Method for removing chromium in ammonium polyvanadate through reduction precipitation

Abstract

The invention discloses a method for removing chromium in ammonium polyvanadate through reduction precipitation, according to the method, the effect of efficient chromium removal can be achieved only through simple dissolution and chemical precipitation methods, the chromium content of a product is smaller than 300 ppm, and the requirement for the content of impurity elements is avoided; the method is high in chromium removal efficiency, low in energy loss, high in vanadium recovery rate, clean, environmentally friendly and suitable for industrial production; and the prepared high-purity ammonium metavanadate is relatively high in purity and low in impurity element content, and can meet the requirements on the purity of vanadium products in the aspects of electrolyte and aeronautical manufacturing.

Description

Technical Field

[0001] The invention relates to the technical field of ammonium metavanadate preparation, in particular to a method for removing chromium from ammonium polyvanadate by reduction precipitation. Background Art

[0002] Ammonium polyvanadate (NH4VO3) is an important vanadium compound, which is widely used in the production of various vanadium products, such as vanadium aluminum alloy, titanium alloy, metallic vanadium, etc. In the production process of vanadium products, ammonium polyvanadate is a key intermediate product, and its purity directly affects the quality of the final product. In the vanadium extraction process, the vanadium slag obtained after vanadium-titanium magnetite is smelted in a blast furnace and oxidized and blown in a converter contains 0.8-6.5% chromium (in terms of Cr2O3) and 6-25% vanadium (in terms of V2O5). After the vanadium slag is oxidized for vanadium extraction and roasted to become a clinker, it is leached with water to obtain a solution containing vanadium and chromium. In the process of precipitating ammonium polyvanadate, due to the similar properties of vanadium and chromium, a small amount of chromium will enter the ammonium polyvanadate along with the vanadium precipitation reaction, resulting in an increase in the chromium content in the ammonium polyvanadate.

[0003] The development of these technologies is of great significance for improving the quality of vanadium products and reducing production costs. High-purity ammonium metavanadate (NH4VO3) is an important chemical raw material, widely used in the production of high-purity vanadium oxides, catalysts, vanadium electrolytes and other fields. With the advancement of science and technology and the improvement of environmental protection requirements, the demand for high-purity ammonium metavanadate has steadily increased, especially in vanadium energy storage projects. The development of all-vanadium liquid flow batteries has brought new market opportunities for high-purity ammonium metavanadate.

[0004] In the background technology of preparing high-purity ammonium metavanadate, the traditional vanadium extraction process has some problems, such as high energy consumption, harsh production environment, large ammonia absorption loss, etc. In order to solve these problems, some new technologies have been developed to improve production efficiency and product quality while reducing costs and environmental impact.

[0005] In practical applications, the production technology of high-purity ammonium metavanadate has reached the domestic leading level, and the product quality can meet the needs of the high-end market. With the gradual commercialization of vanadium energy storage projects, the market demand for high-purity ammonium metavanadate is expected to grow further, and the market development prospects are very optimistic.

[0006] Chinese patent document CN112010347A discloses a method for improving the yield of vanadium precipitation from ammonium salt of high chromium vanadium solution, wherein the method first cools the vanadium solution after impurity removal and purification, adds concentrated sulfuric acid and vanadium precipitation agent to precipitate vanadium; the vanadium precipitation solution is allowed to stand and then filtered to obtain ammonium polyvanadate. Although this method can obtain ammonium polyvanadate, it does not undergo impurity removal, and the ammonium polyvanadate product has a high impurity content, a complex process flow, a high purification cost, and a large amount of process wastewater, which does not meet actual production conditions.

[0007] Chinese patent document CN106186064A discloses a method for producing high-purity ammonium metavanadate by deep chromium removal. The method first adjusts the pH of the vanadium-chromium solution to 10-14, adds a precipitant and adds acid to reduce the pH to 9-12, adds a reducing agent to precipitate most of the chromium into the slag, continues to add acid after filtration to reduce the pH of the solution to 7-9 points, adds a precipitant for solid-liquid separation, and deeply removes chromium precipitation to obtain ammonium metavanadate. This method can deeply remove chromium, and the purity of the ammonium metavanadate produced is extremely high. The high-chromium vanadium raw material is converted into ammonium metavanadate that can be used for the production of vanadium battery electrolytes, but the method must be adjusted by multiple pH adjustments and multiple classifications to remove chromium, and different types of precipitants are consumed at the same time, and the acid consumption is high. In addition, a large amount of ammonia nitrogen wastewater is generated at the same time and needs to be treated, and the environmental protection treatment cost is high.

[0008] In recent years, the recycling and purification technology of vanadium resources and environmentally friendly purification processes have received more and more attention. The method of preparing high-purity ammonium metavanadate from ammonium polyvanadate needs further research to achieve the goal of simplifying, diversifying and cleaning the vanadium recycling and purification process. Summary of the invention

[0009] The purpose of the present invention is to provide a method for removing chromium from ammonium polyvanadate by reduction precipitation, to prepare ammonium metavanadate by removing chromium efficiently and cleanly, and to promote the industrialization of vanadium recovery and purification technology.

[0010] In view of this, the scheme of the present invention is:

[0011] A method for removing chromium from ammonium polyvanadate by reduction precipitation, comprising the following steps:

[0012] a) adding aqueous ammonia to the ammonium polyvanadate solution to carry out a transammonium reaction to obtain an transammonium solution;

[0013] b) adding citric acid to the ammonium conversion solution to reduce hexavalent chromium and pentavalent vanadium to trivalent chromium and tetravalent vanadium, respectively, to obtain a reduction solution;

[0014] c) adding ammonia water to the reducing solution, adjusting the pH to alkaline to carry out chromium precipitation reaction, adding a flocculant to carry out sedimentation, and filtering to obtain a chromium removal solution;

[0015] d) adding acid to the chromium removal solution to adjust the pH, and then adding an oxidant to oxidize the tetravalent vanadium to pentavalent vanadium;

[0016] e) adjusting the pH of the solution obtained by oxidation, cooling and crystallizing, and filtering to obtain high-purity ammonium metavanadate.

[0017] Further, the content of V2O5 in the ammonium polyvanadate is 85-90wt%, for example, it can be 85.0%, 85.5%, 86.0%, 86.5%, 87.0%, 87.5% or 90.0%, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable; the Cr content is 0.1-0.5wt%, for example, it can be 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.35% or 0.50%, etc.

[0018] It should be noted that the content of V2O5 in the ammonium polyvanadate used in the method for removing chromium to prepare high-purity ammonium metavanadate of the present invention is essentially not limited. For higher content of V2O5, the same is applicable under the premise of the presence of similar impurities; for lower content of V2O5, in the presence of higher content of impurities, only the amount of reagent needs to be adaptively adjusted.

[0019] Further, in step a):

[0020] The solvent of the ammonium polyvanadate solution is water or the crystallization mother liquor obtained in step e);

[0021] The liquid-to-solid ratio of the ammonium conversion reaction is (15-20):1, for example, it can be 15.0:1, 15.5:1, 16.0:1, 16.5:1, 17.0:1, 17.5:1, 18.0:1, 18.5:1, 19.0:1, 19.5:1 or 20.0:1, etc.; the temperature of the dissolution process is 70-90°C;

[0022] The pH value of the ammonium transfer is 8-8.5, for example, it can be 70°C, 72°C, 74°C, 76°C, 78°C, 80°C, 82°C, 84°C, 86°C, 88°C or 90°C, etc.

[0023] Further, in step b):

[0024] The dosage of citric acid is 2.1-2.4 times of the theoretical amount of tetravalent vanadium for reducing pentavalent vanadium, for example, it can be 2.1 times, 2.13 times, 2.26 times, 2.38 times, 2.39 times or 2.4 times of the theoretical amount; the function of citric acid is to reduce Cr(VI) and V(V) to Cr(II) and V(IV). The reason for adding excess citric acid is that citric acid belongs to a macromolecular complex, which can complex V(IV) to form stable ions, so that V(IV) will not precipitate when Cr(III) is precipitated later, which is beneficial to the selective precipitation and separation of Cr.

[0025] After the citric acid is added, the pH value is adjusted to 3.2-4.5 for reduction reaction, for example, it can be 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4 or 4.5; the present invention preferably controls the pH of the ammonium polyvanadate ammonium conversion reaction within the above range, too high a reduction pH (pH>4.5) will lead to incomplete reduction of Cr(VI) and V(V), and too low a reduction pH (pH<3.2) will easily lead to polyvanadic acid precipitation of part of V(V).

[0026] The reduction reaction time is 3-5h, such as 3.0h, 3.2h, 3.4h, 3.6h, 3.8h, 4.0h, 4.2h, 4.4h, 4.6h, 4.8h or 5.0h; the reduction reaction temperature is 30-42°C, for example, it can be 30°C, 31°C, 32°C, 33°C, 34°C, 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C or 42°C.

[0027] Furthermore, in step c), the concentration of the aqueous ammonia is 10-15%, such as 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5% or 15.0%, etc.; the chromium precipitation reaction step includes the following two stages:

[0028] The first stage: add ammonia water to pH 3.2-7.8, the addition rate is 1.0-2.2mL / min, and the chromium precipitation reaction temperature is 80-90℃;

[0029] The second stage: continue to add ammonia water until the pH is 7.8-10.0, the addition rate is 0.5-0.8mL / min, and the chromium precipitation reaction temperature is 90-100℃.

[0030] Preferably, in the first stage of the Cr precipitation reaction, the endpoint pH may be 7.5, 7.55, 7.6, 7.68, 7.73 or 7.8, and the rate of addition of ammonia water may be 1.0 mL / min, 1.1 mL / min, 1.2 mL / min, 1.3 mL / min, 1.4 mL / min, 1.5 mL / min, 1.6 mL / min, 1.7 mL / min, 1.8 mL / min, 1.9 mL / min, 2.0 mL / min, 2.1 mL / min or 2.2 mL / min, and the reaction temperature may be 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, 86°C, 87°C, 88°C, 89°C or 90°C, etc.; the stirring rate of the chromium precipitation reaction in the first stage of chromium precipitation reaction is 400-500r / min, for example, it can be 400r / min, 410r / min, 420r / min, 430r / min, 440r / min, 450r / min, 460r / min, 470r / min, 480r / min, 490r / min or 500r / min, etc., but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0031] Preferably, the endpoint pH of the second stage of the Cr precipitation reaction can be 9.5, 9.54, 9.6, 9.66, 9.7, 9.74, 9.8, 9.89, 9.94 or 10.0, etc.; the addition rate of ammonia water in the second stage of the Cr precipitation reaction can be 0.50mL / min, 0.52mL / min, 0.54mL / min, 0.56mL / min, 0.58mL / min, 0.60mL / min, 0.62mL / min, 0.64mL / min, 0.66mL / min, 0.68mL / min, 0.70mL / min, 0.72mL / min, 0.74mL / min, 0.76mL / min n, 0.78mL / min or 0.80mL / min; the chromium precipitation reaction temperature in the second stage of the chromium precipitation reaction can be 90°C, 91°C, 92°C, 93°C, 94°C, 95°C, 96°C, 97°C, 98°C, 99°C or 100°C, etc.; the stirring rate of the chromium precipitation reaction is 200-300r / min, for example, it can be 200r / min, 210r / min, 220r / min, 230r / min, 240r / min, 250r / min, 260r / min, 270r / min, 280r / min, 290r / min or 300r / min, etc., but is not limited to the listed values, and other values ​​not listed in this range are also applicable

[0032] It is worth noting that different ammonia addition rates need to be controlled at different chromium precipitation stages. In the first stage, only acid-base neutralization reaction occurs. The excess acid after reduction is neutralized with ammonia, and no Cr(OH)3 precipitation reaction occurs. It is necessary to control a faster ammonia addition rate and stirring rate to improve mass transfer and accelerate the acid-base reaction rate. If the ammonia addition rate is too low, it will lead to a long reaction time and high reaction energy consumption; in the second stage, Cr(OH)3 precipitation reaction occurs, and it is necessary to control a lower ammonia addition rate and stirring rate, while ensuring the crystallization boiling rate, so as to ensure the generation of large particles of Cr(OH)3 solid precipitate. If the ammonia addition rate is too fast, or the stirring rate is too fast, the supersaturation of Cr(OH)3 will increase, causing the Cr(OH)3 crystal particles to explode into nuclei, generating a large number of extremely fine particles of precipitation, making it difficult for Cr(OH)3 to settle and filter, and it is impossible to obtain a clear chromium removal solution. If the ammonia addition rate is too slow, or the stirring rate is too slow, the local Cr(OH)3 supersaturation will increase, reducing the reaction and precipitation efficiency.

[0033] Furthermore, in step c), the flocculant is anionic polyacrylamide with a concentration of 1-3 g / L, the flocculation sedimentation temperature is 60-80° C., and the flocculation sedimentation time is 20-24 h.

[0034] Preferably, the molecular weight of the flocculant is 5 million to 40 million, such as 5 million, 10 million, 15 million, 20 million, 25 million, 30 million, 35 million or 40 million, etc.; the added amount is 5-10 mL / L, such as 5.0 mL / L, 5.5 mL / L, 6.0 mL / L, 6.5 mL / L, 7.0 mL / L, 7.5 mL / L, 8.0 mL / L, 8.5 mL / L, 9.0 mL / L, 9.5 mL / L or 10.0 mL / L.

[0035] Furthermore, in step d), the oxidant is hydrogen peroxide, and the pH is adjusted to 4.5-6.5, such as adjusted to a pH value of 4.5, 4.6, 4.8, 5.0, 5.5, 5.7, 5.9, 6.2, 6.4 or 6.5.

[0036] Preferably, the amount of hydrogen peroxide added is 1.1-1.3 times the theoretical amount of oxidizing tetravalent vanadium to pentavalent vanadium; for example, the amount added is 1.1 times, 1.12 times, 1.14 times, 1.16 times, 1.22 times, 1.24 times, 1.26 times, 1.28 times or 1.3 times.

[0037] Preferably, the oxidation reaction temperature is 60-80°C, for example, 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, 72°C, 74°C, 76°C, 78°C or 80°C, etc.; the oxidation reaction time is 0.5-1.0h, such as 30min, 35min, 40min, 45min, 50min, 55min, 1.0h, etc.

[0038] The purpose of the hydrogen peroxide oxidation is to oxidize V(IV) in the solution to V(V), while destroying the complexation of citrate to vanadium ions. Therefore, hydrogen peroxide is both an oxidant and a decomplexer, thereby creating conditions for the cooling crystallization of ammonium metavanadate. If the amount of hydrogen peroxide added is insufficient, V(IV) cannot be completely oxidized, and the complexation of citrate to vanadium ions cannot be completely destroyed, resulting in a low subsequent crystallization rate of ammonium metavanadate.

[0039] Further, in step e), the initial pH value of the chromium removal solution crystallization is 8-8.5, which can be 8.00, 8.05, 8.10, 8.15, 8.20, 8.25, 8.30, 8.35, 8.40, 8.45 or 8.50, etc.; the crystallization of ammonium metavanadate needs to be maintained within the appropriate range of pH 8-8.5, and too high or too low pH will reduce the crystallization rate of ammonium metavanadate. The crystallization endpoint temperature is 30-50°C, such as 30°C, 32°C, 34°C, 36°C, 38°C, 40°C, 42°C, 44°C, 46°C, 48°C or 50°C, etc.; the crystallization time is 2-4h, such as 2.0h, 2.2h, 2.4h, 2.6h, 2.8h, 3.0h, 3.2h, 3.4h, 3.6h, 3.8h or 4.0h.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The chromium removal method of the present invention can achieve the effect of efficient chromium removal only through simple dissolution and chemical precipitation methods, and has no requirements on the content of impurity elements; the method has high chromium removal efficiency, low energy consumption loss, high vanadium recovery rate, is clean and environmentally friendly, is more beneficial to environmental protection, and can realize industrial production.

[0042] The method of the present invention adopts a method of acidic vanadium-chromium simultaneous reduction-alkaline chromium precipitation-acidic vanadium oxidation, and realizes efficient removal of chromium through multiple steps, thereby improving the selectivity of the chromium impurity removal process. The prepared high-purity ammonium metavanadate has high purity and low impurity element content, and can meet the purity requirements of vanadium products used in electrolytes and aviation manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 The present invention is a flow chart of a method for removing chromium from ammonium polyvanadate by reduction precipitation according to a specific embodiment of the present invention. DETAILED DESCRIPTION

[0044] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0045] In one embodiment, a method for removing chromium from ammonium polyvanadate by reduction precipitation is provided, and its flow diagram is as follows: Figure 1 As shown, the steps include:

[0046] S1. Ammonia is added to the ammonium polyvanadate solution to perform a transammonium reaction to obtain an ammonium solution;

[0047] S2. Add citric acid to the ammonium solution to reduce hexavalent chromium and pentavalent vanadium to trivalent chromium and tetravalent vanadium, respectively, to obtain a reduced solution;

[0048] S3. Ammonia was added to the reducing solution, the pH was adjusted to alkaline for chromium precipitation reaction, a flocculant was added for sedimentation, and a chromium removal solution was obtained after filtration;

[0049] S4. Adding acid to the chromium removal solution to adjust the pH, and then adding an oxidant to oxidize the tetravalent vanadium to pentavalent vanadium;

[0050] S5. Adjust the pH of the solution obtained by oxidation, cool it down and crystallize it, and filter it to obtain high-purity ammonium metavanadate.

[0051] In the above embodiment, a method of acidic vanadium-chromium simultaneous reduction-alkaline chromium precipitation-acidic vanadium oxidation is adopted to achieve efficient removal of chromium through multiple steps, thereby improving the selectivity of the chromium impurity removal process, and the purification process is clean and environmentally friendly. The prepared high-purity ammonium metavanadate has a purity higher than 99.5% and a low content of impurity elements, which can meet the purity requirements of vanadium products used in electrolytes and aviation manufacturing and is suitable for industrial production.

[0052] In the above embodiment, by adopting appropriate reagents and removal methods, chromium can be removed efficiently and ammonium metavanadate can be recovered efficiently, achieving low cost and high profit.

[0053] The following are preferred implementation examples. For those reagents or instrument manufacturers not specified in the examples, conventional products can be purchased on the market. For those without specific conditions, conventional conditions or manufacturer recommended conditions can be followed. Unless otherwise specified, the data indicating the proportion of components such as "content" and "concentration" are all weight proportions. The ammonium polyvanadate used in the example is a primary product prepared by roasting, leaching, and vanadium precipitation of raw materials such as vanadium slag and stone coal. The content of V2O5 in ammonium polyvanadate is 85-90%, and the content of Cr is 0.1-0.5%.

[0054] Example 1

[0055] Ammonium polyvanadate with a V2O5 content of 89.5% and a Cr content of 0.2% was selected as the purification raw material, mixed with hot water at a ratio of 16:1, the dissolution temperature was 70°C, and ammonia water was added to adjust the solution pH to 8.0. The ammonium polyvanadate was completely dissolved into ammonium metavanadate solution. The V(V) concentration in the solution was 31.35 g / L, the V(IV) concentration was 0 g / L, and the Cr(VI) concentration was 0.188 g / L.

[0056] In the ammonium metavanadate solution, citric acid was added according to the reduction of V(V) to 2.1 times the theoretical amount of V(IV), and sulfuric acid was added to adjust the pH to 3.6 for reduction reaction. The reduction temperature was 36°C and the reduction reaction time was 3.6 h. V(V) and Cr(VI) in the solution were reduced to V(IV) and Cr(III). At this time, the concentration of Cr(III) in the solution was 0.188 g / L, the concentration of V(V) was 0 g / L, and the concentration of V(IV) was 31.35 g / L.

[0057] After the reduction is completed, 12% ammonia water is added to adjust the solution pH. The first stage end point pH is 7.5, the ammonia water addition rate is 1.6 mL / min, the chromium precipitation reaction temperature is 86°C, and the reaction stirring rate is 460 r / min; the second stage end point pH is 9.5, the ammonia water addition rate is 0.6 mL / min, the chromium precipitation reaction temperature is 96°C, the chromium precipitation reaction stirring rate is 280 r / min, and after chromium precipitation, 1.6 g / L of anionic polyacrylamide with a molecular weight of 5.6 million is added 6 mL / L, the flocculation sedimentation temperature is 66°C, the flocculation sedimentation time is 22 hours, and then filtered to obtain the chromium-free solution. The Cr(III) concentration in the chromium-free solution is reduced to 0.018 g / L, the V(IV) concentration in the solution is 31.35 g / L, and the V(V) concentration is 0 g / L.

[0058] Sulfuric acid was added to the chromium-removed solution to adjust the pH to 5.6, and then hydrogen peroxide was added to oxidize V(IV) to V(V). The oxidation temperature was 77°C and the oxidation time was 0.5 h. The amount of hydrogen peroxide added was 1.22 times the theoretical amount of V(IV) oxidized to V(V). At this time, the Cr(III) concentration in the solution was 0.018 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 31.35 g / L.

[0059] Ammonia water was added to the oxidized solution with a pH of 8.2 for cooling and crystallization. The key crystallization temperature was 36°C and the crystallization time was 2.6 hours. After crystallization, the slurry obtained high-purity ammonium metavanadate crystals and crystallization mother liquor. The Cr(III) concentration in the crystallization mother liquor was 0.016 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 2.60 g / L. The purity of the high-purity ammonium metavanadate was 99.75%, of which the Cr content was 0.025%.

[0060] Example 2

[0061] Ammonium polyvanadate with a V2O5 content of 90% and a Cr content of 0.1% was selected as the purification raw material, mixed with hot water at a ratio of 15:1, the dissolution temperature was 80°C, and ammonia water was added to adjust the solution pH to 8.3. The ammonium polyvanadate was completely dissolved into ammonium metavanadate solution. The V(V) concentration in the solution was 33.63 g / L, the V(IV) concentration was 0 g / L, and the Cr(VI) concentration was 0.067 g / L.

[0062] In the ammonium metavanadate solution, citric acid was added according to the reduction of V(V) to 2.18 times the theoretical amount of V(IV), and sulfuric acid was added to adjust the pH to 4.0 for reduction reaction. The reduction temperature was 39°C and the reduction reaction time was 3.6 h. V(V) and Cr(VI) in the solution were reduced to V(IV) and Cr(III). At this time, the concentration of Cr(III) in the solution was 0.067 g / L, the concentration of V(V) was 0 g / L, and the concentration of V(IV) was 33.63 g / L.

[0063] After the reduction is completed, 10% ammonia water is added to adjust the solution pH. The first stage end point pH is 7.5, the ammonia water addition rate is 1.0 mL / min, the chromium precipitation reaction temperature is 80°C, and the reaction stirring rate is 450 r / min; the second stage end point pH is 9.5, the ammonia water addition rate is 0.5 mL / min, the chromium precipitation reaction temperature is 90°C, the chromium precipitation reaction stirring rate is 260 r / min, and after chromium precipitation, 1.0 g / L of anionic polyacrylamide with a molecular weight of 5 million is added 5 mL / L, the flocculation sedimentation temperature is 60°C, the flocculation sedimentation time is 20 hours, and then filtered to obtain the chromium-free solution. The Cr(III) concentration in the chromium-free solution is reduced to 0.012 g / L, the V(IV) concentration in the solution is 33.63 g / L, and the V(V) concentration is 0 g / L.

[0064] Sulfuric acid was added to the chromium-removed solution to adjust the pH to 4.56, and then hydrogen peroxide was added to oxidize V(IV) to V(V). The oxidation temperature was 68°C and the oxidation time was 0.5 h. The amount of hydrogen peroxide added was 1.13 times the theoretical amount of V(IV) oxidized to V(V). At this time, the Cr(III) concentration in the solution was 0.012 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 33.63 g / L.

[0065] Ammonia water was added to the oxidized solution with a pH of 8.0 for cooling and crystallization. The key crystallization temperature was 30°C and the crystallization time was 2 hours. After crystallization, the slurry obtained high-purity ammonium metavanadate crystals and crystallization mother liquor. The Cr(III) concentration in the crystallization mother liquor was 0.011 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 2.50 g / L. The purity of the high-purity ammonium metavanadate was 99.65%, and the Cr content was 0.028%.

[0066] Example 3

[0067] Ammonium polyvanadate with a V2O5 content of 85% and a Cr content of 0.5% was selected as the purification raw material, mixed with hot water at a ratio of 15:1, the dissolution temperature was 90°C, and ammonia water was added to adjust the solution pH to 8.5. The ammonium polyvanadate was completely dissolved into ammonium metavanadate solution. The V(V) concentration in the solution was 31.76 g / L, the V(IV) concentration was 0 g / L, and the Cr(VI) concentration was 0.333 g / L.

[0068] In the ammonium metavanadate solution, citric acid was added according to the reduction of V(V) to 2.3 times the theoretical amount of V(IV), and sulfuric acid was added to adjust the pH to 4.4 for reduction reaction. The reduction temperature was 40°C and the reduction reaction time was 3.8 h. V(V) and Cr(VI) in the solution were reduced to V(IV) and Cr(III). At this time, the concentration of Cr(III) in the solution was 0.333 g / L, the concentration of V(V) was 0 g / L, and the concentration of V(IV) was 31.76 g / L.

[0069] After the reduction is completed, 15% ammonia water is added to adjust the solution pH. The first stage end point pH is 8.0, the ammonia water addition rate is 2.2 mL / min, the chromium precipitation reaction temperature is 90°C, and the reaction stirring rate is 460 r / min; the second stage end point pH is 10, the ammonia water addition rate is 0.8 mL / min, the chromium precipitation reaction temperature is 100°C, the chromium precipitation reaction stirring rate is 250 r / min, and after chromium precipitation, 3 g / L of anionic polyacrylamide with a molecular weight of 40 million is added. 10 mL / L, the flocculation sedimentation temperature is 80°C, the flocculation sedimentation time is 24 hours, and then filtered to obtain the chromium-free solution. The Cr(III) concentration in the chromium-free solution is reduced to 0.021 g / L, the V(IV) concentration in the solution is 31.76 g / L, and the V(V) concentration is 0 g / L.

[0070] Sulfuric acid was added to the chromium-removed solution to adjust the pH to 6.3, and then hydrogen peroxide was added to oxidize V(IV) to V(V). The oxidation temperature was 75°C and the oxidation time was 0.8 h. The amount of hydrogen peroxide added was 1.21 times the theoretical amount of V(IV) oxidized to V(V). At this time, the Cr(III) concentration in the solution was 0.021 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 31.76 g / L.

[0071] Ammonia water was added to the oxidized solution with a pH of 8.5 for cooling and crystallization. The key crystallization temperature was 50°C and the crystallization time was 4 hours. After crystallization, the slurry obtained high-purity ammonium metavanadate crystals and crystallization mother liquor. The Cr (III) concentration in the crystallization mother liquor was 0.020 g / L, the V (IV) concentration was 0 g / L, and the V (V) concentration was 2.35 g / L. The purity of the high-purity ammonium metavanadate was 99.83%, of which the Cr content was 0.018%.

[0072] Example 4

[0073] Ammonium polyvanadate with a V2O5 content of 86% and a Cr content of 0.23% was selected as the purification raw material, mixed with hot water at a ratio of 16:1, the dissolution temperature was 76°C, and ammonia water was added to adjust the solution pH to 8.5. The ammonium polyvanadate was completely dissolved into ammonium metavanadate solution. The V(V) concentration in the solution was 31.12 g / L, the V(IV) concentration was 0 g / L, and the Cr(VI) concentration was 0.144 g / L.

[0074] In the ammonium metavanadate solution, citric acid was added according to the reduction of V(V) to 2.3 times the theoretical amount of V(IV), and sulfuric acid was added to adjust the pH to 3.8 for reduction reaction. The reduction temperature was 36°C and the reduction reaction time was 4.6 h. V(V) and Cr(VI) in the solution were reduced to V(IV) and Cr(III). At this time, the concentration of Cr(III) in the solution was 0.144 g / L, the concentration of V(V) was 0 g / L, and the concentration of V(IV) was 31.12 g / L.

[0075] After the reduction is completed, 12% ammonia water is added to adjust the solution pH. The first stage end point pH is 7.5, the ammonia water addition rate is 1.2 mL / min, the chromium precipitation reaction temperature is 86°C, and the reaction stirring rate is 440 r / min; the second stage end point pH is 9.6, the ammonia water addition rate is 0.6 mL / min, the chromium precipitation reaction temperature is 96°C, the chromium precipitation reaction stirring rate is 230 r / min, and after chromium precipitation, 1.8 g / L of anionic polyacrylamide with a molecular weight of 10 million is added 6 mL / L, the flocculation sedimentation temperature is 70°C, the flocculation sedimentation time is 22 hours, and then filtered to obtain the chromium-free solution. The Cr(III) concentration in the chromium-free solution is reduced to 0.014 g / L, the V(IV) concentration in the solution is 31.12 g / L, and the V(V) concentration is 0 g / L.

[0076] Sulfuric acid was added to the chromium-removed solution to adjust the pH to 4.8, and then hydrogen peroxide was added to oxidize V(IV) to V(V). The oxidation temperature was 65°C and the oxidation time was 0.7 h. The amount of hydrogen peroxide added was 1.15 times the theoretical amount of V(IV) oxidized to V(V). At this time, the Cr(III) concentration in the solution was 0.014 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 31.12 g / L.

[0077] Ammonia water was added to the oxidized solution with a pH of 8.25 for cooling and crystallization. The crystallization key temperature was 38°C and the crystallization time was 3.6 hours. After crystallization, the slurry obtained high-purity ammonium metavanadate crystals and crystallization mother liquor. The Cr(III) concentration in the crystallization mother liquor was 0.015 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 2.64 g / L. The purity of the high-purity ammonium metavanadate was 99.59%, of which the Cr content was 0.029%.

[0078] Example 5

[0079] Ammonium polyvanadate with a V2O5 content of 88.9% and a Cr content of 0.21% was selected as the purification raw material, mixed with hot water at a ratio of 18:1, the dissolution temperature was 85°C, and ammonia water was added to adjust the solution pH to 8.3. The ammonium polyvanadate was completely dissolved into ammonium metavanadate solution. The V(V) concentration in the solution was 27.68 g / L, the V(IV) concentration was 0 g / L, and the Cr(VI) concentration was 0.117 g / L.

[0080] In the ammonium metavanadate solution, citric acid was added according to the reduction of V(V) to 2.3 times the theoretical amount of V(IV), and sulfuric acid was added to adjust the pH to 3.6 for reduction reaction. The reduction temperature was 40°C and the reduction reaction time was 4.2 h. V(V) and Cr(VI) in the solution were reduced to V(IV) and Cr(III). At this time, the concentration of Cr(III) in the solution was 0.117 g / L, the concentration of V(V) was 0 g / L, and the concentration of V(IV) was 27.68 g / L.

[0081] After the reduction is completed, 14% ammonia water is added to adjust the solution pH. The first stage end point pH is 7.8, the ammonia water addition rate is 2.1 mL / min, the chromium precipitation reaction temperature is 86°C, and the reaction stirring rate is 500 r / min; the second stage end point pH is 9.9, the ammonia water addition rate is 0.65 mL / min, the chromium precipitation reaction temperature is 96°C, the chromium precipitation reaction stirring rate is 300 r / min, and after chromium precipitation, 2.6 g / L of anionic polyacrylamide with a molecular weight of 20 million is added 6.5 mL / L, the flocculation sedimentation temperature is 76°C, the flocculation sedimentation time is 21 hours, and then filtered to obtain the chromium-free solution. The Cr(III) concentration in the chromium-free solution is reduced to 0.012 g / L, the V(IV) concentration in the solution is 27.68 g / L, and the V(V) concentration is 0 g / L.

[0082] Sulfuric acid was added to the chromium-removed solution to adjust the pH to 5.5, and then hydrogen peroxide was added to oxidize V(IV) to V(V). The oxidation temperature was 70°C and the oxidation time was 0.8 h. The amount of hydrogen peroxide added was 1.2 times the theoretical amount of V(IV) oxidized to V(V). At this time, the Cr(III) concentration in the solution was 0.012 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 27.68 g / L.

[0083] Ammonia water was added to the oxidized solution with a pH of 8.4 for cooling and crystallization. The key crystallization temperature was 46°C and the crystallization time was 3.5 hours. After crystallization, the slurry obtained high-purity ammonium metavanadate crystals and crystallization mother liquor. The Cr(III) concentration in the crystallization mother liquor was 0.012 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 2.42 g / L. The purity of the high-purity ammonium metavanadate was 99.60%, and the Cr content was 0.027%.

[0084] Example 6

[0085] Ammonium polyvanadate with a V2O5 content of 89% and a Cr content of 0.2% was selected as the purification raw material, mixed with hot water at a ratio of 20:1, the dissolution temperature was 85°C, and ammonia water was added to adjust the solution pH to 8.1. The ammonium polyvanadate was completely dissolved into ammonium metavanadate solution. The V(V) concentration in the solution was 24.94 g / L, the V(IV) concentration was 0 g / L, and the Cr(VI) concentration was 0.100 g / L.

[0086] In the ammonium metavanadate solution, citric acid was added according to the reduction of V(V) to 2.4 times the theoretical amount of V(IV), and sulfuric acid was added to adjust the pH to 4.5 for reduction reaction. The reduction temperature was 42°C and the reduction reaction time was 5 hours. V(V) and Cr(VI) in the solution were reduced to V(IV) and Cr(III). At this time, the concentration of Cr(III) in the solution was 0.100 g / L, the concentration of V(V) was 0 g / L, and the concentration of V(IV) was 24.94 g / L.

[0087] After the reduction is completed, 13% ammonia water is added to adjust the solution pH. The first stage end point pH is 7.5, the ammonia water addition rate is 1.8 mL / min, the chromium precipitation reaction temperature is 89°C, and the reaction stirring rate is 400 r / min; the second stage end point pH is 9.58, the ammonia water addition rate is 0.56 mL / min, the chromium precipitation reaction temperature is 96°C, the chromium precipitation reaction stirring rate is 200 r / min, and after chromium precipitation, 1.6 g / L of anionic polyacrylamide with a molecular weight of 30 million is added 8 mL / L, the flocculation sedimentation temperature is 70°C, the flocculation sedimentation time is 22 hours, and then filtered to obtain the chromium-free solution. The Cr(III) concentration in the chromium-free solution is reduced to 0.008 g / L, the V(IV) concentration in the solution is 24.94 g / L, and the V(V) concentration is 0 g / L.

[0088] Sulfuric acid was added to the chromium-removed solution to adjust the pH to 6.5, and then hydrogen peroxide was added to oxidize V(IV) to V(V). The oxidation temperature was 80°C and the oxidation time was 1 h. The amount of hydrogen peroxide added was 1.3 times the theoretical amount of V(IV) oxidized to V(V). At this time, the Cr(III) concentration in the solution was 0.008 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 24.94 g / L.

[0089] Ammonia water was added to the oxidized solution with a pH of 8.2 for cooling and crystallization. The key crystallization temperature was 34°C and the crystallization time was 2.6 hours. After crystallization, the slurry obtained high-purity ammonium metavanadate crystals and crystallization mother liquor. The Cr(III) concentration in the crystallization mother liquor was 0.007 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 2.60 g / L. The purity of the high-purity ammonium metavanadate was 99.70%, of which the Cr content was 0.020%.

[0090] Example 7

[0091] Ammonium polyvanadate with a V2O5 content of 88.5% and a Cr content of 0.22% was selected as the purification raw material, mixed with hot water at a ratio of 16:1, the dissolution temperature was 73°C, and ammonia water was added to adjust the solution pH to 8.4. The ammonium polyvanadate was completely dissolved into ammonium metavanadate solution. The V(V) concentration in the solution was 30.99 g / L, the V(IV) concentration was 0 g / L, and the Cr(VI) concentration was 0.138 g / L.

[0092] In the ammonium metavanadate solution, citric acid was added according to the reduction of V(V) to 2.1 times the theoretical amount of V(IV), and sulfuric acid was added to adjust the pH to 3.2 for reduction reaction. The reduction temperature was 30°C and the reduction reaction time was 3h. V(V) and Cr(VI) in the solution were reduced to V(IV) and Cr(III). At this time, the concentration of Cr(III) in the solution was 0.138 g / L, the concentration of V(V) was 0 g / L, and the concentration of V(IV) was 30.99 g / L.

[0093] After the reduction is completed, 12% ammonia water is added to adjust the solution pH. The first stage end point pH is 7.5, the ammonia water addition rate is 2.0 mL / min, the chromium precipitation reaction temperature is 81°C, and the reaction stirring rate is 460 r / min; the second stage end point pH is 9.7, the ammonia water addition rate is 0.68 mL / min, the chromium precipitation reaction temperature is 93°C, the chromium precipitation reaction stirring rate is 260 r / min, and after chromium precipitation, 1.5 g / L of anionic polyacrylamide with a molecular weight of 35 million is added 6 mL / L, the flocculation sedimentation temperature is 77°C, the flocculation sedimentation time is 22 hours, and then filtered to obtain the chromium-free solution. The Cr(III) concentration in the chromium-free solution is reduced to 0.014 g / L, the V(IV) concentration in the solution is 30.99 g / L, and the V(V) concentration is 0 g / L.

[0094] Sulfuric acid was added to the chromium-removed solution to adjust the pH to 4.5, and then hydrogen peroxide was added to oxidize V(IV) to V(V). The oxidation temperature was 60°C and the oxidation time was 0.5 h. The amount of hydrogen peroxide added was 1.1 times the theoretical amount of V(IV) oxidized to V(V). At this time, the Cr(III) concentration in the solution was 0.014 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 30.99 g / L.

[0095] Ammonia water was added to the oxidized solution with a pH of 8.1 for cooling and crystallization. The crystallization key temperature was 36°C and the crystallization time was 4 hours. After crystallization, the slurry obtained high-purity ammonium metavanadate crystals and crystallization mother liquor. The Cr(III) concentration in the crystallization mother liquor was 0.013 g / L, the V(IV) concentration was 0 g / L, and the V(V) concentration was 2.55 g / L. The purity of high-purity ammonium metavanadate was 99.71%, and the Cr content was 0.021%.

[0096] Comparative Example 1

[0097] For comparison of purification effects, the reduction pH was adjusted to 1.5, and the other conditions were the same as in Example 3. It was found that a large amount of vanadium was precipitated in the reduction process in the form of ammonium polyvanadate, which resulted in a decrease in the vanadium yield in the subsequent process and affected the subsequent chromium removal effect. The purity of high-purity ammonium metavanadate was 99.22%, of which the Cr content was 0.019%.

[0098] Comparative Example 2

[0099] For comparison of purification effects, the reduction pH was adjusted to 6.5, and the other conditions were the same as those in Example 5. At this time, the reduction of vanadium and chromium was not complete, and the purpose of efficient chromium removal could not be achieved. The purity of high-purity ammonium metavanadate was 98.71%, and the Cr content was 0.170%.

[0100] Comparative Example 3

[0101] To compare the purification effects, the pH was adjusted to 8.2 in the first stage of the reaction, and the other conditions were the same as in Example 4. It was found that in the first stage, a large amount of chromium hydroxide had been precipitated, and the particles were very fine, making subsequent filtration difficult, with a filtration phenomenon, and the chromium content in the product was high. The purity of the high-purity ammonium metavanadate was 99.32%, of which the Cr content was 0.080%.

[0102] Comparative Example 4

[0103] To compare the purification effects, the pH was adjusted to 14 in the second stage of the reaction, and the other conditions were the same as in Example 4. It was found that the precipitated chromium hydroxide showed a small amount of re-dissolution, indicating that the alkalinity in the second stage was too high to be conducive to the precipitation and removal of chromium hydroxide. The purity of high-purity ammonium metavanadate was 99.41%, of which the Cr content was 0.078%.

[0104] Comparative Example 5

[0105] For comparison of purification effects, the addition rate of ammonia water in the second stage of the reaction was 5 mL / L, and the other conditions were the same as those in Example 4. It was found that in the second stage, a large amount of chromium hydroxide precipitated, and the particles were very fine, making subsequent filtration difficult, with a filtration phenomenon, and the chromium content in the product was high. The purity of high-purity ammonium metavanadate was 99.21%, of which the Cr content was 0.091%.

[0106] Comparative Example 6

[0107] For comparison of purification effects, the flocculation and sedimentation time in the chromium precipitation stage was 10 hours, and the other conditions were the same as those in Example 4. It was found that due to insufficient sedimentation time, a small amount of fine chromium hydroxide particles were not completely precipitated, and there was a filtration phenomenon after filtration, resulting in a high chromium content in the product. The purity of high-purity ammonium metavanadate was 98.71%, of which the Cr content was 0.110%.

[0108] Comparative Example 7

[0109] For comparison of purification effects, no flocculant was added for flocculation and sedimentation in the chromium precipitation stage, and the other conditions were the same as those in Example 4. It was found that due to insufficient sedimentation time, a small amount of fine chromium hydroxide particles were not completely precipitated, and there was a filtration phenomenon after filtration, resulting in a high chromium content in the product. The purity of high-purity ammonium metavanadate was 98.89%, of which the Cr content was 0.102%.

[0110] The purity and related impurity content of ammonium metavanadate prepared by the chromium removal method of the above Examples 1-7 and Comparative Examples 1-7 are summarized in Table 1.

[0111] Table 1:

[0112] Group Ammonium metavanadate purity % Chromium content% Example 1 99.75 0.025 Example 2 99.65 0.028 Example 3 99.83 0.018 Example 4 99.59 0.029 Example 5 99.60 0.027 Example 6 99.70 0.020 Example 7 99.71 0.021 Comparative Example 1 99.22 0.090 Comparative Example 2 98.71 0.170 Comparative Example 3 99.32 0.080 Comparative Example 4 99.41 0.078 Comparative Example 5 99.21 0.091 Comparative Example 6 98.71 0.110 Comparative Example 7 98.89 0.102

[0113] It is not difficult to see from Table 1 that the purity of ammonium metavanadate obtained in Examples 1-7 is greater than 99.5%, and the chromium content is less than 0.05%; when the pH value of the reduction process is controlled to 1.5 in Comparative Example 1, a large amount of vanadium is precipitated during the reduction process, resulting in a decrease in the vanadium yield of the entire process and affecting the chromium removal effect; when the pH value of the reduction process is controlled to 6.5 in Comparative Example 2, the chromium removal effect is also poor, which confirms that the pH range of the reduction reaction process has an effect on the chromium removal effect. In addition, Comparative Examples 3, 4, and 5 illustrate that the pH value of the first and second stages of the chromium removal reaction and the rate of addition of ammonia water have an effect on the precipitation of chromium hydroxide; Comparative Examples 6 and 7 illustrate that the flocculation time is short or no flocculant is added, resulting in insufficient sedimentation time, and a small amount of fine chromium hydroxide particles are not completely precipitated, resulting in a high chromium content in the product. The above comparison shows that citric acid is particularly important for the sedimentation effect after chromium reduction. At the same time, the addition of ammonia water and the rate of ammonia water addition must be strictly controlled to meet the chromium removal requirements, and the steps in the links cannot be adjusted at will.

[0114] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for removing chromium from ammonium polyvanadate by reduction precipitation, characterized in that the steps include: a) adding aqueous ammonia to the ammonium polyvanadate solution to carry out a transammonium reaction to obtain an transammonium solution; b) adding citric acid to the ammonium conversion solution to reduce hexavalent chromium and pentavalent vanadium to trivalent chromium and tetravalent vanadium, respectively, to obtain a reduction solution; c) adding ammonia water to the reducing solution, adjusting the pH to alkaline to carry out chromium precipitation reaction, adding a flocculant to carry out sedimentation, and filtering to obtain a chromium removal solution; d) adding acid to the chromium removal solution to adjust the pH, and then adding an oxidant to oxidize the tetravalent vanadium to pentavalent vanadium; e) adjusting the pH of the solution obtained by oxidation, cooling and crystallizing, and filtering to obtain high-purity ammonium metavanadate.

2. The method according to claim 1, characterized in that The content of V2O5 in the ammonium polyvanadate is 85-90wt%, and the content of Cr is 0.1-0.5wt%.

3. The method according to claim 1, characterized in that In step a), the solvent of the ammonium polyvanadate solution is water or the crystallization mother liquor obtained in step e); And / or, the liquid-to-solid ratio of the ammonium conversion reaction is (15-20): 1 and the temperature of the dissolution process is 70-90°C; And / or, the pH value of the ammonium transfer is 8-8.

5.

4. The method according to claim 1, characterized in that: In step b), the amount of citric acid used is 2.1-2.4 times the theoretical amount of tetravalent vanadium for reducing pentavalent vanadium; And / or, the reduction reaction adjusts the pH value to 3.2-4.5; And / or, the reduction reaction time is 3-5h, and the reduction reaction temperature is 30-42°C.

5. The method according to claim 1, characterized in that In step c), the concentration of the ammonia water is 10-15%, and the chromium precipitation reaction step includes: 1) Add ammonia water until the pH is 7.5-8.0, the addition rate is 1.0-2.2 mL / min, and the chromium precipitation reaction temperature is 80-90°C; 2) Continue to add ammonia water until the pH is 9.5-10.0, the addition rate is 0.5-0.8 mL / min, and the chromium precipitation reaction temperature is 90-100°C.

6. The method according to claim 1, characterized in that In step c), the flocculant is anionic polyacrylamide with a concentration of 1-3 g / L, the flocculation sedimentation temperature is 60-80° C., and the flocculation sedimentation time is 20-24 h.

7. The method according to claim 6, characterized in that The molecular weight of the flocculant is 5 million to 40 million, and the added amount is 5 to 10 mL / L.

8. The method according to claim 1, characterized in that In step d), the oxidant is hydrogen peroxide, and the pH is adjusted to 4.5-6.

5.

9. The method according to claim 8, characterized in that The amount of hydrogen peroxide added is 1.1-1.3 times the theoretical amount of pentavalent vanadium after the tetravalent vanadium is oxidized; And / or, the oxidation reaction temperature is 60-80° C., and the oxidation reaction time is 0.5-1.0 h.

10. The method according to claim 1, characterized in that In step e), the initial pH value of the chromium removal solution during crystallization is 8-8.5, the crystallization endpoint temperature is 30-50° C., and the crystallization time is 2-4 hours.

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