Desicription：The carbon source of MgO-C refractories can be divided into amorphous carbon and graphitic carbon. Among them, amorphous carbon is derived from the carbonization product of binder phenolic resin or tar pitch, so it is also called bonded carbon. The raw materials added to the ingredients such as flake graphite, expanded graphite and earthy graphite are mostly graphitic carbon. Carbon black is amorphous carbon due to its amorphous nature. Various carbon raw materials have their own physical characteristics, and the influence of different carbons on MgO-C refractories should be analyzed when selecting. For traditional MgO-C refractories, the carbon content is about 10-20wt%. The total carbon content of low-carbon MgO-C refractories is generally ≤8wt%. The performance of low-carbon MgO-C refractories can be improved through the selection and design of carbon raw materials.
Title：Selection and design of carbon source for MgO-C refractories
Keyword：Magnesia carbon brick, refractory, carbon source, selection
Graphite is mainly divided into two categories: natural graphite and artificial graphite.
At present, the carbon raw materials used in the preparation of carbon-containing refractories are mainly natural graphite, and the most widely used is flake graphite. Flake graphite has excellent thermal and mechanical properties, and can form a continuous carbon network in the matrix of MgO-C refractories to relieve thermal stress caused by rapid temperature changes. The purity, particle size and volatile content of graphite directly affect the performance and service life of MgO-C refractories. The higher the purity of graphite, the lower its ash and volatile content. Adding MgO-C refractories can form a dense structure, which can effectively resist the erosion of slag and significantly increase the service life of the material.
The purity of graphite has a significant impact on the high-temperature mechanical properties of MgO-C refractories: For MgO-C refractories prepared by adding low-purity graphite, the impurities in the matrix are likely to react with magnesia and other raw materials to form low-melting phases, which will damage them. The internal structure of the material is reduced, and the high-temperature mechanical properties of MgO-C refractories are reduced. The particle size of graphite also affects the oxidation resistance and thermal shock stability of the prepared MgO-C refractories. The larger the particle size of the flake graphite used in the ingredients, the better its oxidation resistance and thermal shock stability. Flake graphite with large particle size has a more complete crystal structure and higher thermal conductivity, which can slow down the oxidation rate and reduce the damage of thermal stress to MgO-C refractories. Therefore, the particle size of flake graphite used in the industrial production of MgO-C refractories is usually required to be greater than 0.125mm.
- Carbon black
Carbon black is a product obtained by incomplete combustion or thermal decomposition of organic matter under conditions of insufficient oxygen. It belongs to amorphous carbon. Because of its ultra-fine particle size, it can be used to fill the gaps between the internal pores and the matrix of MgO-C refractories. Existing research results show that using other carbon raw materials to partially replace flake graphite can improve the performance of low-carbon MgO-C refractories. Liu et al. used nano-sized carbon black to partially replace flake graphite as a carbon source, and found that nano-sized carbon black instead of 0.4wt% flake graphite can significantly improve the thermal shock stability of the sample, which is significantly better than the sample without carbon black. The thermal shock stability is equivalent to that of the sample with a carbon content of 16wt%.
At the same time, the normal temperature mechanical properties and high temperature mechanical properties of the samples also showed an upward trend with the increase of the content of nano carbon black. At the same time, Tang et al. added nano carbon black to the binder phenolic resin to prepare a composite binder. The results show that the introduction of nano-carbon black significantly improves the mechanical properties of the test. In addition, the mechanical properties of the samples prepared by the unique introduction of carbon black-phenolic resin composite binder are better than those with carbon black directly introduced into the matrix. In addition, the introduction of carbon black not only improves the mechanical properties of the sample, but also reduces the number of pores after carbonization of the phenolic resin, and improves the degree of graphitization after the heat treatment of the binder.
- Allotropes of carbon
Since carbon atoms can form a variety of molecular structures, carbon has a variety of allotropes such as carbon nanotubes, graphene, and fullerenes. For its low-dimensional allotropes such as carbon nanotubes or graphene, due to its unique mechanical properties, adding MgO-C refractories as carbon raw materials can also improve the performance of MgO-C refractories. Studies have shown that adding carbon nanotubes to refractory materials can improve their strength and toughness. Zhu et al. introduced graphene sheets into low-carbon MgO-C refractories and found that the mechanical properties and thermal shock stability of the graphene samples were improved. However, because the above-mentioned materials cannot be mass-produced in industrial applications, their higher cost also limits their application in the field of refractory materials.