Blowing of Oxygen in Converter Steelmaking
Blowing of Oxygen in Converter Steelmaking
Oxygen (O2) is blown on the hot metal in the converter during steel making for removal of impurities such as carbon (C), silicon (Si), manganese (Mn), and phosphorus (P) etc. A water cooled lance is used to inject oxygen at very high velocities onto a liquid bath to produce steel. In the 1950s when the top blown converter process was commercialized and the size of the converter was limited to 50 tons maximum then a lance with a single hole lance tip was being used for the blowing of O2 in the converter. With the passage of time the converter size went on increasing. This has necessitated increase of number of holes in the lance tip for better distribution of O2 over a larger surface of the bath in the converter.
With the increasing demands to produce higher quality steels with lower impurity levels, O2 of very high purity is required for steelmaking in the converter. The O2 needed for steelmaking is to be at least 99.5 % pure, and ideally 99.7 % to 99.8 % pure. The remaining parts are 0.005 % to 0.01 % nitrogen (N2) and the rest is argon (Ar).
In top-blown converters, the O2 is jetted at supersonic velocities with convergent divergent nozzles at the tip of the water cooled lance. A forceful gas jet penetrates the slag and impinges onto the surface of the liquid bath to refine the steel. Today most of the converters operate with lance tips containing 3 to 6 nozzles. Even 8 nozzles lance tips are under use. The axes of each of the nozzles in a lance with a multi hole lance tip are inclined with respect to the lance axes and equally spaced around the tip. The lance tip is made of copper and is welded to the lance steel pipe.
The O2 flow rates and the speed of O2 flow with the lance pipe size for different converter sizes are given in Table 1
|Tab 1 Design flow rates in converter lance|
|Converter heat size in tons||Oxygen flow rate in N Cum/hr||Internal pipe dia. X thickness in mm||Speed of Oxygen flow in m/sec|
In converter steelmaking, O2 at supersonic speed is blown on hot metal to remove the impurities like C, Si, Mn, and P etc. During the blow the lance height is decreased to make the O2 available into the bath for the removal of C. It takes around 15 to 20 minutes to blow the O2 for refining. The tap to tap time varies in between 50 to 60 minutes depending upon O2 flow rate, hot metal composition, lance profile and steel chemistry. It is interesting to note that the O2 blowing time and the tap to tap time do not depend significantly on the capacity of the converter.
O2 is blown in the converter through a water cooled lance (nearly 8 m to 10 m long) with a convergent-divergent nozzle at high pressure (around 11-14 kg/sq cm) and at supersonic velocity (Mach number greater than 1). Supersonic jet of O2 from the nozzle helps higher entrainment of O2 in the liquid bath. During the blow, a three phase dispersion consisting of slag/metal droplets/gas bubbles is formed. The most important part of the lance is the nozzle tip. It is designed to produce non-coalescing free O2 jet. The functions of the nozzle are as follows.
- Supply and distribution of oxygen
- To produce a gaseous jet
- To induce bath agitation
- To produce metal droplets
After the previous heat is tapped and slag is drained, lining is inspected. Scrap and hot metal are charged. Converter is tilted into the vertical position and the lance is lowered in the vessel to start the blowing of O2. Selection of the starting lance height is to be such that the concentration of the force at the bath level should not cause ejection of tiny iron particles (sparking) and at the same time maximum bath surface area is covered by the O2 jet.
Initially O2 is blown soft by keeping lance height higher to promote slag formation and to avoid ejection of small particles, because hot metal is not covered by slag. Lime is normally added at the beginning of the blow and also during the blow. O2 is blown for around 15 to 20 minutes by progressively decreasing the lance height such that slag foaming remains under control and oxidation reactions take place uninterruptedly.
Supply of O2 in the form of free gas jet is an important feature of converter steelmaking both in top blown and different types of combined blown converters. In this form of O2 supply, the total time of blowing of O2 is almost independent of converter capacity, O2 blowing rate and bottom stirring. This is reflected by evaluating dimensionless momentum flow rate vs. ratio of time of blowing /total blowing time for different converter capacities ranging from 30 tons to 400 tons.
It has been seen that dimensionless momentum flow rate describes the action of free O2 jet produced by constant volume flow rate of O2 at various lance heights. The dimensionless momentum flow rate number increases with the decrease in lance height. Decrease in lance height makes the blow hard and increase in lance height makes the blow soft. Lance profile can be considered to generate soft blow initially and progressively harder blow with the progress of the blow.
The fundamental requirements of the lance profile in all converter steel making are formation of FeO (iron oxide) rich slag in the initial stage and then removal of C and P by progressively increasing the availability of O2 in the bath to avoid over oxidation of slag. The first requirement is achieved by ?soft blow? (shallow penetration of jet) and the other requirement is achieved by hardening the blow (deep penetration of jet into bath) progressively. Thus soft and hard blow are essential requirement of refining of hot metal by impinging of O2 jet irrespective of the converter capacity and type of converter steelmaking practices (pure top blowing or combined blowing) as a result the total O2 blow time remains more or less same.
Availability of oxygen
O2 is available energetically during the refining process in the converter. The energetic availability of O2 is obtained by passing a certain flow rate of O2 through the nozzle.
In converter steelmaking O2 is blown through Laval nozzles. A Laval nozzle also called a convergent-divergent nozzle and is characterized by a flow-passage whose cross sectional area decreases in the direction of flow and attains a minimum cross section area and then increases further in the direction of flow. The minimum cross section area of the flow passage is called throat of the nozzle. The Laval nozzle can accelerate the gas to the supersonic velocity (Mach number greater than 1). In fact gas velocity at the exit corresponds to a Mach value of around 2.0 to 2.4.
Behaviour of free gas jet
The behaviour of a gas when it exits a single Laval nozzle in the surrounding which consists of air shows that the gas when exiting through a nozzle spreads in the surrounding and is called ?free gas jet?, because spreading is not confined. A free jet in the surrounding is characterized by the potential core length (PCL) and supersonic core length (SCL). In the potential core no entrainment of the surrounding occurs and hence velocity of gas in both axial and radial direction is that at the exit value. Beyond the potential core both radial and axial velocity begins to decrease due to entertainment of the surrounding. However a point is reached in the free gas jet at which the gas velocity attains a sonic value (M=1). Within the supersonic core length gas velocity is above the supersonic value in both radial and axial direction. Beyond the supersonic core length the gas velocity is subsonic. Thus radial spreading and axial velocity decay beyond the potential core are the main characteristics of a free gas jet.
Due to spreading, mass of the jet increases which means that concentration of the gas at plane P=0 decreases due to entrainment of the surrounding. If O2 is flowing through the nozzle, concentration of O2 at plane P2 is lower than at P1 and at P=0. But mass of jet (jet consists of main fluid + surrounding) at P2 is more than the mass at P1. Axial velocity of the jet is a function of axial distance measured from the nozzle exit.
One of the important property of the free jet is that it carries with it momentum flow rate which on hitting the liquid is converted into force and penetrates into the liquid. Momentum flow rate within the jet is conserved. This is an important property of the jet since it depends only on the upstream variables like pressure, number and diameter of the nozzle. It does not depend on the downstream conditions.
Behaviour of jets produced by multi-nozzle tips depends on number of nozzles and inclination angle of each nozzle with the axis of the lance. Number of nozzles in converter steelmaking varies with the converter capacity but in general it is between 3 and 6. The inclination angle of each nozzle for a three holes lance tip is normally 10 to 12 deg and for five to six holes lance tip is generally 15 to 16 deg with the axis of the lance.
The multi free gas jets downstream the nozzle can coalesce or not would depend on inclination angle and number of nozzles for a given upstream pressure and flow rate of gas. A coalescing jet is similar to that of a single jet. When angle of inclination is 10 ?12 deg for a three hole lance, the multi-jets do not coalesce up to certain distance downstream the nozzle. A non-coalescing jet, when impinged on the liquid will produce penetration equal to number of jets.
The axial velocity decay and radial spreading depend on the ratio of density of surrounding /density of the O2 jet. If the density of the O2 jet is more than the density of the surrounding then such a jet spreads slowly in the surrounding. The speed of spreading depends on the value of the ratio. Hence the velocity of the jet decays slower at any distance downstream the surrounding. In such a situation a cold jet is discharged in the hot metal surrounding and the length of the potential core, PCL and the length of the supersonic core SCL is longer than when the ratio of density of surrounding /density of the O2 jet is one. If the ratio of density of surrounding /density of the O2 jet is less than one then the O2 jet is lighter than surrounding and the O2 jet spreads faster which results in lower the length of the potential core PCL and the length of the supersonic core SCL. In such a case the cold oxygen jet is discharged into the slag.
Action of free oxygen jet
Velocity of the free O2 jet is important. Axial velocity decreases as the distance downstream the nozzle increases due to entrainment of the surrounding. In the converter as the blow begins, the surrounding of O2 jet is hot atmosphere. As the blow continues the jet surrounding changes from carbon monoxide (CO) to slag. For most of the periods the jet is submerged into slag. The surrounding in the converter is dynamic. The velocity of the jet depends on upstream pressure, downstream axial distance and the surrounding. It is difficult to calculate the jet velocity when the surrounding is changing, but the momentum flow rate within the jet is independent of the distance downstream the nozzle and can be calculated.
Jet carries with it momentum flow rate which on hitting the bath is converted into force. Thus action of free jet can be described in terms of dimensionless flow rate number. The dimensionless flow rate number increases with the decrease in the lance height. Thus dimensionless flow rate number is used to describe the dynamic variation of the lance height. The dimensionless momentum flow rate number signifies the action of the O2 jet on the bath at a lance height against the gravity
Dimensionless flow rate number describes the effect of lance height on the penetrability of jet. Shallow jet penetration as obtained at higher lance height is a ?soft jet? as compared to deep penetrating jet as obtained at lower lance height and is termed ?hard jet?.
This means that a constant volume flow rate of O2 supplied at constant pressure when discharged through a nozzle can be made to hit the bath ?soft? and can be made progressively harder. Thus method of O2 supply in converter steelmaking practice through ?free jet? is very effective in terms of physico- chemical reactions.
The effects induced by a reactive soft impinging O2 jet when it hits the liquid metal bath are (i) oxidation of iron (Fe), (ii) shallow penetration, (iii) slag/metal reaction, and (iv) enhancement of slag formation and thus facilitating removal of P. Too long duration of soft O2 jet results into slopping of slag because of over oxidation.
The effects induced by a reactive hard impinging O2 jet when it hits the liquid metal bath are (i) availability of O2 deep in the bath, (ii) enhancement of C oxidation and impairment of P removal, (iii) evolution of CO deep into the bath and its escape through the bath agitates the bath, and (iv) generation of droplets which are then emulsified in the slag.
Types of O2 jets and their effects are shown in Fig 1.
Fig 1 Types of oxygen jets and their effects
Reactions in the converter
In O2 steelmaking, C saturated hot metal is blown with pure O2 at supersonic velocities. The reactions and mixing are intense. O2 reacts with the dissolved Si, dissolved Mn, and the Fe itself to make a liquid FeO-containing slag. The O2 also reacts with the dissolved C to liberate CO gas and thereby decarburize the iron. The path to oxidation of these elements (C, Si, etc.) during steel refining is to blow O2 into the iron solution to the point where its concentration in the liquid bath exceeds the equilibrium level allowed by the particular impurity element. The dissolved O2 and the dissolved impurity element then combine to form CO gas (in the case of C) or liquid silica (SiO2, in the case of Si). Since the solubility of both of these products [CO (gas) and SiO2 (liquid)] is very limited in liquid iron, they quickly nucleate their separate phases, coagulate, consolidate, and are floated-out by the intense stirring action of the process.
Also, during the intense O2 blow of steelmaking, some of the liquid iron is itself oxidized to FeO which then becomes intensely mixed with the liquid metal bath into an emulsion and can react with the dissolved impurities in the molten iron directly according to the following reactions.
2Fe (molten) + O2 (gas) = 2FeO (liquid slag)
FeO (liquid slag) + C (dissolved in the liquid iron solution) = CO (gas) + Fe (molten)
2FeO (liquid slag) + Si (dissolved in the liquid iron solution) = SiO2 (liquid) + Fe (molten)
These oxidation reactions are highly exothermic. The heat released from oxidation of Si and other impurities, together with the enthalpy from oxidation of iron itself, is used to melt cold scrap to increase the converter heat size as well as to increase the temperature of the liquid steel for downstream operations. CO2 is never produced within the bath of a steelmaking converter except in trace quantities. CO2 (if it ever is formed) is quickly turned into CO by reaction with any remaining dissolved C. In case if there is no dissolved C remaining after oxidation of the liquid bath, CO2 is reduced to CO by the oxidation of Fe itself. Thus CO2 is an oxidant at steelmaking temperatures. The equilibrium product of reaction for oxidation of Fe, or C dissolved in iron, by CO2 is strongly towards CO, with trace quantity of CO2 according to the Gibbs free energy calculations. C acts as a reducing agent to FeO according to the above equation, and is another factor which can slow (or interfere) with iron oxidation.
During the steelmaking process, some of the iron is oxidized to the point where it adds to the percentage of liquid FeO in the liquid slag phase which co-exists in the converter with the metal. If the O2 is blown beyond the end-point of impurity oxidation, the oxidation of Fe becomes excessive. This shows up as a measurable yield loss of Fe to the slag, giving predictable, calculable, higher concentrations of liquid iron oxide (FeO) in slag. Once the C is oxidized to CO, any additional O2 combines with the Fe to produce FeO into the slag.