Method for preparing magnetic flocculant from iron-containing sludge and application of magnetic flocculant in enhanced treatment of low-turbidity high-algae-content water

Abstract

The invention relates to the technical field of water treatment, and discloses a preparation method for synthesizing a magnetic flocculant by taking iron-containing sludge of a water purification plant as a raw material and an application of the magnetic flocculant in enhanced pollution removal in low-turbidity and high-algae-content water. The method comprises the following steps: carrying out pretreatment such as drying, grinding, screening and the like on the iron-containing sludge of the water purification plant, taking ethylene glycol as a reducing solvent, adding a proper amount of alkaline buffer substance, and carrying out carbon thermal reduction reaction in a polytetrafluoroethylene-lined stainless steel reaction kettle through a solvothermal method to obtain a magnetic flocculant (Fe3O4 at C) of an iron-carbon compound; a coagulant polyaluminum chloride (PACl) is combined to enhance the pollution removal efficiency of the low-turbidity high-algae-content water, effective magnetic separation is realized through the action of an external magnetic field, the algae removal efficiency is enhanced, the flocculation efficiency is improved, and the precipitation time is saved. The method has the advantages that the main components and structural characteristics of the iron-containing sludge of the water plant are fully utilized to realize resource utilization of wastes, the preparation process is simple and convenient, and the cost is low.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and particularly to a method for preparing a magnetic flocculant from iron-containing sludge and its application in enhanced treatment of low-turbidity and high-algae water. Background Art

[0002] Due to the increasingly serious eutrophication problem, a large number of algae reproduce to form water blooms. Eutrophic water bodies often exhibit the characteristics of low turbidity and high algae. The algae in the water bloom can produce toxic metabolites, which pose a hazard to the stability of the water ecosystem and human life and health. Therefore, effectively controlling water eutrophication and strengthening the treatment of low-turbidity and high-algae water pollution are important tasks for improving water quality and protecting water resources.

[0003] Traditional water treatment technologies face many challenges in removing pollutants from low-turbidity and high-algae water. In the traditional coagulation-sedimentation process for treating low-turbidity and high-algae water bodies, since the surface of algae is negatively charged, it is easy to form a stable colloidal system, and the flocs generated during the coagulation process are loose, have low density and poor sedimentation performance. In addition, conventional coagulants such as polyaluminum chloride (PACl), although they can effectively remove suspended solids and organic matter, when treating low-turbidity and high-algae water, there will be problems such as large consumption of coagulant, unsatisfactory coagulation effect, low density of the flocs, and low sedimentation efficiency.

[0004] Magnetic flocculation technology refers to adding a certain amount of magnetic materials while using conventional coagulants such as iron salts and aluminum salts, which can significantly shorten the flocculation-sedimentation time and improve the flocculation efficiency, and recover the magnetic flocculant and recycle it under the condition of an external magnetic field. In the preparation and research of magnetic flocculants, most synthesis methods use iron salts of Fe 3+ and Fe 2+ as raw materials and adopt methods such as coprecipitation, thermal decomposition, hydrothermal, and polyol to prepare nano-magnetic materials. The preparation process is relatively complex and the raw material cost is relatively high.

[0005] As a by-product generated during the water treatment process using iron salt coagulants, the iron sludge in the water treatment plant is rich in a large amount of elements such as iron, oxygen, and carbon, and has great potential for synthesizing iron-based magnetic materials. Therefore, the present invention uses iron-containing sludge as a raw material to research and develop a simple and feasible preparation method to synthesize a magnetic flocculant, which can not only achieve enhanced coagulation for algae removal and rapid precipitation separation, but also realize the resource recycling of iron sludge. Summary of the Invention

[0006] The purpose of the present invention is to provide a method and application for preparing a magnetic flocculant using the iron-containing sludge in the water treatment plant as a raw material. The preparation method is simple, feasible and has a low cost; by combining with traditional flocculants and using magnetic flocculation technology, the enhanced pollution removal efficiency of low-turbidity and high-algae water is realized, and rapid precipitation and separation are achieved through an external magnetic field, while realizing the resource utilization of waste.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] One of the purposes of the present invention is to provide a method for preparing a magnetic flocculant from iron-containing sludge, comprising the following steps:

[0009] (1) The iron-containing sludge cake of the water treatment plant is pretreated to obtain the iron-containing sludge powder, which is placed in a drying dish and sealed for storage;

[0010] (2) adding the iron-containing sludge powder to ethylene glycol, and then adding an appropriate amount of alkaline buffer substance, and fully mixing and stirring under ultrasonic conditions to obtain a brown suspension of iron sludge mixed liquid;

[0011] (3) The iron mud mixture is transferred to a reactor for high temperature and high pressure solvent thermal reaction, and a magnetic flocculant is obtained after the reaction is completed.

[0012] The present invention pre-treats the iron-containing sludge in a water treatment plant by drying, grinding, screening, etc., uses ethylene glycol as a reducing solvent, and adds an appropriate amount of alkaline buffer substance to maintain an alkaline environment, and then performs a carbothermal reduction reaction in a polytetrafluoroethylene-lined stainless steel reactor by a solvent thermal method to obtain a magnetic flocculant (Fe2O3) of an iron-carbon composite. 3 O 4 @C).

[0013] Preferably, the iron-containing sludge cake from the water treatment plant in the aforementioned step (1) is derived from the sludge water produced by the coagulation-sedimentation process using iron salt as a coagulant in an actual water treatment plant, and then the iron-containing cake is formed by the dehydration-drying-filter pressing process, and the iron content of the cake is 20-30%.

[0014] Preferably, the pretreatment method described in the aforementioned step (1) includes air drying, grinding, drying, and screening processes, specifically, the iron-containing sludge is naturally air-dried until the moisture content is not higher than 10%, crushed into granules, and then ground into powder using a ceramic grinding body, placed in a forced air drying oven, and dried at 100-110°C for 24 hours; after drying, it is sieved using a 200-mesh copper mesh to obtain a dry iron-containing sludge powder sample.

[0015] Preferably, the feeding amount of the iron-containing sludge powder in the aforementioned step (2) is calculated as iron / ethylene glycol, and its concentration range is 6.7-8.0 g / L, and more preferably 2.0 g of iron-containing sludge powder / 75 mL of ethylene glycol.

[0016] Preferably, the alkaline buffer substance in the aforementioned step (2) is at least one of carbonate, phosphate, and borate, more preferably sodium acetate; the amount of the alkaline buffer substance added is calculated to maintain the pH of the system at 8 to 9, more preferably, the iron-containing sludge powder is calculated as iron / sodium acetate, and its mass ratio range is 1:12.5 to 15, and more preferably the mass ratio of the iron-containing sludge powder to sodium acetate is 2:7.5.

[0017] Preferably, in the foregoing step (2), the ultrasonic frequency is 80 - 100 kHz, the stirring speed is 200 - 300 rpm, and the time is 1.5 - 2 h.

[0018] More preferably, in the foregoing step (2), the specific contents of the ethylene glycol solution and the iron sludge powder are as follows: Weigh 75 mL of ethylene glycol solution into a 200 mL polytetrafluoroethylene reactor. Weigh 2.0 g of the iron sludge obtained in step (1) with an electronic balance, add it to the ethylene glycol and stir evenly. Weigh 7.5 g of sodium acetate with an electronic balance and add it to the mixed solution of ethylene glycol and iron sludge. Under ultrasonic conditions, mechanically stir and mix for 1.5 - 2 h to obtain a yellowish-brown suspension, and the rotation speed is 300 rpm.

[0019] Preferably, in the foregoing step (3), the reaction conditions for preparing the iron sludge-based magnetic coagulant by solvothermal reaction are as follows: The calcination temperature in the muffle furnace is 180 - 220 °C, the heating rate is 4 - 10 °C / min, and calcination is carried out at this temperature for 8 - 12 h. More preferably, it is 200 - 220 °C and calcination is carried out for 10 - 12 h.

[0020] Preferably, in the foregoing step (3), after the reaction is completed, the reaction kettle is cooled to room temperature, and the black precipitate at the bottom of the inner liner is collected by magnetic separation; after washing treatment, it can be obtained as Fe 3 O 4 @C through vacuum drying and placed in a drying dish for storage and standby.

[0021] More preferably, the washing treatment is to wash repeatedly with deionized water and absolute ethanol for more than 3 times. More preferably, the washing method is to rinse repeatedly with deionized water for more than 3 times first, and then wash repeatedly with absolute ethanol for more than 3 times. During the process, magnetic recovery and separation are used.

[0022] More preferably, the vacuum drying is carried out at a drying temperature of 60 °C under vacuum conditions, and the drying time is not less than 12 h.

[0023] Another object of the present invention is to provide an application of the magnetic flocculant prepared by any of the above preparation methods in removing pollutants in the enhanced treatment of low-turbidity and high-algae water.

[0024] Preferably, in the above application in the implementation manner, the turbidity of the raw water is configured to be 5 - 10 NTU, according to the algal cell concentration during the actual algal bloom phenomenon in the water body, the algal cell concentration is configured to be 10 5 - 10 6 cells / mL, according to the actual pH range in natural water bodies, the pH of the algal solution is adjusted to 7.0 - 8.0, and the dosage of Fe 3 O 4 @C is 0.8 g / L, and the dosage of the coagulant PACl is 10 - 12 mg / L, more preferably 10 mg / L.

[0025] More preferably, the enhanced coagulation reaction parameters are as follows: Take 1 L of the prepared low-turbidity algae-containing water in a beaker under a six-blade agitator, and sequentially add Fe 3 O 4 @C and PACl. During enhanced coagulation, the agitator is rapidly stirred (200 - 250 rpm) for 1 min, then slowly stirred (30 - 50 rpm) for 15 min, followed by static sedimentation or sedimentation with an external magnetic field (magnetic field strength of 50 - 80 mT) for 30 min. After sedimentation, the supernatant is taken 2 cm below the liquid surface to measure the turbidity and algal cell concentration.

[0026] Compared with the prior art, the advantages of the present invention are as follows:

[0027] 1. The iron-containing sludge utilized in the present invention is a by-product generated from a water purification plant using iron salts as coagulants. By fully utilizing components such as iron and carbon, through a simple and feasible solvothermal reaction, it can effectively realize the reduction production of magnetic Fe 3 from the iron salt hydrolysis product Fe(OH) in the iron sludge. 3 O 4 @C. The one-step process is simple and has a low preparation cost. The raw material iron sludge has a wide source and a large output. This method can not only turn waste into treasure and realize the resource utilization of waste, but also reduce the treatment and disposal cost of sludge waste. It has good economic and environmental benefits and is suitable for large-scale industrial production applications.

[0028] 2. It effectively improves the flocculation sedimentation efficiency and solid waste separation efficiency. The flocs have a high density and good sedimentation performance. The magnetic nuclei can be recycled through an external magnetic field, showing a broad market application prospect.

[0029] 3. It can solve problems such as poor floc density, floc floating, and high coagulant dosage in the treatment of low-turbidity and high-algae water. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 FIG. is the X-ray diffraction (XRD) pattern of the iron sludge-based magnetic flocculant (Fe 3 O 4 @C) prepared with different reaction solvents (ethylene glycol, isopropanol, and ethanol) in the present invention.

[0031] Figure 2 FIG. shows the performance of the iron sludge-based magnetic flocculant (Fe 3 O 4 @C) prepared with different reaction solvents (ethylene glycol, isopropanol, and ethanol) in the present invention in enhancing the removal of pollutants from low-turbidity algae-containing water.

[0032] Figure 3For the iron sludge-based magnetic flocculant (Fe 3 O 4 @C) prepared at different solvothermal reaction temperatures (180 °C, 200 °C, and 220 °C) in the present invention, the hysteresis loop is shown as follows.

[0033] Figure 4 For the evaluation of the enhanced coagulation algae removal efficiency and the residual turbidity with different solvothermal reaction times (8 h, 10 h, and 12 h) in the present invention.

[0034] Figure 5 For the transmission electron microscope (TEM) image of the iron sludge-based magnetic flocculant (Fe 3 O 4 @C) prepared under the conditions that ethylene glycol is used as the solvent, the reaction temperature is 200 °C, and the reaction time is 10 h in the present invention.

[0035] Figure 6 For the comparative evaluation of the algae removal rate and the residual turbidity under different PACl dosages in two systems of Fe 3 O 4 @C / PACl and PACl used alone in the present invention.

[0036] Figure 7 For the monitored values of the fractal dimension (D 3 O 4 @C) of the flocs during the floc formation process in the coagulation stage in two systems of Fe f @C / PACl and PACl used alone in the present invention.

[0037] Figure 8 For the monitored values of the residual turbidity at different times during the sedimentation stage in two systems of Fe 3 O 4 @C / PACl and PACl used alone in the present invention. Detailed Embodiments

[0038] In order to better clarify and understand the purpose, process scheme, and advantages of the present invention, the technical scheme and implementation mode of the present invention will be further clearly, completely, and detailedly described below through specific embodiments and in combination with the accompanying drawings. It should be noted that the embodiments described in the present invention are implemented on the premise of the technical scheme of the present invention, and the detailed implementation mode and specific operation process are given, but they are only a part of the embodiments of the present invention, rather than all the embodiments. The specific implementation mode described is only for explaining and interpreting the present invention and does not limit the present invention. Based on the embodiments in the present invention, all other implementation modes obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0039] In the embodiments of the present invention, unless otherwise specified, the experimental methods and conditions used are conventional methods and conventional conditions. The materials, reagents, instruments, etc. used in the embodiments can be obtained from commercial sources or prepared by conventional methods unless otherwise specified. The reaction conditions reflected in the content of the present invention can all achieve the reactions and obtain the products with expected effects. Due to space limitations, some embodiments are listed below to further illustrate the advantages of the technical solutions of the present invention.

[0040] The principle of the present invention is as follows:

[0041] Iron salts are a type of coagulant commonly used in the water treatment process of water purification plants. During the flocculation stage, polymeric hydroxyl iron [Fe(OH) 3 or FeOOH] is generated through hydrolysis, and it co-precipitates with colloidal particles and natural organic matter in the raw water to form iron-containing sludge. Under high temperature and high pressure conditions, through solvothermal reduction reaction, Fe(OH) 3 or FeOOH can achieve lattice reorganization to produce magnetic iron oxides (Fe 3 O 4 ).

[0042] The possible chemical reaction equations may be:

[0043] 6Fe(OH) 3 +2OHCH 2 -CH 2 OH→H 3 C-CO-CO-CH 3 +2Fe 3 O 4 +12H 2 O

[0044] The experimental steps of the present invention are as follows:

[0045] Step 1: Analysis of the source of iron sludge and sample pretreatment

[0046] The iron-containing sludge in the water purification plant is mainly generated from the sedimentation tank effluent sludge produced during the coagulation-sedimentation process using iron salts [such as FeCl 3 , FeSO 4 and polyferric chloride (PFCl), etc.] as coagulants. By adding polyacrylamide (PAM) to make it re-flocculate, and then further dehydrating, concentrating and filtering through a centrifuge and a filter press to form a mud cake with a water content of about 70%-75% and an iron content of about 25-30%.

[0047] Pretreatment process of iron-containing sludge: First, air-dry naturally to make the moisture content not higher than 10%; then crush it into granular form, put it into a forced-air drying oven, and dry it at 100 - 110 °C for 24 h; after drying, sieve it with a 200-mesh copper mesh to obtain a dried iron-containing sludge powder sample.

[0048] Step 2: Determination and analysis of the composition of the original iron sludge

[0049] For the determination of iron element, the pickling method is adopted, and the specific steps are as follows: Weigh 1 g of the iron sludge obtained in Step 1 with an electronic balance, disperse it into a capped bottle containing 100 mL of ultrapure water, then add HCl solution with a stock solution concentration of 0.5 mol / L, adjust the pH value of the iron sludge mixture to be lower than 3, and stir magnetically for more than 24 h to fully dissolve Fe ions from the iron sludge. After filtering out the sludge impurities by vacuum filtration, take out 5 mL of the liquid after diluting it by an appropriate multiple, and determine the Fe ion concentration by atomic absorption spectrometry; finally, convert the Fe element content in the iron sludge to be: 150 ± 5.0 mg / g.

[0050] Step 3: Solvothermal reaction of iron sludge

[0051] For the specific steps and conditions, see each example and comparative example.

[0052] Step 4: Recovery and drying of magnetic flocculant

[0053] After the reaction in each example and comparative example is completed, cool the reaction kettle to room temperature, take out the inner liner, collect the black precipitate at the bottom of the inner liner with a magnet, and wash it repeatedly 4 times with absolute ethanol and deionized water; place it in a vacuum drying oven for drying, set the temperature to 60 °C, and the time to 12 h, and finally obtain Fe 3 O 4 @C; After grinding the dry powder, place it in a centrifuge tube, seal it, and store it in a drying dish for standby.

[0054] Step 5: Application of magnetic flocculant

[0055] Taking the laboratory-prepared water body as an example, in the above application embodiment, prepare low-turbidity algae-containing water with a volume of 1 L. Use kaolin to prepare raw water with a turbidity of 5 ± 0.5 NTU, and use Microcystis aeruginosa to prepare algae-containing water with an algae cell concentration of about 3×10 5 cells / mL (OD 680 = 0.035 cm -1 ), and adjust the pH to 7.0 - 8.0 with hydrochloric acid and sodium hydroxide. Add the iron sludge-based magnetic flocculant (Fe 3 O 4@C) 0.8 g / L, the dosage of the coagulant PAC is 10 mg / L. The parameters of the flocculation reaction conditions are as follows: rapid stirring with a mechanical stirrer for 1 min at a rotation speed of 150 rpm, slow stirring for 15 min at a rotation speed of 30 rpm. After the stirring stops, the flocs are completely precipitated within 5 min by applying an external magnetic field (the magnetic field strength is selected as 60 mT).

[0056] Among them, the monitoring and evaluation method of the algal cell concentration is as follows: the algal cell concentration is counted by the hemocytometer method to obtain the algal cell density. Then, a functional relationship is established with the absorbance value (OD 680 ) of the algal solution at a wavelength of 680 nm, and a standard curve between the algal quantity and the absorbance is drawn. During sample measurement, the OD 680 of the algal solution is detected, and then the algal cell concentration can be calculated through the standard curve.

[0057] The specific steps of the hemocytometer method: Shake the algal solution well, dilute it by a certain multiple, use a pipette to take the algal solution and drop it on the edge of the glass slide, and use the siphon principle to make the algal solution evenly distributed in the counting chamber, ensuring that there are no air bubbles remaining in the counting chamber. Count the algal cells under an electron microscope, and follow the principle of counting the upper ones but not the lower ones, and counting the left ones but not the right ones during counting.

[0058] OD 680 The specific steps of the detection: Centrifuge the algal solution to be measured at a rotation speed of 6000 rpm for 20 min, take the supernatant and place it in a quartz cuvette, place it in an ultraviolet spectrophotometer, adjust the detection wavelength to 680 nm, and read the value.

[0059] Example 1

[0060] Weigh 75 mL of ethylene glycol solution and place it in a 200 mL polytetrafluoroethylene reactor; use an electronic balance to weigh 2.0 g of the iron-containing sludge powder sample obtained in the first step. The concentration range of iron elements dissolved in ethylene glycol is 6.7 - 8.0 g / L. Place it in the ethylene glycol solution and stir evenly; use an electronic balance to weigh 7.5 g of sodium acetate powder and add it to the mixed solution of ethylene glycol and iron sludge. The mass ratio of iron to sodium acetate ranges from 1 / 15 to 1 / 12.5. Mechanically stir for 1.5 - 2 h at a rotation speed of 300 rpm under ultrasonic conditions to obtain a yellowish-brown suspension; seal the polytetrafluoroethylene liner containing the suspension into a stainless steel autoclave and place it in a muffle furnace for high-temperature calcination. Adjust the heating rate to 4 °C / min, set the calcination temperature to 200 °C, and the holding time to 10 h.

[0061] The microstructure of the Fe 3 O 4 @C magnetic flocculant prepared through Example 1 is characterized by transmission electron microscopy TEM, as Figure 5 shown. Fe 3 O4 @C presents a nano - microsphere structure with a particle size of 100 - 180 nm and is loaded in the iron - containing sludge carbon structure layer.

[0062] Example 2

[0063] Weigh 75 mL of isopropyl alcohol solution and place it in a 200 - mL polytetrafluoroethylene reactor; weigh 2.0 g of the iron - containing sludge powder sample obtained in the first step with the iron element dissolved in ethylene glycol in the concentration range of 6.7 - 8.0 g / L, place it in the ethylene glycol solution and stir evenly; weigh 7.5 g of sodium acetate powder and add it to the mixed solution of ethylene glycol and iron sludge, with the iron / sodium acetate ratio in the range of 1 / 15 - 1 / 12.5, mechanically stir for 1.5 - 2 h under ultrasonic conditions at a rotation speed of 300 rpm to obtain a yellow - brown suspension; seal the polytetrafluoroethylene liner containing the suspension into a stainless - steel autoclave, put it into a muffle furnace for high - temperature calcination, adjust the heating rate to 4 °C / min, set the calcination temperature at 200 °C, and the holding time is 10 h.

[0064] Example 3

[0065] Weigh 75 mL of ethanol solution and place it in a 200 - mL polytetrafluoroethylene reactor; weigh 2.0 g of the iron - containing sludge powder sample obtained in the first step with the iron element dissolved in ethylene glycol in the concentration range of 6.7 - 8.0 g / L, place it in the ethylene glycol solution and stir evenly; weigh 7.5 g of sodium acetate powder and add it to the mixed solution of ethylene glycol and iron sludge, with the iron / sodium acetate ratio in the range of 1 / 15 - 1 / 12.5, mechanically stir for 1.5 - 2 h under ultrasonic conditions at a rotation speed of 300 rpm to obtain a yellow - brown suspension; seal the polytetrafluoroethylene liner containing the suspension into a stainless - steel autoclave, put it into a muffle furnace for high - temperature calcination, adjust the heating rate to 4 °C / min, set the calcination temperature at 200 °C, and the holding time is 10 h.

[0066] Ethylene glycol, isopropyl alcohol, and ethanol are selected as reducing solvents in the preparation process of magnetic flocculants, and the crystal structure of the finally obtained samples is as Figure 1 shown. According to the XRD results, the Fe 3 O 4 crystallinity is the best when ethylene glycol is used as the reducing solvent in the solvothermal reaction of iron - containing sludge.

[0067] In addition, by comparing the enhanced algae - removal efficiency of the three samples, as Figure 2As shown in the figure, the removal rates of algal cells by the magnetic flocculants formed under three solvent conditions were 98.7%, 85.3% and 83.6% respectively within 30 min. Under the action of an external magnetic field, the turbidity of the effluent after precipitation within 5 min was 1.63, 3.82 and 8.57 NTU respectively. The results show that the magnetic flocculant prepared with ethylene glycol as the solvent thermal reaction solvent has the best effect in enhancing coagulation and algae removal, and the sedimentation property of the formed flocs is also the best.

[0068] Comparative Example 1

[0069] Using the solvent thermal modification method of iron mud in Example 1, only by adjusting the calcination temperature of the muffle furnace to 180 °C, iron mud modified samples at different solvent thermal reaction temperatures were prepared, and other steps and parameters were the same as those in Example 1.

[0070] Comparative Example 2

[0071] Using the solvent thermal modification method of iron mud in Example 1, only by adjusting the calcination temperature of the muffle furnace to 220 °C, iron mud modified samples at different solvent thermal reaction temperatures were prepared, and other steps and parameters were the same as those in Example 1.

[0072] The magnetic flocculant samples obtained at different calcination temperatures of 180 °C, 200 °C and 220 °C were respectively labeled as Fe 3 O 4 @C-180, Fe 3 O 4 @C-200 and Fe 3 O 4 @C-220. The influence of the calcination temperature on the magnetic characteristics of the samples is as Figure 3 shown. The results show that the calcination temperature will directly affect the magnetic characteristics of the samples. The saturation magnetization intensity of the sample Fe 3 O 4 @C-180 at the calcination temperature of 180 °C is 4.33 mu / g, compared with Fe 3 O 4 @C-200 and Fe 3 O 4 @C-220 generated at the calcination temperatures of 200 °C and 220 °C, with saturation magnetization intensities of 16.5 mu / g and 13.4 mu / g respectively. The results show that Fe 3 O 4 @C obtained at the solvent thermal reaction temperatures of 180 °C, 200 °C and 220 °C all have good magnetism, and the magnetism at 200 °C and 220 °C is better, and can achieve efficient separation under the action of an external magnetic field. Considering the energy consumption of the calcination temperature, 200 °C is preferably selected as the calcination temperature for the solvent thermal reaction of the iron mud-based magnetic flocculant.

[0073] Comparative Example 3

[0074] Using the solvothermal modification method of iron mud in Example 1, by only adjusting the calcination holding time to 8 h, iron mud modified samples under different solvothermal reaction times were prepared, and other steps and parameters were the same as those in Example 1.

[0075] Comparative Example 4

[0076] Using the solvothermal modification method of iron mud in Example 1, by only adjusting the calcination holding time to 12 h, iron mud modified samples under different solvothermal reaction times were prepared, and other steps and parameters were the same as those in Example 1.

[0077] The effect of different holding times on the enhanced algae removal efficiency of the magnetic flocculant samples is as Figure 4 shown. The results show that during the enhanced coagulation and algae removal process of Fe 3 O 4 @C-8, the algae cell removal rate at 30 min was 89.4%, which was lower than that of Fe 3 O 4 @C-10 (98.7%) and Fe 3 O 4 @C-12 (99.0%) samples under the same reaction conditions. In addition, in the enhanced coagulation process of Fe 3 O 4 @C-8, Fe 3 O 4 @C-10 and Fe 3 O 4 @C-12 combined with PACl, under the action of an external magnetic field, the turbidity of the effluent after precipitation within 5 min was 5.36, 1.63 and 1.57 NTU respectively, indicating that Fe 3 O 4 @C-10 and Fe 3 O 4 @C-12 magnetic flocculants showed better enhanced coagulation and turbidity removal effects and floc sedimentation properties. Considering the energy consumption of the reaction time, 10 h was selected as the solvothermal reaction time of the iron mud-based magnetic flocculant.

[0078] Comparative Example 5

[0079] Using the iron mud-based magnetic flocculant Fe 3 O 4 @C in Example 1 for the application in Step 5, and changing the PACl dosage to 4 - 12 mg / L.

[0080] Set up a system using PACl alone: On the basis of the application in Step 5, without adding Fe 3 O 4 @C, only adding PACl with a dosage of 4 - 12 mg / L.

[0081] Fe3 O 4 Comparison of enhanced coagulation efficiency for algae removal and residual turbidity of sedimentation effluent in the @C / PACl system is as Figure 6 shown. Fe 3 O 4 The @C / PACl system can effectively improve the algae removal efficiency compared with using PACl alone. As the dosage of PACl increases from 4 mg / L to 12 mg / L, the algae removal rate increases from 35.3% to over 97%. When the dosage of PACl is 10 mg / L, the algae cell removal rate reaches the highest value of 98.7%, and the turbidity of sedimentation effluent can be reduced to 0.92 NTU within 5 min. When using a 10 mg / L PACl system alone, the algae removal rate is 83.2%, and the turbidity of sedimentation effluent after 30 min is 1.23 NTU. It shows that the iron sludge-based magnetic flocculant can rapidly achieve floc precipitation while efficiently removing algae.

[0082] Comparative Example 6

[0083] The iron sludge-based magnetic flocculant (Fe 3 O 4 @C) in Example 1 was used for the application in Step 5. During the 15-min slow stirring for flocculation, by on-line monitoring the change of floc fractal dimension (D f ), the compactness of flocs formed in the enhanced coagulation process of Fe 3 O 4 @C was evaluated. After stopping stirring, during the sedimentation process, samples were taken at time points of 0, 1, 2, 5, 10, 15, 20, 25, and 30 min to detect the turbidity, so as to evaluate the sedimentation property of flocs during sedimentation. By comparing the change of D 3 O 4 @C / PACl and D f in the single PACl system, the function of Fe 3 O 4 @C in enhancing coagulation to improve floc compactness was analyzed. As Figure 7 shown, within 15 min (900 s) of the flocculation reaction stage, D 3 O 4 of flocs in the @C / PACl system gradually increased to about 2.5, while the maximum D f of flocs in the single PACl system was only 2.1. The results show that the magnetic flocculant Fe f O 3 O 4 @C can effectively improve floc compactness during the enhanced coagulation process, which is helpful for improving sedimentation performance.

[0084] Among them, D f reflects the compactness of reaction flocs in the coagulation system. The larger D f is, the higher the floc compactness is. Df The measurement is carried out by using the immersion optical sampling technology. An online digital camera is used to capture the online photos of the flocs at different reaction time points, and then the floc particle size and the fractal dimension are analyzed and calculated through the image processing software. The fractal dimension D of the irregularly shaped flocs f can be calculated by the following formula:

[0085]

[0086] where: P represents the perimeter of the floc aggregate, and A p represents the projected area of the floc, and D f represents the two-dimensional fractal dimension of the floc.

[0087] Compare the residual turbidity of the supernatant in the coagulation systems of Fe 3 O 4 @C / PACl and PACl alone systems at different time points, and analyze the function of Fe 3 O 4 @C in enhancing the sedimentation property of the flocs. As Figure 8 shown, in the sedimentation process of the Fe 3 O 4 @C / PACl system for 30 minutes, when no external magnetic field is used, the residual turbidity of the supernatant is 12.3 NTU at 5 minutes and can be reduced to 1.32 NTU at 20 minutes. When a 60 mT external magnetic field is added, the turbidity of the sedimented water can be reduced to 0.92 NTU within 5 minutes, realizing the rapid sedimentation of the flocs.

[0088] The above-described embodiments are only the preferred solutions of the present invention, and do not impose any form of limitation on the present invention. There are other variations and modifications without exceeding the technical solutions described in the claims.

Claims

1. A method for preparing a magnetic flocculant from iron-containing sludge, characterized in that: The following steps are involved: (1) Pre-treating iron-containing sludge cake from a water treatment plant to obtain iron-containing sludge powder; (2) adding iron-containing sludge powder to ethylene glycol, and then adding an appropriate amount of alkaline buffer substance, and fully mixing and stirring under ultrasonic conditions to obtain an iron sludge mixed liquid; (3) The iron mud mixture is transferred to a reactor for high temperature and high pressure solvent thermal reaction, and a magnetic flocculant is obtained after the reaction is completed.

2. The method for preparing a magnetic flocculant from iron-containing sludge according to claim 1, characterized in that: In step (1), the iron-containing sludge cake is derived from the sludge water produced by the coagulation-precipitation process using iron salt as a coagulant in an actual water treatment plant, and then the iron-containing sludge cake is formed by the dehydration-drying-filter pressing process, and the iron content is 20-30%.

3. The method for preparing a magnetic flocculant from iron-containing sludge according to claim 1, characterized in that: In step (1), the pretreatment method includes air drying, grinding, drying, and screening processes; Specifically, the iron-containing sludge is naturally air-dried until the moisture content is less than 10%, crushed into granules, and then ground into powder using a ceramic mortar, placed in a blast drying oven, and dried at 100-110°C for 24 hours; after drying, it is sieved using a 200-mesh copper mesh to obtain a dry iron-containing sludge powder sample.

4. The method for preparing a magnetic flocculant from iron-containing sludge according to claim 1, characterized in that: In step (2), the feeding amount of the iron-containing sludge powder is calculated as iron / ethylene glycol, and its concentration range is 6.7 to 8.0 g / L.

5. The method for preparing a magnetic flocculant from iron-containing sludge according to claim 1, characterized in that: In step (2), the amount of alkaline buffer substance added is calculated to maintain the pH of the system at 8-9.

6. The method for preparing a magnetic flocculant from iron-containing sludge according to claim 1, characterized in that: In step (2), the ultrasonic frequency is 80 to 100 kHz, the stirring speed is 200 to 300 rpm, and the time is 1.5 to 2 hours.

7. The method for preparing a magnetic flocculant from iron-containing sludge according to claim 1, characterized in that: In step (3), the solvent thermal reaction conditions are a calcination temperature of 180 to 220° C., a heating rate of 4 to 10° C. / min, and calcination at this temperature for 8 to 12 hours.

8. The method for preparing a magnetic flocculant from iron-containing sludge according to claim 1, characterized in that: In step (3), in step (3), after the reaction is completed, the reactor is cooled to room temperature, and the black precipitate at the bottom of the liner is collected by magnetic separation; after washing, the magnetic flocculant (Fe3O4@C) is obtained by vacuum drying.

9. Use of a magnetic flocculant prepared according to the preparation method according to any one of claims 1 to 8 in enhanced treatment of low-turbidity and high-algae water.

10. The use of the magnetic flocculant in enhanced treatment of low-turbidity and high-algae water according to claim 9, characterized in that: The raw water turbidity is set to 5-10 NTU. According to the actual algae cell concentration during algae bloom in the water body, the algae cell concentration is set to 10 5 ~10 6 / mL, according to the actual pH range in natural water bodies, the pH of the algae liquid was adjusted to 7.0-8.0, the dosage of the iron mud-based magnetic flocculant was 0.8g / L, and the dosage of the coagulant polyaluminium chloride was 10-12mg / L.

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