Polyacrylamide flocculant for oilfield sewage and preparation method thereof

Abstract

The present application relates to the technical field of flocculants, in particular to a polyacrylamide flocculant for oilfield sewage and a preparation method thereof. The present application provides a preparation method of a polyacrylamide flocculant for oilfield sewage, which comprises: uniformly mixing acrylamide, a functional monomer and a cationic monomer, initiating a polymerization reaction, and obtaining a flocculant; wherein the functional monomer comprises a dithio carbamate group. The present application provides a polyacrylamide flocculant for oilfield sewage and a preparation method thereof, which can provide an environment-friendly flocculant with excellent flocculation effect and strong synergistic removal capacity.

Description

Technical Field

[0001] This invention relates to the field of flocculant technology, and in particular to a polyacrylamide flocculant for oilfield wastewater and its preparation method. Background Technology

[0002] Oilfield wastewater is one of the main pollutants generated during oil extraction. Its composition is complex, containing large amounts of crude oil, suspended solids, salts, heavy metal ions, and recalcitrant organic matter. It is characterized by high oil content, high turbidity, high salinity, and large fluctuations in water quality. Direct discharge would severely damage the soil and aquatic ecosystems; therefore, it must undergo advanced treatment before it can be discharged in compliance with standards or reinjected into the formation.

[0003] Flocculation and sedimentation are core components of oilfield wastewater treatment. Polyacrylamide (PAM), as a highly efficient flocculant, is widely used in water treatment due to its large molecular weight and strong adsorption bridging ability. However, traditional polyacrylamide flocculants have the following drawbacks: poor salt tolerance to the high-salt characteristics of oilfield wastewater, prone to molecular chain coiling, leading to decreased flocculation effect; insufficient synergistic removal capacity of crude oil and suspended solids, slow sedimentation speed, and high sludge moisture content; some preparation processes use highly toxic initiators, posing environmental risks, and the product stability is poor.

[0004] Therefore, in order to address the above problems, there is an urgent need for an environmentally friendly flocculant with excellent flocculation effect and strong synergistic removal capability. Summary of the Invention

[0005] This invention provides a polyacrylamide flocculant for oilfield wastewater and its preparation method, which can provide an environmentally friendly flocculant with excellent flocculation effect and strong synergistic removal ability.

[0006] In a first aspect, embodiments of the present invention provide a method for preparing a polyacrylamide flocculant for oilfield wastewater, comprising: Acrylamide, functional monomers and cationic monomers are mixed evenly and then a polymerization reaction is initiated to obtain a flocculant; wherein the functional monomers include dithiocarbamate groups.

[0007] Optionally, the reaction environment for the polymerization reaction includes: pH 4.0~5.0, and an anaerobic environment; The polymerization reaction is carried out at a temperature of 15-20°C for 3-5 hours.

[0008] Optionally, the method for preparing the functional monomer includes: S1: Mix hydroxyethyl acrylate and dithiocarbamate; S2: Adjust the temperature to 0~5℃, add the carboxyl activator in batches, and stir the reaction at room temperature for 12~24h; S3: Add an equal volume of dilute hydrochloric acid, let stand to separate the layers, discard the aqueous phase, add an equal volume of saturated NaHCO3 solution to the organic phase, let stand to separate the layers, discard the aqueous phase, add an equal volume of saturated NaCl solution, shake for 1 min, let stand to separate the layers, discard the aqueous phase; S4: Mix the organic phase and anhydrous MgSO4, react and filter. After removing the solvent from the filtrate, the product is obtained.

[0009] Optionally, in S4, 1-2 g of anhydrous MgSO4 is added for every 10 mL of organic phase.

[0010] Optionally, S1 also includes a catalyst, a solvent, and a polymerization inhibitor; The catalyst is 4-dimethylaminopyridine, the solvent is tetrahydrofuran, and the polymerization inhibitor is hydroquinone.

[0011] Optionally, in S2, the carboxyl activator is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride.

[0012] Optionally, the molar ratio of hydroxyethyl acrylate to dithiocarbamate is 1:0.8~1.2, the molar ratio of hydroxyethyl acrylate to carboxyl activator is 1:0.8~1.2, the molar ratio of hydroxyethyl acrylate to catalyst is 1:0.03~0.06, and the amount of polymerization inhibitor added is 50-100 ppm.

[0013] Optionally, the cationic monomer is one or more of methacryloyloxyethyltrimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, (meth)acryloyloxyethyldimethylbenzylammonium chloride, diallyldimethylammonium chloride, and dimethylaminopropylacrylamide; The initiators for the polymerization reaction include redox initiators and azo initiators: The redox initiator is a combination of potassium persulfate and sodium bisulfite, a combination of ammonium persulfate and sodium bisulfite, or a combination of hydrogen peroxide and sodium bisulfite. The azo initiator is at least one of azobisisobutyramidine hydrochloride, azobisisobutyramidine imidazoline hydrochloride, azobisisobutyronitrile, and azobisisobutyronitrile; wherein the mass ratio of the azo initiator to the redox initiator is 1:(1~1.5).

[0014] Optionally, the reactants and their mass fractions in the polymerization reaction include 180-220 parts of acrylamide, 100-130 parts of cationic monomer, 20-50 parts of functional monomer, 0.1-0.3 parts of initiator, and 600-700 parts of water.

[0015] Secondly, the present invention also provides a polyacrylamide flocculant for oilfield wastewater, prepared according to any of the preparation methods described above.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: 1. Highly efficient chelation of heavy metals in oil fields The dithiocarbamate group can react with Fe 2+ Pb 2+ It forms stable chelates, and the chelate structure does not decompose even in high-salt, high-temperature environments (oilfield wastewater temperatures can reach 40-80℃), thus preventing heavy metal ions from corroding and poisoning subsequent wastewater treatment equipment (such as reverse osmosis membranes). Compared to traditional heavy metal removal agents (such as sodium sulfide), it does not produce toxic hydrogen sulfide gas, making it safer.

[0017] 2. Enhanced demulsification and flocculation of emulsified oils The bridging effect of PAM molecular chains can adsorb oil droplets and suspended solids, forming large-particle flocs; the hydrophobicity of dithiocarbamate groups can also promote oil droplet aggregation and improve demulsification efficiency. The oil content of the treated effluent can be reduced to below 10 mg / L, meeting the standards for oilfield reinjection water or discharge, and the flocs settle quickly, reducing the residence time in the sedimentation tank.

[0018] 3. Improve the dewatering performance of oily sludge Oilfield sludge is highly viscous and has a high water content (typically >95%), resulting in high filtration resistance after traditional PAM treatment. This product chelates heavy metals while reducing the sludge zeta potential, making the sludge flocs denser and reducing the sludge specific resistance by 50%~70%. After pressure filtration, the sludge water content can be reduced to 60%~65%, significantly reducing sludge disposal costs. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a flowchart of the method provided by the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0022] This invention provides a method for preparing a polyacrylamide flocculant for oilfield wastewater, comprising: Acrylamide, functional monomers and cationic monomers are mixed evenly and then a polymerization reaction is initiated to obtain a flocculant; wherein the functional monomers include dithiocarbamate groups.

[0023] In this embodiment, a flocculant is prepared by polymerization of a functional monomer with a dithiocarbamate group. The dithiocarbamate group has two sulfur atoms, and the lone pair electrons of the sulfur atoms can interact with heavy metal ions (Fe). 2+ Pb 2+ Cu 2+ Ni 2+ (etc.) form coordinate bonds to generate stable chelates.

[0024] This chelation process has two advantages: Heavy metal removal: The dissolved heavy metal ions are captured from the water and settled together with the flocs; Protect downstream equipment: Prevent heavy metal ions from poisoning downstream equipment such as reverse osmosis membranes.

[0025] Furthermore, the dithiocarbamate group possesses a certain degree of hydrophobicity. When it is attached to the PAM molecular chain, the flocculant exhibits both hydrophilic and hydrophobic properties. The hydrophobic portion can adsorb onto the surface of emulsified oil droplets, promoting droplet aggregation (demulsification), which is then captured and settled by the adsorption bridging effect of PAM.

[0026] In addition, after the dithiocarbamate groups chelate heavy metals, they can neutralize the negative charge on the surface of sludge particles, reduce the zeta potential, and make the sludge flocs more compact.

[0027] In some embodiments of the present invention, the reaction environment of the polymerization reaction includes: pH value 4.0~5.0, and an anaerobic environment; The polymerization reaction is carried out at a temperature of 15-20°C for 3-5 hours.

[0028] Please refer to Figure 1 In some embodiments of the present invention, the method for preparing the functional monomer includes: S1: Mix hydroxyethyl acrylate and dithiocarbamate; S2: Adjust the temperature to 0~5℃, add the carboxyl activator in batches, and stir the reaction at room temperature for 12~24h; S3: Add an equal volume of dilute hydrochloric acid, let stand to separate the layers, discard the aqueous phase, add an equal volume of saturated NaHCO3 solution to the organic phase, let stand to separate the layers, discard the aqueous phase, add an equal volume of saturated NaCl solution, shake for 1 min, let stand to separate the layers, discard the aqueous phase; S4: Mix the organic phase and anhydrous MgSO4, react and filter. After removing the solvent from the filtrate, the product is obtained.

[0029] In this application, the self-made functional monomer has an ester bond linking chain, and the vinyl group at the hydroxyethyl acrylate end participates in free radical copolymerization, integrating the entire functional monomer into the PAM backbone. Without this double bond, the functional monomer cannot become part of the polymer and can only exist as a free small molecule, significantly reducing its effectiveness.

[0030] The -CH2-CH2-O-CO- segment acts as a spacer arm, separating the dithiocarbamate group from the PAM backbone. This serves to: reduce steric hindrance of the functional groups from the backbone; facilitate contact between the dithiocarbamate group and heavy metal ions in water; and maintain the freedom of movement of the functional groups, thereby improving chelation efficiency.

[0031] In this embodiment, to prevent the DTC group from acting as a RAFT chain transfer agent, which would lead to a decrease in molecular weight, a reduction in polymerization rate, and a broadening of the molecular weight distribution, the DTC group is attached to hydroxyethyl acrylate via an ester bond. This allows the DTC group to enter the polymer as a side chain suspension rather than as an independent RAFT agent. In this way, the DTC group primarily plays the role of a "functional group to be copolymerized" during polymerization. Its chain transfer activity is passivated by the connection between the ester bond and the main chain—because RAFT activity requires the reversible breakage of the S-R bond of DTC. When R is a large group such as -COO-CH2-CH2-O-CO-CH=CH2- (in the process of copolymerization), the free radicals formed after breakage have extremely poor re-initiation ability, effectively inhibiting the chain transfer reaction.

[0032] In summary, the commercially available DTC-containing polymerizable monomers do not have a structural mismatch in the reaction of this application. Therefore, a self-made DTC-containing monomer was prepared.

[0033] In this embodiment, the dithiocarbamate can be a metal salt (copper, sodium, or zinc), an ammonium salt, or an active derivative of dithiocarbamate.

[0034] In some embodiments of the present invention, in S4, 1-2 g of anhydrous MgSO4 is added per 10 mL of organic phase.

[0035] In some embodiments of the present invention, S1 further includes a catalyst, a solvent, and a polymerization inhibitor; The catalyst is 4-dimethylaminopyridine, the solvent is tetrahydrofuran, and the polymerization inhibitor is hydroquinone.

[0036] In some embodiments of the present invention, in S2, the carboxyl activator is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride.

[0037] In some embodiments of the present invention, the molar ratio of hydroxyethyl acrylate to dithiocarbamate is 1:0.8~1.2, the molar ratio of hydroxyethyl acrylate to carboxyl activator is 1:0.8~1.2, the molar ratio of hydroxyethyl acrylate to catalyst is 1:0.03~0.06, and the amount of polymerization inhibitor added is 50-100 ppm.

[0038] In some embodiments of the present invention, the cationic monomer is one or more of methacryloyloxyethyltrimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, (meth)acryloyloxyethyldimethylbenzylammonium chloride, diallyldimethylammonium chloride, and dimethylaminopropylacrylamide; The initiators for the polymerization reaction include redox initiators and azo initiators: The redox initiator is a combination of potassium persulfate and sodium bisulfite, a combination of ammonium persulfate and sodium bisulfite, or a combination of hydrogen peroxide and sodium bisulfite. The azo initiator is at least one of azobisisobutyramidine hydrochloride, azobisisobutyramidine imidazoline hydrochloride, azobisisobutyronitrile, and azobisisobutyronitrile; wherein the mass ratio of the azo initiator to the redox initiator is 1:(1~1.5).

[0039] In some embodiments of the present invention, the reactants and their mass fractions in the polymerization reaction include 180-220 parts of acrylamide, 100-130 parts of cationic monomer, 20-50 parts of functional monomer, 0.1-0.3 parts of initiator, and 600-700 parts of water.

[0040] This invention also provides a polyacrylamide flocculant for oilfield wastewater, prepared according to any of the preparation methods described in the above embodiments.

[0041] To more clearly illustrate the technical solution and advantages of the present invention, the following describes the solution of this application in detail through several embodiments.

[0042] Example 1 1.1 Preparation of Functional Monomers In a 500 mL three-necked flask, add hydroxyethyl acrylate (HEA, 0.1 mol, 11.6 g), dithiocarbamate (0.11 mol, 12.9 g), catalyst 4-dimethylaminopyridine (DMAP, 0.005 mol, 0.61 g), solvent tetrahydrofuran (THF, 100 g, solid-liquid ratio 1:8, mass ratio), purge with nitrogen for 30 min to remove oxygen, and add polymerization inhibitor hydroquinone (80 ppm, based on total monomer mass, 0.0021 g).

[0043] The mixture was cooled to 0-5°C in an ice bath, and the carboxyl activator 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC, 0.11 mol, 21.1 g) was added in batches. The mixture was stirred at room temperature for 18 h.

[0044] Transfer the reaction solution to a separatory funnel, add an equal volume of 5wt% dilute hydrochloric acid (100mL), shake for 1.5min, allow to stand and separate into layers, and discard the aqueous phase; add an equal volume of saturated NaHCO3 solution (100mL) to the organic phase, shake gently, allow to stand and separate into layers, and discard the aqueous phase; add an equal volume of saturated NaCl solution (100mL), shake for 1min, allow to stand and separate into layers, and discard the aqueous phase.

[0045] The organic phase was poured into an Erlenmeyer flask, anhydrous MgSO4 (15g) was added, and the mixture was stirred for 5 min and filtered. The filtrate was evaporated under reduced pressure at a temperature below 40℃ to remove THF, yielding a pale yellow oily functional monomer (yield 18.2g, purity 88%).

[0046] 1.2 Flocculant Preparation In a 2000 mL reactor, acrylamide (200 parts, 200 g), the functional monomer prepared above (30 parts, 30 g), the cationic monomer acryloyloxyethyltrimethylammonium chloride (DAC, 115 parts, 115 g), and deionized water (650 parts, 650 g) were added. After stirring and mixing evenly, the pH was adjusted to 4.5 with dilute hydrochloric acid, and nitrogen was purged for 30 min to remove oxygen. Under nitrogen protection, the temperature was raised to 18 °C, and potassium persulfate (0.04 parts, 0.04 g), sodium bisulfite (0.02 parts, 0.02 g), and azobisisobutyramidine hydrochloride (0.06 parts, 0.06 g) were added as initiators. The reaction was carried out for 4 h.

[0047] After the reaction is complete, the gel block is removed, cut into pieces, and dried at 60℃ for 30 minutes. Then, the temperature is raised to 90℃ and dried for 75 minutes. After grinding, it is passed through a 100-mesh sieve to obtain the target flocculant product.

[0048] Example 2 2.1 Preparation of Functional Monomers The same method for preparing the functional monomer as in Example 1 was used, except that the amount of dithiocarbamate was adjusted to 0.11 mol, the amount of EDC was adjusted to 0.11 mol, and the reaction time was adjusted to 24 h, finally yielding a pale yellow oily functional monomer (yield 17.8 g, purity 87%).

[0049] 2.2 Flocculant Preparation In a 2000 mL reactor, acrylamide (180 parts, 180 g), the functional monomer prepared above (20 parts, 20 g), the cationic monomer methacryloyloxyethyltrimethylammonium chloride (DMC, 100 parts, 100 g), and deionized water (600 parts, 600 g) were added. After stirring and mixing evenly, the pH was adjusted to 4.0 with dilute hydrochloric acid, and nitrogen was purged for 30 min to remove oxygen. Under nitrogen protection, the temperature was raised to 15 °C, and potassium persulfate (0.04 parts, 0.04 g), sodium bisulfite (0.02 parts, 0.02 g), and azobisisobutyramidine hydrochloride (0.06 parts, 0.06 g) were added as initiators. The reaction was carried out for 5 h.

[0050] After the reaction is complete, the gel block is removed, cut into pieces, and dried at 60℃ for 30 minutes. Then, the temperature is raised to 90℃ and dried for 90 minutes. After grinding, it is passed through an 80-mesh sieve to obtain the target flocculant product.

[0051] Example 3 3.1 Preparation of Functional Monomers The same method for preparing the functional monomer as in Example 1 was used, except that the solid-liquid ratio was adjusted to 1:10 (THF dosage 120g), the amount of polymerization inhibitor was 100ppm, and the reaction time was 12h, finally yielding a pale yellow oily functional monomer (yield 18.5g, purity 89%).

[0052] 3.2 Flocculant Preparation In a 2000 mL reactor, acrylamide (220 parts, 220 g), the functional monomer prepared above (50 parts, 50 g), the cationic monomer diallyl dimethyl ammonium chloride (DADMAC, 130 parts, 130 g), and deionized water (700 parts, 700 g) were added. After stirring and mixing evenly, the pH was adjusted to 5.0 with dilute hydrochloric acid, nitrogen was purged for 30 min to remove oxygen, and the temperature was raised to 20 °C under nitrogen protection. The initiator azobisisobutyramidine hydrochloride and ammonium persulfate-sodium bisulfite (mass ratio 1:1, total amount 0.3 parts, 0.3 g) were added, and the reaction was carried out for 3 h.

[0053] After the reaction is complete, the gel block is removed, cut into pieces, and dried at 60℃ for 30 minutes. Then, the temperature is raised to 90℃ and dried for 60 minutes. After grinding, it is passed through a 120-mesh sieve to obtain the target flocculant product.

[0054] Comparative Example 1 (Missing Functional Monomer) In a 2000 mL reactor, acrylamide (200 parts, 200 g), cationic monomer acryloyloxyethyltrimethylammonium chloride (DAC, 115 parts, 115 g), and deionized water (650 parts, 650 g) were added. After stirring and mixing thoroughly, the pH was adjusted to 4.5 with dilute hydrochloric acid, and nitrogen was purged for 30 min to remove oxygen. Under nitrogen protection, the temperature was raised to 18 °C, and potassium persulfate (0.04 parts, 0.04 g), sodium bisulfite (0.02 parts, 0.02 g), and azobisisobutyramidine hydrochloride (0.06 parts, 0.06 g) were added as initiators. The reaction was carried out for 4 h.

[0055] After the reaction was completed, the gel block was removed, cut into pieces, dried at 60℃ for 30 minutes, then heated to 90℃ for 75 minutes, ground, and passed through a 100-mesh sieve to obtain the comparative flocculant product (without DTC groups).

[0056] Comparative Example 2 (Replacement Functional Unit) The functional monomer in Example 1 was replaced with hydroxyethyl acrylate (without dithiocarbamate), and the amounts of other raw materials and the preparation process were the same as in Example 1, to obtain a comparative flocculant product (containing hydroxyl groups, without DTC groups).

[0057] Application Evaluation: Test method: 1. Basic index testing: Molecular weight was determined by gel permeation chromatography (GPC); cationicity was determined by colloidal titration; and dissolution time was determined by static observation method (0.1g of sample was weighed and dissolved in 100mL of deionized water, and the time required for complete dissolution was recorded).

[0058] 2. Oilfield wastewater treatment effect test: Test object: Wastewater from a certain oilfield (oil content 85mg / L, Fe...) 2+ The concentration was 12 mg / L, suspended solids content was 150 mg / L, salinity was 15000 mg / L, and pH was 7.8. 10 mg / L of the above-mentioned examples and comparative products were added to 500 mL of oilfield wastewater, stirred for 3 min, and allowed to stand for 30 min. The oil content and Fe content of the effluent were then measured. 2+ Removal rate, suspended solids removal rate, and sludge moisture content.

[0059] Table 1 Table 2 Conclusion: 1. Basic indicators: The molecular weights of the products from Examples 1-3 were all within 7.8 × 10⁻⁶. 6 ~9.2×10 6 Within the specified range, the cationicity is 25.3%~32.1%, the dissolution time is ≤52min, and it exhibits good water solubility and application compatibility. 2. Treatment effect: The products of Examples 1-3 are significantly better than the comparative examples in terms of oil removal, heavy metal chelation, suspended solids flocculation and sludge dewatering of oilfield wastewater, proving that the functional monomer (containing DTC group) of the present invention is the key to improving product performance, while not affecting the basic application performance of flocculant.

[0060] Finally, it should be noted that the above embodiments and comparative examples are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments and comparative examples, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments and comparative examples, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments and comparative examples of the present invention.

Claims

1. A method for preparing a polyacrylamide flocculant for oilfield wastewater, characterized in that, include: Acrylamide, functional monomers and cationic monomers are mixed evenly and then a polymerization reaction is initiated to obtain a flocculant; wherein the functional monomers include dithiocarbamate groups.

2. The preparation method according to claim 1, characterized in that, The reaction environment for the polymerization reaction includes: pH 4.0~5.0, and an anaerobic environment; The polymerization reaction is carried out at a temperature of 15-20°C for 3-5 hours.

3. The preparation method according to claim 1, characterized in that, The preparation method of the functional monomer includes: S1: Mix hydroxyethyl acrylate and dithiocarbamate; S2: Adjust the temperature to 0~5℃, add the carboxyl activator in batches, and stir the reaction at room temperature for 12~24h; S3: Add an equal volume of dilute hydrochloric acid, let stand to separate the layers, discard the aqueous phase, add an equal volume of saturated NaHCO3 solution to the organic phase, let stand to separate the layers, discard the aqueous phase, add an equal volume of saturated NaCl solution, shake for 1 min, let stand to separate the layers, discard the aqueous phase; S4: Mix the organic phase and anhydrous MgSO4, react and filter. After removing the solvent from the filtrate, the product is obtained.

4. The preparation method according to claim 3, characterized in that, In S4, add 1-2 g of anhydrous MgSO4 per 10 mL of organic phase.

5. The preparation method according to claim 3, characterized in that, S1 also includes a catalyst, a solvent, and a polymerization inhibitor; The catalyst is 4-dimethylaminopyridine, the solvent is tetrahydrofuran, and the polymerization inhibitor is hydroquinone.

6. The preparation method according to claim 3, characterized in that, In S2, the carboxyl activator is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride.

7. The preparation method according to claim 5, characterized in that, The molar ratio of hydroxyethyl acrylate to dithiocarbamate is 1:0.8~1.2, the molar ratio of hydroxyethyl acrylate to carboxyl activator is 1:0.8~1.2, the molar ratio of hydroxyethyl acrylate to catalyst is 1:0.03~0.06, and the amount of polymerization inhibitor added is 50-100 ppm.

8. The preparation method according to claim 1, characterized in that, The cationic monomer is one or more of the following: methacryloyloxyethyltrimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, (meth)acryloyloxyethyldimethylbenzylammonium chloride, diallyldimethylammonium chloride, and dimethylaminopropylacrylamide; The initiators for the polymerization reaction include redox initiators and azo initiators: The redox initiator is a combination of potassium persulfate and sodium bisulfite, a combination of ammonium persulfate and sodium bisulfite, or a combination of hydrogen peroxide and sodium bisulfite. The azo initiator is at least one of azobisisobutyramidine hydrochloride, azobisisobutyramidine imidazoline hydrochloride, azobisisobutyronitrile, and azobisisobutyronitrile; wherein the mass ratio of the azo initiator to the redox initiator is 1:(1~1.5).

9. The preparation method according to claim 1, characterized in that, The reactants and their mass fractions for the polymerization reaction include 180-220 parts acrylamide, 100-130 parts cationic monomer, 20-50 parts functional monomer, 0.1-0.3 parts initiator, and 600-700 parts water.

10. A polyacrylamide flocculant for oilfield wastewater, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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