EP0291493B1 - Process for electrolytic pickling of chrome containing stainless steel - Google Patents

Description

oxidiert wird, während Fe gelöst sofort als Fe(OH)₃ ausfällt. Das gebildete CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4 }}$

bleibt dabei in Lösung und wird nur beim Entschlammen bzw. mit dem Edelstahlband ausgeschleppt und erst dann durch Reduktionsmittel entgiftet. Dafür hat sich hauptsächlich die Reduktion mit einer wässrigen FeSO₄-Lösung im pH-Bereich von 0-2 bzw. 7-8 durchgesetzt. In beiden Fällen muß anschließend nochmals neutralisiert werden, um alle Metallionen in der Lösung als Hydroxide zu fällen. Außerdem ist bei sehr großem, plötzlichem Anfall von CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4 }}$

-hältigen Lösungen immer die Gefahr eines Durchbruches von CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4 }}$

gegeben, das dann in das Abwasser gelangen kam. Ein weiterer Nachteil des Verfahrens ist, daß nur ein Bruchteil des gebildeten CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4 }}$

aus der wässrigen Lösung von Na₂SO₄ entfernt wird, der Rest jedoch die Konzentration in der Lösung erhöht und zu verstärktem Angriff auf Kunststoffrohrleitungen und Pumpen führt.The invention relates to a process for the electrolytic pickling of chromium-containing stainless steel, in which firstly in an aqueous Na₂SO₄ solution and then in acid, preferably mixed acid, optionally pickled without current. Such a method is described in AT-PS 252.685, hydrofluoric acid plus nitric acid being used as the mixed acid. In the first step, the scale is removed, while in the second step the chrome-depleted layer underneath the scale, which is formed during the annealing process, is detached. This method has established itself worldwide, for example for pickling stainless steel strips, but has the disadvantage that the Cr in the scale through the current to the CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

is oxidized, while Fe dissolved as Fe (OH) ₃ precipitates immediately. The CrO formed $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

remains in solution and is only dragged out when desludging or with the stainless steel belt and only then detoxified by reducing agents. For this, reduction with an aqueous FeSO₄ solution in the pH range of 0-2 or 7-8 has mainly prevailed. In both cases, neutralization must then be carried out again in order to precipitate all metal ions in the solution as hydroxides. In addition, if there is a very large, sudden attack of CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

solutions always contain the risk of a breakthrough of CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

given, which then came into the sewage. Another disadvantage of the process is that only a fraction of the CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

is removed from the aqueous solution of Na₂SO₄, but the rest increases the concentration in the solution and leads to increased attack on plastic pipes and pumps.

In the reference "Chemical Abstracts", Vol. 87, No. 14, October 3, 1977, page 396, Abstract No 108322s, the formation of CrO₄ '' or Cr₂O₇ '' is described, but there is no solution to this problem.

The object of the invention is to provide a method of type mentioned, which avoids the disadvantages mentioned.

-freie Beizlösung erzielt wird. In Ausgestaltung der Erfindung ist vorgesehen, daß der pH-Wert der Lösung kleiner als 3, vorzugsweise 1,5 bis 2,5, zweckmäßig 2, durch Zugabe von H₂SO₄ eingestellt wird. In weiterer Ausgestaltung der Erfindung ist vorgesehen, daß das Redoxpotential der Lösung gemessen gegen eine Kalomelelektrode durch Zugabe von Säure und Reduktionsmittel um 50 bis 100mV verringert wird, wobei als Reduktionsmittel eine Substanz aus der Gruppe Na_x H_y S_z O_v verwendet wird, wobei x = 0 bis 2, y = 0 bis 2, z = 1 bis 6 und v = 2 bis 6 gilt, und daß bei der Reaktion auch Na₂SO₄ gebildet wird.This object is achieved in that the aqueous Na₂SO₄ solution acid and reducing agent are added according to the pH and redox potential, so that a CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

-free pickling solution is achieved. In an embodiment of the invention it is provided that the pH of the solution is less than 3, preferably 1.5 to 2.5, advantageously 2, by adding H₂SO₄. In a further embodiment of the invention it is provided that the redox potential of the solution, measured against a calomel electrode, is reduced by 50 to 100 mV by adding acid and reducing agent, a substance from the group Na _x H _y S _z O _{v being} used as the reducing agent, wherein x = 0 to 2, y = 0 to 2, z = 1 to 6 and v = 2 to 6, and that Na₂SO₄ is also formed in the reaction.

• Reaktion 1:
  3 Na₂SO₃ + 3H₂SO₄ + 2 H₂CrO₄ → Cr₂(SO₄)₃ + 3 Na₂SO₄ + 5 H₂O
• Reaktion 2:
  6 Na₂S₂O₃+ 6H₂SO₄+2H₂CrO₄ → Cr₂(SO₄)₃+3Na₂SO₄ +3Na₂S₄O₆ + 8H₂O
• Reaktion 3:
  3Na₂S₂O₄+3H₂SO₄+2H₂CrO₄ → Cr₂(SO₄)₃+3Na₂S₂O₅ +5H₂O
  3Na₂S₂O₅ + 3H₂O ⇄ 6 NaHSO₃
  6 NaHSO₃ + 2H₂CrO₄ → Cr₂(SO₃)₃ + 3Na₂SO₄ +5H₂O
  3 Na₂S₂O₄ +3H₂SO₄ + 4H₂CrO₄ → Cr₂(SO₄)₃+Cr₂ (SO₃)₃ +3Na₂SO₄+ 7 H₂O
• Reaktion 4:
  3 Na₂S₂O₅+2H₂CrO₄ → Cr₂(SO₃)₃+3Na₂SO₄ +2H₂O
• Reaktion 5:
  3Na₂S₂O₆+ 3H₂O ⇆ 3NaHSO₃ +3NaHSO₄
  3Na₂S₂O₆+ 2H₂CrO₄ → Cr₂(SO₄)₃ +3Na₂SO₄+2H₂O

Zusätzlich dazu wird auch das gelöste Fe₂(SO₄)₃ durch das Reduktionsmittel reduziert werden, wozu folgendes Beispiel dient:
   Fe₂(SO₄)₃ +Na₂SO₃+H₂O → 2FeSO₄+2NaHSO₄
und das gebildete FeSO₄ weiter mit H₂CrO₄ nach folgender Reaktionsgleichung reagieren:
   2H₂CrO₄+ 6 FeSO₄ + 6H₂SO₄ → Cr₂(SO₄)₃ +3Fe₂(SO₄)₃ + 8H₂O,
womit aber in Summe wieder die Reaktion 1 beschrieben ist.The following reaction mechanisms can be formulated for the reduction process:

• Response 1:
  3 Na₂SO₃ + 3H₂SO₄ + 2 H₂CrO₄ → Cr₂ (SO₄) ₃ + 3 Na₂SO₄ + 5 H₂O
• Response 2:
  6 Na₂S₂O₃ + 6H₂SO₄ + 2H₂CrO₄ → Cr₂ (SO₄) ₃ + 3Na₂SO₄ + 3Na₂S₄O₆ + 8H₂O
• Response 3:
  3Na₂S₂O₄ + 3H₂SO₄ + 2H₂CrO₄ → Cr₂ (SO₄) ₃ + 3Na₂S₂O₅ + 5H₂O
  3Na₂S₂O₅ + 3H₂O ⇄ 6 NaHSO₃
  6 NaHSO₃ + 2H₂CrO₄ → Cr₂ (SO₃) ₃ + 3Na₂SO₄ + 5H₂O
  3 Na₂S₂O₄ + 3H₂SO₄ + 4H₂CrO₄ → Cr₂ (SO₄) ₃ + Cr₂ (SO₃) ₃ + 3Na₂SO₄ + 7 H₂O
• Response 4:
  3 Na₂S₂O₅ + 2H₂CrO₄ → Cr₂ (SO₃) ₃ + 3Na₂SO₄ + 2H₂O
• Response 5:
  3Na₂S₂O₆ + 3H₂O ⇆ 3NaHSO₃ + 3NaHSO₄
  3Na₂S₂O₆ + 2H₂CrO₄ → Cr₂ (SO₄) ₃ + 3Na₂SO₄ + 2H₂O

In addition, the dissolved Fe₂ (SO₄) ₃ will be reduced by the reducing agent, which is shown in the following example:
Fe₂ (SO₄) ₃ + Na₂SO₃ + H₂O → 2FeSO₄ + 2NaHSO₄
and the FeSO₄ formed further react with H₂CrO₄ according to the following reaction equation:
2H₂CrO₄ + 6 FeSO₄ + 6H₂SO₄ → Cr₂ (SO₄) ₃ + 3Fe₂ (SO₄) ₃ + 8H₂O,
which describes Reaction 1 in total.

The oxidation of these substances also forms Na₂SO₄, which again serves as the conductive salt in the Na₂SO₄ solution and by suitable choice of the pH of this solution it is possible to let the dissolved Fe³⁺ precipitate as Fe (OH) ₃ after the solubility has been exceeded , whereby the solution does not have to be completely discarded at a certain Fe concentration, but only has to be freed from the Fe (OH) ₃ sludge. The concentration of Na₂SO₄ in the solution is 10 to 250g / l, preferably 170 to 200g / l.

hältigen Lösung um 50 bis 100mV größer als in der CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4 }}$

-freien Lösung. Selbstverständlich können neben dem Redoxpotential auch andere, analyt. Methoden zur Bestimmung des CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4 }}$

-Gehaltes der Lösung herangezogen werden, doch hat sich diese Methode als einfachste und kostengünstigste herausgestellt.According to the invention, the pH for these reactions is chosen to be less than 3, preferably 1.5 to 2.5, advantageously 2. At this pH the reaction rate in the solution is sufficiently high and the redox potential measured against a calomel electrode in the CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

containing solution 50 to 100mV larger than in the CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

-free solution. Of course, in addition to the redox potential, other, analyt. Methods for determining the CrO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4th}}$

Content of the solution can be used, but this method has proven to be the simplest and cheapest.

Example 1:

In an electrolytic pickling line with an aqueous solution of Na₂SO₄, a stainless steel belt 1000 x 6.0 mm was pickled at a belt speed of 8.4 m / min and then the chromium-depleted layer located underneath the scale layer was removed in a mixed acid tub with nitric acid. With a freshly prepared aqueous Na₂SO₄ solution, the increase in Cr⁶⁺ concentration was 0.2 g Cr⁶⁺ / l over a period of eight hours.
After adjusting the pH to 2.0 by adding H₂SO₄ 96%, the entire Cr⁶⁺ was by further addition of 8.8 ml of 10% Na₂SO₃ solution and 3.7 ml of 96% H₂SO₄ per liter of the aqueous solution reduced, the redox potential of the solution changing from previously 620 mV to 530 mV, measured against a calomel electrode.

During the next eight hours, this redox potential was kept constant by further, constant addition of Na₂SO₃ solution and sulfuric acid. At the end of the eight hours, no Cr⁶⁺ could be detected analytically in the aqueous solution.

Example 2:

Following the example 1, the addition of reducing agent was stopped until the redox potential again 620 mV had risen. After about another 4 hours the analyte was. certain Cr⁶⁺ concentration o, 11 g Cr⁶⁺ / l. By adding solid Na₂S₂O₅ in an amount of 0.9 g Na₂S₂O₅ (62%) per liter, the redox potential could be set to 520 mV again, measured against a calomel electrode, and no Cr⁶⁺ could be detected analytically. During the addition of Na₂S₂O₅, the pH of the solution dropped from 2.0 to 1.9.

Example 3:

After the addition according to Example 2, the addition of a reducing agent was interrupted again in the aqueous solution until a Cr⁶ ⁺ concentration of 0.16 g Cr⁶⁺ / l was reached again. By adding 3.9 ml of 10% Na₂S₂O₄ solution and 1.3 ml of 96% H₂SO₄ per liter of solution, the redox potential was reset to 515 mV and no Cr⁶⁺ could be detected analytically.

In all examples, the stainless steel strip was free of scale and silvery after treatment with acid or mixed acid.

Claims (3)

1. A method for electrolytic pickling of chromium-containing high-grade steel, wherein pickling firstly takes place in an aqueous Na₂SO₄ solution, and subsequently in acid, preferably mixed acid, optionally in a currentless manner, characterised in that the pH value of the aqueous Na₂SO₄ solution is adjusted by the addition of H₂SO₄ so as to be less than 3, preferably between 1.5 and 2.5, advisably 2, the redox potential of the solution is measured against a calomel electrode, and acid and reducing agents are added, the hexavalent chromium thus being reduced, until the redox potential is reduced to 50 to 100 mV, and the redox potential is maintained at the value reached, so that a pickling solution is obtained which is free of CrO₄²⁻.
2. A method in accordance with claim 1, characterised in that a substance from the group Na_x H_y S_z O_v is used as a reducing agent, wherein x = 0 to 2, y = 0 to 2, z = 1 to 6 and v = 2 to 6, and Na₂SO₄ is also formed during the reaction.
3. A method in accordance with claim 1, characterised in that the Na₂SO₄ concentration in the solution is adjusted so as to be from 100 to 250 g/l, preferably 170 to 200 g/l.