BLAST FURNACE

 

For thousands of years, the blast furnace has provided the main means of iron production.  In the furnace, iron ore is heated by burning coke (a by-product of coal) in a blast of air.  The heat causes chemical reactions to take place and the iron ore is converted into iron.

 

 

STRUCTURE AND SITING:

 

   Modern blast furnaces are large, tower-like buildings, often more than 100 feet (30meters) high.  Above the furnace, another 100 feet or so is taken up by gas pipes and equipment for loading the furnace.  The hearth of the furnace measures up to about 45 feet (14 meters).

 

   Blast furnaces used to be built close to supplies of iron ore and coal suitable for making into coke.  But now the raw materials often have to be brought in from other countries.  The furnaces are mostly sited near ports or major waterways, which avoids the high cost of having to transport the raw materials over land.  The overall cost of producing the iron is greatly reduced.

 

 

The blast furnace works on a counter-current principle.  Due to the higher economy of bigger furnaces, the number of high capacity units are in operation has been increasing during the descent, the furnace diameter is enlarged from the top downward to avoid “scaffolding”, i.e. clogging of the furnace.

 

Since in the lower parts of the furnace the charge materials or their products of transformation melt with volume contraction, it is possible to reduce the furnace diameter.

 

For charging, coke and burden (ore and flux) are fed in layers.  This burdening is effected either by means of a lift system where a container (“bell type distributing gear”) is put on the furnace and emptied or by means of a permanent installed container, which is filled with a conveyor belt.  A rotating runway allows continuous feeding and even charging. 

Another advantage of the latter system is that less energy-rich top gas is lost during burdening.  The flux is necessary to transform the gangue (iron-free stone of the ore, usually silicondioxide or aluminium (III) oxide) in well melting calcium – aluminium – silicates.  This slag also dissolves the ash contains in the coke.

 

Below the largest diameter the tuyere line is located in which air, preheated to 900-1300^oC and sometimes enriched with oxygen is blown through 6 – 12 tuyeres (water cooled nozzles of copper).

The room in front of these tuyeres is the hottest place (1850 – 2200 ^oC) in the furnace.  In the lowest part of the blast furnace (“hearth”) the liquid pig iron is collected, which is covered by the molten slag preventing the reoxidation of the iron by the wind.

 

The blast furnace itself is composed of a steel coat, which is lined with refractory material (fire clay, mullite, corundum, chromium oxide, carbon below the tuyeres.)  In addition, the warmer parts (>300^oC) of the coat are water-cooled because strength of steel decreases markedly at higher temperatures.  From outside the furnace is propped by a supporting structure.

 

The furnace interior can be separated into different areas, according to the temperatures inside the various areas in the furnace and also the reactions occurring.

 

The following table summarises the reactions occurring in the various areas within the furnace and the main use, or purpose of each part:

 

 

 

AREA IN FURNACE	TEMPERATURE (oC)	USE / REACTIONS OCCURRING
TOP	200 – 250	Drying of the charge, preheating.
UPPER SHAFT	250 – 700	Decomposition of hydrates and carbonates which requires large amounts of energy.
LOWER SHAFT	700 – 1000	“Indirect reduction” of ferrous oxides with carbon monoxide and hydrogen.  A far-reaching indirect reduction is desired for economical reasons.    The reduction of Fe3O4 to FeO.    CO2 formed during the reduction processes is retransformed to CO by means of coke in a temperature – dependent equilibrium reaction.    Recovering of CO loses importance with decreasing temperature, because the reaction velocity becomes too slow and in addition the position of equilibrium shifts to higher carbon dioxide shares.
BOSH	1000 – 6000	“Direct reduction”.  Ferrous oxide and other oxides also (among other manganese, silicon, phosphorus) are reduced directly with coke.    At the same time iron and gangue are melting and the liquid drops through the glowing coke, and the liquid iron drops are further arburised.    Carburizing reduces the melting point to 1100 – 1300oC (pure iron = 1539oC)
HEARTH	1600	(In the combustion area in front of the tuyeres up to       2200oC)   Gasification of coke    Formation of CO and H2    Generation of heat.

 

 

 

 

 

PIG IRON AND SLAG REMOVAL:

 

Pig iron and slag are “tapped” continuously from time to time through a tap hole. 

Flues (stopper where the iron is flowing below) make sure that the slag, which has a distinctive lower density then iron is collected separately.

A clay gun with a plastified mass of kaolin, fine coal and tar effects closing of the tap hole.

The huge furnaces, which produce large amounts of pig iron fill the pig iron into ‘torpedo ladles’.  There are horizontally fixed containers with a cylindrical middle part and a truncated cone-shaped lining inside.  The ladle can be rotated around its longitudinal axis in order to position an opening in the middle part on the top for filling and transportation and at the bottom for emptying.  Due to the closed region only little heat is lost.  The filling capacity amounts to 600 tonnes.  The slag is filled in tiltable railroad cars and dropped in a slag pit where it cools down and solidifies.  After crushing it is used for road construction or for production of mortar, cement or building stones.

 

 

 

USEFUL LINKS RELATED TO THE BLAST FURNACE AND THE MANUFACTURE OF IRON AND STEEL:

 

http://www.elmhurst.edu/~chm/onlcourse/chm110/outlines/steel.html

http://www.steel.org/learning/howmade/eaf.html