Method for recovering ammonium sulfate and application

Abstract

The invention provides a recovery method and application of ammonium sulfate, and the recovery method comprises the following steps: taking a solution simultaneously containing ammonium sulfate and pentamethylene diamine sulfate as a raw material, adjusting the pH value of the solution to 5-8, forming a concentrated solution containing ammonium sulfate crystals through evaporative crystallization, and carrying out solid-liquid separation to obtain an ammonium sulfate solid; at the end point of evaporative crystallization, the proportion a of amine nitrogen in the evaporative concentrated solution to total nitrogen is more than or equal to 0 and less than or equal to 82.5%. Before evaporative crystallization, a decolorization step and a concentration step can be included. According to the method for recovering ammonium sulfate, the wastewater generated in the production process of 1, 5-pentanediamine by a biological method is taken as a raw material, and the ammonium sulfate is recovered, so that the salt content in the wastewater is effectively reduced, the cost of the raw material is saved, and more importantly, downstream pollution can be effectively reduced, and the sewage treatment cost is also remarkably reduced; therefore, the production cost of the pentamethylene diamine is further reduced. Moreover, the recovered ammonium sulfate is high in purity and low in impurity residue, and can be used for fermentation.

Description

Technical Field

[0001] The invention belongs to the field of wastewater recovery, and in particular relates to a method for recovering ammonium sulfate. Background Art

[0002] At present, the biological method for preparing 1,5-pentanediamine (pentanediamine for short) is mainly divided into direct fermentation method and enzyme conversion method. The fermentation method produces a fermentation broth containing pentamethylenediamine salt, or the enzyme conversion liquid containing pentamethylenediamine salt is obtained by reaction under the action of lysine decarboxylase (LDC). Due to the low concentration of the fermentation broth or the enzyme conversion liquid, evaporation and concentration operations are usually required in the process of extracting and purifying pentamethylenediamine. For example, CN1065164101A records: before refining, the fermentation broth of pentamethylenediamine carbonate is evaporated and concentrated; for example, CN105612257A records: before distillation, the aqueous solution of pentamethylenediamine is evaporated and concentrated, etc. These operations are mainly to remove part of the solvent water in the pentamethylenediamine solution to facilitate the purification of pentamethylenediamine.

[0003] A small amount of ammonium salts such as ammonium sulfate used in the biological preparation of pentamethylenediamine will remain in the feed solution to be purified, and will exist in the aqueous phase after extraction and purification of pentamethylenediamine. If discharged into the sewage system, it will increase the burden of deammoniation and nitrogen removal and cause resource waste. Therefore, it is necessary to find a suitable method to recover ammonium sulfate in the wastewater and continue to use it in the fermentation process to form a circular economy. Summary of the invention

[0004] In order to overcome the problem of high wastewater treatment cost caused by high ammonium sulfate content in the existing biological 1,5-pentanediamine production wastewater, the present invention provides a method for recovering ammonium sulfate. The method can be used to obtain high-purity ammonium sulfate, which can be used as the next batch of raw materials for biological fermentation, thereby reducing downstream pollution and saving costs, which is of great significance.

[0005] The first object of the present invention is to provide a method for recovering ammonium sulfate. The method comprises:

[0006] The raw material is a solution containing ammonium sulfate and pentamethylenediamine sulfate at the same time, the pH value of the solution is adjusted to 5-8, a concentrated solution containing ammonium sulfate crystals is formed by evaporation and crystallization, and ammonium sulfate solid is obtained by solid-liquid separation;

[0007] At the end of evaporation crystallization, the ratio of amine nitrogen to total nitrogen in the evaporated concentrate is 0 <a≤82.5%。

[0008] In one embodiment, preferably, at the end point of evaporation crystallization, the proportion of amine nitrogen in the evaporation concentrate to the total nitrogen is 0 < a ≤ 55%, further 5% ≤ a ≤ 55%, further 5% ≤ a ≤ 30%, further 5% ≤ a ≤ 21%, further 5% ≤ a ≤ 15%, further 5% ≤ a ≤ 10%. The percentage is by mass.

[0009] In one embodiment, the pH value of the solution is adjusted to 5 - 8, preferably 6.0 - 6.5.

[0010] In one embodiment, the solution is concentrated before evaporation crystallization. The concentration temperature is 60 - 90°C, further 73 - 90°C. Concentrating the solution before evaporation crystallization can improve the efficiency of evaporation crystallization and reduce energy consumption.

[0011] In one embodiment, the solution is decolorized before evaporation crystallization.

[0012] In one embodiment, a decolorizing agent is used for decolorization, and after decolorization, the decolorizing agent is removed by solid - liquid separation. The method of solid - liquid separation is centrifugation or filtration. Based on the mass of ammonium sulfate contained in the solution to be decolorized, the addition amount of the decolorizing agent is 0.03% - 2%, further 0.03% - 1%. The decolorizing agent is activated carbon. The temperature for decolorization is 50 - 80°C. The time for decolorization is 10 - 80 min, further 15 - 60 min.

[0013] In one embodiment, the solution is decolorized before evaporation crystallization and then concentrated. The concentration treatment and decolorization treatment have the same definitions as described above.

[0014] In one embodiment, the solution is concentrated before evaporation crystallization and then decolorized. The concentration treatment and decolorization treatment have the same definitions as described above.

[0015] In one embodiment, before solid - liquid separation in the recovery method, the mixed system of ammonium sulfate crystals and concentrated solution is cooled to 20 - 40°C. By cooling, the ammonium sulfate crystals continue to grow, the system thickens, which facilitates subsequent smooth solid - liquid separation and also helps to improve the recovery rate of the recovered ammonium sulfate.

[0016] In one embodiment, the methods of solid - liquid separation include centrifugal separation and filtration separation.

[0017] In one embodiment, the ammonium sulfate solid obtained by solid - liquid separation is washed and dried to obtain ammonium sulfate product.

[0018] In one embodiment, the mother liquor after solid-liquid separation is returned to the evaporation crystallization step or any step before the evaporation crystallization step. Returning to any step before the evaporation crystallization step will ultimately enter the evaporation crystallization step to form ammonium sulfate crystals and concentrated liquid. For example, if the pigment content of the mother liquor is high and the color is deep, it can be returned to the decolorization treatment step.

[0019] In one embodiment, the proportion of amine nitrogen in the solution before evaporation crystallization to the total nitrogen is 0 < b ≤ 10%, further 4% ≤ b ≤ 10%. The percentage is by mass.

[0020] In one embodiment, the temperature range of evaporation crystallization is 45 - 95 °C, further 50 - 90 °C.

[0021] In the present invention, there are no special limitations on the concentration treatment equipment and evaporation crystallization equipment.

[0022] In one embodiment, the evaporation crystallization equipment includes a forced circulation concentrator, a crystallization reactor, and a draft tube-baffle evaporation crystallizer.

[0023] Through the research of the inventors, it is found that when the system contains pentamethylenediamine sulfate, the dissolution and crystallization behavior of ammonium sulfate in the system is significantly affected by pentamethylenediamine sulfate. Compared with the system without pentamethylenediamine sulfate, pentamethylenediamine sulfate existing as an impurity can promote the evaporation crystallization of ammonium sulfate earlier and avoid excessive precipitation of impurities.

[0024] At the same time, different from the pure solution system, since the raw material is the wastewater discharged during the purification of 1,5-pentanediamine from the fermentation broth or enzyme conversion solution formed by fermentation or enzyme conversion method to prepare 1,5-pentanediamine, which contains residual pigments and / or proteins and other impurities, the crystallization behavior is also affected by the complex components of the system, different from the crystallization situation in general pure aqueous solutions.

[0025] The inventors have found that when the proportion of amine nitrogen to total nitrogen at the end point of evaporation crystallization is within a suitable range, it can enable ammonium sulfate to crystallize while avoiding the crystallization of pentamethylenediamine sulfate, and the purity of the crystalline product is relatively high. At the same time, the primary yield of ammonium sulfate is also relatively high. The low impurity content in the ammonium sulfate crystals can be better applied to fermentation. For example, we have found that if the residual amount of pentanediamine impurity is too high, it will be toxic to the fermentation strain when applied to fermentation. Amine nitrogen refers to the nitrogen derived from 1,5-pentanediamine.

[0026] In addition, adjusting the pH value of the raw material solution to a suitable range before concentration and crystallization can, on the one hand, avoid the corrosion of the concentration equipment caused by the low pH value and sharp drop of the system during the subsequent concentration and evaporation processes, and on the other hand, avoid the problem that the large change in the pH value of the system affects the existence form and aggregation state of substances in the system, thus affecting the crystallization process.

[0027] In one embodiment, the content of ammonium sulfate in the raw material is 4 wt% - 20 wt%, further 5 wt% - 15 wt%, and further 8 wt% - 15 wt%.

[0028] In one embodiment, the content of pentamethylenediamine sulfate in the raw material is 0.2 wt% - 3.5 wt%, further 0.5 wt% - 2.6 wt%.

[0029] In one embodiment, the light transmittance of the raw material is 70% - 95%, further 75% - 90%.

[0030] In one embodiment, the pH value range of the raw material is 0.5 - 4, further 2.0 - 2.7.

[0031] In one embodiment, the raw material is the wastewater discharged during the purification of 1,5 - pentanediamine from the fermentation broth or enzyme conversion solution formed by the fermentation or enzyme conversion method for preparing 1,5 - pentanediamine. For example, the analytical solution formed after the condensate discharged during the purification of 1,5 - pentanediamine by evaporation, distillation or rectification is adsorbed by resin and analyzed by sulfuric acid solution.

[0032] In one embodiment, the purity of the ammonium sulfate product is 90 wt% or more, further 97 wt% or more, further 99 wt% or more, and / or the pentanediamine residue content of the ammonium sulfate product is 50000 ppm or less, further 8000 ppm or less, further 5500 ppm or less, further 2500 ppm or less, and / or the light transmittance of the ammonium sulfate product is 75% or more, further 95% or more, further 97% or more.

[0033] The second aspect of the present invention is to provide the application of ammonium sulfate recovered by the above - mentioned recovery method in fermentation. For example, the application in the fermentation of long - chain dibasic acids, and the long - chain dibasic acids include decanedioic acid, undecanedioic acid, dodecanedioic acid, tridecanedioic acid, tetradecanedioic acid, pentadecanedioic acid, hexadecanedioic acid, octadecanedioic acid.

[0034] The recovered ammonium sulfate product is used to prepare the culture medium, and the proportion of replacing the commercially available ammonium sulfate product is greater than 0 and less than or equal to 100%, further greater than 0 and less than or equal to 75%, further greater than 0 and less than or equal to 50%, further greater than 0 and less than or equal to 35%.

[0035] The beneficial effects of the present invention are as follows:

[0036] 1. Using the method for recovering ammonium sulfate of the present invention, with the wastewater in the production process of biobased pentamethylenediamine as the raw material, the recovery of ammonium sulfate effectively reduces the salt content in the wastewater, not only saving the raw material cost, but more importantly, it can effectively reduce downstream pollution, thus significantly reducing the sewage treatment cost, and further reducing the production cost of pentamethylenediamine.

[0037] 2. The recovered ammonium sulfate has high purity and low impurity residue, and can be used for fermentation. Specific embodiments

[0038] The Candida viswanathii strain used has a preservation number of CCTCC M 2020048 (see the publicly disclosed Chinese patent CN111748480A).

[0039] The involved test methods are as follows:

[0040] Transmittance: The transmittance at 620 nm is detected by an ultraviolet spectrophotometer.

[0041] Ratio of amine nitrogen to total nitrogen: Detection methods for amine nitrogen content and total nitrogen content: Near-infrared on-line detection. Calculation of the ratio of amine nitrogen to total nitrogen: Ratio of amine nitrogen to total nitrogen = amine nitrogen content / total nitrogen content * 100%. Amine nitrogen refers to the nitrogen derived from 1,5-pentanediamine.

[0042] The preparation method of the resin eluate refers to steps (1) to (2) of Example 3 of Chinese patent CN 108276293 A, and different batches of condensate in the 1,5-pentanediamine concentration process are used to obtain different resin eluates.

[0043] Resin eluate - 1: pH value is 2.5, ammonium sulfate content is 12 wt%, pentamethylene diamine sulfate content is 1 wt%, color is light red, transmittance is 82.3%.

[0044] Resin eluate - 2: pH value is 2.4, ammonium sulfate content is 6.5 wt%, pentamethylene diamine sulfate content is 0.8 wt%, color is light red, transmittance is 87.4%.

[0045] In the following examples, the addition amount of the decolorizing activated carbon is based on the mass of ammonium sulfate contained in the liquid to be decolorized.

[0046] Example 1

[0047] (1) The pH of resin eluate - 1 is adjusted to 6.2 with calcium hydroxide, 0.1 wt% of activated carbon is added, stirred at 70 °C for 30 min, and then the activated carbon is removed by plate and frame filtration. The filtrate is concentrated 3.5 times in a falling film evaporator at 80 °C (the ratio of amine nitrogen to total nitrogen is 5.21%).

[0048] (2) Evaporate and crystallize the solution concentrated in step (1) in a forced circulation concentrator at a temperature of 55°C. As evaporation and crystallization proceed, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue the evaporation and crystallization until the proportion of amine nitrogen in the total nitrogen of the evaporated concentrated liquid reaches 7.25%, which is the end point of evaporation and crystallization.

[0049] (3) Discharge the mixture of the evaporated concentrated liquid and ammonium sulfate crystals, cool it to 28°C, and obtain crystals and mother liquor by centrifugal separation. Wash and dry the crystals to obtain ammonium sulfate products.

[0050] Example 2

[0051] Return the mother liquor separated in Example 1 to the forced circulation concentrator and mix it with the solution concentrated in the falling film evaporator in step (1) for evaporation and crystallization in step (2). The temperature of evaporation and crystallization is 55°C. The end point of evaporation and crystallization is when the proportion of amine nitrogen in the total nitrogen of the evaporated concentrated liquid reaches 19.48%.

[0052] Discharge the mixture of the evaporated concentrated liquid and ammonium sulfate crystals in step (2), cool the mixture to 28°C, and obtain crystals and mother liquor by centrifugal separation. Wash and dry the crystals to obtain ammonium sulfate products.

[0053] Example 3

[0054] Step (1): The same as in Example 1.

[0055] Step (2): Evaporate and crystallize the solution concentrated in step (1) in a forced circulation concentrator at a temperature of 55°C. As evaporation and crystallization proceed, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue the evaporation and crystallization until the proportion of amine nitrogen in the total nitrogen of the evaporated concentrated liquid reaches 82.97%, which is the end point of evaporation and crystallization.

[0056] Step (3): The same as in Example 1.

[0057] Example 4

[0058] Step (1): The same as in Example 1.

[0059] Step (2): Evaporate and crystallize the solution concentrated in step (1) in a forced circulation concentrator at a temperature of 55°C. As evaporation and crystallization proceed, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue the evaporation and crystallization until the proportion of amine nitrogen in the total nitrogen of the evaporated concentrated liquid reaches 67.66%, which is the end point of evaporation and crystallization.

[0060] Step (3): The same as in Example 1.

[0061] Example 5

[0062] (1) Adjust the pH of the resin analytical solution -1 to 6.2 with calcium hydroxide, and then concentrate it 3.5 times at 80 °C in a falling film evaporator. Then add 0.2 wt% of activated carbon, stir at 70 °C for 25 min, and remove the activated carbon by plate and frame filtration to obtain a filtrate (the proportion of amine nitrogen in total nitrogen is 5.35%).

[0063] (2) Evaporate and crystallize the filtrate from step (1) in a forced circulation concentrator at a temperature of 55 °C. As the evaporation and crystallization proceed, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue the evaporation and crystallization until the proportion of amine nitrogen in the total nitrogen of the evaporation concentrate reaches 7.46% as the end point of evaporation and crystallization.

[0064] (3) Drain the mixture of the evaporation concentrate and ammonium sulfate crystals, cool it to 30 °C, and obtain crystals and mother liquor by centrifugal separation. Wash and dry the crystals to obtain ammonium sulfate products.

[0065] Example 6

[0066] Step (1): The same as in Example 1.

[0067] Step (2): Evaporate and crystallize the solution concentrated in step (1) in a forced circulation concentrator at a temperature of 70 °C. As the evaporation and crystallization proceed, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue the evaporation and crystallization until the proportion of amine nitrogen in the total nitrogen of the evaporation concentrate reaches 7.55% as the end point of evaporation and crystallization.

[0068] Step (3): The same as in Example 1.

[0069] Example 7

[0070] Step (1): Adjust the pH of the resin analytical solution -1 to 6.4 with calcium hydroxide, add 0.1 wt% of activated carbon, stir at 75 °C for 30 min, then remove the activated carbon by plate and frame filtration, and concentrate the filtrate 3.2 times at 80 °C in a falling film evaporator (the proportion of amine nitrogen in total nitrogen is 5.42%).

[0071] Step (2): Evaporate and crystallize the solution concentrated in step (1) in a forced circulation concentrator at a temperature of 55 °C. As the evaporation and crystallization proceed, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue the evaporation and crystallization until the proportion of amine nitrogen in the total nitrogen of the evaporation concentrate reaches 8.32% as the end point of evaporation and crystallization.

[0072] Step (3): The same as in Example 1.

[0073] Example 8

[0074] Step (1): The same as in Example 1.

[0075] Step (2): Subject the solution concentrated in step (1) to evaporation crystallization in a forced circulation concentrator at a temperature of 55°C. As evaporation crystallization proceeds, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue evaporation crystallization until the proportion of amine nitrogen in the evaporation concentrate to the total nitrogen reaches 25.76%, which is the end point of evaporation crystallization.

[0076] (3) Drain the mixture of the evaporation concentrate and ammonium sulfate crystals, cool it to 32°C, and obtain crystals and mother liquor by centrifugal separation. Wash and dry the crystals to obtain ammonium sulfate products.

[0077] Example 9

[0078] (1) Adjust the pH of the resin eluate-1 to 6.1 with calcium hydroxide, add 0.1 wt% of activated carbon, stir at 70°C for 35 min, then remove the activated carbon by plate and frame filtration. Concentrate the filtrate 3.5 times in a falling film evaporator at 80°C (the proportion of amine nitrogen in the total nitrogen is 5.44%).

[0079] (2) Subject the solution concentrated in step (1) to evaporation crystallization in a forced circulation concentrator at a temperature of 80°C. As evaporation crystallization proceeds, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue evaporation crystallization until the proportion of amine nitrogen in the evaporation concentrate to the total nitrogen reaches 13.54%, which is the end point of evaporation crystallization.

[0080] (3) Drain the mixture of the evaporation concentrate and ammonium sulfate crystals, cool it to 28°C, and obtain crystals and mother liquor by centrifugal separation. Wash and dry the crystals to obtain ammonium sulfate products.

[0081] Example 10

[0082] (1) Adjust the pH of the resin eluate-2 to 6.3 with calcium hydroxide, add 0.2 wt% of activated carbon, stir at 70°C for 40 min, then remove the activated carbon by plate and frame filtration. Concentrate the filtrate 5 times in a falling film evaporator at 75°C (the proportion of amine nitrogen in the total nitrogen is 7.61%).

[0083] (2) Subject the solution concentrated in step (1) to evaporation crystallization in a forced circulation concentrator at a temperature of 65°C. As evaporation crystallization proceeds, the ammonium sulfate concentration in the solution gradually increases to form ammonium sulfate crystals. Continue evaporation crystallization until the proportion of amine nitrogen in the evaporation concentrate to the total nitrogen reaches 12.38%, which is the end point of evaporation crystallization.

[0084] (3) Drain the mixture of the evaporation concentrate and ammonium sulfate crystals, cool it to 28°C, and obtain crystals and mother liquor by centrifugal separation. Wash and dry the crystals to obtain ammonium sulfate products.

[0085] The ammonium sulfate products obtained from Examples 1-10 were tested and analyzed, as shown in Table 1.

[0086] Table 1

[0087]

[0088] Application Example 1

[0089] The seed solution of Candida viswanathii (preservation number CCTCC NO: M 2020048) (OD 620 value diluted 30 times was 0.62) was inoculated into a 10 L fermenter, and a glucose solution with a concentration of 50% (w / v) was fed into the fermenter. The feeding period of the glucose solution was from the 7th to the 10th h after the start of fermentation. Candida viswanathii cells began to multiply rapidly within 0 h to 15 h of fermentation. When the OD620 value of the cells reached 0.9 (measured after dilution 30 times), the pH value was adjusted to 6.6. At the same time, n-dodecane was fed into the fermenter, and the flow rate of the alkane was controlled so that the concentration of dodecane in the fermentation broth was controlled at about 2% (w / v). The cumulative addition amount of n-dodecane was 25% (w / v). Fermentation was terminated when the content of dodecanedioic acid in the fermentation broth no longer increased.

[0090] Among them, the components of the fermentation medium were: ammonium sulfate 0.24%, magnesium sulfate heptahydrate 0.35%, potassium nitrate 0.4%, glucose 3.3%, yeast extract 0.5%, potassium dihydrogen phosphate 0.45%, corn steep liquor 0.8%, Tween-80 0.1%, polyether defoamer 0.07% (w / v).

[0091] The ammonium sulfate in the fermentation medium used commercially available ammonium sulfate, the ammonium sulfate of Examples 1-10, or a mixture of both. Specifically, it is shown in Table 2.

[0092] Table 2

[0093]

[0094] Application Example 2

[0095] The difference from Application Example 1 was only that the fermentation substrate used n-undecane, and the fermentation product was undecanedioic acid. The results are shown in Table 3.

[0096] Table 3

[0097]

[0098] As can be seen from Table 2 and Table 3, during the biological fermentation process, using the recycled ammonium sulfate as a raw material to replace commercially available sulfuric acid, the fermentation results basically reach the fermentation level of only using commercially available ammonium sulfate. When using the recycled ammonium sulfate as a raw material for fermenting long-chain dibasic acids, the total salt discharge amount of the fermentation plant and the cost of fermentation raw materials can be significantly reduced.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for recovering ammonium sulfate, characterized in that, Including: The raw material is a solution containing both ammonium sulfate and pentamethylenediamine sulfate. The pH value of the solution is adjusted to 5 - 8, and a concentrated solution containing ammonium sulfate crystals is formed by evaporation crystallization. The ammonium sulfate solid is obtained through solid-liquid separation. At the end point of evaporation crystallization, the proportion a of amine nitrogen in the evaporated concentrated solution to the total nitrogen is 0 < a ≤ 82.5%.

2. The method according to claim 1, The solution is concentrated before evaporation crystallization. Preferably, the concentration temperature is 60 - 90 °C, and further 73 - 90 °C.

3. The method according to claim 1 or 2, wherein the solution is decolorized before evaporation crystallization.

4. The method according to claim 1, before solid-liquid separation, the mixed system of ammonium sulfate crystals and concentrated liquid is cooled to 20 - 40 °C.

5. The method according to claim 1, the ammonium sulfate solid is washed and dried to obtain ammonium sulfate product.

6. The method according to claim 1, the ways of solid-liquid separation include centrifugal separation and filtration separation.

7. The method according to claim 1 or 6, the mother liquor after solid-liquid separation is returned to the evaporation crystallization step, or returned to any step before the evaporation crystallization step.

8. The method according to any one of claims 1 - 3, the proportion b of amine nitrogen in the solution before evaporation crystallization to the total nitrogen is 0 < b ≤ 10%, and further 4% ≤ b ≤ 10%.

9. The method as claimed in claim 1, characterized in that, The temperature of evaporation crystallization is 45 - 95 °C, further 50 - 90 °C.

10. The method as claimed in claim 1, characterized in that, At the end point of evaporation crystallization, the proportion of amine nitrogen in the evaporated concentrated solution to the total nitrogen is 0 < a ≤ 55%, further 5% ≤ a ≤ 55%.

11. The method as claimed in claim 3, characterized in that, A decolorizing agent is used for decolorization treatment, and the decolorizing agent is removed by solid-liquid separation after the decolorization treatment; preferably, Based on the mass of ammonium sulfate contained in the solution to be decolorized, the addition amount of the decolorizing agent is 0.03% - 2%, further 0.03% - 1%, and / or The decolorizing agent is activated carbon; and / or The temperature of the decolorization treatment is 50 - 80 °C; and / or The time of the decolorization treatment is 10 - 80 min, further 15 - 60 min.

12. The method as claimed in claim 3, characterized in that, The solution is first subjected to decolorization treatment and then concentration treatment before evaporation crystallization, or first subjected to concentration treatment and then decolorization treatment.

13. The method as claimed in claim 1, characterized in that, The raw material is the wastewater discharged during the purification of 1,5-pentanediamine from the fermentation broth or enzyme conversion solution formed by fermentation or enzyme conversion method for preparing 1,5-pentanediamine.

14. The method as claimed in claim 13, characterized in that, The wastewater is the analytical solution formed after the condensate discharged during the purification of 1,5-pentanediamine by evaporation, distillation or rectification is adsorbed by resin and analyzed by sulfuric acid solution.

15. The method as claimed in claim 1, characterized in that, The evaporation crystallization equipment includes a forced circulation concentrator, a crystallization reactor, and a draft tube-baffle evaporation crystallizer.

16. Use of ammonium sulfate recovered by the method according to any one of claims 1-15 in fermentation.

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