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A0004 Iron Making
by Terry Smith
Iron is the main ingredient of steel and is also a product that is used both as produced and further refined into a number of different types. Iron has been produced in many different ways over the past centuries with improvements made in both processing and the final product. In this article, I will discuss the modern Blast furnace method of producing iron.
A modern day blast furnace can be as tall as a ten story building. The iron produced in a modern day blast furnace is called pig iron. Some of this iron is cast into molds called ‘pigs’ hence the term pig iron. The iron cast as is into pigs is generally sold as is to smaller steel producers and foundries for further processing into different grades of cast iron products or steels. The vast majority of the iron however, is used for primary steel making. The four major ingredients used in the blast furnace for producing iron are, iron ore pellets, coke, limestone (and or dolomite) and air. The iron ore, coke and limestone are introduced through the top of the furnace and the air in through the bottom.
The ore, coke and limestone are held in large hoppers and are dumped into a type of rail car called a skip car. There are at least two skip cars for each furnace and they travel up an inclined rail to the top of the furnace where they are tipped up and dump their load into what is called a bell at the top of the furnace. These ‘bells’ as they are called are cone shaped doors into the furnace. The one the skip cars dump into is the outer bell. After a load is dumped the bell opens and the load of ore, limestone or coke drops down onto an inner bell. The outer bell then closes and the inner bell opens dropping the load into the furnace, this ensures the furnace is sealed at all times. The ore, coke and limestone are added in layers in a controlled amount to ensure peak performance of the iron production. Air is pre-heated in large furnaces called stoves before it is introduced through the bottom of the furnace to create the ‘blast’. A blast furnace is lined with a high temperature brick and once it has been pre-heated and charged it operates continuously for 5 to 6 years until it needs to be re-lined. When in production a blast furnace produces an excess of Carbon Monoxide (CO) that is part of the reduction process to make iron. Not the entire CO is consumed in the chemical reduction process to make iron, and the excess is drawn off the top of the furnace and burnt in the pre-heat stoves. The stoves contain large stacks of bricks inside called checkers. These checkers are heated by the burning gasses drawn from the blast furnace up to 2300 degrees F. Excess gas not burnt in the stoves is directed to a large boiler in the turbine house to produce steam, which in turn operates the turbine of a large blower that produces the air blast for the furnace. These large blowers produce anywhere from 80,000 to 230,000 cubic ft/min. of air for the blast. The cold blast, as it is called, travels through a large pipe leading into the stoves where it is pre-heated to 1600 to 2300 degrees F before entering the blast furnace. The air is directed through a stove until the temperature of the stove drops to about 1600 degrees. At that time the blast is directed into a different hotter stove and the cooler stove is again re-heated to about 2300 deg. F .The air coming from both hotter and cooler stoves is mixed along with some of the cold blast air to try and maintain a relative temperature of the hot blast as it enters the furnace.
The hot air blast main feeds a large pipe that encircles the entire base of the furnace called a bustle pipe. Inside the bustle pipe on the furnace side are contained large water cooled copper tuyeres. The hot air blast is injected into the furnace through these tuyeres. They may also inject oil, tar, powdered coal, natural gas and even oxygen through the tuyeres as well. The hot air blast ignites the coke in the furnace and the temperature in front of the tuyeres can be from 3600 to 4200 degrees F. As the coke is about 95% pure carbon, when it starts to burn in the air blast it produces excess amounts of CO. In the high heat of the furnace this excess co reacts in a chemical process with the iron oxide producing CO2 and pure iron. Further up the furnace the heat generated from the burning coke causes the limestone to under go a chemical change as well changing it to calcium oxide. This calcium oxide reacts with various acidic compounds in the iron ore, mostly silica, as well as sulfur, magnesia, and alumina to form a slag. Both the liquefied iron and slag trickle down through the charge and collect in the bottom of the furnace, where the slag being less dense than iron floats on top. All the excess Co not used up in reaction in the furnace is drawn off the top of the furnace to fuel the stoves and boiler. As the iron trickles down it also absorbs carbon from the co produced by the burning coke and this results in the liquid iron having a carbon content as high as 4.5 to 5%.
Depending on the efficiency and operation of the furnace it may be tapped anywhere from every 45 min to 2 hours. At any rate it is necessary to tap the furnace before the level of molten iron reaches the level of the tuyeres. The furnaces all have a cast house at their base and some modern furnaces may even have two cast houses and up to four tap holes. When a furnace is ready to be tapped a large drill is brought into line with the tap hole, and a refractory plug in the tap hole is drilled out. The iron spills out of the tap hole into a trough in the floor of the cast house. This trough runs to other troughs in the floor that have a refractory brick across and into the trough. This refractory has a small opening in the bottom of it through which the molten iron flows, over a dam and into a runner and then down into brick lined railroad car called a torpedo car. Because the slag is lighter than iron it flows on top and when it reaches the refractory brick in the trough it is re-directed into separate runners that carry it to slag pots on other railroad cars and taken away to be dumped. When the tap hole in the furnace begins to ‘spit’ it means that the liquid iron level in the hearth has fallen below the tap hole level and some of the blast from the tuyeres is coming out the hole and this signals the end of the tap. At this time a large ‘mud gun’ with a large cylinder filled with refractory is swung into place in front of the tap hole. Then a large ram at the back end of the gun pushes the refractory out of the cylinder and into the tap hole sealing it up. The cast house floor is then cleaned up, and the ‘mud gun’ re-filled in preparation for the next tap.
The iron is then taken by torpedo cars to the next processing stage to be desulfurized and dephosphourized as required. Both sulphur and phosphorous can cause steel to be hot or cold short, respectively, as well as making steel brittle and are usually kept to minimum levels. There are certain types of steels that do have a certain level of sulfur added (1100 series) to improve machining properties and others that have phosphorous added to improve corrosion resistance, but these are added at the steel making level. The sulfur and phosphorous levels are kept to a minimum in the iron making process in order to allow for controlled additions are a later stage. At the desulfurizing station an oxygen lance is lowered into the molten iron and oxygen and magnesium are injected through the lance. Calcium oxide (burnt lime) and mill scale are also added to the iron. With the oxygen injected an exothermic reaction results with the magnesium and sulfur and the phosphorus reacts with the mill scale and burnt lime. These reactions result in a slag that floats to the top of the iron that is then skimmed off the top and there is a significant reduction in both sulfur and phosphorous in the iron and it is ready to be processed into steel.
Iron to be used as cast iron can be processed into different grades as well. If used as is from the blast furnace and cast it would be classified as white cast iron. If the carbon content is reduced to a lower level it would be a malleable cast iron and with significant additions of silicon it would be a grey cast iron. Each has their own characteristics and different combinations of carbon, silicon and other minor alloying elements can be used to improve certain characteristics of the cast product. However if the carbon content is 1.5% or less than it is no longer called iron but rather it is steel.
The basis for this article and the one on coke making are form my own experiences working at what was formerly called Algoma Steel in Sault Ste Marie On., Canada. They are a fully integrated primary steel maker with coke making, iron making, steel making, casting, hot rolling and cold rolling facilities. Having worked a number of years at the plant as a laborer, maintenance man, and later as a Blacksmith, I have had the opportunity to see all aspects of all these operations and processes. Other information has been researched by me, as a matter of personal interest. The articles are in no way complete descriptions of all the processes but are done for a simple basic understanding of the process involved.
The next article in this series will be the steel making process using the Basic Oxygen type furnace.