Method and system for recycling bypass exhaust flue gas of cement kiln

Abstract

The invention discloses a method and a system for recycling bypass exhaust flue gas of a cement kiln. The recycling method for the bypass exhaust flue gas of the cement kiln comprises the following steps: cooling, dedusting and washing the bypass exhaust flue gas of the cement kiln to obtain a water-washing salt-extracting solution and washed flue gas; mixing the washing salt extraction liquid with the washing flue gas to react to generate a carbonized liquid, carrying out solid-liquid separation on the carbonized liquid, and removing carbon dioxide from a liquid phase part to obtain a calcium-containing liquid; taking part of the calcium-containing liquid to react with ammonia water to generate crystal nucleus turbid liquid, adding the crystal nucleus turbid liquid into the residual calcium-containing liquid, and adding the ammonia water for reaction in batches to obtain vaterite turbid liquid; and carrying out solid-liquid separation on the vaterite suspension, and drying the obtained slag phase to obtain a vaterite product. According to the invention, a'dissolution-recrystallization 'coupling process is provided for the first time, and efficient and controllable synthesis of vaterite in the waste liquid with ultralow calcium concentration is realized. The method not only realizes collaborative resourceful treatment of industrial wastes, but also promotes high-valued conversion and cyclic utilization of the low-calcium waste liquid.

Description

Technical Field

[0001] The present invention belongs to the technical field of solid waste resource utilization, and in particular relates to a cement kiln bypass venting flue gas resource utilization method and system. Background Art

[0002] In the process of co-disposal of solid waste in cement kilns, the bypass ash will produce a large amount of alkaline water washing liquid containing potassium chloride (KCl) and sodium chloride (NaCl) after being treated by water washing-salt extraction process. 2+ The concentration of calcite is significantly diluted due to the washing process. Conventional treatment involves introducing CO2 into the water wash to generate calcite through carbonization, while simultaneously recovering the chloride salts. However, as calcite is a low-value-added product, this traditional method results in low resource efficiency.

[0003] In recent years, the synthesis technology of high-value-added vaterite calcium carbonate (Vaterite) has attracted attention. Studies have shown that by regulating the crystal stabilizers (such as alcohols, amino acids, and ammonium salts), vaterite can be directed to synthesize from high-calcium systems such as carbide slag extracts and desulfurization gypsum conversion liquids. However, its technical implementation implicitly relies on calcium ion concentration. The calcium dissolution mechanism of raw materials such as carbide slag and desulfurization gypsum is acidification or high-temperature conversion, and the calcium ion concentration is naturally high. For some bypass ash water washes that need to meet the needs of deep chlorine removal, the high liquid-to-solid ratio washing process will cause the calcium ions to be highly diluted, which is much lower than that of other calcium-containing wastewaters. At low calcium concentrations, the introduction of CO2 can easily trigger the reverse dissolution reaction of calcium carbonate. However, vaterite, as a metastable crystal form, will spontaneously transform into calcite if it exists in aqueous solution for a long time. Furthermore, vaterite has a higher solubility than calcite, and its dissolution effect is even more severe in low calcium ion concentration environments, making it extremely difficult to stabilize. Existing technologies (such as CN119018923A) mention the use of water-washed salt solution to synthesize vaterite, but they use a conventional one-step carbonization method, passing carbon dioxide from flue gas into the water wash solution. This does not address the dynamic competition between the reverse dissolution of calcium carbonate and the formation of vaterite at low calcium ion concentrations. The yield and purity of vaterite are uncontrollable. Furthermore, organic compounds such as alcohols, acids, and amino acids used as crystal form control agents are not only non-degradable but also cannot be recycled, reducing the implicit economic and environmental value of the product.

[0004] Based on the above-mentioned technical gaps, it is urgent to develop a process for directional control of the crystal form of carbonization products of low-calcium waste liquid for the brine extracted from bypass ash water with low calcium ion concentration, break through the existing technology's dependence on high calcium concentration system, and realize the coordinated disposal of industrial waste and the high-value conversion of low-calcium waste liquid. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method and system for resource utilization of cement kiln bypass vent gas, which is used to solve the technical problems of low resource utilization efficiency of cement kiln bypass vent gas in the existing technology and difficulty in controlling the purity of the prepared vaterite-type calcium carbonate crystal.

[0006] The purpose of the present invention is achieved through the following technical solutions: A method for recycling cement kiln bypass flue gas, comprising the following steps: S1. After heat exchange treatment, cement kiln bypass vent flue gas is subjected to dust removal to obtain bypass vent ash and dust-removed flue gas; the bypass vent ash is washed with water to obtain a water-washing salt extraction solution, and the dust-removed flue gas is washed with water to obtain washed flue gas; S2, mixing the water-extracted salt solution with the scrubbing flue gas to generate a carbonized liquid, performing solid-liquid separation on the carbonized liquid, and removing carbon dioxide from the liquid phase to obtain a calcium-containing liquid; S3, taking part of the calcium-containing solution and reacting it with ammonia water to generate a crystal nucleus suspension, adding the suspension to the remaining calcium-containing solution, and adding ammonia water in batches to react to obtain a vaterite suspension; S4, performing solid-liquid separation on the vaterite suspension, and drying the obtained slag phase to obtain a vaterite product.

[0007] Preferably, step S2 further comprises recycling the removed carbon dioxide as a carbon source to the carbonization liquid generation process.

[0008] Preferably, step S4 further comprises: performing solid-liquid separation on the vaterite suspension, and concentrating the obtained liquid phase by evaporation, cooling and crystallizing to obtain KCl and NH4Cl products.

[0009] Preferably, the CO2 concentration in the cement kiln bypass flue gas in step S1 is not less than 10%.

[0010] Preferably, the Ca in the water-extracted salt solution in step S1 is 2+ ≤3g / L, K + ≥100g / L, pH≥12.

[0011] Preferably, the volume of the scrubbing flue gas and the water-extracting salt solution in step S2 is 0.1-0.5 m 3 / m 3 .

[0012] In the present invention, during the synthesis of the carbonized liquid in step S2, the generation and dissolution of calcium carbonate follows the following core reaction pathway:

[0013] The following reaction characteristics exist in the high calcium concentration system: Thermodynamic advantage: The calcium ion concentration is much higher than the calcium carbonate solubility product (K sp= 8.7 × 10 -9 ), the positive driving force of reaction (4) is significantly stronger than the reverse dissolution trend of reaction (5); Kinetic inhibition: In a high calcium environment, calcium carbonate rapidly nucleates and grows, reducing the CO2 diffusion rate through surface adsorption and inhibiting the reaction (5).

[0014] In contrast, the low calcium concentration system has the following reaction dilemma: Thermodynamic disadvantage: After precipitation, the calcium ion concentration in the solution further decreases, and the solubility product is much lower than the equilibrium constant of equation (5), resulting in dissolution dominating; Failure of crystal form control: Under low supersaturation, the metastable characteristics of vaterite cannot be maintained, resulting in the production of calcium carbonate precipitation, the product is mainly calcite.

[0015] In general, in the present invention, due to the low calcium ion strength in the solution and the metastable characteristics of vaterite, even if vaterite precipitates in a short time, it is easy to be precipitated by high concentration of HCO3 - So this stage is mainly based on "dissociation" to obtain Ca 2+ / HCO3 - The enriched solution is the carbonized liquid described in the present invention.

[0016] Preferably, the specific operation of removing carbon dioxide from the liquid phase in step S2 is to remove dissolved carbon dioxide using a vacuum decompression method or an inert gas stripping method.

[0017] Preferably, the carbon dioxide partial pressure (pCO2) in the calcium-containing solution in step S2 is less than 0.1 atm.

[0018] Preferably, in step S3, the specific operation of reacting the portion of the calcium-containing solution with ammonia water to generate a crystal nucleus suspension is: adding ammonia water to the calcium-containing solution at a ratio of 2-10 g / L at one time, raising the pH of the system to 9-11 within 1-3 minutes, and stirring the reaction at a rate of 400-1000 rpm for 2-10 minutes to obtain a crystal nucleus suspension.

[0019] Preferably, the specific operation of adding ammonia water in batches to react in step S3 is: adding ammonia water to the system in batches, with each addition amount being 2-3 g / L and the total addition amount being 6-15 g / L, and stirring the reaction at a rate of 400-1000 rpm for 10-20 minutes after the addition is completed.

[0020] Preferably, the concentration of the ammonia water in step S3 is 5-8%.

[0021] Preferably, in step S3, the portion of the calcium-containing solution used to generate the crystal nucleus suspension accounts for 5-10% of the total calcium-containing solution.

[0022] In the present invention, the CO2 partial pressure in the solution is too high due to the "dissociation" process, and its buffering effect on pH will delay the crystal form regulation of ammonia, inhibit the subsequent vaterite recrystallization process, and further reduce the vaterite yield. - / CO3 2- The concentration is stable, the reverse dissolution reaction rate is greatly reduced, and the recrystallization efficiency is improved. To further stabilize the yield and purity of vaterite, a nucleus induction method is adopted. Ammonia water is quickly added to the seed crystallizer to increase the supersaturation, prompting a large amount of vaterite nucleation, which is then refluxed into the main crystallizer as seed crystals. The purpose of adding ammonia water in batches is to maintain an appropriate supersaturation and induce the uniform growth of vaterite.

[0023] Preferably, the drying temperature in step S4 is 100-120°C.

[0024] A system for implementing the above-mentioned cement kiln bypass flue gas resource utilization method comprises: The water washing unit is used to cool, remove dust and wash the cement kiln bypass flue gas, thereby obtaining water-washing salt extraction liquid and washed flue gas; The dissolution-recrystallization unit includes an absorption tower, a filter tower, a degassing tower, a seed crystallizer, and a main crystallizer. The liquid inlet of the filter tower is connected to the liquid outlet of the absorption tower; the feed inlet of the degassing tower is connected to the liquid outlet of the filter tower; the liquid outlet of the degassing tower is respectively connected to the feed inlet of the seed crystallizer and the feed inlet of the main crystallizer; the feed outlet of the seed crystallizer is connected to the feed inlet of the main crystallizer; the absorption tower is used to receive water-extracted salt solution and scrubbing flue gas to generate carbonized liquid; The product recovery unit includes a separator and a dryer, wherein the feed port of the separator is connected to the discharge port of the main crystallizer, and the feed port of the dryer is connected to the slag discharge port of the separator.

[0025] Preferably, the water washing unit includes a heat exchanger, a dust collector, a first washing tower and a second washing tower; the heat medium outlet of the heat exchanger is connected to the feed inlet of the dust collector, the discharge port of the dust collector is connected to the feed inlet of the first washing tower, the air outlet of the dust collector is connected to the air inlet of the second washing tower, the liquid outlet of the first washing tower is connected to the liquid inlet of the absorption tower, and the air outlet of the second washing tower is connected to the air inlet of the absorption tower.

[0026] Preferably, the degassing tower air outlet is connected to the absorption tower air inlet, the absorption tower air outlet is connected to the second washing tower air inlet, and the second washing tower liquid outlet is connected to the first washing tower feed inlet.

[0027] Preferably, the product recovery unit further comprises an evaporator and a cooling crystallizer, the feed port of the evaporator is connected to the liquid outlet of the separator, and the feed port of the cooling crystallizer is connected to the discharge port of the evaporator.

[0028] Preferably, the liquid outlet of the cooling crystallizer is connected to the feed inlet of the first washing tower.

[0029] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention pioneered the "dissolution-recrystallization" coupling process, which truly realized the efficient synthesis of vaterite in ultra-low calcium waste liquid (i.e., water washing salt solution). Specifically, the mud kiln bypass flue gas is first used to dissociate calcium carbonate into soluble Ca 2+ , and then the free CO2 in the system is stripped off through the degassing tower to completely inhibit reverse dissolution. The degassed calcium-containing liquid directly generates vaterite microcrystal suspension in situ in the seed crystallizer and flows back to the main crystallizer as a uniform heterogeneous nucleation site; in the main crystallization stage, ammonia water is added in stages and mechanically stirred to make Ca 2+ It preferentially precipitates on the surface of the seed crystal and grows evenly, with almost no calcite or amorphous calcium carbonate generated. The purity and morphology of the product are stable and controllable.

[0030] (2) In the present invention, the waste heat of the flue gas at the kiln tail is recovered through a heat exchanger and used for drying, evaporation and preheating processes; the high-purity CO2 gas discharged from the degassing tower is refluxed to the absorption tower; the filtrate of the main crystallizer is evaporated, concentrated and cooled and crystallized to obtain KCl / NH4Cl products; the mother liquor after cooling and crystallization and the washing water obtained from washing the flue gas in the second washing tower are purified and used as washing water for the primary washing tower respectively, thereby realizing a closed cycle of heat energy and materials in the entire process.

[0031] (3) The key equipment involved in this system, including heat exchangers, dust collectors, washing towers, absorption towers, filter towers, degassing towers, seed kettles, main crystallization kettles, filter presses and multi-effect evaporation-crystallization systems, can be directly connected or integrated into the cement kiln co-treatment system and the reclaimed water treatment system after modification, so as to realize the sharing of flue gas, steam, vacuum and hot water pipelines; the required ammonia water, heat source and seed crystals are all from the existing facilities in the factory area, and ultimately achieve deep integration with the existing production system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the process flow of the cement kiln bypass flue gas resource utilization method described in an embodiment of the present invention.

[0033] Figure 2 The XRD patterns of the calcium carbonate products prepared in Examples 1-3 and Comparative Examples 1-2 are shown.

[0034] Figure 3 This is an SEM image of the vaterite product prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0036] Figure 1 This is a process flow diagram of the cement kiln bypass flue gas resource utilization method according to an embodiment of the present invention. Figure 1 The embodiment of the present invention provides a system for implementing the above-mentioned cement kiln bypass venting flue gas resource utilization method, comprising: The water washing unit is used to cool, remove dust and wash the cement kiln bypass flue gas, thereby obtaining water-washing salt extraction liquid and washed flue gas; The dissolution-recrystallization unit includes an absorption tower, a filter tower, a degassing tower, a seed crystallizer, and a main crystallizer. The liquid inlet of the filter tower is connected to the liquid outlet of the absorption tower; the feed inlet of the degassing tower is connected to the liquid outlet of the filter tower; the liquid outlet of the degassing tower is respectively connected to the feed inlet of the seed crystallizer and the feed inlet of the main crystallizer; the feed outlet of the seed crystallizer is connected to the feed inlet of the main crystallizer; the absorption tower is used to receive water-extracted salt solution and scrubbing flue gas to generate carbonized liquid; The product recovery unit includes a separator and a dryer, wherein the feed port of the separator is connected to the discharge port of the main crystallizer, and the feed port of the dryer is connected to the slag discharge port of the separator.

[0037] In some embodiments of the present invention, the absorption tower adopts a countercurrent absorption tower. Compared with the traditional stirring-aeration process, the countercurrent absorption tower can achieve gas-liquid contact more efficiently, increase the local CO2 concentration, and obtain Ca-rich 2+ / HCO3 - of carbonized liquid.

[0038] In some embodiments of the present invention, the seed crystal device and the main crystallizer adopt a reactor, which has strong controllability and wide adaptability, so that the conditions can be strictly controlled during the seed crystal generation and crystal growth process, which helps to obtain high-quality crystals; the degassing tower adopts a vacuum degassing tower, and the vacuum decompression method is simpler to operate and more efficient than the inert gas stripping method.

[0039] In some embodiments of the present invention, the separator is a filter press, which can efficiently separate solids from liquids, is particularly suitable for high-concentration suspensions, and has a high degree of automation and is easy to maintain.

[0040] Furthermore, the water washing unit includes a heat exchanger, a dust collector, a first washing tower and a second washing tower; the heat medium outlet of the heat exchanger is connected to the feed inlet of the dust collector, the discharge port of the dust collector is connected to the feed inlet of the first washing tower, the air outlet of the dust collector is connected to the air inlet of the second washing tower, the liquid outlet of the first washing tower is connected to the liquid inlet of the absorption tower, and the air outlet of the second washing tower is connected to the air inlet of the absorption tower.

[0041] Furthermore, the degassing tower's gas outlet is connected to the absorption tower's gas inlet, which in turn is connected to the second scrubber's gas inlet, and the second scrubber's liquid outlet is connected to the first scrubber's feed inlet. The degassing tower's gas outlet is connected to the absorption tower's gas inlet, allowing the removed CO2 to be recycled. Unreacted scrubbing flue gas from the absorption tower is fed through the gas outlet to the second scrubber for further scrubbing, achieving gas recycling. The scrubbing liquid from the second scrubber is then transported through the liquid outlet to the first scrubber for continued scrubbing. Through this process, the present invention achieves a closed-loop material circulation system.

[0042] In some embodiments of the present invention, the dust collector is a cyclone dust collector, which is used to initially capture most of the dust in the cement kiln bypass vent gas to avoid affecting the quality of the final vaterite product; the flue gas treated by the cyclone dust collector is then introduced into the second washing tower for subsequent washing treatment. Furthermore, the collected bypass vent ash and the washing water discharged from the second washing tower are jointly input into the first washing tower, and a water washing salt extracting solution is generated through a water washing reaction; this process simultaneously achieves: (1) removal of soluble calcium ions and alkali metal ions in the bypass vent ash to achieve high value-added product recovery; (2) reduction of chloride ion content to avoid erosion of the production line and products during recycling; because excessive washing water is required to ensure ion removal efficiency, low-calcium waste liquid is generated.

[0043] Furthermore, the product recovery unit also includes an evaporator and a cooling crystallizer. The evaporator feed port is connected to the separator liquid outlet, and the cooling crystallizer feed port is connected to the evaporator outlet. The liquid separated by the separator is evaporated and concentrated to obtain a concentrated liquid, which then enters the cooling crystallizer to precipitate KCl and NH4Cl crystals under cooling conditions, completing the K in the system. + and Cl - recycling.

[0044] In some embodiments of the present invention, the evaporator is a multiple-effect evaporator, which can significantly reduce the demand for external heat sources and improve overall energy utilization.

[0045] Furthermore, the liquid outlet of the cooling crystallizer is connected to the feed inlet of the first washing tower, and the mother liquid in the cooling crystallizer is returned to the first washing tower for reuse, forming a complete closed-loop material circulation.

[0046] The working principle of the system for implementing the above-mentioned cement kiln bypass flue gas resource utilization method is as follows: Cement kiln bypass vent gas first passes through a heat exchanger to reduce its temperature for subsequent processing. The flue gas then enters a dust collector, separating the bypass vent ash and dust-collected flue gas. The bypass vent ash is scrubbed in a first scrubber to produce a water-washed brine solution; the dust-collected flue gas is purified in a second scrubber to produce washed flue gas. Finally, the water-washed brine solution and washed flue gas are fed into an absorption tower, where they react to produce a carbonized liquid. The carbonized liquid is filtered through a filter tower to remove insoluble impurities, and the resulting filtrate is transferred to a degassing tower for carbon dioxide removal, yielding a calcium-containing liquid. A portion of the calcium-containing liquid is placed in a seed crystallizer, where ammonia water is rapidly added under stirring to generate a seed crystal suspension. The seed crystal suspension is then transferred to the main crystallizer, where the remaining calcium-containing liquid is added. Ammonia water is added in stages to produce a crystallization reaction, producing a vaterite suspension. The suspension is separated into solid and liquid by a separator, and the obtained slag phase is dried to finally obtain the vaterite product; the obtained liquid phase is first evaporated to obtain a concentrated liquid, and then cooled and crystallized to obtain a KCl / NH4Cl product.

[0047] Example 1 A method for recycling cement kiln bypass flue gas, comprising the following steps: (1) The flue gas from the bypass vent at the tail of the cement kiln is first heat exchanged through a heat exchanger. The CO2 concentration in the flue gas is 17%, and the remaining heat is used to heat the dryer and multi-effect evaporator. After the waste heat recovery, the dust-laden flue gas enters the bag filter or cyclone dust collector for dust removal. The dust-removed flue gas is sent to the second scrubber for washing and purification. The washed flue gas enters the subsequent absorption section. The washing liquid is returned to the first scrubber to wash the bypass vent ash, thereby obtaining a water-washing salt solution, in which Ca 2+ =2.6g / L, K + =137g / L, solution pH=13; (2) The scrubbing flue gas is introduced into the bottom of the countercurrent absorption tower and made to flow from bottom to top, and to fully contact the gas and liquid with the water washing salt solution from top to bottom. The volume ratio of the scrubbing flue gas to the water washing salt solution is 0.5m 3 / m 3 , the generated Ca-rich 2+ / HCO3 - The carbonized liquid is discharged from the bottom of the absorption tower and sent to the filter tower; (3) The carbonized liquid is filtered to separate the solid and liquid to obtain a calcium-containing filtrate and insoluble impurities; the filtrate enters a vacuum or inert gas degassing tower to strip off the dissolved CO2. After degassing, the pCO2 in the solution is 0.08atm. The resulting tail gas is refluxed to the countercurrent absorption tower, and the degassed calcium-containing liquid is then sent to the crystallization section; (4) 10 wt% of the degassed calcium-containing solution was transferred to the seeding kettle, and ammonia water (concentration 8%) was added at a ratio of 5.3 g / L at one time to raise the pH of the solution to 10. After rapid stirring for 5 minutes, a vaterite crystal nucleus suspension was generated, which was directly refluxed to the main crystallization kettle, and the remaining 90 wt% of the degassed calcium-containing solution was added. Subsequently, ammonia water of the same concentration was added four times, each adding a ratio of 2 g / L. The mixture was mechanically stirred for 20 minutes to selectively precipitate calcium carbonate under the action of the seeding crystals, and finally a vaterite-type calcium carbonate suspension was obtained. (5) The vaterite suspension is passed through a filter press for solid-liquid separation, and the resulting filter residue is transported to a dryer and dried at 110°C to obtain a vaterite-type calcium carbonate product. After testing, the vaterite content in the product is 99.5%; the filtrate obtained by filtration enters the subsequent process, wherein Ca 2+ =0.13g / L; (6) The filtrate is sent to a multi-effect evaporator and concentrated at 80℃. The concentrated liquid is then cooled to 30℃ at a rate of 0.5℃ / min in a cooling crystallizer and kept warm for 60 minutes to collect KCl crystals. It is then cooled to 5℃ at a rate of 0.1℃ / min and kept warm for 120 minutes to collect NH4Cl crystals. The K in the desalted liquid produced by evaporation is + =3g / L, and after simple purification, it is recycled back to the first washing tower to achieve a closed-loop cycle of energy and materials in the entire process.

[0048] Example 2 A method for recycling cement kiln bypass flue gas, comprising the following steps: (1) The flue gas from the bypass vent at the tail of the cement kiln is first heat exchanged through a heat exchanger. The CO2 concentration in the flue gas is 17%, and the remaining heat is used to heat the dryer and multi-effect evaporator. After the waste heat recovery, the dust-laden flue gas enters the bag filter or cyclone dust collector for dust removal. The dust-removed flue gas is sent to the second scrubber for washing and purification. The washed flue gas enters the subsequent absorption section. The washing liquid is returned to the first scrubber to wash the bypass vent ash, thereby obtaining a water-washing salt solution, in which Ca 2+ =2.9g / L, K + =145g / L, solution pH=13; (2) The scrubbing flue gas is introduced into the bottom of the countercurrent absorption tower and made to flow from bottom to top, and to fully contact the gas and liquid with the water washing salt solution from top to bottom. The volume ratio of the scrubbing flue gas to the water washing salt solution is 0.5m 3 / m 3 , the generated Ca-rich 2+ / HCO3 - The carbonized liquid is discharged from the bottom of the absorption tower and sent to the filter tower; (3) The carbonized liquid is filtered to separate the solid and liquid to obtain a calcium-containing filtrate and insoluble impurities; the filtrate enters a vacuum or inert gas degassing tower to strip off the dissolved CO2. After degassing, the pCO2 in the solution is 0.08atm. The resulting tail gas is refluxed to the countercurrent absorption tower, and the degassed calcium-containing liquid is then sent to the crystallization section; (4) 10 wt% of the degassed calcium-containing solution was transferred to the seeding kettle, and ammonia water (concentration 8%) was added at a ratio of 3.75 g / L at one time to raise the pH of the solution to 9.5. After rapid stirring for 5 minutes, a vaterite crystal nucleus suspension was generated, which was directly refluxed to the main crystallization kettle, and the remaining 90 wt% of the degassed calcium-containing solution was added. Subsequently, ammonia water of the same concentration was added three times, each adding ratio was 3 g / L, and mechanical stirring was performed for 15 minutes to selectively precipitate calcium carbonate under the action of the seeding crystals, and finally a vaterite-type calcium carbonate suspension was obtained; (5) The vaterite suspension is passed through a filter press for solid-liquid separation, and the resulting filter residue is transported to a dryer and dried at 110°C to obtain a vaterite-type calcium carbonate product. After testing, the vaterite content in the product is 97.3%; the filtrate obtained by filtration enters the subsequent process, wherein Ca 2+ =0.15g / L; (6) The filtrate is sent to a multi-effect evaporator and concentrated at 80℃. The concentrated liquid is then cooled to 30℃ at a rate of 0.5℃ / min in a cooling crystallizer and kept warm for 60 minutes to collect KCl crystals. It is then cooled to 5℃ at a rate of 0.1℃ / min and kept warm for 120 minutes to collect NH4Cl crystals. The K in the desalted liquid produced by evaporation is + =3g / L, and after simple purification, it is recycled back to the first washing tower to achieve a closed-loop cycle of energy and materials in the entire process.

[0049] Example 3 A method for recycling cement kiln bypass flue gas, comprising the following steps: (1) The flue gas from the bypass vent at the tail of the cement kiln is first heat exchanged through a heat exchanger. The CO2 concentration in the flue gas is 17%, and the remaining heat is used to heat the dryer and multi-effect evaporator. After the waste heat recovery, the dust-laden flue gas enters the bag filter or cyclone dust collector for dust removal. The dust-removed flue gas is sent to the second scrubber for washing and purification. The washed flue gas enters the subsequent absorption section. The washing liquid is returned to the first scrubber to wash the bypass vent ash, thereby obtaining a water-washing salt solution, in which Ca 2+ =2.9g / L, K + =145g / L, solution pH=13; (2) The scrubbing flue gas is introduced into the bottom of the countercurrent absorption tower and made to flow from bottom to top, and to fully contact the gas and liquid with the water washing salt solution from top to bottom. The volume ratio of the scrubbing flue gas to the water washing salt solution is 0.5m 3 / m3 , the generated Ca-rich 2+ / HCO3 - The carbonized liquid is discharged from the bottom of the absorption tower and sent to the filter tower; (3) The carbonized liquid is filtered to separate the solid and liquid to obtain a calcium-containing filtrate and insoluble impurities; the filtrate enters a vacuum or inert gas degassing tower to strip off the dissolved CO2. After degassing, the pCO2 in the solution is 0.06atm. The resulting tail gas is refluxed to the countercurrent absorption tower, and the degassed calcium-containing liquid is then sent to the crystallization section; (4) 10 wt% of the degassed calcium-containing solution was transferred to the seeding kettle, and ammonia water (concentration 8%) was added at a ratio of 2.5 g / L at one time to raise the pH of the solution to 9. After rapid stirring for 5 minutes, a vaterite crystal nucleus suspension was generated, which was directly refluxed to the main crystallization kettle, and the remaining 90 wt% of the degassed calcium-containing solution was added. Subsequently, ammonia water of the same concentration was added three times, each adding ratio was 3 g / L, and mechanical stirring was performed for 18 minutes to selectively precipitate calcium carbonate under the action of the seeding crystals, and finally a vaterite-type calcium carbonate suspension was obtained; (5) The vaterite suspension is passed through a filter press for solid-liquid separation, and the resulting filter residue is transported to a dryer and dried at 110°C to obtain a vaterite-type calcium carbonate product. After testing, the vaterite content in the product is 96%; the filtrate obtained by filtration enters the subsequent process, wherein Ca 2+ =0.1g / L; (6) The filtrate is sent to a multi-effect evaporator and concentrated at 80℃. The concentrated liquid is then cooled to 30℃ at a rate of 0.5℃ / min in a cooling crystallizer and kept warm for 60 minutes to collect KCl crystals. It is then cooled to 5℃ at a rate of 0.1℃ / min and kept warm for 120 minutes to collect NH4Cl crystals. The K in the desalted liquid produced by evaporation is + =3g / L, and after simple purification, it is recycled back to the first washing tower to achieve a closed-loop cycle of energy and materials in the entire process.

[0050] Comparative Example 1 A method for recycling cement kiln bypass flue gas, comprising the following steps: (1) The flue gas from the bypass vent at the tail of the cement kiln is first heat exchanged through a heat exchanger. The CO2 concentration in the flue gas is 17%, and the remaining heat is used to heat the dryer and multi-effect evaporator. After the waste heat recovery, the dust-laden flue gas enters the bag filter or cyclone dust collector for dust removal. The dust-removed flue gas is sent to the second scrubber for washing and purification. The washed flue gas enters the subsequent absorption section. The washing liquid is returned to the first scrubber to wash the bypass vent ash, thereby obtaining a water-washing salt solution, in which Ca 2+ =2.6g / L, K + =137g / L, solution pH=13; (2) The scrubbing flue gas is introduced into a stirring device containing aqueous washing salt solution by aeration. The introduction rate of the scrubbing flue gas is 1.2 L / min. Ammonia water is added during this process to control the crystal form. The ammonia water concentration is 8%, and the total amount of ammonia water added is 13.3 g / L. Mechanical stirring is continued until the pH of the solution reaches 8-9, and the reaction is terminated to obtain a calcium carbonate suspension. (3) The calcium carbonate suspension is passed through a filter press for solid-liquid separation, and the resulting filter residue is transported to a dryer and dried at 110°C to obtain a finished calcium carbonate product. After testing, the vaterite content in the product is 37.3%; the filtrate obtained by filtration enters the subsequent process, wherein Ca 2+ =2.33g / L.

[0051] Comparative Example 2 A method for recycling cement kiln bypass flue gas, comprising the following steps: (1) The flue gas from the bypass vent at the tail of the cement kiln is first heat exchanged through a heat exchanger. The CO2 concentration in the flue gas is 17%, and the remaining heat is used to heat the dryer and multi-effect evaporator. After the waste heat recovery, the dust-laden flue gas enters the bag filter or cyclone dust collector for dust removal. The dust-removed flue gas is sent to the second scrubber for washing and purification. The washed flue gas enters the subsequent absorption section. The washing liquid is returned to the first scrubber to wash the bypass vent ash, thereby obtaining a water-washing salt solution, in which Ca 2+ =2.6g / L, K + =137g / L, solution pH=13; (2) The scrubbing flue gas is introduced into the bottom of the countercurrent absorption tower and made to flow from bottom to top, and to fully contact the gas and liquid with the water washing salt solution from top to bottom. The volume ratio of the scrubbing flue gas to the water washing salt solution is 0.5m 3 / m 3 , the generated Ca-rich 2+ / HCO3 - The carbonized liquid is discharged from the bottom of the absorption tower and sent to the filter tower; (3) performing solid-liquid separation in a carbonized liquid filtration tower to obtain a calcium-containing filtrate and insoluble impurities, thereby obtaining a calcium-containing filtrate; (4) 10 wt% of the total calcium-containing solution was transferred to the seeding kettle, and ammonia water (concentration 8%) was added at a ratio of 5 g / L at one time. After rapid stirring for 5 minutes, a vaterite crystal nucleus suspension was generated, which was directly refluxed to the main crystallization kettle, and the remaining 90 wt% of the calcium-containing solution was added. Then, ammonia water of the same concentration was added five times, each adding ratio was 3 g / L. Mechanical stirring was carried out for 34 minutes to selectively precipitate calcium carbonate under the action of the seeding crystals, and finally a vaterite-type calcium carbonate suspension was obtained; (5) The vaterite suspension is passed through a filter press for solid-liquid separation, and the resulting filter residue is transported to a dryer and dried at 110°C to obtain a vaterite-type calcium carbonate product. After testing, the vaterite content in the product is 84.5%; the filtrate obtained by filtration enters the subsequent process, wherein Ca 2+ =2.05g / L.

[0052] The whiteness statistics of the calcium carbonate products prepared in Examples 1 to 3 and Comparative Examples 1 to 2 are shown in Table 1.

[0053] Table 1 Whiteness of calcium carbonate products

[0054] From the results in Table 1, it can be seen that the calcium carbonate product produced by the present invention has a higher whiteness than that produced by the traditional aeration process. This advantage is mainly due to the dissociation-filtration process adopted in the dissolution-recrystallization unit. The excessive introduction of CO2 lowers the pH of the solution, causing the non-ferrous metal ions in the system to be converted from complexes to hydroxide precipitates, which are then removed through solid-liquid separation, thereby effectively improving the whiteness of the product.

[0055] Figure 2 The XRD patterns of the calcium carbonate products prepared in Examples 1 to 3 and Comparative Examples 1 to 2 are as follows: Figure 2 Can find out: except vaterite, only contain minute amount of calcite in the embodiment product, this is mainly owing to the crystal transformation that the thermodynamic instability of vaterite itself causes.Yet, if adopt traditional aeration process that low-calcium waste liquid is carried out carbonization treatment (as comparative example 1), not only can cause calcium carbonate product to be based on calcite, also be accompanied by the reverse dissolution reaction of calcium carbonate in low calcium concentration solution simultaneously, thereby significantly reduce carbonization efficiency.In addition, the result of comparative example 2 has further verified the necessity of degassing step among the present invention, and compared with Example 1, comparative example 2 has consumed more ammoniacal liquor in recrystallization process, and has prolonged the recrystallization time, finally caused the productive rate and the purity synchronous reduction of vaterite.

[0056] Figure 3 The SEM image of the vaterite finished product prepared in Example 1 of the present invention is shown in FIG. Figure 3 It can be seen that the vaterite has a regular crystal form and a uniform particle size distribution, and is expected to be applied to high-end fields such as biomedicine, environmental adsorption, and 3D printing. In addition, the SEM images of the vaterite finished products prepared in Examples 2 and 3 are similar to those in Example 1 and are not given separately here.

[0057] The specific embodiments of the present invention described above do not limit the scope of protection of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A cement kiln bypass flue gas resource recovery method, characterized in that: The steps include: S1. After heat exchange treatment, cement kiln bypass vent flue gas is subjected to dust removal to obtain bypass vent ash and dust-removed flue gas; the bypass vent ash is washed with water to obtain a water-washing salt extraction solution, and the dust-removed flue gas is washed with water to obtain washed flue gas; S2, mixing the water-extracted salt solution with the scrubbing flue gas to generate a carbonized liquid, performing solid-liquid separation on the carbonized liquid, and removing carbon dioxide from the liquid phase to obtain a calcium-containing liquid; S3, taking part of the calcium-containing solution and reacting it with ammonia water to generate a crystal nucleus suspension, adding the suspension to the remaining calcium-containing solution, and adding ammonia water in batches to react to obtain a vaterite suspension; S4, performing solid-liquid separation on the vaterite suspension, and drying the obtained slag phase to obtain a vaterite product.

2. The cement kiln bypass flue gas resource utilization method according to claim 1, characterized in that: Step S2 also includes recycling the removed carbon dioxide as a carbon source to generate the carbonized liquid; and / or Step S4 also includes performing solid-liquid separation on the vaterite suspension, and concentrating the obtained liquid phase by evaporation, cooling and crystallizing to obtain KCl and NH4Cl products.

3. The cement kiln bypass flue gas resource utilization method according to claim 1, characterized in that: The CO2 concentration in the cement kiln bypass flue gas in step S1 is not less than 10%; and / or The Ca in the water-extracted salt solution in step S1 2+ ≤3g / L, K + ≥100g / L, pH≥12.

4. The cement kiln bypass flue gas resource utilization method according to claim 1, characterized in that: The volume of the scrubbing flue gas and water-extracted salt solution in step S2 is 0.1-0.5m 3 / m 3 and / or The specific operation of removing carbon dioxide from the liquid phase in step S2 is: removing dissolved carbon dioxide by vacuum decompression or inert gas stripping; and / or The carbon dioxide partial pressure (pCO2) in the calcium-containing solution in step S2 is less than 0.1 atm.

5. The cement kiln bypass flue gas resource utilization method according to claim 1, characterized in that: In step S3, the specific operation of reacting the portion of the calcium-containing solution with ammonia water to form a crystal nucleus suspension is: adding ammonia water to the calcium-containing solution at a ratio of 2-10 g / L at one time, raising the pH of the system to 9-11 within 1-3 minutes, stirring the reaction at a rate of 400-1000 rpm for 2-10 minutes to obtain a crystal nucleus suspension; and / or The specific operation of adding ammonia water in batches to react in step S3 is as follows: adding ammonia water to the system in batches, with each addition amount being 2-3 g / L and the total addition amount being 6-15 g / L. After the addition is completed, stirring the reaction at a rate of 400-1000 rpm for 10-20 minutes.

6. The cement kiln bypass flue gas resource utilization method according to claim 5, characterized in that: The concentration of the ammonia water in step S3 is 5-8%; and / or In step S3, the portion of the calcium-containing solution used to generate the crystal nucleus suspension accounts for 5-10% of the total calcium-containing solution; and / or The drying temperature in step S4 is 100-120°C.

7. A system for implementing the cement kiln bypass flue gas resource utilization method according to any one of claims 1 to 6, characterized in that: include: The water washing unit is used to cool, remove dust and wash the flue gas from the cement kiln bypass, obtaining water-washing salt solution and washing flue gas; The dissolution-recrystallization unit comprises an absorption tower, a filtration tower, a degassing tower, a seed crystallizer and a main crystallizer, wherein the liquid inlet of the filtration tower is connected to the liquid outlet of the absorption tower; the feed inlet of the degassing tower is connected to the liquid outlet of the filtration tower; and the liquid outlet of the degassing tower is respectively connected to the feed inlet of the seed crystallizer and the feed inlet of the main crystallizer; The seed crystal device outlet is connected to the main crystallizer feed port; the absorption tower is used to receive the water-extracted salt solution and the washing flue gas to generate carbonized liquid; The product recovery unit includes a separator and a dryer, wherein the feed port of the separator is connected to the discharge port of the main crystallizer, and the feed port of the dryer is connected to the slag discharge port of the separator.

8. The system according to claim 7, characterized in that: The water washing unit includes a heat exchanger, a dust collector, a first washing tower and a second washing tower; the heat medium outlet of the heat exchanger is connected to the feed inlet of the dust collector, the discharge port of the dust collector is connected to the feed inlet of the first washing tower, the air outlet of the dust collector is connected to the air inlet of the second washing tower, the liquid outlet of the first washing tower is connected to the liquid inlet of the absorption tower, and the air outlet of the second washing tower is connected to the air inlet of the absorption tower.

9. The system according to claim 8, characterized in that The degassing tower air outlet is connected to the absorption tower air inlet, the absorption tower air outlet is connected to the second washing tower air inlet, and the second washing tower liquid outlet is connected to the first washing tower feed inlet; The product recovery unit further comprises an evaporator and a cooling crystallizer, wherein the feed port of the evaporator is connected to the liquid outlet of the separator, and the feed port of the cooling crystallizer is connected to the discharge port of the evaporator.

10. The system according to claim 9, characterized in that: The liquid outlet of the cooling crystallizer is connected to the feed inlet of the first washing tower.

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