A method for washing sodium-based fly ash from waste incineration

Abstract

The present invention discloses a method for washing sodium-based fly ash from waste incineration, which washes the fly ash generated by using sodium-based deacidification to purify waste incineration flue gas. Through the washing of sodium-based deacidification fly ash, the quantity of washing residues can be minimized to the greatest extent, and a ash-water ratio of 1:3 can meet the co-processing standard of fly ash washing in a cement kiln. A method for washing sodium-based fly ash from waste incineration includes the following processes: the fly ash is subjected to primary washing, and the primary washing separates three phases of heavy solid phase, light solid phase, and liquid. The water used in the primary washing is the mother liquor in the secondary washing; the heavy solid phase in the wet residues from the primary washing undergoes solid-liquid separation in the secondary washing, and the secondary washing separates two phases of solid and liquid; the water used in the secondary washing is the mother liquor in the tertiary washing; the wet residues from the secondary washing enter the tertiary washing for solid-liquid separation, and the tertiary washing separates two phases of solid and liquid; fresh water is used in the tertiary washing process; the inlet temperature of the fresh water is 37-46 °C, the ratio of powder ash to fresh water > 1:3, and the residence time in the washing process is greater than 30 min.

Description

Technical Field

[0001] The present invention relates to the field of environmental protection technologies, and particularly relates to a method for washing sodium-based fly ash from waste incineration with water. Background Art

[0002] Currently, most of the fly ash from waste incineration is fly ash obtained by flue gas purification with calcium-based (calcium hydroxide). There are mainly two methods for its disposal. One is to add a chelating agent and cement to the fly ash and then landfill it after solidification. The other is to wash the fly ash with water, and the washed residue enters the cement kiln for co-disposal, while the salts after washing with water have not been well disposed of.

[0003] Traditional calcium-based (calcium hydroxide) flue gas deacidification technology generates fly ash accounting for 3-5% of the waste incineration volume, with a huge total amount. Chelating and solidifying for landfill not only requires a large amount of land resources, but also has a risk of secondary pollution due to reasons such as stability failure and leakage. It brings greater difficulties to the subsequent disposal after chelating and solidifying for landfill.

[0004] Based on the fact that the reduction of calcium-based fly ash by washing with water is not significant, the washed residue accounts for 60%-70% of the total fly ash. At the same time, calcium sulfate in the fly ash cannot be washed out, and harmful gas sulfur dioxide is discharged again during the co-disposal process in the cement kiln, forming new pollution. A large amount of calcium salts washed out from calcium-based fly ash have little utilization value, and the subsequent disposal is difficult and costly.

[0005] Therefore, in the prior art, there is a choice to use sodium-based (sodium bicarbonate) to purify waste incineration flue gas, and the generated fly ash is washed with water to separate the soluble salts therein. The residue after washing with water is used as a raw material for cement production, and the separated salts are used as chemical raw materials.

[0006] Three-stage countercurrent water washing is a commonly used water washing process for fly ash washing, and its feature is to wash more salts with the least amount of water. Based on the general technical direction of three-stage countercurrent water washing, many calcium-based three-stage countercurrent water washing processes have been disclosed in the prior art, but there is no disclosure of a sodium-based three-stage countercurrent water washing process in the prior art.

[0007] During the research process by the applicant, it was found that if the method disclosed in the calcium-based fly ash water washing process is used, when washing with the ratio of ash to water of 1:3, the sodium-based fly ash cannot be washed clean, because the proportion of soluble salts in sodium-based fly ash is more than 20% higher than that in calcium-based fly ash. The soluble salts in calcium-based fly ash are mainly calcium chloride and potassium chloride. The components of soluble salts in sodium-based fly ash are mainly sodium sulfate, sodium chloride and potassium chloride. Therefore, directly using the calcium-based water washing process cannot achieve the washing effect. Summary of the Invention

[0008] The object of the present invention is to provide a method for washing sodium-based fly ash from waste incineration, which washes the fly ash generated by using sodium-based (sodium bicarbonate) to remove acid and purify waste incineration flue gas. Through the washing of sodium-based deacidified fly ash, the amount of washing residue can be minimized, and a solid-liquid ratio of 1:3 can meet the standard for co-disposal of fly ash washing and cement kilns.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] A method for washing sodium-based fly ash from waste incineration includes the following processes:

[0011] The fly ash is subjected to primary washing. The primary washing separates three phases: heavy solid phase, light solid phase, and liquid. The water used in the primary washing is the mother liquor from the secondary washing.

[0012] The heavy solid phase in the wet residue from the primary washing is subjected to solid-liquid separation in the secondary washing. The secondary washing separates two phases: solid and liquid. The water used in the secondary washing is the mother liquor from the tertiary washing.

[0013] The wet residue from the secondary washing enters the tertiary washing for solid-liquid separation. The tertiary washing separates two phases: solid and liquid. Fresh water is used in the tertiary washing process.

[0014] The inlet temperature of the fresh water is 37 - 46°C, the ratio of fly ash to fresh water is > 1:3, and the residence time in the washing process is greater than 30 minutes.

[0015] As a preferred technical solution, a PAM solution and a sodium sulfide solution are added to the mother liquor of the primary washing for suspension sedimentation treatment and removal of high-valent metal ions. The sedimented mother liquor of the primary washing is subjected to solid-liquid separation, coarse filtration, and fine filtration and then sent to the next process section.

[0016] As a preferred technical solution, the ratio of fly ash to fresh water is 1:3.

[0017] As a preferred technical solution, the inlet temperature of the fresh water is 40°C.

[0018] As a preferred technical solution, the residence time in the washing process is 30 minutes.

[0019] As a preferred technical solution, the moisture content of the wet residue from the tertiary washing is ≤ 25%. Description of the Drawings

[0020] Figure 1 It is the phase diagram of the mixed salt in the present invention;

[0021] Figure 2 It is the comparison of the moisture content and chloride ion content of the residue from the tertiary washing. Detailed Embodiments

[0022] The object of the present invention is to overcome the defects of the prior art and provide a method for washing sodium-based fly ash from waste incineration. The present invention is further described in detail below in conjunction with embodiments.

[0023] Example 1

[0024] A method for washing sodium-based fly ash from garbage incineration, comprising the following steps:

[0025] Primary water washing: fly ash enters the primary water washing process (washed in a washing tank), primary water washing solid-liquid separation The primary water washing separates the solid-solid-liquid three phases at 40°C; the heavy solid phase and light solid phase in the solution are separated at one time;

[0026] PAM solution and sodium sulfide solution are added to the primary water washing mother liquor to carry out suspension sedimentation treatment and remove high-valent metal ions. The settled primary water washing mother liquor is subjected to solid-liquid separation, coarse filtration, and fine filtration before being sent to the next stage. The next stage is to carry out salt separation treatment on the mother liquor.

[0027] Secondary water washing: The wet residue separated by primary water washing enters the secondary water washing process, and solid-liquid separation is performed after washing.

[0028] Three-stage water washing: The wet residue separated from the second-stage water washing enters the third-stage water washing process, and the solid-liquid separation is carried out after washing. The third-stage water washing centrifuge uses a filter-type plate centrifuge to ensure that the moisture content of the separated solid phase is ≤25%. The water used in the third-stage water washing process is fresh water;

[0029] The water washing process of the present invention can achieve a chloride ion content of less than 1% in the residue after washing.

[0030] Since sodium-based waste incineration fly ash contains light solid phase and heavy solid phase, the content of light solid phase is less than 1%. Therefore, in the primary water washing process, it is necessary to separate the water washing mother liquor, heavy solid phase and light solid phase. The light solid phase is mainly organic components such as activated carbon and dioxins added in the flue gas purification process.

[0031] The heavy solid phase and the second wash mother liquor were separated by secondary centrifugation.

[0032] The three-stage centrifuge uses a flat plate centrifuge for separation. The moisture content of the filter residue after treatment by the flat plate centrifuge will be lower. Chloride ions exist in water, and low water content will make the chloride ion content in the filter residue lower.

[0033] Furthermore, in this embodiment, the washing temperature is controlled. The fly ash after the sodium-based flue gas treatment contains mixed salt. According to the phase diagram of mixed salt, 40°C is the peak of mixed salt dissolution, and the solubility of mixed salt is the best at this time.

[0034] Furthermore, the longer the washing time, the cleaner the washing will be. However, the longer the washing time, the higher the energy consumption and the lower the processing volume. Therefore, it is necessary to find a balance.

[0035] Furthermore, the ash-water ratio is a very important indicator. Generally speaking, the larger the water consumption, the greater the dissolution of fly ash in water and the cleaner the water washing. However, a large water consumption will lead to a higher energy consumption in the subsequent treatment process of the washing liquid. Therefore, on the premise of minimizing water consumption, how to wash the fly ash clean (according to the technical indicators, it means that the chloride ion content in the slag is 1%) is a very important point.

[0036] Furthermore, in the present invention, separating the heavy solid phase and the light solid phase during the first-stage water washing is a very core process step, because it is crucial for the subsequent sufficient dissolution of washing salts after separating the light solid phase and the heavy solid phase respectively. Because if the light solid phase is not separated, the light solid phase will adsorb chloride ions during the subsequent water washing process, resulting in difficulty in washing out chloride ions.

[0037] In this embodiment, it is very important to separate three phases during the first washing separation, as shown in the following table:

[0038] Table 1:

[0039]

[0040] In this embodiment, the moisture content of the filter residue is controlled, and the control of the moisture content is closely related to the ash-water ratio, water washing temperature, washing duration, and the selection of the centrifuge. The moisture content of the slag during the water washing process is shown in the following table:

[0041] Table 2: Washing Parameters and Effects

[0042]

[0043] When the ash-water ratio, washing temperature, residence time, and moisture content of the slag discharged from the third-stage water washing change during the water washing process, the washing effects change as shown in the following table.

[0044] Table 3: Influence of Temperature on Washing Effect

[0045]

[0046] Table 4: Influence of Washing Time on Washing Effect

[0047]

[0048] Table 5: Influence of Moisture Content on Washing Effect

[0049]

[0050] Table 6: Influence of Ash-Water Ratio on Washing Effect

[0051]

[0052] As can be seen from Table 3, when the water washing time is 30 min, the moisture content of the third-stage filter residue is controlled at 25%, and the ash-water ratio is 1:3 and these three parameters remain unchanged, and the water washing temperature varies from 37 to 46 °C, the chloride ion content in the third-stage filter residue is <1%. And when the temperature is 40 °C, due to its maximum solubility, the washing effect is the best, and the effect changes little when the temperature exceeds 40 °C. Therefore, the temperature of 40 °C can be determined as the optimal washing temperature.

[0053] As can be seen from Table 4, when the water washing temperature is 40 °C, the moisture content of the third-stage filter residue is controlled at 25%, and the ash-water ratio is 1:3 and remains unchanged, and the water washing time is changed. When the water washing time is 30 min, the chloride ion content is 0.84%. The washing time exceeding 30 minutes cannot significantly improve the washing effect. Therefore, the washing time of 30 minutes is determined to be an economical and effective washing time.

[0054] As can be seen from Table 5, when the water washing temperature is 40 °C, the water washing residence time is 30 min, and the ash-water ratio is 1:3 and remains unchanged, and the moisture content of the third-stage water washing residue is changed. When the moisture content is 25%, the chloride ion in the residue is 0.84%. It is more difficult to further reduce the moisture content, and at the same time, the chloride ion content does not decrease significantly.

[0055] As can be seen from Table 6, when the water washing temperature is 40 °C, the water washing residence time is 30 min, and the moisture content of the third stage is controlled at 25% and remains unchanged, and the ash-water ratio is changed. When the ash-water ratio is above 1:3, the chloride ion content is 0.85%. The ash-water ratio exceeding 1:3 has little effect on the chloride ion content in the filter residue. However, the greater the water consumption, the higher the energy consumption for the subsequent treatment of the water washing mother liquor. Therefore, the ash-water ratio of 1:3 is determined as the optimal point.

[0056] Generally speaking, for the fly ash water washing process, it is necessary to comprehensively consider the energy consumption, treatment volume, and washing as much fly ash as possible with as little water as possible. Therefore, it is necessary to comprehensively control the water washing residence time, water washing temperature, moisture content of the third-stage wet residue, and ash-water ratio.

[0057] The following table shows the corresponding water consumption (ash-water ratio) when the water washing time, water washing temperature, and moisture content of the third-stage wet residue change, and if the chloride ion content of the third-stage wet residue is to be lower than 1%:

[0058] Table 7: Influence of Temperature, Time and Moisture Content on Ash-Water Ratio

[0059]

[0060] As can be seen from this table, if the residence time, water washing temperature, and moisture content of the third-stage wet residue are not well controlled, more water needs to be used and the energy consumption is higher.

[0061] It should be noted that on the premise of the above structural design, in order to solve the same technical problems, even if some non-substantive changes or polish are made to the present invention, the essence of the technical solution adopted is still the same as that of the present invention, so it should also be within the protection scope of the present invention.

Claims

1. A method for washing sodium-based fly ash from waste incineration, characterized in that, It consists of the following processes: The fly ash undergoes primary water washing. The primary water washing separates three phases: heavy solid phase, light solid phase, and liquid. The water used in the primary water washing is the mother liquor from the secondary water washing; The heavy solid phase in the wet slag from the primary water washing undergoes solid-liquid separation in the secondary water washing. The secondary water washing separates two phases: solid and liquid. The water used in the secondary water washing is the mother liquor from the tertiary water washing; The wet slag from the secondary water washing enters the tertiary water washing for solid-liquid separation. The tertiary water washing separates two phases: solid and liquid. Fresh water is used in the tertiary water washing process; The inlet temperature of the fresh water is 37 - 46 °C, the ratio of fly ash to fresh water > 1:3, and the residence time in the water washing process is greater than 30 min; The moisture content of the wet slag from the tertiary water washing ≤ 25%; 2. The sodium-based fly ash washing method for waste incineration according to claim 1, wherein PAM solution and sodium sulfide solution are added to the mother liquor from the primary water washing for suspension sedimentation treatment and removal of high-valent metal ions. The sedimented mother liquor from the primary water washing is subjected to solid-liquid separation, coarse filtration, and fine filtration and then sent to the next process section; 3. A sodium-based fly ash washing method for waste incineration according to claim 1, characterized in that, The ratio of fly ash to fresh water is 1:3; 4. A method for washing sodium-based fly ash from waste incineration according to claim 1, characterized in that, The inlet temperature of the fresh water is 40 °C; 5. A method for washing sodium-based fly ash from waste incineration according to claim 1, characterized in that, The residence time in the water washing process is 30 min.

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