Slopping of Slag in Basic Oxygen Furnace

Slopping of Slag in Basic Oxygen Furnace

Steelmaking in basic oxygen furnace (BOF) is a batch process in which steel is made from liquid iron. The concentration of elements such as carbon (C), manganese (Mn), and phosphorous (P) have an impact on the steel quality. For the steel to be cast, it needs to be at a pre-defined temperature. To achieve the predefined temperature and composition, oxygen (O2) is blown into the refractory lined converter which contains the liquid iron. The O2 oxidizes the different elements within the bath causing an increase in temperature and a reduction in the concentration of the undesirable elements. The formed liquid oxides float to the top of the bath forming a slag layer. To make the BOF steelmaking process effective, a significant slag volume is needed in the vessel. However, the slag volume is bounded by the limited size of the converter

The fast reaction rates in the converter are due to the extremely large surface area available for the reactions. When O2 is injected onto the metal bath, a huge quantity of gas is evolved forming an emulsion with the liquid slag and with metal droplets sheared from the bath surface by the impingement of the O2 jet. The formed gaseous oxides such as carbon monoxide (CO) and carbon dioxide (CO2) rise through this slag layer making it foamy.

The slag foam formation takes place near the middle of the O2 blowing period and there is a decrease in the absorption of iron oxide (FeO) by the slag. This almost coincides with the period of maximum rate of decarburization. Hence, the maximum foam height is reached shortly before the first half of the blowing (around 35 %) due to the high volume of gas generated by decarburization and the high foamability of the slag at that time.

The distinctive characteristic of the steelmaking process in the BOF is the formation of multi-phased foam, consisting of liquid slag, metal droplets, solid ‘second phase’ particles such as undissolved fluxes, and process gases. This is because (i)  the high-velocity O2 jet impinges the melt, ejecting a considerable part of the melt in the form of metal droplets into the upper part of the converter, (ii) the lumpy fluxes are added in batches, resulting in a slow flux dissolution, and hence a slow liquid slag formation, (iii) the liquid slag, undissolved fluxes and metal droplets form a more or less viscous emulsion, intercepting the process gases on the way up towards the vessel mouth, and (iv) a large portion of the process gases is formed within the emulsion itself due to the reaction between the carbon (C) in metal droplets and FeO in the liquid slag. To make the BOF steelmaking process effective, a large volume of foam (gas-metal-slag emulsion) is needed in the converter. This gas-metal-slag emulsion, shown in Fig 1 is bounded by the limited size of the converter. If the volume of emulsion increases beyond vessel capacity, then a portion of the slag is expelled through the converter throat.

Fig 1 Physical state of the BOF in the middle of the blow

Slag foaming is beneficial as it assists the refining process in different ways, for example, by providing an increased surface area for the refining reactions, protecting the molten metal bath from the direct contact of the atmosphere, protecting the refractory lining from extreme combustion effects, and forming the medium for post-combustion and heat transfer. On the other hand, slag foaming can become disadvantageous and hazardous when formed in huge quantities, and overflow from the mouth of the vessel, which is termed slopping of the bath.

The ejection of slag out of the converter is called the slopping which is visually identified as occurrence by which from the flaming converter the piece of melt and slag are thrown out through throat of converter during a blowing process. Slopping is an irregular phenomenon. Heavy slopping can be accompanied by large ejection of dust. The consequences of the slopping include loss of yield, interruptions to the continuous production, environmental pollution, health and safety costs, and damage to fume hoods and converter mouth. Further, when the slopping takes place, the steel production is be stopped for cleaning of the ejected slag from the area below the converter and on the converter mouth. This increases the converter heat time and hence, has adverse effect on the converter productivity. The slopping also disturbs the heat balance of the converter which affects the final temperature of the liquid steel after the blow has been completed. On the other hand, if slag foaming is prevented or limited to subtle amounts, the dust generation and heat loss from the converter via radiation tend to increase.

Traditionally, it is the responsibility of the operator to visually monitor the converter, and exercise necessary corrective actions to prevent any hazardous slopping incidences. Since it is of great importance to drive the process without any interruptions for a yield with requisite quality and quantity, the need to suppress slag foam from ejection from the converter is significant.

The requirements for an increase in production and a decrease in the occurrence of slopping seem to be contradicting. While an increase in production can be achieved by increasing the O2 blowing rate, the same increase in the O2 blowing rate increases the gas generation rate inside the converter. Under steady state conditions, an increase in gas generation rate increases the foam height and the chance of the occurrence of slopping.

Initially, it has been understood that the slopping occurs due to two main factors namely (i) the evolution of slag with the characteristics such as low basicity, high viscosity, and low surface tension,  and (ii) decarburization encouraged by high bath temperature and increased iron content in slag. However, the slopping phenomenon is more complex, dynamic and dependent on many process variables. The process variables causing slopping include (i) violent course of melting, (ii) slag viscosity, (iii) slag surface tension, (iv) slag density, (v) size of the gas bubbles generated in the decarburization process, (vi) weak or unsteady circulation of melting, (vii)  converter working lining height, volume, and shape, (viii) lance height above the bath, (ix) O2 flow rate through the lance, (x) wear of lance tip hole, (xi) chemistry of the hot metal (HM) and the scrap, and (xii) decarburization rate. There are some more process variables which are not very common.  The large number of process variables influencing the slopping incidences in the BOF converter explains the reasons for the common belief that the slopping incidences are chaotic in nature and unpredictable. The persistence of slopping problems has given rise to a search for ways to maintain a suitable foam volume while preventing slopping from occurring. Unfortunately, this has proved to be a rather challenging task.

Slag foaming

Slag foaming is beneficial as it assists the refining process in many ways, for example, by providing an increased surface area for refining reactions, protecting the molten metal bath from the direct contact of the atmosphere, protecting the refractory lining from extreme combustion effects, and forming the medium for post-combustion and heat transfer. On the other hand, slag foaming can become disadvantageous and hazardous when formed in huge quantities, and overflow from the mouth of the converter.

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 O2 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 decarburization 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 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 foaming index is time which is normally in the range of 0.6 seconds to 1.3 seconds. Hence, the foaming index 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 foaming index.

A very important property in regard to the foaming index is the apparent viscosity of the emulsion. The higher is the apparent viscosity, the higher is the foaming index. 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.

Gas generation

According to the foaming index expression, 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 decarburization reaction is the one in the melt between C and dissolved O2 as per the reaction under (iii). Hence, the decarburization 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 3.

Fig 3 Principle reactions involved in the decarburization of the melt in the BOF converter

As shown in the right of the Fig 3, the maximum decarburization 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.

Blowing regimes

In practical operation of making steel in a BOF converter, achieving perfect balance of O2 supplied to the bath and slag respectively is not an easy task. Two terms are used to describe a deviation from a balanced O2 state in the converter. These terms are (i) hard blowing, and (ii) soft blowing. The ‘hard blowing’ (harder impact of the O2 jet on the metal surface) stands for the case when the O2 lance is closer to the bath, promoting decarburization in the hot spot as per the reaction under (i) above and later in the bath as per the reaction under (iii), resulting in an under-oxidized slag. The ‘soft blowing’ (softer impact of the O2 jet on the metal surface) stands for the case when the O2 lance is farther from the bath, increasing the supply of O2 to the slag as per the second reaction under (ii) above, resulting in reduced decarburization rate and an over-oxidized slag. In an ideal process situation with the lance at an optimum position above the metal surface, the fresh O2 supplied to the slag balances the consumption of FeO for the decarburization of ejected metal droplets. The decarburization rate is high but controlled, creating stable foam which fills a large portion of the converter volume above the bath, minimizing lining wear and skulling. Yield as well as blowing result with such decarburization rate is good.

If the lance is positioned too deep, the supply of O2 to the slag is not enough to balance the consumption of FeO for the decarburization of ejected metal droplets and the slag is starved of O2. The decarburization is still high due to a more intense harder contact between the O2 jet and the bath. At lower FeO content, the slag does not foam at all. Instead, the emulsion becomes viscous and shrinks. A reduced emulsion height not only leads to an easy passage of process gases by channelling but also an intensified spitting, sending droplets high up and out of the converter. More spitting leads to reduced yields and also leads to the skulling of the lance, the converter cone, and the mouth. Another effect of hard blowing is an increased rate of bottom wear.

If the lance is too high, the O2 level in the slag gets elevated, not only due to a softer contact between O2 and metal bath, but also due to a slower consumption of FeO in the slag as less metal droplets are ejected from the bath. Hence, the decarburization rate is lower, reducing the foam height and promoting lining wear. A shallower impact of the O2 jet reduces the bath mixing, creating dead zones and causing bottom build-up.

The phenomena of slopping

The combination of a low decarburization rate and an over-oxidized slag can be compared to a time-bomb. The descriptive term used is ‘hyper-reactive conditions’ which constitute excess O2 and C not reacting due to poor mixing. In this state, any minor change in conditions triggers a drastic increase in the gas generation and foam growth, leading to violent slopping.

Slopping is the general term used when, due to excessive foam growth, the foam cannot be contained within the converter and the foam flows down the outer side of the converter with the pace depending upon the oxidizing state of the slag. The pace is slow in case of a thick under-oxidized slag and fast in the case of a runny over-oxidized slag. The avoidance of the slopping needs a tight control on the slag composition and, hence, the oxidizing state of the foam. If the slag is under-oxidized, the apparent viscosity becomes too high, which occurs if the FeO content at the start of the main decarburization period is too low. This results in ‘dry’, very viscous foam during the middle part of the blow. If the slag is over-oxidized, the gas generation rate and, hence, the gas velocity within the foam becomes too high.

The slopping causes can be divided in two groups depending of the type namely (i) static or (ii) dynamic. Static causes are related to the pre-blow operational conditions, such as the design of the converter and the volume and characteristics of slag, the quality of the charge materials, especially HM and scrap, blow patterns which control the positioning of the O2 lance, time of additions and O2 flow. The dynamic causes are related to types of blow, such as deflection of blow patterns and the extent of agitation at the bottom of the converter. The slopping can occur due to the excessive growth of the gas-slag-metal emulsion. The foam can flow out depending on the degree of oxidation of the slag, i.e., it can happen slowly in the case of a dense and low-oxidized slag, and rapidly in the case of a highly oxidized slag.

The low silicon contents in the HM can cause an increase in the slopping at the beginning of the blowing, due to the formation of a slag layer thick enough to protect the bath from the O2 jet. In this situation, the slopping occurs due to the very rapid burning of the Si (silicon), which anticipates the start of the decarburizing phase and, thus, the slopping can be violent, generating even the formation of skulling on the lance. In this case for the prevention of slopping, normally the rate of decarburization is reduced by raising the level of the lance or decreasing the O2 flow rate or both and adding fluxes to accelerate the formation of the slag. The low Si content can also affect the removal of P and S (sulphur) due to the reduced volume of the slag.

Slopping can also be present when Si content in the HM is high, as the slag volume increases and also the Fe content of the slag. A solution in this case consists of the adequate addition of lime during the blowing, avoiding low basicities, adjusting the velocity of decarburization by reducing the O2 flow during the first stage of the blow and choosing a blow pattern which prevents a large amount of FeO in the slag in this step, so that excessive decarburization does not occur.

Prevention of slopping

All methods of controlling the slopping can be categorized as either on-line or off-line methods. In on-line control methods, corrections are made during the blow in response to some ‘real time’ signal which indicates whether the heat is about to slop. On the other hand, in off-line control methods, the corrective actions are taken before the blow starts and they are based on past experience of how various operating conditions has affected slopping in earlier blows.

Off-line methods for controlling slopping seek to reduce the number of heats which slop by either eliminating or minimizing the effect of factors which have been found to cause slopping. The factors which have been reported as causes of slopping are (i) Si content of the HM above 1.2 %, (ii) manganese (Mn) content of HM outside the range of 0.5 % to 0.9 %, (iii) charging more than the designed capacity of the converter, (iv) use of fluorspar to speed up lime dissolution, (v) use of iron oxide (iron ore, sinter etc.) as a coolant, (vi) use of a newly relined furnace since it has low internal volume of the converter, (vii) the design of the O2 lance, and (viii) blowing with the lance higher up above the steel bath than the normal. Since the operating conditions vary from BOF shop to BOF shop, some of these factors are more of a problem in one BOF shop than in the other BOF shop.

A variety of actions can constitute off-line control of slopping, for example, restricting the aim Mn content of the HM for the BOF, and regularly checking (calibrating) the lance to bath separation. Since the problems differ, off-line control practices vary from BOF shop to BOF shop. Off-line control practices can often only be implemented to a limited extent because they sometimes conflict with other aspects of steel plant operation.

Slopping in a converter during the O2 steelmaking process is considered to be costly, dangerous and unpredictable. Although the slopping is expected when extreme foaming of slag occurs during the process, the exact moment of the slopping can be hardly predicted when the process is in operation. The traditional method of preventing a slopping incidence is by observing the converter, once the signs of slopping are visible, take corrective actions manually. However, this approach of controlling and minimizing the occurrence of slopping has been proven to be less effective. The reason is that, once the slopping signs are visible, the slopping has already begun and the damage has been already sustained by the converter and other equipment, simultaneously with a yield lost. Further, if the operator is less attentive due to some reasons or not experienced and less aware of corrective actions, the damage and the loss is extensive. Hence, the attentiveness and the level of experience of the operator are crucial factors, even when a computerized control system is in use.

The next stage of identifying the start of slopping is the experience of acoustics and light intensity behaviour of the converter at the situation, and different models have been developed depending on the acoustic and light intensity signals to predict any slopping. The slopping in a steelmaking converter can be predicted by image processing of the in-furnace environment obtained via an image fiberscope. The sound signals from the converter are used to study the dynamic foaming, and the suggested corrective action for changing the lance height. Sound signals captured from a microphone in the exit-gas ducting have been used to estimate the slag level in the converter. In addition to sound and image analysis, intelligent computational techniques such as fuzzy logic, genetic algorithms and neural networks, lance vibration analysis, and microwave method are also used to control the slopping in the converter.

The slag foam suppression techniques have been in use to control the excessive foaming during the process of O2 blowing in the converter. One common technique is to sprinkle carbonaceous materials like coke on the foaming slag. It is revealed from the X-ray fluoroscopy that these coke particles promote the coalescence of foam bubbles which then destabilize the foam. However, the size of the particles is to be larger than the foam bubbles, as the foam is stabilized when the particles are smaller than the foam bubbles.  Other methods in use include injection of aluminum powder, changing the lance height and manipulating the gas flow rates.

However, even though, the prevention, prediction, and mitigation of slopping are a long-standing issue in steelmaking, it is essential for cost-efficient steel production with high quality, optimal converter design, and minimum environmental impact. Further, the techniques mentioned above, which depend on the on-line physical measurements from the operating converter, are indirect, low in accuracy and reliability, especially at high temperatures.

The efforts to develop a system for slopping warning and mitigation have been focused on three major areas namely (i) modelling of slopping and its potential to occur, (ii) measurement devices which detect the onset of slopping, and (iii) mitigation measures undertaken in real-time to prevent the development of full-blown slopping incidences.

An example of the first group is the results presented in a study, where an optimal blow profile has been calculated based on the initial composition of the melt. A similar system based on a calculation of the slopping potential for each heat was developed in another study.

Examples of the second group include a microwave gauge which has been utilized for measuring the slag surface level relative to the converter mouth, use of a detection system involving O2 lance vibration measurements, and employing of radio waves for the slag depth measurements. .

A promising direction of different studies on the slopping phenomenon in the converter has been to combine on-line measurement devices for early slopping detection and use it for initiating process interventions to mitigate slopping. Extensive studies of slag forming have been performed to this end. To gain insight, different kinds of empirical equations describing the change in foam height have been suggested in the past. A model with a physical background is derived by using results of cold and hot model experiments. The area of dynamic modeling of slag foaming is taken even further by the results of these experiments. On the basis of a physical model, a system for control of dynamic foaming is also developed. A water model of the converter process is used to validate the results. The approach is also further refined.

In one of the study, slopping is detected by a combination of the sonic-meter and gas analysis. Another system utilizing the idea of combining several measurements has been presented in another study, where adaptive filtering and change detection algorithms are utilized to construct an on-line alarm system providing warnings to the operator.

Camera for slopping detection – For evaluation purposes, an objective way of quantifying slopping is preferable. In one of the study, a person with a stop watch noted the times for slopping observations during the blow. VCR (video-cassette recorder) cameras have been employed while an attempt to use IR (infra-red) camera devices has been unsuccessful due to software issues.

For the experiments described in the sequel, a camera system has been implemented on-site to monitor the process. When slopping occurs, molten metal fall from the top of the converter onto the floor below the converter. The camera position makes it possible to capture images of the falling slag. Each frame in the video sequence is segmented using gray-level thresholding. A brightness constant or threshold is determined to separate the molten metal from the darker background. The ratio between bright and dark image pixels gives an indication of how severe the slopping incidence is. This ratio is averaged over a sampling period of 2 seconds and saved in real-time along with other process data in the database.

The sonic-meter for slopping detection – Since the beginning of the 1970s, a device called sonic-meter has been employed in many converter shops for indirect monitoring of slag foam level. The basic idea is that as the foam level increases, the sound emission from the converter under blowing decreases at certain frequency bands. The sonic-meter signal is usually used by the operator for monitoring slag level changes but it has also been employed as a controller input.

A warning system for slopping detection – The idea of combining key measurements with advanced signal processing has been is developed. A system identification model is updated by recursive parameter estimation, and is employed to provide early warnings to the operator at the onset of slopping. The system identification model is fed with the off-gas flow rate as well as the CO content in the off-gas. The sonic meter signal is utilized as an output signal of the model. The system has been shown to work reasonably well on a limited number of charges.

The warning system is further refined, where the CO content is changed in favour of a pressure measurement due to difficulties stemming from time delays in the off-gas analysis. A microphone in the exit-gas ducting (actually the same one as used by the sonic-meter) provides high resolution audio data to a slag foam height estimator based on the intensity of the signal at certain frequencies.

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