Minerals Information
Iron is the fourth most abundant rock-forming element and composes about 5% of the Earth's crust. Astrophysical and seismic evidence indicate that iron is even more abundant in the interior of the Earth and has apparently combined with nickel to make up the bulk of the planet's core. Geologic processes have concentrated a small fraction of the crustal iron into deposits that contain as much as 70% of the element. The principal ore minerals of iron are hematite, magnetite, siderite, and goethite. An estimated 98% of the ore shipped in the world is consumed in the manufacture of iron and steel. The remaining 2% is used in the manufacture of cement, heavy-medium materials, pigments, ballast, agricultural products, or specialty chemicals. As a result, demand for iron ore is tied directly to the production of raw steel and the availability of high-quality ferrous scrap.
World production of raw steel was at a record-high in 1989 and would have been even greater in 1990 and 1991 if political and socioeconomic events had not led to the disintegration and dissolution of the U.S.S.R. The U.S.S.R. had been the leading producer of iron ore for more than three decades and traditionally accounted for one-fourth to one-third of the world's annual output. Other major producers include Australia, Brazil, China, India, and the United States. Since 1980, demand for steel has stabilized and even slackened in many industrialized countries. However, demand continues to escalate in the developing and newly industrialized countries. Much of the recent growth has been in the Far East.
Iron ore mining and beneficiation in the United States has changed significantly since World War II. At that time, natural ores were the mainstay of the domestic mining industry. The natural ores, which consisted primarily of hematite and goethite, were extracted from near-surface zones of enrichment in the Precambrian banded iron ore formations of Minnesota, Michigan, and Wisconsin. The ores came from areas where part of the silica had been leached from the underlying low-grade taconite by weathering or movement of ground water. The old Lake Superior ores averaged 50% to 60% Fe and could be shipped directly to the steelworks without prior beneficiation. In 1944, at the peak of the war, 103 natural ore mines were operating in Minnesota and another 43 in Michigan. That year, the two States shipped 65.5 million metric tons of crude ore directly to consumers and another 23.9 million metric tons to beneficiation plants. Demand for steel during the Korean War accelerated the depletion of these reserves, and the mining companies in the Lake Superior District began turning more and more to magnetic taconite. This trend can be seen in figure 1. By the end of the Vietnam Conflict, most of the natural ore in the district had been mined out, with the last mine of this type closing in 1991.
Figure 1 Consumption of iron ore and agglomerates at U.S. iron and steel plants, by type of product.
During the 1960's and 1970's, massive pelletizing complexes were built in the Lake Superior District to compensate for the shutdown of the natural ore mines. Today, the district still produces the bulk of the Nation's iron ore, but almost all of the ore being recovered is magnetite. Pellets made from finely ground magnetite concentrate now account for 97% of U.S. usable production (fig. 2). In the late 1980's, blast furnace operators began switching to fluxed pellets in response to environmental restrictions on cokemaking and higher energy costs. This more easily reducible type of pellet is created by adding limestone and/or dolomite to the iron ore concentrate during the balling stage. In 1990, fluxed pellets accounted for 39% of U.S. pellet production.
Since colonial times, the blast furnace has been the principal instrument for converting the ore to molten iron and is expected to remain the mainstay of the steel industry for at least another 30 years. Nevertheless, because of increasing environmental concerns and sharp increases in energy prices, companies have begun evaluating several novel ironmaking and steelmaking processes. In the late 1970's, Venezuela, Mexico, and other countries with surplus natural gas began making significant quantities of a product called direct-reduced iron (DRI). Since then, DRI has become a competitive substitute for high-quality scrap. In 1990, the world produced 29.37 million metric tons of DRI, which typically averages 90% to 94% Fe. The p29 million metric tons of DRI falls far short of the 500 million metric tons of hot metal and pig iron being produced annually by the blast furnace, but a number of other promising technologies are under development that could help fill the gap.
Figure 2 Usable iron ore production, by type of product.
In 1988, the first commercial Corex plant was commissioned at Pretoria in the Republic of South Africa. Many of the technical problems associated with the startup of this 300,000- metric-ton-per-year demonstration plant have since been solved, and several steel companies are now considering building much larger units in the United States and Western Europe. The proposed Corex plants are still significantly smaller than existing blast furnaces but can be brought up to full operation much quicker with less cost. A key feature of the Corex process is that it uses untreated raw coal in place of coke. The ability to operate without coke gives the Corex plant two environmental advantages over the conventional blast furnace. First, because coke ovens are not needed, all of the problems associated with the generation of benzene and other coal tar byproducts are eliminated. Second, the dust problems associated with blast furnaces are also eliminated because the offgas is used as fuel. Joint COREX and DRI plants are now on the drawing board, with the offgas from the Corex plant being used to fuel the adjoining DRI plant. Direct steelmaking, a much more revolutionary process, is still in the early stages of development. A pilot plant, funded by the American Iron and Steel Institute and the U.S. Department of Energy, has been operating near Pittsburgh since 1990.
