Iron carbide is a high melting point, non-pyrophoric, strongly magnetic synthetic compound obtained in granular or powder form. It is composed of three atoms of Fe and one atom C and its chemical formula is Fe3C. The commercial iron carbide consists of around 90 % total iron and around 6 % to 6.5 % of total carbon. The primary use of the product is as a metallic charge during steelmaking for substitution of hot metal, direct reduced iron, or steel scrap.
Iron carbide is an intermetallic compound of iron and carbon. It is, more precisely, intermediate transition metal carbide. Its stoichiometric composition consists of 6.67 % carbon and 93.3 % iron (Fe) by weight. It has an orthorhombic crystal structure (Fig 1). It is a hard, brittle material and normally classified as a ceramic in its pure form. It is a frequently found and important constituent in ferrous metallurgy. While iron carbide is present in most steels and cast irons, it is produced as a raw material by the iron carbide process, which belongs to the family of alternative ironmaking technologies.
Fig 1 Crystal of iron carbide
Iron carbide is a premium quality feed for steelmaking in electric arc furnaces and basic oxygen furnaces. It is available as dark gray granules or powder. It offers matchless metallurgical advantages and outstanding cost savings. It has a density of 7.64 kg/cu m and is thus slightly denser than the liquid iron, which has a density of 6.98 kg/cu m.
The iron carbide is composed of three atoms of Fe and one atom C and is also known as cementite. Cementite is an intermetallic compound which is hard, brittle, and metastable because it tends to decompose in ferrite (or austenite) and graphite according to the reaction Fe3C = 3 Fe + C. In fact, this transformation is not established without sufficiently high temperatures and reasonably long residence times. In fact, iron carbide is stable at temperatures below 200 deg C.
In the Fe-C system (plain C steels and cast irons), it is a common constituent because ferrite can contain at most 0.02 % of uncombined carbon. Hence, in C steels and cast irons which are slowly cooled, a portion of the C is in the form of cementite (iron carbide).
Iron carbide is the second phase which is formed when carbon exceeds the limit of solubility which defines the point where the solubility of C in the Fe is at its maximum as shown in the Fe-C phase diagram in Fig 2, which is a graph of the balance for the combinations of C in a solid solution with iron.
Fig 2 Iron-carbon phase diagram
In the iron–carbon system, cementite forms directly from the melt in the case of white cast iron. In carbon steel, cementite precipitates from austenite as austenite transforms to ferrite on slow cooling, or from martensite during tempering. It is an intimate mixture with ferrite, the other product of austenite, known as pearlite and which has a lamellar structure.
While cementite is thermodynamically unstable, eventually being converted to austenite (low carbon level) and graphite (high carbon level) at higher temperatures, it does not decompose on heating at temperatures below the eutectoid temperature (723 deg C) on the metastable iron-carbon phase diagram. In pure form cementite changes from ferromagnetic to paramagnetic at its curie temperature of around 210 deg C.
Natural iron carbide (containing minor amounts of nickel and cobalt) occurs in iron meteorites and is called cohenite after the German mineralogist Emil Cohen, who first described it. As carbon is one of the possible minor light alloy components of metallic planetary cores, the high-pressure/high-temperature properties of cementite (Fe3C) as a simple proxy for cohenite are studied experimentally.
The Fe-C system suggests that an increase of C in the Fe changes the properties of the latter. When the C is added to the Fe, it improves the hardness and strength, even if it increases the fragility. Fe3C has a relatively high melting point of 1837 deg C which is normally higher than the temperature of the bath of liquid iron to which it is added. Adding Fe3C to a bath of liquid iron is much like adding sugar to tea. If the tea is hot enough to melt the sugar it would be too hot to drink. The sugar dissolves in the tea it does not melt. Likewise, the Fe3C dissolves in the liquid iron, it does not melt.
Iron carbide is produced by iron ores which are screened for smaller dimensions of less than 1.0 mm to greater than 0.1 mm. The size of the transition to the 80 % is 0.4 mm – 0.5 mm. The iron carbide feed does not need to be pelletized and the product does not need to be stabilized or briquetted. The grains of iron carbide dissolve instantly in both liquid iron and liquid steel, and also remove nitrogen and hydrogen more effectively than any other material. The iron carbide is completely free of sulphur and other metal residues, such as: copper, zinc, tin, and chromium.
The iron carbide manufacturing process typically produces a product which has been converted 93 % to iron carbide. The typical mineralogical and elemental composition of the product consists of Fe3C- 91 % to 96 %, Fe (total) – 89 % to 93 %, Fe (met) – 0.5 % to 1 %, SiO2 + A12O3 – 2 % to 5 %, Fe3O4 -2 % to 5 %, C (as Fe3C) – 6 % to 6.5 %, and O (as Fe3O4) – 0.5 % to1.5 %. Iron carbide is magnetic, so if the gangue is be physically liberated, either before, during or after the process of carburation, then a dry magnetic separation can be carried out to lower the gangue content in the final product and therefore increase its iron grade.
Although the analysis of the product can vary depending on the type of ore used, there is no significant sulphur present in any case. Phosphorous level depends on the type of ore used and is usually present in the product as P2O5. But most of the phosphorous gets transferred to the furnace slag, not in the product.
The residual elements in the ore are normally present in the product as oxides, but since most iron-ores have very low levels of copper, nickel, chromium, molybdenum or tin, there are no significant amounts of these elements in the final product. As a result, iron carbide produced is very clean and provides an effective method of diluting the tramp residual metals during steelmaking, while avoiding the sulphur which generally comes with some virgin iron sources.
The iron carbide is much more effective and less costly than any other means to produce high-quality steel. A 2004 report by the Department of Energy Technology Roadmap Program of United States of America has recognized the iron carbide as the best material for the control of the nitrogen in the production of steel with the electric arc furnaces. Iron carbide is hard, thick, chemically stable, granular, material which is manageable and easy to transport. Being dense and heavy, the steel melting shops can easily enter it into electric arc furnaces using submerged lances.
Iron carbide is also very environmental friendly and provides large environmental advantages. The process achieves the lowest C emission of all virgin-iron steelmaking processes, producing only 1.09 kg of CO2 for each kg of steel produced. This is far less than the 2.01 kg for the conventional blast furnace -basic oxygen furnace route of steelmaking, 3.09 kg for coal based DRI, and 1.87 kg for natural gas based DRI – EAF route of steelmaking. Only steel totally made from scrap achieves a lower emission.
As a further advantage, the iron carbide produces much of the carbon dioxide in a concentrated stream, which is then easy to sequester and/or use advantageously for other purposes.
Use of iron carbide in electric arc furnaces
The novelty is in the use of iron carbide, a new substance for the production of steel. This new material has several advantages which includes the metallurgical, economic and environmental advantages.
The iron carbide material is ideal for the electric arc furnaces since it is granular, non-pyrophoric and dissolves immediately in molten steel. This makes it easy to transport and easy to introduce into the electric arc furnace.
Iron carbide is free from residual metals and sulphur, contains excess carbon, more than any other reduced iron product, and the surplus carbon reduces the residual iron oxides, which would otherwise decrease the yield.
Being dense and heavy, the steel melting shop can easily inject it into the electric arc furnace using submerged lances. This is an advantage over use of DRI, HBI and cast iron. Rates of injection can be reached up to 2,000 kg/minute. After adding the grains of iron carbide into the molten metal bath with gas injection, which may be nitrogen or air, the gases rises to the surface without reacting in a considerable way with metal. A separate degassing operation is rather slow and expensive, however, the iron carbide form a large number of bubbles with a different mechanism.
When the iron carbide enters the electric arc furnace, it dissolves instantly. Then, the uniformly dissolved carbon reacts with the small amount of iron oxide which remains in iron carbide product. The carbon and iron oxide form carbon monoxide. This generates a large quantity of tiny bubbles of carbon monoxide, which create the boiling of the metal and rapidly homogenize the molten metal bath, absorbing nitrogen and hydrogen and creating a foamy slag, which in turn allows the removal of the unwanted nitrogen and hydrogen in the steel.
For the achievement of a higher removal of nitrogen and hydrogen, the injection of iron carbide can start as soon as the electric arc furnace has melted enough steel to submerge the lance. The heat of the furnace does not damage the injection pipe, because the carrier gas effectively cools the lance. The widespread generation of carbon monoxide bubbles thoroughly mixes the bath, avoids temperatures gradients, and removes the nitrogen from the bath. The mixing is more efficient and faster with the iron carbide, with which the mixing time is just one minute, and with the argon, the time is as high as four minutes. Also, the foamy slag produced promotes the metallurgical reactions, isolates the molten metal, improves energy efficiency since it reduces heat losses to the side walls, and reduces wear of the refractory cover and the electrodes.
DRI, HBI and cast iron are unable to provide mixing and reduction of nitrogen and hydrogen. Studies have shown that the process provides high yields. In some cases, the yield of iron carbide for the degree of metallization reach 100 %, since the powerful chemical reducing action of carbon monoxide reduces the iron oxide in the slag, which has been separated from the molten metal, to iron. This is not the case for scrap or with DRI, where the performance is 92 % to 95 %. For all these reasons the iron carbide is the better material for the electric arc furnaces.
Advantages of iron carbide consist of the following.
- It is the better charge material than other materials for the electric arc furnace since it contains around 6.0 % to 6.5 % C and is produced from virgin iron ore and hence it contains negligible tramp elements. Use of iron carbide in steelmaking processes results into a low content of N2 and H2 in the steel.
- It is not pyrophoric and hence it is safe and easy to handle.
- It is a dense, granular powder which that dissolves easily in liquid steel. It can be easily injected into a basic oxygen furnace and/or an electric arc furnace, where it dissolves instantly.