Refractory lining of a Basic Oxygen Furnace
Refractory lining of a Basic Oxygen Furnace
The purpose of a refractory lining in a basic oxygen furnace (BOF) is to provide maximum furnace availability during operation of the converter in order to meet production requirements and to ensure lowest possible specific refractory consumption. For achieving this, it is essential (i) to optimize the design of the lining, (ii) to optimize the lining maintenance practices, and (iii) to have good technological discipline during converter operation. Typical refractory lining is shown in the cross-section of a converter in Fig 1.
Fig 1 Typical arrangement of the refractory lining in a BOF
Wear of the refractory lining of the BOF is due to either the individual or the combined effect of the several agents which have their influence on the wear mechanism. These agents are (i) heat duration determining the residence time of the slag and the metal in the converter, (ii) corrosion due to chemical attack of the slag because of its chemistry, (iii) temperature of the liquid steel and the slag, (iv) corrosion due to the state of oxidation of the melt, (v) erosion due to slag and metal during the blowing of oxygen (O2) and during the tilting of the BOF, (vi) impact and abrasion of dust and gases, (vii) impact of scrap and metal during charging, (viii) impact and penetration of O2 jet, (ix) thermal cycling during the heat, and (x) mechanical damage during the deskulling of the converter. Due to varying action of these agents, there are many wear areas in a converter.
The converter operation as well as the lining configuration has a huge effect on the lining life of a converter. The tapping temperature of the liquid steel varies significantly with the superheat needed to cast a particular grade of steel, and the secondary steelmaking facilities available to a particular plant. The state of oxidation of the melt in the converter is extremely significant in terms of BOF lining wear and is aggravated by the high temperature. Slag chemistry is important in several ways. Magnesia-carbon (Mag-C) bricks are basic refractories which require a basic slag. The basic slags formed also attempt to dissolve upto their saturation level of magnesia (MgO) from the brick. Typical MgO saturation occurs at around 8 % again depending on the temperature and state of oxidation, so if MgO is not added, usually in the form of calcined dolomite, the slag dissolves the lining preferentially and thereby increase lining wear. Slag chemistry is again related to state of oxidation and temperature as the basicity and the MgO content are diluted by high levels of FeO and temperature increases the kinetic reaction rates. Erosion and abrasion effects are much related to the practices adopted during the steelmaking.
Theoretically the refractory lining of a converter is to be designed by the refractory type and different thicknesses so that no material is wasted at the end of the converter campaign. This means that all the zones (Fig 2) of the converter lining is to worn out to the stopping thickness at the same time. But in practice this does not happen and the refractories in some zones are worn out faster than the refractories in other zones. A balanced lining design is always aimed to improve lining life at the optimum refractory cost. A balanced lining is the lining where different qualities and thicknesses of refractories are used in different zones of the converter after careful study of the wear pattern of the refractories in the converter. This type of lining is also called zonal lining since in such type of lining the refractories are zoned such that a given segment of lining having lesser wear is assigned a lower quality or lesser 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.
Fig 2 Different zones of the converter
Refractories qualities normally used for converter lining ranges from tar bonded dolomite, pitch bonded dolomite, and pitch bonded magnesia to the advanced refractories which are made with resin bonds, metallics, graphites and sintered and/or fused MgO which is 99 % pure. Refractories are made to have combination of properties to withstand high temperatures and rapid changing environment condition during a heat in a converter. A balance of properties such as hot strength, oxidation resistance, and slag resistance are expected from the converter refractories. Presently Mag-C refractories are most popular refractories for converter lining.
The different variables associated with the Mag-C refractories for improving the quality in order to have an improved lining life of the converter are (i) type of magnesia grain used such as chemistry, crystal size, and density, (ii) bonding type and size of brick press (friction / hydraulic), (iii) bonding agent such as pitch, resin, quantity, and re-impregnation, (iv) type of graphite used such as purity, sizing, and quantity, (v) anti-oxidants such as type, quantity, and sizing, (vi) brick physical properties such as density, porosity, and strength (hot and cold). The type and size of the press be it friction or hydraulic has an influence on the brick properties.
Magnesia grain – Since the largest component of a Mag-C brick is the MgO grain, the composition and properties of the grain play an important role in the characteristics of the brick. There are several types of MgO grains which are available. Higher quality of MgO grains are needed for withstanding basic slags, erosion, abrasion, and temperature etc. The grain density, size, and chemistry are vital. In terms of the chemistry, the lime (CaO) /silica (SiO2) ratio of the grain is important. CaO/SiO2 ratio is to be either zero or in excess of 2:1 to ensure the formation of di-calcium silicate, a high melting point phase. Some MgO grains have a CaO/SiO2 ratio as high as 6:1 but then these grains become more susceptible to hydration. Low CaO/SiO2 ratio results in low melting point phases and the loss of hot strength can be catastrophic. The amount of secondary minerals formed in the grain is also important and hence, the SiO2 content is to be as low as possible (less than 0.3 %). High content of boron (B) is also very critical since it destroys the hot strength of the grain.
Grain density – Grain density can vary from 3.2 grams per cubic centimeter (g/cc) to higher than 3.5 g/cc. Low grain density means high porosity making the grain susceptible to slag penetration.
Crystal size – Large grained crystals normally outperform the grains with low crystal size due to a reduction in the 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 the high temperature service. The reduction process destroys both the C in the brick and the MgO in the grain producing magnesium (Mg) metal vapour and CO gas. Crystal size is normally considered to be large when it is higher than 140 microns. Fused MgO grain size can exceed 1000 microns. However the fused grain material can have variation in chemistry, and crystal size.
Bonding agent – Mag-C bricks are C bonded bricks with having the residue of finely divided C remaining after the coking of the binder. This residue holds the brick together.
Type of graphite used – Graphite is non wetting to steelmaking slags and hence prevents the slag penetration into the brick and subsequent dissolution of the MgO grains. The graphite also is very thermally conductive transferring heat away from the brick surface thereby reducing the kinetics of aggressive reaction. Chemically, all graphites are pure C but all contain some ash (clay minerals found in the graphite deposits). Impure graphite adds fluxes such as SiO2 and Al2O3 (alumina) to the brick which generates only negative effects. Flake graphite is usually used as it has a higher resistance to oxidation than amorphous graphite and a higher thermal conductivity. Generally the amount of graphite used can vary from 5 % to 25 % and when everything else being equal then with the higher graphite content the higher is the slag resistance and thermal conductivity of the brick.
Anti-oxidants – Metal powders are added to Mag-C bricks since they act as scavengers for O2 delaying the oxidation of the graphite and C-bond. The powders improve hot strength markedly by forming complex metallic- carbide- oxide bonds in the brick.
Design of a BOF lining varies from plant to plant each with the intention of generating a lining which achieves desired life and availability, and with an attempt to equalize wear from the different wear mechanisms in the different areas of the BOF. A typical zonal lining of the converter is given in Fig 3.
Fig 3 Typical zonal lining of the converter
Lining maintenance practices
Several lining maintenance practices are employed to enhance the refractory lining life in a converter. These are given below.
Measurement of refractory wear by a laser beam – It is a technique which is used for the measurement of lining thickness with the help a laser beam (Fig 4). It is currently a widely used method. In the technique, a laser beam is rebounded off calibrated points on the converter proper and compared to the points in the worn lining. A computer analysis is then used to plot the remaining lining thickness. This information is also useful in comparing the wear rates for different refractories and avoiding of the shell damage. The usefulness of this technique is in determining and controlling the needed furnace maintenance by gunning. By using laser beam, the areas actually requiring gunning maintenance can be identified for carrying out the required gunning maintenance. The amount of gunning material which is needed can also be controlled.
Fig 4 Measurement of refractory wear by laser beam
Magnesia levels – MgO level of slag is a very important factor for improving the lining life. The objective is to charge more MgO than the saturation level of the slag at the operating temperature. Higher MgO in slag also improves the coating characteristic of slag during slag coating as well as the sticking characteristics of the slag during slag splashing. The MgO sources are normally the low cost magnesia source. Used basic lining material can also be used for this purpose. MgO levels in the slag are to be based on the tapping temperatures.
Slag coating – It 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. Requirements for the slag coating practice to succeed are (i) selecting the right type of slag, (ii) conditioning of the slag with right and proper amount of additions, (iii) correct rocking of the converter, (iv) disposing of the slag when necessary, (v) coating when it is the best time. These items are to be well planned and correctly executed for proper coating of the slag.
Slag splashing – Slag splashing technique contributes to major enhancement in the converter lining life. Slag splashing as the name suggests, utilizes residual slag from the steelmaking process, which is conditioned, to provide a coating on the refractory surface to act as a wear lining in the subsequent heat. Liquid viscous slag is blown by means of high pressure nitrogen (N2) into the different parts of the converter 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 heat. It has become a powerful tool for increasing of the lining life of the converter. It entails the use of O2 lance to blow N2 on the residual slag. Slag splashing needs 2 minutes to 3 minutes and is done with converter in vertical condition. N2 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 gunning refractory material normally a monolithic on the areas which encounter severe wear out such as trunnions, and the slag line. Gunning is usually done only on the selective areas. 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 studies have been made 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 steelmaking.
Lining of converter and lining materials
In a converter lining usually two layers of refractory bricks are used. This consist of a thinner safety lining to protect the shell usually of thickness ranging from 150 mm to 225 mm and a thicker working lining usually of thickness ranging from 450 mm to 750 mm. A large part of the safety lining normally lasts a number of the working lining campaigns. The safety lining is held in place with steel retainer rings and mortar. The lining of converter is simple and consists of the following steps.
- The lining of the converter is cooled after the campaign is over and the remaining spent working lining is removed by mechanical means generally using a de-bricking machine. The removed refractories are allowed to fall by rotating the converter to an inverted position.
- The repair of the damaged portion of the safety lining is carried out and the working lining is installed without mortar.
- In principle the working lining is installed with minimum brick cutting in a ringed keyed construction where the brick is held in place by the brick taper (smaller hot face than cold face)
The wear conditions in the different zones of the converter and the type of the refractories recommended for use is given below.
Bottom – The erosion of refractories in this area is by molten metal, slag and gases. Thermo-mechanical stresses are developed in case of a combined blown converter as a result of thermal gradients between the tuyeres cooled by the gas and the bottom lining. Mag-C lining does not provide an enhanced performance in the area of converter bottom. In case of deep blowing during the heat, there is faster wear out of the bottom. The bottom repairs during the campaign are normally carried out by building of the bottom with dolomite enriched viscous slag or patching the bottom with mixture of liquid slag and broken basic bricks. The combined blowing operation in the converter causes high stress on the bottom and for this reason fired MgO refractory blocks with modifiers and pitch impregnation are normally used in this area.
Bottom tuyeres and its surroundings – Bottom blowing and bath agitation through tuyeres contributes to localized wear of the refractory materials in the surrounding area. The wear is due to the turbulent flow of molten steel giving rise to erosion of the refractory and also due to the thermal stress caused by the flow of the cold gases. High dense and with low porosity pitch bonded and impregnated Mag-C refractory blocks based upon fused MgO are preferred for this application.
Charge pad – Charge pad in the converter is normally directly opposite the tap-hole. The charge pad is subjected to impact load by falling scrap, sometimes of heavy nature such as bloom ends. Molten steel also cause erosion of the charge pad. In addition, sampling probes and temperature probes are introduced from this side of the tilted vessel and this leads to slag at high temperature carrying out the washing of the charge pad area. Initial refractory solutions to the charge pad were found with pitch impregnated fired MgO bricks. As converter lives have increased owing to the use of Mag-C materials, the charge pad became a major wear area, and resin bonded Mag-C with metal additions are being used to get good performance. These materials offer good resistance to impact, provide resilience coupled with strength, and resistance to slag ingress owing to the presence of graphite.
Tapping area – Tapping area is subjected to erosion by liquid steel at high temperature and subject to corrosion by liquid slag. Refractory development in the tapping area has been on the lines of the development of the charge pad area. The earlier materials are being replaced with pitch bonded and impregnated Mag-C with metal additions. Since the dominant wear process in the tapping area involves high temperature slag attack, refractory blocks manufactured from large crystal sized MgO are rapidly becoming the standard. Pitch bonded and impregnated refractories have been found to give superior performance in the tapping area due to the reduction of penetrating slag oxides by the action of C in the refractory block porosity.
Slag zone cross-over – This area of the converter lining, which is situated at the intersection between the lower tapping area and upper level of the static bath, is very complex since it is subjected to several modes of attacks such as slag attack, high temperature, and erosion. All these attacks take place during the tapping of the converter. Refractory materials with resistance to high temperature slag attack along with oxidation resistance are required for the lining of this area. Pitch bonded, impregnated Mag-C based upon large crystal size MgO, high purity flake graphite and containing metal additions has given good results. Use of high purity graphite flakes is particularly useful since it limits ‘self oxidation’ by the impurity oxides inherently present in graphite flakes.
Trunnion zone – Trunnion zone of the converter is the most difficult to maintain since it is on the rotational axis and thus incapable of slag washing. Wear of the refractory material is primarily by loss of the working face by oxidation of the C bond. Gunniting of this area of the converter is the only practicable method of maintaining this area and refractory blocks with high resistance to oxidation are preferred.
Main barrel – Main barrel of the converter can be conveniently divided into two areas namely (i) the knuckle, and (ii) the upper sidewall.
The knuckle area in the converter is often a major wear area, although design changes are frequently effective in improving performance. The knuckle area is mostly subjected to severe slag attack, particularly when the converter is not being blown. It requires refractory materials with very good resistance to slag corrosion and erosion.
Upper sidewall of the converter is subjected to slag attack, but also to the extremes of temperature cycling. Materials which have good slag resistance are used in this area. However, the refractory lining in this area is also to be capable of taking and retaining a slag coating.
Sampling slag line – Areas of the cone distributed either side of the charge pad are subjected to A preferential attack by slag when the vessel is tilted for temperature measurement and for sample probe dips. Pitch bonded impregnated Mag-C give good performance in this area of the converter.
Cone – The cone area is subjected to high temperature erosion by high velocity gases carrying with them entrapped particles. There is oxidizing atmosphere. Resin bonded Mag-C materials give good performance in the cone area.
Upper cone – In the upper cone, the top six to ten rings, the wear mechanism is modified by the loss of refractory material during skull removal. Damage to refractory bricks in the upper cone occurs during mechanical deskulling either progressively by loss of brick ends or by dislodgement of whole bricks.
Refractory materials with high resilience combined with strength and oxidation resistance are a prerequisite for this area of the converter. Pitch bonded impregnated MgO, sometimes provided with co-moulded metal plates, which expand during oxidation and tighten the brick rings, is the most successful refractory material in this area.