IRON ORE PREP CRITICAL TO KEEPING MINNESOTA MINES OPEN
Eastern edition technology article by Harold Hough
In an era when ore preparation is seen as a way to milk an extra percentage point in recovery rates, it’s easy to forget that ore preparation and new refining methods can mean the difference between keeping a mine open or closed.
There is no better example that in the iron ore deposits of the “Range” as it is known to Minnesota miners. The range actually covers four different iron ore ranges – the Mesabi Range (the best known one), the Vermilion Range, the Gunflint Range, and the Cuyuna Range. They have been producing the bulk of America’s steel for over a century.
Like many other mineral deposits, the Range was discovered as a result of a gold discovery on Vermilion Lake at the end of the Civil War. The gold didn’t hold out long, but prospectors discovered a world class iron ore deposit. The timing of the discovery coincided with the post war expansion of the young nation.
In those early years, the ore of choice was Hematite. This ore, which was up to 70% iron, was the heart of the Mesabi Range. The ore was so rich in iron; it could be profitably shipped via the Great Lakes to Pittsburgh, where the metallurgical coal was to be found. This, in turn, encouraged the building of the locks on the Great Lakes to facilitate the shipping.
The hematite mining continued up to the middle of the 20th Century. However, the massive demand for steel during World War Two for tanks, ships, and artillery used up much of the remaining deposits of hematite, leaving the poorer grade taconite.
The problem was that taconite isn’t as easy to process into iron. Taconite is a low-grade ore containing about 30% iron and steel mills need iron ore with at least 50% iron. The tiny iron particles are scattered throughout a very tough variety of quartz called chert, and, when the Iron Range was opened up, this ore was considered waste because it was technologically impossible to use in steel production. Unless someone could find a way to process it, Minnesota would lost a major industry.
Thanks to Dr. Edward W. Davis of the University of Minnesota and others, a way was found to profitably prep the taconite ore. The new method for processing and prepping the ore began in the ground. Since it was harder than hematite, blasting was used to wrench it from the earth instead of using heavy equipment to dig it out.
The ore is then crushed and milled until it has the consistency of powder. The iron ore is separated from the taconite using magnetism. The remaining rock is waste material and is dumped into tailings basins. The taconite powder with the iron in it is the concentrate.
The wet taconite powder is then rolled with bentonite clay, limestone and dolomite inside large rotating cylinders. The cylinders cause the powder to roll into marble-sized balls. The balls are then dried and heated until they are white hot. The balls become hard as they cool. The finished product is taconite pellets that contains about 65% iron as well as its own binder and flux.
These pellets are now ready to be shipped to steel mills around the country. In many cases, they still travel by Great Lakes cargo ships, however, they can also be shipped via rail to mills far from the Great Lakes.
This process, called beneficiation, allowed the steel mills of the United States to continue using Minnesota iron ore, even though the composition of the ore being mined had changed dramatically. Today, these pellets are the most desired form of ore for steel production. They also provide a consistent quality, unlike the raw iron ore.
But, it doesn’t stop there. Most of the pellets are produced for a specific type of iron and smelter that it will go to. Some chemicals are deliberately added - such as flux which makes a blast furnace more efficient. Others are added because they make the iron more fluid, harder, or give it some other desirable quality. The choice of ore, fuel, and flux determine how the slag behaves and the operational characteristics of the iron produced.
Thanks to beneficiation and iron ore prep, Minnesota has remained America’s major iron ore producer for over a century. It has preserved the mining community, and it has made the production of steel downstream cheaper and better.