JP3171912B2 - Detoxification method for waste liquid containing hexavalent chromium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly uses a waste aqueous solution containing hexavalent chromium generated in a manufacturing process of an iron alloy steel strip containing at least chromium, and a waste solution generated in a manufacturing process of an iron alloy steel strip. And detoxification methods.

[0002]

2. Description of the Related Art A manufacturing process of a steel strip made of an iron alloy containing at least chromium or a steel plate obtained by shearing the steel strip (hereinafter, these are collectively simply referred to as an iron alloy steel strip) represented by a stainless steel strip manufacturing process. Has a step of generating hexavalent chromium which is generally harmful to the environment. That is, in general, when an iron alloy steel strip is heated or electrolytically oxidized in a highly basic environment, chromium contained in the iron alloy steel strip is oxidized to a hexavalent valence, and the hexavalent chromium is further converted to hexavalent chromium. By contacting the produced iron alloy steel strip with water, a hexavalent chromium-containing waste liquid is generated.
The place where such hexavalent chromium-containing waste liquid is generated is, for example, in a cold rolling process of an iron alloy steel strip, a salt bath, a sodium sulfate electrolytic bath, a caustic soda electrolytic bath in the pickling process, and a position subsequent to these baths. Cleaning tank. Conventionally, the following method has been used as a method of detoxifying the hexavalent chromium-containing waste liquid because it can perform a relatively inexpensive and reliable treatment and can effectively utilize the generated sludge.

That is, a hexavalent chromium-containing waste liquid that has undergone a step of adding sulfuric acid so as to maintain the pH within a range of 2 to 3 (pH adjusting step) is used as an reducing agent, and an aqueous solution mainly containing ferrous sulfate is used as a reducing agent. The oxidation-reduction potential of the waste liquid containing hexavalent chromium is 500 ± 10 mv with respect to the silver chloride reference electrode.
(Reduction treatment step) to maintain the above range
And then adjust the pH of the waste liquid containing hexavalent chromium to 6 to
After adding the aqueous solution containing magnesium hydroxide as a neutralizing agent so as to maintain the range of 8 and supplying the aqueous solution mainly to the step of generating metal hydroxide and sulfate (neutralization treatment step), gravity sedimentation Solid-liquid separation using a thickener or the like based on the principle of water-recycling and solids containing water are reused as steelmaking raw materials after dehydration and drying, and the separated liquid is harmless and is discharged into public waters (solid-liquid separation step). And detoxifying the hexavalent chromium-containing waste liquid through the above step.

In such a method for detoxifying a hexavalent chromium-containing waste liquid, the sulfuric acid used in the pH adjusting step is mainly purchased fresh acid, but the sulfuric acid waste liquid used for surface treatment of the iron alloy steel strip is used. Was sometimes used. However, ferrous sulfate used in the reduction treatment step and magnesium hydroxide used in the neutralization treatment step were purchased. The reason why expensive magnesium hydroxide is used as a neutralizing agent in the neutralization treatment step is that the sulfate (magnesium sulfate) generated during the neutralization treatment has a relatively high solubility in water. In other words, if inexpensive calcium hydroxide or sodium hydroxide, which is widely used as a neutralizing alkali, is used, the solubility of calcium sulfate (gypsum) and sodium sulfate generated in the neutralization treatment in water is low, so that This is to cause such a problem.

When the waste liquid after the reduction treatment step and the neutralization treatment step is transferred by a pipe to a thickener located at a location distant from the treatment equipment for solid-liquid separation, a waste liquid containing a large amount of calcium sulfate may be used. As a result of the generation of scale in the piping and the blockage of the piping, production activities sometimes stop. Since a large amount of calcium sulfate is contained in the slurry obtained by solid-liquid separation of the waste liquid after the reduction treatment step and the neutralization treatment step using a thickener or the like, when this is used as a steelmaking raw material after dehydration and drying, it is used as a steelmaking raw material. Since a useful metal hydroxide is treated together with a large amount of calcium sulfate, labor and energy required for the treatment are wasted. As described above, as a method of detoxifying a hexavalent chromium-containing waste liquid, when sulfuric acid is used as a pH adjusting agent and ferrous sulfate is used as a reducing agent, calcium hydroxide, which is an inexpensive neutralizing agent, is used as a neutralizing agent. Since the amount of solid product increases with the use of, the labor and cost required for the treatment become enormous, so use of expensive magnesium hydroxide to reduce the amount of solid product after neutralization Was necessary.

[0006]

SUMMARY OF THE INVENTION Accordingly, in the present invention, a hexavalent chromium-containing waste liquid generated in an iron alloy steel strip manufacturing process is also generated in an iron alloy steel strip manufacturing process and conventionally mainly treated alone. Various waste liquids are treated to make them harmless by utilizing their respective properties to reduce the amount of waste and reduce the cost of waste treatment by eliminating the use of expensive magnesium hydroxide. The task is to achieve this.

[0007]

Means for Solving the Problems The present inventor has solved the above-mentioned problems of the prior art, and has utilized the waste liquid generated in the iron alloy steel strip manufacturing process for the detoxification treatment of the hexavalent chromium-containing waste liquid. As a method for mutual harmless treatment of the waste liquids, after adding waste hydrochloric acid also generated in the iron alloy steel strip manufacturing process to the hexavalent chromium-containing waste liquid generated in the iron alloy steel strip manufacturing process to adjust the pH to ≦ 1, Iron waste is added to a waste aqueous solution mainly composed of ferrous chloride and ferric chloride generated in an iron alloy steel strip manufacturing process to produce a solution mainly composed of ferrous chloride. Hexavalent chromium containing hydrochloric acid adjusted to pH ≤ 1 by adding, as a reducing agent, an aqueous solution of ferrous chloride acidic with hydrochloric acid produced through a process of sequentially adding waste hydrochloric acid generated in the production process to acidify to pH ≤ 1 as a reducing agent. Oxidation of waste liquid Source potential 50 of silver chloride reference electrode
A reduction treatment is performed so as to maintain the range of 0 ± 10 mv, and a neutralization treatment is performed so that the hexavalent chromium-containing waste liquid similarly reduced with the waste alkali generated in the iron alloy steel strip manufacturing process maintains the range of 7 to 8. After that, the present inventors completed the present invention by investigating that it is possible to detoxify the hexavalent chromium-containing waste liquid by solid-liquid separation of the neutralized hexavalent chromium-containing waste liquid using a centrifuge.

The following describes a current treatment method for waste liquid generated in the iron alloy steel strip manufacturing process, and a method of using the waste liquid for detoxifying a hexavalent chromium-containing waste liquid. (1) Ferric chloride waste liquid In recent years, ferric chloride aqueous solution has been widely used for surface treatment and descaling treatment of iron alloy steel strip. When the iron alloy steel strip is immersed in this ferric chloride aqueous solution, the iron alloy steel strip is subjected to the oxidizing dissolution action, while the ferric chloride is gradually reduced in ferrous chloride by the reduction action ( Hereinafter, the solution containing a large amount of ferrous chloride may be simply referred to as aged iron chloride aqueous solution). Waste liquid treatment of the aged iron chloride aqueous solution is also troublesome. That is, if the aqueous ferric chloride solution alone is used, it is neutralized with inexpensive calcium hydroxide to obtain iron hydroxide and water-soluble calcium chloride, and the iron hydroxide is solid-liquid separated and then reused as a steelmaking raw material. Utilizing it, the aqueous solution of calcium chloride is harmless and can be discharged directly into public waters. However, since the aged iron chloride aqueous solution contains a large amount of ferrous chloride, it is necessary to once neutralize the ferrous ion into ferric ion by air aeration or the like, and then perform a neutralization treatment. This is because the formation of hydroxide of ferric ion is about pH
2 and ends at about pH 7, whereas the formation of ferrous ion hydroxide starts at about pH 6 and ends at about pH 10, so the pH 6 that meets drainage standards
This is because in the range of ８8, it is impossible to separate ferrous ions from the solution by hydroxide conversion. Ferrous ions are not harmful, but when discharged into public waters while remaining in wastewater, they are converted to ferric ions due to the oxidizing action of oxygen in water and air, and the public waters The problem of discoloration to brown occurs. Thus, the labor and cost involved in treating the aged iron chloride aqueous solution were enormous.

Therefore, attention was paid to the strong reducing power of ferrous chloride, and the use of this aged aqueous solution of iron chloride as a reducing agent was examined. That is, since the aged iron chloride aqueous solution is a mixed aqueous solution of ferrous chloride and ferric chloride, an attempt was made to reduce this ferric chloride to ferrous chloride. As a reducing agent, abundant iron scraps were used in the steel industry, which manufactures iron alloy steel strips. FIG. 3 shows the concentration of ferrous chloride due to the addition of iron scrap to the aged iron chloride aqueous solution and its reduction efficiency. This FIG.
This graph shows the change over time in the concentration of ferric chloride when an equivalent amount of ferric chloride is added to a prepared aqueous solution composed of 0 g / l ferrous chloride and 100 g / l ferric chloride. In addition, the graph shows the reduction efficiency calculated from the change in the concentration of ferric chloride before and after the addition of iron scrap, depending on whether or not the solution was stirred. It can be seen that the reduction efficiency is 90% or more within 5 minutes after the addition of iron scrap.

(2) Hydrochloric acid waste liquid Recently, for surface treatment and descaling treatment of iron alloy steel strip,
Hydrochloric acid came to be used. This hydrochloric acid is mainly used for chromium stainless steel among iron alloy steel strips,
Also in this treatment process, as the iron content increases in the bath, an aging phenomenon occurs in which the treatment capacity is reduced.
Therefore, a regenerating device for hydrochloric acid is often installed, but when the amount used is small, it is often discarded. This aging hydrochloric acid waste liquid also contains a large amount of ferrous chloride, and has the same problem as the aging iron chloride aqueous solution in terms of waste liquid treatment.

On the other hand, in order to bring out the reducing ability of ferrous chloride, it is necessary to make the aqueous ferrous chloride solution acidic with hydrochloric acid. The reason for this is to separate the iron ion and the chloride ion of ferrous chloride and to facilitate the contact between the divalent iron ion and the hexavalent chromium ion. PH of waste liquid containing hexavalent chromium
FIG. 4 shows the effect of ferrous chloride on the ability to reduce ferrous chloride. FIG. 4 shows that the same hexavalent chromium-containing waste liquid was subjected to OR by adding ferrous chloride as a reducing agent to which various amounts of hydrochloric acid had been added.
It is a result of investigating the concentration of residual hexavalent chromium at the time when there is a change in P (redox potential) and a change in ORP. This indicates that at least the pH of the hexavalent chromium-containing waste liquid needs to be 1 or less in order to bring out the reducing ability of ferrous chloride.

(3) Alkaline Waste Liquid In the process of producing an iron alloy steel strip, a degreasing treatment is performed. As the degreasing agent, a chlorine-based organic solvent represented by trichloroethane and an alkali represented by sodium hydroxide or sodium silicate are generally used.
However, the use of alkali is gradually becoming mainstream due to environmental problems. The principle of degreasing using an alkali is based on dispersion of a mineral oil in a degreasing solution or adsorption on an alkali colloid in degreasing mineral oil. Therefore, when the mineral oil becomes saturated in the degreasing solution, there arises a problem of a decrease in the degreasing ability and a problem of reverse contamination such that oil and dirt are re-adhered to the degreasing material. Therefore, at this point, the degreasing solution is renewed, but the treatment of the waste alkali is also troublesome. That is, since waste alkali contains a large amount of oil, it is necessary to reduce oil content and COD (chemical oxygen demand) under drainage regulations. As a method for removing oil from waste alkali, a salting-out method using an acid, an oil collection method using static electricity, and a pressurized flotation method using air aeration are known. Recently, oil removal processing using a centrifugal separator has been known. And the amount of waste of alkali has been reduced. However, in the alkali recycled for a long period of time, accumulation of fine oil and metal abrasion powder (such as those generated in the rolling process and mixed in the rolling oil) occurs. Therefore, it is necessary to periodically renew the degreasing solution.

Conventionally, the neutralization treatment of waste alkali has been performed together with other cleaning waste liquid and waste acid (such as nitric acid, hydrofluoric acid, and sulfuric acid) generated in the manufacturing process of the iron alloy steel strip. But,
When the waste alkali containing a small amount of oil is diluted, the oil is diffused, and as a result, the waste alkali is sometimes drained while remaining in the waste water. That is, the waste acid has a dissolved metal and forms a large amount of hydroxide in the process of neutralizing the same. But,
When the waste liquid is diluted, the concentration of the waste liquid decreases, and as a result, the amount of oil adsorbed by the hydroxide decreases. Therefore, when the oil content in waste alkali is removed by utilizing the oil inclusion effect of the metal hydroxide, the higher the metal hydroxide concentration, the better. Therefore, by using this waste alkali as a neutralizing alkali after the detoxification treatment of the hexavalent chromium-containing waste liquid using ferrous chloride and hydrochloric acid, the acid is detoxified and contained in the waste alkali at the same time. Oil removal can be performed. At this time, it is a preferable condition that these treatment liquids containing metal ions are not unnecessarily diluted with water.

[0014]

According to the present invention, as described above, the detoxification treatment of the hexavalent chromium-containing waste aqueous solution generated in the manufacturing process of the iron alloy steel strip is mainly performed by the waste in the manufacturing process of the iron alloy steel strip. The present invention is characterized by utilizing a certain aging iron chloride water waste liquid, aging hydrochloric acid and waste alkali. The apparatus and conditions suitable for carrying out the method of the present invention will be described below.

FIG. 1 is an explanatory view showing an apparatus for producing ferrous chloride using a ferric chloride waste liquid. Each waste liquid discharged from the iron alloy steel strip manufacturing process is temporarily stored in the storage tanks 1a and 2a. These storage tanks 1a and 2a serve as buffers for adjusting the discharge amount of each waste liquid discharged from the iron alloy steel strip manufacturing process and the supply amount of the hexavalent chromium-containing waste liquid to the detoxification processing device. It is to fulfill. The aged iron chloride aqueous solution 1 supplied from the storage tank 1a for the aged iron chloride aqueous solution is supplied to the reaction tank 3 via the pump 1b. The concentration of the aging iron chloride aqueous solution 1 is not particularly limited, but is preferably higher in order not to increase the capacity of the reaction tank 3, and the higher the concentration of ferrous chloride contained is required for the reduction of ferric chloride. It is economical because less iron scrap is used. The aging iron chloride aqueous solution 1 contains 100 g / l to 650 g / l of ferric chloride in consideration of the disposal conditions at the generation source, and the sum of the ferrous chloride concentration and 650 g / l or less. It has been confirmed by the inventor's investigation that it can be sufficiently used. Here, the reason for limiting the concentration of ferrous chloride to a maximum of 650 g / l is that if the concentration is higher than this, ferrous chloride becomes iron hydroxide or iron oxide, which blocks pipes and the like and makes transfer difficult. It is because it becomes. The higher the liquid temperature, the more favorable the reaction with iron scrap is. However, if the temperature is too high, hydrochloric acid is added to a solution mainly composed of ferrous chloride by adding iron scrap to the aged iron chloride aqueous solution 1 as described later. In this case, a large amount of hydrogen chloride gas is generated, which is not preferable. Preferred liquid temperature is 25-85 ° C.

The supply hopper 4a is replenished with iron scraps 4 from a steel scrap storage area 4c by a belt conveyor 4b. The finer the form of the iron scraps 4, the greater the surface area and the higher the reactivity with acid,
If it is too fine, it will be scattered by wind and the like, which is not preferable. In the investigation of the inventor, a waste shot (particle size: 0.1 to 0.6 mm) of shot blast used in an annealing and pickling facility in a ferrous alloy steel strip manufacturing process was appropriate. However, since the amount of the waste shot varies, stable supply is difficult.
Attempts to use iron scraps such as Dalai powder were also found, but there was a problem that clogging or undissolved components remained in the hopper 4a. As a result of various investigations, there was no problem such as transfer in the hopper 4a or undissolved in the reaction tank 3 as long as iron chips were sheared with a chopper or the like to a size of 10 mm square or less.

Aged iron chloride aqueous solution 1 supplied to reaction tank 3
The iron scraps 4 react with each other while being stirred by the stirrer 3a in the reaction tank 3 to form a concentrated solution of ferrous chloride, and the overflow liquid is sent to the next storage tank 5. The reason for providing the storage tank 5 is to avoid mixing of undissolved iron scraps 4. Further, since the ferrous chloride solution has a risk of remaining undissolved iron scraps 4 and containing sludge due to alteration of ferrous chloride, a centrifuge 6 is provided. The solid content separated by the centrifuge 6 is returned to the reaction tank 3. The ferrous chloride solution purified by solid-liquid separation is supplied to a dilution tank 7 via a pump 7a, where the waste hydrochloric acid 2 generated in the iron alloy steel strip manufacturing process and stored in the storage tank 2a is removed by a pump 2b. , And water 7b for dilution is added, and each solution is stirred in the dilution tank 7 by the stirrer 7c. The amount of the waste hydrochloric acid 2 is added so that the pH of the solution in the dilution tank 7 becomes 1 or less. The adjustment of the addition amount is performed by a valve 7f via a controller 7e based on the output measured by a pH meter 7d described later. Further, it is desirable to measure the concentration of ferrous chloride in the solution in the dilution tank 7 and control the amount of the water 7b for dilution based on this output. This is because it is easier to control the hexavalent chromium-containing waste liquid detoxification processing apparatus described later without greatly changing the ferrous chloride concentration. As a method for measuring the concentration of ferrous chloride in the solution in the dilution tank 7, a method based on the principle of measuring the ultrasonic wave propagation velocity, a method for measuring a generally used oxidation-reduction potential, and the like are appropriate. . The produced hydrochloric acid-ferrous chloride solution 8 is stored in a storage tank 8a, and then supplied to a detoxification treatment apparatus for hexavalent chromium-containing waste liquid described below.

FIG. 2 is an explanatory view showing the structure of a detoxifying apparatus for a hexavalent chromium-containing waste liquid. The detoxification apparatus for the hexavalent chromium-containing waste liquid includes a pH adjustment tank 10, a reduction tank 11, a neutralization tank 12, and a solid-liquid separator 14. In the pH adjustment tank 10,
The hexavalent chromium-containing waste liquid 9 is continuously or intermittently charged, and the waste hydrochloric acid 2 is automatically added from the storage tank 2a via the pump 2b so that the pH measured by the pH meter 10b is maintained at 1 or less. You. Here, the reason for adjusting the pH to 1 or less is as follows.
If the waste liquid containing hexavalent chromium was sent to the next reduction tank 11 while the pH was high, the iron ion of ferrous chloride and the chloride ion were separated, and the contact between divalent iron ion and hexavalent chromium ion was made. This is because it is not possible to expect a sufficient reduction action by simplifying the above, which is clear from the results of FIG. The hexavalent chromium-containing waste liquid after the pH adjustment is continuously or intermittently charged into the reduction tank 11, and the hydrochloric acid acid chloride is added so that the measurement output of the ORP meter (oxidation-reduction potentiometer) 11b maintains 500 ± 10 mv. Ferrous solution 8 is automatically added from storage tank 8a via pump 8b. Measured output of ORP meter 11b is 500 ± 10mv
The reason for this depends on the properties of the hexavalent chromium-containing waste liquid, the reducing agent used, and the like. However, according to experiments performed by the present inventors, hexavalent chromium in the treated waste liquid is sufficiently reduced, and the remaining divalent iron ions are removed. This is because this control value, which is a small condition, was appropriate.

The waste liquid after detoxification of hexavalent chromium reduced by contacting ferrous chloride divalent iron ions and hexavalent chromium ions in the reduction tank 11 is continuously or neutralized in the neutralization tank 12. The waste alkali 13 is pumped from the storage tank 13a into the pump 1 so as to be intermittently charged and maintain the pH measured by the pH meter 12b at 6 to 8.
Added automatically via 3b. Here, ions of trivalent chromium, trivalent iron, and other dissolved metals in the waste liquid become hydroxides, and in the production process, adsorb the oil contained in the waste alkali, and as a result, the treated waste liquid contains The oil content will be very low. The waste liquid after the neutralization treatment is subjected to solid-liquid separation using a thickener based on the principle of natural gravity sedimentation or a solid-liquid separator 14 (thickener in the illustrated embodiment) comprising a centrifuge for forcibly performing solid-liquid separation. Is performed. Since the liquid waste liquid is harmless, it is discharged to public water bodies as it is. on the other hand,
The solid content mainly composed of metal hydroxide is reused as a raw material for producing an iron alloy steel strip after dehydration drying. When the supply amount of waste alkali is insufficient, a calcium hydroxide solution is used. This is because the anions in the solution to be neutralized are mainly chloride ions, so that the generated salt is calcium chloride having high solubility in water, and rarely generates solids unlike conventional calcium sulfate.

The detoxification treatment of the hexavalent chromium-containing waste liquid described above may be performed continuously, but is preferably performed intermittently for more reliable processing. That is, when the treatment amount of the waste liquid containing hexavalent chromium is small and the concentration is high, after adjusting the pH in the pH adjusting tank 10, the liquid is sent to the reducing tank 11 and the reducing agent (hydrochloric acid chloride) By adding ferrous aqueous solution and stopping the addition of the reducing agent when the change in ORP changes sharply, the addition of the reducing agent required for reducing hexavalent chromium can be saved, and at the same time, the remaining 2 The amount of valence iron ions can be reduced. In addition, in order to make the change of ORP abruptly clearer, the operation is facilitated by performing an operation of differentiating the amount of change and stopping the addition of the reducing agent at that time.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Aged iron chloride solution 1 (ferric chloride concentration 180 g / l, ferrous chloride concentration 1
500 g of 80 g / l, specific gravity 1.2, liquid temperature 40 ° C.) was sent, and 17 kg (equimolar amount of ferric chloride in waste liquid) of iron scrap (waste shot) 4 was added to about 10 kg. The mixture was stirred with the stirrer 3a for minutes.
At this time, the concentration of ferrous chloride in this solution was about 300 g / l.
Met. After diluting this solution 10-fold with 7b of water for dilution, 750 l of waste hydrochloric acid 2 was added thereto to adjust the pH to 1.
The waste hydrochloric acid 2 is composed of 60 g / l free hydrochloric acid and 10 g / l
Of dissolved metal.

Next, 500 l of sodium sulfate electrolyte (hexavalent chromium concentration: 21 g / l, pH: 5.2, liquid temperature: 50 ° C.) was sent to the pH adjusting tank 10 of the apparatus shown in FIG. Liquid. The waste hydrochloric acid 2 was added to the mixture to adjust the pH to 0.1, and the hydrochloric acid-acidic ferrous chloride solution 8 was further added. When the ORP meter output reached 500 mv, the waste liquid was analyzed. However, the hexavalent chromium ion concentration was 0.5 mg / l, and the divalent iron ion concentration was 1 mg / 1 or less. At this time, the amount of O added to the amount of the reducing agent (hydrochloric acid ferrous chloride solution 8) added
FIG. 5 shows the amount of change in RP and the properties of the waste liquid after reduction. Next, a waste alkali was added to the solution to adjust the pH to 7.
The waste alkali contained 50 g / l NaOH, 100 mg / l oil. The oil concentration in the waste liquid after the neutralization treatment was 5 mg / l, and the metal hydroxide concentration was 10 g / l.

[0023]

As described in detail above, the method for detoxifying a hexavalent chromium-containing waste liquid according to the present invention includes a hexavalent chromium-containing waste liquid generated in a steel alloy steel strip manufacturing process represented by a stainless steel strip manufacturing process. Since waste liquids can be mutually detoxified mainly by using waste liquids generated in the iron alloy steel strip manufacturing process, which has been conventionally treated as a waste detoxification treatment alone, not only can the cost of this treatment be reduced, By replacing the expensive ferrous sulfate / sulfuric acid type reducing agent used conventionally with the ferrous chloride / hydrochloric acid type reducing agent which is a waste liquid generated in the iron alloy steel strip manufacturing process, This makes it easy to use slaked lime which is inexpensive as an alkali used for a neutralization treatment, and its industrial value is very large.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an apparatus for producing ferrous chloride using a ferric chloride waste liquid.

FIG. 2 is an explanatory view showing a configuration of a detoxification treatment apparatus for a hexavalent chromium-containing waste liquid.

FIG. 3 is an explanatory diagram showing the concentration of ferrous chloride by adding iron scrap to an aged iron chloride aqueous solution and its reduction efficiency.

FIG. 4 is an explanatory diagram showing the effect of the pH of a waste liquid containing hexavalent chromium on the ability to reduce ferrous chloride.

FIG. 5 is a diagram showing a change amount of ORP with respect to an added amount of a hydrochloric acid-acidic ferrous chloride solution as a reducing agent, and a property of a waste liquid after reduction.

[Explanation of symbols]

 1 Aged iron chloride aqueous solution 1a Storage tank 1b pump 2 Waste hydrochloric acid 2a Storage tank 2b pump 3 Reaction tank 3a Stirrer 4 Iron scrap 4a Hopper 4b Belt conveyor 4c Scrap iron storage 5 Storage tank 6 Centrifugal separator 7 Dilution tank 7a Pump 7b Water 7c Stirring 7d pH meter 7e Controller 7f Valve 8 Hydrochloric acid ferrous chloride solution 8a Storage tank 8b Pump 9 Hexavalent chromium-containing waste liquid 10 pH adjustment tank 10a Stirrer 10b pH meter 11 Reduction tank 11a Stirrer 11b ORP meter 12 Medium Japanese tank 12a Stirrer 12b pH meter 13 Waste alkali 13a Storage tank 13b Pump 14 Solid-liquid separator

Claims (4)

(57) [Claims] Claims: 1. A solution containing hexavalent chromium containing at least chromium-containing iron alloy steel strip, to which waste hydrochloric acid also generated in the iron alloy steel strip manufacturing step is added to adjust the pH to ≤1. Iron scrap is added to a waste aqueous solution mainly composed of ferrous chloride and ferric chloride generated in the alloy steel strip production process to produce a solution mainly composed of ferrous chloride, and the iron alloy steel strip is similarly produced in the solution. Hexavalent chromium-containing waste liquor having pH ≤ 1 by adding, as a reducing agent, an aqueous solution of ferrous chloride acidic with hydrochloric acid produced by sequentially adding waste hydrochloric acid generated in the process to acidify to pH ≤ 1 Oxidation-reduction potential of 500 with respect to silver chloride reference electrode
Reduction treatment was performed so as to maintain the range of ± 10 mv, and neutralization treatment was also performed using the waste alkali generated in the iron alloy steel strip manufacturing process so that the hexavalent chromium-containing waste liquid that had been reduced was maintained in the pH range of 7 to 8. A method for detoxifying hexavalent chromium-containing waste liquid, wherein the neutralized hexavalent chromium-containing waste liquid is subsequently subjected to solid-liquid separation with a centrifuge. 2. A waste aqueous solution comprising ferrous chloride and ferric chloride, which contains 100 g / l to 650 g / l of ferric chloride, and whose sum with the ferrous chloride concentration is 650 g / l or less. The method for detoxifying hexavalent chromium-containing waste liquid according to claim 1, wherein a waste aqueous solution is used. 3. The hexavalent chromium-containing waste liquid according to claim 1, wherein iron scrap having a size of 10 mm square or less is used as the iron scrap added to the waste aqueous solution comprising ferrous chloride and ferric chloride. Detoxification method. 4. The method according to claim 1, wherein a calcium hydroxide solution is used when the supply amount of waste alkali for neutralizing the hexavalent chromium-containing waste liquid so as to maintain the pH in the range of 7 to 8 is insufficient. The method for detoxifying a hexavalent chromium-containing waste liquid according to claim 1 or 2.