Method for producing high purity vanadium compounds from vanadium raw materials having high molybdenum content

Abstract

The present invention is a method for producing high purity vanadium compounds from vanadium raw materials having a high molybdenum content, in which molybdenum is selectively extracted over vanadium from an alkaline vanadate solution using the following method steps: providing a molybdenum-containing alkaline vanadate solution at a temperature of up to 70°C, preferably about 60°C; -Maintaining the pH value at a constant level between 6 and 7 using acid, calcium hydroxide is added multiple times as a precipitant; mixing the solution; - separating the resulting suspension into solid and liquid; and - The low molybdenum alkaline vanadate solution is further processed to produce a high purity vanadium compound having a molybdenum content of up to 500 ppm.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a high purity vanadium compound from a vanadium raw material having a high molybdenum content. [Background technology]

[0002] With respect to the background of the present invention, it should be noted that the objective is to develop a chemical separation method for selectively extracting molybdenum from alkaline vanadate solutions, in particular from sodium vanadate solutions with a neutral salt content. This method is used prior to vanadium precipitation in order to enable the use of vanadium raw materials with a high molybdenum content for the production of high-purity vanadium compounds. These high-purity vanadium compounds meet the requirements for use in catalysis and in the aviation industry, as well as for use as energy storage materials.

[0003] The raw materials used for this purpose can be spent catalysts and gasification residues (gasification cakes) of the petrochemical industry.

[0004] There is a need for this type of separation method because molybdenum is filtered from the aforementioned raw materials with the same efficiency as vanadium and is partially precipitated along with the vanadium. If vanadium precipitation occurs before molybdenum precipitation, the higher the molybdenum content in the raw material, the more the vanadium compounds will be contaminated with molybdenum. This makes the vanadium compounds unusable for the aforementioned applications.

[0005] In particular, the relevant prior art provides various techniques for extracting molybdenum from vanadate solutions, namely precipitation, ion exchange or liquid-liquid extraction - see Non-Patent Document 1 and Non-Patent Document 2.

[0006] For example, it was only possible to selectively extract molybdenum from vanadate solutions by precipitation after the vanadium content in the mother liquor had been reduced by prior precipitation and the remaining vanadium was reduced to tetravalent vanadium with sulfur dioxide at 80-90°C. Molybdic acid was then precipitated with strong hydrochloric acid at a pH of 1.1 or less - see Patent Document 1 (1997, ZR Llanos, GF Provoost, WG Deering, FJ Debaene, Integrated process for the recovery of metals and fused alumina from spent catalysts).

[0007] Patent Document 2 discloses the precipitation of vanadium from a purified vanadate solution containing 25.3 g / L molybdenum and 2.1 g / L vanadium, obtained from a raw material containing 9.7% molybdenum and 1.5% vanadium, using ammonium chloride at a pH of 7-8 at 90°C. This precipitates 1.5% molybdenum and more than 95% vanadium. Considering the volume of ammonium chloride solution added, 18.7 g / L molybdenum and less than 0.08 g / L vanadium are finally obtained, and then molybdenum is precipitated as calcium molybdate using calcium chloride solution at a pH of 7-8 and 90°C. Unless otherwise disclosed in this document, this amount of calcium chloride solution is apparently added all at once. Unlike that of Patent Document 2, when an alkaline vanadate solution containing more vanadium than molybdenum is treated in the same manner, a significant amount of calcium metavanadate precipitates in addition to calcium molybdate, despite the lower solubility of calcium molybdate described in Patent Document 3. This is because in solutions with high concentrations of various salts (e.g., calcium molybdate, calcium metavanadate, and sodium sulfate), precipitation of the minor component, calcium molybdate, is kinetically suppressed as a result of the salting effect and completely ceases once the molybdenum content drops below a certain level. In contrast, because the salt effect is less important for components with higher concentrations, precipitation of the major component, calcium metavanadate, is kinetically desirable despite its higher solubility. When the kinetically desirable precipitation of calcium metavanadate is not significantly suppressed by the reaction process, as is the case using the method described in Patent Document 2, the selectivity for molybdenum over vanadium is significantly shifted in favor of vanadium. Therefore, more molybdenum remains in the precipitation solution, thereby compromising the purity of subsequent products such as ammonium metavanadate or vanadium pentoxide.

[0008] In the vanadium extraction method according to the aforementioned Patent Document 3, calcium molybdate, calcium metavanadate, and sodium metavanadate exist as solids in the presence of sodium sulfate before the ammonium carbonate solution is added for dissolution. This results in the selective dissolution of calcium metavanadate over calcium molybdate, and does not result in the selective precipitation of molybdenum from the alkaline vanadate solution. If the indication of the lower solubility of calcium molybdate compared to calcium metavanadate in Patent Document 3 is used for selective molybdenum removal, this will only be successful for solutions whose vanadium content is significantly lower than their molybdenum content, since any competing precipitation of calcium metavanadate cannot be suppressed. In contrast, the mere indication of the lower solubility of calcium molybdate compared to calcium metavanadate in solutions with higher vanadium than molybdenum content does not result in the selective molybdenum extraction required to produce high-purity vanadium compounds. Control of reaction kinetics is even more necessary when this type of solution is present.

[0009] Examples show that it is known to precipitate molybdenum from solutions in which vanadium has previously been precipitated and which have been significantly depleted. However, based on the current state of knowledge, there are no reports of successful selective precipitation of molybdenum from vanadate solutions having a higher vanadium content than the molybdenum content and at the same time having a high neutral salt content where competitive precipitation of vanadium must be suppressed.

[0010] In ion exchange units, hexavalent molybdenum and pentavalent vanadium are equally absorbed and eluted, so separation does not occur here. This is utilized for complete purification of the waste liquid and is only economically viable for low vanadium and molybdenum contents (0.1 g / L to 1 g / L). By ion exchange, molybdenum can only be effectively separated from pentavalent vanadium in the strongly acidic range (pH 1), which also requires first reducing the vanadium with sulfite - see non-patent document 1 of the same work.

[0011] The most widely used method for extracting molybdenum from vanadate solutions is liquid-liquid extraction (see Non-Patent Document 1, Non-Patent Document 2, and Patent Document 4). High-purity vanadium compounds can be obtained by extracting molybdenum from vanadate solutions with similar vanadium and molybdenum contents (approximately 10 g / L each) (see Patent Document 4). However, this method is technically very complex due to the use of organophosphoric acid or alkylamine-based extractants dissolved in organic solvents such as kerosene or xylene, followed by subsequent back-extraction with aqueous ammonia, and is very expensive due to the use of special compounds. In addition, molybdenum is extracted from these vanadate solutions at a strongly acidic pH, usually in the hydrochloric acid pH range. For example, organophosphoric acid extractants di-(2-ethylhexyl)phosphoric acid (DEHPA) (see Non-Patent Document 3) or trioctylphosphine oxide (TOPO) (see Non-Patent Document 4) dissolved in kerosene may be used, which separate cationic molybdenum species from vanadium species at pH < 1. Molybdenum must then be separated using aqueous ammonia. When D2EHPA is used, a second organic phase can form as a result of increased viscosity, making it difficult to reuse the extractant (see Non-Patent Document 5). Higher amines such as trioctylamine (TOA) in Alamine® 336 dissolved in toluene (see Non-Patent Document 6) or TOA combined with tributyl phosphate (TBP) (see Non-Patent Document 7) extract molybdenum at pH < 1 only via tetravalent vanadium species, which must first be prepared using sulfite or sulfur dioxide.

[0012] What all these known extraction methods have in common is that the vanadium content in the solutions to be treated is similar to or lower than the high molybdenum content (usually <1 g / L vanadium and 1-10 g / L molybdenum). [Prior art documents] [Patent documents]

[0013] [Patent Document 1] US5702500A

Patent document 2

Patent Document 3

Patent document 4

Non-licensed literature

[0014] [Non-licensed document 1] L. Zeng, CY Cheng, A literature review of the recovery of molybdenum and vanadium from spent hydrodesulphurisation catalysts, Part II: Separation and purification, Hydrometallurgie, 2009, 98, 10-20 [Non-licensed document 2] TH Nguyen, MS Lee, A review on the separation of molybdenum, tungsten, and vanadium from leach liquors of diverse resources by solvent extraction, Geosystem Engineering, 2016, 19, 247-259 [Non-licensed document 3] RK Biswas, Recovery of vanadium and molybdenum from heavy oil desulfurization waste catalyst, Hydrometallurgy, 1985, 14, 219-230

Non-licensed Document 4

[0015] Based on the aforementioned problems, it is an object of the present invention to provide a simpler, more environmentally friendly yet highly efficient method for selectively extracting molybdenum from alkaline vanadate solutions, in particular those having a high molybdenum content and, conversely, a higher vanadium content and, at the same time, a high neutral salt content, in order to produce vanadium compounds of high purity. [Means for solving the problem]

[0016] This object is achieved by a method having the features of claim 1, in that molybdenum is selectively extracted over vanadium from an alkaline vanadate solution by applying the following method steps: - preparing a molybdenum-containing alkaline vanadate solution at a temperature of up to 70°C, preferably about 60°C; - Add calcium hydroxide as a precipitant in multiple portions while maintaining the pH value constant in the range of 6-7 using acid, - Mixing the solution, - separating the resulting suspension into solid and liquid; and -Further processing of the low-molybdenum alkaline vanadate solution to produce high-purity vanadium compounds with a molybdenum content of up to 500 ppm.

[0017] The method according to the present invention solves the main problem of avoiding competitive precipitation of vanadium when dealing with alkaline vanadate solutions with a higher vanadium content than molybdenum content and simultaneously a high neutral salt content by controlling the kinetics of the proceeding reaction in a defined manner and ensuring that the correct vanadium and molybdenum species are present in solution throughout the precipitation process. Thus, the kinetics of the reaction to form calcium metavanadate is controlled by the multiple (portionwise) addition of calcium hydroxide; in other words, it is slowed down (thermodynamic reaction control) so that the thermodynamically favored product, calcium molybdate, which is kinetically inhibited by the salt effect, nevertheless preferentially forms. For example, if there is an excess of vanadium, a faster addition will always precipitate the thermodynamically favored product, calcium metavanadate. Adjusting the pH between each addition of calcium oxide prevents the precipitation reaction from drifting into the basic pH range, ensuring as little calcium metavanadate as possible and mostly the more water-soluble calcium decavanadate, ultimately supporting the selective precipitation of calcium molybdate. Additionally, a lower pH limit should be controlled, as this would result in an equilibrium shift between molybdate and heptamolybdate. Here, drift below pH 6.2 is best avoided, as otherwise the molybdate fraction would be shifted in favor of heptamolybdate. However, only molybdate ions can form calcium molybdate, which is poorly soluble in water, whereas calcium heptamolybdate is very readily soluble in water. If the molybdate fraction decreases with decreasing pH, the remaining molybdate will no longer precipitate due to a prevailing salt effect. As a result of the above, a pH range of 6.2 to 6.9 is preferred (claim 8).

[0018] Only by combining these two steps of multiple additions of calcium hydroxide and maintaining a constant pH value, the selectivity of molybdenum over vanadium achieved according to the present invention becomes possible, especially from solutions having a higher vanadium content than the molybdenum content and at the same time a high neutral salt content, so that only in this way can high purity vanadium compounds be obtained with a molybdenum content of up to 500 ppm.

[0019] Further preferred developments of the method according to the invention are specified in the dependent claims.

[0020] Thus, the object of the present invention is essentially achieved by precipitating molybdenum, for example as calcium molybdate (CaMoO4), from an alkaline vanadate solution having a higher vanadium content than the molybdenum content and simultaneously a high neutral salt content (see claim 2). The molybdenum content is at least 6.5 g / L, and the neutral salt content, particularly the sodium sulfate content, is preferably in the range of 70 g / L to 120 g / L (see claim 3). The precipitation of molybdenum occurs before the precipitation of vanadium (see claim 4). This results in a residual solubility of molybdenum of 1 g / L to 2 g / L, ensuring the consistent production of high-purity vanadium compounds with a molybdenum content of up to 500 ppm from the above-mentioned raw materials. The use of this type of raw material with a high molybdenum content is only possible by applying this method.

[0021] Precipitation of molybdenum as calcium molybdate is carried out in an aqueous medium without the addition of organic auxiliary substances by gradually adding calcium hydroxide in a stoichiometric amount relative to the molybdenum (see claim 5). The pH value is maintained constant by measuring sulfuric acid (see claim 7). Under the above conditions, molybdenum is precipitated over vanadium with a selectivity of 85% to 90%, which is expressed by a molybdenum / vanadium molar ratio in the calcium molybdate of 85:15 to 90:10. Precipitation of sparingly soluble calcium sulfate is negligible. The achievable purity of calcium molybdate allows its sale as a product in the molybdenum industry.

[0022] The process according to the invention therefore makes it possible to produce the aforementioned high purity vanadium compounds from vanadium feedstocks with a high molybdenum content, such as spent catalysts, preferably vanadium-containing Ni-Mo catalysts, or vanadium-containing residues from oil refineries - see claim 9.

[0023] A crucial advantage of the present invention over the prior art is that molybdenum can be selectively extracted by precipitation as calcium molybdate from alkaline vanadate solutions having a vanadium content (approximately 35 g / L to 50 g / L) significantly higher than the molybdenum content (approximately 7 g / L to 10 g / L) and simultaneously a high neutral salt content. According to the present invention, molybdenum precipitation can occur before vanadium precipitation, so that the molybdenum content is already consistently reduced to low levels (1 g / L to 2 g / L) in the mother liquor for the subsequent precipitation of ammonium metavanadate (AMV). Finally, and unlike the prior art, the production of high-purity vanadium compounds with molybdenum contents of up to 500 ppm from raw materials with high molybdenum contents is made possible by the molybdenum precipitation reaction as a purification step.

[0024] By using the method according to the present invention, ion exchange and liquid-liquid extraction can be avoided, which constitutes an additional economic, safety, and environmental advantage over the prior art. Molybdenum is precipitated as calcium molybdate from alkaline vanadate solution in an exclusively aqueous medium and at a system-friendly, slightly acidic pH range up to a neutral pH of 6-7. Only inexpensive, non-toxic calcium hydroxide and small amounts of sulfuric acid are used in the present invention, which is acceptable from the standpoint of occupational safety and the environment. In addition, when calcium hydroxide is used, there is no problematic contamination of high-purity vanadium compounds with chloride ions, as is the case when calcium chloride is used. The possibility of secondary contamination of vanadium compounds and wastewater discharge with organic substances is eliminated. Compound-intensive pH jumps and oxidation or reduction steps are eliminated using this method. Similarly, complex extraction and stripping steps or stripping equipment are dispensed with; only a heatable stirred tank is required. Additionally, precipitation of CaMoO4 at about 60°C requires less energy than, for example, precipitation of molybdic acid at 80°C-90°C and precipitation of calcium molybdate at 90°C as reported in Patent Document 2. A preferred temperature range of about 60°C according to the present invention may be 58°C-62°C.

[0025] Further features, details and advantages of the invention will become apparent from the following description of embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0026] Example 1 In this example, one liter of sodium vanadate solution (NaV solution) was obtained by filtering a roasted Ni-Mo catalyst containing 18.7% V and 2.4% Mo. The NaV solution, containing 8.5 g / L molybdenum, 42.8 g / L vanadium, and 105 g / L sodium sulfate, was heated to 60°C. Next, a total of 6.55 g of calcium hydroxide (anhydrous), stoichiometric to the molybdenum, was added. The calcium hydroxide was added in four portions of 1.64 g each, spaced 20 minutes apart. During the addition of the calcium hydroxide portions, the pH was maintained constant at 6.4 by slowly metering in concentrated sulfuric acid (96%). After adding the final portion and adjusting the pH, the suspension was stirred at 60°C for 5 minutes. During the stirring period, the pH was checked and, if necessary, adjusted back to 6.4. The precipitant was filtered and washed with water. Ammonium sulfate was added to a low-molybdenum sodium vanadate solution at a pH of 8-9, which was stirred for 2 hours. The precipitated ammonium metavanadate (AMV) was filtered and washed with water. As an alternative to AMV, ammonium polyvanadate (APV) or sodium polyvanadate (NPV) can be obtained by precipitation at a pH of 2-3, and ammonium sodium vanadate (NAV) can be obtained at a pH of 5-6. Vanadium pentoxide (VO2) can be routinely produced from AMV or APV by calcination in an air atmosphere, vanadium dioxide (VO2) can be produced by calcination with a mild reducing agent such as natural gas, or vanadium trioxide (VO2) can be produced by calcination with hydrogen. Table 1 shows the molybdenum, vanadium and sulfur contents of the sodium vanadate solution before and after molybdenum removal, the contents in the calcium molybdate precipitant, and the contents of ammonium metavanadate (AMV) obtained from each NaV solution:

[0027] Table 1: Molybdenum, vanadium and sulfur contents of NaV solutions before and after molybdenum removal, of calcium molybdate, and of AMV solutions without and with Mo removal.

[0028] [Table 1]

[0029] Example 1 shows that after the aforementioned method is applied, a NaV solution having 8.5 g / L Mo and 42.8 g / L V contains 0.8 g / L Mo and 42.6 g / L V. The precipitated product, calcium molybdate, contains Mo and V in a molar ratio of 88:12, reflecting the selectivity of the method. AMV subsequently precipitated from the low-molybdenum NaV solution contains 0.008% Mo (corresponding to 0.010% Mo in VO and 0.013% Mo in VO), while the AMV without prior molybdenum removal contains 0.051% Mo (corresponding to 0.065% Mo in VO, 0.073% Mo in VO, and 0.080% Mo in VO). This example shows that the method of the present invention for selective molybdenum removal from sodium vanadate solutions is successful and makes it possible to produce high purity vanadium compounds such as AMV from feedstocks with high molybdenum content (2.4% Mo). This example also shows that the precipitation of calcium molybdate is selective to the possible precipitation of calcium sulfate.

[0030] Example 2 In this example, one liter of sodium vanadate solution (NaV solution) was obtained by filtering a roasted Ni-Mo catalyst containing 13.5% V and 5.5% Mo. The NaV solution, containing 7.9 g / L molybdenum, 36.3 g / L vanadium, and 79 g / L sodium sulfate, was heated to 60°C. Next, a total of 6.09 g of calcium hydroxide (anhydrous), stoichiometric to the molybdenum, was added. The calcium hydroxide was added in four portions of 1.52 g each at 20-minute intervals. During the addition of the calcium hydroxide portions, the pH was maintained constant at 6.8 by slowly metering in concentrated sulfuric acid (96%). After adding the final portion and adjusting the pH, the suspension was stirred at 60°C for 5 minutes. During the stirring period, the pH was checked every hour and adjusted to 7.0 if necessary. The precipitant was filtered and washed with water. Ammonium sulfate was added to a low-molybdenum sodium vanadate solution at a pH of 8-9, which was stirred for 2 hours. The precipitated ammonium metavanadate (AMV) was filtered and washed with water. As an alternative to AMV, precipitation at a pH of 2-3 can yield ammonium polyvanadate (APV) or sodium polyvanadate (NPV), and precipitation at a pH of 5-6 can yield sodium ammonium vanadate (NAV). Vanadium pentoxide (VO) can be routinely produced from AMV or APV by calcination in an air atmosphere; vanadium dioxide (VO) can be produced by calcination with a mild reducing agent such as natural gas; or vanadium trioxide (VO) can be produced by calcination with hydrogen.

[0031] Table 2 shows the molybdenum, vanadium and sulfur contents of the sodium vanadate solution before and after molybdenum removal, the contents in the calcium molybdate precipitant, and the contents of ammonium metavanadate (AMV) obtained from each NaV solution:

[0032] Table 1: Molybdenum, vanadium and sulfur contents of NaV solutions before and after molybdenum removal, of calcium molybdate, and of AMV solutions without and with Mo removal.

[0033] [Table 2]

[0034] Example 2 shows that after the aforementioned method is applied, a NaV solution with 7.9 g / L Mo and 36.3 g / L V contains 2.2 g / L Mo and 35.9 g / L V. The precipitated product, calcium molybdate, contains Mo and V in a molar ratio of 83:17, reflecting the selectivity of the method. AMV subsequently precipitated from the low-molybdenum sodium vanadate solution contains 0.009% Mo (corresponding to 0.012% Mo in VO; 0.013% Mo in VO; and 0.014% Mo in VO), while the AMV without prior molybdenum removal contains 0.075% Mo (corresponding to 0.096% Mo in VO; 0.107% Mo in VO; and 0.117% Mo in VO). This example demonstrates that the method of the present invention for selective molybdenum removal from sodium vanadate solutions is successful, making it possible to produce high purity vanadium compounds such as AMV from feedstocks with high molybdenum content (5.5% Mo). This example also demonstrates that the precipitation of calcium molybdate is selective to the possible precipitation of calcium sulfate.

Claims

1. A method for producing a high-purity vanadium compound from a vanadium raw material having a high molybdenum content, comprising: Molybdenum is selectively extracted over vanadium from an alkaline vanadate solution by the following method steps: providing a molybdenum-containing alkaline vanadate solution at a temperature of up to 70°C, preferably about 60°C; - Add calcium hydroxide as a precipitant multiple times while maintaining the pH value constant in the range of 6-7 using acid; - mixing the solutions, - separating the resulting suspension into solid and liquid, and - further processing the low-molybdenum alkaline vanadate solution to produce a high-purity vanadium compound having a molybdenum content of up to 500 ppm.

2. 2. The method according to claim 1, wherein the molybdenum is precipitated in the form of one or several molybdenum-containing calcium compounds, preferably as calcium molybdate.

3. 3. The method according to claim 1 or claim 2, characterized in that the alkaline vanadate solution provided has a vanadium content higher than its molybdenum content and at the same time a high neutral salt content, the molybdenum content being at least 6.5 g / L.

4. 4. The method according to claim 1, wherein the precipitation of molybdenum occurs before the precipitation of vanadium from the alkaline vanadate solution.

5. 5. The method according to claim 1, wherein calcium hydroxide is added multiple times stoichiometrically with respect to molybdenum in at least four steps within a period of 90 minutes.

6. 6. The method according to claim 1, wherein the precipitation of the molybdenum-containing calcium compounds occurs in an aqueous medium without the addition of organic auxiliary substances.

7. 7. The method according to claim 1, wherein the pH value is maintained constant in this range by adding sulfuric acid in multiple measurements.

8. 8. The method according to claim 1, wherein the pH value is kept constant in the range of 6.2 to 6.

9.

9. 9. The method according to any one of claims 1 to 8, characterized in that the vanadium raw material having a high molybdenum content is a spent catalyst, preferably a vanadium-containing Ni-Mo catalyst, or a vanadium-containing residue from an oil refinery.

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