Preparation of Vanadium Nitride Using a Thermally Processed Precursor with Coating Structure

: A new effective method is proposed to prepare vanadium nitride (VN) via carbothermal reduction–nitridation (CRN) of the precursor, obtained by adding carbon black (C) to the stripping solution during the vanadium recovery from black shale. VN was successfully prepared at a low temperature of 1150 ◦ C for only 1 h with a C/V 2 O 5 mass ratio of 0.30 in N 2 atmosphere, but a temperature of 1300–1500 ◦ C is required for several hours in the traditional CRN method. The low synthesis temperature and short period for the preparation of VN was due to the vanadium-coated carbon structure of the precursor, which enlarged the contact area between reactants signiﬁcantly and provided more homogeneous chemical composition. In addition, the simultaneous direct reduction and indirect reduction of the interphase caused by the coating structure obviously accelerated the reaction. The phase evolution of the precursor was as follows: (NH 4 ) 2 V 6 O 16 · 1.5H 2 O → V 2 O 5 → V 6 O 13 → VO 2 → V 4 O 7 → V 2 O 3 → VC → VN. The precursor converted to V 6 O 13 and VO 2 completely after being calcined at 550 ◦ C, indicating that the pre-reduction of V 2 O 5 in the traditional CRN method can be omitted. This method combined the synthesis of VN with the vanadium extraction creatively, having the advantages of simple reaction conditions, low cost and short processing time.


Introduction
Vanadium nitride (VN) has received increasing attention in recent years due to its typical properties including extreme hardness, high melting point, wear and corrosion resistance, good electric and thermal conductivity, high-temperature stability, as well as good catalytic activity. It can be widely used as iron and steel additive [1,2], electrode [3,4], catalyst [5,6], superconductor [7], and coating [8,9]. The frequent application of VN, as an important steel additive, can be ascribed to its beneficial effect on fine grain and precipitation strengthening, which can improve the comprehensive performance of steel and reduce the cost of the smelting process by using nitrogen to reduce the vanadium content of steel [10].
The traditional procedure for the synthesis of VN is the process of carbothermal reduction-nitridation (CRN) of V 2 O 5 in N 2 at a high temperature of 1500 • C for 3 h with a N 2 flow rate of 13.3 × 10 −6 m 3 · s −1 [11], and at 1400 • C with a N 2 flow rate of 50 L·h −1 , using a microwave method [12]. The low melting point (670 • C) and the high saturation vapor pressure of V 2 O 5 will result in a volatile loss of V 2 O 5 near its melting point. Hence, the pre-reduction during transformation from V 2 O 5 to a high melting point and low-valent vanadium oxides below the melting point of V 2 O 5 is necessary [13], resulting inevitably in a long reaction period. However, using the low-valent vanadium oxides will increase the production cost due to the higher price compared to V 2 O 5 . In addition to the traditional CRN method, VN was prepared by thermal liquid-solid reaction [14]: Ammonolysis of precursor compounds of metal [7], self-propagating high-temperature synthesis [15], sol-gel method [16], chemical vapor deposition [17], mechanical alloying [18] and other methods [19,20]. However, most methods are plagued by the presence of air-sensitive or toxic reagents, expensive equipment and raw materials, elevated temperatures and long reaction time, which limit its large-scale production and application. Therefore, there is a need for a simple synthesis route of VN. Thermal processing precursor, as a novel method to prepare metal nitrides, has gradually become a research hotspot. Zhao et al. [21] synthesized VN nanopowders by thermal nitridation of the precursor obtained by physically drying an aqueous solution of ammonium vanadate (NH 4 VO 3 ) and nanometer carbon black. (Cr,V) 2 (C,N) solid solution powders were synthesized by CRN of the precursor, prepared by heating admixtures of ammonium dichromate ((NH 4 ) 2 Cr 2 O 7 ), NH 4 VO 3 , and carbon black in distilled water with continuous stirring [22]. These methods can reduce the synthesizing temperature greatly, but the production cost remained high due to the use of de-ionized water, NH 4 VO 3 and nanometer carbon black, indicating the strong demand of a simple, low-cost and low-temperature approach to synthesize VN. Moreover, the formation process and microstructure of the precursor and its promoting effect on the preparation of metal nitrides were not discussed.
In China, most of the vanadium resources exist in the form of black shale, so the utilization and exploitation of black shale is an essential way to recover vanadium [23]. The vanadium recovery from black shale was investigated using a pyro-hydrometallurgical process specifically including roasting, acid leaching, purification, and chemical precipitation [24][25][26]. Solvent extraction was the most commonly adopted process to purify the acid leaching solution for better selectivity and economy [27,28]. After solvent extraction and stripping, vanadium can be precipitated from the stripping solution with acidic ammonium salt [29]. In view of the preparation method of precursor [7,21,22] in the synthesis of VN, and the application of the precipitation method [30] to synthesize Pb(Mg 1/3 Nb 2/3 )O 3 ceramic, it may be feasible to prepare the precursor during the vanadium precipitation from the stripping solution in the process of vanadium extraction from black shale, to synthesize VN. This method revealed the merits of a low reaction temperature, short period, and a widely available, cheap vanadium source, which make it more practical, economical and efficient in industrial applications.
In this study, we combined the synthesis of VN with vanadium extraction from black shale for the first time by adding carbon black (C) to the stripping solution. First, the precursor containing the vanadium source and reducing material formed gradually during the vanadium precipitation, then the precursor was reduced and nitrided in the N 2 atmosphere to yield VN. The effects of the main reaction conditions on the nitrogen content (N content) in VN product were investigated. In order to explain the lower temperature and shorter time for the preparation of VN in comparison with the conventional CRN method: The phase and microstructure of the precursor were analyzed. In addition, the mechanism of preparing VN from the precursor was discussed through phase analysis, thermodynamic calculation, and thermogravimetric (TG) experiment.

Materials
The stripping solution was obtained by blank roasting, acid leaching, solvent extraction and stripping of black shale obtained from Tongshan, China. The main chemical composition of the stripping solution is given in Table 1, the concentration of V in the stripping solution is 20.63 g·L −1 , while the major impurity ion is Al with the content of 5.78 g·L −1 . The pH of the stripping solution is 0.03. Carbon black containing 94.80% of carbon with the particle size of less than 0.074 mm was used as the reducing agent.

Typical Procedure for the Preparation of VN
The process flow sheet to prepare VN is shown in Figure 1. Typically, sodium chlorate (NaClO 3 , AR (analytical reagent), 2.95 g) was added to the stripping solution (200 mL) to oxidize vanadium (IV) to vanadium (V) in a temperature-controlled magnetic stirrer (SZCL-2A, Wuhan Keer Instrument Co., Ltd., Wuhan, China) at room temperature. After complete oxidation, a certain amount of carbon black was added to the solution and dispersed by stirring. The pH of the solution was adjusted to 1.8 with ammonia (AR), and then the solution was stirred to precipitate ammonium polyvanadate (APV) at 90 • C for 1 h. After solid-liquid separation by vacuum filtration (model SHB-III, Wuhan Keer Instrument Co., Ltd., Wuhan, China), the precursor was washed by water, dried in an oven, and then compressed into cylindrical blocks of 30 mm diameter at a pressure of 14 MPa. The cylindrical block was placed into the tubular atmosphere furnace (SGL-1700, Shanghai Institute of Optics and Fine Mechanics, Shanghai, China) with a certain flow rate of nitrogen (99.999%). The sample was calcined to deaminize at 550 • C for 20 min and pre-reduced at 650 • C for 2.5 h, and then heated to the desired temperature for reduction and nitridation for a certain of time. Subsequently, the product was cooled to room temperature in nitrogen atmosphere.

Typical Procedure for the Preparation of VN
The process flow sheet to prepare VN is shown in Figure 1. Typically, sodium chlorate (NaClO3, AR (analytical reagent), 2.95 g) was added to the stripping solution (200 mL) to oxidize vanadium (IV) to vanadium (V) in a temperature-controlled magnetic stirrer (SZCL-2A, Wuhan Keer Instrument Co., Ltd., Wuhan, China) at room temperature. After complete oxidation, a certain amount of carbon black was added to the solution and dispersed by stirring. The pH of the solution was adjusted to 1.8 with ammonia (AR), and then the solution was stirred to precipitate ammonium polyvanadate (APV) at 90 °C for 1 h. After solid-liquid separation by vacuum filtration (model SHB-III, Wuhan Keer Instrument Co., Ltd., Wuhan, China), the precursor was washed by water, dried in an oven, and then compressed into cylindrical blocks of 30 mm diameter at a pressure of 14 MPa. The cylindrical block was placed into the tubular atmosphere furnace (SGL-1700, Shanghai Institute of Optics and Fine Mechanics, Shanghai, China) with a certain flow rate of nitrogen (99.999%). The sample was calcined to deaminize at 550 °C for 20 min and pre-reduced at 650 °C for 2.5 h, and then heated to the desired temperature for reduction and nitridation for a certain of time. Subsequently, the product was cooled to room temperature in nitrogen atmosphere.

Material Characterization
The nitrogen content (N content) was determined employing distillation-acid base titration analysis. The X-ray diffraction (XRD) patterns were obtained by using a Rigaku D/MAX-RB x-ray diffractometer (Rigaku, Akishima, Japan) with Cu Kα radiation to analyze the phase compositions in the products. Microscopic observation and elemental analysis (SEM-EDS; SEM: Scanning electron microscope; EDS: Energy-dispersive X-Ray spectroscope) was conducted using a JSM-IT300 scanning electron microscope (JEOL, Tokyo, Japan), equipped with an X-ACT energy dispersive spectrometer (Oxford Instruments, Oxford, UK). The thermogravimetric (TG) experiment was carried out by utilizing a STA449C analyzer (Netzsch, Selb, Germany) to change from room temperature to 1400 °C with a linear heating rate of 10 °C·min −1 under N2, and a gas flow rate of 50 mL·min −1 .

Material Characterization
The nitrogen content (N content) was determined employing distillation-acid base titration analysis. The X-ray diffraction (XRD) patterns were obtained by using a Rigaku D/MAX-RB X-ray diffractometer (Rigaku, Akishima, Japan) with Cu Kα radiation to analyze the phase compositions in the products. Microscopic observation and elemental analysis (SEM-EDS; SEM: Scanning electron microscope; EDS: Energy-dispersive X-Ray spectroscope) was conducted using a JSM-IT300 scanning electron microscope (JEOL, Tokyo, Japan), equipped with an X-ACT energy dispersive spectrometer (Oxford Instruments, Oxford, UK). The thermogravimetric (TG) experiment was carried out by utilizing a STA449C analyzer (Netzsch, Selb, Germany) to change from room temperature to 1400 • C with a linear heating rate of 10 • C·min −1 under N 2 , and a gas flow rate of 50 mL·min −1 .

Effect of C/V 2 O 5 Mass Ratio
The effect of C/V 2 O 5 mass ratio (m(C:V 2 O 5 )) on the N content in VN product is shown in Figure 2, obtained at a reaction temperature of 1250 • C, a reaction time of 3 h, and a N 2 flow rate of 300 mL· min −1 .

Effect of C/V2O5 Mass Ratio
The effect of C/V2O5 mass ratio (m(C:V2O5)) on the N content in VN product is shown in Figure  2, obtained at a reaction temperature of 1250 °C, a reaction time of 3 h, and a N2 flow rate of 300 mL·min −1 . However, there were no peaks that could be indexed to C or VC in Figure 2b, which was due to the amorphous state of carbon black and very similar diffraction patterns of VC and VN [31], respectively. Therefore, 0.30 was considered as the most suitable C/V2O5 mass ratio for the reaction.

Effect of Reaction Temperature
To investigate the effect of the reaction temperature on the N content in VN product, several experiments were performed at a C/V2O5 mass ratio of 0.30, a reaction time of 3 h and a N2 flow rate of 300 mL·min −1 . The experimental results are depicted in Figure 3. However, there were no peaks that could be indexed to C or VC in Figure 2b, which was due to the amorphous state of carbon black and very similar diffraction patterns of VC and VN [31], respectively. Therefore, 0.30 was considered as the most suitable C/V 2 O 5 mass ratio for the reaction.

Effect of Reaction Temperature
To investigate the effect of the reaction temperature on the N content in VN product, several experiments were performed at a C/V 2 O 5 mass ratio of 0.30, a reaction time of 3 h and a N 2 flow rate of 300 mL·min −1 . The experimental results are depicted in Figure 3. As shown in Figure 3a, the N content in VN product increased rapidly from 4.69% to 16.70% as the reaction temperature increased from 950 °C to 1150 °C; the N content then stayed at a constant level over the elevated reaction temperature. Figure 3b exhibits XRD patterns of the products obtained at different temperatures. At 950 °C, the peaks were identified as V2O3 and VN, indicating that the reduction and nitridation of the precursor were not complete at a temperature as low as 950 °C. The XRD patterns were identical and all diffraction peaks corresponded to VN at 1150 °C and 1250 °C, revealing that the precursor was reduced and nitrided almost completely. Hence, a reaction temperature of 1150 °C was chosen as the optimal condition in this experiment.

Effect of Reaction Time
The effect of reaction time on the N content in VN product was investigated under the conditions of a C/V2O5 mass ratio of 0.30, a reaction temperature of 1150 °C and a N2 flow rate of 300 mL·min −1 . The results are shown in Figure 4. It can be observed that the N content in VN product increased strongly from 13.80% to 16.38% as the reaction time increased from 0.5 h to 1 h and then remained almost constant beyond 1 h, which As shown in Figure 3a, the N content in VN product increased rapidly from 4.69% to 16.70% as the reaction temperature increased from 950 • C to 1150 • C; the N content then stayed at a constant level over the elevated reaction temperature. Figure 3b exhibits XRD patterns of the products obtained at different temperatures. At 950 • C, the peaks were identified as V 2 O 3 and VN, indicating that the reduction and nitridation of the precursor were not complete at a temperature as low as 950 • C. The XRD patterns were identical and all diffraction peaks corresponded to VN at 1150 • C and 1250 • C, revealing that the precursor was reduced and nitrided almost completely. Hence, a reaction temperature of 1150 • C was chosen as the optimal condition in this experiment.

Effect of Reaction Time
The effect of reaction time on the N content in VN product was investigated under the conditions of a C/V 2 O 5 mass ratio of 0.30, a reaction temperature of 1150 • C and a N 2 flow rate of 300 mL·min −1 . The results are shown in Figure 4.  As shown in Figure 3a, the N content in VN product increased rapidly from 4.69% to 16.70% as the reaction temperature increased from 950 °C to 1150 °C; the N content then stayed at a constant level over the elevated reaction temperature. Figure 3b exhibits XRD patterns of the products obtained at different temperatures. At 950 °C, the peaks were identified as V2O3 and VN, indicating that the reduction and nitridation of the precursor were not complete at a temperature as low as 950 °C. The XRD patterns were identical and all diffraction peaks corresponded to VN at 1150 °C and 1250 °C, revealing that the precursor was reduced and nitrided almost completely. Hence, a reaction temperature of 1150 °C was chosen as the optimal condition in this experiment.

Effect of Reaction Time
The effect of reaction time on the N content in VN product was investigated under the conditions of a C/V2O5 mass ratio of 0.30, a reaction temperature of 1150 °C and a N2 flow rate of 300 mL·min −1 . The results are shown in Figure 4. It can be observed that the N content in VN product increased strongly from 13.80% to 16.38% as the reaction time increased from 0.5 h to 1 h and then remained almost constant beyond 1 h, which It can be observed that the N content in VN product increased strongly from 13.80% to 16.38% as the reaction time increased from 0.5 h to 1 h and then remained almost constant beyond 1 h, which indicated that the reduction and nitridation of the precursor was almost complete at 1 h. Thus, the optimum reaction time was determined to be 1 h.

Effect of N 2 Flow Rate
The effect of N 2 flow rate on the N content in VN product was investigated under the conditions of a C/V 2 O 5 mass ratio of 0.30, a reaction temperature of 1150 • C and a reaction time of 1 h. The results are shown in Figure 5. indicated that the reduction and nitridation of the precursor was almost complete at 1 h. Thus, the optimum reaction time was determined to be 1 h.

Effect of N2 Flow Rate
The effect of N2 flow rate on the N content in VN product was investigated under the conditions of a C/V2O5 mass ratio of 0.30, a reaction temperature of 1150 °C and a reaction time of 1 h. The results are shown in Figure 5.  Figure 5 indicates that the N2 flow rate substantially influenced the N content in VN product. The N content first rose from 13.73% to 16.38% and then decreased to 15.43% with an increasing N2 flow rate. There was an optimum N2 flow rate of 300 mL·min −1 , at which a maximum N content was achieved. The flowing nitrogen can increase N2 partial pressure and decrease the CO partial pressure, which might accelerate the reduction and nitridation reactions. However, the rapid flowing nitrogen could not be sufficiently heated, and the contact time with vanadium oxides was too short to react adequately, resulting in decreased N content as the flow rate increased further. Therefore, 300 mL·min −1 was the optimal N2 flow rate.

Phase and Microstructure Analyses of the Precursor
Although an appropriate flow rate of nitrogen can promote the synthesis of VN, high reaction temperature and long reaction time were still required in the synthesis of VN with the flow of nitrogen in the tubular furnace [11,12]. Compared with a temperature of 1300-1500 °C required to prepare VN by the traditional CRN method, this study permitted a lower temperature and a shorter time by as much as several hundred degrees centigrade and several hours, respectively. The traditional raw materials were a mixture of V2O5 and C, while a precursor with V-coated C structure was applied in our research, a fact we consider an essential difference. Hence, we had to investigate the precursor. To understand the phase compositions of the precursor, XRD analysis was conducted. The result shown in Figure 6 revealed that the precursor consists of (NH4)2V6O16·1.5H2O with a small amount of NH4Al(SO4)2·12H2O. No peaks can be indexed to C, due to the amorphous state of carbon black.  Figure 5 indicates that the N 2 flow rate substantially influenced the N content in VN product. The N content first rose from 13.73% to 16.38% and then decreased to 15.43% with an increasing N 2 flow rate. There was an optimum N 2 flow rate of 300 mL·min −1 , at which a maximum N content was achieved. The flowing nitrogen can increase N 2 partial pressure and decrease the CO partial pressure, which might accelerate the reduction and nitridation reactions. However, the rapid flowing nitrogen could not be sufficiently heated, and the contact time with vanadium oxides was too short to react adequately, resulting in decreased N content as the flow rate increased further. Therefore, 300 mL·min −1 was the optimal N 2 flow rate.

Phase and Microstructure Analyses of the Precursor
Although an appropriate flow rate of nitrogen can promote the synthesis of VN, high reaction temperature and long reaction time were still required in the synthesis of VN with the flow of nitrogen in the tubular furnace [11,12]. Compared with a temperature of 1300-1500 • C required to prepare VN by the traditional CRN method, this study permitted a lower temperature and a shorter time by as much as several hundred degrees centigrade and several hours, respectively. The traditional raw materials were a mixture of V 2 O 5 and C, while a precursor with V-coated C structure was applied in our research, a fact we consider an essential difference. Hence, we had to investigate the precursor. To understand the phase compositions of the precursor, XRD analysis was conducted. The result shown in Figure 6 revealed that the precursor consists of (NH 4 ) 2 V 6 O 16 ·1.5H 2 O with a small amount of NH 4 Al(SO 4 ) 2 ·12H 2 O. No peaks can be indexed to C, due to the amorphous state of carbon black. To have a visualized comprehension of the distribution of (NH4)2V6O16·1.5H2O and carbon black in the precursor, SEM-EDS analysis was conducted and the results are presented in Figure 7. As shown in Figure 7a, there was C in the center and V was plentiful around C in a single particle, which indicated that the carbon black was surrounded by vanadium during the process of vanadium precipitation. Figure 7b is a partially enlarged view of the edge of the particle shown in Figure 7a. It can be clearly seen that V is located at the edge of the particle, with C being in the interior. There were many vanadium-coated carbon structures (V-coated C structures), as shown in the red circles in Figure 7c. The formation process of the V-coated C structure is schematically illustrated in Figure 8, where the black parts and orange parts represent carbon black and APV, respectively. Due to the presence of the dispered carbon black, the nuclei of the vanadium precipitates in the slurry grow on the surface of carbon black, which served as the heterogeneous nucleation site, instead of forming discrete precipitates. As the precipitation proceeded, APV particles grew gradually and coated carbon black completely, the typical coated structure formed with the core and the coating layer was carbon black and APV, respectively; the uncoated carbon black located around APV. The overall distribution of the precursor was the V-coated C structure which was surrounded by the uncoated carbon black. To have a visualized comprehension of the distribution of (NH 4 ) 2 V 6 O 16 ·1.5H 2 O and carbon black in the precursor, SEM-EDS analysis was conducted and the results are presented in Figure 7. As shown in Figure 7a, there was C in the center and V was plentiful around C in a single particle, which indicated that the carbon black was surrounded by vanadium during the process of vanadium precipitation. Figure 7b is a partially enlarged view of the edge of the particle shown in Figure 7a. It can be clearly seen that V is located at the edge of the particle, with C being in the interior. There were many vanadium-coated carbon structures (V-coated C structures), as shown in the red circles in Figure 7c. To have a visualized comprehension of the distribution of (NH4)2V6O16·1.5H2O and carbon black in the precursor, SEM-EDS analysis was conducted and the results are presented in Figure 7. As shown in Figure 7a, there was C in the center and V was plentiful around C in a single particle, which indicated that the carbon black was surrounded by vanadium during the process of vanadium precipitation. Figure 7b is a partially enlarged view of the edge of the particle shown in Figure 7a. It can be clearly seen that V is located at the edge of the particle, with C being in the interior. There were many vanadium-coated carbon structures (V-coated C structures), as shown in the red circles in Figure 7c. The formation process of the V-coated C structure is schematically illustrated in Figure 8, where the black parts and orange parts represent carbon black and APV, respectively. Due to the presence of the dispered carbon black, the nuclei of the vanadium precipitates in the slurry grow on the surface of carbon black, which served as the heterogeneous nucleation site, instead of forming discrete precipitates. As the precipitation proceeded, APV particles grew gradually and coated carbon black completely, the typical coated structure formed with the core and the coating layer was carbon black and APV, respectively; the uncoated carbon black located around APV. The overall distribution of the precursor was the V-coated C structure which was surrounded by the uncoated carbon black. The formation process of the V-coated C structure is schematically illustrated in Figure 8, where the black parts and orange parts represent carbon black and APV, respectively. Due to the presence of the dispered carbon black, the nuclei of the vanadium precipitates in the slurry grow on the surface of carbon black, which served as the heterogeneous nucleation site, instead of forming discrete precipitates. As the precipitation proceeded, APV particles grew gradually and coated carbon black completely, the typical coated structure formed with the core and the coating layer was carbon black and APV, respectively; the uncoated carbon black located around APV. The overall distribution of the precursor was the V-coated C structure which was surrounded by the uncoated carbon black. The V-coated C structure evidently promoted the reaction between vanadium oxide and carbon black due to the following reasons. First, on the internal interphase of the V-coated C structure, CO produced by direct reduction of vanadium oxides by carbon black could not escape due to the coated structure; hence, it continued to react with vanadium oxide by indirect reduction. Subsequently, CO2 produced by indirect reduction will react with carbon black to produce CO at high temperature. That is to say, inside the vanadium oxides, the direct reduction and indirect reduction occur simultaneously at the internal interphase. Secondly, outside the vanadium oxides, the uncoated C reacted with vanadium oxides simultaneously on the external interphase. Lastly, the V-coated C structure increased the contact area between carbon black and vanadium oxides remarkably and provided a more homogeneous chemical composition than traditional mechanical blending. APV decomposed into V2O5 and V2O5 was reduced and nitrided gradually during the synthesis of VN. The coating structure always existed with the coating layer changing from APV to lower-valent vanadium oxides, vanadium carbide and vanadium nitride, until the precursor converted into VN completely. Hence, the V-coated C structure obtained by adding carbon black to the stripping solution before the precipitation of vanadium significantly reduced the reaction temperature and shortened the reaction time.

Phase Evolution of Preparing VN from the Precursor
The progress of the phase formation during the preparation of VN was monitored by interrupting the reaction intermittently at different temperatures and then the samples were analyzed by XRD analysis. Figure 9 shows the XRD patterns of the precursor and products at different temperatures. The precursor was composed of (NH4)2V6O16·1.5H2O and a small amount of NH4Al(SO4)2·12H2O as shown in curve (a). The diffraction peaks were identified as V6O13 and VO2 [32][33][34][35] in curve (b), attributed to the decomposition of (NH4)2V6O16·1.5H2O into V2O5 and NH3; then, the V2O5 was initially reduced to V6O13 and VO2 by NH3 under 550 °C. As shown in curve (c), all peaks corresponded to The V-coated C structure evidently promoted the reaction between vanadium oxide and carbon black due to the following reasons. First, on the internal interphase of the V-coated C structure, CO produced by direct reduction of vanadium oxides by carbon black could not escape due to the coated structure; hence, it continued to react with vanadium oxide by indirect reduction. Subsequently, CO 2 produced by indirect reduction will react with carbon black to produce CO at high temperature. That is to say, inside the vanadium oxides, the direct reduction and indirect reduction occur simultaneously at the internal interphase. Secondly, outside the vanadium oxides, the uncoated C reacted with vanadium oxides simultaneously on the external interphase. Lastly, the V-coated C structure increased the contact area between carbon black and vanadium oxides remarkably and provided a more homogeneous chemical composition than traditional mechanical blending. APV decomposed into V 2 O 5 and V 2 O 5 was reduced and nitrided gradually during the synthesis of VN. The coating structure always existed with the coating layer changing from APV to lower-valent vanadium oxides, vanadium carbide and vanadium nitride, until the precursor converted into VN completely. Hence, the V-coated C structure obtained by adding carbon black to the stripping solution before the precipitation of vanadium significantly reduced the reaction temperature and shortened the reaction time.

Phase Evolution of Preparing VN from the Precursor
The progress of the phase formation during the preparation of VN was monitored by interrupting the reaction intermittently at different temperatures and then the samples were analyzed by XRD analysis. Figure 9 shows the XRD patterns of the precursor and products at different temperatures. The V-coated C structure evidently promoted the reaction between vanadium oxide and carbon black due to the following reasons. First, on the internal interphase of the V-coated C structure, CO produced by direct reduction of vanadium oxides by carbon black could not escape due to the coated structure; hence, it continued to react with vanadium oxide by indirect reduction. Subsequently, CO2 produced by indirect reduction will react with carbon black to produce CO at high temperature. That is to say, inside the vanadium oxides, the direct reduction and indirect reduction occur simultaneously at the internal interphase. Secondly, outside the vanadium oxides, the uncoated C reacted with vanadium oxides simultaneously on the external interphase. Lastly, the V-coated C structure increased the contact area between carbon black and vanadium oxides remarkably and provided a more homogeneous chemical composition than traditional mechanical blending. APV decomposed into V2O5 and V2O5 was reduced and nitrided gradually during the synthesis of VN. The coating structure always existed with the coating layer changing from APV to lower-valent vanadium oxides, vanadium carbide and vanadium nitride, until the precursor converted into VN completely. Hence, the V-coated C structure obtained by adding carbon black to the stripping solution before the precipitation of vanadium significantly reduced the reaction temperature and shortened the reaction time.

Phase Evolution of Preparing VN from the Precursor
The progress of the phase formation during the preparation of VN was monitored by interrupting the reaction intermittently at different temperatures and then the samples were analyzed by XRD analysis. Figure 9 shows the XRD patterns of the precursor and products at different temperatures. The precursor was composed of (NH4)2V6O16·1.5H2O and a small amount of NH4Al(SO4)2·12H2O as shown in curve (a). The diffraction peaks were identified as V6O13 and VO2 [32][33][34][35] in curve (b), attributed to the decomposition of (NH4)2V6O16·1.5H2O into V2O5 and NH3; then, the V2O5 was initially reduced to V6O13 and VO2 by NH3 under 550 °C. As shown in curve (c), all peaks corresponded to The precursor was composed of (NH 4 ) 2 V 6 O 16 ·1.5H 2 O and a small amount of NH 4 Al(SO 4 ) 2 ·12H 2 O as shown in curve (a). The diffraction peaks were identified as V 6 O 13 and VO 2 [32][33][34][35] in curve (b), attributed to the decomposition of (NH 4 ) 2 V 6 O 16 ·1.5H 2 O into V 2 O 5 and NH 3 ; then, the V 2 O 5 was initially reduced to V 6 O 13 and VO 2 by NH 3 under 550 • C. As shown in curve (c), all peaks corresponded to V 4 O 7 , which indicated that the further reduction of V 6 O 13 and VO 2 to V 4 O 7 may be by the progressive reduction of V 6 O 13 to VO 2 and VO 2 to V 4 O 7 below 650 • C. The appearance of V 2 O 3 and VN in curve (d) illustrates that V 4 O 7 was reduced to V 2 O 3 continuously and the VN phase was formed from V 2 O 3 below 950 • C. There was only a single phase of VN at 1150 • C. Based on the above recorded phase changes, the possible mechanism of preparing VN could be formulated as follows: According to the standard Gibbs free energy changes (∆G θ (T)) of Reactions (8)-(10), calculated by the "Reaction" module of Factsage 7.1 software (Factsage 7.1, Thermfact/CRCT, Montreal, QC, Canada; GTT-Technologies, Aachen, Germany), the VN phase can be formed at 16,575 • C via V 2 O 3 reacting directly with nitrogen under standard state conditions, while the VC phase can be formed at a lower temperature of about 1092 • C. The transformation of VC to VN was spontaneous below 1257 • C. This means that intermediate formation of VC can occur and subsequently convert to VN when the temperature varies between 1092 • C and 1257 • C under standard state conditions and in an extended range of temperatures in flowing nitrogen atmosphere, indicating that 1150 • C satisfies the thermodynamic conditions. Therefore, the phase evolution of (NH 4 ) 2 V 6 O 16 ·1.5H 2 O to VN took place in the following sequential order: It is worth noting that after being heated at 550 • C for 20 min, the precursor converted completely to low-valent vanadium oxides V 6 O 13 and VO 2 without the appearance of residual unreacted V 2 O 5, which has a low melting point (670 • C) and high saturation vapor pressure. V 6 O 13 can decompose to VO 2 at 700 • C, and VO 2 has a high melting point of 1542 • C [36]. Thus, the pre-reduction at 650 • C for 2.5 h can be eliminated in this process.
TG experiments were carried out under N 2 atmosphere to reflect the physical and chemical phenomena in the preparation process of VN. TG and DTG (differential thermogravimetric) curves of the precursor are shown in Figure 10. There were four major weight losses indicated by four DTG peaks in the DTG curve. The first major weight loss between room temperature and 136 • C was attributed to the evaporation of physically absorbed and crystalline water. The second major weight loss from 380 • C to 447 • C was due to the decomposition of APV and the initial reduction of V 2 O 5 to V 6 O 13 and VO 2 , which corresponded to the Reactions (1)-(3). The third weight loss corresponded to the further reduction of V 6 O 13 and VO 2 to V 4 O 7 according to Reactions (4)- (6). In this stage, the absolute value of weight loss rates first increased, then decreased and eventually increased again at the temperatures of 600 • C-653 • C, 653 • C-660 • C and 660 • C-700 • C, respectively. The absolute value of weight loss rate escalated with increasing temperature because the reduction reaction was endothermic and increasing the temperature promoted the reduction. The absolute value of weight loss rate decreased as the reaction progressed. The increase of the weight loss rate was due to the decomposition of V 6 O 13 to VO 2 ; the maximum, absolute value of the weight loss rate at 700 • C corresponded to the decomposition temperature of V 6 O 13 . The last weight loss was assigned to the phase transformation from V 4 O 7 to V 2 O 3 and V 2 O 3 to the desired phase VN, which corresponded to Reactions (7), (9) and (10). corresponded to the decomposition temperature of V6O13. The last weight loss was assigned to the phase transformation from V4O7 to V2O3 and V2O3 to the desired phase VN, which corresponded to Reactions (7), (9) and (10).

Conclusions
A novel synthesis method of a (NH4)2V6O16·1.5H2O precursor to prepare VN was proposed, characterized by adding carbon black to the stripping solution during the vanadium recovery from black shale. VN with an N content of 16.38% was successfully prepared by thermal processing of the precursor at 1150 °C for 1 h with a C/V2O5 mass ratio of 0.30 and a N2 flow rate of 300 mL·min −1 . Low synthesis temperature and short period for the preparation of VN was due to the V-coated C structure of the precursor, which enlarged the contact area between reactants significantly and provided a more homogeneous chemical composition. In addition, the simultaneous direct reduction and indirect reduction at the interphase caused by the coating structure evidently accelerated the reaction. XRD patterns and thermodynamic analysis indicated that the phase evolution from the precursor to VN took place in the following order (NH4)2V6O16·1.5H2O → V2O5 → V6O13 → VO2 → V4O7 → V2O3 → VC → VN. The precursor converted to V6O13 and VO2 completely after being calcined at 550 °C, indicating that the pre-reduction of V2O5 in the traditional CRN method can be dispensed with. Therefore, this method is an effective, simple and low-cost route to prepare VN.

Conclusions
A novel synthesis method of a (NH 4 ) 2 V 6 O 16 ·1.5H 2 O precursor to prepare VN was proposed, characterized by adding carbon black to the stripping solution during the vanadium recovery from black shale. VN with an N content of 16.38% was successfully prepared by thermal processing of the precursor at 1150 • C for 1 h with a C/V 2 O 5 mass ratio of 0.30 and a N 2 flow rate of 300 mL·min −1 . Low synthesis temperature and short period for the preparation of VN was due to the V-coated C structure of the precursor, which enlarged the contact area between reactants significantly and provided a more homogeneous chemical composition. In addition, the simultaneous direct reduction and indirect reduction at the interphase caused by the coating structure evidently accelerated the reaction. XRD patterns and thermodynamic analysis indicated that the phase evolution from the precursor to VN took place in the following order (NH 4 The precursor converted to V 6 O 13 and VO 2 completely after being calcined at 550 • C, indicating that the pre-reduction of V 2 O 5 in the traditional CRN method can be dispensed with. Therefore, this method is an effective, simple and low-cost route to prepare VN.