Factors affecting Lining Life of a Basic Oxygen Converter
Factors affecting Lining Life of a Basic Oxygen Converter
The life, reliability and costs of lining in a basic oxygen converter are vital for the smooth operations of the steel melting shop utilizing basic oxygen process for steel production. Higher lining life results into improved availability of the converter which in turn improves its productivity.
Three important factors for achieving higher lining life of the basic oxygen converter (Fig 1) are (i) qualities of refractories and their laying pattern in the converter, (ii) operating practices followed, and (iii) monitoring of the lining wear and practices for the maintenance of the refractory lining. Development of improved refractory materials in combination with improved process control and better maintenance during campaigns make it possible to increase the lining life of the basic oxygen converter.
Fig 1 Factors affecting lining life of the basic oxygen converter
These days without exception, basic oxygen converters are lined with magnesia – carbon (MgO-C) refractories because of their superior properties than other types of converter lining materials. However zoned refractory lining practices are followed by using MgO-C refractories of different qualities in different areas of the converter.
The causes of wear of refractories in the basic oxygen converter are either due to chemical reasons or due to the physical reasons. Chemical causes for the wear of the converter lining are mainly due to gaseous materials (oxidizing gases, reducing gases, and water vapour), liquid materials (slag. hot metal, and liquid steel melt), and solid materials (fluxes, and carbon disintegration). Physical causes for the wear of the converter lining are excessive temperatures (poor dissipation, and hot spots), static mechanical stresses (spalling, and expansion), and dynamic mechanical stresses (abrasion, impact, and vibrations). The key wear mechanisms of the refractory lining of basic oxygen converter can be summarized as follows.
- Maximum temperature of liquid metal and liquid slag in the converter
- Residence time of high temperature materials in the converter
- State of oxidation of the melt (wear due to corrosion)
- Slag chemistry (wear due to corrosion)
- Impact and penetration of oxygen jet during oxygen blowing
- Erosion of refractories due to slag and metal during blowing and tilting of converter.
- Abrasion of refractories ( due to generation of dust and gases)
- Impact of scrap and hot metal during charging of the converter
- Thermal cycling
- Mechanical damage by cleaning equipment
Corrosion of the refractories takes place due to the chemical attack while erosion involves both the chemical attack (erosion) and the mechanical abrasion of the refractory. Erosion frequently arises from turbulent flows generated in the converter or from mechanical erosion (e.g. the feeding of scrap to the converter). Enhanced erosion tends to occur in furnace linings at the metal/slag and slag/atmosphere interfaces. This type of erosion is known as ‘slag line erosion’.
Quality of refractory
Important parameters determining the quality of the MgO-C refractories are as follows.
- Magnesia -The main raw material for making the MgO- C brick is either Periclase mineral or sintered sea water magnesia. The important properties are its purity, sintered or fused grade or combination, density, crystal size, grain structure, and grain size distribution.
- Carbon – The relevant properties of carbon are purity, grain size distribution (flake graphite), grain structure (flake graphite), amount of carbon black, and amount of flake graphite
- Bonding agent – The important parameters are amount and type of resin, glassy carbon, amount and type of pitch (graphite carbon), and re-impregnation.
- Metallic additions – Metallic additions (anti-oxidants) are aluminum, silicon, aluminum- magnesium etc. There can be combination of several metallic additions. Important parameters are amount and the grain size and type of the metallic additions.
- Brick physical properties – Important properties are density, porosity, hot and cold strength
- Reinforcement – Reinforcement is done with fibres. Kind and amount of fibres are the important factors.
The type and size of brick making press (whether friction or hydraulic) influences the properties of the bricks.
MgO content of the magnesia is to be a minimum of 99 %. Minerals formed in the grain are also important. Overall SiO2 is to be as low as possible (less than 0.3 %). High boron (B) content is also very critical and destroys the grain’s hot strength.
Grain density usually varies in the range from 3.2 to over 3.5 g/cc. Low grain density means high porosity making the grain susceptible to slag penetration.
Large crystallite size is generally considered to be over 140 microns in size. Fused MgO grain can exceed 1000 microns. Large grained crystallite normally outperforms low crystal size due to a reduction in interstitial porosity thereby reducing the chance of slag penetration into the grain boundaries and by lowering the susceptibility of the MgO to reduction by the C present in the brick during high temperature service. The reduction process destroys both the C in the brick and the MgO in the grain producing magnesium metal vapour and CO gas.
Bricks are carbon bonded with the residue of finely divided C remaining after the coking of the binder. This is what holds the brick together. Graphite is non-wetting to steelmaking slags preventing slag penetration into the brick and subsequent dissolution of the magnesia grains. The graphite is also very thermally conductive transferring heat away from the brick surface thereby reducing the kinetics of aggressive reaction. Chemically all graphites are pure carbon but all contain some ash (clay minerals found in the graphite deposits). Impure graphite adds impurities such as silica and alumina to the brick which generates only negative effects. Flake graphite is normally used as it has a higher resistance to oxidation than amorphous graphite and a higher thermal conductivity. Generally the amount of graphite used varies from 5 % to 25 %. Everything else being equal the higher the graphite content the higher is the slag resistance and thermal conductivity of the brick.
Metal powders added to Mag-C bricks act as scavengers for oxygen delaying oxidation of the graphite and C-bond. The powders improve hot strength markedly by forming complex metallic-carbide- oxide bonds in the brick.
Refractories in different zones of basic oxygen converter are subject to different conditions due to which their wear rates vary. Hence different qualities of refractories are required in different zones of the converter to have a uniform rate of wear. This type of lining is known as a balanced lining or zonal lining. In the zonal lining pattern a given segment of lining having lesser wear is assigned a lower quality or less thickness of refractory. Similarly refractories of greater wear resistance and normally having higher cost are assigned to those segments of the converter lining which are having higher wear pattern so as to have longer life of these severe wear areas.
Good control of slag development, oxygen flow and lance practice, and use of bottom stirring and limited use of re-blow practice are key features of the operating practices which influences the lining life of the basic oxygen converter. Knowledge of interactions between process chemistry, blowing dynamics and converter lining wear can achieve both efficient steelmaking as well as long converter lining life.
The most important factors which have maximum effect on the wear rate of the basic oxygen converter refractories are the high bath temperature at the end of the blow and high content of FeO in the slag. Further converter waiting for the tapping for a long time after the end of the blow has a big negative influence on the refractory lining. Other factors which have negative influence on the refractory lining of the basic oxygen converter include (i) high silicon content of the hot metal, (ii) high manganese content of the hot metal, (iii) high frequency of the reblows, (iv) poor reactivity and low quality of lime additions, (v) inadequate addition of lime specially in the initial period of the blow, (vi) converter slag unsaturated with MgO during different period of the blow due to low additions of MgO additives such as calcined dolomite or calcined magnesite, and (vii) low slag basicity.
Also important factors affecting the converter lining life are (i) titanium content of hot metal and titanium oxide content of the slag, (ii) duration of time for which the converter bath has liquid material in it, (iii) high amount of addition of iron ore, and (iv) frequency of cleaning of converter mouth.
Most important factors which have for positive effect on the lining wear rate of the basic oxygen converter include (i) high frequency of slag splashing, (ii) high frequency of slag coating, (iii) appropriate addition of calcined dolomite and/or calcined magnesia, (iv) frequent action for bottom care such as brick patching, and (v) frequent repair measures such as gunning of the worn out areas etc.
A slag saturated with lime is not only important for steelmaking but also to prevent excessive wear of the converter lining. Lime added before and during the blow is to ensure a slightly lime super saturated slag at the end of the blowing process.
The slag development path for different silicon percentage of hot metal show that starting from the high FeO containing initial slag, the SiO2 and CaO contents of the slag rise, as a result of increasing silicon oxidation and lime dissolution. The higher the initial hot metal silicon content, the higher the SiO2 content early on in the blowing process. At the end of the blow slags need to be slightly lime supersaturated in order to avoid excessive refractory wear. In order to achieve this goal a lime addition rate is necessary which is to be adapted to suit the silicon content in the hot metal and to the target slag FeO content.
Since the basic oxygen converter has an MgO-C lining, the slag should be both CaO and MgO saturated in order to minimize lining wear. The solubility of MgO in the slag is dependent upon its basicity, temperature, and FeO content. MgO solubility in the slag is high when the basicity and FeO level of slag is low and its temperature is high. Thus the MgO solubility increases with increasing SiO2 content. Slags with low basicity, equivalent to low FeO content in the slag have the highest MgO solubility, therefore, a magnesite lining is most heavily attacked early in the blow when the slag basicity is still low. MgO solubility decreases with increasing basicity and FeO.
Above the saturation line, all the MgO cannot remain liquid, and hence with an MgO saturated slag, a further increase in slag basicity causes MgO to be precipitated and increase the viscosity of the slag, with the result that build-ups on the converter bottom and walls occur. These buildups prolong lining life.
Lining life is influenced by the slag analysis throughout the blow. In the boundary system FeO-SiO2, there is a compound fayalith (2FeO.SiO2) with a very low melting point of 1,205 deg C. The higher the hot metal silicon content the longer the time period that is required to pass through the area of fayalith-containing slags. This area together with the high MgO solubility at the low basicities that exist at this part of the blow, have a very unfavourable influence on the lining life. Therefore, it is very important that the added lime dissolves quickly in order to raise the slag basicity as early as possible.
The use of soft burnt lime and a sufficiently large lance height to the metal bath at the beginning of the blow (which enhances Fe oxidation and therefore lime dissolution), are favourable to achieving this aim. Also to facilitate early lime dissolution the lime addition should be complete within three to four minutes of blow start.
Lime is not pure CaO. It contains impurities such as SiO2 and Al2O3 which must be compensated for in the additions calculations. Also, its metallurgical efficiency is affected by the particle size and reactivity (or degree of burning). The normal particle size is 8 mm to 40 mm (some prefers 10 mm to 50 mm), as particles below 6 mm are extracted from the converter, together with the waste gas; up to 30 % in some cases. If this happens the slag produced can be under-saturated, causing additional converter lining wear.
Lime with a wide particle size range also separates when charging into storage bunkers such that coarse material travels to the outer side of the cone shaped charging pile with the fine grained material remaining in the inner area. Thus, when charging the lime from the bunker, lime quality is generally variable, with negative consequences for steel chemistry, temperature control and converter lining life.
Re-blowing for final adjustment of temperature or analysis is often required, but at the expense of increased iron oxidation and hence higher refractory wear. For instance, a re-blow of less than one minute raises the temperature by 20 deg C, but it also increases the slag FeO by 5 %. Although theoretically lime is to be added during re-blowing in order for it to stay on the saturation line (as a result of the FeO increase), in most cases this is not done, and especially not in cases when the re-blow is required to raise temperature, as the temperature increase by the Fe oxidation gets compensated for, to a large extent, by the heat consumption for lime dissolution. Although under-saturated slags with quite high FeO contents and temperatures are acceptable for metallurgical reasons, they are extremely detrimental to lining life and the damage is greater the longer the liquid steel is kept in the converter between blow end and tapping.
Another important factor to achieve consistency and controllability of blowing behaviour, and a low rate of variation of the results after the end of the blow is a sufficient bath movement during blowing. During the main decarburization period there is good bath movement as a result of CO formation. With the decrease of the C contents below 0.30 % resulting in reduced CO gas formation, bath movement decreases considerably. During this period of the blow, bath movement task is to be fulfilled by lowering the blowing oxygen lance. Although the stirring effect, induced by the lance, is much less than with CO formation, it ensures that bath stirring is maintained to the end of the blow. This is one reason why bottom stirring with inert gases was introduced. Although the gas quantity blown through the converter bottom via plugs (typically in the range 0.01 to 0.05 N cum/t/min) is small compared to the top blown oxygen, its stirring effect has multiple benefits in ensuring the slag and bath are in greater equilibrium and in producing lower and more controllable FeO levels in turndown slags which are beneficial for the converter lining life.
Iron ore lumps, which are added to cool the converter bath, also have an influence on lining life due to the increase in the FeO content. Excessive amount of added ore is to be avoided because the additional amount of oxygen introduced by the ore leads to an uncontrollable blowing behaviour. Ore addition is preferably to be completed during the main decarburization period otherwise there may be insufficient carbon available to reduce the melted ore. If ore is always charged to the same side of the converter through the charging chute, the FeO-rich slag which is formed locally at the trunnion area causes localized lining wear. For this reason it is necessary to vary the ore addition side to the converter.
Lining wear monitoring and maintenance of lining
Monitoring of the lining wear is done using laser technology measurement of the lining thickness. This technique uses the measurement of lining thickness with the help a laser beam. For this purpose special laser measuring machines are available. It is advisable to measure the lining thickness once a day to know the lining profile of the converter during its operation. When the lining thickness is reduced to a certain level then the implementation of lining maintenance techniques are to start.
Several lining maintenance practices are employed to enhance the life of lining in a converter. These are given below.
- Slag coating – Slag coating is basically a technique of rocking the converter for creating a working lining of slag. It is an art which requires a considerable attention during converter operation. Actions needed for the slag coating practice to succeed are (i) selecting the right type of slag, (ii) slag conditioning after right and proper amount of additions, (iii) correct rocking of the converter, (iv) disposing of the slag when necessary, and (v) coating when it is best time. These items are to be well planned and correctly executed for proper slag coating. The key to the successful slag coating is to follow the established rules. The slag coating takes around 1-2 minutes.
- Slag splashing – Slag splashing technique, a relatively recent development, has contributed to major enhancement in the converter lining life. Slag splashing as the name suggests, utilizes residual slag from the steel making process, which is conditioned and cooled to increase its refractoriness for providing a coating on the refractory surface to act as a wear lining in the subsequent blow. Liquid viscous slag is blown by means of high pressure nitrogen into the upper part of the converter (cone) where it sticks to the converter working lining. Slag splashing technique needs few minutes of the converter time after the tapping of the previous heat and before the start of the next heat. Slag splashing technique has been developed to counter the erosion and produce a freeze lining in a converter. Splashed slag acts as a working lining during subsequent heats. It is become a powerful tool for increasing of the lining life of the converter. It entails the use of oxygen lance to blow nitrogen on the residual slag. Two slag splashing practices are known namely i) with the converter empty of steel and all the slag inside the converter ii) with both the molten steel bath and slag in the converter. The second method is mainly used for coating the trunnions and the upper portion of the converter. The blowing practice is different in two techniques. Slag splashing needs 2-3 minutes and is done with converter in vertical condition. Nitrogen flow is controlled based on the lance height and is usually automated.
- Gunning – This technique helps to attain an extended life on a lining. It consists of MgO based gunning refractory material normally a monolithic on the areas that encounter severe wear out such as trunnions, scrap impact area and the slag line. Gunning is usually done only on the selective areas and after steel and slag tapping. A shooter type of gun is used for the gunning process to encounter hostile environment of the process. Gunning materials are normally water based. A lot of research has been done on the gunning materials and their quality is being improved continuously. Since gunning material has a cost. The amount of gunning is to be balanced with the specific cost of the refractories during steel making.
- Brick patching – This technique is normally used for building up the eroded bottom. After steel tapping and slag tapping is over, some slag is kept in the converter. Coarse or broken spent converter refractory bricks are added to the liquid slag. A total of 30 to 60 minutes extra time is needed to accomplish the solidification of the slag. The spent refractory is to be coarse to be able to reinforce the liquid slag.