JPH0564240B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for recovering zinc from an acidic electrolytic galvanizing waste solution containing zinc sulfate as a main component and reusing it in a plating bath.

(Conventional technology) Compared to hot-dip galvanized steel sheets, electrogalvanized steel sheets have better workability, less plating, and a smoother surface, so they can be used indoors where workability and surface appearance are required. It is mainly used for this purpose, and an acidic bath containing zinc sulfate as a main component is mainly used for this plating.

Generally, when performing electroplating, in order to maintain a constant bath composition, it is necessary to pump out the plating solution when replenishing the plating solution, or in some cases, to replace aging plating solution, which may cause plating that contains harmful heavy metals. Waste liquid is produced. Alternatively, waste liquid is generated to maintain water balance. It is well known that in order to protect the environment, such plating waste liquid must be neutralized and rendered harmless before being disposed of.

Conventionally, waste liquid from acidic electrogalvanizing baths containing zinc sulfate as a main ingredient has been treated by simple neutralization using Ca(OH) ₂ or the like. As a result, zinc precipitates as zinc hydroxide,
At present, the separated sludge is discarded and the waste is not used.

On the other hand, as a method for recovering zinc from zinc-containing by-products such as by-product zinc dust or ash, basic zinc carbonate is precipitated by adding soda ash (Na ₂ CO ₃ , i.e., sodium carbonate) to a zinc sulfate solution. Although it is known that the zinc content can be separated and recovered, there have been few proposals regarding a method for recovering the zinc content from electrogalvanizing waste liquid.

Furthermore, in JP-A-59-88319, when precipitating zinc carbonate with ammonium carbonate from a zinc salt solution in which Fe and Pb coexist, metal zinc is added in advance to precipitate dissolved heavy metal ions. It is stated that it should be separated.

(Problem to be solved by the invention) It would be possible to recover Zn, which is presently contained in a high concentration in electrogalvanizing waste liquid that is discarded after detoxification treatment, and reuse it in a plating bath. For example, this is advantageous because it not only makes effective use of the Zn content in the waste liquid, but also leads to a reduction in the unit consumption of the zinc source.
It goes without saying that it is desirable to reduce the cost of treating plating waste liquid.

(Means for Solving the Problems) Therefore, the present inventors have developed an economical waste liquid treatment method that can recover zinc from the waste liquid of acidic electrogalvanizing baths that are often used for electrogalvanizing steel plates, etc. As a result of repeated research to develop the method, we focused on the fact that since the plating solution uses purified raw materials, the plating waste solution also has relatively few dissolved impurities, and we decided to precipitate zinc as basic zinc carbonate. They discovered that by neutralizing with alkali carbonate and alkali hydroxide, the precipitated basic zinc carbonate can be reused as a zinc source for acid electrogalvanizing, and that this does not have any negative effects on plating operations, and has completed the present invention. did.

Here, the present invention adds powdered zinc after adding an oxidizing agent to acidic bath electrogalvanizing waste liquid to precipitate and remove dissolved heavy metal impurities that have a lower ionization tendency than zinc, and then The zinc plating waste liquid is neutralized with sodium carbonate or sodium carbonate and sodium hydroxide to precipitate basic zinc carbonate, and the precipitated crystals are washed with water to remove water-soluble impurities. A method for recovering and utilizing zinc from electrogalvanizing waste liquid, which is characterized in that it is supplied as a slurry or as a solid powder obtained by separation and drying to an acidic zinc electroplating bath and reused as a zinc source. be.

(Function) The object of the present invention is an acid bath, particularly waste liquid of an electrolytic galvanizing solution containing zinc sulfate as a main component (hereinafter also simply referred to as plating waste liquid). In addition to zinc sulfate, such plating solutions generally contain small amounts of sulfates such as ammonium, sodium, magnesium, and aluminum, and have a pH of 1 to 4.5.
is within the range of In addition to the Fe components eluted from the plated substrate, trace amounts of heavy metal ions such as Pb and Cd are generally dissolved in the waste plating solution. However, as mentioned above, the plating solution generally uses purified raw materials, and therefore the amount of impurities in the plating waste solution is quite low. Therefore, acid bath electrogalvanizing waste liquid can be directly subjected to neutralization treatment according to the present invention.

However, in cases where the plating waste liquid contains many dissolved ions of heavy metal impurities such as Pb and Fe, or when it is desired to remove such impurities as much as possible,
Appropriate treatment to remove such impurities is performed before neutralization. For this reason, the method is to first add an oxidizing agent to the plating waste liquid, then add zinc powder, and use the difference in ionization tendency to precipitate and remove heavy metal ions, which have a lower ionization tendency than zinc, as metals. . This takes advantage of the high ionization tendency of zinc, as shown below.

K＞Ca＞Na＞Mg＞Al＞Zn＞Cr＞Fe ²⁺ ＞Cd＞
Co＞Ni＞Sn＞Pb＞Fe ³⁺ The reason for adding the oxidizing agent first is to oxidize Fe ²⁺ ions to Fe ³⁺ with a lower ionization tendency and ensure the precipitation of Fe. . H ₂ O ₂ is preferred as this oxidizing agent. Other oxidizing agents, e.g.
Although HNO ₃ , KMnO ₄ , HCrO ₄ , etc. can also achieve the above purpose, they introduce new impurities into the waste liquid to be treated. The finer the zinc powder, the better, in order to promote the ion exchange reaction in which Zn is ionized and dissolved heavy metal ions are precipitated as metal, and it is usually preferable to use a fine powder of 88 μm or less. Impurities precipitated by this are removed by separation means such as filtration.

The waste liquid from which impurities have been removed as described above,
According to the present invention, sodium carbonate or sodium carbonate and sodium hydroxide are added to neutralize the plating waste liquid and simultaneously precipitate the zinc component as basic zinc carbonate. The reaction formula when the zinc content in the plating waste liquid is zinc sulfate is shown below.

5ZnSO ₄ +6NaOH+2Na ₂ CO ₃ → [Zn(OH) ₂ ] ₃
[ZnCO ₃ ] ₂ ↓+5Na ₂ SO _4Although the composition of basic zinc carbonate is not constant, the typical one has the above composition. To generate this salt, NaOH:Na ₂
The molar ratio of CO ₃ should be 3:1. but,
When Na ₂ CO ₃ is dissolved in water, it becomes
Generates NaOH.

Na ₂ CO ₃ + H ₂ O → NaOH + NaHCO ₃ Therefore, the amount of NaOH added may be less than the above molar ratio, and even if only Na ₂ CO ₃ is added without adding NaOH, basic zinc carbonate It forms and precipitates. In addition, in the case of a plating waste liquid that has a high content of free strong acid and therefore exhibits strong acidity, NaOH may be added in an amount greater than the above molar ratio. In this way, even when adding NaOH,
The proportion of NaOH and Na ₂ CO ₃ can be varied within a wide range depending on the circumstances, especially the acidity of the effluent. It is preferable to adjust the pH of the plating waste liquid to about 7.0 to 7.5 by neutralization treatment.

The precipitated basic zinc carbonate is then washed with water and then recovered in an aqueous slurry or solid state and recycled to the plating bath.

The washing and recovery step can be carried out in a conventional manner. for example,
Separate the precipitate by filtration, or repeat repulping washing (i.e., removing water-soluble impurities by dispersing it in a large amount of water and filtration) once or several times as necessary. Obtain a slurry of zinc carbonate. The slurry-like basic zinc carbonate thus obtained is concentrated using, for example, a separation thickener, and is recycled as it is to the electrogalvanizing bath to be reused as a zinc source. Alternatively, the slurry may be filtered and the resulting cake dried to provide a powdered basic zinc carbonate product that is recycled to the plating bath. Of course, the powder product thus recovered can also be used as a zinc source for other applications.

Basic zinc carbonate is insoluble in water but soluble in acid. That is, when it is added as a Zn source to an acidic plating bath, it reacts with the free acid present in the plating solution, such as sulfuric acid, to produce a soluble zinc salt (e.g., zinc sulfate), which is added to the plating bath. dissolve. Basic zinc carbonate is therefore a suitable compound to supply as a Zn source to acidic plating baths.
Furthermore, even if basic zinc carbonate, recovered from plating waste liquid, is added as a Zn source to a plating liquid mainly composed of zinc sulfate, there will be no reduction in plating efficiency or other negative effects on operations. Without eyelashes, Zn
It was found that it can be effectively used as a source.

Electroplating waste liquid usually contains unreacted free acids, such as sulfuric acid. Utilizing this free acid, a metallic zinc-containing source, such as zinc-containing dust, is added to the waste liquid, and the zinc is dissolved by the reaction between zinc and the acid, increasing the zinc concentration in the waste liquid, and then the method of the present invention is carried out. May be implemented. For this purpose, the concentration of free acid in the waste liquid must be at least
It is preferably 0.25 mol/or more. Zinc-containing dust is generated, for example, in hot-dip galvanizing lines and dust-reduced iron equipment in steel plants, and its composition is as follows:
For example, Zn: 44.5%, Fe: 15.7%, Pb: 6.1%,
C: 3.8%, Cl: 3.0%, _SiO2 : 2.0% and Ca:
It is 1.9%. When such zinc-containing dust is added to the plating waste liquid to enrich the Zn concentration, as can be seen from the above composition example, other impurities will be mixed in with zinc. Before performing the neutralization treatment, it is preferable to perform an impurity removal treatment using the above-mentioned ion exchange reaction.

Next, the present invention will be specifically explained using examples.

Example: A waste solution discharged from an electrolytic galvanizing bath for steel sheets containing zinc sulfate as the main component (Zn content: metal
73.1g/) 500c.c. as Zn, 30% H ₂ O ₂ aqueous solution
30 c.c. was added and stirred at room temperature for 30 minutes. Next, 50 g of zinc powder was added and stirred for an additional 30 minutes at room temperature. This solution was suction filtered to separate undissolved zinc powder and precipitated impurities.

This 500 c.c. of filtrate contains the necessary amount for neutralizing the filtrate.
500 containing 30.5g NaOH and _{27.0g Na2CO3}
cc of alkaline aqueous solution was added and stirred at room temperature for 30 minutes to precipitate basic zinc carbonate crystals. This reaction solution was suction-filtered, and the recovered cake was further added with 1
Repulp washing with pure water was performed three times to remove water-soluble impurities. Repulp washing was carried out by adding 1 part of pure water each time, stirring at room temperature for 30 minutes, and filtering. Finally dried at 105℃, 69.3g
A solid powder was obtained. The analysis results of this powder were as follows.

Zn: 58.52% Fe: 4.5ppm Pb: 0.8ppm Cd: 0.1ppm Cl: 10.3ppm This powder can be used satisfactorily for acid electrogalvanizing without adversely affecting plating operations.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, the zinc contained in the acid bath electrogalvanizing waste liquid can be recovered and effectively reused for plating. In addition to reducing the unit consumption of zinc sources required for neutralization, the amount of waste generated by neutralization is also reduced, leading to a reduction in waste treatment costs. In addition, the method of the present invention can be carried out economically, since the sodium carbonate (soda ash) and sodium hydroxide used for neutralization are both relatively inexpensive substances that can be obtained as by-products within steel mills. It also reduces waste liquid treatment costs.
Further, as shown in the examples, since the zinc content is recovered in high purity, even if it is recycled to the electrogalvanizing bath according to the present invention, there will be no problem in the plating operation.

Claims (1)

[Claims] 1. An oxidizing agent,
Zinc powder is then added to precipitate and remove dissolved heavy metal impurities that have a lower ionization tendency than zinc, and the acid bath electrogalvanizing waste liquid is then neutralized with sodium carbonate or sodium carbonate and sodium hydroxide. After precipitating basic zinc carbonate and washing the precipitated crystals with water to remove water-soluble impurities, the obtained basic zinc carbonate is applied as a slurry or as a solid powder obtained by separation and drying to an acidic zinc electroplating bath. A method for recovering and utilizing zinc from electrolytic galvanizing waste solution, characterized by supplying the zinc to a zinc source and reusing it as a zinc source. 2. The method of claim 1 further comprising adding zinc-containing dust to the acid bath electrogalvanizing effluent to increase the zinc concentration of the effluent before adding the oxidizing agent and zinc powder.