Heavy metal sequestering agents and methods for their preparation

Abstract

The application discloses a heavy metal chelating agent and a preparation method thereof. The heavy metal chelating agent is obtained by controlling the synthesis of melamine formaldehyde resin prepolymer and introducing a dithiocarbamate salt group with a chelating function into the structure of the melamine formaldehyde resin prepolymer. The heavy metal chelating agent has the characteristics of high efficiency and stability, high heavy metal removal rate, fast settling speed of chelation products, low leaching, and low cost, and is suitable for industrialized treatment of heavy metal pollution in fly ash and industrial wastewater.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heavy metal chelating agents, in particular to a heavy metal chelating agent and a preparation method thereof. BACKGROUND

[0002] In recent years, the heavy metal pollution problem of industrial wastewater and waste incineration fly ash has become an important factor restricting the green and circular development of industries such as machining, electroplating, mining, smelting, petrochemical industry, and safe disposal of household waste. How to effectively control heavy metal pollution has become a key and difficult problem in current environmental governance.

[0003] For the treatment of heavy metal ions in industrial wastewater, common treatment methods include adsorption, ion exchange, reverse osmosis, biological method, chelation precipitation method, etc. Among them, the chelation precipitation method has become the mainstream technology for removing heavy metal ions in industrial wastewater because of its simple operation, high heavy metal removal efficiency, and low cost. The key is to develop efficient chelating agents.

[0004] For the treatment of heavy metal in waste incineration fly ash, common treatment methods include cement solidification method, lime solidification method, high temperature melting method, and chelation stabilization method. Among them, the chelation stabilization method has gradually become the main method for stabilization treatment of domestic waste incineration fly ash in recent years because of its simple operation, high heavy metal stabilization efficiency, and low cost. The key is also to develop efficient chelating agents.

[0005] Common heavy metal chelating agents are mainly derivatives of dithiocarbamic acid compounds, such as polyamine dithiocarbamate, polyethylene imine xanthate, polyamine-epichlorohydrin copolymer xanthate, and polyacrylamide xanthate. Currently, sodium dimethyldithiocarbamate, 1.3.5-triazine-2.4.6-trithiol trisodium salt (TMT-15), and piperazine chelating agents are more commonly used in China. However, sodium dimethyldithiocarbamate has many by-products in the production process, forms small flocs, has poor settling performance, needs to add a flocculant, and needs to maintain an appropriate excess to achieve good results. 1.3.5-triazine-2.4.6-trithiol trisodium salt (TMT-15) and piperazine chelating agents have a relatively stable chelation structure due to the presence of a cyclic structure, have a high heavy metal removal rate, but are relatively expensive and cannot be used on a large scale. SUMMARY

[0006] In order to solve the problems of poor settling performance and high price of existing heavy metal chelating agents, one of the purposes of the present application is to provide a preparation method of a heavy metal chelating agent.

[0007] The technical solution of the present application to solve the above technical problems is as follows:

[0008] The preparation method of the heavy metal chelating agent comprises the following steps:

[0009] Step 1, synthesis of prepolymer

[0010] First, add the alcohol and aldehyde to the mixture of melamine and water, and fully stir to obtain a mixture I;

[0011] Then, adjust the pH of the mixture I to 11-12 with NaOH solution, and then start heating the reaction, and control the reaction time to obtain a clear solution, i.e. the prepolymer;

[0012] Step 2, synthesis of heavy metal chelator

[0013] Add the strong base and carbon disulfide to the prepolymer obtained in step 1, and start the reaction at room temperature for a period of time to obtain the heavy metal chelator.

[0014] The chelator prepared by the present application retains the stable six-membered ring structure of melamine, has a moderate molecular weight, and forms larger flocs after treating heavy metals, and has a faster settling speed without the need to add a flocculant. At the same time, the chelator prepared by the present application contains multiple dithiocarboxyl active sites, which can effectively improve the heavy metal removal efficiency. In addition, the raw materials in the present application are characterized by low price and wide source, so the heavy metal chelator prepared by the present application has the characteristics of low price and is conducive to industrial use.

[0015] On the basis of the above technical solution, the present application can also be improved as follows:

[0016] Further, the mass ratio of melamine to water in step 1 is 1:7-10.

[0017] Further, the molar ratio of melamine to alcohol and aldehyde in the reaction mixture I in step 1 is 1:(1.5-2):(1.5-2).

[0018] The beneficial effects of the above further technical solution are that the molar ratio in this range allows the amino group of melamine to react with formaldehyde to increase water solubility, while ensuring that the amino group reacts with carbon disulfide to obtain a heavy metal chelator.

[0019] Further, the alcohol includes methanol, ethanol or propanol; and the aldehyde includes formaldehyde or paraformaldehyde.

[0020] Further, the temperature after heating in step 1 is 40-60℃, and the reaction time of the mixture I in step 1 is 0.5-2h.

[0021] Further, the molar ratio of melamine to carbon disulfide in step 2 is 1:1.5-3.

[0022] Further, the strong alkaline substance in step 2 comprises NaOH, KOH or tetramethyl guanidine; the molar ratio of melamine to the strong alkaline substance is 1:1-2.

[0023] Further, the reaction condition in step 2 is: the reaction temperature is room temperature, and the reaction time is 2.5-3.5 h.

[0024] The second object of the present application is to provide a heavy metal chelating agent prepared by the preparation method in the first object.

[0025] The present application has the following beneficial effects:

[0026] 1. The chelating agent in the present application can form stable complex precipitates with heavy metals rapidly due to containing multiple dithioamino groups; meanwhile, the chelating agent has a melamine six-membered ring stable structure, a moderate molecular weight, and a fast settling speed after chelating with heavy metals without adding flocculants; and the heavy metal chelating agent prepared by the method in the present embodiment also has the characteristics of low cost, which is suitable for industrial use and used for treating heavy metal pollution problems in fly ash and industrial wastewater.

[0027] 2. The temperature of the heavy metal chelating agent in the present application is controllable, which avoids the volatilization of carbon disulfide and reduces the generation of by-products, thereby improving the yield of the chelating agent to a certain extent and reducing the price of the heavy metal chelating agent in the present application. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is an infrared spectrum of the heavy metal chelating agent prepared in the embodiment 1 in the present application;

[0029] Figure 2 It is a removal rate distribution diagram of the heavy metal chelating agent prepared by different molar ratios of melamine and carbon disulfide in the present application;

[0030] Figure 3 It is a removal rate distribution diagram of the heavy metal chelating agent prepared by different molar ratios of melamine and formaldehyde in the present application;

[0031] Figure 4 It is a removal rate distribution diagram of the heavy metal chelating agent prepared by different molar ratios of carbon disulfide and sodium hydroxide in the present application;

[0032] Figure 5 It is a removal rate distribution diagram of the heavy metal chelating agent prepared by different pH values in the present application. DETAILED DESCRIPTION

[0033] The heavy metal chelating agent and the preparation method thereof in the present application will be described below in combination with embodiments.

[0034] The present application can be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein, rather the purpose of providing these embodiments is to make the present application will be thorough and complete, and will be to convey the scope of the present application to those skilled in the art.

[0035] In addition, the embodiments of the present application are not specified in the specific conditions, according to the conventional conditions or manufacturer's recommended conditions. The reagents or instruments used are not specified by the manufacturer, which are conventional products that can be obtained by commercial purchase.

[0036] The inventors found that the existing heavy metal has the problems of poor settling performance and high price after studying the commonly used heavy metal capture agent.

[0037] Based on this, the inventors synthesize an intermediate (i.e. methylol melamine prepolymer) by modifying melamine formaldehyde resin, introduce dithiocarbamate salt group with chelating function on the methylol melamine prepolymer, and then prepare a heavy metal chelating agent.

[0038] The embodiments of the first aspect of the present application provide a preparation method of a heavy metal chelating agent, comprising the following steps:

[0039] Step 1, synthesis of prepolymer

[0040] First, add alcohol and aldehyde to the mixture of melamine and water, and fully stir to mix evenly to obtain mixture I; then, adjust the pH value of mixture I to 11-12 with NaOH solution, and start heating reaction, and control the reaction time to obtain a clear solution, i.e. prepolymer;

[0041] Step 2, synthesis of heavy metal chelating agent

[0042] Add strong alkaline substance and carbon disulfide to the prepolymer prepared in step 1 in sequence, and start reaction at room temperature for a period of time, and then the heavy metal chelating agent can be obtained.

[0043] The heavy metal chelator prepared in the embodiment has the characteristics of high efficiency and stability, high heavy metal removal rate, fast settling speed and difficulty in precipitation of chelation products. The heavy metal chelator prepared by the method in the embodiment also has the characteristics of low cost, is suitable for industrial use, and can be used for treating heavy metal pollution in fly ash and industrial wastewater. In addition, the heavy metal chelator is further prepared by modifying the prepolymer in the embodiment. In the embodiment, the prepolymer is a substance with a large molecular weight, so the temperature of the heavy metal chelator can be controlled during synthesis (i.e., the heat released by the exothermic reaction in step 2 causes little change in the reaction temperature), which avoids excessive heat release by the exothermic reaction in step 2 (the addition reaction in step 2 is an exothermic reaction), thereby preventing the volatilization of carbon disulfide, improving the utilization rate of carbon disulfide, reducing the generation of by-products, and to some extent, improving the yield of the heavy metal chelator in the embodiment.

[0044] Specifically, in step 1, a prepolymer (hydroxymethyl melamine, which is an intermediate in the synthesis of melamine formaldehyde resin) is prepared by using melamine and aldehyde substances to undergo an addition reaction, and then an etherification reaction occurs between the prepolymer and alcohol substances to prevent the prepolymer from further condensation, and then a dithiocarbamate group with chelation function is introduced onto the prepolymer by an addition reaction, and finally a heavy metal chelator is prepared.

[0045] In addition, in the embodiment, the pH value of the reaction mixture I in step 1 is adjusted to 11-12 to facilitate the addition reaction in steps 1 and 2, and a strong alkaline substance is added to improve the activity of the addition reaction in step 2 to facilitate the reaction between the prepolymer and carbon disulfide.

[0046] In addition, in some embodiments, the mass ratio of melamine to water in the mixture in step 1 is 1:7-10.

[0047] In addition, in some embodiments, the molar ratio of melamine to alcohol substances and aldehyde substances in the reaction mixture I in step 1 is 1:(1.5-2):(1.5-2). In the embodiment, the molar ratio of melamine to aldehyde substances in this range ensures that melamine reacts with alcohol substances to obtain water-soluble hydroxymethyl melamine, and at the same time, ensures that amino groups react with carbon disulfide to undergo an addition reaction, thereby ensuring that a heavy metal chelator can be obtained. In the embodiment, the alcohol substances herein include methanol or ethanol, and methanol is preferred; the aldehyde substances herein include formaldehyde, acetaldehyde or paraformaldehyde; of course, in actual processes, the alcohol substances herein can also be other substances that can prevent the prepolymer from further condensation and improve the stability of the prepolymer; the aldehyde substances herein can also be other substances that can react with primary amino groups and increase the hydrophilicity of the prepolymer, and the present application will not be described in detail.

[0048] In some embodiments, the pH of the reaction mixture I in step 1 is adjusted by NaOH. In this embodiment, the pH adjustment of the reaction mixture I by NaOH is conducive to the addition reaction between melamine and aldehyde. In practice, other substances can also be used to adjust the pH of the reaction mixture I, such as KOH or Ca(OH)2.

[0049] In some embodiments, the temperature of the heated reaction mixture I in step 1 is 40-60°C, and the reaction time of the reaction mixture I in step 1 is 1-2h.

[0050] In some embodiments, the molar ratio of melamine to carbon disulfide in step 2 is 1:1.5-3, and the dropping speed of carbon disulfide is 1.30-1.60 mL / min. In this embodiment, the heavy metal chelating agent prepared from carbon disulfide and melamine within the above molar ratio range has a good removal rate. The dropping speed of carbon disulfide within the above range can further control the reaction system in step 2, avoiding the local high temperature in the addition reaction process in step 2, which causes the volatilization of carbon disulfide.

[0051] In some embodiments, the strong alkaline substance in step 2 includes NaOH, KOH or tetramethyl guanidine, and the molar ratio of melamine to the strong alkaline substance is 1:1-2. In this embodiment, the addition of the strong alkaline substance provides an alkaline environment for the addition reaction between the prepolymer and carbon disulfide, which is conducive to the addition reaction between the prepolymer and carbon disulfide, and the formation of dithiocarbamate, and improves the hydrophilicity of the prepared heavy metal chelating agent.

[0052] In some embodiments, the reaction conditions in step 2 are as follows: the reaction temperature is room temperature, and the reaction time is 2.5-3.5h.

[0053] The embodiment of the second aspect of the present application provides a heavy metal chelating agent prepared by the embodiment of the first aspect.

[0054] Embodiment

[0055] Embodiment 1

[0056] A method for preparing a heavy metal chelating agent, comprising the following steps:

[0057] Step 1, synthesis of hydroxymethyl melamine prepolymer

[0058] A mixture was formed by dissolving 15.75 g of melamine in 120 mL of water, and then methanol 12.15 mL and formaldehyde 17.33 mL were added to the mixture and mixed thoroughly to obtain a reaction mixture I, and then the pH of the reaction mixture I was adjusted to 12 with NaOH and heated to 40-60°C, and after the reaction mixture I was clarified, the reaction was carried out for 1 hour, and after the reaction was completed, the reaction mixture was cooled to room temperature to obtain a hydroxymethyl melamine prepolymer; the reaction equation is as follows:

[0059]

[0060] Step 2, synthesis of heavy metal chelating agent

[0061] To the hydroxymethyl melamine prepolymer prepared in step 1, 10.8 mL of carbon disulfide was slowly added to obtain a reaction mixture II, and then 6 g of NaOH was added to the reaction mixture II and reacted at room temperature for 2.5 h, and finally a yellowish heavy metal chelating agent was obtained; wherein the time for adding 10.8 mL of carbon disulfide is 7-8 min; the reaction equation is as follows:

[0062]

[0063] 1 mL of the heavy metal chelating agent prepared in this example was taken and diluted 50 times, and 5 mL of the diluted heavy metal chelating agent was added to 40 mL of copper nitrate solution with a concentration of 100 mg / L, and the chelating agent in this example had a Cu 2+ removal rate of 99.8%.

[0064] Example 2

[0065] The preparation method of the heavy metal chelating agent in this example is the same as that in example 1, except that in step 2, the NaOH added to the reaction mixture II is 4 g.

[0066] 1 mL of the heavy metal chelating agent prepared in this example was taken and diluted 50 times, and 5 mL of the diluted heavy metal chelating agent was added to 40 mL of copper nitrate solution with a concentration of 100 mg / L, and the chelating agent in this example had a Cu 2+ removal rate of 98.7%.

[0067] Example 3

[0068] The preparation method of the heavy metal chelating agent in this example is the same as that in example 1, except that in step 2, the NaOH added to the reaction mixture II is 4.5 g.

[0069] 1 mL of the heavy metal chelating agent prepared in this example was taken and diluted 50 times, and 5 mL of the diluted heavy metal chelating agent was added to 40 mL of copper nitrate solution with a concentration of 100 mg / L, and the chelating agent in this example had a Cu 2+The removal rate was 99.7%.

[0070] Example 4

[0071] The preparation method of the heavy metal chelating agent in this example is the same as that in Example 1, except that the amount of carbon disulfide added dropwise to the reaction mixture I in step 2 is 8 mL, and the amount of NaOH added to the reaction mixture II in step 2 is 6 g.

[0072] Take 1 mL of the heavy metal chelating agent prepared in this example and dilute it 50 times. Take 5 mL of the diluted heavy metal chelating agent and add it to 40 mL of copper nitrate solution with a concentration of 100 mg / L. The removal rate of the chelating agent in this example for Cu 2+ The removal rate was 89.5%.

[0073] Example 5, heavy metal chelating agent prepared by different molar ratios of melamine and carbon disulfide

[0074] The preparation method of the heavy metal chelating agent in this example is the same as that in Example 1, except that the molar ratio of carbon disulfide to melamine added dropwise to the reaction mixture I in step 2 is 1:1, 1:1.5, 1:2, 1:2.5, and 1:3, respectively.

[0075] Take 1 mL of the heavy metal chelating agent prepared in this example and dilute it 50 times. Take 5 mL of the diluted heavy metal chelating agent and add it to 40 mL of copper nitrate solution with a concentration of 100 mg / L. The removal rate of the chelating agent in this example for Cu 2+ As Figure 2 shown, the removal rates are 72.5%, 99.8%, 81.4%, 79.5%, and 75.8%, respectively.

[0076] Example 6, heavy metal chelating agent prepared by different molar ratios of melamine and formaldehyde

[0077] The preparation method of the heavy metal chelating agent in this example is the same as that in Example 1, except that the molar ratio of melamine to formaldehyde in the reaction mixture I in step 1 is 1:2, 1:2.5, and 1:3, respectively.

[0078] Take 1 mL of the heavy metal chelating agent prepared in this example and dilute it 50 times. Take 5 mL of the diluted heavy metal chelating agent and add it to 40 mL of copper nitrate solution with a concentration of 100 mg / L. The removal rate of the chelating agent in this example for Cu 2+ The removal rate is as Figure 3 described, and the removal rates are 99.5%, 68.4%, and 41.3%, respectively.

[0079] Example 7, heavy metal chelating agent prepared by different molar ratios of carbon disulfide and sodium hydroxide

[0080] The preparation method of the heavy metal chelating agent in the embodiment is the same as that in Embodiment 1, except that the molar ratio of carbon disulfide to NaOH in step 2 is 1:0.5, 1:0.75, 1:1, 1:1.25, and 1:1.5, respectively.

[0081] The heavy metal chelating agent prepared in the embodiment is diluted 50 times, and 5 mL of the diluted heavy metal chelating agent is added to 40 mL of a copper nitrate solution with a concentration of 100 mg / L. The chelating agent in the embodiment has a Cu 2+ removal rate of 99.7%. Figure 4 The removal rate is 38.7%, 68.4%, 87.6%, 99.7%, and 95.8%, respectively.

[0082] Embodiment 8, preparation of heavy metal chelating agents at different pH values

[0083] The preparation method of the heavy metal chelating agent in the embodiment is the same as that in Embodiment 1, except that in step 1, the Cu 2+ solution is adjusted to a pH of 1-14 by using NaOH and HCl solutions.

[0084] The heavy metal chelating agent prepared in the embodiment is diluted 50 times, and 5 mL of the diluted heavy metal chelating agent is added to 40 mL of a copper nitrate solution with a concentration of 100 mg / L. The chelating agent in the embodiment has a Cu 2+ removal rate of 99.7%. Figure 5 The Cu 2+ removal rate is higher than 90% under strong acidic conditions, and the removal rate is close to 100% under neutral and alkaline conditions.

[0085] Test analysis

[0086] 1. Infrared test analysis

[0087] The heavy metal chelating agent prepared in Embodiment 1 is tested and analyzed by using Fourier variable infrared, and the test results are shown in Table 1. Figure 1 As can be seen from Table 1, Figure 1 it can be seen that the heavy metal chelating agent is successfully prepared. Specifically, on the infrared curve of Figure 1 3387 cm -1 appears, which is the peak of symmetric stretching vibration of —NH2 in primary amine, 1441 cm -1 appears, which is the characteristic absorption peak of C=N stretching vibration, and 990 cm -1 appears, which is the characteristic absorption peak of C—O bending vibration. The above characteristic peaks prove that the heavy metal chelating agent prepared in the embodiment contains a hydroxymethyl structure.

[0088] In addition, Figure 1 1537 cm -1The peak appearing at 690cm -1 and 580cm -1 is a peak generated by C-S stretching vibration in dithioamino (—C(=S)—S—), which further indicates that the heavy metal chelator prepared by the present application introduces dithioamino having chelating function.

[0089] In conclusion, the present application successfully synthesizes a heavy metal chelator in which dithioamino having chelating function is introduced into the hydroxymethyl melamine prepolymer.

[0090] 2. Performance test analysis

[0091] The heavy metal chelator prepared in Example 1 is freeze-dried to obtain a solid powder chelator.

[0092] The test is as follows: the fly ash sample is crushed and passed through a 100-mesh sieve, and is dried in an oven at 105℃ to constant weight. The pretreated fly ash sample is weighed and added to deionized water at a solid-liquid ratio of 1:1. Then, 1%, 3%, and 5% of the chelator based on the mass of the fly ash is added, and the mixture is stirred thoroughly and air-dried naturally. The leaching toxicity is tested according to the national standard HJ / T 300-2007 "Solid waste leaching toxicity leaching method-acetic acid buffer solution method". The test results are shown in Table 1.

[0093] Table 1. Heavy metal detection results of fly ash

[0094]

[0095] As can be seen from Table 1, the content of lead and arsenic elements in the original fly ash exceeds the control standard of domestic waste landfill sites. When the amount of the chelator is 3%, both of them can meet the pollution control standard of domestic waste landfill sites.

[0096] The above merely describes the preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a heavy metal chelating agent, characterized in that, Includes the following steps: Step 1: Synthesis of the prepolymer First, alcohols and aldehydes are added to the mixture of melamine and water, and the mixture is stirred thoroughly to obtain mixture I. Then, the pH of mixture I was adjusted to 11-12 with NaOH solution, and the reaction was started by heating. The reaction time was controlled to obtain a clear solution, i.e., the prepolymer. Step 2: Synthesize heavy metal chelating agents A heavy metal chelating agent can be obtained by sequentially adding a strong alkaline substance and carbon disulfide to the prepolymer obtained in step 1 and reacting it at room temperature for a period of time. In step 1, the molar ratio of melamine to alcohols and aldehydes in mixture I is 1:(1.5~2):(1.5~2). In step 1, the alcohols include methanol, ethanol, or propanol; the aldehydes include formaldehyde or paraformaldehyde. In step 2, the molar ratio of melamine to carbon disulfide is 1:1.5~3; The strong alkaline substance in step 2 includes NaOH, KOH, or tetramethylguanidine; the molar ratio of melamine to the strong alkaline substance is 1:1~2.

2. The preparation method according to claim 1, characterized in that, In step 1, the mass ratio of melamine to water is 1:7~10.

3. The preparation method according to claim 1, characterized in that, The temperature after heating in step 1 is 40~60℃, and the reaction time of mixture I in step 1 is 0.5~2 h.

4. The preparation method according to any one of claims 1-3, characterized in that, The reaction conditions in step 2 are: the reaction temperature is room temperature and the reaction time is 2.5~3.5 h.

5. The heavy metal chelating agent prepared by the preparation method according to any one of claims 1-4.

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