Steel Mini Mill. A short introduction

Steel Making Process

Steel is an iron alloy that has specific additives that give it increased strength, resistant to corrosion and other properties (depending on the metals added in it).

Steel gives a better life and more durability and has replaced nearly all kinds of iron works. An ancient art, steel manufacturing goes back to ancient Rome, China and even India.

Steel production, however, was done on a limited basis as the process of refining iron ore and the tight quality control required was difficult.

This changed with the Bessemer process, where concentrated oxygen (99% purity) replaced normal air. Since air is nearly 78% nitrogen and is chemically neutral in the process, it would take a lot of time and effort to refine the ore.

Introduction of pure oxygen through the Bessemer process largely increased the efficiency and reduced the time required, making steel a commercial viability. Overtime, other processes have been introduced, which are much more efficient.

Steel Mini Mill & Electric Arc Furnace

Once a secondary process, minimills mostly consists of recycling scrap metal to create steel. Modern steel making industry has changed this, with many industries using it exclusively.

The most common method of steel making in minimills is using and Electric Arc Furnace (EAF). EAFs consist of a large vat known as the furnace, which is lined with corrosion resistant chemicals.

Scrap iron placed inside the furnace in a layered fashion (bottom layer of small pieces, middle of large and then the top of smaller scrap pieces).

How Graphite Electrodes are used

Three thick graphite rods are lowered into the furnace and are electrically charged. When the graphite cores touch the metal, they create an electric arc which heats the scrap.

As the top scrap melts, the graphite rods are lowered and the electric potential increased. After a certain depth, the rods are slightly raised, allowing the liquid metal to pool around it and conduct the heat and electricity more efficiently.

The process is highly controlled and to decrease the time for smelting, the furnace can be with heated through a gas/fuel fire or even charged with pre melted material from the last batch.

Once the scrap is melted and impurities float on top, the furnace is tilted and tapped, allowing for the metal to flow down into a ladle furnace, where further chemicals can be added according to customer requirements.

Since the electric arc furnace is a batch process that can be started up or shut down with ease, it has become a very popular choice as it allows steel manufacturers to follow the market demand and cut down on production without incurring process losses.

Sources,melt%20(primarily)%20scrap%20steel.&text=Minimills%20make%20steel%20from%20scrap,into%20the%20semi%2Dfinished%20forms. (link below)

Click to access TM18-4_as_published.pdf





Primary Steelmaking for Beginners

Steelmaking history

Steelmaking has existed for nearly a thousand years, with modern techniques introduced in the 19th century. The process of manufacturing steel involves removal of impurities such as sulfur and silicon, with introduction of chromium and nickel to produce different grades of steel.

The history of steel making is very old, with it being found in ancient China, India, Iran and even Rome. Before the invention of the Bessemer process in 1850s, steel was only produced in small quantities, suitable for demands of small cities and states. The industrial revolution fueled the need of large scale production, with the Bessemer the only method of making steel in such large volumes.

Primary steel making either involves the use of pig iron or converted into steel, or the use of electric arc furnaces to melt steel scrap and recycle the material.

Oxygen Steelmaking

For using pig iron, the basic oxygen steelmaking method is used. This involves blowing oxygen in a carbon heavy melted iron. The oxygen is blown inside the furnace using a hollow pipe called the lance. The lance is liquid cooled to prevent its melting and its mouth is placed a few feet above the surface of the molten iron. The pressurized oxygen reacts with the carbon. The reaction ignites the carbon and an exothermic reaction raises the temperature to around 1,600 Celsius. Burnt lime or dolomite is introduced, which reacts with other impurities such as silicon and forms a layer on the top of the molten liquid, called slag.

After the purification is complete, the vessel is tilted and the molten iron is poured into another ladle furnace, where other metals and chemicals are added (nickel, chromium etc.) and mixed to produce the exact grade of steel required.

Electric Arc Furnace

Electric arc furnaces are usually used for melting scrap iron and steel. The process involves the use of a lined vessel that is fed with scrap and a three graphite rods are lowered, touching the surface. A high voltage electrical current is passed, which creates arcs and produce heat, melting the steel. To assist the process, some pre melted steel may also be added prior to the arcing and sometimes even gas burners are used to bring the temperature up to speed.

As in all steel making, after the melting is complete, the vessel is tilted to remove the liquefied steel, ensuring that the impurities that are floating on top stay behind to be removed later.