Salt resource recovery process

Abstract

The application provides a salt resourceization process, which can realize the double effects of carbon capture and waste resourceization. The salt resourceization process comprises an organic waste treatment step and a waste salt treatment step. The organic waste treatment step comprises the following steps: anaerobic digestion treatment of organic waste to obtain biogas and a digestion liquid; combustion of the biogas in a combustion chamber to obtain high-temperature flue gas containing carbon dioxide; separation treatment of the digestion liquid to obtain ammonia water; reaction of the carbon dioxide in the high-temperature flue gas with the ammonia water to obtain ammonium bicarbonate. The waste salt treatment step comprises the following steps: pyrolysis and water rinsing of waste salt as solid waste to obtain purified salt; double decomposition reaction of the purified salt with the ammonium bicarbonate to obtain sodium bicarbonate crystal slurry; centrifugal dewatering and drying of the sodium bicarbonate crystal slurry.

Description

Technical Field

[0001] This invention relates to a salt resource recovery process for carbon capture. Background Technology

[0002] In cities or industrial parks, an environmental industrial park or treatment center is usually built to centrally manage various types of solid waste. Organic waste and industrial waste salts are centrally managed to achieve economies of scale and synergistic effects.

[0003] Organic waste includes, for example, kitchen waste, expired food, food industry waste, municipal sludge, and sewage. The treatment of organic waste typically employs anaerobic digestion technology to convert its organic matter into biogas. However, biogas slurry contains high concentrations of ammonia nitrogen, and traditional aerobic biological treatment requires significant energy consumption and does not produce any valuable products. Furthermore, although biogas is often used for power generation or combined heat and power (CHP), the carbon dioxide produced after combustion is usually directly released into the atmosphere, thus also generating no value.

[0004] Industrial waste salt mainly originates from industrial production or zero-discharge wastewater treatment. Its main components are NaCl or Na₂SO₄, along with small amounts of organic or inorganic impurities. Industrial waste salt may be identified as hazardous waste; direct landfilling would be extremely costly, making it a difficult-to-dispose-of waste. If treated with purification methods, it can produce high-purity sodium chloride or sodium sulfate. However, because the market supply of these two products is already saturated, it is difficult to realize their commercial value. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a salt resource recovery process that couples the treatment methods for organic waste and waste salt. Specifically, carbon dioxide and ammonia nitrogen generated during organic waste treatment are chemically synthesized into ammonium bicarbonate, which is then reacted with purified waste salt via a metathesis reaction to produce sodium bicarbonate. This process transforms low-value byproducts from organic waste and waste salt treatment into high-value sodium bicarbonate and ammonium fertilizer, achieving a dual effect of carbon capture and waste resource recovery.

[0006] The salt resource recovery process of the present invention includes an organic waste treatment step and a waste salt treatment step.

[0007] The organic waste treatment process includes the following steps:

[0008] Step (1-1) involves anaerobic digestion of organic waste to obtain biogas and digestate.

[0009] Steps (1-2) involve burning biogas in a combustion chamber to obtain high-temperature flue gas containing carbon dioxide.

[0010] Steps (1-3) involve separating the digestive fluid to obtain ammonia water;

[0011] In steps (1-4), the carbon dioxide in the high-temperature flue gas obtained in step (1-2) reacts with the ammonia water obtained in step (1-3) to produce ammonium bicarbonate.

[0012] The waste salt treatment process includes the following steps:

[0013] Step (2-1) involves pyrolyzing and rinsing the waste salt, which is a solid waste, to obtain purified salt.

[0014] In steps (2-2), the purified salt obtained in step (2-1) undergoes a metathesis reaction with the ammonium bicarbonate obtained in step (1-4) to obtain sodium bicarbonate slurry.

[0015] Step (2-3): The sodium bicarbonate slurry obtained in step (2-2) is centrifuged, dehydrated, and dried.

[0016] In some embodiments, preferably, the separation treatment of the digestive fluid in steps (1-3) further includes the following steps:

[0017] Step (1-3-1): The digestion liquid is separated into biogas residue and biogas slurry by a centrifuge, and the biogas slurry is stored in a biogas slurry storage tank;

[0018] Step (1-3-2): Add lime to the biogas slurry and remove the resulting precipitate;

[0019] Step (1-3-3): The effluent after removing the precipitate in step (1-3-2) is sent to a stripping tower to obtain ammonia vapor;

[0020] Steps (1-3-4) involve condensing ammonia vapor to obtain ammonia water.

[0021] In some embodiments, it is preferred that, in step (1-3-2), the pH is adjusted to above 10.5 by adding lime to the biogas slurry.

[0022] In some embodiments, it is preferred that, in step (1-3-3), the carrier gas of the stripping tower is low-pressure saturated steam generated in the waste heat boiler.

[0023] In some embodiments, it is preferable to allow the inlet and outlet water of the stripping tower to exchange heat outside the stripping tower in order to preheat the inlet water of the stripping tower.

[0024] In some embodiments, preferably, in step (1-3-4), the condensation of ammonia vapor includes primary condensation and secondary condensation.

[0025] After primary condensation, the temperature of the ammonia vapor is reduced to 70℃~80℃, and the condensate is sent back to the stripping tower.

[0026] After two-stage condensation, the temperature of the ammonia vapor is further reduced to 30℃~35℃.

[0027] In some embodiments, it is preferred that step (1-3-4) yields an ammonia concentration of 15% to 20% at a temperature of 30°C to 35°C.

[0028] In some embodiments, it is preferred that the non-condensable gas in step (1-3-4) is sent to the combustion chamber in step (1-2).

[0029] In some embodiments, preferably, the high-temperature flue gas generated in step (1-2) further undergoes the following treatment steps:

[0030] Step (1-2-1): The high-temperature flue gas discharged from the combustion chamber is sent into the pyrolysis furnace for pyrolysis in step (2-1) to indirectly heat the waste salt;

[0031] Step (1-2-2): After the high-temperature flue gas leaves the pyrolysis furnace, it enters the waste heat boiler for waste heat recovery to generate low-pressure saturated steam.

[0032] Steps (1-2-3): After the high-temperature flue gas leaves the waste heat boiler, it enters the circulating spray cooling tower.

[0033] In step (1-2-4), the gas discharged from the circulating spray cooling tower is pressurized and sent to step (1-4) to react with ammonia water to obtain ammonium bicarbonate.

[0034] In some embodiments, it is preferred that, in steps (1-4), the mass ratio of carbon dioxide to ammonia is 2.7 to 3.0:1, the reaction temperature is controlled at 32°C to 38°C, and the endpoint pH is 8.2 to 8.8.

[0035] In some embodiments, it is preferred that the heating temperature in the pyrolysis furnace is controlled at 600°C to 700°C.

[0036] In some embodiments, it is preferred that the low-pressure saturated steam generated by the waste heat boiler in step (1-2-2) is also used for drying the sodium bicarbonate slurry in step (2-3).

[0037] In some embodiments, preferably, the condensate formed by the condensation of water vapor in the high-temperature flue gas is used as the spray liquid of the circulating spray cooling tower, and is cooled by circulating cooling water through an external heat exchanger and then recycled.

[0038] In some embodiments, preferably, the condensate formed by the condensation of water vapor in the high-temperature flue gas is also used for water rinsing in step (2-1).

[0039] In some embodiments, it is preferred that the pyrolysis gas generated from the waste salt pyrolysis in step (2-1) is fed into the combustion chamber in step (1-2) for combustion.

[0040] In some embodiments, preferably, in step (2-1), after water rinsing, the salt is further separated into rinsing water and the purified salt by a hydrocyclone separator, and the rinsing water is recycled back to the rinsing contact tank used for water rinsing.

[0041] In some embodiments, it is preferred that, in the metathesis reaction of step (2-2), when the main component of the purified salt is sodium sulfate, the concentration of sodium sulfate is controlled at 290-340 g / L, the mass ratio of sodium sulfate to ammonium bicarbonate is about 1:1.05-1.25, and the reaction temperature is 35℃-40℃.

[0042] In some embodiments, preferably, in the metathesis reaction of step (2-2), when the main component of the purified salt is sodium chloride, the concentration of sodium chloride is controlled at about 280 to 330 g / L, the mass ratio of sodium chloride to ammonium bicarbonate is about 1:1.4 to 1.5, and the reaction temperature is 20°C to 30°C.

[0043] In some embodiments, it is preferred that at least a portion of the ammonium bicarbonate obtained in steps (1-4) is recovered after centrifugation, dehydration, and natural drying.

[0044] In some embodiments, it is preferred that the waste gas from the reaction of carbon dioxide and ammonia in steps (1-4) and the waste gas generated from the drying of sodium bicarbonate in steps (2-3) be combined, purified by a low-temperature plasma purification device, and then discharged in a centralized manner.

[0045] In some embodiments, it is preferred that the mother liquor obtained by centrifugation and dehydration of ammonium bicarbonate in steps (1-4) and the mother liquor obtained by centrifugation, dehydration and drying of sodium bicarbonate in steps (2-3) be combined and discharged into a high-efficiency sedimentation tank, lime is added to generate precipitate, the precipitate is dehydrated, and the high-efficiency sedimentation and the dehydrated mother liquor are sent to a stripping tower. Attached Figure Description

[0046] Figure 1 This is a process flow diagram of the salt resource utilization process according to an embodiment of the present invention. Detailed Implementation

[0047] To make the inventive objectives, technical solutions, and beneficial technical effects of this application clearer, this application will be described in detail below. It should be noted that the various aspects, features, embodiments, and advantages described in this application can be compatible and / or combined together.

[0048] Unless otherwise specified, the technical terms used in this specification have the same meaning as commonly understood by those skilled in the art. Furthermore, unless expressly stated otherwise, the words “comprising,” “including,” or “containing” will be understood to include the stated elements but not exclude any other elements.

[0049] The salt resource utilization process of this invention includes an organic waste treatment step and a waste salt treatment step.

[0050] The organic waste treatment process includes the following steps:

[0051] Step (1-1) involves anaerobic digestion of organic waste to obtain biogas and digestate.

[0052] Steps (1-2) involve burning biogas in a combustion chamber to obtain high-temperature flue gas containing carbon dioxide.

[0053] Steps (1-3) involve separating the digestive fluid to obtain ammonia water;

[0054] In steps (1-4), the carbon dioxide in the high-temperature flue gas obtained in step (1-2) reacts with the ammonia water obtained in step (1-3) to obtain ammonium bicarbonate.

[0055] The waste salt treatment process includes the following steps:

[0056] Step (2-1) involves pyrolyzing and rinsing the waste salt, which is a solid waste, to obtain purified salt.

[0057] In steps (2-2), the purified salt obtained in step (2-1) undergoes a metathesis reaction with the ammonium bicarbonate obtained in step (1-4) to obtain sodium bicarbonate slurry.

[0058] Step (2-3): The sodium bicarbonate slurry obtained in step (2-2) is centrifuged, dehydrated, and dried.

[0059] The aforementioned salt resource recovery process couples the treatment processes for organic waste with those for waste salt, which is solid waste. Anaerobic digestion of organic waste provides green energy (biogas) for salt resource recovery and raw materials (carbon dioxide and ammonia) for the process. Carbon dioxide and ammonia can be synthesized into ammonium bicarbonate. This ammonium bicarbonate, after centrifugation and dehydration, can undergo a metathesis reaction with purified salt to produce sodium bicarbonate, a high-value product. Additionally, a portion of the ammonium bicarbonate, after centrifugation, dehydration, and natural drying, can become agricultural-grade ammonium fertilizer. Therefore, the salt resource recovery process of this invention achieves the dual effects of carbon capture and waste resource recovery.

[0060] The following is a detailed explanation of each step in the salt resource utilization process.

[0061] (a) Organic waste treatment steps

[0062] Organic waste includes, for example, kitchen waste, food waste, expired food, food industry waste, urban sludge, and sewage.

[0063] In step (1-1), the organic waste is subjected to anaerobic digestion.

[0064] The carbon in the decomposed organic matter is largely converted into biogas, a green energy source, whose main components are methane and carbon dioxide. In some implementations, the biogas can be further desulfurized and stored in biogas tanks for later use.

[0065] Subsequently, in steps (1-2), the biogas is fed into the combustion chamber for combustion, producing high-temperature flue gas. The main components of the high-temperature flue gas are carbon dioxide, water, and nitrogen. This high-temperature flue gas can provide a heat source for processes such as waste salt pyrolysis, and can also serve as a carbon dioxide supply source for the subsequent reaction to prepare ammonium bicarbonate.

[0066] Next, the treatment of high-temperature flue gas will be explained.

[0067] In steps (1-2), the temperature of the high-temperature flue gas generated by the combustion of biogas in the combustion chamber can reach 900℃ to 1000℃. In some embodiments, this high-temperature fuel gas is fed into a pyrolysis furnace to indirectly heat the waste salt, i.e., to achieve the pyrolysis in step (2-1). The pyrolysis of waste salt will be explained in detail later.

[0068] The temperature of the high-temperature flue gas after leaving the pyrolysis furnace can reach 750°C to 850°C. In some embodiments, the high-temperature flue gas after leaving the pyrolysis furnace enters a waste heat boiler for waste heat recovery, thereby generating low-pressure saturated steam (below 0.4 MPa). This low-pressure saturated steam can be used as a carrier gas in a stripping tower or for drying sodium bicarbonate crystals.

[0069] The temperature of the high-temperature flue gas decreases to approximately 150°C after leaving the waste heat boiler. In some embodiments, the high-temperature flue gas leaving the waste heat boiler enters a circulating spray cooling tower for further cooling. At this point, the temperature of the high-temperature flue gas can be further reduced to 30°C to 35°C. Additionally, the condensate formed by the condensation of water vapor in the high-temperature flue gas can be used as the spray liquid in the circulating spray cooling tower. Alternatively, a circulating flow path can be provided outside the circulating spray cooling tower, allowing the spray liquid to be cooled by circulating cooling water through an external heat exchanger and then recycled.

[0070] Additionally, in some embodiments, when there is sufficient spray liquid, excess condensate can also be discharged into a water rinsing tank for water rinsing in step (2-1). Water rinsing of waste salt will be described in detail later.

[0071] After being cooled by the circulating spray cooling tower, the exhaust gas discharged from the high-temperature flue gas mainly consists of carbon dioxide and nitrogen, and contains small amounts of oxygen and water. A centrifugal compressor can be used to pressurize the discharged exhaust gas to above 0.1 MPa. The pressurized exhaust gas is then sent to steps (1-4) as a carbon dioxide supply source for ammonia carbonization.

[0072] In steps (1-4), the compressed carbon dioxide gas and the received ammonia water react to synthesize ammonium bicarbonate. The synthesis of ammonium bicarbonate will be explained in detail later.

[0073] Through the above steps, biogas, one of the products of anaerobic digestion of organic waste gas, can provide heat as a green energy source. The high-temperature flue gas produced by biogas combustion serves as a heat transfer medium, first used in a pyrolysis furnace to pyrolyze waste salts, and then used in a waste heat boiler to generate low-pressure saturated steam. After the high-temperature flue gas has completed its thermal energy utilization, it is cooled by circulating spraying to provide carbon dioxide for the carbonization of ammonia water, and then discharged after the synthesis of ammonium bicarbonate. Compared to existing technologies that directly emit high-temperature flue gas from biogas combustion, this method not only fully utilizes the thermal energy carried by the high-temperature flue gas but also synthesizes a large amount of carbon dioxide into ammonium bicarbonate, achieving carbon capture.

[0074] Next, the treatment of digestive juices will be explained.

[0075] On the other hand, in step (1-1), after the organic waste undergoes anaerobic digestion, the nitrogen in the decomposed organic matter is basically converted into ammonia nitrogen and released into the digestate. In step (1-3), ammonia can be extracted from the digestate by separation treatment to prepare ammonia water.

[0076] The separation and treatment of the digestive fluid in steps (1-3) may include the following steps:

[0077] Step (1-3-1): The digestion liquid is separated into biogas residue and biogas slurry by a centrifuge, and the biogas slurry is stored in a biogas slurry storage tank;

[0078] Step (1-3-2): Add lime to the biogas slurry and remove the resulting precipitate;

[0079] Step (1-3-3): The effluent after removing the precipitate in step (1-3-2) is sent to a stripping tower to obtain ammonia vapor;

[0080] Steps (1-3-4) involve condensing ammonia vapor to obtain ammonia water.

[0081] In step (1-3-1), the digestate is separated into solid and liquid phases by a centrifuge. The solid phase is biogas residue, and the liquid phase is biogas slurry. Ammonia nitrogen is mainly present in the biogas slurry, with a concentration that can reach several thousand mg / L.

[0082] Since centrifuges typically have a solids rejection rate of 97%–99%, the separated biogas slurry will still contain a certain amount of suspended solids, as well as a certain amount of carbonates and phosphates. Furthermore, if there is reflux liquid, ammonium sulfate or ammonium chloride may also be present.

[0083] Therefore, in step (1-3-2), lime is added to the biogas slurry. Lime can react with carbonates, phosphates, etc., to form precipitates such as calcium carbonate and calcium phosphate. Afterwards, the precipitates and most of the suspended solids can be rapidly removed together in a high-efficiency sedimentation tank, and the sludge is discharged as sludge and sent to a sludge treatment plant for further processing. At this point, the suspended solids concentration in the effluent from the high-efficiency sedimentation tank can be below 20 mg / L.

[0084] In some embodiments, it is preferred that, in step (1-3-2), the pH is adjusted to above 10.5 by adding lime to the biogas slurry. By making the pH above 10.5, the ammonia nitrogen in the effluent after precipitation exists mainly in the form of free ammonia, thus providing good stripping conditions.

[0085] In step (1-3-3), the effluent after sediment removal is fed into the stripping tower, which becomes the feed water for the stripping tower. Alternatively, low-pressure saturated steam (below 0.4 MPa) generated in the waste heat boiler can be used as the carrier gas in the stripping tower.

[0086] In some implementations, the inlet and outlet water of the stripping tower can exchange heat outside the tower to preheat the inlet water. By preheating the inlet water to 60°C–70°C, the amount of ammonia removal steam used can be reduced.

[0087] As an example, the stripping tower can be a sieve plate tower type. Preheated feed water enters from above the top sieve plate of the tower, and an overflow distributor ensures even distribution across the first sieve plate. The feed water then overflows and descends layer by layer from top to bottom, forming a liquid layer on each sieve plate. Simultaneously, low-pressure saturated steam, acting as the carrier gas, enters tangentially and evenly from the steam inlet at the bottom of the tower. A steam distributor within the tower ensures that the steam passes evenly across the entire cross-section. The steam rises from the bottom of the tower, bubbling through the small holes of each sieve plate. The countercurrent contact between gas and liquid, along with the high pH and high temperature conditions, causes free ammonia in the feed water to rapidly transfer from the liquid phase to the gas phase and be carried away by the steam stripping. The ammonia vapor carrying the free ammonia is then discharged from the top of the stripping tower.

[0088] After gas-liquid heat exchange, the effluent temperature of the stripping tower is reduced to 35℃~45℃. Ammonia nitrogen is largely transferred to the gas phase through steam stripping, thus reducing the ammonia nitrogen content of the effluent to 200mg / L~300mg / L. This effluent can then be directly sent to an organic waste system wastewater treatment plant for treatment, or the influent to the stripping tower can be preheated before being sent to such a plant. Because the ammonia nitrogen content in the stripping tower effluent has been reduced, the wastewater treatment plant no longer needs to bear the significant energy consumption associated with treating high ammonia nitrogen levels.

[0089] In steps (1-3-4), ammonia vapor carrying free ammonia is discharged from the top of the stripping tower and then condensed to obtain ammonia water. In some embodiments, the condensation of the ammonia vapor includes primary condensation and secondary condensation.

[0090] After primary condensation, using circulating cooling water as the refrigerant, the ammonia vapor temperature is reduced to 70℃~80℃. At this point, most of the water vapor condenses into liquid water. Additionally, a small amount of ammonia remains dissolved in the condensate, which can be returned to the stripping tower.

[0091] The ammonia vapor then undergoes a secondary condensation process, still using circulating cooling water as the refrigerant, to lower the steam temperature to 30℃~35℃. At this point, all the remaining water vapor condenses into liquid water, and the remaining ammonia is also basically dissolved in the liquid water, thus producing concentrated ammonia water with a concentration of 15%~20% and a temperature of 30℃~35℃.

[0092] In some embodiments, it is preferred that the non-condensable gas from step (1-3-4) is sent to the combustion chamber of step (1-2). The main component of the non-condensable gas is ammonia. When sent to the biogas combustion chamber, it can be used as a reducing agent in the SNCR denitrification process to control nitrogen oxides generated by high-temperature combustion and reduce nitrogen oxides to nitrogen.

[0093] Next, the carbonization of ammonia will be explained.

[0094] The concentrated ammonia obtained from the separation and treatment of the digestate, as well as the compressed tail gas after the treatment of the high-temperature flue gas, are both sent to step (1-4) to react the ammonia with carbon dioxide to synthesize ammonium bicarbonate. In step (1-4), the mass ratio of carbon dioxide to ammonia is 2.7 to 3.0:1, the reaction temperature inside the tower is controlled at 32℃ to 38℃, and the final pH is 8.2 to 8.8.

[0095] In this embodiment, the reactions in steps (1-4) can be carried out in a carbonization tower. The carbonization tower can be divided into an upper absorption section, a middle exothermic reaction section, and a lower crystallization section. Ammonia water enters from the top of the tower, and compressed tail gas containing carbon dioxide enters from the middle and lower parts of the tower. Multiple horizontal cooling water tanks are arranged in sections inside the carbonization tower, and the temperature is controlled by circulating cooling water.

[0096] The slurry discharged from the bottom of the carbonization tower contains synthesized ammonium bicarbonate crystals. After centrifugation and dehydration, this slurry yields wet ammonium bicarbonate with a moisture content of 8-10%. This wet ammonium bicarbonate can be sent to a metathesis reactor to react with purified salt to obtain sodium bicarbonate slurry. Alternatively, in some embodiments, a portion of the wet ammonium bicarbonate can be naturally dried to reduce the moisture content to below 5%, thereby obtaining agricultural-grade ammonium bicarbonate.

[0097] The exhaust gas discharged from the carbonization tower after ammonia carbonization contains carbon dioxide, ammonia, oxygen, nitrogen, nitrogen oxides, etc. It can be purified by a low-temperature plasma treatment device before being discharged and then discharged centrally through the exhaust stack to avoid adverse environmental impact.

[0098] (II) Waste Salt Treatment Steps

[0099] Waste salt, such as industrial waste (miscellaneous) salt and other solid wastes, can come from industrial production or zero-discharge wastewater treatment. Its main components are sodium chloride or sodium sulfate, and it also contains a small amount of organic or inorganic impurities.

[0100] First, the waste salt needs to be purified. In step (2-1), purified salt is obtained by pyrolyzing the waste salt, which is a solid waste, and rinsing it with water.

[0101] Next, the pyrolysis of waste salt will be explained.

[0102] Waste salt is discharged quantitatively from the storage tank to the pyrolysis furnace. The high-temperature flue gas generated by biogas combustion in step (1-2) is sent to the pyrolysis furnace to indirectly heat the waste salt.

[0103] As an example, the pyrolysis furnace adopts a spiral design. The shell of the pyrolysis furnace is a hollow jacketed structure. As the spiral rotates, waste salt can be transported from the feed end at one end to the discharge end at the other. At the same time, high-temperature flue gas enters from the discharge end and exits from the feed end. The high-temperature flue gas indirectly heats the waste salt by conducting heat through the shell.

[0104] Preferably, the heating temperature is controlled at 600℃~700℃. Within this temperature range, it is below the melting point of salt, but it allows all organic impurities in the waste salt to volatilize. The volatilized pyrolysis gas exits from the feed end, and its main components are volatile organic compounds and water vapor. This pyrolysis gas can be sent into the combustion chamber of step (1-2) for complete treatment through high-temperature oxidation and decomposition.

[0105] Organic impurities can be removed by pyrolysis of waste salt. The waste salt discharged from the pyrolysis furnace is cooled to 40℃~50℃ by a cooler and then rinsed with water.

[0106] Next, the water rinsing of waste salt will be explained.

[0107] The rinsing of waste salt can be carried out in a rinsing contact tank equipped with a stirring function. As an example, the salt and rinsing water come into contact in the same direction of flow. Minor components in the waste salt can be transferred to the rinsing solution, while the major components remain. By reducing the content of minor components and other impurities in the waste salt, the purity of the major components can be increased, thereby ensuring the purity of the metathesis reaction product.

[0108] In some implementations, the brine slurry after water rinsing can be separated into rinse water and purified salt by a hydrocyclone separator.

[0109] The rinse water is recycled back to the rinse contact tank until the concentration reaches the limit, at which point it is discharged to the organic waste system wastewater treatment plant. Alternatively, the rinse water can come from the drainage of the downstream circulating spray cooling tower of the flue gas waste heat boiler after biogas combustion, i.e., the condensate formed by the condensation of water vapor in the high-temperature flue gas is used for water rinsing in step (2-1).

[0110] The salt slurry discharged from the bottom of the hydrocyclone separator is sent to the metathesis reactor for reaction.

[0111] Next, we will explain the double displacement reaction.

[0112] In step (2-2), the purified salt obtained in step (2-1) undergoes a metathesis reaction with the ammonium bicarbonate obtained in step (1-4) to obtain sodium bicarbonate slurry.

[0113] The conditions of a metathesis reactor can be set according to the main components of the salt.

[0114] For example, when the main component of the salt is sodium sulfate, the concentration of sodium sulfate is controlled at approximately 290 g / L to 340 g / L, the mass ratio of sodium sulfate to ammonium bicarbonate is approximately 1:1.05 to 1.25, and the reaction temperature is 35℃ to 40℃. The reaction equation is as follows: Na₂SO₄ + 2NH₄HCO₃ = 2NaHCO₃ + (NH₄)₂SO₄.

[0115] In addition, when the main component of the salt is sodium chloride, the concentration of sodium chloride is controlled at approximately 280 g / L to 330 g / L, the mass ratio of sodium chloride to ammonium bicarbonate is approximately 1:1.4 to 1.5, and the reaction temperature is 20℃ to 30℃. The reaction equation is as follows: NaCl + NH4HCO3 = NaHCO3 + NH4Cl.

[0116] As an example, the metathesis reactor can adopt the DTB crystallizer type, which integrates metathesis reaction and physical crystallization in one device. The residence time is set to 120-240 minutes. The crystal growth rate is optimized by controlling local supersaturation through segmented multi-point feeding. The fine crystals are eliminated by external circulation heating and melting through fine crystal overflow. This produces high-quality sodium bicarbonate crystals.

[0117] Sodium bicarbonate, in crystalline slurry form, is discharged from the bottom of the metathesis reactor and then concentrated in a thickener. The separated clarified liquid can be sent to the metathesis reactor for further reaction. The concentrated slurry can then be centrifuged to remove water. As an example, the centrifuge can be a piston pusher type or a siphon scraper type, with an outlet moisture content of 8%–12%.

[0118] Sodium bicarbonate slurry is centrifuged and dehydrated to obtain wet sodium bicarbonate. This wet sodium bicarbonate can be fed into a steam-tube rotary dryer for low-temperature drying to produce solid sodium bicarbonate. In some embodiments, low-pressure saturated steam generated by a waste heat boiler is used for drying the sodium bicarbonate slurry. As an example, in the dryer, the bed temperature does not exceed 50°C, the drying time is 90–150 minutes, and the output moisture content is not higher than 0.2%, reaching the Class III grade of industrial sodium bicarbonate.

[0119] In addition, the exhaust gas emitted by the rotary dryer contains water vapor, carbon dioxide, ammonia, etc., which can be combined with the exhaust gas from the carbonization tower and then purified by a low-temperature plasma purification device before being centrally discharged through the exhaust stack to avoid adverse environmental impacts.

[0120] In some embodiments, the mother liquor from the ammonium bicarbonate produced by the carbonation tower, obtained through centrifugal dehydration, and the mother liquor from the sodium bicarbonate slurry produced by metathesis, obtained through centrifugal dehydration, can be combined and fed into a high-efficiency sedimentation tank. Lime is added to the high-efficiency sedimentation tank to precipitate carbonates and sulfates, and the pH is adjusted to above 10.5. The precipitated calcium carbonate and calcium sulfate can be dehydrated using a plate and frame dehydrator to produce solid byproducts that can be comprehensively utilized. Furthermore, the mother liquor from the high-efficiency sedimentation tank and the mother liquor after plate and frame dehydration still contain free ammonia, which can then be sent to a stripping tower as feed water for the preparation of ammonia water.

[0121] [Example]

[0122] An embodiment of the present invention is a waste disposal center that receives approximately 200 tons of kitchen waste and approximately 30 tons of industrial waste salt, mainly sodium sulfate, daily.

[0123] The food waste has a moisture content of 88%, an organic matter content of 85%–90%, and a nitrogen content of 5%–6% in the organic matter. After anaerobic digestion, the organic matter decomposition rate is 75%–80%, and the daily biogas production is approximately 16,000 m³. 3 The biogas contains 60% methane and 40% carbon dioxide.

[0124] After desulfurization treatment, the H2S content of biogas is reduced to below 20 ppm, ensuring that the sulfur dioxide concentration in the flue gas after combustion is below the emission limit. (Daily production: 16,000 m³) 3 After the biogas is burned, methane is converted into carbon dioxide. Together with the original carbon dioxide, a total of about 31 tons of carbon dioxide is contained in the high-temperature flue gas after combustion.

[0125] High-temperature flue gas is fed into a pyrolysis furnace for indirect heating of waste salt. The temperature of the flue gas exiting the pyrolysis furnace drops to 750℃–850℃, and after waste heat recovery in a waste heat boiler, it generates over 100 tons of 0.4MPa saturated steam for subsequent processes. After heat recovery in the waste heat boiler, the temperature of the high-temperature flue gas drops to 150℃–200℃, and then it undergoes circulating spray cooling, reducing the temperature to 35℃–40℃. Most of the water vapor in the flue gas condenses into condensate, which is used as rinsing water for the waste salt. The cooled flue gas is then sent to a carbonation tower to produce ammonium bicarbonate as a carbon dioxide supply source.

[0126] After dehydration, the digestive liquid produced by anaerobic digestion yields approximately 30 tons of biogas residue and 170 tons of biogas slurry per day. The ammonia nitrogen content in the biogas slurry is approximately 5000 mg / L, which means that 0.85 tons of ammonia nitrogen are produced daily.

[0127] Lime is added to the biogas slurry to adjust the pH to above 10.5, so that the ammonium ions in the liquid phase are converted into free ammonia. Then, through precipitation, suspended solids and precipitates are separated, and the effluent containing free ammonia is sent to the stripping tower.

[0128] The influent and effluent of the stripping tower undergo heat exchange to raise the influent temperature to 60℃~70℃ before stripping. The stripping tower uses saturated steam with a pressure not exceeding 0.4MPa as the carrier gas to strip free ammonia from the influent. The tower temperature reaches 85℃~95℃, transferring free ammonia from the liquid phase to the gas phase, achieving an ammonia stripping rate of over 98%. The mixture of ammonia and steam is discharged from the top of the stripping tower, first undergoing a primary condenser to rapidly condense most of the water vapor and a small portion of the ammonia, and then a secondary condenser to condense the remaining water vapor and ammonia. The primary condensate is returned to the stripping tower as its influent, while the secondary condensate becomes a 15%~20% ammonia solution stored in a tank for later use.

[0129] Ammonia water is fed from the ammonia storage tank into the carbonization tower. Simultaneously, the carbonization tower receives the carbon dioxide-containing tail gas emitted after biogas combustion. Inside the carbonization tower, the mass ratio of carbon dioxide to ammonia is 2.7–3.0:1, the reaction temperature is controlled at 32℃–38℃, and the final pH is 8.2–8.8. Inside the carbonization tower, ammonia reacts with carbon dioxide to produce ammonium bicarbonate.

[0130] The volumetric content of carbon dioxide in the exhaust gas from the carbonization tower is less than 20%, therefore the carbon dioxide conversion rate is approximately 70%, meaning about 20 tons of carbon dioxide are converted, producing approximately 36 tons of agricultural-grade ammonium bicarbonate. In subsequent processing, no more than 4.8 tons of ammonium bicarbonate are recovered as ammonium fertilizer, and approximately 31.2 tons of ammonium bicarbonate undergo a metathesis reaction with salt to produce sodium bicarbonate.

[0131] Industrial waste salt is stored in waste salt storage tanks with a water content of 8%. The main component on a dry basis is sodium sulfate, with a content of about 94%, and the minor component is sodium chloride, with a content of about 4%. The content of organic impurities is about 1%, and the content of other inorganic impurities is about 1%.

[0132] To utilize waste salt as a resource, a two-stage purification process is employed. First, the waste salt is fed into an oxygen-free pyrolysis furnace, using the high-temperature flue gas from biogas combustion as the heating medium. Indirect heating is performed in an indirect pyrolysis furnace, reaching a temperature of 700℃. At this temperature, the salt will not melt, but all organic impurities and moisture will volatilize into the gas phase. The volatilized gaseous organic matter and water vapor are then sent to the biogas combustion chamber. The organic impurities, being combustible, undergo complete high-temperature oxidation and decomposition within the combustion chamber, while the water vapor remains in the high-temperature flue gas. The waste salt, after removing organic impurities, is first cooled to 30℃–35℃ and then rinsed with fresh water at a ratio of 1.2–1.5:1. After rinsing, the dry basis sodium chloride content decreases to approximately 0.2%–0.3%, while the content of the main component, sodium sulfate, increases to 99.5%.

[0133] The purified waste salt, containing approximately 26 tons of sodium sulfate, was fed into a metathesis reactor to react with ammonium bicarbonate. The concentration of sodium sulfate was controlled at approximately 330 g / L, and the mass ratio of sodium sulfate to ammonium bicarbonate was approximately 1:1.1. The metathesis reactor used a DTB crystallizer type, with the temperature controlled between 35℃ and 40℃. The generated sodium bicarbonate precipitated out in crystalline particle form, ultimately producing approximately 30.7 tons of sodium bicarbonate and approximately 1.4 tons of unreacted ammonium bicarbonate. The sodium bicarbonate slurry discharged from the metathesis reaction was centrifuged to reduce the water content to approximately 10%, and then dried in a steam pipe rotary dryer to a water content below 0.2%. The product met the industrial sodium bicarbonate Class III standard.

[0134] Approximately 24.2 tons of ammonium sulfate are produced daily through the metathesis reaction. About 1.4 tons of unreacted ammonium bicarbonate are retained in the mother liquor from the centrifugation and dehydration of sodium bicarbonate. When lime is added to adjust the pH, approximately 24.9 tons of calcium sulfate, 3 tons of calcium carbonate, and 5.4 tons of ammonia nitrogen are produced. After precipitation, the ammonia nitrogen remains in the liquid phase and is sent to a stripping tower for recycling.

[0135] The liquid fed into the stripping tower includes effluent from anaerobic digester treatment of organic waste, containing approximately 1.0 ton of ammonia, and effluent from the combined treatment of mother liquor from ammonium bicarbonate and sodium bicarbonate centrifugation dehydration, containing approximately 10.5 tonnes of ammonia. After passing through the stripping tower, approximately 11 tonnes of ammonia are converted into ammonia water with a concentration of 15%–20%. Steam stripping is used, with all steam derived from the high-temperature flue gas waste heat boiler after biogas combustion, eliminating the need for external steam supplementation. The steam-to-influent ratio is 0.18–0.25 steam / kg influent. The ammonia nitrogen content of the effluent is reduced to 200 mg / L–300 mg / L and discharged to the wastewater treatment plant of the organic waste treatment step, where it can be treated using traditional biological methods.

[0136] By coupling the anaerobic digestion process for food waste with the resource recovery process for waste salt, a total of 4.8 tons of ammonium bicarbonate and 30.7 tons of sodium bicarbonate are recovered daily, and a total of 19 tons of carbon dioxide are captured. Therefore, a maximum of approximately 11,000 tons of sodium bicarbonate, approximately 1,700 tons of ammonium bicarbonate, and approximately 6,800 tons of carbon dioxide can be captured annually, demonstrating a significant carbon capture effect.

[0137] In addition, although a small amount of ammonia is lost in the salt resource recovery process, this can be balanced by reducing the amount of ammonium bicarbonate recovered, so there is no need to supplement the nitrogen source from outside the treatment system.

[0138] In addition, the combustible gas volatilized from the pyrolysis furnace undergoes high-temperature oxidation and decomposition in the biogas combustion chamber. During the condensation process after the ammonia stripping tower, a small amount of ammonia is sent to the biogas combustion chamber as non-condensable vapor to reduce nitrogen oxides produced during combustion, ensuring that nitrogen oxide emissions from biogas combustion meet standards. After waste heat recovery and ammonia-water carbonization, the flue gas from biogas combustion is combined with the exhaust gas from the sodium bicarbonate dryer. The mixed gas contains ammonia, carbon dioxide, nitrogen, nitrogen oxides, oxygen, and water, which is then treated by a low-temperature plasma purification unit. The main components are carbon dioxide, nitrogen, oxygen, and water, ensuring safe and environmentally friendly emissions. Through these methods, all process tail gases within the entire treatment system can be safely and environmentally treated without the need for additional chemical additives.

Claims

1. A salt resource utilization process, comprising an organic waste treatment step and a waste salt treatment step, The organic waste treatment process includes the following steps: Step (1-1) involves anaerobic digestion of organic waste to obtain biogas and digestate. Steps (1-2) involve burning biogas in a combustion chamber to obtain high-temperature flue gas containing carbon dioxide. Steps (1-3) involve separating the digestive fluid to obtain ammonia water; In steps (1-4), the carbon dioxide in the high-temperature flue gas obtained in step (1-2) reacts with the ammonia water obtained in step (1-3) to produce ammonium bicarbonate. The waste salt treatment process includes the following steps: Step (2-1) involves pyrolyzing and rinsing the waste salt, which is a solid waste, to obtain purified salt. In steps (2-2), the purified salt obtained in step (2-1) undergoes a metathesis reaction with the ammonium bicarbonate obtained in step (1-4) to obtain sodium bicarbonate slurry. Step (2-3): The sodium bicarbonate slurry obtained in step (2-2) is centrifuged, dehydrated, and dried.

2. The salt resource utilization process according to claim 1, wherein, The separation and treatment of the digestive fluid in steps (1-3) further includes the following steps: Step (1-3-1): The digestion liquid is separated into biogas residue and biogas slurry by a centrifuge, and the biogas slurry is stored in a biogas slurry storage tank; Step (1-3-2): Add lime to the biogas slurry and remove the resulting precipitate; Step (1-3-3): The effluent after removing the precipitate in step (1-3-2) is sent to a stripping tower to obtain ammonia vapor; Steps (1-3-4) involve condensing ammonia vapor to obtain ammonia water.

3. The salt resource utilization process according to claim 2, wherein, In step (1-3-2), the pH is adjusted to above 10.5 by adding lime to the biogas slurry.

4. The salt resource utilization process according to claim 2, wherein, In step (1-3-3), the carrier gas of the stripping tower is low-pressure saturated steam generated in the waste heat boiler.

5. The salt resource utilization process according to claim 2, wherein, The inlet and outlet water of the stripping tower undergo heat exchange outside the stripping tower to preheat the inlet water.

6. The salt resource utilization process according to claim 2, wherein, In step (1-3-4), the condensation of ammonia vapor includes primary condensation and secondary condensation. After primary condensation, the temperature of the ammonia vapor is reduced to 70℃~80℃, and the condensate is sent back to the stripping tower. After two-stage condensation, the temperature of the ammonia vapor is further reduced to 30℃~35℃.

7. The salt resource utilization process according to any one of claims 2 to 6, wherein, Steps (1-3-4) yield ammonia water with a concentration of 15% to 20% at a temperature of 30℃ to 35℃.

8. The salt resource utilization process according to any one of claims 2 to 6, wherein, The non-condensable gas in step (1-3-4) is sent to the combustion chamber in step (1-2).

9. The salt resource utilization process according to claim 1, wherein, The high-temperature flue gas generated in step (1-2) further undergoes the following treatment steps: Step (1-2-1): The high-temperature flue gas discharged from the combustion chamber is sent into the pyrolysis furnace for pyrolysis in step (2-1) to indirectly heat the waste salt; Step (1-2-2): After the high-temperature flue gas leaves the pyrolysis furnace, it enters the waste heat boiler for waste heat recovery to generate low-pressure saturated steam. Steps (1-2-3): After the high-temperature flue gas leaves the waste heat boiler, it enters the circulating spray cooling tower. In step (1-2-4), the gas discharged from the circulating spray cooling tower is pressurized and sent to step (1-4) to react with ammonia water to obtain ammonium bicarbonate.

10. The salt resource utilization process according to claim 1 or 9, characterized in that, In steps (1-4), the mass ratio of carbon dioxide to ammonia is 2.7 to 3.0:1, the reaction temperature is controlled at 32℃ to 38℃, and the final pH is 8.2 to 8.

8.

11. The salt resource utilization process according to claim 9, characterized in that, The heating temperature in the pyrolysis furnace is controlled at 600℃~700℃.

12. The salt resource utilization process according to claim 9, characterized in that, The low-pressure saturated steam generated by the waste heat boiler in step (1-2-2) is also used for drying the sodium bicarbonate slurry in step (2-3).

13. The salt resource utilization process according to claim 9, characterized in that, The condensate formed by the condensation of water vapor in the high-temperature flue gas is used as the spray liquid of the circulating spray cooling tower. After being cooled by the circulating cooling water through an external heat exchanger, it is recycled.

14. The salt resource utilization process according to claim 9, characterized in that, The condensate formed by the condensation of water vapor in the high-temperature flue gas is also used for water rinsing in step (2-1).

15. The salt resource utilization process according to claim 1, wherein, The pyrolysis gas generated from the waste salt pyrolysis in step (2-1) is sent to the combustion chamber in step (1-2) for combustion.

16. The salt resource utilization process according to claim 1, characterized in that, In step (2-1), after rinsing with water, the salt is separated into rinse water and purified salt by a hydrocyclone separator. The rinse water is then recycled back to the rinse contact tank used for rinsing.

17. The salt resource utilization process according to claim 1, characterized in that, In the metathesis reaction in step (2-2), when the main component of the purified salt is sodium sulfate, the concentration of sodium sulfate is controlled at 290-340 g / L, the mass ratio of sodium sulfate to ammonium bicarbonate is about 1:1.05-1.25, and the reaction temperature is 35℃-40℃.

18. The salt resource utilization process according to claim 1, characterized in that, In the metathesis reaction in step (2-2), when the main component of the purified salt is sodium chloride, the concentration of sodium chloride is controlled at about 280 to 330 g / L, the mass ratio of sodium chloride to ammonium bicarbonate is about 1:1.4 to 1.5, and the reaction temperature is 20℃ to 30℃.

19. The salt resource utilization process according to claim 1 or 9, characterized in that, At least a portion of the ammonium bicarbonate obtained in steps (1-4) is recovered after centrifugation, dehydration, and natural drying.

20. The salt resource utilization process according to claim 1 or 9, characterized in that, The waste gas from the reaction of carbon dioxide and ammonia in step (1-4) and the waste gas from the drying of sodium bicarbonate in step (2-3) are combined, purified by a low-temperature plasma purification device, and then discharged in a centralized manner.

21. The salt resource utilization process according to any one of claims 2 to 6, characterized in that, The mother liquor obtained by centrifugation and dehydration of ammonium bicarbonate in step (1-4) and the mother liquor obtained by centrifugation, dehydration and drying of sodium bicarbonate in step (2-3) are combined and discharged into a high-efficiency sedimentation tank. Lime is added to generate precipitate, and the precipitate is dehydrated. The high-efficiency precipitate and the dehydrated mother liquor are sent to a stripping tower.

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