At the bottom of the furnace where temperatures rise well over Fahrenheit, molten slag floats on a pool of molten iron which is four or five feet deep. Because the slag floats on top of the iron it is possible to drain it off through a slag notch in the furnace. The molten iron is released from the hearth of the furnace through a tap hole. The tapping of iron and slag is the major factor permitting additional materials to be charged at the furnace top.
This brief summary of the complex operations of a blast furnace is presented here to provide a point of reference for the actual flow of operations. Very often, several blast furnaces may be arranged in a single plant so that the most efficient possible use can be made of fuels, internal rail facilities, etc.
During this era, wrought iron was produced by blacksmiths who would heat the iron and hammer out impurities over an anvil. The resulting iron was tough but malleable. In the Middle Ages, a new type of iron was developed using higher temperatures. This was known as cast iron, which was harder than wrought iron but more brittle.
Iron formed the material basis of human civilization for over three thousand years until the mass production of steel in AD. Steel is an alloy of iron and carbon. It can contain small quantities of silicon, phosphorus, sulfur and oxygen. The carbon content of steel is between 0. This makes it harder than wrought iron but not as brittle as cast iron. Steel has a unique balance of hardness, flexibility and tensile strength. At the same time it resists shock and tension better than the more brittle cast iron.
To make steel, iron ore is heated and melted in furnaces where the impurities are removed and carbon added. Blast furnaces use mainly raw materials iron ore, limestone and coke with some scrap steel to make steel, whereas Electric Arc Furnaces use mainly scrap steel. The blast furnace was invented by an Englishman named Henry Bessemer in the mid s. The slag is mostly calcium silicate and contains most of the commercially unimportant components of the ore:. Just below the middle of the furnace, the temperature is high enough to melt both the iron and the slag.
They collect in layers at the bottom of the furnace; the less dense slag floats on the iron and protects it from oxidation.
Several times a day, the slag and molten iron are withdrawn from the furnace. Much of the iron produced is refined and converted into steel. Steel is made from iron by removing impurities and adding substances such as manganese, chromium, nickel, tungsten, molybdenum, and vanadium to produce alloys with properties that make the material suitable for specific uses.
Most steels also contain small but definite percentages of carbon 0. However, a large part of the carbon contained in iron must be removed in the manufacture of steel; otherwise, the excess carbon would make the iron brittle. However, there is not just one substance called steel - they are a family of alloys of iron with carbon or various metals.
Impurities in the iron from the Blast Furnace include carbon, sulfur, phosphorus and silicon, which have to be removed. Cast iron has already been mentioned above. This section deals with the types of iron and steel which are produced as a result of the steel-making process.
Austin State University with contributing authors. The limestone and ore form a slag that floats on the surface. Impurities, including carbon, are oxidized and float out of the iron into the slag. When the carbon content is right, you have carbon steel. Another way to create steel from pig iron is the Bessemer process , which involves the oxidation of the impurities in the pig iron by blowing air through the molten iron in a Bessemer converter.
The heat of oxidation raises the temperature and keeps the iron molten. As the air passes through the molten pig iron, impurities unite with the oxygen to form oxides. Carbon monoxide burns off and the other impurities form slag. However, most modern steel plants use what's called a basic oxygen furnace to create steel.
The advantage is speed, as the process is roughly 10 times faster than the open-hearth furnace. In these furnaces, high-purity oxygen blows through the molten pig iron, lowering carbon, silicon, manganese and phosphorous levels.
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