Reagents for Desulphurization of Hot Metal...

Reagents for Desulphurization of Hot Metal Sulphur (S), present in solid steel as iron sulphide (FeS) inclusions, has several detrimental effects on steel processing and on steel’s physical properties. During deformation, the iron sulphide inclusions act as crack initiation sites and zones of weakness. Such inclusions from sulphur adversely affect steel’s toughness, ductility, formability, weldability, and corrosion resistance. An increase in manganese (Mn) however, helps prevent formation of iron sulphide, which is highly detrimental to steel’s hot workability and also leads to severe cracking. Sulphur is such an undesirable element in steel that its removal is desired. The ever increasing requirements to steel properties and the growing demand for steel qualities and quantities with lowest sulphur contents of down to 0.001 %, has made it necessary for the steel makers to carry out the desulphurization of hot metal. Presently hot metal is regularly being desulphurized to below 100 ppm, and in some steel plants, to 10 ppm. Besides the increased requirements of steel quality, other reasons which necessitate desulphurization of hot metal are reduced scrap quality and increasing cost of high quality iron ores. In the desulphurization process, powdered desulphurization reagents are injected into the hot metal through an immersed lance using an inert carrier gas such as argon or nitrogen, as shown in Fig 1. Since desulphurization is a diffusion-controlled reaction, and related to the reactive surface area available for reaction, the desulphurization reagents are to be as fine grained as possible. However, flowability is reduced with very fine grains and hence it is necessary to find an optimum between efficiency and conveying ability. In order to obtain good flow characteristics, normally a fluxing agent is added during the grinding operation, so that pneumatic transport during injection does not pose any problems. Fig...

Coal based Direct Reduction Rotary Kiln Process Feb14

Coal based Direct Reduction Rotary Kiln Process...

Coal based Direct Reduction Rotary Kiln Process The coal based direct reduction rotary kiln process was developed for converting iron ore directly into metallic iron without the melting of the materials. The process has the advantage of low capital expenditure and no requirement of coking coal. The metallic iron in this process is produced by the reduction of iron oxide below the fusion temperature of iron ore (1535 deg C) by utilizing carbonaceous material present in the non-coking coal. As the iron ore is in direct contact with the reducing agent throughout the reduction process, it is often termed as direct reduced iron (DRI). The reduced product having high degree of metallization shows a ‘honeycomb structure’, due to which it is often called sponge iron. Coal based DRI plants are flexible with respect to plant location since non-coking coal is widely distributed in large deposits and is easy to transport. Most plants employ reduction process which is carried out in rotary kilns. These plants use wide variety of raw materials and non-coking coal. The quality of these materials has direct bearing on the process as well as the product. Some plants do not use iron ore directly. These plants use iron ore pellets in the rotary kiln. Raw material mix consisting of iron ore, dolomite and non-coking coal is fed at the one end of the rotary kiln and is heated by coal burners to produce DRI. The product DRI along with char (sometimes called dolo char) is taken out from the other end of the kiln. Apart from this, primary air and secondary air are supplied to the kiln to initiate the combustion and sustain the reaction process in the kiln. Raw materials The main raw materials for the production of DRI by...

Desulphurization of Liquid Steel Jul30

Desulphurization of Liquid Steel...

Desulphurization of Liquid Steel Solubility of sulphur (S) in liquid iron (Fe) is quite high. But the solubility of S in solid iron is limited. It is 0.002 % in ferrite at room temperature and 0.013 % in austenite at around 1000 deg C. Hence, when liquid steel cools down, sulphur is liberated from the solution in the form of iron sulphide (FeS) which forms a eutectic with the surrounding iron. The eutectic is segregated at the iron grain boundaries. The eutectic temperature is comparatively low at around 988 deg C. Fe-FeS eutectic weakens the bonding between the grains and causes sharp drop in the properties of steel at the temperatures of hot deformation. During the continuous casting of liquid steel, sulphur present in liquid steel (i) causes the formation of undesirable sulphides which promotes granular weaknesses and cracks in steel during solidification, (ii) lowers the melting point and inter-granular strength, (iii) contributes to the brittleness of steel and thus acts as stress raiser in steel, and (iv) results in the hot shortness. Sulphur, present in solid steel as FeS inclusions, has several detrimental effects on steel processing. During deformation, FeS inclusions act as crack initiation sites and zones of weakness. Such inclusions from sulphur adversely affect the toughness, ductility, formability, weldability, and corrosion resistance of steel. An increase in manganese (Mn) content (not less than 0.2 %) however, helps prevent formation of FeS. Sulphur is thus an undesirable element in steel. Manganese actively reacts with iron sulphides during solidification of steel transforming FeS to MnS according to the following reaction. FeS (slag) + Mn (steel) = MnS (slag) + Fe The melting temperature of manganese sulphide (MnS) is comparatively high (around 1610 deg C). Hence steel containing manganese can be deformed in hot state. However...

Manganese in Steels

Manganese in Steels  Manganese (Mn) (atomic number 25 and atomic weight 54.93) has density of 7.44 gm/cc. Melting point of Mn is 1244 deg C and boiling point is 2095 deg C. The phase diagram of the Fe-Mn binary system is at Fig 1. Fig 1 Fe-Mn phase diagram  Mn is present in most commercially made steels. Mn plays a key role in steel because of its two important properties namely (i) its ability to combine with sulphur (S), and (ii) its powerful deoxidation capacity. Mn is undoubtedly the most prevalent alloying agent in steels, after carbon (C). Mn is intentionally present in many grades of steel and is a residual constituent of virtually all others. Mn has played a key role in the development of various steel making processes and its continuing importance is indicated by the fact that about 85 % to 90 % of all Mn consumed in the world annually goes into iron and steel making as well as in steel as an alloying element. No satisfactory substitute for Mn in steel has been identified which combines its relatively low price with outstanding technical benefits. Available forms Mn is used in steel industry in an extensive variety of product forms. These can be classified into three major groups namely (i) ferro-manganese (Fe-Mn), (ii) silico-manganese (Si-Mn), and (iii) Mn ore. There are several standard grades within each group. Fe-Mn and Si-Mn are used mainly during steel making while Mn ore is mainly used in iron making. Types of Fe-Mn and Si-Mn produced are given in article having link http://ispatguru.com/717/. High density Mn containing 96 % or 97 % Mn, depending on grade and Iron (Fe) as the principal impurity, is also used as a desirable addition agent for super alloys, stainless...

Sulphur in Steels

Sulphur in Steels  Sulphur (S) (atomic number 16 and atomic weight 32.066) has density of 2.05 gm/cc. Monoclinic S melts at 119.25 deg C and boils at 444.6 deg C. However, S and iron (Fe) are miscible, and Fe-S binary system at one atmosphere of pressure forms a liquid at temperatures as high as 1800 deg C, far above the boiling point of S alone. Fig 1 is the phase diagram of the Fe-S binary system at 1 atmosphere of pressure.  Fig 1 Fe-S phase diagram S is an element which is always present in steel in small quantities. S in steel is introduced through iron ore and fuel (coal and coke). The removal of S during steel making is a tedious and difficult process. S is normally regarded as an impurity in steel and is required to be reduced to the limits of practicality. However steels which are to be machined need a certain minimum S content for proper chip formation. Where machining constitutes a major fraction of the end products cost, many types of steel (carbon, alloy, and less often stainless) are intentionally resulphurized just for this reason. (refer http://ispatguru.com/free-cutting-steels/) Except in those cases where it is added for machinability, or where residual S content of around 0.040 % maximum is tolerable, the usual aim during iron and steel making is to reduce S to low levels, consistent with mechanical property requirements. For high strength (HS) steel plates and for some special bar quality (SBQ) steel products, this may mean removing the S to a level of 0.005 % maximum. There are several methods which are widely used for achieving this level of S. Further, efficient removal of S from liquid steel or iron depends on specific metallurgical and thermodynamic conditions. Though...