JP2933727B2 - Aging control method of neutral salt electrolytic bath for descaling stainless steel strip - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for easily managing a neutral salt electrolyte for descaling a stainless steel strip, which is easy to implement.

[0002]

2. Description of the Related Art A scale (metal oxide) is formed on a surface of a stainless steel strip by an annealing treatment or the like in a manufacturing process. Then, in a state where the scale remains, there are various inconveniences, so that the descaling process is performed. As the descaling method, an electrolytic treatment in an aqueous solution of a neutral salt such as sodium sulfate (hereinafter sometimes simply referred to as a neutral salt electrolytic treatment) or a mixed molten salt of caustic soda and sodium nitrate is used. After performing a pretreatment such as an immersion treatment, immersion in an aqueous solution of nitric hydrofluoric acid, electrolytic treatment in an aqueous solution of nitric acid, and a combination thereof are widely performed.

[0003] Of these, neutral salt electrolytic treatment is disclosed in JP-B-38-12.
This processing method has been widely adopted in recent years since it was disclosed in JP-A-162. The bath management in the neutral salt electrolysis generally includes the neutral salt concentration, pH, liquid temperature, liquid volume,
It is performed by measuring the concentration of valent chromium ions, the concentration of sludge, and the like.

Of these, hexavalent chromium ions are aging in neutral salt electrolytic baths as described below. The effect of neutral salt electrolysis on descaling of stainless steel strip is as follows.
It is considered as shown in Equation 1 below. That is, mainly oxides of trivalent chromium and oxides of ferrous iron in the scale on the surface of the stainless steel strip are subjected to anodic electrolysis to produce 6%.
As a result of transfer into the bath as a hydroxide of trivalent chromic acid and ferric iron, the scale of the stainless steel strip surface becomes coarse, and in the subsequent pickling treatment, the penetration action of nitric hydrofluoric acid or the like into the metal base is enhanced. It is.

Cr ₂ O ₃ + 2FeO + 8H ₂ O-8e → H ₂ Cr ₂ O ₇ + 2Fe (OH) ₃ ↓ + 4H ₂ ↑ (1)

Therefore, hexavalent chromium ions accumulate as aging in the neutral salt electrolytic bath. As the accumulation progresses, there arises a problem that the processing capacity of the neutral salt electrolytic bath is lowered due to the chemical equilibrium relationship, and that the product quality is lowered. As a technique for avoiding such adverse effects, Japanese Unexamined Patent Publication No.
JP-A-63-286600 discloses a method of adding a reducing agent in a bath.
The reduction of chromium ions is also described in JP-A-1-964.
No. 00 discloses that the concentration of hexavalent chromium ions is controlled to 8 g / liter or less. Generate 6
As a method for directly removing valent chromium ions from the neutral salt electrolytic bath, a method of installing an ion exchange apparatus and disposing of the bath separately is also known. In the latter method, when the hexavalent chromium ion concentration in the bath reaches the upper limit of the control value, a part of the bath is discarded and a new solution is supplied, and a neutral salt electrolytic bath having a predetermined concentration is newly constructed. (Hereinafter, rejuvenating the aged electrolyte in this way is sometimes referred to as regeneration treatment), but the wastewater treatment equipment for detoxifying hexavalent chromium ions contained in the aged solution to be discarded. In order to avoid a large load (which would also necessitate an increase in the size of the equipment), if the upper limit of the control value is set as low as possible and the frequency of the bath disposal is small or continuous, the The accumulation of hexavalent chromium ions will be small.

[0007] In order to maintain the processing performance of the neutral salt electrolytic bath by adopting any of the above-mentioned methods, in particular, the divided waste method, it is necessary to grasp the progress of aging of the neutral salt electrolytic bath, It is important that the aging control of the plant can be easily performed.

Conventionally, a method for determining hexavalent chromium ion as a method for measuring the degree of aging of an electrolytic solution is described in, for example, diphenylcarbazide absorption spectrophotometry described in 65.2.1 of JIS K0102 and 65 method of JIS K0102. .
Atomic absorption spectrometry described in 2.2 or reduction titration generally in bath management of a chromium plating bath (for example, JIS K1402: 4.
2.2) and so on, however, all of these methods have problems such as the need for human labor, the relatively long analysis time, and the need for specialized knowledge. Quantification of hexavalent chromium ions was difficult.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a method for easily controlling the aging of the neutral salt electrolytic bath.

[0010]

Means for Solving the Problems The present inventor has investigated the change of the hexavalent chromium ion concentration in the bath immediately after the neutral salt electrolysis bath was constructed in accordance with the neutral salt electrolysis treatment of the stainless steel strip. The solution color was observed. As a result, when the hexavalent chromium ion is almost nonexistent immediately after the neutral salt electrolytic bath is formed, the solution is pale yellow, and the reddish brown color peculiar to the hexavalent chromium ion increases with the subsequent electrolytic treatment. The spectral characteristics of the neutral salt electrolytic bath were investigated. As a result, it was determined that the aged neutral salt electrolytic bath exhibited strong absorption at a specific wavelength in the visible region, and that the absorbance and the hexavalent chromium ion concentration had a linear function. The present invention has been completed.

That is, the present invention is characterized in that the aging in which the neutral salt electrolytic bath generates hexavalent chromium during use of descaling of the stainless steel strip is controlled by measuring the absorbance of the bath sample at a wavelength of 370 nm. The present invention relates to a method for controlling the aging of a neutral salt electrolytic bath for descaling a stainless steel strip.

Hereinafter, the method for controlling the aging of the neutral salt electrolytic bath for descaling stainless steel strip according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing the absorption characteristics of one example of an aged neutral salt electrolytic bath, FIG. 2 is a diagram showing the relationship between the hexavalent chromium ion concentration in the neutral salt electrolytic bath and the absorbance at a wavelength of 370 mm, and FIG. FIGS. 4 and 5 are flow charts showing an example of the implementation of the method of the present invention, and FIGS. 4 and 5 are diagrams showing the absorbances measured by intermittently charging bath samples at the beginning and after half a year of running bath water, respectively.

The neutral salt electrolytic bath to which the method of the present invention is applied may be one in which the neutral salt is a sodium salt or a potassium salt such as sulfuric acid, nitric acid, hydrochloric acid or the like alone or as a mixture.
An aqueous solution of sodium sulfate is preferably used from the viewpoint of economy and scale removal effect, and the concentration of the aqueous solution of sodium sulfate is generally 170 to 230 g / liter at the beginning of bathing.

Using such a neutral salt electrolytic bath for the neutral salt electrolytic treatment of a stainless steel strip, the spectroscopic characteristics of a sample having a hexavalent chromium ion concentration were examined. Is 60 grams / liter (J
According to the reduction titration method described in 4.2.2 of ISK1402. As shown by the solid line in FIG. 1, the electrolytic bath in which hexavalent chromium ions were generated showed strong absorption at a wavelength of 370 nm, and the hexavalent chromium ion concentration was different from the above 60 g / liter. The same point was observed in that strong absorption was observed at a wavelength of 370 nm.
On the other hand, in the case where the hexavalent chromium ion has not yet been generated immediately after the bathing (blank), no special absorption at a wavelength of 370 nm is recognized as shown by the broken line in FIG.

Here, such an absorbance [log ₁₀ (I ₀ /
I); where I ₀ : incident light intensity, I: transmitted light intensity]. An absorbance meter is used for the measurement of the absorbance. The absorbance meter may be originally a self-made device or a commercially available product, but a commercially available product is generally used. Since such commercially available products are generally designed for general use, when measuring the absorbance of the electrolytic solution as described above, the hexavalent chromium ion concentration in the measured solution is measured by using an absorbance meter to be used. It is necessary to adjust it to be within the measurement range. Very standard commercial absorbance meters, such as "UV24
When "0" (manufactured by Shimadzu Corporation) is used, if the limit value of the hexavalent chromium ion concentration in the aging control of the electrolytic bath is set to, for example, 50 g / liter and measurement is made up to this limit, hexavalent chromium can be used as it is. It is necessary to dilute the electrolytic bath because the ion concentration is too high to exceed the measurement range of the absorbance meter and measurement cannot be performed. A cell is used to apply the measurement solution to the absorbance meter.Cells with cell lengths of 10 mm and 0.5 mm are generally prepared, and light is projected in the cell length direction. If the absorbance is measured by using the former and the latter, the hexavalent chromium ion concentration ratio is 1:20.
Is the same condition. In the method of the present invention, in consideration of the above, when the above standard absorbance meter is used, an electrolyte solution (hereinafter, sometimes referred to as a bath sample) to be subjected to absorbance measurement is diluted 2500 to 5000 times. Is the standard. In the following, unless otherwise specified, the standard absorbance meter as described above is used for the absorbance measurement.

As described above, the aging electrolyte has a wavelength of 37.
A remarkable absorbance at 0 nm was observed, and the relationship between the absorbance at this wavelength of 370 nm (for a dilute solution of a fixed magnification) and the hexavalent chromium ion concentration (gram / liter) in the electrolytic solution (according to the reductive titration method). FIG. 2 shows an example of the results obtained by examining samples having different degrees of aging. From Fig. 2, the concentration of hexavalent chromium ion in the electrolytic bath (gram / liter)
And absorbance are linearly related (relationship of linear function) [For this dilution ratio, hexavalent chromium ion concentration (gram / liter)]
= 75 x absorbance]. Therefore, take a bath sample, dilute and mix it to an appropriate constant magnification, and
By measuring the absorbance at 370 nm, the hexavalent chromium ion concentration, that is, the degree of aging can be known from the above relationship (figure or formula).

The aging of the neutral salt electrolytic bath is relatively slow, and once the bath is built, the equipment scale and operating conditions such as the amount of electrolytic treatment of the stainless steel strip with respect to the amount of electrolytic bath during the period until the next regeneration treatment. Although it depends on the aging process, it is roughly a monthly unit, so as described above, the bath sample is sometimes sampled from the electrolytic cell and diluted and mixed as described above, and the absorbance of the obtained homogeneous diluent is measured. It is easy to manage aging by doing.

[0018] However, sampling of the bath sample from the electrolytic cell, although occasionally, requires manpower, and this bath sample is harmful because it contains hexavalent chromium ions. May be considered low, and other tasks may take precedence. Therefore, it is preferable that the bath sample diluted during the neutral salt electrolysis treatment can automatically pass through the cell of the absorbance meter without the need for manual sampling. This makes it possible to manage the aging of the electrolytic bath by measuring the absorbance continuously or at appropriate time intervals, and it is easy to increase the measurement frequency according to this method. Some anxiety about the fact that the bath sample in (1) represents an electrolytic bath of a large electrolytic cell can also be solved.

According to the above-mentioned method, a bath sample of a constant flow rate is injected into running water of a constant flow rate while being stirred at a fixed position at a stirring position so as to have a predetermined dilution ratio, and a bath sample downstream thereof is diluted and mixed. The absorbance of the homogenous dilution is measured. In this case, the bath sample of a fixed magnification is taken out of the electrolytic bath by using an existing path for guiding a part of the electrolytic bath out of the system and returning it to its original state, and is taken after being branched off. Back to.

At an appropriate time during which the aging of the neutral salt electrolytic bath has progressed, the concentration of hexavalent chromium ion as an aged material is reduced by a regeneration treatment, for example, by divisional disposal, and the neutral salt such as sodium sulfate is removed. What is necessary is just to perform the process of newly building a bath with the concentration of the water being predetermined.

Next, a specific example for carrying out the method of the present invention will be described with reference to FIG. In FIG. 3, the stainless steel strip S is sent to the electrolytic cell 1 via the deflector roll 1a and the immersion roll 1b, and is immersed in the neutral salt electrolyte 1c in the electrolytic cell 1 to form a group of electrodes 1d while passing. A neutral salt electrolytic treatment is performed by the action.
The neutral salt electrolytic bath 1c in the electrolytic cell 1 is stirred by being circulated between the circulation tank 2 via the pipe 3 and the pipe 5 provided with the pump 4, thereby maintaining uniformity. The electrolytic cell 1 is
The bath sample is connected to a bath surface measuring tank 7 by a communication pipe 6, and a bath sample is supplied from the bath liquid level measuring tank 7 to a pipe 9 provided with a filter 8.
The liquid is sent to the mixer 11 at a constant flow rate by the pump 10. On the other hand, water is separately sent to the mixer 11 by a pipe 12 and a pump 13 at a constant flow rate, so that each constant flow rate of the bath sample and water is adjusted to a predetermined dilution ratio. The uniform diluent diluted and mixed at a predetermined ratio by the mixer 11 joins the circulating electrolyte in the pipe 3 through the pipe 14, and the pipe 15 branches off from the middle of the pipe 14 and is pumped by the pump 16. Part of the uniform diluent is sent to the absorbance meter 17. The absorbance meter 17 mainly includes a light source 17a, a light receiving body 17b, and a flow cell 17a.
c, and an arithmetic unit 17d. The uniform diluent passes through the flow cell 17c, returns to the pipe 14 via the pipe 15, and joins the original uniform diluent, and all the diluent finally flows into the electrolytic cell 1 via the circulation tank 2. Under such conditions, the absorbance meter 17
Is operated to measure the absorbance of the uniform diluent in the flow cell 17c as an electrical output. The measured electrical output is
It is converted into an absorbance by the arithmetic unit 17d, and this arithmetic unit 1
In 7d, the relationship between the absorbance at a wavelength of 370 nm and the hexavalent chromium ion concentration is stored in advance in relation to the dilution ratio, so that the absorbance is converted into the hexavalent chromium ion concentration in the bath based on this relationship. The hexavalent chromium ion concentration can be converted from a molecular weight ratio to a chromate ion concentration. The hexavalent chromium ion concentration or chromate ion concentration thus measured is displayed on the display 18 in an analog or digital manner.

The flow rate of the above-mentioned diluent into the electrolytic cell 1 is actually about 5 liters / minute because there is an evaporation amount of water over the entire electrolyzer and an amount taken out by adhesion to the stainless steel strip S. There is no problem in terms of concentration or quantity.

The above description is an example of the case where a uniform diluent is continuously prepared. As described above, water for dilution having a large dilution ratio and a large difference in flow rate is always mixed with a bath sample to be diluted. In order to obtain a stable and uniform diluent, precision on the apparatus is required. Therefore, the following simple method can be used. That is, a fixed amount of a bath sample is intermittently charged at the stirring position into a constant flow rate of water flowing while being stirred at a fixed position, and the absorbance of a non-uniform diluent in which a bath sample downstream thereof is diluted and mixed is measured. Make measurements continuously,
Aging is controlled by the average value of a plurality of peak values of absorbance obtained at each intermittent injection.

To explain this more specifically with reference to FIG. 3, there is no change in sending a constant flow of water for dilution to the mixer 11 by the pump 12, but a fixed amount of the bath sample is sent to the mixer 11 intermittently. is there. Therefore, a pump for such a fixed amount of intermittent liquid feeding is used as the pump 10. In this case, the certain amount is the dilution ratio (2500 to 5000 times) with respect to the amount of water for dilution sent to the mixer 11 within the time period during which the total amount of the intermittent liquid supply in succession is repeated a plurality of times. In this case, the term dilution ratio is used in this sense.However, as described below, since the concentration temporarily becomes high, the dilution ratio is lower than that in the case of uniform dilution. A slightly higher magnification is preferred. As described above, when the absorbance is continuously measured while the bath sample is intermittently charged into the running water for dilution, there is a peak value of the absorbance every time the bath sample is intermittently charged, and the increase and decrease before and after the measurement are repeated. It is relatively easy to input such a fixed amount every time under the same conditions, so that by averaging a plurality of successive peak values, absorbance data with relatively few errors can be obtained. By examining the relationship between the absorbance thus obtained and the hexavalent chromium ion concentration in advance, it is possible to know the hexavalent chromium ion concentration in the electrolyte from the peak average absorbance and control the aging. is there.

In each of the above-mentioned embodiments of the method of the present invention, if the relationship between the absorbance and the hexavalent chromium ion concentration is examined in advance, the aging can be directly controlled by the absorbance itself.

[0026]

EXAMPLE In the flow sheet shown in FIG.
Water was supplied at a flow rate of 5 liters / minute to 11 and the bath sample was intermittently charged at 1 ml every 1 minute, and the average value of 10 absorbance peak values obtained in 10 minutes was taken as a single absorbance measurement value for half a year. Absolutely, the absorbance at a wavelength of 370 nm was continuously measured. The cell length of the flow cell is 10 mm, the dilution ratio is 5,000 times, and a small-capacity 1 ml pumping pump is used as the pump 10, and a metering pump set to a capacity of 5 l / min is used as the pump 13. Pump for branch installed in 15
As 16, a metering pump of 0.1 liter / min was used in consideration of the capacity of the flow cell 17c. As the absorbance meter 17, UV240 manufactured by Shimadzu Corporation was modified and used. Some of the results obtained are shown in FIGS.

FIG. 4 shows that the neutral salt electrolytic bath is relatively new.
FIG. 5 shows the case where the concentration of the valent chromium ion is low, and FIG. 5 shows the case where the concentration of the hexavalent chromium ion is increased by continuously using the neutral salt electrolytic bath. The average values of the ten absorbance peak values in FIGS. 4 and 5 were 0.06 and 0.38, respectively.
Then, from the relationship between the absorbance (average of peak values) and the hexavalent chromium ion concentration under the above-mentioned conditions, which were previously checked, the hexavalent chromium ion concentration was 4.5 g / liter in FIG.
In FIG. 5, 28 grams / liter was obtained. On the other hand, the hexavalent chromium ion concentration by the reductive titration method was 5.1 g / l in FIG. 4 and 31 g / l in FIG. As described above, according to the method of the present invention, aging can be controlled by the absorbance supported by the hexavalent chromium ion concentration which is well suited to the reduction titration method.

[0028]

As described above in detail, the present invention measures the absorbance of a neutral salt electrolytic bath at a wavelength of 370 nm, and determines the degree of aging from the relationship with the hexavalent chromium ion concentration which has been checked in advance. The following effects can be obtained by configuring the neutral salt electrolytic bath to manage aging.

(1) Conventionally, the work of measuring the degree of aging was performed manually. However, the present invention enables the management of aging with little manual work, so that human resources can be effectively used. (2) The aged neutral salt electrolytic bath contains harmful hexavalent chromium ions, but the sampling of the bath sample from the electrolytic bath can be carried out without human intervention, thereby improving the work safety. (3) The conventional method for measuring the degree of aging of the neutral salt electrolytic bath is difficult to measure frequently because of human labor, but it is difficult to implement due to human problems. Since it was possible to know the degree of aging, it was possible to separate and dispose of the electrolytic bath at a fine frequency corresponding to the generation rate of hexavalent chromium ions as aging. That is, 6
It became easy to maintain the valent chromium ion concentration within the control value. (4) Since the aging control of the neutral salt electrolytic bath is facilitated, the neutral salt electrolytic treatment action and the descaling ability can be stabilized by appropriately and appropriately treating the waste liquid. (5) Since the electrolytic bath can be discarded at a fine frequency corresponding to the generation rate of aging, the load on the waste liquid treatment equipment is stabilized, and untreated waste liquid is discharged from the waste treatment equipment. Danger has been reduced. (6) Since no reagent is required for carrying out the method of the present invention,
It is economical.

[Brief description of the drawings]

FIG. 1 is a diagram showing the absorption characteristics of an example of an aged neutral salt electrolytic bath.

FIG. 2 Concentration of hexavalent chromium ion in neutral salt electrolytic bath and wavelength
It is a figure which shows the relationship with the light absorbency in 370mm.

FIG. 3 is a flow sheet showing an example of an implementation state of the method of the present invention.

FIG. 4 and FIG. 5 are diagrams showing absorbances measured by intermittently charging bath samples at the beginning and after half a year, respectively, in running water.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electrolyzer 1a Deflector roll 1b Immersion roll 1c Neutral salt electrolysis bath 1d Electrode 2 Circulation tank 3 Piping 4 Pump 5 Piping 6 Communication pipe 7 Bath surface measuring tank 8 Filter 9 Piping 10 Pump 11 Mixer 12 Piping 13 Pump 14 Piping 15 Piping 16 Pump 17 Absorbance meter 17a Light source 17b Photoreceptor 17c Flow cell 17d Arithmetic unit 18 Display S Stainless steel strip

Claims (3)

(57) [Claims] A stainless steel characterized in that the aging of a neutral salt electrolytic bath while forming hexavalent chromium during use of descaling of a stainless steel strip is controlled by measuring the absorbance of a bath sample at a wavelength of 370 nm. Aging control method for neutral salt electrolytic bath for descaling steel strip. 2. The aging control method for a neutral salt electrolytic bath for descaling a stainless steel strip according to claim 1, wherein the absorbance measurement is performed on a sample obtained by diluting and mixing the bath sample 2500 to 5000 times. 3. A method of measuring absorbance, wherein a fixed amount of a bath sample is intermittently charged at a predetermined flow rate into a stream of water flowing while being stirred at a fixed position, and continuously measured for a certain period of time at a stream downstream of the bath sample. 3. The aging management method for a neutral salt electrolytic bath for descaling a stainless steel strip according to claim 1 or 2, wherein aging is controlled by a plurality of average values of the absorbance peak values obtained at each intermittent charging.