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Improvement of oxidation resistance of carbon composite refractories

wallpapers Tech 2020-12-30
Regardless of whether it is a magnesia-carbon brick or an aluminum-magnesium-carbon brick, graphite is the key to inhibiting slag penetration and slag erosion resistance. Its biggest weakness is that it is easy to oxidize. There are three main oxidation methods for carbon in refractories. The first is gas-phase oxidation, that is, carbon and O2 in the air are directly oxidized into COx gas; the second is liquid-phase oxidation, that is, the reaction of carbon and FeO in slag; the third is solid-phase oxidation , That is, carbon and MgO or AL2O3 inside the material undergo a carbothermal reduction reaction. Therefore, improving the oxidation resistance of the material is the research hotspot and focus of promoting the application of materials.
 
Adding antioxidants
The method of adding antioxidants is the most common and effective method to improve the oxidation resistance of carbon composite refractories. The protection mechanism is mainly based on two considerations: thermodynamics and kinetics. In terms of thermodynamics, the use of additives with greater affinity for oxygen makes them oxidized prior to carbon, thereby indirectly protecting the carbon; in terms of kinetics, the use of additives and oxidation source gas or carbon generated by the reaction The compound changes the microstructure of the carbon composite refractory (such as increasing the density and blocking the pores), thereby hindering the subsequent erosion and penetration of oxygen and slag. Currently commonly used antioxidants are divided into metallic antioxidants and non-metallic antioxidants. The former mainly includes Al, Si, etc., while the latter mainly includes SiC, B4C, etc.
 
Antioxidant mechanism process
Al powder, the most used metal antioxidant, first reacts with carbon to form Al4C3 at high temperature, and Al4C3 reacts with CO(g) and so on. As the metal Al or Al4C3 participates in the reaction, the oxygen partial pressure in the brick decreases, and carbon materials such as graphite are protected. The anti-oxidation mechanism of metal Si is similar. Based on this, Shi Yongwu and others added metal Al powder and Si powder as antioxidants when designing a new slag line magnesia carbon brick, which has a longer service life than the original traditional slag line magnesia carbon brick. Zhang et al. went further and observed and discussed the structure of magnesia-carbon bricks added with Al and Si from the perspective of microstructure, and analyzed the anti-oxidation mechanism with thermodynamics.
 
After reacting at 1200 and 1500°C for 3 hours, Al4C3, AlN, AL2O3 and magnesia-aluminum spinel MA are present in the Al-added magnesia carbon bricks. There are SiC, Si3N4, SiO2 and M2S structures in the magnesia carbon bricks with Si added. However, it should be pointed out that the formation of Al4 C3 is accompanied by a larger volume effect, so the addition of metal Al powder The amount should be controlled below 3% by mass; Si powder generates M2S due to SiO2 generated by oxidation, which will reduce the high temperature performance of the material, and these shortcomings limit their application to a certain extent.

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