Phosphogypsum repair material for ecological restoration of mine and preparation method thereof

Abstract

The application discloses a kind of phosphogypsum repair materials for mine ecological restoration and preparation method thereof, it is related to solid waste resource utilization and mine ecological restoration technical field.The present application includes four core links of slurry conditioning buffer, nine-zone rotary filter cascade washing, aging stabilization and modification treatment;Through the partition function design and process parameter accurate control of nine-zone rotary filter, the graded removal of phosphorus and fluorine and pH gradient regulation are realized;Through the design of staged washing of clarified calcium oxide water and unclarified calcium oxide concentrated water, closed-loop reuse of washing water is realized;Through the synergistic effect of aging stabilization and solidification / decomposition modification treatment, the long-term dissolution risk of monobasic calcium phosphate is eliminated.The phosphorus removal rate of the repair material of the present application is greater than 99.99%, the fluorine removal rate is greater than 99.96%, the monobasic calcium phosphate decomposition rate is greater than 98%, the phosphorus leaching concentration is less than 0.1mg / L within 10 years or more leaching period, the washing water reuse rate reaches 85%, reduces the processing cost, and is suitable for mine roadway filling and slope re-greening and other scenes.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and mine ecological restoration technology, and in particular relates to a phosphogypsum remediation material for mine ecological restoration and its preparation method. Background Technology

[0002] Phosphogypsum is a major industrial by-product solid waste in the wet-process phosphoric acid production process. Approximately 4 to 5 tons of phosphogypsum are generated for every ton of phosphoric acid produced. The cumulative stockpile of phosphogypsum in my country has exceeded 800 million tons, with an annual increase in emissions of about 80 million tons. The large stockpile not only occupies land resources, but the acidic substances, phosphorus, fluorine, and trace heavy metals it contains can also pollute soil, surface water, and groundwater through leachate, threatening ecological and environmental safety. Mine ecological restoration is one of the core tasks of my country's "dual carbon" goals and ecological civilization construction. Utilizing phosphogypsum as a backfill and revegetation substrate for mines is an important way to realize the resource utilization of solid waste.

[0003] In the prior art, for example, CN121266925A discloses a method for preparing phosphogypsum-based ecological restoration materials, which uses a belt filter with four-stage washing combined with membrane treatment technology; another example is CN116462479B, which discloses a method for ecological reconstruction of mines using harmless phosphogypsum, which uses water washing for deep purification followed by chemical treatment.

[0004] However, traditional single-stage or limited-stage washing cannot deeply remove phosphorus, fluoride, and heavy metals. The pH value of raw phosphogypsum residue is only 1 to 2, the phosphate content is greater than 10,000 mg / L, and the fluoride content is greater than 5,000 mg / L. Direct use in mine remediation will lead to soil acidification, water eutrophication, and heavy metal pollution. The water resource utilization rate of existing washing processes is less than 30%, and the consumption of calcium oxide agents accounts for more than 45% of the total pretreatment cost. Some membrane treatment technologies have large equipment investment and high operating costs. The residual dicalcium phosphate (CaHPO4·2H2O) in phosphogypsum is slightly soluble in water and will slowly release phosphate ions under long-term leaching in the mine, causing phosphorus exceedances to reappear within 3 to 5 years after remediation. The independent operation of each process results in fluctuations of more than ±20% in indicators such as moisture content, pH value, and pollutant content of the remediation material, which cannot meet the standardized requirements of mine backfilling or revegetation scenarios.

[0005] To address these issues, we provide a phosphogypsum remediation material for mine ecological restoration and its preparation method. Summary of the Invention

[0006] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0007] This invention provides a method for preparing phosphogypsum remediation material for mine ecological restoration. It achieves deep removal of phosphorus and fluoride through a nine-zone rotary disc filter cascade washing process, reduces treatment costs through graded reuse of wash water, eliminates the long-term leaching risk of dicalcium phosphate through the synergistic effect of aging stabilization and modification treatment, and ensures stable product quality through fully automated control of the entire process. This invention solves the problems of incomplete pollutant removal, high treatment costs, poor long-term stability, and large fluctuations in product quality in existing technologies.

[0008] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution: This invention relates to a method for preparing phosphogypsum remediation material for mine ecological restoration, comprising the following steps: S1. Slurry preparation and buffering: The raw phosphogypsum residue is sent into the slurry preparation and buffering tank, and circulating wash water is added to adjust the slurry until the water content is greater than 80%, forming phosphogypsum residue slurry. S2. Nine-zone rotary disc filter stage washing: The phosphogypsum slurry obtained in step S1 is fed into a nine-zone rotary disc filter, and sequentially passes through nine zones: slurry zone, primary filtration and drying zone, first washing zone, first washing and filtration and drying zone, second washing zone, second washing and filtration and drying zone, final filtration and drying zone, slag roll-up zone, and filtration system washing zone. Slurry zone: Add anionic flocculant to cause solid particles to settle into a cake on the filter cloth, and control the filter cake thickness to 5 to 8 mm; Primary filtration and drying zone: Vacuum filtration is performed at a pressure of -0.06 to -0.08 MPa; Washing Zone 1: Washed by spraying with clear calcium oxide water with a pH greater than 12.50; Washing, filtration, and drying zone: Vacuum filtration is performed at a pressure of -0.07 to -0.09 MPa; Second washing zone: The filter cake is washed by spraying with concentrated calcium oxide solution with a pH greater than 12.80 to form a calcium oxide slurry coating on the surface of the filter cake. Second washing, filtration and drying zone: The calcium oxide slurry sealing layer on the surface of the filter cake is broken by scraping and then vacuumed at -0.08 to -0.10 MPa; Total filtration and drying zone: Vacuum filtration is performed at a pressure of -0.09 to -0.11 MPa; Filter cake collection area: After the filter cake is broken up, it is transported to the aging warehouse; Filtration system washing area: Regularly rinse the filter cloth; S3. Aging and stabilization: The phosphogypsum obtained in step S2 is transported to the aging warehouse, statically stored and turned over regularly, with an aging time of 15 days or more. S4. Modification treatment: Modify the aged phosphogypsum; S5. Obtain the finished phosphogypsum repair material.

[0009] The present invention is further configured such that the circulating wash water in step S1 comes from the wastewater in the primary filtration and drying zone; the pH of the phosphogypsum slurry is 1 to 2, the phosphorus content is greater than 10000 mg / L, and the fluorine content is greater than 5000 mg / L.

[0010] The present invention is further configured such that the anionic flocculant in the slurry zone is polyacrylamide with a molecular weight of 8 million to 12 million, an addition amount of 0.5 to 1.0 mg / L, and a cake formation time of 10s to 30s; after initial filtration and drying, the filter cake has a moisture content of less than 30% and a phosphorus and fluorine content of less than 300 mg / L.

[0011] The present invention is further configured such that the source of the clarified calcium oxide water in the first washing zone is: lime with a CaO content greater than or equal to 90% and the supernatant after the wastewater from the primary filtration and drying zone has been treated by an 8-stage inclined plate settling tank, with a residence time of 2 hours in each settling tank; a spray intensity of 2 to 3 m³ / (m²·h); and a phosphorus and fluoride content of less than 100 mg / L and a pH greater than 5 after the first washing.

[0012] The present invention is further configured such that the source of the unclarified calcium oxide concentrate in the secondary washing zone is: a mixed slurry of lime with a CaO content of 70% to 80% and wastewater from the primary filtration and drying zone; a spraying intensity of 3 to 4 m³ / (m²∙h); and a phosphorus content of less than 0.20 mg / L, a fluoride content of less than 20 mg / L, a pH greater than 11.50, and a filter cake moisture content greater than 40% after the secondary washing.

[0013] The present invention is further configured such that the scraper in the secondary washing, filtration and drying zone is a stainless steel scraper with a rotation speed of 5 to 8 r / min; the moisture content of the filter cake after total filtration and drying is less than 28%; and the particle size of the filter cake in the slag rolling zone is less than 50 mm.

[0014] The present invention is further configured such that the static stacking height in step S3 is 3 to 5 m, and the periodic turning of the stack is once every 5 days; after aging, the pH of the material is less than 9, and the active calcium oxide is completely consumed.

[0015] The present invention is further configured such that the curing scheme in step S4 is as follows: a water glass solution with a modulus of 3.0 to 3.2 is diluted at a ratio of 1:5 to 1:10 and then sprayed onto the surface of the filled phosphogypsum repair material, with a compaction density greater than 1.7 t / m³, and cured for 7 days, with a curing agent dosage of 2 to 5 kg / m³; the decomposition scheme is as follows: low-quality lime with a CaO content of 50% to 60% and a pH less than 8.80 is added, with a dosage of 3% to 5% of the phosphogypsum mass, and the mixture is stirred twice for 5 to 10 minutes using a twin-shaft mixer.

[0016] The present invention also provides a phosphogypsum remediation material for mine ecological restoration, wherein the remediation material has a pH value of less than 9, a leached fluoride content of less than 10 mg / L, and a leached phosphate content of less than 0.5 mg / L.

[0017] The present invention further specifies that the remediation material has a pH of 8.2 to 8.8, fluoride of 3.2 to 6.5 mg / L, phosphate of 0.12 to 0.28 mg / L, total chromium of 0.3 to 0.8 mg / L, total cadmium of 0.02 to 0.04 mg / L, and total lead of 0.1 to 0.3 mg / L.

[0018] The present invention has the following beneficial effects: 1. This invention utilizes a nine-zone rotary disc filter with a tiered washing process, achieving a phosphorus removal rate of over 99.99% and a fluorine removal rate of over 99.96%. All indicators of the repair material meet the requirements of GB18599-2020 and local standards. Compared to the existing four-stage belt filter washing technology, the nine-zone design of this invention achieves more refined washing grading and more thorough removal of pollutants.

[0019] 2. This invention achieves the tiered utilization of calcium oxide and closed-loop reuse of wash water through a graded washing design of first washing with clarified calcium oxide water and second washing with concentrated unclarified calcium oxide water. The wash water reuse rate reaches 85%, the consumption of calcium oxide reagent is reduced by 35%, and the treatment cost is reduced from 80 yuan / ton to 52 yuan / ton. Compared with the water reuse schemes in the prior art that use complex equipment such as membrane treatment, this invention can achieve efficient water circulation through simple graded sedimentation and graded reuse, reducing equipment investment and operating costs.

[0020] 3. Through the synergistic effect of aging stabilization and modification treatment, the decomposition rate of dicalcium phosphate is greater than 98%, and the phosphorus leaching concentration in the 10-year leaching test is less than 0.1 mg / L, with no long-term pollution risk. Compared with the existing technology that does not undergo aging treatment or has insufficient aging time, the present invention solves the hidden danger of long-term leaching of dicalcium phosphate by fully aging for more than 15 days in combination with targeted modification.

[0021] 4. By achieving fully automated control of the entire process, the key indicators of the remediation material (pH, moisture content, phosphorus, and fluorine) fluctuate by less than ±5%, meeting the standardized requirements of different mine ecological restoration scenarios.

[0022] 5. For every 10,000 tons of phosphogypsum remediation material used in this invention, the mining of natural sand and gravel can be reduced by 8,000 tons, 0.5 mu of land can be saved for stockpiling, and CO2 emissions can be reduced by about 200 tons. The production cost of the remediation material is lower than that of natural sand and gravel and traditional hardener filling materials, which can reduce the remediation cost for mining enterprises. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart illustrating the preparation method of phosphogypsum remediation material for mine ecological restoration. Detailed Implementation

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0026] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0027] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0028] Secondly, the present invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of the present invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not according to the usual scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. In addition, actual fabrication should include three-dimensional spatial dimensions of length, width, and depth.

[0029] Example 1: Preparation of phosphogypsum repair material for mine roadway filling Raw materials: The phosphogypsum residue was taken from a phosphoric acid plant in Yunnan Province. The initial parameters were: pH=1.5, phosphorus=11200mg / L, fluorine=5370mg / L, total chromium=2.1mg / L, and total cadmium=0.15mg / L.

[0030] Preparation steps: S1. Slurry preparation and buffer: The raw phosphogypsum residue is sent to the slurry preparation and buffer tank, and wastewater from the primary filtration and drying zone is added to adjust the slurry to a moisture content of 85% and SS=135g / L. Dynamic stirring and liquid level interlock control ensure stable concentration.

[0031] S2, Nine-zone rotary filter cascade washing: Slurry zone: Add 0.8 mg / L anionic polyacrylamide flocculant (molecular weight 10 million), cake thickness 6 mm, cake forming time 25 s; Primary filtration and drying zone: vacuum degree -0.07MPa, filter cake moisture content 27%, phosphorus = 265mg / L, fluorine = 238mg / L; First washing zone: sprayed with clear calcium oxide water (lime with 92% CaO content + primary filtered wastewater after 8 stages of sedimentation) with a pH of 12.6, spray intensity 2.5 m³ / (m²·h), after the first washing, phosphorus = 68 mg / L, fluoride = 32 mg / L, pH = 5.9; Washing, filtration, and drying zone: vacuum degree -0.08MPa, filter cake moisture content 29%; Second washing zone: Spraying unclarified calcium oxide concentrate (low-grade lime with 75% CaO content + primary filtered wastewater) with pH=13.0, spray intensity 3.5m³ / (m²·h), after the second washing phosphorus=0.15mg / L, fluoride=14mg / L, pH=11.7. Second washing, filtration and drying zone: scraper speed 6r / min, vacuum degree -0.09MPa after breaking the calcium oxide slurry layer; Total filtration drying zone: vacuum degree -0.10MPa, filter cake moisture content 26%; Slag-collecting zone: The blade spiral shaft rotates at a speed of 12 r / min, and the crushing particle size is less than 50 mm.

[0032] S3. Aging and Stabilization: Transport to the aging warehouse for storage, pile height 4m, aging for 15 days, turning over once every 5 days (3 times in total). After aging, pH=8.6, phosphorus=0.28mg / L, fluorine=5.2mg / L, total chromium=0.5mg / L, total cadmium=0.03mg / L.

[0033] S4. Modification treatment: Using a curing method, water glass with a modulus of 3.1 is diluted at a ratio of 1:8 and evenly poured onto the surface of the filled phosphogypsum repair material, compacted to a density of 1.75t / m³, and cured for 7 days.

[0034] Performance testing: compressive strength 1.8MPa; leaching test was conducted according to the "Solid Waste Leaching Toxicity Leaching Method Acetic Acid Buffer Solution Method" (HJ / T 300-2007), with a phosphorus leaching concentration of 0.03mg / L; all indicators meet the requirements of the "Technical Specification for Mine Roadway Backfilling" (GB / T 51446-2021).

[0035] Example 2: Preparation of phosphogypsum remediation material for mine revegetation Raw materials: Same as in Example 1.

[0036] Preparation steps: Steps S1 to S3 are the same as in Example 1.

[0037] S4. Modification treatment: A decomposition scheme is adopted, and 4% low-quality lime (CaO content 55%, pH=8.6) is added. The mixture is then stirred twice for 8 minutes using a twin-shaft mixer (speed 35r / min).

[0038] Performance testing: pH=8.7, dicalcium phosphate decomposition rate 99.2%; leaching test according to HJ / T 300-2007, phosphorus leaching concentration 0.02mg / L; after mixing with humus at a 1:1 ratio, plant germination index is greater than 85%, which can be used for mine slope revegetation.

[0039] Comparative Example 1: Phosphogypsum repair material prepared by traditional single-stage water washing process Process: The raw phosphogypsum residue is washed directly with water 3 times, with a water-to-solid ratio of 3:1 each time, and then air-dried.

[0040] Indicators: pH=5.2, phosphorus=320mg / L, fluorine=180mg / L, total chromium=1.8mg / L; leaching test was conducted according to HJ / T 300-2007, with a phosphorus leaching concentration of 1.2mg / L. All indicators do not meet the requirements of GB 18599-2020 and the standards for mine ecological restoration.

[0041] Comparative Example 2: Unaged Modified Phosphogypsum Remediation Material Process: Same as steps S1 to S2 in Example 1, but without steps S3, aging and stabilization, and steps S4 and modification treatment.

[0042] Indicators: pH=11.3, dicalcium phosphate content 0.8%; Leaching test was conducted according to HJ / T 300-2007, with an initial phosphorus leaching concentration of 0.85 mg / L, which increased to 1.5 mg / L after 30 days. There is a significant risk of secondary pollution.

[0043] Comparative Example 3: Traditional Four-Stage Belt Filter Washing Process Process: Following the method in CN121266925A, a four-stage washing process using a belt filter (countercurrent clean water + alkaline washing + phosphorus and fluoride cleaning) is adopted, without aging treatment.

[0044] Indicator: A leaching test was conducted according to HJ / T 300-2007. The initial phosphorus leaching concentration was 0.35 mg / L; after 90 days, the phosphorus leaching concentration rose to 0.72 mg / L, exceeding the limit requirement of GB18599-2020 (less than 0.5 mg / L). This indicates that without aging and stabilization treatment, the long-term leaching risk of dicalcium phosphate was not effectively controlled.

[0045] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0046] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention.

Claims

1. A method for preparing phosphogypsum remediation material for mine ecological restoration, characterized in that, Includes the following steps: S1. Slurry preparation and buffering: The raw phosphogypsum residue is sent into the slurry preparation and buffering tank, and circulating wash water is added to adjust the slurry until the water content is greater than 80%, forming phosphogypsum residue slurry. S2. Nine-zone rotary disc filter stage washing: The phosphogypsum slurry obtained in step S1 is fed into a nine-zone rotary disc filter, and sequentially passes through nine zones: slurry zone, primary filtration and drying zone, first washing zone, first washing and filtration and drying zone, second washing zone, second washing and filtration and drying zone, final filtration and drying zone, slag roll-up zone, and filtration system washing zone. Slurry zone: Add anionic flocculant to cause solid particles to settle into a cake on the filter cloth, and control the filter cake thickness to 5 to 8 mm; Primary filtration and drying zone: Vacuum filtration is performed at a pressure of -0.06 to -0.08 MPa; Washing Zone 1: Washed by spraying with clear calcium oxide water with a pH greater than 12.50; Washing, filtration, and drying zone: Vacuum filtration is performed at a pressure of -0.07 to -0.09 MPa; Second washing zone: The filter cake is washed by spraying with concentrated calcium oxide solution with a pH greater than 12.80 to form a calcium oxide slurry coating on the surface of the filter cake. Second washing, filtration and drying zone: The calcium oxide slurry sealing layer on the surface of the filter cake is broken by scraping and then vacuumed at -0.08 to -0.10 MPa; Total filtration and drying zone: Vacuum filtration is performed at a pressure of -0.09 to -0.11 MPa; Filter cake collection area: After the filter cake is broken up, it is transported to the aging warehouse; Filtration system washing area: Regularly rinse the filter cloth; S3. Aging and stabilization: The phosphogypsum obtained in step S2 is transported to the aging warehouse, statically stored and turned over regularly, with an aging time of 15 days or more. S4. Modification treatment: Modify the aged phosphogypsum; S5. Obtain the finished phosphogypsum repair material.

2. The preparation method according to claim 1, characterized in that, The circulating wash water in step S1 comes from the wastewater in the primary filtration and drying zone; the phosphogypsum slurry has a pH of 1 to 2, a phosphorus content greater than 10,000 mg / L, and a fluorine content greater than 5,000 mg / L.

3. The preparation method according to claim 1 or 2, characterized in that, The anionic settling agent in the slurry zone is polyacrylamide with a molecular weight of 8 million to 12 million, an addition amount of 0.5 to 1.0 mg / L, and a cake formation time of 10 to 30 seconds. After initial filtration and drying, the filter cake has a moisture content of less than 30% and a phosphorus and fluorine content of less than 300 mg / L.

4. The preparation method according to any one of claims 1, 2, and 3, characterized in that, The source of the clarified calcium oxide water in the first washing zone is: lime with a CaO content of ≥90% and the supernatant after the wastewater from the primary filtration and drying zone has been treated by an 8-stage inclined plate settling tank, with a residence time of 2 hours in each settling tank; spray intensity of 2 to 3 m³ / (m²·h); phosphorus and fluoride content of less than 100 mg / L and pH greater than 5 after the first washing.

5. The preparation method according to claims 1 to 4, characterized in that, The source of the unclarified calcium oxide concentrate in the secondary washing zone is: a mixed slurry of lime with a CaO content of 70% to 80% and wastewater from the primary filtration and drying zone; a spraying intensity of 3 to 4 m³ / (m²∙h); and after secondary washing, the phosphorus content is less than 0.20 mg / L, the fluoride content is less than 20 mg / L, the pH is greater than 11.50, and the filter cake moisture content is greater than 40%.

6. The preparation method according to claim 1, characterized in that, The scraper in the second washing, filtration and drying zone is a stainless steel scraper with a rotation speed of 5 to 8 r / min; the moisture content of the filter cake after total filtration and drying is less than 28%; and the particle size of the broken filter cake in the slag rolling zone is less than 50 mm.

7. The preparation method according to claim 1, characterized in that, The static stacking height in step S3 is 3 to 5 m, and the periodic turning of the stack is once every 5 days; after aging, the pH of the material is less than 9, and the active calcium oxide is completely consumed.

8. The preparation method according to claim 1, characterized in that, The curing scheme in step S4 is as follows: a water glass solution with a modulus of 3.0 to 3.2 is diluted at a ratio of 1:5 to 1:10 and then sprayed onto the surface of the filled phosphogypsum repair material. The compaction density is greater than 1.7 t / m³, and the curing is carried out for 7 days. The amount of curing agent used is 2 to 5 kg / m³. The decomposition scheme is as follows: low-quality lime with a CaO content of 50% to 60% and a pH of less than 8.80 is added at a dosage of 3% to 5% of the mass of phosphogypsum. The mixture is then stirred twice for 5 to 10 minutes using a twin-shaft mixer.

9. A phosphogypsum remediation material for mine ecological restoration, characterized in that, The phosphogypsum remediation material for mine ecological restoration is prepared by the preparation method described in claim 1; the remediation material has a pH value of less than 9, a leached fluoride content of less than 10 mg / L, and a leached phosphate content of less than 0.5 mg / L.

10. The phosphogypsum repair material according to claim 9, characterized in that, The remediation material has a pH of 8.2 to 8.8, fluoride content of 3.2 to 6.5 mg / L, phosphate content of 0.12 to 0.28 mg / L, total chromium content of 0.3 to 0.8 mg / L, total cadmium content of 0.02 to 0.04 mg / L, and total lead content of 0.1 to 0.3 mg / L.

Images