Synergistic thallium removal and rare earth recovery method for rare earth mine wastewater

Abstract

The invention belongs to the technical field of rare earth mine water environment comprehensive treatment, and particularly discloses a collaborative thallium removal and rare earth recovery method for rare earth mine wastewater. The collaborative thallium removal and rare earth recovery method comprises the following steps: concentrating and separating ore leaching wastewater through a nanofiltration membrane to obtain a concentrated solution and a filtrate; carrying out rare earth recovery treatment on the concentrated solution, and carrying out heavy metal removal treatment on the filtrate; wherein the rare earth recovery treatment comprises the following steps: adding an organic acid solution to carry out precipitation reaction to obtain rare earth salt precipitate and supernate, carrying out secondary concentration separation on the supernate through a nanofiltration membrane to obtain concentrated solution and filtrate, and then carrying out rare earth recovery treatment and heavy metal removal treatment again. The method provided by the invention can realize heavy metal removal and rare earth resource recovery, effectively improves the rare earth recovery rate and the heavy metal removal efficiency, and has important application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of comprehensive treatment of rare earth mine water environment, in particular to a method for collaboratively removing thallium from rare earth mine wastewater and recovering rare earths. Background Art

[0002] In recent years, the use of in-situ leaching technology for ionic rare earth mining has been widely used, but this method will produce leaching wastewater, which may cause a series of environmental problems. Therefore, it is necessary to remove heavy metals from the leaching wastewater. In addition, the recycling of rare earth metal resources in the leaching wastewater is also of great practical significance, which is often ignored in the conventional treatment process. The leaching wastewater of abandoned ionic rare earth mines has a high content of heavy metals, especially thallium, which can reach 400μg / L, hundreds of times higher than the emission standard. At the same time, the wastewater also contains high concentrations of heavy rare earth elements such as neodymium, yttrium, and praseodymium, which are of high grade and great value.

[0003] For rare earth mine wastewater, the current conventional treatment method only focuses on the removal of heavy metals, usually using pre-enrichment or conventional sodium sulfide precipitation treatment methods. This method requires a large amount of excessive dosage of reagents, and when sulfides are added, toxic H2S gas will also be produced. If not handled properly, it will cause secondary pollution and significantly increase the treatment cost.

[0004] Therefore, there is an urgent need to further improve the rare earth mine wastewater treatment process to achieve low-cost and efficient heavy metal removal and recycling of rare earth metal resources. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a method for synergistic thallium removal and rare earth recovery. The present invention uses a nanofiltration device to pre-enrich the rare earth metals in the leaching wastewater, adds a rare earth recovery process for the residual liquid, and the concentrated liquid is used for recovery after organic acid precipitation, thereby eliminating the waste of rare earth metals; the secondary concentration process used in the present invention can reduce the processing cost required for the initial concentration, facilitate centralized processing, and effectively reduce the processing load of heavy metals and rare earth elements in the wastewater, and improve the recovery rate of rare earth elements; in addition, the present invention recovers the rare earth in the wastewater and then performs thallium removal, which can achieve a heavy metal ion removal rate of more than 99%.

[0006] The present invention also proposes the application of the above-mentioned coordinated thallium removal and rare earth recovery method.

[0007] The first aspect of the present invention provides a method for collaboratively removing thallium and recovering rare earths, comprising the following steps:

[0008] S1, concentrating and separating the leaching wastewater through a nanofiltration membrane to obtain a concentrate and a filtrate;

[0009] S2, recovering rare earth from the concentrated solution and removing heavy metals from the filtrate;

[0010] Wherein, the rare earth recovery treatment is to add an organic acid solution to perform a precipitation reaction to obtain a rare earth salt precipitate and a supernatant;

[0011] The supernatant is subjected to secondary concentration and separation through a nanofiltration membrane to obtain a concentrate and a filtrate, and then the process of step S2 is performed again.

[0012] According to the specific implementation mode of the present invention, the synergistic thallium removal and rare earth recovery method provided by the present invention has at least the following beneficial effects: the present invention combines and improves the thallium removal and rare earth resource recovery process of rare earth mine leaching wastewater, and realizes simpler and more efficient synergistic thallium removal and rare earth recovery; the present invention first uses a nanofiltration device to pre-enrich the rare earth metals in the leaching wastewater, adds a residual liquid rare earth recovery process, and the concentrated liquid is used for recovery after organic acid precipitation, thereby eliminating the waste of rare earth metals and realizing effective resource recovery; the secondary concentration process also used in the present invention can reduce the processing cost required for the initial concentration, facilitate centralized treatment, reduce the subsequent wastewater treatment volume, reduce the amount of reagents required for treatment, is not easy to cause secondary pollution, and can effectively reduce the processing load of heavy metals and rare earth elements in the wastewater and improve the recovery rate of rare earth elements; in addition, the present invention recovers the rare earth in the wastewater and then performs thallium removal treatment, which can more efficiently remove heavy metals, achieve a heavy metal ion removal rate of more than 99%, and after thallium removal, the liquid thallium content is reduced to below 0.1μg / L, meeting the emission standard.

[0013] According to some embodiments of the present invention, the leaching wastewater is ionic rare earth mine leaching wastewater.

[0014] According to some embodiments of the present invention, the thallium (Tl) element content in the leaching wastewater is 150-250 μg / L, and the heavy metal elements yttrium (Y), neodymium (Nd), praseodymium (Pr), gadolinium (Gd) and dysprosium (Dy) content is 10-200 mg / L.

[0015] According to some embodiments of the present invention, the pore size of the nanofiltration membrane is 2 to 3 nm.

[0016] According to some embodiments of the present invention, the pressure of the nanofiltration membrane concentration separation is 0.2-0.5 MPa.

[0017] According to some embodiments of the present invention, the membrane flux of the nanofiltration membrane concentration separation is 4 to 8 L / m 2 ·h.

[0018] According to some embodiments of the present invention, the nanofiltration membrane performs concentration and separation to obtain a concentrate and a filtrate in a volume ratio of 1 to 4:2.

[0019] According to some embodiments of the invention, the organic acid comprises at least one of oxalic acid, citric acid, tartaric acid or malic acid.

[0020] According to some embodiments of the invention, the organic acid is oxalic acid.

[0021] According to some embodiments of the present invention, the adding of organic acid for precipitation reaction is specifically: firstly adding sulfuric acid solution to adjust the pH of the concentrated solution to 2-4, and then adding 4wt%-6wt% organic acid aqueous solution to mix with the concentrated solution.

[0022] During the exploration of the present invention, it was found that compared with the traditional method of using oxalic acid solution prepared with sulfuric acid solution directly for rare earth precipitation in wastewater, this scheme first uses sulfuric acid solution to adjust the pH of the wastewater to the optimal pH range of the oxalic acid precipitation reaction, and then adds an appropriate amount of oxalic acid aqueous solution. This can achieve more efficient oxalic acid utilization with less oxalic acid usage, thereby improving the recovery efficiency of rare earth metal elements.

[0023] According to some embodiments of the present invention, the molar ratio of the organic acid in the rare earth recovery treatment in step S2 to the rare earth elements in the leaching wastewater after the primary concentration and separation is 1 to 1.5:2.

[0024] According to some embodiments of the present invention, the molar ratio of the organic acid in the rare earth recovery treatment in step S2 to the rare earth elements in the leaching wastewater after the primary concentration and separation is 1.1 to 1.5:2.

[0025] The present invention finds that when the above method is used to carry out the oxalic acid precipitation reaction of rare earth metals, an optimal precipitation recovery effect can be achieved by using an appropriate excess of oxalic acid.

[0026] In the present invention, when rare earth recovery treatment is performed after secondary concentration and separation, a small amount of residual rare earth elements can be saturated with a small amount of oxalic acid to achieve more efficient precipitation collection.

[0027] According to some embodiments of the present invention, the step further comprises calcining the rare earth salt precipitate to obtain rare earth oxide powder.

[0028] According to some embodiments of the present invention, the calcination treatment is carried out at a temperature of 800 to 900° C. and for a time of 1 to 3 hours.

[0029] According to some embodiments of the present invention, the heavy metal removal treatment is specifically: adjusting the pH of the filtrate to 10-12, adding a stabilizer for reaction, then adding a biological agent for reaction, and finally adding a flocculant to accelerate solid-liquid separation to obtain a thallium-containing precipitate.

[0030] According to some embodiments of the present invention, the pH of the filtrate is adjusted by adding lime.

[0031] According to some embodiments of the present invention, the stabilizer includes at least one of a sodium sulfide solution, a potassium sulfide solution, a sodium hydrosulfide solution or a potassium hydrosulfide solution.

[0032] According to some embodiments of the present invention, the stabilizer includes at least two of a sodium sulfide solution, a potassium sulfide solution, a sodium hydrosulfide solution or a potassium hydrosulfide solution.

[0033] According to some embodiments of the present invention, the concentration of the stabilizer is 5 wt% to 15 wt%, and the amount of the stabilizer added is 0.01% to 0.1% of the volume of the filtrate.

[0034] According to some embodiments of the present invention, the time for adding the stabilizer to react is 10 to 30 minutes.

[0035] According to some embodiments of the present invention, the thallium removal biological agent is a biological agent conventionally used for thallium removal in the art; the biological agent of the present invention can effectively remove low-concentration and highly toxic metal ions such as Tl and Mn.

[0036] According to some embodiments of the present invention, the preparation method of the thallium-removing biological agent is as follows: adding manganese salt to the composite bacterial flora of Thiobacillus ferrooxidans and Thiobacillus thiooxidans in 9K medium for acclimation, Mn 2+ The amount of addition is 1-5 g / L, and the culture medium is cultured until Fe 2+ The concentration is ≤0.2g / L, and aeration is maintained during the culture process to obtain the thallium-removing biological preparation.

[0037] According to some embodiments of the present invention, the thallium-removing biological agent adopts the liquid biological agent for removing thallium from wastewater disclosed in the invention patent No. 202110736036.6 "A method for preparing a biological agent and a method for treating thallium-containing wastewater".

[0038] According to some embodiments of the present invention, the amount of the thallium-removing biological agent added is 0.01% to 0.1% of the volume of the filtrate.

[0039] According to some embodiments of the present invention, the time for reacting the adding of the dethallium biological agent is 10 to 30 minutes.

[0040] According to some embodiments of the present invention, the flocculant is at least one of a polyacrylamide (PAM) solution or a polyaluminium sulfate (PAS) solution.

[0041] The second aspect of the present invention provides the use of the synergistic thallium removal and rare earth recovery method described in the first aspect of the present invention in the recovery or purification of rare earth mine leaching wastewater.

[0042] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention will be further described below with reference to the accompanying drawings and embodiments, wherein:

[0044] Figure 1 This is a processing flow chart of leaching wastewater according to Example 1 of the present invention;

[0045] Figure 2 This is a SEM image of the rare earth metal oxide recovered in Example 1 of the present invention at a magnification of 500;

[0046] Figure 3 This is a SEM image of the rare earth metal oxide recovered in Example 1 of the present invention at a magnification of 5000. DETAILED DESCRIPTION

[0047] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0048] If the specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If the manufacturers of the reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0049] Example 1

[0050] This embodiment provides a method for synergistically removing thallium and recovering rare earth metal resources in rare earth mine wastewater. The processing flow chart is as follows: Figure 1 As shown, the specific steps are as follows:

[0051] 1) Take 2000 mL of leaching wastewater from a mine, concentrate it with a nanofiltration device, and obtain 1000 mL of concentrate and 1000 mL of filtrate;

[0052] The leaching wastewater was tested and found to contain 213.19 μg / L of thallium (Tl), 144.19 mg / L of yttrium (Y), 161.80 mg / L of neodymium (Nd), 28.50 mg / L of praseodymium (Pr), 46.58 mg / L of gadolinium (Gd), and 35.69 mg / L of dysprosium (Dy).

[0053] The nanofiltration device uses a nanofiltration membrane with a pore size of 2.5 nm, a set pressure of 0.35 MPa, and a membrane flux of 6 L / m 2 h; use nanofiltration equipment to enrich rare earth metals and concentrate them by one time; the concentrated solution contains a small amount of thallium, and the filtrate contains almost no rare earth metals.

[0054] 2) Rare earth metal recovery:

[0055] The concentrated solution is transferred to a rare earth reaction tank, 10 wt % sulfuric acid is added to adjust the pH to 2.5, and after sufficient mixing, 17 mL of 5 wt % oxalic acid (oxalic acid solution prepared with deionized water, the added amount is the theoretical amount calculated according to the rare earth metal content) is added to carry out precipitation reaction (the pH value of the solution is now in the optimal pH range of 2 to 4 for oxalic acid precipitation reaction). After the reaction is completed, it is transferred to a rare earth separation tank for solid-liquid separation, and the supernatant is circulated into a nanofiltration device for secondary concentration to obtain 400 mL of concentrated solution and 400 mL of filtrate; the concentrated solution after secondary concentration is placed in a rare earth reaction tank, and 5 mL of 5 wt % oxalic acid solution is continuously added to recover rare earths (the pH value of the solution is now in the optimal pH range of 2 to 4 for oxalic acid precipitation reaction) to obtain the final precipitation product of the two precipitation reactions; the precipitation product is subjected to high-temperature roasting at 850° C. and kept warm for 2 hours to recover the rare earth oxides; the supernatant of the precipitation reaction meets the domestic water discharge standard and is discharged.

[0056] 3) Strengthening Thallium Removal:

[0057] The filtrates of step 1) and step 2) are mixed and transferred to a reaction tank, lime is added to adjust the pH to 11, and after reacting for 15 minutes, a stabilizer solution is added to the mixed solution in an amount of 0.05% of the volume of the mixed solution, and the reaction is continued for 15 minutes. A biological agent is then added in an amount of 0.05% of the volume of the mixed solution, and the reaction is continued for 15 minutes. Then, a 0.002 mg / L polyacrylamide (PAM) solution is added in an amount of 0.1% of the volume of the mixed solution, and solid-liquid separation is performed to obtain purified water and sediment; the purified water meets the domestic water discharge standard and is discharged, and the sediment is post-treated as hazardous waste.

[0058] Among them, the stabilizer solution used is: sodium sulfide and sodium hydrosulfide are mixed in a mass ratio of 1:2 and then configured into a 10wt% solution; the biological agent used is the biological agent prepared in Example 1 of the invention patent No. 202110736036.6 "A method for preparing a biological agent and a method for treating thallium-containing wastewater".

[0059] Comparative Example 1

[0060] The difference between this comparative example and Example 1 is only in step 2). This comparative example performs rare earth metal recovery in step 2) according to the conventional method of rare earth recovery in the art:

[0061] 2) The concentrated solution is transferred to a rare earth reaction tank, and a 5wt% oxalic acid solution prepared with 10wt% sulfuric acid is added, and a precipitation reaction is carried out after the pH is adjusted to 2.5 (the amount of oxalic acid solution added is 28.82mL). After the reaction is completed, the concentrated solution is transferred to a rare earth separation tank for solid-liquid separation, and the supernatant is circulated into a nanofiltration device for secondary concentration to obtain 400mL of concentrated solution and 400mL of filtrate; the concentrated solution after the secondary concentration is placed in a rare earth reaction tank, and 5mL of a 5wt% oxalic acid solution prepared with 10wt% sulfuric acid is continued to be added for rare earth recovery (at this time, the pH value of the solution is within the optimal pH range of 2 to 4 for the oxalic acid precipitation reaction), to obtain the final precipitation product of the two precipitation reactions; the precipitation product is subjected to high-temperature roasting at 850°C and kept warm for 2h to recover the rare earth oxides.

[0062] The other steps are the same as those in Example 1.

[0063] Comparative Example 2

[0064] The difference between this comparative example and Example 1 is that in step 2), no secondary concentration is performed in this comparative example; the other steps are the same as those in Example 1.

[0065] Comparative Example 3

[0066] The difference between this comparative example and Example 1 is only in step 2). This comparative example performs rare earth metal recovery in step 2) according to the conventional method of rare earth recovery in the art:

[0067] 2) The concentrated solution is transferred to a rare earth reaction tank, and a 5 wt % oxalic acid solution prepared with 10 wt % sulfuric acid is added, and the pH is adjusted to 2.5 to carry out a precipitation reaction (the added oxalic acid solution is 28.82 mL). After the reaction is completed, it is transferred to a rare earth separation tank for solid-liquid separation to obtain a precipitation product of the precipitation reaction; the precipitation product is calcined at 850° C. and kept warm for 2 hours to recover the rare earth oxides.

[0068] The other steps are the same as those in Example 1.

[0069] Comparative Example 4

[0070] This comparative example provides a method for removing thallium from leaching wastewater, and the specific steps are as follows:

[0071] 2000 mL of leaching wastewater from the same batch as in Example 1 was added to the reaction tank, lime was added to adjust the pH to 11, and after reacting for 15 minutes, a stabilizer solution was added to the mixed solution in an amount of 0.05% of the volume of the mixed solution, and the reaction was continued for 15 minutes. A biological agent was then added in an amount of 0.05% of the volume of the mixed solution, and the reaction was continued for 15 minutes. PAM was then added in an amount of 0.1% of the volume of the mixed solution, and solid-liquid separation was performed to obtain purified water and sediment; the purified water was discharged after meeting the domestic water discharge standard.

[0072] The stabilizer solution, biological agent and PAM solution are the same as those in Example 1.

[0073] Comparative Example 5

[0074] This comparative example provides a method for recovering heavy metals from leaching wastewater, and the specific steps are as follows:

[0075] 2000 mL of the same batch of leaching wastewater as in Example 1 was added to the rare earth reaction tank, and 10 wt % of sulfuric acid was added to adjust the pH to 2.5 according to the volume ratio of Example 1. After being fully mixed, 5 wt % of oxalic acid (oxalic acid solution prepared with deionized water) was added to carry out precipitation reaction. After the reaction was completed, it was transferred to the rare earth separation tank for solid-liquid separation;

[0076] The separated supernatant is subjected to the above-mentioned oxalic acid precipitation step to obtain the final precipitation product of the two precipitation reactions; the precipitation product is subjected to high-temperature roasting at 850° C. and kept warm for 2 hours to recover the rare earth oxide.

[0077] Detection of leaching wastewater after treatment:

[0078] 1. XRF analysis was performed on the rare earth oxides obtained in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 5, and the sediment obtained in Comparative Example 4, respectively, to calculate the recovery rate of rare earth metals, wherein the XRF analysis results of the rare earth oxides in Example 1 are shown in Table 1, and the recovery rate of rare earth metals calculated based on the XRF analysis results are shown in Table 2.

[0079] Table 1 XRF analysis results of rare earth oxides recovered in Example 1

[0080]

[0081]

[0082] XRF analysis of the rare earth oxides finally obtained by this scheme can intuitively show the content of rare earth elements in the rare earth oxides. It is found that there are relatively high contents of rare earth elements such as neodymium, yttrium, gadolinium, dysprosium and praseodymium, which can be used as high-quality rare earth resources for subsequent utilization. Therefore, this scheme can reduce environmental pollution and improve the production efficiency of enterprises.

[0083] 2. All purified water finally obtained in Example 1 and Comparative Examples 1 to 5 was tested for thallium element, and the thallium concentration of the effluent water was obtained as shown in Table 2.

[0084] Table 2 Rare earth metal recovery rate and effluent thallium concentration after leaching wastewater treatment

[0085]

[0086]

[0087] From the above test results, it can be seen that the synergistic thallium removal and rare earth recovery method of the present invention achieves a removal rate of more than 99.9% of heavy metal thallium in leaching wastewater, reduces the thallium concentration in the final purified water to below 0.1 μg / L, and achieves a high recovery efficiency of rare earth elements such as yttrium, neodymium, praseodymium, gadolinium and dysprosium, with a minimum recovery rate of more than 72%. This scheme simultaneously achieves efficient recycling of resources and environmental protection, and has a good application prospect.

[0088] The rare earth recovery rate of comparative example 1 is low, mainly because of its oxalic acid addition method, which leads to low utilization rate of oxalic acid and difficulty in achieving more efficient rare earth recovery. In the method of comparative example 2, only one concentration is used, and some rare earth metal elements that are not recovered are still present in the supernatant after oxalic acid precipitation, and a small amount of thallium is also present in the concentrated solution after one concentration, resulting in a higher concentration of thallium in the final purified water. In comparative example 3, oxalic acid is used inefficiently and secondary concentration is not used, resulting in significantly lower rare earth metal recovery efficiency. In comparative example 4, only enhanced thallium removal treatment is performed on the leaching wastewater. Due to the excessive load pressure to be treated and the large number of impurity elements, the thallium removal effect is weakened compared to the embodiment, and it is difficult to achieve a higher level of thallium removal effect, and there is almost no rare earth recovery effect, and the valuable part of the wastewater is not recovered, resulting in a waste of resources. In comparative example 5, only oxalic acid precipitation treatment is performed on the leaching wastewater to recover rare earth metals, but it cannot achieve a good effect of removing heavy metals such as thallium, resulting in serious excess thallium concentration in the effluent. If the discharge is to meet the standard, a weight removal process must be added again, and the treatment process is more complicated, with greater requirements on equipment and cost.

[0089] 3. The rare earth oxide obtained in Example 1 was observed by scanning electron microscope. The SEM images at 500 times and 5000 times magnification were as follows: Figure 2 and Figure 3 shown.

[0090] The above is a detailed description of the embodiments of the present invention, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge of ordinary technicians in the relevant technical field without departing from the purpose of the present invention. In addition, the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A method for synergistic thallium removal and rare earth recovery, characterized in that: The following steps are involved: S1, concentrating and separating the leaching wastewater through a nanofiltration membrane to obtain a concentrate and a filtrate; S2, recovering rare earth from the concentrated solution and removing heavy metals from the filtrate; Wherein, the rare earth recovery treatment is to add an organic acid solution to perform a precipitation reaction to obtain a rare earth salt precipitate and a supernatant; The supernatant is concentrated and separated twice by a nanofiltration membrane to obtain a concentrate and a filtrate, and then the process of step S2 is performed again.

2. The method according to claim 1, characterized in that The pore size of the nanofiltration membrane is 2 to 3 nm; And / or, the pressure of the nanofiltration membrane concentration separation is 0.2-0.5 MPa.

3. The method according to claim 1, characterized in that The organic acid comprises at least one of oxalic acid, citric acid, tartaric acid or malic acid; Preferably, the organic acid is oxalic acid.

4. The method according to claim 1, characterized in that: The adding of organic acid to carry out precipitation reaction specifically comprises: firstly adding sulfuric acid solution to adjust the pH of the concentrated solution to 2-4, and then adding 4wt%-6wt% organic acid aqueous solution to mix with the concentrated solution.

5. The method according to claim 4, characterized in that The molar ratio of the organic acid in the rare earth recovery treatment in step S2 after the primary concentration and separation to the rare earth elements in the leaching wastewater is 1 to 1.5:

2.

6. The method according to claim 1, characterized in that The step also includes calcining the rare earth salt precipitate to obtain rare earth oxide powder.

7. The method according to claim 1, characterized in that The heavy metal removal treatment is specifically as follows: adjusting the pH of the filtrate to 10-12, adding a stabilizer for reaction, then adding a thallium removal biological agent for reaction, and finally adding a flocculant to accelerate solid-liquid separation to obtain a thallium-containing precipitate.

8. The method according to claim 7, characterized in that The stabilizer includes at least one of a sodium sulfide solution, a potassium sulfide solution, a sodium hydrosulfide solution or a potassium hydrosulfide solution.

9. The method according to claim 8, characterized in that The concentration of the stabilizer is 5wt% to 15wt%, and the amount of the stabilizer added is 0.01% to 0.1% of the volume of the filtrate.

10. Use of the synergistic thallium removal and rare earth recovery method according to any one of claims 1 to 9 in the recovery or purification of rare earth mine leaching wastewater.

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