Taconite

[coalminer3]

In response to a question in the "Ore Cars" thread elsewhere.

Easily minable supplies of soft iron ore were running out by the early 1950s due to demands imposed by American economic growth, two world wars and the Korean War. Minnesota-based iron mines were also having to compete in a world market with other countries who could mine ore more cheaply. There were, however, large supplies of taconite on the iron range. Taconite is a siliceous (contains lots of quartz) iron formation that has to be concentrated to produce useable iron ore.

People had known of taconite for years but had not been interested in mining it because of processing problems associated with its hardness. Mining engineers conducted some experiments to develop ways to process taconite and came up with some good ideas. These processes proved too costly for taconite to compete with "soft" iron ores and taconite development stopped in the early 1920s. The picture for taconite changed in the 1950s as supplies of soft ore became depleted. An amendment to the Minnesota state constitution preventing discriminatory taxation against new taconite operations was passed in 1963, clearing the way for the investement of capital needed to develop taconite operations and processing plants. Today, virtually all of the iron produced in the U.S. comes from taconite.

Taconite is mined in open pit operations that use special drilling and blasting techniques to produce the smallest size possible ore. Electric shovels load the material into trucks that carry the ore to the railroad where it is laoded into ore trains. Ore trains move the material to the mill where it is dumped from the train either into the primary crushers or stockpiles where the ore is stored.

The primary crusher reduces the ore into chunks that are about 6 inches or less; secondary crushers further reduce the ore down to chunks that are 3 inches or less. The ore is screened after secondary crushing and fines (less than 3/4") go to another mill. Larger material is crushed again.

The ore is now ready for concentrating. The fines go through a grinder and then pass througha magnetic separator called a "cobber" that concetrates the iron-bearing particles and rejects other material as tailings. The concentrated iron particles pass thorugh another set of mills and separators that further concentrates the finely ground particles. The ore is now the consistency of baby powder. Agglomeration is the final step in processing taconite. The ore concentrate is dried and mixed with a small amount of betonite (clay) and rolled into pellets. The moist pellets are then dried and stockpiled for shipment.

Two companies (Reserve Mining and Erie Mining) were the first to become involved with taconite productiion as they were in operation before the passage of the taconite amendment.

Probably more than you wanted to know, but it is an interesting process; with more steps than you think.

work safe

[ndbprr]

Pellets aren't dried they are roasted at 1800 deg. F. to skin them. if they were just dried they would basically be dried mudballs that would crush in transit or storage. Efforts have been made to also include limestone in the pellet to allow better slag-limestone interaction but it has had limited success. The furnace grates used are typically hi-temp alloys that are not subject to corrosive attack in acidic atmospheres but highly corrosive in basic atmospheres which the limestone brings to the equation. The Silver Bay site used by Reserve and Erie is the only one I know of located away from the taconite mining operation. All others are fed by conveyer belt and ship pellets by rail to a port. The northern Michigan mines ship to either MArquette or Escanaba but both are a shadow of what once was.

[dknelson]

Interesting. Back in the days when ore was shipped "raw" they needed heaters because the loads would freeze. A lot of old steam locomotives lasted into the early 1960s in this service. Unfortunately the C&NW did not save one of its streamlined Hudsons even though they were in ore thawing service for some time.

Dave Nelson

[ndbprr]

Both coal and taconite freeze because the railroads or companies wet down the load before weighing it. The logic is that it controls the dust. I always thought they were trying to charge the customer for weight they would never get in the form of water but that is my opinion. Then when it is shipped it freezes into a big block. Most of theore docks had steam pipes for thawing the loads. In Proctor Minnesota there were thawing yards and buildings where the cars would be thawed out using steam before running out onto the ore dock for dumping.