JP6724433B2 - Wastewater treatment method - Google Patents

Description

The present invention relates to a wastewater treatment method. Specifically, it relates to a method for removing cadmium from an aqueous solution containing cadmium and lead and having a high chloride ion concentration.

Heavy metals are often contained in the wastewater generated from the smelting process of various metals. In particular, cadmium and lead are contained in trace amounts in many metal raw materials including not only mineral resources but also secondary resources, so the wastewater generated from the smelting process of many metal smelters is , Contains trace amounts of cadmium and lead. Wastewater containing such heavy metals is generally rendered harmless by being treated in a wastewater treatment facility, and the treated wastewater that meets the discharge standards is discharged to public water bodies. Regarding the concentration of heavy metals in the discharged wastewater, statutory emission standards are strictly defined. In particular, cadmium and lead are required to be treated to a very low concentration of ppb order in the waste water after being discharged into public water bodies. More specifically, the emission standard values for cadmium have been tightened, and in December 2014, the emission standard value was strengthened from 0.1 mg/l or less to 0.03 mg/l or less.

A typical example of wastewater containing cadmium and lead generated in the metal smelting process is wastewater generated in the process of recovering zinc from steel dust. Iron and steel dust contains impurities such as cadmium, lead and halogens in a certain amount or more in addition to metals to be recovered such as iron and zinc. The cleaning water after removing such impurities such as halogens is an aqueous solution containing mainly chlorine and having a high chloride ion concentration, and contains a trace amount of cadmium and lead.

Neutralization treatment is mentioned as a treatment method conventionally used for the treatment of wastewater containing cadmium and lead. As a specific example of the neutralization treatment, after adding silicate ions to a waste water containing cadmium ions and lead ions so that the weight concentration ratio of Si/Pb is 1 or more, the pH of the waste water is 11 or more and less than 12 Patent Document 1 discloses a method for treating wastewater containing cadmium ions and lead ions, in which the cadmium ion and the lead ion are simultaneously precipitated by the above adjustment to separate and remove the formed precipitate. In the neutralization treatment, the required neutralizing agent is generally inexpensive and easily available, and heavy metals can be precipitated and separated only by strictly adjusting the pH, and no special equipment or special technical operation is required. The point is its preferable feature.

However, when the wastewater to be treated has an aqueous chloride solution containing cadmium and lead, especially when the chloride ion concentration is a certain concentration or more, cadmium forms a stable chloro complex ion in the solution. Therefore, when such an aqueous solution having a high chloride ion concentration is treated, there is a problem that cadmium cannot be sufficiently removed by the neutralization treatment.

As another conventional method for removing cadmium in waste water, a sulfurization treatment for producing cadmium sulfide by a sulfurization reaction is also performed. As a specific example of the sulfurization treatment, in removing the heavy metal from the sulfurous acid gas generated in the copper smelting, by controlling the amount of hydrogen sulfide that does not contribute to the reaction, to improve the reaction efficiency of the heavy metal and sulfide ions, In order to increase the removal rate, sodium hydroxide is added to the waste acid after the gypsum step to perform sulfurization until the oxidation-reduction potential becomes −5 mV to −110 mV, and the remaining heavy metal is removed as a sulfide. Patent Document 2 discloses a heavy metal removing method for removing heavy metals in a removing step.

The sulfide obtained by the sulfurization treatment (cadmium sulfide) has a lower solubility than the neutralized precipitate obtained by the neutralization treatment (cadmium hydroxide). Therefore, according to the sulfurization treatment, cadmium in the wastewater can be removed from the wastewater with a higher separation recovery rate than the neutralization treatment.

However, the hydrogen sulfide gas derived from the sulfiding agent generated in the sulfiding treatment is highly toxic, and in order to safely carry out the sulfiding treatment, a special reaction device such as a closed reaction tank or an exhaust device of the gas phase of the reaction tank is required. The installation of exhaust gas abatement equipment and strict operation management were required. As described above, the problem of safety and the heavy burden of facility cost and operating cost for coping with it have been factors that prevent the introduction of sulfurization treatment as a wastewater treatment method at many wastewater treatment sites.

JP, 08-309368, A JP, 2015-20103, A

The present invention was devised in view of the above problems, and is a method for treating wastewater for the purpose of reducing the cadmium concentration in an aqueous solution of high chloride ion concentration containing cadmium and lead. It is an object of the present invention to provide a treatment method that can realize a sufficient reduction of cadmium concentration at a safe and low cost.

The present inventors, in the treatment of an aqueous solution containing a high chloride ion concentration containing cadmium and lead, a neutralization treatment, and a sulphidization treatment to be auxiliary, by combining as a series of processes in a unique aspect of the present invention, The inventors have found that the above problems can be solved by an original processing method that performs these as a series of complex processes, and have completed the present invention. Specifically, the present invention provides the following.

(1) A method for treating wastewater containing cadmium and lead and having a chloride ion concentration of 0.2 mol/l or more and 1.5 mol/l or less, wherein a neutralizer is added to the wastewater to adjust the pH to 10 By adjusting the above to 11 or less, a neutralizing treatment step of performing a neutralizing treatment for producing a neutralized precipitate, and a sulfurizing agent is added to the wastewater after the neutralizing treatment to add a redox potential (Ag/AgCl). By adjusting the electrode standard) to -200 mV or more and -40 mV or less, a sulfurization treatment step of performing a sulfurization treatment to generate a metal sulfide, and a solid-liquid separation of a slurry containing the neutralized precipitate and the metal sulfide. A solid-liquid separation step, and a method for treating wastewater.

In the invention of (1), the wastewater before the treatment, that is, the aqueous solution having a high chloride ion concentration containing cadmium and lead is first subjected to a neutralization treatment as a preceding treatment, and then the wastewater after the neutralization treatment. That is, the composite process is performed in which the auxiliary sulphidizing treatment is interlocked in this order with the wastewater during the treatment in which the pH is increased by the same treatment.

Thus, according to the invention of (1), from the aqueous solution of high chloride ion concentration containing at least cadmium and lead as heavy metals, which was conventionally difficult to separate with a sufficient removal rate only by the neutralization treatment, It has become possible to perform the separation of cadmium at a sufficiently high separation recovery rate. In addition, by incorporating the sulfurization treatment, which had been a problem of difficulty in ensuring safety and increasing the cost of implementation, into a complex process that performs this in the high pH state after neutralization treatment, While avoiding the danger of generation of hydrogen sulfide gas during the treatment, only the effect of improving the separation and recovery rate of heavy metals by the sulfurating treatment can be enjoyed safely and at low cost.

Specifically, by adopting a process that can suppress the generation of hydrogen sulfide gas during sulphation treatment, the reaction tank for sulphation treatment does not need to have an airtight structure as in the past, and the gas phase part There is no need to suck in and remove the gas. Therefore, it can be carried out by providing only the facility for adding and supplying the sulfiding agent to the reaction tank on the downstream side of the conventional neutralization treatment facility. As described above, according to the invention of (1), the facility cost for safety measures can be significantly reduced as compared with the conventional facility for sulfurating treatment.

Further, in the invention of (1), most of the cadmium is hydroxide-precipitated by the neutralizing agent, and the sulfurating treatment requiring a relatively expensive sulfurizing agent is performed only as an auxiliary treatment. As a result, the chemical cost can be significantly reduced as compared with the case where the same separation/recovery rate is achieved only by the sulfurating treatment.

(2) The method for treating wastewater according to (1), wherein the neutralization treatment is performed in a first stirring tank,
A method for treating wastewater, wherein the sulfurization treatment is performed in a tank that is connected to the first stirring tank directly or via another stirring tank.

According to the invention of (2), by performing the wastewater treatment method of (1) while sequentially flowing through a plurality of stirring tanks, the cadmium separation treatment from the wastewater can be continuously and efficiently performed. Although the invention of (1) can be carried out as a batch process in a single tank, such a continuous process using a plurality of tanks can further improve the processing efficiency. In addition, since the existing neutralization treatment facility is usually operated in a facility in which a plurality of such agitating tanks are connected to each other, such an existing neutralization treatment facility is used as it is for carrying out the invention of (2). It can also be diverted to. That is, according to the invention of (2), the invention of (1) can be efficiently implemented by directly utilizing it in a general existing neutralization treatment facility without incurring a special additional facility cost.

(3) The method for treating wastewater according to (1) or (2), wherein the neutralizing agent is slaked lime and the sulfiding agent is sodium hydrosulfide.

According to the invention of (3), water that is easily available and relatively inexpensive and uses slaked lime as a neutralizing agent as a main chemical agent for wastewater treatment makes handling easy and further secures safety. By using sodium sulfide as a sulfiding agent added as an auxiliary agent, it is possible to further improve the safety and economical efficiency in the implementation of the wastewater treatment method described in (1) or (2).

(4) The drainage is an overflow liquid after cementation treatment of a treatment liquid discharged from a wet process performed in the process of producing zinc oxide from steel dust, (1) to (3) Wastewater treatment method.

According to the invention of (4), a typical smelting process that includes, as an indispensable process in the entire process, a process in which an aqueous solution having a high chloride ion concentration containing impurities such as cadmium, lead, and chlorine is generated. The wastewater treatment method of the present invention can be applied to an example of a process for recovering zinc from steel dust. This can greatly contribute to the improvement of the economic efficiency and environmental compatibility of the entire zinc smelting process.

According to the present invention, a method for treating wastewater, which aims to reduce the cadmium concentration in an aqueous solution containing a high chloride ion concentration containing cadmium and lead, is safe and low-cost, and has a sufficient cadmium concentration. It is possible to provide a processing method capable of realizing reduction.

1 is a flow chart showing an overall process for producing zinc oxide ore, which is an example of an overall process including a step of treating wastewater which is an aqueous solution having a high chloride ion concentration containing cadmium and lead by the method for treating wastewater according to the present invention. is there. It is a flow chart which shows the process of the processing method of the wastewater of the present invention. It is a conceptual diagram which shows typically an example of the treatment equipment suitable for implementation of the wastewater treatment method of this invention. It is the dissociation curve of H _{2 S} when changing the pH.

Hereinafter, as a specific example of the metal smelting process to which the present invention can be preferably applied, the whole process relating to the production of zinc oxide ore for separating and recovering zinc components from steel dust is cited, and the wastewater treatment method of the present invention is applied to the same process. Will be described as a preferred embodiment of the present invention. However, the present invention is not limited to the following embodiments. INDUSTRIAL APPLICABILITY The wastewater treatment method of the present invention is a treatment method widely applicable to all processes of a metal smelting process including a treatment process of an aqueous solution containing cadmium and lead and having a high chloride ion concentration.

<Overall process>
As shown in FIG. 1, the overall process for producing zinc oxide ore comprises reducing roasting step S10 for reducing and roasting steel dust to obtain crude zinc oxide, cadmium, etc. from the crude zinc oxide obtained in reducing roasting step S10. The wet process S20 for separating and recovering to obtain a crude zinc oxide cake, the dry heating process S30 for drying and heating the zinc oxide cake obtained in the wet process S20 to obtain zinc oxide ore, and the dry heating process S30 are discharged. The exhaust gas treatment step S40 for performing the solid-gas separation process for separating the exhaust gas into solid and gas, and the treatment liquid discharged from the wet process S20 (hereinafter, this treatment liquid is also referred to as “wet treatment liquid”), metal A cementation step S50 for primary removal of cadmium and/or lead from the wet treatment liquid by utilizing a cementation reaction due to the addition of zinc powder, and solid-liquid separation of the slurry after the cementation reaction. Wastewater treatment step S60 for removing cadmium and lead remaining in the wastewater after removal of cadmium and lead by neutralization treatment or the like.

By performing the wastewater treatment method of the present invention as a method for treating wastewater having a high chloride ion concentration in which cadmium and lead remain after cementation treatment in the wastewater treatment step S60 in the overall process, Drainage of cadmium at the same cost as the conventional wastewater treatment method using only neutralization, and at a significantly higher cost and safety compared to conventional wastewater treatment using only sulfurization, with a sufficiently high separation recovery rate. It is a method that can be removed from.

<Reduction roasting process>
As a specific method for carrying out the reduction roasting step S10 for recovering crude zinc oxide from iron and steel dust, it is common to employ a reduction roasting method using a reduction roasting rotary kiln (RRK). In the kiln, steel dust is reduced and roasted, and the volatilized metallic zinc is reoxidized in the exhaust gas to become powdery zinc oxide. The powdery zinc oxide is introduced into the dust collector together with the exhaust gas from the rotary kiln, captured, and recovered as crude zinc oxide. The general composition of this crude zinc oxide is generally 45 to 55 mass% zinc, 4 to 8 mass% lead, 2 to 6 mass% chlorine, about 1 mass% fluorine, and 0.1 to 0.5 mass% cadmium. (All are based on dry amount).

<Wet process>
In the wet step S20 subsequent to the reducing roasting step, impurities such as chlorine contained in the crude zinc oxide obtained in the reducing roasting step are separated and extracted into the treatment liquid, and further solid-liquid separation treatment is performed to remove the crude zinc oxide. A wet treatment is performed in which water-soluble impurities are removed by a water washing method to obtain a crude zinc oxide cake. More specifically, in a state where halogen-based impurities such as chlorine are removed in the treatment liquid, the wet treatment liquid in which the impurities are distributed is removed from the slurry by solid-liquid separation. This wet treatment liquid contains heavy metals such as cadmium and lead in addition to chlorine separated from crude zinc oxide, and the composition thereof is generally chloride ion 10 to 30 g/l, zinc 0.7 to. It is 1 g/l, cadmium 0.2 to 0.5 g/l, and lead 0.02 to 0.1 g/l. As described above, the wet treatment liquid, which is “an aqueous solution containing cadmium and lead and having a high chloride ion concentration”, is sent to the cementation step S50. Further, the zinc oxide slurry from which impurities have been removed through the wet treatment is dehydrated by a vacuum suction type dehydrator or the like to form a zinc oxide cake, which is then put into the drying and heating step S30.

<Dry heating process>
The crude zinc oxide cake obtained in the wet step S20 is charged into a dry heating device such as a dry heating rotary kiln (DRK) and fired, and in the dry heating step S30, a zinc oxide ore with a further reduced concentration of cadmium or the like is produced. be able to. As a general standard, the content of cadmium in zinc oxide ore, which is a raw material for zinc smelting, is required to be less than 0.1%.

<Exhaust gas treatment process>
In the exhaust gas processing step S40, solid-gas separation processing of the exhaust gas discharged from the DRK in the drying and heating step S30 is performed.

<Cementation process>
In the cementation step S50, from the wet treatment liquid containing cadmium and lead separated in the wet step S20, cadmium and lead are precipitated by a cementation treatment using metal zinc powder or the like, and these are removed from the wet treatment liquid. Remove. The residue containing cadmium and lead removed by this cementation treatment is discharged to the cadmium smelting process. On the other hand, the reaction liquid after the cementation treatment is sent to the next wastewater treatment step S60. A trace amount of cadmium and lead remain in the reaction solution after the cementation treatment, and the compositions thereof are approximately 2 to 20 mg/l of cadmium and 0.5 to 5 mg/l of lead, and the chloride ion concentration Is about 0.28 to 0.85 mol/l, and the pH is about 6 to 8. In the entire process for producing zinc oxide ore, this "reaction liquid after cementation treatment" is the "wastewater" to be treated by the wastewater treatment method of the present invention.

Generally, this cementation treatment is often positioned as a part of a wastewater treatment process in a broad sense, which is performed as a series of processes together with the neutralization treatment and the like. However, the wastewater treatment method of the present invention is a method in which the reaction liquid after the cementation treatment is a representative treatment target, and therefore, in order to clearly explain the technical scope of the present invention, in the present specification, Describes the cementation step S50 as a separate step from the wastewater treatment step in a narrow sense described in detail below, that is, the wastewater treatment step S60 to which the wastewater treatment method of the present invention can be preferably applied. did.

<Wastewater treatment process>
As shown in the flowchart of FIG. 2, in the wastewater treatment step S60, the trace amount of cadmium remaining at the above concentration is removed from the wastewater sent from the cementation step S50 (the reaction solution after the cementation treatment). , Further reduce the concentration of cadmium in wastewater. By using the “wastewater treatment method” of the present invention for the treatment for removing cadmium from the reaction solution after the cementation treatment in the wastewater treatment step S60, it is possible to safely and safely reduce the necessary and sufficient decrease in the concentration of cadmium in the wastewater. It can be realized at low cost.

[Wastewater treatment method]
The wastewater treatment method of the present invention (hereinafter, also simply referred to as “wastewater treatment method”) is a wastewater containing cadmium and lead and having a chloride ion concentration of 0.2 mol/l or more and 1.5 mol/l or less. The processing target. Then, as shown in FIG. 2, the neutralization treatment step ST61, the sulfurization treatment step ST62, and the solid-liquid separation step ST63 are performed in this order as a series of complex processes.

Here, the conventionally known neutralization treatment is a technique based on the principle that the pH of the wastewater to be treated is basically raised to generate a sparingly soluble metal hydroxide. In such a neutralization process, cadmium in the waste water forms a precipitate based on the following formula 1. In this respect, the basic principle is the same in the neutralization treatment step ST61 in the wastewater treatment method of the present invention.

[Formula 1]
Cd ²⁺ +2OH ⁻ ←→ Cd(OH) ₂

Further, under the condition that the pH of the waste water is 13 or more, the precipitate is redissolved as shown in the following formula 2, so that the pH is adjusted to 13 or less in order to sufficiently remove cadmium by the neutralization treatment. It is preferable. This point is also the same as the basic principle in the neutralization treatment step ST61 in the wastewater treatment method of the present invention.

[Formula 2]
Cd(OH) ₂ +2OH ⁻ ←→ [Cd(OH) ₄ ] ²⁻

On the other hand, lead in the waste water tends to be redissolved under the condition that the pH is 10 or more, as in the case of the formula 2. Therefore, in the conventional neutralization treatment for treating wastewater in which cadmium and lead coexist, in order to precipitate cadmium and lead, the pH is adjusted to the optimum pH range (for example, 10 to 11) for the precipitation of both. Then, the method (i) in which cadmium and lead are collectively precipitated is used, or first, the pH is adjusted to about 10 to remove lead, and then the pH is adjusted to about 13 to remove cadmium. It was necessary to select either method (ii) or (i) or (ii) for performing the separation process. However, when the chloride ion concentration in the wastewater is high (specifically, the chloride ion concentration is 0.2 mol/l or more), cadmium in the wastewater produces a chloro complex ion as shown in the following formula 3. As a result, there is a problem that the reaction of forming cadmium hydroxide shown in Formula 1 is difficult to proceed.

[Formula 3]
Cd ₂ ++4Cl ⁻ ←→ CdCl ₄ ²⁻

For example, according to thermodynamic calculation using known equilibrium constants and coefficients, the concentration of cadmium in the waste water at pH 11 is about 0.0004 mg/l in the absence of chloride ion, When the chloride ion concentration is 0.5 mol/l, it becomes 0.04 mg/l. Therefore, when the chloride ion concentration is 0.5 mol/l, it is theoretically impossible to satisfy the above-mentioned emission standard value, that is, the cadmium concentration of 0.03 mg/l or less, only by the neutralization treatment.

Therefore, it is conceivable to use a sulfurization treatment instead of the neutralization treatment. For example, even when the sulfurization treatment is performed using sodium hydrosulfide as a sulfiding agent in the treatment of the wastewater which is an aqueous solution having a high chloride ion concentration as described above, a precipitate can be produced based on the following formula 4. it can. According to this, cadmium sulfide having a solubility lower than that of cadmium hydroxide can be generated to reduce the cadmium concentration to a lower concentration. According to the Chemical Manual, the solubility of Cd(OH) ₂ in water is 2.74×10 ⁻³ g/l, and the solubility of CdS is 2.11×10 ⁻⁸ g/l.

[Formula 4]
Cd ²⁺ +NaHS ←→ CdS+Na ⁺ +H ⁺

However, as described above, this sulfurization treatment has a problem that the additional cost of equipment and operation for avoiding various risks associated with the generation of hydrogen sulfide gas increases. For example, sodium hydrosulfide used as a sulfiding agent generates hydrogen sulfide gas as shown in the following formula 5 in the low pH range.

[Formula 5]
NaHS+H ⁺ ←→ H ₂ S+Na ⁺

Here, hydrogen sulfide dissociates in an aqueous solution as shown in the following formulas 6 and 7. Since hydrogen ions are involved in the reactions of Formula 6 and Formula 7, the equilibrium depends on pH. FIG. 3 is a dissociation curve of H ₂ S when pH is changed. From FIG. 3, it can be seen that at pH 10 to 11, generation of hydrogen sulfide gas does not occur with the execution of the sulfurization treatment.

[Formula 6]
^{_{H 2 S ← → H + +}} SH -

[Formula 7]
SH ⁻ ←→ H ⁺ +S ^2-

The wastewater treatment method of the present invention performs the neutralization treatment step ST61 as a preceding treatment when the aqueous solution having a high chloride ion concentration as described above is used as the wastewater to be treated, and the pH of the wastewater is adjusted to 10 to 11. After being adjusted to a certain degree, the sulfurization treatment step ST62 is supplementarily performed as a subsequent treatment, and as a final treatment, a mixture of the neutralized precipitate and the metal sulfide is separated from the liquid after the sulfurization reaction all at once. The separation step ST63 is performed. By performing these three steps as a series of processes that are functionally linked, the separation and recovery rate of cadmium from the wastewater can be significantly improved at a safe and low cost.

(Neutralization process)
In the neutralization treatment step ST61, a neutralizing agent such as slaked lime is added to the waste water (reaction liquid after cementation treatment) to adjust the pH to 10 to 11. As a result, heavy metals such as cadmium and lead precipitate as hydroxides.

In addition, in the neutralization treatment step ST61, it is preferable to add an aqueous solution of water glass as an adsorbent in addition to the neutralizing agent.

(Sulfidation process)
Then, a sulfurating treatment step ST62 is performed. In this step, a sulfiding agent is further added to the slurry containing the neutralized precipitate, the pH of which has been adjusted to the range of 10 to 11 in the neutralization treatment step ST61. As a result, a small amount of cadmium that could not be completely removed in the neutralization treatment step ST61 is precipitated as a sulfide. Incidentally, the addition of the sulfiding agent in the sulfiding treatment step ST62 is carried out by redissolving the cadmium hydroxide represented by the formula 1, that is, even when the reverse reaction of the production reaction occurs, the cadmium ion is sulfided to obtain the cadmium ion concentration. It also has the effect as a buffer that stabilizes.

In the sulfurization treatment step ST62, the sulfurization reaction conditions may be adjusted so that the oxidation-reduction potential is -200 mV or more and -40 mV or less (Ag/AgCl electrode reference). When it is less than -200 mV, sodium hydrosulfide that does not contribute to the reaction is excessively added. If it exceeds -40 mV, the sulfurization reaction may be insufficient and the cadmium concentration may increase. The redox potentials in this specification are based on Ag/AgCl electrode standards.

Hydrogen sulfide gas can be used as the sulfiding agent, but it is more preferable to use an aqueous solution of sodium hydrosulfide. This is because the aqueous sodium hydrosulfide solution is easy to handle and has a high efficiency of sulfurization reaction.

(Solid-liquid separation process)
After ST62, solid-liquid separation step ST63 is performed. In this step, the mixture of the neutralized precipitate and the metal sulfide produced through the above two steps is separated from the wastewater after the sulfurization reaction at once. In this step, cadmium in the waste water is reduced to 0.001 to 0.01 mg/l and lead is reduced to 0.003 to 0.03 mg/l. Incidentally, the precipitated hydroxide is solid-liquid separated and is repeated in the system. The wastewater after solid-liquid separation is sent to a further final treatment step such as a pH adjustment step and finally treated for detoxification.

The above-mentioned neutralization treatment step ST61 refers to a step of adding a neutralizing agent as a treatment, and the sulfidation treatment step ST62 refers to a step of adding a sulfiding agent, which are distinguished as separate steps. However, this distinction does not necessarily mean that the neutralization reaction actually proceeding in the reaction tank is completely completed when the neutralization treatment step ST61 is performed. It is also within the range of assumption that a part of the neutralization reaction continues in the reaction tank even during the subsequent sulfurization treatment step ST62, and the neutralization treatment step ST61, the sulfurization treatment step ST62, As long as it is a method of performing the liquid separation step ST63 in conjunction with each other under the above conditions in this order, and finally a method of solid-liquid separating the target neutralized precipitate and sulfide from the wastewater, All are within the scope of the present invention regardless of the degree of progress of each reaction during the progress of the treatment.

(Reaction tank for treating wastewater)
The wastewater treatment method is preferably carried out by a reaction tank 10 including a stirrer 3, preferably a plurality of, preferably, first to third stirring tanks 11, 12, and 13 connected to each other, as shown in FIG. Since the reaction tank 10 has a general configuration as a basic configuration of the reaction tank 10 even if it is a conventional reaction tank for performing only neutralization treatment, most of the existing tanks are used as the reaction tank 10 for performing the wastewater treatment method. You can

In the reaction tank 10, slaked lime slurry is added to the first stirring tank 11 as a neutralizing agent (neutralization treatment step ST61). The addition amount of the slaked lime slurry is set so that the pH of the third stirring tank 13, which is the last tank among the three stirring tanks arranged in series, is 10 to 11. The stirrer 3 uniformly mixes the slaked lime slurry in the tank and maintains a desired residence time.

In the second stirring tank 12, the neutralization process of the wastewater sent from the first stirring tank 11 further continues.

The third stirring tank 13, which is the last tank in the reaction tank 10 in FIG. 3, is configured to be able to add about 25 mass% sodium hydrosulfide aqueous solution to the treatment flow rate of waste water. (Sulfidation treatment step ST62). In the third stirring tank 13, a slurry (sulfide) containing cadmium and the like is generated.

The processed slurry that overflows the third stirring tank 13 is sent to a thickener and subjected to solid-liquid separation (solid-liquid separation step ST63), and the overflow liquid of the thickener is subjected to a further final processing step such as a pH adjustment step. It is sent and processed for final detoxification.

(Example)
As shown in FIG. 3, a solution after the cementation reaction with a chloride ion concentration of 26 g/l (0.73 mol/l) and a cadmium concentration of 8 mg/l in a crude zinc oxide production plant was carried out at a flow rate of 60 m ³ /h. A reaction tank equipped with a simple stirrer was supplied to the waste water neutralization reaction tank, which is arranged in series in three tanks. Here, slaked lime slurry adjusted to a concentration of 10 to 15 mass% was added to the first stirring tank so that the pH of the third stirring tank was 10 to 11. In addition, a water glass aqueous solution adjusted to a concentration of 5 to 10% by mass was added to the first stirring tank at a flow rate of 10 l/h. A 25 mass% aqueous solution of sodium hydrosulfide was added to the third stirring tank at a flow rate of 6 l/h. The redox potential in the third stirred tank was −150 to −50 mV (Ag/AgCl electrode). The slurry after the reaction that overflowed the third stirring tank was subjected to solid-liquid separation with a thickener, and the cadmium concentration of the thickener overflow liquid was 0.006 mg/l. The cadmium concentration was measured with an ICP mass spectrometer. Further, no odor was confirmed in the upper part of the third stirring tank, and it was confirmed that hydrogen sulfide gas was not generated during the sulfurating treatment.

(Comparative example)
In a reaction tank having the same configuration as that of the example, except that the 25% by mass aqueous sodium hydrogen sulfide solution was not added to the third stirring tank, using the same equipment as the example, under the same operating conditions as the example, Wastewater neutralization operation was carried out. As a result, the cadmium concentration in the thickener overflow liquid was 0.2 mg/l.

From the above, it is confirmed that the wastewater treatment method of the present invention is a treatment method capable of safely and sufficiently reducing the cadmium concentration in an aqueous solution containing cadmium and lead and having a high chloride ion concentration. It was

S10 Reduction roasting process S20 Wet process S30 Dry heating process S40 Exhaust gas treatment process S50 Cementation process S60 Waste water treatment process ST61 Neutralization treatment process ST62 Sulfidation treatment process ST63 Solid-liquid separation process 10 Reaction tank 11 First stirring tank 12 Second stirring Tank 13 Third stirring tank 3 Stirrer

Claims (3)

An overflow liquid after cementation treatment of a treatment liquid discharged from a wet process performed in the process of producing zinc oxide from steel dust , containing cadmium and lead, and having a chloride ion concentration of 0.2 mol/l or more. A method for treating wastewater having a concentration of 1.5 mol/l or less,
A neutralization treatment step of performing a neutralization treatment to form a neutralized precipitate by adding a neutralizing agent to the waste water and adjusting the pH to 10 or more and 11 or less;
A sulfidizing treatment for producing a metal sulfide is performed by adding a sulfidizing agent to the wastewater after the neutralization treatment to adjust the redox potential (Ag/AgCl electrode standard) to −200 mV or more and −40 mV or less. Processing steps,
A method for treating wastewater, which comprises a solid-liquid separation step of solid-liquid separating a slurry containing the neutralized precipitate and the metal sulfide. The method for treating wastewater according to claim 1, wherein
The neutralization treatment is performed in the first stirring tank,
A method for treating wastewater, wherein the sulfurization treatment is performed in a tank that is connected to the first stirring tank directly or via another stirring tank. The method for treating wastewater according to claim 1 or 2, wherein the neutralizing agent is slaked lime, and the sulfiding agent is sodium hydrosulfide.

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