A kind of preparation method of ammonium heptamolybdate

Abstract

The present application relates to the technical field of preparation of ammonium heptamolybdate, and specifically to a method for preparing ammonium heptamolybdate. The present application is to accelerate the dissolution of molybdenum in molybdenum concentrate by using an efficient catalyst, thereby improving the recovery rate of molybdenum. The preparation method of ammonium heptamolybdate in the present application has the following specific steps: first, the molybdenum concentrate is crushed, and under the catalytic action of the catalyst, the low-valent molybdenum in the molybdenum concentrate is oxidized to a high-valent state by hydrogen peroxide and dissolved, and then ammonia leaching and impurity removal are performed; finally, evaporation concentration, cooling and crystallization are performed. The catalyst used in the preparation process of ammonium heptamolybdate is a carbon-nitrogen-doped metal oxide nanomaterial, which has a large specific surface area, is rich in oxygen vacancies, has a high catalytic activity, and is conducive to the rapid oxidation and dissolution of molybdenum. In addition, a large amount of acid solution and ammonia water are not used in the above-mentioned preparation process of ammonium heptamolybdate, which reduces pollution.

Description

Technical Field

[0001] The present application relates to the technical field of preparation of ammonium heptamolybdate, and in particular to a method for preparing ammonium heptamolybdate. Background Art

[0002] Ammonium heptamolybdate is an important intermediate product in the molybdenum metallurgical process. It is used in the preparation of molybdenum powder and its deep-processing products, catalysts, dyes, pigments, micro-fertilizers, etc. Moreover, with the development of science and technology, it is gradually developing into high-tech fields such as high-end material preparation, nanomaterial synthesis, and detection and analysis.

[0003] At present, the domestic production of ammonium heptamolybdate generally adopts direct oxidation roasting method, firstly, the molybdenum concentrate is oxidized into industrial molybdenum oxide, and the industrial molybdenum oxide is pickled, ammonia leached, and impurities are removed to precipitate ammonium tetramolybdate, and then the ammonium tetramolybdate is dissolved in ammonia water, evaporated and concentrated, cooled and crystallized to obtain the ammonium heptamolybdate product. This production process has a long process, high cost, low output, and relatively poor quality, which cannot meet the market requirements. In addition, the processes such as pickling, acid precipitation and secondary ammonia dissolution produce a large amount of ammonia nitrogen pollutants, which are difficult to handle. Therefore, how to further improve the preparation process of ammonium heptamolybdate, reduce environmental pollution, and improve the quality of ammonium heptamolybdate will be the main development direction of production enterprises.

[0004] The Chinese patent application document with publication number CN112758983A discloses a method for preparing ammonium heptamolybdate. The method does not adopt the method of ammonium tetramolybdate ammonia dissolution and recrystallization, thereby reducing ammonia nitrogen wastewater pollution. However, the method uses molybdenum roasted sand as a raw material. The molybdenum roasted sand consumes a lot of energy during the preparation process, and the low-valent molybdenum that has not been completely oxidized is difficult to be recovered, thereby reducing the recovery rate of molybdenum.

[0005] In order to improve the recovery rate of molybdenum, a Chinese patent application document with publication number CN107601565A discloses a method for preparing ammonium molybdate. In this method, hydrogen peroxide is added to convert low-valent molybdenum into high-valent state, thereby improving the recovery rate of molybdenum. However, a large amount of organic solvent is used in the preparation process, and subsequent processing is difficult. Summary of the invention

[0006] In order to reduce ammonia nitrogen pollution and improve molybdenum recovery rate, the present application provides a method for preparing ammonium heptamolybdate.

[0007] A method for preparing ammonium heptamolybdate comprises the following steps:

[0008] S1: crush the molybdenum concentrate, add water to make a slurry, add a catalyst and a hydrogen peroxide solution, mix and stir evenly, adjust the pH value to 9-11, react at 25-35°C for 1-3h, filter, and obtain a filtrate;

[0009] S2: Add liquid ammonia to the filtrate to adjust the pH value to 8.5-9, react at 60-70°C for 0.5-1h, slowly add ammonium carbonate while stirring, let stand, filter, and obtain an ammonium molybdate solution;

[0010] S3: Evaporating and concentrating the ammonium molybdate solution to a pH value of 6-6.5, cooling and crystallizing, filtering, washing, centrifuging, and drying;

[0011] The method for preparing the catalyst in step S1 above comprises the following steps:

[0012] (1) Dispersing the carbon and nitrogen source organic ligand in water, adding the metal active component precursor, adjusting the pH value to 2-6, reacting at 120-160°C for 0.5-3h, cooling, crystallizing, and grinding to obtain a metal complex powder;

[0013] (2) Calcine the above metal complex powder at 300-450°C for 1-3h under nitrogen atmosphere.

[0014] In the above technical scheme, first, under the catalytic action of the catalyst, the low-valent molybdenum in the molybdenum concentrate is oxidized to a high-valent state by hydrogen peroxide and enters the solution; then liquid ammonia is introduced to generate ammonium molybdate; then ammonium carbonate is added to remove impurities; finally, ammonium heptamolybdate is obtained through evaporation concentration, cooling crystallization, filtration, washing, centrifugation and drying.

[0015] In the process of molybdenum dissolution, the catalyst plays an important role. First, the active component in the catalyst is a metal oxide, which can continuously receive and lose electrons, play the role of electron transfer, and continuously regenerate in a cycle, which not only maintains the activity of the catalyst, but also supports the decomposition of hydrogen peroxide and promotes the oxidation of molybdenum; second, after the metal oxide in the catalyst is doped with carbon and nitrogen, the oxygen vacancies will increase, so the catalytic activity is higher; third, the catalyst is based on carbon and nitrogen, which overcomes the disadvantage of easy agglomeration of metal oxide nanomaterials, and also provides higher stability for the catalyst, preventing the catalyst from deactivating during long-term use.

[0016] In addition, liquid ammonia is directly introduced during the ammonia leaching process instead of using an aqueous ammonia solution, which simplifies ammonia recovery and greatly reduces the amount of ammonia solution used, thereby reducing the discharge of ammonia nitrogen wastewater. It can also provide sufficient ammonia sources, so the amination reaction can be carried out quickly and efficiently, thereby increasing the formation rate of ammonium molybdate and shortening the reaction time.

[0017] In the process of catalyst preparation, firstly, the metal complex is synthesized by hydrothermal method, and then the metal complex is used as a precursor to obtain the catalyst by high temperature roasting in nitrogen atmosphere. Firstly, nitrogen is introduced under high temperature conditions to reduce the oxygen concentration, and the oxygen atoms in the material are separated due to lack of oxygen to form oxygen vacancies; secondly, the reaction of carbon and nitrogen with metal oxides under high temperature conditions will consume the oxygen in the metal oxide to form oxygen vacancies; thirdly, the acidic environment may also increase the desorption of oxygen atoms. Therefore, the catalyst obtained by this method is rich in oxygen vacancies, and oxygen vacancies can adsorb reactant molecules, such as hydrogen peroxide molecules, to decompose them into oxygen active species. The oxygen active species is a strong oxidant, which can further attack molybdenum disulfide in molybdenum concentrate powder, promote the breaking of sulfur-molybdenum bonds of molybdenum disulfide, oxidize molybdenum from a low valence state to a high valence state, accelerate the oxidation process, and increase the oxidation reaction rate. In addition, oxygen vacancies will also cause changes in local electron density, which is conducive to the transfer of electrons. It can not only provide additional electrons to participate in the catalytic reaction, but also accept electrons, thereby increasing the catalytic reaction rate.

[0018] Preferably, in step S1, the mass ratio of the molybdenum concentrate powder, the catalyst and hydrogen peroxide is 100:(0.5-2):(15-30).

[0019] Preferably, in step S1, the solid-liquid ratio of the molybdenum concentrate powder to water is 1:(3-4.5).

[0020] Preferably, the metal complex powder in step (1) is modified by a method comprising the following steps:

[0021] Dissolve the iron salt in water, slowly add sodium hydroxide while stirring, filter, add acetic acid to the precipitate, stir to form a colloidal solution; disperse the metal complex powder in the colloidal solution, centrifuge, wash and dry.

[0022] In the above technical solution, by forming a colloidal solution of iron, the iron oxide nanoparticles formed after calcination are made smaller, the specific surface area of ​​the catalyst nanomaterial is increased, and the dispersion is better, and the agglomeration of nano iron oxide is reduced, thereby further improving the catalytic activity of the catalyst. In addition, nano iron oxide can also be used as a metal active component to synergistically improve the catalytic activity of the catalyst.

[0023] Preferably, the carbon-nitrogen source organic ligand in step (1) is one of imidazole-4,5-dicarboxylic acid, benzimidazole-5,6-dicarboxylic acid, pyridine-3,4-dicarboxylic acid, pyridine-2,6-dicarboxylic acid and pyridine-2,5-dicarboxylic acid.

[0024] Preferably, in step (1), the metal active component is one of copper oxide, cobalt oxide and nickel oxide.

[0025] Preferably, in step (1), the mass ratio of the carbon-nitrogen source organic ligand to the metal active component precursor is 1:(1.2-1.8).

[0026] Preferably, the mass ratio of the iron salt to acetic acid is 1:(0.45-0.7).

[0027] Preferably, the mass ratio of the iron salt to the metal complex powder is 1:(0.5-1.8).

[0028] The above technical solution of the present application includes at least the following beneficial effects:

[0029] 1. The present application uses carbon-nitrogen doped metal oxide nanomaterials as catalysts to catalyze the transformation of low-valent molybdenum to a high-valent state, accelerate the oxidation process of molybdenum, promote the dissolution of molybdenum, and improve the recovery rate of molybdenum.

[0030] 2. The present application first obtains the metal complex powder by a hydrothermal method and then calcines it in a nitrogen atmosphere to increase the specific surface area and the number of oxygen vacancies, thereby improving the catalytic activity of the catalyst.

[0031] 3. In the present application, the metal complex powder is dispersed in a colloidal solution of iron. After calcination, the loaded nano-iron oxide has a larger specific surface area and a more uniform distribution, further improving the catalytic activity of the catalyst.

[0032] 4. In this application, ammonia water and a large amount of acid solution are not used, but liquid ammonia is directly introduced, and ammonia recovery is simple, which greatly reduces the use of ammonia water, thereby reducing the discharge of ammonia nitrogen wastewater, and can provide sufficient ammonia source, increase the formation rate of ammonium molybdate, and shorten the reaction time. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a trend chart of the purity of ammonium heptamolybdate and the recovery rate of molybdenum. DETAILED DESCRIPTION

[0034] The present application is further described in detail below in conjunction with embodiments.

[0035] Unless otherwise specified, the raw materials used in the examples and comparative examples of the present application are all commercially available.

[0036] Example

[0037] Example 1

[0038] The preparation method of ammonium heptamolybdate of the present embodiment comprises the following steps:

[0039] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 300 g of water to make a slurry, add 0.5 g of catalyst and 50 g of 30% hydrogen peroxide solution, mix and stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 9, react at 25°C for 3 h, filter and obtain a filtrate;

[0040] S2: Add liquid ammonia to the filtrate to adjust the pH value to 8.5, react at 60°C for 1 hour, slowly add ammonium carbonate while stirring until the solution is transparent, let stand, filter, and obtain an ammonium molybdate solution;

[0041] S3: evaporating and concentrating the ammonium molybdate solution to a pH value of 6, cooling and crystallizing the concentrated ammonium molybdate solution, filtering, washing, centrifuging, and drying;

[0042] The preparation method of the catalyst described in this embodiment comprises the following steps:

[0043] (1) Weigh 1 g of imidazole-4,5-dicarboxylic acid and disperse it in 50 mL of deionized water, add 1.8 g of cobalt acetate tetrahydrate, add acetic acid-sodium acetate buffer solution to adjust the pH value to 2, react at 120° C. for 3 h, cool and crystallize, grind, and obtain a metal complex powder;

[0044] (2) The above metal complex powder is calcined at 300°C for 3 hours under a nitrogen atmosphere.

[0045] Example 2

[0046] The preparation method of ammonium heptamolybdate of the present embodiment comprises the following steps:

[0047] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 450 g of water to make a slurry, add 0.5 g of catalyst and 50 g of 30% hydrogen peroxide solution, mix and stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 11, react at 35°C for 1 h, filter and obtain a filtrate;

[0048] S2: Add liquid ammonia to the filtrate to adjust the pH value to 9, react at 70°C for 0.5h, slowly add ammonium carbonate while stirring until the solution is transparent, let stand, filter, and obtain an ammonium molybdate solution;

[0049] S3: evaporating and concentrating the ammonium molybdate solution to a pH value of 6.5, cooling and crystallizing the concentrated ammonium molybdate solution, filtering, washing, centrifuging, and drying;

[0050] The preparation method of the catalyst described in this embodiment comprises the following steps:

[0051] (1) Weigh 1 g of pyridine-2,5-dicarboxylic acid and disperse it in 50 mL of deionized water, add 1.2 g of nickel sulfate hexahydrate, add acetic acid-sodium acetate buffer solution to adjust the pH value to 6, react at 160° C. for 0.5 h, cool and crystallize, grind, and obtain a metal complex powder;

[0052] (2) The above metal complex powder is calcined at 450°C for 1 hour under a nitrogen atmosphere.

[0053] Example 3

[0054] The preparation method of ammonium heptamolybdate of the present embodiment comprises the following steps:

[0055] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 400 g of water to make a slurry, add 0.5 g of catalyst and 50 g of 30% hydrogen peroxide solution, mix and stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 10, react at 30°C for 2 h, filter and obtain a filtrate;

[0056] S2: Add liquid ammonia to the filtrate to adjust the pH value to 8.5, react at 65°C for 0.5h, slowly add ammonium carbonate while stirring until the solution is transparent, let stand, filter, and obtain an ammonium molybdate solution;

[0057] S3: evaporating and concentrating the ammonium molybdate solution to a pH value of 6.5, cooling and crystallizing the concentrated ammonium molybdate solution, filtering, washing, centrifuging, and drying;

[0058] The preparation method of the catalyst described in this embodiment comprises the following steps:

[0059] (1) Weigh 1 g of benzimidazole-5,6-dicarboxylic acid and disperse it in 50 mL of deionized water, add 1.5 g of copper acetate tetrahydrate, add acetic acid-sodium acetate buffer solution to adjust the pH value to 4, react at 135°C for 1 h, cool and crystallize, grind, and obtain a metal complex powder;

[0060] (2) The above metal complex powder is calcined at 400°C for 2 hours under a nitrogen atmosphere.

[0061] Example 4

[0062] The preparation method of ammonium heptamolybdate in this embodiment is different from that in Example 3 in that:

[0063] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 400 g of water to make a slurry, add 2 g of catalyst and 100 g of 30% hydrogen peroxide solution, mix and stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 10, react at 30°C for 2 h, filter and obtain a filtrate;

[0064] The remaining steps are the same as those in Example 3;

[0065] The preparation method of the catalyst described in this example is the same as that of Example 3.

[0066] Example 5

[0067] The preparation method of ammonium heptamolybdate in this embodiment is different from that in Example 3 in that:

[0068] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 400 g of water to make a slurry, add 1.5 g of catalyst and 75 g of 30% hydrogen peroxide solution, mix and stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 10, react at 30°C for 2 h, filter and obtain a filtrate;

[0069] The remaining steps are the same as those in Example 3;

[0070] The preparation method of the catalyst described in this example is the same as that of Example 3.

[0071] Example 6

[0072] The preparation method of ammonium heptamolybdate in this embodiment is the same as that in Example 5;

[0073] The preparation method of the catalyst described in this embodiment comprises the following steps:

[0074] (1) Weigh 1 g of benzimidazole-5,6-dicarboxylic acid and disperse it in 50 mL of deionized water, add 1.5 g of copper acetate tetrahydrate, add acetic acid-sodium acetate buffer solution to adjust the pH value to 4, react at 135°C for 1 h, cool and crystallize, grind, and obtain a metal complex powder;

[0075] (2) Weigh 14 g of ferric chloride hexahydrate and dissolve it in 100 mL of deionized water. After stirring to dissolve, slowly add 50 mL of 1 mol / L sodium hydroxide solution while stirring to form a precipitate. Centrifuge and filter. Add 6.3 g of acetic acid to the wet precipitate and stir for 1 hour to form a colloidal solution. Add 7 g of metal complex powder to the colloidal solution and disperse for 0.5 hour. Centrifuge, wash and dry to obtain a modified metal complex powder.

[0076] (3) The modified metal complex powder is calcined at 400°C for 2 h in a nitrogen atmosphere.

[0077] Example 7

[0078] The preparation method of ammonium heptamolybdate in this embodiment is the same as that in Example 6;

[0079] The preparation method of the catalyst described in this embodiment is different from that in Example 6 in that:

[0080] (2) Weigh 20 g of ferric sulfate nonahydrate and dissolve it in 100 mL of deionized water. After stirring and dissolving, slowly add 50 mL of 1 mol / L sodium hydroxide solution while stirring to form a precipitate. Centrifuge and filter. Add 14 g of acetic acid to the wet precipitate and stir for 1 hour to form a colloidal solution. Add 36 g of metal complex powder to the colloidal solution and disperse for 0.5 hour. Centrifuge, wash and dry to obtain a modified metal complex powder.

[0081] The remaining steps are the same as those in Example 6.

[0082] Example 8

[0083] The preparation method of ammonium heptamolybdate in this embodiment is the same as that in Example 6;

[0084] The preparation method of the catalyst described in this embodiment is different from that in Example 6 in that:

[0085] (2) Weigh 14 g of ferric nitrate nonahydrate and dissolve it in 100 mL of deionized water. After stirring to dissolve, slowly add 50 mL of 1 mol / L sodium hydroxide solution while stirring to form a precipitate. Centrifuge and filter. Add 8.4 g of acetic acid to the wet precipitate and stir for 1 hour to form a colloidal solution. Add 14 g of metal complex powder to the colloidal solution and disperse for 0.5 hour. Centrifuge, wash and dry to obtain a modified metal complex powder.

[0086] The remaining steps are the same as those in Example 6.

[0087] Comparative Example

[0088] Comparative Example 1

[0089] The preparation method of ammonium heptamolybdate of this comparative example comprises the following steps:

[0090] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 400 g of water to make a slurry, add 50 g of a 30% hydrogen peroxide solution, mix and stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 10, react at 30°C for 2 h, filter, and obtain a filtrate;

[0091] S2: Add liquid ammonia to the filtrate to adjust the pH value to 8.5, react at 65°C for 0.5h, slowly add ammonium carbonate while stirring until the solution is transparent, let stand, filter, and obtain an ammonium molybdate solution;

[0092] S3: The ammonium molybdate solution is evaporated and concentrated to a pH value of 6.5, and the concentrated ammonium molybdate solution is cooled, crystallized, filtered, washed, centrifuged, and dried.

[0093] Comparative Example 2

[0094] The preparation method of ammonium heptamolybdate of this comparative example comprises the following steps:

[0095] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 400 g of water to make a slurry, add 0.5 g of nano iron oxide and 50 g of 30% hydrogen peroxide solution, mix and stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 10, react at 30°C for 2 h, filter and obtain a filtrate;

[0096] S2: Add liquid ammonia to the filtrate to adjust the pH value to 8.5, react at 65°C for 0.5h, slowly add ammonium carbonate while stirring until the solution is transparent, let stand, filter, and obtain an ammonium molybdate solution;

[0097] S3: The ammonium molybdate solution is evaporated and concentrated to a pH value of 6.5, and the concentrated ammonium molybdate solution is cooled, crystallized, filtered, washed, centrifuged, and dried.

[0098] Comparative Example 3

[0099] The preparation method of ammonium heptamolybdate of this comparative example comprises the following steps:

[0100] S1: Weigh 100 g of molybdenum concentrate (containing 42.56% molybdenum), crush it, add 50 g of 30% sodium hypochlorite solution, stir evenly, add 1 mol / L sodium hydroxide solution to adjust the pH value to 10, react at 30°C for 2 h, filter and obtain a filtrate;

[0101] S2: Add liquid ammonia to the filtrate to adjust the pH value to 8.5, react at 65°C for 0.5h, slowly add ammonium carbonate while stirring until the solution is transparent, let stand, filter, and obtain an ammonium molybdate solution;

[0102] S3: The ammonium molybdate solution is evaporated and concentrated to a pH value of 6.5, and the concentrated ammonium molybdate solution is cooled, crystallized, filtered, washed, centrifuged, and dried.

[0103] Performance testing

[0104] Detection Methods

[0105] 1. Purity of ammonium heptamolybdate

[0106] The purity of ammonium heptamolybdate obtained by the preparation methods of Examples 1-8 and Comparative Examples 1-3 was tested with reference to GB / T 657-2011 "Chemical Reagent Ammonium Molybdate Tetrahydrate (Ammonium Molybdate)". The results are as follows: Figure 1 shown.

[0107] 2. Molybdenum recovery rate

[0108] Elemental analysis was used to determine the molybdenum content in molybdenum concentrate and ammonium heptamolybdate.

[0109] The molybdenum recovery rate is calculated according to the formula ω=m1φ1 / m0φ0×100%, where: ω is the molybdenum recovery rate, %; m1 is the mass of ammonium heptamolybdate, g; φ1 is the molybdenum content in ammonium heptamolybdate, %; m0 is the mass of molybdenum concentrate, g; φ0 is the molybdenum content in molybdenum concentrate, %. The results are as follows Figure 1 shown.

[0110] Results Analysis

[0111] from Figure 1 It can be seen that the purity of ammonium heptamolybdate obtained according to the preparation method of Examples 1-8 can reach more than 95%, or even more than 99%, and the highest recovery rate of molybdenum can also reach more than 98%.

[0112] from Figure 1 It can be seen from the data of Examples 1-3 and Comparative Examples 1-3 that the addition of catalyst is beneficial to improving the recovery rate of molybdenum, and after the metal oxide is doped with carbon and nitrogen, the catalytic activity is higher and the recovery rate of molybdenum is improved.

[0113] from Figure 1 It can be seen from the data of Examples 3-5 that with the increase in the amount of catalyst and hydrogen peroxide solution used, the platinum recovery rate increases and can reach more than 90%, but when it increases to a certain extent, the platinum recovery rate does not increase significantly.

[0114] from Figure 1 It can be seen from the data of Examples 5-8 that the introduction of nano-iron oxide further improves the molybdenum recovery rate, which can even reach more than 98% through adjustment.

[0115] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed, but such modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A method for preparing ammonium heptamolybdate, characterized in that: The steps include: S1: crush the molybdenum concentrate, add water to make a slurry, add a catalyst and a hydrogen peroxide solution, mix and stir evenly, adjust the pH value to 9-11, react at 25-35°C for 1-3h, filter, and obtain a filtrate; S2: Add liquid ammonia to the filtrate to adjust the pH value to 8.5-9, react at 60-70°C for 0.5-1h, slowly add ammonium carbonate while stirring, let stand, filter, and obtain an ammonium molybdate solution; S3: Evaporating and concentrating the ammonium molybdate solution to a pH value of 6-6.5, cooling and crystallizing, filtering, washing, centrifuging, and drying; The method for preparing the catalyst in step S1 above comprises the following steps: (1) dispersing a carbon-nitrogen source organic ligand in water, adding a metal active component precursor, adjusting the pH value to 2-6, reacting at 120-160° C. for 0.5-3 h, cooling and crystallizing, grinding, and obtaining a metal complex powder; the carbon-nitrogen source organic ligand is one of imidazole-4,5-dicarboxylic acid, benzimidazole-5,6-dicarboxylic acid, pyridine-3,4-dicarboxylic acid, pyridine-2,6-dicarboxylic acid and pyridine-2,5-dicarboxylic acid; the metal active component is one of copper oxide, cobalt oxide and nickel oxide; the mass ratio of the carbon-nitrogen source organic ligand to the metal active component precursor is 1:(1.2-1.8); (2) Calcine the above metal complex powder at 300-450°C for 1-3h under nitrogen atmosphere.

2. The method for preparing ammonium heptamolybdate according to claim 1, wherein In step S1, the mass ratio of the molybdenum concentrate powder, the catalyst and the hydrogen peroxide is 100:(0.5-2):(15-30).

3. The method for preparing ammonium heptamolybdate according to claim 1, characterized in that: The metal complex powder in step (1) is modified by a method comprising the following steps: Dissolve the iron salt in water, slowly add sodium hydroxide while stirring, filter, add acetic acid to the precipitate, stir to form a colloidal solution, disperse the metal complex powder in the colloidal solution, centrifuge, wash and dry.

4. The method for preparing ammonium heptamolybdate according to claim 3, characterized in that: The iron salt is one or more of ferric chloride hexahydrate, ferric sulfate nonahydrate and ferric nitrate nonahydrate.

5. The method for preparing ammonium heptamolybdate according to claim 3, characterized in that: The mass ratio of the iron salt to acetic acid is 1:(0.45-0.7).

6. The method for preparing ammonium heptamolybdate according to claim 3, characterized in that: The mass ratio of the iron salt to the metal complex powder is 1:(0.5-1.8).

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