Chemistry of Steelmaking by Basic Oxygen Furnace
Chemistry of Steelmaking by Basic Oxygen Steelmaking
Basic oxygen steelmaking (BOS) is the most widely used primary steelmaking process for the production of crude steel from hot metal (HM). The process vessel is known as converter. It plays a predominant role in integrated steel plants for the production of crude steel. The process involves blowing oxygen (O2) through HM with the help of a top lance to reduce its carbon (C) content by oxidation. At present mixed blowing is adopted in the BOS process which was developed in the late 1970s. In mixed blowing, a limited blowing of neutral gas, argon (Ar) or nitrogen (N2) is done through the bottom of the top blown converter. It provides an efficient stirring.
BOS process has two characteristics. First, the process is autogenous meaning that no external heat source is needed. The oxidation reactions during the O2 blow provide the energy needed to melt fluxes and scrap and to achieve the desired temperature of liquid steel. Second, the process refines HM at high production rates for the production of liquid steel. The fast reaction rates are due to the large surface area available for reactions. Large quantity of gas is evolved when O2 is injected into the bath of metal. This gas forms an emulsion with the liquid slag and metal droplets sheared from the bath surface by the impingement of the O2 jet. The large surface area which is generated by gas- metal- slag emulsion increases the rates of the refining reactions.
Since the impurities are dissolved in the molten metal, reactions between impurities and O2 occurs with the dissolved O2. Further since oxidation of C takes place at higher temperature, C oxidation to carbon mono-oxide (CO) is highly probable and hence majority of C is removed as CO.
During the BOS process, impurities in HM such as C, silicon (Si), manganese (Mn), phosphorus (P) etc. are removed by oxidation for the production of liquid steel. Oxidation is carried out with high purity O2 gas which is blown in the converter. The oxidation reactions result into the formation of CO, CO2 (carbon di-oxide), silica (SiO2), manganese oxide (MnO), and iron oxide (FeO). While CO and CO2 are in gaseous form and removed from the top of the converter as converter gas, other oxides are dissolved with the fluxes added to the converter, to form liquid slag. Liquid slag is able to remove P and S (sulphur) from the liquid metal.
The reactions taking place in the BOS process can be categorized into five categories. The reactions in the first category ‘oxygen pick up by the metal’ are (i) O2(g) = 2O, (ii) (FeO) = Fe + O, (iii) (Fe2O3) = 2(FeO) + O, and (iv) CO2(g) = CO(g) + O. The reactions in the second category ‘oxidation of elements in the metal’ are (i) C + O = CO(g), (ii) Fe + O = (FeO), (iii) Si + 2O = (SiO2), (iv) Mn + O = (MnO), and (v) 2P + 5O = (P2O5). The reactions in the third category ‘oxidation of compounds in the slag’ are (i) 2(FeO) + 1/2O2(g) = (Fe2O3) and (ii) 2(FeO) + CO2(g) = (Fe2O3) + CO. The reactions in the fourth category ‘flux reactions’ are (i) MgO(s) = (MgO), and (ii) CaO(s) = (CaO). The reaction in the fifth category ‘gas reactions’ is CO(g) + ½O2(g) = CO2.
The BOS is a process of very high kinetics, the reactions taking place at multiple locations. The jet liquid interaction and the C-O reaction generating a gaseous product have huge effects on the overall process dynamics. The process is characterized by high reaction rates, the refining process being completed typically in 12 minutes (min) to 15 min. For the control of the process for quality and productivity in this short time frame, a good understanding of the dynamics of the process is important.
A typical BOS converter consists of a cylindrical barrel with a rounded bottom, and a conical top (25 degrees to 30 degrees half cone angle) for directing the gases into the exhaust-gas hood. The body is supported on pivots, called the trunnions, such that the furnace can be rotated around for charging, sampling, tapping, and slag-removal. The inside is typically lined with magnesia-carbon refractory, of different quality and thickness to match the wear pattern. The typical volume provided inside the converter is around 1 cubic meter (cum) per ton of liquid steel produced. If the slag weight is 100 kilograms per ton (kg/t) to 120 kg/t, the freeboard above the inactive bath is more than 80 %. This accommodates the vigorous reactions which take place during the middle part of a typical blow. The bottom of the converter is fitted with several (typically 6 to 8) porous elements, through which Ar gas is passed for bath mixing and aiding slag-metal reactions. A tap hole is provided on one side in the lower part of the cone for tapping the liquid steel. Slag is poured out on the other side through the mouth.
The BOS process is an exceedingly fast refining process needing a good dynamic control and a dynamic model for better understanding of the process. The process is characterized by reactions at multiple scales such as at the scale of the metal bath and the slag and at the scales of droplets and bubbles. The reactions also take place at multiple reaction sites. The presence of the supersonic jet interacting with the metal bath and the slag layer, producing different sizes of drops in the emulsion which on reaction produce copious bubbles at its interface, lime dissolution issues, etc., make the description of the dynamics of the process complex.
The primary raw material is HM at around 1,300 deg C to 1,400 deg C. Since heat generated is more than what is needed, steel scrap along with iron ore is used as coolant. Lime stone (CaCO3) is added in some steel melting shops as a coolant for adjusting the final temperature. Calcined lime (CaO) is used as a flux for achieving high basicity needed for P removal. Scrap is added first to an empty converter (after slag-tapping from the previous heat), on which the requisite quantity of HM is added. Iron ore when used is added in a distributed fashion typically during the first half of the blow.
Part or all of the required lime is added before scrap addition to act as an impact pad for protecting the lining from scrap fall. Rest of the lime is typically added in a distributed fashion during the blow. Some magnesia (MgO) addition takes place in the form of calcined dolomite (CaO.MgO), to minimize refractory dissolution into the slag. The quantities of different charge materials are calculated theoretically by a charge control model based on material and heat balance, taking into consideration the input compositions, HM temperature, and output steel composition and temperature.
The refining reactions are all oxidizing. This is accomplished by blowing tonnage oxygen through a top lance fitted with 3 to 6 supersonic flow nozzles (2.0 Mach to 2.1 Mach, fitted at an angle to the lance axis). The tip of the lance is held at a distance between 1.8 metres (m) to 2.5 m above the level of the quiet metal bath in a large sized converter. The lance height is one of the operating parameters to control the process
A typical tap-to-tap cycle consists of the steps as described here. Charging sequence is lime, scrap, and HM. Once the converter is made upright, the O2 lance is lowered to the required height (initially, highest value, 2.2 m to 2.5 m) and blowing is started. During the initial half of the blow, additional lime if any along with iron ore, calcined dolomite, and any other additives are added. Additions of moisture containing materials are avoided during the latter part of the blow to keep hydrogen (H2) in the produced steel low. The high lance operation is continued (typically 3 min to 4 min) till the slag has enough FeO for facilitating lime dissolution. Thereafter the lance is progressively lowered to achieve the needed rates of refining. The lance height is decreased in 3 steps to 5 steps depending on the individual plant practice.
At around 80 % to 90 % of the blow (based on O2 flow) a sample is taken for analysis and temperature is measured so that when one finishes the blow, the required composition and temperature are achieved simultaneously. Sampling and temperature measurement can be done either manually, i.e. stopping the blow, turning the converter to a near horizontal position and taking a sample through a spoon and measuring the temperature, or through a sub-lance which is lowered into a blowing converter (in-blow sampling). Based on the sample analysis and temperature, the remaining part of the blow is completed with requisite trim additions. Once the blow is completed the converter is turned to the tapping side for pouring the liquid steel out, and then to the other side for slag tapping. In modern practice, some slag is retained, the converter is made upright, some magnesite (MgO) is added and then the slag is splashed on to the inside surface by blowing high velocity N2. Periodically, the empty converter is inspected for refractory damage, either manually or through laser scanners. Damage is repaired by gunning refractory guniting mass. After this the converter is ready for the next blow.
The BOS process is a complex process taking place over a short time span, with very little direct feedback information available as the process progresses. The process consists of several sub-processes which are either ill-understood or of which there have been only semi-quantitative understanding. Since the process is an autogenous process, there is a heat excess even after the input HM is at around 1,350 deg C and the output steel is tapped at 1,650 deg C to 1,700 deg C. Hence, different coolants are used, scrap and iron ore being the primary ones. O2 is delivered to the process through supersonic jets issuing into hot and dust laden gases or under a liquid gas emulsion with the jet behaviour being affected by the ambient environment. Fig 1 shows schematic representation of the BOS process and its basic features of are described below.
Fig 1 Schematic representation of the basic oxygen steelmaking
Carbon oxidation – Decarburization of the C available in the bath is the most extensive and the important reaction during BOS process. There are three distinct stages during this decarburization reaction. In the first stage which occurs during first few minutes of the blow, decarburization takes place at a slow rate, since most of the O2 supplied reacts with the Si of the bath. During the second stage, which occurs at high C content of the bath, decarburization takes place at a higher rate and is controlled by the rate of supplied O2. The third stage occurs when the C content of the bath reaches around 0.3 %. At this stage, the decarburization rate drops since lesser C is available to react with all the O2 supplied. At this stage, the rate is controlled by mass transfer of C, and the O2 mostly reacts with iron (Fe) to form FeO. At this stage, since the rate of CO generation drops, the flame at the mouth of the converter becomes less luminous and practically disappears when the C drops to a level of around 0.1 %.
Silicon oxidation – Conditions favorable for silicon oxidation are (i) low temperature, and (ii) low amount of SiO2 in the slag. A basic slag favours Si oxidation. In basic slags, Si oxidation occurs practically to a very low value since SiO2 reacts with CaO and decreases activity of SiO2 in the slag. Almost all the Si gets oxidized and removed early in the blow because of a strong affinity of O2 for Si. The Si of the HM is oxidized to a very low level (less than 0.005 %) in the first 3 min to 5 min of the blow. The oxidation of Si to SiO2 is exothermic and it produces considerable quantity of heat which raises the bath temperature. It also forms a silicate slag which reacts with the added lime and calcined dolomite to form a basic slag. Since the oxidation of Si is the main heat source, its quantity in HM determines the quantity of the cold charge (scrap, and pig iron etc.) which can be added to the converter. It also determines the volume of slag and hence affects dephosphorization of the bath and the yield. As per thumb rule, higher slag quantity result into lower P but also lesser yield.
Iron oxidation – Oxidation of iron (Fe) is the most important for the BOS process since it controls (i) FeO content of the slag and O2 content in the steel, (ii) loss of Fe in the slag and hence affects the productivity of the steelmaking process, (iii) oxidation potential of the slag, and (iv) FeO helps in the dissolution of CaO in the slag.
Manganese oxidation – Mn oxidation reaction in the BOS process is rather complex. In a top blown converter, Mn is oxidized to MnO oxide in the earlier stages of the blow and after most of the Si is oxidized then Mn reverts back into the bath metal. Finally, at the end of the blow when more O2 is available for the oxidation the Mn gets reduced in the bath metal. In case of the bottom blowing or combined blowing in the converter, the oxidation of Mn has got a similar pattern but the residual Mn content of the liquid steel in the converter bath is higher than the top blown converter.
Phosphorus oxidation – The oxidizing conditions in the converter favour the dephosphorization of bath metal. The reaction of dephosphorization takes place due to the interaction of metal and slag in the bath. Parameters such as lower bath temperatures, higher slag basicity (CaO/SiO2 ratio), higher content of FeO in the slag, higher slag fluidity, and good stirring of the bath favour the dephosphorization reaction. The phosphorus content of the bath metal reduces in the beginning of the blow, then during the main decarburization period when the FeO is reduced, P reverts into the bath metal and finally it reduces again at the blow end. Bath stirring improves the mixing of metal and slag and helps in the rate of dephosphorization. Good stirring with the addition of the fluxing agents such as flour spar etc., also improves the P removal by increasing the dissolution of CaO, resulting in a highly basic and fluid liquid slag.
Sulphur reaction – S removal is not very effective in the BOS process because of the highly oxidizing conditions. S distribution ratio (% S in slag / % S in the metal) is around 4 to 8 which is much lower than in the steel ladle (around 300 to 500) during the secondary steelmaking process. During the BOS process, around 10 % to 20 % of S in the bath reacts with O2 directly to form SO2 (sulphur di-oxide). The remaining S is removed by the slag – metal reaction S + CaO = CaS + FeO. Removal of S by the slag is assisted by high basicity and low Fe content of the slag. S content of the liquid steel is highly influenced by the S contained in the HM and scrap which is charged in the converter.
The reactions taking place during the BOS process are heterogeneous and at different length scales. There are the bulk metal bath phase, the bulk slag phase, and the gas phase. On the other hand, much of the reaction takes place at the scale of fine droplets and bubbles distributed in the slag / metal / gas emulsion phase. The difference in length scales also result in difference in time scales. The metal bath sees changes over the entire heat cycle of 12 minutes to 15 minutes whereas the drops can undergo the complete cycle of refining in about a minute. Hence, the picture of the process dynamics has evolved over several years based on observations and measurements in commercial and pilot plants, carefully designed experiments, and mathematical modelling.
Typical composition of the HM can be C – 4.5 %, Si – 0.3 % to 0.5 %, Mn – 0.2 % to 0.7 %, P – 0.1 % to 0.18 %, S – 0.02 % to 0.03 %, and temperature of 1,350 deg C. Since S can only be removed to the slag in the reduced state in the presence of liquid iron, the oxidation process in the basic oxygen steelmaking does not remove any considerable quantity of S. The overall reactions of significance can be written as (i) [Si] + {O2} = (SiO2), (ii) [Mn] + 1/2 {O2} = (MnO), (iii) [C] + 1/2 {O2} = {CO}, (iv) 2[P] + 5/2 {O2} = (P2O5), (v) Fe (l) + 1/2 O2(g) = (FeO). [-],{-}, and (-) are used for the metalloids dissolved in the metal bath, gas, and constituents in the slag, respectively. Figure 2a gives the progress of the reactions in a 200 t converter. Measurements done in different converters also show similar patterns. Fig 2b gives the corresponding evolution of the slag composition.
Fig 2 Evolution of carbon content in the bath during blow and evolution of slag composition
One notable feature in the evolution of metal composition is the simultaneous removal of considerable quantities of C even before Si has dropped to a very low level. This is also borne out by the observation that the CO flame shoots up at the mouth of the converter within a short time of start of the O2 blow. This is in contrast to the observations in the now obsolete Bessemer converter or the OBM process, where air / O2 are blown from the bottom. In these two processes, appearance of a significant flame takes some time, which is assumed to indicate that the C oxidation does not start till Si has fallen to fairly low values.
Thermodynamically, the order of the solute oxidation reactions at any local location for the input conditions mentioned above is to be Si, Mn, C, and P. That is, at the conditions prevailing in the initial part of the blow, Si is oxidized before C. The low initial temperature itself makes Si reaction to be favourable. In addition the product SiO2 is at very low activity in the highly basic conditions maintained right from the beginning. CO partial pressure on the other hand remains nearly at 0.1 MPa (one atmosphere). For example, if an activity 0.001 for SiO2 is assumed then the pO2 in equilibrium with 4.5 % C and 0.5 % Si is 1MPa to 1.7 MPa, and 1 MPa to 1.9 MPa respectively. This feature makes the analysis of the process dynamics and reaction mechanisms interesting.
Characteristics of the process
Since the feedback information from the process is limited, it necessitates building a model for the process dynamics from the characteristics observed from the information which can be obtained. The important features of the BOS process are described below.
The reaction rates are extremely fast. During peak decarburization, C is removed at the rate of around 0.3 % per minute, that is, around 600 kg of C per minute in 200 t converter (Fig 2a). The C reaction shows three typical periods (Fig 2a) namely (i) an initial period when the rate is building up, (ii) an intermediate period when the rate is relatively constant in spite of the fact that the C content in the bath continuously falls from around 3.5 % to 4.0 % during this period, and a final third period beyond a critical C content when the rate tapers off. The critical C content normally lies in the range of 0.2 % to 0.5 %.
However, the individual heats with identical blowing conditions show wide irreproducibility. Two sequential blows with identical inputs and process parameters can show quite different behaviours, with some blows displaying slopping (metal-slag-gas emulsion boiling over the mouth of the converter) or dry slag and spitting (resulting in lance and mouth build-up). The irreproducibility was much more prevalent in the early days of the BOS process, when bottom blowing of stirring gas was not yet incorporated. Use of less scrap as a coolant can also lead to more reproducibility and decreased slopping.
After a study of several the BOS shops, it has been shown that the rate of peak decarburization is directly proportional to the rate of O2 blowing. It has also been shown during the blow in a laboratory sized converter, that the effects of increasing rate of O2 blowing rate and decreasing the lance height on the rate of peak decarburization are similar.
During the experiments in a pilot converter at MEFOS (a research institute in Sweden), it has been shown that there is a concentration variation along the height of a top blown converter. This indicates that top blowing does not mix the metal bath well in spite of the enormous momentum in the top jet. However, this difference disappeared on blowing a very small quantity of inert gas from the bottom.
It is well known that the slag in the BOS process contains a considerable fraction of the metal in the form of droplets in the slag phase. The quantities vary during the blow, being highest during the middle part of the blow. The estimates vary in the range of 10 % to 25 %. These droplets are quite fine, most being less than 1 mm to 2 mm. The number of droplets in the emulsion falls towards the end of the blow. The droplets are normally in a much more advanced state of refining as compared to the bulk metal bath.
There exists a slag-metal-gas emulsion during most of the blow. By about one third of the blow, the emulsion height exceeds around 2 m, hence submerging the lance tip and muting the sound of the supersonic jet. Sometimes, the emulsion can fill the whole furnace, boiling over the mouth (slopping). Towards the end of the blow beyond a critical C in the bath, the emulsion collapses indicating that the emulsion is transitory needing continuous gas generation for its survival.
As stated, C, Mn, and P are oxidized simultaneously with Si in the initial part of the blow, against the expectation of Si reaction being preferred over the other reactions based on bulk metal bath composition. Mn and P reactions can be explained to some extent by the fact of activities in the slag. C reaction cannot be explained, unless one uses the hypothesis that the bulk metal composition does not prevail at the site of the reaction.
There is reversal of Mn and P during the middle part of the blow (Fig 3a). This is reflected in the slag path too (Fig 2b). However, it is clear that the reversions are correlated with the FeO content in the slag. Dissolution of CaO continues almost till the end in spite of lime being added in the beginning or in the early part of the blow. C determines the overall dynamics of the process and this reaction takes place vigorously. Fig 3a shows change in melt composition during the blow.
Fig 3 Change in melt composition during the blow
The dynamics the BOS process depends on the C reaction taking place vigorously. The complete dynamics can be divided into several sites. Other reactions can be understood on this framework. O2 jet being almost pure, molecules reach the bath surface directly without a considerable mass transfer barrier. When a molecule strikes, it can do one of the following things.
O2 molecule reacts with C at the impact site. The reactions can be [C] + 1/2{O2} = {CO}, and [C] + {O2} = {CO2}. It can dissolve in the metal as [O]. This can then travel elsewhere and react with other oxidizable elements such as O2 = 2[O]. Some of it can react with Fe in the bath producing FeO as per equation Fe + 1/2{O2} = (FeO). FeO can travel to the slag phase and react with metal elsewhere. Each one of these leads to refining reactions to take place at different possible sites in the converter, leading to, on mixing, overall bath refinement. These different sites are shown schematically in Fig 4.
Fig 4 Reaction sites in the basic oxygen steelmaking
It is to be remembered that the C-O reaction is heterogeneous. There is at least one mass transfer step which can be rate limiting. C has to diffuse in the metal to the interface. Transfer of O2 in the gas phase, dissolved O2 in the metal phase, or FeO in the slag phase can also be involved depending on the O2 source for the reaction. O2 dissolved can travel to other parts inside the metal bath and react with dissolved C to release CO to gas-filled pores in the refractory (site 1). CO can also form on solid particles floating in the metal bath by heterogeneous nucleation (site 2). Heterogeneous nucleation can also take place at the slag layer / metal bath interface (site 3). Homogeneous nucleation within the bath is highly improbable unless the CO super-saturation is very high. As described earlier CO reaction can directly take place at the impact site (site 5). Some of the FeO formed at or near the impact site can get under the metal bath surface and travel along the slag / metal interface, reacting with C giving emulsified interface (as in site 3). However, bulk of the FeO formed probably transfers to the slag phase.
This now gives several possibilities. At the interface between slag and metal bath, reactions can take place as described earlier (site 3), O2 now coming from the slag phase and C from the metal. As described earlier, the slag phase contains a large number of metal droplets, being continuously generated by the momentum of the jet at the impact site. Hence, the FeO in the slag can react with these droplets through different mechanisms such as (i) CO bubbles can heterogeneously nucleate at the interface (site 8), (ii) CO can be transferred to a passing bubble which comes in contact with the droplet (site 4), and (iii) CO bubble can homogeneously nucleate inside the droplet, if the super saturation is very high (site 6). If some metal droplet is thrown to the free board, it can react directly with any O2 or CO2 in the gas (site 7).
Though all these sites can be active during the blow to some degree, there is a need to identify the predominant mechanism which determines the overall dynamics. The contribution from each of these sites can be evaluated based on the observations. The fact that the bath shows a concentration gradient in the absence of bottom gas injection, which disappears with as little as 1 % of the inert gas being blown from the bottom in comparison to the top gas flow, the mechanisms at site 1 and site 2 can be discounted as being unimportant.
The temperature at the surface at the impact zone is expected to be above 2,120 deg C. Hence, the rates of the chemical reactions are expected to be very high. The area of the impact site is comparatively small and O2 arrival rate is very high. However, the solutes need to diffuse to the interface and the heat has to conduct into the metal. Fresh metal is brought to the interface which is swept outward by large surface velocity. Under these circumstances, it can be expected that the impact surface is being starved of the solutes leaving behind a layer of Fe reacting with O2. Ultimately it can be reasonable to assume that a layer of metal of the bulk metal composition is entirely oxidized, condensed phase oxides being transferred to the slag layer. When the C content is around 5 % (20 to 25 mol %), this approximation means that around 25 % of the supplied O2 is consumed for C (CO and CO2) at this site. The contribution is estimated to be around 40 % based on calculations assuming metal side mass transfer not to be the rate controlling. At one time, it was considered to be the major mechanism (hot zone or impact zone theory). Metal layer flowing outward at this site can also get saturated with O2 as mentioned earlier.
The reactions in the emulsion seem to contain major sites for reactions (sites 4, 6 and 8). The droplets in the emulsion have extremely large specific surface area. In the presence of reasonable quantities of FeO in the slag, all the refining reactions in a droplet can take place in a matter of tens of seconds, instead of minutes. A 3 mm drop of metal containing 4.5 % C can evolve around 3,000 times its volume CO. This as it escapes through the viscous slag emulsifies it. The complex interactions of emulsion formation, droplet generation, droplet residence time etc., hence contribute largely to the overall dynamics. Reactions of droplets reacting with the gas phase directly are important primarily in the first couple of minutes of the blow when a complete slag layer has not yet formed.
A comprehensive look at this overall process dynamics needs a background on supersonic gas jets, their interaction with a metal / slag bath, droplet generation and their residence times, CaO dissolution, and bath mixing etc.
A gas-in-gas jet entrains the ambient gas in its periphery. The disturbed layer reaches the jet axis a few nozzle diameters downstream (potential core region) beyond which the flow becomes fully developed with self-similar radial velocity profiles. The axial velocity varies inversely with distance to maintain momentum conservation. Typically, the jet expands at a half cone of angle of around 10 degrees to 12 degrees, if the ambient gas has the same density as that of the jet gas. If the ambient is lighter the expansion is less due to the mass effect.
In supersonic jets, compressibility factor affects jet expansion. It has been shown that the jet does not expand much until the axial velocity decelerates to the sonic velocity (supersonic core). Thereafter the jet expands as a subsonic jet as shown in Fig 5. Recent CFD (computational fluid dynamics) study of an O2 jet into a BOS converter has shown that the axial velocity is almost constant for a distance of around 1 m and the temperature of the gas at the axis remains at around -170 deg C in this region. Thereafter the temperature increases steadily due to entrainment of hot gases. O2 lances are hence operated at an exit Mach number of around two so that it can be kept at some distance and still effect good jet / metal interaction.
Fig 5 Supersonic core of an oxygen jet flowing from a convergent-divergent nozzle
It can be noted that if the O2 jet is submerged in an atmosphere of CO as in the BOS converter, the concentration of O2 can come down substantially.
When a high velocity jet hits a metal surface, a crater is formed, the edges of which are highly unstable due to the high velocity of the deflected jet, throwing out metal droplets. At high enough values the jet becomes re-entrant, where some of the droplets are thrown into the jet itself leading to a highly unstable crater oscillating and rotating around. In the presence of a slag layer these droplets are trapped by the slag leading to droplet-in-slag emulsion.
The crater depth can be calculated by performing a momentum balance at the stagnation point at the centre of the crater. In further studies with a constant based on the experiments with various liquids and gases at room temperature and quantitatively studying the emulsification phenomena with the help of a 2-dimension, two phase model of mercury and glycerol, it has been found, as expected, that the droplets in the emulsion are increased with gas flow rate and varies inversely with stand-off distance of the lance tip from the liquid surface (lance height). While experimenting with a 3-dimension model of water to determine the droplet generation rate with a top layer representing slag, it has been found that there are two regions, one at a lower flow rate where the rate increases nonlinearly with flow rate and the second where the rate varies almost linearly with the flow rate. The Weber number has been used to characterize the flow phenomena. Droplet generation rate (kg/second) is correlated experimentally as a function of the blow number. It has been shown that simultaneous bottom gas injection can increase droplet generation especially when they are nearly coaxial with the top jet. The presence of slag phase can change the rate of generation substantially.
In a supersonic jet, say of Mach 2, the exit gas temperature is around -100 deg C. Thereafter, it entrains lower density converter gas. The temperature, velocity, and composition of the gas change as the jet strikes the bath. Hence this correlation has large uncertainties, because of which usefulness of the correlation in the BOS model is less than adequate. Since there is no other correlation, one normally uses the above correlation for generation of droplets and tunes it as needed.
The reaction rates also depend on the droplet sizes. Several studies have obtained emulsion samples from the working converters or laboratory hot models. These studies have found in general the sizes to be in the range of 0.05 mm to 3 mm. In a study experimenting with pig iron and O2, there were large chunks of liquids thrown out, which normally spend negligible quantity of time in the emulsion. Though these approximations and correlations are clearly inadequate, most models use these for lack of better correlations.
One of the studies found large quantities of metallic droplets in the foamy slag formed during high P iron refining. Another study made similar observations by collecting samples ejected through the tap hole in a 230 t converter and analyzing them. Several other studies have also made similar observation.
As mentioned earlier, the droplets are in various states of advanced refining, some of them being almost completely refined, though the bath still had considerable quantity of C. The fraction of metal in the emulsion has been estimated to be large, being almost 25 % of the bath weight. This corresponds to a surface area of around 40,000 square metres (sqm) if one assumes an average size of 1 mm for the droplets. It has been proposed in one of the studies that refining in the converter takes place primarily in the emulsion phase, the bath seeing refining by dilution from droplets falling back (emulsion theory). Emulsion in the converters refers to a slag-metal-gas system. One can visualize it as slag-gas foam in which metal droplets are distributed.
It has been also reported that several of the droplets display high O2 super-saturation and this has postulated that the finer droplets can have been generated by homogeneous nucleation of CO droplet bursting. Some droplets show evidences of being attached to gas bubbles and some are even hollow. There have been several experiments with magnetically levitated and freely falling droplets reacting with oxidizing gases. The results of these experiments are interesting. When the C content is high, one can see reactions taking place at the surface, as evidenced by CO burning. As the C content comes down, small droplets are thrown out indicating sub-surface nucleation. Further lower in C content, the droplets sometimes burst, indicating O2 super-saturation and nucleation deep within the droplet. Super-saturation to the extent of around 5 MPa (for equilibrium CO) had been reported at the time of droplet bursting.
In one of the studies, the residence time of the droplets in a converter has been measured by radioactive gold isotope tracer technique. The maximum residence time of droplets which are in advanced state of decarburization has been estimated to be around 2 minutes. Residence time calculated on the basis of free fall is of the order of a few seconds even while considering the slag to be emulsified to a much greater height. The high residence time hence needs an explanation.
Several experiments using X-rays for visualization of a single Fe-C droplet reacting in molten oxidizing slags have shown that the droplet gets buoyed up to the surface as soon as decarburization starts, and stays at the surface till the CO bubbling subsides. Further, it has been shown that the droplet residence time is dependent on bubble formation which keeps the droplet afloat.
There are two views on how the CO formation keeps the droplet buoyant. One of the studies has formulated a bloated droplet theory wherein CO forms homogeneously inside the droplet and this hollow droplet has a low apparent density, due to which it remains afloat. The other view is that the bubbles form heterogeneously at the droplet / slag interface and as long as the bubbles stay attached to the droplet they keep it afloat. The visual evidences from X-ray fluoroscopic studies cannot clearly distinguish between these two. The fact that there does not seem to be a nucleation barrier during vigorous deoxidation as evidenced by copious evolution of bubbles suggests interface nucleation.
At high C concentrations when C mass transfer within the drop is not rate controlling, the highest CO super-saturation is to be seen at the droplet surface. Hence, it can be expected that for the nucleation to take place heterogeneously at the surface, the bubble is to spend some time at the interface before detaching. Since there can be several bubbles attached, the droplet remains buoyed. As C falls to low values, nucleation at the interface becomes sporadic, and in periods when there is no bubble attached, O2 dissolves into the metal and diffuses in. Hence, the highest super-saturation region moves inward, first to sub-surface and then to deep inside the droplet. One can thus see sub-surface nucleation initially throwing out small droplets and then deep inside. These homogeneous nucleation events are probably sporadic, with a stochastic nature.
Simultaneously, the apparent density of the droplet with no or few bubbles is now high and it falls down into the metal bath. The critical C content when the droplet falls down depends on droplet size, the oxidizing potential of the slag (and the rate of mass transfer of FeO), and the sporadic nucleation event either at the surface or inside the droplet. Empirical work to correctly predict the critical C content is lacking. Evidence from levitated droplet experiments also point towards these series of events, though the stirring due to the electro-magnetic field makes the condition different from that in the converter especially with respect to mass transfer within the droplet.
In the context of converter, the droplet surface is continuously disturbed by the bubbles. This has two counteracting effects. Part of the droplet surface is covered by the gas bubble and is not available for mass transfer from the slag to the droplet. The droplet surface is also vigorously stirred by the formation and detachment of bubbles, enhancing mass transfer locally. Several indirect estimations have been made. In one of the studies, indirect estimation of mass transfer coefficient has been made for FeO in slag for P transfer rate in high temperature single droplet experiments, and the values obtained are between (10)−5 metres per second (m/s) and (10)−4 m/s . Another study estimated similar values. Proper experimental studies, both in cold and hot models, are necessary to get reasonable correlations in terms of dimensionless variables.
Though the slag is very well stirred in the converter due to the gas jet and a large quantity of gas passing through it, the metal bath in top blown converters is comparatively poorly mixed. Measurement of mixing time (t95 which is the time to get 95 % homogeneity) in top blown converter can be as high as 150 seconds (s) to 180 s, as compared with 10 s to 20 s in bottom blown OBM converters. This has consequence on the reaction dynamics, since the metal droplets are removed from the top layer and the refined droplets from the emulsion fall back at the top. Since much of the heat is also released in the slag, the slag and the droplets falling back are also hotter. There can also be composition and temperature stratification due to the scrap at the bottom slowly dissolving into the liquid metal.
High mixing times also correspond to high irreproducibility in mixing times, leading to irreproducible blow behaviour in the absence of bottom blowing. For example, a large eddy of liquid metal containing higher C from the bottom being brought to the surface of the metal bath can suddenly increase the rate of decarburization leading to instabilities. Hence, inert gas injection from the bottom of the converters to bring down the mixing time has become the standard practice.
Since the rate of bottom gas injection and the position of the porous elements through which the gas is introduced have a bearing on the reaction dynamics, it is necessary to quantify the mixing behaviour for quantitative predictions of composition and temperature evolution. A single average t95 value is inadequate for incorporation into a comprehensive model of the converter, since two different mixing curves can give the similar t90 (time to get 90 % homogeneity) and different t95 values. The compromise can hence be a two parameter model, based on estimation of two mixing times (t90 and t95). One can then idealize the metal bath as consisting of two stirred tank reactors (STR), exchanging metal continuously. The bottom part sees only scrap melting and the top part sees all other phenomena explained earlier. The two parameters of this model, ratio of reactor sizes and the metal exchange rate can then be fitted to the mixing times of the converter under various conditions of operation.
Formation of slag and dissolution of fluxes
Fluxes (lime and calcined dolomite) which are charged early in the blow dissolve with the developing oxides to form a liquid slag. The rate of dissolution of these fluxes strongly affects the slag-metal reactions occuring during the blow. At the beginning of the blow, the lance height above the bath is kept high which causes an initial slag rich in SiO2 and FeO. During this period large quantities of fluxes are charged in the converter. The lance is then lowered and the slag starts to foam at around one third of the blow due to the reduction of FeO in the slag in conjunction with CO formation. As the blow progresses, the CaO dissolves in the slag, and the active slag weight increases. After the blow has progressed around three fourth of the time, the FeO content in the slag increases because of a decrease in the rate of decarburization.
During the blow, the temperature of the liquid steel gradually increases from around 1,350 deg C to 1.650 deg C at turndown of the converter, and the slag temperature is around 50 deg C higher than that of the liquid steel. The slag at turndown can contain regions of undissolved lime mixed with the liquid slag, since the dissolution of lime is limited by the presence of dicalcium silicate (2CaO.SiO2) coating, which is solid at steelmaking temperatures and prevents rapid dissolution. The presence of MgO in the flux weakens this coating. Hence, earlier charging of MgO speeds up slag formation due to quicker solution of lime.
The converter needs to maintain a good fluid slag of high basicity (high CaO content) so that the large quantity of CO generated can be handled, and P can be removed efficiently. Hence, the converter operator tries to achieve a CaO / SiO2 ratio in excess of 3.0 in the final slag.
Fig 6a shows the liquidus contours in a CaO-SiO2-FeO ternary diagram. It is clearly seen that a CaO / SiO2 ratio which can be achieved in this system at 1,350 deg C, i.e. at the beginning of a blow this ratio is limited to around 1.6 to 1.7. Marginal improvement can take place with MgO additions (Fig 6b) and some Al2O3 coming from the carry over slag. In the final slag also at 1,650 deg C with 25 % to 30 % FeO, the maximum CaO / SiO2 remain less than 3.0. This is also borne out by the slag analyses which frequently show un-dissolved lime. Apart from the issue of solubility of CaO in the converter slags, the lime particles get passivated in the presence of highly siliceous slags. Since the CaO concentration is the highest at the surface of a dissolving lime particle, di-calcium silicate forms here. This compound is not only highly refractory but it forms an adherent layer retarding further dissolution.
A lime particle remaining undissolved for long at the high temperatures also sinters and becomes less reactive. One way of breaking the adherent layer is to have high FeO content in the slag. This is the reason for the practice of raised lance blowing in the first few minutes of the blow, when the FeO is built up to 25 % to 35 % or higher. Though the effect on solubility of CaO is marginal (Fig 6), this facilitates breaking of the adherent di-calcium silicate layer permitting further dissolution.
Additives like fluorspar (CaF2) can bring about this effect much more efficiently, though this is not an acceptable plant practice in recent times for various reasons. Fig 6 shows phase diagrams with Fig 6a showing liquidus isotherms of CaO-SiO2-FeO system and Fig 6b showing tffect of MgO addition on the liquidus.
Fig 6 Phase diagrams
Process flow and reaction dynamics
The contents of the converter can be divided into several important regions such as (i) the metal bath, which itself can be divided into the bottom and top part between which there is exchange of metal, (ii) the O2 jet and the impact region, and (iii) the slag region which mostly is in the form of a slag-metal-gas emulsion. There are three distinct regimes in the blow. The initial part is characterized by a bare metal bath covered with islands of solid lime and some slag carried over from the previous heat. Jet of O2 hits the metal bath and does two primary things. First, it oxidizes almost an entire layer of the metal giving CO, SiO2, MnO, P2O5 and lots of FeO. Not all O2 is consumed in this location, and the gas above hence can contain high ratio of CO2 / CO and some O2 as seen in exhaust gas analysis. The jet also throws droplets into the gas phase, which after free flight fall back. Since the gas is oxidizing, the droplets get refined during the flight. At the surface of the droplets, the order of reactions is dictated by the thermodynamics.
For each of the solutes, reaction involves mass transfer steps such as mass transfer of CO2 / O2 in the gas phase and of the solutes in the liquid phase. The interfacial chemical reactions are expected to be very fast at this temperature. The order of the reactions can be achieved by solving the mass transfer equations along with free energy minimization for the interface reactions competing for O2. The order is normally Si and Mn followed by C and P. Since the time of flight is typically of the order of a second or lower, the droplets fall back probably completing only part of the Si reaction. Smaller the droplet, further the refining proceeds because of the larger specific surface area. Reaction at the rest of the surface of the metal bath is small because of the smaller surface area compared to that of the droplets.
The mass transfer in the gas phase can easily be calculated by Ranz-Marshall type correlations. At this initial phase of the process droplets are high in solutes, and the gas phase mass transfer is expected to be rate controlling. The small droplets can be considered as rigid and one can assume pure diffusion of solutes inside the droplets. When the droplets fall back, the condensed phase oxide products in the droplets remain at the top of the bath, and on combining with the oxides from the impact site and the fluxes added start forming a liquid slag. As mentioned earlier, good quantity of FeO is formed at the impact site, and hence liquid slag formation is easy. After sometime, there is a liquid slag layer covering the metal bath. Increasingly more and more droplets are thrown to the slag. The droplets ejected into the gas phase now have to pass through the slag phase before reaching the metal bath. Further refining hence takes place in the slag.
Initially when the slag layer is thin and the droplets are high enough in Si and Mn, the droplets fall through before the C reaction starts, that is, with no gas evolution, especially for larger droplets. Smaller droplets high in C can however start to decarburize early releasing CO, and slowly emulsifying the slag. This early phase is characterized by a low flame at the mouth, since CO formation is comparatively low. Once the Si in the metal bath falls down to some extent, the desiliconization progresses considerably, before the droplet has fallen down. C reaction starts and the droplet stays now buoyed in the emulsion till its C content reaches the critical C content as explained earlier. In the slag phase, the rate is expected to be controlled by slag phase mass transfer of FeO, as long as C in the droplet remains high enough. Once a critical C is reached in the droplet, bubbling slows down and then ceases, and the droplet falls down. The critical C is largely determined by the FeO content in the bath. Quickly the emulsion builds up and the second phase of reactions in the emulsion starts. The flame at the converter mouth becomes large. The lance tip gets dipped into the emulsion.
In the second phase almost all of the droplets are ejected into the emulsion, and the gas phase reactions become unimportant. It is to be noted that the residence times of the quiescent droplets in the slag are only of the order of a few seconds unless decarburization reaction starts. Hence, for maximizing the refining, the operator is to quickly reach a stage where the decarburization reaction starts before the droplets fall back. One way to accelerate the reactions is to keep the FeO content in the slag high. Another reason why FeO is to be increased as early as possible is to have a fluid slag by the time decarburization rate reaches its highest value, since the large quantity of gasses are to quickly escape from the slag. Else, the emulsion height gets build up uncontrollably leading to overflow, and slopping.
The FeO content in the slag is a balance between its generation at the point of impact and its consumption by the droplets in the emulsion. The FeO generation is probably weakly dependent of the lance height, whereas a high lance leads to less droplet generation due to lower force with which the jet strikes the metal bath, and vice versa. Hence a raised lance practice, called the soft blow, leads to quick increase in the FeO content in the slag. This facilitates CaO dissolution and formation of a fluid slag. The initial soft blow, normally 3 min to 4 min, is the normal plant practice.
At an optimum moment, the lance is lowered to induce high rates of reactions. Droplet generation rates are high, the bath is already desiliconized, and hence the droplets undergo vigorous decarburization till C goes to low values before falling back. During this period of peak decarburization rates, hence a large part of the metal bath remains in the emulsion as droplets. These droplets have spent different times in the emulsion and hence are in different stages of refinement. The degree of refinement also depends on the droplet size. The droplet are hence characterized by two variables namely the time it has been formed (and hence its age) and the size of the droplet. The droplet starts to fall back when a characteristic C content is reached, which depends on its size, the slag FeO, and the temperature. During this last phase of the droplets, the O2 potential at the interface is also high and hence P is also removed if other conditions are favourable. Falling droplets result in apparent refinement of the top of the metal bath, which on mixing lead to refinement of the rest of the bath. Since the time for refinement of a droplet can be of the order of 0.5 min to 2 min, one sees drop in C, Mn and P in the bulk metal sample even if Si in the sample is still of considerable quantity (Fig 3a).
The overall rate of reaction, to some extent, is self-correcting. If the number of droplets in the emulsion come down decreasing the rate, the level of the metal bath increases leading to lower effective lance distance, which in turn causes droplet generation to increase. This is one of the reasons for the near constant decarburization rate during middle part of the blow. It is however is to be noted that the C content of droplets entering into the emulsion keeps falling down, and hence their residence time. The operator is required to correspondingly increase the droplet generation rate by progressively lowering the lance. Fig 7a indicates the lance height variation during a typical blow.
Fig 7 Variation of lance height and progress of phosphorus partition ratio
As the bath becomes low in C in the final phase of the blow, the rate now gets limited by C diffusion within the droplet even as it enters the emulsion. The CO generation is low and is not able to keep the droplets floated. The residence time drops down to a few seconds, and hence number of droplets in the emulsion comes down even though the lance height has been brought down to the lowest level permissible for lance health. The emulsion dies down. At this time there is falling rates of decarburization and fast buildup of FeO in the slag. Since the rates are low, the FeO content and the O2 dissolved in steel increase much beyond what is dictated by C-O equilibrium. Hence, at this period, the operator raises the argon stirring rate, increasing thereby the droplet generation rate without adding extra O2. This helps to some extent.
Phosphorus removal is sometimes an issue in the BOS process and can result in re-blows, especially when the input P in the hot metal is high (around 0.2 %). Though the conditions are normally favourable in the final slag with high FeO and high basicity, the converter operator can land in adverse situation if the slag regime is not carefully managed throughout the blow. The thermodynamics of P is well known. The reaction is written either in terms of molecular species or in the ionic form. The reaction is 2P + 5/2 O2(g) = P2O5(l), P + 5/4 O2(g) + 3/2 (O)2− = (PO4)3−. In the former case one writes an equilibrium constant, and expresses the Raoultian activity coefficient as a function of slag composition. If one adopts an ionic form of the equation, one instead writes an equation for a phosphate capacity of slag and correlates the phosphate capacity to the slag composition empirically. Both these approaches are conceptually similar. The partial pressures of P and O2 can easily be converted to percent dissolved in metal or activity of FeO in slag with known thermodynamic data. The slag data as a function of composition either as Raoultian activity coefficient or as phosphate capacity have been empirically determined in several studies. The progress of partition coefficient for P between bulk slag and bulk metal can be calculated when the slag analysis during the blow is known.
In the initial period of the blow, the bath C is quite high and also contains Si. Hence at the slag / metal interface, the O2 potential remains low. Therefore, very high rate of dephosphorization at the bulk metal / slag interface is not expected. The metal droplets, on the other hand, get highly refined in a matter of 1 min to 2 min, and before returning to the metal bath, have high O2 potential at its interface. Further the partition coefficient at this time is high since FeO content is high due to soft blow, temperature is low, although with some CaO yet to dissolve. The number of droplets in the emulsion is also very large. Hence, the dephosphorization rate is very high which can be seen in Fig 3a. Towards the end of the blow again, the conditions in the slag are favourable with high FeO and high basicity, though now the temperature has risen substantially. The rate of phosphorus removal however is not very high in this period, since the number of droplets is not very high, surface area is quite small and hence all reactions are slow.
Vigorous Ar stirring is helpful at this time of the blow, and for some time after the O2 flow is stopped, though to a limited extent. It is in the middle part of the blow the operator has the highest opportunity for efficient overall dephosphorization. After the soft blow when the lance is lowered progressively for effecting high rates of decarburization, FeO content in the slag drops considerably and remains low till the emulsion starts collapsing. The slag becomes comparatively ‘dry.’ The partition coefficient becomes adverse, and one can easily get P reversal to the metal. Lower is the FeO level, higher is the reversal. This reversal increases the load on the last part of the blow where the rates of reactions are anyway low as explained earlier.
Hence, close control of the FeO content during the middle part of the blow is necessary if the operator is required to make low P steel. Premature lowering of the lance in each stage can lead to very low FeO content (less than 12 % to 15 %). FeO content is determined by the balance between droplet generation rate (consumption rate) and the FeO generation rate. However, it is to be noted that very low FeO in the slag also lowers the decarbonization rate. Very high FeO on the other hand leads to sloppy conditions.
Higher levels of FeO content can be achieved by modifying the lance practice. The lance height for the intermediate levels can be kept slightly higher than the normal. The operator can also slightly delay lowering of the lance, taking care to see that it does not lead to uncontrolled emulsion build up. The operator can also achieve this by distributed ore addition during this period.
The chemistry of steelmaking in BOS converter is summarized here. From the thermodynamics of the O2 steelmaking process, it can be seen that, at the beginning, the O2 blown onto the HM preferably reacts with the dissolved Si, forming SiO2 which floats on the surface of the metal. From kinetics, it is expected that a part of the O2 blown reacts with the dissolved C and Fe atoms. The formation of CO gas occurs instantaneously on process ignition. Calcined lime is added to neutralize the acid slag, which initially includes a liquid mixture of FexOy and SiO2. Several chemical reactions take place in the BOS converter. The main reactions are dissolution of O2 into the metal from O2 gas, decarburization through dissolved O2, and oxidation of Fe, [Si], [Mn], [P], [V] and [Ti]. Solid or liquid oxides are formed as reaction products during blowing, and they are bound with the lime which is added at the start of blowing to form a liquid slag in the converter. Due to intensive CO gas formation, droplets of liquid metal are introduced into the slag, which tends to foam. Hence, the slag in the converter during O2 blowing is actually an emulsion of liquid slag and metal droplets, foaming because of the influence of gas bubbles. The emulsion is also a favourable site for reactions. For example, a considerable fraction of C oxidation can occur in the metal droplets in the emulsion although the majority takes place in the impact zone of the O2 jets. The rest of the O2 is used to burn Fe into FexOy. During blowing, O2 penetrates the metal droplets and can react with the CO gas. The total slag-gas system behaves as foam and rises quickly to the cone of the converter. Hence, the O2 inflow and the reaction rates have to be adjusted so that foam is not spilled from the converter. Slopping frequently occurs even though the inner volume of the converter is almost nine times larger than the volume of the inactive metal and slag bath.
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