Method for Preparing N-P Compound Fertilizer from Incinerated Sewage Sludge Ash (ISSA)

Abstract

The present disclosure provides a method for preparing an N-P compound fertilizer from incinerated sewage sludge ash (ISSA), and relates to the technical field of compound fertilizer production. In the present disclosure, the ISSA is used as a main raw material, and a phosphorus-rich supernatant is obtained using high-concentration nitric acid at a low liquid-to-solid ratio. The phosphorus-rich supernatant and aqueous ammonia are mixed to obtain a white precipitate. The white precipitate is dried and pulverized to obtain a high-grade N-P compound fertilizer. The present disclosure realizes the resource utilization of excess sludge in sewage treatment plants.

Claims

1. A method for preparing an N-P compound fertilizer from incinerated sewage sludge ash (ISSA), comprising the following steps: (1) weighing the ISSA, drying to a constant weight, and cooling in an airtight environment; (2) heating and mixing a nitric acid solution with the ISSA to obtain a mixed solution; (3) subjecting the mixed solution to a reaction by fully stirring in a constant-temperature stirring device, and conducting centrifugation on an obtained reacted mixed solution to avoid formation of colloids, thereby obtaining a phosphorus-rich supernatant and an ISSA residue; (4) adjusting the phosphorus-rich supernatant to a pH value of 6 to 8 using aqueous ammonia with a mass fraction of 15% to 25%, conducting a reaction fully under stirring to obtain a white turbid liquid, and conducting centrifugation on the white turbid liquid to avoid formation of colloids, thereby obtaining a white precipitate; and (5) subjecting the white precipitate to drying, pulverizing, and sieving by a 100-mesh sieve to obtain the N-P compound fertilizer.

2. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (1), the drying is conducted at 90° C. to 110° C.

3. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (2), the nitric acid solution has a concentration of 3.0 mol/L to 7.0 mol/L and a temperature of 20° C. to 45° C.

4. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (2), the nitric acid solution and the ISSA are at a liquid-to-solid ratio of (2.5-5.0) mL:1 g.

5. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (3), the stirring is conducted at 20° C. to 45° C. and 200 rpm to 400 rpm for 10 min to 30 min.

6. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (3), the centrifugation is conducted at 5,000 rpm to 6,500 rpm for 5 min to 10 min.

7. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (4), the reaction is conducted at 25° C. to 55° C. and 300 rpm to 600 rpm for 25 min to 60 min.

8. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (4), the centrifugation is conducted at 5,000 rpm to 6,500 rpm for 5 min to 10 min.

9. The method for preparing an N-P compound fertilizer by ISSA according to claim 1, wherein in step (4), the drying is conducted at 90° C. to 110° C. for 6 h to 10 h.

10. An N-P compound fertilizer prepared by the method according to claim 1.

11. The N-P compound fertilizer according to claim 10, wherein in step (1), the drying is conducted at 90° C. to 110° C.

12. The N-P compound fertilizer according to claim 10, wherein in step (2), the nitric acid solution has a concentration of 3.0 mol/L to 7.0 mol/L and a temperature of 20° C. to 45° C.

13. The N-P compound fertilizer according to claim 10, wherein in step (2), the nitric acid solution and the ISSA are at a liquid-to-solid ratio of (2.5-5.0) mL:1 g.

14. The N-P compound fertilizer according to claim 10, wherein in step (3), the stirring is conducted at 20° C. to 45° C. and 200 rpm to 400 rpm for 10 min to 30 min.

15. The N-P compound fertilizer according to claim 10, wherein in step (3), the centrifugation is conducted at 5,000 rpm to 6,500 rpm for 5 min to 10 min.

16. The N-P compound fertilizer according to claim 10, wherein in step (4), the reaction is conducted at 25° C. to 55° C. and 300 rpm to 600 rpm for 25 min to 60 min.

17. The N-P compound fertilizer according to claim 10, wherein in step (4), the centrifugation is conducted at 5,000 rpm to 6,500 rpm for 5 min to 10 min.

18. The N-P compound fertilizer according to claim 10, wherein in step (4), the drying is conducted at 90° C. to 110° C. for 6 h to 10 h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present disclosure will be further described below with reference to the accompanying drawings.

[0029] FIG. 1 shows a process flow diagram of the method for preparing an N-P compound fertilizer by ISSA in the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The present disclosure provides a method for preparing an N-P compound fertilizer by ISSA. The method has a simple operation process, a low cost, and a high recovery rate, and includes the following steps: [0031] (1) weighing the ISSA, drying to a constant weight, and cooling in an airtight environment; [0032] (2) heating and mixing a nitric acid solution with the ISSA to obtain a mixed solution; [0033] (3) subjecting the mixed solution to a reaction by fully stirring in a constant-temperature stirring device, and conducting centrifugation on an obtained reacted mixed solution to avoid formation of colloids, thereby obtaining a phosphorus-rich supernatant and an ISSA residue; [0034] (4) adjusting the phosphorus-rich supernatant to a pH value of 6 to 8 using aqueous ammonia with a mass fraction of 15% to 25%, conducting a reaction fully under stirring to obtain a white turbid liquid, and conducting centrifugation on the white turbid liquid to avoid formation of colloids, thereby obtaining a white precipitate; and [0035] (5) subjecting the white precipitate to drying, pulverizing, and sieving by a 100-mesh sieve to obtain the N-P compound fertilizer.

[0036] In one example, in step (1), the drying is conducted at 90° C. to 110° C.

[0037] In one example, in step (2), the nitric acid solution has a concentration of 3.0 mol/L to 7.0 mol/L and a temperature of 20° C. to 45° C.

[0038] In one example, in step (2), the nitric acid solution and the ISSA are at a liquid-to-solid ratio of (2.5-5.0) mL:1 g.

[0039] In one example, in step (3), the stirring is conducted at 20° C. to 45° C. and 200 rpm to 400 rpm for 10 min to 30 min.

[0040] In one example, in step (3), the centrifugation is conducted at 5,000 rpm to 6,500 rpm for 5 min to 10 min.

[0041] In one example, in step (4), the reaction is conducted at 25° C. to 55° C. and 300 rpm to 600 rpm for 25 min to 60 min.

[0042] In one example, in step (4), the centrifugation is conducted at 5,000 rpm to 6,500 rpm for 5 min to 10 min.

[0043] In one example, in step (4), the drying is conducted at 90° C. to 110° C. for 6 h to 10 h.

[0044] An N-P compound fertilizer is prepared by the method according to any one of claims 1 to 9.

[0045] The technical solutions provided by the present disclosure will be further described below with reference to the accompanying examples.

Example 1

[0046] A method for preparing an N-P compound fertilizer by ISSA included the following steps: [0047] (1) ISSA in an urban domestic sewage treatment plant was dried to a constant weight at 105° C.; [0048] (2) a nitric acid solution with a concentration of 5 mol/L was heated to 25° C., and mixed with the ISSA at a liquid-to-solid ratio of 3 mL:1 g to obtain a mixed solution; [0049] (3) the mixed solution was reacted by stirring in a constant-temperature stirring device at 25° C. and 300 rpm for 20 min, a resulting reacted mixed solution was centrifuged in a high-speed centrifugal device at 6,500 rpm for 5 min, and an obtained centrifuged product was subjected to solid-liquid separation to obtain a phosphorus-rich supernatant and an ISSA residue; [0050] (4) the phosphorus-rich supernatant was poured into a reaction vessel, added dropwise with aqueous ammonia at a mass fraction of 25% to adjust a pH value to 7, and fully stirred in the constant-temperature stirring device at 50° C. and 400 rpm for 30 min to obtain a white suspension; the white suspension was centrifuged in the high-speed centrifugal device at 6,500 rpm for 5 min to obtain a white precipitate and an ammonia-neutralized supernatant; and [0051] (5) the white precipitate was dried in a drying device at 90° C. for about 10 h to obtain a white lump, and the white lump was fully cooled in a sealed cooling device; an obtained cooled white lump was pulverized in a pulverizer for 5 min, and sieved by a 100-mesh sieve to obtain the N-P compound fertilizer containing a large amount of phosphorus.

[0052] After testing, the N-P compound fertilizer had N, P.sub.2O.sub.5, and K.sub.2O of 28.00%, 13.70%, and 0.54%, respectively, and had a total nutrient content of 42.24%, thus reaching the level of excellent products (≥42.0%) among nitrophosphate fertilizers and nitric phosphate-potassium fertilizers.

Example 2

[0053] A method for preparing an N-P compound fertilizer by ISSA included the following steps: [0054] (1) ISSA in an urban domestic sewage treatment plant was dried to a constant weight at 105° C.; [0055] (2) a nitric acid solution with a concentration of 3 mol/L was heated to 45° C., and mixed with the ISSA at a liquid-to-solid ratio of 5 mL:1 g to obtain a mixed solution; [0056] (3) the mixed solution was reacted by stirring in a constant-temperature stirring device at 45° C. and 400 rpm for 15 min, a resulting reacted mixed solution was centrifuged in a high-speed centrifugal device at 6,000 rpm for 8 min, and an obtained centrifuged product was subjected to solid-liquid separation to obtain a phosphorus-rich supernatant and an ISSA residue; [0057] (4) the phosphorus-rich supernatant was poured into a reaction vessel, added dropwise with aqueous ammonia at a mass fraction of 20% to adjust a pH value to 6, and fully stirred in the constant-temperature stirring device at 25° C. and 600 rpm for 40 min to obtain a white suspension; the white suspension was centrifuged in the high-speed centrifugal device at 6,000 rpm for 8 min to obtain a white precipitate and an ammonia-neutralized supernatant; and [0058] (5) the white precipitate was dried in a drying device at 105° C. for about 8 h to obtain a white lump, and the white lump was fully cooled in a sealed cooling device; an obtained cooled white lump was pulverized in a pulverizer for 3 min, and sieved by a 100-mesh sieve to obtain the N-P compound fertilizer containing a large amount of phosphorus.

[0059] After testing, the N-P compound fertilizer had N, P.sub.2O.sub.5, and K.sub.2O of 25.54%, 11.24%, and 4.57%, respectively, and had a total nutrient content of 41.35%, thus reaching the level of first-class products (≥40.0%) among nitrophosphate fertilizers and nitric phosphate-potassium fertilizers.

Comparative Example 1

[0060] A method for preparing a compound fertilizer by phosphorus in ISSA included the following steps: [0061] (1) ISSA in an urban domestic sewage treatment plant was dried to a constant weight; [0062] (2) a nitric acid solution with a concentration of 1 mol/L was heated to 35° C., and mixed with the ISSA at a liquid-to-solid ratio of 2 mL:1 g to obtain a mixed solution; [0063] (3) the mixed solution was reacted by stirring in a constant-temperature stirring device at 35° C. and 200 rpm for 50 min, a resulting reacted mixed solution was centrifuged in a high-speed centrifugal device at 6,500 rpm for 5 min, and an obtained centrifuged product was subjected to solid-liquid separation to obtain a phosphorus-rich supernatant and an ISSA residue; [0064] (4) the phosphorus-rich supernatant was poured into a reaction vessel, added dropwise with aqueous ammonia at a mass fraction of 10% to adjust a pH value to 3, and fully stirred in the constant-temperature stirring device at 25° C. and 200 rpm for 10 min to obtain a white suspension; the white suspension was centrifuged in the high-speed centrifugal device at 6,500 rpm for 5 min to obtain a white precipitate and an ammonia-neutralized supernatant; and [0065] (5) the white precipitate was dried in a drying device at 105° C. for about 6 h to obtain a white lump, and the white lump was fully cooled; an obtained cooled white lump was pulverized in a pulverizer for 3 min, and sieved by a 100-mesh sieve to obtain the N-P compound fertilizer containing a large amount of phosphorus. After testing, the N-P compound fertilizer had N, P.sub.2O.sub.5, and K.sub.2O of 19.82%, 7.67%, and 2.35%, respectively, and had a total nutrient content of 29.84%, thus reaching the level of unqualified products (38.0%) among nitrophosphate fertilizers and nitric phosphate-potassium fertilizers.

[0066] It was seen that when using nitric acid to extract phosphorus in ISSA at a low liquid-to-solid ratio, the phosphorus extraction was insufficient to cause low phosphorus content in the extract. Therefore, the subsequently prepared compound fertilizer had a low content of nutrient elements and could not meet the requirements of relevant standards.

Comparative Example 2

[0067] A method for preparing a compound fertilizer by phosphorus in ISSA included the following steps: [0068] (1) ISSA in an urban domestic sewage treatment plant was dried to a constant weight; [0069] (2) a nitric acid solution with a concentration of 5 mol/L was heated to 25° C., and mixed with the ISSA at a liquid-to-solid ratio of 3 mL:1 g to obtain a mixed solution; [0070] (3) the mixed solution was reacted by stirring in a constant-temperature stirring device at 25° C. and 300 rpm for 20 min, a resulting reacted mixed solution was centrifuged in a high-speed centrifugal device at 6,500 rpm for 5 min, and an obtained centrifuged product was subjected to solid-liquid separation to obtain a phosphorus-rich supernatant and an ISSA residue; [0071] (4) the phosphorus-rich supernatant was poured into a reaction vessel, added dropwise with aqueous ammonia at a mass fraction of 15% to adjust a pH value to 4, and fully reaction in the constant-temperature stirring device at 70° C. and 0 rpm (namely without stirring) for 20 min to obtain a white suspension; the white suspension was centrifuged in the high-speed centrifugal device at 6,500 rpm for 5 min to obtain a white precipitate and an ammonia-neutralized supernatant; and [0072] (5) the white precipitate was dried in a drying device at 105° C. for about 10 h to obtain a white lump, and the white lump was fully cooled; an obtained cooled white lump was pulverized in a pulverizer for 5 min, and sieved by a 100-mesh sieve to obtain the N-P compound fertilizer containing a large amount of phosphorus. After testing, the N-P compound fertilizer had N, P.sub.2O.sub.5, and K.sub.2O of 20.59%, 9.88%, and 2.74%, respectively, and had a total nutrient content of 33.21%, thus reaching the level of unqualified products (38.0%) among nitrophosphate fertilizers and nitric phosphate-potassium fertilizers.

[0073] It was seen that the phosphorous in the ISSA was extracted by nitric acid, and the obtained phosphorous-containing extract had a higher phosphorous content. However, during the ammonia neutralization by aqueous ammonia, improper operating conditions led to low phosphorus content in the white precipitate. Therefore, the compound fertilizer obtained by drying and pulverizing had low nutrient element contents and could not meet the requirements of relevant standards. Only under the cooperation of the method designed in the present disclosure, the raw materials, and the technical parameters of each process, could phosphorus be successfully extracted from the ISSA and prepared to obtain the N-P compound fertilizer.

Test Example 1

[0074] The N-P compound fertilizers obtained in Examples 1 to 2 presented a white powder; by observation under an optical microscope, the compound fertilizer was in an amorphous state. Observation under a scanning electron microscope (SEM) showed that the surface and interior of the N-P compound fertilizer were in an amorphous state, and various structures were mixed together to form a compound. Moreover, there were generally clusters of protrusions similar to unit cells on the surface, making the surface of the N-P compound fertilizer not smooth. Occasionally, rod-like structures appeared in these fragmented compounds. It was seen that some substances with better crystallization were also formed in the amorphous state. The N-P compound fertilizer was basically an inorganic compound composed of many amorphous substances.

Test Example 2 Pot Plant Verification Experiment

[0075] The pot plant experiment of ryegrass was conducted with the N-P compound fertilizer obtained in Example 1. The growth as well as fresh weight (dry weight), root length, plant height, leaf length, and chlorophyll content of the ryegrass were all desirable, and compared with a non-fertilization group, there was a significant promotion effect. Moreover, no heavy metal elements (Cu, Cd, Pb, Cr, As, and Hg) were detected from the plants, so it was seen that fertilization did not cause the accumulation of heavy metals in plants. Simultaneously, soil pH, organic matter, total nitrogen, and total phosphorus did not deteriorate, and the heavy metals (Cu, Cd, Pb, Cr, As, and Hg) were not detected, indicating that fertilization had little impact on the soil.

[0076] Specific examples are used herein to explain the principles and implementations of the present disclosure. The foregoing description of the embodiments is merely intended to help understand the method of the present disclosure and its core ideas; besides, various modifications may be made by a person of ordinary skill in the art to specific embodiments and the scope of application in accordance with the ideas of the present disclosure. In conclusion, the content of the present specification shall not be construed as limitations to the present disclosure.