A fly ash-based ferric sulfate flocculant and a method for preparing the same

Abstract

This invention discloses a ferric sulfate flocculant based on fly ash and its preparation method, belonging to the field of water treatment agent preparation technology. The preparation method involves: after acid leaching of fly ash with hydrochloric acid, stepwise pH adjustment is used to achieve Fe... 3+ Al 3+ Fe 2+ and Ca 2+ Selective precipitation separation yields a magnesium-containing filtrate; the iron-enriched filter cake is acid-dissolved, oxidized, and polymerized to obtain ferric sulfate liquid product; the magnesium-containing filtrate can be used to prepare magnesium hydroxide and magnesium-aluminum hydrotalcite particles. This invention is the first to use LDH in a fixed-bed configuration for pH fine-tuning, solving the problem of separating suspended LDH from Fe(OH)2; the acidic flocculent Al(OH)3 has high reactivity and can directly react with the magnesium-containing filtrate to prepare LDH, which can be used as a low-cost adsorbent and pH adjuster for process use; it achieves efficient recovery of iron resources and recycling of magnesium and aluminum resources in fly ash, and the prepared ferric sulfate product meets all national standards.

Description

Technical Field

[0001] This invention relates to the field of water treatment agent preparation technology, and in particular to a ferric sulfate flocculant based on fly ash and its preparation method. Background Technology

[0002] Fly ash is a major solid waste generated by coal-fired power plants, with huge annual emissions. Fly ash contains various valuable metal elements such as iron, aluminum, calcium, and magnesium. In some regions (such as the Junggar coalfield in Inner Mongolia), fly ash is also rich in rare and dispersed metals such as gallium, making it highly valuable for comprehensive recycling. However, existing fly ash resource utilization technologies mostly focus on the extraction of single elements (such as aluminum), paying insufficient attention to other elements, and often generating large amounts of secondary waste liquid, failing to achieve true full-component utilization.

[0003] Ferric sulfate (PFS) is a highly efficient inorganic polymeric flocculant with advantages such as rapid floc formation, fast settling speed, wide applicable pH range, and good turbidity and decolorization effects. It is widely used in water supply and drainage, industrial wastewater treatment, and sludge dewatering. Currently, the main methods for preparing ferric sulfate include ferrous sulfate oxidation and iron ore acid leaching. Utilizing the iron resources in fly ash to prepare ferric sulfate flocculants can achieve both the resource utilization of solid waste and the production of high-value-added water treatment agents.

[0004] However, fly ash acid leaching solution usually contains Fe. 3+ Fe 2+ Al 3+ Ca 2+ Mg 2+ The presence of various metal ions, such as iron, results in the precipitation of their hydroxides within very similar pH ranges. Traditional one-step precipitation methods struggle to achieve selective separation of iron, leading to the introduction of impurities like aluminum, calcium, and magnesium into the product, thus affecting the quality of the ferric sulfate flocculant. Furthermore, existing pH control methods often involve the direct addition of alkali, which presents problems such as localized over-alkaliness, pH over-overshooting, and difficulties in control.

[0005] Magnesium aluminum hydrotalcite (Mg-Al LDH) is a type of anionic clay material with a layered structure, and its general chemical formula is [M]. 2+ 1-x M 3+ x (OH)2] x+ (A n- ) x / n•mH2O. Studies have shown that the isoelectric point (pHpzc) of Mg-Al LDH is approximately 9.7, exhibiting the strongest buffering capacity in the pH range of 8.5-9.5, making it suitable for precise pH control of solutions. Utilizing magnesium and aluminum resources from fly ash to prepare LDH and using it for pH control of acid leaching solutions can realize a circular economy model of "treating waste with waste."

[0006] To address the above problems, this invention provides a ferric sulfate flocculant based on fly ash and its preparation method, achieving Fe... 3+ Al 3+ Ca 2+ Mg 2+ The separation process involves a series of steps, and the separated products are used to prepare magnesium aluminum hydrotalcite. This hydrotalcite is then used in a fixed-bed configuration for precise pH control, ultimately yielding a high-purity ferric sulfate flocculant product. Summary of the Invention

[0007] This invention addresses the technical challenges of traditional processes, such as difficulty in pH control, severe interference from impurities, and the inability to recycle materials. It achieves selective separation of iron from fly ash acid leaching solution through stepwise pH regulation and utilizes the separated aluminum and magnesium sources to prepare magnesium-aluminum hydrotalcite, which is then used in a fixed-bed configuration for precise pH control. This invention proposes a fly ash-based ferric sulfate flocculant and its preparation method.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: The present invention discloses a method for preparing a ferric sulfate flocculant based on fly ash, comprising the following steps: Part 1: Selective Separation of Fly Ash from Acid Leaching Step (1) Acid leaching treatment of fly ash Fly ash was added to a reaction vessel, and hydrochloric acid was added for acid leaching. The reaction temperature was controlled at 60-90℃, the reaction time at 1-3 hours, and the liquid-solid ratio at 3:1 to 6:1, to obtain Fe-containing... 3+ Fe 2+ Al 3+ Ca 2+ Mg 2+ Acid leaching solution containing trace elements. After acid leaching, the solution is separated by pressure filtration to obtain acid leaching filtrate and leaching residue.

[0009] Step (2) Fe 3+ Precipitation separation Add the magnesium hydroxide slurry prepared in step (8) to the acid leaching filtrate obtained in step (1), adjust the pH of the solution to 2.7-3.0, and stir the reaction for 30-60 minutes. Under these pH conditions, Fe 3+ It completely precipitates as Fe(OH)3, while Al 3+ Ca 2+Mg 2+ and Fe 2+ The iron remains in the solution. Pressure filtration is used to separate the iron, yielding iron-enriched filter cake A (Fe(OH)3) and the filtrate after iron removal. Iron-enriched filter cake A is used for subsequent preparation of ferric sulfate products.

[0010] Step (3) Al 3+ Precipitation separation Add the magnesium hydroxide slurry prepared in step (8) to the iron-removed filtrate obtained in step (2), adjust the pH of the solution to 3.4-5.0, control the final pH to 4.5±0.3, and stir the reaction for 1-2 hours. Under these pH conditions, Al 3+ Hydrolysis forms a highly reactive, acidic flocculent aluminum hydroxide precipitate (pH≈4.5), while Ca... 2+ Mg 2+ and Fe 2+ It remains in solution. Pressure filtration separates the aluminum hydroxide flocculent filter cake (aluminum source) and the filtrate after aluminum removal (containing Ca). 2+ Mg 2+ and Fe 2+ ).

[0011] Step (4) Fine-tuning of LDH fixed bed and Fe 2+ precipitation The Mg-Al LDH particles prepared in step (9) were packed into a fixed-bed reactor to form an LDH fixed bed. The aluminum-removed filtrate obtained in step (3) was passed through the LDH fixed bed at a certain flow rate. Utilizing the pH buffering characteristics of LDH (isoelectric point pHpzc≈9.7), the pH of the solution was automatically raised and stabilized at 8.5-9.0. Under these pH conditions, the Fe in the solution... 2+ It precipitates completely as Fe(OH)2, forming a mixture with the solution that flows out of the fixed bed.

[0012] The effluent enters the pressure filtration process, where iron-enriched filter cake B (Fe(OH)2) and Ca-containing filter cake are separated. 2+ Mg 2+ The filtrate. Iron-enriched filter cake B is used for subsequent preparation of ferric sulfate products.

[0013] Step (5) Ca 2+ Precipitation separation To the Ca-containing product obtained in step (4) 2+ Mg 2+ Sodium carbonate (Na₂CO₃) was added to the filtrate to control the pH at approximately 9.0, and the mixture was stirred for 30-60 minutes. Under these conditions, Ca... 2+ It precipitates as CaCO3, while Mg... 2+ It remains in solution. Pressure filtration separates the calcium carbonate filter cake (byproduct) and magnesium-containing filtrate (magnesium source).

[0014] Step (6) Magnesium source diversion The magnesium-containing filtrate obtained in step (5) is divided into two parts: Route A: accounting for 20-40% of the total, used in step (8) to prepare magnesium hydroxide; B route: accounting for 60-80% of the total, is directly used in step (9) to prepare LDH.

[0015] Part 2: Cyclic Preparation of Magnesium-Aluminum Hydrotalcite (Mg-Al LDH) Step (7) Preparation of ferric sulfate flocculant Combine the iron enrichment filter cake A (Fe(OH)3) obtained in step (2) and the iron enrichment filter cake B (Fe(OH)2) obtained in step (4), add dilute sulfuric acid to carry out the dissolution reaction, control the reaction temperature at 40-60℃, and stir until the precipitate is completely dissolved to obtain a ferrous sulfate / ferric sulfate mixed solution.

[0016] Add an oxidizing agent (such as sodium hypochlorite, hydrogen peroxide, or air) to the resulting solution to remove Fe. 2+ Completely oxidized to Fe 3+ Continue until the solution completely changes from green to reddish-brown. Continue controlling the polymerization conditions, adjusting the basicity (OH / Fe molar ratio) to 0.3-0.8, and polymerize at 50-80℃ for 1-3 hours to obtain a reddish-brown viscous ferric sulfate flocculant product.

[0017] Ferrous sulfate flocculant products can be directly bottled or diluted before use.

[0018] Step (8) Preparation of magnesium hydroxide Take the magnesium-containing filtrate from step (6) A, add alkaline solution (sodium hydroxide or waste alkali recovered in the previous step) to adjust the pH to 10.5-11.0, stir and react for 1-2 hours to allow the Mg to react. 2+ Complete precipitation occurs as Mg(OH)2. Pressure filtration separates the precipitate into magnesium hydroxide filter cake (used for pH adjustment in steps 2 and 3) and alkaline filtrate (the alkali can be recovered or discharged).

[0019] Step (9) Preparation of Mg-Al LDH particles Take the magnesium-containing filtrate from step (6) B and the aluminum hydroxide flocculent filter cake (aluminum source) obtained in step (3), and adjust the Mg / Al molar ratio to 2:1 to 4:1 (preferably 3:1). Heat the mixed slurry to 60-80℃ and stir for 4-8 hours. During this process, a co-precipitation reaction driven by neutralization occurs using the acidity of the aluminum hydroxide flocculent (pH≈4.5) and the alkalinity of the magnesium-containing filtrate to generate the Mg-Al LDH precursor. After the reaction is complete, filter, wash, and dry at 80-105℃ to obtain Mg-Al LDH powder.

[0020] Mix LDH powder with 10-15% bentonite (binder), add an appropriate amount of water and stir to form a wet material. Use a granulator to form particles with a diameter of 3-8 mm, and dry at 80℃ to obtain Mg-Al LDH composite particles, which are used in step (4) fixed bed.

[0021] Step (10) Mother liquor circulation Step (9) The mother liquor produced during LDH preparation contains Na + CO3 2- CO2 can be introduced to convert Na2CO3 into NaHCO3, which can then be returned to step (5) as a precipitant to achieve zero discharge of mother liquor.

[0022] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention achieves the cascade separation and full resource utilization of the four main elements Fe, Al, Ca and Mg in fly ash through four steps of precise pH control. Compared with traditional processes (which usually only recover 1-2 elements and discharge the rest as waste residue), the resource utilization rate of all elements is increased from less than 50% to more than 95%.

[0023] 2. This invention is the first to use magnesium aluminum hydrotalcite in a fixed bed for precise pH control of a solution, improving pH control accuracy by 3-5 times and extending the LDH cycle life by 15-20 times. It completely solves the problem of separating suspended LDH from Fe(OH)2 colloidal precipitate. Furthermore, the adsorption capacity of LDH can be fully utilized to reduce heavy metals in the solution (such as when fly ash produced in some heavy metal mining areas is used to prepare ferric sulfate flocculant according to the preparation method of this invention).

[0024] 3. The process of this invention also utilizes the high reactivity of acidic flocculent aluminum hydroxide with a pH of 4.5 to directly react with magnesium-containing filtrate to prepare LDH, thereby increasing the aluminum source utilization rate from 70-80% to over 95%, with zero acid and alkali consumption. This breaks through the conventional understanding of those skilled in the art that Al(OH)3 precipitate needs to be redissolved before it can be used for LDH preparation.

[0025] 4. This invention achieves a complete closed-loop cycle of magnesium elements through a sophisticated magnesium source distribution design. This increases the magnesium utilization rate from 0% (as discharged as waste liquid in traditional processes) to over 85%, saving over 600,000 yuan in magnesium source costs annually. Simultaneously… 5. This invention also achieves the recycling of LDH preparation mother liquor through CO2 conversion technology, reducing wastewater discharge by 100%, Na2CO3 recovery rate >95%, saving more than 400,000 yuan in soda ash costs annually, and simultaneously utilizing industrial CO2 to achieve carbon emission reduction.

[0026] 6. This invention completely removes Al through staged separation. 3+ Ca 2+Mg 2+ Impurities are eliminated to obtain a high-purity polyferric sulfate product. The impurity content of the product is reduced by more than 90% compared with traditional processes, meeting the standards for food-grade water treatment agents. Furthermore, the product of this invention achieves a COD removal rate of 84.99% and a color removal rate of 95-97% for dyeing and printing wastewater, and a turbidity removal rate of 89.7% for domestic sewage, with a settling time of 8-9 minutes. All indicators are superior to commercially available products (COD removal rate 80-83%, settling time 10-12 minutes). This is due to the high purity and low impurity content of the product. 3+ The polymerization morphology is more stable, the molecular chains are longer, the floc particle size increases by 50%, and the sedimentation velocity increases by 20-30%.

[0027] 7. This invention significantly reduces operating costs through a closed-loop cycle of magnesium and aluminum elements, a self-sufficient acid-base process, and mother liquor recycling. While addressing fly ash pollution, it also produces high-quality flocculants: annually disposing of 15,000-20,000 tons of fly ash, reducing land occupation and environmental pollution, and eliminating the impact of fly ash dust on surrounding residents. The product's performance surpasses commercially available products, reducing downstream water treatment companies' operating costs by 10-15%, achieving a green circular model of treating waste with waste. 8. In summary, this invention achieves the full resource utilization of Fe, Al, Ca, and Mg in fly ash, producing high-value-added polyferric sulfate products. Simultaneously, it achieves acid-base self-sufficiency within the process through the recycling of magnesium-aluminum hydrotalcite. It boasts significant advantages such as a simple process flow, precise operation control, high resource utilization rate, excellent product quality, low operating costs, and significant environmental benefits. It provides a feasible technical solution for the high-value utilization of fly ash from coal-fired power plants and is of great significance for promoting the development of a circular economy and the construction of waste-free cities. Attached Figure Description

[0028] Figure 1 This is a process flow diagram for preparing a fly ash-based ferric sulfate flocculant proposed in this invention. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0030] Example 1. A preparation process for a ferric sulfate flocculant based on fly ash: (1) Acid leaching treatment of fly ash One kg of fly ash from a power plant in Inner Mongolia (main components: SiO2 45.3%, Al2O3 28.6%, Fe2O3 5.2%, CaO 3.8%, MgO 1.2%) was added to 4 L of 6 mol / L hydrochloric acid and stirred at 90℃ for 2 hours. The mixture was then separated by pressure filtration, yielding approximately 3.8 L of acid leaching filtrate. ICP analysis showed that the main ion concentration in the filtrate was: Fe... 3+ 3.2 g / L, Fe 2+ 0.5 g / L, Al 3+ 18.5 g / L, Ca 2+ 2.1 g / L, Mg 2+ 0.8 g / L.

[0031] (2) Fe 3+ Precipitation separation (pH 2.7-3.0) Add the magnesium hydroxide slurry (10% solid content) prepared in step (8) to the acid leaching filtrate, adjust the pH to 2.8, and stir for 45 minutes. Filter by pressure to obtain iron-enriched filter cake A (Fe(OH)3, dry weight approximately 42 g) and iron-removed filtrate (volume approximately 4.0 L). Upon testing, the Fe content in the filtrate was... 3+ Residual <0.01 g / L, Fe 3+ Sedimentation rate > 99.5%.

[0032] (3) Al 3+ Precipitation separation (pH 3.4-5.0) Magnesium hydroxide slurry was added to the filtrate after iron removal to adjust the pH to 4.5. The mixture was stirred for 1.5 hours to obtain an acidic flocculent aluminum hydroxide precipitate. Filtration yielded an aluminum hydroxide flocculent filter cake (approximately 75% moisture content, equivalent to approximately 128 g of dry Al(OH)3) and a filtrate after aluminum removal (containing Ca...). 2+ 2.1 g / L, Mg 2+ 0.8 g / L, Fe 2+ 0.5 g / L (approximately 4.2 L in volume). Analysis showed that the filtrate contained Al. 3+ Residual <0.01 g / L, Al 3+ Sedimentation rate > 99.5%.

[0033] (4) Fine-tuning of LDH fixed bed and Fe 2+ precipitation The Mg-Al LDH particles (particle size 3-5 mm, Mg / Al = 3:1) prepared in step (9) were packed into a fixed-bed reactor at a loading rate of 80 kg / m³. 3The bed height is 0.8 m and the diameter is 0.2 m. The filtrate after aluminum removal is pumped into the fixed bed from the bottom at a flow rate of 1.5 L / h, with an empty bed residence time of approximately 50 minutes. The pH of the effluent stabilizes at 8.7, and Fe(OH)₂ precipitate forms. The effluent enters a filter press, yielding iron-enriched filter cake B (Fe(OH)₂, dry weight approximately 4.2 g) and a Ca-containing filter cake. 2+ Mg 2+ Filtrate (approximately 4.3 L). Analysis revealed the presence of Fe in the filtrate. 2+ Residual <0.005 g / L, Fe 2+ Sedimentation rate > 99.0%.

[0034] (5) Ca 2+ Precipitation separation Towards Ca 2+ Mg 2+ Add Na2CO3 (as Ca) to the filtrate 2+ (1.1 times the molar weight), and stirred for 40 minutes. Filter by pressure to obtain a CaCO3 filter cake (dry weight approximately 22 g, purity 98.2%) and a magnesium-containing filtrate (Mg). 2+ The concentration is approximately 0.8 g / L, and the volume is approximately 4.5 L.

[0035] (6) Magnesium source diversion Take 4.5 L of magnesium-containing filtrate and divide it into two streams: 1.5 L (33%) for stream A and 3.0 L (67%) for stream B.

[0036] (7) Preparation of ferric sulfate flocculant (corresponding to step 7 of claim 1 and claim 4) Iron-enriched filter cake A (42 g Fe(OH)3) and iron-enriched filter cake B (4.2 g Fe(OH)2) were combined and dissolved in approximately 150 mL of 20% dilute sulfuric acid at 50°C with stirring, yielding approximately 200 mL of a ferrous sulfate / ferric sulfate mixed solution (total Fe concentration approximately 42 g / L). Approximately 30 mL of 10% sodium hypochlorite solution was then added dropwise to the solution to remove the Fe... 2+ Completely oxidized to Fe 3+ Adjust the alkalinity (OH / Fe molar ratio) of the solution to 0.5, and polymerize at 70℃ for 2 hours to obtain approximately 220 mL of a reddish-brown, viscous ferric sulfate flocculant product. Product test results are as follows: Table 1. Product test results of Example 1 (8) Preparation of magnesium hydroxide Add NaOH solution to the magnesium filtrate (1.5 L) from route A to adjust the pH to 11.0 and stir for 1.5 hours. Filter the solution to obtain magnesium hydroxide filter cake (dry weight approximately 2.3 g), which will be used for pH adjustment in the next batch of steps (2) and (3).

[0037] (9) Preparation of Mg-Al LDH particles Take 3.0 L of magnesium-containing filtrate from route B (containing Mg). 2+ Approximately 2.4 g) and the aluminum hydroxide flocculent filter cake obtained in step (3) (take a portion, containing Al) 3+ Approximately 3.2 g (adjust Mg / Al molar ratio ≈ 3:1). The mixed slurry was heated to 75℃ and stirred for 6 hours. After the reaction, it was filtered, washed, and dried at 90℃ to obtain approximately 12 g of LDH powder in flake form with good crystallinity. It was mixed with 1.5 g of bentonite and granulated to obtain approximately 13 g of LDH particles with a particle size of 3-5 mm, which were used in the next batch step (4) fixed bed.

[0038] (10) Mother liquor circulation In step (9), the mother liquor (approximately 2.5 L) generated from LDH preparation is passed through CO2 gas (flow rate 50 mL / min, time 2 hours) to convert Na2CO3 into NaHCO3, which is then returned to step (5) for use as a precipitant. The conversion rate was measured to be >95%.

[0039] (11) Cyclic stability verification After 10 consecutive batches of operation, the magnesium circulation rate in the system remained stable at 85-90%, and the LDH particles maintained 92% of their initial pH regulation activity after 10 batches of use, verifying the long-term stability of the process.

[0040] Example Group 2 (different Mg / Al molar ratios): Following the same procedure as in Example 1, but adjusting the Mg / Al molar ratio to 2:1, 3:1, and 4:1 in step (9), the LDH performance is compared as follows: Table 2. Effect of different Mg / Al molar ratios on LDH performance The results showed that LDH crystallinity was highest, yield was greatest, and pH control precision was best when Mg / Al = 3:1, which is consistent with existing literature reports.

[0041] Example Group 3 (Different Fixed Bed Parameters): Following the same procedure as in Example 1, the LDH loading amount and empty bed residence time were changed in step (4), and the pH adjustment results are as follows: Table 3. Effects of different fixed-bed parameters on pH control results The results showed that a filling volume of 80 kg / m³ was suitable. 3 A residence time of 50 minutes can achieve the best pH control effect (8.5-9.0±0.1). Excessive filling volume will cause the pH to slightly exceed 9.0.

[0042] Example Group 4 (Different magnesium source split ratios) Following the same process as in Example 1, the A / B traffic splitting ratio is changed in step (6), resulting in the following economic benefits: Table 4. Impact of different magnesium source diversion ratios on economic benefits Therefore, a ratio of 33%:67% can achieve a balance between magnesium supply and demand, and the magnesium recycling rate is the highest; if the ratio of A is too low, the Mg(OH)2 in steps (2) and (3) will be insufficient and needs to be supplemented externally.

[0043] Example Group 5 (Comparison of different oxidants): Following the same process as in Example 1, but using different oxidants in step (7), the flocculant product performance is as follows: Table 5. Effects of different oxidants on the performance of flocculant products The table above shows that NaClO and H2O2 have high oxidation efficiency, while air oxidation takes longer and leaves slightly higher residues.

[0044] Example Group 6 (Different Fly Ash Sources): Example 6-1. Fly ash from a power plant in Shanxi: One kg of fly ash from a power plant in Shanxi Province (main components: SiO2 48.7%, Al2O3 32.4%, Fe2O3 3.8%, CaO 2.5%, MgO 0.9%) was subjected to acid leaching under the same conditions as in Example 1. Approximately 3.9 L of acid leaching filtrate was obtained, with the following ion concentration: Fe... 3+ 2.4 g / L, Fe 2+ 0.3 g / L, Al 3+ 20.1 g / L, Ca 2+ 1.4 g / L, Mg 2+ 0.6 g / L.

[0045] The subsequent steps were the same as in Example 1, yielding approximately 165 mL of ferric sulfate flocculant product with an iron content of 9.1% and a basicity of 13.8%, all of which met national standards.

[0046] Example 6-2. Fly ash from a power plant in Sichuan: One kg of fly ash from a power plant in Sichuan Province (main components: SiO2 42.5%, Al2O3 25.3%, Fe2O3 6.8%, CaO 5.2%, MgO 1.5%) was subjected to acid leaching under the same conditions as in Example 1. Approximately 3.7 L of acid leaching filtrate was obtained, with the following ion concentration: Fe... 3+ 4.1 g / L, Fe 2+ 0.6 g / L, Al3+ 15.8 g / L, Ca 2+ 3.2 g / L, Mg 2+ 1.1 g / L.

[0047] The subsequent steps were the same as in Example 1, yielding approximately 285 mL of ferric sulfate flocculant product with an iron content of 9.3% and a basicity of 14.8%, all of which met national standards.

[0048] Comparative Example 1 (no stepwise precipitation, one-step iron precipitation): The acid leaching solution was obtained according to step (1) of Example 1, without Fe. 3+ And Al 3+ Stepwise precipitation: NaOH is added directly to adjust the pH to 8.5-9.0 to precipitate iron. Pressure filtration yields a mixed filter cake (containing co-precipitates of Fe, Al, Ca, and Mg) and filtrate. The filter cake is dissolved in acid, oxidized, and polymerized to obtain ferric sulfate product.

[0049] Product testing results: Iron content 7.2% (lower than national standard), Al content 2.1%, Ca content 0.8%, Mg content 0.5%, reducing substances 0.15%. Flocculation performance testing showed a removal rate of 82% for wastewater with the same turbidity, lower than the 89.7% in Example 1.

[0050] Conclusion: One-step precipitation cannot achieve selective separation of iron, and the product contains serious excesses of Al, Ca, and Mg impurities, resulting in decreased flocculation performance.

[0051] Comparative Example 2 (pH adjustment outside the range): Comparative Example 2-1. Fe 3+ Precipitation pH=2.5 (below 2.7): Adjust the pH to 2.5 according to step (2) of Example 1 to precipitate Fe. 3+ Detection revealed that Fe in the filtrate 3+ The residue was 0.15 g / L, with a precipitation rate of only 93.8%, and some Fe... 3+ If it enters subsequent processes, it will affect the purity of the product.

[0052] Comparative Example 2-2.Fe 3+ Precipitation pH=3.2 (above 3.0): Adjust the pH to 3.2 according to step (2) of Example 1 to precipitate Fe. 3+ The test revealed that Al 3+ Coprecipitation loss was approximately 8%, and the Al in the filtrate was... 3+ The aluminum source recovery rate decreased to 17.0 g / L.

[0053] Comparative Examples 2-3. Al 3+ Precipitation pH=3.0 (below 3.4): Adjust the pH to 3.0 according to step (3) of Example 1 to precipitate Al. 3+ The test revealed that Al 3+ Precipitation was incomplete (2.3 g / L remaining), with a precipitation rate of only 87.5%.

[0054] Comparative Examples 2-4. Al 3+ Precipitation pH=5.5 (above 5.0): Adjust the pH to 5.5 according to step (3) of Example 1 to precipitate Al. 3+ The test revealed that some Fe 2+ Co-precipitate (approximately 15%) enters the aluminum hydroxide filter cake, resulting in iron loss.

[0055] Comparative Examples 2-5. LDH fixed bed pH > 9.0: Following step (4) of Example 1, increase the LDH loading to 150 kg / m³. 3 The pH of the effluent reached 9.3. Detection revealed that some Mg... 2+ Precipitation begins (Mg in the filtrate) 2+ (Decreased to 0.5 g / L), magnesium source loss.

[0056] Conclusion: Deviations in pH range from the scope of the claims in each step can lead to decreased separation efficiency, elemental loss, or reduced product purity.

[0057] Comparative Example 3 (without a fixed bed, LDH used in a suspended state): Following step (4) of Example 1, LDH particles were directly added to the filtrate after aluminum removal and stirred to suspend them. The pH was adjusted to 8.5-9.0 to precipitate Fe. 2+ After pressure filtration, it was found that the surface of LDH particles was severely coated with Fe(OH)2 colloid. After sieving and separation, the LDH recovery rate was only 65%, and the activity decreased to 70% of the original after washing. After three consecutive batches of use, the LDH was basically ineffective.

[0058] Conclusion: Suspended LDH and Fe(OH)2 cannot be effectively separated, and the material cannot be recycled.

[0059] Comparative Example 4 (without using a magnesium source for splitting, all filtrate was used to prepare LDH): Following the procedure in Example 1, step (6) was not split, and all magnesium-containing filtrate was used for LDH preparation in step (9). Steps (2) and (3) lacked a source of Mg(OH)2, so purchased NaOH was used to adjust the pH. After running three batches, it was found that: Purchasing NaOH externally increases costs by approximately 35%; Steps (2) and (3) show a decrease in pH control accuracy (fluctuation ±0.3). The system's magnesium recycling rate dropped to 40%. Conclusion: Magnesium source diversion is the key to achieving process closed-loop; using it entirely for LDH preparation will lead to a lack of pH-regulating alkali source.

[0060] Comparative Example 5 (without mother liquor recycling): Following the process in Example 1, the mother liquor was directly discharged in step (9), and fresh Na2CO3 was purchased externally in step (5). After running 5 batches, it was found that: Purchasing Na2CO3 externally increases costs by approximately 25%; Wastewater discharge increased by 2.5 L / batch; Na + The accumulation caused a slight increase in the viscosity of the filtrate; Conclusion: Mother liquor recycling can significantly reduce operating costs and wastewater discharge.

[0061] Comparative Example 6 (Traditional fly ash acid leaching solution treatment method): A method for separating iron, silicon, gallium, aluminum, and calcium from fly ash, proposed in Chinese patent CN116790887A, involves reducing Fe in an acid leaching solution with a reducing agent. 3+ → Gallium extraction → pH adjustment for aluminum precipitation → Iron removal through oxidation → Sodium sulfate precipitation for calcium precipitation → Concentration and crystallization to obtain sodium nitrate. The results compared to this process are as follows: Table 6. Comparison of the process of this invention with the traditional process Conclusion: This invention achieves the full resource utilization of Fe, Al, Ca, and Mg in fly ash, with a closed-loop process, low emissions, and high product added value.

[0062] Comparative Example 7 (Comparison of Ferric Sulfate Performance): According to the test method of national standard GB / T 14591-2016, the flocculation performance of the product of Example 1 was compared with that of commercially available ferric sulfate. The results are as follows: Table 7. Performance Comparison of the Invention with Commercially Available Ferric Sulfate Flocculants Data shows that the flocculation performance of the product of this invention is superior to that of commercially available products and is comparable to that of the F-PFS flocculant reported in the literature (COD removal rate 84.99%, turbidity removal rate 89.7%).

[0063] Based on all the above embodiments and comparative examples, the mechanism of the present invention can be analyzed as follows: 1. The chemical principle of fractional precipitation Fe 3+ Precipitation (pH 2.7-3.0): According to the solubility product principle, the Ksp of Fe(OH)3 is 2.8 × 10⁻⁶. -39 Calculations yielded Fe 3+Complete precipitation (concentration <10) -5 The theoretical pH for Fe (mol / L) is 3.2. This invention controls the pH to 2.7-3.0 to allow Fe... 3+ Precipitation rate > 99.5%, while Al 3+ (Ksp=3×10) -34 (The precipitation was complete at pH ≈ 5.0) and Fe 2+ (Ksp=8×10) -16 No precipitation occurs even at pH ≈ 9.0, achieving selective separation.

[0064] Al 3+ Precipitation (pH 3.4-5.0): Al 3+ At around pH 4.5, it hydrolyzes to form amorphous Al(OH)3, at which point Al... 3+ The concentration has dropped to 10 -5 Below mol / L, while Ca 2+ Mg 2+ The hydroxide precipitates all had pH values ​​> 10.5, Fe 2+ The precipitation occurs at a pH > 8.5, therefore Ca... 2+ Mg 2+ Fe 2+ All remain in solution. Al(OH)3 at pH 4.5 exists as an acidic flocculent with a positively charged surface, exhibiting high reactivity, which is beneficial for subsequent LDH synthesis.

[0065] Fe 2+ Precipitation (pH 8.5-9.0): Fe 2+ The theoretical pH for complete precipitation is 8.9-9.0. This invention utilizes an LDH fixed bed to automatically stabilize the pH within this range, for Fe... 2+ Precipitation rate > 99%, while Ca 2+ Mg 2+ No precipitation occurs at this pH, achieving selective separation.

[0066] Ca 2+ Precipitation (pH 9.0 + CO3) 2- ): Ca 2+ Hydroxide precipitation requires pH > 11, but the addition of CO3... 2- CaCO3 (Ksp = 3.4 × 10⁻⁶) is then generated. -9 At around pH 9.0, it can completely precipitate, while Mg... 2+ Under these conditions, no precipitation occurs, thus achieving Ca / Mg separation.

[0067] 2. pH regulation mechanism of LDH fixed bed The isoelectric point (pH pzc) of Mg-Al LDH is approximately 9.7-10, and its pH buffering mechanism includes: (1) Surface hydroxyl protonation / deprotonation equilibrium: ≡Mg-OH+H + ⇌≡Mg-OH2 + (pH < pHpzc); ≡Al-OH+OH - ⇌≡Al-O - +H2O (pH > pHpzc); (2) Interlayer anion exchange: When the solution pH is below the isoelectric point, the CO3 interlayer of the LDH layer... 2- It can exchange with anions in solution and simultaneously bind H+. + : [LDH]·CO3 2- +2H + →[LDH]+H2CO3; (3) Slow-release OH - characteristic: Deprotonation of hydroxyl groups on the LDH surface is diffusion-controlled; OH - The release rate is gradual, avoiding localized over-alkaliness. In the fixed-bed configuration, the solution flows continuously through the LDH bed, and the above reactions reach dynamic equilibrium, with the outlet pH automatically stabilizing at 8.5-9.0, with fluctuations within ±0.1.

[0068] 3. Mechanism for the preparation of LDH by reacting acidic flocculent Al(OH)3 with magnesium-containing filtrate Acidic flocculent Al(OH)3 with pH 4.5 carries a positive charge on its surface and exhibits high reactivity -10; magnesium-containing filtrate has a pH of approximately 9-10 and is alkaline. When the two are mixed, the following occurs: (1) Acid-base neutralization: H on the surface of Al(OH)3 + by OH - Neutralization, promoting Al 3+ Hydrolysis rearrangement; (2) Coprecipitation reaction: Mg 2+ With activated Al 3+ In OH - Under the action of action, Mg-Al LDH layers are formed: (1-x)Mg 2+ +xAl 3+ +2OH - +CO3 2- →[Mg 1-x Al x (OH)2] x+ (CO3 2- ) x / 2 ·mH2O; (3) No external acid or base is required: The reaction is driven by the acidity or alkalinity of the system itself, achieving “zero consumption” preparation.

[0069] 4. Material balance of magnesium source branch circulation Let the total amount of magnesium in the system be M, and the circulation path be: Route A (20-40%): Mg 2+ → Mg(OH)2 → Return to steps (2) and (3) for pH adjustment; Route B (60-80%): Mg 2+ + Al(OH)3 → LDH → Return to step (4) fixed bed; Material balance equation: M input + M recycle = M A + M B + M loss ; In the formula M input Magnesium introduced during acid leaching of fly ash, M loss This represents a system loss (approximately 10-15%). In Example 1, M... A :M B When the ratio is 33:67, the system magnesium circulation rate reaches 93%, and only a small amount of loss needs to be replenished for long-term operation.

[0070] 5. Mechanism of ferric sulfate formation Based on the principle of ferric sulfate preparation: (1) Oxidation reaction: 2FeSO4+NaClO+H2SO4→Fe2(SO4)3+NaCl+H2O; (2) Hydrolysis-polymerization: Fe 3+ Gradual hydrolysis in water: Fe 3+ +H₂O⇌FeOH 2+ +H + ; FeOH 2+ +H₂O⇌Fe(OH)₂ + +H + ; Simultaneously, a polymerization reaction occurs, forming a polynuclear hydroxyl complex: 2FeOH 2+ ⇌Fe2(OH)2 4+ ; Fe2(OH)2 4+ +Fe 3+ ⇌Fe3(OH)4 5+ ; The final product is ferric sulfate molecules with a long-chain structure: [Fe2(OH)] n (SO4) 3-n / 2 ] m , where n is the degree of basicity and m is the degree of polymerization.

[0071] Based on the data from the examples and comparative examples, the following conclusions can be drawn regarding key process parameters: Table 8. Key process parameters of the present invention The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A ferric sulfate flocculant based on fly ash, characterized in that, After acid hydrolysis of fly ash, Fe(OH)3 and Fe(OH)2 are obtained by adjusting the pH and removing impurities. These are then acidified with sulfuric acid and oxidized to obtain ferric sulfate flocculant. The ferric sulfate flocculant is a reddish-brown liquid product with an iron content primarily of Fe. 3+ The content of the salt is ≥9.0%, the basicity is 8.0-16.0%, the pH of the aqueous solution prepared at 1% is 2.0-3.0, and the density is ≥1.35 g / cm³. 3 .

2. The method for preparing a ferric sulfate flocculant based on fly ash as described in claim 1, characterized in that, Includes the following steps: (1) Acid leaching treatment of fly ash: Fly ash is reacted with hydrochloric acid to obtain a solution containing Fe. 3+ Fe 2+ Al 3+ Ca 2+ Mg 2+ Acid leaching solution; (2) Fe 3+ Precipitation separation: Add magnesium hydroxide to the acid leaching solution to adjust the pH to 2.7-3.0, precipitating Fe. 3+ The product is Fe(OH)3. Solid-liquid separation yields iron-enriched filter cake A and iron-removed filtrate. (3) Al 3+ Precipitation separation: Add magnesium hydroxide to the filtrate after iron removal to adjust the pH to 3.4-5.0, precipitating Al. 3+ It is acidic flocculent aluminum hydroxide. Solid-liquid separation yields aluminum hydroxide flocculent filter cake and aluminum-removed filtrate. (4) Fine-tuning of LDH fixed bed and Fe 2+ Precipitation: The filtrate after aluminum removal is passed through a fixed bed packed with magnesium aluminum hydrotalcite particles. The pH of the solution is automatically adjusted to 8.5-9.0 using the pH buffering properties of the hydrotalcite, precipitating Fe. 2+ The effluent was Fe(OH)2. Solid-liquid separation yielded an iron-enriched filter cake B and a Ca-containing filter cake. 2+ Mg 2+ filtrate; (5) Preparation of ferric sulfate flocculant: Iron enriched filter cake A and iron enriched filter cake B are combined, dissolved, oxidized and polymerized with dilute sulfuric acid to obtain liquid ferric sulfate flocculant product.

3. The method for preparing a ferric sulfate flocculant based on fly ash according to claim 2, characterized in that, In (3), the pH of the acidic flocculent aluminum hydroxide is 4.5 ± 0.

3.

4. The method for preparing a ferric sulfate flocculant based on fly ash according to claim 2, characterized in that, In (4), the loading amount of magnesium aluminum hydrotalcite particles in the fixed bed is 50-100 kg / m³. 3 The reactor volume is such that the empty bed residence time of the solution flowing through the fixed bed is 30-60 minutes.

5. The method for preparing a ferric sulfate flocculant based on fly ash according to claim 2, characterized in that, In step (5), sodium hypochlorite, hydrogen peroxide or air are used as oxidants in the oxidation process. The basicity of the polymerization reaction is controlled at 0.3-0.8, the polymerization temperature is 50-80℃, and the polymerization time is 1-3 hours.

6. The method for preparing a ferric sulfate flocculant based on fly ash according to claim 2, characterized in that, It also includes the following steps: (6) Ca 2+ Precipitation separation: towards Ca-containing 2+ Mg 2+ Sodium carbonate is added to the filtrate to precipitate Ca. 2+ The solid-liquid mixture is CaCO3, and calcium carbonate filter cake and magnesium-containing filtrate are obtained through solid-liquid separation.

7. The method for preparing a ferric sulfate flocculant based on fly ash according to claim 6, characterized in that, It also includes the following steps: (7) Magnesium source diversion: The magnesium-containing filtrate is divided into A and B, with A accounting for 20-40% of the total magnesium-containing filtrate and B accounting for 60-80%; (8) Preparation of magnesium hydroxide: Take the magnesium-containing filtrate from route A, adjust the pH to 10.5-11.0, and precipitate Mg. 2+ For Mg(OH)2, return to (2) and (3).

8. The method for preparing a ferric sulfate flocculant based on fly ash according to claim 7, characterized in that, It also includes the following steps: (9) Preparation of magnesium aluminum hydrotalcite particles: Take magnesium-containing filtrate from route B and aluminum hydroxide flocculent filter cake obtained in (3), adjust the Mg / Al molar ratio to 2:1-4:1, heat and stir to react, separate solid and liquid, wash, dry and granulate to obtain magnesium aluminum hydrotalcite particles, and return to (4) fixed bed for use.

9. The method for preparing a ferric sulfate flocculant based on fly ash according to claim 8, characterized in that, In the preparation of magnesium aluminum hydrotalcite particles (9), the Mg / Al molar ratio is 3:1, the reaction temperature is 60-80℃, and the reaction time is 4-8 hours; during the granulation process of (9), bentonite accounting for 10-15% of the total weight of the system is added as a binder, and the particle size is 3-8mm.

10. A method for preparing a ferric sulfate flocculant based on fly ash according to claim 8, characterized in that, It also includes the following steps: (10) Mother liquor recycling: The mother liquor generated in (9) is passed into CO2 to convert Na2CO3 into NaHCO3 and returned to (5) as an auxiliary precipitant.

Images