There are, at present, two iron mines operating in the UP--both are in the Marquette Range.  This page is devoted to the Tilden Mine, although the other (Empire) Mine is equally impressive.  Marquette County's Empire and Tilden mines together produce 20% of North America's iron ore.  When  the Empire and Tilden mines are producing concentrated iron ore pellets at full capacity, they produce (combined) nearly 16 million tons a year. Open-pit iron mining in the 21st century is an expensive proposition that uses explosives, computers, telecommunications equipment, immensely powerful machinery and huge quantities of energy to convert iron-bearing rock into marble-sized pellets of concentrated ore.  To justify the huge investment in such machinery, the mines run 24-hours a day, seven days a week. When they need to cut production, they often shut down entirely for a period of weeks, rather than attempting to operate below capacity.

An old image of the Empire processing plant, and its very large open pit, are shown below.

Source: Image Courtesy of Tilden Mine

Source: Image Courtesy of Tilden Mine

THE TILDEN OPEN PIT IRON ORE MINE

The Tilden mine is a longstanding feature of the UP.  The image below shows that it was in existence in 1930!

Source: Image Courtesy of Tilden Mine

Tilden: Iron Ore Reserves
   In the first century of mining in Michigan’s Upper peninsula, high-grade ore was mined and shipped directly to the steel mills. By the 1950's, high-grade reserves in North America were rapidly being depleted and the industry launched extensive research projects to find methods to utilize the low-grade reserves which were available in abundance.
    The ore being mined today was once considered worthless rock because there was no practical method to unlock the iron bearing material. If those methods had not been devised, North American steel mills would have been forced to look elsewhere in the world for this vital commodity.
    While most North American iron mines use a magnetic separation process to upgrade magnetite ores, the initial Tilden flow sheet was new technology for processing non-magnetic hematite ore. The hematite pit lies within a belt approximately 2 miles long and ½ mile wide. Entering the 1990s, this pit had reserves capable of producing over 300 million tons of pellets.
    In 1989 a new pit, in magnetite ore, was opened. It is a much smaller ore body and is expected to allow for the production of 4 million tons of pellets per year for approximately 15 years.

Mining
   Millions of tons of material are mined each year to produce pellets at capacity levels. Some is rock and overburden which must be removed to gain access to the iron bearing minerals. This process is known in the industry as stripping.
    Marquette Range ore is composed of both magnetite and hematite ores.   All of the Marquette Range ore currently mined is excavated from open pits using diesel-powered off-road trucks loaded by electric and diesel-powered shovels. The ore, which in the ground consists of 25 to 45 % iron by weight, is pelletized at processing plants adjacent to each mine.
    The life-of-mine stripping ratio in the hematite pit is about one-half to one, and in the magnetite pit it is about one to one. That means that a half ton of rock must be removed to gain access to every ton of hematite ore, and one ton of rock must be mined for every ton of magnetite ore. Now, since the ore is about 35% iron, three tons of iron bearing material must be mined for every ton of pellets produced. To produce 6 million tons of pellets, for example, the Tilden mine must remove about 30 million tons of ore and rock combined, from its open pit mine (below)

Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

    The Tilden facility can mine either reserve, but it cannot process hematite and magnetite simultaneously without further changes in the flow sheet.  The production from each area in a given year will depend on owner and customer requirements.
    Transporting the ore to the plant is done using extremely large trucks, such as the one shown below.  On a recent February morning, pit dispatcher Dennis Trudgeon sat before a bank of computer screens in a building on the lip of the Empire Mine, using strokes on a keyboard to direct mining trucks to spots where they were needed in the huge open-pit mine. Though the 900-foot-deep pit was obscured in a sea of pea soup fog, Trudgeon could track the movements of the fleet of trucks, loaders and power shovels. Global positioning satellite units and wireless communication equipment are mounted on each vehicle and linked through a computer system.

Source: Image Courtesy of Tilden Mine

Here's part of a conversation between a reporter and Bob Reynolds, who drives one of those huge trucks in a gold mine in the western US:
Is it safe to say the truck you drive is bigger than your house?
Yes. And I actually sit roughly two stories up to drive it. The tires are about 14 feet tall, and if loaded correctly, the gross weight would be 1.1 million pounds.

So is the fuel tank as big as a bathroom?
Not a great big bathroom, but it’s goat a 1,200-gallon fuel tank. And every 12-hour shift you have to get fuel. I’ve been told by Komatsu, the truck’s manufacturer, that the engines can run that truck at 80 mph. Right now they’ve got it turned down to 40 mph. Someone comes with a laptop and sets the top speed to whatever the mine company wants. It won’t run any faster than that. But even with a payload, it will go 40 mph.

Ever blown a tire?
The other day I was fully loaded going up a ramp, and I had a blowout on the left front tire. It blew a two-foot hole in the ground! Seconds after it did that, you could still see stuff falling down in my headlights. When it blew, the truck dropped right down on one side—that’s a six-foot drop from the middle of the axle down. They couldn’t change the tire right there and they wanted to wait for the day shift to come in, so it just sat there on the ramp with the flashers on.

What else can go wrong?
When I was working at a previous mine, one guy’s truck caught on fire. Somebody hollered to him, "Hey, you’ve got a wheel-motor fire!" By the time the guy realized what was happening, the truck was engulfed underneath. If you have a fire, you need to shut the truck down so the fire isn’t fueled by hydraulic oil. But he didn’t, and he stayed inside the cab until the heat got so bad that he couldn’t take it. When he bailed out of it, he bailed out into flames.

Have you ever seen anybody accidentally run over something small, like a car?
A surveyor one time parked his Bronco pickup in the wrong place, and a guy rolled right over it and didn’t even know he had done it. The car looked like it went through a compactor. Someone had to holler to the guy and say, "Hey, man, you just ran over that pickup!" He had no idea.

In time, it is the cost of transporting the ore from the ever-deepening pit bottom that will close these mines. Economically, it is the cost of transportation (including not only diesel fuel but labor, tires, capital equipment, etc.) that trims the profit margin to zero and causes the mine to close--not the diminished quality of the ore.

Source: Image Courtesy of Tilden Mine

Processing
   Turning crude ore into pellets occurs in the mine’s concentrator and pellet plant.  During concentrating, there are some steps which are identical and others which are unique to hematite or magnetite.  The process in the pellet plant is the same for either type of ore.

Mining Operations
    Mining operations are the same in either of the huge open pit. The open pit of Tilden's "sister" mine (the Empire Mine) is shown below.  In a few years, the bottom of this mine will be the lowest spot in all of Michigan!

Source: Image Courtesy of Tilden Mine

Prior to blasting, rotary drills drill 16-inch holes 50 feet deep in precisely laid out patterns, which can include as few as 30 or as many as several hundred holes. They are filled with explosives and set off in a carefully controlled blast, which breaks up the material for mining.

Source: Image Courtesy of Tilden Mine

    Electric shovels load the broken material into trucks for transport from the pit. Initial pit equipment included shovels with 11 cubic yard buckets and 85 ton production trucks. For the 1990s, the mine is converting its fleet to 170 and 190 ton trucks.

Source: Image Courtesy of Tilden Mine

    The crude ore is hauled to the primary gyratory crusher, where it is reduced to chunks less than 10 inches in size. From the crusher, the ore is conveyed to a covered ore storage building.

Grinding
   Liberating the iron mineral requires that the crude ore be ground to the consistency of face powder. This process begins in the same way for both magnetite (metallic gray, below) and hematite (red-brown, below) as crude ore and water are fed into large primary autogenous mills. The term autogenous means that grinding media like the steel balls and rods used in some mills are not required. Instead, the tumbling action of the ore in the rotating mills is sufficient to reduce it to a consistency of beach sand. Tilden has twelve primary mills that are 27 feet in diameter and 14 ½ feet long.

Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

Ore is dumped from the trucks into crushers (below).

Source: Image Courtesy of Tilden Mine

Further grinding occurs in grinders and pebble mills (below) which also operate autogenously.  Grinders (below) and pebble mills (second photo below) are, put quite simply, cylinders that continuously roll and turn.  Inside them are rocks (ore) of various sizes. 

Source: Image Courtesy of Tilden Mine

Source: Image Courtesy of Tilden Mine

The larger pieces of ore essentially act as grinders, crushing the smaller pieces into powder.  As the larger pieces of ore ("pebbles") get comminuted in size, more large pieces are added.  In the pebble mills shown above, pebbles about 2 inches in size which are screened from the primary mill are used as grinding media. In grinders, larger rocks are used.  Thus, grinders preceed pebble mills in the comminution process.  The Tilden concentrator has twenty-four 15 ½ foot diameter pebble mills, each about 30 feet long.

Concentrating Magnetite
   First ground to a fine powder, the ore is next concentrated using magnetic separation and flotation to 60 to 65 % iron, then rolled into 3/8" dia pellets that are purplish-grey when they emerge, steaming, from the plants.
    Turning low-grade crude ore into high-grade concentrate requires a means of separating the iron particles from the waste rock. In the magnetite operation, magnetic separators known as cobbers and finishers (see image below) help perform that function.

Source: Image Courtesy of Tilden Mine

    The mixture of finely ground crude ore and water enters the separator tanks where stainless steel drums with powerful internal magnet systems attract and recover the magnetic iron particles, while the non-magnetic silica is washed away as waste, known as tailings.
    Tilden has 54 cobbers in the primary grinding circuit and 60 finishers. Magnetite processing also utilizes deslime thickeners for hydraulic concentration prior to magnetic finishing and flotation of silica as the final concentrating step.

Concentrating Hematite
   When processing hematite, Tilden must use a flotation system specially developed for the mine’s fine-grained ore, rather than by magnetic separation.
    The finely ground mineral particles are conditioned by adding caustic soda and a dispersant in the grinding process. Then a cooked corn starch is introduced to selectively flocculate (or gather together) the very fine iron particles. The separation occurs in twenty-four large tanks know as deslime thickeners where the flocculated iron particles settle and are recovered in the underflow while the fine silica tailings are carried away in the overflow. The material is then fed to the flotation circuit consisting of three hundred 500 cubic foot flotation cells. Here, further separation occurs as silica is removed in the froth overflow through a process known as amine flotation, producing a high-grade iron ore concentrate.

Processing-General
   After the material has been concentrated using the magnetic or flotation process, dewatering begins as the material is thickened in large settling tanks. From the thickener, the mixture of high grade iron-ore and water is pumped into concentrate slurry storage tanks.
    To produce fluxed pellets, a mixture of limestone and dolomite is also ground to a very fine size and added to the concentrate slurry tank at the desired rate. If it is not added to the pellets, the limestone and dolomite must be added at the blast furnace. The specific mix and amount of flux material in the pellets can be tailored to the customer’s (i.e., the steel mill’s) specifications.
    Filtering is the final step in the concentrator. Large vacuum disc filters dewater the concentrate in preparation for pelletizing.

Pelletizing
   Iron ore concentrate (known as "filter cake") provides the ingredient for the mine’s final product, but it must be put in a form which is suitable for shipping and for handling in the blast furnace.  The powdery iron ore concentrate is mixed with a small amount of bentonite clay binder and then rolled into marble-sized pellets in balling drums.  The image below shows the inside of a balling drum, with thousands of round pellets within.

Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

Tilden has 14 balling drums which are 12 feet in diameter and 33 feet long.  The rust color of pellets that have laid exposed to the weather is entirely natural, as the iron in the pellets is chemically combined with oxygen to form iron oxide, more commonly known as rust.  Small amounts of silica, alumina, manganese, limestone and bentonite make up the rest of the pellet.  Bentonite (an extremely sticky clay) is added at the rate of 16 lbs per ton as a binder. Limestone, which serves as a flux in the steel-making process, is added to the pellets at the processing plants to simplify blending at the steel mills.     
    Leaving the balling drums, the pellets are the proper size and shape, but they are too soft for handling. The unfired pellets, or "green balls", are thus conveyed to the travelling grate furnace where the temperature is gradually increased to dry and pre-heat them. The pellets then enter the huge rotary kilns where they are hardened by firing at temperatures above 2200 degrees F. The pellets leaving the kiln enter a cooler where they are cooled to a temperature suitable for conveying. 

Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

    As the diagram below shows, the use of iron ore pellets in steel mills is increasing year by year, and soon almost all mills will use pellets exclusively as their source of iron for the blast furnaces.

Source: Unknown

Shipping
   The Lake Superior & Ishpeming Railroad (LS&I), contrary to what has been widely published, does not haul "taconite" pellets, but what are properly referred to as "processed iron ore" pellets.  Magnetic taconite ore, extensively mined on the Mesabi Range of Minnesota, is not found in appreciable quantities on the Marquette Range.  The ore bodies at the Empire and Tilden Mines are instead composed of magnetite and hematite ores; when it occurs in commercial deposits the latter is referred to as jasper.  These pellets are conveyed from the plant to the stockpile or the load-out bin.  The stockpile has a capacity of 3,000,000 tons of pellets, while the load-out bin can fill up to 450 railroad cars per day. 

Source: Image Courtesy of Tilden Mine

Pellets travel by rail to ports in Marquette and Escanaba where they are shipped by Great Lakes ore carriers to steel mills in the United States and Canada. 

Source: Unknown

The image below shows an ore freighter unloading taconite (flux) pellets at a steel mill.  

Source:  Photograph by Randy Schaetzl, Professor of Geography - Michigan State University

    The Empire Mine, in the Marquette Range, has an annual capacity of 8.0 million tons.  Most of its production goes to boats through Escanaba to steel mills in the Chicago area.  By way of comparison, its neighbor the Tilden Mine has a capacity of 7.8 million tons and most of its tonnage goes through the Marquette dock mainly to the Algoma Steel Plant in Sault Ste. Marie, Ontario.

Environment
   The Tilden Mine was designed to include modern pollution control processes and equipment. Water receives careful attention at both ends of the processing operation.
    To meet the need for a dependable source of water, the Greenwood Reservoir was constructed on the middle branch of the Escanaba River. This man-made lake includes 1,400 acres, 26 miles of shoreline and 13 major islands. In addition to providing water for the mine, the Reservoir is open to the public for a variety of recreational activities.
    Nearly all of the water used in the plant is re-circulated through the use of large tailing thickeners and a re-use water pond system. Fresh water use is approximately 5% of the total process water requirements.
    The water used for processing the iron-bearing material through the concentrating process also carries the silica, or tailings, to a large impoundment know as a tailings basin. At the Gribben Basin, which serves the Tilden Mine, the water is decanted and clarified so it can be returned to the watershed meeting all government clean water standards.
    Air quality is also important and Tilden uses modern electrostatic precipitators to remove particulate matter from all waste gas streams entering the environment.

My thanks to the Tilden Mine people for providing much of the text for this page, from their Tilden Mine pamphlet---Prof. Schaetzl

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.