Carbon dioxide high-pressure extraction and decalcification process for fly ash

Abstract

The invention provides a carbon dioxide high-pressure extraction and decalcification process for fly ash. The process comprises the following steps: (i) raw material pretreatment, (ii) additive mixing, (iii) reaction system construction, (iv) high-pressure extraction, (v) product separation and (vi) product refining. The deep reaction is promoted through the high-pressure supercritical state, the passivation effect is overcome through the dynamic cycle operation, the selective protection target component is improved through the compound additive, the balance efficiency and energy consumption of the slurry system are optimized, and the product purity is guaranteed through tail-end fine washing. According to the process disclosed by the invention, the loss of valuable silicon components can be remarkably reduced while the calcium components are efficiently removed.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of industrial solid waste resource utilization, and particularly relates to a carbon dioxide high-pressure leaching decalcification process of fly ash. BACKGROUND

[0002] A large amount of fly ash, such as pulverized coal furnace ash and circulating fluidized bed ash, generated in the industrial coal combustion process is a kind of main solid waste. These fly ashes usually contain rich calcium components, including free calcium oxide, calcium hydroxide and various calcium-containing minerals. The existence form of calcium element in fly ash has a dual influence on its resource utilization. On the one hand, an appropriate amount of calcium component is beneficial to the activation of fly ash and the promotion of its hydration reaction in cementitious materials; on the other hand, too high calcium content, especially too high free calcium oxide (f-CaO) content, may cause stability problems, such as water absorption and cracking of cement hardening products in the later stage when used as cement admixture; when fly ash is used as raw material to extract valuable elements such as aluminum and silicon, calcium element is easy to form insoluble salt on the surface of minerals in the acid leaching process, which hinders the leaching efficiency of target elements.

[0003] At present, the decalcification pretreatment technologies for high-calcium fly ash mainly include two categories: physical separation method and chemical method. The physical separation method such as air separation, magnetic separation and flotation separates calcium minerals based on the differences in density, magnetism or surface hydrophobicity, but its separation precision is limited, especially for fly ash with fine particle size and complex embedding, it is difficult to achieve deep removal of calcium element. The chemical method is relatively more widely used, including acid leaching method, water washing method and the biological method and carbon dioxide method developed in recent years. Although the acid leaching method has high decalcification efficiency, it has problems such as corrosion of equipment, high treatment cost, difficult treatment of salt-containing wastewater, easy leaching of valuable elements such as aluminum and iron, and loss of target components. The water washing method can remove part of the soluble calcium salt, but it has little effect on the main calcium components: free calcium oxide and insoluble calcium minerals, and the water consumption is large.

[0004] Carbon dioxide mineralization technology provides a new way for fly ash decalcification. The technology uses carbon dioxide in industrial exhaust gas to react with alkaline components such as calcium and magnesium oxides / hydroxides in fly ash to form carbonates. Existing decalcification methods based on carbon dioxide mainly include atmospheric water bath method and low-pressure bubbling method. The atmospheric water bath method disperses fly ash in water and introduces carbon dioxide gas at atmospheric pressure for reaction. This method has a slow reaction rate and requires a long processing time, usually several hours to several tens of hours. It has poor selectivity for calcium forms and generally low decalcification efficiency. The treatment of the filtrate after processing is also relatively complex. The low-pressure bubbling method introduces carbon dioxide into the fly ash slurry at a low pressure of less than 1 MPa in an attempt to increase the solubility of carbon dioxide to accelerate the reaction. Although this method is slightly improved over the atmospheric method, the carbon dioxide mass transfer efficiency is limited due to the low system pressure, and the permeability of the carbon dioxide into the internal micropores of the fly ash particles is insufficient. Especially when dealing with fly ash with a large specific surface area, a dense structure, or a large amount of inert components wrapped, the reaction is often limited to the surface of the particles, and deep decalcification is difficult to achieve.

[0005] In addition, existing carbon dioxide decalcification processes can easily lead to excessive dissolution and loss of silicon components while removing target calcium components, significantly reducing the content of silicon dioxide as an important phase in fly ash, and weakening the application value of the treated fly ash in the building materials field. At the same time, for complex fly ash, such as CFB ash containing a large amount of unburned carbon, simple carbon dioxide treatment cannot simultaneously control decalcification and carbon components. More importantly, the reaction rate and decalcification efficiency of existing processes cannot meet the requirements of industrial large-scale application, and the processing capacity is weak.

[0006] Therefore, it is necessary to design a carbon dioxide high-pressure leaching decalcification process for fly ash. SUMMARY

[0007] In order to overcome the defects in the prior art, a carbon dioxide high-pressure leaching decalcification process for fly ash is provided.

[0008] In order to achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0009] A carbon dioxide high-pressure leaching decalcification process for fly ash, the process comprising the following steps:

[0010] (i) Raw material pretreatment: adjusting the water content of fly ash to be between 5% and 15% to obtain fly ash raw material;

[0011] (ii) Additive mixing: adding a water-soluble silicate additive to the fly ash raw material obtained in step (i) and mixing uniformly to obtain modified fly ash;

[0012] (iii) Reaction system construction: loading the modified fly ash obtained in step (ii) into a high-pressure reaction kettle, injecting water at 20% to 100% of the dry basis mass of fly ash to form a slurry system;

[0013] (iv) High pressure leaching: Carbon dioxide gas is introduced into the high pressure reactor to a pressure of 10-25 MPa, and the temperature is raised to 50-100°C. The reaction is carried out under supercritical carbon dioxide for 0.5-6.0 hours at constant pressure. Pressure differential cycling is carried out every 15 minutes;

[0014] (v) Product separation: After the reaction is completed, the high pressure reactor is depressurized, and solid-liquid separation is carried out to obtain decalcified fly ash solids and a liquid phase containing calcium carbonate;

[0015] (vi) Product purification: The decalcified fly ash solids are washed with an alkaline washing liquid, and after drying, the low calcium fly ash product is obtained.

[0016] In step (i), if the initial moisture content of the fly ash raw material is less than 5%, water is sprayed for humidification; if it is higher than 15%, hot air is blown to dry it to the target range;

[0017] A pre-grinding step is added after step (i): the fly ash raw material is treated with a ball mill to make its average particle size between 5-30 microns.

[0018] In step (ii), the water-soluble silicate additive is a mixture of sodium silicate and potassium silicate in a mass ratio of 1:1-2, and the total addition amount is 0.5%-4.0% of the dry basis mass of the fly ash;

[0019] The method of adding the water-soluble silicate additive is to first dissolve the water-soluble silicate additive in water with a dry basis mass of 3%-8% of the fly ash raw material, and then spray it into the fly ash raw material.

[0020] In step (ii), a complex surfactant system is also added, which is a mixture of polyoxyethylene sorbitan monooleate and sodium dodecyl sulfonate in a mass ratio of 3-8:1, and the total addition amount of the complex surfactant system is between 0.05%-0.50% of the dry basis mass of the fly ash; each polyoxyethylene sorbitan monooleate contains 15-20 ethoxyl units.

[0021] In step (iv), the pressure differential cycling operation includes the following steps:

[0022] a) The pressure is reduced to between 30%-60% of the current pressure, and maintained for 1-10 minutes;

[0023] b) The pressure is raised to the original pressure value.

[0024] During the pressure reduction phase, a nitrogen gas pulse with a flow rate of 10-50 standard liters / minute is injected into the reactor, and each pulse lasts for 5-30 seconds.

[0025] In step (vi), the alkaline washing liquid is ammonia solution with a concentration of 0.05-0.5 mol / L; the washing times are 2-3 times, and the dosage of the alkaline washing liquid is 50%-100% of the dry mass of the fly ash each time.

[0026] The calcium carbonate-containing liquid phase obtained in step (v) is added into a sodium oxalate solution to react to generate calcium oxalate precipitate, which is then recovered.

[0027] In step (vi), the drying temperature is 105-120°C, and the water content of the product is ≤1%.

[0028] Compared with the prior art, the application has the following advantages and beneficial effects:

[0029] 1. The application increases the reaction pressure to the range of 10 to 25 MPa and creates a supercritical carbon dioxide state, greatly improving the solubility and mass transfer efficiency of carbon dioxide in the system. This enables carbon dioxide molecules to more effectively diffuse and penetrate into the internal micro-porous structure of fly ash particles, even the surfaces of calcium minerals wrapped by dense inert components or difficult to access at low pressure, so that carbon dioxide can fully contact and react, thereby achieving deeper decalcification. This solves the problem that the low-pressure or atmospheric pressure method is limited to the surface of the particles and has difficulty in deep decalcification.

[0030] 2. The application implements differential pressure cycle operation periodically during the reaction process, combined with optional nitrogen pulse injection. This operation produces controllable pressure fluctuations and gas impact in the middle of the reaction. The core principle is that the pressure drop and recovery process will form micro-vortices and strong physical disturbances in the slurry system, effectively breaking the calcium carbonate product layer that may be attached to the surface of the fly ash particles or blocking the pores, breaking the concentration barrier of the reaction interface. At the same time, nitrogen pulse as a physical flushing means can carry out the displaced reaction by-products into the internal micro-pores.

[0031] 3. In terms of additives, the application pre-introduces water-soluble silicate additives, which can form a protective film or complex structure containing silicon elements on the surface of fly ash particles. This structure can selectively inhibit the dissolution and release of active silicon components in fly ash in the subsequent high-pressure acidic environment. At the same time, by compounding a specific ratio of non-ionic and anionic surfactant systems, the application optimizes the interfacial properties of the reaction system, significantly reducing the interfacial tension of the solid-liquid-gas three phases. The hydrophilic group in the surfactant improves the hydrophilicity of the fly ash surface, improving the wettability and penetration ability of the water medium and carbon dioxide to the fly ash particles. The lipophilic group helps to promote the dissolution and micro-dispersion of supercritical carbon dioxide in the liquid film. The two additives work together to achieve efficient decalcification while minimizing the loss rate of non-target silicon components, solving the key problem of poor selectivity and serious silicon component loss in existing decalcification processes.

[0032] 4. The amount of water added in the process is precisely controlled between 20% and 100% of the dry mass of fly ash, forming a special slurry system. Compared with the large amount of water treatment and energy consumption required by the over-diluted system, this amount of water ensures that the slurry has a high solid content, improving the space utilization of the reactor and the single treatment capacity. More importantly, this humidity level can not only provide the necessary reaction medium environment to promote ion migration and carbonation reaction, but also maintain the effective state of supercritical carbon dioxide in the system, ensuring its excellent dissolution, diffusion and reaction performance. This avoids the drawbacks of the traditional water washing method, which consumes a large amount of water and has poor results, and some extreme dry methods that cannot fully utilize the reaction activity of carbon dioxide.

[0033] 5. In the end treatment, the application selects a specific concentration of ammonia solution as the alkaline washing liquid to treat the decalcified fly ash solids. Ammonia has a mild alkaline environment, which can effectively dissolve and remove trace amounts of calcium carbonate particles and some soluble impurities that may be re-adsorbed or remain in the washing step. Because of its volatility, it is easy to remove in the subsequent drying process without introducing new difficult-to-remove cation impurities, such as sodium or potassium ions, which may have adverse effects on subsequent building material applications. This washing strategy helps to further improve the purity of the product.

[0034] 6. The application promotes deep reaction through high-pressure supercritical state, overcomes passivation effect through dynamic circulation operation, improves selective protection of target components through compound additives, optimizes the balance efficiency and energy consumption of the slurry system, and guarantees product purity through fine end washing. Through the synergistic cooperation of the above technical features, the process of the application can significantly reduce the loss of valuable silicon components while efficiently removing calcium components. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the application will be described below in a clear and complete manner. Obviously, the described embodiments are only part of the embodiments of the application, not all embodiments. Based on the embodiments in the application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of the application.

[0036] In the application, the sources of various raw materials are briefly described as follows:

[0037] Sodium silicate: purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd., CAS No. 6834-92-0, type: industrial grade water glass, modulus range 2.2-3.3. Potassium silicate: purchased from Aldrich Biochemical Technology Co., Ltd., CAS No. 1312-76-1, type: analytical pure liquid potassium silicate, modulus range 2.5-3.5. Polyoxyethylene sorbitan monooleate, also known as Tween 80, purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd., CAS No. 9005-65-6, type: Tween 80, ethylene unit number 15-20. Sodium dodecyl sulfate: purchased from Macklin Biochemical Technology Co., Ltd., CAS No. 2386-53-0, type: SDS surfactant, purity ≥ 99%. Ammonia: purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd., CAS No. 1336-21-6, type: electronic grade ammonia, concentration 25%-28%. Sodium oxalate: purchased from Aldrich Biochemical Technology Co., Ltd., CAS No. 62-76-0, type: analytical pure crystalline powder, purity ≥ 99.8%

[0038] A high-pressure leaching decalcification process of fly ash carbon dioxide, comprising the following steps:

[0039] (i) Raw material pretreatment: control the water content of fly ash between 5%-15% to obtain fly ash raw material;

[0040] (ii) Additive mixing: add water-soluble silicate additive to the fly ash raw material obtained in step (i) and mix uniformly to obtain modified fly ash;

[0041] (iii) Reaction system construction: load the modified fly ash obtained in step (ii) into a high-pressure reaction kettle, inject water at 20%-100% of the dry basis mass of fly ash to form a slurry system;

[0042] (iv) High-pressure leaching: introduce carbon dioxide gas into the high-pressure reaction kettle to a pressure of 10-25 MPa, heat to 50-100°C, and react at constant pressure for 0.5-6.0 hours under supercritical carbon dioxide state; at the same time, differential pressure cycle operation is carried out every 15 minutes;

[0043] (v) Product separation: after the reaction is completed, the high-pressure reaction kettle is depressurized, and solid-liquid separation is carried out to obtain decalcified fly ash solid and calcium carbonate-containing liquid phase;

[0044] (vi) Product refining: wash the decalcified fly ash solid with alkaline washing liquid, and dry to obtain low-calcium fly ash product.

[0045] In step (i), if the initial water content of the fly ash raw material is less than 5%, spray water to humidify; if it is higher than 15%, dry with hot air to the target range;

[0046] A pre-grinding step is added after step (i): the fly ash raw material is treated with a ball mill to reduce its average particle size to between 5 and 30 micrometers.

[0047] In step (ii), the water-soluble silicate additive is a mixture of sodium silicate and potassium silicate in a mass ratio of 1:1-2, and the total amount added is 0.5%-4.0% of the dry weight of fly ash;

[0048] The method for adding the water-soluble silicate additive is as follows: first dissolve the water-soluble silicate additive in water at a dry basis weight of 3%-8% of the fly ash, and then spray it into the fly ash raw material.

[0049] In step (ii), a compound surfactant system is also added, wherein the compound surfactant system is a mixture of polyoxyethylene sorbitan monooleate and sodium dodecyl sulfonate at a mass ratio of 3-8:1, and the total amount of the compound surfactant system added is between 0.05% and 0.50% of the dry weight of fly ash; each polyoxyethylene sorbitan monooleate contains 15-20 ethoxy units.

[0050] In step (iv), the differential pressure cycle operation includes the following steps:

[0051] a) Reduce the pressure to between 30% and 60% of the current pressure and maintain this position for 1 to 10 minutes;

[0052] b) Increase the pressure to the original pressure value.

[0053] During the depressurization phase, nitrogen pulses with a flow rate of 10-50 standard liters per minute are injected into the reactor, each lasting between 5-30 seconds.

[0054] In step (vi), the alkaline washing solution is an ammonia solution with a concentration of 0.05-0.5 mol / L; the washing is performed 2-3 times, and the amount of alkaline washing solution used each time is 50%-100% of the dry weight of fly ash.

[0055] The calcium carbonate-containing liquid phase obtained in step (v) is added to a sodium oxalate solution, and the reaction produces a calcium oxalate precipitate, which is then recovered.

[0056] In step (vi), the drying temperature is 105-120℃ until the moisture content of the product is ≤1%.

[0057] This application promotes deep reaction through high-pressure supercritical state, overcomes passivation effect through dynamic cyclic operation, improves selectivity and protects target components through compound additives, optimizes the balance efficiency and energy consumption of slurry system, and ensures product purity through fine washing at the end. Through the synergistic combination of the above technical features, the process of this application can significantly reduce the loss of valuable silicon components while efficiently removing calcium components.

[0058] The technical solutions of the present application are further illustrated by the following examples and comparative examples, but the scope of protection of the present application is not limited thereto.

[0059] Example 1

[0060] Raw material pretreatment: adjust the water content of fly ash to 5%; the average particle size after ball milling is 17.5 microns.

[0061] Additive mixing: mix sodium silicate and potassium silicate at a mass ratio of 1:1, dissolve in water with a dry basis mass of 3% to spray and add, and the total additive amount is 0.5%; compound polyoxyethylene sorbitan monooleate (containing 15 ethoxy units) and sodium dodecyl sulfonate (mass ratio 3:1), and the additive amount is 0.50%.

[0062] Reaction system: the water injection amount is 100% of the dry basis mass.

[0063] High-pressure leaching: pressure 25 MPa, temperature 100℃, reaction time 0.5 hours; differential pressure cycle every 15 minutes: reduce to 30% of the current pressure for 10 minutes, and inject nitrogen pulse (50 standard liters / minute x 5 seconds) before pressure increase.

[0064] Product treatment: 0.5 mol / L ammonia water washing 2 times (100% dry basis mass each time), and drying at 105℃ to a water content of ≤1%.

[0065] Example 2

[0066] In this example, the same as in Example 1 will not be repeated, and the differences are described as follows:

[0067] Raw material pretreatment: adjust the water content of fly ash to 15%; the average particle size after ball milling is 30 microns.

[0068] Additive mixing: mix sodium silicate and potassium silicate at a mass ratio of 2:1, dissolve in water with a dry basis mass of 8% to spray and add, and the total additive amount is 4.0%; compound polyoxyethylene sorbitan monooleate (containing 20 ethoxy units) and sodium dodecyl sulfonate (mass ratio 8:1), and the additive amount is 0.05%.

[0069] Reaction system: the water injection amount is 60% of the dry basis mass.

[0070] High-pressure leaching: pressure 17.5 MPa, temperature 75℃, reaction time 3.25 hours; differential pressure cycle: reduce to 45% pressure for 5.5 minutes, and nitrogen pulse 30 standard liters / minute x 17.5 seconds.

[0071] Product treatment: 0.275 mol / L ammonia water washing 2 times (75% dry basis mass each time), and drying at 112.5℃.

[0072] Example 3

[0073] In this example, the same as example 1, not described again, the difference is described as follows:

[0074] Raw material pretreatment: adjust the fly ash moisture content to 10%; the average particle size is 5 microns after ball milling.

[0075] Additive mixing: sodium silicate and potassium silicate are mixed in a mass ratio of 1.5:1, dissolved in water with a dry basis mass of 5.5%, sprayed and added, the total additive amount is 2.25%; compound polyoxyethylene sorbitan monooleate (containing 17 ethoxy units) and sodium dodecyl sulfonate (mass ratio 5.5:1), the additive amount is 0.275%.

[0076] Reaction system: water injection amount is 20% of dry basis mass.

[0077] High pressure leaching: pressure 10 MPa, temperature 50℃, reaction time 6.0 hours; differential pressure cycle to 60% pressure for 1 minute, nitrogen pulse 10 standard liters / minute x 30 seconds.

[0078] Product treatment: 0.05 mol / L ammonia water washing 3 times (50% dry basis mass each time), 120℃ drying.

[0079] Comparative example 1

[0080] In this comparative example, the same as example 1, not described again, the difference is described as follows:

[0081] The pressure is reduced to 1 MPa, the reaction time is extended to 10 hours, and no additives are added, and there is no differential pressure cycle.

[0082] Comparative example 2

[0083] In this comparative example, the same as example 2, not described again, the difference is described as follows:

[0084] The water injection amount is increased to 200% of dry basis mass, there is no nitrogen pulse, only silicate is used without surfactant.

[0085] Comparative example 3

[0086] In this comparative example, the same as example 3, not described again, the difference is described as follows:

[0087] The pressure is atmospheric pressure (0.1 MPa), the water addition amount is 100%, the reaction time is 20 hours, there is no additive / differential pressure cycle, and pure water is used instead of ammonia water for washing.

[0088] Comparative example 4

[0089] In this comparative example, the same as example 1, not described again, the difference is described as follows:

[0090] No differential pressure cycle and surfactant, water injection amount 20% dry base mass.

[0091] Comparative Example 5

[0092] In the present comparative example, the same as in Example 1 is not described again, and the difference is described as follows:

[0093] The silicate is replaced with an equivalent amount of sodium chloride, and the washing is performed with a 0.5 mol / L sodium hydroxide solution.

[0094] Performance test results and analysis

[0095] In the present example, the fly ash used as raw material is industrial pulverized coal furnace ash, and the initial calcium oxide content is 18.7%, and the silicon dioxide content is 22.4%. The decalcification of fly ash is carried out according to the parameters of the example and the comparative example, and the test results are shown in Table 1.

[0096] Table 1 Analysis test results

[0097]

[0098] The test results show that the calcium removal rate of Examples 1-3 is 97.5-98.2% under 10-25 MPa supercritical CO2, and the residual calcium is ≤0.46%. The core lies in that the density of supercritical CO2 is close to that of liquid, the high-pressure supercritical state strengthens the decalcification depth, the solubility is increased by more than 10 times, the fly ash micropores are effectively penetrated, and the calcium minerals wrapped by inert are fully reacted. In Comparative Example 1, under a pressure of 1 MPa, due to insufficient CO2 solubility, the internal reaction of the particles is limited, and the decalcification rate is suddenly reduced to 75.6%.

[0099] The differential pressure cycle of 3 examples generates transient pressure difference every 15 minutes, forming micron-scale vortex shear force, and passivating this force can strip the CaCO3 film (thickness about 2-5 μm) generated on the surface of the particles, avoiding its hindering of CO2 diffusion, and the differential pressure cycle breaks the product layer. Compared with Comparative Example 4 without differential pressure cycle, although the same high pressure of 25 MPa is used, due to the accumulation of the product layer, the decalcification rate is reduced by 8.1%, and the residual calcium is increased to 1.84%.

[0100] Silicate / surfactant complex silicon protection: silicate additives form a -Si-O-Si- crosslinking network on the surface of fly ash, covering the surface of active silicon minerals. In an acidic CO2 environment, this network inhibits H + attack on Si-O bonds, so that the silicon retention rate of the example is ≥94.2%. After replacing with sodium chloride in Comparative Example 5, a large amount of silicon lacking a protective layer is dissolved out, and the retention rate is 88.5%. The surfactant improves the wettability of the slurry, the contact angle is reduced, and the CO2 diffusion rate is increased. In the example, the silicon loss is <6%, and compared with the loss of 15-30% in the traditional acid leaching method, the silicon resource utilization benefit can be improved by 28%.

[0101] The semi-solid slurry formed by 20-100% water injection provides ion migration channels with a water molecule layer thickness of about 50 nm and maintains the required density of the supercritical state. The over-diluted Comparative Example 2 has a decreased reaction efficiency due to excessive dilution of CO2. The end ammonia water washing can dissolve residual CaCO3 microcrystals, and the Na + / K + The system will leave alkaline ions, and Comparative Example 5 introduces Na + which leads to a 9.8% reduction in strength, indicating that the slurry system has a synergistic advantage with ammonia water washing.

[0102] Comparative Example 3 completely simulates the atmospheric water bath method in the background technology, and the reaction time of up to 20 hours is too different from the fastest 0.5 hours of the example, and the efficiency is low. The residual calcium of 8.81% is far beyond the upper limit of cement admixture (1.5%), and cannot be used for building materials. The addition of no additives leads to acid dissolution of the silicon component. Comparative Example 3 verifies the three major pain points of the background technology described as "slow reaction rate, low decalcification efficiency, and serious silicon loss" of the atmospheric water bath method, and reversely highlights the necessity of the high-pressure penetration, dynamic circulation, and silicon protection technical route of the present application.

[0103] Through the synergistic mechanism of supercritical state penetration, dynamic passivation breaking, silicon protection film formation, and ammonia water harmless washing, the present process realizes a breakthrough balance between deep decalcification and silicon retention.

[0104] The above is a preferred embodiment of the present application. It should be noted that for ordinary skilled persons in the technical field, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered within the scope of protection of the present application.

Claims

1. A process for decalcification of fly ash by high pressure carbon dioxide leaching, characterized in that, The process comprises the following steps: (i) raw material pretreatment: regulating the water content of fly ash to be between 5% and 15%, obtaining fly ash raw material; (ii) additive mixing: adding water-soluble silicate additive to the fly ash raw material obtained in step (i) and mixing uniformly to obtain modified fly ash; (iii) reaction system construction: loading the modified fly ash obtained in step (ii) into a high-pressure reaction kettle, injecting water at 20%-100% of the dry mass of fly ash, and forming a slurry system; (iv) high-pressure leaching: introducing carbon dioxide gas into the high-pressure reaction kettle to a pressure of 10-25 MPa, heating to 50-100°C, and reacting at a constant pressure for 0.5-6.0 hours in a supercritical carbon dioxide state; simultaneously, differential pressure cycle operation is performed every 15 minutes; (v) product separation: after the reaction is completed, the high-pressure reaction kettle is depressurized, and solid-liquid separation is performed to obtain decalcified fly ash solids and a calcium carbonate-containing liquid phase; (vi) product refining: washing the decalcified fly ash solids with an alkaline washing solution, and drying to obtain low-calcium fly ash product.

2. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1, wherein: In step (i), if the initial water content of the fly ash raw material is less than 5%, water is sprayed for humidification; if it is higher than 15%, hot air is used for drying to the target range; A pre-grinding step is added after step (i): the fly ash raw material is treated with a ball mill to make its average particle size between 5-30 microns.

3. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1, wherein: In step (ii), the water-soluble silicate additive is a mixture of sodium silicate and potassium silicate at a mass ratio of 1:1-2, and the total addition amount is 0.5%-4.0% of the dry mass of fly ash.

4. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1, wherein: The method for adding the water-soluble silicate additive is to first dissolve the water-soluble silicate additive in water at 3%-8% of the dry mass of fly ash, and then spray it into the fly ash raw material.

5. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1, wherein: In step (ii), a compounded surfactant system is also added, which is a mixture of polyoxyethylene sorbitan monooleate and sodium dodecyl sulfonate at a mass ratio of 3-8:1, and the total addition amount of the compounded surfactant system is between 0.05% and 0.50% of the dry mass of fly ash; each polyoxyethylene sorbitan monooleate contains 15-20 ethoxyl units.

6. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1, wherein: In step (iv), the differential pressure cycle operation comprises the following steps: a) reducing the pressure to between 30% and 60% of the current pressure, maintaining for 1-10 minutes; b) increasing the pressure to the original pressure value.

7. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 6 wherein: During the pressure reduction stage, a nitrogen gas pulse with a flow rate of 10-50 standard liters / minute is injected into the reaction kettle, and each pulse lasts for 5-30 seconds.

8. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1, wherein: In step (vi), the alkaline washing solution is an ammonia solution with a concentration of 0.05-0.5 mol / L; The washing is performed 2-3 times, and the amount of alkaline washing solution used each time is 50%-100% of the dry mass of fly ash.

9. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1 wherein: The calcium carbonate-containing liquid phase obtained in step (v) is added to a sodium oxalate solution, and calcium oxalate precipitate is generated by reaction, which is then recovered.

10. A process for decalcification of fly ash by high pressure leaching of carbon dioxide as claimed in claim 1 wherein: In step (vi), the drying temperature is 105-120°C, and the water content of the product is ≤1%.

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