Method for treating wastewater

Abstract

The waste liquid treatment method has: an aluminum dissolving step of dissolving aluminum in an acidic waste liquid to separate into a first treated water and a heavy metal reduction precipitate; a gypsum recovery step of adding a calcium compound to the first treated water at a pH of 4 or less to separate into a second treated water and gypsum; an aluminum and fluorine removal step of adding an alkali to the second treated water to separate into a third treated water and a precipitate containing aluminum and fluorine; and a neutralization step of adding an alkali to the third treated water to separate into an alkali neutralized treated water and a neutralization precipitate of a heavy metal hydroxide.

Description

Technical Field

[0001] This invention relates to a wastewater treatment method that recovers gypsum with low fluoride content from acidic wastewater (e.g., wastewater from a non-ferrous metal smelter) containing heavy metals such as copper, arsenic, and zinc in addition to sulfuric acid, fluoride, and chlorine, and then removes the heavy metals from the residual liquid at low cost.

[0002] This application claims priority based on Japanese Patent Application No. 2017-250885, filed on December 27, 2017, the contents of which are incorporated herein by reference. Background Technology

[0003] Wastewater from non-ferrous metal smelters contains not only sulfuric acid, fluorine, and chlorine, but also large amounts of heavy metals such as copper, arsenic, and zinc. When this wastewater is discharged outside the system, these heavy metals must be thoroughly removed to comply with discharge regulations. Furthermore, because this wastewater is typically strongly acidic and contains sulfate ions, it is usually neutralized by adding calcium compounds. However, it is hoped that the gypsum produced during this neutralization process can be recovered and reused.

[0004] The following methods are known for treating wastewater from non-ferrous metal smelters.

[0005] (a) A method for treating waste acid, comprising: a primary sulfidation step in which a sulfiding agent is mixed in waste acid produced from copper smelting to sulfidate heavy metals, and the mixture is separated into a primary slurry and a primary clarified liquid containing the obtained sulfidation precipitate; a gypsum manufacturing step in which a neutralizing agent is mixed in the primary clarified liquid to convert sulfuric acid into gypsum, and solid-liquid separation is performed to obtain a final gypsum solution; and a secondary sulfidation step in which a sulfiding agent is mixed in the final gypsum solution to sulfidate heavy metals, and the mixture is separated into a secondary slurry and a secondary clarified liquid containing the obtained sulfidation precipitate, wherein the waste acid treatment method returns the secondary slurry from which the secondary clarified liquid is separated in the secondary sulfidation step to the primary sulfidation step and mixes it with the waste acid (Patent Document 1).

[0006] (b) A method for manufacturing waste acid gypsum, wherein a Ca-containing alkaline agent is added to waste acid obtained from waste gas generated from non-ferrous metal smelting to neutralize the waste acid, and water or sulfuric acid is used to wash away fluorine contained in the gypsum generated in the neutralization treatment (Patent Document 2).

[0007] (c) A method for treating waste sulfuric acid, wherein aluminum is added to fluorine-containing waste sulfuric acid at a concentration of more than 0.5 times the amount of fluorine contained in the waste sulfuric acid, and then neutralized with an alkaline agent to a pH of 5.6 or below (Patent Document 3).

[0008] (d) A wastewater treatment method comprising adding aluminum salt to wastewater containing any one or more of fluorine, selenium or their compounds to form flocs and then performing sedimentation separation, adding a liquid chelating agent to the separated supernatant and reacting it, and adding aluminum salt to the reaction solution to cause solids to coagulate and then performing solid-liquid separation (Patent Document 4).

[0009] Patent Document 1: Japanese Patent No. 6206287

[0010] Patent Document 2: Japanese Patent Application Publication No. 2017-105651

[0011] Patent Document 3: Japanese Patent Publication No. 59-34644

[0012] Patent Document 4: Japanese Patent Application Publication No. 9-192675

[0013] In the treatment method of Patent Document 1, dissolved heavy metals are precipitated and removed as sulfides using a sulfiding agent. However, in cases like wastewater from non-ferrous metal smelting plants, adding a sulfiding agent to a strongly acidic solution generates harmful hydrogen sulfide gas, posing a safety hazard. Furthermore, since the sulfiding agent containing the volatilized hydrogen sulfide component does not contribute to the precipitation and removal of heavy metals, the reaction efficiency is low. Additionally, this treatment method manufactures gypsum in a single sulfidation process, but this sulfidation process cannot remove fluoride from the wastewater, resulting in a significant amount of fluoride contaminating the gypsum.

[0014] Patent Document 2 describes a method for cleaning fluoride-contaminated gypsum with water or sulfuric acid. However, in this embodiment, 50 mL of cleaning solution is required for every 10 g of gypsum, resulting in a large amount of wastewater being discharged. This increase in wastewater is detrimental both environmentally and economically. Furthermore, because the gypsum contains a high amount of fluoride, the fluoride content may not be sufficiently reduced due to missed cleaning, agitation, or inadequate cleaning. To stabilize the cleaning process, the amount of solution added to the gypsum can be increased to reduce the concentration of solid components; however, this still increases the volume of cleaning solution or wastewater, requiring further wastewater treatment of the fluoride-containing cleaning water.

[0015] In the treatment method of Patent Document 3, aluminum is added to fluorine-containing waste sulfuric acid to keep the fluorine in the solution, and calcium compounds are added to generate gypsum, followed by solid-liquid separation. However, a large amount of aluminum, fluorine and heavy metals are dissolved in the filtrate from which the gypsum is separated, and the treatment of aluminum, fluorine and heavy metals in the solution becomes a problem.

[0016] In the treatment method of Patent Document 4, aluminum salt is added to the wastewater to adjust the pH to 6-8, thereby causing aluminum hydroxide to precipitate. Suspended solids (SS) components in the wastewater, namely gypsum and calcium fluoride (CaF2), enter the aluminum hydroxide flocculent, and some fluoride ions are adsorbed onto the aluminum hydroxide and removed. However, since the precipitate becomes a mixture of gypsum and fluoride-containing precipitates, it is difficult to effectively utilize the precipitate as a resource. Furthermore, it cannot adequately remove copper or arsenic contained in the wastewater. Summary of the Invention

[0017] This invention solves the above-mentioned problems in existing treatment methods and provides a treatment method with excellent effect in removing fluorine and heavy metals contained in waste liquid.

[0018] The present invention is a waste liquid treatment method that solves the above-mentioned problems according to the following structure.

[0019] [1] A method for treating waste liquid, comprising recovering gypsum with low fluoride content and removing heavy metals from acidic waste liquid containing fluoride and heavy metals, the method comprising: an aluminum dissolution step, wherein aluminum is dissolved in the acidic waste liquid to convert fluoride in the solution into fluoroaluminate ions for stable dissolution, and heavy metal reduction precipitate is generated, thereby separating the waste liquid into first treated water and the heavy metal reduction precipitate; and a gypsum recovery step, wherein after removing the heavy metal reduction precipitate, a calcium compound is added to the first treated water at a pH below 4 to generate gypsum, thereby separating the gypsum into the first treated water. The process involves: a second treatment of water and the gypsum; an aluminum and fluoride removal step, in which, after removing the gypsum, alkali is added to the second treatment water to adjust the pH to 5.5–9.5 to suppress the amount of precipitate, thereby generating a precipitate containing aluminum and fluoride, which is then separated into a third treatment water and the precipitate containing aluminum and fluoride; and a neutralization step, in which, after removing the precipitate containing aluminum and fluoride, alkali is further added to the third treatment water to adjust the pH to 9.5–11.8 and generate a neutralized precipitate of heavy metal hydroxides, thereby separating into alkali-neutralized treated water and the neutralized precipitate of heavy metal hydroxides.

[0020] [2] According to the waste liquid treatment method described in [1] above, in the aluminum and fluorine removal process, the pH of the second treated water is adjusted to pH 5.5 to 7.0 to suppress the amount of precipitate and to suppress the precipitation of arsenic and zinc, thereby causing fluorine and aluminum to precipitate.

[0021] [3] The waste liquid treatment method according to [1] or [2] above, wherein the acidic waste liquid containing fluorine and heavy metals is wastewater from a non-ferrous metal smelting plant.

[0022] According to the waste liquid treatment method of the present invention, a method for treating waste liquid that can more effectively remove fluorine and heavy metals contained in the waste liquid can be provided. Attached Figure Description

[0023] Figure 1 This is a process diagram illustrating the general outline of the waste liquid treatment method of the present invention.

[0024] Figure 2 This is a graph showing the changes in precipitate formation and aluminum concentration in Example 2.

[0025] Figure 3 This is a graph showing the changes in fluorine concentration, arsenic concentration, and zinc concentration in Example 3. Detailed Implementation

[0026] Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the accompanying drawings, but the present invention should not be interpreted as limited to these embodiments.

[0027] The wastewater treatment method of this embodiment recovers gypsum with low fluoride content and removes heavy metals from acidic wastewater containing fluoride and heavy metals. The method is characterized by comprising: an aluminum dissolution step, in which metallic aluminum is dissolved in the acidic wastewater to convert fluoride in the solution into aluminofoate ions for stable dissolution, and to generate a heavy metal reduction precipitate, thereby separating the wastewater into first-treatment water and the heavy metal reduction precipitate; and a gypsum recovery step, in which, after removing the heavy metal reduction precipitate, a calcium compound is added to the first-treatment water at a pH below 4 to generate gypsum, thereby separating the wastewater into... The process involves: a second treatment of water and the gypsum; an aluminum and fluoride removal step, in which, after removing the gypsum, alkali is added to the second treatment water to adjust the pH to 5.5–9.5 to suppress the amount of precipitate, thereby generating a precipitate containing aluminum and fluoride, which is then separated into third treatment water and the precipitate containing aluminum and fluoride; and a neutralization step, in which, after removing the precipitate containing aluminum and fluoride, alkali is further added to the third treatment water to adjust the pH to 9.5–11.8 and generate a neutralized precipitate of heavy metal hydroxides, thereby separating into alkali-neutralized treated water and the neutralized precipitate of heavy metal hydroxides.

[0028] exist Figure 1 The process diagram shows an outline of the waste liquid treatment method of this embodiment.

[0029] The object of treatment in this embodiment is acidic wastewater containing fluorine and heavy metals, such as wastewater generated in processes involving the smelting of sulfide minerals for non-ferrous metals, such as copper smelting. Typically, wastewater from non-ferrous metal smelters is strongly acidic wastewater with a pH of 0.8 to 2.0, containing heavy metals such as copper, arsenic, and zinc, as well as sulfuric acid and fluorine.

[0030] [Aluminum dissolving process]

[0031] The waste liquid treatment method of this embodiment includes the following aluminum dissolution step: aluminum is dissolved in an acidic waste liquid containing fluorine and heavy metals, so that the fluorine in the solution is converted into fluoroaluminate ions and dissolved stably, and a heavy metal reduction precipitate is generated, thereby separating the solid and liquid into first treated water and the heavy metal reduction precipitate.

[0032] By dissolving aluminum, aluminum ions (Al) 3+ The aluminum ions are supplied to the solution as shown in equations (1) to (3) to form a reaction between the aluminum ions and fluoride ions (F) in the solution. - Complexed fluoroaluminate ions (AlF) 2+ AlF2 + AlF3 0 Therefore, fluorine dissolves stably in solution.

[0033] Al 3+ (aq)+F - (aq)→AlF 2+ (aq) (1)

[0034] AlF 2+ (aq)+F - (aq)→AlF2 + (aq) (2)

[0035] AlF2 + (aq)+F - (aq)→AlF3 0 (aq) (3)

[0036] Since fluoride ions in the solution form fluoroaluminate ions and dissolve stably, even if calcium compounds are added in the next process, the formation of calcium fluoride (CaF2) is suppressed, and calcium fluoride can be prevented from mixing into the resulting gypsum, thus obtaining gypsum with low fluoride content.

[0037] Furthermore, when aluminum dissolves, a reduction reaction occurs as shown in equations (4) and (5). For example, since aluminum has a greater tendency to ionize than copper, the copper ions (Cu) in the solution... 2+ The copper is reduced and precipitated through the dissolution of aluminum, or the reduced copper reacts with arsenate ions (AsO3). 3- The reaction produces copper arsenide precipitate. Furthermore, zinc's ionization tendency is similar to that of aluminum, therefore most of the zinc dissolves in the solution and remains.

[0038] 2Al(s) + 3Cu 2+ (aq)→2Al 3+ (aq) + 3Cu(s) (4)

[0039] Al(s) + 3Cu(s) + AsO3 3- (aq)+6H + →Al 3+ (aq) + Cu3As(s) + 6H2O (5)

[0040] The molar ratio of aluminum to fluorine in the solution is preferably in the range of Al / F = 0.4 to 0.8. If the aluminum dissolution is at an Al / F molar ratio of 0.3, the fluorine complexation and ionization are insufficient, thus increasing the fluorine content in the gypsum when it is formed. However, if the Al / F molar ratio is 0.4 or higher, the fluorine content in the gypsum can be significantly reduced.

[0041] Specifically, as shown in Example 1, if the Al / F molar ratio is 0.3, the fluorine content in the gypsum is 0.3% by mass or more. On the other hand, if the Al / F molar ratio is 0.4, the fluorine content in the gypsum can be 0.2% by mass or less.

[0042] The pH of the acidic waste liquid from the aluminum dissolving process is preferably below 4.0. As shown in Example 1, if the pH is above 4.1, the arsenic in the solution will be adsorbed onto the gypsum, and the arsenic content will increase sharply, which is therefore not preferred.

[0043] In this way, aluminum can be dissolved in acidic waste liquid to produce a reduction reaction, thereby causing copper or arsenic in the solution to precipitate as reduced precipitates, which are then removed through solid-liquid separation. This reduction reaction proceeds well if the redox potential is below +400mV (vs. SHE).

[0044] Furthermore, in acidic wastewater containing suspended particles of fine heavy metals, these particles can be coagulated and separated in the reduction precipitate. By removing heavy metals at the initial stage of the treatment process, it is possible to prevent heavy metal contamination in later precipitates and reduce the amount of neutralizing agent required in later treatments compared to conventional methods. The main components of the reduction precipitate are copper and copper arsenide, thus it can be recovered and used as a raw material for copper smelting.

[0045] [Plaster Recycling Process]

[0046] Regarding the filtrate (first-stage treated water) from the reduced precipitate formed by dissolving aluminum, as shown in formula (6), a calcium compound is added to generate gypsum, and the gypsum is then separated into second-stage treated water and gypsum for gypsum recovery. Sulfate ions in the solution are removed by generating gypsum. The calcium compound can be calcium carbonate, calcium hydroxide, calcium oxide, or lime containing these as main components.

[0047] H2SO4(aq)+CaCO3(s)+H2O→CaSO4·2H2O(s)+CO2(g) (6)

[0048] The fluoride ions in the filtrate form complexes with aluminum ions and dissolve stably. Therefore, even with the addition of calcium carbonate, calcium fluoride (CaF2) is difficult to form, preventing fluoride from contaminating the gypsum and resulting in gypsum with low fluoride content. Furthermore, even if suspended particles containing fine heavy metals are present in the wastewater, they are coagulated and separated in the previous process, allowing for the recovery of gypsum with low heavy metal content. It is preferable to generate gypsum at a pH below 4.0. If the pH exceeds 4.0, heavy metals will precipitate as hydroxides or co-precipitate in the gypsum, which is undesirable.

[0049] [Aluminum and Fluorine Removal Process]

[0050] In the residual liquid after gypsum recovery (second treatment water), most sulfate ions are removed, but dissolved aluminum or fluorine or heavy metals originally contained in the wastewater are dissolved. Conventionally, as a method for removing heavy metals from such solutions, it is known to add neutralizing agents such as calcium hydroxide to bring the pH to an alkaline range of 9.5–11.8, thereby generating heavy metal hydroxide precipitates. However, if a neutralizing agent is added to adjust the pH to the range of 9.5–11.8 in one step, in addition to generating heavy metal hydroxides, as shown in equations (7)–(9), Friedel's salt (Ca2Al(OH)6Cl·2H2O) and Kuzel's salt (Ca4Al2(OH)) are also generated. 12 Cl(SO4)·5H2O], Ettringite [Ca6Al2(OH)] 12 Layered double hydroxides such as (SO4)3·26H2O are generated, and precipitates containing a large amount of chlorine or hydroxyl groups and hydrated water in addition to aluminum are produced, increasing the amount of sludge generated.

[0051] 2Ca(OH)2+Al 3+ +2OH - +Cl - +2H2O→Ca2Al(OH)6Cl·2H2O (7)

[0052] 4Ca(OH)2 + 2Al 3+ +4OH - +Cl - +SO4 2- +5H₂O→Ca₄Al₂(OH)₂ 12 Cl(SO4)·5H2O (8)

[0053] 6Ca(OH)2+2Al 3+ +Cl - +3SO42- +26H₂O→Ca₆Al₂(OH)₂ 12 (SO4)3·26H2O (9)

[0054] Wastewater treatment sludge is typically reprocessed during the smelting process or landfilled at the final disposal site. The input of large amounts of high-moisture wastewater treatment sludge increases fuel consumption during the smelting process and leads to insufficient landfill capacity during landfill disposal. Therefore, it is necessary to avoid increasing sludge production.

[0055] In this embodiment, to suppress the formation of layered double hydroxides and avoid an increase in sludge production, the treatment method does not immediately adjust the pH of the second treated water to 9.5–11.8. Instead, it adjusts the pH to a slightly lower range of 5.5–9.5, preferably within the range of 5.5–6.5, to allow for selective precipitation of aluminum. At a pH of 5.5–9.5, the formation of layered double hydroxides is difficult. On the other hand, almost all aluminum in the solution forms hydroxides and precipitates. Therefore, by adjusting the pH to the aforementioned level, excessive sludge production and the formation of aluminum precipitates can be avoided, and aluminum can be effectively removed through solid-liquid separation. Calcium hydroxide, calcium oxide, sodium hydroxide, and potassium hydroxide can be used as neutralizing agents.

[0056] In this process, if a calcium compound is used as a neutralizing agent, it reacts with fluoride in the second-treated water to form calcium fluoride (CaF2). Due to the formation of the fluoride compound, fluoride can be effectively removed from the solution while removing aluminum. Furthermore, the generated calcium fluoride has good filterability, significantly improving solid-liquid separation. Additionally, although the sodium fluoride or potassium fluoride formed when sodium hydroxide or potassium hydroxide is used as a neutralizing agent is easily soluble, fluoride ions in the solution are adsorbed onto the aluminum hydroxide precipitate, thus removing fluoride from the solution while removing aluminum. The precipitates recovered through solid-liquid separation into third-treated water and aluminum- and fluoride-containing precipitates are mainly composed of aluminum or fluoride, and can therefore be utilized as aluminum or fluoride resources.

[0057] In the aluminum and fluoride removal process, to suppress the co-precipitation of arsenic or zinc remaining in the second treated water and to selectively precipitate aluminum and fluoride, it is preferable to adjust the pH to a range of 5.5 to 7.0. For example, fluoride in the solution reacts with calcium hydroxide to form calcium fluoride precipitate. Within the pH range of 4.0 to 5.5, the fluoride concentration in the solution decreases sharply, and near pH 7, the concentration becomes almost zero. On the other hand, arsenic or zinc in the solution adsorbs onto the calcium fluoride precipitate within the pH range of 4.0 to 7.0, thus the concentration in the solution decreases slowly. If the pH exceeds 7.0 and becomes alkaline, some hydroxides or calcium salts begin to form, and the rate of decrease in zinc and arsenic concentrations gradually increases. Therefore, to suppress the formation of zinc or arsenic precipitates and promote the precipitation of aluminum and fluoride, it is preferable to control the pH to a range of 5.5 to 7.0. The aluminum and fluoride precipitates formed in this pH range have low zinc or arsenic contamination, and can therefore be utilized as aluminum or fluorine resources, such as raw materials for cement production.

[0058] [Neutralization process]

[0059] After removing aluminum and fluoride, alkali is further added to the third treated water to adjust the pH to the range of 9.5–11.8, thereby generating a neutralized precipitate of heavy metal hydroxides, which is then removed by solid-liquid separation. If the pH exceeds 11.8, zinc hydroxide redissolves, which is therefore not preferable. By adjusting the pH to the range of 9.5–11.8, residual heavy metals such as zinc, cadmium, and nickel in the solution form hydroxides and precipitate, thus allowing for separation and removal of the alkali-neutralized treated water and the neutralized precipitate of heavy metal hydroxides.

[0060] Through the aforementioned series of treatment processes, gypsum with low levels of fluoride and heavy metals is recovered, reducing the fluoride and heavy metal levels in the wastewater to meet discharge regulations, thus allowing it to be discharged outside the system. Furthermore, the pH of the water treated by the alkali neutralization process in the precipitate removal step is between pH 9.5 and 11.8. Therefore, for effluent discharge, it is suitable to set the pH of the discharge standard value to be between 5.8 and 8.6, allowing for reverse neutralization by adding acid. The recovered gypsum or precipitate can be utilized as cement raw material.

[0061] In this embodiment, the treatment method effectively treats the wastewater by simultaneously dissolving aluminum in it and stabilizing the dissolution of fluoride while generating heavy metal precipitates. Since gypsum is formed while fluoride is stably dissolved in the solution, gypsum with minimal fluoride content can be obtained without introducing fluoride into it. Furthermore, large amounts of chemicals such as sulfuric acid are not required to clean the gypsum, thus reducing wastewater volume. In addition, this embodiment's treatment method does not use sulfiding agents, therefore no hydrogen sulfide is generated, resulting in a safe working environment.

[0062] In the treatment method of this embodiment, instead of adjusting the pH to 9.5–11.8 in one step after gypsum recovery to generate hydroxides, the pH is adjusted to a slightly lower range of 5.5–9.5 to allow aluminum to selectively precipitate. Therefore, layered double hydroxides are not generated, and the amount of sludge generated does not increase. As a result, the burden of sludge treatment is significantly reduced. Specifically, in the sludge smelting process, an increase in fuel consumption can be avoided, and in landfill treatment, the amount of landfill material can be suppressed to extend the life of the final treatment plant.

[0063] Example

[0064] Hereinafter, embodiments and comparative examples of the present invention will be presented together. The concentration was determined according to JIS K 0102:2013, Plant Wastewater Test Method.

[0065] [Example 1]

[0066] Aluminum foil (Made by Mitsubishi Aluminum Co., Ltd., purity ≥ 99.5%, thickness 20μm, width 2mm, length 4mm) was added to 1L of copper smelter waste liquid (fluorine concentration 2.9g / L, arsenic concentration 6.2g / L, copper concentration 1.5g / L, pH 1.1) and stirred for 30 minutes. The resulting precipitate was then subjected to solid-liquid separation.

[0067] Calcium carbonate was added to the filtrate to produce gypsum, and the contents of fluorine, arsenic, and copper in the gypsum recovered through solid-liquid separation were determined. Tables 1-3 show the results of altering the amount of aluminum added (Al / F molar ratio) and the pH relative to the fluorine content in the waste liquid.

[0068] As shown in Tables 1-3, if the Al / F molar ratio is 0.3, the fluorine content in gypsum is 0.3% by mass or more, indicating a high fluorine content. On the other hand, if the Al / F molar ratio is 0.4, the fluorine content in gypsum is 0.2% by mass or less, resulting in a significant reduction in fluorine content. However, if the pH is 4.1 or higher, the amount of arsenic mixed into the gypsum increases dramatically. Therefore, in the dissolution of aluminum, an Al / F molar ratio of 0.4 or higher and a pH of 4 or lower are preferred. By dissolving aluminum under these conditions, the fluorine content mixed into the gypsum can be reduced, and heavy metals contained in the waste liquid will not precipitate as hydroxides or co-precipitate in the gypsum, thus obtaining gypsum that is almost free of arsenic and copper.

[0069] [Table 1]

[0070]

[0071] [Table 2]

[0072]

[0073] [Table 3]

[0074]

[0075] [Example 2]

[0076] In the same copper smelter wastewater as in Example 1, aluminum foil was added to generate a precipitate under an Al / F molar ratio of 0.4, followed by solid-liquid separation. Calcium carbonate was then added to the filtrate at pH 4.0 to generate gypsum. Calcium hydroxide was then gradually added to the filtrate from which the gypsum underwent solid-liquid separation to generate aluminum precipitate.

[0077] Figure 2 The diagram shows the changes in precipitate formation and aluminum concentration corresponding to pH changes associated with the amount of calcium hydroxide added. For example... Figure 2 As shown, it can be confirmed that if the pH is above 5.5, almost all of the aluminum becomes a precipitate. On the other hand, the increase in precipitate formation reaches its limit before pH 6.5, but if the pH exceeds 9.5, the amount of precipitate formation increases sharply again. This is believed to be because the formation of layered double hydroxides increases the penetration of chlorine, hydroxyl groups, and hydrated water into the precipitate. From these results, it can be concluded that the pH range of 5.5 to 9.5 is appropriate for reliable aluminum precipitation without increasing the amount of precipitate, and the preferred range is pH 5.5 to 6.5.

[0078] [Example 3]

[0079] Similar to Example 2, calcium hydroxide was gradually added to the residual liquid (pH 4.0) from which gypsum was separated to generate aluminum precipitate and fluoride precipitate (calcium fluoride). Figure 3 The diagram shows the changes in residual fluoride and residual arsenic concentrations in the solution corresponding to pH changes associated with the amount of calcium hydroxide added. Figure 3 As shown, the fluoride concentration in the solution decreases sharply in the pH range of 4.0 to 5.5, decreasing to about 0.1 g / L near pH 5.5, and becoming almost zero near pH 7.

[0080] On the other hand, the concentration of arsenic or zinc in the solution decreases slowly within the pH range of 4.0 to 7.0, but if the pH exceeds 7.0 and becomes alkaline, the rate of concentration decrease gradually increases. Based on this result, to avoid the formation of fluoride precipitates due to the contamination of zinc or arsenic, it is preferable to control the pH within the range of 4.0 to 7.0.

[0081] [Example 4]

[0082] Wastewater from a copper smelter was heated to 40°C in a water bath. Cut pieces of aluminum foil (manufactured by Mitsubishi Aluminum Co., Ltd., purity ≥ 99.5%, thickness 20μm, width 2mm, length 4mm, Al concentration 2.0g / L, with a molar ratio of Al / F = 0.5) were added, and the mixture was stirred for 30 minutes. After stirring, it was confirmed that all the added aluminum had dissolved and precipitated as a black reduced precipitate. This precipitate was then filtered to obtain treated water A and the reduced precipitate (aluminum dissolution process).

[0083] The treated water A was heated to 55°C in a water bath, calcium carbonate was added, and the mixture was stirred for 2 hours to produce gypsum. The pH after 2 hours was 2.10. The gypsum slurry was filtered to recover the gypsum and treated water B, and the surface of the gypsum was thoroughly washed with pure water (gypsum recovery process).

[0084] The treated water B from which gypsum was separated was heated to 40°C in a water bath, and calcium hydroxide was added as a pH adjuster, followed by stirring for 1 hour. After 1 hour, the pH was 6.00. The slurry containing the generated aluminum precipitate was filtered to recover the precipitate containing aluminum and fluoride, as well as the treated water C, and the surface of the precipitate was thoroughly washed with pure water (AlF removal process).

[0085] Next, in the treated water C from which the aluminum precipitate was separated, calcium hydroxide was added as a pH adjuster at room temperature and stirred for 1 hour to generate a neutralized precipitate. The pH after 1 hour was 11.75. The slurry containing the neutralized precipitate was filtered to obtain the neutralized precipitate and treated water D [neutralization process].

[0086] The results are shown in Table 4. As shown in Table 4, the fluoride content in the recovered gypsum is 0.05% by mass, which is very low. Furthermore, the treated water C after aluminum and fluoride removal contains very little aluminum and fluoride, allowing for effective recovery of aluminum and fluoride as precipitates. In addition, the heavy metal content in the treated water D after the neutralization process is below the discharge regulations, reducing the burden on wastewater treatment. Furthermore, the amount of precipitate (kg-dry / m³) is low. 3 The amount of reduced precipitate was 4.9 kg, aluminum and fluorine precipitate was 15.1 kg, and neutralized precipitate was 11.6 kg (totaling 31.6 kg), which was significantly less than the amount of precipitate in comparative examples 1 and 2.

[0087] [Table 4]

[0088]

[0089] [Comparative Example 1]

[0090] Untreated wastewater with the same composition as in Example 3 was heated to 55°C in a water bath, and calcium carbonate was added and stirred for 2 hours to produce gypsum. The pH after stirring for 2 hours was 1.81. The gypsum-containing slurry was filtered to recover the gypsum and treated water B2, and the surface of the gypsum was thoroughly washed with pure water (gypsum recovery process).

[0091] Next, calcium hydroxide was added to treated water B2 at room temperature as a pH adjuster and stirred for 1 hour. The pH after 1 hour was 11.81 (neutralization process). The slurry containing the precipitate generated in this neutralization process was filtered to recover the neutralized precipitate (41.6 kg) and treated water D2. The recovered gypsum contained 1.52% fluoride by mass, significantly more than the fluoride content in the gypsum recovered in Example 3, and the amount of precipitate was also greater than in Example 3.

[0092] [Comparative Example 2]

[0093] Untreated wastewater with the same composition as in Example 3 was heated to 40°C in a water bath. Cut pieces of aluminum foil (manufactured by Mitsubishi Aluminum Co., Ltd., purity ≥ 99.5%, thickness 20 μm, width 2 mm, length 4 mm) were added at a molar ratio of Al / F = 0.5 and an Al concentration of 2.0 g / L, and stirred for 30 minutes. After stirring, it was confirmed that all the added aluminum had dissolved and precipitated as a black reduced precipitate. This precipitate was then filtered to obtain treated water A2 and the reduced precipitate (aluminum dissolution process).

[0094] The treated water A2 was heated to 55°C in a water bath, calcium carbonate was added, and the mixture was stirred for 2 hours to produce gypsum. The pH after 2 hours was 2.10. The gypsum slurry was filtered to recover the gypsum and treated water B2, and the surface of the gypsum was thoroughly washed with pure water (gypsum recovery process).

[0095] Next, calcium hydroxide was added to treated water B2 at room temperature as a pH adjuster and stirred for 1 hour to generate a neutralized precipitate. The pH after 1 hour was 11.75. The slurry containing the neutralized precipitate was then filtered to obtain the neutralized precipitate and treated water D2 (neutralization process).

[0096] The treated water D2 contained 0.054 g / L of aluminum and 0.090 g / L of fluoride, indicating a higher residual amount of aluminum and fluoride compared to Example 3. Furthermore, the amount of reduced precipitate was 4.9 kg (dry / m³). 3 The amount of neutralized precipitate was 44.1 kg (dry / m³). 3 The total amount of precipitate (49.0 kg) was significantly increased compared to Example 3.

[0097] Industrial availability

[0098] According to the waste liquid treatment method of the present invention, a waste liquid treatment method can be provided that can, during the waste liquid treatment process, suppress the mixing of fluoride into gypsum and the formation of sludge, and more effectively remove fluoride and heavy metals contained in the waste liquid.

Claims

1. A method for treating waste liquid, comprising recovering gypsum with low fluoride content and removing heavy metals from acidic waste liquid containing fluorine, sulfuric acid, and heavy metals, wherein the heavy metals include copper, arsenic, and zinc, the method being characterized by having: The aluminum dissolution process involves dissolving metallic aluminum in the acidic waste liquid to produce a reduction reaction, thereby converting fluorine in the solution into aluminofluorophosphate ions for stable dissolution and precipitating heavy metal reduction precipitates containing copper and copper arsenide, thus separating the first treated water and the heavy metal reduction precipitates. In the gypsum recovery process, after removing the heavy metal reduction precipitate, calcium compounds are added to the first treated water at a pH below 4 to generate gypsum, thereby separating the second treated water and the gypsum. In the aluminum and fluoride removal process, after removing the gypsum, alkali is added to the second treated water to adjust the pH to 5.5–9.5 to suppress the amount of precipitate while generating precipitates containing aluminum and fluoride, thereby separating the third treated water and the precipitates containing aluminum and fluoride; and In the neutralization process, after removing the precipitates containing aluminum and fluoride, alkali is further added to the third treated water to adjust the pH to 9.5–11.8 and generate a neutralized precipitate of heavy metal hydroxides, thereby separating the alkali-neutralized treated water and the neutralized precipitate of heavy metal hydroxides. The redox potential of the reduction reaction is below +400mV. In the aluminum dissolution process, the amount of metallic aluminum dissolved relative to the amount of fluorine in the solution, expressed as a molar ratio Al / F, is 0.4 or more and 0.8 or less. In the aluminum dissolving process, the reactions shown in formulas (1) to (3) are carried out. Al 3+ (aq) + F - (aq) → AlF 2+ (aq) (1) AlF 2+ (aq) + F - (aq) → AlF2 + (aq) (2) AlF2 + (aq) + F - (aq) → AlF3 0 (aq) (3).

2. The waste liquid treatment method according to claim 1, wherein, In the aluminum and fluoride removal process, the pH of the second treated water is adjusted to 5.5-7.0 to suppress the amount of precipitate and to suppress the precipitation of arsenic and zinc, thereby causing fluoride and aluminum to precipitate.

3. The waste liquid treatment method according to claim 1 or 2, wherein, The acidic waste liquid containing fluorine, sulfuric acid and heavy metals is wastewater from a non-ferrous metal smelting plant.

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