What is Steel?
Steel is an alloy made of iron and carbon. Carbon makes iron strong. Without carbon, iron is futile, it is squishy. Compared to other elements, these two elements rank fourth and fifteenth in terms of their abundance in the earth’s crust. Iron ore exists in the form of hematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3) and pyrites (FeS2). Hematite is the kind that goes into the making of iron and steel. (Sittig & Rao 648.)
China; India; Japan; and US are some of the largest steel producers in the world. While, China; Brazil, India; and Australia are some of the largest iron ore producers in the world.
What’s the traditional recipe for making steel?
You will need:
- Iron ore
- Blasting furnace
- Puddling furnace
Step 1: Smelt the iron ore to get pig iron
Mix iron ore and coke. Coke is a fuel here. Smelt the mixture in a blast furnace to get elemental iron by heating it. Smelting is the process of extraction of metal from its ore through heating. Smelting of coke produces carbon monoxide, which in turn reduces the iron ore to elemental iron. Blast furnace is an enclosed structure where smelting takes place. This process gives us pig iron. As funny as it may sound, pig iron is called pig iron because at the end of this process we pour the molten iron into long blocks called ‘pigs’.
Step 2: Remelt pig iron to get cast iron
The cast iron thus obtained is brittle. Even though carbon gives strength to iron, any more than an optimum amount can render it brittle. So, we need to get rid of the excess carbon.
Step 3: Puddle the cast iron to get wrought iron
Take a puddling furnace. Here you get rid of the brittle nature of cast iron. In this furnace, reheat cast iron and mix it with air. This process is known as puddling. Puddling cast iron will produce wrought iron, the kind that can be used to make sturdy armors. Air oxidizes (oxidation) the carbon in the ore, giving off carbon dioxide and carbon monoxide. Oxidation also removes other undesired elements. Impurities in cast iron can be carbon, sulfur and phosphorous. To remove phosphorus, limestone be added. Limestone is also referred to as carbonate flux here. Care should be taken that not much oxygen remains in the iron. One way to deal with this is to add molten spiegel (mixture of manganese + carbon + iron).
Step 4: Enhance the wrought iron to get steel
Various kinds of steel such as Mild Steel; Medium Carbon Steel; and High Carbon Steel can be made by optimizing the puddling process. Steel needs a carbon content ranging from 0.2 to 1.5 percent. This particular amount of carbon makes steel better than wrought iron (makes it stronger) and cast iron (removes brittleness). Large scale steel making began with the advent of Bessemer process. Now, steel is either made in an open hearth furnace (OHF) or electric-arc furnace (EAF) or by basic oxygen furnace (BOF).
Step 5: Shape and size the steel as desired and serve. Don’t serve hot.
Energy and environmental issues in the Steel industry:
The issues discussed in this section will cover an integrated iron and steel industry, as also suggest by the preparation method. Steel industry is by far the largest industrial emitter of greenhouse gases. And these are direct emissions. An example of direct emissions is fuel combustion. An example of indirect emissions is business travel. Investigation of the production of steel stepwise can show us the energy and environmental issues in the steel industry.
Pelletising of iron ore is an alternative to producing pig iron. These pellets can then be fed to the furnace instead of the lump ore. No loss of material is observed since pellets do not break during transport and handling. Similarly, sintering of iron ore is preferred as a blast furnace burden material. Sintering converts fine iron ore into larger particles suitable for furnace. Lesser consumption of coal has been reported when such feeds are used. Thus sintering and pelletising are ways to reduce carbon emissions.
A furnace requires a fuel. Depending on the kind of fuel, the carbon footprint of the process changes. If charcoal from afforested wood is to be used instead of coal, the net carbon dioxide emissions will be -1.1 ton (a negative number). This is because we used afforested wood. If we use coal, the production of one ton of pig iron would emit 1.9 tons of CO2 (a positive number, not good). The Brazilian steel is made using charcoal from afforested wood. Biomass briquettes or natural gas too can be used instead of coal to reduce the carbon footprint of this industry.
Waste gases (blast furnace gas (BFG)) evolve from the furnace which need to be treated before discharging them into the atmosphere. Waste gases however are not totally useless. Heat can be recovered from these waste gases and used back in the processes or used for power generation or be sold off. For example, coke dry quenching (CDQ) is a way to recover heat without the use of water. To further this, better heat recovery units may be created and established.
Remelting consumes energy as every other step during the preparation of steel. Furnace operating efficiencies can be improved.
Limestone is added to remove phosphorous. Limestone (CaCO3) also contributes to the carbon footprint via its calcination (formation of lime).
EAF uses carbon electrodes which give off carbon emissions after being consumed.
For not serving it hot, steel would need cooling. In this case, we can say that steel industry is water intensive as well. To address this issue, water is recycled and reused. Hence, water pollution is not a major issue as compared to air pollution in this industry.
Some advancements towards making a sustainable steel industry that may nullify the above issues are:
- steel can be recycled, all of it
- carbon capture and storage methods (CCS) may be employed
- process that replaces coal by natural gas, a cleaner, energy and water efficient process
- process known as ‘molten oxide electrolysis (MOE)’ that claims to be CO2-emissions free
The steel industry is not perfect and it is easy to forget how steel empowers other industries. The renewable energy sector depends on it. Energy efficiency buildings are made using steel. You could drive in an energy efficient car that is made of steel. Isn’t it a wonder material?
- Marshall Sittig & M. Gopala Rao, Outlines of Chemical Technology – For the 21st Century. 3rd ed. New Delhi. Affiliated East-West Press Pvt Ltd. 2007.
- Spoerl Joseph S., A Brief History of Iron and Steel Production. Saint Anselm College.
- Tata Steel, “Definitions of what is meant by cast iron, wrought iron and steel.”
- Carvalho Anthony de, “Challenges & opportunities for the steel industry in moving towards green growth.” 2010 PDF file.
- World Steel Association, “SUSTAINABLE STEEL At the core of a green economy.” PDF file.
- The Green Steel Revolution: Replacing Fossil Fuels In Steel Production, UNEP