Production technology of silicon carbide saggers

Abstract

Silicon carbide saggers (o.d., 510 mm; i.d., 450 mm; height, 307 mm) are used in the production of iron powder as containers in which the scale is reduded by its interaction with a reducing agent in tunnel kilns at I150-I180~The geometric characteristics of the sagger are inconvenient from the standpoint of its production: H/Rh = i0, where H is the height and R h the hydraulic radius (the ratio of twice the cross-sectional area to the perimeter [I, p. 143]) in addition to which the area of the side (full) surface is 9133 cm 2 so that the production of the saggers is a very difficult and time,consuming process.Before the development work reported in this article was carried out the saggers were formed by pneumatic ramming. The properties of the fired saggers were poor: apparent density, 2.22-2.50 g/cmS; cold-crushing strength, only 400-450 kgf/cm2; and density variation, up to 0.2 g/cm 3. In-service tests with several experimental batches of fired pneumorammed saggers produced from mixes of varied composition showed that their durability was 17 service cycles on average.An analysis was carried out of the known methods of molding and several were tested with a view to determining the most efficient method of producing silicon carbide saggers taking into account their special configuration and the necessity to improve their quality.Experiments were carried out in the press-forming of the saggers in a floating mold on a DA-2238 hydraulic press. The composition for the saggers contained 40% silicon carbides Nos. 125 and 80 in the ratio of i/i, 15% silicon carbides Nos. 50 and 16, 30% silicon carbides Nos. 12 and 6, and 15% ground silicon carbide scrap material of particle size smaller than 0.06 mm. Clay LTI was added in the proportion of 10% on 100% together with 5-6% s.s.l, of density 1.26-1.28 g/cm ~ .The LTI clay was added in the proportion of 10% (in place of the 3% for alumlnosilicatebonded silicon carbide products) in order to increase the plasticity of the mix.Saggers press-molded at i000 kgf/cm 2 contained overpressure cracks and their upper part was crushed while lower molding pressures gave saggers with a crumbling lower part.Next, the ratio H/R h was changed and the saggers were molded at a pressure of 800 kgf/ cm ~. The dimensions of the saggers were as follows: o.d., 530 mm; i.d., 450 mm; and height, 260 mm (H/R h = 6~6). The apparent density of these products is given in Table i.In-service testing of the experimental batch of saggers showed that their durability had increased only slightly, i.e., to 20 cycles (Table 2), while the increase in the thickness of the sagger wall resulted in a higher specific consumption of silicon carbide in the production of iron powder.The results of these experiments, data obtained in earlier research [2][3], and the published findings relating to the vibration-molding of products of complex configuration from materials in powder form [4; 5, pp. 101-106] suggested that it would be advantageous to investigate the feasibility of producing the sagger by vibromolding.All-Unl...