CN102689963A - Organic copolymerized amphoteric polymeric flocculant and preparation method - Google Patents

Abstract

The invention discloses an organic copolymerized amphoteric polymer flocculant and a preparation method thereof. The amphoteric polymer flocculant is composed of methacryloyloxyethyltrimethylammonium halide, N-vinylpyrrolidone and sorbic acid (salt) three A copolymer formed by copolymerization of a monomer and a bifunctional monomer diacrylate. Redox initiators, inorganic peroxide initiators and polymerization aids are used simultaneously in the preparation process, which improves the conversion rate of monomers and reduces residual monomers; the preparation of flocculants is simple and can be used directly; the prepared flocculants It has the advantages of small dosage, large floc diameter and fast settlement.

Description

A kind of organic copolymerized amphoteric macromolecule flocculant and its preparation method

technical field

The invention relates to an organic copolymerized amphoteric polymer flocculant and a preparation method.

Background technique

Water is the basic material for human survival and social development. At present, the water environment in the world is facing the problems of water resource shortage, excessive consumption and serious pollution. The World Health Organization (WHO) survey shows that 80% of human diseases are related to unsafe water quality. China is a country with abundant water resources, but at the same time it is a country with severe water shortage, extremely low utilization rate of water resources and serious water pollution. These three factors seriously restrict people's normal life and sustainable economic development. Therefore, effective treatment and reuse of waste water (sewage) is an inevitable way to solve limited water resources to meet people's growing production and living needs. In the process of waste (sewage) treatment and sludge dehydration, flocculation is an important, effective, economical and convenient method. The proportion of domestic and foreign industrial wastewater and domestic sewage treated by flocculation technology is 55% to 85%. 100% of industrial and sludge dehydration is treated by flocculation; the effect of waste (sewage) water flocculation and sludge dehydration is closely related to the operation of subsequent processes, the discharge of effluent and the cost of treatment, and the flocculant is waste (sewage) water One of the key factors for water treatment and solid-liquid separation, so the research and development of new, non-toxic and efficient flocculants is the focus and direction of water treatment.

Flocculants can be divided into several categories such as microbial flocculants, inorganic flocculants and organic polymer flocculants. Microbial flocculants are relatively safe for the environment and human beings. At present, most microbial flocculants are still in the screening stage of strains, and the cost is high, which cannot meet the requirements of industrial production and application. Due to the large amount of inorganic flocculants, the generation of a lot of sludge, strong corrosion, secondary pollution and the influence of waste (sewage) water quality, the water purification effect is not ideal in some occasions, and the application is greatly limited, and is gradually being used. Organic flocculants instead. Organic polymer flocculant has the advantages of less dosage, fast flocculation speed, less sludge generation, good decolorization and little impact on waste (sewage) water quality, etc. It is divided into natural organic polymer flocculants and synthetic organic polymer flocculants; the advantages of natural organic polymer flocculants are rich sources of raw materials, low price, basically non-toxic, easy biochemical degradation and basically no secondary pollution, and the disadvantage is that the composition is complex , Unstable composition, large performance fluctuations, possible deterioration during storage, and the use of a large amount of organic solvents in the production process, which limit its practical application; the synthetic organic polymer flocculant has stable performance, and the molecular weight of the synthetic product can be controlled according to needs And structure and other advantages, can be divided into non-ionic, anionic, cationic and amphoteric. With the development of society and economy, the discharge of modern industrial and domestic sewage and the increase in chemical oxygen demand (Chemical Oxygen Demand, COD _Cr ), the composition and properties of waste (sewage) water have become increasingly complex. Using conventional flocculants It is difficult to meet the requirements, and the surface of particles in waste (sewage) water usually has a negative charge, and the water contains a certain amount of toxic metal ions, so the use of non-ionic, anionic and cationic polymer flocculants is limited; amphoteric organic high The polymer molecular chain of the molecular flocculant contains both positive and negative charge groups, called amphoteric polymer or amphoteric polyelectrolyte, which has some characteristics due to its special molecular structure, such as electrical neutralization , adsorption bridging and intermolecular "entanglement" encapsulation, suitable for waste (sewage) water where anions and cations coexist, for dispersions, emulsions and various sludges stabilized by anionic surfactants or by cations The various colloidal dispersions stabilized have good flocculation and sludge dehydration effects, and can be used as water treatment flocculants, sludge dehydrating agents, adsorbents and metal ion chelating agents, etc.; due to their good dehydration, The advantages of metal ion removal, wide range of pH requirements and good salt resistance make amphoteric organic polymer flocculants have a wide range of applications, and have become a hot research topic at home and abroad.

At present, the synthetic amphoteric organic polymer flocculants developed at home and abroad are mainly modified by polyacrylamide (pAM), that is, by the chemical reaction of amide groups, such as Mannich reaction, hydrolysis or grafting reaction modification. , and polyacrylonitrile (pAN) as the polymer chain, amphoteric polyacrylonitrile-dicyandiamide (pAN-DCD) modified with dicyandiamide; "Organic Chemical Materials" [M], 1999 The third edition in January 2006, Chemical Industry Press, edited by Si Hang, introduced acrylamide (AM) and acrylonitrile (AN) as highly toxic chemicals; "Acrylamide Polymer" [M], April 2006 No. 1st edition, Chemical Industry Press, edited by Fang Daobin, etc., introduced the hazards of acrylamide to the human body and the environment, "There are three forms of exposure to acrylamide, oral, respiratory inhalation and skin contact. Acrylamide is distributed in body fluids after inhalation. Water is the most important pollutant. Non-occupational exposure to acrylamide mainly comes from drinking water and food contaminated with acrylamide. The toxicity of acrylamide is mainly manifested as skin damage and nervous system damage. The U.S. Environmental Protection Agency (EPA) on propylene The regulations on the toxicity of amide to humans are: short-term and long-term oral or exposure to acryloyl can cause damage to the human nervous system, and it is considered that low-concentration acrylamide is also a substance that is harmful to the nervous system. Acute poisoning is the Irritation of the skin and respiratory tract, rash at the site of contact. Chronic poisoning causes neurotoxic reactions, affecting the central and peripheral nerves." "In 1994, the International Agency for Research on Cancer (IARC) investigated the genotoxicity to animals and raised acrylamide from Group 2B (possibly carcinogenic) to Group 2A (probably carcinogenic) in 1987." "In view of the impact of acrylamide on human beings, the environment and health, WHO, IARC, the US Food and Drug Administration (FDA), EPA, the European Union (EU) and the Chinese government have all made strict regulations on the limit of acrylamide in the environment ". "Countries have strict restrictions on the maximum allowable dosage of polyacrylamide in water treatment and the maximum allowable content of residual monomer acrylamide in polyacrylamide products. For example, WHO stipulates the dosage of polyacrylamide in water purification It shall not exceed 1mg/L. The residue of acrylamide in the product shall not exceed 0.05%, that is, the maximum residue of acrylamide in drinking water is 0.0005 mg/L". With the increasing awareness of environmental protection and the severe situation of waste (sewage) water pollution control, the requirements for water treatment are getting higher and higher. The purpose and significance of the present invention aim at the toxicity of existing monomers for preparing flocculants at home and abroad, select non-(low) toxic monomers, and adopt suitable polymerization methods and process conditions to prepare a non-(low) toxic, A practical, effective and economical new organic copolymerized amphoteric polymer flocculant to meet the needs of waste (sewage) water treatment.

At present, synthetic amphoteric polymer flocculants have the following defects:

1. The synthetic amphoteric polymer flocculant uses highly toxic acrylamide and acrylonitrile monomers, and the residual monomers after polymerization and small molecules after decomposition will cause secondary pollution;

2. The Mannich reaction requires the use of a large amount of organic solvents, which does not conform to the principles of green chemistry;

3. The polyacrylamide amphoteric polymer flocculant modified by Mannich reaction is not suitable for treating wastewater with high pH, and its storage stability is not high;

The present invention can achieve better performance under the premise that the flocculation efficiency of the flocculant is not reduced:

1. The selected monomer has no (low) toxicity, does not cause secondary pollution, and is more in line with the requirements of green chemistry;

2. Copolymerization uses water as a solvent, which conforms to the principle of clean production;

3. Overcome the shortcomings of poor dehydration effect and unstable performance of polyacrylamide flocculants;

4. For copolymerization, redox initiators are used at low temperatures, and inorganic peroxide initiators are used at high temperatures. At the same time, acidity regulators and polymerization aids are used to make the molecular weight of the product meet the requirements of use and reduce residual monomers.

Contents of the invention

Through the new technology, an organic copolymerized amphoteric polymer flocculant with stable performance, wider application range and better flocculation effect and a preparation method with simple operation are provided.

The organic copolymerized amphoteric polymer flocculant is a copolymer formed by copolymerization of methacryloyloxyethyltrimethylammonium halide, N-vinylpyrrolidone, sorbic acid (salt) and bifunctional monomer diacrylate. Characterized in that,

Monomer 1 is methacryloyloxyethyltrimethylammonium halide, the structural formula is:

In the formula, X is Cl, Br;

Monomer 2 is N-vinylpyrrolidone with the structural formula:

 ；

;

Monomer 3 is sorbic acid (salt), the structural formula is:

 

In the formula; Y is H, Na or K;

Bifunctional monomer diacrylate, the structural formula is:

Where 1≦i≦23 (i is a positive integer),

    

    

The structure of the copolymer formed by copolymerization of three kinds of monomers and bifunctional monomer diacrylate according to a certain ratio is shown in the figure:

     

     

In the figure: 1≦n≦38552; 1≦m≦71976; 1≦p≦71341; 1≦q≦47003; 1≦i≦23, n, m , p, q, i are positive integers.

The preparation method of the present invention has no special requirements on the above-mentioned comonomers, bifunctional monomer diacrylates, redox initiators, inorganic peroxide initiators, acidity regulators and polymerization aids. For example, the copolymerization monomer, the general formula can be expressed as CH ₂ =C(CH ₃ )COO(CH ₂ ) ₂ N ⁺ (CH ₃ ) ₃ X ^- dimethyl diallyl ammonium halide; the chemical molecular formula is expressed as C N-vinylpyrrolidone of ₆ H ₉ ON; sorbic acid (salt) whose general formula can be expressed as CH ₃ CH=CHCH=CHCOOY; bifunctional group whose general formula can be expressed as CH ₂ =CHCOO(R) _i OOCCH=CH ₂ Monomer diacrylate. The test results show that in order to achieve the best performance and economic efficiency of the organic copolymerized amphoteric polymer flocculant prepared by the present invention, the ratio of the amounts of the three monomers is controlled at (15-80): (10-80) 70): (5-60) (mol%); the mass of the diacrylate monomer of the difunctional group is 0.8%-10% of the sum of the mass of the other three monomers; the mass fraction of the total mass of the monomer in the reaction system is 10 %~60%.

As mentioned above, the present invention adds a redox initiator at a relatively low temperature in the early stage of the copolymerization for the purpose of obtaining a copolymer with a large molecular weight. For example, the oxidizing agent selected can be any one of hydrogen peroxide, ammonium persulfate, sodium persulfate, potassium persulfate, and the reducing agent is any one of sodium bisulfite, sodium sulfite, potassium bisulfite, sodium thiosulfate kind. The test results show that the ratio of the amount of oxidizing agent to reducing agent is 1: (1-0.2), and the ideal effect can be achieved in the range of 0.1%-2% of the total mass of the monomer.

As mentioned above, the present invention adds an inorganic peroxide initiator at a higher temperature in the later stage of polymerization, the purpose of which is to reduce the residual monomer and increase the conversion rate of the monomer. For example, hydrogen peroxide, ammonium persulfate, sodium persulfate, potassium persulfate are selected. The test results show that the conversion rate of the monomer can achieve the desired effect when the dosage is within the range of 0.02% to 2.0% of the total mass of the monomer.

As mentioned above, the present invention adds an acidity regulator in the copolymerization reaction to adjust the pH of the polymerization system to 2-9, the purpose of which is to ensure that the initiator used can achieve the most ideal initiation efficiency. The substance used to adjust the pH is an aqueous solution of any one of sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, sodium bicarbonate and hydrochloric acid.

As mentioned above, in the present invention, a polymerization assistant is added in the copolymerization reaction, the purpose of which is to prevent the primary free radicals formed by the redox initiator and monomer free radicals from being prematurely terminated. For example, any one of diethanolamine, triethanolamine, ethylenediaminetetraacetic acid and its salts, water-soluble fluorine-containing compounds such as fluorosilicic acid, calcium fluorosilicate, potassium fluoride, and sodium fluoride, or a mixture of two or more is used. The test results show that the dosage is within the range of 0.01% to 1.5% of the total mass of the monomers, and the copolymerization reaction initiated by the redox initiator can obtain a large molecular weight organic copolymerized amphoteric polymer flocculant and the polymerization rate can be controlled.

The method of the present invention comprises the following steps and processing conditions:

Step 1. Add sorbic acid (salt), polymerization aids and distilled water into the four-neck reaction flask, stir and dissolve, vacuumize twice, each time 5 minutes, and pass nitrogen once in the middle, time 5 minutes;

Step 2. Dilute methacryloyloxyethyltrimethylammonium halide with distilled water and put it into the dropping funnel (a), dilute N-vinylpyrrolidone with distilled water and put it into the dropping funnel (b), dilute the bifunctional mono Bulk diacrylate and transfer to dropping funnel (c);

Step 3. Under the conditions of continuous stirring and nitrogen flow, add the redox initiator at one time, raise the temperature to 35°C~45°C, adjust the pH of the aqueous solution to 2~9, add methacryloyloxyethyl dropwise into the dropping funnel (a) The aqueous solution of trimethylammonium halide, the dropping funnel (b) add the aqueous solution of N-vinylpyrrolidone, the dropping funnel (c) drop the aqueous solution of the bifunctional monomer diacrylate to the four-necked reaction flask, three The dripping funnel is added dropwise at the same time, and the dropping speed is controlled. The dropping time is 0.5 to 3 hours, and the temperature is kept constant for 0.5 to 4 hours after the dropping. Keep the pH value at 2-9;

Step 4. Heat up to 75°C-85°C, add inorganic peroxide initiator to continue the reaction, check the pH value of the reaction liquid in the four-neck reaction bottle every 20 minutes, keep the pH at 2-9, and the reaction time is 1-3.5 hours , cooling, and discharging to obtain an organic copolymerized amphoteric polymer flocculant product;

In the method of the present invention, temperature-variable copolymerization is carried out by three kinds of monomers and bifunctional monomer diacrylate, and there are sufficient active free radicals in the reaction system, so that the copolymerization reaction can be continuously carried out; water-soluble redox agent is used as a low-temperature initiator , to ensure that the flocculant has a sufficiently high molecular weight; adjust the pH of the reaction system to make the initiation efficiency more ideal; add a certain amount of polymerization assistant to reduce the impurities in the reaction system and prevent the premature termination of active free radicals; The inorganic peroxide initiator is added in the later stage of the polymerization reaction, which reduces the residual monomer and improves the conversion rate.

Compared with the existing amphoteric polyacrylamide flocculants used for water treatment, the present invention has the following advantages:

1. Compared with the organic copolymerized amphoteric polymer flocculants produced by the prior art, the monomers used are non-toxic (low) and have almost no impact on the environment;

2. Water-soluble redox agent is used as the low-temperature initiator, and water-soluble peroxide initiator is added in the later stage to ensure that the flocculant has a sufficiently high molecular weight, and the conversion rate is higher and the residual monomer content is lower;

3. The preparation method is simple, the reaction conditions are mild, and it is suitable for popularization and application;

4. Stable performance, suitable for a wide range of pH value, many types of wastewater treatment, small input (only 1-200mg/L), and obvious economic benefits.

BRIEF DESCRIPTION OF THE DRAWINGS The attached figure is the chemical formula for the copolymerization reaction of monomer 1, monomer 2, monomer 3 and bifunctional monomer diacrylate in a certain ratio under the action of an initiator and a polymerization aid.

Implementation case The present invention is further described as follows in conjunction with embodiment:

Example 1

Step 1, add the mixture of 1.6g sorbic acid (salt), 0.006g polymerization assistant sodium edetate and potassium fluoride and 15g Stir the distilled water to dissolve, vacuumize twice, each time for 5 minutes, and pass nitrogen once in the middle, for 5 minutes;

Step 2. Dilute 15.7g of methacryloyloxyethyltrimethylammonium halide with 8g of distilled water and put it into the dropping funnel (a), dilute 2.7g of N-vinylpyrrolidone with 15g of distilled water and put it into the dropping funnel (b). 20g of distilled water diluted 1.2g of bifunctional monomer diacrylate into the dropping funnel (c);

Step 3, under the condition of continuous stirring and nitrogen flow, add 0.19 redox initiator potassium persulfate and sodium potassium bisulfite mixture at one time, the ratio of the amount of substance of oxidizing agent and reducing agent is 1: 0.6, be warming up to 45 °C, adjust the pH of the aqueous solution to 3, drop the aqueous solution of methacryloyloxyethyltrimethylammonium halide into the dropping funnel (a), drop the aqueous solution of N-vinylpyrrolidone into the dropping funnel (b), drop the aqueous solution of the dropping funnel (c) Add the aqueous solution of bifunctional monomer diacrylate dropwise to the four-neck reaction flask, and drop the three dropping funnels at the same time to control the dropping speed. The dropping time is 1 hour. After the dropping, keep the temperature for 2 hours. During the addition and constant temperature process, check the pH value of the reaction liquid in the four-neck reaction flask every 10 minutes, and keep the pH at 3;

Step 4. Heat up to 80°C, add 0.05g of inorganic peroxide initiator potassium persulfate to continue the reaction, check the pH value of the reaction liquid in the four-neck reaction flask every 20 minutes, keep the pH at 3, react for 2 hours, cool , discharged to obtain an organic copolymerized amphoteric polymer flocculant product with a molecular weight of 7.3×10 ⁶ and a conversion rate of 97.6%.

Example 2

Step 1, add the mixture of 2.4g sorbic acid (salt), 0.009g polymerization assistant sodium edetate and potassium fluoride and 22g Stir the distilled water to dissolve, vacuumize twice, each time for 5 minutes, and pass nitrogen once in the middle, for 5 minutes;

Step 2. Dilute 24g of methacryloxyethyltrimethylammonium halide with 12g of distilled water and put it into the dropping funnel (a), dilute 4g of N-vinylpyrrolidone with 22g of distilled water and put it into the dropping funnel (b), and put it into the dropping funnel (b) with 30g of distilled water Dilute 1.8g of bifunctional monomer diacrylate into the dropping funnel (c);

Step 3, under the condition of continuous stirring and nitrogen flow, add the mixture of 0.29 redox initiator potassium persulfate and sodium potassium bisulfite at one time, the ratio of the amount of substance of oxidizing agent and reducing agent is 1: 0.5, be warming up to 40 °C, adjust the pH of the aqueous solution to 4, drop the aqueous solution of methacryloyloxyethyltrimethylammonium halide into the dropping funnel (a), drop the aqueous solution of N-vinylpyrrolidone into the dropping funnel (b), drop the dropping funnel (c) Add the aqueous solution of bifunctional monomer diacrylate dropwise to the four-necked reaction flask, and drop the three dropping funnels at the same time to control the dropping speed. The dropping time is 0.5 hours. After the dropping, keep the temperature for 2 hours. During the addition and constant temperature process, check the pH value of the reaction liquid in the four-neck reaction flask every 10 minutes, and keep the pH at 4;

Step 4. Heat up to 80°C, add 0.07g of inorganic peroxide initiator potassium persulfate to continue the reaction, check the pH value of the reaction liquid in the four-neck reaction flask every 20 minutes, keep the pH at 4, react for 2 hours, cool , discharged to obtain an organic copolymerized amphoteric polymer flocculant product with a molecular weight of 7×10 ⁶ and a conversion rate of 96.3%.

Application example

The No.1 and No.2 organic copolymerized amphoteric polymer flocculants prepared in the above Examples 1 and 2 were used for flocculation treatment of printing and dyeing wastewater (see Table 1 and Table 2).

Claims (6)

1. An organic copolymerized amphoteric macromolecule flocculant, characterized in that the molecular weight is 9×10 ⁴ to 8×10 ⁶ , and it is formed by copolymerization of three monomers and bifunctional monomer diacrylate, Monomer 1 is methacryloyloxyethyltrimethylammonium halide, structural formula is

In the formula, X is Cl, Br; Monomer 2 is N-vinylpyrrolidone with the formula

; Monomer 3 is sorbic acid (salt), the structural formula is   In the formula, Y is H, Na or K; Difunctional monomer diacrylate, the structural formula is

Where 1≦i≦23 (i is a positive integer),

     The structural formula of the copolymer formed by copolymerization of three kinds of monomers and bifunctional monomer diacrylate according to a certain amount ratio is

In the formula, 1≦n≦38552; 1≦m≦71976; 1≦p≦71341; 1≦q≦47003; 1≦i≦23, n, m, p, q, i are positive integers. 2. A preparation method of organic copolymer amphoteric polymer flocculant as claimed in claim 1, is characterized in that, the step that this method comprises and processing condition are Step 1. Add sorbic acid (salt), polymerization aids and distilled water into the four-neck reaction flask, stir and dissolve, vacuumize twice, each time 5 minutes, and pass nitrogen once in the middle, time 5 minutes; Step 2, respectively diluting methacryloyloxyethyltrimethylammonium halide, N-vinylpyrrolidone and bifunctional monomer diacrylate with distilled water; Step 3. Under the conditions of continuous stirring and nitrogen flow, add the redox initiator at one time, raise the temperature to 35 ° C ~ 45 ° C, adjust the pH of the aqueous solution to 2 ~ 9, add methacryloyloxyethyl group dropwise through a dropping funnel Aqueous solution of trimethylammonium halide, a dropping funnel to drop N-vinylpyrrolidone aqueous solution, a dropping funnel to drop aqueous solution of bifunctional monomer diacrylate to four-necked reaction flask, three dropping funnels Add dropwise at the same time, control the drop rate, the drop time is 0.5 to 3 hours, keep the temperature for 0.5 to 4 hours after the drop, during the drop and constant temperature, check the pH value of the reaction liquid in the four-necked reaction flask every 10 minutes, Keep the pH between 2 and 9; Step 4. Heat up to 75°C-85°C, add inorganic peroxide initiator to continue the reaction, check the pH value of the reaction liquid in the four-neck reaction bottle every 20 minutes, keep the pH at 2-9, and the reaction time is 1-3.5 hours , cooling, and discharging to obtain an organic copolymerized amphoteric polymer flocculant product; The raw materials used are methacryloyloxyethyltrimethylammonium halide, N-vinylpyrrolidone, sorbic acid (salt), bifunctional monomer diacrylate, redox initiator, inorganic peroxide initiator, polymerization assistant agents and acidity regulators, Wherein methacryloyloxyethyltrimethylammonium halide, N-vinylpyrrolidone, and sorbic acid (salt) are three monomers in a ratio of (15 to 80): (10 to 70): (5 ~60) (mol%), The mass of the bifunctional monomer diacrylate is 0.8% to 10% of the sum of the mass of the other three monomers, The mass fraction of the mass sum of the three monomers and the bifunctional monomer diacrylate in the total mass of the reaction system is 10% to 60%; The mass of the redox initiator is 0.1% to 2% of the total mass of the monomers, and the ratio of the amount of the oxidizing agent to the reducing agent is 1: (1 to 0.2); The mass of the inorganic peroxide initiator is 0.02% to 2.0% of the total mass of the monomers; The mass of the polymerization aid is 0.01% to 1.5% of the total mass of the monomers. 3. the preparation method of organic copolymerization amphoteric macromolecule flocculant as claimed in claim 2, it is characterized in that the redox initiator among the claim 2 is that oxygenant is hydrogen peroxide, ammonium persulfate, sodium persulfate, potassium persulfate Any one of them, the reducing agent is any one of sodium bisulfite, potassium bisulfite, sodium sulfite, sodium thiosulfate. 4. the preparation method of organic copolymerization amphoteric macromolecule flocculant as claimed in claim 2 is characterized in that the inorganic peroxide initiator among the claim 2 is hydrogen peroxide, ammonium persulfate, sodium persulfate, potassium persulfate any of the. 5. the preparation method of organic copolymerization amphoteric polymer flocculant as claimed in claim 2, is characterized in that the acidity regulator in claim 2 is sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, sodium bicarbonate , any one of hydrochloric acid. 6. The preparation method of organic copolymerized amphoteric macromolecule flocculant as claimed in claim 2, it is characterized in that the polymerization assistant in claim 2 is diethanolamine, triethanolamine, ethylenediaminetetraacetic acid and salt thereof, water-soluble containing Fluorine compounds such as any one of fluorosilicic acid, calcium fluorosilicate, potassium fluoride, and sodium fluoride, or a mixture of two or more.

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