KR101046218B1 - Free acid concentration analysis method of mixed acid pickling solution of stainless steel - Google Patents

Abstract

It is an object of the present invention to provide a method for quickly, simply and inexpensively measuring the concentration of each acid in a mixed acid so that it can be easily applied to a stainless pickling process. According to the present invention, a method for analyzing the free acid concentration of a mixed acid mixed with hydrofluoric acid and a strong acid includes: (a) diluting a mixed acid; (b) measuring a first electrical conductivity of the diluted mixed acid; (c) adding an additive containing Fe trivalent to the diluted mixed acid; (d) measuring a second electrical conductivity of the mixed acid to which the additive is added; And (e) calculating the concentration of hydrofluoric acid in proportion to the difference between the second electrical conductivity and the first electrical conductivity.

Mixed Acid, Stainless Steel, Pickling Process

Description

Analysis method of free acid in mixed acid pickling solution for stainless steel}

The present invention relates to a method for analyzing the free acid concentration of a mixed acid pickling solution of stainless steel, and more particularly, free of each acid in a mixed acid (such as nitric acid-fluoric acid or sulfuric acid-fluoric acid) used in pickling stainless steel. The present invention relates to a free acid concentration analysis method, which makes measuring acid concentration very simple and accurate.

Stainless steel is a type of steel made by alloying corrosion-resistant metals such as chromium and nickel with iron. It shows high corrosion resistance by alloys. In the manufacturing process, heat treatment is performed to obtain uniform structure after rolling. To form a scale. The process of chemically removing the oxidized scale is a pickling process, and since the oxidized scale has a high chromium content, it cannot be removed by a general single acid solution. Therefore, a mixed acid such as hydrofluoric acid and nitric acid or hydrofluoric acid and sulfuric acid and hydrogen peroxide is mainly used. . However, since the product quality after pickling varies according to the concentration of mixed acid, it is very important to accurately measure and control the concentration.

To this end, first, a method of separating and analyzing total acid and hydrofluoric acid in an acid solution using a fluorine-selective electrode and a hydrogen-selective electrode (US Pat. No. 4,060,717), in which the concentration of total acid is obtained and fluorine ion It is a method of converting each concentration by removing the concentration value, but the mutual interference is large, the equipment is expensive, and the life of the ion-selective electrode is not long.

Second, there is a method of determining the concentration of hydrofluoric acid, mixed acid and metal by monitoring the temperature and electrical conductivity of the solution simultaneously with neutralization titration by caustic soda (US Pat. No. 5,518,933), but also requires an expensive thermo titrator. It takes a considerable time at the time of titration, there is a disadvantage that takes a lot of time to measure a sample.

Third, J.P. McKaveney published an analysis of absorbance (Analytical Chemistry, Vol. 40, No. 8, 1968, “Spectrophotometric Method for Hydrofluoric Acid in Stainless Steel Pickling Batshs” to automate this method (Masahiko ITO et al., ISIJ, Vol. 37., No. 1, pp. 47-54, 1997, “Development of an Automatic Analyzer for a Mixed Nitric Acid, Hydrofluoric Acid, and Iron Ion in the Pickling Process”), but the automated process is complex and expensive. Requires analytical equipment Moreover, it is a method inferior in effectiveness, such as absorbance changes with time and is influenced by various ionic substances.

In addition, various attempts have been made, but most of them are not practical methods, and all of the above three kinds of applications are practically used.

It is an object of the present invention to provide a method for quickly, simply and inexpensively measuring the concentration of each acid in a mixed acid so that it can be easily applied to a stainless pickling process.

According to the present invention, a method for analyzing the free acid concentration of a mixed acid mixed with hydrofluoric acid and a strong acid includes: (a) diluting a mixed acid; (b) measuring a first electrical conductivity of the diluted mixed acid; (c) adding an additive containing Fe trivalent to the diluted mixed acid; (d) measuring a second electrical conductivity of the mixed acid to which the additive is added; And (e) calculating the concentration of hydrofluoric acid in proportion to the difference between the second electrical conductivity and the first electrical conductivity.

In addition, the method of analyzing the free acid concentration of the mixed acid in which the hydrofluoric acid and the strong acid are mixed according to the present invention includes: (a) diluting the mixed acid; (b) measuring a first electrical conductivity and a measurement temperature of the diluted mixed acid; (c) adding an additive containing Fe trivalent to the diluted mixed acid; (d) measuring a second electrical conductivity of the mixed acid to which the additive is added; (e) calculating a third electrical conductivity which compensates the first electrical conductivity at the measurement temperature with the electrical conductivity at a reference temperature; (f) calculating a fourth electrical conductivity which compensates the second electrical conductivity at the measurement temperature with the electrical conductivity at a reference temperature; (g) The concentration of the strong acid is calculated by [A × (third conductivity) + B], where A and B are the inclination and y-intercept values of the calibration curve of the conductivity and the concentration with respect to the strong acid of the mixed acid, respectively. The determined step; And (h) the concentration of hydrofluoric acid is calculated by [C x (fourth conductivity-third conductivity) + D], wherein C and D are the electrical conductivity for the hydrofluoric acid and the mixed acid of the mixed acid, respectively. It characterized in that it comprises the step of determining the difference between the electrical conductivity for the strong acid and the slope of the calibration curve and the y-intercept value of the concentration.

In the step (a), the mixed acid is diluted by 50 to 100 times, and the additive containing Fe trivalent is Fe (NO ₃ ) ₃ · 9H ₂ O, FeCl ₃ , Fe ₂ (SO ₄ ) ₃ or a mixture thereof The concentration of the additive is preferably 50g / L to 200g / L on the basis of Fe.

In the step (e), the third electrical conductivity is calculated by [α × (first electrical conductivity)], wherein α is determined as the slope of the calibration curve of the temperature difference and the conductivity change rate with respect to the mixed acid, The temperature difference is (measurement temperature-reference temperature), and in the step (f), the fourth electrical conductivity is calculated by [β × (second electrical conductivity)], wherein β is added to the mixed acid to which the additive is added. It is determined by the slope of the calibration curve between the temperature difference and the conductivity change rate, and the temperature difference is preferably (measurement temperature-reference temperature).

Since conventional methods for measuring the concentration of free acid in mixed acid have to be analyzed by expensive analysis equipment and skilled analysts, it takes much time to measure and transfers to a specific place for sample analysis. It was difficult, and there was a disadvantage that a lot of maintenance costs are required.

According to the present invention, it is possible to easily measure the concentration of each free acid in the mixed acid, and it is possible to precisely manage the stainless steel pickling solution, and to analyze by an unskilled person, thereby improving work efficiency. It is very economical and effective because it requires little maintenance cost.

In general, when hydrofluoric acid and other strong acids are mixed, hydrofluoric acid acts as a weak acid, and its dissociation degree is not large. The strong acid sulfuric acid or nitric acid has a large dissociation degree, and when the electrical conductivity of the mixed solution is measured, the value is mainly determined by the concentration of the strong acid. It varies greatly and the change is minimal due to the concentration of hydrofluoric acid. This relationship can be seen by the following formula.

HF + H ₂ SO ₄ (or _{HNO 3) + H 2 O →} HF ( mass) + H ⁺ + F ^- (small) + SO ₄ ^2- (or NO ₃ ^-) (volume)

_{HF + Fe (NO 3) 3} · 9H 2 O → FeF 3 + H + + 3 (NO 3 -) ( relative to the HF conductivities sangseungdoem)

Applying this principle, first dilute the mixed acid in an appropriate proportion (50-100 times) and measure and record its conductivity. The conductivity at this time is called first conductivity.

In the case of hydrofluoric acid, Fe 3 ions are combined to form a FeF ₃ mixture. Therefore, when a solution containing Fe trivalent is added to a solution containing hydrofluoric acid, the F component in hydrofluoric acid, which was present as HF, binds to Fe ^The ions are liberated, and the anions (NO ₃ , Cl, SO _4, etc.) contained in the additive are also released, and the electrical conductivity is greatly increased in proportion to the concentration of hydrofluoric acid.

Using this principle, the electrical conductivity is measured by adding a mixture containing Fe trivalent [Fe (NO ₃ ) ₃ · 9H ₂ O, FeCl ₃ , Fe ₂ (SO ₄ ) _3, etc.] after measuring the first electrical conductivity. Record the change in. The conductivity at this time is referred to as a second electrical conductivity.

Subsequently, by subtracting the first electrical conductivity from the second electrical conductivity, it can be seen that the increase in electrical conductivity increased only by the presence of pure hydrofluoric acid, which is proportional to the concentration of hydrofluoric acid. Therefore, by applying this principle, the concentration of each free acid in the mixed acid solution can be calculated quickly and accurately.

1 shows a flow chart of a method for analyzing the free acid concentration of mixed acid according to the present invention, hereinafter, the present invention will be described using exemplary numerical values.

Looking at the method for analyzing the free acid concentration of the mixed acid of the present invention, for analyzing the free acid concentration in the mixed acid, an electric conductivity meter and an electric conductivity electrode (0 to 200 mS / cm) capable of measuring temperature are prepared.

If the range of the conductivity electrode is too low, it is disadvantageous because the conductivity range of the mixed acid must be exceeded or too much dilution is required, and the conductivity electrode of the too high range decreases the sensitivity and decreases the accuracy.

Subsequently, 100 mL of distilled water is taken into a plastic beaker, and 2 mL of the mixed acid solution is taken and diluted in distilled water. In this embodiment, the dilution ratio is 51 times, but the dilution ratio may be used from 10 to 1000 times. However, if the dilution ratio is too low, the dilution ratio is too high, resulting in high conductivity, shortening the life of the electrode, and adding a large amount of additives into the Fe solution. Not practical. If the dilution ratio is too high, the measurement sensitivity is low and accuracy is difficult to secure.

Therefore, dilution is preferably performed between 50 and 100 times. In this example, a dilution ratio of 51 times was used for convenience of explanation. Accurately measure and record the electrical conductivity (first electrical conductivity) and measured temperature of the diluted acid.

Subsequently, 5 mL of a Fe trivalent solution, such as (Fe (NO ₃ ) ₃ .9H ₂ O, FeCl ₃ , Fe ₂ (SO ₄ ) _3, etc.), is added to the diluted mixed acid. May be prepared by any of the above, but a Fe (NO 3) ₃ · 9H ₂ O solution having a low solubility and high solubility with the pickling solution is most preferable.The concentration of the additive is about 50 to 200 g / L based on Fe. If this concentration is low, the solution will be diluted due to the high volume of the solution, and too high concentrations will be difficult to manufacture and the conductivity change will be so large that the accuracy will be reduced.

Thus, after the addition of the Fe 3 additive, the electrical conductivity (second conductivity) is measured in the same manner, and the temperature compensation and free acid concentration of each acid are calculated using the following correlation equation.

[Temperature compensation of mixed acid solution itself]

3rd conductivity = α × 1st conductivity

[Temperature Compensation of Mixed Acid Solution Added as Additive]

4th conductivity = β × 2nd conductivity

In the above equations, the temperature change constants α and β are constants for converting the electrical conductivity obtained at the measured temperature at which the first and second electrical conductivity are measured to the electrical conductivity at 25 ° C. (reference temperature).

The temperature change constants α and β represent calibration curves where the x axis is the temperature difference (measurement temperature-reference temperature) (° C.) and the y axis is the conductivity change rate (%). It is determined by creating each of the hydrofluoric acid mixed acids and taking the slopes of these curves, respectively.

Figure 2 shows the electrical conductivity change rate according to the temperature of the sulfuric acid-fluoric acid mixed acid before the additive is added, wherein the slope of the curve 1.0212 is determined as α. In the same manner, after drawing the calibration curve of the electrical conductivity change rate according to the temperature of the sulfuric acid-fluoric acid mixed acid after the additive is added, taking the slope of the curve, the value is determined to be β.

Hereinafter, a method for calculating the concentration of free acid will be described. The free acid of the mixed acid is calculated using the following formula.

[Calculation of concentration of strong acid]

Strong Acid Concentration = A × Third Conductivity + B

[Calculation of concentration of hydrofluoric acid]

Hydrofluoric acid concentration = C × (4th conductivity-3rd conductivity) + D

Here, A and B are constants calculated from the calibration curves of the electrical conductivity and the concentration of the strong acid of the mixed acid, and C and D are the calibration curves of the difference between the electrical conductivity of the hydrofluoric acid of the mixed acid and the electrical conductivity of the strong acid of the mixed acid. Calculated constant.

In order to calculate the constants A, B, C, and D, a calibration curve is prepared in which the x-axis is the third electrical conductivity with respect to the strong acid of the mixed acid and the y-axis is the strong acid concentration, and the slopes and y-intercepts of the curve are A and B. Determine each. In addition, for the hydrofluoric acid of the mixed acid, the x-axis is set as the electrical conductivity difference (ie, the fourth electrical conductivity-the third electrical conductivity) due to the addition of additives, and the calibration curve is prepared with the y-axis as the hydrofluoric acid concentration. Determine the sections with C and D, respectively.

FIG. 3A shows the calibration curve for the conductivity and concentration of sulfuric acid at 25 ° C. in sulfuric acid-fluoric acid mixed acid. FIG. 3B shows the calibration curve for the conductivity and concentration of hydrofluoric acid at 25 ° C. in sulfuric acid-fluoric acid mixed acid. To show. 3A and 3B, when A, B, C, and D are found, A = 10.588, B = -19.083, C = 5.0506, and D = -102.29 are determined.

FIG. 4A shows a calibration curve of the electrical conductivity and concentration of nitric acid at 25 ° C. in nitric acid-fluoric acid mixed acid. FIG. 4B shows the calibration curve of the conductivity of hydrofluoric acid at 25 ° C. in nitric acid-fluoric acid mixed acid. To show. In Figs. 4A and 4B, when A, B, C, and D are found, A = 8.5844, B = -6.0634, C = 4.7581, and D = -91.614.

Using the constants A, B, C, D and the temperature-compensated third and fourth electrical conductivity, the concentration of each free acid can be easily and quickly obtained.

The mixed acid standard solution was prepared by the method according to the present invention and the results of analyzing the concentration were compared. The results are shown in Table 1, and the measurement result error rate according to the present invention was able to accurately measure within a maximum of 2.15%.

TABLE 1 Concentration measurement results by standard solution preparation

Sulfuric acid concentration (g / L) Folic acid concentration (g / L) Measurement result Error rate (%) Sulfuric acid (g / L) Foshan (g / L) Sulfuric acid Foshan 20 20 20.14 19.92 0.70 0.40 40 20 40.26 19.95 0.65 0.25 60 20 59.87 20.43 0.22 2.15 80 20 79.84 19.67 0.20 1.65 80 40 80.04 39.87 0.05 0.33 80 60 81.02 60.24 1.28 0.40 80 80 81.01 79.98 1.26 0.02

FIG. 5 is a graph of measurement results of acid concentration change during one month operation in a pickling process using a sulfuric acid-fluoric acid mixed acid. FIG.

Actually, the solution used in the pickling process of stainless steel was analyzed for one month to examine the utility of the present invention. In the case of pickling stainless steel plates, 60 g / L sulfuric acid and 40 g / L hydrofluoric acid were used as initial acid concentrations. The results are shown in Figure 5, the initial concentration of each acid concentration decreases with the progress of pickling, and when the acid is added, the process returns to the original concentration again and decreases again.

As a result of measuring and managing the pickling solution for 1 month by the method according to the present invention, it was possible to operate the product with a very uniform and stable productivity.

Preferred embodiments of the invention described above have been disclosed for purposes of illustration. Therefore, those skilled in the art having ordinary knowledge of the present invention will be capable of various modifications, changes, and additions within the spirit and scope of the present invention, and such modifications, changes and additions should be regarded as belonging to the following claims.

1 is a flowchart of a method for analyzing the free acid concentration of mixed acid according to the present invention.

Figure 2 shows the electrical conductivity change rate according to the temperature of the sulfuric acid-fluoric acid mixed acid before the additive is added.

FIG. 3A shows a calibration curve of the electrical conductivity and concentration of sulfuric acid at 25 ° C. in a sulfuric acid-fluoric acid mixed acid.

FIG. 3B shows calibration curves of the electrical conductivity and concentration of hydrofluoric acid at 25 ° C. in a sulfuric acid-fluoric acid mixed acid.

FIG. 4A shows a calibration curve of the electrical conductivity and concentration of nitric acid at 25 ° C. for nitric acid-fluoric acid mixed acid.

FIG. 4B shows calibration curves for the electrical conductivity and concentration of hydrofluoric acid at 25 ° C. in nitric acid-fluoric acid mixed acid.

FIG. 5 is a graph of measurement results of acid concentration change during one month operation in a pickling process using a sulfuric acid-fluoric acid mixed acid. FIG.

Claims (5)

delete In the analysis method of free acid concentration of mixed acid mixed with hydrofluoric acid and strong acid, used as a pickling solution of stainless steel, (a) diluting the mixed acid; (b) measuring a first electrical conductivity and a measurement temperature of the diluted mixed acid; (c) adding an additive containing Fe trivalent to the diluted mixed acid; (d) measuring a second electrical conductivity of the mixed acid to which the additive is added; (e) calculating a third electrical conductivity that compensates the first electrical conductivity at the measurement temperature with the electrical conductivity at a reference temperature; (f) calculating a fourth electrical conductivity which compensates the second electrical conductivity at the measurement temperature with the electrical conductivity at a reference temperature; (g) The concentration of the strong acid is calculated by [A × (third conductivity) + B], where A and B are the inclination and y-intercept values of the calibration curve of the conductivity and the concentration with respect to the strong acid of the mixed acid, respectively. The determined step; And (h) The concentration of hydrofluoric acid is calculated by [C × (fourth conductivity-third conductivity) + D], where C and D are the electrical conductivity for the hydrofluoric acid of the mixed acid and the A method for analyzing the free acid concentration of a mixed acid mixed with hydrofluoric acid and a strong acid, characterized in that it comprises the step of determining the difference between the electrical conductivity for the strong acid and the slope of the calibration curve of the concentration and the y-intercept value. The method of claim 2, In the step (a), the mixed acid is characterized in that diluted 50 to 100 times, free acid concentration analysis method of the mixed acid mixed with hydrofluoric acid and strong acid. The method of claim 2, The additive containing Fe 3 is Fe (NO ₃ ) ₃ · 9H ₂ O, FeCl ₃ , Fe ₂ (SO ₄ ) ₃ or a mixture thereof, the concentration of the additive is 50g / L to 200g / L on the basis of Fe The free acid concentration analysis method of the mixed acid which hydrofluoric acid and the strong acid were mixed, characterized by the above-mentioned. The method of claim 2, In the step (e), The third electrical conductivity is calculated by [α × (first electrical conductivity)], Α is determined by the slope of the calibration curve of the temperature difference and the conductivity change rate with respect to the mixed acid, The temperature difference is (measurement temperature-reference temperature), In the step (f), The fourth electrical conductivity is calculated by [β × (second electrical conductivity)], Β is determined by the slope of the calibration curve of the temperature difference and the conductivity change rate with respect to the mixed acid to which the additive is added, The temperature difference is (measurement temperature-reference temperature), characterized in that the free acid concentration analysis method of the mixed acid mixed with hydrofluoric acid and strong acid.

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