Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Reexamination Certificate
1996-02-15
2003-01-14
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
C205S712000, C205S717000, C205S741000
Reexamination Certificate
active
06506294
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a process for regeneration of electrolytes, in particular of Na
2
SO
4
from spent pickling liquid, or pickle liquors, for stainless steel, in particular, stainless steel strips.
The “neolyte” process using a neutral electrolyte has become widely accepted all over the world for electrolytically pickling stainless steel strip, in particular cold strip. This process provides for the strip to be de-scaled in a sodium sulphate solution by means of electric current. In the process, metals build up in the electrolyte—i.e., the sodium sulphate solution in this case. Chromium is dissolved in the form of chromate (Cr
6+
), while the other metals such as iron and nickel are present in the form of hydroxides.
The metal content of the pickling acid is kept constant by continuous extraction of electrolyte from the pickling bath and its substitution with fresh sodium sulphate solution. This causes considerable operating expenses for sodium sulphate and for the disposal of the spent electrolyte solution.
SUMMARY OF THE INVENTION
Briefly stated, the invention in a preferred form is a process for regenerating the electrolyte present in the pickle liquors which reduces both the operating cost and the environmental impact of so-called neolyte pickling plants.
In the process according to the invention, this objective is achieved by adding Na
x
H
y
S
z
O
v
in an acid solution, which causes Cr
6+
to be reduced to Cr
3+
, with the suffixes being x=0 to 2, y=0 to 2, z=1 to 6, v=2 to 6. By adding this substance, Na
2
SO
4
is produced in any of the reactions and can be easily recycled to the process.
An advantageous design of the invention is characterized by the fact that no foreign atoms are introduced to the solution. This avoids formation of other, undesirable reaction products apart from Na
2
SO
4
.
An advancement of the invention is characterized by the fact that the reaction takes place at a pH in the range of approx. 2-3. The decrease of the pH is favorably effected with H
2
SO
4
, which also means that no foreign atoms are introduced to the solution. Having the reduction take place in the acid range speeds up the reaction in an advantageous manner.
An advantageous design of the invention is characterized by the fact that the acid solution produced is neutralized by adding NaOH. This neutralization with NaOH causes Na
2
SO
4
, which can be easily recycled to the process, to be formed also in this step.
An advantageous advancement of the invention is characterized by the fact that the metal hydroxides produced by neutralization are separated from the pickling acid (Na
2
SO
4
) and the solution is recycled to the pickling process. By recycling the pickling acid, the high cost of fresh pickling acid is avoided, and by separation of the metal hydroxides from the pickling acid these can be deposited at a higher concentration, i.e., with a lower content of residual acid.
REFERENCES:
patent: 01-184096 (1989-07-01), None
Petkevich et al. “Removal of chromates” from the wastewaters of electroplating factories. (1987) Abstract only Urbas “Removal of hexa- and trialent chromium from wastewaters” (1979) Abstract only.
Braun Edgar
Starcevic Jovan
Alix Yale & Ristas, LLP
Andritz-Patentverwaltungs-Gesselschaft m.b.H.
Phasge Arun S,.
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