The Story of Steel Part #2. The Creation of Steel

This is The Now

In Part 1 of “the Story of Steel” we discussed the source of iron, a key ingredient in steel, and saw that it originated in the stars and is present on Earth in the form of its oxide, iron oxide (FeO). Now we need to discuss how we get from the iron ore to steel and, what the ramifications of this may be.

To date, the first and traditional step, after mining the iron ore and delivering it to so called  “integrated steel mills”, is converting the ore into pig iron in a blast furnace. In addition to the ore, we also need coke and limestone for the reaction in the blast furnace to occur. A blast furnace is shown in Figure 1

Figure 1. Layout of a Blast Furnace

We have seen where iron ore is present in Canada but the country also has significant deposits of metallurgical coal, used to produce coke, and of limestone which chemically is Calcium Carbonate (Ca CO3)

Coal is Canada’s most abundant fossil fuel, with over 6.6 billion tonnes of recoverable coal reserves. Canada has both metallurgical coal and thermal coal deposits used mainly for two purposes, for steel making and power generation. Only metallurgical coal, with its specific coking properties, can be used in the blast furnace.

Limestone is found in the Northwest Territories, British Columbia, Alberta, Manitoba, Ontario, Nova Scotia and Newfoundland. Ontario and Québec produce over 70% of Canada's total lime

Returning to the Blast Furnace, the charge loaded into the top is made up of:

1)   iron ore  2)  limestone 3)  coke

The separation of the metal from the oxide ore requires a reducing agent to combine with the oxygen. The reducing agent is carbon monoxide which forms from coke in the heat of a furnace. Coke is a fuel with very few impurities and a high carbon content.  

A hot blast of air is blown into the lower section of the furnace through a series of pipes called tuyeres The limestone acts as a fluxing agent and removes impurities to form a slag. The molten iron collects at the bottom of the furnace and is run out into a hot metal ladle. This iron contains many impurities such as phosphorus, and a lot of carbon up to about 3% to 4%. It is known as pig iron.

The chemical reaction that occurs in the blast furnace is:

FeO + CO = Fe + CO2

The second step is to take the pig iron from the blast furnace and convert it to steel. The two types of furnace that produce the majority of the worlds steel are the basic oxygen process (BOF) and the electric arc process (EAF). The basic oxygen process, also known as the LD process, is illustrated in Figure 2.

Figure 2. The Basic Oxygen or LD Process for Steel Making

In a basic oxygen furnace, oxygen is blown into liquid metal and heat is generated by the oxidation of elements such as carbon and silicon. The carbon content is lowered to that which we classify as steel,  being less than about 1.7% from the 3-4% in the pig iron. The vessel can be tilted to pour out the steel when ready, requiring less than an hour to produce a 250-ton heat of steel. The term “basic” refers to the chemical nature of the refractory lining of the furnace. Basic materials help in removing impurities such as phosphorus.

So now we have steel, but its production via the two-stage process in so called “integrated steel plants”, of pig iron in the blast furnace and steel in the LD furnace cannot be thought of as “green”. Both of these furnaces produce significant amounts of emissions in the form of greenhouse gases and the future of steel making dictates that changes will need to be made.

An alternate and second way to make steel is to use the Electric Arc Furnace (EAF). The electric furnace uses electricity to supply the heat required for melting and, the refining conditions in these furnaces are generally more controllable than in the basic oxygen furnaces. The refining takes place in an enclosed environment, where temperatures and operating conditions are kept under rigid control.  The charge used to feed the electric furnace is scrap steel and, as such, all steel produced is generally recyclable again and again by using the EAF approach. 

The scrap is carefully analyzed and sorted, as its alloy content will affect the composition of the refined metal. After the furnace is charged, electrodes are placed in close proximity to the metal. The current enters through one of the electrodes, arcs the charge and then flows through the metal back to the next electrode. Heat is generated due to the resistance the charge produces to the current flow. An EAF is depicted in Figure 3

Figure 3. Schematic of and Electric Arc Furnace

This is the Future

To reduce the greenhouse gas effects of the “integrated steel plants” and progress to the production of “green” steel” the ore can be reduced by using the so called “direct reduction iron “or DRI process. The DRI process is comparatively energy efficient as it requires significantly less fuel as the traditional blast furnace is now no longer needed. In the DRI process the objective is to remove the oxygen contained in the various forms of mined iron without melting it, the usual fuel used for reduction being natural gas.

DRI iron is most commonly made into steel using electric arc furnaces to take advantage of the heat produced by the DRI product itself. 

Currently, at least one plant in Canada is transitioning away from the blast furnace-BOF production route to direct-reduced iron (DRI) and EAF production. The switch means the mill may consume more ferrous scrap, and it will carry a “significantly lower carbon footprint”.  An additional plant is now in the process of constructing two new state- of-the-art electric-arc-furnaces (EAF) to replace its existing blast furnace and basic oxygen steelmaking operations. The transformation to “green steel” is expected to reduce the carbon emissions of these plants by approximately 60-70%.

Also in Europe, particularly in Sweden, Germany and Spain, research, development and application is underway to produce green steel. One such evolution is to use hydrogen. This entails the coke, which is currently used to reduce the iron ore to iron, being replaced with hydrogen gas which is produced by electricity from fossil-free sources of energy. 

The result will be fossil-free steel-making technology, from iron to EAF steel, with virtually no carbon footprint. A by-product of this process is water, which can be of course be recovered for production of new hydrogen gas. Other European nations are also expected to replace coal fired blast furnaces with hydrogen powered technology by 2025-27.

So, the move to produce green steel is underway and gaining significant momentum. In Part 3 of the Story of Steel we will go one step further and discuss what influences the steel that is poured from the steel making furnaces.