JP6790561B2 - Heavy metal recovery method - Google Patents

Description

The present invention relates to a method for recovering a heavy metal, and more specifically, a sulfide agent is added to an aqueous solution containing a heavy metal such as mercury in which an oxidizing agent coexists, and the heavy metal is recovered as a solid sulfide. Regarding the method of recovering heavy metals.

For example, mercury, which is a heavy metal, and its water-soluble compound are highly toxic to the human body and living organisms, so that the concentration of emissions in wastewater and exhaust gas is also severely restricted. Therefore, as a method for treating a waste liquid containing mercury, an advanced separation technique is required, and various processes have been developed.

Specifically, for example, a method of adsorbing mercury with a solid adsorbent is mentioned, and Patent Document 1 discloses a method of using an adsorbent containing a dialkylaminoalkyl metaacrylate, and Patent Document 2 Discloses a method of adsorbing with an anion exchange resin and then re-adsorbing mercury remaining in the aqueous solution with a compound forming carbon, silica, and mercaptide. Further, a method of recovering mercury by aggregation and coprecipitation is mentioned, and Patent Document 3 describes a polymer chelating agent, iron chloride, iron sulfate, aluminum sulfate, polyaluminum chloride, and aluminate under conditions of pH 3 to 7. A method of coprecipitating mercury using sodium or the like is disclosed. Further, a method of recovering as metallic mercury with a reducing agent is mentioned, and Patent Document 4 discloses a method of reducing with anthraquinone, hydroquinone, etc. on a porous carrier, and Patent Document 5 temporarily adsorbs to spinel. A method of eluting with ozone and finally reducing and recovering with hydrazine is disclosed.

In addition, as a slightly physical method, for example, a method of recovering by ion flotation using aluminum zanthate, as disclosed in Patent Document 6, is also known. In addition, as a method for selectively precipitating mercury, there is a method in which once crudely separated mercury is concentrated and separated in a single state, and then hydroxylated or sulfurized. For example, Patent Document 7 discloses a method in which a mercury-containing substance is selectively dissolved in a mixture of hydrofluoric acid and nitric acid, and a neutralizing agent and a sulfide agent are added for recovery.

However, in the method of adsorbing and separating mercury using an adsorbent, mercury is recovered as an eluent after being adsorbed. Therefore, in order to finally detoxify it, it is necessary to separately separate and recover it as a solid. In addition, in the method of separating and recovering by coprecipitation, reduction, and flotation, those that can be recovered as a solid from the beginning, coprecipitants and other coexisting metals that are easily reduced, and coexisting metals that are easily floated are separated because they are precipitated at the same time. The grade of mercury is very low and requires secondary concentration for treatment and storage.

On the other hand, in the method of selectively dissolving mercury from the primary concentrate with an oxidizing agent and recovering it with a neutralizing agent, a sulfurizing agent, etc., mercury can be separated as a solid and coexisting substances are separated in advance. Therefore, there is an advantage that the concentration rate is high. In particular, mercury sulfide has extremely low solubility (solubility product Ksp: 4.0 × ^10-53 ), and the sulfurization reaction has high selectivity for mercury, so that it is suitable for recovery as a solid.

However, since mercury has a property of being easily reduced to metallic mercury, an oxidizing agent is often added in order to stabilize it in the supplied aqueous solution, and the oxidizing agent coexists. Therefore, even if an attempt is made to add a sulfurizing agent to such an aqueous solution to perform a sulfurizing treatment, the sulfurizing agent is decomposed by the oxidizing agent, and it becomes difficult to recover mercury in the form of sulfide.

International Publication No. 2012/063305 Tokukousho 48-019793 Japanese Unexamined Patent Publication No. 2003-47828 Japanese Unexamined Patent Publication No. 07-124573 Japanese Unexamined Patent Publication No. 62-294490 Special Publication No. 51-011429 Japanese Unexamined Patent Publication No. 2003-453336

The present invention has been proposed in view of such circumstances, and it is possible to separate the mercury as a solid from an aqueous solution containing mercury in which an oxidizing agent coexists to obtain an aqueous solution having a sufficiently low mercury grade. It is an object of the present invention to provide a method for recovering mercury in an aqueous solution that can be produced.

The present inventors have made extensive studies to solve the above-mentioned problems. As a result, when a sulfurizing agent is added to a mercury-containing aqueous solution in which an oxidizing agent coexists and mercury is recovered as sulfide, the oxidation-reduction potential of the aqueous solution accompanying the addition of the sulfurizing agent is used as a reference, and the oxidation-reduction potential is used. By adding a sulfide agent to the aqueous solution until two inflection points appear in the transition, it was found that mercury can be effectively immobilized as a sulfide and can be recovered with a high recovery rate, and the present invention is completed. I arrived.

(1) The first invention of the present invention is a method for recovering mercury by adding a sulfide agent to an aqueous solution containing mercury coexisting with an oxidizing agent and recovering the mercury as a solid sulfide. A mercury recovery method in which the sulfide is added to the aqueous solution until two inflection points appear in the graph of the oxidation-reduction potential with the silver / silver chloride electrode of the aqueous solution as the reference electrode with respect to the amount of the sulfide added. is there.

(2) The second invention of the present invention is the method for recovering mercury in the first invention, in which the sulfurizing agent is added until the redox potential reaches −200 mV or less.

(3) The third invention of the present invention is the method for recovering mercury in the first or second invention, in which the sulfurizing agent is added until the redox potential reaches the range of −250 mV or less and −400 mV or more. is there.

(4) The fourth invention of the present invention is the method for recovering mercury in any one of the first to third inventions, wherein the redox potential of the aqueous solution in which the oxidizing agent coexists is 800 mV or more.

(5) The fifth invention of the present invention is a method for recovering mercury, wherein hydrogen sulfide or a sulfide having a higher solubility than mercury sulfide is used as the sulfide agent in any of the first to fourth inventions. is there.

(6) The sixth invention of the present invention is a method for recovering mercury using an alkali sulfide as the sulfurizing agent in the fifth invention.

(7) The seventh invention of the present invention is the method for recovering mercury in which the oxidizing agent is sodium hypochlorite in any one of the first to sixth inventions.

According to the present invention, the mercury can be easily and effectively separated as a solid from an aqueous solution containing mercury in which an oxidizing agent coexists, and the aqueous solution can be made into an aqueous solution having a sufficiently low mercury grade. A method for recovering mercury can be provided.

It is a graph which shows the transition of the ORP (reference electrode: silver / silver chloride electrode) in the aqueous solution with respect to the addition equivalent when sodium hydrogen sulfide is added as a sulfide agent to the mercury-containing aqueous solution in which an oxidizing agent coexists.

Hereinafter, a specific embodiment of the present invention (hereinafter, referred to as “the present embodiment”) will be described in detail. The present invention is not limited to the following embodiments, and various modifications can be made without changing the gist of the present invention. Further, in the present specification, the notation of "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less".

The method for recovering mercury according to the present embodiment is a method for recovering mercury from an aqueous solution containing mercury (Hg) in which an oxidizing agent coexists (hereinafter, also simply referred to as "aqueous solution" or "mercury-containing aqueous solution"). , A method of recovering mercury as a solid sulfide (mercury sulfide) by adding a sulfide agent to the aqueous solution.

The mercury-containing aqueous solution is not particularly limited, and is, for example, a waste liquid discharged from a non-ferrous metal smelting process containing mercury and mercury, a cleaning liquid / adsorbent for gas discharged from the non-ferrous metal smelting process, or an organic synthesis catalyst. , A bactericide, a heavy liquid used for specific gravity beneficiation, a waste treatment liquid for mercury electrolysis, and the like.

In general, when mercury is selectively adsorbed as a solid from a mercury-containing aqueous solution and eluted and concentrated, or when mercury is separated and concentrated as an aqueous solution from a solid or gas containing metallic mercury or an insoluble mercury compound, mercury is finally concentrated. Is recovered as an aqueous solution containing a high concentration of. However, since the standard redox potential (ORP) between Hg ²⁺ / Hg is as high as 0.85 V, mercury in the aqueous solution easily returns to the metallic form. For this reason, in an industrial treatment facility having a plurality of steps, when such an aqueous solution is handled, metallic mercury is separated as a precipitate before reaching the final treatment step as the ORP of the aqueous solution decreases. In addition, it diffuses and dissipates into the atmosphere as metal vapor, and a situation occurs in which concentrated separation cannot be performed intensively in the final treatment process.

In order to prevent such a situation, it is effective to coexist the oxidizing agent in the mercury-containing solution. However, even if a sulfide agent is added to fix mercury as a sulfide in the presence of an oxidant, the sulfide agent is consumed by the oxidant in the water-soluble machine and is sufficiently recovered as a sulfide. I can't.

Specifically, when a sulfide agent for immobilizing mercury as a sulfide is added to a mercury-containing aqueous solution in which an oxidant coexists, first, in the stage where an excess amount of the oxidant is present in the first stage. , The added sulfurizing agent is oxidized to sulfuric acid by the oxidizing agent. Next, in the second step, in which the amount of the oxidizing agent is gradually reduced, the added sulfurizing agent is oxidized to elemental sulfur.

The following reaction formulas (1) and (2) show the states in the aqueous solution in the first and second steps described above, and an oxidizing agent is added to the aqueous solution containing mercury (II) chloride as a mercury compound. As a specific example, the case where sodium hypochlorite coexists and sodium hydrogen sulfide as a sulfide agent is added to the aqueous solution is shown.

4 NaClO + NaSH → Na ₂ SO ₄ + 3 NaCl + HCl
(When the oxidizing agent is in excess of the sulfurizing agent) ... (1)
NaClO + NaCl → S + NaOH + NaCl
(When the oxidizing agent is slightly excessive with respect to the sulfurizing agent) ... (2)

Finally, after the reactions of (1) and (2), the oxidizing agent in the aqueous solution is consumed and decomposed by the addition of the sulfurizing agent, and only then, as shown by the following reaction formula (3). , The sulfurization reaction of mercury in the aqueous solution will occur.

NaSH + HgCl ₂ → HgS + NaCl + HCl ・ ・ ・ (3)

Here, FIG. 1 shows an ORP (reference electrode:: reference electrode:) in the aqueous solution in which sodium hydrogen sulfide is added as a sulfide to a mercury-containing aqueous solution in which an oxidizing agent (sodium hypochlorite) coexists. It is a graph which shows the transition of (silver / silver chloride electrode). The processing conditions and the like will be described in Example 1.

As can be seen from the graph of FIG. 1, when a sulfide agent is added to a mercury-containing aqueous solution in which an excess amount of an oxidizing agent coexists, and the mercury is immobilized as sulfide (mercury sulfide), the sulfide is formed. It can be seen that two turning points (X and Y in FIG. 1) appear in the ORP of the aqueous solution as the amount of addition increases. Here, the "inflection point" means a point when the slope of the graph changes significantly. Then, even if the amount of the sulfurizing agent added is increased after the two inflection points appear, the change in the ORP of the aqueous solution is almost eliminated. Although details will be described in Examples, the recovery rate of mercury as a sulfide will be at a high level after the appearance of two inflection points.

Such two inflection points shown in FIG. 1 can be explained based on the above-mentioned reaction formula. That is, in the first plateau region up to the first inflection point X, the ORP was almost constant at 800 mV or more, and no precipitation of mercury sulfide (black) was observed even when a sulfide agent was added. It is presumed that only the sulfate ion production reaction represented by the above reaction formula (1) is proceeding.

After that, in the second plateau region from the appearance of the first inflection point X to the second inflection point Y, the ORP was substantially constant at around 400 mV, and with the addition of the sulfurizing agent, the ORP was substantially constant. The formation of elemental sulfur (yellow) was observed instead of the precipitation of black mercury sulfide, and it is presumed that the oxidation reaction of the sulfurizing agent to sulfur is proceeding as shown in the above reaction formula (2).

Then, when the ORP falls below about −200 mV after the appearance of the second inflection point Y, mercury sulfide, which is a black precipitate, is generated for the first time with the addition of the sulfide agent. That is, it is presumed that the mercury sulfide production reaction represented by the above reaction formula (3) is proceeding after the appearance of the second inflection point Y.

For this reason, in the treatment of recovering mercury as a solid sulfide by adding a sulfurizing agent to a mercury-containing aqueous solution in which an excessive amount of oxidizing agent coexists, the ORP of the aqueous solution with respect to the addition amount of the sulfurizing agent A sulfurizing agent is added to the mercury-containing aqueous solution until "two turning points" appear in the graph. As a result, the mercury can be efficiently immobilized as a sulfide while preventing the mercury ions from being reduced to metallic mercury in the aqueous solution, and the mercury can be effectively recovered in the form of mercury sulfide.

In recovering mercury from a mercury-containing aqueous solution in which an oxidizing agent coexists, it is practically difficult to accurately quantitatively analyze the concentration of the oxidizing agent and the concentration of mercury in the aqueous solution immediately before the reaction. Further, in the case where mercury is complexly formed or in an acidic region, even if a sulfurizing agent corresponding to the reaction equivalent is added, mercury precipitation is not completely formed. In such a case, as described above, each reaction in the aqueous solution is performed by measuring the transition of the ORP with respect to the addition equivalent of the sulfide agent and confirming the appearance of two inflection points based on the ORP of the aqueous solution. The progress of the above can be grasped by a simple method of accurately and measuring the ORP.

When the specific value of the ORP of the aqueous solution is used as a reference, the ORP of the aqueous solution is −200 mV or less with respect to the mercury-containing aqueous solution in which an excess amount of the oxidizing agent coexists, based on the graph shown in FIG. , More preferably -250 mV or less, and even more preferably -300 mV or less. In this way, by adding the sulfurizing agent until the ORP becomes −200 mV or less, the sulfurization reaction of mercury can be almost completely completed through the decomposition of the oxidizing agent.

Here, when the concentration of mercury ions in the aqueous solution is low, or when the content of the oxidizing agent present with mercury in the aqueous solution is low, the inflection point as shown in FIG. 1 cannot be clearly confirmed. is there. Even in such a case, as described above, the ORP of the aqueous solution is used as a specific reference, and the ORP is set to a predetermined ORP, specifically -200 mV or less, more preferably -250 mV or less, still more preferably -300 mV or less. By adding a sulfurizing agent up to the point, a sulfurization reaction of mercury can be effectively caused.

When a specific value of ORP is used as a reference, the lower limit is not particularly limited, but it is preferable to add the sulfurizing agent within that range with −400 mV or more as the lower limit. Mercury sulfide generated by the sulfurization reaction due to the addition of a sulfurizing agent is dissolved by a large excess of alkali sulfide or the like. In addition, unreacted sulfurizing agents may cause water pollution. From these facts, it is preferable that the lower limit value is −400 mV as a guide.

Strictly speaking, ORP depends on temperature and pH, but in actual processing, it can be sufficiently dealt with by setting an optimum value in advance.

The rate of addition of the sulfide agent is not particularly limited, but when metal ions other than mercury coexist in the mercury-containing aqueous solution, if the sulfide agent is added rapidly, sulfide is more sulfided than mercury sulfide. It is preferable to note that metals with high solubility of substances may also be sulfurized, impairing the selectivity of mercury as a target of the sulfurization reaction, and the sulfurizing agent may be wasted.

The sulfide agent is not particularly limited as long as it is a sulfide that exchanges cations with mercury ions, but it is preferable to use hydrogen sulfide (gas) or a sulfide having a higher solubility than mercury sulfide. Such hydrogen sulfide or a sulfide having a higher solubility than mercury sulfide is easily dissolved in water, so that the ORP of the aqueous solution can be strictly controlled.

Specifically, it is preferable to use an alkali metal sulfide that is soluble in water. For example, sodium sulfide, potassium sulfide, or hydrogen salts thereof (sodium hydrogen sulfide, etc.) can be mentioned. These sulfides are preferable from the viewpoint of economy and the ability to suppress nitrogen contamination in an aqueous solution, and among them, sodium hydrogen sulfide is more inexpensive than other sulfides and is particularly preferable. Although it is possible to use heavy metal sulfides as sulfides, there is a risk that such sulfides will cause new heavy metals to elute into the aqueous solution as a result of ion exchange, resulting in new contamination. Therefore, it is not preferable.

Specific examples to which the present invention is applied will be described below, but the present invention is not limited to these examples.

[Example 1]
Sodium hypochlorite (12%) as an oxidant containing 110 mg / L of mercury in the form of mercury (II) chloride and until the redox potential (ORP) with the silver / silver chloride electrode as the reference electrode reaches 819 mV. The solution) was allowed to coexist, and the stabilized aqueous solution was used as the undiluted solution (treatment undiluted solution). In addition, sodium hypochlorite was allowed to coexist by adding an excess amount from the viewpoint of preventing mercury from being reduced and precipitating as metallic mercury.

2.5% sodium hydrogen sulfide (NaSH) was gradually added to this undiluted solution as a sulfide agent, and the mercury in the aqueous solution was recovered as solid mercury sulfide. In this treatment, the amount of the sulfurizing agent added, the ORP of the aqueous solution, and the residual mercury concentration in the aqueous solution were confirmed.

FIG. 1 is a graph showing the transition of the ORP of the aqueous solution with respect to the addition equivalent of NaSH. In addition, Table 1 below shows the residual mercury concentration (mg / L) in the aqueous solution when the predetermined amount of NaHS added (against Hg, mol / mol) was reached, and the mercury removal rate (%) at that time. Shown. The mercury removal rate refers to the ratio of the decrease (concentration) of mercury after treatment (after adding a predetermined amount of sulfurizing agent) to the concentration of mercury contained in the undiluted solution.

As shown in FIG. 1, two inflection points appeared with the addition of the sulfurizing agent (X, Y in the figure). In the plateau region where the ORP was 800 mV or more until the first inflection point X appeared, no precipitation was observed even if the amount of the sulfurizing agent added was increased. After that, after the appearance of the first inflection point X, in the plateau region where the ORP became almost constant at around 400 mV, the formation of a yellow precipitate was confirmed as the amount of the sulfide added increased. Then, after the appearance of the second inflection point Y, the formation of a black precipitate was confirmed for the first time when the ORP fell below −200 mV.

From the results of the quantitative analysis, it was confirmed that the reaction for producing black mercury sulfide is proceeding. Further, as shown in Table 1, when the ORP is -250 mV and -315 mV, the residual mercury concentration is reduced to 0.32 mg / L, the mercury removal rate is 99% or more, and mercury is recovered as a solid at a high rate. I was able to.

Claims (6)

A method for recovering mercury by adding a sulfurizing agent to an aqueous solution containing mercury in which an oxidizing agent coexists and recovering the mercury as a solid sulfide.
After starting the addition of the sulfurizing agent to the aqueous solution,
The relative amount of sulfide agent, two inflection points appear in the graph of redox potential of silver / reference silver chloride electrode the electrode of the aqueous solution, and the redox potential is optimum until below -200 mV, A method for recovering mercury by adding the sulfide agent to the aqueous solution. Until said oxidation reduction potential reaches a range of more than or less -400 mV -250 mV, the recovery method of mercury according to claim 1, adding the sulfurizing agent. The method for recovering mercury according to claim 1 or 2 , wherein the redox potential of the aqueous solution in which the oxidizing agent coexists is 800 mV or more. The method for recovering mercury according to any one of claims 1 to 3 , wherein hydrogen sulfide or a sulfide having a higher solubility than mercury sulfide is used as the sulfide agent. The method for recovering mercury according to claim 4 , wherein an alkali sulfide is used as the sulfurizing agent. The method for recovering mercury according to any one of claims 1 to 5 , wherein the oxidizing agent is sodium hypochlorite.

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