A method for recovering phosphorus resources from blue iron ore synthesized by pressurized hydrothermal synthesis of urban sewage sludge

Abstract

The present invention discloses a method for recovering phosphorus resources by pressurized hydrothermal synthesis of blue iron ore from urban sewage sludge: wet sludge is mixed with deionized water and then added to a hydrothermal reactor; gas is introduced into the hydrothermal reactor to increase the initial pressure inside it, and after the hydrothermal temperature and hydrothermal residence time are set, a pressurized hydrothermal reaction is performed to synthesize blue iron ore to obtain a hydrothermal product; the obtained hydrothermal product is subjected to solid-liquid separation to obtain a hydrothermal product containing blue iron ore; the obtained hydrothermal product containing blue iron ore is subjected to magnetic separation to obtain blue iron ore, and freeze-dried to obtain pure blue iron ore powder. The present invention uses a pressurized hydrothermal method to prepare blue iron ore, which can accelerate the decomposition of organic matter such as carbohydrates and proteins in sludge in an inert environment, promote the reduction of trivalent iron to divalent iron, and form blue iron ore with phosphorus crystals, effectively shortening the blue iron ore preparation cycle, the process is controllable, and the preparation process does not require the use of acid and alkali reagents, with low production cost and green environmental protection.

Description

Technical Field

[0001] The invention relates to a method for recovering phosphorus resources from urban sewage sludge, belonging to the technical field of resource utilization of organic solid wastes. Background Art

[0002] Phosphorus is a non-renewable resource as a nutrient element necessary for the growth of plants and animals. At the same time, excessive phosphorus entering the water body will cause eutrophication pollution. Therefore, phosphorus resource recovery is of great significance. Urban sewage treatment plants are one of the important "phosphorus sinks", and the phosphorus in sewage is eventually enriched in sewage sludge. Therefore, how to efficiently recover phosphorus resources from sludge has become one of the research hotspots. At the same time, sludge contains Ca 2+ Mg 2+ 、Al 3+ and Fe 3+ Various metal ions such as phosphorus and iodine affect the form of phosphorus in sludge, and its form directly determines the quality of phosphorus recovery products in sludge.

[0003] Currently, phosphorus resources can be recovered from sludge or sludge incineration ash by acid-base dissolution, electrodialysis, incineration, pyrolysis, thermal hydrolysis, anaerobic digestion and hydrothermal treatment, or a combination of the above methods. The main phosphorus recovery products are struvite, cyanite, hydroxyapatite and phosphate-rich ash / biochar. Blue iron ore has high economic value and can be used as a phosphate fertilizer and a raw material for the production of lithium iron phosphate batteries. Therefore, there is a broad prospect for recovering phosphorus resources from sludge in the form of blue iron ore.

[0004] At present, some studies have been conducted to extract the original sludge or sludge incineration ash with acid and alkali solution to obtain phosphorus-rich extract, and then form cyanite through crystallization; or to generate cyanite through microbial reduction and crystallization during anaerobic digestion. However, the above technologies have the disadvantages of long microbial reduction reaction cycle (20 to 30 days), low reduction rate, high drug consumption and high production cost. Therefore, it is urgent to develop a new method to recover sludge phosphorus resources in the form of cyanite. Summary of the invention

[0005] The technical problem to be solved by the present invention is: how to recover phosphorus resources in sludge in the form of blue iron ore.

[0006] In order to solve the above problems, the present invention provides a method for recovering phosphorus resources of blue iron ore by pressurized hydrothermal synthesis of urban sewage sludge, comprising the following steps:

[0007] Step 1): mixing wet sludge with deionized water and adding the mixture into a hydrothermal reactor;

[0008] Step 2): Use a high-pressure explosion-proof pipe to connect the gas cylinder and the valve, introduce gas into the hydrothermal reactor to increase the initial pressure inside the reactor, set the hydrothermal temperature and the hydrothermal residence time, and then perform a pressurized hydrothermal reaction to synthesize blue iron ore to obtain a hydrothermal product;

[0009] Step 3): performing solid-liquid separation on the hydrothermal product obtained in step 2) to obtain a hydrothermal product containing blue iron ore;

[0010] Step 4): The hydrothermal product containing cyanite obtained in step 3) is subjected to magnetic separation to obtain cyanite, which is then freeze-dried to obtain pure cyanite powder.

[0011] Preferably, in step 1), the wet sludge is mechanically dewatered sludge from a sewage treatment plant with a moisture content of 70-90%.

[0012] Preferably, in step 1), the deionized water is nitrogen or argon aerated deionized water, and the dissolved oxygen content is less than 5%.

[0013] Preferably, in step 1), the solid-liquid mass ratio of the wet sludge to deionized water is 1:1 to 1:20.

[0014] Preferably, in step 2), the initial pressure inside the hydrothermal reactor is 0.1-6.0 MPa, the temperature of the hydrothermal reaction is 70-200° C., and the hydrothermal residence time is 0.5-8 hours.

[0015] Preferably, in step 2), the solid-liquid separation is centrifugal separation, vacuum filtration separation or mechanical filter press separation.

[0016] Preferably, the recovery method is specifically as follows: wet sludge with a moisture content of 80% and nitrogen-aerated deionized water (dissolved oxygen content less than 5%) are mixed in a solid-liquid mass ratio of 1:10 and added to a hydrothermal reactor, a high-pressure explosion-proof pipe is used to connect the gas cylinder and the valve, nitrogen is introduced to increase the initial pressure in the reactor to 1 MPa, the hydrothermal temperature is set to 110°C and the hydrothermal residence time is set to 4 hours, pressurized hydrothermal synthesis of cyanite is performed to obtain a hydrothermal product, and centrifugal separation is performed at a speed of 9000 r / min for 15 minutes to obtain a hydrothermal product containing cyanite; then cyanite is obtained by magnetic separation, and vacuum freeze-drying is performed for 24 hours to obtain pure cyanite powder.

[0017] The present invention aims at the current technical problems of low microbial reduction rate of blue iron ore, long preparation cycle, large consumption of acid and alkali reagents, etc., and proposes a pressurized hydrothermal method to synthesize blue iron ore, improve the decomposition rate of organic matter such as carbohydrates and proteins in sludge, promote the production of organic matter with reducing ability such as organic acids, thereby promoting the reduction of trivalent iron to divalent iron in an inert atmosphere, and crystallizing to generate blue iron ore. This method can effectively shorten the preparation cycle of blue iron ore (0.5 to 8 hours), and the reaction process does not use any acid and alkali reagents, has the characteristics of green environmental protection and low energy consumption, and provides a new way for sludge phosphorus resource recovery.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The sludge pressurized hydrothermal method provided by the present invention, the phosphorus form distribution and XRD spectrum of the obtained hydrothermal product indicate that the present invention can effectively prepare cyanite, which makes up for the shortcomings of the existing cyanite preparation method, such as long cycle and large amount of acid and alkali reagents used, and provides a new method for sludge phosphorus resource recovery. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the principle of the present invention;

[0021] Figure 2 XRD pattern and phosphorus speciation distribution of blue iron ore prepared hydrothermally under saturated pressure of sludge model to adjust protein ratio;

[0022] Figure 3 XRD pattern and phosphorus speciation distribution of blue iron ore prepared by pressurized hydrothermal method for adjusting protein content of sludge model;

[0023] Figure 4 XRD pattern and phosphorus speciation distribution of blue iron ore prepared by pressurized hydrothermal method for actual municipal sludge. DETAILED DESCRIPTION

[0024] In order to make the present invention more clearly understood, preferred embodiments are described in detail below with reference to the accompanying drawings.

[0025] like Figure 1 As shown, it is a schematic diagram of the method for preparing viorite by pressurized hydrothermal treatment of sludge in Examples 1-3, which includes Example 1 for preparing viorite by hydrothermal treatment of a sludge model with adjusted protein ratio under saturated pressure, Example 2 for preparing viorite by pressurized hydrothermal treatment of a sludge model with adjusted protein ratio, and Example 3 for preparing viorite by pressurized hydrothermal treatment of actual municipal sludge.

[0026] Example 1

[0027] Glucose, protein, goethite and potassium dihydrogen phosphate were used as sewage sludge modeling materials, and the mass ratio of protein to glucose was adjusted to 3:1 to 1:3, respectively. Aerated deionized water with dissolved oxygen below 5% was added to the hydrothermal reactor at a solid-liquid mass ratio of 1:10, and the mixture was stirred and tightened. The hydrothermal reaction temperature was set to 110 °C, the reaction residence time was 4h, and the saturated pressure was set. After the reaction, the reactor was cooled to room temperature. The hydrothermal product was centrifuged for 15min at a speed of 9000r / min, and the solid phase product was vacuum freeze-dried for 24h to obtain hydrothermal charcoal containing cyanite; the solid phase product after centrifugation can also be separated from the hydrothermal charcoal by magnetic separation, and then freeze-dried to obtain pure cyanite powder.

[0028] Example 2

[0029] Glucose: protein: goethite: potassium dihydrogen phosphate in a mass ratio of 5.6:8.4:3.37:3.43 and aerated deionized water with dissolved oxygen less than 5% are added to the hydrothermal reactor in a solid-liquid mass ratio of 1:10, stirred and mixed evenly, and then tightened. Use a high-pressure explosion-proof pipe to connect the gas cylinder and the reactor valve, and introduce nitrogen to increase the initial pressure in the reactor to 1.0MPa. Set the hydrothermal reaction temperature to 110°C and the reaction residence time to 4h. After the reaction, cool the reactor to room temperature. The hydrothermal product is centrifuged for 15min at a speed of 9000r / min, and the solid phase product is vacuum freeze-dried for 24h to obtain hydrothermal charcoal containing blue iron ore; after centrifugation, blue iron ore can also be separated from the hydrothermal charcoal by magnetic separation, and then freeze-dried to obtain pure blue iron ore powder.

[0030] Example 3

[0031] Municipal sewage sludge and aerated deionized water with dissolved oxygen less than 5% are added to the hydrothermal reactor at a solid-liquid mass ratio of 1:10, stirred and mixed evenly, and then tightened; a high-pressure explosion-proof pipe is used to connect the gas cylinder and the reactor valve, and nitrogen is introduced to increase the initial pressure in the reactor to 1.0MPa. The hydrothermal reaction temperature is set to 110°C and the reaction residence time is 4h. After the reaction, the reactor is cooled to room temperature. The hydrothermal product is centrifuged for 15min at a speed of 9000r / min, and the solid phase product is vacuum freeze-dried for 24h to obtain hydrothermal charcoal containing blue iron ore; after centrifugation, blue iron ore can also be separated from the hydrothermal charcoal by magnetic separation, and then freeze-dried to obtain pure blue iron ore powder.

[0032] The freeze-dried samples prepared in Examples 1-3 were sieved through a 200-mesh stainless steel sieve for X-ray diffraction analysis and phosphorus speciation analysis. The phosphorus speciation analysis method was based on the literature (Wang Qian et al., Water Research, 2021, 202, 117411), as follows: Figure 2 and Figure 3 In Example 1, the mass ratio of protein to glucose was 1.5:1, and the characteristic peak of blue iron ore was found at 27.81°. At the same time, the mass ratio of ferrous phosphorus to total phosphorus in the hydrothermal charcoal reached 40.14% ( Figure 2 ); In Example 2, the ratio of protein to glucose was adjusted and nitrogen was used for pressurization. The characteristic diffraction peaks of blue iron ore were present at 18.14° and 27.81° in the XRD spectrum and the peak intensity was significantly enhanced. At the same time, the proportion of ferrous phosphorus in the hydrothermal charcoal to the total phosphorus reached 63.09% ( Figure 3 ); In Example 3, Figure 4 As shown, after nitrogen pressurized hydrothermal treatment, a characteristic diffraction peak of blue iron ore appeared at 27.81°, and the peak intensity increased significantly with increasing pressure. The proportion of ferrous phosphorus in the hydrothermal charcoal to total phosphorus was 9.30%, which was 5 times higher than that of the original sludge.

[0033] The above results prove the feasibility of a method for recovering phosphorus resources by synthesizing cyanite from municipal sewage sludge by pressurized hydrothermal method. Compared with the existing method, the preparation cycle of cyanite is shorter and no chemical reagents are consumed in the preparation process, which provides a new method for recovering phosphorus resources from sewage sludge.

Claims

1. A method for recovering phosphorus resources from blue iron ore synthesized by pressurized hydrothermal synthesis of municipal sewage sludge, characterized in that: The following steps are involved: Step 1): mixing wet sludge with deionized water and adding the mixture into a hydrothermal reactor; Step 2): Use a high-pressure explosion-proof pipe to connect the gas cylinder and the valve, introduce gas into the hydrothermal reactor to increase the initial pressure inside the reactor, set the hydrothermal temperature and the hydrothermal residence time, and then perform a pressurized hydrothermal reaction to synthesize blue iron ore to obtain a hydrothermal product; the initial pressure inside the hydrothermal reactor is 1.0 to 6.0 MPa, and the temperature of the hydrothermal reaction is 110 to 200° C.; Step 3): performing solid-liquid separation on the hydrothermal product obtained in step 2) to obtain a hydrothermal product containing blue iron ore; Step 4): The hydrothermal product containing cyanite obtained in step 3) is subjected to magnetic separation to obtain cyanite, which is then freeze-dried to obtain pure cyanite powder.

2. The recycling method according to claim 1, characterized in that: In step 1), the wet sludge is mechanically dehydrated sludge from a sewage treatment plant with a moisture content of 70-90%.

3. The recycling method according to claim 1, characterized in that: In step 1), the deionized water is nitrogen or argon aerated deionized water, and the dissolved oxygen content is less than 5%.

4. The recycling method according to claim 1, characterized in that: In step 1), the solid-liquid mass ratio of the wet sludge to deionized water is 1:1 to 1:

20.

5. The recycling method according to claim 1, characterized in that: In step 2), the hydrothermal residence time is 0.5 to 8 hours.

6. The recycling method according to claim 1, characterized in that: In step 3), the solid-liquid separation is centrifugal separation, vacuum filtration separation or mechanical filter press separation.

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