Iron is one of the most common elements on earth. Mankind has been converting earthen materials to usable iron forms for over 3000 years. The earliest objects formed by man are tools, ornaments and weaponry. In a pure state, iron is a soft, grayish-white metal.
While iron is a common element, pure iron is almost never found in nature. The only pure iron known to exist naturally comes from fallen meteorites. The iron we extract from earth-based ores is found in minerals formed by the combination of iron with other elements. Iron oxides are the most common. The iron-bearing minerals in the earth’s crust (refereed to as iron ore) have the highest concentration of iron content and are commercially mined.
More than 85% of the taconite iron ore mined in the United States is mined in northeastern Minnesota to supply the steel and foundry industry. Because the hard taconite iron ores are located near the surface, the most common mining style is open-pit mining that uses explosive blasting to loosen the taconite into smaller chunks that enables large electric shovel machines to load the rocks into large transport trucks. Each shovel can scoop up to 85 tons of ore. The transport trucks, capable of moving up to 240 tons, take the taconite chunks to a rotary crushing unit.
At the processing plant, the taconite is crushed into very small pieces by rock crushing machines. The crushers keep crushing the rock until it is about the size of a marble. The crushed rock is then mixed with water and further ground up in rotating mills until the taconite ore is as fine as powder.
By flowing the water-based liquid slurry of powderized taconite passed rotating drums with magnets located inside, the iron ore is separated from the taconite powder. Depending on the ore concentration in the taconite, 20-30% of the taconite is separated from the base ore. The remaining material is considered waste and is dumped into tailings basins and will be used to reconstruct the mine site after it has been depleted of ore. The taconite powder with the iron in it is called concentrate.
The concentrate (the wet taconite powder) is rolled with clay inside large rotating cylinders. The cylinders cause the powder to roll into marble-sized balls. The process has been decribed as being similar to rolling sticky snow into balls to build a snowman. The balls are then dried and heated until they are white hot. The balls become hard as they cool. The finished product is referred to as taconite pellets, a stable media ready for shipment. These taconite pellets are loaded into ore ships. These ships sail on the Great Lakes to Gary, Indiana, Cleveland, Ohio and other steel-making towns. The taconite pellets are brought to the steel mills to be melted down into steel.
While most of the taconite pellets produced in the iron ranges of Minnesota is shipped to steel mills located in the great lakes region, there is some market for Pig Iron as a feed stock to cast iron foundries. Pig iron (know also as Crude Iron) is an intermediate product of the iron industry, which is obtained by smelting iron ore in a blast furnace. Pig iron has a very high carbon content, typically 3.8–4.7%, along with the dross silica pickup from the molding sand. The high carbon and and non-metallic inclusion contents make pig iron brittle and not useful as an engineered material until it can be further refined.
Pig iron, is frequently used produce gray iron, but it’s considered an expensive feed stock to the iron foundry. Cast iron foundry melting shops will use substantial quantities of lower cost scrap steel and scrap iron, but still need to use pig iron for supplemental volume. By knowing the chemistry for each of the scrap-related feed stocks and pig iron, the iron quality in the melting furnace is achieved by adding the needed alloys and adjusting the carbon content. Some pig iron grades are also suitable for producing ductile iron. These are high purity pig irons and depending on the grade of ductile iron being produced these pig irons may be low in the elements silicon, manganese, sulfur and phosphorus. These high purity forms of pig iron are used to dilute all the elements found in the scrap feed stocks (except carbon) in a ductile iron charge.
Recycled Ferrous Materials for the Foundry
Since most of the iron mining materials are being used in the steel industry, the most significant source of material for cast iron plants is actually from scrap. Much of the automotive and industrial cast iron material is collected and recycled right back into to the foundry. This material has alloy compositions that are very close to grey or ductile grades. In addition, steel scrap, particularly the heavier steel clippings and any rail-road steel is melted along with the scrap iron. To minimize the expense of adding primary elements, the experienced melt-shop superintendent will know how much scrap metal to charge from each type of scrap source and supplement the remainder from the pig iron pile.