JP2017192916A - Wastewater treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wastewater treatment method allowing for efficient and stable reduction of cadmium in wastewater containing cadmium ion, an organic compound and lead ion and/or zinc ion.SOLUTION: The wastewater treatment method has at least a pH adjustment step S2 of adjusting pH of wastewater containing cadmium ion, an organic compound and lead ion and/or zinc ion to the range of 10 to 12 by adding a neutralizer to the wastewater, a sulfurization step S3 of performing a sulfurization treatment on the pH-adjusted wastewater by adding a sulfidizing agent to the pH-adjusted wastewater and a solid-liquid separation step S5 of removing a precipitation in a slurry obtained after the sulfurization treatment by solid-liquid separation. In the sulfurization step S3, and added amount of the sulfidizing agent is controlled so that oxidation reduction potential is -40 mV or less as a value with a silver chloride silver electrode as a reference electrode.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a wastewater treatment method. Specifically, the present invention relates to a method for treating wastewater containing cadmium, an organic compound, and lead ions and / or zinc ions.

  In the process of smelting non-ferrous metals such as copper, nickel, lead and zinc, heavy metals such as cadmium ion, lead ion and zinc ion, organic compounds, etc. are included in the wastewater generated by smelting There may be. Wastewater containing these substances cannot be discharged into the sea as it is, and the contents containing heavy metals such as cadmium, lead, and zinc in the wastewater are separated by an appropriate method to a concentration below a certain standard. Reduction processing is required. For example, as described in Non-Patent Document 1, the cadmium concentration in the wastewater discharged into the sea area needs to be reduced to 0.03 mg / L.

  Examples of the method for separating cadmium include the following methods. That is, first, wastewater containing cadmium ions is placed in a reaction tank, and an alkaline agent such as slaked lime or sodium hydroxide is added to the reaction tank to raise the pH to obtain a slurry containing starch containing cadmium. Next, this slurry is allowed to stand with a thickener or the like, so that the cadmium-containing starch is precipitated and solid-liquid separated. The supernatant liquid from which cadmium has been separated and removed in this manner is discharged as waste water.

  Here, in order to make the cadmium concentration in the wastewater discharged into the sea area 0.03 mg / L, it is necessary to maintain the pH of the wastewater before the discharge to 11 or more.

  However, when lead ions and zinc ions are also contained in the wastewater, these elements are amphoteric metals, so if the pH is raised excessively, these starches will be dissolved again to prevent dissolution. It is necessary to control the upper limit value. Specifically, in the case of lead ions, pH 8 to 11 is suitable for precipitation generation, and when the pH exceeds 11, the tendency of re-dissolution increases. Similarly, with zinc ions, pH 8 to 11.5 is suitable for precipitation, and when the pH exceeds 11.5, the tendency to redissolve increases. Thus, in the case of wastewater containing lead and zinc in addition to cadmium, an operation for accurately controlling the pH in a narrow region is required to separate all of them.

  In general, in the smelting process, the operation level changes finely according to the raw material circumstances. Therefore, wastewater components and concentrations are likely to fluctuate, and it is not easy to stably treat wastewater discharged in large quantities industrially. In recent years, drainage regulations tend to be further strengthened, and wastewater management has become more difficult.

  Therefore, in the above method, a method has been used in which an iron compound such as ferrous sulfate or ferric chloride is added to waste water before pH adjustment, and then an alkaline agent is added to raise the pH. . In this method, the added iron compound is converted into a precipitate of hydroxide, and cadmium is separated by precipitation using the coprecipitation effect at that time.

  In addition, a treatment method has been proposed in which a reducing iron compound is added to heavy metal-containing water such as waste water to precipitate heavy metals, and the precipitate is solid-liquid separated to remove heavy metals. For example, in Patent Document 1, a step of adding a reducing iron compound to heavy metal-containing water (iron compound addition step), a step of generating a precipitate from heavy metal-containing water added with a reducing iron compound (precipitation step). And a step of solid-liquid separation of the generated precipitate (sludge) (solid-liquid separation step), a step of making all or part of the separated sludge alkaline and returning it to the reaction tank (sludge return step), and precipitation In the process, wastewater added with reducing iron compound and alkaline sludge are mixed and reacted in a non-oxidizing atmosphere and under alkali to produce a reducing iron compound precipitate, and heavy metals are incorporated into the precipitate. A method for treating heavy metal-containing water to be removed outside the system is disclosed.

JP 2006-263699 A

New Pollution Prevention Technology and Regulations 2013 Water Quality (Industrial Environmental Management Association Maruzen Publishing) p444-445

  However, in the method of adjusting the pH of the wastewater as described above or adjusting the pH by adding an iron compound to the wastewater, particularly when the organic compound is contained in the wastewater, the cadmium concentration can be stabilized. It becomes difficult to reduce, and it is difficult to fully satisfy the drainage standards that have been strengthened in recent years. This is because phosphorus (P), nitrogen (N), carbon (C), chloride ions (Cl), etc. resulting from organic compounds form a complex with heavy metals such as cadmium in the wastewater, and the heavy metal This is considered to increase the solubility of the species.

  In addition, as an organic compound contained in waste water, the fats and oils, surface treatment agent, etc. which were apply | coated to the surface of the steel material at the time of steel processing are mentioned, for example. These organic compounds are recovered together with the steel dust generated during the steel processing, and when these recovered materials are treated as smelting raw materials for recycling, they are volatilized, collected and washed and contained in the waste water.

  In addition, the method described in Patent Document 1 is a method of reducing the concentration of heavy metals by sealing a reaction vessel using a reducing iron compound, and in an open system that has been widely used conventionally, Since the ferrous iron compound reacts with oxygen in the atmosphere and oxidizes, it cannot be used. For this reason, in order to adopt the method described in Patent Document 1, a new investment is required, which is not efficient due to problems such as high costs.

  The present invention has been made in view of such circumstances, and can efficiently and stably reduce cadmium in waste water containing cadmium ions, organic compounds, and lead ions and / or zinc ions. It aims at providing the processing method of waste water.

  This inventor repeated earnest examination in order to solve the subject mentioned above. As a result, in wastewater containing cadmium ions, organic compounds, and lead ions and / or zinc ions, after adjusting the pH to a predetermined range, a sulfurizing agent is added to reduce the oxidation-reduction potential of the wastewater to the predetermined range. The inventors have found that cadmium ions can be efficiently and stably reduced by performing a sulfidation treatment so as to be controlled, and the present invention has been completed.

  (1) In the first invention of the present invention, a neutralizer is added to wastewater containing cadmium ions, organic compounds, and lead ions and / or zinc ions to adjust the pH to a range of 10 or more and 12 or less. a pH adjustment step, a sulfidation step of adding a sulfiding agent to the pH-adjusted waste water and subjecting it to a sulfidation treatment, and a solid-liquid separation step of separating and removing the precipitate in the slurry obtained after the sulfidation treatment. In the sulfiding step, the amount of the sulfiding agent added is controlled so that the oxidation-reduction potential is -40 mV or less with a silver / silver chloride electrode as a reference electrode. .

  (2) According to a second aspect of the present invention, in the first aspect, before the pH adjustment step, a silicate addition step of adding 5 mg / L or more and 200 mg / L or less of a silicate compound to the waste water. It has a wastewater treatment method.

  According to the present invention, cadmium in waste water containing cadmium ions, organic compounds, and lead ions and / or zinc ions can be efficiently and stably reduced.

It is a flowchart which shows the processing method of waste_water | drain.

  Hereinafter, a specific embodiment of the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, In the range which does not change the summary of this invention, it can change suitably. Further, in this specification, the expression “x to y” (x and y are arbitrary numerical values) means “x or more and y or less” unless otherwise specified.

  The wastewater treatment method according to the present embodiment includes a pH adjustment step of adjusting pH to a predetermined range by adding a neutralizing agent to wastewater containing cadmium ions, organic compounds, and lead ions and / or zinc ions. And a sulfiding step of adding a sulfiding agent to the pH-adjusted waste water to sulfidize, and a solid-liquid separation step of solid-liquid separating the treated slurry.

  In addition, this wastewater treatment method may include a silicate addition step of adding a silicate compound to the wastewater at a predetermined ratio prior to the pH adjustment step.

  Further, the wastewater treatment method may include a coagulation step of adding a coagulant to the slurry obtained after the sulfurization treatment after the sulfurization treatment in the sulfurization step and prior to the solid-liquid separation step.

  Here, as described above, the “drainage” to be treated contains cadmium ions and organic compounds, and also contains any one or more of lead ions and zinc ions. This wastewater includes various types of wastewater generated naturally and artificially. For example, industrial wastewater, sewage, seawater, river water, water in swamps and lakes, surface pool water, rivers, Water, underground running water, pool water, culvert water and so on. In addition to cadmium ions, lead ions, and zinc ions, the waste water may contain heavy metals such as selenium, hexavalent chromium, copper, nickel, arsenic, and antimony. The wastewater treatment method according to the present embodiment has a removal effect on heavy metals other than cadmium ions, lead ions, and zinc ions contained in the wastewater.

  In addition, the organic compound contained in the wastewater is not particularly limited and may be contained naturally, such as oils and fats and surface treatment agents applied to the surface of steel materials during steel processing. In addition, it may be artificially added.

  Specifically, FIG. 1 is a flowchart showing an example of the flow of the wastewater treatment method according to the present embodiment. As shown in FIG. 1, this waste water treatment method includes a silicate addition step S1, a pH adjustment step S2, a sulfurization step S3, an agglomeration step S4, and a solid-liquid separation step S5. . Below, each process is demonstrated in detail.

[Silicate addition process]
In the silicate addition step S1, a silicate compound is added to the wastewater to be treated prior to pH adjustment. In wastewater containing lead ions and zinc ions, the lead and zinc may be redissolved during pH adjustment in the next step. Therefore, this silicate addition process S1 is provided as needed, and a silicate compound is added with respect to the waste_water | drain before pH adjustment.

  In this way, by adding a silicate compound to the wastewater, the silicate dissolves in water and becomes silicate ions, which are combined with lead and / or zinc, thereby re-dissolving lead and zinc. Suppress.

  Although it does not specifically limit as a silicate compound, For example, water glass etc. can be used. The addition amount of the silicate compound is preferably 5 mg / L to 200 mg / L, more preferably 30 mg / L to 150 mg / L, and particularly preferably 50 mg / L to 100 mg / L. preferable. When the addition amount of the silicate compound is less than 5 mg / L, re-dissolution of lead and / or zinc cannot be sufficiently suppressed. On the other hand, even if the addition amount of the silicate compound is more than 200 mg / L, the effect does not change, but the drug cost increases because it is excessively added.

[PH adjustment step]
In the pH adjustment step S2, a neutralizing agent is added to the wastewater to be treated, and the pH of the wastewater is adjusted to a predetermined range. When the silicate addition step S1 is provided prior to the pH adjustment step S2, a neutralizing agent is added to the wastewater obtained by adding the silicate compound to adjust the pH. Although it does not specifically limit as a neutralizing agent, For example, alkaline agents, such as slaked lime, caustic soda, quicklime, and limestone, can be used.

  Specifically, in pH adjustment process S2, the pH of waste water is adjusted to the range of 10-12 by adding a neutralizing agent. In the wastewater adjusted to a pH of 10 to 12, the solubility of cadmium is low, and most cadmium precipitates as hydroxide. Also, zinc and lead, which are amphoteric metals, can be converted into hydroxide precipitates at a certain rate. Furthermore, by adjusting the pH to a range of 10 to 12, generation of hydrogen sulfide gas can be suppressed in the subsequent sulfurization step S3.

  On the other hand, when the pH is less than 10, the solubility of cadmium becomes high, and cadmium precipitate cannot be generated effectively. On the other hand, if the pH is higher than 12, the solubility of zinc or lead becomes too high, and a precipitate of zinc or lead is hardly formed.

  In addition, in the pH adjustment in this pH adjustment step S2, the solubility of zinc and lead is not sufficiently lowered, so that only a part of the precipitate is precipitated, but zinc and lead are separated and removed to the extent that the drainage standard is satisfied. be able to.

[Sulfurization process]
In the sulfidation step S3, a sulfide is added to the wastewater after the pH adjustment in the pH adjustment step S2 to perform a sulfidation treatment to generate a metal sulfide. As the sulfiding agent, a hydrogen sulfide salt or a sulfide salt can be used. For example, sodium hydrosulfide (NaSH), hydrogen sulfide gas, sodium sulfide, or the like can be used.

  When organic compounds are contained in the wastewater, cadmium in the wastewater forms a complex with the organic compound, and due to the increase in solubility, it remains dissolved in the wastewater even after pH adjustment. . At this time, by adding a sulfiding agent to the drained water after pH adjustment, the cadmium dissolved in the drained water after pH adjustment can be precipitated as a metal sulfide. Of cadmium can be reliably precipitated. In addition, by performing sulfuration treatment in the sulfurization step S3, heavy metals such as zinc and lead can be precipitated as metal sulfides, and can be effectively removed from the waste water.

  Furthermore, in the sulfidation step S3, sulfidation is performed on the wastewater adjusted to a pH range of 10 to 12 in the pH adjustment step S2, so that heavy metal such as cadmium is avoided while avoiding generation of hydrogen sulfide gas accompanying the sulfidation. Can be precipitated as metal sulfides.

  Specifically, in this sulfidation step S3, the oxidation-reduction potential (ORP) of the wastewater is −40 mV or less, preferably −200 mV to −40 mV, more preferably − The sulfiding treatment is performed by controlling the amount of the sulfiding agent added so as to be in the range of 150 mV to -30 mV.

  When the oxidation-reduction potential exceeds -40 mV, cadmium dissolved in the waste water, and in some cases, zinc or lead cannot be sufficiently precipitated. In order to lower the oxidation-reduction potential, the addition amount of the sulfiding agent is preferably increased.

  On the other hand, when the oxidation-reduction potential is less than −200 mV, the addition amount of the sulfiding agent tends to be excessive, which is not preferable. The wastewater treated through the sulfidation step S3, the coagulation step S4, and the solid-liquid separation step S5 is finally discharged through a final step of adjusting the pH to a neutral region using sulfuric acid or the like. If the addition amount of the sulfiding agent in the sulfiding step S3 is excessive, hydrogen sulfide gas is likely to be generated along with the pH adjustment in the final step, which is not preferable.

  Here, when the sulfiding agent is added, the sulfiding agent in an amount of, for example, 2 equivalents to 70 equivalents can be added to the cadmium ions in the waste water. When the addition amount of the sulfiding agent is less than 2 equivalents with respect to cadmium ions, cadmium dissolved in the waste water, and in some cases, zinc or lead may not be sufficiently precipitated. On the other hand, when the addition amount of the sulfurizing agent is more than 70 equivalents with respect to the cadmium ions, an excessive amount of the sulfurizing agent is added to the cadmium ions dissolved in the waste water, and cadmium is precipitated. The sulfurizing agent that does not contribute to the residue remains, and the chemical cost increases.

  However, as described above, particularly when wastewater from a smelting process is treated, the cadmium concentration in the wastewater may not be known beforehand or may vary greatly depending on the raw material composition or treatment. Therefore, in the sulfidation step S3, if an attempt is made to determine the addition amount of the sulfidizing agent without referring to the oxidation-reduction potential, the cadmium concentration in the wastewater is added after waiting for an analytical value to be obtained. Processing takes time.

  In contrast, as in the wastewater treatment method according to the present embodiment, when the sulfurizing agent is added, the oxidation-reduction potential of the wastewater is monitored, and the oxidation-reduction potential of the sulfurization agent is maintained so that the oxidation-reduction potential is −40 V or less. By controlling the amount of addition, even if the cadmium concentration in the wastewater is not known in advance or when the fluctuation is large, the cadmium concentration can be controlled efficiently and accurately without requiring concentration analysis. it can. Further, it is possible to reliably prevent the cadmium from being discharged as it is.

  Furthermore, since this wastewater treatment method can suppress the generation of hydrogen sulfide gas during the sulfidation treatment, there is no need to make the reaction tank for performing the sulfidation treatment an airtight structure as in the prior art. There is no need to remove it by suction. Therefore, it can be carried out by providing only the equipment for adding and supplying the sulfiding agent to the reaction tank on the downstream side of the equipment for adjusting the pH, and the equipment cost is greatly reduced compared with the conventional equipment for sulfiding treatment. can do.

  In this wastewater treatment method, most of the cadmium is neutralized with a neutralizing agent in the pH adjustment step S2 through a two-step process of pH adjustment step S2 and sulfurization step S3, which is relatively expensive. Since the sulfidation process in the sulfidation step S3, which requires a sulfidizing agent, is performed only as an auxiliary process, the cost of the drug is greatly reduced compared with the case where an equivalent separation recovery rate is achieved only by the sulfidation process. be able to.

[Aggregation process]
In the wastewater treatment method according to the present embodiment, after performing the sulfidation treatment in the sulfidation step S3, the aggregation step S4 can be provided as necessary.

  In the agglomeration step S4, an aggregating agent is added to the slurry obtained after the sulfidation treatment, and a starch containing heavy metals such as cadmium, zinc, lead, etc. in the slurry is agglomerated. As described above, the starch in the slurry is agglomerated by the aggregating agent, so that the residue can be efficiently separated in the solid-liquid separation step S5 of the next step.

  The flocculant is not particularly limited, and for example, an inorganic flocculant or a polymer flocculant can be used. For example, as a polymer flocculant, trade name: Sumiflock manufactured by MT Aqua Polymer Co., Ltd. is commercially available. The amount of the flocculant added can be appropriately determined according to the type of flocculant and the amount of the precipitate.

[Solid-liquid separation process]
In the solid-liquid separation step S5, the slurry is separated and recovered from the slurry by subjecting the slurry to a solid-liquid separation process. By the solid-liquid separation treatment, waste water from which cadmium, lead, and zinc are substantially separated and removed can be obtained.

  In the solid-liquid separation step S5, a general solid-liquid separation method can be employed. For example, the solid-liquid separation step S5 can be performed by a method using a filtration separation device, a centrifugal dehydration device, a sedimentation separation device such as a thickener.

  As described above in detail, according to the wastewater treatment method according to the present embodiment, even if the wastewater contains lead ions or zinc ions other than cadmium ions, or contains organic compounds, As will be described later, heavy metals such as cadmium can be stably reduced at low cost by simple pH adjustment through two steps of a pH adjustment step and a sulfurization step. Moreover, according to the wastewater treatment method according to the present embodiment, the amount of cadmium in the wastewater is determined in advance because the addition amount of the sulfiding agent is controlled so that the oxidation-reduction potential is maintained at -40 V or less. Even when it is not performed or when the fluctuation is large, a large amount of waste water can be treated efficiently without requiring concentration analysis. Furthermore, according to the processing method according to the present embodiment, since it can be performed with a release system facility, a conventional release system facility can be used, which is efficient.

[Example 1]
A waste water containing 17 mg / L of cadmium, 1.2 mg / L of lead, 670 mg / L of zinc, and 50 mg / L of an nitrogen-containing organic amine compound as an organic compound was prepared.

  After adding sodium silicate to the drainage so as to have a concentration of 50 mg / L, neutralization is performed by adjusting the pH of the drainage to 10.5 using slaked lime as a neutralizing agent. A slurry containing the product was obtained.

  Next, a 25 wt% sodium hydrosulfide (NaHS) solution was added to this slurry so that the oxidation-reduction potential (ORP) was −45 mV with a silver / silver chloride electrode as a reference electrode. Treated. Subsequently, a polyacrylamide polymer flocculant (trade name: Sumifloc FA-70) was added to the slurry after the sulfurization treatment at a rate of 10 mg / L. The slurry after the addition of the flocculant was allowed to stand, and then filtered using a filter bottle and Nutsche to obtain a starch and a liquid after filtration.

  As a result of analyzing the obtained filtered liquid using high frequency inductively coupled plasma (ICP) emission spectroscopy, the cadmium concentration is 0.01 mg / L or less, the lead concentration is 0.01 mg / L, zinc The concentration was 0.5 mg / L, and good results satisfying drainage standards were obtained. Here, the drainage standards are cadmium concentration 0.03 mg / L or less, lead concentration 0.1 mg / L or less, and zinc concentration 2 mg / L or less.

[Comparative Example 1]
The treatment was performed in the same manner as in Example 1 except that sodium hydrosulfide was not added. That is, after neutralizing with slaked lime, without adding a sulfiding treatment, a flocculant is added to the resulting slurry to agglomerate the neutralized starch and filtered to obtain a starch and a post-filtration liquid. It was.

  The obtained post-filtration liquid had a lead concentration of 0.01 mg / L and a zinc concentration of 0.6 mg / L, which satisfies the drainage standards, but the cadmium concentration was 1.2 mg / L.

[Comparative Example 2]
The treatment was carried out in the same manner as in Example 1 except that a 25 wt% sodium hydrosulfide (NaHS) solution was added so that the ORP of the wastewater was -20 mV.

  The obtained post-filtration liquid had a lead concentration of 0.01 mg / L and a zinc concentration of 0.6 mg / L, which satisfies the drainage standards, but the cadmium concentration was 0.04 mg / L.

  As described above, in Example 1 to which the present invention was applied, cadmium was almost removed from the waste water, and at the same time, zinc and lead could be effectively removed, and a treatment satisfying the waste water standard could be performed. On the other hand, in Comparative Example 1 and Comparative Example 2, cadmium could not be sufficiently removed from the waste water, and the treatment satisfying the waste water standard could not be performed.

S1 Silicate addition process S2 pH adjustment process S3 Sulfurization process S4 Aggregation process S5 Solid-liquid separation process

Claims (2)

A pH adjusting step of adjusting the pH to a range of 10 to 12 by adding a neutralizing agent to wastewater containing cadmium ions, organic compounds, and lead ions and / or zinc ions;
a sulfidation step of adding a sulfidizing agent to the pH-adjusted wastewater to perform sulfidation,
A solid-liquid separation step of removing the precipitate in the slurry obtained after the sulfurization treatment by solid-liquid separation, and at least,
In the sulfiding step, the amount of the sulfiding agent added is controlled such that the oxidation-reduction potential is −40 mV or less with a silver / silver chloride electrode as a reference electrode. The wastewater treatment method according to claim 1, further comprising a silicate addition step of adding 5 mg / L or more and 200 mg / L or less of a silicate compound to the wastewater before the pH adjustment step.

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