Slag splashing technique in converter operation
Slag splashing technique in converter operation
It is known that the wear of refractory lining in an oxygen steelmaking converter for the production of liquid steel is a factor which greatly influences the production costs. The wear of the refractory lining of the oxygen converter results from a combination of thermal, chemical and mechanical phenomena which are occurring inside the converter. The thermal effects are linked to temperature fluctuations and thermal shock, while the degradation of the linings can be caused by chemical interactions between the refractory, slag and the gases in the converter. Those of mechanical origin are associated with erosion due to scrap charging, liquid movement between refractory surface and the metal, oxygen blowing and effect of gas movement at high temperatures in the vicinity of the refractory lining.
Through wear profile monitoring technique of the refractory lining by laser beam, it is possible to obtain a map of the lining wear profile in each region of the converter. The regions where the liquid steel flows out of the converter after each blow as well as the zone of impact due to scrap charging are the most susceptible to degradation. With this knowledge, one can establish a suitable repair strategy for the worn out regions to extend the campaign life of the linings of the converter.
Maintenance and repair techniques of refractory lining include coating with slag. This is done through guniting of the refractory material or by the slag splashing on the damaged refractory lining. Since last few decades, slag splashing has emerged as one of the leading technology to extend the life of the converter refractory lining. Slag splashing technology reduces the wear associated to thermal and chemical attack by slag and mechanical impact. The splashed layer on the refractory lining acts as a working lining layer in subsequent heat/heats and hence protects the original refractory.
Today slag splashing has become a powerful tool not only for increasing of the lining life of the converter but for increasing of the converter availability and maximizing of production besides reducing of the refractory and gunning costs. Due to its simple operation, and because of small investment needed for it, slag splashing has become most popular method for the increase of the converter lining life.
Slag splashing technique was first developed in 1970 but was not put to large scale use. The Indiana Harbour plant of LTV steel was first to report success in 1992 with respect to improvement in the lining life by the use of this technique. Slowly this technique was used in the other steel melting shops of the world. Inland no. 4 BOF shop has reported a lining life of plus 60,000 heats
Principle and theoretical aspects of slag splashing
The slag splashing technology consists in the splashing of the left over slag from previous heat by blowing of nitrogen gas on it towards the hot face of the converter through a blowing lance. It consists of coating the slag on the converter lining by freezing the liquid slag on the walls of the converter. The nitrogen supply parameters namely pressure and flow rate, general slag condition and consistency of operation are the three major factors for the success of slag splashing.
A higher superheat of the slag (difference between tapping temperature and the liquidus temperature of the final slag) results into thinner slag and faster melt back of the protective slag layer. Hence, reasonable steel tapping temperature and reasonable control of the converter slag are the keys to success of the slag splashing.
As the interaction between the slag and the nitrogen occurs, the slag temperature is gradually reduced, which substantially impacts the phase distribution and, consequently, the effective viscosity of the slag. Basic principle of lining protection by slag splashing technology (Fig 1) is based on the adjustment of the slag viscosity. Good slag splashing needs composition adjustment of end slag with respect to FeO and MgO concentration and basicity.
Fig 1 Basic principle of slag splashing
The amount and the location of the slag coating is dependent on the mass and the size of the slag globules, its velocity and trajectory angle and the pattern of gas flow in the converter. The heat transfer affects the adhesion of the slag on refractory.
In order to achieve an effective process, many variables are required to be controlled. These variables are related to slag physico-chemical properties (e.g. basicity, viscosity, and surface tension etc.), operational aspects (e.g. lance height, nitrogen blow flow, and static lance or in motion etc.), and geometric aspects (e.g. number of lance holes, hole angle, and dimensions of the converter etc.). The pressure and flow rate of the nitrogen gas are the key factors influencing the slag splashing effect. The lance position control is to be based on the fluidity of slag and the splash position.
In slag splashing, the quantity of slag is not only an important technical parameter, but it also determines the thickness of slag splashing layer. The required amount of slag for satisfactory coverage during the slag splashing is dependent on the size of the converter. It increases with size. With the increase in the amount of slag left in the converter from the previous heat, the splash ability of the slag is enhanced. It results in a higher mass flux of the slag in all the region of the converter but normally it is more in the central region. This is beneficial to the operation of slag splashing.
The lance height has a marked effect on the slag splashing process since the lance height affects the shape of the cavity and the wave form (Fig 2). It also influences the position of the converter lining which receives slag layer (region getting coated). Small lance height causes a deep cavity in the molten slag and a large recirculation zone which favours the prevalence of the wash coating mechanism. Large lance height promotes drop generation and the ejection coating mechanism is favoured. Splashing is increased as the lance height is increased, however, beyond a certain value of the lance height, splashing decreases. There is an increase in the splashed volume as the lance height is reduced, and this increase occurs until a maximum value which culminates when an excessive penetration of the jet begins, i.e., the jet exceeds the slag layer and reaches the refractory. From this point, the projection decreases. Also, the lower parts of converter tend to have a more effective covering when the lance is higher, and the opposite trend happens in the upper parts, which have a better covering when the lance is at a lower position.
Fig 2 Effect of lance height on slag splashing
The angle of lance nozzle hole, in turn, has an opposite behaviour, i.e., the larger is the angle of the hole, the more intensive is the splashing in the lower parts of the converter. When the lance nozzle hole is inclined with respect to the vertical then it generates larger shearing forces, alters the shape of the cavity, increases the slag transferred during ejection mechanism, reduces the vertical component of the velocity but increases its horizontal component. This results into higher amount of slag hitting the wall and lower amount of slag going to the upper area of the converter.
In case of the inclined lance nozzle hole, the nitrogen jet leaves the nozzle and enters to the converter with an angle (alpha degrees in Fig 2). As the exit angle is increased, the vertical component of the jet velocity is decreased and the horizontal component is increased. In this case the shearing forces are increased and an overall increase in the amount of slag splashed is observed. Of course, when the angle becomes too large and goes beyond a critical value, the jet does not impact the slag anymore. The value of the critical angle depends on the lance height and the molten slag depth.
The lance hole angle, however, is a geometric parameter which cannot be easily changed, being defined according to the holes number and the size of the converter. It is worth noting that the increase in the number of lance holes can favour a uniform layer of projected slag which adheres on the converter refractory.
Normally same lance nozzle is used for both oxygen blowing during steelmaking and for nitrogen blowing during slag splashing. These lance nozzles are generally designed to match the converter shape and requirements of steelmaking. If the lance nozzle hole angle is narrow then there is a possibility that slag splashing can cause build up at the cone and mouth of the converter. It also causes a build up on the lance surface. If the lance nozzle hole angle is wide then the slag is only splashed onto the lower part of the converter. Further, the increase in the number of holes in the nozzle results into lesser amount of slag splashed from the central region of the nozzle but has more uniform slag splashing. Hence for successful result during slag splashing it is necessary to have an optimum design of the nozzle.
The velocity and the angle of the ejected slag are dependent on the jet characteristics (momentum, flow rate, height, angle and nozzle) and the nature of the cavity. As regards to the effect of the nitrogen flow rate, it is seen that the amount of the splashed slag increases with the blowing flow rate. Besides, the increase in the flow rate results in a greater deposition of slag in all the regions of the converter.
The depth of the cavity formed in the molten slag depends on the momentum of the nitrogen jet. In turn, the jet momentum depends on the nitrogen mass flow rate and the velocity. As the jet velocity is increased the slag splashing is increased. Low jet velocity promotes the stirring of the slag, and the wash coating mechanism dominates. High velocity nitrogen gas results in both agitation and the formation of crater. When the crater depth reaches a critical value slag droplets are ejected. This is due to the high shear forces generated by the high velocity jet. High jet velocity also causes high drop generation. Hence, in the case of high jet velocity, the ejection mechanism becomes dominant. Anyway, with the jet velocity increase, the slag coating efficiency is increased.
The slag density affects the slag splashing process. Low density of the slag yields high efficiency of the process. However, low density promotes the emergence of undesirable spitting phenomenon during the heat blowing too. High density decreases the efficiency of the splashing process. Nevertheless, this negative effect can be counteracted by increasing the nitrogen jet velocity so that additional inertial forces are provided.
The control of the slag viscosity is very important for the effectiveness of the slag splashing. When, the viscosity is increased, the efficiency of slag coating is decreased. This is explained by the fact that for higher viscosities, higher shearing forces are needed for drop generation and the formation of a standing wave. From this point of view, a low viscosity of the molten slag is desirable. But, unfortunately slags with low viscosity have little adherence to the converter sidewalls and tend to flow down. On the other hand, the viscosity of a molten slag mainly depends on temperature, composition and the presence of solid phases. Hence, slag is normally subjected to a conditioning process for optimizing its viscosity through the addition of materials such as magnesia and ferrous oxide. The influence of the slag viscosity on the slag splashing process can be summarized as (i) ejection mechanism becomes dominant as viscosity is decreased, (ii) washing mechanism becomes dominant as viscosity is increased, and (iii) efficiency of the slag splashing process is increased as slag viscosity decreases.
Slag splashing is assisted by the high basicity, high MgO content and low FeO content of the slag. The low melting phase of the slag is rich in FeO, which acts as a binder and contains most of the sulphur present, while the high melting phase of the slag provides the necessary protection to the refractories.
The high melting temperature and slag viscosity are conducive to the slag splashing effect. Slag is to be saturated with MgO to produce a high temperature phase and to increase the slag viscosity. High temperature phases of the slag and their metallurgical properties, and the quantity of these phases are of great importance. In addition, the knowledge of the chemical composition of the liquid slag and the solid phases at the process temperatures is helpful in developing a good slag. Further, in order to predict the coating character of the slag, the physical properties of the slags play an important role. Physical properties of the slag and especially the apparent viscosity are greatly influenced by the total amount of the solid phases.
The phenomenon which takes place during the slag splashing is the slag layer attached to the lining surface has non- uniform composition of the phases. When the temperature increases during the next heat of the converter after the slag splashing, the low melting point phases in the slag splash layer first melts and separates from the high melting point components, and moves slowly from the splash layer. This molten slag layer flows downward. The slag layer retained on the lining surface is that of high melting point phases, which in turn improve the high temperature resistance of slag splash layer. This phenomenon of the sub-melting of slag, also known as selective melting causes the MgO crystallization of slag splash layer. The high melting point components such as C2S (di-calcium silicate) etc. gradually accumulate, and improves the high temperature resistance of slag splash layer and protects the furnace lining.
It is important that the slag for splashing contains the right blend of low-melting and high-melting phases. The low-melting (FeO-rich) phases ensure good adhesion between the slag and refractory whereas the high-melting phases provide erosion resistance and a thermal barrier. Good slag properties are obtained with a FeO content of around 13 % and with a super-saturated MgO content (more than 8 % MgO) for ensuring that the splash slag is MgO-saturated rather than CaO-saturated. The slag basicity (CaO/SiO2) in the range of 2.5 is good for slag splashing.
The bonding mechanism between slag splashing layer and the magnesium carbon lining brick of the converter can be divided into three layers consisting of (i) slag splashing layer, (ii) adhesive layer, and (iii) sintering layer. The main three methods of bonding are through (i) chemical bond, (ii) mechanical and chemical bond, and (iii) condensation sintering. During the slag splashing, after the splash, the slag penetrates and fills the gap between the surfaces of the brick, or reacts with the surrounding MgO particles, or form a consolidated solid solution through a sintering layer. Because of sintering layer, magnesite is no longer loose and thus prevents the corrosion of lining bricks. Also, the slag layer reduces the direct erosion damage of the liquid slag during converter blowing on the surface of the lining brick.
The slag splashing layer has good corrosion resistance to the slag of the converter. Due to the low basicity of the initial slag during the blowing of the converter, the slag phase is mainly calcium silicate. During this period, the high melting point component of C2S in the splashed layer of the slag provides the corrosion protection to the lining.
Typical relationships of the slag splashing parameters are given in Fig 3. These include (i) slag behaviour as a function of temperature, (ii) solid fraction as a function of temperature, (iii) phase distribution as function of temperature, and (iv) effective viscosity as a function of the solid fraction.
Fig 3 Typical relationships between the slag splashing parameters
The effects of various parameters on the slag splashing are shown in Fig 4.
Fig 4 Effects of various parameters on the slag splashing
There are three stages which are present in the formation of the slag splashing. These are (i) transport of molten slag to the converter walls, (ii) adherence of the molten slag to the sidewalls, and (iii) freezing and hardening of the slag layer. As regards to the transport of the molten slag to the converter sidewalls, two mechanisms are identified namely (i) washing coating, and (ii) ejection coating. The first one occurs due to the bulk movement of the molten slag to rise above the initial level, and the second one due to the ejection of slag droplets which adhere to the vessel sidewalls.
After the previous heat is tapped, a part of the final slag is retained while the slag is drained. Then, the slag conditioner is added, the viscosity is adjusted, and the appropriate composition of the slag is achieved. The slag conditioner makes up the MgO content of the slag and produces chemical reaction to generate a series of high melting point compounds. Then the high pressure nitrogen is blown into the furnace with the oxygen lance, and this causes the splashing and the coating of converter lining, Slag splashing is to be done only after ensuring there is no steel is left in the slag in the converter otherwise heavy skulling can result.
The various steps in the slag splashing process steps are (i) at the end of the previous heat the liquid steel is tapped in steel teeming ladle and part of the molten slag is tapped in the slag pot with the remaining slag left in the converter, (ii) the converter operator visually inspects the slag condition to determine the quantity of slag conditioner to be added, (iii) the converter operator visually inspects the converter lining to determine if any specific area of the lining needs special attention, (iv) the molten slag is conditioned with respect to its temperature, FeO and MgO contents by the addition of the conditioner in the required quantity, (v) the converter is rocked for slag coating of the charge pad and tapping pad, (vi) the oxygen lance is lowered to a predetermined level and the nitrogen flow is started, (vii) nitrogen gas at high pressure is blown through the lance nozzle over the conditioned molten slag for it to splash over the lining of the converter and for the splashed layer to get deposited on the lining surface, (viii) the height of the lance is changed by the converter operator to get slag coating on entire converter or it is kept in fixed position to slag coat a particular area, (ix) the converter operator determines the time of nitrogen blowing which generally ranges from 2 minutes to 4 minutes, (x) after the nitrogen gas blowing, the gas flow is stopped and lance is lifted, and (xi) the remaining slag is dumped to avoid excess build of converter bottom and converter is ready for next heat.
Advantage of slag splashing
The advantages of the slag splashing are as follows.
- Increase in the lining life of the converter.
- Yield improvement due to lesser slopping because of increase in the converter volume
- Lesser consumption of flux because of dissolution of basic slag during the steel making process.
- The melting of the low melting phase of the slag lining results in the rapid formation of a basic slag and the rapid dissolution of the CaO from slag coating by SiO2 in converter slag. This leads to rapid de- phosphorization.
- Slag splashing helps in recycling of the steelmaking slag.