Extracting iron from iron ore using a Blast Furnace

Introduction

The common ores of iron are both iron oxides, and these can be reduced to iron by heating them with carbon in the form of coke. Coke is produced by heating coal in the absence of air.

Coke is cheap and provides both the reducing agent for the reaction and also the heat source - as you will see below.

Iron ores

The most commonly used iron ores are haematite (US: hematite), Fe2O3, and magnetite, Fe3O4.

The heat source

The air blown into the bottom of the furnace is heated using the hot waste gases from the top. Heat energy is valuable, and it is important not to waste any.

The coke (essentially impure carbon) burns in the blast of hot air to form carbon dioxide - a strongly exothermic reaction. This reaction is the main source of heat in the furnace.

The reduction of the ore

At the high temperature at the bottom of the furnace, carbon dioxide reacts with carbon to produce carbon monoxide.

It is the carbon monoxide which is the main reducing agent in the furnace.

In the hotter parts of the furnace, the carbon itself also acts as a reducing agent. Notice that at these temperatures, the other product of the reaction is carbon monoxide, not carbon dioxide.

The temperature of the furnace is hot enough to melt the iron which trickles down to the bottom where it can be tapped off.

The function of the limestone

Iron ore isn't pure iron oxide - it also contains an assortment of rocky material. This wouldn't melt at the temperature of the furnace, and would eventually clog it up. The limestone is added to convert this into slag which melts and runs to the bottom.

The heat of the furnace decomposes the limestone to give calcium oxide.

This is an endothermic reaction, absorbing heat from the furnace. It is therefore important not to add too much limestone because it would otherwise cool the furnace.

Calcium oxide is a basic oxide and reacts with acidic oxides such as silicon dioxide present in the rock. Calcium oxide reacts with silicon dioxide to give calcium silicate.

The calcium silicate melts and runs down through the furnace to form a layer on top of the molten iron. It can be tapped off from time to time as slag.

Slag is used in road making and as "slag cement" - a final ground slag which can be used in cement, often mixed with Portland cement.

Cast iron

The molten iron from the bottom of the furnace can be used as cast iron.

Cast iron is very runny when it is molten and doesn't shrink much when it solidifies. It is therefore ideal for making castings - hence its name. However, it is very impure, containing about 4% of carbon. This carbon makes it very hard, but also very brittle. If you hit it hard, it tends to shatter rather than bend or dent.

Cast iron is used for things like manhole covers, cast iron pipes, valves and pump bodies in the water industry, guttering and drainpipes, cylinder blocks in car engines, Aga-type cookers, and very expensive and very heavy cookware.

At the time of writing (2015), world production of iron castings was about 75 million tonnes per year.

Steel

Most of the molten iron from a Blast Furnace is used to make one of a number of types of steel. There isn't just one substance called steel - they are a family of alloys of iron with carbon or various metals. More about this later . . .

Steel-making: the basic oxygen process

Impurities in the iron from the Blast Furnace include carbon, sulphur, phosphorus and silicon. These have to be removed.

Removal of sulphur

Sulphur has to be removed first in a separate process. Magnesium powder is blown through the molten iron and the sulphur reacts with it to form magnesium sulphide. This forms a slag on top of the iron and can be removed.

Removal of carbon etc

The still impure molten iron is mixed with scrap iron (from recycling) and oxygen is blown on to the mixture. The oxygen reacts with the remaining impurities to form various oxides.

The carbon forms carbon monoxide. Since this is a gas it removes itself from the iron! This carbon monoxide can be cleaned and used as a fuel gas.

Elements like phosphorus and silicon react with the oxygen to form acidic oxides. These are removed using quicklime (calcium oxide) which is added to the furnace during the oxygen blow. They react to form compounds such as calcium silicate or calcium phosphate which form a slag on top of the iron.

Types of iron and steel

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.

Wrought iron

If all the carbon is removed from the iron to give high purity iron, it is known as wrought iron. Wrought iron is quite soft and easily worked and has little structural strength. It was once used to make decorative gates and railings, but these days mild steel is normally used instead.

Mild steel

Mild steel is iron containing up to about 0.25% of carbon. The presence of the carbon makes the steel stronger and harder than pure iron. The higher the percentage of carbon, the harder the steel becomes.

Mild steel is used for lots of things - nails, wire, car bodies, ship building, girders and bridges amongst others.

High carbon steel

High carbon steel contains up to about 1.5% of carbon. The presence of the extra carbon makes it very hard, but it also makes it more brittle. High carbon steel is used for cutting tools and masonry nails (nails designed to be driven into concrete blocks or brickwork without bending). You have to be careful with high carbon steel because it tends to fracture rather than bend if you mistreat it.

Iron and steel industry in India

 In 1874,  the Bengal Iron Works (BIW) came into being at Kulti, near Asansol in West Bengal. In 1889 the Bengal Iron and Steel Company acquired the plant and by the turn of the century the Kulti plant became a success story. It produced 40,000 tonnes of pig iron in 1900 and continued to produce the metal until it was taken over by Indian Iron and Steel Company (IISCO) in 1936.

 For modern India’s iron and steel industry August 27, 1907 was a red-letter day when the Tata Iron and Steel Company (TISCO) was formed as a Swadeshi venture to produce 120,000 tonnes of pig iron. The TISCO plant at Sakchi (renamed Jamshedpur) in Bihar, started pig iron production in December 1908 and rolled out its first steel the following year.  TISCO had expanded its production capacity to one million tonnes ingot by the time the country achieved freedom.

In 1918, soon after the war, Indian Iron and Steel Company (IISCO) was formed

Government decided to start a chain of steel plants all over the country in the public sector. The first such plant was set up at Rourkela in Orissa. The second came up at Bhilai in Madhya Pradesh. It was followed by a third at Durgapur in West Bengal. Each of these three plants had an initial production capacity of one million tonne ingot. 

As a matter of fact, the country was dotted with steel and steel-related plants in public and private sectors, like Alloy Steel Plant, Salem Steel Plant, Kalinga Iron Works, Malavika Steel Ltd., Jindal Vijaynagar Steel Ltd., to name only a few. About the same time TISCO launched its two-million-tonne expansion programme

The finished steel pdroduction in India has gone up from mere 1.1 million tonnes in 1951 to 23.37 million tonnes in 1997-98 despite overall economic slow-down in the country. 

The year 2004-05 proved to be a fortunate year for the Indian steel

industry because many of the steel making units were able to earn profits or

reduce their previous debts due to the increased demand in steel consumption and increase in steel prices. In 2004-05 the finished steel production was 40 million tons which was again increased to 49.39 million tones in the year 2006-07.

Iron and steel industry

Certain metals, notably tin, lead and (at a higher temperature) copper, can be recovered from their ores by simply heating the rocks in a fire or blast furnace, a process known as smelting.  It was discovered that by combining copper and tin, a superior metal could be made,  an alloy called bronze 

The extraction of iron from its ore into a workable metal is much more difficult than for copper or tin.  In order to convert a metal oxide or sulphide to a purer metal, the ore must       be reduced physically, chemically, or electrolytically.  

 After mining, large pieces of the ore feed are broken through crushing and/or grinding in order to obtain particles small enough where each particle is either mostly valuable or mostly waste. Concentrating the particles of value in a form supporting separation enables the desired metal to be removed from waste products.  A concentrate may contain more than one valuable metal. That concentrate would then be processed to separate the valuable metals into individual constituents. 

 Leaching dissolves minerals in an ore body and results in an enriched solution. The solution is collected and processed to extract valuable metals.  

The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation with air being blown through the molten iron. The oxidation also raises the temperature of the iron mass and keeps it molten. 

The blowing of air through the molten pig iron introduces oxygen into the melt which results in oxidation, removing impurities found in the pig iron, such as silicon, manganese, and carbon in the form of oxides. These oxides either escape as gas or form a solid slag 

The Bessemer process revolutionized steel manufacture by decreasing its cost, from £40 per long ton to £6–7 per long ton, along with greatly increasing the scale and speed of production of this vital raw material. The process also decreased the labor requirements for steel-making. Before it was introduced, steel was far too expensive to make bridges or the framework for buildings and thus wrought iron had been used throughout the Industrial Revolution. After the introduction of the Bessemer process, steel and wrought iron became similarly priced, and some users, primarily railroads, turned to steel. 

     Iron extraction 

 A process known as potting and stamping was devised in the 1760s and improved in the 1770s, and seems to have been widely adopted in the West Midlands from about 1785. However, this was largely replaced by Henry Cort's puddling process, patented in 1784, but probably only made to work with grey pig iron in about 1790. These processes permitted the great expansion in the production of iron that constitutes the Industrial Revolution for the iron industry.[65] 

In the early 19th century, Hall discovered that the addition of iron oxide to the charge of the puddling furnace caused a violent reaction, in which the pig iron was decarburised, this became known as 'wet puddling'. It was also found possible to produce steel by stopping the puddling process before decarburisation was complete. 

The efficiency of the blast furnace was improved by the change to hot blast, patented by James Beaumont Neilson in Scotland in 1828. This further reduced production costs. Within a few decades, the practice was to have a 'stove' as large as the furnace next to it into which the waste gas (containing CO) from the furnace was directed and burnt. The resultant heat was used to preheat the air blown into the furnace.[66 

The problem of mass-producing cheap steel was solved in 1855 by Henry Bessemer, with the introduction of the Bessemer converter at his steelworks in Sheffield, England. (An early converter can still be seen at the city's Kelham Island Museum). In the Bessemer process, molten pig iron from the blast furnace was charged into a large crucible, and then air was blown through the molten iron from below, igniting the dissolved carbon from the coke. As the carbon burned off, the melting point of the mixture increased, but the heat from the burning carbon provided the extra energy needed to keep the mixture molten. After the carbon content in the melt had dropped to the desired level, the air draft was cut off: a typical Bessemer converter could convert a 25-ton batch of pig iron to steel in half an hour. 

Finally, the basic oxygen process was introduced at the Voest-Alpine works in 1952; a modification of the basic Bessemer process, it lances oxygen from above the steel (instead of bubbling air from below), reducing the amount of nitrogen uptake into the steel. The basic oxygen process is used in all modern steelworks; the last Bessemer converter in the U.S. was retired in 1968. Furthermore, the last three decades have seen a massive increase in the mini-mill business, where scrap steel only is melted with an electric arc furnace. These mills only produced bar products at first, but have since expanded into flat and heavy products, once the exclusive domain of the integrated steelworks. 

  Basic oxygen steelmaking (BOS, BOP, BOF, and OSM), also known as Linz–Donawitz-steelmaking or the oxygen converter process^[1]is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic because fluxes of burnt lime or dolomite, which are chemical bases, are added to promote the removal of impurities and protect the lining of the converter.^[2]