Method for separating salt from sodium-based fly ash in waste incineration flue gas

Abstract

The present invention discloses a method for separating salts from sodium-based fly ash in waste incineration flue gas. The flue gas generated by waste incineration is purified by sodium-based deacidification, and the fly ash generated by the flue gas is washed with water to obtain salts that can be used as chemical raw materials in the salt solution with high purity and high yield. The method for separating salts from sodium-based fly ash in waste incineration flue gas is implemented in the following steps: S1: Sodium bicarbonate is used as a raw material for waste incineration flue gas purification, and the fly ash produced includes sodium salt and potassium salt; S2: The fly ash is washed with water, and the sodium ions and potassium ions are dissolved in water; S3: The salt solution is purified so that Na⁺, K⁺, Cl⁻, SO4 2⁻ The total ion concentration in the solution is not less than 95%; S4: The salt solution is evaporated and concentrated until the mass concentration of sodium chloride in the salt solution is 15-24%; the concentrated salt solution contains sodium chloride, sodium sulfate, and potassium chloride; S5: The salt solution is crystallized, and Na⁺ is crystallized and separated in the form of sodium chloride, and K⁺ is crystallized and separated in the form of potassium chloride.

Description

Technical Field

[0001] The invention relates to the technical field of environmental protection, and in particular to a method for separating salt from sodium-based fly ash in waste incineration flue gas. Background Art

[0002] With the development of urbanization, waste incineration power generation has become a trend replacing landfill. The amount of fly ash generated during incineration is increasing with the amount of incineration. The safe and economical disposal of fly ash from waste incineration has always been a challenge for the industry. Traditional fly ash chelation and solidification landfills require a large amount of land and pose the risk of secondary pollution.

[0003] Currently, waste incineration fly ash is primarily obtained through calcium-based (calcium hydroxide) flue gas purification. There are two primary methods for its disposal: adding chelating agents and cement to the fly ash, solidifying it and then landfilling it. The other method involves washing the fly ash with water, with the washed residue then being co-processed in cement kilns. However, the salt from the wash is not properly disposed of.

[0004] Sodium-based fly ash refers to the fly ash obtained by treating flue gas with sodium hydroxide, sodium bicarbonate, and sodium carbonate.

[0005] Traditional calcium-based (calcium hydroxide) flue gas deacidification technology produces a significant amount of fly ash, representing 3-5% of the total waste incineration volume. Chelation solidification landfill not only requires significant land resources but also carries the risk of secondary pollution due to stability failure and leakage. This also makes subsequent disposal more difficult.

[0006] Calcium-based water-washed fly ash only produces a minimal reduction in volume, with washed slag accounting for 60%-70% of the total fly ash volume. Furthermore, the calcium sulfate in the fly ash cannot be washed out, leading to the re-emission of harmful sulfur dioxide gases during the cement kiln co-processing process, creating new pollution. The large amount of calcium salts washed out of calcium-based fly ash is of limited utility value, and subsequent disposal is difficult and costly.

[0007] Therefore, in the prior art, sodium base (sodium bicarbonate) is selected as the purification raw material, and the fly ash produced is washed with water to separate the soluble salt therein. The residue after washing is used as a raw material for cement production, and the separated salt is recycled as a chemical raw material to obtain sodium bicarbonate again through chemical reaction.

[0008] However, the existing technical solutions still have the following technical problems:

[0009] Sodium-based circulation and salt separation and crystallization are very important processes. It is necessary to obtain sodium ions and potassium ions in fly ash in the form of salt, and then obtain the required chemical substances through chemical reactions. Although the yield and purity of sodium salts and potassium salts obtained by salt separation and crystallization in the technical solutions disclosed in the prior art are already relatively high, the applicant found that they can be further improved during the subsequent research and development process. Summary of the Invention

[0010] The object of the present invention is to provide a method for separating salt from sodium-based fly ash in waste incineration flue gas. The flue gas generated by waste incineration is purified by deacidification using a sodium-based (sodium bicarbonate) solution, and the fly ash generated is washed with water to obtain a salt solution. Salt that can be used as a chemical raw material in the salt solution is then separated with high purity and high yield.

[0011] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0012] The method for separating salt from sodium-based fly ash in waste incineration flue gas is implemented by the following steps:

[0013] S1: Sodium bicarbonate is used as the raw material for waste incineration flue gas purification, and the fly ash produced includes sodium salt and potassium salt;

[0014] S2: Wash the fly ash with water, and dissolve the sodium and potassium ions in the water;

[0015] S3: Purify the salt solution so that the total ion concentration of Na⁺, K⁺, Cl⁻, and SO42⁻ in the solution is not less than 95%;

[0016] S4: Evaporating and concentrating the salt solution until the mass concentration of sodium chloride in the salt solution is 15-24%; the concentrated salt solution contains sodium chloride, sodium sulfate, and potassium chloride;

[0017] S5: Salt solution is separated and crystallized, Na⁺ is crystallized and separated in the form of sodium chloride, and K⁺ is crystallized and separated in the form of potassium chloride;

[0018] The specific implementation of step S5 includes the following process:

[0019] The concentrated salt solution obtained in step S4 is subjected to cooling crystallization of sodium sulfate by the nanofiltration concentrate of the mother liquor separated in the following sodium sulfate cooling crystallization process, at an operating temperature of 5 to -10°C, to precipitate sodium sulfate decahydrate;

[0020] The nanofiltration clear liquid of the mother liquor separated in the mirabilite cooling crystallization process and the mother liquor separated in the following sodium chloride evaporation crystallization process are subjected to sodium chloride evaporation crystallization. The evaporation crystallization temperature is 60℃~110℃, and the secondary steam temperature is 60℃~80℃. It enters the steam compressor and is heated and pressurized to saturated steam at 80℃~100℃ and recycled as heating steam.

[0021] Sodium chloride slurry is subjected to solid-liquid separation to obtain sodium chloride crystals. The potassium ion concentration is controlled at 6-15% during the sodium chloride evaporation and crystallization process;

[0022] The mother liquor obtained from the solid-liquid separation of the sodium chloride slurry is returned to the evaporation crystallization to control the slurry content during the evaporation crystallization process, and the rest is used for potassium chloride cooling crystallization; the operating temperature of potassium chloride cooling crystallization is -10~5℃, and potassium chloride crystals are precipitated.

[0023] As a preferred technical solution, in the above step S1, the purification process is:

[0024] The amount of sodium bicarbonate added to the flue gas is 16 kg / h of 600 mesh sodium bicarbonate sprayed into every 10,000 Nm3 / h of flue gas; ensure that the purification residence time of sodium bicarbonate in the flue gas is 2 to 3 seconds; the flue gas temperature is not lower than 200°C during purification.

[0025] As a preferred technical solution, the sodium sulfate content in the nanofiltration clear liquid separated in the mirabilite cooling crystallization process is less than 700 ppm.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The present invention achieves high yields of sodium sulfate, sodium chloride, and potassium chloride by controlling the concentration parameters of potassium and sodium and the parameters of the cooling crystallization and evaporation crystallization processes. This eliminates the need to further remove residual sulfate ions after obtaining sodium sulfate through cooling crystallization in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a phase diagram of the salt solution obtained after the flue gas is purified by sodium bicarbonate according to the present invention. DETAILED DESCRIPTION

[0029] The object of the present invention is to overcome the defects of the prior art and provide a method for separating salt from sodium-based fly ash in waste incineration flue gas. The present invention is further described in detail below with reference to the embodiments.

[0030] Example 1

[0031] A method for separating salt from sodium-based fly ash from waste incineration flue gas comprises the following steps:

[0032] S1: Sodium bicarbonate is used as the raw material for waste incineration flue gas purification, and the fly ash produced includes sodium salt and potassium salt;

[0033] The purification process is as follows:

[0034] The amount of sodium bicarbonate added to the flue gas is per 10000Nm 3 / h of flue gas is sprayed with 16kg / h of 600-mesh sodium bicarbonate; the purification residence time of sodium bicarbonate in the flue gas is ensured to be no less than 2s; the flue gas temperature is no less than 200℃ during purification.

[0035] S2: Fly ash is washed with water, and sodium ions are dissolved in water.

[0036] S3: Purification of salt solution to make Na⁺, K⁺, Cl⁻, SO4 2⁻ The total ion concentration in the solution is not less than 95%.

[0037] S4: Evaporate and concentrate the salt solution until the mass concentration of sodium chloride in the salt solution is 15-24%.

[0038] In the present invention, the concentration control during the evaporation and concentration of the salt solution in step S4 has an important influence on the recovery of sodium sulfate. Below this concentration, the yield of sodium sulfate decreases, while above this concentration, the purity of sodium sulfate increases.

[0039] S5: Salt solution is separated and crystallized, Na⁺ is crystallized in the form of sodium chloride, and k + It is crystallized and separated in the form of potassium chloride to obtain sodium chloride with a purity of not less than 90%.

[0040] The specific implementation process of step S5 salt separation and crystallization is as follows:

[0041] The refined concentrated solution obtained in step S4 (sodium chloride content 15-24%, sodium sulfate 2-10%, potassium chloride 2.5-11%) and the nanofiltration concentrate of the mother liquor separated in the following thenardite cooling crystallization process are delivered to the thenardite continuous cooling crystallization system (i.e., the thenardite cooling crystallization process) through a delivery pump. The operating temperature is 5--10° C., and sodium sulfate decahydrate is precipitated, achieving a sodium sulfate recovery rate of more than 99% and a purity of more than 98.5%.

[0042] The nanofiltration supernatant (sodium sulfate content less than 700 ppm) from the mother liquor separated during the mirabilite cooling crystallization process, along with the mother liquor separated during the sodium chloride evaporation crystallization process described below, is continuously fed to the OSLO vacuum evaporation crystallization system (i.e., the sodium chloride evaporation crystallization process) for sodium chloride crystallization. The evaporation crystallization temperature is 60°C to 110°C. The sodium chloride slurry is pumped to a horizontal spiral filter centrifuge for solid-liquid separation. The product is sodium chloride crystals (with a moisture content of approximately 2-3%), achieving a sodium chloride recovery rate exceeding 99% and a purity exceeding 98%. It is particularly important to control the potassium ion concentration during the sodium chloride evaporation crystallization process between 6% and 15%. Above this concentration, potassium ions precipitate along with sodium ions, affecting sodium purity. Below this concentration, sodium yield is affected. The potassium ion concentration is controlled by controlling the evaporation rate and / or circulation rate.

[0043] The mother liquor separated by the horizontal spiral filter centrifuge is returned to the OSLO vacuum evaporation crystallization system to control the slurry content in the crystallization system, and the rest is transported to the potassium chloride cooling crystallization system;

[0044] The mother liquor obtained from the sodium chloride evaporation crystallization is separated by a centrifuge and then continuously cooled and crystallized to precipitate potassium chloride crystals. The operating temperature is -10~5℃, and potassium chloride crystals are precipitated, achieving a potassium chloride recovery rate of over 99%. The centrifuged mother liquor is returned to the sodium chloride evaporation crystallization system.

[0045] The salt solution after the fly ash salt solution separates sodium sulfate is used to separate sodium chloride and potassium chloride. The solution mainly contains sodium ions, potassium ions, chloride ions and a small amount of sulfate ions, etc. Due to the very close solubility characteristics of sodium chloride and potassium chloride, and the influence of ions such as a small amount of sulfate ions is also contained in the solution at the same time, the separation difficulty of sodium ions and potassium ions is very large. After multiple experimental tests, we obtained the phase diagram of the mixed salt solution after washing this kind of waste incineration fly ash. On this theoretical basis, a sodium-potassium separation process design was carried out to obtain high-purity, high-yield sodium chloride and potassium chloride. In the present invention, by controlling the sodium ion concentration, potassium ion concentration and other process parameters of the coordination to achieve the whole salt separation crystallization process, higher sodium sulfate, sodium chloride, potassium chloride yield and purity are obtained. Although the prior art discloses obtaining sodium chloride by evaporation, concentration, and crystallization, and obtaining potassium chloride by freezing and crystallization, the different components and concentrations in the mixed salt solution result in significant differences in their phase diagrams, leading to significant differences in their processes. A variety of processes are available, and the yields and purities of the sodium chloride and potassium chloride ultimately separated by different processes vary. The processes disclosed in the prior art yield lower yields of potassium chloride and sodium chloride than those of the present invention. The present invention, however, clarifies how to control the concentrations of sodium and potassium ions and the specific process parameters to achieve higher yields and purities of potassium chloride and sodium chloride.

[0046] In the above step S4, the salt solution is evaporated and concentrated, and the concentration of sodium chloride in the salt solution is controlled. The effects on the yield and purity of sodium sulfate, sodium chloride, and potassium chloride are shown in the following table (the sodium sulfate yield in the table is based on sodium sulfate, and the purity is based on the purity of sodium sulfate decahydrate):

[0047] Table 1 Effect of sodium chloride concentration on salt yield and purity

[0048]

[0049] In the above step S5, the potassium ion degree is controlled to change, and the yield and purity of sodium chloride and potassium chloride are affected as shown in the following table:

[0050] Table 2 Effect of potassium ion concentration on sodium-potassium separation

[0051]

[0052] According to the experimental data obtained in Tables 1 and 2, when the NaCl mass concentration is controlled between 15% and 24%, the purity and yield of the sodium chloride, potassium chloride, and sodium sulfate obtained by salt separation are relatively high. However, when the NaCl mass concentration is controlled below 15% and above 24%, the phase diagram of the mirabilite crystallization approaches the edge of the crystallization zone, which will reduce the purity of the crystallization. In order to maintain a certain purity, the yield will be reduced. The yield and purity are inversely proportional (for example, if all sodium sulfate is to be precipitated, sodium chloride and potassium chloride will also precipitate, resulting in a lower purity). Therefore, it is necessary to find a balance value to maximize the yield at a certain purity. The same applies to potassium chloride and sodium chloride.

[0053] It is worth noting that, based on the premise of the above-mentioned structural design, in order to solve the same technical problem, even if some non-substantial changes or improvements 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 scope of protection of the present invention.

Claims

1. A method for separating salt from sodium-based fly ash from waste incineration flue gas, characterized in that: The implementation steps are as follows: S1: Sodium bicarbonate is used as the raw material for waste incineration flue gas purification, and the fly ash produced includes sodium salt and potassium salt; S2: Wash the fly ash with water, and dissolve the sodium and potassium ions in the water; S3: Purification of salt solution to make Na + , K + , Cl - , SO4 2- Accounting for not less than 95% of the total ion concentration in the solution; S4: Evaporating and concentrating the salt solution until the mass concentration of sodium chloride in the salt solution is 15-24%; the concentrated salt solution contains 15-24% sodium chloride, 2-10% sodium sulfate, and 2.5-11% potassium chloride; S5: Salt solution is separated and crystallized, and Na + It is crystallized in the form of sodium chloride and K + It is separated by crystallization in the form of potassium chloride; The specific implementation of step S5 includes the following process: The concentrated salt solution obtained in step S4 is subjected to cooling crystallization of sodium sulfate by the nanofiltration concentrate of the mother liquor separated in the following sodium sulfate cooling crystallization process, at an operating temperature of 5 to -10°C, to precipitate sodium sulfate decahydrate; The nanofiltration clear liquid of the mother liquor separated in the mirabilite cooling crystallization process and the mother liquor separated in the following sodium chloride evaporation crystallization process are subjected to sodium chloride evaporation crystallization. The evaporation crystallization temperature is 60℃~110℃, and the secondary steam temperature is 60℃~80℃. It enters the steam compressor and is heated and pressurized to saturated steam at 80℃~100℃ and recycled as heating steam. Sodium chloride slurry is subjected to solid-liquid separation to obtain sodium chloride crystals. The potassium ion concentration is controlled at 6-15% during the sodium chloride evaporation and crystallization process; The mother liquor obtained from the solid-liquid separation of the sodium chloride slurry is returned to the evaporation crystallization to control the slurry content during the evaporation crystallization process, and the rest is used for potassium chloride cooling crystallization; the operating temperature of potassium chloride cooling crystallization is -10~5℃, and potassium chloride crystals are precipitated.

2. The method for separating salt from sodium-based fly ash from waste incineration flue gas according to claim 1, characterized in that: In the above step S1, the purification process is: The amount of sodium bicarbonate added to the flue gas is 16 kg / h of 600 mesh sodium bicarbonate sprayed into every 10,000 Nm3 / h of flue gas; ensure that the purification residence time of sodium bicarbonate in the flue gas is not less than 2s; and the flue gas temperature is not less than 200°C during purification.

3. The method for separating salt from sodium-based fly ash from waste incineration flue gas according to claim 1, characterized in that: The sodium sulfate content in the nanofiltration clear liquid separated in the cooling crystallization process of Glauber's salt is less than 700ppm.

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