Kinetics of the combustion of carbon in graphite chamotte articles

Abstract

Graphite-containing refractories have become widely used in recent years [1][2].The technical solution in the field of production of these refractories envisages special conditions for the slowing down of the carbon-oxidation process in the products: firing in a neutral or reducing medium; glazing of the articles [3]; the addition of ferrosilicon or silicon to the batch to bind the carbon as silicon carbide; additives to produce a liquid phase [4]; and so on.One of the ways of controlling the resistance of the materials to oxidation is to chemically modify the surface of the SiC powder by molecular modification of the surface [5,6].The extent of the combustion of carbon in graphite-containing refractories depends largely on the structure of the refractory and the operational temperature.The kinetics of the carbon combustion in unfired graphite-containing high-alumina articles heated from 800 to 1600~ was studied and also the character of the carbon combustion in various directions and at various distances from the surface of the articles was investigated.Specimens in the form of cylinders 25 mmindiameter and height were molded from low-and high-carbon masses* [7] using a semidry pressing method at a pressure of 40 MPa. The batch of the low-carbon articles included 15% of Polozhsk kaolin, 20% of laminar graphite, 45% of highalumina chamotte (-82% AI~03), and 20% of SI-65 ferrosilicon; the batch for the high-carbon articles contained the same components but in the following amounts, respectively: 15, 50, 15, and 20%, To i00 parts of the batch we added 8 parts each of AKPS and 4 parts of distillery waste. The specimens were dried at II0~ and heat treated at 600~ with a dwell of 4 h. One specimen each in a corundum beaker was placed in the isothermal zone of a Kryptol furnace heated to a specified temperature and in a weakly oxidizing medium.We used a mathematical method to plan the experiment: the central composite orthogonal planning (CCOP) 22 [8]. We took the following as the factors: heating temperature X~, ~ and duration of heating X2, min. The optimization parameter was the degree of combustion of the carbon, % (relative).It is well known [9, i0] that the most intense oxidation of graphite starting at 750-800~ occurs in three stages: penetration of oxygen from the air into the particles of graphite; interaction between graphite and the atmospheric oxygen in accordance with the reactions:the removal of the interaction products.In service, complex physicochemical processes of the formation of a liquid phase as well as a change in the chemical and phase compositions and in the structure of the refractory occur in unfired and fired graphite-containing refractories. These processes have a significant effect on the extent of combustion of the graphite.*The designation of the masses was arbitrary.L. I. Brezhnev Dnepropetrovsk Metallurgical Institute.