A method for fixing volatile heavy metals in secondary fly ash

Abstract

The invention discloses a method for fixing volatile heavy metals in secondary fly ash, belongs to the technical field of heavy metal treatment, and comprises the following steps: modifying alginate with dopamine hydrochloride to obtain modified alginate, mixing the modified alginate with CaCl2 and 1,2-propylene oxide to obtain a crosslinking promoter; preparing SiO2-Al2O3 gel, adding the crosslinking promoter to the SiO2-Al2O3 gel to obtain a gel adsorbent; adding chloride to the secondary fly ash, performing high-temperature pretreatment after sufficient mixing, collecting volatiles, using a gel adsorbent for loading, and then performing ultrasonic hydrothermal treatment. Dopamine hydrochloride is grafted on the modified alginate to carry functional groups such as bidentate hydrogen bonds and benzene rings, promoting the polymerization of gel, and leaving heavy metals in the gel system, performing high-temperature pretreatment with chloride, and then performing high-energy ultrasonic hydrothermal treatment on the gel system to achieve fixation of volatile heavy metals.

Description

Technical Field

[0001] The invention relates to the technical field of heavy metal treatment, in particular to a method for fixing volatile heavy metals in secondary fly ash. Background Art

[0002] Fly ash contains heavy metals such as Pb, Cu, Zn and Cd, as well as highly toxic dioxins, and is classified as hazardous waste management. Therefore, how to properly dispose of incineration fly ash has become a difficult problem that many scholars are eager to solve. Existing methods for treating fly ash include cement solidification, reagent stabilization, extraction and high-temperature heat treatment. During the heat treatment process, the flue gas will produce new fly ash byproducts after condensation and purification, which is called secondary fly ash. The heavy metal content contained in it is 1-10 times that of fly ash. Due to its high metal content, it is still a usable material, but its treatment method is different from that of fly ash. The high heavy metal content causes the heavy metal components in it to volatilize during the heating process and will be added to the biological cycle, causing environmental pollution, so direct hydrothermal treatment or high-temperature treatment is no longer applicable. In addition, the high chlorine content in secondary fly ash not only increases the leaching toxicity of heavy metals, but also seriously limits the harmlessness and resource utilization of fly ash. Summary of the invention

[0003] The purpose of the present invention is to provide a method for fixing volatile heavy metals in secondary fly ash to solve the problems raised in the background technology. Double network gel is prepared by using AlCl3·6H2O and tetraethyl orthosilicate, and a crosslinking promoter is added to prepare a gel adsorbent for adsorbing heavy metals volatilized during the high-temperature pretreatment of secondary fly ash. Modified alginate, CaCl2 and 1,2-propylene oxide are used as components of the crosslinking promoter. Dopamine hydrochloride is grafted on the modified alginate to have bidentate hydrogen bonds and benzene rings and other functional groups. On the one hand, it can promote the polymerization of alginate gel, and on the other hand, it can retain the heavy metals in the gel system in the form of metal-ion coordination bonds with the heavy metals entering the gel system to achieve the adsorption effect. High-temperature pretreatment is performed with chloride salt, and then the gel system that collects the heavy metals is subjected to high-energy ultrasonic hydrothermal treatment to achieve the fixation of volatile heavy metals.

[0004] To achieve the above objectives, the present invention provides the following technical solutions:.

[0005] A method for fixing volatile heavy metals in secondary fly ash comprises the following steps:

[0006] S1, modifying alginate with dopamine hydrochloride to obtain modified alginate, and mixing the modified alginate with CaCl2 and 1,2-propylene oxide to obtain a cross-linking accelerator;

[0007] S2, using AlCl3·6H2O and ethyl orthosilicate to prepare a double network gel to obtain SiO2-Al2O3 gel, which was stored at 4°C for future use;

[0008] S3, adding a cross-linking accelerator to the SiO2-Al2O3 gel, stirring evenly to obtain a gel adsorbent, adding chloride salt to the secondary fly ash, mixing thoroughly, and performing high-temperature pretreatment, collecting volatiles, passing them into the gel adsorbent, and connecting the tail pipe to a fume hood;

[0009] S4. The adsorption system loaded with volatiles is subjected to ultrasonic hydrothermal treatment. The specific operation is as follows: after the volatile system is stirred evenly at room temperature, it is treated at 240 W and 35 kHz for 10-30 minutes.

[0010] Further, the modified alginate is prepared by the following steps:

[0011] A1. Add 0.585 g 0.1 mol / L 2-morpholineethanesulfonic acid and 0.0876 g 0.05 mol / L NaCl prepared with sterile deionized water to 30 mL sterile deionized water, and adjust the pH value of the system to 5.1 with NaOH solution to obtain 2-morpholineethanesulfonic acid buffer for standby use.

[0012] A2. Dissolve 0.2 g of alginate, 0.192 g of 1-ethyl-(3-dimethylaminopropyl)carbodiimide, 0.115 g of N-hydroxysuccinimide and 0.948 g of dopamine hydrochloride in 100 mL of 2-morpholineethanesulfonic acid buffer, stir well for 24 hours at room temperature under an inert gas atmosphere, and then freeze-dry in a freeze dryer to obtain modified alginate.

[0013] In the above reaction process, the carboxyl groups on dopamine hydrochloride and alginate are grafted by 1-ethyl-(3-dimethylaminopropyl)carbodiimide / N-hydroxysuccinimide esterification method to obtain modified alginate.

[0014] Furthermore, the alginate is potassium alginate.

[0015] Furthermore, the mass ratio of modified alginate, CaCl2 and 1,2-propylene oxide in the cross-linking accelerator is (0.1-0.5):(0.5-1):(1-3).

[0016] Further, the SiO2-Al2O3 gel is prepared by the following steps:

[0017] B1. Add 3 g of AlCl3·6H2O into 10 mL of 90% ethanol solution and stir at 50°C and 30 rpm for 2 hours to obtain Al2O3 sol;

[0018] B2, adding tetraethyl orthosilicate to the ethanol solution, stirring evenly, adding nitric acid, raising the system temperature to 50°C, and then stirring at a rate of 30 rpm for 2 hours to obtain SiO2 gel;

[0019] B3. At room temperature, add SiO2 gel dropwise into Al2O3 sol and stir for 5-20 hours to obtain SiO2-Al2O3 gel.

[0020] In the above reaction process, in step B1, AlCl3·6H2O is added to an ethanol solution. 3+ Ions and water molecules form a complex [Al(OH2)6] 3+ The complex forms R1-OH-R2 dimer through hydroxyl bridge in the system and forms R1-O-R2 dimer through oxygen bridge, thus forming a cross-linking system. In step B2, ethyl orthosilicate is hydrolyzed in the system to form Si(OH)4 molecules, and Si(OH)4 containing a large number of hydroxyl groups dehydrates and condenses to form dimers, forming a gel system.

[0021] Furthermore, in step B1, the mass fraction of the ethanol solution is 90%, and the mass volume ratio of AlCl3·6H2O to ethanol is (1-5) g: (20-40) mL.

[0022] Furthermore, in step B2, the mass fraction of the ethanol solution is 90%, and the mass ratio of tetraethyl orthosilicate, the ethanol solution and the nitric acid is (1-5): (5-15): (0.5-1.5).

[0023] Furthermore, in step S3, the mass ratio of the cross-linking accelerator to the SiO2-Al2O3 gel is (2.2-4.0) g:100 mL.

[0024] Furthermore, in step S3, the specific operation of high temperature pretreatment of the secondary fly ash is:

[0025] Nitrogen was introduced into the vacuum tube furnace, and then the secondary fly ash was pushed into the furnace. The temperature of the system was increased to 700°C-1000°C at a heating rate of 5°C / min and kept at this temperature for 60 minutes.

[0026] Furthermore, in step S3, the chloride salt is one or more of CaCl2, FeCl3, AlCl3 and MgCl2.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) In the technical solution of the present invention, AlCl3·6H2O and ethyl orthosilicate are used to prepare a double network gel, and a cross-linking promoter is added to prepare a gel adsorbent for adsorbing heavy metals volatilized during the high-temperature pretreatment of secondary fly ash. Compared with traditional solid adsorbents, the consumption of the adsorbent can be clearly seen by using a gel adsorbent, and the gel can absorb the heat in the volatile vapor, thereby avoiding the influence of temperature on the adsorption effect.

[0029] (2) In the technical scheme of the present invention, modified alginate, CaCl2 and 1,2-propylene oxide are used as components of the cross-linking promoter. Dopamine hydrochloride is grafted on the modified alginate to have bidentate hydrogen bonds and functional groups such as benzene rings. On the one hand, it can promote the polymerization of alginate gel, and on the other hand, it can bind heavy metals entering the gel system in the form of metal-ion coordination bonds to retain the heavy metals in the gel system. By using CaCl2 to introduce Ca ions into the system, the circulation and absorption of various heavy metals in the environment can be inhibited. Potassium alginate is used as a material for preparing gel. On the one hand, potassium can also form coordination bonds with bidentate hydrogen bonds, while reducing the circulation of radioactive heavy metals in the environment, so as to achieve the antagonistic effect between elements, reduce the possibility of heavy metal absorption by plants, and prevent the circulation of heavy metal pollutants. Due to the high chlorine content in the secondary fly ash, the addition of 1,2-propylene oxide can react with chloride ions to form alkane chains, reduce the reaction between chloride ions and heavy metals, reduce their migration form, avoid the dechlorination treatment step of the secondary fly ash, and reduce the treatment cost. The alkane chain can react with Al in the system. 3+ It can also combine with modified alginate through hydrophobic interactions to promote the cross-linking of the gel network.

[0030] (3) In the technical solution of the present invention, chloride salt is added to the secondary fly ash, and the mixture is subjected to high-temperature pretreatment, and the chloride salt is used to promote the volatilization of heavy metals to reduce the residual amount of heavy metal substances in the fly ash after treatment. Then, the gel system that collects heavy metals is subjected to high-energy ultrasonic hydrothermal treatment, replacing the traditional high-temperature sintering adsorbent, so that Al2O3 and SiO2 in the gel can form aluminosilicate, which is a cage-like solidified material structure similar to zeolite, plays a bridging role, and seals the heavy metals therein, thereby achieving the fixation of heavy metals. DETAILED DESCRIPTION

[0031] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] Example 1

[0033] The modified alginate is prepared by the following steps:

[0034] A1. Add 0.585 g 0.1 mol / L 2-morpholineethanesulfonic acid and 0.0876 g 0.05 mol / L NaCl prepared with sterile deionized water to 30 mL sterile deionized water, and adjust the pH value of the system to 5.1 with NaOH solution to obtain 2-morpholineethanesulfonic acid buffer for standby use.

[0035] A2. Dissolve 0.2 g potassium alginate, 0.192 g 1-ethyl-(3-dimethylaminopropyl)carbodiimide, 0.115 g N-hydroxysuccinimide and 0.948 g dopamine hydrochloride in 100 mL 2-morpholineethanesulfonic acid buffer, stir well for 24 hours at room temperature under an inert gas atmosphere, and then freeze-dry in a freeze dryer to obtain modified alginate.

[0036] Example 2

[0037] SiO2-Al2O3 gel is prepared by the following steps:

[0038] B1. Add 1 g of AlCl3·6H2O into 20 mL of 90% ethanol solution and stir at 50°C and 30 rpm for 2 hours to obtain Al2O3 sol;

[0039] B2. Add 1 g of tetraethyl orthosilicate to 5 mL of 90% ethanol solution, stir evenly, then add 0.5 mL of 50% nitric acid, raise the system temperature to 50°C, and then stir at 30 rpm for 2 hours to obtain SiO2 gel;

[0040] B3. At room temperature, add SiO2 gel dropwise into Al2O3 sol and stir for 10 hours to obtain SiO2-Al2O3 gel.

[0041] Example 3

[0042] The difference between this embodiment and embodiment 2 is that in step B1, the amount of AlCl3·6H2O used is 3 g, and the amount of ethanol solution used is 30 mL; in step B2, the amount of tetraethyl orthosilicate used is 3 g, the amount of ethanol solution used is 10 mL, and the amount of nitric acid used is 1.0 mL.

[0043] Example 4

[0044] The difference between this embodiment and embodiment 2 is that in step B1, the amount of AlCl3·6H2O used is 5 g, and the amount of ethanol solution used is 40 mL; in step B2, the amount of tetraethyl orthosilicate used is 5 g, the amount of ethanol solution used is 15 mL, and the amount of nitric acid used is 1.5 mL.

[0045] Example 5

[0046] A method for fixing volatile heavy metals in secondary fly ash comprises the following steps:

[0047] S1. Mixing the modified alginate prepared in Example 1 with CaCl2 and 1,2-propylene oxide in a mass ratio of 0.1:0.5:1 to obtain a crosslinking accelerator;

[0048] S2, storing the SiO2-Al2O3 gel prepared in Example 2 at 4°C for future use;

[0049] S3, FeCl3 and AlCl3 are mixed in equal proportions to obtain chloride; 2.2g of cross-linking accelerator is added to 100mL SiO2-Al2O3 gel, and the mixture is stirred to obtain a gel adsorbent; 2g of chloride is added to 10g of secondary fly ash, and the mixture is mixed thoroughly and then sent into a vacuum tube furnace for high-temperature pretreatment; nitrogen is introduced into the vacuum tube furnace, the air inlet valve of the tube furnace is opened, the power of the vacuum pump is turned on, the air outlet valve is opened, the corundum tube and the gas pipeline are evacuated, and then when the vacuum gauge of the tube furnace reaches -0.1, the air outlet valve is closed, the air inlet valve is closed, the knob is turned counterclockwise to open the main valve of the gas cylinder, and the knob is turned clockwise to slowly open the outlet pressure reducing valve to make the outlet The air pressure at the inlet is 0.004MPa. Turn the knob counterclockwise to slowly open the air inlet valve of the tubular furnace and confirm that the air pressure is below 0.08MPa. When the air pressure is stable, close the air inlet valve of the tubular furnace, open the air outlet valve, evacuate to -0.1, close the air outlet valve of the tubular furnace, turn off the mechanical pump, open the air inlet valve of the tubular furnace, and then push the secondary fly ash into the furnace. Raise the temperature of the system to 700℃ at a heating rate of 5℃ / min, keep warm for 60 minutes, collect volatiles, pass them into the gel adsorbent, and connect the tail pipe to the fume hood;

[0050] S4. The adsorption system loaded with volatiles is subjected to ultrasonic hydrothermal treatment. The specific operation is as follows: after the volatile system is stirred evenly at room temperature, it is treated at 240 W and 35 kHz for 10 minutes.

[0051] Example 6

[0052] The difference between this embodiment and embodiment 5 is that in step S1, the cross-linking accelerator is obtained by mixing the modified alginate prepared in embodiment 1 with CaCl2 and 1,2-propylene oxide in a mass ratio of 0.3:0.8:2; in step S3, the SiO2-Al2O3 gel is prepared in embodiment 3, and the mass volume ratio of the cross-linking accelerator to the SiO2-Al2O3 gel is 3.1 g:100 mL; in step S3, the temperature of the high temperature pretreatment is 850°C; in step S4, the time of the ultrasonic hydrothermal treatment is 20 minutes.

[0053] Example 7

[0054] The difference between this embodiment and embodiment 5 is that in step S1, the cross-linking accelerator is obtained by mixing the modified alginate prepared in embodiment 1 with CaCl2 and 1,2-propylene oxide in a mass ratio of 0.5:1:3; in step S3, the SiO2-Al2O3 gel is prepared in embodiment 4, and the mass volume ratio of the cross-linking accelerator to the SiO2-Al2O3 gel is 4.0 g:100 mL; in step S3, the temperature of the high temperature pretreatment is 1000°C; in step S4, the time of the ultrasonic hydrothermal treatment is 30 minutes.

[0055] Example 8

[0056] The difference between this embodiment and embodiment 5 is that in step S3, the chloride salt is obtained by mixing FeCl3 and MgCl2 in equal proportions.

[0057] Comparative Example 1

[0058] The difference between this comparative example and Example 6 is that the alginate is not modified.

[0059] Comparative Example 2

[0060] The difference between this comparative example and Example 6 is that the modified alginate prepared in Example 1 is used as a cross-linking accelerator.

[0061] Comparative Example 3

[0062] The difference between this comparative example and Example 6 is that high temperature treatment is used instead of the ultrasonic hydrothermal treatment in step S4, the temperature is set to 1000° C., and the treatment time is 90 minutes.

[0063] Comparative Example 4

[0064] The difference between this comparative example and Example 6 is that in step S3, no chloride salt is added.

[0065] The fixation effect of the heavy metals fixed in Examples 5-8 and Comparative Examples 1-4 was tested. The collected secondary fly ash was tested for heavy metal content, and then the fixed silica-alumina salt was tested for heavy metal content to calculate the fixation rate. Five readings were obtained for each sample and the average value was taken. The test results are shown in Table 1 below.

[0066] Table 1 Test of the effect of heavy metal fixation in Examples 5-8 and Comparative Examples 1-4

[0067]

[0068] As can be seen from the results in Table 1, the results in Examples 5-8 and Comparative Example 3 all show a good fixation effect on volatile heavy metals. As can be seen from the results in Comparative Example 1, the modified alginate has an enhancing effect on the cross-linking degree of the system, and the functional groups thereon can achieve the preservation and adsorption of metals. The results in Comparative Example 2 verify that CaCl2 and 1,2-propylene oxide in the cross-linking promoter can promote the cross-linking of the system and reduce the influence of high-temperature steam on the adsorption effect of the gel during the adsorption process. As can be seen from the results in Comparative Example 4, the addition of chloride salts can promote the volatilization of heavy metals. From the fixation rate of Cr, it can be seen that chloride salts have a better volatilization promoting effect on difficult-to-volatilize heavy metals such as Cr.

[0069] In the description of the specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0070] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the invention or exceed the scope defined by the claims, they shall all fall within the protection scope of the present invention.

Claims

1. A method for fixing volatile heavy metals in secondary fly ash, characterized in that: The following steps are involved: S1. Modifying alginate with dopamine hydrochloride to obtain modified alginate, and mixing the modified alginate with CaCl2 and 1,2-propylene oxide to obtain a cross-linking accelerator; the mass ratio of the modified alginate, CaCl2 and 1,2-propylene oxide in the cross-linking accelerator is (0.1-0.5): (0.5-1): (1-3); S2, using AlCl3·6H2O and ethyl orthosilicate to prepare a double network gel to obtain SiO2-Al2O3 gel, which was stored at 4°C for future use; S3, adding a crosslinking accelerator to SiO2-Al2O3 gel, stirring evenly to obtain a gel adsorbent, adding chloride salt to the secondary fly ash, mixing thoroughly and performing high-temperature pretreatment, collecting volatiles, passing them into the gel adsorbent, and connecting the tail pipe to a fume hood; the mass volume ratio of the crosslinking accelerator to the SiO2-Al2O3 gel is (2.2-4.0) g: 100 mL; S4. The adsorption system loaded with volatiles is subjected to ultrasonic hydrothermal treatment. The specific operation is as follows: after the volatile system is stirred evenly at room temperature, it is treated at 240 W and 35 kHz for 10-30 minutes.

2. The method for fixing volatile heavy metals in secondary fly ash according to claim 1, characterized in that: The modified alginate is prepared by the following steps: A1, add 2-morpholineethanesulfonic acid and NaCl prepared by sterile deionized water into sterile deionized water, use NaOH solution to adjust the pH value of the system to 5.1, and obtain 2-morpholineethanesulfonic acid buffer for standby use; A2. Dissolve alginate, 1-ethyl-(3-dimethylaminopropyl)carbodiimide, N-hydroxysuccinimide and dopamine hydrochloride in 2-morpholineethanesulfonic acid buffer, stir well for 24 hours at room temperature under an inert gas atmosphere, and then freeze-dry in a freeze dryer to obtain modified alginate.

3. The method for fixing volatile heavy metals in secondary fly ash according to claim 2, characterized in that: The alginate is potassium alginate.

4. The method for fixing volatile heavy metals in secondary fly ash according to claim 1, characterized in that: SiO2-Al2O3 gel is prepared by the following steps: B1. Add AlCl3·6H2O into ethanol solution, stir at 50°C and 30 rpm for 2 hours to obtain Al2O3 sol; B2, adding tetraethyl orthosilicate to the ethanol solution, stirring evenly, adding nitric acid, raising the system temperature to 50°C, and then stirring at a rate of 30 rpm for 2 hours to obtain SiO2 gel; B3. At room temperature, add SiO2 gel dropwise into Al2O3 sol and stir for 5-20 hours to obtain SiO2-Al2O3 gel.

5. The method for fixing volatile heavy metals in secondary fly ash according to claim 4, characterized in that: In step B1, the mass fraction of the ethanol solution is 90%, and the mass ratio of AlCl3·6H2O to ethanol is (1-5) g: (20-40) mL.

6. The method for fixing volatile heavy metals in secondary fly ash according to claim 4, characterized in that: In step B2, the mass fraction of the ethanol solution is 90%, and the mass ratio of tetraethyl orthosilicate, the ethanol solution and the nitric acid is (1-5): (5-15): (0.5-1.5).

7. The method for fixing volatile heavy metals in secondary fly ash according to claim 4, characterized in that: In step S3, the specific operation of high temperature pretreatment of secondary fly ash is as follows: Nitrogen was introduced into the vacuum tube furnace, and then the secondary fly ash was pushed into the furnace. The temperature of the system was increased to 700°C-1000°C at a heating rate of 5°C / min and kept at this temperature for 60 minutes.

8. The method for fixing volatile heavy metals in secondary fly ash according to claim 4, characterized in that: In step S3, the chloride salt is one or more of CaCl2, FeCl3, AlCl3 and MgCl2.

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