A method for recovering nutrients from biogas slurry by activating low-grade phosphate ore through pyrolysis of alkali residue

Abstract

The present invention relates to a method for recovering biogas nutrient by activating low-grade phosphate ore by pyrolysis of alkali residue, wherein the method utilizes pyrolysis of alkali residue to activate low-grade phosphate ore, thereby increasing the content of citric acid-soluble phosphorus and water-soluble phosphorus in the low-grade phosphate ore; then, the activated phosphate ore 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, nitrogen and phosphorus nutrients in biogas slurry are synergistically recovered by MAP reaction and physical adsorption of activated phosphate ore. The method of the present invention utilizes alkali pyrolysis to activate low-grade phosphate ore to recover biogas nutrient process, and the recovery effect is obvious, 99% of phosphorus and 96% of nitrogen in biogas slurry are recovered, and struvite-rich slow-release nitrogen-phosphate fertilizer can be produced. The method solves the problem of disposal of biogas slurry, low-grade phosphate ore and industrial sodium / potash waste residue in agricultural pig farms, provides an innovative process for resource utilization of agricultural and industrial wastes, and belongs to the technical field of resource utilization of agricultural and industrial wastes.

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 low-grade phosphate ore through pyrolysis of alkali residue. Background Art

[0002] With the rapid development of animal husbandry in my country, the amount of livestock and poultry manure is also huge. How to deal with this large amount of livestock and poultry manure is also a problem faced today. At present, most large-scale farms generally use biogas projects to treat manure, but a large amount of biogas slurry and biogas residue will also be produced after treatment. There are two traditional ways to treat biogas slurry and biogas residue. One is to return the biogas slurry and biogas residue to the field, and the other is to carry out deep treatment to meet the discharge standards. If the return to the field method is adopted, since the amount of biogas slurry and biogas residue produced by a large-scale farm is relatively large, there is not enough farmland around the farm area for consumption, and there is a high cost problem in transporting it to other places. The deep treatment mode will lead to the waste of nitrogen and phosphorus resources in biogas slurry and biogas residue. Therefore, how to achieve efficient recovery of nitrogen and phosphorus nutrients in biogas slurry and biogas residue in farms and reduce the amount of waste in farms is conducive to reducing treatment costs and improving the resource utilization of farm manure, which is of great significance for environmental protection and biological resource utilization.

[0003] In terms of nitrogen and phosphorus recovery technology, the current mainstream recovery technologies mainly include biological treatment, membrane separation, chemical precipitation, electrochemistry, physical adsorption, etc. Among them, magnesium ammonium phosphate chemical precipitation (MAP) is considered to be the most promising nitrogen and phosphorus recovery process. It can not only recover nitrogen and phosphorus in water, but also produce a valuable slow-release fertilizer, struvite crystals. Previous studies have shown that the MAP reaction can be used to recover nitrogen and phosphorus in farm biogas slurry, and the recovery effect is obvious. However, in the actual process, due to the high concentration of ammonia nitrogen in farm biogas slurry, the molar ratio of N:P:Mg in the biogas slurry is much greater than 1:1:1, so magnesium and phosphorus elements in the reaction system are limiting factors. In the past, in order to improve the recovery rate of nitrogen and phosphorus, researchers usually need to add phosphate and magnesium salts as phosphorus and magnesium sources for the MAP reaction to improve the recovery of nitrogen and phosphorus. However, these added phosphate and magnesium salts are often pure chemicals with high prices, which leads to high nitrogen and phosphorus recovery costs, limiting the large-scale application of MAP reaction in aquaculture wastewater.

[0004] At present, the most commonly used technical means to activate low-grade phosphate ore to prepare high-quality phosphate fertilizers are: (1) Flotation separation technology: Utilizing the difference in affinity between phosphate and foam, phosphate is floated by adding reagents and flotation equipment, thereby achieving flotation separation. This technology is one of the more mature technologies for the development of low-grade phosphate ore, but it has a greater risk of phosphate pollution and there is a partial waste of effective phosphate resources; (2) Wet activation technology: Phosphate is purified through wet metallurgical processes such as acid leaching, salt melting and alumina. This method can overcome the problem of solid waste treatment of low-grade phosphate ore, but a large amount of reagents need to be added during the production process, which is costly and also poses an environmental risk of leaching of harmful heavy metals. The existing preparation method of calcium-magnesium-rich citric acid-soluble phosphate fertilizer uses low-grade phosphate ore that is difficult to float and wet phosphoric acid sludge acid as the main raw materials for activation reaction to prepare a mixed slurry with high citric acid-soluble phosphorus content, and then uses a conversion agent to prepare a high citric acid-soluble calcium-magnesium-rich phosphate fertilizer product, which contains 24-37% effective phosphorus, 10-30% calcium oxide, 14-30% magnesium oxide, and 50-100% citric acid-soluble phosphorus. This fertilizer is characterized by multiple elements, long fertilizer effect, and strong specificity. However, after the wet phosphoric acid sludge acid mixing reaction, some harmful heavy metals will also be activated and leached, posing a risk of heavy metal pollution; at the same time, the product pH is low, and applying it to acidic soil will aggravate soil acidification. (3) Mechanochemical activation technology: Through different modes of mechanical force, such as grinding, friction, impact, shearing, etc., mechanical energy is introduced and accumulated, so that the physical, chemical properties and structure of phosphate rock powder change, its reaction activity is improved, and chemical reactions are stimulated and accelerated. This technology has the advantages of simple process, clean production, no flotation or chemicals, etc., but it has the problems of low activation efficiency and long activation time. Therefore, the activation of low-grade phosphate ore requires a technical method that can not only effectively increase the effective phosphorus content in low-grade phosphate ore, but also has simple process, low cost, and low environmental pollution risk such as heavy metals.

[0005] Industrial alkali waste refers to solid waste formed in the process of producing, processing and using alkaline substances (such as alkali residue). 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 residue 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 waste residues will bring considerable economic and environmental pressure to enterprises. For example, indigo alkali residue 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 residue is produced. The previous alkali residue 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 residue has always been a concern of environmental protection practitioners. The current treatment methods of alkali residue mainly include sea discharge, damming and storage treatment, and wet oxidation treatment. Among them, the sea discharge method and the accumulation method not only waste alkali slag 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 slag will cause certain corrosion to the equipment. Therefore, how to economically and environmentally friendly, and how to make resourceful use of alkali slag is also a top priority.

[0006] In summary, the existing methods for recovering nutrient elements from biogas slurry and biogas residue have high costs and low efficiency in recovering nutrients from biogas slurry, and cannot be applied on a large scale; the treatment methods for low-grade phosphate ore have high costs and environmental pollution risks; the treatment methods for alkaline residue have high costs and environmental pollution risks. Summary of the invention

[0007] 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 low-grade phosphate ore through pyrolysis of alkaline slag, so as to solve the problems that the existing methods have high costs for recovering nutrient elements from biogas slurry and biogas slag, low efficiency for recovering nutrients from biogas slurry, and cannot be applied on a large scale; the treatment methods for low-grade phosphate ore have high costs and environmental pollution risks; the treatment methods for alkaline slag have high costs and environmental pollution risks.

[0008] In order to achieve the above object, the present invention adopts the following technical solution:

[0009] A method for recovering nutrients from biogas slurry by activating low-grade phosphate ore through pyrolysis of alkaline residues, comprising the following steps:

[0010] S1. The low-grade phosphate ore or phosphate tailings are pre-treated by crushing, screening and homogenization to obtain a first powdery material;

[0011] The alkali residue containing alkali metal elements is pre-treated by crushing, screening and homogenizing to obtain a second powdery material;

[0012] 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;

[0013] 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;

[0014] S3. The activated clinker obtained in step S2 is cooled, cooled to room temperature, and then crushed and ground to obtain an alkaline phosphate fertilizer having a phosphorus-citrate solubility of 45 to 50%;

[0015] S4. Add alkaline phosphate fertilizer to oxalic acid and leach it for 1 to 2 hours to dissolve the phosphate; then pour the filtrate and phosphate fertilizer obtained from the dissolution into a struvite fluidized bed crystallization reactor, add magnesium sulfate and livestock and poultry biogas, 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.

[0016] As a preferred embodiment, in step S1, the fineness of the first powdery material and the second powdery material are both in the range of 80-100 meshes; the mixed powdery material is obtained by mixing the first powdery material and the second powdery material in a mass ratio of 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.

[0017] As a preferred method, in step S1, the method of drying the dry mixed powder material is: using hot air drying 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 dried to be 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.

[0018] As a preferred embodiment, in step S2, the gas flow rate of nitrogen is 7 to 7.5 L / h. The gas flow rate range can ensure the efficiency of the pyrolysis process. Exceeding the gas flow rate may result in heat energy loss, and less than the gas flow rate may result in low pyrolysis efficiency. Experiments have shown that when the gas flow rate of nitrogen is 7 to 7.5 L / h, the pyrolysis efficiency is the best.

[0019] As a preferred embodiment, in step S2, the pyrolysis temperature is 500-700°C, the heating rate of the pyrolysis temperature is 10-20°C / min, and the pyrolysis time is 1-2h. The range of temperature, heating and time can ensure the efficiency of the pyrolysis process, and the pyrolysis time exceeding 1-2h may result in no increase in pyrolysis efficiency and increased costs, and less than 1-2h may result in low pyrolysis efficiency.

[0020] As a preferred embodiment, in step S3, the activated clinker is cooled by a cooling drum, and 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 MAP; 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 subsequent recovery of MAP.

[0021] 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.

[0022] As a preferred embodiment, in step S4, the mass ratio of magnesium sulfate to activated clinker is 1 to 1.3: 1. Experiments show that the pH of the corresponding mixed solution can be accurately adjusted by the mass ratio of magnesium sulfate to activated clinker set above, which is beneficial to the efficiency of the subsequent MAP reaction.

[0023] 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.

[0024] As a preferred embodiment, the method further comprises the following steps: S5. The supernatant obtained in step S4 is pumped into a biochemical pool as the inlet water for biochemical treatment, and the phosphate fertilizer mixture precipitate containing struvite is pumped into a MAP precipitation drying pool, and a biogas residue carbon-based slow-release nitrogen-phosphate fertilizer rich in struvite is obtained through biological drying. The coupling of treatment technologies among the three wastes of biogas slurry, biogas residue, low-grade phosphate rock and alkaline residue is utilized to reduce environmental pollution and simplify the process flow, so as to realize the use of industrial waste as a phosphate rock activator, accelerate the efficient conversion of industrial waste resources, improve the effectiveness of phosphate rock phosphorus, reduce costs, realize the harmlessness, resource utilization and high value of biogas slurry and biogas residue, accelerate the efficient conversion of internal resources, improve the recovery efficiency of nitrogen and phosphorus, and achieve the new environmental protection concept of "treating waste with waste", providing an economically feasible innovative method for the actual application of magnesium ammonium phosphate chemical precipitation method in the recovery of nutrients from pig farm waste.

[0025] Principle of the invention:

[0026] The method of the present invention is divided into three parts: crushing, screening, mixing, drying pretreatment of low-grade phosphate ore / phosphate tailings and sodium / potassium alkali slag, high-temperature pyrolysis activation and subsequent biogas slurry MAP reaction. The strong affinity of sodium / potassium alkali metals in industrial waste alkali slag and phosphorus in phosphate ore at high temperature is utilized to replace metal ions such as iron, aluminum, and calcium in phosphate salts to form sodium / potassium phosphate with stronger water solubility. By cooling and crushing, an alkaline phosphate fertilizer product with superior quality is made. Finally, the alkaline phosphate fertilizer product is used as a MAP phosphorus source, and the nitrogen and phosphorus nutrients in biogas slurry and biogas residue are recovered through the chemical precipitation reaction of magnesium ammonium phosphate and the physical adsorption of phosphate ore. By coupling the treatment technologies of three kinds of waste, namely, biogas slurry and biogas residue, low-grade phosphate rock and alkaline residue, we can reduce environmental pollution and simplify the process flow, realize the use of industrial waste as phosphate rock activator, accelerate the efficient transformation of industrial waste resources, improve the effectiveness of phosphorus, reduce costs, realize the harmlessness, resource utilization and high value of biogas slurry and biogas residue, 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", providing an economically feasible innovative method for the actual application of magnesium ammonium phosphate chemical precipitation method in nutrient recovery from pig farm waste.

[0027] In the present invention, the reaction of the alkali metal ions in the sodium / potassium alkali slag in the pyrolysis activated phosphate ore / phosphate ore tailings is mainly completed through the reaction equation of formula (1-18), converting the insoluble phosphates such as iron, aluminum, calcium, etc. into more water-soluble sodium / potassium phosphates, thereby increasing the water-soluble phosphorus content of the low-grade phosphate ore and forming a high-quality alkaline phosphate fertilizer;

[0028] 2FePO4(s)+6NaOH(s)=2Na3PO4(s)+Fe2O3(s)+3H2O(g), formula (1);

[0029] 2AlPO4(s) + 6NaOH(s) = 2Na3PO4(s) + Al2O3(s) + 3H2O(g), Equation (2);

[0030] Ca3(PO4)2(s) + 6NaOH(s) = 2Na3PO4(s) + 3CaO(s) + 3H2O(g), Equation (3);

[0031] 2FePO4(s) + 6KOH(s) = 2K3PO4(s) + Fe2O3(s) + 3H2O(g), Equation (4);

[0032] 2AlPO4(s) + 6KOH(s) = 2K3PO4(s) + Al2O3(s) + 3H2O(g), Equation (5);

[0033] Ca3(PO4)2(s) + 6KOH(s) = 2K3PO4(s) + 3CaO(s) + 3H2O(g), Equation (6);

[0034] 2FePO4(s) + 3Na2CO3(s) = 2Na3PO4(s) + Fe2O3(s) + 3CO2(g), Equation (7);

[0035] 2AlPO4(s) + 3Na2CO3(s) = 2Na3PO4(s) + Al2O3(s) + 3CO2(g), Equation (8);

[0036] Ca3(PO4)2(s) + 3Na2CO3(s) = 2Na3PO4(s) + 3CaCO3(s), Equation (9);

[0037] 2FePO4(s) + 3K2CO3(s) = 2K3PO4(s) + Fe2O3(s) + 3CO2(g), Equation (10);

[0038] 2AlPO4(s) + 3K2CO3(s) = 2K3PO4(s) + Al2O3(s) + 3CO2(g), Equation (11);

[0039] Ca3(PO4)2(s) + 3K2CO3(s) = 2K3PO4(s) + 3CaCO3(s), Equation (12);

[0040] 2FePO4(s) + 6NaHCO3(s) = 2Na3PO4(s) + Fe2O3(s) + 3H2O(g) + 6CO2(g), Equation (13);

[0041] 2AlPO4(s)+6NaHCO3(s)=2Na3PO4(s)+Al2O3(s)+3H2O(g)+6CO2(g), formula (14);

[0042] Ca3(PO4)2(s)+6NaHCO3(s)=2Na3PO4(s)+3CaCO3(s)+3H2O(g)+3CO2(g), formula (15);

[0043] 2FePO4(s)+6KHCO3(s)=2K3PO4(s)+Fe2O3(s)+3H2O(g)+6CO2(g), formula (16);

[0044] 2AlPO4(s)+6KHCO3(s)=2K3PO4(s)+Al2O3(s)+3H2O(g)+6CO2(g), formula (17);

[0045] Ca3(PO4)2(s)+6KHCO3(s)=2K3PO4(s)+3CaCO3(s)+3H2O(g)+3CO2(g), formula (18).

[0046] 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).

[0047] Mg 2+ +NH4 + +PO4 3- +6H2O→MgNH4 PO4·6H20↓, formula (19);

[0048] Mg 2+ +NH4 + +HPO4 2- +6H2O→MgNH4PO4·6H20↓+H + , formula (20);

[0049] Mg 2+ +NH4 + +H2PO4 - +6H2O→MgNH4PO4·6H20↓+2H + , formula (21).

[0050] In general, the present invention has the following advantages:

[0051] 1. The method of the present invention comprises the following steps: mixing low-grade phosphate ore / phosphate tailings with sodium / potassium alkali slag, and under high-temperature pyrolysis conditions, sodium / potassium alkali metal ions react with metal ions such as iron, aluminum, and calcium to convert insoluble phosphates such as iron, aluminum, and calcium into more water-soluble sodium / potassium phosphates. The phosphorus solubility in the low-grade phosphate ore / phosphate tailings is 45-50%. Then, the activated phosphate ore is acid-leached pre-treated with oxalic acid to further increase the water-soluble phosphorus content as a phosphorus source for MAP, while slowing down the inhibitory effect of calcium ions on the MAP reaction. Finally, through the MAP reaction and the physical adsorption of the activated phosphate ore, nitrogen and phosphorus nutrients in the biogas slurry are synergistically recovered to form a superior alkaline phosphate fertilizer. By coupling the treatment technologies of three types of waste, namely, biogas slurry, biogas residue, low-grade phosphate rock and alkaline residue, 99% of the phosphorus and 96% of the nitrogen in the biogas slurry can be recovered, which not only reduces environmental pollution but also simplifies the process flow, realizes the use of industrial waste as phosphate rock activator, accelerates the efficient transformation of industrial waste resources, improves the effectiveness of phosphorus in phosphate rock, reduces costs, realizes the harmlessness, resource utilization and high value of biogas slurry and biogas residue, accelerates the efficient transformation of internal resources, improves the recovery efficiency of nitrogen and phosphorus, and achieves the new environmental protection concept of "treating waste with waste", providing an economically feasible and innovative method for the actual application of magnesium ammonium phosphate chemical precipitation method in nutrient recovery from pig farm waste.

[0052] 2. The present invention utilizes high-temperature pyrolysis of alkali slag as a phosphate ore activation treatment, which can not only promote the increase of effective P2O5 content in low-grade phosphate ore / phosphate ore tailings, but also the high-temperature pyrolysis can pyrolyze organic matter in the alkali slag to form biochar and passivate heavy metals in the alkali slag; different from traditional flotation, wet acid leaching and other activation treatments, the present invention does not require flotation and chemicals, and does not dissolve other harmful substances and heavy metal ions, avoiding the risk of environmental pollution.

[0053] 3. The present invention provides a large amount of phosphorus source for MAP by activating phosphate rock powder through alkaline pyrolysis. At the same time, since the phosphate rock powder activated by alkaline is alkaline, the alkali solution required for MAP reaction is reduced. The process utilizes MAP reaction and physical adsorption of phosphate rock powder to synergistically recover nitrogen and phosphorus nutrients in biogas slurry, thereby improving nitrogen and phosphorus recovery efficiency and reducing costs compared with existing methods. At the same time, low-grade phosphate ore / phosphate tailings and sodium / potassium alkali slag can also be treated simultaneously.

[0054] 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+ The effect on the formation of struvite in MAP improves the recovery rate of nutrient elements in slurry and residue. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 The present invention is a process flow chart of the method of activating phosphate ore to recover nitrogen and phosphorus from biogas slurry.

[0056] Among them: 1 is feed (low-grade phosphate rock / phosphate tailings), 2 is feed (sodium / potassium alkali slag), 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 (high-quality alkaline 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 nitrogen and phosphorus slow-release fertilizer.

[0057] Figure 2 This is a graph showing the influence of pyrolysis temperature, pyrolysis time, and amount of alkali slag added on the mass ratio of total P2O5, effective P2O5, and water-soluble P2O5 in phosphate rock powder in Example 1.

[0058] Figure 3 This is a graph showing the effects of pyrolysis temperature, pyrolysis time, and alkali slag addition on the phosphorus solubility rate and the ratio of water-soluble phosphorus to total phosphorus in phosphate rock powder in Example 1.

[0059] Figure 4 This is a graph showing the effect of the amount of phosphate rock added after alkali activation on the recovery of total phosphorus and ammonia nitrogen in Example 1.

[0060] 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.

[0061] Figure 6 This is a graph showing the effect of the mass ratio of oxalic acid to phosphate rock powder added after alkali activation on the recovery of total phosphorus and ammonia nitrogen in Example 1.

[0062] Figure 7 This is a graph showing the effect of reaction pH on the recovery of total phosphorus and ammonia nitrogen in Example 1.

[0063] 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

[0064] The present invention will be further described in detail below in conjunction with specific implementation methods.

[0065] Example 1

[0066] 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:

[0067] 1. The feed (low-grade phosphate ore / phosphate tailings) 1 and the feed (sodium / potassium alkali slag) 2 are crushed by a crusher 3 and a crusher 5 respectively; after crushing, they are screened by a screener 4 and a screener 6 respectively, and screened to a powder material with a fineness of 80 to 100 mesh (i.e., a first powder material formed by low-grade phosphate ore or phosphate tailings and a second powder material formed by alkali slag containing alkali metal elements); and the screened feed (low-grade phosphate ore / phosphate tailings) 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 to obtain a mixed powder material.

[0068] 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.

[0069] 3. The mixed powdery 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 500-700°C, and the pyrolysis time is controlled to be 1-2h to form clinker after pyrolysis and activation of phosphate rock.

[0070] 4. The pyrolysis activated clinker is cooled through 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 phosphate fertilizer) 12 with a phosphorus solubility rate of 45 to 50%.

[0071] 5. The alkaline phosphate fertilizer 12 obtained in step 4 is transported to oxalic acid pre-leaching treatment 13, wherein the concentration of oxalic acid is 0.5-0.6 mol / L, the mass ratio of oxalic acid to alkaline phosphate fertilizer is 7-10:1, and the leaching time is 1-2 h.

[0072] 6. The filtrate and residue after the acid leaching treatment are added to the struvite fluidized bed crystallization reactor 19, and the biogas slurry in the storage tank 18 is pumped into the biogas slurry and magnesium sulfate is added to carry out MAP reaction. The reaction pH is 9-9.5, the mass ratio of magnesium sulfate to alkaline phosphate fertilizer is 1-1.3:1, and the MAP reaction time is 0.5-1.0h.

[0073] The basic physical and chemical properties of the pig farm biogas slurry, feed (low-grade phosphate rock / phosphate tailings) 1, feed (sodium / potassium alkali slag) 2 and discharge (high-quality alkaline phosphate fertilizer) are shown in Tables 1-4. By operating in the above manner, under the optimal mixing ratio of low-grade phosphate rock / phosphate tailings and sodium / potassium alkali slag and the optimal process conditions of pyrolysis activation, the effective P2O5 content in the low-grade phosphate rock / phosphate tailings can be increased by 2 to 4 times, and the water-soluble P2O5 content can be increased by 150 to 200 times, and the activation effect is very obvious; at the same time, combined with the attached Figure 2 As shown in the figure, after low-grade phosphate rock is activated by alkaline pyrolysis, its citric acid solubility can be increased from 8-9% to 45-50%, an increase of 5-6 times; the proportion of water-soluble phosphorus P2O5 in the total P2O5 increases from 0.04% to 13%-19%, which improves the effectiveness of phosphorus in phosphate rock. Figure 3-Figure 8 The experimental results of the exploration of MAP reaction conditions found that: the addition amount of activated phosphate rock powder was 20-30 g / L, the concentration of oxalic acid was 0.5-0.6 mol / L, the mass ratio of oxalic acid to activated clinker was (7-10):1, the mass ratio of magnesium sulfate to activated clinker was (1-1.3):1, the pH of the MAP reaction was 9.0-9.5, and the MAP reaction time was 0.5-1.0 h. Under the optimal leaching conditions of oxalic acid and the optimal process conditions of MAP, 99% of the phosphorus and 96% of the nitrogen in the biogas slurry can be recovered, and the nutrient recovery effect is very obvious, and finally a slow-release nitrogen-phosphorus fertilizer rich in struvite is formed.

[0074] Table 1. Biogas slurry quality in pig farms

[0075]

[0076] Note: The values ​​shown in the table are mean ± standard error, n = 3.

[0077] Table 2 Basic physical and chemical properties of low-grade phosphate ore

[0078]

[0079] Table 3 Basic physical and chemical parameters of sodium / potassium alkali slag

[0080]

[0081] Table 4 Basic physical and chemical parameters of the output (high quality alkaline phosphate fertilizer)

[0082]

[0083] 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 low-grade phosphate ore through pyrolysis of alkaline residue, characterized in that: The following steps are involved: S1. The low-grade phosphate ore or phosphate tailings are pre-treated by crushing, screening and homogenization to obtain a first powdery material; The alkali residue containing 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, cooled to room temperature, and then crushed and ground to obtain an alkaline phosphate fertilizer having a phosphorus-citrate solubility of 45 to 50%; S4. Add alkaline phosphate fertilizer to oxalic acid and leach it for 1 to 2 hours to dissolve the phosphate; then pour the filtrate and phosphate fertilizer obtained from the dissolution into a struvite fluidized bed crystallization reactor, add magnesium sulfate and livestock and poultry biogas, 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 low-grade phosphate ore by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S1, the fineness of the first powdery material and the second powdery material are both in the range of 80-100 meshes; the mixed powdery material is obtained by mixing the first powdery material and the second powdery material in a mass ratio of 1:0.3-0.

5.

3. The method for recovering nutrients from biogas slurry by activating low-grade phosphate ore by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S1, the method for drying the dry mixed powder material is: using hot air drying 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 dried to be 10%-15%.

4. The method for recovering nutrients from biogas slurry by activating low-grade phosphate ore by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S2, the flow rate of nitrogen is 7 to 7.5 L / h.

5. The method for recovering nutrients from biogas slurry by activating low-grade phosphate ore by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S2, the pyrolysis temperature is 500-700°C, the heating rate of the pyrolysis temperature is 10-20°C / min, and the pyrolysis time is 1-2h.

6. The method for recovering nutrients from biogas slurry by activating low-grade phosphate ore by pyrolysis of alkaline residue according to claim 1, characterized in that: In step S3, the activated clinker is cooled by a cooling drum, and the cooling time of the activated clinker is 1 to 1.5 hours.

7. The method for recovering nutrients from biogas slurry by activating low-grade phosphate ore 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 low-grade phosphate ore 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 to 1.3:

1.

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

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