JP2022155483A - Method for producing valuable material-containing sludge - Google Patents

Abstract

To provide a method for producing a sludge which is a treatment method for treating heavy metal to an effluent standard value or less and suppressing occurrence of hydrogen sulfide gas in effluent containing heavy metal such as copper, and can improve a percentage content of variable metal in a sludge using the method.SOLUTION: A method for producing a valuable material-containing sludge includes immobilizing heavy metal in effluent with a pH from a neutral region to a weak alkali region using an alkali metal hydroxide and a salt of a dithiocarbamic acid, then adding a polymer coagulant, and solid-liquid separating a sludge containing a heavy metal solid matter.SELECTED DRAWING: None

Description

The present invention relates to a method for producing sludge containing heavy metals such as copper.

Known methods for treating aqueous solutions containing heavy metals include an alkali precipitation method using calcium hydroxide, sodium hydroxide, etc., and a coprecipitation method using iron salts such as ferric chloride and aluminum salts such as aluminum sulfate. . These methods are easy to control reactions through pH and have high process safety. On the other hand, since the sludge separated from the aqueous solution contains a large amount of water of crystallization and salt, it is generally dumped in landfills, etc. However, sludge containing copper, nickel, etc. can be recycled as a metal raw material. is.

From the viewpoint of recycling valuable metals such as copper, a method using a sulfiding agent such as sodium hydrosulfide has been reported as a method for improving the content of valuable metals in sludge (for example, Patent Document 1 reference).

Japanese Patent Application Laid-Open No. 2007-69068

The method described in Patent Document 1 is characterized by adding a sulfiding agent such as sodium hydrosulfide while maintaining the pH at 1 to 5 from the viewpoint of reactivity with heavy metals. However, addition under such acidic conditions causes generation of hydrogen sulfide gas, which is highly toxic, and therefore conditions that cause a decrease in pH should generally be avoided.

In addition, it is said that the same pH range is desirable for the next step of oxidizing agent treatment such as sodium hypochlorite, and if an excessive amount of sulfiding agent is added in the previous step, hydrogen sulfide gas is generated in this step as well. Accompanied by Furthermore, sulfiding agents such as sodium hydrogen sulfide have the property of violently reacting with heat when coming into contact with oxidizing agents, and there is room for improvement in terms of safety in a series of these steps.

That is, it is an object of the present invention to provide a production method that enables heavy metals to be treated in a pH range capable of suppressing the generation of hydrogen sulfide gas and that improves the content of valuable metals in the sludge.

As a result of intensive studies to solve the above problems, the present inventors have found that using an alkali metal hydroxide and a salt of dithiocarbamic acid, heavy metals in wastewater are immobilized at a pH in the neutral to weakly alkaline range. After that, a polymer flocculant is added, and the solid-liquid separation of the sludge containing heavy metal solids can suppress the generation of hydrogen sulfide gas and at the same time improve the content of valuable metals in the sludge. The present inventors have found that it is possible to complete the present invention.

That is, the present invention has the following gists.

[1] A salt of alkali metal hydroxide and dithiocarbamic acid is added to wastewater containing heavy metals to immobilize the heavy metals, and then a polymer flocculant is added to separate the solid-liquid sludge containing heavy metal solids. A method for producing heavy metal-containing sludge, characterized by:

[2] The heavy metal-containing sludge according to [1] above, wherein the heavy metal is copper, and the heavy metal-containing wastewater is wastewater containing copper at a concentration of 20 mg/L or more and 1500 mg/L or less. manufacturing method.

[3] The above [1] or [2], wherein the salt of dithiocarbamic acid is a reaction product of piperazine or tetraethylenepentamine, carbon disulfide, and an alkali metal hydroxide. A method for producing heavy metal-containing sludge.

[4] The method for producing heavy metal-containing sludge according to any one of [1] to [3] above, wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

[5] The heavy metal-containing sludge according to any one of [1] to [4] above, wherein the pH when the salt of dithiocarbamic acid is added to the heavy metal-containing wastewater is in the range of 6 to 11. manufacturing method.

The method for producing heavy metal-containing sludge according to the present invention can treat heavy metals to below the standard value for wastewater, at the same time suppressing the generation of hydrogen sulfide gas, and can improve the content of valuable metals in the sludge after fixing heavy metals. It has the effect of being able to Furthermore, since the treatment can be performed at a pH in the neutral region, there is an effect that the step of adjusting the pH after immobilizing heavy metals to a pH that allows discharge of the wastewater can be omitted, which is extremely useful industrially.

The present invention will be described in detail below.

The method of the present invention is applicable to cadmium, chromium, copper, iron, mercury, nickel, lead, zinc, palladium, gold, silver, platinum, cobalt, indium, molybdenum, antimony, It is effective for producing sludge containing heavy metals such as tin, titanium, zirconium, manganese and tungsten, and is particularly excellent for treating waste water containing copper.

In the method for producing sludge of the present invention, a salt of alkali metal hydroxide and dithiocarbamic acid is added to heavy metal-containing wastewater to immobilize heavy metals, and then a polymer flocculant is added to produce sludge containing heavy metal solids. is characterized by solid-liquid separation.

Alkali metal hydroxides used in the present invention include sodium hydroxide, potassium hydroxide, lithium hydroxide and the like, but sodium hydroxide or potassium hydroxide is preferable in terms of easy availability.

The salt of dithiocarbamic acid used in the present invention is not particularly limited as long as it is a compound having a dithiocarbamyl group in the molecule. Examples thereof include compounds obtained by reacting an amine compound having at least one amino group selected from the group consisting of a primary amino group and a secondary amino group, carbon disulfide, and an alkali metal hydroxide. A compound obtained by reacting an amine compound having two or more amino groups selected from the group consisting of a primary amino group and a secondary amino group, carbon disulfide, and an alkali metal hydroxide is more preferable.

Specific examples of the amine compound having at least one amino group selected from the group consisting of a primary amino group and a secondary amino group include diethylamine, piperazine, pyrrolidine, piperidine, diethylenetriamine, and N-(2-aminoethyl). Examples include piperazine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, heptaethyleneoctamine, polyethyleneimine, and condensates of polyethyleneimine and benzyl chloride.

Among these, compounds obtained by reacting piperazine, tetraethylenepentamine, or diethylamine with carbon disulfide and an alkali metal hydroxide are preferred from the viewpoint of heavy metal treatment performance and stability as a compound. However, in the dithiocarbamic acid salt of tetraethylenepentamine, the raw material tetraethylenepentamine is a linear form [see formula (1) below] as a main component, and analogs [see formulas (2) to (4) below]. ] is industrially produced, the obtained dithiocarbamic acid salt is also a composition, which has the drawback of complicating quality control. On the other hand, piperazine or diethylamine dithiocarbamic acid salts do not have such drawbacks and are particularly preferred.

Examples of the alkali metal hydroxide used to obtain the salt of dithiocarbamic acid used in the present invention include sodium hydroxide, potassium hydroxide, lithium hydroxide and the like. Potassium oxide is preferred.

Inorganic sulfides, polyamines having a weight average molecular weight of 300 or more, polyamines having 3 to 8 nitrogen atoms, hydroxyaminocarboxylic acids and/or salts thereof may be added to the dithiocarbamic acid salts.

Examples of sulfide salts include sodium sulfide, sodium hydrogen sulfide, potassium sulfide, potassium hydrogen sulfide, calcium sulfide, calcium hydrogen sulfide, magnesium hydrogen sulfide, and ammonium sulfide.

Polyamines having a weight average molecular weight of 300 or more include, for example, polyethyleneimines having a weight average molecular weight of 300 or more, and polyetheramines having a weight average molecular weight of 300 or more (polypropylene glycol, compounds in which terminal hydroxyl groups such as polyethylene glycol are converted to primary amino groups). ) and the like.

Polyamines having 3 to 8 nitrogen atoms include, for example, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, heptaethyleneoctamine and the like.

Examples of hydroxyaminocarboxylic acids and/or salts thereof include hydroxyethylethylenediaminetriacetic acid (HEDTA), diaminohydroxypropanetetraacetic acid (DPTA), hydroxyiminodisuccinic acid (HIDS) and salts thereof.

In the present invention, the method of adding an alkali metal hydroxide and a salt of dithiocarbamic acid is not particularly limited, but for example, an aqueous solution, a slurry, a dispersion, or a solution can be used.

Moreover, the alkali metal hydroxide and the salt of dithiocarbamic acid may be added separately, or may be added after being mixed in advance. When adding them separately, it is preferable to add the alkali metal hydroxide first from the viewpoint of pH adjustment. Furthermore, the mass ratio of the alkali metal hydroxide and the salt of dithiocarbamic acid is not particularly limited.

The pH at which the dithiocarbamic acid salt is added is preferably in the neutral to alkaline range (pH 6 to 14), and in the neutral to weakly alkaline range (pH 6 to 11), since the generation of hydrogen sulfide gas can be suppressed. is more preferable, and a neutral region (pH 6 to 8) is more preferable from the viewpoint of the pH at which the wastewater after immobilizing the heavy metal can be discharged.

In the present invention, the temperature at which the alkali metal hydroxide and the salt of dithiocarbamic acid are added and the stirring time after the addition vary depending on the type of wastewater containing heavy metals, but are usually -10°C to 40°C and 40°C, respectively. It is selected from the range of several minutes to 2 hours.

The method for solid-liquid separation of the sludge containing heavy metal solids is not particularly limited, but examples thereof include filtration, centrifugation, and a method of sedimenting solids and then separating them from the supernatant liquid.

After immobilizing the heavy metals, a polymer flocculant is added. By adding the polymer flocculant, the content of valuable metals in the sludge containing heavy metal solids is increased, and at the same time, the heavy metal solids in the post-process to facilitate solid-liquid separation of sludge containing

Commercially available polymer flocculants can be used as the polymer flocculant, and are not particularly limited. Examples include acrylic acid polymer, acrylamide polymer, dimethylaminoethyl methacrylate polymer, and the like. Of these, weakly anionic acrylic acid polymers are preferred from the viewpoint of aggregation performance.

On the other hand, although not particularly limited, inorganic flocculants such as iron compounds such as ferric chloride and ferrous sulfate, and aluminum compounds such as aluminum sulfate and polyaluminum chloride are useful metals in sludge containing heavy metal solids. It is preferred not to use it as it lowers the content.

When copper is contained in the wastewater containing heavy metals, the copper concentration is not particularly limited, but is preferably 20 mg/L or more in order to increase the copper content in the sludge, and the treatment can achieve the amount of heavy metals below the wastewater standard value. The concentration is more preferably 20 mg/L or more and 1500 mg/L or less, and further preferably 20 mg/L or more and 1000 mg/L or less, in that sludge with high performance and valuable substance content can be produced.

In a series of sludge production methods, a pH adjuster may be used when adjusting the aqueous solution to be acidic or alkaline. Examples of pH adjusters include inorganic salts such as hydrochloric acid, sulfuric acid, phosphoric acid, and nitric acid, and their aqueous solutions, when adjusting to acidity, and sodium hydroxide, potassium hydroxide, calcium hydroxide, potassium carbonate, etc. when adjusting to alkalinity. and an aqueous solution thereof.

A series of sludge production methods may be carried out in a continuous manner or in a single manner.

The present invention will be specifically described below, but the present invention should not be construed as being limited to these examples.

(analysis method)
The concentration of heavy metal ions in the aqueous solution was measured with an ICP emission spectrometer (ICPE-9800, manufactured by Shimadzu Corporation).

The amount of hydrogen sulfide gas generated from the aqueous solution during wastewater treatment was measured with a gas detector tube 4LK manufactured by Gastech.

The amount of sludge generated [g/L] was measured after drying the sludge containing heavy metal solids after solid-liquid separation at 105°C for 2 hours.

The water content of sludge (%) was calculated by the following formula.

(1-amount of sludge generated [g/L]/sludge containing heavy metal solids after solid-liquid separation [g/L]) x 100
The copper content (%) in the sludge was calculated by the following formula.

(Copper concentration before wastewater treatment [mg/L]−Copper concentration after wastewater treatment [mg/L])×10 ⁻³ /Amount of sludge generated [g/L]×100
Preparation example 1
Salts of dithiocarbamic acid used in the examples were prepared according to the following methods.

(Preparation of salt A of dithiocarbamic acid)
After mixing 112 g of piperazine (manufactured by Tosoh Corporation) and 386 g of pure water, 306 g of 48% by weight potassium hydroxide (manufactured by Kishida Chemical Co., Ltd.) and 196 g of carbon disulfide (manufactured by Kishida Chemical Co., Ltd.) were mixed at 25 ° C. with stirring in a nitrogen stream. ) was divided into four parts and dropped alternately. After stirring for 1 hour, an aqueous solution containing 40% by weight of the compound represented by the following formula (5) was obtained.

(Preparation of salt B of dithiocarbamic acid)
After mixing 159 g of tetraethylenepentamine (manufactured by Tosoh Corporation) and 331 g of pure water, 281 g of 48% by weight sodium hydroxide (manufactured by Kishida Chemical Co., Ltd.) and 230 g of carbon disulfide (manufactured by Kishida Chemical Co., Ltd.) were mixed at 25° C. while stirring in a nitrogen stream. Kagaku Co., Ltd.) was divided into four portions and dropped alternately. After stirring for 1 hour, an aqueous solution containing 40% by weight of the compound represented by the following formula (6) was obtained.

(Preparation of salt C of dithiocarbamic acid)
Pure water was added to 52.6 g of sodium N,N-diethyldithiocarbamate trihydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) until the total amount reached 100 g, and 40% by weight of sodium N,N-diethyldithiocarbamate was added. An aqueous solution containing

(Polymer flocculant)
As a polymer flocculant, OA-23 (weak anionic polymer) manufactured by Organo Co., Ltd. was used.

Example 1
1000 mL of A company's waste water (pH 2, containing 210 mg/L of copper concentration) was added to a 2000 mL beaker set in a Jar Tester. Then, while stirring at 150 rpm, 11.5 g/L of 10% aqueous sodium hydroxide solution was added to adjust the pH to 8, then 300 mg/L of dithiocarbamic acid salt A was added and stirred at 150 rpm for 10 minutes. Furthermore, 2000 mg/L of 0.1% by weight OA-23 aqueous solution was added as a polymer flocculant and stirred at 50 rpm for 5 minutes. After stirring, the mixture was allowed to stand for 10 minutes, placed in a desktop centrifuge (Kubota Shoji S700T), and centrifuged at 3800 rpm for 20 minutes to separate the aqueous solution and sludge. The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 1 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Example 2
The same operation as in Example 1 was performed except that 300 mg/L of dithiocarbamic acid salt B was added instead of dithiocarbamic acid salt A. The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 1 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Example 3
The same operation as in Example 1 was performed except that 300 mg/L of dithiocarbamic acid salt C was added instead of dithiocarbamic acid salt A. The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 1 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Example 4
A stirrer was installed in an 8-L container, and 6 L of waste water from Company B (pH 3, containing copper concentration of 70 mg/L) was added. Then, while stirring at 250 rpm, 1.7 g/L of 10% aqueous sodium hydroxide solution was added to adjust the pH to 7, and then 50 mg/L of dithiocarbamic acid salt A and 2 mg of polyethyleneimine (average molecular weight: 1800) as a polyamine. /L and stirred at 200 rpm for 10 minutes. Furthermore, 2000 mg/L of 0.1% by weight OA-23 aqueous solution was added as a polymer flocculant, and the mixture was stirred at 100 rpm for 5 minutes. After stirring, the mixture was allowed to stand for 10 minutes, placed in a desktop centrifuge (Kubota Shoji S700T), and centrifuged at 3800 rpm for 20 minutes to separate the aqueous solution and sludge. The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 2 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Example 5
The same operation as in Example 4 was performed except that 2 mg/L of polyethyleneimine (average molecular weight: 10,000) was added instead of polyethyleneimine (average molecular weight: 1,800). The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 2 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Example 6
The same operation as in Example 4 was performed except that 2 mg/L of polyethyleneimine (average molecular weight: 2,000,000) was added instead of polyethyleneimine (average molecular weight: 1,800). The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 2 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Comparative example 1
1000 mL of A company's waste water (pH 2, containing 210 mg/L of copper concentration) was added to a 2000 mL beaker set in a Jar Tester. Then, while stirring at 150 rpm, 11.5 g/L of 10% sodium hydroxide aqueous solution was added to adjust the pH to 8, and then stirred at 150 rpm for 1 hour. Furthermore, 2000 mg/L of 0.1% by weight OA-23 aqueous solution was added as a polymer flocculant and stirred at 50 rpm for 10 minutes. After stirring, the mixture was allowed to stand for 10 minutes, placed in a desktop centrifuge (Kubota Shoji S700T), and centrifuged at 3800 rpm for 20 minutes to separate the aqueous solution and sludge. Table 1 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Comparative example 2
A stirrer was installed in an 8-L container, and 6 L of waste water from Company B (pH 3, containing copper concentration of 70 mg/L) was added. Then, while stirring at 250 rpm, 1.7 g/L of 10% sodium hydroxide aqueous solution was added to adjust the pH to 7, and then stirred at 200 rpm for 10 minutes. Furthermore, 2000 mg/L of 0.1% by weight OA-23 aqueous solution was added as a polymer flocculant, and the mixture was stirred at 100 rpm for 10 minutes. After stirring, the mixture was allowed to stand for 5 minutes, placed in a desktop centrifuge (Kubota Shoji S700T), and centrifuged at 3800 rpm for 20 minutes to separate the aqueous solution and sludge. Table 2 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Reference example 1
Before adding the polymer flocculant, 1000 mg/L of an aqueous polyaluminum chloride solution (10% as aluminum oxide) was added as an inorganic flocculant, and the mixture was stirred at 150 rpm for 5 minutes. . The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 1 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Example 7
To a 2000 mL beaker placed in a Jar Tester, 1000 mL of waste water from Company C (pH 1.5, containing copper concentration of 500 mg/L) was added. Next, while stirring at 150 rpm, a 20% aqueous sodium hydroxide solution was added to adjust the pH to 9, then 500 mg/L of dithiocarbamic acid salt A was added, and the mixture was stirred at 150 rpm for 10 minutes. Furthermore, 2000 mg/L of 0.1% by weight OA-23 aqueous solution was added as a polymer flocculant and stirred at 50 rpm for 5 minutes. After stirring, the mixture was allowed to stand for 10 minutes, placed in a desktop centrifuge (Kubota Shoji S700T), and centrifuged at 3800 rpm for 20 minutes to separate the aqueous solution and sludge. The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 3 shows the results of copper concentration, amount of sludge generated, water content, and copper content in the sludge in the aqueous solution after treatment.

Examples 8-10
The same operation as in Example 7 was performed, except that the chemicals to be added were changed to the conditions shown in Table 3. These results are also shown in Table 3. The amount of hydrogen sulfide gas generated during the series of treatment steps was less than the work environment evaluation standard value.

Comparative example 3
To a 2000 mL beaker placed in a Jar Tester, 1000 mL of waste water from Company C (pH 1.5, containing copper concentration of 500 mg/L) was added. Next, while stirring at 150 rpm, a 20% aqueous sodium hydroxide solution was added to adjust the pH to 9, then 500 mg/L of dithiocarbamic acid salt A was added, and the mixture was stirred at 150 rpm for 10 minutes. After stirring, the mixture was allowed to stand for 10 minutes, placed in a desktop centrifuge (Kubota Shoji S700T), and centrifuged at 3800 rpm for 20 minutes to separate the aqueous solution and sludge. The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 3 also shows the copper concentration, sludge generation amount, water content, and copper content in the sludge of the aqueous solution after the treatment.

When no polymer flocculant was added, the copper concentration after treatment exceeded the effluent standard value, the copper content was low, and the water content was high.

Comparative Examples 4-6, Reference Example 2
The same operation as in Example 7 was performed, except that the chemicals to be added were changed to the conditions shown in Table 3. These results are also shown in Table 3.

Comparative Examples 4 to 6 are examples in which no salt of dithiocarbamic acid is used. The copper concentration after treatment exceeded the wastewater standard of 3 mg/L. Moreover, the moisture content exceeded 90%. When the same amount of heavy metal-containing wastewater is treated, the amount of sludge produced in Comparative Examples 4-6 (sludge containing heavy metal solids immediately after solid-liquid separation) is the same as the sludge produced in Examples 7-10. about twice the amount. In the case of such a comparative example, there is a problem that the transportation cost of sludge is higher than that of the example. Moreover, when the water content is high, there is a problem in that the heavy metal recovery cost rises.

Example 11
To a 500 mL beaker placed in a Jar Tester, 300 mL of D company's waste water (pH 1.0, containing copper concentration of 1000 mg/L) was added. Then, while stirring at 150 rpm, a 20% aqueous sodium hydroxide solution was added to adjust the pH to 8, then 1000 mg/L of dithiocarbamic acid salt A was added and stirred at 150 rpm for 10 minutes. Furthermore, 2000 mg/L of 0.1% by weight OA-23 aqueous solution was added as a polymer flocculant and stirred at 50 rpm for 5 minutes. After stirring, the mixture was allowed to stand for 10 minutes, placed in a desktop centrifuge (Kubota Shoji S700T), and centrifuged at 3800 rpm for 20 minutes to separate the aqueous solution and sludge. The amount of hydrogen sulfide gas generated during the series of treatment steps was below the work environment evaluation standard value. Table 4 shows the copper concentration of the aqueous solution after the treatment, the amount of sludge generated, and the copper content in the sludge.

Comparative Example 7, Reference Example 3
The same operation as in Example 11 was performed except that the chemicals to be added were changed to the conditions shown in Table 4. These results are also shown in Table 4.

Comparative Example 7 is an example in which no salt of dithiocarbamic acid is used, but the copper concentration after treatment exceeded the effluent standard of 3 mg/L, and the water content of the sludge was as high as 94%.

Reference Example 3 is an example in which calcium hydroxide is used instead of sodium hydroxide and no salt of dithiocarbamic acid is used. The copper concentration in the sludge was also low at 9%.

The method for producing heavy metal-containing sludge of the present invention has the potential to be used as a method for treating wastewater discharged from manufacturing plants for metal surface treatment, plating, printed circuit boards, electrical parts, mechanical parts, automobiles, etc. ing.

Claims (5)

Salts of alkali metal hydroxide and dithiocarbamic acid are added to wastewater containing heavy metals to immobilize the heavy metals, and then a polymer flocculant is added to separate the solid-liquid sludge containing heavy metal solids. A method for producing heavy metal-containing sludge. 2. The method for producing heavy metal-containing sludge according to claim 1, wherein said heavy metal is copper, and said heavy metal-containing waste water is waste water containing copper at a concentration of 20 mg/L or more and 1500 mg/L or less. 3. The method for producing heavy metal-containing sludge according to claim 1 or 2, wherein the salt of dithiocarbamic acid is a reaction product of piperazine or tetraethylenepentamine, carbon disulfide and an alkali metal hydroxide. . 4. The method for producing heavy metal-containing sludge according to any one of claims 1 to 3, wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide. 5. The method for producing heavy metal-containing sludge according to any one of claims 1 to 4, wherein the pH of the heavy metal-containing waste water when the salt of dithiocarbamic acid is added is in the range of 6 to 11.