Method and system for cooperatively treating fluorine-containing wastewater by using steel slag

Abstract

The invention relates to a method and system for cooperatively treating fluorine-containing wastewater by using steel slag, and the method comprises the following steps: adding a fluorine removal agent into the fluorine-containing wastewater, and carrying out preliminary coagulation to form a first fluorine-containing mixed solution; adding steel slag into the first fluorine-containing mixed solution, and stirring to form a second fluorine-containing mixed solution; adding a flocculating agent into the second fluorine-containing mixed solution, and carrying out flocculation treatment; and precipitating the mixed solution subjected to flocculation treatment to realize mud-water separation. By means of the efficient adsorption performance of the steel slag on fluoride and the chemical precipitation effect of active ingredients such as calcium and magnesium contained in the steel slag and fluorine ions, the treatment effect of a conventional fluorine removal technology is doubly enhanced. And moreover, the steel slag is recycled to replace part of the defluorination agent, so that the agent consumption cost is remarkably reduced, and the total amount of solid waste which finally needs to be treated is reduced as the steel slag participates in the treatment process as solid waste, so that the circular economy concept of treating waste with waste is really realized.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, and in particular to a method and system for co-treating fluoride-containing wastewater using steel slag. Background Technology

[0002] The hazards of fluoride-containing wastewater cannot be ignored. Excessive fluoride in water not only disrupts the balance of aquatic ecosystems, inhibiting or even killing plant and animal growth, but also harms human health through drinking water and the food chain. Long-term intake of excessive fluoride can easily lead to dental fluorosis, manifested as tooth discoloration and enamel defects; in severe cases, it can also cause skeletal fluorosis, resulting in bone pain, deformities, and even paralysis, causing irreversible damage to bodily functions. Therefore, the effective treatment of fluoride-containing wastewater is crucial.

[0003] Currently, common methods for treating fluoride-containing wastewater mainly include adsorption, chemical precipitation, ion exchange, and membrane separation. While each of these methods has its applications, they all have certain limitations.

[0004] Adsorption method: It is simple to operate and relatively low in cost, but it generally suffers from problems such as limited adsorption capacity of adsorbent, difficulty in regeneration after saturation, and the easy formation of secondary solid waste from waste adsorbent.

[0005] Chemical precipitation is a widely used defluorination technology in industry, with a large processing capacity. However, it requires the addition of large amounts of chemical agents such as calcium and aluminum salts, which not only produces a large amount of difficult-to-treat fluoride-containing sludge, but is also limited by chemical equilibrium, making it difficult to stably meet the strict discharge standards for effluent fluoride concentration.

[0006] Ion exchange method: It has a good effect on treating low-concentration fluoride wastewater, but ion exchange resins are easily contaminated by other anions in the water and become ineffective. The regeneration process is frequent, which requires a large amount of acid and alkali reagents, resulting in high operating costs and the potential for secondary pollution.

[0007] Membrane separation methods (such as nanofiltration and reverse osmosis) have high separation efficiency and good effluent quality, but membrane modules are expensive, have high requirements for feed water quality, are prone to membrane fouling and clogging, and have high maintenance and replacement costs, which restricts their application in large-scale industrial wastewater treatment.

[0008] Therefore, although these methods each have their own application scenarios, they all have certain limitations. Thus, developing efficient, low-cost, and environmentally friendly defluoridation technologies remains an important research direction in the current water treatment field. Summary of the Invention

[0009] The purpose of this invention is to provide a method and system for co-treating fluoride-containing wastewater using steel slag, which features high defluorination efficiency, low operating cost, and low environmental impact, effectively overcoming the limitations of traditional defluorination technologies where cost and effectiveness are difficult to balance.

[0010] To achieve the above objectives, the present invention provides a method for co-treating fluoride-containing wastewater using steel slag, comprising the following steps:

[0011] A defluorinating agent is added to the fluoride-containing wastewater for preliminary coagulation, forming the first fluoride-containing mixed liquid.

[0012] Steel slag is added to the first fluorine-containing mixture and stirred to form a second fluorine-containing mixture;

[0013] Add flocculant to the second fluorinated mixture for flocculation treatment;

[0014] The mixture after flocculation treatment is then precipitated to achieve mud-water separation.

[0015] Optionally, prior to the initial coagulation, the method further includes:

[0016] The pH value of the fluoride-containing wastewater is adjusted to 6-10.

[0017] Optionally, the defluorinating agent includes aluminum sulfate, which is added in solution form, and the mass concentration of aluminum sulfate in the solution is not less than 6%.

[0018] Optionally, the mass content of calcium oxide in the steel slag is not less than 30%.

[0019] Optionally, in the step of adding steel slag to the first fluorine-containing mixture and stirring, the stirring time is 5-20 minutes.

[0020] Optionally, the flocculant is polyacrylamide, which is added in the form of a solution with a mass concentration of 0.2%-0.4%.

[0021] Optionally, in the flocculation treatment step, the stirring time is 3-5 minutes.

[0022] Optionally, in the precipitation step, the settling time is 15-30 minutes.

[0023] Based on the same inventive concept, this invention also provides a system for co-treating fluoride-containing wastewater using steel slag, comprising sequentially connected components:

[0024] The first reaction tank is used to receive fluoride-containing wastewater and add defluorinating agents for preliminary coagulation.

[0025] The second reaction tank is used to receive the first fluorine-containing mixture and add steel slag;

[0026] The third reaction tank is used to receive the second fluorine-containing mixture and add flocculants.

[0027] A sedimentation tank is used to settle the mixture after the flocculation treatment to achieve mud-water separation.

[0028] Optionally, the first reaction tank, the second reaction tank, and the third reaction tank are all equipped with a stirring device.

[0029] The method and system for co-treating fluoride-containing wastewater using steel slag provided by this invention have at least one of the following beneficial effects:

[0030] (1) Synergistic effect to improve defluorination efficiency: By leveraging the high adsorption performance of steel slag for fluorides and the chemical precipitation effect of active components such as calcium and magnesium in steel slag with fluoride ions, the treatment effect of conventional defluorination technology is enhanced in two ways. The combination of steel slag and flocs generated in the conventional defluorination process can increase the weight and density of flocs, effectively solving the problems of poor flocculation effect and light and difficult-to-settle flocs when using coagulants such as aluminum sulfate alone. The defluorination efficiency is improved by 12-25% compared with the traditional single aluminum sulfate coagulation process.

[0031] (2) "Treating waste with waste" to reduce treatment costs: By introducing steel slag into the defluorination process, not only can its adsorption and precipitation efficiency for fluorides be fully utilized to improve defluorination efficiency, but the total amount of solid waste to be disposed of can also be reduced by more than 20%, fundamentally solving the prominent problem of large solid waste production in traditional defluorination processes. At the same time, the resource utilization of steel slag can also reduce the cost of treating fluoride-containing wastewater. This defluorination process can not only directly replace the calcium chloride reagent used in the original process, but also reduce the amount of aluminum sulfate added by 20-40%, which can significantly reduce the operating burden of enterprises while achieving a synergistic improvement in environmental and economic benefits.

[0032] (3) Simple process and easy to implement: The process of this invention is clear and easy to operate. It does not require complex and expensive equipment (such as membrane modules and ion exchange columns). The main facilities are reaction tank and sedimentation tank. It is suitable for the renovation or construction of existing sewage treatment plants and is easy to promote and apply in engineering. Attached Figure Description

[0033] Those skilled in the art will understand that the accompanying drawings are provided to better understand the invention and do not constitute any limitation on the scope of the invention. Wherein:

[0034] Figure 1 A flowchart of a method for co-treating fluoride-containing wastewater using steel slag according to an embodiment of the present invention;

[0035] Figure 2 This is a system block diagram of a method for co-treating fluoride-containing wastewater using steel slag, provided in an embodiment of the present invention.

[0036] The attached figures are labeled as follows:

[0037] 100 - First reaction tank; 200 - Second reaction tank; 300 - Third reaction tank; 400 - Sedimentation tank. Detailed Implementation

[0038] As described in the background section, existing methods for treating fluoride-containing wastewater (such as adsorption, chemical precipitation, ion exchange, and membrane separation) each have their own applications, but all have certain limitations. On the other hand, steel slag, as a solid waste, not only occupies land resources when it is piled up in large quantities, but also easily causes environmental pollution. Its resource utilization has always been a focus of industry attention.

[0039] Therefore, if steel slag can be applied to the treatment of fluoride-containing wastewater to achieve synergistic defluorination, it can not only solve the problem of steel slag disposal and reduce solid waste pollution, but also reduce the treatment cost of fluoride-containing wastewater, thus achieving both environmental and economic benefits.

[0040] Based on this, the present invention proposes an innovative approach of "treating waste with waste", which introduces steel slag as a functional material into the treatment process of fluoride-containing wastewater, aiming to develop a new method for synergistic fluoride removal that is efficient, low-cost, and environmentally friendly.

[0041] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0042] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0043] In the description of this invention, it should be understood that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used to facilitate the description of this invention and to simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0044] Furthermore, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the article or apparatus that includes said element. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0045] Please refer to Figure 1 This invention provides a method for co-treating fluoride-containing wastewater using steel slag, comprising the following steps:

[0046] S100. Add a defluorinating agent to the fluoride-containing wastewater for preliminary coagulation to form the first fluoride-containing mixed liquid;

[0047] S200. Add steel slag to the first fluorine-containing mixture and stir to form the second fluorine-containing mixture.

[0048] S300. Add flocculant to the second fluorinated mixture for flocculation treatment;

[0049] S400: The flocculated mixture is precipitated to achieve mud-water separation.

[0050] First, in step S100, a defluorinating agent is added to the fluoride-containing wastewater to be treated. The agent is stirred to ensure thorough mixing with the wastewater and to initiate a preliminary coagulation reaction. During this process, the flocs produced by the hydrolysis of the defluorinating agent initially remove some fluorides through adsorption and charge neutralization, forming a first fluoride-containing mixture containing the initial flocs.

[0051] In a preferred embodiment, a pH adjustment pretreatment step is included before the initial coagulation step: adjusting the pH of the fluoride-containing wastewater to the range of 6-10. This pH range is crucial to ensure efficient subsequent coagulation and co-precipitation reactions. If the pH is too low, the hydrolysis products of the defluorinating agent will have poor forms, affecting coagulation; if the pH is too high, it may inhibit certain precipitation reactions, and the dissolution efficiency of the active components of the steel slag may change. Common acids (such as hydrochloric acid, sulfuric acid) or alkalis (such as sodium hydroxide, lime slurry) can be used to adjust the pH. It is understood that if the raw water pH is already within this ideal range, this adjustment step can be omitted.

[0052] Regarding the defluorinating agent, aluminum sulfate is preferred in this embodiment due to its advantages of good coagulation effect, wide availability, and low cost. For ease of addition and rapid mixing, aluminum sulfate is usually prepared as a solution. Experiments have shown that when the mass concentration of aluminum sulfate in the solution is not less than 6%, it ensures effective coagulation and defluorination while also facilitating agent storage and metered dosing. Too low a concentration may result in an excessively large volume of liquid added, affecting treatment efficiency; too high a concentration may affect usability due to excessive viscosity or low-temperature crystallization. Of course, in addition to aluminum sulfate, other aluminum salts (such as polyaluminum chloride, aluminum chloride) or iron salts (such as ferric chloride, polyferric sulfate) can also be used as alternative defluorinating agents under suitable pH conditions.

[0053] Next, in step S200, steel slag of a specific specification is added to the first fluoride-containing mixture obtained above, and the mixture is thoroughly stirred. This is a key step in this invention. The addition of steel slag not only utilizes its porous structure to physically adsorb fluoride ions and the initially formed flocs, but also utilizes its rich content of active calcium, magnesium, and other components (existing in the form of calcium oxide, etc.) to react chemically with fluoride ions, generating stable precipitates such as calcium fluoride. At the same time, the steel slag particles, acting as "nuclei" or "weighting agents," can combine with the fine flocs in the water to form larger and denser co-precipitated flocs. After this step, a second fluoride-containing mixture containing steel slag and reinforced flocs is formed.

[0054] Preferably, the calcium oxide content in the steel slag is not less than 30% by mass. A higher calcium oxide content means the steel slag has higher alkalinity and more active calcium sources. This not only allows it to more effectively react with fluoride ions to form calcium fluoride precipitate, but also continuously provides hydroxide ions during the reaction, helping to maintain a suitable alkalinity in the reaction system, thereby enhancing the defluorination effect. Before use, the steel slag can be crushed and screened to a suitable particle size to increase its specific surface area and improve its reactivity.

[0055] In the co-treatment of fluoride with steel slag, the duration of agitation significantly impacts the reaction efficiency. The optimal agitation time is 5-20 minutes. If the time is less than 5 minutes, the contact between the steel slag and wastewater is insufficient, resulting in incomplete adsorption and precipitation reactions and failing to fully realize their synergistic effect. If the agitation time exceeds 20 minutes, although the reaction may be more thorough, it prolongs the overall hydraulic retention time, increasing energy consumption without significantly improving treatment efficiency, making it unnecessary from an economic perspective. Therefore, 5-20 minutes represents an optimized range that balances treatment effectiveness and operating costs.

[0056] Then, execute S300, add flocculant to the second fluorinated mixture, and gently stir. The long molecular chains of the flocculant can quickly bridge and trap the fine flocs, steel slag particles, and newly formed precipitates dispersed in the water, agglomerating them into large, dense flocs that are visible to the naked eye. This process is called flocculation treatment.

[0057] Regarding the flocculant, PAM (polyacrylamide) is preferably used in this embodiment. As a highly efficient polymeric flocculant, PAM can significantly accelerate the formation and growth of flocs. It is typically prepared as a dilute solution and added, with a preferred mass concentration of 0.2%-0.4%. This concentration range ensures that the flocculant molecules fully extend and exert their bridging effect, while avoiding problems such as uneven mixing, incomplete encapsulation, or incomplete dissolution caused by an excessively concentrated solution. PAM can be non-ionic, cationic, or anionic, and the optimal type can be selected based on the water quality characteristics. Anionic PAM generally performs well in treating fluoride-containing wastewater.

[0058] In the flocculation process, the intensity and duration of agitation must be strictly controlled. The optimal agitation time is 3-5 minutes. The purpose of agitation at this stage is primarily to ensure the flocculant is rapidly and evenly dispersed throughout the system and to achieve sufficient contact with the suspended particles. Excessive agitation or prolonged agitation will break down the already formed, fragile flocs, hindering sedimentation; insufficient agitation or too short a time will result in uneven mixing of the flocculant and inadequate bridging. Therefore, gentle agitation for 3-5 minutes is the best choice.

[0059] Finally, S400 is executed, introducing the flocculated mixture into a sedimentation facility for settling. Under gravity, the large flocs settle rapidly, achieving efficient separation of sludge and water. The supernatant is the treated effluent, and the bottom sediment is fluoride-containing sludge.

[0060] Sufficient settling time is crucial in the sedimentation process. The optimal settling time is 15-30 minutes. Within this time, the large, dense flocs formed by flocculation can settle fully under gravity, ensuring clear effluent. Less than 15 minutes may result in some small flocs not settling completely, affecting effluent quality; while more than 30 minutes, although settling is more thorough, the improvement in effluent quality is not significant, and it greatly increases the footprint of the sedimentation tank or the treatment cycle, making it uneconomical.

[0061] Based on the same inventive concept, this invention also proposes a system for co-treating fluoride-containing wastewater using steel slag, for implementing the above-described method for co-treating fluoride-containing wastewater using steel slag, such as... Figure 2 As shown, the system comprises four main units connected sequentially along the treatment flow to form a fluid channel: a first reaction tank 100, a second reaction tank 200, a third reaction tank 300, and a sedimentation tank 400. The first reaction tank 100 receives raw water and adds a defluorinating agent to complete preliminary coagulation; its effluent flows to the second reaction tank 200, where steel slag is added and undergoes a synergistic reaction; the effluent from the second reaction tank 200 enters the third reaction tank 300, where a flocculant is added to complete flocculation; the effluent from the third reaction tank 300 finally enters the sedimentation tank 400 to achieve mud-water separation. The tanks are connected by pipes, channels, or overflow weirs to ensure smooth water flow.

[0062] To enhance the mixing and reaction effects within each unit, stirring devices are preferably installed in the first reaction tank 100, the second reaction tank 200, and the third reaction tank 300. These stirring devices can be mechanical stirrers, air stirring systems, or other equivalent mixing equipment, and their stirring intensity can be independently controlled to meet the mixing intensity requirements of different stages such as coagulation, synergistic reaction, and flocculation.

[0063] Since the system for co-treating fluoride-containing wastewater using steel slag provided by this invention belongs to the same inventive concept as the method for co-treating fluoride-containing wastewater using steel slag described above, the system for co-treating fluoride-containing wastewater using steel slag provided by this invention has all the advantages of the method for co-treating fluoride-containing wastewater using steel slag described above. Therefore, the beneficial effects of the system for co-treating fluoride-containing wastewater using steel slag provided by this invention will not be described in detail here.

[0064] The following example illustrates the method for co-treating fluoride-containing wastewater using steel slag, as provided in this embodiment of the invention.

[0065] The water quality characteristics of the fluoride-containing wastewater are: pH approximately 7.3, and fluoride concentration approximately 61.98 mg / L.

[0066] First, a defluorinating agent is used for preliminary defluorination. Since the pH of the raw water is within the required range, there is no need to adjust the pH. The defluorinating agent is added to coagulate and precipitate the fluorides in the fluoride-containing wastewater, and the mixture is stirred to produce the first fluoride-containing mixed solution.

[0067] The defluorinating agent used is an aluminum sulfate solution with a concentration of 6.3-6.58%, a dosage of 500-800 ppm, and a stirring time of approximately 5 minutes.

[0068] Then, steel slag is added to the first fluorinated mixture, and the mixture is stirred to obtain a second fluorinated mixture. The main component of the steel slag is calcium oxide, with a calcium oxide content of approximately 43%, and the dosage is 500-1500 ppm. The stirring time is 15 minutes.

[0069] Next, PAM was added to the second fluorinated mixture, and the mixture was stirred for 3 minutes to induce flocculation and precipitation.

[0070] Finally, after standing and settling for 20 minutes, mud and water are separated.

[0071] After the above process, the fluoride content in the effluent is less than 10 mg / L, with a removal rate of up to 90%, meeting the defluorination requirements.

[0072] In summary, this invention provides a method and system for co-treating fluoride-containing wastewater using steel slag. By leveraging the high adsorption capacity of steel slag for fluorides and the chemical precipitation effect of active components such as calcium and magnesium in the steel slag with fluoride ions, the treatment effect of conventional defluorination technologies is enhanced. This method utilizes steel slag resources to replace part of the defluorination agents, significantly reducing agent consumption costs. Furthermore, because the steel slag itself participates in the treatment process as solid waste, the total amount of solid waste requiring final disposal is reduced, truly realizing the circular economy concept of "treating waste with waste." This method combines high defluorination efficiency, low operating costs, and low environmental impact, effectively overcoming the limitations of traditional defluorination technologies where cost and effectiveness are difficult to balance.

[0073] The above description is merely a description of preferred embodiments of the present invention and is not intended to limit the scope of the invention in any way. Any changes or modifications made by those skilled in the art based on the above disclosure are within the protection scope of the present invention. Obviously, those skilled in the art can make various modifications and variations to the present invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the present invention and its equivalents, the present invention also intends to include these modifications and variations.

Claims

1. A method for co-treating fluoride-containing wastewater using steel slag, characterized in that, Includes the following steps: A defluorinating agent is added to the fluoride-containing wastewater for preliminary coagulation, forming the first fluoride-containing mixed liquid. Steel slag is added to the first fluorine-containing mixture and stirred to form a second fluorine-containing mixture; Add flocculant to the second fluorinated mixture for flocculation treatment; The mixture after flocculation treatment is then precipitated to achieve mud-water separation.

2. The method for co-treating fluoride-containing wastewater using steel slag according to claim 1, characterized in that, Prior to the initial coagulation, the method further includes: The pH value of the fluoride-containing wastewater is adjusted to 6-10.

3. The method for co-treating fluoride-containing wastewater using steel slag according to claim 1 or 2, characterized in that, The defluorinating agent includes aluminum sulfate, which is added in solution form, and the mass concentration of aluminum sulfate in the solution is not less than 6%.

4. The method for co-treating fluoride-containing wastewater using steel slag according to claim 1, characterized in that, The mass content of calcium oxide in the steel slag is not less than 30%.

5. The method for co-treating fluoride-containing wastewater using steel slag according to claim 1, characterized in that, In the step of adding steel slag to the first fluorine-containing mixture and stirring, the stirring time is 5-20 minutes.

6. The method for co-treating fluoride-containing wastewater using steel slag according to claim 1, characterized in that, The flocculant is polyacrylamide, which is added in the form of a solution with a mass concentration of 0.2%-0.4%.

7. The method for co-treating fluoride-containing wastewater using steel slag according to claim 1 or 6, characterized in that, In the flocculation treatment step, the stirring time is 3-5 minutes.

8. The method for co-treating fluoride-containing wastewater using steel slag according to claim 1, characterized in that, In the precipitation step, the settling time is 15-30 minutes.

9. A system for co-treating fluoride-containing wastewater using steel slag, for implementing the method for co-treating fluoride-containing wastewater using steel slag according to any one of claims 1-8, characterized in that, Including sequential connections: The first reaction tank is used to receive fluoride-containing wastewater and add defluorinating agents for preliminary coagulation. The second reaction tank is used to receive the first fluorine-containing mixture and add steel slag; The third reaction tank is used to receive the second fluorine-containing mixture and add flocculants. A sedimentation tank is used to settle the mixture after the flocculation treatment to achieve mud-water separation.

10. The system for co-treating fluoride-containing wastewater using steel slag according to claim 9, characterized in that, The first reaction tank, the second reaction tank, and the third reaction tank are all equipped with stirring devices.

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