Aluminum electrolysis flue gas treatment and recovery method based on microalgae staged culture

Abstract

The invention provides an aluminum electrolysis flue gas treatment and recovery method based on microalgae staged culture, and belongs to the field of biological purification and recycling of industrial flue gas. The method comprises the following steps: pretreating the aluminum electrolysis flue gas to obtain purified flue gas; introducing the purified flue gas into a photobioreactor inoculated with fluorine-resistant microalgae, and carrying out I-stage tolerance period culture to obtain an algae solution; transferring the algae liquid into a photobioreactor, adding CaCl2 into the algae liquid and limiting the supply of a nitrogen source, and introducing purified flue gas into a second photobioreactor to carry out II-stage accumulation period culture, so as to obtain algae slurry; carrying out solid-liquid separation on the algae slurry to obtain algae mud and algae-containing filtrate; the algae mud is subjected to wall breaking treatment and supercritical extraction, and algae oil and algae residues are obtained. According to the application, the pollutant tolerance mechanism and the product synthesis path of the microalgae are triggered by regulating and controlling environmental factors, so that efficient purification of flue gas, fixation of fluorine in the form of biominerals, step-by-step extraction of high-value products from algae and cyclic utilization of algae residues can be synchronously realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of industrial flue gas biological purification and resource utilization, and particularly relates to a method for treating and recycling aluminum electrolysis flue gas based on microalgae staged culture. BACKGROUND

[0002] The non-ferrous industry is the third largest carbon emission industry in China after the building materials and steel industries. According to statistics, the total carbon dioxide emissions of the non-ferrous industry in 2020 were about 660 million tons, accounting for 6.7% of the total emissions in China. In the process of carbon emissions of the non-ferrous industry, the carbon emissions of the aluminum smelting industry are about 500 million tons, accounting for 76% of the total emissions of the non-ferrous industry, and are the focus of the non-ferrous industry's "double carbon" work. The CO2 sources of China's aluminum industry are wide, including CO2 emissions from aluminum industry self-provided power plants, aluminum industry furnaces and electrolytic cells, etc. Among them, the annual emissions of electrolytic aluminum are nearly 100 million tons, accounting for about 70% to 75% of the amount of flue gas after dust removal, and have a great contribution value to CO2 emissions.

[0003] The aluminum electrolysis flue gas has a high temperature and contains high-concentration fluorides. The existing alkali liquid spraying method will produce fluorine-containing hazardous waste, which has a high disposal cost and cannot recycle CO2. The biological method is the most important and effective way of carbon sequestration on earth. Compared with other higher plants, microalgae have the advantages of high photosynthetic rate, fast reproduction, strong environmental adaptability and controllability, can realize the fixation and utilization of 1.83 times the weight of carbon, and the carbon sequestration efficiency is 10 to 50 times that of ordinary higher plants. The nutritional substances of microalgae are 99% from CO2, which can convert the fixed CO2 into organic carbon skeletons in cells to generate lipids, proteins and carbohydrates, etc. for producing biofuels, protein feed, health food and other products, and realizing the high-value utilization of CO2. However, there are the following bottlenecks in the treatment of fluorine-containing flue gas by microalgae: fluorides will destroy the chloroplasts of microalgae, interfere with enzyme activity and inhibit the growth of algae; traditional single-stage culture cannot balance the pollutant tolerance and biomass accumulation efficiency; and fluorine cannot be recycled. Therefore, how to simultaneously realize the efficient treatment and resource recycling of aluminum electrolysis flue gas based on microalgae is a technical problem that needs to be solved at present. SUMMARY

[0004] The present application provides a method for treating and recycling aluminum electrolysis flue gas based on microalgae staged culture, to solve the technical problem of how to simultaneously realize the efficient treatment and resource recycling of aluminum electrolysis flue gas based on microalgae.

[0005] The present application provides a method for treating and recycling aluminum electrolysis flue gas based on microalgae staged culture, which comprises the following steps: Pretreating the aluminum electrolysis flue gas to obtain purified flue gas; The purified flue gas is introduced into a first photobioreactor inoculated with fluorine-resistant microalgae to carry out a first-stage tolerance period culture under first conditions to obtain an algal liquid; the first conditions include a first set flue gas introduction amount, a first set temperature, a first set culture time, a first set light intensity, and a first set fluorine concentration; The algal liquid is transferred to a second photobioreactor, CaCl2 is added to the algal liquid, nitrogen source supply is limited, and the purified flue gas is introduced into the second photobioreactor to carry out a second-stage accumulation period culture under second conditions to obtain an algal slurry; the second conditions include a second set flue gas introduction amount, a second set temperature, a second set culture time, a second set light intensity, and a second set fluorine concentration; The algal slurry is subjected to solid-liquid separation to obtain algal mud and algal-containing filtrate; The algal mud is subjected to wall breaking treatment and supercritical extraction to obtain algal oil and algal residue; The first set flue gas introduction amount > the second set flue gas introduction amount, the first set temperature > the second set temperature, and the first set fluorine concentration > the second set fluorine concentration. The fluorine-resistant microalgae contain a fluorine ion efflux pump protein gene.

[0006] Optionally, the fluorine-resistant microalgae include one or more of Spirulina, fishy algae, Synechococcus, and Synechocystis.

[0007] Optionally, the first photobioreactor is a gas-lift photobioreactor or a bubble photobioreactor, and the culture solution in the first photobioreactor is Zarrouk or BG-11.

[0008] Optionally, the first set flue gas introduction amount is 60% to 100% of the total flow of the purified flue gas, the first set temperature is 30°C to 45°C, the first set culture time is 24h to 72h, the first set light intensity is 100μmol / m 2 / s to 200μmol / m 2 / s, and the first set fluorine concentration is 50mg / L to 200mg / L.

[0009] Optionally, the second photobioreactor is a flat-plate photobioreactor or a pipeline photobioreactor, and the culture solution in the second photobioreactor is Zarrouk or BG-11.

[0010] Optionally, the second set flue gas introduction amount is 30% to 50% of the total flow of the purified flue gas, the second set temperature is 25°C to 30°C, the second set culture time is 48h to 96h, the second set light intensity is 50μmol / m 2 / s to 250μmol / m2 s, the second set fluorine concentration < 10 mg / L.

[0011] Optionally, the molar amount of CaCl2 in the algal liquid is 0.5 mmol / L-1 mmol / L.

[0012] Optionally, the method further comprises: The algal residue is calcined at high temperature to obtain calcium fluoride and carbon-rich ash residue.

[0013] Optionally, the temperature of the high-temperature calcination is 600-1000℃, and the time is 1-4h.

[0014] Optionally, the method further comprises: The algal-containing filtrate is recycled for the phase I tolerance period culture; The algal oil is processed into high-value products; the high-value products include one or both of biodiesel and Omega-3.

[0015] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art: The embodiments of the present application provide a method for treating and recycling aluminum electrolysis flue gas based on microalgae phased culture, which provides a phased culture coupled with directional induction process in view of the characteristics of high temperature, high concentration of fluoride and strong corrosion of aluminum electrolysis flue gas, triggers the pollutant tolerance mechanism and product synthesis path of microalgae by regulating environmental factors. In phase I (tolerance period), by controlling the environmental factors such as flue gas input amount, fluorine concentration, temperature, light intensity, etc., the fluoride ion efflux pump protein gene of microalgae is activated, and the secretion of extracellular polysaccharide chelating fluoride ion is promoted, realizing the rapid fixation of CO2 and the removal of fluorine in aluminum electrolysis flue gas. In phase II (accumulation period), by reducing the flue gas input amount, reducing the culture temperature, limiting nitrogen nutrient stress and adding CaCl2, the up-regulation of oil / fat / crustacean pigment synthesis genes of microalgae is promoted, the alkaline microenvironment generated by the metabolism of microalgae is used to promote the combination of fluoride ions with calcium ions and phosphate, and the precipitation of fluoride in the form of biological minerals on the surface of algal cells is induced to reduce the intracellular toxicity. The harvested algal sludge can be further processed into biodiesel, Omega-3 and other high-value products, and the residual algal residue is calcined to recover calcium fluoride for processing back to the aluminum electrolysis cell, and the carbon-rich ash residue is processed into a flue gas adsorbent. In summary, the present application can achieve zero waste discharge while achieving both microalgae tolerance and product accumulation. BRIEF DESCRIPTION OF DRAWINGS

[0016] The accompanying drawings, which are incorporated into and form part of the specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the application.

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings required by the embodiments or prior art description will be briefly introduced as follows. Obviously, those skilled in the art can obtain other drawings according to these drawings without any creative effort.

[0018] Figure 1 A flowchart of a method for treating and recycling aluminum electrolysis flue gas based on microalgae staged culture is provided for the embodiments of the present application. DETAILED DESCRIPTION

[0019] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without any creative effort fall within the scope of protection of the present application.

[0020] The range descriptions described herein, such as numerical range, ratio range, etc., include all possible sub-ranges and single values within the range, for example, the range description of "1 to 6" or "1-6" covers all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "include", "contain" and the like used herein mean "include but not limited to"; the relationship terms "first", "second" and the like are only used to distinguish different entities or operations, and do not imply actual sequence or relationship; "and / or" means that multiple cases can exist independently or simultaneously; "at least one", "multiple", "at least one" and the like refer to any combination of the corresponding objects, including single or multiple combinations of objects. The ratio relationship described herein, such as mass ratio, molar ratio, etc., should be understood as the corresponding relationship between the front and the rear in the ratio. The raw materials, reagents, instruments and equipment used herein can be purchased or prepared by existing methods.

[0021] Figure 1 A flowchart of a method for treating and recycling aluminum electrolysis flue gas based on microalgae staged culture is provided for the embodiments of the present application.

[0022] As Figure 1 shown, the embodiments of the present application provide a method for treating and recycling aluminum electrolysis flue gas based on microalgae staged culture, which comprises: S1, pretreating aluminum electrolysis flue gas to obtain purified flue gas; S2, introducing the purified flue gas into a first photobioreactor inoculated with the fluoride-tolerant microalgae to perform a phase I tolerance period culture under first conditions to obtain an algal liquid; the first conditions include a first set flue gas introduction amount, a first set temperature, a first set culture time, a first set light intensity, and a first set fluoride concentration; S3, transferring the algal liquid to a second photobioreactor, adding CaCl2 to the algal liquid and limiting nitrogen source supply, and introducing the purified flue gas into the second photobioreactor to perform a phase II accumulation period culture under second conditions to obtain an algal slurry; the second conditions include a second set flue gas introduction amount, a second set temperature, a second set culture time, a second set light intensity, and a second set fluoride concentration; S4, performing solid-liquid separation on the algal slurry to obtain algal mud and algal-containing filtrate; S5, performing cell wall breaking treatment and supercritical extraction on the algal mud to obtain algal oil and algal residue; The first set flue gas introduction amount > the second set flue gas introduction amount, the first set temperature > the second set temperature, and the first set fluoride concentration > the second set fluoride concentration. The fluoride-tolerant microalgae contains a fluoride ion efflux pump protein gene.

[0023] It should be noted that the aluminum electrolysis flue gas has the characteristics of high temperature, high fluoride concentration, and strong corrosion. Direct introduction into the photobioreactor will damage the cell structure of the fluoride-tolerant microalgae, inhibit its metabolic activity, and even cause the death of the microalgae. At the same time, impurities such as dust in the flue gas will also affect the mass transfer efficiency of the reactor and the subsequent culture stability. Therefore, the pretreatment in step S1 is the basic guarantee of the entire process. By using a step-by-step cooling device to reduce the temperature of the flue gas to approach the suitable temperature range for subsequent culture, the thermal damage of high temperature to the microalgae is avoided. By using a dust removal device to remove solid impurities such as dust, the wear and tear of the internal components of the reactor and the interference with the growth environment of the microalgae are reduced. By using a fluoride adsorption device and a desulfurization device, the concentration of fluorides and sulfides in the flue gas is initially reduced, and the initial impact of high-concentration pollutants on the microalgae is alleviated. The purified flue gas obtained finally can meet the requirements of the subsequent phased culture on the inlet conditions, and lays a foundation for the stable growth of the microalgae and the treatment of pollutants.

[0024] S2 step is the first stage of tolerance period culture, the core goal of this stage is to let the fluoride-resistant microalgae (Spirulina, fishy algae, etc. containing fluoride ion efflux pump protein gene microalgae) quickly adapt to the complex environment of aluminum electrolysis flue gas, activate its anti-fluorine mechanism, and initially achieve the removal of pollutants in the flue gas. From the reactor selection, the gas lift or bubble type photobioreactor has the characteristics of high mass transfer efficiency and low energy consumption, which can uniformly disperse the purified flue gas into the culture medium, ensuring the sufficient contact of microalgae with CO2, fluoride and other substances in the flue gas, and the Zarrouk or BG-11 medium provides the basic nutrients such as nitrogen, phosphorus and potassium required for the growth of microalgae, meeting their initial metabolic needs.

[0025] S3 step is the second stage of accumulation period culture, after the first stage of tolerance period, the microalgae has certain anti-fluorine ability, the core goal of the second stage is to further purify the flue gas while promoting the accumulation of high-value products (oil, astaxanthin, etc.) in the body of microalgae, and to realize the stable fixation of fluorine. The flat or pipeline photobioreactor is selected, which has the advantage of high light utilization efficiency, can provide better light conditions for the photosynthesis of microalgae to synthesize high-value products, and the structural design is beneficial to the precise regulation of culture environment parameters, which is suitable for the fine requirements of the accumulation period for environmental conditions, and the culture medium still uses Zarrouk or BG-11, which ensures the basic nutrition supply of microalgae, and guides the metabolic direction by regulating the nutritional ingredients (such as limiting nitrogen source).

[0026] S4 step is the solid-liquid separation stage, after the second stage of culture, the algae slurry contains a large amount of algal cells that have accumulated high-value products and fixed fluorine, as well as unused culture medium ingredients, a small amount of free fluoride ions, etc. Through centrifuge, filter and other equipment, the obtained algae mud is the core raw material for subsequent extraction of high-value products and recovery of fluorine, while the algae-containing filtrate still contains part of the microalgae cells, nutrients and metabolites, which can be recycled for the first stage of culture, realizing efficient use of resources, reducing the consumption cost of culture medium, and reducing wastewater discharge, which meets the process goal of "zero waste discharge". This step is the key link between the culture process and the product recovery process, and its separation efficiency directly affects the efficiency of subsequent algae mud treatment and the effect of filtrate recycling.

[0027] S5 step is the algae mud treatment stage, the microalgae cells in the algae mud have a tough cell wall, and the efficiency of directly extracting internal algal oil and other products is very low, so it needs to be broken first. Water bath heating, ultrasonic assisted breaking, etc. can destroy the cell wall structure of microalgae through physical or chemical action, so that the algal oil and other substances in the cells are released, creating conditions for subsequent extraction. Different breaking methods are suitable for different scale production needs, for example, ultrasonic assisted breaking is suitable for laboratory or small scale production, and high pressure homogenization breaking is more suitable for large scale industrial production.

[0028] The supercritical extraction technology in step S5 uses supercritical CO2 as an extractant, has the advantages of high extraction efficiency, no solvent residue, environmental friendliness, etc., can efficiently separate algal oil from the algal slurry, and can maximize the retention of active ingredients (such as EPA and DHA in Omega-3) in the algal oil, thereby ensuring the quality of high-value products in subsequent processing. The algal residue obtained by separation has removed most of the algal oil, but still contains fixed fluorides, residual cell wall components and carbon elements, which are important raw materials for subsequent calcination and recovery of fluorine and preparation of adsorbents.

[0029] In some embodiments, the pretreatment system is a one-stage or multi-stage treatment device, including but not limited to a cascade cooling device, a dust removal device, a fluoride adsorption device, and a desulfurization device.

[0030] In some embodiments, the fluoride-resistant microalgae include one or more of Spirulina, Anabaena, Synechococcus, and Synechocystis.

[0031] It should be noted that the fluoride ion efflux pump protein gene (cyanobacterial resistance to fluoride protein B, referred to as crcB gene) is a functional gene widely existing in microorganisms (especially prokaryotes such as blue algae), which encodes fluoride ion efflux pump protein, and helps organisms resist fluoride stress environment through active transport mechanism to expel excess fluoride ions in cells to the outside.

[0032] The protein encoded by the fluoride ion efflux pump protein gene crcB can allow fluoride ions to pass through the cell membrane and participate in the fluoride ion efflux process of microorganisms, helping microorganisms adapt to fluoride-containing environments. Blue algae such as Spirulina, Anabaena, Synechococcus, and Synechocystis gradually formed a fluoride resistance mechanism including the crcB gene during long-term evolution to cope with fluoride stress in the environment. The protein expressed by the gene expels fluoride ions in the cells to the outside, thereby reducing the toxic effects of fluoride ions on cells.

[0033] In some embodiments, the first photobioreactor is an air-lift photobioreactor or a bubble column photobioreactor, and the culture solution in the first photobioreactor is Zarrouk or BG-11.

[0034] It should be noted that the Zarrouk medium is a synthetic culture medium developed by a French scientist in 1966 for Spirulina, and its core feature is to contain a high concentration of sodium bicarbonate (usually 16.8 g / L), which not only provides sufficient carbon source for Spirulina to support efficient photosynthesis, but also stabilizes the pH of the culture system in the weak alkaline range of 8.5-10.0, which is suitable for the growth habit of Spirulina and inhibits the competition of miscellaneous bacteria. The proportion of nutrients such as sodium nitrate and potassium dihydrogen phosphate in its formula is optimized to maximize the promotion of Spirulina biomass accumulation, so it is widely used in laboratory research and large-scale cultivation of Spirulina. In the aluminum electrolysis flue gas treatment process, if Spirulina is selected as a fluoride-tolerant microalgae, this medium can help it quickly adapt to the environment and improve the efficiency of pollutant treatment.

[0035] BG-11 medium is a general synthetic medium for blue-green algae developed by the U.S. Algal Culture Collection Center, and its formula has universality. Its carbon source mainly depends on the external CO2 (such as CO2 in aluminum electrolysis flue gas purification), and only a small amount of sodium bicarbonate is added to assist pH adjustment; At the same time, it contains basic nutrients such as sodium nitrate and potassium hydrogen phosphate, and also forms an "iron-chelating system" through citric acid and EDTA-2Na to avoid iron ion precipitation to ensure the synthesis of blue-green algae chlorophyll. This medium can stabilize the pH in the neutral to slightly alkaline range of 7.1-7.5, which is suitable for the growth of most freshwater blue-green algae. In the flue gas treatment process, it is suitable for non-Spirulina fluoride-tolerant microalgae such as Anabaena, Synechococcus, and Synechocystis, and is also commonly used for basic experiments such as blue-green algae physiological research and algae screening.

[0036] In some embodiments, the first set of flue gas input amount is 60%-100% of the total flow of the purified flue gas, the first set of temperature is 30°C-45°C, the first set of culture time is 24h-72h, the first set of light intensity is 100μmol / m 2 2 / s-200μmol / m

[0037] It should be noted that in the S2 step (I phase tolerance period culture), the amount of purified flue gas input into the first photobioreactor needs to be controlled within 60%-100% of the total flow of the purified flue gas generated in the S1 step (flue gas pretreatment). The present application does not make a clear limitation on the specific flow value of the flue gas in the first photobioreactor, and in actual application, the volume specification of the first photobioreactor can be adjusted accordingly according to the actual total amount of purified flue gas produced in the S1 step, so as to ensure that the flue gas input can be uniformly diffused inside the reactor and fully contact with the fluoride-tolerant microalgae in the culture system, thereby meeting the requirements of the I phase tolerance period culture for flue gas mass transfer efficiency and microalgae growth environment.

[0038] ​The first set flue gas input amount is limited to 60% to 100% of the total purified flue gas flow. A high input amount can quickly increase the CO2 concentration in the reactor, providing sufficient raw materials for microalgae photosynthesis, and allowing microalgae to gradually activate tolerance mechanisms in a higher pollutant concentration environment. For example, the first set flue gas input amount can be 60%, 65%, 70%, 75%, 80%, 85%, 90%, 100%, etc. of the total purified flue gas flow.

[0039] The first set temperature is limited to 30°C to 45°C. On the one hand, this temperature range is close to the temperature of the pretreated aluminum electrolysis flue gas, reducing the energy consumption of additional temperature control. On the other hand, this temperature range can promote the expression of microalgae crcB genes, enhance the activity of fluoride efflux pump proteins, and improve the microalgae's ability to expel fluorides. For example, the first set temperature can be 30°C, 33°C, 35°C, 37°C, 40°C, 42°C, 44°C, 45°C, etc.

[0040] The first set culture time is limited to 24h to 72h. This ensures that microalgae have enough time to adapt to the environment and accumulate a certain amount of biomass, while avoiding excessive enrichment of fluorides in the reactor, which can inhibit the growth of microalgae. For example, the first set culture time can be 24h, 32h, 40h, 48h, 56h, 64h, 70h, 72h, etc.

[0041] The first set light intensity is limited to 100μmol / m 2 / s to 200μmol / m 2 / s. This provides suitable energy for microalgae photosynthesis, promoting the synthesis of extracellular polysaccharides, which can fix fluoride ions in flue gas through chelation, achieving preliminary removal of fluorides. For example, the first set light intensity can be 100μmol / m 2 / s, 120μmol / m 2 / s, 140μmol / m 2 / s, 150μmol / m 2 / s, 160μmol / m 2 / s, 180μmol / m 2 / s, 190μmol / m 2 / s, 200μmol / m 2 / s, etc.

[0042] The first set fluorine concentration is limited to 50 mg / L-200 mg / L, which is within the tolerance range of fluorine-resistant microalgae and can effectively stimulate the activation of anti-fluorine metabolic pathways in microalgae, preparing for the subsequent accumulation period in low fluorine concentration. For example, the first set fluorine concentration can be 50 mg / L, 70 mg / L, 100 mg / L, 120 mg / L, 150 mg / L, 170 mg / L, 190 mg / L, 200 mg / L, etc.

[0043] In some embodiments, the second photobioreactor is a flat plate photobioreactor or a pipeline photobioreactor, and the culture solution in the second photobioreactor is Zarrouk or BG-11.

[0044] In some embodiments, the second set flue gas input amount is 30%-50% of the total flow of the purified flue gas, the second set temperature is 25°C-30°C, the second set culture time is 48h-96h, the second set light intensity is 50μmol / m 2 / s-250μmol / m 2 / s, and the second set fluorine concentration is <10 mg / L.

[0045] The second set flue gas input amount is limited to 30%-50% of the total flow of the purified flue gas. On the one hand, after the I stage treatment, the pollutant concentration in the flue gas has been reduced, and a high input amount is not needed to achieve further purification. On the other hand, a low input amount can reduce gas disturbance in the reactor, providing a relatively stable environment for microalgae to accumulate products, while reducing energy consumption. For example, the second set flue gas input amount can be 30%, 33%, 35%, 38%, 40%, 43%, 45%, 50% of the total flow of the purified flue gas, etc.

[0046] The second set temperature is limited to 25°C-30°C, which is lower than that in the I stage. This temperature range is more suitable for the synthesis and accumulation of products such as oil and astaxanthin in microalgae. High temperature will accelerate the respiration of microalgae and consume organic matter, which is not conducive to the retention of products. For example, the second set temperature can be 25°C, 26°C, 27°C, 28°C, 29°C, 29.5°C, 30°C, 30°C (precise temperature control), etc.

[0047] The second set culture time is limited to 48h-96h, which is longer than that in the I stage. Because product accumulation is a slow metabolic process, sufficient time can allow microalgae to continuously synthesize and store high-value products in a stable environment, while ensuring sufficient fixation of fluorine. For example, the second set culture time can be 48h, 56h, 64h, 72h, 80h, 88h, 92h, 96h, etc.

[0048] The second set light intensity is limited to 50μmol / m2 / s ~ 250 μmol / m 2 / s, the wider light range can adapt to the light requirements of different microalgae to synthesize products, for example, some microalgae are more likely to synthesize astaxanthin under higher light, and some microalgae have higher oil accumulation efficiency under medium light, and flexible light regulation can maximize product yield. For example, the second set light intensity can be 50 μmol / m 2 / s, 80 μmol / m 2 / s, 120 μmol / m 2 / s, 150 μmol / m 2 / s, 180 μmol / m 2 / s, 220 μmol / m 2 / s, 240 μmol / m 2 / s, 250 μmol / m 2 / s, etc.

[0049] The second set fluorine concentration is limited to be less than 10 mg / L, and a low fluorine concentration can reduce the metabolic burden of microalgae, avoid the inhibition of fluorine on the enzyme activity related to product synthesis, and create conditions for stable fluorine fixation.

[0050] In some embodiments, the molar amount of CaCl2 in the algal liquid is 0.5 mmol / L to 1 mmol / L.

[0051] 0.5 mmol / L to 1 mmol / L of CaCl2 is added to the algal liquid, Ca 2+ will combine with fluorine ions in the alkaline microenvironment produced by microalgae metabolism to form stable calcium fluoride precipitates attached to the surface of algal cells, realizing the biological mineral form fixation of fluorine, reducing the intracellular toxicity of fluorine to microalgae, and the concentration of CaCl2 can not only ensure efficient fixation of fluorine, but also will not cause osmotic pressure imbalance of microalgae cells due to excessive calcium ions. For example, the molar amount of CaCl2 in the algal liquid can be 0.5 mmol / L, 0.6 mmol / L, 0.7 mmol / L, 0.75 mmol / L, 0.8 mmol / L, 0.85 mmol / L, 0.9 mmol / L, 1 mmol / L, etc.

[0052] In some embodiments, the method further comprises: S6, high-temperature calcining the algal residue to obtain calcium fluoride and carbon-rich ash residue.

[0053] In some embodiments, the high-temperature calcining temperature is 600°C to 1000°C, and the time is 1 h to 4 h.

[0054] The algal residue is calcined at 600-1000°C for 1-4h. The organic components in the algal residue are fully decomposed in the high-temperature environment, while the fluoride (calcium fluoride) previously fixed in the form of biological minerals has high thermal stability and can be completely retained. Through subsequent processing, high-purity calcium fluoride can be obtained, which is processed into fluorine salt and then used in aluminum electrolysis cells, realizing the recycling of fluorine resources and reducing the raw material cost of the aluminum electrolysis industry. At the same time, the remaining carbon-rich ash after calcination has a large specific surface area and adsorption activity, which can be processed into flue gas adsorbent and used for the pretreatment or deep purification of aluminum electrolysis flue gas, further improving the flue gas treatment efficiency, forming a closed loop of "flue gas treatment-resource recovery-assisted flue gas treatment", and truly realizing zero waste discharge. For example, the high-temperature calcination temperature can be 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 950°C, 1000°C, etc., and the high-temperature calcination time can be 1h, 1.5h, 2h, 2.5h, 3h, 3.2h, 3.5h, 4h, etc.

[0055] In some embodiments, the method further comprises: circulating the algal-containing filtrate for the phase I tolerance period culture; processing the algal oil into high-value products; the high-value products include one or both of biodiesel and Omega-3.

[0056] The present application provides a staged culture coupled with a directed induction process. A microalgae naturally possessing a fluoride ion efflux pump protein gene is selected, and the pollutant tolerance mechanism and product synthesis pathway of the microalgae are triggered by regulating environmental factors, so that the following can be achieved simultaneously: ① efficient flue gas purification; ② fixation of fluoride in the form of biological minerals; ③ step-by-step extraction of high-value products from algal bodies; and ④ recycling of algal residue.

[0057] The aluminum electrolysis flue gas treatment and recovery method based on microalgae staged culture provided in the present application is aimed at the core characteristics of "high temperature, high fluoride, and strong corrosion" of aluminum electrolysis flue gas. Through innovative process design and precise parameter regulation, the bottleneck of traditional microalgae treatment of flue gas containing fluoride is broken, and a closed-loop solution with both environmental benefits and resource value is formed. The advantages of the present application are mainly reflected in the following aspects: First, the flue gas purification is efficient and has strong adaptability. The process is based on pretreatment, and through multi-stage treatment such as stepwise cooling, dust removal, and fluorine adsorption, the flue gas is adjusted to meet the adaptation conditions of microalgae cultivation, avoiding direct damage of high temperature and high concentration of fluorides to microalgae. In the subsequent phased cultivation, in the tolerance stage of phase I, the high mass transfer efficiency of the gas lifting / bubbling reactor is combined with 60%-100% high-flux flue gas input, 30°C-45°C temperature, and 50 mg / L-200 mg / L fluoride concentration to activate the expression of microalgae crcB gene and the secretion of extracellular polysaccharide, realizing rapid CO2 fixation and preliminary removal of fluorine. In the accumulation stage of phase II, the flat plate / tube reactor with higher light utilization efficiency is switched to, with 30%-50% low flue gas input, 25°C-30°C suitable temperature, and <10 mg / L low fluoride concentration, to further purify the flue gas. The whole process realizes efficient removal of aluminum electrolysis flue gas pollutants and meets the scale demand of industrial flue gas treatment.

[0058] Second, the microalgae tolerance and product accumulation are coordinated. Unlike the traditional single-stage cultivation which is difficult to balance the contradiction between “pollution resistance” and “product value”, the present application solves this problem through phased regulation: phase I focuses on “stress adaptation”, and the microalgae fluoride resistance mechanism is triggered by high fluoride and suitable temperature, so that it can survive stably and accumulate basic biomass in complex flue gas environment; phase II turns to “product synthesis”, and through nitrogen-limited nutrient stress, addition of 0.5 mmol / L-1 mmol / L CaCl2, and optimization of temperature and light conditions, the synthesis genes of high-value products such as microalgae oil and astaxanthin are up-regulated, and the alkaline microenvironment of microalgae metabolism is used to promote the combination of fluoride and calcium to form stable biological mineral precipitates, which not only reduces the intracellular toxicity of fluoride, but also creates a safe environment for product accumulation, finally realizing the dual goals of “microalgae survival” and “high-quality products”.

[0059] Third, resource full-chain recycling and recycling. The process establishes a complete conversion path from flue gas pollutants to high-value resources: the fluorine in the flue gas is converted into calcium fluoride that can be reused in aluminum electrolysis cells through biological mineral fixation and algal residue calcination at 600°C-1000°C, realizing fluoride resource closed loop; microalgae are broken and supercritical CO2 extraction is used to obtain algal oil, which can be processed into biodiesel, Omega-3, and other high-value products, improving economic benefits; the algal-containing filtrate produced by solid-liquid separation is recycled for phase I cultivation, reducing the consumption of culture medium; the remaining carbon-rich ash residue after calcination can be made into flue gas adsorbent and returned to the pretreatment link, forming a full-process cycle of “flue gas treatment-resource recovery-assisted treatment”, and truly achieving “zero waste discharge”, which meets the trend of green and low-carbon industrial development.

[0060] Fourth, the technology is highly practical and flexible. In terms of core material selection, the fluoride-resistant microalgae includes various cyanobacteria such as Spirulina and Anabaena, and can be used with two culture media: Zarrouk (suitable for Spirulina) and BG-11 (suitable for multiple types of cyanobacteria), allowing for flexible adjustments based on the specific algae species. The equipment and parameter design caters to different scenario requirements; for example, the cell disruption method can be either ultrasound-assisted (small-scale) or high-pressure homogenization (large-scale). Parameters such as temperature, light intensity, and flue gas flow rate at each stage are provided with reasonable ranges and specific values, facilitating personalized adaptation by enterprises based on flue gas composition and production capacity, thus reducing the technical barriers and debugging costs for industrial applications.

[0061] In summary, this application deeply integrates environmental governance with resource utilization, which not only efficiently solves the problem of aluminum electrolysis flue gas pollution, but also realizes economic benefits through the high-value utilization of microalgae and the resource utilization of pollutants, providing an innovative technical path for industrial flue gas treatment that achieves "environmental compliance, resource value-added, and circular sustainability".

[0062] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0063] Example 1 S1 (Flue Gas Pretreatment): The aluminum electrolysis flue gas of a certain enterprise is successively treated by stepped cooling, electrostatic dust removal, and fluorine adsorption by activated Al2O3. The flue gas temperature is reduced to 40℃, and purified flue gas is obtained.

[0064] S2 (Stage I tolerance phase culture): The purified flue gas is passed through the inoculated Spirulina ( Spirulina platensis The first photobioreactor (bubble-type photobioreactor), FACHB-834, was purchased from the Institute of Hydrobiology, Chinese Academy of Sciences. The culture medium in the reactor was Zarrouk medium. The initial conditions were set as follows: initial flue gas flow rate (80% of the total purified flue gas flow rate), initial temperature (40℃, depending on the purified flue gas temperature), initial incubation time (72 h), and initial light intensity (100 μmol / m²). 2 / s), the first set fluoride concentration (50mg / L), and the algal solution was obtained after cultivation.

[0065] S3 (Phase II accumulation period culture): the algae liquid is transferred to a second photobioreactor (flat plate photobioreactor), CaCl2 is added to the algae liquid (addition amount 0.5 mmol / L) and the supply of nitrogen source is limited, and the purified flue gas is introduced into the second photobioreactor. The second condition is set as: the second set flue gas introduction amount (30% of the total flow of the purified flue gas), the second set temperature (30°C), the second set culture time (48 h), the second set light intensity (250 μmol / m 2 / s), and the second set fluorine concentration (8 mg / L). After culture, algae slurry is obtained.

[0066] S4 (solid-liquid separation): the algae slurry is subjected to solid-liquid separation by centrifugation to obtain algae mud and algae-containing filtrate.

[0067] S5 (wall breaking treatment and supercritical extraction): the algae mud is subjected to wall breaking treatment in an ultrasonic assisted manner, followed by supercritical CO2 extraction to obtain algae oil and algae residue.

[0068] S6 (algae residue treatment): the algae residue is calcined at 600°C for 4 h to obtain calcium fluoride.

[0069] Example 2 S1 (flue gas pretreatment): the flue gas of a certain enterprise is sequentially subjected to stepwise temperature reduction, electrostatic precipitation, and active Al2O3 adsorption fluorine treatment, and the flue gas temperature is reduced to 40°C to obtain purified flue gas.

[0070] S2 (phase I tolerance period culture): the above purified flue gas is introduced into a first photobioreactor (air-lift photobioreactor) inoculated with a small, long, and spherical algae (Chlorella sp. Synechococcus elongatus , FACHB-410, purchased from the Chinese Academy of Sciences Institute of Hydrobiology) to culture the BG-11 medium in the reactor. The first condition is set as: the first set flue gas introduction amount (100% of the total flow of the purified flue gas), the first set temperature (40°C, the temperature of the purified flue gas), the first set culture time (72 h), the first set light intensity (150 μmol / m 2 / s), and the first set fluorine concentration (100 mg / L). After culture, algae liquid is obtained.

[0071] S3 (phase II accumulation period culture): the algae liquid is transferred to a second photobioreactor (pipe photobioreactor), CaCl2 is added to the algae liquid (addition amount 0.8 mmol / L) and the supply of nitrogen source is limited, and the purified flue gas is introduced into the second photobioreactor. The second condition is set as: the second set flue gas introduction amount (50% of the total flow of the purified flue gas), the second set temperature (30°C), the second set culture time (48 h), the second set light intensity (50 μmol / m 2 / s), and a second set fluoride concentration (5mg / L), after which algal slurry was obtained.

[0072] S4 (Solid-Liquid Separation): The algal slurry is centrifuged to separate the solid and liquid components, resulting in algal mud and algae-containing filtrate.

[0073] S5 (cell wall breaking treatment and supercritical extraction): The algal mud is subjected to cell wall breaking treatment by high pressure homogenization, followed by supercritical CO2 extraction to obtain algal oil and algal residue.

[0074] S6 (Algae residue treatment): The algae residue is calcined at 800℃ for 2 hours to obtain calcium fluoride.

[0075] Example 3 S1 (Flue Gas Pretreatment): The aluminum electrolysis flue gas of a certain enterprise is successively treated by stepped cooling, electrostatic dust removal, and fluorine adsorption by activated Al2O3. The flue gas temperature is reduced to 40℃, and purified flue gas is obtained.

[0076] S2 (Stage I tolerance phase culture): The purified flue gas is passed through the inoculated *Synechococcus slenderus*. Synechococcus elongatus The first photobioreactor (airlift photobioreactor), FACHB-410, purchased from the Institute of Hydrobiology, Chinese Academy of Sciences, used BG-11 medium. The initial conditions were: flue gas flow rate (60% of the total purified flue gas flow rate), initial temperature (40℃, consistent with the purified flue gas temperature), initial incubation time (72 h), and initial light intensity (200 μmol / m²). 2 / s), with a first set fluoride concentration (200 mg / L), and algal solution was obtained after cultivation.

[0077] S3 (Stage II Accumulation Phase Cultivation): The algal solution is transferred to the second photobioreactor (pipeline photobioreactor). CaCl2 is added to the algal solution (1 mmol / L), and the nitrogen source supply is restricted. Simultaneously, the purified flue gas is introduced into the second photobioreactor. The second conditions are set as follows: second set flue gas flow rate (40% of the total purified flue gas flow rate), second set temperature (30℃), second set cultivation time (48 h), and second set light intensity (100 μmol / m²). 2 The algal slurry was obtained after incubation with a second set fluoride concentration (5 mg / L) and a fluoride concentration of 5 mg / L.

[0078] S4 (Solid-Liquid Separation): The algal slurry is centrifuged to separate the solid and liquid components, resulting in algal mud and algal-containing filtrate.

[0079] S5 (cell wall breaking treatment and supercritical extraction): The algal mud is subjected to cell wall breaking treatment with ultrasonic assistance, followed by supercritical CO2 extraction to obtain algal oil and algal residue.

[0080] S6 (algae residue treatment): the algae residue was high-temperature calcined at 1000℃ for 1h to obtain calcium fluoride.

[0081] Comparative Example 1 S1 (flue gas pretreatment): the flue gas of a certain enterprise was treated by stepwise temperature reduction, electrostatic precipitation and active Al2O3 adsorption of fluorine in sequence, and the flue gas temperature was reduced to 35℃ to obtain purified flue gas.

[0082] S2 (single-stage culture): the purified flue gas was introduced into a photobioreactor (air-lift photobioreactor) inoculated with Anabaena sp. (FACHB-588, purchased from the Institute of Hydrobiology, Chinese Academy of Sciences), and the culture medium in the reactor was BG-11 medium. CaCl2 was added to the system (addition amount 1 mmol / L). The culture conditions were set as follows: flue gas introduction amount (60% of the total flow of purified flue gas), temperature (35℃), culture time (120h), light intensity (200μmol / m2 / s), and fluorine concentration (150mg / L). After culture, algae slurry was obtained. 2 Anabaena

[0083] S3 (solid-liquid separation): the algae slurry was subjected to solid-liquid separation by centrifugation to obtain algae mud and algae-containing filtrate.

[0084] S4 (cell wall breaking treatment and supercritical extraction): the algae mud was subjected to cell wall breaking treatment by ultrasonic assistance, followed by supercritical CO2 extraction to obtain algae oil and algae residue.

[0085] S5 (algae residue treatment): the algae residue was high-temperature calcined at 1000℃ for 1h to obtain calcium fluoride.

[0086] The aluminum electrolysis flue gas treatment and resource recovery effects in each example and comparative example are shown in Table 1.

[0087] Table 1 aluminum electrolysis flue gas treatment and resource recovery effects in examples and comparative examples

[0088] It should be noted that the algae oil content in Table 1 represents the proportion of the weight of algae oil to the dry weight of algae.

[0089] ​​From Table 1, from the flue gas purification effect, the CO2 fixation rate of Examples 1-3 is 90.2%-97.6%, and the fluorine removal rate is 90.9%-94.7%; Comparative Example 1 uses single-stage culture, is in a constant temperature environment of 35°C with a fixed fluorine concentration of 150 mg / L for a long time, and the microalgae always face high fluorine stress, the metabolic activity is inhibited, and it is difficult to efficiently start the tolerance mechanism to cope with high-concentration pollutants and to continuously play a purification role in a stable environment, resulting in a significant decrease in flue gas treatment efficiency.

[0090] In terms of resource recovery quality, the algal lipid content of Examples 1-3 is 40.2%-43.5%, and the calcium fluoride purity is 92.4%-96.3%. In the single-stage culture of Comparative Example 1, the fluorine concentration is maintained at 150 mg / L for a long time, which not only inhibits the enzyme activity related to algal lipid synthesis, but also causes fluorine to be combined in an unstable form in the algal body, thereby reducing the algal lipid accumulation and making the calcium fluoride obtained by subsequent calcination contain more impurities, and the purity decreases.

[0091] In addition, one or more technical solutions in the embodiments of the present application have at least the following technical effects or advantages: In the embodiments of the present application, the harvested algal slurry can be further used to produce biodiesel, Omega-3 and other high-value products, the residual algal residue is calcined to recover calcium fluoride for processing back to an aluminum electrolysis cell, and the carbon-rich ash residue is processed to produce a flue gas adsorbent.

[0092] In the embodiments of the present application, by regulating environmental factors to trigger the pollutant tolerance mechanism and product synthesis pathway of microalgae, the following are simultaneously achieved: ① efficient flue gas purification; ② fluorine is fixed in the form of biological minerals; ③ algal bodies are stepwise extracted to obtain high-value products; and ④ algal residue is recycled.

[0093] The above merely describes specific embodiments of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features applied herein.

Claims

1. A method for treating and recycling aluminum electrolysis flue gas based on phased cultivation of microalgae, characterized by, The method comprises: Pretreating aluminum electrolysis flue gas to obtain purified flue gas; Passing the purified flue gas into a first photobioreactor inoculated with fluorotolerant microalgae to carry out phase I tolerance period culture under first conditions to obtain algal liquid; the first conditions comprise: first set flue gas passing amount, first set temperature, first set culture time, first set light intensity and first set fluorine concentration; Transferring the algal liquid to a second photobioreactor, adding CaCl2 to the algal liquid and limiting nitrogen source supply, and passing the purified flue gas into the second photobioreactor to carry out phase II accumulation period culture under second conditions to obtain algal slurry; the second conditions comprise: second set flue gas passing amount, second set temperature, second set culture time, second set light intensity and second set fluorine concentration; Carrying out solid-liquid separation on the algal slurry to obtain algal mud and algal-containing filtrate; Carrying out cell wall breaking treatment and supercritical extraction on the algal mud to obtain algal oil and algal residue; The first set flue gas passing amount > the second set flue gas passing amount, the first set temperature > the second set temperature, and the first set fluorine concentration > the second set fluorine concentration. The fluorotolerant microalgae contain a fluorine ion efflux pump protein gene.

2. The method of claim 1, wherein, The fluorotolerant microalgae comprise one or more of the following: Spirulina, fishy algae, Synechococcus and Synechocystis.

3. The method of claim 1, wherein, The first photobioreactor is a gas-lift photobioreactor or a bubble column photobioreactor, and the culture solution in the first photobioreactor is Zarrouk or BG-11.

4. The method of claim 1, wherein, The first set flue gas input amount is 60% to 100% of the total flow of the purified flue gas, the first set temperature is 30°C to 45°C, the first set culture time is 24h to 72h, the first set light intensity is 100μmol / m 2 2 / s to 200μmol / m 2 2 / s, and the first set fluorine concentration is 50mg / L to 200mg / L.

5. The method of claim 1, wherein, The second photobioreactor is a flat-plate photobioreactor or a pipeline photobioreactor, and the culture solution in the second photobioreactor is Zarrouk or BG-11.

6. The method of claim 1, wherein, The second set flue gas input amount is 30% to 50% of the total flow of the purified flue gas, the second set temperature is 25°C to 30°C, the second set culture time is 48h to 96h, the second set light intensity is 50μmol / m 2 / s to 250μmol / m 2 / s, and the second set fluorine concentration is <10mg / L.

7. The method of claim 1, wherein, The molar amount of CaCl2 in the algal liquid is 0.5 mmol / L to 1 mmol / L.

8. The method of claim 1, wherein, The method further comprises: Carrying out high-temperature calcination on the algal residue to obtain calcium fluoride and carbon-rich ash residue.

9. The method of claim 8, wherein, The temperature of the high-temperature calcination is 600°C to 1000°C, and the time is 1h to 4h.

10. The method of claim 1, wherein, The method further comprises: Cycling the algal-containing filtrate for phase I tolerance period culture; Processing the algal oil into high-value products; the high-value products comprise one or both of the following: biodiesel and Omega-3.

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