Role of MgO in prevention of lining corrosion in basic oxygen furnaces
Role of MgO in prevention of lining corrosion in basic oxygen furnaces
The purpose of a refractory lining in a basic oxygen furnace (BOF) is to provide maximum furnace availability during operation of the BOF in order to meet production requirements and to ensure lowest possible specific refractory consumption. The increase of the lining life of a BOF improves its availability and hence has positive effect on the BOF productivity and reduction in the specific refractory consumption.
In the present day practice for steelmaking in the BOF, magnesia- carbon (MgO-C) resin bonded bricks with metallic additions are used for lining the BOF. These bricks are made using mix of fused and sintered high purity MgO, high purity graphite with carbon content in the range of 5 % to 15 %. Zoned lining of the BOF is normally done with various grades of bricks to get the desired cost effective lining. The steelmaking slag is aimed to be saturated with MgO. Slag splashing is a common practice used now to obtain higher lining life. Use of laser is also made to measure lining thickness 360 degrees. Lining thickness/profile maintenance is regularly carried out with MgO based guniting mixes. In order to satisfactorily perform its roles, slag composition is to be compatible with the refractories used in the BOF.
The temperature in the BOF, while producing steel, rises because of exothermic reactions which are taking place during the process of steelmaking. The tapping temperature of liquid steel is normally maintained at a level of 1660 deg C and above. At these temperatures the steelmaking slag, if it is unsaturated with MgO, takes MgO from the lining material of the BOF during the process of steelmaking and try to get saturated. In the process, it depletes MgO from the lining and results into faster wear of the lining. For minimising the chemical wear of the MgO based refractory lining, it is necessary that the steelmaking slag is made saturated or even super-saturated with MgO by adding materials, which are rich in MgO, in the BOF during the process of steelmaking. The most common materials containing good percent of MgO are calcined dolomite and calcined magnesite. The MgO containing materials which are charged in the BOF are to be highly reactive and not to be dead burnt so that they dissolve in the slag quickly.
The lining life differs from plant to plant because of the existence of different interacting parameters in the different plants. These interacting parameters are related to (i) steelmaking process, (ii) properties of the refractories, and (iii) refractory maintenance practices as shown in Fig 1.
Fig 1 Interacting parameters affecting BOF lining life
The effects of different parameters on the lining life are shown in Tab 1.
|Tab 1 Effects of different parameters on BOF lining life|
|Sl. No.||Parameter||Effect on lining life||Severity of influence|
|Total Fe content||Negative||High|
|End point temperature||Negative||High|
|Production rate (heats/day)||Positive||Medium|
|Delay in charging lime||Negative||Medium|
|4||Design of BOF|
|Multi-hole blow lance||Positive||high|
The relationship between various parameters influencing the lining life of the BOF is shown in Fig 2.
Fig 2 Relationship between the parameters affecting the BOF lining life
The main wear mechanisms for refractories in the BOF are impact, corrosion, thermo-mechanical stresses, and erosion during the process of steelmaking. Though all the parameters are important but corrosion due to dissolution of refractory material in the slag has a major effect on the lining life. When steel is made in the BOF, various oxides are produced which are fluxed with calcined lime to produce steelmaking slag. This steelmaking slag is corrosive in nature and is in continuous touch with the surface of the converter lining. If the slag is not compatible with the lining material, and when the conditions are favourable to it, then the dissolution of lining takes place at the surface of the lining where the slag is touching the lining.
Slag chemistry is important in several ways. MgO-C bricks are basic refractories and require a basic slag, which is also required for the removal of phosphorus. The basicity ratio (CaO/SiO2) required in the slag depends on the phosphorus content of hot metal and the steel grade to be made. Normally it is maintained in the range of 3 to 3.5.
Physico-chemical properties of slag influence the productivity of the steelmaking processes to a great extent. The rapid formation of physically and chemically active slag facilitates removal of sulphur and phosphorus from the melt. It reduces metal loss and formation of metal-regulus in the slag; furthermore it decreases wear in the refractory lining. Viscous slag is physically not very active and has a low refining capacity, thus, reaction processes of slag with metal slow down. Such a viscous slag leads to an increase in the metal losses due to the regulus formation and splashing of slag which takes place. It often leads to the lance becoming clogged as well as formation of the skulls on the neck of the vessel.
Very high slag fluidity of the slag is also not very desirable because of the increased refractory wear of the BOF lining. Hence, it is necessary to obtain at the end of the blowing, a sufficiently flowable, physically and chemically active homogeneous slag with basicity ratio in the range of 3 to 3.5 during the process of steelmaking in the BOF.
As it is known, the chemical composition of the BOF slag and the intensity of lining destruction vary in different stages of the steelmaking process in the BOF. The highest rate of destruction of the lining is observed during the formation of slag with the basicity ratio in the range of 1 to 1.5 and having a high oxidation state (upto 30 % FeO). Hence, it is necessary to form a slag with maximum concentration of MgO, closer to the saturation for a desired temperature conditions in the initial period of blowing.
In order to increase the MgO content in the slag, it is essential to use fluxes containing MgO. The consumption of slag-forming materials is determined by calculation, in accordance with the raw material (hot metal and scrap) composition and the desired slag. During the process of melting, a slag sample’s composition can differ from the calculated value, and pieces of undissolved lime can be present in the sample of slag. It can also happen that a melting operation is over and the slag of the desired composition is not formed and detrimental impurities are not fully removed.
A simulation study of the kinetics of slag saturation with MgO and the process of interaction of the refractory material MgO–C and slag with addition of MgO flux has shown a significant decrease in the rate of dissolution of the MgO from the bricks in the slag with the increase of the MgO percent in the slag. This decrease has been in the range of 2 to 2.25 times.
The basic slag formed also attempts to dissolve upto their saturation level of MgO from the brick. Typically MgO saturation occurs at a level of around 8 % MgO in the slag which depends on the temperature and state of oxidation existing within the BOF. Hence, if MgO is added, which is usually done in the form of calcined dolomite or calcined magnesite then the slag dissolving the MgO from the lining preferentially get reduced to a great extent and even the dissolution does not take place at all if all the conditions are favourable within the BOF. This thereby decreases the lining wear of the BOF. Slag chemistry is again related to state of oxidation and temperature since the basicity and MgO slag content are diluted by high levels of FeO and temperature increases the kinetic reaction rates.
One of the industrial studies carried out enables the dissolution degree of basic refractory in the slag to be estimated, depending on the content of MgO in the refractory. In this study, the proportion of MgO transiting into the slag from the lining is found out by calculating the material balance of the slag. The study has shown a clear tendency towards a decrease in MgO dissolution from the lining in the slag with the increase of the saturation of slag with MgO.
In an another study on the investigation of the dissolution rate of MgO, different oxides containing MgO have been added into the melt and the change in the MgO content has been defined. It has been found that the solid formation of MgO and FeO occurs at the interface of the FeO – CaO – SiO2 slag and sintered MgO.
The quantity of MgO containing materials to be charged depends on the tapping temperature. Higher the temperature higher the percent of MgO is needed to make the slag saturated. At 1660 deg C to 1680 deg C tapping temperature, it is better if the MgO in the steelmaking slag is kept above 10 %.
The addition of calcined dolomite in the converter has other advantages also since it reduces the consumption of calcined lime during steelmaking. Further 10 % to 12 % MgO in the slag do not have any appreciable effect on slag viscosity. However increase in slag viscosity, if there, can be controlled by using slag thinning agents. The only feared adverse affect of high MgO slag on steel making is poor phosphorus removal during the steelmaking.