Method for preparing ammonium sulfate from biogas slurry wastewater

Abstract

The invention discloses a method for preparing ammonium sulfate by utilizing biogas slurry wastewater, and relates to the technical field of ammonium sulfate manufacturing, coagulant, calcium oxide and flocculant are added into the biogas slurry wastewater for flocculent precipitation treatment, then solid-liquid separation is performed to obtain first-stage wastewater, polymeric ferric sulfate is selected as the coagulant, and PAM is selected as the flocculant; adding an alkaline substance into the first-stage wastewater to carry out pH regulation treatment, and then carrying out solid-liquid separation to obtain second-stage wastewater; and then carrying out ammonia nitrogen removal treatment on the second-section wastewater to obtain the ammonium sulfate solution and third-section wastewater. And adding an ethanol solution into the ammonium sulfate solution at a speed of 5-20 mL / min, continuously stirring until crystals are completely separated out, then carrying out solid-liquid separation, and washing and drying the obtained solid substance by using the ethanol solution to obtain the ammonium sulfate. The purity of the prepared ammonium sulfate crystal reaches 99.8%.

Description

Technical Field

[0001] The invention relates to the field of wastewater treatment, and in particular to a method for preparing ammonium sulfate by utilizing biogas slurry wastewater. Background Art

[0002] As an important inorganic chemical product, ammonium sulfate is widely used in agriculture, industry and other fields. In agriculture, ammonium sulfate is a nitrogen fertilizer with stable properties and high fertilizer efficiency. As a high-quality nitrogen fertilizer, ammonium sulfate plays an important role in the agricultural field. Its ammonium ions can be adsorbed by soil colloids and then slowly released for crops to absorb and utilize. The sulfur element contained in it is also a medium-amount nutrient element necessary for the growth of some crops. It is of great significance to improve the quality of agricultural products and enhance the stress resistance of crops. It is especially suitable for sulfur-loving crops such as cruciferous vegetables, onions and garlic. In areas where the soil is sulfur-deficient, the use of ammonium sulfate has an outstanding effect on supplementing and balancing soil fertility. In industry, ammonium sulfate is used as a raw material in the preparation of various compounds, such as the preparation of strong oxidants such as ammonium persulfate for batteries, textile printing and dyeing and other industries; in water treatment, it can assist the flocculation and precipitation process, adjust the pH of water quality, and help remove impurities and heavy metal ions; in textile dyeing, it acts as a dyeing promoter to enhance the adhesion of dyes to fabrics and improve dyeing effects and fastness. . It also has specific uses in the medical field, such as reaction raw materials or auxiliary agents in the synthesis of certain drugs.

[0003] Some people have studied and developed a natural gypsum method for producing ammonium sulfate, that is, using natural gypsum as a raw material to react with synthetic ammonia and carbon dioxide to obtain ammonium sulfate.

[0004] The traditional methods for preparing ammonium sulfate are mainly the following:

[0005] One method is to use sulfuric acid and synthetic ammonia as raw materials through a neutralization reaction. This method is costly and has poor market competitiveness. The production of synthetic ammonia is an energy-intensive process, which usually requires a large amount of energy such as natural gas or coal to provide hydrogen, and the fixation of nitrogen also needs to be carried out under high temperature and high pressure conditions. For example, in the Haber-Bosch process for synthesizing ammonia, an iron catalyst is required to promote the reaction of nitrogen and hydrogen to produce ammonia at about 400-500°C and 10-30MPa. These harsh production conditions lead to high production costs for synthetic ammonia.

[0006] The second is the gypsum method for ammonium sulfate, the principle of which is to use natural gypsum or industrial by-product gypsum to react with ammonium carbonate solution to generate calcium carbonate precipitate and ammonium sulfate solution, and then obtain ammonium sulfate products through a series of subsequent treatments such as filtration, evaporation and crystallization. The advantage of this method is that it can realize the resource utilization of industrial by-product gypsum, reduce the pollution of gypsum waste to the environment, turn waste into treasure, and conform to the concept of sustainable development. However, the process is relatively complicated, the reaction conditions are difficult to control, and it involves the coordinated conduct of multi-step reactions and the separation and purification of multiple substances. For example, the filtration and washing of calcium carbonate precipitates require a large amount of water resources and energy, and if the impurity content in the gypsum raw material is high, it will affect the quality of the ammonium sulfate product, and a more cumbersome raw material pretreatment process is required before the reaction. This method has the disadvantages of slow reaction, long process, and high energy consumption, and is rarely used. At present, only a few manufacturers in countries such as India and Indonesia, which are extremely short of sulfur resources, use this method.

[0007] The second is the coke oven gas ammonia recovery method. During the coking process, coke oven gas contains a certain amount of ammonia. Through absorption, desorption and other process steps, the ammonia in the coke oven gas is recovered and converted into ammonium sulfate. This method can effectively utilize the ammonia resources in the coking industrial waste gas, reduce production costs, and reduce the ammonia pollution caused by the direct emission of coke oven gas to the environment. However, the composition of coke oven gas is complex. In addition to ammonia, it also contains a variety of impurity gases, such as hydrogen sulfide and tar. These impurities will interfere with the recovery process of ammonia. It is necessary to adopt a complex purification process for pretreatment, which increases the complexity of the process and equipment investment. The output and ammonia content of coke oven gas are affected by many factors such as coking process and raw coal quality, which makes the output of ammonium sulfate unstable and the continuity of production difficult to guarantee. In addition, there are many problems in this production process, such as the reaction heat is difficult to control, which is easy to cause temperature fluctuations to affect the reaction process; the equipment faces a greater risk of corrosion, which reduces the service life and reliability of the equipment; the product quality is difficult to guarantee and fluctuates greatly; the energy consumption is high, which increases the production cost; the process flow is complex, involving multiple processes and links, and it is difficult to manage and operate.

[0008] It is worth noting that biogas wastewater contains ammonia nitrogen. If it can be effectively utilized, it will be a very promising direction for the preparation of ammonium sulfate. Biogas wastewater is usually a by-product of the biogas production process, and the ammonia nitrogen component in it is a potential raw material for the preparation of ammonium sulfate. However, the actual utilization process faces many difficulties. The composition of biogas wastewater is extremely complex. In addition to ammonia nitrogen, it also contains a large amount of impurities such as organic matter, microorganisms, suspended matter, and other inorganic salts. The presence of these impurities seriously interferes with the process of ammonia nitrogen reacting with sulfuric acid to form ammonium sulfate, which can easily lead to side reactions and affect the purity and quality of the product. For example, some organic substances may be carbonized or compete with ammonia nitrogen during the reaction, causing the ammonium sulfate product to be mixed with impurities that are difficult to separate.

[0009] Despite these problems, given its huge potential value, if the above difficulties can be overcome through technological innovation and process optimization, and the ammonia nitrogen in biogas wastewater can be successfully used for the preparation of ammonium sulfate, it will not only expand the raw material source of ammonium sulfate and reduce production costs, but also realize the resource utilization of biogas wastewater and reduce environmental pollution, which will have a profound positive impact in the fields of agriculture, industry and environmental protection. Summary of the invention

[0010] In view of this, the object of the present invention is to provide a method for preparing ammonium sulfate using biogas wastewater.

[0011] The objective of the present invention is achieved through the following technical solutions:

[0012] <First aspect>

[0013] A method for preparing ammonium sulfate using biogas wastewater comprises the following steps:

[0014] Adding an ethanol solution to the ammonium sulfate solution at a rate of 5 to 20 mL / min, stirring continuously until crystals are completely precipitated, then performing solid-liquid separation, and washing and drying the obtained solid matter with the ethanol solution to obtain the ammonium sulfate;

[0015] The ammonium sulfate solution is obtained by absorbing ammonia-containing waste gas generated during the process of removing ammonia nitrogen from biogas wastewater through dilute sulfuric acid.

[0016] As an embodiment, the components of the ammonium sulfate solution are as follows, by mass: 40 to 50 parts of ammonium sulfate and 50 to 60 parts of water.

[0017] As an implementation scheme, the biogas wastewater is treated by air stripping to remove ammonia nitrogen.

[0018] As an implementation scheme, before the biogas wastewater is treated for ammonia nitrogen removal, it is first subjected to flocculation and sedimentation, and the pH is adjusted to 11 to 12, and then subjected to ammonia nitrogen removal treatment.

[0019] As an implementation scheme, the flocculation and precipitation are as follows: a coagulant, calcium oxide and a flocculant are sequentially added to the biogas wastewater to carry out flocculation and precipitation treatment, and then a first solid-liquid separation is carried out to obtain a first section of wastewater; an alkaline substance is added to the first section of wastewater to carry out pH adjustment treatment, and then a second solid-liquid separation is carried out to obtain a second section of wastewater, and the second section of wastewater is used for ammonia nitrogen removal treatment.

[0020] As an embodiment, the concentration of the ethanol solution is not less than 75 vol.%.

[0021] In some embodiments, the concentration of the ethanol solution is 95%.

[0022] As an embodiment, the ethanol-water solution after the solid-liquid separation is separated by a pervaporation membrane to obtain ethanol solution and water that can be recycled.

[0023] As an embodiment, the ethanol solution is added at a rate of 5 to 10 mL / min.

[0024] In some embodiments, the ethanol solution is added at a rate of 5 mL / min.

[0025] In some embodiments, the mass ratio of the added ethanol solution to the ammonium sulfate solution is 1.5:1.

[0026] As an embodiment, when the coagulant is polyferric sulfate and the flocculant is PAM, the precipitated mud obtained during the first solid-liquid separation is iron-calcium mud.

[0027] As an embodiment, when calcium oxide is selected as the alkaline substance, the precipitated mud obtained by the second solid-liquid separation is white calcium mud.

[0028] As an embodiment, the biogas wastewater also includes alkalinity (mainly including CO3 2- 、HCO3 - ).

[0029] As an embodiment, the amount of polyferric sulfate added is 25 to 40 L per cubic meter of wastewater.

[0030] In some embodiments, the amount of polyferric sulfate added is 33 L per cubic meter of wastewater.

[0031] As an embodiment, the amount of calcium oxide added is 5-10 kg per cubic meter of wastewater.

[0032] In some embodiments, the amount of calcium oxide added is 6 kg per cubic meter of wastewater.

[0033] As an implementation scheme, in the treatment of the biogas slurry wastewater, the amount of PAM added is 10 to 15 L of PAM aqueous solution per cubic meter of wastewater, and the concentration of the PAM aqueous solution is 2 to 3 wt.‰.

[0034] In some embodiments, the amount of PAM added is 13 L of PAM aqueous solution per cubic meter of wastewater, and the concentration of the PAM aqueous solution is 2.5 wt.‰.

[0035] As an embodiment, the method of obtaining calcium-iron mud by solid-liquid separation includes plate and frame pressing.

[0036] In some embodiments, the plate and frame pressing time is 3 to 4 hours.

[0037] As an implementation plan, the second stage wastewater is subjected to ammonia nitrogen removal treatment to obtain the third stage wastewater, the pH value of the third stage wastewater is adjusted, and then biochemical treatment is carried out or the biochemical treatment process is connected with NF nanofiltration and other membrane treatment processes to make the wastewater meet the discharge standard requirements.

[0038] As an embodiment, the amount of the alkaline substance added is 5-10 kg per cubic meter of wastewater.

[0039] In some embodiments, the amount of alkaline substance added is 6 kg per cubic meter of wastewater.

[0040] As an embodiment, the pH of the second stage wastewater is 11-12.

[0041] As an embodiment, carbon dioxide is introduced into the third stage wastewater to adjust the pH to 8-9.

[0042] The biochemical treatment process is a multi-stage AO+MBR membrane treatment process, or a multi-stage AO+MBR membrane treatment process connected with a NF nanofiltration treatment process. In the biochemical treatment process, according to the requirements of the biogas slurry treatment standard, the NF nanofiltration treatment process can be connected to meet the requirements of the biogas slurry discharge standard.

[0043] As an implementation scheme, the biogas slurry wastewater includes COD, SS (suspended solids), TP (total phosphorus), and ammonia nitrogen. The content of each component is: COD 8000-20000 mg / L, ammonia nitrogen 1500-4000 mg / L (total nitrogen ~ 5000 mg / L), TP (total phosphorus) ~ 150 mg / L.

[0044] As an implementation plan, in the first section of wastewater, the treatment efficiency of COD is 70-90%, the treatment efficiency of SS is 90-96%, the treatment efficiency of TP is 85-95%, the treatment efficiency of ammonia nitrogen is 15%-25%, and the treatment efficiency of total nitrogen is 15%-25%.

[0045] As an implementation plan, in the second section of wastewater, the treatment efficiency of COD is 5-15%, the treatment efficiency of SS is 80%-90%, the treatment efficiency of TP is 40-60%, the treatment efficiency of ammonia nitrogen is 2-6%, and the treatment efficiency of total nitrogen is 2-5%.

[0046] As an embodiment, in the third stage wastewater, the treatment efficiency of ammonia nitrogen is 85-95%, and the treatment efficiency of total nitrogen is 80-95%.

[0047] As an embodiment, the calcium iron mud is used to prepare a hydroxy-containing iron oxide desulfurization agent.

[0048] As an embodiment, the iron-calcium mud is used to prepare a carbon / gamma iron oxide chimera desulfurizer.

[0049] As an embodiment, the white lime mud is used to prepare calcium sulfate.

[0050] <Second Aspect>

[0051] The invention provides ammonium sulfate, which is prepared by adopting the method for preparing ammonium sulfate by utilizing biogas slurry wastewater, and the purity of the ammonium sulfate exceeds 99%.

[0052] <Third Aspect>

[0053] The present invention provides a method for preparing a desulfurizing agent containing iron oxide containing hydroxyl, which is prepared by using the iron calcium mud produced in the method for preparing ammonium sulfate using biogas slurry wastewater, that is, a method for preparing ammonium sulfate using biogas slurry wastewater and co-producing a desulfurizing agent containing iron oxide containing hydroxyl. The method comprises the following steps:

[0054] The dried calcium iron mud powder is aerobically calcined at 150-200° C. for 2-3 hours, and then cooled to room temperature to obtain calcium iron mud aerobically calcined;

[0055] The aerobic calcined calcium iron mud is placed in a strong alkali aqueous solution for washing, then rinsed with deionized water until neutral, and dried to obtain the calcium iron mud after alkali washing, which is the hydroxyl-containing iron oxide desulfurizer;

[0056] The iron-calcium mud is obtained by utilizing biogas wastewater.

[0057] As an embodiment, the iron-calcium mud contains, by mass, 14 to 24 parts of calcium sulfate, 4 to 7 parts of calcium carbonate, 26 to 43 parts of iron oxyhydroxide, 1.1 to 1.9 parts of silicon oxide, 0.5 to 0.9 parts of magnesium carbonate and 27 to 53 parts of organic matter.

[0058] As an embodiment, the dried calcium iron mud is first ground to obtain dry calcium iron mud with a mesh size below 200, and then subjected to the aerobic calcination.

[0059] As an embodiment, the ferric calcium mud after alkali washing is ground to obtain a hydroxyl-containing iron oxide desulfurizer with a mesh size of less than 200.

[0060] As an embodiment, the mass ratio of the aerobic calcined calcium iron mud and the aqueous solution of the strong alkali is 3:(6-8), and the concentration of the aqueous solution of the strong alkali is 8 mol / L to saturation.

[0061] In some embodiments, the mass ratio of the aerobic calcined calcium iron mud to the aqueous solution of strong alkali is 3:7, and the concentration of the aqueous solution of strong alkali is 10 mol / L to 15 mol / L.

[0062] As an embodiment, the strong base is NaOH or KOH.

[0063] In some embodiments, the strong base is NaOH.

[0064] In some embodiments, the temperature of the aerobic calcination is 200° C., and the calcination time is 2 hours.

[0065] <Fourth Aspect>

[0066] The present invention provides a hydroxyl-containing iron oxide desulfurizer, which is prepared by the preparation method of the hydroxyl-containing iron oxide desulfurizer, and the components are as follows by weight:

[0067] Calcium sulfate: 28-35 parts;

[0068] Iron oxyhydroxide: 50-62 parts;

[0069] Impurities: 11 to 14 parts.

[0070] As an embodiment, the impurities include SiO2, CaCO3, and MgCO3.

[0071] As an embodiment, the components of the oxyhydroxide-containing iron desulfurizer are: calcium sulfate: 31.35 parts; oxyhydroxide iron: 56.08 parts; impurities: 12.57 parts.

[0072] <Fifth Aspect>

[0073] The present invention also provides a method for preparing a carbon / γ-iron oxide chimera desulfurizer, which is prepared by using the iron-calcium mud produced in the above-mentioned method of preparing ammonium sulfate using biogas slurry wastewater, that is, a method for preparing ammonium sulfate using biogas slurry wastewater and co-producing a carbon / γ-iron oxide chimera desulfurizer.

[0074] A method for preparing a carbon / gamma iron oxide chimera desulfurizer using biogas wastewater comprises the following steps:

[0075] The dried calcium iron mud is subjected to oxygen-free calcination at 550-650° C. for 2.5-3.5 hours, and then subjected to aerobic calcination at 200-500° C. for 1.5-2.5 hours. After cooling to room temperature, the carbon / γ-iron oxide chimera desulfurizer is obtained.

[0076] As an embodiment, the carbon / γ iron oxide chimera desulfurization agent is ground and pulverized to obtain a carbon / γ iron oxide chimera desulfurization agent with a size not exceeding 200 meshes.

[0077] As an embodiment, during the oxygen-isolated calcination treatment, the dried calcium iron mud is pressed to a density of 3.0 to 3.5 g / cm3, and the crucible cover is bonded to the crucible and to the powder in the crucible.

[0078] In some embodiments, the temperature of the oxygen-isolated calcination is 600° C., and the calcination time is 3 hours.

[0079] In some embodiments, the temperature of the aerobic calcination is 400° C., and the calcination time is 2 hours.

[0080] <Sixth Aspect>

[0081] The present invention also provides a method for preparing a carbon / γ-iron oxide chimera desulfurizer using biogas slurry wastewater, which is a method for preparing a carbon / γ-iron oxide chimera desulfurizer using the iron calcium mud produced in the preparation of a hydroxyl-containing iron oxide desulfurizer using biogas slurry wastewater, that is, a method for preparing ammonium sulfate using biogas slurry wastewater and co-producing a carbon / γ-iron oxide chimera desulfurizer. The steps are as follows:

[0082] The dried calcium iron mud is subjected to a hydrothermal reaction at 180-220° C. for 2.5-3.5 hours, and then subjected to an aerobic calcination treatment at 200-500° C. for 1.5-2.5 hours. After cooling to room temperature, the carbon / γ-iron oxide chimera desulfurizer is obtained.

[0083] As an embodiment, the solid-liquid mass ratio during the hydrothermal reaction is 1:3-5.

[0084] In some embodiments, the solid-liquid mass ratio of the hydrothermal reaction is 1:5.

[0085] As an embodiment, during the hydrothermal reaction and the aerobic calcination, the heating rate is 5 to 10°C / min.

[0086] In some embodiments, the heating rate is 10° C. / min.

[0087] In some embodiments, the temperature of the hydrothermal reaction is 200° C. and the reaction time is 3 h.

[0088] In some embodiments, the temperature of the aerobic calcination is 280° C. and the reaction time is 2 h.

[0089] As an embodiment, the product after oxygen calcination and cooling is ground to obtain the carbon / γ-iron oxide chimera with a size of less than 200 meshes.

[0090] <Seventh Aspect>

[0091] The present invention provides a carbon / γ-iron oxide chimera desulfurizer, which is prepared by the preparation method of the carbon / γ-iron oxide chimera desulfurizer, and the components are as follows by weight:

[0092] Carbon: 13-17 parts;

[0093] Gamma iron oxide: 40-50 parts;

[0094] Calcium sulfate: 25-32 parts;

[0095] Calcium carbonate: 7-9 parts;

[0096] Impurities: 2.5 to 3.5 parts.

[0097] As an embodiment, the impurities include SiO2 and MgCO3.

[0098] As an embodiment, the carbon / γ iron oxide chimera desulfurizer comprises the following components by mass: carbon: 15-16.5 parts; γ iron oxide: 45-46 parts; calcium sulfate: 27-28.5 parts; calcium carbonate: 8-9 parts; and impurities: 3-3.2 parts.

[0099] In some embodiments, a carbon / γ iron oxide chimera desulfurizer comprises the following components by mass: carbon: 15.18 parts; γ iron oxide: 45.33 parts; calcium sulfate: 28.19 parts; calcium carbonate: 8.16 parts; and impurities: 3.14 parts.

[0100] In some embodiments, a carbon / γ iron oxide chimera desulfurizer comprises the following components by mass: carbon: 16.28 parts; γ iron oxide: 45.33 parts; calcium sulfate: 27.19 parts; calcium carbonate: 8.11 parts; impurities: 3.09 parts, including SiO2 and MgCO3.

[0101] <Eighth Aspect>

[0102] The invention provides a method for preparing calcium sulfate, which is to prepare the white lime mud produced in the process of preparing ammonium sulfate by utilizing biogas slurry wastewater, that is, a method for preparing ammonium sulfate by utilizing biogas slurry wastewater and co-producing calcium sulfate.

[0103] The white lime mud comprises, by mass, 52 to 64 parts of calcium sulfate, 12 to 16 parts of calcium carbonate, 3 to 5 parts of ferric hydroxide, 0.3 to 0.5 parts of silicon oxide, 2.3 to 3 parts of magnesium carbonate and 18 to 24 parts of organic matter.

[0104] A method for preparing calcium sulfate powder comprises the following steps:

[0105] The dried white lime mud powder and deionized water are prepared into a suspension having a solid content of 15-25%;

[0106] Adding dilute sulfuric acid to the suspension at 60-100° C. to fully react, and filtering, washing, and drying the reaction product to obtain an intermediate sample;

[0107] The intermediate sample is calcined at 300-400° C. for 1.5-2.5 hours, and after cooling, the calcium sulfate having a purity of 99.1 wt.% and a whiteness of more than 95% is obtained.

[0108] In some embodiments, the dried lime mud is ground and pulverized to obtain lime mud powder with a size of less than 200 meshes, and then the suspension is prepared.

[0109] In some embodiments, the solid content of the suspension is 20%.

[0110] In some embodiments, the reaction temperature of adding dilute sulfuric acid to the suspension is 80°C.

[0111] As an embodiment, the concentration of the dilute sulfuric acid is 4 to 6 mol / L.

[0112] In some embodiments, the concentration of dilute sulfuric acid is 5 mol / L.

[0113] As an embodiment, the dilute sulfuric acid is added in a stepwise manner, with 1 to 2 mL added each time, and the pH value is tested after sufficient stirring.

[0114] In some embodiments, the amount of the dilute sulfuric acid added each time is 1 mL.

[0115] As an implementation scheme, the pH value is used to judge whether the reaction is complete. When the pH value of the reaction system is close to 7, the pH value suddenly changes after adding dilute sulfuric acid again, indicating that the reaction is complete.

[0116] In some embodiments, the calcination temperature is 350° C. and the calcination time is 2 h.

[0117] Compared with the prior art, the present invention has the following beneficial effects:

[0118] The present invention provides a method for preparing ammonium sulfate by purifying an ammonium sulfate solution, wherein the ammonium sulfate solution produced by treating biogas slurry is used as a raw material, and ammonium sulfate crystals are prepared through a series of processing. The results show that the ammonium sulfate crystals prepared by using the ammonium sulfate solution produced by treating biogas slurry have a good crystal form and high purity.

[0119] The invention provides a process for preparing ammonium sulfate crystals by utilizing the ammonium sulfate solution generated by biogas slurry treatment with good stability and economy, and provides a new idea for the treatment of the ammonium sulfate solution generated by biogas slurry treatment and the preparation method of ammonium sulfate crystals. BRIEF DESCRIPTION OF THE DRAWINGS

[0120] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0121] Figure 1 A schematic diagram of a processing system used in step S11 in Example 1 of the present invention;

[0122] Figure 2 This is a flow chart of the method for treating biogas slurry wastewater in Example 1 of the present invention;

[0123] Figure 3 The calcium iron mud before aerobic calcination in step S22 of Example 2 of the present invention ( Figure 3 (a)) and after aerobic calcination ( Figure 3 (b)) Sample photos;

[0124] Figure 4 This is a SEM image of the iron oxide desulfurizer containing hydroxyl prepared in Example 2 of the present invention, wherein Figure 4 (a) scale bar: 100 nm, Figure 4 (b) scale bar 500nm, Figure 4 (c) Scale bar: 1 μm, Figure 4 Scale bar in (d) 5 μm;

[0125] Figure 5 This is a SEM image of the iron oxide desulfurizer containing hydroxyl prepared in Example 2 of the present invention, wherein Figure 5 (a) scale bar: 100 nm, Figure 5 (b) scale bar 500nm, Figure 5 (c) Scale bar: 1 μm, Figure 5 Scale bar in (d) 2 μm;

[0126] Figure 6 The calcined calcium iron mud in step S22 of Example 2 of the present invention ( Figure 6 (a)) and the iron oxyhydroxide desulfurizing agent ( Figure 6 (b)) spectrum;

[0127] Figure 7 is the amount before calcination in step S32 in Example 3 of the present invention ( Figure 7 (a)) and after calcination ( Figure 7 (b)) Sample photos;

[0128] Figure 8 This is a photo of the calcined product in step S33 in Example 3 of the present invention;

[0129] Fig. 9 This is a SEM image of the carbon / γFe2O3 chimera desulfurizer prepared in Example 3 of the present invention, wherein Fig. 9The scale bar in (a) is 100 nm. Fig. 9 The scale bar in (b) is 100 nm. Fig. 9 The scale bar in (c) is 200 nm. Fig. 9 The scale bar in (d) is 300 nm;

[0130] Fig.10 This is a picture of the pickling and drying process in Example 4 of the present invention, wherein a picture of a state after adding dilute sulfuric acid in step S43 ( Fig.10 (a)) and photos after water bath heating ( Fig.10 (b)) and the sample after drying in step S44 ( Fig.10 (c));

[0131] Fig.11 is the amount before calcination in step S45 in Example 4 of the present invention ( Fig.11 (a)) and after calcination ( Fig.11 (b)) Sample photos;

[0132] Fig.12 This is the XRD test diagram of the calcium sulfate sample prepared in Example 4 of the present invention;

[0133] Fig.13 This is a SEM test image of the calcium sulfate sample prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0134] The present invention is described in detail below in conjunction with embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, some adjustments and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0135] Example 1

[0136] The present embodiment provides a method for preparing ammonium sulfate using biogas wastewater. The wastewater comes from a biogas station in Funan County, Fuyang City, Anhui Province. The wastewater is first pretreated, including intercepting large solid impurities with a grid and removing larger particles in the wastewater by gravity sedimentation. This is a routine operation in wastewater treatment and will not be described in detail here. In the present embodiment, the wastewater with obvious particles removed is used as the source wastewater.

[0137] A method for preparing ammonium sulfate using biogas wastewater comprises the following steps:

[0138] S11, coagulating, flocculating and precipitating the source wastewater, and performing a first solid-liquid separation to obtain a first section of wastewater, performing a pH adjustment treatment on the first section of wastewater, and performing a second solid-liquid separation to obtain a second section of wastewater, performing ammonia nitrogen removal treatment on the second section of wastewater, absorbing the ammonia-containing waste gas with dilute sulfuric acid to obtain an ammonium sulfate solution, adding an ethanol solution with a concentration greater than 75 vol.% (the solvent is water, and in this embodiment, industrial ethanol with a concentration of 95 vol.%) to the ammonium sulfate solution at a rate of 10 mL / min, and continuously stirring until crystals are completely precipitated, then performing a third solid-liquid separation, and washing the filtered crystals with the ethanol solution to remove impurities remaining on the crystal surfaces;

[0139] S12, drying the washed crystals to constant weight;

[0140] S13, the filtrate produced by the third solid-liquid separation in step S11 and the washing liquid produced in the subsequent washing can be separated from ethanol and water through a pervaporation membrane to obtain ethanol and water that can be recycled.

[0141] In step S11, due to the small solubility of ammonium sulfate in ethanol, with the addition of ethanol, the solution will gradually become turbid, ammonium sulfate crystals begin to precipitate, and stratification phenomenon begins to appear in the mixed solution at the same time. Continuously stir with a glass rod, continue to add ethanol solution until stratification phenomenon no longer occurs in the mixed solution, and the ammonium sulfate crystals in the solution are now completely precipitated. The solution after crystallization is filtered through a funnel, and filter paper is used to separate the ammonium sulfate crystals from the solution. During filtering, it should be noted that the end of the funnel neck is close to the inner wall of the receiving container to prevent the filtrate from splashing. After solid-liquid separation is completed, the ammonium sulfate crystals on the filter paper are washed with a small amount of ethanol to remove impurities remaining on the crystal surface.

[0142] In this embodiment, the mass ratio of the added ethanol solution to the ammonium sulfate solution is 1.5:1.

[0143] In step S12, the washed ammonium sulfate crystals are transferred to an evaporating dish and dried naturally at room temperature until the mass of the ammonium sulfate crystals no longer changes, thereby obtaining relatively pure ammonium sulfate crystals.

[0144] The drying method can also be drying at 50°C or vacuum drying, which will not be described in detail here.

[0145] In step S13, the solution produced by solid-liquid separation is a mixture of ethanol and water, which are separated by a pervaporation membrane. The obtained ethanol can be recycled and the water can also be reused to reduce operating costs and reduce water resource waste.

[0146] In step S11, the process from source wastewater to first-stage wastewater is as follows: Figure 1The biogas slurry wastewater treatment system shown in the figure comprises a polyferric sulfate reaction tank 1, a lime milk reaction tank 12, and a PAM reaction tank 18 connected in sequence;

[0147] The polyferric sulfate reaction tank 1 is provided with a ferric sulfate reaction tank agitator 6, and is externally connected to a polyferric sulfate dosing barrel 2. A valve 3, a polyferric sulfate dosing pump 4, and a valve 5 are sequentially provided on the connection line; and a biogas slurry addition port is provided on the polyferric sulfate reaction tank 1;

[0148] A lime milk reaction tank mixer 11 is provided inside the lime milk reaction tank 12, and the lime milk dosing tank 7 is connected to the outside. A valve 8, a lime milk metering pump 9, and a valve 10 are provided in sequence on the connection line.

[0149] The PAM reaction tank 18 is provided with a PAM reaction tank agitator 17 inside, and is connected to a PAM dosing barrel 13 outside. The connecting line is provided with a valve 14, a PAM dosing metering pump 15, and a valve 16 in sequence;

[0150] The outlet of the PAM reaction pool 18 is connected to a plate-and-frame feed box 19 , a valve 20 , and a plate-and-frame feed pump 21 in sequence.

[0151] Pump the source wastewater from the biogas slurry inlet to the polyferric sulfate reaction tank 1, open valves 3 and 5, and start the polyferric sulfate dosing pump 4 (the dosage of the polyferric sulfate solution is 33L / m 3 The biogas slurry has an iron content of 11%, and the national standard for polyferric sulfate is GB / t14591-2016. The wastewater continuously enters and exits the polyferric sulfate reaction tank 1, and the hydraulic retention reaction time is 15 minutes. Then it overflows into the lime milk reaction tank 12.

[0152] The wastewater overflows into the lime milk reaction tank 12, valves 8 and 10 are opened, and the lime milk dosing pump 9 is started at the same time (the mass fraction of lime milk is 25%, and the dosage is 6 kg / m3 based on lime (dry basis)). 3 The biogas slurry is continuously introduced into and discharged from the lime milk reaction tank 12, and the hydraulic retention reaction time is 15 minutes.

[0153] The wastewater overflows from the lime milk reaction tank 12 to the PAM reaction tank 18, and valves 14 and 16 are opened. At the same time, the PAM dosing pump 16 is started (the coagulant is added as a liquid PAM solution with a mass fraction of 2.5‰, 13L / m 3 The wastewater continuously enters and exits the PAM reaction tank 18, and the hydraulic retention reaction time is 15 minutes.

[0154] Wastewater overflows from the PAM reaction tank 18 to the plate-frame feed box 19 (PP water tank), and is temporarily stored in the plate-frame feed barrel. Open valve 20, start the plate-frame feed pump 21, and complete the plate-frame feeding. The plate-frame feeding time is 3 to 4 hours. Finally, the plate-frame feed pump is used for transportation and pressing to perform the first solid-liquid separation, and the first stage of wastewater (temporarily stored in the PP water tank) is obtained.

[0155] It should be noted that the lime is added in the form of lime milk in this embodiment, but in actual implementation, it can also be added in the form of dry CaO.

[0156] The first stage wastewater is transferred to a lime reactor, and CaO is added to the wastewater according to the amount of 9 kg of CaO per cubic meter of wastewater, and the pH is adjusted to 11-12. After sufficient mixing, a second solid-liquid separation is performed in an inclined tube sedimentation tank to obtain the second stage wastewater. In this embodiment, the CaO is added by dry addition.

[0157] The second stage wastewater is sprayed downward from the top of the ammonia nitrogen stripping tower, and air is introduced from the aeration device at the bottom of the tower. The gas-liquid ratio is controlled at 2000-3000:1, and the pH is maintained at 11-12 through the automatic alkali addition system. The wastewater stays in the stripping tower for 0.3h to remove ammonia nitrogen. The ammonia-containing tail gas is introduced into the dilute sulfuric acid absorption tower through a pipeline, and is absorbed by a dilute sulfuric acid solution with a concentration of 20-45% to obtain an ammonium sulfate solution.

[0158] The composition of the ammonium sulfate solution was tested and is shown in Table 4.

[0159] Table 4 Main components of ammonium sulfate

[0160] substance Ammonium sulfate water content(%) 43.70 56.30

[0161] like Figure 2 As shown, in the above step S11, the precipitated sludge obtained by the first solid-liquid separation is ferric calcium sludge, and the precipitated sludge obtained by the second solid-liquid separation is white calcium sludge. The wastewater obtained after the removal of ammonia nitrogen is the third-stage wastewater, which can be further treated to adjust the pH of the third-stage wastewater to 8-9, and then perform biochemical treatment to make the wastewater meet the requirements of the biogas slurry discharge standard.

[0162] The composition of the calcium iron mud is shown in Table 2, and it can be used as a processing raw material for the desulfurizer, which is introduced in Examples 2 to 4.

[0163] Table 2 Main components of iron calcium mud

[0164]

[0165] The sludge wastewater contains high alkalinity, colloids, suspended matter, etc. The polyferric sulfate is hydrolyzed and reacts with the sludge to form ferric hydroxide, which plays an adsorption bridging and electrical neutralization role. Organic matter and colloids will be adsorbed and removed. At the same time, iron ions will react with the phosphorus element in the water to produce iron phosphate, which will remove the phosphorus element.

[0166] Ca2+ formed by the added lime 2+ It reacts with sulfate to generate nano-scale calcium sulfate. Lime also reacts with alkalinity to generate calcium carbonate. Organic matter and colloidal substances will coat calcium sulfate and calcium carbonate. In addition, the unreacted lime and the generated calcium sulfate and calcium carbonate will play a certain adsorption and co-precipitation role, making the suspended matter easier to precipitate. It greatly reduces the color of the biogas slurry, greatly improves the sedimentation performance of the biogas slurry, and greatly removes COD and other substances in the biogas slurry.

[0167] Biogas slurry contains a lot of alkalinity. Since the polyferric sulfate solution is acidic and Fe 3+ Hydrolysis also produces H + , first add polyferric sulfate, which can remove most of the alkalinity (Fe 3+ It can react with the alkalinity to generate ferric hydroxide) and reduce the subsequent coagulant dosage, and can react with the lime milk formed by the subsequent addition of lime to obtain ferric hydroxide (excess Fe 3+ reacts with OH- provided by lime to generate calcium sulfate (SO4 2- With lime providing Ca 2+ The reaction generates a mixture mainly composed of CaSO4) and organic matter. After burning at a certain temperature, the organic matter is carbonized and can be used as a raw material for a desulfurizer. The material characterization of the desulfurizer shows that the main components are Fe2O3 and CaSO4. After burning, the iron calcium mud, Fe2O3 and CaSO4 components are close to or even better than the iron content of commercial desulfurizers.

[0168] If lime is added first, calcium carbonate will be generated due to the high alkalinity in the biogas slurry, and the lime dosage is high, which is costly. If polyferric sulfate is added later, the dosage of polyferric sulfate will increase, and the mixed sludge will be mainly composed of calcium carbonate, calcium sulfate and organic matter, which changes the composition. The presence of calcium carbonate reduces the content of Fe2O3, the effective ingredient of the desulfurizer, which is not conducive to subsequent resource recovery. It also increases the cost of biogas slurry treatment.

[0169] The composition of white lime mud is shown in Table 3, and can be used to prepare calcium sulfate, which is further introduced in Example 5.

[0170] Table 3 Main components of white calcium mud

[0171] substance <![CDATA[CaSO4]]> <![CDATA[CaCO3]]> <![CDATA[Fe(OH)3]]> <![CDATA[SiO2]]> <![CDATA[MgCO3]]> Organic matter content(%) 58.07 14.44 3.48 0.40 2.61 21

[0172] In the third section, CO2 is introduced into the wastewater to adjust the pH to 8-9, and the wastewater is temporarily stored in the biochemical water inlet tank before entering the subsequent biochemical treatment unit. The biochemical treatment process is a multi-stage AO+MBR membrane treatment process connected with a NF nanofiltration treatment process to meet the requirements of the biogas slurry discharge standards.

[0173] The main pollutant components in the source wastewater and the pollutant components of the wastewater in each section in this embodiment are shown in Table 1.

[0174] Table 1 Pollutant components in each stage of wastewater treatment in biogas slurry

[0175]

[0176] The composition of the discharged wastewater is shown in Table 1.

[0177] The biogas slurry discharged by the present invention meets the Class A standard of urban sewage treatment plants. Different customers implement different standards, and the implementation standards change according to environmental impact assessments. The present invention also meets other standards, such as the pollutant discharge standards for livestock and poultry farming, the Class B water quality standard for sewage discharged into urban sewers, and the comprehensive sewage discharge standard.

[0178] In the field of biogas slurry wastewater treatment, PAC (polyaluminium chloride) is traditionally used as a coagulant to achieve the flocculation and sedimentation process. However, PAC has some limitations during use. For example, the stability of the aluminium hydroxide colloid produced by its hydrolysis is easily affected by water quality conditions. A certain amount of chloride ions may remain in the water after treatment. In the long run, there are potential risks to the environment and subsequent reuse, and the treatment cost is relatively high.

[0179] The present invention abandons the traditional use of PAC, and innovatively selects polyferric sulfate and calcium oxide as alternative coagulants for combination in the treatment of biogas slurry wastewater. Polyferric sulfate has excellent coagulation performance. It can quickly hydrolyze in water to form a large amount of iron hydroxide oxide and iron hydroxide colloids. These colloids have a large specific surface area and a strong adsorption capacity. They can effectively adsorb pollutants such as suspended particles, organic matter and some heavy metal ions in the wastewater, so that they can condense to form larger flocs, which are convenient for subsequent precipitation and separation. On the one hand, calcium oxide can adjust the pH value of the wastewater and create an acid-base environment that is conducive to the best coagulation effect of polyferric sulfate; on the other hand, the calcium ions produced by the hydrolysis of calcium oxide can also participate in the flocculation process, and through adsorption and electrical neutralization with the colloidal particles in the wastewater, the flocculation and precipitation effect is further enhanced, and the removal of pollutants is promoted.

[0180] Detection and analysis

[0181] The ammonium sulfate sample prepared in this embodiment was tested for composition according to standard GB / T 1396-2015, and the testing environment was 20-23° C. and 49-51% RH. The results are shown in Table 7.

[0182] Table 7 ammonium sulfate sample test results

[0183]

[0184] The purity of the ammonium sulfate crystals obtained in step S12 is tested, and it is found that the purity reaches 99.8%. The purity of most ammonium sulfate crystals on the market is between 90% and 98%. The ammonium sulfate crystals prepared by the present invention have a higher purity. At the same time, compared with the traditional ammonium sulfate crystal preparation method, the present method has the advantages of greatly reducing energy consumption in the production process, simple production process flow and less environmental pollution, and also has the benefits of recycling and reducing resource waste.

[0185] This specific embodiment also provides a method for preparing a desulfurizer, which is prepared by using the iron calcium mud in the above-mentioned method for preparing ammonium sulfate using biogas wastewater, and is specifically introduced through Examples 2 to 4.

[0186] Example 2

[0187] This example provides a method for preparing a hydroxyl-containing iron oxide desulfurizer using the iron calcium mud obtained in Example 1.

[0188] S21, drying the calcium iron mud in an oven at 100° C. to constant weight, cooling naturally, and then crushing it with a crusher, passing it through a 200-mesh sieve, and retaining the sieved material;

[0189] S22. Put the sieved iron calcium mud into a corundum crucible, place it in a muffle furnace, calcine it at 200°C in the presence of oxygen for 2 hours, cool it to room temperature, and after measurement, the calcination weight loss is 27.3% The pictures of samples before and after calcination are as follows: Figure 3 As shown;

[0190] S23, adding the calcined calcium iron mud to a 12.5 mol / L NaOH aqueous solution to obtain a calcium iron mud slurry with a solid content of 30%, stirring the slurry with a magnetic stirrer at a speed of 500 rpm for 1 hour, and then separating the solid and liquid, and then fully rinsing with deionized water to neutrality, separating the solid and liquid again, and then drying in an oven at 105°C to constant weight to obtain the calcium iron mud after alkali washing, and the alkali washing weight loss is 28.49% The weights here are all dry weights);

[0191] S24, grinding the ferric calcium mud after alkali washing, passing it through a 200-mesh sieve, retaining the sieve residue, and obtaining a hydroxy-containing iron oxide desulfurizer.

[0192] Detection and analysis

[0193] The morphology of the iron oxyhydroxide desulfurizer measured by SEM on the sample ground and sieved through 200 mesh is as follows: Figure 4 and Figure 5 As shown. The brighter substance is iron oxyhydroxide, and the darker substance is calcium sulfate. During the calcination process, in the presence of high temperature and oxygen, the organic matter undergoes volatilization, combustion and other processes to generate gaseous products such as CO2 and water. These gaseous products form a certain pressure inside the inorganic salt and then escape, leaving pores in the inorganic salt.

[0194] The composition of the desulfurizer containing iron oxide hydroxide measured by XRF is shown in Table 5.

[0195] Table 5 XRF results of desulfurizer containing iron oxyhydroxide

[0196] substance <![CDATA[SiO2]]> <![CDATA[SO3]]> CaO <![CDATA[Fe2O3]]> other content(%) 2.06 19.88 21.56 52.95 3.55

[0197] XRD is mainly used to analyze crystal structure. For materials with poor crystallinity and amorphous state, the diffraction peaks produced are weak, diffuse or even absent. The spectra of the calcined iron calcium mud in step S22 and the oxyhydroxide-containing iron desulfurizer in step S24 were measured by XRD as follows: Figure 6 As shown, it can be seen that the main components are calcium sulfate, calcium carbonate and iron oxyhydroxide. Among them, iron oxyhydroxide mainly exists in amorphous form, so it cannot be clearly shown in the XRD spectrum. Colloidal iron oxyhydroxide is difficult to show clearly in XRD mainly because the particle size is small in the amorphous state and there may be a large amount of amorphous parts.

[0198] The composition of the iron oxyhydroxide-containing desulfurizing agent prepared in this embodiment is as follows:

[0199] Calcium sulfate: 31.35 parts;

[0200] Iron oxyhydroxide: 56.08 parts;

[0201] Impurities: 12.57 parts.

[0202] As an embodiment, the impurities include SiO2, CaCO3, and MgCO3.

[0203] In this embodiment, the samples obtained in each step were subjected to a desulfurization performance evaluation test, and the H2S standard gas concentration used was 20000 ppm, and the balance gas was N2.

[0204] Although the dried calcium iron mud in step S21 also contains iron oxyhydroxide, its surface is covered by a large amount of organic matter, resulting in poor desulfurization effect, and the breakthrough sulfur capacity is only 0.41%;

[0205] The calcined but not alkali-washed iron-calcium mud was ground and passed through a 200-mesh sieve for a desulfurization performance evaluation test. The penetration sulfur capacity was 10.12%. It can be seen that after calcination, the organic matter volatilized and decomposed by about 27.3%, allowing the iron oxyhydroxide to contact the sulfur in the gas, thereby achieving a desulfurization effect.

[0206] The breakthrough sulfur capacity of the ferric oxide desulfurizer containing hydroxyl obtained in step S24 is 20.10%. It can be seen that after the alkaline washing treatment, the removal rate of organic matter is higher, which further improves the desulfurization effect of the ferric oxide desulfurizer containing hydroxyl.

[0207] The present embodiment provides a method for preparing a desulfurizer containing hydroxyl iron using biogas wastewater, which not only retains the active substance of hydroxyl iron, but also realizes the removal of organic matter, and in the process of removing organic matter, a pore is generated in the crystal of Ca-containing inorganic salt, which is suitable for use in biogas and biomass natural gas desulfurization and other related fields. From the perspective of structural stability, calcium sulfate and calcium carbonate can be filled between hydroxyl iron particles to play a supporting and stabilizing role. In the desulfurization process, the desulfurizer needs to withstand complex working conditions such as high temperature and gas scouring. The addition of calcium salt can prevent the agglomeration and sintering of hydroxyl iron particles, maintain the good pore structure of the desulfurizer, and ensure that the gas can smoothly enter the interior of the desulfurizer and contact with the active ingredients. The desulfurizer containing hydroxyl iron prepared by the present invention has excellent desulfurization efficiency and service life.

[0208] Example 3

[0209] This example provides a method for using the iron-calcium mud obtained in Example 1 to prepare a carbon / γ-iron oxide chimera desulfurizer.

[0210] S31, drying the calcium iron mud in an oven at 105° C. to constant weight, and cooling naturally;

[0211] S32, place the dried calcium iron mud in a corundum crucible, fully shake and level the powder surface, and manually compact it with the crucible cover. In this embodiment, the density after pressing is 3.1 g / cm 3 Then, the crucible (with crucible cover) was placed in a muffle furnace and calcined at 600°C for 3 hours without oxygen to obtain a black solid product. After measurement, the calcination weight loss was 33.22%. The photos before and after calcination are shown in FIG. Figure 7 As shown;

[0212] S33, the black solid product was placed in a box-type electric furnace, calcined at 400°C in air for 2 hours, and then cooled to room temperature to obtain a black-red carbon / γFe2O3 chimera crude product. The photo after calcination is as follows: Figure 8 As shown;

[0213] S34, grinding the crude product, passing it through a 200-mesh sieve, retaining the sieve residue, and obtaining a carbon / γFe2O3 chimera desulfurizer.

[0214] In step S32, since the crucible cover and the crucible, and the crucible cover and the powder in the crucible are tightly connected, a relatively closed space is formed, which acts as a physical barrier to the air. During the calcination process, the amount of air entering will be very small. When the organic matter in the iron calcium mud in the crucible is calcined, the organic matter decomposes into gases such as ammonia and hydrogen, leaving carbonaceous components, which will also reduce the amount of air entering when the gas escapes. FeOOH decomposes into Fe2O3 at high temperature, and since the decomposed gas is reducing, during the oxygen-isolated calcination process, Fe2O3 is reduced to Fe3O4, and the carbonaceous components are coated on the surface of Fe3O4, that is, the obtained black solid product.

[0215] In step S33, since the calcination temperature is not high, some carbon produced by carbonization of organic matter still adheres to the surface of γ-Fe2O3, forming a crude carbon-γ iron oxide chimera product.

[0216] Detection and analysis

[0217] By using XRD to detect the main components of the carbon / γ iron oxide chimera desulfurizer, it can be seen that the crystals in the sample are CaCO3, CaSO4, and γFe2O3, and the mass ratio between the three is 1:3.45:5.55.

[0218] The carbon / γ-iron oxide chimera desulfurizer prepared in this example has the following components by weight:

[0219] Carbon: 15.18 parts;

[0220] Gamma iron oxide: 45.33 parts;

[0221] Calcium sulfate: 28.19 parts;

[0222] Calcium carbonate: 8.16 parts;

[0223] Impurities: 3.14 parts, including SiO2 and MgCO3.

[0224] The morphology of the carbon / γFe2O3 chimera desulfurizer was measured by SEM. Fig. 9 As shown, it can be seen that the better crystal form is the γ iron oxide crystal, the activated carbon material and the γ iron oxide are embedded together, the γ iron oxide has a higher sulfur removal effect, and the activated carbon material has a large specific surface area and rich pore structure, which can adsorb H2S in the gas; the γ iron oxide also has a higher sulfur removal effect, so the carbon / γ iron oxide mosaic desulfurizer can be used as a desulfurizer. The desulfurization performance evaluation test was carried out, and the test method was the same as in Example 1, and the breakthrough sulfur capacity of the carbon / γ iron oxide mosaic desulfurizer reached 15.6%.

[0225] When the dried calcium iron mud in this embodiment is calcined at 600°C in an oxygen-free environment, FeOOH is first decomposed into Fe2O3, and then reduced to Fe3O4 by the gas formed by the decomposition and carbonization of organic matter, while the carbon material decomposed by the organic matter is coated around Fe3O4. After further aerobic calcination at 400°C, Fe3O4 is converted into γ iron oxide, which forms a carbon / γ iron oxide chimera with the carbon generated by the decomposition of organic matter, and can be used as a desulfurizer.

[0226] The present invention uses iron calcium mud to prepare a carbon / γ iron oxide chimera desulfurizer. First, the iron calcium mud is calcined in an oxygen-free state. The reducing gas generated during the decomposition of organic matter reduces the iron oxide in the iron calcium mud to ferroferric oxide, and the decomposed carbon component is coated on the ferroferric oxide particles, and then calcined in an oxygen-free state to obtain a carbon / γ iron oxide chimera. This structure can not only improve the stability of the material and prevent the loss of active ingredients, but also give the material good adsorption properties and the like.

[0227] Example 4

[0228] This example provides another method for using the iron-calcium mud obtained in Example 1 to prepare a carbon / γ-iron oxide chimera desulfurizer.

[0229] The dried calcium iron mud was placed in a hydrothermal reactor, deionized water was added, and the solid-liquid mass ratio was 1:5, and the reactor was sealed; the reactor was then placed in an oven at 200°C, the pressure in the reactor increased, and after the hydrothermal treatment for 4 hours, the reactor was cooled to room temperature;

[0230] After the hydrothermal treatment, the ferric calcium mud slurry is separated from the solid and liquid. After solid-liquid separation, drying, grinding and passing through a 200-mesh sieve, the hydrothermal ferric calcium mud, i.e., ferric calcium mud containing Fe3O4, is obtained. A certain amount of ferric calcium mud containing Fe3O4 is weighed into a corundum crucible, and placed in a muffle furnace for aerobic calcination. The temperature is raised to 280°C at a rate of 10°C / min, and after keeping warm for 2h, it is cooled to room temperature. The obtained product is ground and crushed to obtain a carbon / γ-iron oxide chimera desulfurizer below 200 mesh.

[0231] The carbon / γ-iron oxide chimera desulfurizer prepared in this example has the following components by weight:

[0232] Carbon: 16.28 parts;

[0233] Gamma iron oxide: 45.33 parts;

[0234] Calcium sulfate: 27.19 parts;

[0235] Calcium carbonate: 8.11 parts;

[0236] Impurities: 3.09 parts, including SiO2 and MgCO3.

[0237] In terms of economic benefits, the production cost of the desulfurizer is significantly reduced by using waste as raw materials. Compared with the desulfurizer prepared by the traditional pure chemical synthesis method, the desulfurizer prepared by the present invention has obvious advantages in raw material procurement cost and preparation process. In the traditional process, calcium sulfate and other catalysts need to be mixed, which is complicated and costly.

[0238] This specific embodiment also provides a method for preparing calcium sulfate, which is prepared using the white calcium mud obtained in the above-mentioned method for preparing ammonium sulfate using biogas wastewater, and is specifically introduced through Example 5.

[0239] Example 5

[0240] In this embodiment, a method for preparing calcium sulfate using the white lime mud obtained in Example 1 is provided.

[0241] S51, drying the white lime mud in an oven at 100° C. to constant weight, grinding and passing through a 200-mesh sieve, retaining the sieved material, with a whiteness of 81.30;

[0242] S52, adding deionized water to the white lime mud after step S31 to prepare a suspension with a solid content of 20%, and testing the pH value of the suspension to find that it is 11.92;

[0243] S53, heat the suspension in a water bath at 80°C for 3h. During the water bath, stir the suspension continuously at 300r / min using a stirrer, and slowly add 5mol / L dilute sulfuric acid until the pH value drops below 7, then stop adding dilute sulfuric acid. Fig.10 (a) shows a state photo after adding dilute sulfuric acid. Fig.10 (b) shows the photograph after heating in a water bath for 3 h;

[0244] S54, the pH-adjusted suspension was filtered using a vacuum filtration device to obtain a solid substance, which was then washed with deionized water for multiple times until the supernatant was clear to obtain a washed solid, which was then dried in an oven at 105° C. to a constant weight to obtain a calcium sulfate intermediate sample such as Fig.10 As shown in (c), the whiteness is 91.11;

[0245] S55. Grind the intermediate sample into powder, place it in a box-type electric furnace, calcine it at 350°C for 2 hours, and then cool it to room temperature to obtain a calcium sulfate sample with a whiteness of 96.00. The photos of the samples before and after calcination are as follows: Fig.11 shown.

[0246] Compared with the sample in step S51, the whiteness of the intermediate sample after acid washing in step S34 increased by about 12%, indicating that impurities were reduced. After calcination in step S55, the whiteness increased again, indicating that impurities were further removed.

[0247] It should be noted that in step S53, 1 mL of dilute sulfuric acid is added to the suspension every 1 to 2 min, and the pH value is measured. In the present embodiment, the suspension made of 10.42 g of white lime mud after step S31 is added with 6 mL of 5 mol / L dilute sulfuric acid, and the pH value is close to 7. After adding 1 mL of dilute sulfuric acid again, the pH value suddenly changes, indicating that impurities such as calcium carbonate in the white lime mud are completely reacted.

[0248] Detection and analysis

[0249] The main components of calcium sulfate samples were tested by XRD, and the results are as follows: Fig.12 It can be seen that the organic impurities attached to the white lime mud are significantly removed after pickling and calcination, and the impurities in the calcium sulfate sample are significantly reduced. The main component is calcium sulfate, with a content of 99.1wt.%, with excellent whiteness value and high product purity.

[0250] The calcium sulfate samples were tested according to the industry standard HG / T 5346-2018 in an environment of 20-23°C and 49-51% RH. The results are shown in Table 6.

[0251] Table 6 Calcium sulfate sample test results

[0252]

[0253] The morphology of calcium sulfate samples was examined by SEM. Fig.13 As shown, it can be seen that the calcium sulfate crystals are better. Compared with the traditional calcium sulfate preparation method, the product purity and whiteness of this embodiment have great advantages.

[0254] The present invention discloses a method for preparing calcium sulfate by purifying white lime mud. Organic matter attached to the white lime mud is removed by pickling and calcining. The purity of the prepared calcium sulfate reaches 99.1wt.%, and the whiteness exceeds 95%. The calcium sulfate can be used as an additive for biogas residue fertilizer, and can also be processed into building materials and the like.

[0255] In summary, from the perspective of resource recovery, the present invention not only utilizes the biogas slurry wastewater to prepare a variety of products, such as hydroxyl iron oxide desulfurizer, carbon / γ iron oxide chimera desulfurizer, calcium sulfate, ammonium sulfate, humus, etc., but also effectively treats the biogas slurry wastewater during the preparation process to meet the emission standards, thereby realizing the resource recycling of waste.

[0256] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing ammonium sulfate using biogas wastewater, characterized in that: Adding an ethanol solution to the ammonium sulfate solution, stirring continuously until crystals are completely precipitated, then performing solid-liquid separation, and washing and drying the obtained solid matter with the ethanol solution to obtain the ammonium sulfate; The ammonium sulfate solution is obtained by absorbing ammonia-containing waste gas generated during the process of removing ammonia nitrogen from biogas wastewater through dilute sulfuric acid.

2. The method for preparing ammonium sulfate using biogas slurry wastewater according to claim 1, characterized in that: The components of the ammonium sulfate solution are as follows, by mass: 40 to 50 parts of ammonium sulfate and 50 to 60 parts of water.

3. The method for preparing ammonium sulfate using biogas slurry wastewater according to claim 1, characterized in that: The following steps are involved: The biogas wastewater is treated by a stripping method to remove ammonia nitrogen.

4. The method for preparing ammonium sulfate using biogas slurry wastewater according to claim 1, characterized in that: Before the ammonia nitrogen removal treatment is carried out on the biogas wastewater, flocculation and sedimentation and pH adjustment treatment to 11-12 are carried out, and then the ammonia nitrogen removal treatment is carried out.

5. The method for preparing ammonium sulfate using biogas slurry wastewater according to claim 4, characterized in that: The flocculation and precipitation is as follows: a coagulant, calcium oxide and a flocculant are sequentially added to the biogas slurry wastewater to perform flocculation and precipitation treatment, and then a first solid-liquid separation is performed to obtain the first stage of wastewater; Alkaline substances are added to the first stage wastewater for pH adjustment treatment, and then a second solid-liquid separation is performed to obtain a second stage wastewater, and the second stage wastewater is used for ammonia nitrogen removal treatment.

6. The method for preparing ammonium sulfate using biogas slurry wastewater according to claim 1, characterized in that: The concentration of the ethanol solution is not less than 75 vol.%.

7. The method for preparing ammonium sulfate using biogas slurry wastewater according to claim 1, characterized in that: The ethanol-water solution after the solid-liquid separation is separated by a pervaporation membrane to obtain ethanol solution and water that can be recycled.

8. The method for preparing ammonium sulfate using biogas slurry wastewater according to claim 1, characterized in that: The ethanol solution is added at a rate of 5 to 20 mL / min.

9. The method for preparing ammonium sulfate using biogas slurry wastewater according to any one of claims 1 to 8, characterized in that: It is also possible to co-produce one or more of a hydroxyl-containing iron oxide desulfurizer, a carbon / γ iron oxide desulfurizer, and calcium sulfate.

10. Ammonium sulfate, characterized in that: The ammonium sulfate is prepared by the method for preparing ammonium sulfate by utilizing biogas slurry wastewater as claimed in any one of claims 1 to 9, and the purity of the ammonium sulfate is not less than 99.8%.

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