A phosphogypsum leachate treatment unit and treatment method

Abstract

The application belongs to the technical field of wastewater treatment, and particularly relates to a phosphogypsum leachate treatment unit and a treatment method. The phosphogypsum leachate treatment unit comprises a cylinder, a central shaft, an iron electrode plate and an aluminum electrode plate. The method for treating phosphogypsum leachate by using the phosphogypsum leachate treatment unit comprises the following steps: simultaneously performing ozone oxidation and electrolysis coupling reaction on the phosphogypsum leachate to be treated in the phosphogypsum leachate treatment unit, adjusting the pH value of the effluent from the treatment unit, and performing solid-liquid separation on the precipitate. The supernatant is the effluent. The method simultaneously removes phosphoric acid ions, fluoride ions and organic pollutants in the leachate by simultaneously performing ozone oxidation and electrolysis coupling reaction on the phosphogypsum leachate to be treated in the phosphogypsum leachate treatment unit, so that the COD, total phosphorus and fluoride in the wastewater meet the discharge standards, the total phosphorus removal rate can reach 99%, the fluoride ion removal rate is greater than 99%, and the COD removal rate is greater than 76%.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a phosphogypsum leachate treatment unit and treatment method. Background Technology

[0002] Phosphogypsum is an acidic waste residue produced in the phosphoric acid and ammonium phosphate industries. Its main chemical reaction formula is:

[0003] Ca5F(PO4)3 (phosphate rock) + 5H2SO4 + 5nH2O = 3H2PO4 + 5CaSO4·nH2O + HF

[0004] Statistics show that approximately seven tons of phosphogypsum (wet basis) are produced for every ton of phosphoric acid produced. Phosphogypsum leachate is formed by the accumulation and leaching of phosphogypsum generated during the wet-process phosphoric acid production. For every ton of phosphoric acid produced, 5-7 tons of phosphogypsum are generated, and the comprehensive utilization rate of phosphogypsum is very low. Currently, the stockpiled amount exceeds 700 million tons, and it is increasing at a rate of approximately 80 million tons per year. Phosphogypsum is generally a light gray, light yellow, or blackish-gray fine powder solid with a moisture content of 20%-30% and a bulk density of 0.73-0.88 g / cm³. 3 It has high viscosity, poor fluidity, and is acidic (pH generally between 1.5 and 4.5), with a slight odor. The main component of phosphogypsum is calcium sulfate dihydrate, and it also contains undecomposed phosphate rock powder, unwashed phosphoric acid, iron phosphate, aluminum phosphate, fluorosilicate, calcium fluoride, acid-insoluble matter, organic matter, etc. Because it contains unwashed acid, phosphogypsum waste is acidic. The characteristics of phosphogypsum leachate are as follows: (1) High total phosphorus and total fluoride concentrations, with a total phosphorus concentration of 2000 mg / L and a total fluoride concentration of 525 mg / L; (2) High total salt content, with a total salt concentration of 8140 mg / L; (3) Poor biodegradability. Although the concentration of organic matter is not high, with the COD of the leachate being only 100-300 mg / L, the organic pollutants are ethylene glycol methyl ether acetate, isothiocyanate, 3-methoxy-n-pentane, and 2-ethyl-1,3-dioxolane. Furthermore, under the inhibitory effect of high salt, the wastewater is almost non-biodegradable.

[0005] Chinese invention patent CN118026131A describes a method and application for recovering phosphorus from phosphogypsum leachate for the preparation of lithium iron phosphate. However, the recovered element content is low, resulting in limited practicality and economic viability, and thus low practical value. Chinese invention patent CN220951484U, "A Resource Utilization Device for Phosphogypsum Leachate," combines chemical sedimentation, biological treatment, and gelation treatment, resulting in a cumbersome and complex process unsuitable for large-scale application. Chinese invention patent CN117069230A discloses a "Method for Reducing the Phosphorus and Fluorine Content in Phosphogypsum Leachate," which uses calcium salt precipitation to remove phosphorus and fluorine, generating a large amount of solid waste. Chinese invention patent CN114920417A discloses a device and method for deep purification treatment of leachate from a phosphogypsum slag storage facility. It couples traditional two-stage lime slurry neutralization and chemical precipitation treatment, crystallization fluidized bed, and magnetic coagulation technologies. However, this method has a cumbersome process flow, complex operation, and is unsuitable for large-scale application. It also suffers from low utilization rates due to excessive lime slurry usage. Chinese utility model CN218811210U removes phosphorus and fluoride ions from wastewater through multi-stage lime slurry reaction and separate multi-stage precipitation separation. It further treats nitrate nitrogen, ammonia nitrogen, and fluoride ions through the oxidation of sodium hypochlorite and the adsorption of highly selective nitrate ion exchange resin. However, this method has a complex process flow, high slag production, poor resin adsorption chemical stability, and poor regeneration performance. In summary, current treatment methods for phosphogypsum leachate mainly include chemical precipitation, chemical precipitation-biochemical combination processes, and physical treatment methods. Chemical precipitation involves adding phosphorus flocculants and defluorination flocculants to the leachate separately, removing phosphorus and fluoride through two-stage precipitation. While simple to operate and effective at removing phosphorus and fluoride, it generates a large amount of solid waste, causing secondary pollution, and offers almost no removal of organic pollutants. The two-stage coagulation-biological combined process is lengthy, with poor removal efficiency in the biological stage, making it difficult to achieve compliant discharge without dilution. Physical methods, primarily membrane separation, suffer from poor membrane stability and high maintenance costs due to the complex composition of wastewater containing silicates and other pollutants that easily cause membrane fouling, hindering large-scale engineering applications. Summary of the Invention

[0006] In view of the problems of large sludge production, complex process, high operating cost, and difficulty in achieving stable discharge standards in the existing technology, the purpose of this invention is to provide a method for treating phosphogypsum leachate with a short process, stable compliance, low treatment cost, and suitability for large-scale engineering applications.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A phosphogypsum leachate treatment unit includes a cylinder 1, a central shaft 2, iron electrode plates 3, and aluminum electrode plates 4. The cylinder 1 has a gas-liquid mixing inlet, a gas outlet, and a liquid outlet. The central shaft 2 is installed inside the cylinder, and an electrode plate frame is connected to the central shaft 2. Multiple iron electrode plates 3 are axially connected to one side of the central shaft 2 and the electrode plate frame, and multiple aluminum electrode plates 4 are axially connected to the other side of the central shaft and the electrode plate frame. The number of iron electrode plates 3 and aluminum electrode plates 4 are the same and correspond one-to-one. The corresponding iron electrode plate 3 is a positive electrode or a negative electrode, and the corresponding aluminum electrode plate 4 is a negative electrode or a positive electrode.

[0009] Preferably, the central shaft is rotatably connected to the cylinder.

[0010] The gas-liquid mixing inlet is tangentially opened along the cylinder body 1, and the axial centerline of the gas-liquid mixing inlet is inclined upward at 45 to 60 degrees relative to the horizontal plane. The gas-liquid mixture rises through the inlet along a spiral path around the cylinder wall under inertia.

[0011] The iron electrode plate 3 is a wire mesh with a wire diameter of 2-3 mm and a mesh size of 10-18 mm.

[0012] The aluminum electrode plate 4 is an aluminum wire mesh with a wire diameter of 0.5 to 2 mm and a mesh size of 10 to 15 mm.

[0013] The iron electrode plates 3 are arranged at equal intervals, and the aluminum electrode plates 4 are arranged at equal intervals. The corresponding iron electrode plates 3 and aluminum electrode plates 4 are arranged alternately or coplanarly, preferably alternately.

[0014] The distance between any two iron plates or any two aluminum plates is 10–20 mm.

[0015] The aluminum electrode plate is activated with 0.5-1% (m / v) hydrochloric acid for 60 minutes to remove surface oxides.

[0016] The iron or aluminum plates are connected to an external DC regulated power supply via wires.

[0017] A method for treating phosphogypsum leachate using the aforementioned phosphogypsum leachate treatment unit, wherein the phosphogypsum leachate to be treated undergoes simultaneous ozone oxidation and electrolytic coupling reactions within the phosphogypsum leachate treatment unit, the pH value of the effluent from the treatment unit is adjusted, the precipitate is subjected to solid-liquid separation, and the supernatant is the treated effluent.

[0018] Furthermore, the pH of the effluent from the treatment unit is adjusted to 6-7, and then adjusted to 8.5-9, allowing for solid-liquid separation through sedimentation.

[0019] Specifically, the steps include:

[0020] 1) Turn on the power supply and supply power to the iron plate 3 and the aluminum plate 4 through the DC power supply;

[0021] 2) Ozone and phosphogypsum leachate are simultaneously jetted into the phosphogypsum leachate treatment unit after passing through a premixing and pressurizing device.

[0022] 3) Continuous power supply allows for simultaneous ozone oxidation and electrolytic coupling reactions within the phosphogypsum leachate treatment unit;

[0023] 4) The pH of the effluent from the treatment unit is adjusted to 6-7 with sodium hydroxide solution, and then further adjusted to 8.5-9 with 5% quicklime solution. The precipitate is used for solid-liquid separation, and the supernatant is the treated effluent. The filter residue can be further recycled and utilized for phosphorus resource recovery.

[0024] The reaction time for ozone oxidation and electrolysis coupling is 45–75 min.

[0025] The current density of the electrolytic coupling reaction within the phosphogypsum leachate treatment unit is 25–40 mA / cm². 2 .

[0026] The ozone concentration for ozone oxidation is 100–120 mg / L.

[0027] The positive and negative terminals of the external current connected to the phosphogypsum leachate treatment unit are switched every 10 minutes to prevent the formation of a metal oxide passivation film on the electrode surface, improve electrode utilization efficiency, and extend electrode life.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] 1. In the method of the present invention, the phosphogypsum leachate to be treated undergoes ozone oxidation and electrolytic coupling reaction simultaneously in the phosphogypsum leachate treatment unit, followed by precipitation separation, so that multiple pollutants such as phosphate ions, fluoride ions, and organic matter in the leachate can be removed simultaneously, achieving the standard discharge of COD, total phosphorus, and fluoride in the wastewater. The total phosphorus removal rate can reach 99%, the fluoride ion removal rate is >99%, and the COD removal rate is >76%. The process flow is short, and the sludge production is reduced by two-thirds compared with the traditional chemical coagulation and sedimentation sludge production.

[0030] 2. The device of the present invention can realize ozone oxidation and electrolytic coupling reaction simultaneously. The gas-liquid mixing inlet is set at an angle to feed the material, so that the liquid inside the processing unit rises in a swirling state. This can not only increase the full contact between ozone and phosphogypsum leachate and prolong the residence time of ozone, but also remove hydrogen and oxygen generated by electrolysis on the electrode surface in time through the shear force of the fluid.

[0031] 3. The treatment method of the present invention utilizes the combined effect of ozone and electrolysis. The electrode plate provides electrons, which promotes the decomposition of ozone into hydroxyl radicals, thus ensuring the removal efficiency of organic matter.

[0032] 4. This invention reduces the sludge production in leachate treated with traditional coagulation and sedimentation for phosphogypsum removal. It employs a composite dissolving electrode, generating active iron and aluminum that bind more firmly with phosphorus and fluoride ions. The resulting flocs exhibit stronger charge coagulation, significantly reducing sludge production. Because no other ions are introduced, the sludge can be used for phosphorus resource utilization or for saline-alkali land remediation. The effluent treatment effect is also better. Attached Figure Description

[0033] Figure 1 This is a structural diagram of the phosphogypsum leachate treatment unit of the present invention, wherein 1-cylinder, 2-central shaft, 3-iron electrode plate, and 4-aluminum electrode plate. Detailed Implementation

[0034] The following specific examples are used to further illustrate the technical solution of the present invention. However, the present invention is by no means limited to this and is also applicable to the treatment of wastewater with similar water composition in industries such as electroplating.

[0035] To control the environmental pollution caused by phosphogypsum and address the practical problem of unacceptable emissions of total phosphorus, fluoride ions, and COD from phosphogypsum leachate, this invention provides a phosphogypsum leachate treatment unit and method. The treatment method of this invention is applicable to the treatment of wastewater with similar water quality composition from industries such as electroplating. The total phosphorus removal rate can reach 99%, the fluoride ion removal rate is >99%, and the COD removal rate is >76%. The treatment process does not introduce other anions. The sludge production is reduced by two-thirds compared to traditional coagulation, and the problem of poor biochemical feasibility and difficulty in meeting COD standards in phosphogypsum leachate is solved.

[0036] Example 1

[0037] like Figure 1 As shown, the phosphogypsum leachate treatment unit includes a cylinder 1, a central shaft 2, iron electrode plates 3, and aluminum electrode plates 4. The cylinder 1 has a gas-liquid mixing inlet, a gas outlet, and a liquid outlet. The central shaft 2 is installed inside the cylinder, and an electrode plate frame is connected to the central shaft 2. Multiple iron electrode plates 3 are axially connected to one side of the central shaft 2 and the electrode plate frame, and multiple aluminum electrode plates 4 are axially connected to the other side of the central shaft and the electrode plate frame. The number of iron electrode plates 3 and aluminum electrode plates 4 are the same and correspond one-to-one. The corresponding iron electrode plate 3 is the positive electrode or the negative electrode, and the aluminum electrode plate 4 is the negative electrode or the positive electrode.

[0038] The central shaft 2 is rotatably connected to the cylinder 1.

[0039] Four gas-liquid mixing inlets are located at the bottom of the processing unit. Each inlet is tangentially positioned along the cylinder 1, and the axial centerline of each inlet is inclined upwards at a 45-degree angle to the horizontal plane. The gas-liquid mixture rises spirally along the cylinder wall due to inertia through the inlets, creating a swirling flow inside the reactor. The bottom of the electrode frame is 1000mm from the center of the inlet. The iron electrode 3 is made of Q235B steel wire mesh with 10mm aperture and 2mm wire diameter; the aluminum electrode 4 is made of aluminum wire mesh with 1mm wire diameter and 10mm mesh aperture; the electrode spacing is 20mm.

[0040] The iron electrode plates 3 are arranged at equal intervals, the aluminum electrode plates 4 are arranged at equal intervals, and the corresponding iron electrode plates 3 and aluminum electrode plates 4 are arranged alternately.

[0041] Aluminum electrode plate 4 needs to be activated with 0.5% (m / v) hydrochloric acid for 60 minutes to remove surface oxides.

[0042] The iron or aluminum plates are connected to an external DC regulated power supply via wires.

[0043] Example 2

[0044] The phosphogypsum leachate was treated using the phosphogypsum leachate treatment unit of Example 1:

[0045] 1) Turn on the power supply and supply power to the iron plate 3 and aluminum plate 4 via DC power, with a current density of 25 mA / cm². 2 The positive and negative terminals are automatically switched every 10 minutes.

[0046] 2) Ozone and phosphogypsum leachate are simultaneously jetted into the phosphogypsum leachate treatment unit after passing through a premixing and pressurizing device, with an ozone dosage of 180 mg / L.

[0047] 3) With continuous power supply, ozone oxidation and electrolytic coupling reaction are carried out simultaneously in the phosphogypsum leachate treatment unit for 45 minutes.

[0048] 4) The pH of the effluent from the treatment unit is adjusted to 7.0 with sodium hydroxide solution, and then further adjusted to 9.0 with 5% quicklime solution. The precipitate is used for solid-liquid separation, and the supernatant is the treated effluent. The filter residue can be further recycled and utilized for phosphorus resource recovery.

[0049] The influent and effluent analysis indicators are as follows:

[0050]

[0051]

[0052] Example 3

[0053] A phosphogypsum leachate treatment unit includes a cylinder 1, a central shaft 2, iron electrode plates 3, and aluminum electrode plates 4. The cylinder 1 has a gas-liquid mixing inlet, a gas outlet, and a liquid outlet. The central shaft 2 is installed inside the cylinder, and an electrode plate frame is connected to the central shaft 2. Multiple iron electrode plates 3 are axially connected to one side of the central shaft 2 and the electrode plate frame, and multiple aluminum electrode plates 4 are axially connected to the other side of the central shaft and the electrode plate frame. The number of iron electrode plates 3 and aluminum electrode plates 4 are the same and correspond one-to-one. The corresponding iron electrode plate 3 is the positive electrode or the negative electrode, and the aluminum electrode plate 4 is the negative electrode or the positive electrode.

[0054] The central shaft 2 is rotatably connected to the cylinder 1.

[0055] Four gas-liquid mixing inlets are provided at the bottom of the processing unit. Each gas-liquid mixing inlet is tangentially opened along the cylinder 1, and the axial centerline of each gas-liquid mixing inlet is inclined upward at 45 degrees to the horizontal plane. The gas-liquid mixture rises through the inlet along a spiral path around the cylinder wall under the action of inertia, and the liquid inside the reactor is in a swirling state.

[0056] The bottom of the electrode frame is 1000mm from the center of the feed inlet. The iron electrode 3 is made of Q235B steel wire mesh with a 10mm round hole diameter and a wire diameter of 2mm. The electrode spacing is 15mm. The aluminum electrode 4 is made of aluminum wire mesh with a wire diameter of 0.5mm and a mesh size of 15mm. The electrode spacing is also 15mm.

[0057] The iron electrode plates 3 are arranged at equal intervals, the aluminum electrode plates 4 are arranged at equal intervals, and the corresponding iron electrode plates 3 and aluminum electrode plates 4 are arranged alternately.

[0058] Aluminum plate 4 needs to be activated with 1% (m / v) hydrochloric acid for 30 minutes to remove surface oxides. Iron or aluminum plates are connected to an external DC regulated power supply via wires.

[0059] Example 4

[0060] The phosphogypsum leachate was treated using the phosphogypsum leachate treatment unit described in Example 3:

[0061] 1) Turn on the power supply and supply power to the iron plate 3 and aluminum plate 4 via DC power, with a current density of 35 mA / cm². 2 The positive and negative terminals are automatically switched every 10 minutes.

[0062] 2) Ozone and phosphogypsum leachate are simultaneously jetted into the phosphogypsum leachate treatment unit after passing through a premixing and pressurizing device, with an ozone dosage of 170 mg / L.

[0063] 3) With continuous power supply, ozone oxidation and electrolytic coupling reaction are carried out simultaneously in the phosphogypsum leachate treatment unit for 60 minutes.

[0064] 4) The pH of the effluent from the treatment unit is adjusted to 7.0 with sodium hydroxide solution, and then further adjusted to 9.0 with 5% quicklime solution. The precipitate is used for solid-liquid separation, and the supernatant is the treated effluent. The filter residue can be further recycled and utilized for phosphorus resource recovery.

[0065] The influent and effluent analysis indicators are as follows:

[0066] <![CDATA[PO4 3+ (mg / L)]]> <![CDATA[F - (mg / L)]]> COD (mg / L) Water ingress 5800 1100 143 Out of water 0.2 1.8 35 Removal rate 99.9％ 99.7％ 75％ .

[0067] Comparative Example 1

[0068] The difference from Example 1 is that the wastewater first undergoes ozone oxidation and then enters the phosphogypsum leachate treatment unit for electrolytic coupling reaction. Otherwise, it is the same as Example 1, specifically:

[0069] 1) The wastewater is first subjected to ozone oxidation reaction for 30 minutes, with an ozone dosage of 180 mg / L;

[0070] 2) The effluent from the reaction enters the phosphogypsum leachate treatment unit;

[0071] 3) Turn on the power supply and supply power to the iron plate 3 and aluminum plate 4 via DC power, with a current density of 25 mA / cm². 2 The positive and negative terminals are automatically switched every 10 minutes.

[0072] 5) Continuously apply electricity to carry out an electrolytic coupling reaction in the phosphogypsum leachate treatment unit for 45 minutes;

[0073] 6) The pH of the effluent from the treatment unit is adjusted to 7.0 with sodium hydroxide solution, and then further adjusted to 9.0 with 5% quicklime solution. The precipitate is used for solid-liquid separation, and the supernatant is the treated effluent. The filter residue can be further recycled and utilized for phosphorus resource recovery.

[0074] The influent and effluent analysis indicators are as follows:

[0075] <![CDATA[PO4 3+ (mg / L)]]> <![CDATA[F - (mg / L)]]> COD (mg / L) Water ingress 2370 830 124 Out of water 30.1 15.3 45 Removal rate 98.7％ 98.2％ 63.7％ .

[0076] Comparative Example 2

[0077] The difference from Example 1 is that the wastewater undergoes electrolysis first, and then enters the ozone oxidation unit for ozone oxidation. Otherwise, it is the same as Example 1.

[0078] 1) The phosphogypsum leachate treatment unit of Example 1 was used:

[0079] 2) Turn on the power supply and supply power to the iron plate 3 and aluminum plate 4 via DC power, with a current density of 25 mA / cm².2 The positive and negative terminals are automatically switched every 10 minutes.

[0080] 3) After the wastewater enters the phosphogypsum leachate treatment unit, it is continuously powered on and undergoes an electrolytic reaction within the phosphogypsum leachate treatment unit for 45 minutes.

[0081] 4) The effluent from the treatment unit is subjected to ozone oxidation for 30 minutes with an ozone dosage of 180 mg / L. The pH of the effluent is adjusted to 6.0-7.0 with sodium hydroxide solution, and then further adjusted to 8.5-9.0 with 5% quicklime solution. The sediment is separated into solid and liquid, and the supernatant is the treated effluent. The filter residue can be further recycled and utilized for phosphorus resource recovery.

[0082] The influent and effluent analysis indicators are as follows:

[0083]

[0084]

[0085] As can be seen from the above, the method of the present invention simultaneously carries out ozone oxidation and electrolytic coupling reactions in the phosphogypsum leachate treatment unit. By utilizing the combined effect of ozone and electrolysis, the electrode plates provide electrons, promoting the decomposition of ozone into hydroxyl radicals, ensuring the removal efficiency of organic matter, and achieving the standard discharge of COD, total phosphorus, and fluoride in wastewater. The total phosphorus removal rate can reach 99%, the fluoride ion removal rate is >99%, and the COD removal rate is >76%.

Claims

1. A phosphogypsum leachate treatment unit, characterized in that: The device includes a cylinder (1), a central shaft (2), iron electrode plates (3) and aluminum electrode plates (4). The cylinder (1) has a gas-liquid mixing inlet, a gas outlet and a liquid outlet. The central shaft (2) is installed inside the cylinder. An electrode plate frame is connected to the central shaft (2). Multiple iron electrode plates (3) are axially connected to one side of the central shaft (2) and the electrode plate frame. Multiple aluminum electrode plates (4) are axially connected to the other side of the central shaft and the electrode plate frame. The number of iron electrode plates (3) and aluminum electrode plates (4) are the same and correspond one-to-one. The corresponding iron electrode plate (3) is the positive electrode or the negative electrode, and the aluminum electrode plate (4) is the negative electrode or the positive electrode.

2. The phosphogypsum leachate treatment unit according to claim 1, characterized in that: The central shaft (2) is rotatably connected to the cylinder (1).

3. The phosphogypsum leachate treatment unit according to claim 1, characterized in that: The gas-liquid mixing inlet is opened tangentially along the cylinder (1), and the axial center line of the gas-liquid mixing inlet is inclined upward at 45 degrees to 60 degrees relative to the horizontal plane.

4. The phosphogypsum leachate treatment unit according to claim 1, characterized in that: The iron plate (3) is a wire mesh with a wire diameter of 2-3 mm and a mesh size of 10-18 mm. The aluminum electrode plate (4) is an aluminum wire mesh with a wire diameter of 0.5 to 2 mm and a mesh size of 10 to 15 mm.

5. The phosphogypsum leachate treatment unit according to claim 1, characterized in that: The iron plates (3) are arranged at equal intervals, the aluminum plates (4) are arranged at equal intervals, and the corresponding iron plates (3) and aluminum plates (4) are arranged alternately or coplanarly.

6. A method for treating phosphogypsum leachate using the phosphogypsum leachate treatment unit according to claim 1, characterized in that: The phosphogypsum leachate to be treated undergoes simultaneous ozone oxidation and electrolytic coupling reactions within the phosphogypsum leachate treatment unit. The pH value of the effluent from the treatment unit is adjusted, and the precipitate is subjected to solid-liquid separation. The supernatant is the treated effluent.

7. The method according to claim 6, characterized in that: The reaction time for ozone oxidation and electrolysis coupling is 45–75 min.

8. The method according to claim 6, characterized in that: The current density of the electrolytic coupling reaction within the phosphogypsum leachate treatment unit is 25–40 mA / cm². 2 .

9. The method according to claim 6, characterized in that: The ozone concentration for ozone oxidation is 100–120 mg / L.

10. The method according to claim 6, characterized in that: The positive and negative terminals of the external current connected to the phosphogypsum leachate treatment unit are switched every 10 minutes.

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