New environmentally friendly processes and reduced pollutant emissions are the new driving force in the steelmaking industry. Magnesia-chrome bricks fired at above 1700°C are used for permanent lining of ladles for the production of stainless steel due to their high physical and mechanical properties and dimensional stability. Tempered refractory bricks use a special binder that eliminates the burning of fossil fuels and have no carbon dioxide emissions from the combustion process.
The new refractory bricks are based on MgO-ZnO with free chromium and carbon. This article describes the properties and results of sintered and tempered bricks used for safe refractory lining of ladles for stainless steel production.
During the secondary metallurgical refining of stainless steel, the molten steel is fed into ladles for transport and alloying. Ladles are made of different refractory grades, depending on the erosion mechanisms present during operation. In this case, the common state of the ladles used for the production of stainless steel is to use calcined dolomite refractory bricks in the working lining and magnesia-chrome bricks in the permanent lining.
Calcined dolomite brick
Calcined dolomite bricks, whose two main components are CaO and MgO, are commonly used in silicon-killed steel production due to their high compatibility with basic slags and low cost. Direct-bonded calcined dolomite bricks are fired at high temperatures in a tunnel kiln until a ceramic bond is formed. Although the presence of CaO makes the brick hydration sensitive, it plays a crucial role in the chemical behavior of the material. The slag resistance of calcined dolomite bricks is due to free lime, which is able to form a dense Ca2SiO4 (C2S) layer, which has a high refractoriness (2154°C) and protects the material from further penetration and erosion by slag.
Different types of stainless steel are produced in the same ladle, which makes the process demand a wide range of adaptability in refractory materials. Due to the high chromium content of this type of stainless steel, magnesia-chrome bricks are often used for the safe permanent lining of stainless steel ladles as it is compatible with the process. These bricks are known for their ability to form a ceramic bond during sintering due to secondary spinel formation, which is responsible for high mechanical properties, corrosion resistance, erosion and thermal stability.
Magnesia Chrome Refractory Brick
Magnesia-chrome refractory bricks are divided into direct bonded type, rebound type, semi-rebound type, and a new type of tempered zero-carbon material has been developed in recent years. Regardless of the production process, all grades have a high chromium content. Therefore, under normal industrial temperature and oxidation conditions (in air), the trivalent chromium (Cr3+) present in MgO-Cr2O3 refractory materials can be oxidized to Cr6+. The formation of hexavalent chromium is catalyzed by alkaline environment, slow cooling rate, small grains, the presence of CaF2 and low temperature. This component is soluble in water and is known to be carcinogenic. Due to stricter environmental regulations, the steelmaking industry is aware of this problem and has therefore made great efforts to recycle and reduce solid emission waste.
Steelmaking consumes a lot of refractory materials. Most of the refractory materials commonly used in ladles in stainless steel production are fired at high temperatures, which releases a lot of carbon dioxide. In order to reduce pollutant emissions, a new zero-carbon environmentally friendly refractory brick binder system has been developed as a substitute for the current fired refractory materials.
In this context, zero carbon tempered MgO-ZnO refractory bricks were developed as an alternative solution for the permanent lining of ladles in stainless steel production. Due to its rapid sintering capabilities, this new material is able to develop ideal properties using the energy already present in the process, allowing the refractory bricks to be delivered to customers in a tempered condition of 200°C. The new refractory bricks contain no chromium, are easier to recycle, and have less CO2 emissions due to the elimination of the firing process and shorter delivery times.
Customer Trial
Several tests were carried out for the safety lining of stainless steel ladles for customer A. Both types of refractory bricks were always used in the same ladle.
Test A: 93 MgO-ZnO tempered bricks (230×65×70 mm) were assembled and laid in the same equipment ladle container as the MgO-Cr2O3 refractory bricks to ensure the same operating conditions. The assembly is shown in the figure. During the entire production process, a total of 8 different grades of stainless steel were produced. Due to the erosion of the calcined dolomite refractory bricks used, they were ended in 42 furnaces. After the test, no abnormality was found in the MgO-ZnO bricks.
Test B: 300 pieces of MgO-ZnO tempered bricks (230×65×70 mm) were assembled on the same ladle as the fired MgO-Cr2O3 bricks to ensure the same operating conditions. The assembly is shown in Figure 2. During the production process, four different grades of stainless steel were produced, which ended after 30 furnaces due to the erosion of the calcined dolomite bricks used in the slag working lining. The MgO-ZnO bricks used for the safe permanent lining were removed and inspected, and the appearance was good after sintering, as shown in Figure 3.
in conclusion
The tempered bricks after heat treatment show good properties due to the presence of the binder system and zinc oxide, which diffuses into the magnesium oxide to form a solid solution. The combination of zero carbon binder and zinc oxide improves the efficiency of the sintering process.
Using a unique zero carbon, zero chromium binder system, tempered magnesia bricks for stainless steel safety linings have been developed. The newly developed product is an environmentally friendly alternative for stainless steel ladle safety linings. The customer trials presented in this article have shown very good results, and new trials are underway to consolidate the technology.
