Notice that there is more earth exposed on the right (north) side of the photo than on the left side of the peninsula. |
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About 2 billion years ago a shallow fresh water sea covered the area. Sediments and chemical precipitates were deposited to form the Biwabik Iron Formation. In the Mahoning-Hull-Rust area the thickness of the formation is about 500 feet. The sediments were extremely rich in iron which had been in solution in the sea. Several thousand feet of clay was then deposited on top of the iron formation to form the Virginia Slates. This weight compressed the iron formation and formed it into rock. The Lake Superior basin then formed with the extrusion of hundreds of lava flows causing the iron formation to tilt south into the basin. 500 million years of erosion then exposed the iron formation and seas encroached on the area and deposited the younger Cretaceous ores and fossils. Glaciation deposited the gravels on top of the iron deposits and finally fracturing and water movement then oxidized the taconites to form the natural ore bodies that makes Hull-Rust famous.
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