Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
1999-11-30
2001-04-24
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S483000, C205S497000, C205S746000, C205S747000, C204S232000, C204S263000, C204S230200, C204S252000, C204S292000, C204S293000, C204S294000, C204S296000
Reexamination Certificate
active
06221234
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
The present application is the national stage under 35 U.S.C. 371 of PCT/IT98/00038, filed Feb. 25, 1998.
DESCRIPTION
The present invention relates to a method for pickling products made of a metal alloy containing iron and, more specifically, to a pickling process for stainless steels characterized in that it avoids the use of nitric acid as an oxidizing agent and for the recovery of the exhausted solutions deriving from the pickling bath. Object of the present invention is also an apparatus thereof.
The present invention can also be applied for pickling of titanium and alloys thereof, of nickel and alloys thereof, of superstainless steels and for the related recovery of the exhausted solutions from the bath.
It is known that pickling is the process used to remove the layer of oxidation that forms as a result of heat treating of steel, to eliminate the layer depleted in chrome (dechromized layer) below the scale and to allow an efficient final passivation of the surface. In order to achieve an effective pickling process for stainless steels and titanium, a mixture of nitric acid (HNO
3
) and hydrofluoric acid (HF) is normally used, at a temperature that generally varies between 60 and 75° C.
However, the use of nitric acid causes serious environmental problems, which result from the following:
a considerable presence in the vapours over the pickling bath of nitrogen oxides (NO
x
), which are developed by the pickling bath itself;
the formation of exhausted solutions which generate nitrate-rich sludge to be disposed of;
the high cost of disposal for waste products containing nitrates.
To overcome the above difficulties, a number of different methods have been drawn up in which reduction or elimination of the use of nitric acid in chemical pickling processes has been foreseen, and which are based on the use of a number of oxidants, added to the bath as reactive agents, among which, for example, it is possible to include permanganates, persulphates, ferric chloride, hydrogen peroxide (H
2
O
2
), or mixtures thereof. Hydrofluoric acid is always used in the pickling bath, in combination with various mineral acids (generally mixtures of acids), among which: sulphuric acid, hydrochloric acid, phosphoric acid.
The typical concentration of the above mentioned acids and compounds normally used in said pickling methods in absence of nitric acid can be summarized as following (for the stainless steel):
free HF=5~50 g/l;
free H
2
SO
4
=50~200 g/l;
free HCl=0~50 g/l;
Fe
3+
>20 g/l (obtained by adding calculated quantities of hydrogen peroxide);
Fe
tot
(Fe
2+
+Fe
3+
)=80 g/l.
From JP-A-50133125 (see Abstract Nr. 139369y, in CHEMICAL ABSTRACTS, Vol. 84, Nr. 20, May 17, 1976, Columbus, Ohio, US) and from EP-A-585207 is known a process for the elctrolytical recover of pickling solutions for metal surfaces. However in the above references the pickling solution is only in one compartment of the cell. From EP-A-435382 is known a process for the electrolytical recover of pickling solutions for metal surfaces, in which process is provided a step of recovering the pickling oxidizers by sending the pickling solution to the catholyte and then to the anolyte of the electrolytic cell. However, there is no hint of how to recover the acids and the compounds from the pickling bath.
Furthermore, a method for pickling products made of metal alloys containing iron and titanium and alloys thereof is known as an alternative and described in the Italian patent application No. RM96A000849. Said method foresees as oxidizer directly the Fe
3+
ion (in case of alloys of iron) or the Ti
3+
and Ti
4+
ions (in case of titanium alloys). In this case, the required concentration of Fe
3+
is not obtained by adding precise quantities of H
2
O
2
, but by electrochemical oxidation of the ion Fe
2+
to Fe
3+
in an electrolytic cell, using the same pickling bath as electrolyte.
Therefore, the object of the present invention is to provide a method for pickling products of a metal alloy containing iron, and products of titanium and alloys thereof, and products of nickel and alloys thereof in absence of nitric acid as oxidizing agent, the method being characterized in that it provides the recovery of exhausted pickling solutions.
More specifically, the present invention provides a method that foresees the recovery of the total hydrofluoric acid in the exhausted solutions coming from the pickling baths.
Furthermore, the present invention provides a method that foresees the recovery of free sulphuric acid from the exhausted solutions coming from the pickling baths.
Furthermore, the present invention provides a method that foresees the recovery of total hydrochloric acid (and/or the other possible acids) of the exhausted solutions coming from the pickling baths.
Another object of the present invention is to provide a method for the recovery of the Fe
3+
ions coming from the exhausted solutions of the metal products from the pickling baths containing iron and alloys thereof (or Ti
3+
and Ti
4+
for titanium and alloys thereof).
A further object of the present invention is to provide a method that foresees the separation and the possible precipitation of metal ions Fe
2+
, Cr
3+
, Ni
2 +
and Ti
2+
, that are to be disposed of.
According to the present invention, a method for pickling products of a metal alloy containing iron, and of titanium products and alloys thereof, in absence of nitric acid as oxidizing agent is provided, and for the recovery of exhausted pickling solutions, comprising the step of dipping the product to be pickled in an aqueous solution of sulphuric acid, hydrofluoric acid and, optionally, phosphoric and hydrochloric acid and in absence of nitric acid, the oxidizing agent of the pickling solution being the ferric ion, or titanium(III) and titanium(IV) ions,
the method being characterized in that the recovery of the exhausted pickling solutions comprises the following steps:
sending the pickling solution, both as catholyte and as anolyte, in an electrolytic cell optionally of the membrane type in order to:
a) separate the Fe
2+
, or Ti
2+
, ions to be disposed of, from the Fe
3+
(or Ti
3+
and Ti
4+
) ions to be recovered, obtained by reduction at the cathode of the Fe
3+
ions to Fe
2+
, or Ti
3+
and Ti
4+
ions to Ti
2+
, and by oxidation at the anode of Fe
2+
, or Ti
2+
, ions to Fe
3+
or to Ti
3+
and Ti
4+
ions;
b) recover F
−
as HF, complexed with Fe
3+
in the catholyte, by reduction of the Fe
3+
ion to Fe
2+
with consequent dissolution of the complex and release of the F
−
ion;
treating the catholyte coming out of the cell and enriched in Fe
2+
, or Ti
2+
, ions as to allow the separation in two phases, a first phase containing the metal cations Fe
2+
, Cr
3+
, Ni
2+
, or Ti
2+
, to be disposed of and a second liquid phase deprived of said metal cations to be sent in the pickling bath; and
sending the anolyte, coming out of the cell and enriched in Fe
3+
ions or in Ti
3+
and Ti
4+
ions in the pickling bath.
According to an embodiment of the invention, the method foresees that the anodic reaction and the corresponding cathodic one in the cell are potentiostatically or galvanostatically controlled.
In case of alloys containing iron, the electrode potential at the anode is preferably comprised between 771 SHE (corresponding to Erev in the pair Fe
2+
/Fe
3+
) and 1229 mV SHE (corresponding to Erev in the oxygen development reaction) (SHE=standard electrode of hydrogen), in order to avoid development of oxygen.
In case of titanium and alloys thereof, the electrode potential at the anode is preferably comprised between −368 (corresponding to Erev pair Ti
3+
/Ti
2+
) and 1229 mV SHE.
The electrode potential at the cathode is preferably ≧0 mV SHE (to avoid the dev
Fortunati Sandro
Mancia Franco
Acciai Speciali Terni S.p.A.
Browdy and Neimark
Gorgos Kathryn
Keehan Christopher
LandOfFree
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