The traditional magnesia-carbon bricks made with synthetic tar binder according to the cold mixing process harden during the tar damage process and obtain the necessary strength, thus forming isotropic glassy carbon. This kind of carbon does not show thermoplasticity, which can relieve a large amount of stress in a timely manner during the baking or operation of the lining. Magnesia-carbon bricks produced with asphalt binders have high high-temperature plasticity due to the formation of anisotropic graphitized coke structures during the asphalt carbonization process.
Therefore, the crack resistance of bricks made by asphalt bonding is higher than that of asphalt bonded bricks. However, one of the disadvantages of asphalt-bonded bricks is that thermal mixing is required during production, and a brick mold preheating device is required. In this paper, the method of manufacturing magnesia carbon bricks by cold mixing process is described. The made brick has a low elastic modulus, so that it can relieve stress during operation and use. Ordinary coal tar pitch is harmful to environmental protection due to the presence of benzodiazepine. A special binder with low content of benzodiazepines that meets environmental protection requirements is selected.
Title：Method for manufacturing magnesia carbon brick-cold mixing process
Keyword：Magnesia carbon brick, manufacturing, process method, cold mixing
Magnesia carbon bricks were developed 35 years ago, and the bricks are now widely used in the construction of steel furnaces. Magnesia-carbon bricks are also widely used in other kilns, such as double-chamber steelmaking furnaces, and Rll-type circulating vacuum treatment devices. Traditionally, such bricks are manufactured using synthetic tar binders in accordance with the cold mixing process, and asphalt binders in accordance with the hot mixing process. Both binding agents have their own advantages and disadvantages. The main advantage of magnesia-carbon bricks produced with tar binders is that it is possible to use cold mixing technology for production, which is different from the use of asphalt binders and does not require heavy equipment and manual labor. On the other hand, bricks produced with asphalt binder have better thermoplasticity. In the course of use, it has high crack resistance because it can absorb the stress generated during operation. The main content of this research work is to make comprehensive use of the advantages of the two systems and develop a cold mixing process.
2.1 Raw materials
As raw materials, fused magnesia with a purity of 97.5%, FC grade flake graphite with a purity of 95%, and standard coal tar pitch are used. Due to the presence of polyaromatic hydrocarbons, such as benzopyridine, it is harmful to the environment. The content of benzopyridine in coal tar pitch is between 10000～15000mg·kg-1, while the international standard stipulates that the content of benzopyridine should be 50mg·kg-1 (see German standard TGRS 551). In order to comply with this strict requirement, a special binder with a benzopyridine content of 300 mg·kg-1 was selected. The data on the viscosity performance obtained from the supplier shows that the dynamic viscosity of this binder is between the resol and novolac resins, which is completely acceptable for production.
2.2 Preparation of molding mixture and molding of samples
Usually in order to make tar-bonded bricks, we use 2% dilute tar. In the course of the experiment, the thin asphalt was gradually replaced with thin tar, and the changes in the formability and performance of the bricks were observed. In order to conduct a thorough comparative study, five molding mixtures were manufactured.
Prepare the molding mixture in the inclined high-strength mixer of Eirih Company and leave it for 0.5h. The bricks are formed under a pressure of 200 mpa. The formed bricks were dried at 200°C for 20 hours, and then cooled under natural conditions. Cut the sample according to the following various test requirements for the sample: open porosity, bulk density, compressive strength at room temperature and the residual carbon rate in the brick.
2.3 Test of specimen
Experiment in our laboratory. Choose 3 bricks randomly from each ingredient, and calculate the average index in order to reduce the error.
The effect of dehydrating (water absorbing) agents is well known. As such an additive, sulfur powder is selected. Experiments were conducted with different amounts of sulfur. It was found that after adding sulfur, the strength increased by 20%, but the open porosity and bulk density of the material after carbonization treatment did not improve. However, when 10% dehydrating agent and thin pitch are added, both indexes of compressive strength after forming and after carbonization are improved. When the added amount of sulfur is further increased, the strength is not improved at all.
- Industrial test
In order to carry out industrial trials, the ingredient CZ was selected and made into bricks. Three ladle linings were built with bricks made of the above ingredients. The service life of the lining is 109 times, 103 times and 105 times (an average of 109 times). The service life of traditional magnesia carbon bricks combined with tar is 103 times.
Choosing a suitable bonding agent can make the brick achieve higher performance. The bonding agent used meets the requirements of environmental protection and safety (see standard TGRS 551). Obviously, the improvement of the lining life is good for users. In terms of cost, the special asphalt binder used is similar to tar.