JPS6035995B2 - Concentration control method for nitric acid pickling bath - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling the concentration of nitric acid and hydrofluoric acid (hereinafter referred to as nitric acid) used for pickling high alloy steel and the like.

When removing oxidized scale generated by hot working or heat treating steel materials, nitric hydrofluoric acid is usually used to pickle steel materials with stable scale, such as stainless steel.

The acid concentration of the pickling solution must be within a range suitable for pickling, and the concentration is usually adjusted by supplementing nitric acid and hydrofluoric acid as appropriate. Additionally, if the pickling solution becomes dirty or scale is deposited in the solution, the solution is drained and refilled with new solution. If the pickling process is continued while adjusting the concentration by replenishing nitric acid or hydrofluoric acid, non-scale precipitates may be formed in the pickling solution and deposited in the tank.

When such precipitates accumulate, it requires a great deal of effort to remove them from the pot, and there is a concern that the bath will be medicated by acid. Particularly in the case of spray pickling, the pickling process may not be carried out normally due to clogging of drains or piping. In addition, even if the acid is degraded, the pickling solution discharged when precipitates are formed or the amount of scale buildup increases contains a considerable amount of nitric acid and hydrofluoric acid. Therefore, it cannot be disposed of as is, and must be neutralized by adding carbide mosaic, slaked lime, etc. In the present invention, when pickling steel materials such as stainless steel using nitric acid,
Acid concentration control to suppress the formation of hard deposits as described above, recover nitric acid and hydrofluoric acid from the discharged pickling solution, and remove iron ions for effective pickling. The purpose is to carry out.

The method of the present invention monitors the iron ion concentration and fluorine ion concentration in the nitric acid used for pickling to ensure that it is maintained within limits that do not produce iron fluoride precipitates, and prevents the concentration from exceeding a set concentration. When this happens, drain the pickling solution appropriately, recover nitric acid and hydrofluoric acid from the pickling solution using an ion exchange membrane, and reuse the recovered acid to adjust the concentration to the appropriate concentration for pickling. It is characterized by monitoring the nitric acid and hydrofluoric acid concentrations in the pickling solution.

Conventionally, ion exchange membranes were thought to be weak against nitric acid, and it was thought that acid recovery using this method was impossible, but in recent years, membranes that can withstand nitric acid have become commercially available, making the present invention possible. became.

When a steel material is pickled with nitric and hydrofluoric acid, the iron first dissolves according to the '11 formula.

Then, according to equations '2} and [3}, the dissolved iron ions react with hydrofluoric acid to form an iron fluoride complex, which finally becomes iron fluoride and precipitates according to equation [41]. is e
+ Mother H + → is e3 13th day 2↑ '1
'Fe310HF→FeF20H+
'21FeF20HF→FeF210days10
'3'FeF210HF→FeF3
↓10 days ago {41 The present inventors conducted experiments with liquids with various acid concentrations and iron ion concentrations, and by dissecting the deposits in the pickling tank, the above FeF
3 revealed that eF3/insect LO was formed through a hydration reaction.

In addition, the formation of precipitates is hardly affected by nitric acid concentration;
It was confirmed that precipitation does not occur as long as the concentration of iron ions (F30) and total fluorine ions (T-F-) is within the range below curve A in FIG. 1. Within this range, the diagonally shaded range in Figure 1 was set as the concentration control limit as the range where the pickling effect was large. That is, iron ion concentration 3
If the fluorine ion concentration is below 0 Tanok and the fluorine ion concentration is 5 to 60, the pickling effect will be high and no iron fluoride precipitate will be formed. The amount of iron fluoride produced increases as curves B and C appear in Figure 1, with curve B at 10 t/s and curve C at 100 t/s.
Evening/precipitation. Among the above formulas '1} to {4), only formula [1'] is effective for pickling.

Nitric acid also dissolves iron and is effective for pickling. However, in a state where iron ions are contained, if acid is simply replenished, hydrofluoric acid reacts with iron ions and is consumed to form iron fluoride precipitates through the reaction of formula 21 [3}'41. You won't be able to expect the effect of the additional amount. Therefore, the iron ion concentration and total fluorine ion concentration in the pickling solution must be controlled to prevent the formation of iron fluoride precipitates and the wasteful addition of hydrofluoric acid. If the iron ion concentration and total fluorine ion concentration are below curve A in FIG. 1, no iron fluoride precipitate will be formed. However, due to the formula [2', '3}, iron and fluorinated rust are produced, and the added hydrofluoric acid is wasted. The produced flanges were as high as 20 FeF in the lower right region of the curve D in FIG. 1 and up to FeF in the upper left region. Therefore, it is desirable that the iron ion concentration be as low as possible even in a region where iron fluoride does not precipitate. It is also desirable that the total fluorine ion concentration be as low as possible, but a maximum water limit is required for pickling treatment. It is desirable to maintain the iron ion concentration and total fluorine ion concentration within the diagonal lines in FIG. 1 from such a point. In other words, the iron ion concentration and total fluorine ion concentration in the pickling solution were measured, and the acid concentration was adjusted by draining the solution and replenishing it with nitric acid and hydrofluoric acid so that each concentration did not exceed the shaded range in Figure 1. Adjust to a range suitable for pickling.

The concentration of nitric acid and hydrofluoric acid varies depending on the steel type and scale condition, but for example, in the case of hot-rolled stainless steel, HN03 is 20 to 100 m/d, HF is 5 to 5
0 evening/evening is suitable. The nitric acid and hydrofluoric acid to be replenished are those recovered from the wastewater using an ion exchange membrane, and the shortage is supplemented with new acid. The method of the present invention will be explained below with reference to FIG.

The nitric acid pickling tank 1 performs pickling treatment by soaking the steel material in a casing or by spraying a liquid in the casing onto the steel material and circulating it. Iron ions and total fluorine ions in the pickling tank 1 are analyzed and monitored using a spectrometer 5. In order to always maintain iron ions and total fluorine ions within the range shown by the diagonal lines in Figure 1, when Fe30 exceeds 30 T/so, when F- exceeds 60 T, some or all of the inside of the tank is The pickling solution is discharged and introduced into the waste acid storage tank 2, and the recovered acid storage tank 4 and, if necessary, a new acid tank 6 and water 8 are added to adjust the acid concentration to a suitable level for pickling. The waste acid in the waste acid storage tank 2 is introduced into the diffusion filtration tank 3,
Hydrofluoric acid and nitric acid are recovered by an anion exchange membrane. In the diffusion filtration tank 3, the waste acid S and water W are caused to flow in countercurrents across the anion exchange membrane 7, as shown in FIG.
The obtained recovered acid R is introduced into a recovered acid storage tank 4, and the filtered liquid F, which is mainly composed of metal ions mainly composed of Fe30, is introduced to a waste acid treatment facility. The filtered liquid contains a small amount of unrecovered nitric acid and hydrofluoric acid, so neutralization treatment is necessary. The recovered acid in the recovered acid storage tank 4 is subjected to measurement of nitric acid concentration and hydrofluoric acid concentration by a spectrometer 5, and then transferred to the pickling tank 1.
Prepare for the introduction of The acid can be discharged from the pickling tank 1 in its entirety at once when the pickling process is carried out intermittently, but in cases where the pickling process is carried out continuously, such as continuous pickling of strips. During this time, a portion of the acid is discharged to the extent that it does not interfere with the pickling process, and recovered acid and new acid are introduced as necessary. In addition, by continuously discharging the acid from the nitric acid pickling tank 1 and introducing the recovered acid and new acid, the amount of the pickling solution, Fe30, F-concentration, and H suitable for pickling treatment
In order to maintain the N03 and HF concentrations, it is desirable to adjust the amounts of discharge and introduction based on the measurement results of the spectrometer 5.
Note that the recovered acid storage tank 4 and the waste acid storage tank 2 may be omitted when processing is performed continuously.

Next, examples of the present invention will be shown.

Example A pickling solution with an iron ion concentration of 279/unit and a total fluorine ion concentration of 23.6/unit as monitored with a spectrometer.
10 pieces were pulled out from the pickling tank and processed at a rate of 417〆/day,
It was introduced into a diffuser spectroscopy device.

When the acid was recovered by flowing clean water at a rate of 512 〆/hr, recovered acid with the following concentration (■) was obtained at a rate of 410 〆/day. The recovery rates of nitric acid and hydrofluoric acid at this time were 7 each.
1% and 57%. The recovered acid was replenished with 62% nitric acid and 56% fluoric acid while checking the instructions on the spectrometer to give 25.5% nitric acid/14.4% sulfuric acid. The amount of replenishment at this time was 3.6 evenings/day of nitric acid and 4.0 nights/day of hydrofluoric acid. When the prepared pickling solution was returned to the pickling tank and pickling was performed, effective pickling was achieved. Pickling causes the base metal and scale of the stainless steel strip to elute, increasing the concentration of iron ions in the pickling solution and reducing the amount of fluoric acid and nitric acid, which are effective for pickling. The acid concentration was monitored and acid was added so that the concentration of nitric acid was 255 t/night and the concentration of nitric acid was 14.4 t/d.
This allowed the pickling to continue effectively. In addition, by continuously measuring the iron ion concentration with a spectrometer and repeating the above operation every 30 days/Z',
It became possible to carry out continuous pickling without causing precipitation. ■ Pickling liquid composition ■ Recovered acid composition HN0325.5 9/2 HN03 18.9
9/multi HF 14・4 9/multi HF 8
-4 9/Poly Fe 27 g/Poly Fe
l. 4 9/ As stated above, according to the present invention, there is no occurrence of iron fluoride tri-precipitates in the nitrate pickling solution, so there is no particular trouble during spray pickling, and there is no hard precipitate build-up. There is no need to remove objects.

Furthermore, pickling costs are reduced because nitric acid and hydrofluoric acid are recovered and recycled from waste acid that was conventionally discarded.
In addition, since the acid concentration of waste acid is reduced compared to conventional methods, there are great advantages in waste acid treatment, such as a reduction in the amount of neutralizing agent.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the concentration range that does not generate iron fluoride precipitates, Figure 2 is a system diagram for carrying out the method of the present invention, and Figure 3 is a diagram showing the concentration range in which iron fluoride precipitates are not generated.
It is a figure which shows the detail inside the diffusion-transmission tank 3 in a figure. Figure 3 Figure 1 Figure 2

Claims (1)

[Claims] 1. The pickling solution in the pickling tank is continuously or intermittently led out of the pickling tank, nitric acid and hydrofluoric acid are recovered by an ion exchange membrane, and the recovered acid is By introducing new acid into the pickling tank, the iron ion concentration and total fluorine ion concentration in the pickling tank can be maintained within the critical concentration at which iron fluoride precipitates are formed, and the acid A concentration control method for a nitric acid pickling bath characterized by adjusting the acid concentration in the washing tank to a concentration suitable for pickling. 2. When the iron ion concentration and total fluorine ion concentration in the pickling tank exceed a certain concentration within the limit for producing iron fluoride precipitates, some or all of the pickling solution in the tank is removed from the pickling tank. Nitric acid and hydrofluoric acid are recovered using an ion exchange membrane, and the recovered acid (replenishing new acid as necessary) is introduced into the pickling tank to reduce the acid concentration in the pickling tank. A method for controlling the concentration of a nitric acid pickling bath, which is characterized by adjusting the concentration to a concentration suitable for pickling.