Electric Steel Making Process:
Nowadays, about 23% of the total worldwide crude steel production is made by the Electric Steelmaking processing route which mainly involves the following stages:
- RAW MATERIALS
- PRIMARY METALLURGY
- SECONDARY METALLURGY
- CONTINUOUS CASTING
Raw material may be up to 100% scrap or various mixes of hot metal, pig iron, DRI (hot and/or cold) and HBI.
Worldwide, scrap covers around 75% of the raw materials for EAF, while DRI and HBI covers around 15% and the balance (10%) is covered by pig iron and hot metal.
Scrap is the mostly used raw materials for the EAF charge. It classification depends on the grades of steel manufactured and the sorts of charge scrap available commercially in the market. The following properties should be entailed for the sake of the scrap material classifications:
- Content of alloying elements possible to be reclaimed in the steel smelting process;
- Content of harmful elements (including sulphur, phosphorus, copper, zinc);
- Geometry of scrap (relevant density and size). The knowledge of the scrap material geometry enables optimum filling of the EAF charge baskets.
DIRECT REDUCED IRON (DRI)
Direct Reduced Iron (DRI) is the product of the direct reduction of iron ore in the solid state by carbon monoxide and hydrogen derived from natural gas or coal.
Most gas-based direct reduction plants are part of integrated steel mini-mills, located adjacent to the electric arc furnace (EAF) steel plant. DRI can be either hot or cold charged to the EAF. Some steel companies ship DRI from their captive direct reduction plants to their remote steel mills and a small volume of DRI is sold to third parties. There are many small rotary kiln furnaces producing DRI, known as sponge iron, using coal as energy and reductant source. Some of the sponge iron plants are captive to steel mills.
HOT BRIQUETTED IRON (HBI)
Hot Briquetted Iron (HBI) is a premium form of DRI that has been compacted at a temperature greater than 650° C at time of compaction and has a density greater than 5,000 kilograms per cubic metre (5,000 kg/m3).
HBI is compact and the briquettes are easy to feed into the furnace. In contrast, scrap is most unwieldy and sometimes needs cutting down, an expensive process. It also frequently breaks the electrodes. By using HBI both the availability of the smelting furnace and the safety of employees while charging can be increased.
Electric Arc Furnace (EAF) is a central part of the production route of the Electric Steelmaking process that is an alternative to the dominant Basic Oxygen Steelmaking (BOS) route.
EAF is generally used to produce carbon steels and alloy steels primarily by recycling ferrous scrap. In the EAF, scrap and sometimes other raw materials such as DRI, HBI and pig iron are melted and converted into high quality steel by using high-power electric arcs formed between a cathode and one (for direct current DC) or three (for alternating current AC) graphite electrodes (anodes).
The EAF processing route could be summarized as follows:
- CHARGING OF RAW MATERIALS
CHARGING OF RAW MATERIALS
The charging mix (scrap plus other raw materials as indicated above) is put into the furnace from the top using a steel basket. Burnt lime and fluorspar are also added to help the slag formation. Usually more than one basket are used to charge the furnace completely. Once the charge of the first basket has completely melted down and a flat bath is reached, another bucket of scrap can be charged into the furnace and melted down. Some furnaces are equipped with continuous charging.
Melting is carried out by supplying energy to the furnace interior. This energy can be electrical or chemical. Electrical energy is supplied through the graphite electrodes and normally has a very large contribution in the melting phase. Chemical energy is supplied through many sources which include oxy-fuel burners and oxygen lances.
The entire duration of the process can be divided into three main stages:
Heating stage: The scrap is heated by the arc and oxy-fuel burners at the top of the furnace. The heating is done by radiation, convection (due to the hot gas going through the scrap porosities) and conduction (due to contact points between scraps). The temperature at the top increases to reach the melting temperature of the scrap. In this stage the gas – solid phases exist.
Melting stage: The melting starts at the top surface of the scrap pile, and molten liquid penetrate towards the bottom of the furnace and its height increases whereas the height of the scrap pile decreases. In this stage, there are gas – solid and solid –liquid phases in the bath.
Finishing stage: The scrap is completely covered by the melt and only the solid –liquid phase exists until all the scrap is completely melted. At this point, usually bath temperature and bath sample is taken. The analysis of the bath chemistry allows the furnace operator to determine the amount of O2 needed to be blown during refining.
Lower voltages (short arc) are selected for the first stage of melting to protect the roof and the walls from the arcs. Subsequently voltage can be increased (long arc) and the electrodes raised slightly increasing the length of the arc and increasing power to the melt.
Refining operation is carried out following the melting phase after the flat bath conditions are achieved. At this stage, excessive carbon and other elements (such as phosphorous, silicon, manganese) oxidize. The oxidation of the carbon is also known as decarburization.
At the start of the melting process, the arcs are covered by the scrap, so the radiation from the arcs to the lateral walls is low, but when the scrap starts melting, the heat efficiency decreases since the radiation to the walls increases. Thus, a foaming slag is used to cover the arcs as more heat can be transferred to the bath and scrap.
Foaming slag is formed by injection of carbon and oxygen, First, the carbon inside the bath is oxidized by oxygen blowing. Oxygen also oxidized certain amount of iron and FeO is formed. The carbon injected reduces iron oxide. These two reactions generate bubbles of CO which go upward and foam the slag.
The function of the slag (mainly composed by lime CaO) is to refine the steel from the sulphur (desulfurization) and absorbs the oxides, formed as a result of deoxidation (also known as killing process).
The typical composition of EAF slag is CaO (45 % to 58 %) SiO2 (5 % to 15 %) FeO (10 % to 28 %) MgO (5% to 8%) and MnO (2 % to 5 %), Besides it also contains CaF2, S, and P.
The furnace is tilted backwards and slag is poured out of the furnace through the slag door. Removal of the slag eliminates the possibility of P reversion.
Once the desired steel composition and temperature have been achieved in the furnace, the tap hole or the EBT (eccentric bottom tapping) is opened, the furnace is tilted, and the steel is poured into a teeming ladle for transfer to the secondary steel making unit. During the tapping process, ferro alloy additions are made based on the bath analysis and the desired steel grade. Deoxidizing agent are added to the steel to lower the O2 content prior to further processing. Common deoxidizers are Al, ferrosilicon and silico manganese. While making C steel heats a minimum of slag carry over is aimed. A new slag cover is built during tapping. For ladle furnace operations, a calcium aluminate slag is a good choice for the control of S. Slag forming compounds are added in the ladle at tapping so that a slag cover is formed prior to transfer to the ladle furnace.
The ladle is transferred to a device called Ladle Furnace (LF), where it’s places under a cover equipped with three graphite electrodes columns which are connected to an AC transformer.
The primary functions of the LF is the reheating of liquid steel through electric power conducted by the graphite electrodes and homogenization of steel temperature and chemistry through inert gas stirring. In fact the ladle bottom has a porous plug which allows the gas inert argon supply. Moreover the LF stand is also equipped with an addition hopper mounted on the cover (ferroalloys addition) and a lance for injection of desulfurizing agent.
During the treatment, molten steel is covered by a layer of slag. The fumes formed during the operation are extracted through a suction system installed at the cover.
Additional devices, such as VD (vacuum degasser) could be adopted in the secondary metallurgy in order to produce special kind of steel.
The molten metal is now cast through a mold, the casting takes the two dimensional profile of the mold but its length is indeterminate. The casting will keep traveling downward and its length increases with time. New molten steel is constantly supplied to the mold with a specific rate to keep up with the solidifying casting.
Molten metal, from the ladle, is poured into a tundish, which is a container that is located above the mold, it holds the liquid metal for the casting. This particular casting operation uses the force of gravity to fill the mold and to help move along the continuous metal casting. The tundish is constantly supplied with molten steel to keep the process going. The whole process is controlled to ensure there is smooth flow of molten steel through tundish.
Further, the impurities and slag are filtered in tundish before they move into the mold. The entrance of the mold is filled with inert gases to prevent reaction of molten steel with the gases in the environment like oxygen. The molten metal moves swiftly through the mold and it does not completely solidify in it. The entire mold is cooled with water that flows along the outer surface.
The metal casting moves outside the mold with the help of different sets of rollers. While one set of rollers bend the metal cast, another set will straighten it. This helps to change the direction of flow of the steel slab from vertical to horizontal.
Author: Eng. Matteo Sporchia
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