Mine wastewater treatment method

Abstract

To provide a treatment method which has a good effect of removing heavy metal, has a high density of neutralized sediment, and has a good effect of reducing a volume of a sediment.SOLUTION: There is provided a mine wastewater treatment method, which is a treatment method in which a neutralizing agent containing magnesium oxide is added to heavy metal-containing mine wastewater to generate a sediment containing heavy metal, and the sediment is removed by solid-liquid separation, the method including: a neutralization step of adding the above-mentioned neutralizing agent to the mine wastewater to produce heavy metal-containing sediment under a liquid condition of pH 6.5 to 7.5; and a sediment return step of separating a heavy metal-containing sediment generated in the neutralization step into solid and liquid, adding magnesium oxide to part or all of it to make an alkaline sediment, and using this alkaline sediment as a neutralizing agent in the neutralization step.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for neutralizing wastewater (called mine wastewater) discharged from closed and abandoned mines, thereby removing heavy metals and reducing the volume of the neutralized precipitate. [Background technology]

[0002] Mine wastewater contains large amounts of heavy metals such as manganese and iron, so a treatment method has been known in the past to neutralize the mine wastewater by adding slaked lime to the water and remove the manganese, iron, etc. by converting them into hydroxide precipitates (Patent Document 1). However, the neutralized precipitates produced by this treatment method are bulky, which poses a significant burden for landfill disposal. In addition, the slaked lime treatment is only effective in removing fluorine from the mine wastewater.

[0003] Thus, a treatment method is known in which manganese oxide is used as a neutralizing agent, manganese oxide is added to mine wastewater to neutralize it, a precipitate is recovered by solid-liquid separation, manganese oxide is added to the precipitate to form an alkaline precipitate, and the alkaline precipitate is used as a neutralizing agent, and the steps are repeated: a primary neutralization step, and a secondary neutralization step in which magnesium oxide and an aluminum source are added to the liquid fraction separated from the solid after the primary neutralization to produce a fluorine-containing precipitate, and the precipitate is subjected to solid-liquid separation to remove fluorine (Patent Document 2). The treatment method in Patent Document 2 has the advantages of being highly effective in removing manganese, iron, and zinc, being able to remove fluorine by secondary neutralization, and having a high precipitate density due to the use of manganese oxide as a neutralizing agent.

[0004] On the other hand, mine wastewater contains heavy metals such as copper, arsenic, lead, and cadmium in addition to manganese, iron, and zinc, and it is necessary to improve the removal efficiency of these heavy metals. Also, in order to reduce the burden of landfill disposal, it is required that the density of the neutralized precipitate is high and that the volume reduction effect of the precipitate is good. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-160496 [Patent Document 2] Patent Publication No. 2021-166981 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention provides a treatment method that is effective in removing manganese, iron, zinc, as well as copper, arsenic, lead, cadmium, etc. contained in mine wastewater, and that produces a neutralized precipitate with a high density and an effective volume reduction of the precipitate. [Means for solving the problem]

[0007] [1] Mine wastewater containing one or more heavy metals from the group consisting of copper, lead, arsenic, and cadmium a neutralizing step in which the neutralizing agent containing magnesium oxide is added to the mine wastewater to form a precipitate containing the heavy metals, and the precipitate is then removed by solid-liquid separation, the method comprising: a neutralization step in which the neutralizing agent is added to the mine wastewater to form a precipitate containing the heavy metals at a liquid pH of 6.5 to 7.5; and a precipitate return step in which the heavy metal-containing precipitate formed in the neutralization step is solid-liquid separated, magnesium oxide is added to part or all of it to form an alkaline precipitate, and the alkaline precipitate is used as the neutralizing agent in the neutralization step. (2) A method for treating mine wastewater according to the above item [1], in which a neutralizing agent consisting of an alkaline precipitate in which the amount of magnesium oxide in the alkaline precipitate is 0.15 to 0.30 g is added to 1,000 g of mine wastewater having a pH of 2.8 to 3.2, to adjust the pH to 6.5 to 7.5. (3) A method for treating mine wastewater according to [1] or [2] above, in which the neutralization step and the sediment return step are repeated.

[0008] [Specific explanation] The processing method of the present invention will now be described in detail. The treatment method of the present invention is a treatment method in which a neutralizing agent containing magnesium oxide is added to mine wastewater containing heavy metals including one or more of copper, lead, arsenic and cadmium to produce a precipitate containing the heavy metals, and the precipitate is removed by solid-liquid separation, characterized in that it comprises: a neutralization step in which the neutralizing agent is added to the mine wastewater to produce a heavy metal-containing precipitate at a liquid pH of 6.5 to 7.5; and a precipitate return step in which the heavy metal-containing precipitate produced in the neutralization step is separated into solids and liquids, magnesium oxide is added to part or all of it to form an alkaline precipitate, and this alkaline precipitate is used as the neutralizing agent in the neutralization step.

[0009] The treatment method of the present invention includes a neutralization step in which a neutralizing agent containing magnesium oxide is added to mine wastewater containing heavy metals, including one or more of copper, lead, arsenic, and cadmium, to produce a heavy metal-containing precipitate at a pH of 6.5 to 7.5. By using a neutralizing agent containing magnesium oxide, a layered double hydroxide (LDH) precipitate is produced at the above pH, and copper, lead, arsenic, and cadmium are incorporated into the precipitate. These heavy metals can be removed from the mine wastewater by solid-liquid separation of the precipitate.

[0010] In the method of Patent Document 1, the pH of the mine wastewater is adjusted to 9.5 to 10.5 to form a normal hydroxide precipitate, but the treatment method of the present invention forms layered double hydroxides (LDHs) under the above-mentioned liquid conditions, making it possible to separate and remove heavy metals at a lower pH range than the method of Patent Document 1.

[0011] It should be noted that using a neutralizing agent containing magnesium oxide to raise the pH to above 7.5 is undesirable because it takes a long time for neutralization, and the LDH produced at a pH of 7.5 or less is altered to a normal hydroxide at a pH above 7.5, forming a bulky precipitate, and the capacity of the neutralization equipment increases.

[0012] In the neutralization step, to adjust the liquid to pH 6.5 to 7.5, for example, a neutralizing agent consisting of an alkaline precipitate in which the amount of magnesium oxide in the alkaline precipitate is 0.15 to 0.30 g may be added to 1000 g of mine wastewater having a pH of 2.8 to 3.2.

[0013] The concentration of the precipitate produced in the neutralization step is preferably 1000 g / L or more. The precipitate concentration can be increased by repeating the neutralization step and the precipitate return step. Specifically, for example, a precipitate with a concentration of 1000 g / L or more can be obtained by repeating the neutralization step 40 times. However, in the method of Patent Document 1, aluminum sulfate is added to remove fluorine, so the precipitate concentration remains at around 180 g / L. [Effects of the Invention]

[0014] According to the treatment method of the present invention, manganese, iron, and zinc as well as copper, lead, arsenic, and cadmium can be effectively precipitated and removed to levels below the wastewater standards. By repeating the neutralization step and the precipitate return step, the processing time can be gradually halved in the initial, middle, and final stages of processing, thereby shortening the processing time. Specifically, for example, the stirring time when adding and stirring the neutralizing agent can be roughly halved to 30-60 minutes after 1-3 repetitions, 15-30 minutes after 4-5 repetitions, and 8-15 minutes after 6 repetitions. Furthermore, the amount of magnesium oxide used can be gradually reduced. Furthermore, by repeating the neutralization step and the precipitate return step, the concentration of the neutralized precipitate can be increased to 1000 g / L or more, which is significantly smaller in volume than precipitates prepared using slaked lime, thereby significantly reducing the burden on landfill disposal. DETAILED DESCRIPTION OF THE INVENTION

[0015] Examples of the present invention will be described below. The mine wastewater (hereinafter referred to as raw water) with the heavy metal content shown in Table 1 was used, and the experiment was carried out in accordance with the Metal Mining Agency of Japan's "Guidelines for Repeated Neutralization of Sediments" (1990 edition).

[0016] [Table 1]

[0017] [Example] 0.21 g of a neutralizing agent (MgO powder) was added to 1 L of raw water and stirred for 10 to 60 minutes to achieve a target pH of 7.1, producing a precipitate (neutralization step). The supernatant liquid of this precipitate was removed, and 98.6 g / L of precipitate slurry was recovered. 0.21 g of MgO powder was added to this to form a precipitate neutralizer, which was then added to 1 L of untreated raw water (precipitate return step). This neutralization step and precipitate return step were repeated 40 times, followed by solid-liquid separation, yielding a concentrated precipitate with a dry concentration of 1011.0 g / L. The results are shown in Table 2. The heavy metal concentrations in the liquid fraction after 40 solid-liquid separations were measured using ICP-AES. The results are shown in Table 3.

[0018] Comparative Example Instead of the MgO powder used in the above example, 0.224 g of 2.0 wt% slaked lime milk was used as the neutralizing agent, adjusting the target pH to 7.4 to produce a precipitate (neutralization step). The supernatant of this precipitate was removed, and 5.5 g / L of precipitate slurry was recovered. 0.224 g of the above slaked lime milk was added to this to form a precipitate neutralizing agent. This precipitate neutralizing agent was returned to the neutralization step and added to the raw water (precipitate return step). This neutralization step and precipitate return step were repeated 40 times, followed by solid-liquid separation, yielding a concentrated precipitate with a dry concentration of 82.0 g / L. The results are shown in Table 2. The heavy metal concentrations in the separated liquid were measured using ICP-AES. The results are shown in Table 3.

[0019] [Table 2]

[0020] [Table 3]

[0021] As shown in Table 2, comparing the concentrated sediment concentrations, the Example was 1011.0 g / L, while the Comparative Example was only 82.0 g / L, indicating that the concentration of the sediment was much greater. Furthermore, comparing the sediment volumes, the Example was very small at 12.1 mL, while the Comparative Example was 83.3 mL, resulting in a bulky precipitate. This is because the MgO powder in the Example dissolves in water more slowly than hydrated lime, and some of it remains undissolved, forming nuclei for aggregation and contributing to the concentration and volume reduction of the sediment. Furthermore, the remaining MgO powder acts again as a neutralizing agent in the next repetition, allowing for a reduction in the amount of newly added MgO with each repetition. According to the heavy metal concentrations in Table 3, the Mn concentration, Zn concentration, and Cd concentration in the liquid of the Example were significantly lower than those of the Comparative Example, and the Cu concentration, Pb concentration, and As concentration were also at the same level as those of the Comparative Example. It has been reduced.

Claims

1. A treatment method for mine wastewater containing one or more heavy metals, copper, lead, arsenic, and cadmium, by adding a neutralizing agent containing magnesium oxide to the mine wastewater to produce a precipitate containing the heavy metals, and then removing the precipitate by solid-liquid separation, characterized by comprising: a neutralization step in which the neutralizing agent is added to the mine wastewater to produce a precipitate containing the heavy metals at a liquid pH of 6.5 to 7.5; and a precipitate return step in which the heavy metal-containing precipitate produced in the neutralization step is separated into solid and liquid, magnesium oxide is added to some or all of it to make an alkaline precipitate, and the alkaline precipitate is used as the neutralizing agent in the neutralization step.

2. 2. A method for treating mine wastewater according to claim 1, wherein a neutralizing agent comprising an alkaline precipitate having a magnesium oxide content of 0.15 to 0.30 g is added to 1,000 g of mine wastewater having a pH of 2.8 to 3.2 to adjust the pH of the liquid to 6.5 to 7.

5.

3. 3. A method for treating mine wastewater according to claim 1 or 2, wherein the neutralization step and the sediment return step are repeated.

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