666.762.852:669.162.221 In recent years in a number of countries silicon carbide refractories have become widely used for lining blast furnaces [1][2][3][4]. At each level of the furnace lining the refractory is subjected to the aggressive action of physicochemical, thermomechanical, and thermotechnical factors. In the tuyere zone the most important are thermal shocks and the action of alkali and zinc vapors and heated gases containing particles of dust and carbon. The tuyere noses, which, in addition to the above factors, undergo the mechanical and abrasive actions of solid, liquid, and gaseous reaction products, are under even more complex service conditions 9A chemically inert, heat resistant, and mechanically strong refractory capable of resisting the action of these factors is necessary for lining the tuyere nose. In this case the problem of development of parts of self-bonded silicon carbide, which would meet the requirements imposed to the greatest degree, has arisen. The essence of obtaining self-bonded silicon carbide parts is synthesis of the silicon carbide parts is synthesis of the silicon carbide binder during firing of the compacts. There are a number of methods of formation of silicon carbide binders from secondary fl-SiC [5][6][7]. The most common is production of /3-SIC from crystalline silicon and carbon by siliciding the solid component by Si vapors or liquid. Formation of the structure and phase composition occurs in reaction sintering, which occurs under conditions of nonoxidizing heating.To study the processes occurring in firing of silicon carbide compacts containing crystalline silicon and carbon investigations were made of specimens of mixtures of Si and C, Sit 2 and C, and also SiC) 2, Si and C. The compositions of the charges were based on the possibility of formation of SiC as the result of the following reactions between the components: Si + C --, SiC; Sit 2 + 3C --, SiC + 2CO; Si + Sit 2 + 4C --, 2SiC + 2CO.Crystalline silicon, quartzite, and petroleum coke were used for preparation of the binder. All of the materials were ground until obtaining of the freer than 0.06-mm fraction. The pressed and dried specimens were fired in argon, nitrogen, and carbon monoxide (crushed coke) atmospheres. In firing of specimens in argon they did not contract since the volume effect of the reaction (zSq) Siso 1 + Cso I --, SiCso I is negative (Aq = -29.8%). In all of the investigated compacted specimens the density after firing dropped by about 0.06-0.09 g/cm 3 and a direct relationship of density of the fired specimens to density of the green specimen is established.In firing of specimens in nitrogen and CO other reactions occur: 3Si + N 2 + Sit = Si2ON 2 -Aq (+41,8 %), 3SiC + 2CO = Sit 2 + 3C -Aq (+247,4 %), 3SiC + 2N 2 = Si.~Na + 3C -/~q (+64,4 %).These reactions occur with a positive effect with compaction and strengthening of the specimens. The results presented show that by changing the charge composition, temperature, and gaseous atmosphere and compaction of the compacts it is possible to ...
Blast furnace shafts are lined of dense aluminosilicate refractories primarily of chamotte composition [i]. In some cases high-alumina parts are used for lining the shafts [2]. In recent years silicon carbide parts produced by different methods have been used in the lower portion of the shaft, the bosh, and the bosh parallel.These parts are produced with binders of silicon nitride or silicon oxynitride or are self-bonded, that is, with a silicon carbide binder [3][4][5][6][7].Theparts with a silicon oxynitride binder possess higher heat resistance than parts containing a silicon nitride binder [8].At present the direction of making linings of unformed materials by methods of guniting, ramming, and vibration is promising [9]. The use of these methods makes it possible to completely mechanize the processes in production of the powders and their use.In the USSR there is positive experience in the use of powder materials for lining the upper portion of blast furnaces [i0]. Production experience in the use of gunited concrete for repair of the shaft lining is also known.Concretes are used for repair at low temperatures and also concretes hardening at high temperature, which are used for repair of the shaft lining without shutting down the furnace for major repairs [12].Such concretes contain 76-94% A1203, 0.2-14% Si02, and appropriate hardening additions.The bend strength at low temperatures (100~ is 2.5-9.0 N/mm 2 and at II00-1300~ does not exceed 2-11 N/mm 2 [13]. After 6 months of operation in the upper portion of the stack the bend strength of chamotte concrete with a binder of alumina cement (25%) was 1.4 N/mm 2 [14].In this work mullite-corundum chamotte with 77.5 wt.% A1203 from Zaporozh'e Refractory Plant, black silicon carbide, and high-alumina cement were used for production of the concrete.Potassium carbonate was used as the reagent in determination of alkali-resistance. During selection of the grain size composition it was established that a high bulk density of the compound (2.11-2.16 g/cm 3) is obtained with the use of chamotte of four fractions.The silicon carbide was added in a quantity providing sufficiently high alkali resistance under high temperature conditions and having little influence on the change in mechanical properties of the concrete.The maximum quantity of high-alumina cementdid not exceed 15%. This is the result of the necessity of providing high strength of the concrete in hardening and preservation of sufficient strength at the service temperature [14].In heating of concrete containing high-alumina cement its strength drops up to 1000-1100~ At temperatures of intense sintering the strength increases somewhat.The concrete specimens for conducting the investigations were prepared in the form of a normal brick on a vibrotable with a vibration frequency of 2800 per min and an amplitude of imm.After 7 days of hardening the open porosity of the concrete was about 20%, the apparent density 2.32 g/cm 3, and the compressive strength more than 45 N/mm =. Hardening of the concrete occurs as the r...
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