A method for recovering nutrients from biogas slurry by activating anaerobic biogas slag through pyrolysis of alkaline slag

Abstract

The present invention relates to a method for recovering nutrients from biogas slurry by activating anaerobic digester residue by pyrolysis of alkali residue. The method utilizes sodium / potassium alkali residue to activate anaerobic digester residue by high temperature pyrolysis, thereby increasing the content of citric acid-soluble phosphorus and water-soluble phosphorus in the anaerobic digester residue; then, the activated anaerobic digester residue is acid-leached pre-treated with oxalic acid, thereby further increasing the content of water-soluble phosphorus as a phosphorus source for MAP reaction, while slowing down the inhibitory effect of calcium ions on MAP reaction; finally, the nitrogen and phosphorus nutrients in the anaerobic digester residue are synergistically recovered by the physical adsorption of the MAP reaction and the activated anaerobic digester residue. The method has an obvious process recovery effect for recovering nutrients from anaerobic digester residue, wherein 99% of phosphorus and 57% of nitrogen in the anaerobic digester residue are recovered, and a slow-release nitrogen-phosphorus fertilizer rich in struvite can be produced. The method solves the two waste disposal problems of anaerobic digester residue in agricultural pig farms and industrial sodium / potassium alkali waste residue, provides an innovative process for the resource utilization of agricultural and industrial wastes, and belongs to the field of agricultural and industrial waste resource utilization technology.

Description

Technical Field

[0001] The invention relates to the technical field of agricultural and industrial waste resource utilization, and in particular to a method for recovering nutrients from biogas slurry by activating anaerobic biogas slag through pyrolysis of alkaline slag. Background Art

[0002] Large-scale pig farm biogas projects also produce a large amount of solid waste (commonly known as sludge). For example, a large-scale pig farm with a pig inventory of 4,000 heads has accumulated sludge of up to 350 tons in three years. If a large amount of sludge waste is not properly disposed of, it will have a bad impact on the living environment of surrounding residents. During the anaerobic fermentation process, most of the phosphorus will be precipitated in the sludge. The phosphorus content in pig manure sludge can reach 2-5% (w / w); it is a phosphorus-rich waste; but the proportion of water-soluble phosphorus and citrate-soluble phosphorus in untreated anaerobic sludge is relatively large, and most of it is insoluble phosphorus. Traditional phosphorus activation technology also has some problems, such as the inability to effectively increase the solubility of phosphorus, the inability to remove harmful heavy metals or other impurities, and the physical or chemical methods that may require high energy consumption, which increases production costs and affects the overall environmental benefits.

[0003] Industrial alkali waste refers to solid waste formed in the process of producing, processing and using alkaline substances (such as alkali slag). The chemical composition of this type of waste is usually extremely corrosive and toxic, and has potential hazards to the environment and human health. Sodium-potassium alkali slag is an industrial waste, including sodium hydroxide, sodium carbonate, potassium hydroxide, etc., which is usually used to produce building materials such as cement. The removal of these wastes will bring considerable economic and environmental pressure to enterprises, and the standardized treatment process needs to strictly follow relevant laws, regulations and standards. For example, indigo alkali slag refers to the yellow alkaline sludge discharged during the alkali concentration stage in the indigo production process, which contains a large amount of KOH, NaOH, Na2CO3, K2CO3 and a large amount of polycyclic organic matter produced in the dye production process. For every ton of indigo product produced, nearly one ton of alkali slag is produced. The previous alkali slag treatment mode not only wastes resources, but also causes environmental pollution, and directly affects the quality and cost of indigo products. How to treat and dispose of indigo alkali slag has always been a concern for environmental protection practitioners. At present, the main methods for treating alkali residues are sea discharge, damming and stockpiling, and wet oxidation. Among them, sea discharge and stockpiling not only waste alkali residue resources, but also cause great harm to the surrounding environment. The wet oxidation method has problems such as high equipment and operating costs, and long-term treatment of alkali residues will cause certain corrosion to the equipment. Therefore, how to make economical and environmentally friendly use of alkali residues as resources is also a top priority.

[0004] In summary, the existing anaerobic biogas residue activation phosphorus technology has the problem of high cost and affects the overall environmental benefits, and the industrial alkaline waste residue treatment method has the problem of high cost and inability to effectively utilize alkaline residue resources. Summary of the invention

[0005] In view of the technical problems existing in the prior art, the purpose of the present invention is to provide a method for recovering nutrients from biogas slurry by activating anaerobic biogas slag through pyrolysis of alkaline slag, so as to solve the problems that the existing phosphorus activation technology of anaerobic biogas slag is high in cost and affects the overall environmental benefits, and the industrial alkaline waste residue treatment method is high in cost and cannot effectively utilize the alkaline residue resources.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A method for recovering nutrients from biogas slurry by activating anaerobic biogas slag through pyrolysis of alkaline slag, comprising the following steps:

[0008] S1. Pre-treating the anaerobic digestate by crushing, screening and homogenizing to obtain a first powdery material;

[0009] The alkali residue containing sodium / potassium alkali metal elements is pre-treated by crushing, screening and homogenizing to obtain a second powdery material;

[0010] Mixing the first powdery material with the second powdery material to obtain a mixed powdery material, and drying the mixed powdery material to obtain a dry mixed powdery material;

[0011] S2. The dry mixed powder material obtained in step S1 is added to a pyrolysis furnace for pyrolysis; nitrogen is then introduced into the pyrolysis furnace to keep the pyrolysis furnace in an oxygen-free state, so that the dry mixed powder material undergoes an activation reaction to obtain an activated clinker;

[0012] S3. The activated clinker obtained in step S2 is cooled by a cooling drum, and then the activated clinker is cooled to room temperature and then crushed and ground to obtain an alkaline biogas residue carbon-based phosphate fertilizer having a phosphorus solubility rate of 30 to 40% and a water-soluble phosphorus ratio of 2 to 3% of the total phosphorus;

[0013] S4. Add the alkaline biogas residue charcoal-based phosphate fertilizer to oxalic acid to leach for 1 to 2 hours to dissolve the phosphorus; then pour the obtained filtrate and the alkaline biogas residue charcoal-based phosphate fertilizer after leaching into a struvite fluidized bed crystallization reactor, add magnesium sulfate and pig farm biogas slurry, adjust the pH to 9 to 9.5 with 1 mol / L hydrochloric acid / sodium hydroxide, and carry out MAP reaction to obtain a phosphate fertilizer mixture precipitate containing struvite and a supernatant.

[0014] As a preferred embodiment, in step S1, the crushing and screening fineness of the first powdery material and the second powdery material are both 80-100 meshes; the mixing ratio of the first powdery material and the second powdery material by mass ratio is 1:0.3-0.5. Through comparative experiments, when the mass of the second powdery material is lower than 0.3 relative to the first powdery material, the phosphorus activation effect is poor; when the mass of the second powdery material is higher than 0.5 relative to the first powdery material, the alkali residue ratio is too high, resulting in a low total phosphorus content in the mixture, and when the mass of the second powdery material is in the range of 0.3-0.5 relative to the first powdery material, the phosphorus activation effect is best.

[0015] As a preferred embodiment, in step S1, the mixed powder material is dried with hot air at a drying condition of 100-200°C; the drying time is 30-45 minutes; the moisture content of the mixed powder material is 10%-15%. The drying process is to facilitate the packaging, transportation and sales of the product, and to make full use of the residual heat of the pyrolysis process, thereby realizing full utilization of resources; the moisture content of 10-15% can fully increase the phosphorus content of the mixture.

[0016] As a preferred embodiment, in step S2, the gas flow rate of nitrogen is 7-7.5 L / h; the heating rate of the pyrolysis furnace is 10-20°C / min. The range of gas flow rate can ensure the efficiency of the pyrolysis process. Exceeding the range may cause heat energy loss, and less than the range may cause low pyrolysis efficiency. Through experiments, it can be found that when the gas flow rate of nitrogen is 7-7.5 L / h, the pyrolysis efficiency is the best.

[0017] As a preferred embodiment, in step S2, the pyrolysis temperature is 500-600°C; the pyrolysis time is 0.5-1h. The range of temperature, heating and time can ensure the efficiency of the pyrolysis process, and the pyrolysis time exceeding 0.5-1h may result in no increase in pyrolysis efficiency and increased costs, while less than 0.5-1h may result in low pyrolysis efficiency.

[0018] As a preferred embodiment, in step S3, the cooling time of the activated clinker is 1 to 1.5 hours. A cooling time within a certain range can prevent overheated products from entering the MAP reaction stage and reduce the recovery efficiency of the MAP reaction; a too long cooling time will increase the processing cost. Experiments show that a cooling time of 1 to 1.5 hours has a promoting effect on the recovery of the subsequent MAP reaction.

[0019] As a preferred embodiment, in step S4, the concentration of oxalic acid is 0.5-0.6 mol / L; the mass ratio of oxalic acid to activated clinker is 7-10:1. Experiments show that the above-set oxalic acid and mass ratio can achieve the best phosphorus dissolving effect of alkaline phosphate fertilizer. If the oxalic acid concentration is lower or higher than the above conditions, the phosphorus dissolving effect will decrease. If the mass ratio of oxalic acid to activated clinker is higher or lower than the above ratio, the phosphorus dissolving effect will decrease.

[0020] As a preferred embodiment, in step S4, the mass ratio of magnesium sulfate to activated clinker is 1.3 to 1.4: 1. Experiments show that the pH of the corresponding mixed solution can be accurately adjusted by the mass ratio between magnesium sulfate and activated clinker set as above, which is beneficial to the efficiency of the subsequent MAP reaction.

[0021] As a preferred embodiment, in step S4, the MAP reaction time is 0.5 to 1.0 h. Experiments show that the above-set MAP reaction time can allow the corresponding mixed solution to fully precipitate struvite phosphate fertilizer mixture precipitation and supernatant.

[0022] As a preferred embodiment, the method further comprises the following steps: the supernatant after the MAP reaction obtained in step S4 is pumped into a biochemical tank as the inlet water for biochemical treatment, and the phosphate fertilizer mixture containing struvite is precipitated and pumped into a MAP reaction drying tank, and a sludge charcoal-based slow-release nitrogen-phosphate fertilizer rich in struvite is obtained through biological drying.

[0023] Principle of the invention:

[0024] The method of the present invention is divided into three parts: crushing, screening, mixing, drying pretreatment of anaerobic biogas residue and alkali residue containing sodium / potassium alkali metal elements, high-temperature pyrolysis activation, and subsequent MAP reaction of biogas slurry. The strong affinity of sodium / potassium alkali metals in industrial waste alkali residue and phosphorus in biogas residue at high temperature is utilized to replace metal ions such as iron, aluminum, and calcium in biogas residue phosphate salts to form sodium / potassium phosphate with stronger water solubility. By cooling and crushing, an alkaline biogas residue charcoal-based phosphate fertilizer product with a high content of citric acid-soluble phosphorus is prepared. Finally, the alkaline biogas residue charcoal-based phosphate fertilizer product is used as a phosphorus source for MAP reaction, and the nitrogen and phosphorus nutrients in the anaerobic biogas residue are recovered through the synergistic reaction of magnesium ammonium phosphate chemical precipitation method and physical adsorption of biogas residue. It mainly achieves the reduction of environmental pollution and simplification of process flow by coupling the treatment technologies of anaerobic digestate and alkaline digestate containing sodium / potassium alkali metal elements, and uses industrial waste as anaerobic digestate activator to accelerate the efficient transformation of industrial waste resources, improve the effectiveness of anaerobic digestate phosphorus, reduce costs, achieve the harmlessness, resource utilization and high value of anaerobic digestate, accelerate the efficient transformation of internal resources, improve the recovery efficiency of nitrogen and phosphorus, and achieve the new environmental protection concept of "treating waste with waste". It provides an economically feasible innovative method for the actual application of magnesium ammonium phosphate chemical precipitation method in nutrient recovery of pig farm waste, and also solves the problem of treating alkaline digestate containing sodium / potassium alkali metal elements.

[0025] In the present invention, the reaction of the alkali metal ions in the sodium / potassium alkali slag in the pyrolysis activation of the phosphate salts of the anaerobic digestate is mainly completed through the reaction equation of formula (1-18), converting the insoluble phosphates such as iron, aluminum, and calcium into more water-soluble sodium / potassium phosphates, thereby increasing the water-soluble phosphorus content of the anaerobic digestate and forming a high-quality alkaline digestate carbon-based phosphate fertilizer;

[0026] 2FePO4(s) + 6NaOH(s) = 2Na3PO4(s) + Fe2O3(s) + 3H2O(g), Equation (1);

[0027] 2AlPO4(s) + 6NaOH(s) = 2Na3PO4(s) + Al2O3(s) + 3H2O(g), Equation (2);

[0028] Ca3(PO4)2(s) + 6NaOH(s) = 2Na3PO4(s) + 3CaO(s) + 3H2O(g), Equation (3);

[0029] 2FePO4(s) + 6KOH(s) = 2K3PO4(s) + Fe2O3(s) + 3H2O(g), Equation (4);

[0030] 2AlPO4(s) + 6KOH(s) = 2K3PO4(s) + Al2O3(s) + 3H2O(g), Equation (5);

[0031] Ca3(PO4)2(s) + 6KOH(s) = 2K3PO4(s) + 3CaO(s) + 3H2O(g), Equation (6);

[0032] 2FePO4(s) + 3Na2CO3(s) = 2Na3PO4(s) + Fe2O3(s) + 3CO2(g), Equation (7);

[0033] 2AlPO4(s) + 3Na2CO3(s) = 2Na3PO4(s) + Al2O3(s) + 3CO2(g), Equation (8);

[0034] Ca3(PO4)2(s) + 3Na2CO3(s) = 2Na3PO4(s) + 3CaCO3(s), Equation (9);

[0035] 2FePO4(s) + 3K2CO3(s) = 2K3PO4(s) + Fe2O3(s) + 3CO2(g), Equation (10);

[0036] 2AlPO4(s) + 3K2CO3(s) = 2K3PO4(s) + Al2O3(s) + 3CO2(g), Equation (11);

[0037] Ca3(PO4)2(s) + 3K2CO3(s) = 2K3PO4(s) + 3CaCO3(s), Equation (12);

[0038] 2FePO4(s)+6NaHCO3(s)=2Na3PO4(s)+Fe2O3(s)+3H2O(g)+6CO2(g), formula (13);

[0039] 2AlPO4(s)+6NaHCO3(s)=2Na3PO4(s)+Al2O3(s)+3H2O(g)+6CO2(g), formula (14);

[0040] Ca3(PO4)2(s)+6NaHCO3(s)=2Na3PO4(s)+3CaCO3(s)+3H2O(g)+3CO2(g), formula (15);

[0041] 2FePO4(s)+6KHCO3(s)=2K3PO4(s)+Fe2O3(s)+3H2O(g)+6CO2(g), formula (16);

[0042] 2AlPO4(s)+6KHCO3(s)=2K3PO4(s)+Al2O3(s)+3H2O(g)+6CO2(g), formula (17);

[0043] Ca3(PO4)2(s)+6KHCO3(s)=2K3PO4(s)+3CaCO3(s)+3H2O(g)+3CO2(g), formula (18).

[0044] The phosphorus source and magnesium source in the present invention are the Mg in the reaction (Formula 19-21) that provides struvite. 2+ and PO4 3- content, thereby increasing NH4 + recovery effect, generating more struvite (magnesium ammonium phosphate).

[0045] Mg 2+ +NH4 + +PO4 3- +6H2O→MgNH4 PO4·6H20↓, formula (19);

[0046] Mg 2+ +NH4 + +HPO4 2- +6H2O→MgNH4PO4·6H20↓+H + , formula (20);

[0047] Mg 2+ +NH4 + +H2PO4 - +6H2O→MgNH4PO4·6H20↓+2H + , formula (21).

[0048] In general, the present invention has the following advantages:

[0049] 1. The method of the present invention is the first to innovatively propose to use industrial waste sodium / potassium alkali slag as an activator for anaerobic biogas residue, and utilize the strong affinity of sodium / potassium alkali metals in industrial waste alkali slag and phosphorus in anaerobic biogas residue at high temperature to replace metal ions such as iron, aluminum, and calcium in anaerobic biogas residue phosphate salts to form sodium / potassium phosphate with stronger water solubility. By cooling and crushing, an alkaline biogas residue charcoal-based phosphate fertilizer product with a high content of citric acid-soluble phosphorus is prepared. Finally, the alkaline biogas residue charcoal-based phosphate fertilizer product is used as a phosphorus source for the MAP reaction, and the nitrogen and phosphorus nutrients in the biogas slurry and anaerobic biogas residue are recovered through the coordinated reaction of magnesium ammonium phosphate chemical precipitation method and physical adsorption of anaerobic biogas residue. The method of the present invention mainly reduces environmental pollution and simplifies the process flow by coupling the treatment technologies of anaerobic biogas residue and alkaline residue containing sodium / potassium alkali metal elements, and uses industrial waste as a biogas residue activator to accelerate the efficient conversion of industrial waste resources, improve the effectiveness of biogas residue phosphorus, reduce costs, achieve harmlessness, resource utilization and high value of biogas slurry and biogas residue, accelerate the efficient conversion of internal resources, improve nitrogen and phosphorus recovery efficiency, and achieve the new environmental protection concept of "treating waste with waste". It provides an economically feasible innovative method for the actual application of magnesium ammonium phosphate chemical precipitation method in nutrient recovery of pig farm waste, and also solves the problem of difficult treatment of alkaline residue containing sodium / potassium alkali metal elements.

[0050] 2. The present invention utilizes high-temperature pyrolysis of alkali residue as an activation treatment for anaerobic digestate residue, which can not only promote the increase of effective P2O5 content in anaerobic digestate residue, but also the high-temperature pyrolysis can pyrolyze organic matter in anaerobic digestate residue and alkali residue to form biochar and passivate heavy metals in alkali residue residue. Different from traditional activation treatments such as ball milling and wet acid leaching, the present invention does not require chemicals and will not dissolve other harmful substances and heavy metal ions, thereby reducing costs. It can treat both alkali residue and anaerobic digestate residue.

[0051] 3. The present invention provides a large amount of phosphorus source for MAP by activating anaerobic biogas residue powder through alkaline pyrolysis. At the same time, since the alkaline-activated anaerobic biogas residue powder is alkaline, the alkali solution required for MAP reaction is reduced. The process utilizes MAP reaction and physical adsorption of biogas residue powder to synergistically recover nitrogen and phosphorus nutrients in biogas slurry, thereby improving the nitrogen and phosphorus recovery efficiency and being able to recover 99% of phosphorus and 57% of nitrogen in biogas slurry.

[0052] 4. The present invention uses oxalic acid as a leaching agent, which can not only promote the dissolution of phosphorus, but also inhibit the Ca 2+ Effects on struvite formation in MAP. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 The present invention is a process flow chart of the method of activating biogas residue to recover nitrogen and phosphorus from biogas liquid.

[0054] Among them: 1 is feed (sodium / potassium alkali residue), 2 is feed (sludge), 3 is crusher, 4 is screening machine, 5 is crusher, 6 is screening machine, 7 is mixer, 8 is drying drum, 9 is continuous pyrolysis furnace, 10 is cooling drum, 11 is crusher, 12 is discharge (alkaline sludge carbon-based phosphate fertilizer), 13 is oxalic acid acid leaching pretreatment, 14 is black film biogas digester in breeding farm, 15 is solid-liquid separation treatment, 16 is biogas slurry, 17 is biogas slurry transportation pipeline, 18 is liquid storage tank, 19 is struvite fluidized bed crystallization reactor, and 20 is sludge carbon-based slow-release nitrogen and phosphorus fertilizer.

[0055] Figure 2 This is a graph showing the influence of pyrolysis temperature, pyrolysis time, and alkali residue addition amount on the mass ratio of total P2O5, effective P2O5, and water-soluble P2O5 in biogas residue powder in Example 1.

[0056] Figure 3 This is a graph showing the effects of pyrolysis temperature, pyrolysis time, and alkali residue addition amount on the phosphorus solubility rate and the ratio of water-soluble phosphorus to total phosphorus in biogas residue powder in Example 1.

[0057] Figure 4 This is a graph showing the effect of the amount of sludge powder added after alkali activation on the recovery of total phosphorus and ammonia nitrogen in Example 1.

[0058] Figure 5 This is a graph showing the effect of magnesium sulfate addition on the recovery of total phosphorus and ammonia nitrogen in Example 1.

[0059] Figure 6 This is a graph showing the influence of the mass ratio of oxalic acid and alkali-activated biogas residue powder added in Example 1 on the recovery of total phosphorus and ammonia nitrogen.

[0060] Figure 7 This is a graph showing the effect of reaction pH on the recovery of total phosphorus and ammonia nitrogen in Example 1.

[0061] Figure 8 This is a graph showing the effect of reaction time on the recovery of total phosphorus and ammonia nitrogen in Example 1. DETAILED DESCRIPTION

[0062] The present invention will be further described in detail below in conjunction with specific implementation methods.

[0063] Example 1

[0064] The method for recovering nitrogen and phosphorus from pig farm biogas slurry under actual production conditions is as follows: Figure 1 As shown, the specific steps are as follows:

[0065] 1. The feed (anaerobic digestate) 1 and the feed (sodium / potassium alkali slag) 2 are crushed by a crusher 3 and a crusher 5 respectively; after crushing, they are sieved by a sieving machine 4 and a sieving machine 6 respectively, and sieved into powdery materials with a fineness of 80 to 100 mesh; then the sieved feed (anaerobic digestate) 1 and the feed (sodium / potassium alkali slag) 2 are fully mixed by a mixer 7 at a mass ratio of 1:0.3 to 0.5.

[0066] 2. The mixed powdery material is then dried by a drying drum 8, the hot air drying temperature is controlled at 100-200°C, the drying time is 30-45 minutes, and the dry mixed powdery material is dried to a moisture content of 10%-15%; the heat source of the drying drum 8 comes from the hot exhaust gas generated by the continuous pyrolysis furnace 9, and the resources are reused.

[0067] 3. The mixed powdered material dried by the drying drum 8 is conveyed to the continuous pyrolysis furnace 9, and the air flow rate of N2 is controlled to be 7-7.5 L / h, and the heating rate is controlled to be 10-20°C / min; the optimal pyrolysis temperature is controlled to be 500-600°C, and the pyrolysis time is controlled to be 0.5-1h to form anaerobic biogas residue pyrolysis activated clinker.

[0068] 4. The pyrolysis activated clinker is cooled by a cooling drum 10, and the cooling time is controlled within 1 to 1.5 hours; the cooled clinker is then transported to a crusher 11 for crushing to obtain an output (high-quality alkaline biogas residue carbon-based phosphate fertilizer) 12 with a phosphorus solubility rate of 30 to 40% and a water-soluble phosphorus ratio of 2 to 3% of the total phosphorus.

[0069] 5. The alkaline biogas residue charcoal-based phosphate fertilizer 12 obtained in step 4 is transported to oxalic acid acid leaching pretreatment 13, wherein the concentration of oxalic acid is 0.5-0.6 mol / L, the mass ratio of oxalic acid to alkaline biogas residue charcoal-based phosphate fertilizer is 7-10:1, and the leaching time is 1-2 h.

[0070] 6. The filtrate and residue after the acid leaching treatment are added to the struvite fluidized bed crystallization reactor 19, pumped into the biogas slurry in the storage tank 18 and added with magnesium sulfate for MAP reaction, the reaction pH = 9-9.5, the mass ratio of magnesium sulfate to alkaline biogas residue carbon-based phosphate fertilizer is 1.3-1.4:1, and the MAP reaction time is 0.5-1.0h.

[0071] The basic physical and chemical properties of pig farm biogas slurry, feed (biogas residue) 1, feed (sodium / potassium alkali residue) 2 and output (high-quality alkaline biogas residue carbon-based phosphate fertilizer) are shown in Table 1-4. The above operation is carried out. Figure 2-Figure 3As shown in the figure, under the optimal mixing ratio of biogas residue and sodium / potassium alkali residue and the optimal process conditions of pyrolysis activation, the citric acid solubility rate can be increased from 20% to 30-40%, an increase of 1-2 times; the proportion of water-soluble phosphorus P2O5 in the total P2O5 increased from 0.06% to 2.3-2.9%, an increase of 46-58 times; and the effectiveness of biogas residue phosphorus was improved. Figure 4-Figure 8 The experimental results of the MAP process conditions exploration found that: the addition amount of activated sludge charcoal was 20-30 g / L, the concentration of oxalic acid was 0.5-0.6 mol / L, the mass ratio of oxalic acid and activated clinker was (7-10):1, the mass ratio of magnesium sulfate and activated clinker was (1.3-1.4):1, the pH of MAP reaction was 9.0-9.5, and the MAP reaction time was 0.5-1.0 h. Under the optimal oxalic acid leaching conditions and the optimal MAP process conditions, 99% of the phosphorus and 57% of the nitrogen in the biogas slurry can be recovered, and the nutrient recovery effect is very obvious. Finally, a slow-release nitrogen-phosphorus fertilizer rich in struvite is formed.

[0072] Table 1. Biogas slurry quality in pig farms

[0073]

[0074] Note: The values ​​shown in the table are mean ± standard error, n = 3.

[0075] Table 2 Basic physical and chemical parameters of anaerobic digestate

[0076]

[0077] Note: The values ​​shown in the table are mean ± standard error, n = 3.

[0078] Table 3 Basic physical and chemical parameters of sodium / potassium alkali slag

[0079]

[0080] Table 4 Basic physical and chemical parameters of the output (high quality alkaline biogas residue phosphate fertilizer)

[0081]

[0082] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for recovering nutrients from biogas slurry by activating anaerobic biogas slag by pyrolysis of alkaline slag, characterized in that: The following steps are involved: S1. Pre-treating the anaerobic digestate by crushing, screening and homogenizing to obtain a first powdery material; The alkali residue containing sodium / potassium alkali metal elements is pre-treated by crushing, screening and homogenizing to obtain a second powdery material; Mixing the first powdery material with the second powdery material to obtain a mixed powdery material, and drying the mixed powdery material to obtain a dry mixed powdery material; S2. The dry mixed powder material obtained in step S1 is added to a pyrolysis furnace for pyrolysis; nitrogen is then introduced into the pyrolysis furnace to keep the pyrolysis furnace in an oxygen-free state, so that the dry mixed powder material undergoes an activation reaction to obtain an activated clinker; S3. The activated clinker obtained in step S2 is cooled by a cooling drum, and then the activated clinker is cooled to room temperature and then crushed and ground to obtain an alkaline biogas residue carbon-based phosphate fertilizer having a phosphorus solubility rate of 30 to 40% and a water-soluble phosphorus ratio of 2 to 3% of the total phosphorus; S4. Add the alkaline biogas residue charcoal-based phosphate fertilizer to oxalic acid to leach for 1 to 2 hours to dissolve the phosphorus; then pour the obtained filtrate and the alkaline biogas residue charcoal-based phosphate fertilizer after leaching into a struvite fluidized bed crystallization reactor, add magnesium sulfate and pig farm biogas slurry, adjust the pH to 9 to 9.5 with 1 mol / L hydrochloric acid / sodium hydroxide, and carry out MAP reaction to obtain a phosphate fertilizer mixture precipitate containing struvite and a supernatant.

2. A method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S1, the first powdery material and the second powdery material are crushed and screened to a fineness of 80-100 mesh; the first powdery material and the second powdery material are mixed in a mass ratio of 1:0.3-0.

5.

3. The method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S1, the mixed powder material is dried with hot air at a drying condition of 100-200° C.; the drying time is 30-45 minutes; and the moisture content of the mixed powder material is 10%-15%.

4. The method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S2, the flow rate of nitrogen is 7-7.5 L / h; the heating rate of the pyrolysis furnace is 10-20° C. / min.

5. The method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S2, the pyrolysis temperature is 500-600°C; the pyrolysis time is 0.5-1h.

6. The method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S3, the cooling time of the activated clinker is 1 to 1.5 hours.

7. The method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S4, the concentration of oxalic acid is 0.5-0.6 mol / L; the mass ratio of oxalic acid to activated clinker is 7-10:

1.

8. The method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S4, the mass ratio of magnesium sulfate to activated clinker is 1.3-1.4:

1.

9. The method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S4, the MAP reaction time is 0.5 to 1.0 h.

10. A method for recovering nutrients from biogas slurry by activating anaerobic biogas residue by pyrolysis of alkaline residue according to any one of claims 1 to 9, characterized in that: The following steps are also included: S5. The supernatant after the MAP reaction obtained in step S4 is pumped into the biochemical pool as the inlet water for biochemical treatment, and the phosphate fertilizer mixture containing struvite is precipitated and pumped into the MAP reaction drying pool to obtain a struvite-rich biogas charcoal-based slow-release nitrogen-phosphate fertilizer through biological drying.

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