A composite coagulant for eucalyptus mechanical pulp wastewater with gradient chelation-free radical activation synergy and its preparation method

Abstract

The present invention discloses a composite coagulant for eucalyptus chemi-mechanical pulp wastewater with gradient chelation-radical activation synergy and a preparation method thereof, belonging to the technical field of eucalyptus chemi-mechanical pulp wastewater treatment. The present invention uses ferrous sulfate heptahydrate, aluminum hydroxide and magnesium sulfate as raw materials, and through gradient chelation reaction, radical activation treatment, functional component composite reaction, pH and specific gravity regulation, and aging treatment, a water purifying agent with a viscosity of 35-45 mPa·s is prepared. The present invention utilizes the synergistic effect of polydimethyldiallylammonium chloride and formaldehyde dicyandiamide resin to significantly enhance the electro-neutralization ability of macromolecular colloidal substances in wastewater and effectively remove bio-refractory macromolecular substances in water; when treating eucalyptus chemi-mechanical pulp wastewater, the dosage of the composite coagulant is small, and it has excellent COD reduction and decolorization effects.

Description

Technical Field

[0001] The present invention belongs to the technical field of eucalyptus chemi-mechanical pulp wastewater treatment. More specifically, it relates to a composite coagulant for eucalyptus chemi-mechanical pulp wastewater with gradient chelation-free radical activation synergy and a preparation method thereof. Background Technique

[0002] Eucalyptus chemi-mechanical pulp is one of the main pulp types that key pulp mills at home and abroad focus on developing. The production wastewater of this pulp type is characterized by high chromaticity, high chemical oxygen demand (COD), and high water temperature. At present, the mainstream process flow for treating eucalyptus chemi-mechanical pulp wastewater includes preliminary precipitation, hydrolysis, anaerobic treatment, aerobic treatment, Fenton oxidation, and final discharge. When treating eucalyptus chemi-mechanical pulp wastewater, COD becomes the main limiting pollution index.

[0003] Taking a project with an annual output of 200,000 tons of eucalyptus chemi-mechanical pulp as an example, generally after aerobic treatment, the effluent COD concentration fluctuates between 1550 mg / L and 1950 mg / L, with an average value of 1776 mg / L and an average COD removal rate of 42.7%. In order to reduce the COD concentration from about 1776 mg / L to below 50 mg / L of the discharge standard, in actual operation, usually secondary Fenton treatment is adopted. The first-stage treatment reduces COD to about 300 mg / L, and the second-stage treatment further reduces COD to 40 mg / L to 50 mg / L. The cost of Fenton treatment is about 8 yuan / m 3 to 11 yuan / m 3 between. However, the Fenton reaction process has extremely strict requirements for reaction conditions and must be carried out under acidic pH conditions. After the reaction is completed, a large amount of alkaline substances must be added to adjust the pH to neutral. In addition, the use of ferrous sulfate results in the generation of a large amount of chemical sludge during the treatment process, which not only makes the Fenton treatment process cumbersome and costly, but also may cause equipment corrosion and bring a large number of chemical sludge treatment problems.

[0004] Traditional water purification agent products (such as polyaluminum chloride, polyferric sulfate) have deficiencies such as large dosage, unsatisfactory COD removal effect, and poor decolorization effect when treating eucalyptus chemi-mechanical pulp wastewater, and it is difficult to meet the treatment requirements of chemi-mechanical pulp wastewater. The COD removal rate disclosed in the Chinese invention patent with the patent publication number CN109133303A is 65% - 70%, but its chromaticity removal rate for eucalyptus chemi-mechanical pulp wastewater is only 58% - 62%, and the sludge moisture content ≥ 85%. Although the chromaticity removal rate is increased to 75% in the Chinese invention patent with the patent publication number CN112062257A, it needs to be used in combination with the Fenton process, resulting in a higher cost (>8 yuan / m 3). The Chinese invention patent with the publication number CN110563184A points out that due to the difference in charge density between PDMDAAC and formaldehyde resin, phase separation is likely to occur. Summary of the Invention

[0005] In view of the above problems existing in the prior art, the technical problems to be solved by the present invention are as follows: to provide a preparation method of a composite coagulant for eucalyptus chemi-mechanical pulp wastewater with gradient chelation-radical activation synergy, and the preparation process is green and simple. Another technical problem to be solved by the present invention is to provide a composite coagulant prepared by the above preparation method, which shows excellent performance in treating eucalyptus chemi-mechanical pulp wastewater. There is also a technical problem to be solved by the present invention is to provide the application of the above composite coagulant in the treatment of eucalyptus chemi-mechanical pulp wastewater.

[0006] To solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0007] A preparation method of a composite coagulant for eucalyptus chemi-mechanical pulp wastewater with gradient chelation-radical activation synergy, using ferrous sulfate heptahydrate, aluminum hydroxide and magnesium sulfate as raw materials, through gradient chelation reaction, radical activation treatment, functional component composite reaction, regulating pH and specific gravity, and aging treatment, to obtain a water purification agent with a viscosity of 35-45 mPa·s; wherein, the molar ratio of ferrous sulfate heptahydrate, aluminum hydroxide and magnesium sulfate is (8-10):(0.5-1.5):(0.5-1).

[0008] Preferably, the specific process of the gradient chelation reaction is as follows: first add water, and then add sulfuric acid into the reaction kettle in three gradients, dissolve ferrous sulfate heptahydrate, aluminum hydroxide and magnesium sulfate in sequence, control the reaction temperature at 50-60 °C, and stir and react at a stirring speed of 700-900 revolutions per minute for 3 h.

[0009] Preferably, the ratio of the three additions of sulfuric acid is 6:3:1, and the interval between each addition is 10-15 min.

[0010] Preferably, the specific process of the radical activation treatment is as follows: after the gradient chelation reaction, add an appropriate amount of sodium persulfate, carry out an oxidation polymerization activation reaction at a temperature of 60-70 °C, control the dissolved oxygen concentration at 3-7 mg / L, until the molar ratio of Fe 3+ :Fe 2+ is (9-19):1, stop adding sodium persulfate and end the reaction.

[0011] Preferably, the addition amount of sodium persulfate is 0.8%-2.5% of the total system mass.

[0012] Preferably, the specific process of the composite reaction of the functional components is as follows: The solution after radical activation treatment is cooled down and maintained at 40-50°C, and poly(dimethyldiallylammonium chloride)-formaldehyde dicyandiamide resin copolymer is added, followed by low-speed stirring and mixing reaction to obtain a homogeneous composite solution.

[0013] The addition amount of the poly(dimethyldiallylammonium chloride)-formaldehyde dicyandiamide resin copolymer is 1%-3%.

[0014] Preferably, the preparation process of the poly(dimethyldiallylammonium chloride)-formaldehyde dicyandiamide resin copolymer is as follows: Poly(dimethyldiallylammonium chloride) and formaldehyde dicyandiamide resin are mixed in a mass ratio of 5-20:1, and reacted at 40-50°C for 2-4 hours with a stirring speed of 200-300 r / min. The poly(dimethyldiallylammonium chloride) is a solution with an effective content of 40% and a molecular weight of 800,000-1,200,000 daltons.

[0015] Preferably, the specific process of regulating pH and specific gravity is as follows: The pH of the system is adjusted to 2-3 with concentrated sulfuric acid, and the specific gravity is diluted to 1.3-1.4 with deionized water; the ripening process is as follows: The solution after regulating pH and specific gravity is subjected to gradient temperature-controlled ripening treatment. The first stage: Cool down to 20°C at a rate of 1°C / min and maintain for 3 h; the second stage: Heat up to 38°C at a rate of 0.3°C / min and maintain for 21 h.

[0016] The composite coagulant prepared by the preparation method of the gradient chelation-radical activation synergistic eucalyptus chemi-mechanical pulp wastewater composite coagulant.

[0017] Preferably, the composite coagulant comprises the following components in mass percentage: (Fe 3+ / Fe 2+ ): 8%-10%, Al 3+ : 0.5%-1.5%, Mg 2+ : 0.1%-0.5%, organic polymer: 1.5%-3.5%, and the balance is water.

[0018] The application of the composite coagulant in the treatment of eucalyptus chemi-mechanical pulp wastewater.

[0019] For the above application, according to the concentration of the eucalyptus chemi-mechanical pulp biochemical tail water, the dosage of the composite coagulant is controlled at 2-4 g / L, and aeration operation is carried out, with the aeration volume being 0.3-0.6 m 3 / h, and the aeration reaction is carried out for 15-30 min to effectively treat the eucalyptus chemi-mechanical pulp wastewater. After treatment, the moisture content of the sludge is ≤80%, and the reduction amount of the sludge volume is ≥30%

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1) The present invention adopts a gradient chelation-free radical activation composite process to achieve the molecular self-assembly of inorganic-organic components by controlling the reaction conditions in stages. The gradient chelation ensures the orderly complexation of metal ions and avoids competitive precipitation. Creatively, sodium persulfate is introduced as a free radical initiator to cooperate with metal ions to construct a Fenton-like system, generating hydroxyl radicals (·OH), which cooperate with metal ions to degrade macromolecular organic substances. Then, a two-phase flocculant synergistic enhancement technology (PDMDAAC and formaldehyde dicyandiamide resin form a charge complementary structure) is adopted to prepare a liquid composite coagulant;

[0022] 2) The present invention utilizes the charge complementary mechanism of poly(dimethyldiallylammonium chloride) (PDMDAAC) and formaldehyde dicyandiamide resin (PDMDAAC provides positive charges to neutralize the negative charges of colloids, and formaldehyde dicyandiamide resin strengthens the adsorption bridging effect), and it is found that PDMDAAC and formaldehyde resin can form a stable interpenetrating network structure within a specific mass ratio range (10-15:1), and maintain good stability in the system of the present invention, significantly enhancing the electro-neutralization ability for macromolecular colloidal substances in wastewater and effectively removing bio-refractory macromolecular substances in water;

[0023] 3) The composite coagulant of the present invention combines metal ions such as aluminum, iron, and magnesium with high molecular polymers to form a composite coagulant, significantly improving the removal efficiency of organic pollutants and suspended solids, and showing excellent performance in treating eucalyptus chemimechanical pulp wastewater; under the condition of the same dosage, its treatment effect significantly exceeds other similar products on the market, such as polyaluminum chloride (PAC) or polyferric sulfate (PFS);

[0024] 4) The product of the present invention synergistically acts with the Fenton process, can significantly enhance the efficiency of Fenton treatment, and greatly reduce the treatment cost; through the synergistic effect of gradient chelation, free radical activation and two-phase flocculation, a COD removal rate of ≥83.5% and a chroma removal rate of ≥88.7% are achieved in a single reagent system;

[0025] 5) The present invention mainly uses industrial by-products as raw materials, the production process is simple, and no additional environmental pollution is generated, having good economic benefits. Description of the Drawings

[0026] Figure 1 SEM image of the composite coagulant prepared in Example 3. Detailed Embodiments

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described below in conjunction with specific embodiments. Unless otherwise specified in the following embodiments, the technical means used are all conventional means well-known to those skilled in the art. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0028] In the following embodiments, the poly(dimethyldiallylammonium chloride) selected is a solution with an effective content of 40%, and the molecular weight is 800,000 - 1,200,000 daltons. The charge density ratio of poly(dimethyldiallylammonium chloride) to formaldehyde dicyandiamide resin is (15 - 18) meq / g : (8 - 12) meq / g.

[0029] Example 1

[0030] A preparation method of a composite coagulant for eucalyptus mechanical pulp wastewater with gradient chelation - free radical activation synergy, comprising the following steps:

[0031] (1) Gradient chelation reaction: Add 400 mL of deionized water to the reaction kettle, and then add 150 mL of 98% concentrated sulfuric acid in three portions (90 mL for the first time, 45 mL after an interval of 12 min, and 15 mL after another interval of 15 min). After each addition of acid, 200 g of ferrous sulfate heptahydrate, 80 g of aluminum hydroxide, and 15 g of magnesium sulfate are added in sequence. Control the temperature at 55 ± 1 °C through the jacket steam, and the stirring speed at 800 ± 50 rpm. React for 3 h, and continuously monitor the pH to be stable at 1.8 - 2.2 until a homogeneous solution is formed;

[0032] (2) Free radical activation treatment: Add 15 g of sodium persulfate to the homogeneous solution obtained in step (1), control the dissolved oxygen concentration at 5 mg / L through aeration, heat up to 70 °C for oxidation activation reaction for 1 h. The solution turns from light green to reddish brown until the molar ratio of Fe 2+ / Fe 3+ reaches 1:19;

[0033] (3) Mix 10 g of poly(dimethyldiallylammonium chloride) and 0.5 g of formaldehyde dicyandiamide resin, react at 45 °C for 2 h, and the stirring speed is 200 - 300 r / min to obtain a poly(dimethyldiallylammonium chloride) - formaldehyde dicyandiamide resin copolymer;

[0034] (4) Composite of functional components: Cool the solution obtained in step (2) and maintain the system temperature at 45 °C. Add 1.0% (w / w) of the poly(dimethyldiallylammonium chloride) - formaldehyde dicyandiamide resin copolymer prepared in step (3) to the solution, and stir and mix at a low speed of 200 - 300 rpm for 1 h to obtain a homogeneous composite solution;

[0035] (5) pH and specific gravity regulation: Additionally, concentrated sulfuric acid is used to adjust the pH of the homogeneous composite solution obtained in step (4) to 2.5, and the specific gravity is diluted to 1.35 with deionized water. Then, the solution with regulated pH and specific gravity is subjected to aging treatment. First stage: The temperature is decreased to 20 °C at a rate of 1 °C / min and maintained for 3 h. Second stage: The temperature is increased to 38 °C at a rate of 0.3 °C / min and maintained for 21 h to obtain a dark brown transparent liquid aluminum-iron-magnesium composite coagulant with a viscosity of 37.5 mPa·s.

[0036] Example 2

[0037] A preparation method of a gradient chelation-free radical activation synergistic eucalyptus mechanical pulp wastewater composite coagulant, comprising the following steps:

[0038] (1) Add 400 mL of deionized water to the reaction kettle, and then add 100 mL of 98% concentrated sulfuric acid to the reaction kettle in three gradients (60 mL for the first time, 30 mL after an interval of 12 min, and 10 mL after another interval of 15 min). After each addition of acid, 250 g of ferrous sulfate heptahydrate, 20 g of aluminum hydroxide, and 10 g of magnesium sulfate are added in sequence. Control the temperature at 60 °C and the stirring speed at 800 rpm, and react for 3 h. Monitor the pH in real time and keep it stable at 1.8 - 2.2 until a homogeneous solution is formed;

[0039] (2) Add 18 g of sodium persulfate to the homogeneous solution obtained in step (1), control the dissolved oxygen concentration at 5 mg / L by aeration, and raise the temperature to 70 °C for oxidation activation reaction for 1 h. The solution turns from light green to reddish brown until the molar ratio of Fe 2+ / Fe 3+ is 1:10, and the reaction ends;

[0040] (3) Mix 20 g of polydimethyldiallylammonium chloride and 2 g of formaldehyde dicyandiamide resin, and react at 45 °C for 2 h with a stirring speed of 200 - 300 r / min to obtain a polydimethyldiallylammonium chloride-formaldehyde dicyandiamide resin copolymer;

[0041] (4) Cool the solution obtained in step (2) by lowering the temperature and maintain the system temperature at 40 °C. Add 2.0% (w / w) of the polydimethyldiallylammonium chloride-formaldehyde dicyandiamide resin copolymer obtained in step (3) to the solution, and stir and mix at a low speed at 200 - 300 rpm for 2 h to obtain a homogeneous composite solution;

[0042] (5) Additionally, adjust the pH of the homogeneous composite solution obtained in step (4) to 2.1 with concentrated sulfuric acid, dilute the specific gravity to 1.38 with deionized water, and then carry out the ripening treatment on the solution after adjusting the pH and specific gravity. First stage: cool down to 20 °C at a rate of 1 °C / min and maintain for 3 h; second stage: heat up to 38 °C at a rate of 0.3 °C / min and maintain for 21 h to obtain a high-iron composite coagulant in dark red-brown liquid form, with a viscosity of 41.9 mPa·s.

[0043] Example 3

[0044] A preparation method of a composite coagulant for eucalyptus mechanical pulp wastewater with gradient chelation-free radical activation synergy, comprising the following steps:

[0045] (1) Add 400 mL of deionized water to the reaction kettle, and then add 130 mL of 98% concentrated sulfuric acid to the reaction kettle in three gradients (78 mL for the first time, add 39 mL after an interval of 12 min, and then add 13 mL after an interval of 15 min). After adding acid each time, add 160 g of ferrous sulfate heptahydrate, 10 g of aluminum hydroxide, and 20 g of magnesium sulfate in sequence. Control the temperature at 60 °C and the stirring speed at 800 rpm, and react for 3 h. Monitor the pH in real time and keep it stable at 1.8 - 2.2 until a homogeneous solution is formed;

[0046] (2) Add 12 g of sodium persulfate to the homogeneous solution obtained in step (1), control the dissolved oxygen concentration at 5 mg / L by aeration, heat up to 70 °C and carry out the oxidation activation reaction for 1 h. The solution turns from light green to red-brown until the Fe 2+ / Fe 3+ ratio is 1:10, and the reaction ends;

[0047] (3) Mix 20 g of polydimethyldiallylammonium chloride and 4 g of formaldehyde dicyandiamide resin, and react at 40 °C for 3 h with a stirring speed of 200 - 300 r / min to prepare a polydimethyldiallylammonium chloride-formaldehyde dicyandiamide resin copolymer;

[0048] (4) Cool down the solution obtained in step (2) and maintain the system temperature at 40 - 50 °C. Add 2.0% (w / w) of the polydimethyldiallylammonium chloride-formaldehyde dicyandiamide resin copolymer obtained in step (3) to the solution, and stir and mix at a low speed at 200 - 300 rpm for 1 - 2 h to obtain a homogeneous composite solution;

[0049] (5) Additionally, adjust the pH of the homogeneous composite solution obtained in step (4) to 2.5 with concentrated sulfuric acid, and dilute the specific gravity to 1.3 with deionized water. Then, subject the solution with adjusted pH and specific gravity to aging treatment. The first stage: cool down to 20 °C at a rate of 1 °C / min and maintain for 3 h; the second stage: heat up to 38 °C at a rate of 0.3 °C / min and maintain for 21 h to obtain a high decolorization functional composite coagulant in the form of a brownish-red viscous liquid with a viscosity of 43.6 mPa·s.

[0050] It can be seen from Figure 1 that the three-dimensional network structure formed by the PDMDAAC-resin copolymer enhances the flocculation efficiency through adsorption bridging.

[0051] Example 4

[0052] Compare the composite coagulants prepared in Examples 1-3 with representative coagulants on the market, such as polyaluminum chloride (PAC), aluminum sulfate, polyferric sulfate, etc., using the effluent from the biochemical treatment of eucalyptus CTMP wastewater in a certain factory for a coagulation comparison test. The dosage of the composite coagulant is 3000 mg / L, and aeration operation is carried out in combination. The aeration volume is 0.3 - 0.6 m 3 / h, and the aeration time is 15 - 30 min. The results are shown in Table 1.

[0053] Table 1 Coagulation effect of the effluent from the biochemical treatment of eucalyptus CTMP wastewater

[0054]

[0055] It can be seen from Table 1 that under the same dosage of the composite coagulant of the present invention, the removal rates of COD and chromaticity in the three examples are significantly improved, and the sludge moisture content decreases significantly, indicating that the composite coagulant prepared by the present invention can significantly improve the removal efficiency of COD and chromaticity in eucalyptus wastewater.

[0056] Comparative Example 1 (without radical activation treatment)

[0057] A preparation method of a composite coagulant for eucalyptus chemi-mechanical pulp wastewater, comprising the following steps:

[0058] (1) Add 400 mL of deionized water to the reaction kettle, and then add 100 mL of 98% concentrated sulfuric acid to the reaction kettle in three gradients (60 mL for the first time, add 30 mL after an interval of 12 min, and then add 10 mL after an interval of 15 min). After adding acid each time, add 250 g of ferrous sulfate, 20 g of aluminum hydroxide, and 10 g of magnesium sulfate in sequence. Control the temperature at 60 °C, the stirring speed at 800 rpm, and stir and react for 3 h. Monitor the pH in real time and keep it stable at 1.8 - 2.2 to form a homogeneous solution;

[0059] (2) Oxygen is introduced, and the system undergoes a catalytic oxidation reaction at a temperature of 90 - 100 °C for 3 h. The solution turns from light green to reddish brown until the molar ratio of Fe 2+ / Fe 3+ is 1:10. Then oxygen supply is stopped and the reaction ends;

[0060] (3) 20 g of polydimethyldiallylammonium chloride and 2 g of formaldehyde dicyandiamide resin are mixed and reacted at 45 °C for 2 h with a stirring speed of 200 - 300 r / min to obtain a polydimethyldiallylammonium chloride - formaldehyde dicyandiamide resin copolymer;

[0061] (4) By cooling down and maintaining the system temperature at 40 - 50 °C, 2.0% of the polydimethyldiallylammonium chloride - formaldehyde dicyandiamide resin copolymer obtained in step (3) is added to the solution, and homogeneous compounding is achieved by stirring and mixing at a low speed of 200 - 300 rpm for 1 - 2 h;

[0062] (5) The pH of the homogeneous compounded solution obtained in step (4) is adjusted to 2.1 with concentrated sulfuric acid, and the specific gravity is diluted to 1.38 with deionized water. Then the solution after adjusting pH and specific gravity is subjected to a curing treatment. The first stage: cooling down to 20 °C at a rate of 1 °C / min and maintaining for 3 h; the second stage: heating up to 38 °C at a rate of 0.3 °C / min and maintaining for 21 h to obtain a dark brown liquid water purifying agent with a viscosity of 41.1 mPa·s.

[0063] Example 5

[0064] The composite coagulant prepared in Comparative Example 1 and the composite coagulant obtained in Example 2 are used for a coagulation comparison test with the effluent from the biochemical treatment of eucalyptus CTMP wastewater in this factory (appearance is reddish brown, pH value is 8.31, COD Cr is 1586 mg / L, and the chromaticity is 1250 times). The results are shown in Table 2.

[0065] Table 2 Comparison data of the treatment effects of Comparative Example 1 and Example 2

[0066]

[0067] As can be seen from Table 2, under the same dosage of the composite coagulant prepared in Example 2 of the present invention and the composite coagulant prepared in Comparative Example 1, the removal effects on COD and chromaticity are better, and the effluent COD and chromaticity are lower, indicating that the lack of hydroxyl radicals leads to an obvious decrease to varying degrees in the removal of macromolecular organic substances. The free radical activation process contributes 12.1% of the COD removal rate and 32.5% of the chromaticity removal rate, and it is the core for treating refractory substances. Tracking by in - situ infrared spectroscopy shows that in Example 2, the C - O - C bond (1260 cm -1The strength decreased by 78%, while that of Comparative Example 1 only decreased by 23% at the same time, indicating that the absence of free radicals significantly delays the decomposition process of macromolecules.

[0068] Example 6

[0069] The process of the free radical concentration-treatment effect correlation experiment was as follows: 400 mL of deionized water was added to the reaction kettle, and then 100 mL of 98% concentrated sulfuric acid was added to the reaction kettle in three gradients (60 mL for the first time, 30 mL was added after an interval of 12 min, and 10 mL was added after another interval of 15 min). After each addition of acid, 250 g of ferrous sulfate, 20 g of aluminum hydroxide, and 10 g of magnesium sulfate were added in sequence. The temperature was controlled at 60 °C, the stirring speed was 800 rpm, and the stirring reaction was carried out for 3 h. The pH was monitored in real time and stabilized at 1.8 - 2.2 to form a homogeneous solution; Sodium persulfate accounting for 0 - 2% of the total system mass was added to the obtained homogeneous solution, and the dissolved oxygen concentration was controlled at 5 mg / L by introducing air, and the reaction was carried out at 70 °C for 1 h. Samples were taken to detect the Fe 2+ / Fe 3+ molar ratio (o-phenanthroline spectrophotometry), and the fluorescence probe method was used to detect the concentration of hydroxyl radicals. The results are shown in Table 3. Among them, when the addition amount of sodium persulfate was 0, it was Comparative Example 1 (control group), and when the addition amount of sodium persulfate was 1.5%, it was Example 2.

[0070] Table 3 Treatment effects at different free radical concentrations

[0071]

[0072] As can be seen from Table 3, when the addition amount of sodium persulfate ≥ 1.5%, the concentration of ·OH breaks through 0.28 μmol / L, and the COD removal rate increases sharply (from 79.6% to 86.9%), proving that 1.2% - 1.8% is the technical critical range. It is proved that the free radical activation process can effectively break through the limitation of traditional flocculants that only rely on electro-neutralization / adsorption, realize the oxidation-flocculation synergistic treatment mechanism, and solve the technical bottleneck of the low removal rate of colored and refractory substances in existing traditional production technologies. [[ID=**21**]]

[0073] Comparative Example 2 (without using the polymer copolymer)

[0074] A preparation method of a composite coagulant for eucalyptus chemi-mechanical pulp wastewater, comprising the following steps:

[0075] (1) Add 400 mL of deionized water to the reaction kettle, and then add 130 mL of 98% concentrated sulfuric acid to the reaction kettle in three gradients (78 mL for the first time, 39 mL after an interval of 12 min, and 13 mL after another interval of 15 min). After adding the acid each time, add 160 g of ferrous sulfate heptahydrate, 10 g of aluminum hydroxide, and 20 g of magnesium sulfate in sequence. Control the temperature at 60 °C, the stirring speed at 800 rpm, and stir and react for 3 h. Monitor the pH in real-time and keep it stable at 1.8 - 2.2 to form a homogeneous solution;

[0076] (2) Add 12 g of sodium persulfate to the homogeneous solution obtained in step (1), control the dissolved oxygen concentration at 5 mg / L by aeration, heat up to 70 °C and carry out the oxidation activation reaction for 1 h. The solution changes from light green to reddish-brown until the molar ratio of Fe 2+ / Fe 3+ is 1:10, and then end the reaction;

[0077] (3) Add 24 g of deionized water to the solution obtained in step (2), adjust the pH of the solution to 2.5 with concentrated sulfuric acid additionally, dilute the specific gravity to 1.3 with deionized water, and then carry out the aging treatment on the solution after adjusting the pH and specific gravity. The first stage: cool down to 20 °C at a rate of 1 °C / min and maintain for 3 h; the second stage: heat up to 38 °C at a rate of 0.3 °C / min and maintain for 21 h to obtain a composite coagulant with a viscosity of 30.4 mPa·s.

[0078] Example 7

[0079] Compare the composite coagulant prepared in Comparative Example 2 with the composite coagulant obtained in Example 3 by using the effluent from the biochemical treatment of eucalyptus CTMP wastewater in this factory (appearance is reddish-brown, pH value is 8.31, COD Cr is 1586 mg / L, and the chromaticity is 1250 times) for a coagulation comparison test. The results are shown in Table 4.

[0080] Table 4 Comparison data of the treatment effects of Comparative Example 2 and Example 3

[0081]

[0082] As can be seen from Table 4, the COD removal rate decreases by 10.2%, the chromaticity removal rate decreases by 24.9%, and the electro-neutralization ability decreases by 49.7%. The removal rate of macromolecular substances with chromaticity decreases, verifying the specific capture of substances containing chromogenic groups such as lignin by the adsorption bridging effect of the polymer copolymer.

[0083] Comparative Example 3 (only without gradient chelation)

[0084] A preparation method of a composite coagulant for eucalyptus chemi-mechanical pulp wastewater, comprising the following steps:

[0085] (1) Add 400 mL of deionized water to the reaction kettle, and add 150 mL of 98% concentrated sulfuric acid to the reaction kettle all at once. At the same time, add 200 g of ferrous sulfate heptahydrate, 80 g of aluminum hydroxide, and 15 g of magnesium sulfate. Control the temperature at 55 °C and the stirring speed at 800 rpm, and stir and react for 3 h to form a homogeneous solution;

[0086] (2) Add 15 g of sodium persulfate to the homogeneous solution obtained in step (1). Control the dissolved oxygen concentration at 5 mg / L by aeration, heat up to 70 °C, and carry out an oxidation activation reaction for 1 h. The solution changes from light green to reddish brown until the Fe 2+ / Fe 3+ molar ratio reaches 1:19;

[0087] (3) Mix 10 g of polydimethyldiallylammonium chloride and 0.5 g of formaldehyde dicyandiamide resin, and react at 45 °C for 2 h with a stirring speed of 200 - 300 r / min to obtain a polydimethyldiallylammonium chloride-formaldehyde dicyandiamide resin copolymer;

[0088] (4) Cool down the solution obtained in step (2) and maintain the system temperature at 45 °C. Add 1.0% of the polydimethyldiallylammonium chloride-formaldehyde dicyandiamide resin copolymer obtained in step (3) to the solution, and stir and mix at a low speed of 200 - 300 rpm for 1 h to obtain a homogeneous composite solution;

[0089] (5) Adjust the pH of the homogeneous composite solution obtained in step (4) to 2.5 with concentrated sulfuric acid, dilute the specific gravity to 1.35 with deionized water, and then carry out a ripening treatment on the solution after adjusting the pH and specific gravity. The first stage: cool down to 20 °C at a rate of 1 °C / min and maintain for 3 h; the second stage: heat up to 38 °C at a rate of 0.3 °C / min and maintain for 21 h to obtain an aluminum-iron-magnesium composite water purifying agent in the form of a dark brown liquid with a viscosity of 37.3 mPa·s.

[0090] Example 8

[0091] Compare the composite coagulant prepared in Comparative Example 3 with the composite coagulant obtained in Example 1 by carrying out a coagulation comparison test on the effluent from the biochemical treatment of eucalyptus CTMP wastewater in this factory (appearance is reddish brown, pH value is 8.31, COD Cr is 1586 mg / L, and the chromaticity is 1250 times). The results are shown in Table 5.

[0092] Table 5 Comparison data of treatment effects of Comparative Example 3 and Example 1

[0093]

[0094] As can be seen from Table 5, the gradient chelation process increases the absolute value of the COD removal rate by 6.2% and the relative value by 7.9%. The non-gradient process results in the dissolution of metal ions exceeding about twice, posing a risk of secondary pollution. The staged addition of acid control extends the shelf life of the reagent from 6 months to 12 months, doubling the stability. Gradient ion chelation can minimize the competitive precipitation of metal ions, achieve pH buffer control to maintain the optimal reaction environment, and realize ordered assembly to construct a stable composite structure.

[0095] Comparing Example 1 and Comparative Example 3, through on-line pH monitoring, it was found that: during gradient chelation, the pH of the system was stable at 1.8 - 2.2, while during non-gradient chelation, the pH dropped suddenly (<1.5), resulting in the premature dissolution of aluminum hydroxide to form Al 3+ instead of Al(OH)3 colloid. Chelation degree analysis by EDTA titration method: the metal chelation degree of Example 1 was 98.2%, and that of Comparative Example 3 was 72.5%. Gradient chelation increased the complexation efficiency of metal ions by 35.5%. The particle size distribution results detected by a laser particle size analyzer are shown in Table 6.

[0096] Table 6 Particle size distribution results of Comparative Example 3 and Example 1

[0097]

[0098] As can be seen from Table 6, the non-gradient process easily leads to incomplete dissolution and reaction of raw materials, reducing the effective flocculation particles (1 - 100μm) by 25.4%.

[0099] Example 9 (Comparative experiment on different sulfuric acid ratios in gradient chelation)

[0100] Adopt the preparation steps of Example 1, fix the total amount of sulfuric acid at 150 mL, and compare the effects of different sulfuric acid distribution ratios (6:3:1, 4:4:2, 7:2:1) on the chelation degree, COD removal rate, and metal dissolution amount. The results are shown in Table 7.

[0101] Table 7 Influence results of different sulfuric acid distribution ratios on performance

[0102]

[0103] As can be seen from Table 7, adopting the ratio of 6:3:1 can significantly improve the metal chelation degree (an increase of 21.9% compared with the ratio of 4:4:2), and perform better in terms of the removal rate of chemical oxygen demand (COD). In addition, this ratio can effectively reduce the risk of secondary pollution, specifically manifested as a 65.7% reduction in the dissolution amount of Fe³+.

[0104] In summary, from the results of the above Examples 1-3 and Comparative Examples 1-3, it can be seen that: 1) The necessity of gradient chelation: The metal chelation degree of Comparative Example 3 (non-gradient acid addition) is only 72.5%, resulting in the Fe 3+ dissolution amount exceeding 2.1 times that of the present invention (2.5 mg / L vs 1.2 mg / L); 2) The critical value of free radical activation: When the addition amount of sodium persulfate < 1.2% (Comparative Example 1), hydroxyl free radicals were not detected, and the COD removal rate decreased by 9.4%; 3) The copolymer synergistic effect: The color removal rate of Comparative Example 2 (without adding copolymer) decreased by 23.3%, proving that the charge complementarity of PDMDAAC-resin is the core mechanism for decolorization.

[0105] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a composite coagulant for eucalyptus mechanical pulp wastewater with gradient chelation-free radical activation synergy, characterized in that, Using ferrous sulfate heptahydrate, aluminum hydroxide, and magnesium sulfate as raw materials, a water purifying agent with a viscosity of 35 - 45 mPa·s is prepared through gradient chelation reaction, free radical activation treatment, functional component compound reaction, pH and specific gravity regulation, and aging treatment; wherein, the molar ratio of the ferrous sulfate heptahydrate, aluminum hydroxide, and magnesium sulfate is (8 - 10):(0.5 - 1.5):(0.5 - 1). The specific process of the gradient chelation reaction is as follows: First, add water, and then add sulfuric acid into the reaction kettle in three gradients. Sequentially dissolve ferrous sulfate heptahydrate, aluminum hydroxide, and magnesium sulfate, control the reaction temperature at 50 - 60 °C, and carry out stirring reaction for 3 h at a stirring speed of 700 - 900 revolutions per minute. The specific process of the free radical activation treatment is as follows: after the gradient chelation reaction, an appropriate amount of sodium persulfate is added, and an oxidation polymerization activation reaction is carried out at a temperature of 60-70 °C, and the dissolved oxygen concentration is controlled at 3-7 mg / L until the molar ratio of Fe 3+ :Fe 2+ is (9-19):1, stop adding sodium persulfate and end the reaction; The specific process of the functional component compound reaction is as follows: Cool the solution after free radical activation treatment and maintain it at 40 - 50 °C, add a copolymer of polydimethyldiallylammonium chloride - formaldehyde dicyandiamide resin, and carry out low - speed stirring and mixing reaction to obtain a homogeneous composite solution. The specific process of the pH and specific gravity regulation is as follows: Adjust the pH of the system to 2 - 3 with concentrated sulfuric acid, and dilute the specific gravity to 1.3 - 1.4 with deionized water; the aging treatment process is as follows: Carry out gradient temperature - controlled aging treatment on the solution after pH and specific gravity regulation. The first stage: Cool down to 20 °C at a rate of 1 °C / min and maintain for 3 h; the second stage: Heat up to 38 °C at a rate of 0.3 °C / min and maintain for 21 h.

2. The preparation method of the composite coagulant for eucalyptus mechanical pulp wastewater with gradient chelation-free radical activation synergy according to claim 1, characterized in that The addition ratio of the three - time sulfuric acid is 6:3:1, and the interval between each addition is 10 - 15 min.

3. A composite coagulant prepared by the preparation method of the gradient chelation - free radical activation synergistic eucalyptus chemical pulp wastewater composite coagulant according to any one of claims 1 - 2.

4. The composite coagulant according to claim 3, characterized in that, The composite coagulant comprises the following components in mass percentage: (Fe 3+ / Fe 2+ ): 8% - 10%, Al 3+ : 0.5% - 1.5%, Mg 2+ : 0.1% - 0.5%, organic polymer: 1.5% - 3.5%, and the balance is water.

5. Application of the composite coagulant according to claim 3 or 4 in the treatment of eucalyptus chemical pulp wastewater.

6. The application according to claim 5, wherein According to the concentration of the biochemical tail water of eucalyptus chemi-mechanical pulp, the dosage of the composite coagulant is controlled at 2 - 4 g / L, and aeration operation is carried out. The aeration volume is 0.3 - 0.6 m 3 / h, and the aeration reaction is carried out for 15 - 30 min to effectively treat the eucalyptus chemi-mechanical pulp wastewater. After treatment, the moisture content of the sludge ≤ 80%, and the reduction of the sludge volume ≥ 30%.

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