Foaming of Slag in Basic Oxygen Furnace
Foaming of Slag in Basic Oxygen Furnace
In basic oxygen furnace (BOF) steelmaking, a supersonic oxygen (O2) stream is injected onto the surface of the hot metal bath. The impurity elements in the bath such as carbon (C), silicon (Si), manganese (Mn), and phosphorus (P) are oxidized and removed from the bath in the form of CO (carbon mono-oxide), CO2 (carbon di-oxide), SiO2 (silica), MnO (manganese oxide), P2O5 (phosphorus pentoxide), and iron oxides by the injected O2. The gaseous oxides, containing 90 % CO and 10 % CO2, escape the furnace from the top with small amounts of iron oxide (FeO) and lime (CaO) dust. The other liquid oxides dissolve with the added fluxes forming the liquid slag which further facilitates the refining of the bath.
Foaming is an important phenomenon which is commonly encountered when gas is blown through a viscous liquid. It is beneficial in the BOF steelmaking as it assists the refining process in different ways. It provides an increased surface area for the refining reactions and protects the liquid metal bath from the direct contact of the atmosphere. It enhances the kinetics of the reactions, heat transfer, and energy efficiency of the process. It forms the medium for post-combustion and heat transfer. It protects the refractory lining from extreme combustion effects by providing a shield for the refractory and hence extends the life of the refractory lining. It acts as a sink for the oxides of impurities such as Mn, Si, and P, which have been oxidized from the liquid bath. In addition, slag foaming prevents the liquid bath from oxidizing and enables control of its composition. It also acts as a thermal insulator between the hot bath and the surroundings and thus prevents major energy losses. However, the foaming of slag in the BOF steelmaking is considered to be a phenomenon which is needed to be controlled to a limited volume to get the benefits.
There are basically two requirements for foaming of slag. These are (i) reactions or processes which generate small gas bubbles, and (ii) suitable slag properties to keep the bubbles as stable foam. Normally, gases resulting from chemical reactions tend to foam the slag with smaller bubbles, whereas the injection of gas phases such as O2 and Ar (argon) etc. results in larger bubbles and less stable foams.
Foams are a common occurrence in the BOF which is produced by trapping the gases in the slag layer. With the progression of the blow, the quantity of slag as well as the gases generated increase, and consequently, the slag foaming also increases. Slag foaming can become disadvantageous and hazardous when formed in huge quantities, and overflow from the mouth of the converter, which is termed slopping of the bath. Hence, slag foaming has to be properly controlled for a continuous and efficient production process. Thus, a thorough understanding of foams and the foaming process is necessary to optimize the process by minimizing the slag foaming.
Slag is the non-metallic material produced from the products of oxidizing reactions (SiO2, MnO, P2O5, and FeO) and dissolving fluxes (lime and calcined dolomite) during the steelmaking process. Slag can also contain the oxide compounds, dissolved gases, dissolved refractory lining and solid particles of undissolved fluxes or precipitated oxides/oxide compounds.
Slag has a low density and hence floats on the liquid metal. The main requirement of the slag in the BOF steelmaking is to function as a pool for the oxides of the oxidized and removed impurities to collect. Further, the composition and temperature of this slag pool assist the progress of refining reactions in the converter. The other advantages of having a slag layer in the operating converter comprise of shielding the metal bath from the ambient air, retarding the dissolution of the refractory lining of the converter, and controlling the bath temperature.
Slag composition (usually in the system of CaO–MgO–SiO2–FeO–Al2O3) directly affects its viscosity, thermal conductivity, density and other properties, causing an impact on the ability of the slag to remove impurities from the liquid bath. Slag is generally high in viscosity and low in density, causing slag to float on the bulk metal bath. As a result of its physical properties, slag has the capability to trap the gas bubbles produced by the chemical reactions and injected O2, and produce slag foam.
The slag foam (Fig 1) is produced as the CO gas is generated and trapped in the slag. The de-carburization of the steel is the source for generating the foam. As the O2 jet hits the bath surface, metal droplets are torn off due to the high momentum and end up in the foam creating an emulsion of slag, gas and metal droplets. In the emulsion, the slag has a lot of metal droplet surface to react with, which enhance the kinetics of the reactions. The process is expected to be considerable longer if there has been no emulsion. The emulsion is of great importance for the process and it is important to have a proper slag composition to keep the foaming under control, with a neither too high nor too low foaming level. The residence time determines the possible reaction time between the slag and the moving droplet in the emulsion phase.
The gas generation rate plays an important part in the formation and growth of the foam. The gas is a product of the decarburization process. It proceeds (i) by direct oxidation at the metal surface in the hot spot as per the equation [C] + 1/2O2(g) = CO(g), (ii) in the foam, indirectly by iron oxide reacting with metal droplets as per the equation [C] + (FeO) = CO(g) + {Fe} where the (FeO) is a product of oxidation of iron (Fe) by pure O2 as per equation {Fe} + 1/2 O2(g) = (FeO), and (iii) in the melt, by reaction between dissolved O2 and C as per the equation [C] + [O] = CO(g)..
Decarburization as per the reaction under (i), and also the oxidation of Fe as per the second reaction under (ii), begins immediately and continue throughout the blow, although in the first case with a diminishing pace, due to the continuous decrease in the C content at the metal surface. The rate of Fe oxidation is more constant, but the resulting FeO content of the slag eventually decreases due to an increased consumption as per the first reaction under (ii). At the end of the blowing period the FeO content in the slag starts to increase again as the participation of FeO in the decarburization process is reduced due to the low C content of the melt. At the very end of the blow the controlling de-carburization reaction is the one in the melt between C and dissolved O2 as per the reaction under (iii). Hence, the de-carburization rate at the end of the blow is dependent on mass transfer of C from the lower to the upper part of the melt and of dissolved O2 in the opposite direction. The principle reactions involved in the decarburization of the melt in the BOF converter are shown in Fig 1.
Fig 1 Principle reactions involved in the decarburization of the melt in the BOF converter
As shown in the right of the Fig 1, the maximum de-carburization rate, and hence the maximum gas generation rate, is reached 25 % to 30 % into the blow, and proceeds to a great extent within the foam as per the first reaction under (ii). The rate is fairly constant with the level depending on the availability of FeO and the supply of metal droplets ejected from the O2 impingement zone. At about 80 % of the blow the gas generation rate quickly drops off due to a low C content in the melt.
Slag foam is beneficial for the steelmaking process in terms of the large surface area formed and the protection provided against the direct contact of the melt with the atmosphere. Normally, the physical properties of slag evolve in favour of foaming during the blow, and when coincided with high gas production rates from chemical reactions, slag foaming can become uncontrollable and overflow the converter creating a slopping incidence. Slopping is a detrimental incidence, and its consequences include yield loss, health and safety costs, damage to equipment and environmental pollution.
The process variables which affect foaming in the BOF are slag composition, superficial gas velocity, bath temperature, bubble size, slag basicity, slag density, slag viscosity, and slag surface tension. Superficial gas velocity is normally measured in meter per second (m/s) and is the true gas velocity multiplied by the volume fraction of the gas.
The composition of slag is one of the most important process variables which affect its foaming, which evolves throughout the blow, generally, in favour of foaming. This owes to the fact that the physico-chemical properties of the slag such as the density, viscosity, surface tension, and basicity, vary with the composition of the slag. The foaming at such high superficial gas velocities as encountered in BOF steelmaking (i.e. greater than 1 m/s), the liquid is held up by the gas flow. It is argued that in this situation the void fraction (VF) strongly depends on the superficial gas velocity, while weakly dependent on the physical properties of slag and liquid. Further, formation and existence of this gas hold up are governed by the gravity and the drag forces on the liquid exerted by the gas.
Slag foam is formed when the gases injected and generated by the refining reactions are trapped by the slag during the process. For slag foams, the amount of gas trapped by the slag is measured by the VF or the gas fraction, and the VF generally varies in the range of 0.7 to 0.9. Fig 2 shows a typical foam column with different foam layers according to the VF. The combined effect of evolving physical properties of slag during the blow is to be in favour of foam stabilization, and when coincided with the high rate of de-carburization in the first half of the blow, the volume of the slag foam increases rapidly.
Fig 2 Typical foam column showing structure of its layers
The foaming index (FI) is an indication of the extent of foaming and it is the ratio between the foam height and the superficial gas velocity. Hence, the unit of the FI is time which is normally in the range of 0.6 seconds (s) to 1.3 s. Thus, the FI can be interpreted as a measure of the time it takes for the process gases to vertically pass through the foam. With a constant O2 supply rate the gas velocity can be assumed to be fairly constant during the main decarburization period of the blow, i.e. the foaming height is directly proportional to the FI.
A very important property in regard to the FI is the apparent viscosity of the emulsion. The higher is the apparent viscosity, the higher is the FI. The obvious consequence is that an increased apparent viscosity automatically leads to an increased foam height, and with a sufficiently high apparent viscosity, the foam eventually begins to flow over the converter, i.e. slopping occurs. One parameter which strongly influences the apparent viscosity is the presence of solid particles. As per a study, increasing of the fraction of solid particles by only 10 %, there is a 50 % increase in apparent viscosity and at least an equivalent increase in foam height.
The effect of slag composition on its foaming was the subject of an experimental study, using a CaO-SiO2-FeO slag in the temperature range of 1,250 deg C to 1,400 deg C. The slag was contained in an alumina crucible, and argon (Ar) gas was injected to foam the slag. The observed results on the effect of different parameters on the slag foaming were expressed using FI, which was first used for aqueous systems. The FI has the meaning of average traveling time of gas through the foam layer and can be expressed by equation FI = delta h/delta Vg, and Vg = Qg/A. Here delta h is the change of slag height, and Vg is the superficial gas velocity, Qg is the gas flow rate, and A is the cross-sectional area of the container. The FI was observed to change between 0.3 s to 56 s for metallurgical slags with different compositions. The above relationship was considered to be valid when the VF of the foam is independent of foam height. The extent of slag foaming has since been defined frequently by using the FI.
Another method of calculating the FI can be expressed as ‘rate of change of foam volume = rate of gas generation or injection – rate of volume change due to bubble rupture’. Hence, the FI can also be expressed in the form of equation FI = 1/k.e, where, ‘k’ is the rate constant for bubble decay, and ‘e’ is the average VF. However, these definitions of the FI are for a steady state system, which has an almost constant gas flow rate and chemical composition. However, in order to explain the foaminess of the BOF slag, a dynamic FI which takes into account the dynamic nature of the gas generation and the slag composition is needed which is represented by FI(d) = f(r).FI where f(r) = (rate of gas generation + rate of change of slag volume)/ rate of change of slag volume. The FI is a property of the slag which changes with time throughout the blow and which pass through a maximum.
In another study, it has been shown that the FI is related to the physical properties of slag. This relationship shows that the FI is increased with the increase of slag viscosity, while it is decreased with the increase of density and the surface tension of the slag.
Effect of superficial gas velocity on slag foaming
The effect of superficial gas velocity on the foaming of CaO-SiO2-FeO slag, where the foam was produced by injecting Ar gas has been studied. It has been observed that the foam height increases linearly with the increasing superficial gas velocity.
In the smelting reduction processes, the converters operate around 0.3 m/s to 3.0 m/s of superficial gas velocities. The foam observed at low superficial gas velocities is different to that observed at high gas velocities. The foam at high superficial gas velocity (i.e. higher than 1 m/s) is the expanded slag, where the entire volume of liquid is expanded uniformly in a turbulent and churning method. The mixture height and the VF of this expanded slag increase with increasing superficial gas velocity, and it collapses immediately with the stopping of the gas flow. On the other hand, the foam produced at low superficial gas velocities (i.e. 0.01 m/s to 0.1 m/s) is like soap foam, and this foam collapses gradually with the stopping of the gas flow. These observations are found to be valid for higher superficial gas velocities typical for the smelting reduction processes, when there are no undissolved oxides which alter the viscosity of the slag.
On the other hand, in one of the study, it was suggested that the foaming at such higher superficial gas velocities as encountered in BOF steelmaking (i.e. higher than 1 m/s), the liquid is held up by the gas flow. It is argued that in this situation the VF strongly depends on the superficial gas velocity, while weakly dependent on the physical properties of slag and liquid. Further, formation and existence of this gas hold up are governed by the gravity and the drag forces on the liquid exerted by the gas.
Effect of slag composition on slag foaming
The composition of slag is one of the most important factors that affect its foaming, which evolves throughout the blow, generally, in favour of foaming. This owes to the fact that the physico-chemical properties of slag such as the density, viscosity, surface tension, and basicity, vary with the composition of the slag.
In one of the study, it has been noticed that the addition of P2O5 slightly decreases the FI, while the addition of S marginally decreases the FI. The FI is found to decrease with the addition of CaF2 (calcium fluoride) significantly, while it has increased notably with the addition of MgO. These observations are made in the study for a CaO-SiO2-FeO slag at 1,400 deg C, where the Ar gas injection has produced the foam. In this study, it has also been observed that the FI is increased with the increasing particle concentration.
The effect of the addition of FeO and MgO on the FI of CaO-SiO2-FeO-MgO slag has been the subject of one other study. The FI has been observed to decrease with increasing FeO content upto around 20 % of FeO mass in the slag and then stayed constant upto a mass concentration of around 32 % FeO in the slag. MgO (magnesium oxide) addition has also shown a similar trend to that of FeO by decreasing the foaming index with its addition to a 35 %CaO-35 %SiO2-30 %FeO slag.
When analyzing the FI evolution with the composition of slag, the effect of the addition of MnO and P2O5 on the FI is also worth considering. The effect of MnO and P2O5 addition to the CaO-SiO2-30FeO-MgOsaturated slag respectively has been the subject of one study. It has been observed that the FI slightly decreases with increasing MnO in the slag, while it increases with the addition of P2O5 upto 3 % and then decreases with the further addition of P2O5.
Effect of temperature on slag foaming
Increasing the temperature of slag has been observed to decrease the foam index of 35 %CaO-35 %SiO2-30 %FeO-10 %MgO slag during a study carried out in the temperature range of 1,400 deg C to 1,550 deg C. This observation can be attributed to the positive temperature coefficient of surface tension and the negative temperature coefficient of viscosity. The effect of surface tension and the viscosity on the foaming of the slag is such that increase in surface tension destabilizes the foam while the increase in viscosity stabilizes the foam. Hence, the combined effect of the above two properties reduces the FI with the increase of temperature.
Effect of bubble size on slag foaming
The common observation is that the slag foaming is inversely proportional to the size of the bubbles. In other words, foam consisting of larger bubbles collapses earlier compared to that consists of smaller bubbles. This observation has been put under investigation in a study by injecting Ar gas through a multi-orifice nozzle into a bath smelting type slag. The foaming of the same slag by the bubbles generated from inter-facial reactions has been studied. The average bubble diameter of bubbles produced from the single orifice nozzle has been 13.5 mm, while that from the multi-orifice nozzle has been around 7.5 mm. It has been observed that the measured foam height when the gas injection has been through the multi-orifice nozzle was around 70 % greater than that measured when the gas injection was through a single orifice nozzle. The bigger bubbles produced from injecting Ar gas through the single orifice nozzle, were of polyhedral shape. On the other hand, when the bubbles have been produced from the CO generated by the interfacial reactions, the bubbles were observed to be fine spherical gas bubbles, and the foam produced was comparatively more stable. However, in both cases, the size of bubbles was inversely proportional to the FI (Fig 3).
Fig 3 Effect of the bubble diameter on the FI
This observation has been explained by the fact that increasing the bubble size increases the drainage. It is seen that the transversal area of the plateau borders as shown in Fig 3 affects the drainage velocity. In other words, the transversal area is larger for larger bubbles increasing the drainage compared to that for smaller bubbles. This influence of bubble diameter on the FI shows the inverse proportionality between the bubble size and the FI, despite the different degree of influence. However, in the BOF steelmaking, the slag foam is primarily produced by trapping CO gas in the slag layer, and in this case, the rate and quantity of CO production, turbulent fluid flow, and surface tension are the primary effects on the size of the bubbles.
Effect of slag basicity on slag foaming
In a study carried out on the CaO-SiO2-FeO slags to investigate the factors which affect foaming, the effect of basicity of the slag on its foaming was studied. The observations made in the studies are that the FI goes through a minimum with increasing basicity. There is an initial decrease of the FI with the increasing basicity which is due to the decreasing viscosity and increasing surface tension, since they have a negative effect on the FI. The CaO composition at the minimum FI refers to that of the liquidus composition, which precipitates if exceeded. Hence, after the liquidus composition, further addition of CaO precipitates as solid 2CaO.SiO2 particles, increasing the viscosity and thereby stabilizing the foam.
Effect of slag density on slag foaming
Density is another important physical property, which has a considerable effect on the foaming of slags. As shown in Fig 4, the density of slags increases with the accumulation of slag components like FeO, MnO, and MgO, and decreases with the increasing SiO2 and temperature.
Fig 4 Densities of slags
Therefore, the FI decreases with the accumulation of FeO, MnO, and MgO, and increases with increasing SiO2 content in the slag. Further, FeO is also considered capable of lowering the viscosity with its increasing content in the slag. This inverse proportionality between the FI and the slag density is shown in several studies and it is evident that foaming index reduces with increasing slag density, despite the different degree of influence found in different studies.
Effect of slag viscosity on slag foaming
Slag viscosity and its evolution during the blow also play a significant role in slag foaming. The viscosity of a slag is primarily governed by the network formers like SiO2, and the addition of metal oxides, such as FeO, MgO, MnO, and CaO, breaks the network structures and reduces the slag viscosity.
It is the common observation that the slopping occurs in the first few minutes of the blow during the BOF steelmaking process. This is because of the increasing slag viscosity due to lower temperature and the presence of undissolved lime particles at the beginning of the process, simultaneously with the high rate of decarburization. Further, if the slag path, goes below the liquidus temperature of the slag, second phase particles precipitate. These second phase particles, which are smaller in size compared to the foam bubbles, increase the viscosity of the liquid slag stabilizing the foam. However, the foam volume increases with the increase of viscosity only until a critical viscosity value, after which the gas starts to channel through the slag without foaming. It has been shown in several studies that the FI increases with the increasing slag viscosity.
Basically, the effect of viscosity is on the drainage and rupture of bubble films in the foam. The increase in viscosity stabilizes the foam by increasing the thickness of bubble films to retard the bubble coalescence and by reducing the downward flow of the liquid (i.e. liquid drainage) from the films through the plateau borders.
Effect of slag surface tension on slag foaming
The surface tension of a liquid exists because of the cohesive forces exerted on the molecules on the liquid surface by other molecules in the liquid. As a result, surface tension is responsible for the formation of droplets and for retarding the liquid molecules from escaping the liquid. In the case of BOF steelmaking, the formation of slag/metal emulsion and foam is affected by the slag surface tension. Furthermore, the mass transfer between slag and metal is also affected by the slag surface tension to a considerable extent. The surface tension of pure liquid iron is about 1.8 N/m at 1,550 deg C, which is around 25 times higher than that of water. On the contrary, the surface tension of pure liquid oxides and slags are very low, ranging between 0.20 N/m and 0.70 N/m.
The surface tension of slags tends to vary with the temperature and the composition of the slag. The non-metals such as S, P, O2, and N2 (nitrogen) are surface active with different strengths in liquid iron. On the other hand, the oxides including SiO2, P2O5, and MnO, decrease the surface tension of the slag, while Al2O3 (alumina) slightly increases the surface tension.
Inter-facial tension between metal and slag is also an important factor in the BOF steelmaking process, similar to the surface tension. Hence, the inter-facial tension affects the formation of metal / slag emulsion and mass transfer between metal and slag similar to the behaviour of surface tension. When the inter-facial tension at the slag / metal interface is low, the refining process is encouraged via assisting the inter-facial mass transfer and foam / emulsion formation, which is advantageous. However, low inter-facial tension can also encourage the entrapment of slag droplets by the liquid metal, and the strong adhesion between the slag and the metal, which makes the physical separation of slag from metal more difficult at the tapping stage.
Various elements added to the liquid iron exert different levels of influence on the interfacial tension between the liquid metal and slag. In general, almost all of the added elements decrease the inter-facial tension of liquid iron. In BOF steelmaking, O2 and S are considered to be the strongest surface active elements in metal, which decreases the inter-facial tension remarkably. Further, FeO and MnO are considered to be the surface active oxide components in slag which reduces the interfacial tension between the metal and slag.
In order for a bath smelting slag to foam, the presence of a surface active component such as P2O5, CaF2, Fe2O3 (ferric oxide), V2O5 (vanadium pentoxide) , and Na2O (sodium oxide) is essential. Surface active compounds as such have the capability to reduce the viscosity, and most importantly can reduce the surface tension of the slag, allowing the slag to trap the gases and produce foam.
One aspect of the influence of surface / inter-facial tension is its capability to determine the size of the bubbles generated at the slag /metal interface. Hence, increasing slag surface tension and slag /metal inter-facial tension increases the diameter of the bubbles, which destabilize the foam. On the other hand, the foam is stabilized by small bubbles produced when the surface tension of the metal is increased. Further, the bubbles of an already produced foam experience increased drainage of their films with the increase of surface tension. This is due to the increased suction of the liquid in the films towards the plateau borders as the curvature of the bubble films increases with the increasing surface tension.
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