Study on the Behavior Mechanism of K and Na during the Synthesis of VN Alloy

Abstract

In this paper, the sources of K and Na in vanadium-nitrogen (VN) alloys and their effects on the furnace structure were studied. The results show that K and Na are mainly present as counter-ions of decavanadate and dodecanadate in ammonium polyvanadate (APV) and vanadium trioxide (V₂O₃). In the production process of VN, the concentration of K in the scab produced in the kiln is 25 times higher than that of the vanadium raw material, the concentration of Na is 15 times higher than that of the raw material, and the degree of enrichment of K is stronger than that of Na. Additionally, in the VN products, K and Na are mainly distributed uniformly in the form of KCN and NaCN. The results also found that the K and Na vapors during the production of VN promoted the formation of VN. However, K, Na steam, KCN, NaCN, K₂CO₃, Na₂CO₃ and other potassium-sodium high-temperature corrosive substances cause strong corrosion of the furnace lining, graphite sagger, push plate, and heating elements. Therefore, in the process of synthesizing VN, the raw materials of K and Na content of impurity elements must be reasonably controlled. The results not only analyzed the source of K and Na, but also analyzed the characteristics of their enrichment and the influence on the equipment.

1. Introduction

In the process of steelmaking, nitrogen in steel is considered as a harmful element and strictly controlled. According to the research of material science, when some steels contain a certain number element of metal elements and a certain amount of nitrogen, the performance of the steel can be greatly improved after heat treatment. For example, nitrogen can stabilize austenitic stainless steel and improve its corrosion performance. In V-containing or V and C steel, if a certain amount of N is contained, nitride precipitates are produced after heat treatment, the hardening of the steel is promoted, and the strength of the steel is increased.

In low-alloy high-strength steel, vanadium is formed into fine vanadium carbide or vanadium nitride precipitates to effectively promote the grain refinement and strengthening of the steel [1,2,3]. The vanadium carbide or vanadium nitride phase can hinder the migration of dislocations and grain boundaries, and the strength of the steel is improved. At the same time, the existence of vanadium carbide and vanadium nitride phases can also improve the recrystallization temperature and high-temperature performance of the material. It is reported that the addition of vanadium carbide and vanadium nitride to steel can also improve the wear resistance, corrosion resistance, toughness, ductility, and hardness of the steel, as well as the comprehensive mechanical properties of heat resistance and fatigue resistance [4,5,6]. The nitrogen-rich vanadium carbonitride can provide a convenient method to add V and N at the same time. In high-strength low-alloy steels, the nitrogen in vanadium nitride is more effective in strengthening and refining grains than vanadium carbide.

The method for preparing vanadium carbonitride reported in the literature is a two-step method, that is, after the vanadium carbide is prepared by reduction under vacuum conditions, the vanadium carbide is nitrided to obtain a vanadium carbonitride product [7,8,9]. V₂O₅ and NH₄VO₃ are used, and V₂O₃ or V₂O₅ and NH₄VO₃ are also used as raw materials. It is easy to involve liquid phase components in the preparation process, which is not helpful to the mass transfer of the gas phase, so it needs to undergo a low-temperature pre-forming treatment in advance, and then carry out the subsequent preparation process.

In the process of high-temperature reduction and nitridation, low melting-point volatiles will be produced. When the content of Na₂O + K₂O in the vanadium raw material is high, these substances will affect the normal production. First, these volatiles have a strong corrosive effect on the refractory material, graphite saggar, push plate, and heating element in the furnace, and their service life is shortened. Secondly, these volatiles will cause the vanadium nitride product to be bonded at high temperature, which will increase the powder content in the product crushing process, so the product quality will be significantly affected.

The content of (Na₂O + K₂O) in the vanadium pentoxide raw material specified by the industry is less than or equal to 1.0%. However, the (Na₂O + K₂O) content of the vanadium pentoxide raw materials used in actual production is mostly between 0.6% and 1.5%, and the content of sodium and potassium fluctuates greatly.

Therefore, this paper studies the occurrence law and behavior mechanism of K and Na in the production process of VN from raw materials to final products, so as to obtain the corrosion mechanism and optimization method of K and Na in the production process of VN. Finally, it provides a feasible solution for the selection of VN raw materials in industrial production, and for controlling the behavior of K and Na in the process of VN production and reducing its impact on equipment and product quality.

2. Experimental Method

2.1. Experimental Materials and Instruments

The production raw materials of vanadium nitride are composed of industrial grade vanadium trioxide (V ≥ 63.5%), nitrogen, carbon powder (C ≥ 99.5%), and triethanolamine, the equipment is shown in Figure 1. The morphology of vanadium-nitrogen alloy products was observed by scanning electron microscope (JEOL 5800SV, Tokyo, Japan), the phase structure was analyzed by D/Max-RC fixed copper target X-ray diffractometer (RIGAKU, Tokyo, Japan), and the elemental composition was analyzed by ICP-AES.

2.2. Vanadium-Nitrogen Alloy Production Process

V₂O₃ and graphite carbon powder were mixed according to a certain mass ratio, and a certain proportion of triethanolamine was added to the mixture and mixed for 2–3 h. The mixture was subjected to high-temperature reduction reaction and nitridation reaction after compression molding at 20 MPa. During the reduction and nitridation process, the temperature was controlled between 1000–1500 °C, and the nitrogen pressure was controlled at 700–1000 Pa. After 4–5 h of reaction, the sample was cooled to room temperature under nitrogen atmosphere, and the chemical composition of the product was 77~81% V, 14~18% N, C ≤ 6.0%, P ≤ 0.06%, S ≤ 0.10%. The appearance of the product is shown in Figure 2.

3. Results and Discussion

3.1. Analyze the Source of Impurities K and Na

During the wet extraction of vanadium, the existing forms of vanadium in solutions with different pH values have complex changes, which mainly exist in the ionic forms of V₃O₉³⁻, V₄O₁₂⁴⁻, HV₆O₁₇³⁻, HV₁₀O₂₈⁵⁻, and H₂V₁₀O₂₈⁴⁻ [10,11,12,13]. Part of the vanadium in the solution was converted to dodecanadate when the pH of the solution was lowered to 2. When alkali metal ions and ammonium ions are present at the same time, the order in which dodecavanadate (Na₄V₁₂O₃₂) will be selected is as follows [14]:

K⁺ ≥ NH₄⁺ ≥ Na⁺ ≥ H⁺ ≥ Li⁺

This shows that the binding capacity of K⁺, Na⁺, and vanadate ion is greater than that of H⁺. Therefore, in the precipitation process of ammonium polyvanadate (ammonium oligovanadate) is mainly ammonium hexavanadate (NH₄)₂V₆O₁₆, ammonium decavanadate (NH₄)₆V₁₀O₂₈, ammonium dodecanadate (NH₄)₂V₁₂O₃₁, etc. In the solution, since the pH of the solution is controlled between 2 and 2.5, a part of K⁺ and Na⁺ will combine with vanadate ions and precipitate. Sodium ions can be replaced when the added ammonium salt is excessive, but potassium ions cannot be replaced. Therefore, the final APV contains a small amount of K and Na impurities.

3.2. Occurrence Rule of K and Na in Tablet Vanadium Pentoxide

APV is dried and calcined to form flake vanadium pentoxide. Figure 3 shows the backscattered electron microscope image of flake vanadium pentoxide and the corresponding element surface distribution. It can be seen from the figure that the phase distribution is relatively uniform; there is no obvious phase dissociation, no doped phase, and Na and K elements (Na₂O and K₂O) are evenly distributed in the entire area of vanadium sheet.

The phase and composition of the vanadium sheet were further analyzed, and the results are shown in Figure 4. The results show that the main phase in the vanadium sheet is V₂O₅ phase, but there is also a small amount of tetravalent vanadium. This is because the 5-valent vanadium is reduced to 4-valent vanadium in the APV deamination-melting process, and its phase is V₆O₁₂. At the same time, through energy spectrum analysis and ICP analysis, it is found that there are certain amounts of K and Na elements in the vanadium flakes, and the content of K is higher than that of Na, which is basically consistent with the theoretical analysis results.

Evans and Xu found that decavanadate did not decompose at lower temperatures when studying the synthesis of K₂ZnV₁₀O₂₈·16H₂O, K₆V₁₀O₂₈·9H₂O and Na₆V₁₀O₂₈·12H₂O [15,16,17,18]. When using APV as a raw material to prepare tablet V₂O₅, the temperature in the melting furnace is close to the melting point of V₂O₅ at 670 °C, Therefore, decavanadate, dodecanavanate (potassium, sodium salt) from APV will be melted and uniformly doped in the vanadium flakes when producing vanadium pentoxide in the flakes. Finally, the existence of Na and K impurities in the vanadium sheet is caused.

3.3. Sources and Occurrence Rules of K and Na in Vanadium Trioxide

Figure 5 is the phase analysis result of vanadium trioxide. It can be seen that the main phase is the V₂O₃ phase, but there is also a small amount of V₃O₅ and VO substances, and no phase formed by other impurity elements has been detected. Further analysis of its composition, as shown in Figure 5, shows that vanadium trioxide contains a small amount of K and Na impurities, and the K content is higher than the Na content. This is consistent with the distribution law of K and Na in the vanadium sheet, indicating that the above impurities are also from the raw material APV.

Similarly, if we want to study the occurrence forms of K and Na in vanadium trioxide, we need to study the reduction behavior of polyvanadate (K, Na). Therefore, NaVO₃ and KVO₃ were used as the research objects to carry out reduction experiments, in order to explain the phase change law of decavanadate and dodecavanadate in the process of V₂O₅ reduction to produce V₂O₃.

Figure 6a,b shows that the phase structures of the products obtained after KVO₃ and NaVO₃ were reduced by hydrogen at 800 °C. The analysis results show that KVO₃ and NaVO₃ are directly reduced to low-valent vanadium oxide V₂O₃ under high-temperature hydrogen atmosphere. However, K and Na were not detected in the product, so it is speculated that the content of the compound formed is low.

3.4. Characterization of Vanadium Nitride Samples and Occurrence Rules of Internal K and Na

Figure 7 is the phase analysis result of the vanadium nitride product. It can be seen that the phase in the vanadium nitride is single, which is the VN phase. Figure 8 shows the microstructure and main component distribution of vanadium nitride. It can be seen from the figure that there are many irregularly distributed voids at the edge of VN, and the voids contain a certain amount of Al, as shown in the black area in Figure 8. The distribution rules of elements V, C, and N are basically the same, and they are distributed in the gray area in Figure 8. However, for the distribution of Na and K, they are uniformly dispersed in the whole VN in the form of non-oxide, and the content of K in VN is higher than that of Na.

Figure 9 shows the microstructure and composition analysis results of the central position of the VN sample. It can be concluded from the figure that the main elements V and N in the sample are uniformly distributed, and there are many irregular voids in the middle area of the alloy. These voids appear as gray areas, and the impurity Si is mainly concentrated in these areas in the sample. Impurities Na and K are dispersed in the sample, and the content of Na is higher than that of K.

3.5. Analysis of Volatile Compounds in the Preparation of VN

During the preparation of VN, some volatile substances will be produced, and these volatile substances will form a kiln scale inside the kiln. Figure 10 shows the phase analysis results of the volatiles, which shows that the main components of the volatiles include the MgO, NaCN, KCN, SiO₂ and K₂CO₃ phases, in which the K and Na contents are relatively high. K and Na impurities mainly come from vanadium oxide raw materials. Compared with the contents of K and Na in the raw materials, they are significantly enriched. The content of K is enriched by 25 times, and the content of Na is enriched by 15 times. Moreover, the enrichment degree of K is obviously stronger than that of Na.

After the volatilization condenses in the low-temperature area, large crystal granular scales are formed and adhere to the surface of the furnace refractory material, and the microscopic analysis is carried out, which is shown in Figure 11. The results show that the black area contains higher Na, mainly the Na₂CO₃ phase and the NaCN phase, of which NaCN is not easily decomposed (its melting point is 563.7 °C, and its boiling point is 1496 °C). The gray area is the substance with higher K and Zn, and the main phases are K₂CO₃ and ZnCO₃. It can be concluded that the K and Na impurities remaining in the vanadium oxide raw material are volatilized at high temperature, and the formed volatiles condense near the refractory and adhere to the surface of the refractory, eventually forming a large amount of scale.

3.6. Study on the Behavior Mechanism of K and Na in the Synthesis of VN

Volatile substances containing K and Na are condensed in the kiln, which will cause corrosion to the structural parts in the kiln and affect its service life. Therefore, it is necessary to study the migration behavior of K and Na during the production of vanadium nitride. Vanadium oxide (V₂O₃, V₂O₅), graphite, and nitrogen are the main raw materials for the preparation of VN, and K and Na oxides are both derived from vanadium oxide (V₂O₃, V₂O₅). At high temperature, the above substances are reduced to elemental Na and K by C and form steam. The reaction equation is shown in Equations (1) and (2):

K₂O(s) + C(s) → 2K(g) + CO(g)   ΔG^Θ = 98,500 − 89.25T

(1)

Na₂O(s) + C(s) → 2Na(g) + CO(g)  ΔG^Θ = 118,050 − 90.9T

(2)

The results of thermodynamic analysis are shown in the figure, and the results show that K₂O is more easily reduced, so the content of K in the volatiles is also higher. These basic metals are highly corrosive, and form compounds with O, N, C and other elements in the system when they condense on the surface of the refractory. This process has a strong erosive effect on the refractory and other devices in the furnace. When the temperature in the furnace body is controlled at 1000–1300 °C (the stage where the vanadium oxide is reduced by carbon), the production of K and Na increases, which not only increases the corrosion of the components in the kiln, but also affects the exhaust gas of the kiln due to the condensation of these substances.

When the temperature in the kiln is controlled at 1400–1500 °C for the nitriding, due to the existence of flowing nitrogen and C and O, the K₂CO₃ and Na₂CO₃ generated by the reaction of the volatile substances K and Na are precipitated into the kiln and the exhaust duct, as shown in Equations (3)–(6). It can be seen from this that when the reaction temperature is controlled within 1500 °C, the standard Gibbs free energies of the following four reaction equations are all negative values, that is, the formation reactions of K₂CO₃ and Na₂CO₃ in the standard state are thermodynamically acceptable, which will help to analyze the formation process of K and Na compounds in the production of VN alloys.

At the same time, nitrogen will also undergo nitridation reaction with K and Na, and the reaction equation is as shown in Equations (7) and (8). The thermodynamic analysis of the formation of KCN and NaCN in the nitridation reaction is shown in the Figure 12. It means that KCN is more likely to be formed than NaCN when the temperature is within 1500 °C and the ΔG < 0 [19,20]. The above theoretical analysis is consistent with the XRD analysis results of the actual fouling substances. Therefore, in order to reduce the influence of K and Na alkaline substances on the corrosion of components in the kiln and exhaust gas, it is necessary to reduce the content of K and Na from the raw materials or clean the fouling substances in the kiln and flue in time to prolong the service life of the kiln.

2K(g) + C(s) + 1.5O₂(g) = K₂CO₃(s)   ΔG^Θ = −11,277,100 + 410.35T

(3)

2K(g) + C(s) + 1.5O₂(g) = K₂CO₃(l)   ΔG^Θ = −1,204,750 + 353.69T

(4)

2Na(l) + C(s) + 1.5O₂(g) = Na₂CO₃(s)  ΔG^Θ = −1,127,500 + 273.64T

(5)

2Na(g) + C(s) + 1.5O₂(g) = Na₂CO₃(l)  ΔG^Θ = −1,229,600 + 362.47T

(6)

2K(g) + N₂(g) + 2C(s) = 2KCN(s)

(7)

2Na(g) + N₂(g) + 2C(s) = 2NaCN(s)

(8)

4. Conclusions

(1)

The elements K and Na in APV exist in the form of decavanadate and dodecanadate, and due to the adsorption capacity of polyamide K⁺ > Na⁺, the content of the K element in APV is higher than that of Na. At the same time, K and Na exist in the same form and regularity in the vanadium sheet obtained by APV dehydration-deamination-melting.

(2)

The concentration of K in the alloy product is 25 times higher than that of the vanadium raw material, the concentration of Na is 15 times higher than that of the raw material, and the degree of enrichment of K is stronger than that of Na.

(3)

During the preparation of the VN alloy, some K and Na vapors promote the reduction in low-valent vanadium oxides to metal V, thereby promoting the formation process of vanadium nitride and improving the formation efficiency of vanadium nitride.

(4)

Impurities K and Na are converted into corrosive substances such as KCN, NaCN, K₂CO₃, Na₂CO₃, etc. These substances cause the equipment to be corroded; and these impurities are easily deposited on the surface of the product, which affects the equipment life cycle and product quality.