JP3598324B2 - Method and apparatus for recovering zinc and iron in acid waste liquid as precipitate - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for treating molten zinc waste liquid.
[0002]
[Prior art]
Hot-dip galvanizing is widely used for various steel structures because it is inexpensive to apply and has corrosion resistance and durability superior to painting. Various acid waste liquids are discharged in the process of hot-dip galvanizing.Hydrochloric acid waste liquid and sulfuric acid containing zinc and iron, which are generated when dissolving the plating layer of defective products generated when hot-dip galvanizing steel products, in particular There is no effective treatment method for the waste liquid other than the neutralization method, which is a problem.
When the acid waste liquid is neutralized with an alkali, not only a large amount of alkali is consumed, but also a large amount of hydroxide sludge is formed as a result of the neutralization. Since the sludge is mixed with iron and zinc, it cannot be effectively used even if it is separated from water, so that there is a problem that a large amount of sludge must be discarded.
[0003]
[Problems to be solved by the invention]
An object of the present invention is to provide a method and an apparatus for efficiently and efficiently treating an acid waste liquid containing zinc and iron discharged in a hot-dip galvanizing process with a reduced amount of sludge.
[0004]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to solve the above problems, and as a result, completed the present invention.
That is, according to the present invention, in a method for treating an acid waste liquid containing zinc and iron discharged in a hot-dip galvanizing step as a liquid to be treated, the liquid to be treated is treated with a calcium hydroxide suspension having a pH of 9 or more and 50 ° C. or more. The mixture is added dropwise to the suspension to form a mixture having a temperature of 50 ° C. or more and a pH of 6 to 11, and if necessary, an oxidizing agent is added to make the dissolved zinc ions and iron ions hardly soluble. A method for recovering zinc oxide from an acid waste solution containing zinc and iron discharged in a hot dip galvanizing step, wherein the zinc oxide is precipitated as a substance and recovered is provided.
Furthermore, according to the present invention, there is provided an apparatus used in a method for treating an acid waste liquid containing zinc and iron discharged in the above-described hot-dip galvanizing step as a liquid to be treated, and the zinc discharged in the hot-dip galvanizing step. Raw water tank 1 for acid waste liquid containing iron and iron, unit 2 for storing and supplying calcium hydroxide suspension, unit 3 for supplying hot water, and acid waste liquid containing zinc and iron and calcium hydroxide suspension A reaction tank 4 for mixing, a unit 5 for adding an oxidizing agent, a unit 6 for adding a coagulant, a coagulation tank 7, a unit 8 for separating the precipitate obtained in the coagulation tank, and the obtained precipitate A unit for recovering zinc oxide from an acid waste liquid containing zinc and iron discharged in a hot-dip galvanizing step, comprising a unit 9 for concentrating and dewatering the wastewater and a unit 10 for dewatering the sludge.
[0005]
BEST MODE FOR CARRYING OUT THE INVENTION
One embodiment of the method for treating an acid waste liquid containing zinc and iron discharged in the hot-dip galvanizing step of the present invention is to drop-mix the liquid to be treated with a calcium hydroxide suspension at pH 9 or higher and 50 ° C. or higher. to form a mixed solution having a pH of temperatures above 50 ° C. and 6 to 11, further with an oxidizing agent added as necessary, the solution presence of zinc ions and iron ions, wherein the precipitation as sparingly soluble substance It is assumed that.
[0006]
The pH of the mixture is generally in the range of 6 to 11, preferably in the range of 8 to 11. The temperature of the mixture is 50 ° C. or higher, preferably 50 to 60 ° C.
[0007]
In the calcium hydroxide suspension used in the present invention, the concentration of calcium hydroxide is 0.05 to 10%, preferably 0.1 to 8%. The mixing temperature of the liquid to be treated and the suspension of calcium hydroxide is 50 ° C. or higher, usually 50 to 60 ° C. To obtain this mixing temperature, the mixture of the liquid to be treated and calcium hydroxide is heated. There is a method of heating to 50 ° C. or more, a method of heating the liquid to be treated and / or the calcium hydroxide suspension to 50 ° C. or more, and mixing them to obtain a mixed solution of 50 ° C. or more. In the case of the present invention, if the mixing temperature is lower than 50 ° C., problems such as an increase in the amount of sludge occur, which is not preferable.
[0008]
In the present invention, it is preferable to add an oxidizing agent to the liquid to be treated. Dissolved zinc and iron can be recovered as a precipitate without using an oxidizing agent, but the amount of the precipitate can be significantly reduced by using an oxidizing agent. The oxidizing agents used are those used in the treatment of wastewater, and include hydrogen peroxide, chlorine-based oxidizing agents, ozone, and the like. The amount of the oxidizing agent used is 1 to 200 mg / L, preferably 1 to 100 mg / L, as the concentration in water.
The oxidizing agent may be added after adding the calcium hydroxide suspension, simultaneously with the calcium hydroxide suspension, or before adding the calcium hydroxide suspension.
[0009]
In the present invention, a coagulant is preferably used in combination. The flocculant in this case is one used for flocculation of floc, such as a cationized modified product of polyacrylamide, poly (dimethylaminoethyl acrylate), poly (dimethylaminoethyl methacrylate), Includes cationic organic coagulants such as polyethyleneimine and chitosan; nonionic organic coagulants such as polyacrylamide; anionic organic coagulants such as polyacrylic acid, copolymers of acrylamide and acrylic acid, and salts thereof. Is done. The amount of the coagulant used is 1 to 200 mg / L, preferably 1 to 100 mg / L, in water.
[0010]
The liquid to be treated including the flocs of the precipitate after the aggregation step is subjected to a solid-liquid separation treatment. As the solid-liquid separation method in this case, a conventional method, for example, filtration separation, centrifugation, sedimentation and the like can be mentioned.
By this solid-liquid separation treatment, precipitates in the liquid are removed and sludge is formed. This sludge is mainly composed of zinc.
[0011]
Next, the present invention will be described with reference to the drawings.
FIG. 1 shows an example of an apparatus system diagram in the case of carrying out the present invention, in which an acid waste liquid containing zinc and iron discharged in a hot-dip galvanizing step is treated and sludge is collected.
In FIG. 1, 1 is a raw water tank, 2 is a calcium hydroxide suspension supply unit, 3 is a hot water supply unit, 4 is a reaction tank for mixing a waste liquid and a calcium hydroxide suspension, 5 is an oxidizing agent addition unit, Reference numeral 6 denotes a coagulant addition unit, 7 denotes a coagulation tank, 8 denotes a sedimentation separation unit, 9 denotes a unit for concentrating and dewatering the precipitate, and 10 denotes a sludge dewatering unit.
11 is a raw water tank, 12 and 13 are pH meters, and 14 is a hot water tank.
[0012]
The method for supplying the acid waste liquid containing zinc and iron is not particularly limited. For example, a waste liquid discharge pipe may be provided, the waste liquid may be introduced into the raw water tank 1, or a waste liquid storage tank may be separately provided. From there, the waste liquid may be introduced into the raw water tank 1.
The acid waste liquid containing zinc and iron is mixed with the calcium hydroxide suspension heated in the reaction tank 4 to 50 ° C. or higher.
[0013]
As a method for supplying the heated calcium hydroxide suspension, a calcium hydroxide suspension having a predetermined concentration may be supplied after being heated to a predetermined temperature, or hot water and a hydroxide having a concentration higher than the predetermined concentration may be supplied. The calcium suspension may be separately added to the reaction tank so as to have a predetermined concentration. In this case, the concentration of the calcium hydroxide suspension must be higher than a predetermined concentration, but the ratio is not particularly limited as long as the temperature of the reaction tank 4 can be maintained at 50 ° C. or higher. .
[0014]
An oxidant is added from the oxidant supply unit 5 to the reaction solution in the reaction tank 4. The reaction liquid is further transferred to a coagulation tank 7 and a coagulant is added thereto. Then, the reaction liquid is transferred to a precipitation / separation unit (solid-liquid separation tank) 8 to be subjected to solid-liquid separation. The sediment is transferred from the bottom of the sedimentation / separation unit 8 to the sludge concentration / dehydration unit 9 and further concentrated / dewatered. The concentrated and dewatered solids are dewatered in the sludge dewatering unit 10 and recovered and reused as zinc resources.
[0015]
The acid waste solution containing zinc and iron and the calcium hydroxide suspension are dropped into the reaction vessel 4 of the calcium hydroxide suspension maintained at 50 ° C. or higher so that the pH becomes 6 to 11, preferably 9 to 11. Mix. In order to maintain the calcium hydroxide suspension at 50 ° C. or higher, a reactor equipped with a heater may be used, or a calcium hydroxide suspension previously heated to 50 ° C. or higher may be dropped. good.
[0016]
The concentration of the calcium hydroxide suspension added to the reaction tank 4 is 0.01 to 10%, preferably 0.1 to 5%.
[0017]
The flocculant used in the flocculation tank 7 is used for floc flocculation. Examples of such flocculants include cationically modified polyacrylamide, dimethylaminoethyl polyacrylate, and dimethylamino methacrylate. Cationic organic flocculants such as ethyl ester, polyethyleneimine and chitosan; nonionic organic flocculants such as polyacrylamide; anionic organic flocculants such as polyacrylic acid, copolymers of acrylamide and acrylic acid and salts thereof Agents are included. The amount of the coagulant used is 1 to 200 mg / L, preferably 1 to 100 mg / L, in water.
[0018]
As the concentration and dehydration unit 9 employed in the present invention, a conventional and commonly used dehydrator, for example, a centrifugal device may be used, but in the present invention, a sludge concentration and dehydration tank is preferably used for automation.
[0019]
As the sludge dewatering unit 10 employed in the present invention, a conventional general sludge dewatering device such as a filter press may be used, but in the present invention, a screw press is preferably used for automation.
[0020]
【Example】
Next, the present invention will be described in more detail with reference to examples.
[0021]
Example 1
The acid waste liquid containing zinc and iron discharged in the hot-dip galvanizing process having a pH of -1.83, a zinc concentration of 242.5 g / L, an iron concentration of 48 g / L, and a chlorine concentration of 381 g / L is 255 mL / min. 6.4 L / min of a calcium hydroxide suspension (2%) heated to 55 ° C. was sent to the reaction vessel and mixed by stirring (to have a pH of about 9.5 to 10), and an aqueous hydrogen peroxide solution (concentration: 30%) 25.5 mL / min is added (pH becomes about 9 to 9.5) and sent to a flocculation tank with a residence time of 20 minutes, where a 0.1% aqueous solution of polymer flocculant Diafloc KP208BH is added at 320 mL / min. The mixture was added, stirred, and sent to a sedimentation tank at a residence time of 2 minutes. At a residence time of 20 minutes, supernatant water was discharged as treated water. The SV value of the settled sludge was 35%. This was discharged from the bottom, sent to a sludge thickening and dewatering tank, concentrated and dewatered, and sent to a sludge dewatering unit with a residence time of 20 minutes to be dewatered. The concentrations of zinc, iron and chlorine in the treated water were 0.56 mg / L, 0.14 mg / L and 5200 mg / L, respectively. The amount of sludge cake was 635 g per liter of waste liquid. The sludge was dissolved in nitric acid, and zinc, iron and calcium were measured by an ICP emission spectrometer and chloride ions were measured by ion chromatography. As a result, the compositions (% by weight) of the sludge were 47.9% and 9.0%, respectively. , 7.5% and 1.1%.
[0022]
Example 2
When the same treatment as in Example 1 was carried out except that an aqueous solution of hydrogen peroxide as an oxidizing agent was not added, the SV value of the sludge was about 45%. The composition (% by weight) of the sludge was 41% zinc, 8.2% iron, 11% calcium, and 3.4% chlorine.
[0023]
Comparative Example 1
When the treatment was performed in the same manner as in Example 1 except that the temperature was changed to normal temperature, the SV value of the sludge was about 90%. The composition of the sludge was 34% zinc, 7.5% iron, 14% calcium and 17% chlorine.
[0024]
Example 3
The waste liquid of Example 1 was heated to 255 mL / min, 3.2 L / min of water heated to 55 ° C. and 3.2 L / min of a calcium hydroxide suspension (4%) were sent to the reaction tank and mixed by stirring. Minutes, the mixture is fed to a flocculation tank, where a 0.1% solution of polymer flocculant Diafloc KP208BH is added at 300 mL / min and stirred, sent to a sedimentation tank with a residence time of 2 minutes, and supernatant water is added for a residence time of 20 minutes. Was discharged as treated water. The precipitated sludge was discharged from the bottom, sent to a sludge thickening / dewatering tank, concentrated and dewatered, and dewatered with a sludge dewatering unit. The obtained results were almost the same as in Example 1.
[0025]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the acid waste liquid containing zinc and iron can be treated efficiently and with the reduced amount of sludge generated.
[Brief description of the drawings]
FIG. 1 shows an example of an apparatus system diagram for implementing a method of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Raw water tank 2 Calcium hydroxide suspension supply unit 3 Hot water supply unit 4 Reaction tank 5 Oxidant supply unit 6 Coagulant addition unit 7 Coagulation tank 8 Precipitation tank 9 Concentration dewatering unit 10 Sludge dewatering unit 11 Raw water tank 12, 13 pH 14 hot water tanks

Claims (5)

In a method of treating an acid waste liquid containing zinc and iron discharged in a hot-dip galvanizing step as a liquid to be treated, the liquid to be treated is dropped and mixed with a calcium hydroxide suspension at pH 9 or higher and 50 ° C. or higher, A hot-dip galvanizing step characterized by forming a mixed solution having a temperature of 50 ° C. or more and a pH of 6 to 11 to precipitate the dissolved zinc ions and iron ions as hardly soluble substances and recover them. A method for treating acid waste liquid containing zinc and iron discharged. The method according to claim 1, wherein an oxidizing agent is used in combination. 3. The method of claim 2 wherein said oxidizing agent is hydrogen peroxide. 3. The method according to claim 1, wherein a coagulant is added to the mixture. An apparatus used in a method for treating an acid waste liquid containing zinc and iron discharged in the hot-dip galvanizing step according to any one of claims 1 to 4 as a liquid to be treated, for an acid waste liquid containing zinc and iron. A raw water tank 1, a unit 2 for storing and supplying a calcium hydroxide suspension, a unit 3 for supplying hot water, a reaction tank 4 for mixing an acid waste liquid containing zinc and iron and a calcium hydroxide suspension, A unit 5 for adding an oxidizing agent, a unit 6 for adding a coagulant, a coagulation tank 7, a unit 8 for separating a precipitate obtained in the coagulation tank, and a unit 9 for concentrating and dehydrating the obtained precipitate. And a unit 10 for dewatering sludge, wherein zinc oxide is recovered from an acid waste liquid containing zinc and iron discharged in a hot dip galvanizing step.

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