KR100345694B1 - Method for measuring free acidity of pickling solution - Google Patents

Abstract

PURPOSE: Provided is a method for measuring the free acidity of a pickling solution, by which it is possible to promptly and precisely measure the free acidity of a pickling solution which is used in pickling of steel plates. CONSTITUTION: The method includes the steps of preparing a pickling solution of any predetermined concentration and determining a lower limit amount, a target amount and an upper limit amount of sodium hydroxide needed for titration of the pickling solution, mixing a sample pickling solution to be measured for its free acidity with each of the determined lower limit amount, target amount and upper limit amount of sodium hydroxide, followed by measuring of pH, and processing the obtained data to determine the free acidity.

Description

Method for Measuring Free Acidity of Pickling Solution {Method for Measuring Free Acidity of Pickling Solution}

The present invention relates to a method for measuring the free acidity of the pickling solution used for removing the surface oxide of the steel sheet.

The pickling treatment is a treatment performed to remove oxides formed on the surface, and is mainly used in the steel industry to remove surface oxides before cold rolling the surface oxides formed during the manufacturing of hot rolled sheets.

In the pickling process, nitric acid, hydrochloric acid, or sulfuric acid are frequently used. The pickling efficiency is more effective when the hydrogen ion concentration in the solution is higher. However, when sulfuric acid or nitric acid exceeds a certain concentration, the corrosion rate tends to decrease. This requires an appropriate concentration.

In the case of sulfuric acid solution, the influence of the temperature of pickling solution has an absolute effect, but in the case of hydrochloric acid, the influence of temperature is small.

In a continuous pickling line that uses hydrochloric acid as the pickling solution, the concentration of hydrochloric acid in the pickling tank determines the quality of the steel sheet and the basic unit of hydrochloric acid, making it one of the most important management items in operation. The pickling solution is a high concentration hydrochloric acid solution, the solution temperature is also high temperature 80 ℃ or more can not be directly measured through the pH sensor. Therefore, conventionally, various indirect measuring methods such as colorimetric + specific gravity method or conductivity + specific gravity method have been put into practical use, but the hydrochloric acid concentration is always subsidiaryly managed by manual analysis. In addition, an automatic analysis device using titration method has been developed for ease of operation.However, when the concentration of solution changes rapidly, the actual solution on-line cannot be accurately measured due to the dead time of 10 minutes or more. In addition, there is a problem that the analysis device is complicated for this, as the feedback control is set to the amount of the titrant to obtain the equivalent point. Due to this problem, it has not been put to practical use on-line.

The indirect measurement method based on conductivity and specific gravity uses the principle that the conductivity of the solution increases with the ion concentration in the solution and the principle that the specific gravity increases as the iron ion concentration in the solution increases. It is a method frequently used for concentration measurement. However, since this method is an indirect measurement and affected by coexistence components, the difference between the measured value and the actual value is greater than or equal to 2 wt% HCl, so the operator uses this measurement only as a reference value. The concentration of pickled liquor is managed by incorporating experiences.

An object of the present invention is to provide a new pickling solution concentration measuring method having excellent measurement accuracy and short measuring time (real time) for the control of the concentration of pickling solution.

Figure 1 is a graph showing the relationship between the pH of the pickling solution and the appropriate amount of sodium hydroxide

Figure 2 is a measure of the free acidity of the pickling solution with a concentration of 9.5% HCl in accordance with the present invention

Results

Figure 3 is measuring the free acidity of the pickling solution with a concentration of 10.8% HCl in accordance with the present invention

Results

Figure 4 measures the free acidity of the pickling solution with a concentration of 11.5% HCl in accordance with the present invention

Results

Figure 5 is a measure of the free acidity of the pickling solution with a concentration of 8.8% HCl in accordance with the present invention

Results

The present invention is a method for measuring the free acidity of the pickling solution,

Measuring a pH change while titrating sodium hydroxide to a pickling solution having a varying concentration of hydrochloric acid, and obtaining a relationship between the measured pH and the amount of sodium hydroxide added during titration;

A plurality of pickling solutions are selected from pickling solutions having different concentrations of hydrochloric acid, and one of these pickling solutions is arbitrarily set as a pickling solution having a target concentration, and the rest is compared with the hydrochloric acid concentration of the pickling solution having the target concentration. Classifying a pickling solution having a lower limit and an upper limit;

Determining a proper dose of the required sodium hydroxide corresponding to the pickling solution having the target, the lower limit, and the upper limit from the relationship between the pH obtained above and the dose of sodium hydroxide added at the time of titration;

Adding an appropriate amount of sodium hydroxide determined above to any pickling solution to be measured and measuring the pH in each pickling solution; And

The present invention relates to a method for measuring free acidity in a pickling solution comprising the step of comparing the pH value of the pickling solution measured above to determine the hydrochloric acid concentration of the pickling solution.

Hereinafter, the present invention will be described in detail.

The present invention is a new solution management method for concentration measurement using rapid pH change during acid-alkali neutralization reaction by applying acid base neutralization titration reaction.

That is, the present invention mixes a plurality of pickling solutions having a certain hydrochloric acid concentration and sodium hydroxide (NaOH) having a certain titration amount in order to measure the free acidity of the pickling solution, and the pH of the solution is measured. The free acidity is measured by combining the output values.

To this end, first prepare a relationship (curve) between the pH of the pickling solution with the concentration of hydrochloric acid and the amount of sodium hydroxide added in advance.

The relationship between the pH and the amount of sodium hydroxide (curve) was measured by titrating sodium hydroxide in a pickling solution with varying concentrations of hydrochloric acid, and measuring the change in pH, using the measured pH and the amount of sodium hydroxide added at the titration. Can be saved.

Then, an arbitrary pickling solution is set according to the hydrochloric acid concentration of the pickling solution, one of which is a pickling solution having a target concentration, and a pickling solution having a lower value than the concentration of the pickling solution having the target concentration. Pickling solution having a high value, and pickling solution having a high value is a pickling solution having an upper limit concentration.

At least one pickling solution having such an upper limit and a lower limit is preferable.

Thereafter, the hydrochloric acid concentration value of the pickling solution, that is, the amount of sodium hydroxide required to neutralize the pickling solution having a target, a lower limit, and an upper limit concentration was determined using the relationship (curve) between the pH and the amount of sodium hydroxide, and Sodium is added to each pickling solution to be measured.

1 is a diagram schematically illustrating the concept of the present invention. That is, the pH change measured by adding NaOH to the pickling solution is shown. The lower limit NaOH (1-e), target NaOH (1-f) and upper limit NaOH (1-g) shown in the figure are the upper limit, the target and the lower limit. It means the amount of NaOH required to neutralize the pickling liquid having a hydrochloric acid concentration.

That is, after mixing the pickling solution having an arbitrary concentration with the lower limit NaOH amount, the target NaOH amount and the upper limit NaOH amount in each of three pH measuring cells, the pH of the solution is measured. , At the intersection indicated by 1-c or 1-d. That is, since the hydrochloric acid concentration of the pickling solution is neutralized in all three measuring cells in the solution (1-a) having exceeded the upper limit setting value, the hydrochloric acid remaining in the pickling solution shows strong acidity.

In the case of pickling solution with a higher concentration than the target hydrochloric acid concentration and below the upper limit, (1-b) is neutralized in the measuring cell to which the upper limit of NaOH is added, so the pH value of the solution indicates alkali. In the measurement cell to which the lower limit NaOH was added, the amount of NaOH added was insufficient to neutralize all of the hydrochloric acid, so that hydrochloric acid remained and thus the pH value indicated acidity.

In the case of the pickling solution having a higher pickling concentration than the lower limit hydrochloric acid concentration and lower than the target hydrochloric acid concentration (1-c), in the measurement cell to which the lower limit NaOH was added, the amount of NaOH added was insufficient to neutralize all of the hydrochloric acid, so that hydrochloric acid remained. The value indicates acidity, and in the measuring cell to which the target and the upper limit of NaOH were added, the pH value of the solution indicates alkali because the remaining NaOH was present. In the solution (1-d) where the hydrochloric acid concentration of the detail solution is less than the lower limit, all three measuring cells are neutralized and the remaining NaOH is present, so the pH value of the solution indicates alkali. In this way the three pH values can be measured and combined to determine the free acidity of the pickling solution.

That is, in the case of the present invention, if the measured concentration of the pickling solution belongs to the pickling solution having the target and the lower limit concentration, it is possible to quickly determine the intermediate value between the target concentration and the lower limit concentration.

On the other hand, since the hydrochloric acid washing solution applied to the present invention is a high temperature strong acid, when diluted with pure water at a predetermined ratio, it can reduce the life span of the pH and the error range.

In addition, in the pickling liquid in the pickling tank, Fe ²⁺ and Fe ³⁺ coexist. In the case of Fe ³⁺ it reacts from pH 2.0 or higher to form Fe (OH) ₃ , and in the case of Fe ²⁺ it starts to react at pH 6.4 to form Fe (OH) ₂ .

Therefore, the pickling solution is preferably set to pH 4 as the equivalent point of free acidity in a titration curve of pH 6.4 or lower, which is mostly Fe ²⁺ in the solution and is not affected by Fe ²⁺ .

That is, when measuring the concentration of pickling liquid by using the control concentration measurement method according to the present invention, sodium hydroxide is supplied by diluting the pickling solution with pure water at a predetermined ratio to improve the reliability of the electrode and the measured value and to use the electrode for a long time. On the other hand, it is preferable to set the equivalent point of free acidity to pH 4 in a pH titration curve.

Hereinafter, the effects of the present invention will be described in detail through examples.

(Example)

First, a titration curve was calculated using 1N-sodium hydroxide for hydrochloric acid having a varying concentration of hydrochloric acid, and the concentration of hydrochloric acid was measured by varying the input amount of the titrant in each pH meter. Then, the experiment was conducted under the same conditions under a mixed solution of FeCl ₂ and hydrochloric acid. At the time of concentration measurement, the pickling solution was sucked 10 ml per minute by each metering pump and sent to three measuring mixers. At the same time, the titration solution 1N NaOH solution was sent to the measuring mixer by each metering pump with different dosages. At this time, the input amount of 1N-NaOH is the amount that can be neutralized to pH 4 on the basis of 10ml per minute pickling solution having the upper limit, the target and the lower limit hydrochloric acid concentration. Water is also supplied together with the measuring mixer to protect the pH electrode with a high concentration of pickling and sodium hydroxide solution. The measured pH value of each measuring mixer was converted to a value in the range of 1-5 Voltaga using a data wireless logging system with an output of 4-20 mA DC. That is, the output value is set to 1Volt when the pH value is 6.4, and 5Volt when the pH value is 2.

In this example, the target hydrochloric acid concentration was generally set to 10% HCl, which is a management standard in pickling lines, and the remaining upper and lower concentrations were 11% HCl and 9% HCl, respectively.

2, 3, 4, and 5 are results obtained by measuring pickling solutions having different hydrochloric acid concentrations under conditions in which 9, 10, and 11% were set as lower, target, and upper limits of hydrochloric acid.

FIG. 2 shows that when the concentration of the pickling solution is 9.5% (Fe ²⁺ : 80 g / l), since the actual hydrochloric acid concentration is greater than the set lower limit, the output value is closer to 5 Volt, and both are smaller than the set target and upper limit concentrations. It showed almost 1 Volt.

3 is a solution having a pickling solution of 10.8% (Fe ²⁺ : 80 g / l), and the actual hydrochloric acid concentration is higher than the lower limit and the target hydrochloric acid concentration. Since the concentration was lower than the upper limit hydrochloric acid, the upper limit monitoring cell showed 4 Volt.

FIG. 4 shows that the pickling solution has a hydrochloric acid concentration of 11.5% (Fe ²⁺ : 100 g / l), which is larger than the upper limit of the hydrochloric acid concentration setting. Since all hydrochloric acid is present in all concentration monitoring cells, the output values are all close to 5 volts. .

5 shows that the concentration of hydrochloric acid in the pickling solution is 8.8 (Fe ²⁺ : 100 g / l), and the actual concentration of hydrochloric acid is smaller than the lower limit hydrochloric acid concentration.

Therefore, when the lower limit, target and upper limit concentration values are actually narrower concentration values, more accurate acidity of the pickling solution can be measured quickly.

As described above, the measuring method according to the present invention has the advantage of being able to quickly and accurately manage the free acidity of the pickling solution used in the pickling treatment of the steel sheet.

Claims (4)

In the method for measuring the free acidity of the pickling solution, Measuring a pH change while titrating sodium hydroxide to a pickling solution having a varying concentration of hydrochloric acid, and obtaining a relationship between the measured pH and the amount of sodium hydroxide added during titration; A plurality of pickling solutions are selected from pickling solutions having different concentrations of hydrochloric acid, and one pickling solution is arbitrarily set as a pickling solution having a target concentration, and the rest is compared with the hydrochloric acid concentration of the pickling solution having the target concentration. Classifying with a pickling solution having a lower limit and an upper limit; Determining a proper dose of the required sodium hydroxide corresponding to the pickling solution having the target, the lower limit and the upper limit from the relationship between the pH obtained above and the dose of sodium hydroxide added at the time of titration; Adding an appropriate amount of sodium hydroxide determined above to any pickling solution to be measured and measuring the pH in each pickling solution; And Comparing the pH value of the pickling solution measured above to determine the hydrochloric acid concentration of the pickling solution free acidity measurement method The method of measuring free acidity in a pickling solution according to claim 1, wherein the pickling solution having a lower limit and an upper limit and at least one sodium hydroxide solution corresponding thereto are at least one. The method of measuring free acidity in a pickling solution according to claim 1, wherein sodium hydroxide is added while the pickling solution is diluted with a predetermined ratio of pure water when sodium hydroxide is added to any pickling solution to be measured. The method of measuring free acidity in a pickling solution according to claim 1, wherein the neutralization peak is set at pH 4 in relation to the pH and the amount of sodium hydroxide added.

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