Refractory materials used in alkaline oxygen furnaces

Refractories for alkaline oxygen furnaces

 The main objective of developing refractories for alkaline oxygen furnaces (BOFs) was to achieve a longer wear lining life in order to obtain maximum availability of the BOF. Longer lining life not only reduces refractory costs， but also increases productivity by improving furnace availability.

The following are the basic requirements for BOF refractories

Resistance to thermal spalling

Corrosion resistance

Abrasion resistance

Resistance to oxidation

Modulus of thermal fracture

Steel mills usually use a permanent liner， on top of which there is a wear-resistant lining. The thickness of the permanent lining can vary from 100 mm to 120 mm and is made of a chrome-magnesium permanent lining， which is given over the entire height of the converter.

Refractory materials that can be used for BOF wear liners include tar or pitch-bonded dolomite or magnesium oxide (MgO)， chrome magnesite or magnesium-chromium refractories， and advanced refractories made of resin-bonded， metallic， graphite， sintered and/or fused magnesia up to 99% purity. The bricks are designed to incorporate key physical properties to withstand high temperatures and rapidly changing conditions/environments throughout the BOF thermal cycle. Balancing different properties， such as thermal strength， oxidation resistance and slag resistance， is necessary for good performance of BOF refractories.

When the BOF steelmaking process was introduced in the 1950s， the converter was lined with tarred dolomite bricks and stabilized burnt dolomite bricks. These refractories were later replaced by semi-stabilized tar dolomite bricks and bricks fired with tar-bonded synthetic magnesia dolomite clinker. Chromium-magnesite， or magnesium-chromium refractories were used in the linings of some converters. High purity sintered magnesia bricks were also used for the linings of some BOFs. In the late 1970s， magnesia-carbon bricks with resistance to corrosion and spalling were developed and were quickly used in converter linings. These bricks take advantage of the corrosiveness of magnesite to high alkalinity slag and the high thermal conductivity and low wettability of graphite (carbon). Today， the use of magnesia-carbon refractories as BOF liners is very common.

The stability of magnesia carbon bricks can be improved by preventing the oxidation of graphite and by increasing the corrosion resistance of magnesia clinker. Oxidation of graphite is prevented by the addition of metals prone to oxidation， such as aluminum and magnesium-aluminum， carbides， such as silicon carbide (SiC) and boron carbide (B4C)， and borides， such as calcium boride (CaB6). Oxidation can also be prevented by using high purity graphite. The purity of magnesium clinker can be improved by using electrofused magnesium or seawater magnesium， which improves the corrosion resistance of magnesium. It can also be improved by optimizing the particle size distribution of the magnesium oxide clinker. Magnesia carbon bricks with the addition of zircon (ZrSiO4) have also been developed to reduce the thermal stress in use.

Modern high purity magnesium oxide is produced by well controlled processes. The main source of magnesium is brine， usually from deep wells or seawater. Magnesium hydroxide， Mg(OH)2， is precipitated from these sources by reaction with calcined dolomite or limestone. The resulting magnesium hydroxide slurry is filtered to increase its solids content. The filter cake is then fed directly into a rotary kiln to produce refractory grade magnesium oxide. On these days， the filter cake is calcined in a multi-bore furnace at approximately 900 degrees C to 1000 degrees C to convert the magnesium hydroxide into reactive magnesium oxide. After calcination， the magnesium oxide is briquetted or granulated and fired into dense refractory grade magnesium oxide， usually in a vertical kiln at about 2000 degrees Celsius. The end product is sintered magnesium oxide. Fused magnesium oxide is produced by melting refractory grade magnesium oxide or other magnesium oxide precursors in an electric arc furnace. The melt is then removed from the furnace， cooled， and broken up for use in the manufacture of refractory materials. Impurities in magnesite are controlled by the composition of the original source of magnesite (brine or seawater)， the composition of the calcined dolomite or limestone， and the processing technology. In particular， the percentages and ratios of CaO and SiO2 are effectively controlled and B2O3 is kept at very low levels. The high grade refractory magnesite produced in this way can be used for the production of magnesia refractory materials.

Different factors contribute to the wear of the BOF lining in different areas of the BOF. Therefore， zoned lining of BOF means installing different types of magnesia carbon bricks or other bricks in different areas of BOF to ensure the wear balance and through it the lining life of BOF is extended. Figure 1 shows a typical BOF zoned lining.

Fig. 1 Typical zonal lining of a converter

In order to optimize the design of the wear-resistant lining， a balanced lining had to be developed， i.e.， different refractory masses and thicknesses were assigned to the various zones of the converter lining based on a careful study of the wear patterns. In a balanced lining， the refractory is divided into different zones， with the parts of the lining known to wear less being assigned to refractories of lower quality or thickness， while the more resistant and costly refractories are reserved for those parts of the lining that will suffer the most severe wear. The wear conditions and recommended refractory materials for the different sections of the BOF for zonal lining are given in Table 1 below.

The cost of refractory materials varies greatly due to the wide variety of brick qualities available. The more expensive bricks can cost up to six times as much as traditional tar/asphalt-bonded dolomite bricks. As furnace lining designs are upgraded， higher cost refractories are increasingly being used for converter linings today. However， the use of higher cost refractories must justify the overall technical economy of the converter plant.

For example， when lining costs increase by 25% for a converter plant with an average of 4，000 furnaces， the lining life needs to increase to 5，000 furnaces in order to maintain refractory costs. However， in shops where productivity requires higher furnace availability during periods of high production demand， a smaller increase in lining life and higher refractory costs may be justified.

As lining designs are upgraded to optimize performance and cost， it is important to understand the impact of operating variables on lining wear. With this information， the potential for controlling parameters that adversely affect lining wear can be better evaluated， as well as the economic trade-offs of increasing operating costs to extend lining life. In general， improved process control methods， such as cannon splitting， benefit lining life. In addition， providing magnesium oxide in the slag by adding dolomite ash， minimizing the addition of fluorspar， controlling the addition of fluxes， and reducing the iron oxide content in the slag by blowing gas all contribute to liner life extension. These practices need to be optimized to produce the most cost effective liner performance.

Even though many operating conditions have been improved， liner designs optimized to balance wear， and the best brick technology is used， wear does not occur uniformly and maintenance practices including refractory spraying and slag coating are often used to extend liner life.