Foaming of Slag in Electric Arc Furnace
Foaming of Slag in Electric Arc Furnace
Slag foaming has become an important and critical practice for the modern electric arc furnace (EAF) since a good foaming slag contributes considerable efficiency and other benefits to the process. The quality of the slag foaming is an important factor in the EAF steelmaking process since it ensures that the heat from the liquid bath is not lost through the EAF walls. Slag foam is beneficial for the EAF steelmaking process in terms of the large surface area formed and the protection provided against the direct contact of the melt with the atmosphere.
The slag foaming is a technique which is both useful as well as economical. It is a widely used technique, not only since it allows energy to be saved, but due to the several advantages it offers which includes (i) increased energy efficiency, since the heat from the arc is captured by the slag, (ii) protection of the water panels and the roof from radiation, (iii) decreased vibrations and noise pollution, and (iv) decreased nitrogen incorporation by the bath.
Foamy slag formation can be divided into 3 steps. These are (i) step 1 which is oxygen injection in liquid steel phase, (ii) step 2 which is carbon injection into the slag, and (iii) step 3 which is when oxygen is injected into the slag. Initially, oxygen is injected into the molten metal (step 1). This oxygen reacts with the existing carbon forming CO (carbon monoxide) bubbles. The oxygen also reacts with the iron present in the bath. As the iron is lost in the form of iron oxide, carbon is injected into the slag (step 2), performing the iron oxide reduction reaction. This step generates CO gas, and also causes iron to return to the bath, improving furnace performance. In step 3, oxygen is injected into the slag, to cause oxidation of the carbon present in the slag. In this step, carbon and oxygen can also be injected simultaneously, allowing better generation of CO and better foaming.
There is normally a dense layer of slag beneath the foam. However, it is possible that all of the slag present is required to produce the foam and no dense layer exists. In such a case, the amount of slag present is insufficient to produce all the foam which the slag and gas can form.
It has also been reported that the slag foaming process can save 3 % to 10 % and 25 % to 63 % of energy and refractory consumption, respectively. There are basically two requirements for foaming namely (i) reactions or processes that 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 (oxygen, argon, etc.) results in larger bubbles and less stable foams.
A certain foamability of the slags used for the EAF process is desired, for a number of reasons. During the process, the foaming slag protects the graphite electrodes from wear and covers the arcs. This allows for a higher productivity in the furnace, since it increases the heat transfer between electrode and the molten metal. It also reduces radiation losses, since the slag isolates the light beams. This in turn protects the refractories from wear, which reduces the amount of down-time for maintenance work. The foaming slag also contributes to stabilization of the arc, ensuring a higher efficiency. In addition to this, the foam helps to reduce noise from the EAF, which provides a better working environment. Another advantage with foaming slag is the reduction of power and voltage fluctuations.
The consequences of a good foaming slag in the EAF and its proper control include (i) the decrease in harmonics due to electrode immersion in the slag can increase power at the same transformer settings by 6 % to 9 %, (ii) an increase in power by 15 % when the electrode tips are 300 mm to 600 mm deep in the slag because of the hotter arc plasma around the electrode tip, and (iii) a good foaming slag allows oxygen-fuel burners (when used) to blow into the slag, increasing energy efficiency of the burners from 40 % to 70 %. However, to get the most benefit out of a foaming slag, the slag is to be built early in the heat and that slag chemistry is to be tightly controlled by limiting slag flushing and by putting lime or mill scale in the bottom of the first charge bucket. The thermal benefits of a good foaming slag are due to the shift in the heat transfer path to the bath and away from the sidewalls. As per a study, the variations in foaming slag quality can account for + /- 0 kWh/t in demand for the electrical energy
It is useful to know what is meant by slag foaming. Gas generation is indispensable in order to succeed with a foaming slag practice. Gas is mainly generated by the reduction of iron oxide (FeO) with carbon forming CO-gas. Gas causes foam bubbles to form on top of a dense layer of slag. The foam can be relatively small foam bubbles like foam on beer or larger bubbles like soap bubble foam on water. The small bubbles result from chemical reactions and the resulting foam is fairly stable. Gas injection, however, produces larger bubbles and less stable foam. True foaming is not to be confused with simple gas hold-up of bubbles in a liquid. In the case of gas hold-up, the gas bubbles are distributed throughout the entire liquid and the expansion of the slag is due to the gas bubbles in the liquid. For gas hold-up, expansion decays rapidly after the gas stops. True foam can be fairly stable and remain so for several minutes after gas generation stops.
The gas bubble generation reaction, the reduction of FeO in slag by added carbon and carbon dissolved in metal. In the slag foaming process, carbon is injected into the slag, reacts with iron oxide in the slag to produce CO gas, which foams the slag. The reaction is given by C (injected) + (FeO) = Fe + CO. CO gas for foaming is also produced by decarburization of the metal given by the reaction C (in metal) + 1/2 O2 = CO. The FeO is generated in situ as the major oxidation product of the oxygen blow and is hence the major component in the slag (higher than 20 %). If the consistency of the slag is suitable for sustaining foam, the simple injection of carbon into slag causes the slag to foam. The relatively high reduction rate gives FeO the potential of generating a large amount of gas inside the foam. The reduction rate of FeO by carbon is considerably fast. Additionally, the reduction reaction also consumes heat, which results in a local increase in the viscosity of the slag.
The process variables which affect foaming 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 foaming index (FI) is an important parameter for the slag foaming. It can be viewed as the time for the gas to pass through the slag. It is an indication of the extent of the foaming and is the ratio between the foam height and the superficial gas velocity. Hence, the unit of the FI is time which is normally in seconds. Thus, the FI can be interpreted as a measure of the time it takes for the process gases to vertically pass through the foam. The FI is related to the slag properties such as viscosity. The higher is the viscosity, the higher is the FI. The obvious consequence is that an increased viscosity automatically leads to an increased foam height.
The FI decreases with increasing bubble size. It is generally seen that small bubbles are formed by the first reaction given above and a relatively stable foam results. In EAF steelmaking, it has been found that, towards the end of the process, the foam becomes less stable and the foam height decreases. This happens because initially as the FeO content increases, the rate of fist reaction and CO generation increases and, hence, the foaming increases. However, as the FeO content increases, the slag viscosity decreases and the density increases causing the foam bubbles to drain more rapidly, to decrease and the foam to decay. Hence, there is a critical FeO content below which foaming increases with FeO content and above which the foam is less stable. This occurs at around 20 % to 40 % FeO depending on other conditions.
Slag foaming when it was introduced was assisted by injection of graphite powder made from electrodes (80 % of -200 mesh size) to the metal-slag interface using suitable injection equipment with air as a carrier gas. Through previous calculations and trial and error procedures, it was found that a mass flow rate of graphite equal to 25 kg/min was good enough to produce stable foam. Higher values than this, at the beginning of a heat, promoted heavy slag slopping affecting operations in the working floor and making unstable electric arcs hindering operations with long arcs of high voltage and low current.
In the EAF steelmaking process, long arc operation occurs in conjunction with a foamy slag practice. By foaming the slag through carbon injection, the height of the slag layer can be raised until it covers the arcs completely, intercepting the arc radiation and flare, transferring the heat to the bath and reducing the heat load on the sidewalls. If a good foaming slag cannot be sustained then however, it is preferable to reduce the secondary voltage and thus the arc length in order to avoid the risk of damage to the furnace structure.
The slag foaming phenomenon is dependent on the properties of the slag and the gas evolution rate, due to reactions in the slag. Thus, to obtain foam in a slag, a gas flow is needed. The gas flow is generated when carbon (C) and gaseous oxygen (O2) is added to the slag and the metal bath respectively. The control of foaming height is needed to maintain a steady operation.
Depending on the physical properties of the slag (which are influenced by composition and temperature), foaming then takes place to varying degrees. In EAF steelmaking it is quite common for carbon (in the form of coke or coal) to be added to the slag layer. Carbon consumption takes place through the Boudouard reaction, and thus the rate of CO generation is proportional to the rate of addition of carbon to an EAF slag, once the reaction has reached steady state.
The foaming of the slag is significantly affected by the gas flow into the furnace, but is also affected by the slag properties, the latter controlled by the viscosity, density and surface tension. These physical properties are, in turn, dependent of the temperature of the system and the slag composition.
For the EAF process, the composition of the slag depends on steel grade as well as the refinement method being used. When choosing a composition for the slag, a number of things have to be considered. The slag consists of various oxides, which can be divided into three categories, namely (i) basic oxides (such as CaO, MgO and FeO), acidic oxides (such as SiO2) and amphoteric oxides (such as Al2O3). For basic slags, the content of FeO is generally considered at optimum level ranging from 15 % to 25 %. In order to build up foam in a slag, the viscosity needs to be high enough to constitute a hindrance for the ascent of the bubbles. To obtain a suitable viscosity, it is important to have an appropriate basicity of the slag. The basicity is generally referred to as the ratio of the basic components in the slag to the acid components. The basic components provide the O2 ions to the melt and the acid components bind them. The viscosity can be altered through a change of composition, which causes a change in ratio of CaO/SiO2, thus lowering or increasing of the basicity of the slag.
A change of viscosity can also be achieved through an alteration of the content of FeO. When increasing the amount of FeO, the viscosity is reduced. Another way to modify the slag viscosity is to increase the amount of solid particles in the slag, such as by an addition of lime. It has been observed that slag viscosity decreases with an increase in basicity, but when it passes a critical value and a solid phase is precipitated, it increases once again. Thus, the presence of solid particles contributes to an increase of the apparent viscosity. The solid particles also act as nucleation sites for the bubbles. However, an excessive amount of solid particles prevents the bubbles from ascending through the slag.
In a system of liquids, a gradient in surface or interfacial tension can induce motion. This is called the Marangoni effect. In the interface between slags and liquid metals there is generally a large gradient in interfacial tension. The concentration of FeO is considered to be lower than in the rest of the slag at the interface between slag and metal, where reactions which form CO occur. This enables bubbles to form, since less FeO content results in a lower surface tension, which gives rise to the Marangoni effect. Thus, the ability to create new surfaces, which occurs when bubbles are formed in the slag, is facilitated when the surface tension is low.
The surface tension also affects the degradation of the foam, referring to the stability of the bubbles. Various studies have shown that stability can be improved by the addition of a surface active component. It has also been observed in the steelmaking process that the presence of surface active components promote foaming. However, the Marangoni flow, which helps the removal of CO bubbles from the interface, can be generated without surface active elements. The density of the slag affects the foam height. A low density means that there is less weight to uphold, which allows a greater height of the foam, compared to a slag with high density. In order to withstand load, the surface needs to have elastic properties. This can be achieved through addition of surface active elements which results in a variable surface tension.
Foaming slag in steelmaking operations results from the generation of CO bubbles, through the reduction reactions , (i) CO2 + C(s) = 2CO (the Boudouard reaction), (ii) CO + Fe2O3 = CO2 + 2FeO, (iii) CO + FeO = CO2 + Fe, and (iv) C(s) + FeO = CO + Fe. The Boudouard reaction and the reduction of FeO by solid carbon are highly endothermic reactions, while the reduction of Fe2O3 by CO is energy-neutral and the reduction of FeO by CO is only slightly exothermic. It is important to note that iron oxide is the source for the oxygen component required for the combustion of carbon, not gaseous oxygen, and thus some oxidation of the steel bath is needed for slag foaming.
Driving force for slag foaming is a pneumatic energy provided by CO generation during the process of the melting-refining in the EAF. In the slag foaming process, carbon is injected into the slag, reacts with iron oxide (FeO) in the slag to produce carbon monoxide (CO), which foams the slag. The reaction is given by C (injected) + (FeO) = Fe + CO. CO for foaming is also produced by decarburization of the metal given by the reaction C (in metal) + 1/2 O2 = CO. These reactions are responsible for the CO formation. The second reaction represents a direct reaction between gaseous oxygen and carbon in bath, whereas the first reaction is a direct iron oxide reduction reaction by carbon in slag. All these reactions are CO formers giving stirring energy for mass transfer between metal slag, carbon, and gas phases and they can be considered as a necessary condition for slag foaming. Some reactions are consumers of CO such as (i) CO + Fe2O3 = CO2 + 2FeO, and (ii) CO + FeO = CO2 + Fe, which can be controlled by the Boudouard reaction CO2 + C(s) = 2CO.
Sites of these reactions can be located as (i) at the slag-metal interface, (ii) at the gas-metal interface, (iii) at the carbon slag interface, and (iv) at slag-gas and carbon-gas interfaces. Fig 1 shows a schematic diagram of chemical reactions for slag foaming in EAF. The diagram shows the reaction sites during injection of carbon during steel melting-refining processes in an EAF. It can be seen from the diagram that CO formation is a very complex function of several process variables consisting of size, type and mass flow rate of carbonaceous material, flow rate of carrier gas, carbon content in bath, slag chemistry, and slag-metal interfacial area etc.
Fig 1 Schematic diagram of chemical reactions for slag foaming in EAF
On the other hand, another condition for foaming of slag is required to be fulfilled. This condition is directly related to the transport properties (which affect foam stability) of steelmaking slags at refining temperatures. FI of slag determines these properties. FI is measured in time units i.e. seconds and is a function of slag chemistry. The stability curves for FI of the slags are shown in Fig 2. The inferences from the curves are given below.
- Acid slags have the highest foaming index forming capabilities which can be called as homogeneous foaming.
- Slags with low contents of iron oxide (less than 10 %) form more stable foam than highly oxidized slags (more than 40 % FeO at any basicity) which, therefore, can be called non-foaming slags.
- At low iron oxide contents slag basicity influences strongly the foaming stability. At higher basicities, under a given iron oxide content, FI decreases. At high iron oxide contents slag basicity plays no role.
- Slags with intermediate iron oxide contents and high basicities near the lime saturation zone have also intermediate FIs and the presence of solid particles of lime solution can assist to stabilize foam formation. Owing to this reason this zone can be called as heterogeneous foaming.
Fig 2 Foamability of steelmaking slags
Fig 2 also shows slag composition and foaming. Solid particles which are suspended in liquid slag affect its viscosity. It is normally seen that with only 20 % volume fraction of solid particles in the liquid slag, there is a two fold increase in the slag viscosity and zone for heterogeneous foaming can be considerably enlarged, partially embracing the non-foaming zone, as is shown by the dotted line in Fig 2.
To have a more complete view of slag foaming during carbon injection, iron oxide activity in complex slags has been the subject of several studies. These studies have shown the following.
- Homogeneous foaming zone with low iron oxide activities (less than 10 %) shows a strong resistance to iron reduction by carbon not only because of low iron oxide activities but mainly owing to the surface active nature of silica in slags. The iron reduction in this zone is mixed controlled by mass transfer and chemical reaction mechanisms.
- Highly oxidized slags are easily reduced by carbon since iron oxide activities are high enough but they do not form stable foams.
- Normal compositions of steelmaking slags observe an intermediate behaviour. Although, final slags have the tendency to be very oxidized.
Critical to maintaining the foaming slag is the monitoring of its condition. When slag foaming was first introduced, it was manually monitored, the furnace operators using their eyes and ears, coupled with their experience, to judge whether the slag was right or not, and the steps necessary to correct it. Beginning in the mid-1980s, electronic monitoring and computer control of slag foaming was introduced, with several possible signals studied such as arc distortion, arc noise, and light emissions. Currently, arc distortion appears to be the most widely utilized signal in AC furnace operations for monitoring foam condition, as the sensors and programs required are often integrated with the sophisticated electrode regulator. Many EAFs utilize harmonics generated in the electrical supply by the arcs as a foaming slag monitor, as a better foaming slag leads to a more stable arc and fewer harmonics. Monitoring through sound and light emissions from the EAF has also been attempted as a method of foam monitoring.
Foaming in EAF, however, has become integral to the steelmaking process, especially during the refining stage of the heat. The current long-arc process maximizes the energy transfer to the scrap surrounding the electrodes and arcs. Thus, the scrap intercepts most of the arc radiation and little passes through to the sidewalls.
However, once the scrap melts, all of the arc radiation can impinge on the sidewalls. By covering as much of the exposed arc as possible, a foaming slag intercepts this energy, prevents it from reaching the walls, and passes it to the steel bath instead. Foaming slag can be slowly built up as the scrap melts and the furnace sidewalls and roof become progressively exposed to heat radiating from the arcs, reaching a maximum height during the refining stage of the heat. The injected carbon and CO also helps reduce any iron oxide in the slag back to metallic iron.