A method for treating epitaxial high-concentration arsenic-containing wastewater

Abstract

The application provides a treatment method of epitaxial high-concentration arsenic-containing wastewater, comprising the following steps: adding calcium hypochlorite into the epitaxial high-concentration arsenic-containing wastewater, and stirring and aerating; then adding ferric chloride, and stirring and aerating; then using lime milk solution to adjust the pH of the system to 6.5-7, and continuing to stir and aerate; performing pressure filtration to obtain primary arsenic-containing wastewater; adding calcium chloride solution and ferric chloride solution into the primary arsenic-containing wastewater obtained by pressure filtration, using lime milk solution to adjust the pH of the system to 6.5-7, and then standing, taking supernatant to test, discharging the supernatant after reaching the standard; if the supernatant does not reach the standard, continuously adding calcium chloride solution and ferric chloride solution for cyclic treatment until the supernatant reaches the standard and is discharged. The method has the advantages of small reagent dosage, low cost, high arsenic removal rate, high stability of arsenic-containing hazardous waste and the like, and after being treated by the method, the arsenic content is lower than 0.3 mg / L, which meets the national discharge standard.

Description

Technical Field

[0001] This invention relates to a method for treating high-concentration arsenic-containing wastewater from epitaxial growth, belonging to the field of MOCVD wastewater treatment technology. Background Technology

[0002] MOCVD technology is used to prepare III-V compound semiconductor epitaxial materials, and is widely used in the growth of epitaxial materials such as light-emitting diodes (LEDs) and laser diodes (LDs). The reaction gases required during epitaxial material growth mainly include phosphine and arsine. These gases must be treated before being emitted into the atmosphere, typically through chemical scrubbing to adsorb and degrade toxic substances, bringing the exhaust gas concentration below national emission standards. After adsorbing and degrading all emitted gases, the MOCVD equipment's exhaust gas treatment system generates a large amount of arsenic-containing wastewater. This wastewater is usually highly acidic and contains harmful elements such as arsenic and phosphorus, especially arsenic, with concentrations reaching up to 4000 mg / L, far exceeding the national emission standard (0.5 mg / L). Arsenic and its compounds are highly toxic carcinogens; therefore, if left uncontrolled, they can easily pollute the environment, and once formed, pollution is difficult to eliminate. In particular, if arsenic pollutes water and soil, it can enter the human body through the food chain or surface and groundwater, harming human and animal health and causing poisoning. With the increasing global attention to the environment in recent years, researching and developing an efficient and economical method for treating arsenic-containing wastewater is of great social, economic, and environmental significance.

[0003] Currently, the main methods for effectively treating arsenic-containing wastewater fall into three categories: chemical methods, physicochemical methods, and biochemical methods. Physicochemical methods generally employ ion exchange, membrane methods, electrodialysis, photocatalytic oxidation, and adsorption to remove arsenic from wastewater. These methods are mostly relatively new and developed in recent years, with limited practical applications and high processing costs, thus rarely used in industry. Biochemical methods include extracellular microbial transformation, plant absorption, intracellular microbial transformation, and dead microbial cell adsorption. However, biochemical methods are mostly used for treating low-concentration arsenic-containing wastewater, and the cultivation of microorganisms requires the addition of nutrient sources, often leading to COD exceeding standards.

[0004] Chemical methods are commonly used for arsenic removal in industrial production, including chemical precipitation and flocculation precipitation. Chemical precipitation is further subdivided into lime precipitation, ferric salt precipitation, ferric salt-lime co-precipitation, and sulfide precipitation. Lime precipitation utilizes the reaction of lime with arsenate and arsenite ions in water to form calcium arsenate and calcium arsenite precipitates, thus removing arsenic from the water. However, calcium arsenate and calcium arsenite have high solubility in water, making it difficult for arsenic-containing wastewater treated by lime alone to meet discharge requirements. Sulfide precipitation involves adding sulfides to form arsenic sulfide precipitates, thus removing arsenic from the water. However, sulfide precipitation is only effective under acidic conditions, and in acidic environments, it easily generates toxic and irritating H2S gas, resulting in a harsh working environment and high operating costs, thus limiting its widespread industrial application.

[0005] However, current chemical methods for treating arsenic-containing wastewater have the following drawbacks: a large number of reagents need to be added, low reagent utilization rate, low sedimentation efficiency, complex treatment facilities, and poor arsenic removal effect. Therefore, developing an economical, efficient, and stable method for treating arsenic-containing wastewater is of great significance. This invention is proposed for this purpose. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for treating high-concentration arsenic-containing wastewater from epitaxy. The method of this invention features a simple treatment device, low reagent usage, low cost, high arsenic removal rate, and minimal arsenic residue. The treated wastewater meets national discharge standards for arsenic-containing wastewater, with arsenic concentrations below 0.3 mg / L.

[0007] Terminology Explanation:

[0008] High-concentration arsenic-containing epitaxial wastewater: When growing III-V compound semiconductor epitaxial materials using MOCVD technology, the high-concentration arsenic-containing wastewater generated by the waste gas treatment system of the MOCVD equipment is called high-concentration arsenic-containing epitaxial wastewater. Its pH is 3-5 and the arsenic content is 3000-4000 mg / L.

[0009] The technical solution of the present invention is as follows:

[0010] A method for treating high-concentration arsenic-containing wastewater includes the following steps:

[0011] (1) Calcium hypochlorite was added to the high-concentration arsenic-containing wastewater and stirred and aerated; then ferric chloride was added and stirred and aerated; then lime milk solution was used to adjust the pH of the system to 6.5-7 and stirred and aerated; after pressure filtration, primary arsenic-containing wastewater was obtained.

[0012] (2) Add calcium chloride solution and ferric chloride solution to the primary arsenic-containing wastewater obtained by pressure filtration, adjust the pH of the system to 6.5-7 using lime milk solution, let it stand, take the supernatant for testing, if the supernatant meets the standard, filter and discharge; if the supernatant does not meet the standard, continue to add calcium chloride solution and ferric chloride solution for circulation treatment according to the measured arsenic content, until the supernatant meets the standard and is discharged.

[0013] According to a preferred embodiment of the present invention, the molar ratio of calcium hypochlorite to arsenic in arsenic-containing wastewater in step (1) is 1-1.4:1.

[0014] According to a preferred embodiment of the present invention, in step (1), the stirring and aeration time after adding calcium hypochlorite is 5-10 minutes.

[0015] According to a preferred embodiment of the present invention, the molar ratio of ferric chloride to arsenic in arsenic-containing wastewater in step (1) is 1.4-2:1.

[0016] According to a preferred embodiment of the present invention, in step (1), the stirring and aeration time after adding ferric chloride is 15-25 minutes.

[0017] According to a preferred embodiment of the present invention, the mass fraction of the lime slurry solution in step (1) is 20-30%; and the stirring and aeration time after adding the lime slurry solution is 10-15 minutes.

[0018] According to the present invention, the arsenic content of the primary arsenic-containing wastewater obtained in step (1) is less than 3 mg / L.

[0019] According to a preferred embodiment of the present invention, the mass concentration of the calcium chloride solution in step (2) is 10-20 g / L; the molar ratio of calcium chloride in the calcium chloride solution to arsenic in the primary arsenic-containing wastewater is 90-120:1.

[0020] According to a preferred embodiment of the present invention, the mass concentration of the ferric chloride solution in step (2) is 10-20 g / L; the molar ratio of ferric chloride to arsenic in the primary arsenic-containing wastewater is 70-85:1.

[0021] According to a preferred embodiment of the present invention, in step (2), the arsenic content in the supernatant is less than 0.3 mg / L, which is considered to meet the standard.

[0022] According to a preferred embodiment of the present invention, in step (2), when adding calcium chloride solution and ferric chloride solution for cyclic treatment, the mass concentration of calcium chloride solution is 10-20 g / L, and the molar ratio of calcium chloride in calcium chloride solution to arsenic in arsenic-containing wastewater is 90-120:1; the mass concentration of ferric chloride solution is 10-20 g / L, and the molar ratio of ferric chloride in ferric chloride solution to arsenic in arsenic-containing wastewater is 70-85:1.

[0023] According to the present invention, after treatment by the method of the present invention, the arsenic content in the supernatant can be lower than 0.3 mg / L.

[0024] According to the present invention, the wastewater treatment system used in the above-mentioned method for treating high-concentration arsenic-containing wastewater includes a primary reaction tank and a secondary reaction tank. The primary reaction tank is connected in sequence to a sludge pump a, a sludge tank, a sludge pump b, and a filter press via pipelines. The secondary reaction tank is connected to a ferric chloride solution tank via a diaphragm pump a, and to a calcium chloride solution tank via a diaphragm pump b. The secondary reaction tank is connected to the sludge tank via a wastewater pump. The secondary reaction tank is connected to the filter press. The secondary reaction tank is connected in sequence to a water collection tank, a booster pump, a sand filter, a fine filter, and a drainage tank via pipelines.

[0025] According to the present invention, the filter press is connected to a sludge tank for storing the sludge obtained from the filter press.

[0026] According to a preferred embodiment of the present invention, the steps include the following:

[0027] (1) Add the high-concentration arsenic-containing wastewater from the outer layer to the primary reaction tank, add calcium hypochlorite to the primary reaction tank and stir and aerate; then add ferric chloride and stir and aerate; then add lime milk solution to the primary reaction tank under stirring conditions to adjust the pH of the system to 6.5-7, and continue stirring and aerating; then pump the resulting mud-water mixture into the sludge tank through sludge pump a, and pump the mud-water mixture in the sludge tank into the filter press through sludge pump b. After filtration, the primary arsenic-containing wastewater is obtained, the filter residue is transported to the sludge storage tank, and the primary arsenic-containing wastewater is discharged into the secondary reaction tank.

[0028] (2) Use diaphragm pump a and diaphragm pump b to pump the ferric chloride solution in the ferric chloride solution tank and the calcium chloride solution in the calcium chloride solution tank into the secondary reaction tank respectively. Add lime slurry solution to the secondary reaction tank to adjust the pH of the system to 6.5-7, stop stirring, let it stand, measure the supernatant, let it stand again, take the supernatant for testing. If the supernatant meets the standard, drain the water into the water collection tank, pump it into the sand filter through the pressure pump, and then filter it through the sand filter and the fine filter before discharging it into the drainage tank. The resulting sludge is pumped into the sludge tank by the sewage pump. If the arsenic content in the supernatant does not meet the standard, continue to pump ferric chloride solution and calcium chloride solution into the secondary reaction tank for circulation treatment according to the measured arsenic content until it meets the standard before discharging.

[0029] The principle of this invention is as follows:

[0030] The method of this invention first involves adding an appropriate amount of calcium hypochlorite based on the arsenic content in the wastewater. Calcium hypochlorite dissolves in water to produce calcium hydroxide and hypochlorous acid. Hypochlorous acid has strong oxidizing properties and can remove arsenic from the wastewater. 3+ Partially oxidized to As 5+Therefore, after adding calcium hypochlorite, slightly soluble calcium arsenate and a small amount of calcium arsenite are generated in the system. The reaction equations are shown in equations (1) and (2) below. Then, ferric chloride is added, and the pH is adjusted to 6.5-7 with lime milk. The iron ions react with arsenate and arsenite ions to generate stable and insoluble ferric arsenate and a small amount of ferric arsenite. The reaction equations are shown in equations (3) and (4) below. Then, primary arsenic-containing wastewater is obtained by pressure filtration.

[0031] 3Ca(OH)2+2AsO3 3– =Ca3(AsO3)2↓+6H2O (1)

[0032] 3Ca(OH)2+2AsO4 3– =Ca3(AsO4)2↓+6H2O (2)

[0033] AsO4 3– +Fe(OH)3=FeAsO4↓+3(OH) – (3)

[0034] AsO3 3– +Fe(OH)3=FeAsO3↓+3OH – (4)

[0035] Add appropriate amounts of calcium chloride and ferric chloride to the primary arsenic-containing wastewater obtained by pressure filtration, adjust the pH to 6.5-7, and calcium and iron ions react with arsenate ions and arsenite ions to generate arsenate and arsenite. After sedimentation by filter screen, sand filtration and precision filtration, the wastewater meets the discharge standards. The reaction formulas are shown in equations (5)-(8) below.

[0036] AsO4 3– +Fe(OH)3=FeAsO4↓+3(OH) – (5)

[0037] AsO3 3– +Fe(OH)3=FeAsO3↓+3OH – (6)

[0038] 3Ca(OH)2+2AsO3 3– =Ca3(AsO3)2↓+6H2O (7)

[0039] 3Ca(OH)2+2AsO4 3– =Ca3(AsO4)2↓+6H2O (8)

[0040] Everything not described in detail in this invention is existing technology in the field.

[0041] The technical features and beneficial effects of this invention are as follows:

[0042] 1. The method for treating arsenic-containing wastewater according to the present invention employs a two-stage treatment process to improve arsenic removal efficiency. The first stage uses calcium hypochlorite to pre-oxidize arsenic ions in the wastewater, as pentavalent arsenates are more stable. Ferric chloride is used as a flocculant to further enhance arsenic removal. The second stage uses calcium chloride and ferric chloride solutions, pumped in small doses into a secondary treatment tank using a diaphragm pump for continuous circulation until the wastewater meets standards. The present invention utilizes both calcium chloride and ferric chloride for simultaneous secondary treatment of arsenic-containing wastewater, resulting in more effective removal of arsenite ions, better sedimentation, clearer water, and significantly higher arsenic removal efficiency than ferrous salts.

[0043] 2. The method of this invention has low operating costs, is convenient to operate and easy to implement, is environmentally friendly, and causes no secondary pollution. The acidic wastewater generated during epitaxial growth meets the national emission standards after treatment by this process, with an arsenic content of less than 0.3 mg / L. At the same time, the method of this invention has simple facilities, low initial investment, convenient reagent addition, short process flow, and higher treatment efficiency.

[0044] 3. The calcium hypochlorite + iron salt + calcium chloride method used in this invention to treat arsenic-containing industrial wastewater not only improves the arsenic removal effect, but also has the advantages of requiring fewer types of reagents, high reagent utilization, high precipitation efficiency, simple treatment facilities, and convenient operation. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the wastewater treatment system used in the method for treating high-concentration arsenic-containing wastewater according to the present invention.

[0046] The components include: 1. Primary reaction tank; 2. Secondary reaction tank; 3. Sludge pump a; 4. Sludge tank; 5. Sludge pump b; 6. Filter press; 7. Sludge storage tank; 8. Wastewater pump; 9. Ferric chloride solution tank; 10. Diaphragm pump a; 11. Calcium chloride solution tank; 12. Diaphragm pump b; 13. Water collection tank; 14. Booster pump; 15. Sand filter; 16. Fine filter; 17. Drainage tank. Detailed Implementation

[0047] The present invention will be further described below with reference to specific embodiments and accompanying drawings, but is not limited thereto. Any aspects not described in detail herein are existing technologies in the art.

[0048] In addition, unless otherwise specified, the instruments and equipment used in the following embodiments are all commonly used instruments and equipment in the field and can be purchased commercially; unless otherwise specified, the raw materials used are all commonly used raw materials in the field and ordinary commercially available products.

[0049] Example 1

[0050] A method for treating high-concentration arsenic-containing wastewater, employing methods such as... Figure 1The wastewater treatment system shown includes the following steps:

[0051] (1) Primary treatment: First, the arsenic content in the high-concentration arsenic-containing wastewater was measured to be 4000 mg / L. 1 m3 of arsenic-containing wastewater was added to the primary reaction tank 1. 10 kg of calcium hypochlorite was added to the primary reaction tank 1 and stirred and aerated for 5 minutes to fully dissolve the calcium hypochlorite. After uniform mixing, 15 kg of ferric chloride was added, and stirring and aeration continued for 20 minutes to ensure uniform mixing of the added reagents and complete oxidation of trivalent arsenic to pentavalent arsenic, generating more stable and insoluble arsenates and arsenites. Finally, a 20% lime slurry solution (Ca(OH)2) was added while stirring and uniformly mixed in the primary reaction tank 1. The pH of the wastewater was measured. When the pH of the mixture reached 6.8, the addition of lime slurry solution was stopped, and stirring and aeration continued for 10 minutes. The resulting mixture was pumped into the sludge tank 4 through sludge pump a3. The sludge-water mixture in the sludge tank 4 was then pumped into the sludge tank through sludge pump b. 5. The wastewater is fed into a filter press 6 and filtered to obtain primary arsenic-containing wastewater. The filter residue is transported to the sludge storage tank 7, and the primary arsenic-containing wastewater is discharged into the secondary reaction tank 2.

[0052] After primary treatment, the arsenic content of the primary arsenic-containing wastewater is 2.2 mg / L, and the volume of the primary arsenic-containing wastewater is 1000 L.

[0053] Secondary treatment: Add 1.5 kg of ferric chloride to ferric chloride solution tank 9, add 100 L of water and stir for 5 minutes to fully dissolve the ferric chloride. Add 1.5 kg of calcium chloride to calcium chloride solution tank 11, add 100 L of water and stir for 5 minutes to fully dissolve the calcium chloride. Pump the ferric chloride solution and calcium chloride solution into the secondary reaction tank 2 using diaphragm pump a 10 and diaphragm pump b 12 respectively, while simultaneously starting the agitator. After pumping in 25L of ferric chloride solution and 25L of calcium chloride solution, the pH is adjusted to 6.8 with 20% lime milk. Stirring is stopped, and after settling, the supernatant is measured. The arsenic content in the supernatant is 0.2mg / L. The supernatant that meets the discharge standard is discharged into the collection tank 13, pumped into the sand filter 15 by the pressurization pump 14, and then filtered by the sand filter 15 and the fine filter 16 before being discharged into the drainage tank 17. The sludge obtained from settling is pumped into the sludge tank 4 by the sewage pump 8, and then pumped into the filter press 6 by the sludge pump b 5 for filter pressing. The sludge obtained from filter pressing is sent to the sludge storage tank 7 for storage, and the filtrate is discharged into the secondary reaction tank 2.

[0054] Project implementation and results:

[0055] After a phased experimental demonstration, the new process has been officially put into operation. The new process employs a two-stage treatment of "calcium hypochlorite + iron salt + calcium chloride." Through follow-up analysis, the final determined molar ratio of iron to arsenic in the primary treatment is 1.4-2:1, and the molar ratio of calcium hypochlorite to arsenic is 1-1.4:1. Under normal circumstances, for every cubic meter of secondary arsenic-containing wastewater treated, the dosage of ferric chloride is 0.3-0.5 kg, and the dosage of calcium chloride is 0.3-0.5 kg. Generally, one secondary treatment is sufficient to meet the standards; any waste that does not meet the standards can be recycled to achieve complete compliance with discharge requirements.

[0056] This treatment process employs a two-stage process, which can efficiently treat arsenic-containing wastewater with a high arsenic removal rate. The residual arsenic concentration in the treated solution is less than 0.3 mg / L, meeting national emission standards. The arsenic-containing sludge produced is also difficult to dissolve in water, significantly improving safety. The equipment is simple to operate and easy to use.

Claims

1. A method for treating high-concentration arsenic-containing wastewater, comprising the following steps: (1) Calcium hypochlorite was added to the high-concentration arsenic-containing wastewater and stirred and aerated; then ferric chloride was added and stirred and aerated; then lime milk solution was used to adjust the pH of the system to 6.5-7, and stirring and aeration continued; after pressure filtration, primary arsenic-containing wastewater was obtained; the molar ratio of calcium hypochlorite to arsenic in the arsenic-containing wastewater was 1-1.4:1; the molar ratio of ferric chloride to arsenic in the arsenic-containing wastewater was 1.4-2:1; (2) Add calcium chloride solution and ferric chloride solution to the primary arsenic-containing wastewater obtained by pressure filtration, adjust the pH of the system to 6.5-7 using lime milk solution, let it stand, and take the supernatant for testing. If the supernatant meets the standard, it is filtered and discharged; if the supernatant does not meet the standard, calcium chloride solution and ferric chloride solution are added and circulated according to the measured arsenic content until the supernatant meets the standard and is discharged. The molar ratio of calcium chloride in the calcium chloride solution to arsenic in the primary arsenic-containing wastewater is 90-120:1; the molar ratio of ferric chloride in the ferric chloride solution to arsenic in the primary arsenic-containing wastewater is 70-85:

1.

2. The method for treating high-concentration arsenic-containing wastewater according to claim 1, characterized in that, In step (1), after adding calcium hypochlorite, the stirring and aeration time is 5-10 minutes.

3. The method for treating high-concentration arsenic-containing wastewater according to claim 1, characterized in that, In step (1), the stirring and aeration time after adding ferric chloride is 15-25 minutes.

4. The method for treating high-concentration arsenic-containing wastewater according to claim 1, characterized in that, The mass fraction of the lime slurry solution in step (1) is 20-30%; the stirring and aeration time after adding the lime slurry solution is 10-15 minutes.

5. The method for treating high-concentration arsenic-containing wastewater according to claim 1, characterized in that, The mass fraction of the calcium chloride solution in step (2) is 10-20 g / L.

6. The method for treating high-concentration arsenic-containing wastewater according to claim 1, characterized in that, The mass fraction of the ferric chloride solution in step (2) is 10-20 g / L.

7. The method for treating high-concentration arsenic-containing wastewater according to claim 1, characterized in that, In step (2), when adding calcium chloride solution and ferric chloride solution for recycling, the mass concentration of calcium chloride solution is 10-20 g / L, and the molar ratio of calcium chloride in calcium chloride solution to arsenic in arsenic-containing wastewater is 90-120:1; the mass concentration of ferric chloride solution is 10-20 g / L, and the molar ratio of ferric chloride in ferric chloride solution to arsenic in arsenic-containing wastewater is 70-85:

1.

8. The wastewater treatment system used in the method for treating high-concentration arsenic-containing wastewater according to any one of claims 1-7, characterized in that, The wastewater treatment system includes a primary reaction tank and a secondary reaction tank. The primary reaction tank is connected in sequence to a sludge pump a, a sludge tank, a sludge pump b, and a filter press via pipelines. The secondary reaction tank is connected to a ferric chloride solution tank via diaphragm pump a, and to a calcium chloride solution tank via diaphragm pump b. The secondary reaction tank is connected to the sludge tank via a wastewater pump. The secondary reaction tank is also connected to the filter press via pipelines. Finally, the secondary reaction tank is connected in sequence to a water collection tank, a booster pump, a sand filter, a fine filter, and a drainage tank via pipelines.

Images