Method and system for washing, dechlorinating and protecting calcium of fly ash based on carbonation pretreatment

Abstract

The invention relates to a method and a system for washing, dechlorinating and protecting calcium of fly ash based on carbonation pretreatment, and belongs to the technical field of resource utilization of dangerous solid wastes. The method comprises the following steps: exposing fly ash with the water content of 10-20% in air to carry out natural carbonation pretreatment, so that the pH value of the fly ash is not higher than 9; and washing the fly ash subjected to natural carbonation pretreatment by adopting a citric acid solution. According to the method provided by the invention, the defects of traditional water washing are overcome, the Ca-based component in the fly ash can be retained while the chlorine reaches the standard, and the mass loss caused in the fly ash elution process is reduced to a certain extent, so that the resource property of the fly ash is well retained, and the resource utilization value of the fly ash is improved.

Description

Technical Field

[0001] The invention relates to a method and system for removing chlorine and retaining calcium by water washing of fly ash based on carbonation pretreatment, and belongs to the technical field of resource utilization of hazardous solid wastes. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention, and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] The fly ash from garbage incineration contains a large amount of CaO-SiO 2 -Al 2 O 3 -Fe 2 O 3 Beneficial ingredients, with great potential as auxiliary cementitious materials. Due to the high content of chloride salts in fly ash, the amount of replacement allowed by fly ash is severely limited, and appropriate pretreatment must be carried out to eliminate chloride salts.

[0004] Water washing dechlorination technology has gradually become a widely used pretreatment method in the resource utilization of fly ash due to its simple process and controllable cost. Simple water washing is difficult to meet the requirements of soluble chlorine content. In order to achieve a better dechlorination effect, it is often chosen to increase the water washing liquid-solid ratio, but a large amount of harmful wastewater will be generated, increasing the cost of subsequent wastewater treatment. Adding reagents for acid washing can further dissolve insoluble chlorides, but the introduction of acid will also cause a large loss of beneficial elements such as Ca in fly ash, thereby increasing the quality loss of water washing products and greatly reducing the resource utilization value of water washing products. At present, most of the research only focuses on whether the chloride salts meet the standards, and there are fewer studies on the retention of beneficial elements such as Ca, ignoring the retention of the resource attributes of fly ash. Therefore, how to achieve efficient dechlorination and simultaneously retain Ca-based components is an urgent problem to be solved in the process of fly ash treatment and disposal. Summary of the invention

[0005] In order to address the deficiencies in the prior art, the purpose of the present invention is to provide a method and system for water washing chlorine removal and calcium retention in fly ash based on carbonation pretreatment. The method provided by the present invention overcomes the disadvantages of traditional water washing, can retain the Ca-based components in the fly ash while achieving chlorine standards, and to a certain extent reduces the mass loss caused by the fly ash elution process, thereby better retaining the resource properties of the fly ash and improving the value of fly ash resource utilization.

[0006] In order to achieve the above object, the technical solution of the present invention is:

[0007] In the first aspect, a method for removing chlorine and retaining calcium from fly ash by water washing based on carbonation pretreatment comprises the following process:

[0008] The fly ash with a moisture content of 10-20% is exposed to air for natural carbonation pretreatment, so that the pH value of the fly ash is not higher than 9; the fly ash after the natural carbonation pretreatment is washed with a citric acid solution.

[0009] The present invention performs natural carbonation pretreatment on the fly ash before washing. Since Ca(OH) 2 The presence of alkaline substances such as carbon dioxide makes it have a higher acid neutralization ability. When the acidic water washing liquid is mixed with fly ash, the high alkalinity of the fly ash will neutralize part of the acid, resulting in the pH of the water washing environment not being low enough, and only the soluble salts and a small part of Friedel's salt in the fly ash can be removed. However, in an environment rich in carbon dioxide, CaO, Ca(OH) 2 , CaClOH reacts easily to form thermodynamically stable calcium carbonate and reduce the alkalinity of fly ash. The present invention has experimentally found that the soluble chlorine content of fly ash pretreated by natural carbonation is reduced by 13.7-17% compared with the original fly ash. This shows that pre-treating fly ash by natural carbonation first is beneficial to the subsequent dechlorination of water washing and the retention of Ca-based components.

[0010] The present invention further uses citric acid solution as a washing agent to wash the naturally carbonated pretreated fly ash. Although the soluble chlorine content in the washed slag washed with a simple citric acid solution can meet the requirements of the "Technical Specifications for Pollution Control of Fly Ash from Incineration of Municipal Waste (Trial)" (HJ 1134-2020), a large amount of calcium-based components are lost. Through experiments, the present invention unexpectedly found that the soluble chlorine content in the washed slag of the naturally carbonated pretreated fly ash eluted with a citric acid solution is more significantly reduced, and at the same time, the calcium-based components are better retained, which greatly improves the resource utilization value of the fly ash.

[0011] In a second aspect, a fly ash water washing pretreatment product is obtained by the above method.

[0012] A third aspect provides an application of the fly ash water washing pretreatment product in the field of building materials.

[0013] In a fourth aspect, a fly ash water washing dechlorination and calcium preservation system based on carbonation pretreatment comprises:

[0014] A pretreatment device, used to expose fly ash with a moisture content of 10 to 20% to air for natural carbonation pretreatment;

[0015] The water washing device is used to wash the fly ash after natural carbonation pretreatment with citric acid solution.

[0016] The beneficial effects of the present invention are:

[0017] (1) The carbonation pretreatment of the present invention is carried out by directly exposing the waste incineration fly ash to the air, without consuming industrial gas, thus reducing the cost.

[0018] (2) The present invention performs carbonation pretreatment on fly ash to convert the easily eluted calcium-based components into a more stable state, thereby reducing the loss of calcium in the subsequent pickling process. Compared with simple citric acid washing, the Ca loss rate is reduced by 14.32%, the mass loss is reduced by 8.23%, and its resource recovery attribute is improved.

[0019] (3) The present invention uses citric acid solution as a water washing agent, which can effectively remove chloride salts from fly ash. Compared with traditional water washing, the washing effect is more obvious, and the soluble chlorine content can be reduced to less than 1%. Moreover, compared with inorganic acids, citric acid is an organic acid that is degradable and has less impact on the environment.

[0020] (4) The method provided by the present invention retains most of the calcium in the fly ash, so that the calcium ion concentration in the water wash liquid obtained after solid-liquid separation is greatly reduced, which significantly reduces the hardness removal cost in the subsequent water wash liquid treatment process. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0022] Figure 1 This is a flow chart of fly ash water washing, dechlorination and calcium preservation based on carbonation pretreatment in an embodiment of the present invention;

[0023] Figure 2 The mineral composition diagram of the original fly ash, lightly carbonated fly ash, and heavily carbonated fly ash in the embodiments of the present invention;

[0024] Figure 3 This is a surface morphology diagram of the original fly ash in an embodiment of the present invention;

[0025] Figure 4 This is a surface morphology diagram of lightly carbonated fly ash in an embodiment of the present invention;

[0026] Figure 5 This is a surface morphology diagram of heavily carbonated fly ash in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0028] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0029] In view of the fact that the existing water washing technology causes a large loss of the beneficial component Ca in the process of washing fly ash, reduces the resource attribute of fly ash, and produces a large quality loss, the present invention proposes a fly ash water washing dechlorination and calcium retention method and system based on carbonation pretreatment.

[0030] A typical embodiment of the present invention provides a method for removing chlorine and retaining calcium from fly ash by water washing based on carbonation pretreatment, comprising the following process:

[0031] The fly ash with a moisture content of 10-20% is exposed to air for natural carbonation pretreatment, so that the pH value of the fly ash is not higher than 9; the fly ash after the natural carbonation pretreatment is washed with a citric acid solution.

[0032] The present invention controls the moisture content of the fly ash to 10-20%, which is beneficial to retaining the Ca-based components in the fly ash.

[0033] In some embodiments, natural carbonation pretreatment is performed until the pH value of the fly ash is between 7 and 9. Studies have shown that this condition is beneficial for retaining the Ca-based components in the fly ash.

[0034] In some embodiments, the natural carbonation pretreatment time is 5 to 8 days. Studies have shown that this condition can allow the natural carbonation pretreatment to reach a pH value of 7 to 9 in the fly ash.

[0035] In some embodiments, the liquid-to-solid ratio of the citric acid solution to the fly ash is 3 to 20 mL / g, and the concentration of the citric acid solution is 1 to 12 g / L. Specifically, the liquid-to-solid ratio is 6 to 12 mL / g. Specifically, the concentration of the citric acid solution is 2 to 6 g / L. Studies have shown that the removal of chloride salts is better under this condition.

[0036] In some embodiments, the washing time is 10 to 90 minutes, and the washing temperature is 25 to 80° C. Specifically, the washing time is 30 to 60 minutes. Specifically, the washing temperature is 25 to 50° C. Studies have shown that the removal of chloride salts is better under these conditions.

[0037] Another embodiment of the present invention provides a fly ash water washing pretreatment product obtained by the above method.

[0038] A third embodiment of the present invention provides an application of the above-mentioned fly ash water washing pretreatment product in the field of building materials.

[0039] The application of the present invention in the field of building materials is divided into high-temperature building material utilization and non-high-temperature building material utilization according to the specific building material preparation process. Among them, high-temperature building material utilization mainly refers to building material utilization under high temperature conditions, such as 1300-1450℃ cement kiln co-treatment to prepare cement clinker, 700-1100℃ high-temperature sintering to prepare ceramsite, 1000-1400℃ high-temperature melting to prepare microcrystalline glass, etc., that is, high temperature means sintering temperature above 700℃; non-high-temperature building material utilization, such as the production of concrete admixtures, unfired bricks, geopolymers, zeolite materials, etc.

[0040] A fourth embodiment of the present invention provides a fly ash water washing dechlorination and calcium preservation system based on carbonation pretreatment, comprising:

[0041] A pretreatment device, used to expose fly ash with a moisture content of 10 to 20% to air for natural carbonation pretreatment;

[0042] The water washing device is used to wash the fly ash after natural carbonation pretreatment with citric acid solution.

[0043] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

[0044] The chemical composition of the fly ash used in the following examples is shown in Table 1.

[0045] Table 1 Chemical composition of raw fly ash (wt / %)

[0046]

[0047] Example 1

[0048] Fly ash water washing dechlorination and calcium retention method based on carbonation pretreatment, such as Figure 1 As shown, the steps are as follows:

[0049] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0050] (2) A certain amount of fly ash is weighed and spread evenly and thinly on the surface of a tray, and deionized water is evenly applied to the surface of the fly ash in a spray form to obtain a carbonated initial fly ash sample with a moisture content of 15%.

[0051] (3) Directly expose to air at room temperature for 8 days until the pH of the fly ash drops to 7, thereby obtaining a heavily carbonated fly ash sample.

[0052] (4) Dry the heavily carbonated fly ash sample at 80°C, grind it into powder, seal it and store it for later use. Measure its soluble chlorine content.

[0053] (5) Weigh 10 g of heavily carbonated fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 10 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 30 °C for 40 min.

[0054] (6) After standing for 3 minutes, the mixture obtained in step (5) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0055] (7) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0056] Example 2

[0057] Fly ash water washing dechlorination and calcium retention method based on carbonation pretreatment, such as Figure 1 As shown, the steps are as follows:

[0058] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0059] (2) A certain amount of fly ash is weighed and spread evenly and thinly on the surface of a tray, and deionized water is evenly applied to the surface of the fly ash in a spray form to obtain a carbonated initial fly ash sample with a moisture content of 15%.

[0060] (3) Directly expose to air at room temperature for 8 days until the pH of the fly ash drops to 7, thereby obtaining a heavily carbonated fly ash sample.

[0061] (4) Dry the heavily carbonated fly ash sample at 80°C, grind it into powder, seal it and store it for later use. Measure its soluble chlorine content.

[0062] (5) Weigh 10 g of heavily carbonated fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 8 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 40 °C for 50 min.

[0063] (6) After standing for 3 minutes, the mixture obtained in step (5) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0064] (7) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0065] Example 3

[0066] Fly ash water washing dechlorination and calcium retention method based on carbonation pretreatment, such as Figure 1 As shown, the steps are as follows:

[0067] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0068] (2) A certain amount of fly ash is weighed and spread evenly and thinly on the surface of a tray, and deionized water is evenly applied to the surface of the fly ash in a spray form to obtain a carbonated initial fly ash sample with a moisture content of 15%.

[0069] (3) Directly expose to air at room temperature for 8 days until the pH of the fly ash drops to 7, thereby obtaining a heavily carbonated fly ash sample.

[0070] (4) Dry the heavily carbonated fly ash sample at 80°C, grind it into powder, seal it and store it for later use. Measure its soluble chlorine content.

[0071] (5) Weigh 10 g of heavily carbonated fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 12 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 25 °C for 30 min.

[0072] (6) After standing for 3 minutes, the mixture obtained in step (5) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0073] (7) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0074] Example 4

[0075] Fly ash water washing dechlorination and calcium retention method based on carbonation pretreatment, such as Figure 1 As shown, the steps are as follows:

[0076] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0077] (2) A certain amount of fly ash is weighed and spread evenly and thinly on the surface of a tray, and deionized water is evenly applied to the surface of the fly ash in a spray form to obtain a carbonated initial fly ash sample with a moisture content of 15%.

[0078] (3) Directly expose to air at room temperature for 5 days until the pH of the fly ash drops to 9, thereby obtaining a slightly carbonated fly ash sample.

[0079] (4) Dry the lightly carbonated fly ash sample at 80°C, grind it into powder, seal it and store it for later use. Measure its soluble chlorine content.

[0080] (5) Weigh 10 g of lightly carbonated fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 10 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 30 °C for 40 min.

[0081] (6) After standing for 3 minutes, the mixture obtained in step (5) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0082] (7) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0083] Example 5

[0084] Fly ash water washing dechlorination and calcium retention method based on carbonation pretreatment, such as Figure 1 As shown, the steps are as follows:

[0085] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0086] (2) A certain amount of fly ash is weighed and spread evenly and thinly on the surface of a tray, and deionized water is evenly applied to the surface of the fly ash in a spray form to obtain a carbonated initial fly ash sample with a moisture content of 15%.

[0087] (3) Directly expose to air at room temperature for 5 days until the pH of the fly ash drops to 9, thereby obtaining a slightly carbonated fly ash sample.

[0088] (4) Dry the lightly carbonated fly ash sample at 80°C, grind it into powder, seal it and store it for later use. Measure its soluble chlorine content.

[0089] (5) Weigh 10 g of lightly carbonated fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 8 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 40 °C for 50 min.

[0090] (6) After standing for 3 minutes, the mixture obtained in step (5) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0091] (7) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0092] Example 6

[0093] Fly ash water washing dechlorination and calcium retention method based on carbonation pretreatment, such as Figure 1 As shown, the steps are as follows:

[0094] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0095] (2) A certain amount of fly ash is weighed and spread evenly and thinly on the surface of a tray, and deionized water is evenly applied to the surface of the fly ash in a spray form to obtain a carbonated initial fly ash sample with a moisture content of 15%.

[0096] (3) Directly expose to air at room temperature for 5 days until the pH of the fly ash drops to 9, thereby obtaining a slightly carbonated fly ash sample.

[0097] (4) Dry the lightly carbonated fly ash sample at 80°C, grind it into powder, seal it and store it for later use. Measure its soluble chlorine content.

[0098] (5) Weigh 10 g of lightly carbonated fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 12 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 25 °C for 30 min.

[0099] (6) After standing for 3 minutes, the mixture obtained in step (5) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0100] (7) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0101] Example 7

[0102] Fly ash water washing dechlorination calcium preservation method, such as Figure 1 As shown, the steps are as follows:

[0103] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0104] (2) Weigh 10 g of the original fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 10 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 30 °C for 40 min.

[0105] (3) After standing for 3 minutes, the mixture obtained in step (2) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0106] (4) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0107] Example 8

[0108] The method for removing chlorine and retaining calcium by washing fly ash based on carbonation pretreatment has the following steps:

[0109] (1) Dry the original waste incineration fly ash at 105°C for 24 hours and measure its pH and soluble chlorine content.

[0110] (2) A certain amount of fly ash is weighed and spread evenly and thinly on the surface of a tray, and deionized water is evenly applied to the surface of the fly ash in a spray form to obtain a carbonated initial fly ash sample with a moisture content of 15%.

[0111] (3) The sample obtained in step (2) is loaded into a glass column. The bottom of the glass column is connected to one end of a rubber tube, and the other end is connected to a carbon dioxide cylinder via a rotor flowmeter. The flow rate of carbon dioxide is controlled at 0.5 L / min until the pH of the fly ash sample drops to 7.

[0112] (4) Dry the heavily carbonated fly ash sample at 80°C, grind it into powder, seal it and store it for later use. Measure its soluble chlorine content.

[0113] (5) Weigh 10 g of heavily carbonated fly ash sample, mix it evenly with 4 g / L citric acid solution at a liquid-to-solid ratio of 8 mL / g, place it in a glass beaker, place it on a magnetic stirrer (speed of 400 r / min), and stir it at 40 °C for 50 min.

[0114] (6) After standing for 3 minutes, the mixture obtained in step (5) is removed and vacuum filtered to separate the solid and liquid to obtain a water washing liquid and a water washing residue.

[0115] (7) Dry the washed residue at 105°C, weigh it, grind it into powder, and seal it for storage. Determine the soluble chlorine content in the washed residue. Determine the Ca ion content in the washed liquid by EDTA titration, and determine the Cl ion content in the washed liquid by silver nitrate titration.

[0116] After testing Examples 1 to 8, the obtained data are shown in Table 2.

[0117] Table 2 Experimental data table of Examples 1, 2, 3, 4, 5, 6, 7, and 8

[0118]

[0119] As can be seen from Table 2, better dechlorination and calcium retention effects are achieved by washing with citric acid solution after carbonation pretreatment. The soluble chlorine content in the washed slag under this method meets the requirements of HJ 1134-2020 standard. Compared with simple citric acid washing, the soluble chlorine content is lower and the Ca-based component retention effect is better. The degree of carbonation also affects the dechlorination and calcium retention effect of the washing process. Under the same conditions, heavy carbonation has better dechlorination effect and is more conducive to the retention of Ca-based components in fly ash than light carbonation. In addition, the dechlorination and calcium retention effect and quality loss of fly ash pretreated with natural carbonation are better than those of directly forcing the introduction of carbon dioxide. The soluble chlorine content in the washed slag washed with citric acid solution after carbonation pretreatment can be reduced to less than 1% and still belongs to SiO 2 -Al 2 O 3 -CaO-Fe 2 O 3 system, retaining the resource utilization properties of fly ash.

[0120] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for removing chlorine and retaining calcium from fly ash by water washing based on carbonation pretreatment, characterized in that it comprises: The process is as follows: The fly ash with a moisture content of 10-20% is exposed to air for natural carbonation pretreatment, so that the pH value of the fly ash is not higher than 9; the fly ash after the natural carbonation pretreatment is washed with a citric acid solution.

2. The fly ash water washing and chlorine removal and calcium retention method as claimed in claim 1, characterized in that: Natural carbonation pretreatment until the pH value of the fly ash is 7-9.

3. The fly ash water washing and chlorine removal and calcium retention method as claimed in claim 1, characterized in that: The time for natural carbonation pretreatment is 5 to 8 days.

4. The fly ash water washing and chlorine removal and calcium retention method as claimed in claim 1, characterized in that: The liquid-to-solid ratio of the citric acid solution to the fly ash is 3-20 mL / g, and the concentration of the citric acid solution is 1-12 g / L.

5. The fly ash water washing and chlorine removal and calcium preservation method as claimed in claim 4, characterized in that: The liquid-to-solid ratio is 6 to 12 mL / g; or, the concentration of the citric acid solution is 2 to 6 g / L.

6. The fly ash water washing and chlorine removal and calcium retention method as claimed in claim 1, characterized in that: The washing time is 10 to 90 minutes, and the washing temperature is 25 to 80°C.

7. The fly ash water washing and chlorine removal and calcium retention method as claimed in claim 6, characterized in that: The washing time is 30 to 60 minutes; or, the washing temperature is 25 to 50°C.

8. A fly ash washing pretreatment product, characterized in that: The fly ash is obtained by the fly ash water washing, chlorine removal and calcium retention method according to any one of claims 1 to 7.

9. An application of the above-mentioned fly ash water washing pretreatment product in the field of building materials.

10. A fly ash water washing dechlorination and calcium preservation system based on carbonation pretreatment, characterized in that: The method for removing chlorine and retaining calcium by water washing of fly ash according to any one of claims 1 to 7 comprises: A pretreatment device, used to expose fly ash with a moisture content of 10 to 20% to air for natural carbonation pretreatment; The water washing device is used to wash the fly ash after natural carbonation pretreatment with citric acid solution.

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