Method for recovering nitrogen and phosphorus elements from municipal sludge

Abstract

The invention relates to the technical field of municipal sludge treatment, in particular to a method for recycling nitrogen and phosphorus elements from municipal sludge. According to the method, the municipal sludge is subjected to hydrothermal carbonization treatment, the treated solid-phase product is dried and ground to a specific particle size, the solid-phase product is subjected to acid leaching under a specific condition to extract the phosphorus element, and then heavy metal and metal elements such as Al and Fe in the acid leaching liquid are removed by adopting strong-acid ion exchange resin and a chemical precipitant, so that the phosphorus element is obtained. And finally, mixing the struvite with a hydrolyzate which is subjected to hydrothermal carbonization treatment and is rich in ammonia nitrogen, and precipitating by adjusting the element proportion and the pH value to obtain a solid-phase product struvite crystal. According to the method, external nitrogen-containing substances do not need to be added, the content of heavy metal in the solid-phase product struvite can be effectively reduced, the recovery rate of phosphorus can reach 93% or above, the recovery rate of ammonia nitrogen can reach 63% or above, the recovery rate of nitrogen and phosphorus is high, energy consumption is low, and the treatment cost is low.

Description

Technical Field

[0001] The invention relates to the technical field of municipal sludge treatment, and in particular to a method for recovering nitrogen and phosphorus elements from municipal sludge. Background Art

[0002] Municipal sludge is the impurities left after the treatment of urban domestic wastewater and industrial wastewater, and contains rich nitrogen and phosphorus elements. At present, the sludge treatment process in my country usually aims at harmlessness, and the resource utilization of nutrients such as nitrogen and phosphorus has not been achieved. Phosphorus is a non-renewable resource. The recycling of phosphorus is of great significance to solving the phosphorus resource crisis and phosphorus pollution problems. Municipal sludge is a potential phosphorus resource.

[0003] In recent years, some studies have reported the use of direct chemical acid leaching for phosphorus, but this method has the problems of low phosphorus release efficiency and simultaneous leaching of heavy metals. If the phosphorus recovery product is directly used as a slow-release fertilizer, there will be a certain risk of heavy metal pollution. There are also studies reporting the use of thermochemical methods to pyrolyze and carbonize sludge before acid leaching. Although this method can increase the phosphorus release rate, the pyrolysis temperature is as high as 400-600℃, the drying pretreatment consumes a lot of energy, and it is impossible to achieve nitrogen recovery.

[0004] In summary, the existing sludge phosphorus recovery technology generally has the disadvantages of low phosphorus release efficiency leading to low phosphorus recovery rate, heavy metal interference affecting the purity of phosphorus recovery products, high energy consumption and failure to achieve simultaneous recovery of nitrogen and phosphorus. Therefore, it is necessary to develop a sludge treatment method that is efficient, low energy consumption and can achieve simultaneous recovery of nitrogen and phosphorus. Summary of the invention

[0005] The invention provides a method for recovering nitrogen and phosphorus elements from municipal sludge. The method has high nitrogen and phosphorus recovery rate, low heavy metal content in solid phase products, low energy consumption and low processing cost.

[0006] The present invention achieves the above technical objectives through the following technical solutions:

[0007] A method for recovering nitrogen and phosphorus from municipal sludge, comprising:

[0008] Step 1, adjusting the solid-liquid ratio of municipal sludge to 1: (8-10), heating it to 180-240°C at a heating rate of 5-10°C / min, 250-350rpm, and hydrothermal carbonization reaction for 1-3h under saturated vapor pressure conditions;

[0009] Step 2, separating the hydrothermal carbonization reaction product into solid and liquid to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate, drying the hydrothermal carbon and grinding it to a particle size of less than 0.15 mm to obtain hydrothermal carbon powder;

[0010] Step 3, extracting phosphorus from the sludge by acid leaching the hydrothermal carbon powder, wherein the liquid-to-solid ratio during leaching is 50-100 mL / g, the concentration of the acid solution is 0.1-1.0 mol / L, and the leaching time is not less than 10 min under stirring conditions;

[0011] Step 4, after the leaching is completed, solid-liquid separation is performed to obtain a liquid phase product acid leaching solution and a solid phase product;

[0012] Step 5, adding a strong acid ion exchange resin to the acid leaching solution, stirring and reacting for 2-4 hours, wherein the addition amount of the strong acid ion exchange resin is 0.5-5 g / g based on the dry weight of the raw material municipal sludge;

[0013] Step 6, after the reaction is completed, separate the strongly acidic ion exchange resin to obtain filtrate 1, add a sulfide or hydroxide precipitant to the filtrate 1, stir and react for 0.5-1h, remove the precipitate, and obtain filtrate 2, wherein the amount of the precipitant added is 1-5 mol / L;

[0014] Step 7, mixing the filtrate 2 in step 6 with the hydrolyzate in step 2 to ensure that the molar ratio of nitrogen and phosphorus in the solution is (1-1.5):1, then adding magnesium salt to adjust the molar ratio of magnesium and phosphorus to (1-1.3):1, adjusting the pH of the solution to 9.0-9.5, stirring the reaction sufficiently, letting it stand and precipitate, filtering, and recovering the solid phase product, thereby completing the recovery of nitrogen and phosphorus.

[0015] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, the strongly acidic ion exchange resin in step 5 is a strongly acidic styrene-based cation exchange resin.

[0016] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, in step 3, the acid used for acid leaching extraction is an inorganic acid or an organic acid.

[0017] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, the inorganic acid is selected from nitric acid or hydrochloric acid, and the organic acid is selected from citric acid or oxalic acid.

[0018] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, the method for separating the strongly acidic ion exchange resin in step 6 is: screening using a 50-mesh sieve.

[0019] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, the solid-liquid separation method in step 2 and / or the solid-liquid separation method in step 4 and / or the filtration and removal of precipitates in step 6 is centrifugation or vacuum filtration.

[0020] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, in step 7, the filtrate 2 and the hydrolyzate are mixed in a volume ratio of 1:(0.8-2).

[0021] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, the magnesium salt added in step 7 is magnesium chloride.

[0022] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, the precipitant in step 6 is sodium sulfide or sodium hydroxide.

[0023] According to the above-mentioned method for recovering nitrogen and phosphorus from municipal sludge, the solid phase product obtained after acid leaching is modified and used as a carbon-based adsorption material or dried and used as a biomass fuel.

[0024] The method for recovering nitrogen and phosphorus from municipal sludge provided by the present invention comprises the following steps: subjecting municipal sludge to hydrothermal carbonization treatment, drying and grinding the treated solid phase product to a specific particle size, extracting phosphorus by acid leaching under specific conditions, and then effectively reducing the concentration of metal ions such as calcium, aluminum, iron, zinc, copper, manganese, and chromium in the extract by adding a strong acid ion exchange resin and combining chemical precipitation treatment. Finally, the extract is mixed with a hydrolysis product after hydrothermal carbonization treatment, and precipitation is performed by adjusting the element ratio and pH to obtain a solid phase product, struvite crystals. The method does not require the addition of exogenous nitrogen-containing substances, reduces the content of heavy metals in the solid phase product, effectively improves the safety of the phosphorus recovery product, and the phosphorus recovery rate can reach more than 93%, the ammonia nitrogen recovery rate can reach more than 63%, the nitrogen and phosphorus recovery rate is high, the energy consumption is low, and the treatment cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention provides a process flow chart of a method for recovering nitrogen and phosphorus from municipal sludge according to an embodiment of the present invention. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] The present invention provides a method for recovering nitrogen and phosphorus elements from municipal sludge, which specifically comprises the following steps:

[0028] Step 1, adjusting the solid-liquid ratio of municipal sludge to 1: (8-10), heating it to 180-240°C at a heating rate of 5-10°C / min, 250-350rpm, and hydrothermal carbonization reaction for 1-3h under saturated vapor pressure conditions;

[0029] Step 2, separating the hydrothermal carbonization reaction product into solid and liquid to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate, drying the hydrothermal carbon and grinding it to a particle size of less than 0.15 mm to obtain hydrothermal carbon powder;

[0030] Step 3, extracting phosphorus from the sludge by acid leaching the hydrothermal carbon powder, wherein the liquid-solid ratio during leaching is 50-100 mL / g, the concentration of the acid solution is 0.1-10 mol / L, and the stirring condition is that the leaching time is not less than 10 min;

[0031] Step 4, after the extraction is completed, solid-liquid separation is performed to obtain a liquid phase product extract and a solid phase product;

[0032] Step 5, adding a strong acid ion exchange resin to the acid leaching solution, stirring and reacting for 2-4 hours, wherein the addition amount of the strong acid ion exchange resin is 0.5-5 g / g based on the dry weight of the raw material municipal sludge;

[0033] Step 6, after the reaction is completed, separate the strongly acidic ion exchange resin to obtain filtrate 1, add a sulfide or hydroxide precipitant to the filtrate 1, stir and react for 0.5-1h, remove the precipitate, and obtain filtrate 2, wherein the amount of the precipitant added is 1-5 mol / L;

[0034] Step 7, mixing the filtrate 2 in step 6 with the hydrolyzate in step 2 to ensure that the molar ratio of nitrogen to phosphorus in the solution is (1-1.5):1, then adding magnesium salt to control the molar ratio of magnesium to phosphorus to (1-1.3):1, adjusting the pH of the solution to 9.0-9.5, stirring the reaction sufficiently, letting it stand for precipitation and filtering, recovering the solid phase product and calculating the nitrogen and phosphorus recovery rate.

[0035] The present invention performs hydrothermal carbonization treatment on municipal sludge, dries and grinds the treated solid phase product to a specific particle size, then performs acid leaching under specific conditions to extract nitrogen and phosphorus elements, then uses a strong acid ion exchange resin and a chemical precipitant to remove heavy metals and elements such as Al and Fe in the acid leaching solution, and finally mixes the sludge with a hydrolyzate rich in ammonia nitrogen after the hydrothermal carbonization treatment, and performs precipitation by adjusting the element ratio and pH to obtain a solid phase product, struvite crystals. The method does not require the addition of exogenous nitrogen-containing substances, reduces the content of heavy metals in the solid phase product, effectively improves the safety of the phosphorus recovery product, and the phosphorus recovery rate can reach more than 93%, the ammonia nitrogen recovery rate can reach more than 63%, the nitrogen and phosphorus recovery rate is high, the energy consumption is low, and the treatment cost is low.

[0036] The present invention firstly uses a hydrothermal carbonization reaction to treat municipal sludge. The hydrothermal carbonization reaction can realize the conversion of organic phosphorus into inorganic phosphorus and the conversion of apatite into non-apatite. Because non-apatite phosphorus in inorganic phosphorus is more easily acid-leached than apatite phosphorus, the proportion of non-apatite phosphorus in the sludge increases after the sludge is subjected to a hydrothermal carbonization reaction. By optimizing the acid leaching and metal removal steps, a high recovery rate of phosphorus can be ensured. In addition, in the hydrothermal carbonization reaction, polyphosphates and organic phosphates in the sludge microbial cells are also released to undergo a hydrolysis reaction. The released orthophosphates and dissolved metal ions or ammonium ions are precipitated or adsorbed to form other inorganic phosphate compounds, which are also beneficial to the extraction of phosphorus. That is, as long as the inorganic phosphorus in the hydrothermal carbon is extracted, the phosphorus in the sludge can be extracted efficiently. In addition, compared with the pyrolysis carbonization reaction, the hydrothermal carbonization used in the present invention does not require drying treatment, the reaction process is simple and fast, and the energy consumption is lower. Compared with the hydrothermal liquefaction reaction (reaction temperature is 260-320°C) and the pyrolysis reaction, the reaction temperature of the hydrothermal carbonization reaction is also lower, and the energy consumption is also lower.

[0037] In the present invention, the conditions for the hydrothermal carbonization reaction are: a solid-liquid ratio of 1: (8-10), a heating rate of 5-10°C / min to 180-240°C, stirring and reacting for 1-3h under saturated vapor pressure conditions. It can be understood that the saturated vapor pressure conditions mentioned above refer to the saturated vapor pressure of water at different reaction temperatures, that is, the reaction pressure. In other words, the reaction pressure depends on the saturated vapor pressure of water at different reaction temperatures. Under the reaction temperature conditions of the present invention, the reaction pressure is usually 0.8-3Mpa.

[0038] In the present invention, the municipal sludge after the solid-liquid ratio is adjusted is heated to the hydrothermal carbonization reaction temperature at a heating rate of 5-10°C / min. This is because the heating rate of the sludge hydrothermal carbonization reaction has an important influence on the product distribution, pore structure and reaction process. Too high a heating rate may lead to incomplete cracking of organic matter, affecting the quality and yield of solid carbon.

[0039] After the hydrothermal carbonization reaction is completed, the hydrothermal carbonization reaction product is subjected to solid-liquid separation. It is understood that the solid-liquid separation can be carried out by conventional centrifugation or vacuum filtration. For example, in some specific embodiments, the solid phase product hydrothermal carbon and the liquid phase product hydrolyzate are obtained by centrifugation at 2000-4000rpm for 15-30min. In other specific embodiments, a qualitative filter paper with a pore size of 1-3μm is used as a filter medium for vacuum filtration to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate.

[0040] The hydrothermal charcoal is ground to a particle size of less than 0.15 mm and then subjected to acid leaching. During the acid leaching, inorganic acids such as nitric acid, hydrochloric acid, etc., or organic acids such as citric acid, oxalic acid, etc. can be used for acid leaching.

[0041] By strictly controlling the acid leaching conditions, the acid leaching effect can be guaranteed. In the present invention, the liquid-solid ratio is controlled to be 50-100mL / g during acid leaching, the concentration of the acid solution is 0.1-1.0mol / L, the leaching time is controlled to be more than 10min, and magnetic stirring or shaking is used for stirring to ensure that the leaching is more sufficient. In the present invention, the liquid-solid ratio during acid leaching can be 50mL / g, 60mL / g, 70mL / g, 80mL / g, 90mL / g, 100mL / g and the range between any two of the above values. The concentration of the acid solution can be 0.1mol / L, 0.2mol / L, 0.3mol / L, 0.4mol / L, 0.5mol / L, 0.6mol / L, 0.7mol / L, 0.8mol / L, 1.0mol / L and the range between any two of the above values. The leaching time is not less than 10min to ensure sufficient acid leaching, and is usually controlled within 10-100min.

[0042] After the extraction is completed, solid-liquid separation is performed, and the same solid-liquid separation can be performed by conventional methods such as centrifugation or vacuum filtration. For example, in some specific embodiments, the liquid phase product acid leaching solution and the solid phase product are obtained by centrifugation at 2000-4000rpm for 15-30min. In other specific embodiments, a qualitative filter paper with a pore size of 1-3μm is used as a filter medium for vacuum filtration to obtain a liquid phase product acid leaching solution and a solid phase product. The obtained solid phase product can be further modified as a carbon-based adsorption material, or directly used as a biomass fuel after drying.

[0043] The present invention uses a strongly acidic ion exchange resin for adsorption treatment. This type of resin easily dissociates H⁺ in the solution, so it has a strong ion exchange capacity. They can exchange cations in the solution, such as metal ions such as sodium, potassium, calcium, and iron, and release hydrogen ions. In some specific embodiments of the present invention, the strongly acidic ion exchange resin is a strongly acidic styrene-based cation exchange resin. This type of resin is mainly obtained by copolymerization of styrene and divinylbenzene, and then a subsequent sulfonation reaction. Because it has a sulfonic acid group, it has a high cation exchange capacity and can quickly and effectively perform ion exchange. In addition, the resin also has good chemical stability, mechanical strength, and easy regeneration, and can maintain stable performance in a variety of chemical environments, and resist certain pressure and wear.

[0044] In the present invention, the dosage of the strongly acidic ion exchange resin is 0.5-5 g / g based on the dry weight of the original sludge, that is, 0.5-5 g is added for every 1 g of dry weight of the municipal sludge, for example, it can be 0.5 g, 1 g, 2 g, 3 g, 4 g, 5 g.

[0045] After the strong acid ion exchange resin is adsorbed, it can be separated by conventional filtration. For example, in some specific embodiments, the method for separating the strong acid ion exchange resin is to use a 50 mesh (0.315 mm) screen for sieving. The separated strong acid ion exchange resin can be eluted with a strong acid (such as hydrochloric acid, nitric acid, etc.) and then regenerated and reused, and the eluted metals (Al, Fe, etc.) can be further recycled.

[0046] Sulfide or hydroxide (such as sodium sulfide, sodium hydroxide, etc.) is added to the filtrate obtained after separation by the strong acid ion exchange resin for chemical precipitation treatment, so that the residual metal ions such as calcium, aluminum, iron, etc. in the leaching solution are formed into insoluble precipitates and removed.

[0047] The same method of removing the precipitate can be conventional methods such as centrifugation or vacuum filtration. For example, in some specific embodiments, centrifugation is performed at 2000-4000 rpm for 15-30 minutes to obtain filtrate 2. In other specific embodiments, vacuum filtration is performed using qualitative filter paper with a pore size of 1-3 μm as the filter medium to obtain filtrate 2.

[0048] The hydrolyzate after hydrothermal carbonization treatment contains a large amount of ammonia nitrogen. In the past, the treatment of sludge often ignored the recovery of nitrogen in the hydrolyzate. The present invention mixes the hydrolyzate with an acid leaching solution rich in phosphate to form a mixed solution rich in nitrogen and phosphorus elements. Only magnesium salt needs to be added to achieve simultaneous recovery of nitrogen and phosphorus, and obtain struvite (MgNH4PO4•6H2O) slow-release fertilizer, which not only realizes the resource utilization of solid waste, but also reduces the energy and material consumption of additional nitrogen source and hydrolyzate treatment, greatly reduces costs, and reduces carbon emissions. Specifically, by detecting the element content in the filtrate 2 and the hydrolyzate, the two are mixed and the nitrogen and phosphorus molar ratio is controlled to (1-1.5):1. The two are usually mixed in a volume ratio of 1:(0.8-2).

[0049] After the nitrogen-phosphorus ratio is adjusted, the magnesium-phosphorus molar ratio is adjusted by adding magnesium salt, and the ratio of the two is controlled to be (1-1.3):1. At this time, the magnesium salt is calculated as magnesium chloride, and its addition amount is usually 13-25wt% of the dry weight of the raw material municipal sludge to meet this ratio. Finally, the pH is adjusted to 9.0-9.5 by adding a strong base (sodium hydroxide or potassium hydroxide, etc.), and after stirring the reaction, the solid phase product is filtered after standing and settling, and the recovery of nitrogen and phosphorus is completed. The solid phase product obtained is struvite, which can be used as a slow-release fertilizer after drying.

[0050] It is understood that in the present invention, in order to ensure sufficient reaction, stirring conditions are adopted during hydrothermal carbonization reaction, acid leaching, strong acid ion exchange resin adsorption and chemical precipitation. There is no special restriction on the stirring speed, and it is sufficient to ensure sufficient stirring reaction. The stirring speed is usually controlled between 150-500rpm, preferably between 150-300rpm.

[0051] The method for recovering nitrogen and phosphorus from municipal sludge according to the present invention is described in detail below with reference to specific embodiments.

[0052] The municipal sludge used in the following embodiments is the same batch of municipal sludge, which is taken from the residual sludge dewatering cake treated by A2 / O process in Beijing Qinghe Reclaimed Water Plant. After the sludge is retrieved, it is placed in a refrigerated storage at 4°C and experimentally analyzed within one month. The moisture content of the original sludge is 86.35%, the pH value is 6.68, and the organic matter content is 39.98%. The original sludge (by dry weight) has a C content of 25.01%, a N content of 7.06%, and a P content of 4.29%. The metal content of the original sludge (by dry weight) is 606.06 mg / kg for Zn, 306.84 mg / kg for Cu, 61.27 mg / kg for Cr, 132.23 mg / kg for Mn, and 26.49 mg / kg for Ni. Other metal elements include Ca at 2212.21 mg / kg, Al at 2079.06 mg / kg, Fe at 11118.96 mg / kg, and Mg at 203.53 mg / kg.

[0053] Example 1

[0054] This embodiment provides a method for recovering nitrogen and phosphorus from municipal sludge. Figure 1 As shown, the specific steps are as follows:

[0055] (1) Municipal sludge was placed in a hydrothermal reactor, the solid-liquid ratio was adjusted to 1:9, and the temperature was raised to 220°C at a heating rate of 10°C / min. The hydrothermal carbonization reaction was carried out for 2 h under the autogenous pressure condition of the reaction temperature (i.e., under the saturated vapor pressure state), during which the stirring speed was 300 rpm.

[0056] (2) After the reaction is completed, circulating cooling water is immediately introduced to cool the reactor to room temperature, the mixed liquid in the reactor is taken out, and the solid-liquid separation is performed by centrifugation at 3000 rpm for 20 minutes to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate. The hydrothermal carbon is dried at 105°C to a constant weight, ground to pass through a 100-mesh sieve (<0.15 mm), and the hydrolyzate is stored for later use.

[0057] (3) The hydrothermal carbon powder was subjected to acid leaching to extract phosphorus from the sludge, wherein the liquid-to-solid ratio during leaching was 80 mL / g, the acid solution was 1.0 mol / L hydrochloric acid solution, the leaching time was 60 min, and the stirring speed was 200 rpm.

[0058] (4) After the extraction is completed, centrifuge at 3000 rpm for 20 minutes to separate the solid and liquid, and obtain the liquid phase product acid extract and the solid phase product. The solid phase product can be further modified as a carbon-based adsorption material, or directly used as a biomass fuel after drying.

[0059] (5) Strong acid ion exchange resin (H type) and strong acid styrene cation exchange resin (purchased from Shanghai MacLean Biochemical Co., Ltd., model 001×7 (732)) were added to the above acid extract, and the two were allowed to react fully at room temperature for 3 h using a shaker at a rotation speed of 220 rpm. The amount of strong acid ion exchange resin added was 3 g / g based on the original sludge dry weight.

[0060] (6) After the reaction is completed, a 50-mesh stainless steel screen (pore size 0.315 mm) is used to separate the strong acid ion exchange resin in the mixture to obtain filtrate 1, and sodium sulfide precipitant is added to filtrate 1 to make the residual metal ions such as calcium, aluminum, and iron in the leaching solution form insoluble precipitates. The precipitates are removed by centrifugation at 3000 rpm for 20 minutes to obtain filtrate 2. The amount of sodium sulfide added is 3 mol / L. The strong acid ion exchange resin can be regenerated by elution with a strong acid (such as hydrochloric acid, nitric acid, etc.) and reused, and the eluted metals (Al, Fe, etc.) can be further recycled.

[0061] (7) The filtrate 2 obtained in step (6) is mixed with the hydrolyzate obtained in step (2) to ensure that the molar ratio of nitrogen to phosphorus in the solution is (1-1.2):1, and then magnesium chloride is added to control the molar ratio of magnesium to phosphorus to (1-1.1):1, and then a KOH solution is added to adjust the pH of the mixed solution to 9.2, and magnetic stirring is performed for half an hour to allow it to react fully, and then the mixture is allowed to stand for precipitation for half an hour, and the solid phase product struvite crystals are obtained by filtration. The solid phase product struvite crystals mainly consist of nitrogen and phosphorus, and can be used as a slow-release fertilizer after drying.

[0062] Example 2

[0063] This embodiment provides a method for recovering nitrogen and phosphorus from municipal sludge, and the specific steps are as follows:

[0064] (1) Municipal sludge was placed in a hydrothermal reactor, the solid-liquid ratio was adjusted to 1:8, and the temperature was raised to 240°C at a heating rate of 10°C / min. The saturated vapor pressure was maintained and the hydrothermal carbonization reaction was carried out for 1.5 hours. The stirring speed was 320 rpm.

[0065] (2) After the reaction is completed, circulating cooling water is immediately introduced to cool the reactor to room temperature, the mixed liquid in the reactor is taken out, and the solid-liquid separation is performed by centrifugation at 4000 rpm for 15 minutes to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate. The hydrothermal carbon is dried at 105°C to a constant weight, ground to pass through a 100-mesh sieve (<0.15 mm), and the hydrolyzate is stored for later use.

[0066] (3) The hydrothermal carbon powder was subjected to acid leaching to extract phosphorus from the sludge, wherein the liquid-to-solid ratio during leaching was 50 mL / g, the acid solution was 1.0 mol / L nitric acid solution, the leaching time was 40 min, and the stirring speed was 200 rpm.

[0067] (4) After the extraction is completed, centrifuge at 4000 rpm for 15 minutes to separate the solid and liquid, and obtain the liquid phase product acid extract and the solid phase product. The solid phase product can be further modified as a carbon-based adsorption material, or directly used as a biomass fuel after drying.

[0068] (5) Strong acid ion exchange resin (H type) and strong acid styrene cation exchange resin (purchased from Shanghai MacLean Biochemical Co., Ltd., model 001×7 (732)) were added to the above acid extract, and the two were allowed to react fully at room temperature for 4 h using a shaker at a rotation speed of 250 rpm, wherein the amount of strong acid ion exchange resin added was 1 g / g based on the original sludge dry weight.

[0069] (6) After the reaction is completed, a 50-mesh stainless steel screen (pore size 0.315 mm) is used to separate the strong acid ion exchange resin in the mixture to obtain filtrate 1, and sodium sulfide precipitant is added to filtrate 1 to make the residual metal ions such as calcium, aluminum, and iron in the leaching solution form insoluble precipitates. The precipitates are removed by centrifugation at 4000 rpm for 15 minutes to obtain filtrate 2. The amount of sodium sulfide added is 5 mol / L. The strong acid ion exchange resin can be regenerated by elution with a strong acid (such as hydrochloric acid, nitric acid, etc.) and reused, and the eluted metals (Al, Fe, etc.) can be further recycled.

[0070] (7) The filtrate 2 obtained in step (6) is mixed with the hydrolyzate obtained in step (2) to ensure that the molar ratio of nitrogen to phosphorus in the solution is (1.2-1.3):1, and then magnesium chloride is added to control the molar ratio of magnesium to phosphorus to (1.2-1.3):1, and then KOH solution is added to adjust the pH of the mixed solution to 9.4, and magnetic stirring is performed for half an hour to allow it to react fully, and then it is allowed to stand for precipitation for half an hour, and the solid phase product struvite crystals are obtained by filtration. The components of the solid phase product struvite crystals are mainly nitrogen and phosphorus, and can be used as a slow-release fertilizer after drying.

[0071] Example 3

[0072] This embodiment provides a method for recovering nitrogen and phosphorus from municipal sludge, and the specific steps are as follows:

[0073] (1) Municipal sludge was placed in a hydrothermal reactor, the solid-liquid ratio was adjusted to 1:10, and the temperature was raised to 190°C at a heating rate of 8°C / min. The saturated vapor pressure was maintained and the hydrothermal carbonization reaction was carried out for 3 hours. The stirring speed was 280 rpm.

[0074] (2) After the reaction is completed, circulating cooling water is immediately introduced to cool the reactor to room temperature, the mixed liquid in the reactor is taken out, and the solid-liquid separation is performed by centrifugation at 2000 rpm for 30 minutes to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate. The hydrothermal carbon is dried at 105°C to a constant weight, ground to pass through a 100-mesh sieve (<0.15 mm), and the hydrolyzate is stored for later use.

[0075] (3) The hydrothermal carbon powder was subjected to acid leaching to extract phosphorus from the sludge, wherein the liquid-to-solid ratio during leaching was 90 mL / g, the acid solution was 1.0 mol / L citric acid solution, the leaching time was 80 min, and the stirring speed was 200 rpm.

[0076] (4) After the extraction is completed, centrifuge at 2000 rpm for 30 minutes to separate the solid and liquid, and obtain the liquid phase product acid extract and the solid phase product. The solid phase product can be further modified as a carbon-based adsorption material, or directly used as a biomass fuel after drying.

[0077] (5) Strong acid ion exchange resin (H type) and strong acid styrene cation exchange resin (purchased from Shanghai MacLean Biochemical Co., Ltd., model 001×7 (732)) were added to the above acid extract, and the two were allowed to react fully at room temperature for 3 h using a shaker at a rotation speed of 200 rpm, wherein the amount of strong acid ion exchange resin added was 5 g / g based on the original sludge dry weight.

[0078] (6) After the reaction is completed, a 50-mesh stainless steel screen (pore size 0.315 mm) is used to separate the strong acid ion exchange resin in the mixture to obtain filtrate 1, and sodium sulfide precipitant is added to filtrate 1 to make the residual metal ions such as calcium, aluminum, and iron in the leaching solution form insoluble precipitates. The precipitates are removed by centrifugation at a speed of 2000 rpm for 30 minutes to obtain filtrate 2. The amount of sodium sulfide added is 1 mol / L. The strong acid ion exchange resin can be regenerated by elution with a strong acid (such as hydrochloric acid, nitric acid, etc.) and reused, and the eluted metals (Al, Fe, etc.) can be further recycled.

[0079] (7) The filtrate 2 obtained in step (6) is mixed with the hydrolyzate obtained in step (2) to ensure that the molar ratio of nitrogen to phosphorus in the solution is (1.4-1.5):1, and then magnesium chloride is added to control the molar ratio of magnesium to phosphorus to (1-1.1):1, and then a KOH solution is added to adjust the pH of the mixed solution to 9.0, and magnetic stirring is performed for half an hour to allow it to fully react, and then it is allowed to stand for precipitation for half an hour, and the solid phase product struvite crystals are obtained by filtration. The components of the solid phase product struvite crystals are mainly nitrogen and phosphorus, and can be used as a slow-release fertilizer after drying.

[0080] Comparative Example 1

[0081] This comparative example provides a method for recovering nitrogen and phosphorus from municipal sludge. The main difference between the method and Example 1 is that the resin used in step (5) is a (sodium type) cation exchange resin (purchased from Sinopharm Group Shanghai Co., Ltd., product number: Dowex Marathon C, Sigma-Aldrich), specifically as follows:

[0082] (1) Municipal sludge was placed in a hydrothermal reactor, the solid-liquid ratio was adjusted to 1:9, and the temperature was raised to 220°C at a heating rate of 10°C / min. The saturated vapor pressure was maintained and the hydrothermal carbonization reaction was carried out for 2 h. The stirring speed was 300 rpm.

[0083] (2) After the reaction is completed, circulating cooling water is immediately introduced to cool the reactor to room temperature, the mixed liquid in the reactor is taken out, and the solid-liquid separation is performed by centrifugation at 3000 rpm for 20 minutes to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate. The hydrothermal carbon is dried at 105°C to a constant weight, ground to pass through a 100-mesh sieve (<0.15 mm), and the hydrolyzate is stored for later use.

[0084] (3) The hydrothermal carbon powder was subjected to acid leaching to extract phosphorus from the sludge, wherein the liquid-to-solid ratio during leaching was 80 mL / g, the acid solution was 1.0 mol / L hydrochloric acid solution, the leaching time was 60 min, and the stirring speed was 200 rpm.

[0085] (4) After the extraction is completed, centrifuge at 3000 rpm for 20 minutes to separate the solid and liquid, and obtain the liquid phase product acid extract and the solid phase product. The solid phase product can be further modified as a carbon-based adsorption material, or directly used as a biomass fuel after drying.

[0086] (5) Add (sodium type) cation exchange resin (purchased from Sinopharm Group Shanghai Co., Ltd., product number: Dowex Marathon C, Sigma-Aldrich) to the above acid extract, and use a shaker at 220 rpm to allow the two to react at room temperature for 3 h. The amount of (sodium type) cation exchange resin added is 3 g / g based on the original sludge dry weight.

[0087] (6) After the reaction is completed, a 50-mesh stainless steel sieve (pore size 0.315 mm) is used to separate the (sodium type) cation exchange resin in the mixture to obtain filtrate 1, and sodium sulfide precipitant is added to filtrate 1 to make the residual metal ions such as calcium, aluminum, and iron in the extract form insoluble precipitates. The precipitates are removed by centrifugation at 3000 rpm for 20 minutes to obtain filtrate 2. The amount of sodium sulfide added is 3 mol / L.

[0088] (7) The filtrate 2 obtained in step (6) is mixed with the hydrolyzate obtained in step (2) to ensure that the molar ratio of nitrogen to phosphorus in the solution is (1-1.2):1, and then magnesium chloride is added to control the molar ratio of magnesium to phosphorus to (1-1.1):1, and then a KOH solution is added to adjust the pH of the mixed solution to 9.2, and magnetic stirring is performed for half an hour to allow it to react fully, and then the mixture is allowed to stand for precipitation for half an hour, and the solid phase product struvite crystals are obtained by filtration. The solid phase product struvite crystals mainly consist of nitrogen and phosphorus, and can be used as a slow-release fertilizer after drying.

[0089] Comparative Example 2

[0090] This comparative example provides a method for recovering nitrogen and phosphorus from municipal sludge. The main difference between the method and Example 1 is that the acid leaching conditions in step (3) are different, as follows:

[0091] (1) Municipal sludge was placed in a hydrothermal reactor, the solid-liquid ratio was adjusted to 1:9, and the temperature was raised to 220°C at a heating rate of 10°C / min. The saturated vapor pressure was maintained and the hydrothermal carbonization reaction was carried out for 2 h. The stirring speed was 300 rpm.

[0092] (2) After the reaction is completed, circulating cooling water is immediately introduced to cool the reactor to room temperature, the mixed liquid in the reactor is taken out, and the solid-liquid separation is performed by centrifugation at 3000 rpm for 20 minutes to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate. The hydrothermal carbon is dried at 105°C to a constant weight, ground to pass through a 100-mesh sieve (<0.15 mm), and the hydrolyzate is stored for later use.

[0093] (3) The hydrothermal carbon powder was subjected to acid leaching to extract phosphorus from the sludge, wherein the liquid-to-solid ratio during leaching was 30 mL / g, the acid solution was 1.0 mol / L hydrochloric acid solution, the leaching time was 120 min, and the stirring speed was 200 rpm.

[0094] (4) After the extraction is completed, centrifuge at 3000 rpm for 20 minutes to separate the solid and liquid, and obtain the liquid phase product acid extract and the solid phase product. The solid phase product can be further modified as a carbon-based adsorption material, or directly used as a biomass fuel after drying.

[0095] (5) Strong acid ion exchange resin (H type) and strong acid styrene cation exchange resin (purchased from Shanghai MacLean Biochemical Co., Ltd., model 001×7 (732)) were added to the above acid extract, and the two were allowed to react fully at room temperature for 3 h using a shaker at a rotation speed of 220 rpm, wherein the amount of strong ion exchange resin added was 3 g / g based on the original sludge dry weight.

[0096] (6) After the reaction is completed, a 50-mesh stainless steel screen (pore size 0.315 mm) is used to separate the strong acid ion exchange resin in the mixture to obtain filtrate 1, and sodium sulfide precipitant is added to filtrate 1 to make the residual metal ions such as calcium, aluminum, and iron in the leaching solution form insoluble precipitates. The precipitates are removed by centrifugation at 3000 rpm for 20 minutes to obtain filtrate 2. The amount of sodium sulfide added is 3 mol / L. The strong acid ion exchange resin can be regenerated by elution with a strong acid (such as hydrochloric acid, nitric acid, etc.) and reused, and the eluted metals (Al, Fe, etc.) can be further recycled.

[0097] (7) The filtrate 2 obtained in step (6) is mixed with the hydrolyzate obtained in step (2) to ensure that the molar ratio of nitrogen to phosphorus in the solution is (1-1.2):1, and then magnesium chloride is added to control the molar ratio of magnesium to phosphorus to (1-1.1):1, and then a KOH solution is added to adjust the pH of the mixed solution to 9.2, and magnetic stirring is performed for half an hour to allow it to react fully, and then the mixture is allowed to stand for precipitation for half an hour, and the solid phase product struvite crystals are obtained by filtration. The solid phase product struvite crystals mainly consist of nitrogen and phosphorus, and can be used as a slow-release fertilizer after drying.

[0098] Comparative Example 3

[0099] This comparative example provides a method for recovering nitrogen and phosphorus from municipal sludge. The main difference between the method and Example 1 is that the hydrothermal charcoal in step (2) is ground and then passed through an 80-mesh sieve, as follows:

[0100] (1) Municipal sludge was placed in a hydrothermal reactor, the solid-liquid ratio was adjusted to 1:9, and the temperature was raised to 220°C at a heating rate of 10°C / min. The saturated vapor pressure was maintained and the hydrothermal carbonization reaction was carried out for 2 h. The stirring speed was 300 rpm.

[0101] (2) After the reaction is completed, circulating cooling water is immediately introduced to cool the reactor to room temperature, the mixed liquid in the reactor is taken out, and the solid-liquid separation is performed by centrifugation at 3000 rpm for 20 minutes to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate. The hydrothermal carbon is dried at 105°C to a constant weight, ground to pass through an 80-mesh sieve (<0.18 mm), and the hydrolyzate is stored for later use.

[0102] (3) The hydrothermal carbon powder was subjected to acid leaching to extract phosphorus from the sludge, wherein the liquid-to-solid ratio during leaching was 80 mL / g, the acid solution was 1.0 mol / L hydrochloric acid solution, the leaching time was 60 min, and the stirring speed was 200 rpm.

[0103] (4) After the extraction is completed, centrifuge at 3000 rpm for 20 minutes to separate the solid and liquid, and obtain the liquid phase product acid extract and the solid phase product. The solid phase product can be further modified as a carbon-based adsorption material, or directly used as a biomass fuel after drying.

[0104] (5) Strong acid ion exchange resin (H type) and strong acid styrene cation exchange resin (purchased from Shanghai MacLean Biochemical Co., Ltd., model 001×7 (732)) were added to the above acid extract, and the two were allowed to react fully at room temperature for 3 h using a shaker at a rotation speed of 220 rpm. The amount of strong acid ion exchange resin added was 3 g / g based on the original sludge dry weight.

[0105] (6) After the reaction is completed, a 50-mesh stainless steel screen (pore size 0.315 mm) is used to separate the strong acid ion exchange resin in the mixture to obtain filtrate 1, and sodium sulfide precipitant is added to filtrate 1 to make the residual metal ions such as calcium, aluminum, and iron in the leaching solution form insoluble precipitates. The precipitates are removed by centrifugation at 3000 rpm for 20 minutes to obtain filtrate 2. The amount of sodium sulfide added is 3 mol / L. The strong acid ion exchange resin can be regenerated by elution with a strong acid (such as hydrochloric acid, nitric acid, etc.) and reused, and the eluted metals (Al, Fe, etc.) can be further recycled.

[0106] (7) The filtrate 2 obtained in step (6) is mixed with the hydrolyzate obtained in step (2) to ensure that the molar ratio of nitrogen to phosphorus in the solution is (1-1.2):1, and then magnesium chloride is added to control the molar ratio of magnesium to phosphorus to (1-1.1):1, and then a KOH solution is added to adjust the pH of the mixed solution to 9.2, and magnetic stirring is performed for half an hour to allow it to react fully, and then the mixture is allowed to stand for precipitation for half an hour, and the solid phase product struvite crystals are obtained by filtration. The solid phase product struvite crystals mainly consist of nitrogen and phosphorus, and can be used as a slow-release fertilizer after drying.

[0107] Result analysis:

[0108] The volume and ammonia nitrogen concentration of the hydrolyzate were determined (Determination of Ammonia Nitrogen in Water Quality HJ 535-2009); the recovered struvite (MAP) was digested with nitric acid and hydrogen peroxide at 98°C, and the magnesium ions and other metal ions were determined by inductively coupled plasma atomic emission spectrometry (Prodigy, Leeman Labs, USA), and the phosphate concentration was determined by molybdenum blue spectrophotometry. Then, the ratio of each element (nitrogen, phosphorus and magnesium) in the precipitate was calculated, and the recovery rates of nitrogen and phosphorus elements and the purity of the recovered product MAP were analyzed.

[0109] (1) Recovery rate

[0110] The calculation formula of phosphorus recovery rate is as follows:

[0111]

[0112] Wherein, m2 is the mass of the recovered product (i.e., struvite MAP), g;

[0113] C2 is the mass percentage of total phosphorus in the recovered product, %;

[0114] m1 is the original sludge dry weight, g;

[0115] C1 is the mass percentage of total phosphorus in sludge, %.

[0116] The nitrogen recovery rate is calculated as follows:

[0117]

[0118] Wherein, m2 is the mass of the recovered product (i.e., struvite MAP), g;

[0119] C3 is the mass percentage of total nitrogen in the recovered product, %;

[0120] V is the volume of the hydrolyzate, L;

[0121] C4 is the ammonia nitrogen concentration in the hydrolyzate, g / L.

[0122] The dry weight of sludge used in each batch of this experiment was 10 g, and the total phosphorus and total nitrogen in the sludge were 4.29% and 7.06%.

[0123] Table 1 Results of mass and recovery of MAP generated under different reaction conditions

[0124]

[0125] From the above results, it can be seen that under the conditions of Examples 1-3, the phosphorus recovery rate can reach more than 93%, and the ammonia nitrogen recovery rate can reach more than 63%. Under the conditions of Comparative Examples 1-3, the phosphorus recovery rate is 2%~86%, and the ammonia nitrogen recovery rate is 51%~61%.

[0126] (2) Struvite purity test results

[0127] Table 2 Metal element content in MAP generated under different reaction conditions (unit: wt%)

[0128]

[0129] From the above results, it can be seen that in the solid phase product, except for magnesium, the calcium content is about 2%, the aluminum content is about 1%, the iron content is about 2%, and the other impurity metals are less than 1.2%. Heavy metal elements such as Cr, Mn, Ni, and Pb are not detected, indicating that the product struvite MAP recovered by the method of the present invention is safe to be used as a slow-release fertilizer.

[0130] 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 it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, 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 nitrogen and phosphorus from municipal sludge, characterized in that: include: Step 1, adjusting the solid-liquid ratio of municipal sludge to 1: (8-10), heating it to 180-240°C at a heating rate of 5-10°C / min, 250-350rpm, and hydrothermal carbonization reaction for 1-3h under saturated vapor pressure conditions; Step 2, separating the hydrothermal carbonization reaction product into solid and liquid to obtain a solid phase product hydrothermal carbon and a liquid phase product hydrolyzate, drying the hydrothermal carbon and grinding it to a particle size of less than 0.15 mm to obtain hydrothermal carbon powder; Step 3, extracting phosphorus from the sludge by acid leaching the hydrothermal carbon powder, wherein the liquid-to-solid ratio during leaching is 50-100 mL / g, the concentration of the acid solution is 0.1-1.0 mol / L, and the leaching time is not less than 10 min under stirring conditions; Step 4, after the leaching is completed, solid-liquid separation is performed to obtain a liquid phase product acid leaching solution and a solid phase product; Step 5, adding a strong acid ion exchange resin to the acid leaching solution, stirring and reacting for 2-4 hours, wherein the addition amount of the strong acid ion exchange resin is 0.5-5 g / g based on the dry weight of the raw material municipal sludge; Step 6, after the reaction is completed, separate the strongly acidic ion exchange resin to obtain filtrate 1, add a sulfide or hydroxide precipitant to the filtrate 1, stir and react for 0.5-1h, remove the precipitate, and obtain filtrate 2, wherein the amount of the precipitant added is 1-5 mol / L; Step 7, mixing the filtrate 2 in step 6 with the hydrolyzate in step 2 to ensure that the molar ratio of nitrogen and phosphorus in the solution is (1-1.5):1, then adding magnesium salt to adjust the molar ratio of magnesium and phosphorus to (1-1.3):1, adjusting the pH of the solution to 9.0-9.5, stirring the reaction sufficiently, letting it stand and precipitate, filtering, and recovering the solid phase product, thereby completing the recovery of nitrogen and phosphorus.

2. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1, characterized in that: The strongly acidic ion exchange resin in step 5 is a strongly acidic styrene-based cation exchange resin.

3. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1 or 2, characterized in that: In step 3, the acid used for acid leaching is an inorganic acid or an organic acid.

4. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 3, characterized in that: The inorganic acid is selected from nitric acid or hydrochloric acid, and the organic acid is selected from citric acid or oxalic acid.

5. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1 or 2, characterized in that: The method for separating the strongly acidic ion exchange resin in step 6 is: screening using a 50-mesh sieve.

6. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1 or 2, characterized in that: The method for solid-liquid separation in step 2 and / or the method for solid-liquid separation in step 4 and / or the method for filtering and removing precipitates in step 6 is centrifugation or vacuum filtration.

7. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1 or 2, characterized in that: In step 7, the filtrate 2 and the hydrolyzate are mixed in a volume ratio of 1:(0.8-2).

8. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1 or 2, characterized in that: The magnesium salt added in step 7 is magnesium chloride.

9. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1 or 2, characterized in that: The precipitating agent in step 6 is sodium sulfide or sodium hydroxide.

10. The method for recovering nitrogen and phosphorus from municipal sludge according to claim 1 or 2, characterized in that: The solid phase product obtained after acid leaching is modified and used as a carbon-based adsorption material or dried and used as a biomass fuel.

Images