Effect of the silicon content in the initial alloy on synthesis of silicon nitride in burning ferrosilicon in nitrogen

Abstract

Nitriding of the industrial alloy FS 65 in conditions of self-propagated high-temperature synthesis was investigated. The effect of the phase composition of the initial alloy on the combustion process was established. The dependence of the degree of nitriding of the products of combustion on the nitrogen pressure, sample diameter, and dilution of the initial batch with inert and activating additives was investigated. Silicon nitride with a 0.07% residual iron content was obtained by the method of acid concentration.The possibility of obtaining silicon nitride by the method of self-propagated high-temperature synthesis (SHS) using industrial ferrosilicon FS 75 or the industrial powder (PUD 75) that is obtained when this alloy is ground was previously demonstrated (RF Patent No. 2257338) [1]. Calculation of the economic effectiveness of obtaining silicon nitride from ferrosilicon powder wastes (PUD 75) showed that the cost of the silicon nitride obtained is several times lower than the stabilized market price. Ferroalloy plants currently manufacture other high-silicon alloys in addition to ferrosilicon FS 75, including ferrosilicon with a 63 -68% 3 silicon content (FS 65). When FS 65 is crushed, industrial powder with a 65 -69% silicon content is produced and must be utilized. If we consider that hundreds of tons of powdered ferrosilicon wastes are formed each year at ferroalloy plants, organization of more exhaustive processing of the wastes to obtain a valuable expensive product should be considered economically expedient.An analysis of the published data showed that the mechanisms of combustion of FS alloy FS 65 in nitrogen have not been established. RF Patent No. 2218440 showed that the product of combustion of FS 65 in nitrogen is multiphase, with a low degree of nitriding. The product obtained (nitrided ferrosilicon) is an alloying material and can be used for manufacturing high-nitrogen steels.The results of a study of the mechanisms of combustion of ferrosilicon FS 65 in gaseous nitrogen are reported and the conditions of synthesis of combustion products with the maximum possible degree of nitriding are determined in the present article.Industrial ferrosilicon FS 65 contains 66.7% silicon, and the remainder is iron and impurities (GOST 1415-93). The alloy is a two-phase material consisting of silicon and lebeauite, FeSi 2 . As the phase diagram of the Fe -Si system indicates, the FeSi 2 -Si eutectic melts at 1206°C and lebeauite melts at 1220°C. On the whole, the melting point of different brands of ferrosilicon is a function of the silicon content in them. The melting point of alloy FS 65 with a 63 -68% silicon content is 1210 -1250°C, while it is slightly higher (1210 -1315°C) for alloys FS 75 and FS 90 with a 74 -80 and over 89% silicon content. The melting point of silicon is 1415°C.The initial alloy was crushed to dispersion of less than 200 mm and poured into cylindrical tubes 40 -60 mm in diameter with gas-permeable walls. Before synthesis, the initial ferrosilicon was dried in a vacuum drying chamb...

Fabrication and Characterization of Short Silicon Nitride Fibers from Direct Nitridation of Ferrosilicon in N2 Atmosphere

“…Themicroscopic feature revealed the characteristics of the VLS mechanism [ 14 , 19 , 20 , 21 ]. The liquid phase of ferrosilicon alloy necessary for VLS growth appearedwhen FeSi 2 -Si eutectic melts at 1206 °C which is much lower than melting point of pure silicon [ 22 ]. Therefore, the possible growth mechanism of silicon nitride fiber is VLS mechanism.…”

“…In the initial stage, the liquid eutectic Fe-Si phase formed as a droplet at 1206 °C [ 22 ] which is much lower than the nitradation temperature. Such a liquid droplet is a preferred site for deposition of Si vapor and N 2 [ 19 ], then the Si vapor and N 2 in the high temperature diffused into Fe-Si droplets, further forming eutectic Fe-Si-N liquid droplets [ 23 ].…”