JP7351199B2 - Treatment method for arsenic-containing wastewater - Google Patents

Description

The present invention relates to a treatment method for stably reducing the arsenic concentration of arsenic-containing wastewater, such as mine wastewater, to below the wastewater standard.

Mine wastewater includes water that comes from mine shafts (inside mine water) and water that comes from outside the mine, such as ore processing plants, smelters, or rubble and slag accumulation sites. Mine wastewater includes mine water and outside mine water. This mine wastewater is acidic wastewater containing heavy metals such as iron, manganese, copper, zinc, lead, arsenic, and cadmium, and when mine wastewater is discharged outside, it must be treated to prevent these from becoming a source of pollution. .

Regarding wastewater containing arsenic, since arsenic is incorporated into ferric hydroxide and co-precipitates, conventionally, the wastewater is neutralized in the presence of iron and the arsenic is turned into a precipitate to reduce the arsenic concentration. . For example, the treatment methods described in Patent Documents 1 to 3 are known for wastewater containing arsenic.

Patent Document 1 discloses that mine wastewater containing arsenic and iron sulfide is neutralized by adding magnesium hydroxide or sodium hydroxide as a neutralizing agent and stirring it together with magnesium sulfate precipitate or sodium sulfate precipitate. , describes a method for treating mine wastewater characterized by co-precipitating arsenic precipitates and separating these precipitates.

Patent Document 2 discloses that a ferric salt with Fe/As = 2 or more and a calcium salt with Ca/F = 20 or more are added to wastewater containing As and F to cause coagulation and precipitation in the pH range of 8.0 to 8.5. The first step is to perform coagulation and precipitation in the pH range of 6.5 to 7.0 by adding an aluminum salt with Al/As = 30 or more and Al/F = 2 or more to the supernatant separated water. A method for treating arsenic- and fluorine-containing wastewater is described, which comprises a second stage treatment step.

Patent Document 3 discloses that a soluble ferric salt is added to wastewater containing gallium and arsenic, the pH is adjusted with an alkaline agent, gallium and arsenic are coprecipitated with a precipitate of ferric hydroxide, and this precipitate is added to wastewater. Furthermore, the separated precipitate is suspended in water, the pH is adjusted to the alkaline side to elute gallium, and the solid-liquid separated liquid is evaporated to dryness to obtain gallium. The process for collecting the is described.

Japanese Patent Application Publication No. 2006-116468 Japanese Unexamined Patent Publication No. 185375/1983 Japanese Patent Application Publication No. 59-95991

In the treatment methods of Patent Documents 1 to 3, in which wastewater containing arsenic is neutralized in the presence of iron and arsenic is co-precipitated with iron hydroxide precipitate to form a precipitate, the iron content contained in the wastewater is low. Regarding wastewater in which the weight ratio of iron to arsenic (Fe/As ratio) varies significantly, there is a problem that arsenic cannot be sufficiently converted into a precipitate, making it difficult to enhance the arsenic removal effect.

Furthermore, since the treatment method of Patent Document 2 uses aluminum salt in the second treatment step, the treatment step is complicated and the cost is high. Furthermore, the treatment method of Patent Document 3 has a problem in that it is difficult to sufficiently remove arsenic due to the influence of coexisting gallium.

The treatment method of the present invention solves the above-mentioned conventional problems in the treatment of wastewater containing arsenic, and stably reduces the arsenic concentration to below the wastewater standard even in wastewater where the arsenic and iron contents vary widely. Provides a processing method that can.

The treatment method of the present invention is a treatment method for arsenic-containing wastewater having the following configuration.
[1] In a treatment method in which acidic wastewater containing arsenic is neutralized in the presence of iron and the arsenic is turned into a precipitate to reduce the arsenic concentration, the weight ratio of iron to arsenic (Fe/As ratio) is 29. The sludge containing arsenic precipitates produced in the neutralization process was collected from the wastewater whose concentration ranged from 55 to 66 and the solid content was 60 to 120 g/L. Arsenic-containing wastewater characterized by returning the collected sludge to the neutralization treatment and adding it to the wastewater and subjecting it to neutralization treatment, thereby alleviating fluctuations in the Fe/As ratio and promoting the formation of arsenic as a precipitate. processing method.
[2] The method for treating arsenic-containing wastewater according to [1] above, wherein the arsenic-containing wastewater is mine wastewater.
[3] The recovered sludge with the above Fe/As ratio and the above solids concentration is returned to the above neutralization treatment, and is added at a ratio of 0.02 to 0.2 L to 1 L of the above wastewater for the neutralization treatment. 1] or the method for treating arsenic-containing wastewater described in [2] above.

[Specific explanation]
The treatment method of the present invention involves neutralizing acidic wastewater containing arsenic in the presence of iron to reduce the arsenic concentration by turning the arsenic into a precipitate. The sludge containing arsenic precipitates produced in the neutralization process is collected from the wastewater whose Fe/As ratio varies in the range of 29 to 110 , and the Fe/As ratio is 55 to 66 and the solids concentration is 60. The recovered sludge of ~120g/L is returned to the neutralization treatment, and is added to the wastewater and subjected to the neutralization treatment, thereby alleviating the fluctuation of the Fe/As ratio and promoting the conversion of arsenic into precipitate. This is a method for treating arsenic-containing wastewater.

When acidic wastewater containing ferric ions is neutralized to pH 5.8 to pH 8.6 by adding slaked lime, a neutralized precipitate of ferric hydroxide is produced. If this wastewater contains arsenic ions, the arsenic ions will be incorporated into the neutralized precipitate and co-precipitate, forming an arsenic-containing precipitate and reducing the arsenic concentration.

The treatment method of the present invention collects sludge containing arsenic-containing precipitates generated through neutralization treatment of wastewater in which the weight ratio of iron to arsenic (Fe/As ratio) varies, and removes Fe/As from the wastewater. A treatment method in which sludge having an Fe/As ratio in the middle of the ratio fluctuation range is returned to the neutralization treatment to be neutralized, thereby alleviating the fluctuation in the Fe/As ratio and promoting the formation of arsenic as a precipitate. be.

In the treatment method of the present invention, sludge having an intermediate Fe/As ratio in the Fe/As ratio fluctuation range of the wastewater is defined as a sludge having an Fe/As ratio of about 1.9 times the lower limit of the Fe/As ratio of the wastewater. This refers to sludge having a Fe/As ratio that is approximately 0.6 times the upper limit of the Fe/As ratio. Specifically, for example, when the Fe/As ratio of the wastewater varies in the range from 29 to 110, the sludge has a Fe/As ratio in the range from 55 to 66.

The treatment method of the present invention includes a neutralization step of adding slaked lime or the like to wastewater in which the Fe/As ratio fluctuates, and a step of recovering sludge containing arsenic-containing precipitate produced by the neutralization treatment. The sludge has a Fe/As ratio that is intermediate in the Fe/As ratio fluctuation range of the wastewater, and the sludge is returned to the neutralization step and added to the wastewater together with slaked lime.

By returning sludge with an Fe/As ratio that is intermediate to the Fe/As ratio fluctuation range of wastewater to the neutralization process and adding it to the wastewater, fluctuations in the Fe/As ratio of wastewater are alleviated, and the return of this sludge is By repeating the process, an arsenic-containing precipitate having a stable arsenic concentration is formed in the neutralization treatment in which slaked lime is added, and it is possible to perform the neutralization treatment in which the arsenic concentration is stably kept low. In addition, when the sludge containing arsenic-containing sediment recovered after the neutralization treatment deviates from the intermediate Fe/As ratio of the Fe/As ratio fluctuation range of the wastewater, when returning it to the neutralization process, An iron arsenic source may be added to adjust the Fe/As ratio within the above range. As the iron arsenic source used for this adjustment, sludge containing arsenic-containing sediment collected separately can be used.

The amount of sludge to be returned to the neutralization step may be, for example, 0.02 to 0.2 L per 1 L of wastewater for sludge with a solid content concentration of 60 to 120 g/L.

The treatment method of the present invention can be suitably used for neutralizing mine wastewater containing iron and arsenic. FIG. 1 shows an example in which the treatment method of the present invention is applied to treatment of mine wastewater.

For example, a plurality of mine wastewaters (hereinafter referred to as wastewaters) having a pH of 3 to 4 and a Fe/As ratio varying in the range of about 29 to about 110 are collected in the raw water receiving tank 10. The wastewater in the raw water receiving tank 10 is sent to a neutralization reaction tank 11. Slaked lime used for neutralization is put into a slaked lime dissolving tank 12, converted into a slaked lime liquid, and sent to a conditioned tank 13, from which it is supplied to the neutralization reaction tank 11. In this neutralization reaction tank 11, the wastewater is neutralized with slaked lime, and by this neutralization treatment, iron in the wastewater forms a ferric hydroxide precipitate, and arsenic in the wastewater is taken into this ferric hydroxide precipitate. and coprecipitates to form an arsenic-containing precipitate (neutralization step). Slaked lime is supplied in an amount to neutralize the wastewater to, for example, around pH 8.

The neutralized wastewater is introduced from the neutralization reaction tank 11 to the coagulation tank 14. A polymer flocculant solution is supplied to the flocculant tank 14 from a polymer flocculant dissolving tank 15 , and the neutralized wastewater to which the polymer flocculant solution has been added is sent to a sedimentation separation tank 16 . In the settling tank 16, the arsenic-containing sediment settles and neutralized sludge is deposited on the bottom of the tank. This neutralized sludge is collected (recovery step), returned to the conditioned tank 13 (returned sludge), and added to the slaked lime solution in the conditioned tank 13 to produce Fe/As that is intermediate in the Fe/As ratio fluctuation range of the wastewater. The sludge having the same ratio is sent from the condition tank 13 to the neutralization reaction tank 11 and supplied thereto (return process). By adding the returned sludge, fluctuations in the Fe/As ratio of the wastewater are alleviated, and the neutralization process proceeds stably. Note that, if necessary, the Fe/As ratio of the returned sludge is adjusted in the condition tank 13 so that it falls within a Fe/As ratio range that is intermediate to the Fe/As ratio fluctuation range of wastewater.

On the other hand, the remaining sludge (surplus sludge) after recovering the returned sludge is extracted from the bottom of the sedimentation separation tank 16 and sent to a collection site for final disposal. In the wastewater from which sludge has been separated in the sedimentation separation tank 16, most of the arsenic is taken into the sludge and separated, so the arsenic concentration can be reduced to below the wastewater standard (0.1 mg/L or less), This treated wastewater is passed from the sedimentation separation tank 16 to the filtration equipment 17 to remove suspended particles, and is discharged to the outside.

According to the treatment method of the present invention, the returned sludge is repeatedly supplied to the neutralization reaction tank and the neutralization process progresses, so even in wastewater where the Fe/As ratio fluctuates widely, arsenic can be stably reduced to below the effluent standard. Since the sludge can be removed and the sludge is further concentrated, the sludge volume can be reduced.

FIG. 2 is a treatment process diagram of the present invention applied to treatment of mine wastewater.

[Example 1]
In the treatment process shown in FIG. 1, 1 L of mine wastewater was collected as a sample and its pH was measured. Next, 36 ml of returned sludge was taken, 4.0 ml of 3.0 wt% slaked lime slurry was added, and the mixture was stirred for 10 minutes to prepare conditioned sludge (solid concentration 70 g/L, Fe/As ratio 57). The entire amount of this conditioned sludge was put into the mine wastewater, stirred for 15 minutes, and neutralized to pH 8.0. If the pH did not reach 8.0, it was adjusted by adding a small amount of slaked lime slurry. Thereafter, 0.4 ml of a 0.05% solution of a polymer flocculant (Diafloc AP-825B, manufactured by Mitsubishi Chemical Corporation) was added to the neutralized slurry and stirred, and the resulting precipitate was separated by sedimentation. After sedimentation and separation, the supernatant liquid was collected, and the iron concentration and arsenic concentration were quantitatively analyzed using an inductively coupled plasma emission spectrometer. The results are shown in Table 1.
As shown in Table 1, the Fe/As ratio in the neutralization step was stable in the range of 55 to 58, and the solids concentration of the sludge was concentrated to 2.59 g/L to 2.66 g/L. In addition, the iron concentration of the supernatant liquid is 0.1 to 0.3 mg/L, and the arsenic concentration is reduced to 0.06 mg/L or less.

[Example 2]
Mine wastewater was neutralized in the same manner as in Example 1, except that the solid concentration, Fe/As ratio, and amount of returned sludge in the returned sludge were adjusted as shown in Table 2. The results are shown in Table 2.
As shown in Table 2, when returning sludge with an Fe/As ratio of 56 to 65 is used, the Fe/As ratio in the neutralization step is stable in the range of 53 to 64, and the solids concentration of the sludge is 2.58. It is concentrated to ~2.64. Furthermore, the iron concentration of the supernatant liquid is 0.1 to 0.2 mg/L, and the arsenic concentration has been reduced to 0.08 mg/L or less.

[Comparative example 1]
Mine wastewater was neutralized in the same manner as in Example 1, except that the sludge for neutralization was not returned. The results are shown in Table 3. The Fe/As ratio in the neutralization process fluctuates in the same way as the Fe/As ratio in raw mine wastewater, so the iron concentration in the supernatant liquid is as high as 3.4 to 6.2 mg/L, and the Fe/As ratio is high. The arsenic concentration of mine wastewater below 51 cannot be reduced to below the wastewater standard (0.1 mg/L or below). Furthermore, the solid content concentration of the sludge produced in the neutralization step is about 0.16 g/L or less, and the sludge is bulky without being concentrated.

10-raw water receiving tank, 11-neutralization reaction tank, 12-slaked lime dissolution tank, 13-conditioning tank, 14-coagulation tank, 15-polymer flocculant dissolution tank, 16-sedimentation separation tank, 17-filtration equipment.

Claims (3)

Acidic wastewater containing arsenic is neutralized in the presence of iron, and the arsenic is turned into a precipitate to reduce the arsenic concentration . The sludge containing arsenic precipitates produced in the neutralization process was collected from the wastewater that fluctuated within a range , and the sludge with a Fe/As ratio of 55 to 66 and a solid concentration of 60 to 120 g/L was recovered. A method for treating arsenic -containing wastewater, characterized in that the sludge is returned to the neutralization treatment, and the sludge is added to the wastewater and subjected to the neutralization treatment, thereby alleviating fluctuations in the Fe/As ratio and promoting the formation of arsenic as a precipitate. . The method for treating arsenic-containing wastewater according to claim 1, wherein the arsenic-containing wastewater is mine wastewater. The recovered sludge having the above Fe/As ratio and the above solids concentration is returned to the above neutralization treatment, and is added at a ratio of 0.02 to 0.2 L to 1 L of the waste water for neutralization treatment. A method for treating arsenic-containing wastewater as described in Item 2.

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