Medium-temperature co-precipitation vanadium extraction method for sodium aluminate solution with low vanadium concentration

Abstract

The invention discloses a medium-temperature co-precipitation vanadium extraction method for a sodium aluminate solution with low vanadium concentration, and belongs to the field of aluminum industry. The method comprises the following steps: by taking a sodium aluminate solution with low vanadium concentration as a raw material, adding a seed crystal, a settling aid and a crystallization aid into the sodium aluminate solution, and carrying out coprecipitation under a medium-temperature condition to obtain coprecipitated vanadium slag; part of the coprecipitation vanadium slag serves as a seed crystal to react again, the other coprecipitation vanadium slag is subjected to impurity removal and purification to obtain filter residues and vanadium-rich filtrate, the filter residues return to a system to serve as a settling aid, and the vanadium-rich filtrate is subjected to a secondary vanadium precipitation procedure to obtain a vanadium pentoxide product. The method solves the problems of large heat loss during low-temperature vanadium extraction of the sodium aluminate solution with low vanadium concentration and low vanadium precipitation rate during medium-temperature crystallization, vanadium extraction and purification are performed synchronously, the vanadium content in the mother liquor can be reduced to 80-120ppm, the vanadium precipitation rate is high, energy is saved, consumption is reduced, a vanadium settling aid is recycled, waste discharge is reduced, environmental protection is facilitated, and the method is suitable for industrial production. Meanwhile, the influence of other impurity salts on the process of producing aluminum oxide by the Bayer process is eliminated.

Description

Technical Field

[0001] The invention relates to the field of aluminum industry, and in particular to a method for extracting vanadium through medium-temperature co-precipitation from a sodium aluminate solution with a low vanadium concentration. Background Art

[0002] Bauxite is the raw material for the production of alumina, which contains a small amount of vanadium. During the high-temperature dissolution of the Bayer process, the vanadium element in the bauxite enters the sodium aluminate mother liquor, and with the recycling of the mother liquor, the vanadium in the production system continues to accumulate. Due to the huge amount of circulating mother liquor in the alumina production process, the reserves of vanadium in the mother liquor are considerable, and the conditions for extraction and utilization are met. Since the enrichment of vanadium in the mother liquor is not conducive to the seed decomposition process of the Bayer process for the production of alumina, it seriously affects the quality of alumina products. At the same time, metallic vanadium is a widely used and high-priced metal, so extracting vanadium from the Bayer process for the production of alumina has become a win-win trend.

[0003] At present, the known methods for extracting vanadium from the Bayer process of alumina production include crystallization, precipitation, and adsorption extraction. For the Bayer process sodium aluminate solution system with a high vanadium concentration, the crystallization method is generally used in the industry to recover vanadium. This method has a mature process and simple equipment, and can obtain crystalline vanadium slag with a high vanadium content. The vanadium slag can be further used to produce vanadium pentoxide.

[0004] However, as far as the precipitation of vanadium by crystallization is concerned, the temperature of the sodium aluminate solution and the concentration of caustic soda are two important factors that affect the vanadium precipitation rate. For this alumina production process, the vanadium extraction process generally uses the evaporated mother liquor with high caustic soda and high vanadium concentration as the vanadium extraction raw material to obtain the crystallized vanadium slag. For the sodium aluminate solution with high vanadium concentration, in order to reduce the heat loss caused by the large temperature drop, the mother liquor temperature is usually reduced to a medium temperature of about 45°C, so that a certain amount of vanadium salt can be crystallized from the solution, but the vanadium concentration in the vanadium extraction tail liquid will be higher. As described in patent CN116199575A, the evaporated mother liquor caustic soda is adjusted to 180-210g / L, wherein the V content is as high as 600ppm or more. It is only necessary to reduce the mother liquor temperature to 45°C and react for a certain period of time, and the V content in the mother liquor can be reduced to about 250ppm. For sodium aluminate solution with low vanadium concentration, in order to reduce the vanadium concentration in the vanadium extraction tail liquid to a certain concentration and obtain a higher vanadium precipitation rate, an external refrigeration device is usually used to reduce the reaction temperature to 15-35°C. However, this will make it difficult to recover a large amount of heat, increase the cost of vanadium extraction, and at the same time, the purification and impurity removal filter residue in the process of preparing vanadium pentoxide from vanadium slag is also difficult to use. For example, in patent CN103159259A, when the V concentration in the evaporated mother liquor is 237ppm, in order to promote the precipitation of vanadium from the solution, the crystallization temperature is reduced to 28-32°C to obtain vanadium slag, and the vanadium slag is impurity-free by calcium and magnesium salts to obtain impurity filter residue and vanadium-rich filtrate, the filter residue is discharged from the system, and the vanadium-rich filtrate is subjected to the ammonium method secondary vanadium precipitation process to produce vanadium pentoxide. Summary of the invention

[0005] In view of this, the present invention provides a method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration at medium temperature by co-precipitation. The method solves the problems of large heat loss in low-temperature vanadium extraction from a sodium aluminate solution with a low vanadium concentration and low vanadium precipitation rate through medium-temperature crystallization. Vanadium extraction and purification are carried out simultaneously, so that the vanadium content in the mother liquor can be reduced to 80-120ppm, the vanadium precipitation rate is high, energy is saved and consumption is reduced, the vanadium precipitation aid can be recycled, waste discharge is reduced, and environmental protection is beneficial.

[0006] The method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration by co-precipitation at medium temperature of the present invention comprises the following steps:

[0007] (1) cooling the sodium aluminate solution with low vanadium concentration to 40-50° C., adding a precipitation aid, a crystallization aid and a seed crystal, stirring and reacting for 6-12 hours, and filtering to obtain a co-precipitated vanadium slag;

[0008] (2) taking part of the co-precipitated vanadium slag obtained in step (1) and returning it to step (1) as a seed crystal, and adding an acid solution to the remaining co-precipitated vanadium slag to obtain a vanadium slag solution;

[0009] (3) adding metal salt to the vanadium slag solution, controlling the concentrations of metal elements and P and As in the solution, stirring the reaction, aging, and filtering to obtain a filter residue and a vanadium-rich filtrate;

[0010] (4) adding ammonium sulfate to the vanadium-rich filtrate, stirring for reaction, and filtering to obtain an ammonium metavanadate precipitate;

[0011] (5) The ammonium metavanadate is calcined to obtain vanadium pentoxide.

[0012] Preferably, the V content in the low vanadium concentration sodium aluminate solution described in step (1) is 220.4-320.6 ppm.

[0013] Preferably, the seed crystal in step (1) is vanadium slag; more preferably, the seed crystal is the co-precipitated vanadium slag obtained in step (1).

[0014] Preferably, the precipitation aid described in step (1) is the filter residue obtained in step (3).

[0015] Preferably, the concentration of the seed crystals added to the low vanadium concentration sodium aluminate solution in step (1) is 1-5 g / L, the concentration of the precipitation aid is 15-20 g / L; and the concentration of the crystallization aid is 15-30 ppm.

[0016] Preferably, the specific steps of adding dissolution in step (2) are: adding acid solution according to the liquid-to-solid ratio of acid solution to co-precipitated vanadium slag of 2-10, until the pH of the solution is 7-10, reacting at a temperature of 30-95°C for 50-60 minutes to obtain a vanadium slag solution; the acid solution is sulfuric acid or hydrochloric acid.

[0017] Preferably, in step (3), the concentration ratio of the metal element to P and As is metal element / (As+P)=1.2-2.0; and the metal salt is aluminum salt or iron salt.

[0018] Preferably, the stirring reaction time in step (3) is 1 to 1.5 hours, and the reaction temperature is 30-95°C; the aging time is 1 to 1.5 hours, and the aging temperature is 30-95°C.

[0019] Preferably, the ammonium sulfate in step (4) is NH4 + Add at a concentration of 10-20g / L.

[0020] Preferably, the stirring reaction in step (4) is carried out at a temperature of 25-30° C. and for a time of 5-8 h.

[0021] Preferably, the calcination temperature in step (5) is 500-600° C., and the calcination time is 2-3 hours.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention provides a method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration at a medium temperature by coprecipitation. The method uses a sodium aluminate solution with a low vanadium concentration (Bayer process evaporation mother liquor) as a raw material, adds crystal seeds, a precipitation aid and a crystallization aid thereto, and coprecipitates under medium temperature conditions to obtain a coprecipitated vanadium slag. Part of the coprecipitated vanadium slag is used as a crystal seed to react again, and the remaining coprecipitated vanadium slag is purified after impurities removal to obtain a filter residue and a vanadium-rich filtrate, wherein the filter residue is returned to the system as a precipitation aid, and the vanadium-rich filtrate is subjected to a secondary vanadium precipitation process to obtain a vanadium pentoxide product. The invention solves the problems of large heat loss in low-temperature vanadium extraction from sodium aluminate solution with low vanadium concentration and low vanadium precipitation rate in medium-temperature crystallization, and simultaneously performs vanadium extraction and purification, thereby reducing the vanadium content in the mother liquor to 80-120ppm, achieving high vanadium precipitation rate, energy saving and consumption reduction, recycling of vanadium precipitation aids, reduced waste discharge, and being beneficial to environmental protection. Meanwhile, the vanadium extraction process can eliminate the influence of other impurity salts such as V, F, P, As, Na2C2O4, sodium carbonate, sodium sulfate, potassium perchlorate, humic acid, etc. on the Bayer process for producing alumina. DETAILED DESCRIPTION

[0024] The present invention provides a method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration by co-precipitation at medium temperature, the steps of which are as follows:

[0025] (1) cooling the low vanadium concentration sodium aluminate solution to 60° C. through a primary heat exchanger, and then cooling it to 45° C. through a secondary heat exchanger together with cold water, the cooled low vanadium concentration sodium aluminate solution enters a retention tank, and then a precipitation aid (15-20 g / L), a crystallization aid (a mixture of chitosan and sodium polyacrylate in a mass ratio of 1:1, 15-30 ppm) and a vanadium slag seed (1-5 g / L) are added thereto, stirring and reacting in a sedimentation tank for 6-12 hours, and filtering through a filter press to obtain a co-precipitated vanadium slag and a vanadium extraction liquid; wherein the vanadium extraction liquid is returned to the system for the primary heat exchange of the low vanadium concentration sodium aluminate solution, and the vanadium extraction liquid is returned to the production system after heat exchange;

[0026] (2) taking part of the co-precipitated vanadium slag obtained in step (1) and returning it to step (1) as a seed crystal, placing the remaining co-precipitated vanadium slag in a dissolution tank, adding sulfuric acid according to a liquid-solid ratio of sulfuric acid to the co-precipitated vanadium slag of 2-10 until the pH value of the solution is 7-10, reacting at a temperature of 30-95° C. for 50-60 minutes, dissolving the slag phase, and obtaining a vanadium slag solution;

[0027] (3) adding aluminum sulfate to the vanadium slag solution, controlling the concentration ratio of Al to P and As in the solution to be Al / (As+P)=1.2-2.0, stirring the reaction at 30-95° C. for 1 h, aging at 30-95° C. for 1 h, and filtering through a filter press to obtain a filter residue and a vanadium-rich filtrate; wherein the filter residue is returned to step (1) as a precipitation aid;

[0028] (4) Place the vanadium-rich filtrate in a crystallization tank and heat it according to NH4 + Add ammonium sulfate at a concentration of 10-20 g / L to the vanadium-rich filtrate, stir and react at a temperature of 25-30° C. for 5-8 hours, filter through a filter press to obtain ammonium metavanadate precipitate; return the filtrate to the system;

[0029] (5) The ammonium metavanadate precipitate is calcined at 500° C. for 2 h to obtain vanadium pentoxide.

[0030] The present invention will be further described below in conjunction with the embodiments.

[0031] Example 1

[0032] A method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration by co-precipitation at medium temperature, comprising the following steps:

[0033] (1) cooling the low vanadium concentration sodium aluminate solution to 60° C. through a primary heat exchanger, and then cooling it to 45° C. through a secondary heat exchanger together with cold water. The cooled low vanadium concentration sodium aluminate solution enters a retention tank, and then a precipitation aid (15 g / L), a crystallization aid (a mixture of chitosan and sodium polyacrylate in a mass ratio of 1:1, 15 ppm) and a vanadium slag seed (3 g / L) are added thereto. After stirring and reacting in a sedimentation tank for 8 hours, the solution is filtered through a filter press to obtain a co-precipitated vanadium slag and a vanadium extraction liquid; wherein the vanadium extraction liquid is returned to the system for the primary heat exchange of the low vanadium concentration sodium aluminate solution, and the vanadium extraction liquid is returned to the production system after heat exchange;

[0034] The specific composition of the low vanadium concentration sodium aluminate solution is shown in Table 1:

[0035] Table 1

[0036] <![CDATA[Na2O / (g / L)]]> V / ppm P / ppm As / ppm F / (g / L) <![CDATA[C2O4 2- / (g / L)]]> 240.6 310.6 100.3 90.4 0.65 1.86

[0037] The precipitation aid is derived from the filter residue of step (3), and its specific components are: P2O5 13.46%, As2O3 11.88%, V2O5 5.27%, Na2O 34.16%, Al2O3 28.67%, and F 6.56%.

[0038] The specific composition of the seed crystal is: V2O58.36%, P2O59.86%, As2O36.85%, Na2O55.36%, Al2O3 content is 16.45%, F 3.12%, and the balance is inevitable impurities.

[0039] The composition of the co-precipitated vanadium slag was analyzed, and the specific composition is shown in Table 2:

[0040] Table 2

[0041] Element <![CDATA[V2O5]]> <![CDATA[P2O5]]> <![CDATA[As2O3]]> <![CDATA[Al2O3]]> <![CDATA[Na2O]]> F content(%) 6.36 7.02 6.15 17.86 60.36 2.25

[0042] The composition of the vanadium-extracted liquid was analyzed, and the specific composition is shown in Table 3:

[0043] Table 3

[0044] <![CDATA[Na2O / (g / L)]]> V / ppm P / ppm As / ppm F / (g / L) <![CDATA[C2O4 2- / (g / L)]]> 245.4 91.3 82.6 59.5 0.20 0.83

[0045] The calculated vanadium precipitation rate is 70.61%.

[0046] (2) taking part of the co-precipitated vanadium slag obtained in step (1) and returning it to step (1) as a seed crystal, placing the remaining co-precipitated vanadium slag in a dissolution tank, adding sulfuric acid according to a liquid-solid ratio of sulfuric acid to the co-precipitated vanadium slag of 5 until the pH value of the solution is 8.2, reacting at a temperature of 90° C. for 60 minutes, and dissolving the slag phase to obtain a vanadium slag solution;

[0047] (3) adding aluminum sulfate to the vanadium slag solution, controlling the concentration ratio of Al to P and As in the solution to be Al / (As+P)=1.5, stirring the reaction at 90° C. for 1 h, aging at 90° C. for 1 h, and filtering through a filter press to obtain a filter residue and a vanadium-rich filtrate; wherein the filter residue is returned to step (1) as a precipitation aid;

[0048] (4) Place the vanadium-rich filtrate in a crystallization tank and heat it according to NH4 + Add ammonium sulfate at a concentration of 12 g / L to the vanadium-rich filtrate, stir and react at a temperature of 30° C. for 8 hours, and filter through a filter press to obtain ammonium metavanadate precipitate; divide the difference in vanadium concentration of the vanadium-rich filtrate before and after the reaction by the original vanadium concentration of the vanadium-rich filtrate to obtain a secondary vanadium precipitation rate of 97.88%; return the filtrate to the system;

[0049] (5) The ammonium metavanadate precipitate is calcined at 500° C. for 2 h to obtain vanadium pentoxide with a purity of 98.97%.

[0050] Example 2

[0051] A method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration by co-precipitation at medium temperature, comprising the following steps:

[0052] (1) cooling the low vanadium concentration sodium aluminate solution to 60° C. through a primary heat exchanger, and then cooling it to 30° C. through a secondary heat exchanger together with cold water. The cooled low vanadium concentration sodium aluminate solution enters a retention tank, and then a precipitation aid (15 g / L), a crystallization aid (a mixture of chitosan and sodium polyacrylate in a mass ratio of 1:1, 15 ppm) and a vanadium slag seed (3 g / L) are added thereto. After stirring and reacting in a sedimentation tank for 8 hours, the solution is filtered through a filter press to obtain a co-precipitated vanadium slag and a vanadium extraction liquid; wherein the vanadium extraction liquid is returned to the system for the primary heat exchange of the low vanadium concentration sodium aluminate solution, and the vanadium extraction liquid is returned to the production system after heat exchange;

[0053] The specific composition of the low vanadium concentration sodium aluminate solution is shown in Table 4:

[0054] Table 4

[0055] <![CDATA[Na2O / (g / L)]]> V / ppm P / ppm As / ppm F / (g / L) <![CDATA[C2O4 2- / (g / L)]]> 245.8 320.6 83.3 88.4 0.58 1.90

[0056] The precipitation aid is derived from the filter residue of step (3), and its specific components are: P2O5 17.46%, As2O3 15.47%, V2O5 4.27%, Na2O 30.15%, Al2O3 23.09%, and F 9.56%.

[0057] The specific composition of the seed crystal is: V2O5 10.12%, P2O 5 8.86%, As2O3 7.85%, Na2O 5 3.36%, Al2O3 content is 15.63%, and F 4.18%.

[0058] The composition of the co-precipitated vanadium slag was analyzed, and the specific composition is shown in Table 5:

[0059] Table 5

[0060] Element <![CDATA[V2O5]]> <![CDATA[P2O5]]> <![CDATA[As2O3]]> <![CDATA[Al2O3]]> <![CDATA[Na2O]]> F content(%) 9.57 8.12 9.15 15.24 53.36 4.56

[0061] The composition of the vanadium-extracted liquid was analyzed, and the specific composition is shown in Table 6:

[0062] Table 6

[0063] NK / (g / L) V / ppm P / ppm As / ppm F / (g / L) <![CDATA[C2O4 2- / (g / L)]]> 244.4 43.7 30.6 25.5 0.20 0.66

[0064] The calculated vanadium precipitation rate is 86.37%.

[0065] (2) taking part of the co-precipitated vanadium slag obtained in step (1) and returning it to step (1) as a seed crystal, placing the remaining co-precipitated vanadium slag in a dissolution tank, adding sulfuric acid according to a liquid-solid ratio of sulfuric acid to the co-precipitated vanadium slag of 5 until the pH value of the solution is 8.2, reacting at a temperature of 90° C. for 60 minutes, and dissolving the slag phase to obtain a vanadium slag solution;

[0066] (3) adding aluminum sulfate to the vanadium slag solution, controlling the concentration ratio of Al to P and As in the solution to be Al / (As+P)=1.5, stirring the reaction for 1 hour, aging for 1 hour, and filtering through a filter press to obtain a filter residue and a vanadium-rich filtrate; wherein the filter residue is returned to step (1) as a precipitation aid;

[0067] (4) Place the vanadium-rich filtrate in a crystallization tank and heat it according to NH4 + Add ammonium sulfate at a concentration of 12 g / L to the vanadium-rich filtrate, stir and react at a temperature of 30° C. for 8 hours, and filter through a filter press to obtain ammonium metavanadate precipitate; divide the difference in vanadium concentration of the vanadium-rich filtrate before and after the reaction by the original vanadium concentration of the vanadium-rich filtrate to obtain a secondary vanadium precipitation rate of 97.45%; return the filtrate to the system;

[0068] (5) The ammonium metavanadate precipitate is calcined at 500° C. for 2 h to obtain vanadium pentoxide with a purity of 98.97%.

[0069] Comparative Example 1

[0070] A method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration by co-precipitation at medium temperature, comprising the following steps:

[0071] (1) According to the method of Example 1, the low vanadium concentration sodium aluminate solution was cooled to 45° C., 3 g / L vanadium slag was added as seed crystals, and after stirring for 8 hours, the co-precipitated vanadium slag was filtered;

[0072] The specific components of the low vanadium concentration sodium aluminate solution are the same as those in Example 1.

[0073] The specific composition of the seed crystal is: V2O5 0.89%, P2O5 0.15%, As2O3 0.22%, Al2O3 29.49%, Na2O 6 8.23%, F 1.02%, and the remainder is inevitable impurities.

[0074] The composition of the co-precipitated vanadium slag was analyzed, and the specific composition is shown in Table 7:

[0075] Table 7

[0076] Element <![CDATA[V2O5]]> <![CDATA[P2O5]]> <![CDATA[As2O3]]> <![CDATA[Al2O3]]> <![CDATA[Na2O]]> F content(%) 0.98 0.11 0.33 25.46 70.60 0.82

[0077] The balance is inevitable impurities.

[0078] The composition of the vanadium-extracted liquid was analyzed, and the specific composition is shown in Table 8:

[0079] Table 8

[0080] NK / (g / L) V / ppm P / ppm As / ppm F / (g / L) <![CDATA[C2O4 2- / (g / L)]]> 245.2 289.7 95.2 81.5 0.45 0.78

[0081] In Comparative Example 1, the vanadium precipitation rate was only 6.73%, and the V2O5 content in the product vanadium slag was 0.98%.

[0082] Comparative Example 2

[0083] A method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration by co-precipitation at medium temperature, comprising the following steps:

[0084] (1) According to the method of Example 1, the low vanadium concentration sodium aluminate solution was cooled to 30° C., 3 g / L vanadium slag was added as seed crystals, and after stirring for 8 hours, the co-precipitated vanadium slag was filtered;

[0085] The specific components of the low vanadium concentration sodium aluminate solution are the same as those in Example 2.

[0086] The specific composition of the seed crystal is: V2O5 10.30%, P2O 5 4.30%, As2O3 2.58%, Al2O3 28.89%, Na2O 51.31%, and F 2.62%.

[0087] The composition of the co-precipitated vanadium slag was analyzed, and the specific composition is shown in Table 9:

[0088] Table 9

[0089] Element <![CDATA[V2O5]]> <![CDATA[P2O5]]> <![CDATA[As2O3]]> <![CDATA[Al2O3]]> <![CDATA[Na2O]]> F content(%) 12.40 4.68 3.36 19.42 55.35 2.46

[0090] The balance is inevitable impurities.

[0091] The composition of the vanadium-extracted liquid was analyzed, and the specific composition is shown in Table 10:

[0092] Table 10

[0093] NK / (g / L) V / ppm P / ppm As / ppm F / (g / L) <![CDATA[C2O4 2- / (g / L)]]> 245.8 110.2 13.7 25.4 0.15 0.51

[0094] The calculated vanadium precipitation rate is 65.62%, and the V2O5 content in the product vanadium slag is 9.40%.

[0095] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for extracting vanadium from a sodium aluminate solution with a low vanadium concentration by coprecipitation at medium temperature, characterized in that: The following steps are involved: (1) cooling the sodium aluminate solution with low vanadium concentration to 40-50° C., adding a precipitation aid, a crystallization aid and a seed crystal, stirring and reacting for 6-12 hours, and filtering to obtain a co-precipitated vanadium slag; (2) taking part of the co-precipitated vanadium slag obtained in step (1) and returning it to step (1) as a seed crystal, and adding sulfuric acid to the remaining co-precipitated vanadium slag to obtain a vanadium slag solution; (3) adding aluminum sulfate to the vanadium slag solution, controlling the concentrations of Al, P and As in the solution, stirring the reaction, aging and filtering to obtain a filter residue and a vanadium-rich filtrate; (4) adding ammonium sulfate to the vanadium-rich filtrate, stirring for reaction, and filtering to obtain an ammonium metavanadate precipitate; (5) The ammonium metavanadate is calcined to obtain vanadium pentoxide.

2. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature according to claim 1, characterized in that: The content of V in the low vanadium concentration sodium aluminate solution described in step (1) is 220.4-320.6 ppm.

3. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature by coprecipitation according to claim 1, characterized in that: The seed crystal in step (1) is vanadium slag; more preferably, the seed crystal is the co-precipitated vanadium slag obtained in step (1).

4. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature by coprecipitation according to claim 1, characterized in that: The precipitation aid in step (1) is the filter residue obtained in step (3).

5. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature by coprecipitation according to claim 1, characterized in that: In step (1), the concentration of seed crystals added to the sodium aluminate solution with low vanadium concentration is 1-5 g / L, the concentration of precipitation aid is 15-20 g / L; and the concentration of crystallization aid is 15-30 ppm.

6. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature by coprecipitation according to claim 1, characterized in that: The specific steps of adding sulfuric acid to dissolve in step (2) are: adding sulfuric acid according to a liquid-to-solid ratio of sulfuric acid to the co-precipitated vanadium slag of 2-10 until the pH of the solution is 7-10, reacting at a temperature of 30-95°C for 50-60 minutes to obtain a vanadium slag dissolved solution.

7. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature by coprecipitation according to claim 1, characterized in that: In step (3), the concentration ratio of Al to P and As is Al / (As+P)=1.2-2.0; The stirring reaction time is 1 to 1.5 hours, and the reaction temperature is 30-95°C; the aging time is 1 to 1.5 hours, and the aging temperature is 30-95°C.

8. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature by coprecipitation according to claim 1, characterized in that: The ammonium sulfate in step (4) is NH 4+ Add at a concentration of 10-20g / L.

9. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature by coprecipitation according to claim 1, characterized in that: The stirring reaction in step (4) is carried out at a temperature of 25-30° C. and for a time of 5-8 h.

10. The method for extracting vanadium from sodium aluminate solution with low vanadium concentration at medium temperature according to claim 1, characterized in that: The calcination temperature in step (5) is 500-600° C. and the calcination time is 2-3 hours.