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.






ABC of graphite electrodes (2) – what are electrodes made of?

this is part 2 of our series. As promised, the first article covers the raw materials of graphite electrodes.

Raw materials of graphite electrodes

We only mention here the raw materials which become a part of the electrode end product. Apart from those we can mention auxiliary materials such as Quartz Sand for the baking process. The raw materials that make up an electrode recipe are as follows.

Petroleum coke & needle coke

Petroleum coke is produced using the route of crude oil vacuum distillation and delayed coking. Needle coke is a highly crystalline petroleum coke type with clear striped texture with directionality to allow for high thermal expansion coefficient and electrical conductivity. Therefore, needle coke is the most ideal material for the production of graphite electrodes.  Interesting for us is the ratio of petroleum coke and needle coke. In practice of electrode making, petroleum and needle coke contents have an inverse relationship: the lower the content of needle coke, the higher that of petroleum coke and vice versa. As we will see in the section about graphite electrode grading system, we can denote an electrode formula superior in quality, the higher. The leading manufacturers for needle coke by annual output are as follows: Seadrift coke LP (USA), Conoco Philips (Britain), Mizushima Ferroalloy Co., Ltd (Japan), Jinzhou CNPC (Jinzhou, China), Gaoqiao CNPC (Shanghai, China), Mitsubishi Chemical (Japan), Hongte (Shanxi, China), Sinosteel Anshan Research Institute of Thermo-energy Co., Ltd (Anshan, China)

Metallurgical coke

In the form of powder & particles, metallurgical coke is a coal with a low ash and sulfur content. It is beneficial to add this ingredient to increase hardness and decrease the release of burned ash during electrode use.

Coal tar pitch

Used during the kneading and the impregnation phases of the production process. When kneading, the liquid coal tar pitch is added to combine the otherwise loose particles to form a soft mass. Like metallurgical coke, it is produced from coal over the route coal -> coal tar -> coal tar pitch utilizing fractional distillation. For the impregnation stage (which is the production step between baking and graphitization; more on this process later), it is used to decrease the porosity by filling the many small holes inside the semi-finised electrode. Thereby, the product gains in density.

In the next article, we will be concerning ourselves with the question: how are graphite electrodes produced? We are going to focus on the production process overview.

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