Microwave-assisted Carbothermal Reduction The Prepared Vanadium Nitride And Its Mechanism

Vanadium nitride is widely used as an important steelmelting additive in steelmaking industry of III high strength steel. It takes the place of vanadium iron for its high utilization ratio of vanadium which can reduce the usage of construct steel for 6 million ton every year. Compared with the convention method of vanadium nitride preparation, microwave-high temperature technique has many advantages such as high reaction rate, high efficiency, short process and stable quality of product et al. In the present work, microwave carbonthermal nitridation method under the condition of nitrogen at atmospheric pressure was used to the synthesis of vanadium nitride from vanadium pentoxide and carbon black. Several factors were discussed on the apparent density of the vanadium nitride; Response surface methodology (RSM) based on the central composite design (CCD) was introduced to study the effects on nitride content and vanadium content of the samples; Thermogravimetric Analysis was conducted to study the procedure of the deoxidization process. The main contents and conclusions are as follow:The effects of synthesis temperature, flowing rate of nitrogen, heating rate and soaking time on the apparent density and the nitrogen content of the microwave sintered samples were studied. The experimental data indicated that the apparent density and the nitrogen content of the microwave sinter sample was 4.1 g/cm3 and 13.8%, respectively under the optimum conditions of flowing rate of nitrogen 50 L/h, heating rate of 6℃/min to 1400℃and soaking time of 50 min. The denser pellet was preferred to generated in the centre of the samples of which was beneficial to the expulsion of stoma and the diffusion of nitrogen due to the opposite temperature gradient compared with traditional heating. Densification of the sample depends on the nitrogen diffusion and vanadium nitride production rate. Microwave heating causes different temperature gradient in the sample, so the pellet inside is denser than that of the outside, which helps the expulsion of stoma from the sample and also enhances the diffusion of nitrogen, which accelerates the reaction rate. No density addition agent is used in the present study and the apparent density of vanadium nitride is 4.1g/cm3 that is applicable in micro alloy steel industry.Response surface methodology (RSM) based on the central composite design (CCD) was conducted to study the effects on nitride content and vanadium content of the samples, such as reaction temperature (1300-1400℃), reaction time (230-300min) and carbon addition (0.25-0.29). Quadratic models were developed correlate the synthesis variables to the two responses. According to analysis of variance (ANOVA) results, the significant factors on each experimental design response were found to be carbon addition and reaction temperature. The process optimization was then analyzed by "Design Expert" software and the optimum conditions were obtained as follows:reaction temperature of 1400℃, reaction time of 300 min and carbon addition of 0.29, resulting in 15.83 wt.% of nitride content and 80.99 wt.% of vanadium content which was close with the predicted value.Based on thermogravimetric analysis, reduction rates of vanadium pentoxide by convention heating and microwave heating were studied and compared. The results indicated that there are four stages of reduction while using convention heating, the apparent activation energy of them are 85.70 KJ/mol,130.90 KJ/mol,251.77 KJ/mol, 323.96KJ/mol, respectively. Compared with others, it is small offset of the reduction rate peak value. Secondly, different microwave input power were studied on the effect of vanadium pentoxide reduction. The results proved that reduction rate was increased as the input power and microwave irradiation time increased. At last, microwave carbonthermal nitridation process was observed under the condition of 16 input power. While the sample was irradiation for 115min, the carbonthermal nitridation took place. From the XRD analysis, vanadium nitride was obtained by the reduction and nitridation of vanadium trioxide. Under this input power, the sample contained 76wt.% of vanadium nitride and 24wt.% of vanadium trioxide. It means the nitride content was 16.36wt.%.In conclusion, the present work introduces RSM and TGA to optimize the synthesis process, reaction time was shorten to 50% and the nitride content was improved 2wt.%. The simulating results also show the course of microwave carbonthermal nitridation which provides a theoretical basis for the optimization of the synthesis process.