Refractories for Basic Oxygen Furnace
Refractories for Basic Oxygen Furnace
The main objective in the development of refractories for basic oxygen furnace (BOF) is to get a useful lining life of the wear lining so as to obtain maximum availability of the BOF. Longer lining life not only results in lower refractories cost but it also enables higher productivity through increased furnace availability.
The following are the basic requirements from the refractories of BOF.
- Thermal spalling resistance
- Corrosion resistance
- Abrasion resistance
- Oxidation resistance
- Hot modulus of rupture
BOF is lined normally with a permanent lining and above it there is a wear lining. Permanent lining thickness may vary from 100 mm to 120 mm and is made of chrome-magnesite permanent lining which is given on the full height of the BOF.
The refractories available for use in wear linings of BOF range from tar or pitch bonded dolomite or magnesia (MgO), chrome magnesite, or magnesium chrome refractories to the advanced refractories that are made with resin bonds, metallics, graphites, and sintered and/or fused magnesia that can be with a purity of 99 %. Bricks are designed with a combination of critical physical properties to withstand the high temperatures and rapidly changing conditions/environment throughout the BOF heat cycle. A balance of different properties such as hot strength, oxidation resistance, and slag resistance is necessary from the BOF refractories for good performance.
When BOF process of steelmaking was introduced in 1950s, converters were lined with tar dolomite bricks and stabilized burnt dolomite bricks. These refractories were then replaced by semi stabilized burnt dolomite bricks and tar bonded and fired bricks made of synthetic magnesia dolomite clinker. Chrome magnesite, or magnesium chrome refractories were used for lining of some BOFs. High purity burnt magnesia bricks were also used in some of BOF linings. During late 1970s, magnesia-carbon bricks with corrosion and spalling resistance were developed and rapidly brought into use for lining of the BOF. These bricks utilize the resistance of magnesia to the corrosive high basicity slag and the high thermal conductivity and low wettability of graphite (carbon). Today the practice of using magnesia- carbon refractories for the lining of BOFs has become very common.
The stability of magnesia – carbon bricks can be increased by preventing the oxidation of graphite and by improving the corrosion resistance of magnesia clinker. Oxidation of graphite is prevented by adding easy to oxidize metals such as aluminum and magnesium – aluminum, carbides such as silicon carbide (SiC) and boron carbide (B4C), and borides such as calcium boride (CaB6). Use of high purity graphite also prevents the oxidation. Corrosion resistance of magnesia is improved by raising the purity level of magnesia clinker either by using electro-fused magnesia or sea water magnesia. It is also improved by optimizing particle size distribution of magnesia clinker. Magnesia carbon brick to which zircon (ZrSiO4) is added for thermal stress alleviation in service has also been developed.
Modern high purity magnesia is produced by well controlled processes. The principal sources of magnesia are brines often from deep well or from seawater. Magnesium hydroxide, Mg(OH)2, is precipitated from these sources by reaction with calcined dolomite or limestone. The resultant magnesium hydroxide slurry is filtered to increase its solids content. The filter cake is then fed directly to a rotary kiln to produce refractory grade magnesia. These days the filter cake is calcined at about 900 deg C to 1000 deg C in multiple hearth furnaces for converting the magnesium hydroxide to active magnesia. This calcined magnesia is then briquetted or pelletized for firing into dense refractory grade magnesia, usually in shaft kilns at around 2000 deg C temperatures. The end product is sintered magnesia. Fused magnesia is produced by melting refractory grade magnesia or other magnesia precursor in an electric arc furnace. The molten mass is then removed from the furnace, cooled, and broken up for its use in making of refractories. The impurities in magnesia are controlled by the composition of the original source of the magnesia (brine or seawater), the composition of the calcined dolomite or limestone, and the processing techniques. In particular the percentages and ratio of CaO and SiO2 are effectively controlled, and the B2O3 is held to very low levels. High grade refractory magnesia produced such is used for the production of magnesia refractories.
Different factors are responsible for the wear of BOF lining in different zones of the BOF. Therefore zonal lining of BOF is practiced where different type of magnesia carbon bricks or other bricks are installed in different zones of the BOF to ensure wear balance and through it extension of the lining life of the BOF is achieved. A cross section of BOF cut vertically at the tap hole showing the concept of zonal lining is at Fig 1.
Fig 1 Cross section of a BOF showing concept of zonal lining
To optimize the design of wear lining, it is essential to develop a balanced lining, that is, a lining in which different refractory qualities and thicknesses are assigned to various zones of the converter lining on the basis of a careful study of the wear patterns. In a balanced lining, the refractories are zoned such that a given segment of lining known to receive less wear is assigned a lower quality or less thickness of refractory, whereas refractories of greater wear resistance and generally of higher costs are reserved for those segments of the furnace that will be subjected to the most severe wear. The wear conditions and proposed refractories in different zones of BOF for the zonal lining are given in Tab 1 below.
|Tab 1 Converter zones wear conditions and proposed refractories|
|Converter zone||Wear conditions||Proposed refractories|
|Cone||1) Oxidizing atmosphere||1) Standard quality magnesia – carbon bricks containing anti oxidants|
|2) Mechanical abuse||2) Pitch bonded magnesia bricks|
|3) Thermo mechanical stress||3) Resin bonded low carbon bricks with anti oxidants|
|4) High temperature|
|Trunnions||1) Oxidizing atmosphere||1) Premium quality magnesia – carbon bricks containing anti oxidants|
|2) Slag corrosion||2) Premium quality magnesia – carbon bricks containing fused MgO and anti oxidants|
|3) Slag and metal erosion||3) High strength premium quality magnesia – carbon bricks|
|Charge pad||1) Mechanical impact||1) Pitch impregnated burned magnesia bricks|
|2) Abrasion from scrap and hot metal||2) Standard quality high strength magnesia – carbon bricks containing anti oxidants|
|3) High strength low carbon magnesia bricks containing anti oxidants|
|Tap pad||1) Slag erosion||1) Premium quality magnesia – carbon bricks containing anti oxidants|
|2) High temperature||2) High strength low carbon magnesia bricks with metallic additives|
|3) Mechanical erosion||3) Standard quality magnesia – carbon bricks containing anti oxidants|
|Turndown slaglines||1) Severe slag corrosion||1) Premium quality magnesia – carbon bricks containing anti oxidants|
|2) High temperature||2) Premium quality magnesia – carbon bricks containing fused magnesia and anti oxidants|
|Bottom and Stadium (bottom stirred vessels)||1) Erosion by moving metal, slag and gases||1) High strength standard quality magnesia – carbon bricks containing anti oxidants|
|2) Thermo-mechanical stresses as a result of expansion||2) Magnesia – carbon bricks without metallic additives characterized by low thermal expansion and good thermal conductivity|
|3) Internal stresses as a result of thermal gradients between gas cooled tuyeres and surrounding lining||3) Pitch impregnated burned magnesia bricks|
With the wide variety of available brick qualities, there is a wide range of costs of the refractories. The more expensive brick can cost as much as six times that of a conventional tar/pitch bonded dolomite brick. With the upgradation of lining designs, more of the refractories with higher costs are used in a BOF lining these days. However use of the high costs refractories has to justify the overall techno economics of the BOF shop.
For example, when the cost of a lining is increased by 25 % in a BOF shop that is averaging 4000 heats, the lining life will need to increase to 5000 heats for the refractories costs to be maintained. However, in shops where furnace availability is needed for productivity, a lesser increase in lining life and a higher refractory cost may be justified if the furnace availability is greater during periods of high production needs.
As lining designs are upgraded to optimize performance and costs, the effects of operating variables on lining wear are important to know. With this information, the possibility of controlling those parameters that affect lining wear adversely and the economic tradeoffs of increasing operating costs to extend lining life can be better evaluated. In general, the practices that improve process control, such as sub-lances, benefits lining life. In addition, lining life is helped by charging dolomitic lime to provide slag MgO, minimizing the charge levels of fluorspar, controlling flux additions and blowing practices to yield low FeO levels in the slags. These practices need to be optimized to yield the most cost effective lining performance.
Even when many operating conditions are improved, lining designs are optimized for balanced wear, and the best brick technology is used, wear does not occur uniformly, and, generally, maintenance practices that involve gunning of refractories and coating with slag are used to extend the life of a lining.