Study On Synthesis Of Vanadium Carbonitride By A One-Step Method

Both precipitating and precipitation strengthening of vanadium in steel can be facilitated by increase of nitrogen content, as a result, the steel property is enhanced obviously and the additive quantity of vanadium in steel is reduced from 20 to 40 percent. Therefore, the adding vanadium carbonitride to steel is a reliable and economic method. The purpose of present study is to develop a continuous, high efficiency and energy saving technology for synthesis vanadium carbonitride by an one-step method and based on thermodynamic analysis in the reaction system. The phase change in V₂O₃ carbothermal reduction and nitridation process, and the reaction mechanism as well as kinetic regularity were systematically studied by modern analysis and testing methods. The feed stock flow in rotary reactor was simulated and a large-scale experiment was carried out in order to synthesize vanadium carbonitirde in the rotary reactor. The theory guide for improving technique is provided by the experimental results.It is known by thermodynamic analysis that the V₂O₃ co-existed with C can be reduced and nitrided to VN at high temperature under nitrogen atmosphere with lower partial pressure of oxygen. It can be seen by analyzing the phase equilibrium and predominance area diagrams in V-C-O ternary system that oxide VO can not be co-existed with C and final product is VC if additive carbon is exceeded to chemical stoichiometry. One step method for producing VN can be regarded as coupling process between V₂O₃ carbonization and VC nitrification, by which the synthesis temperature can be decreased. So the one step method is feasible and available.It is showed by analysis of phase equilibrium in V-C-O-N quarternary system and predominance area diagrams that the vanadium carbonitride is formated if the partial pressure of N₂ and CO are controlled strictly, namely, the maxium partial pressure of CO should be less than 89.6kPa. When the VN and VC solid solution is formed, the increasing partial pressure of N₂ or the decreasing partial pressure of CO can improve the VN content in the solid solution, while the temperature increasing is favourable to carbonization but opposite to nitrification.It was showed by TG experiment that the carbothermal reduction of V₂O₃ started in the range of temperature 1140～1540K under argon atmosphere, and the mass loss ratio was 38% which nears to the theoretic ratio of 40.58%; however under nitrogen atmosphere the formation of vanadium carbonitirde from V₂O₃ occurred in the range of 1160～1500K ,and the mass loss ratio was 27.4%. The final temperature of reaction under nitrogen atmosphere was lower than that under argon atmosphere, namely under atmospheric pressure to synthesize vanadium carbonitirde, the one-step method, is in favour of efficency improvement.The phase composition in the system will be converted from V₂O₃ directly to V₈C₇ with the temperature increasing. The oxide VO reduced from V₂O₃ was unstable and then carbonized to form V₈C₇. It was proved by energy spectrum analysis that the oxygen content in the product was decreased with increasing of reduction degree of V₂O₃, and the carbon content was not increased proportionably, since the product was vanadium carbide with vacancy defect. During the carbothermal reduction and nitriding process, the V₂O₃ was disappeared at temperature 1373K, but with the temperature increasing the carbon content was increased, while nitrogen content was increased firstly and then gradually dropped.It was showed by single factor experiment of one-step method under atmospheric pressure that the nitrogen content in the product can be approached more than 10% as reaction parameters as following: raw material ratio (?): 0.276, reactiontemperature: 1400℃, reaction time: 120 min, average granularity of reactants: 1.25mm. Additives such as Fe, CaCO₃, CaO, NH₄Cl and La₂O₃ can increase the nitrogen content in the products.During carbothermal reduction and nitriding process of V₂O₃ in the fixed-bed, the gas diffusion is not rate-limiting step in the reaction process. It was verified by TG experimental data and kinetic analysis that the carbothermal reduction and nitriding process meet the case of first order - phase boundary reaction (Rn model) and kinetic equations at different temperature stage. The kinetic rate equations are expressed as following:The residence times in rotary reactor are consistent with normal distribution approximately, though physical properties of the raw materials have much difference. The effect of operation condition (such as rotation speed and slope angle) and structural characteristics (dam) on average residence time of material is consistent with that of fine homogeneous distribution. Based on particle movement model and introducing correction factorε, the average residence time can be represented as:(?), the calculated value matched experimental value well.The large-scale experiment has the nitrogen content more than 7.8% and the content of carbon and nitrogen in the product conformed with standard Nitrovan product, thereby under atmospheric pressure in the rotary reactor, the one-step method is feasible. Comparing with product of other technics, the vanadium content in product was less than that of standard Nitrovan product, even less consumption of nitrogen and energy. So the basic parameters of the one step method need to be adjusted and optimized.