Understanding Blast Furnace Production

The two main raw materials in the refining of iron ore to steel are iron ore and cooking coal. The process involved is called the blast furnace production. Pig iron is the product of smelting iron ore with a high – carbon fuel and reductant such as coke, usually with limestone as a flux. Pig iron is produced by smelting iron ore in blast furnaces or by smelting ilmenite in electric furnaces. In the blast furnace, counter current gas/solids reactor in the descending column of burden materials reacts with the ascending hot gases. The process is continuous with raw materials being regularly charged to the top of the furnace and the molten iron and slag being tapped from the bottom of the furnace at regular intervals.

The key steps of the process are:

Upper part of the furnace-the free moisture is driven off from the burden materials and hydrates and carbonates are dissociated.

Lower part of the blast furnace shaft-indirect reduction of the iron oxides by carbon monoxide and hydrogen occurs at 700-1000 degrees.

Both area of the furnace where the burden starts to soften and melt-direct reduction of the iron oxides and carbonization by coke occurs at 1000-1600 degrees. Molten iron and slag start to drip through the bottom of the furnace.

Between the bosh and the hearth are the tubers through which the blast-combustion air, preheated to 900-1300 degrees, often enriched with oxygen is blown into the furnace. In front of the tuyers is the combustion zone, where coke reacts with oxygen and steam in the blast to form carbon monoxide and hydrogen. The molten iron and slag collect in the furnace hearth. Being less dense, the slag floats on top of the iron. Slag and iron are tapped at regular intervals through tap holes. The slag is transferred to slag pits for further processing into usable materials.

The raw materials involved in this process are; iron ore, which is treated in some way after it comes from the mine. Coal which must be converted to coke is also used, limestone, steel scrap, steel producer, fluxing materials, refractory materials and alloys. The equipment’s used in this experiment are washing machine magnetic separator, pelletizer, mixer, hot blast cupola blower, pouring ladle, thermocouple, hammer mill and vibrating screen.

The chemical process taking place in the blast furnace are referenced bellow. Composite pellets with about 10, 15, and 20mm diameter are rapidly smelted to produce pig iron. The refining of iron to make steel is where the carbon content of hot metal is lowered usually to less than 1% by an oxidation process in steelmaking furnace. At the time alloying materials are added to the furnace to achieve the required chemical composition of the final product. The Bessemer processes was the first inexpensive process for the mass production of steel from molten pig iron. The open hearth furnace is the process where excess carbon and other impurities are burnt out of pig iron to produce steel.

Impurities removed are nitrogen, silicon, phosphorous, Sulphur, and excess carbon. The slag from the blast furnace is poured from ladle into the BOF and water- cooled lance is lowered into the vessel. As additional scrap steel are charged into the basic oxygen furnace, oxygen is blown through the lance at high pressure reacting chemically with carbon to burn off impurities. This technique is called electric arc when furnace is charged by an electric arc. Blowing oxygen forces impurities to react with flux to form slag or escape through the top of the furnace as fumes. Flux acts as the chemical cleaning agent absorbing the iron impurities and leaving behind molten steel. The result is separate levels of slag and molten steel.

Reference

Bugayev, K., and E. Tretyakov. Iron and steel production. The Minerva Group, Inc., 2001.

Bhardwaj, B. P. Steel and Iron Handbook. NIIR project consultancy services, 2014.

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