Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to methods for pickling iron or
steel ob~ects, b~fore a subsequent surface treatment or a
mechanical processing.
Before the mechanical processing of ob~ects of iron or
steel, e.g. by drawing, milling or pressing, or before
application of any anti-corrosive film, the surface coating
usually must be removed. This coating can comprise various
oxides, e.g. rust, being formed by chemical corrosion. Other
coatings can consist of scale from a preliminary heat treatment
or rolling skin from a preliminary rolling. These different
coatings are normally removed by pick~ing in acid baths.
Normally, either sulphuric acid or hydrochloric acid is
used for pickling carbon steel or cast iron. The former is less
expensive to buy, but the hydrochloric acid presents several
technical advantages, often ma~ing the total economy for this
acid the most profitable.
A pickling bath intended for pickling with hydrochloric
acid normally comprise about 200 g HCl per liter. During
pickling, normally performed at a temperature of about 20C, iron
is solved as Fe2~. The proportion of iron in the pickling bath
rises gradually until it, after some use, reaches about 80-85 g
per litre pickling solution. The proportion of acid in the
pickling bath is now about 80-100 g per litre pickling solu-tion.
The pickling continues very slowly during these circumstances
making it necessary to exchange the pickling solution for
regenerating or dumping. In total, 7-8 kg of hydrochloric acid
is used per kg of dissolved iron~
The depositing or regenerating of used pickling
solution is very important from various points of view. Partly
because these pickling baths constitute a dangerous waste, which
has to be destructed before deposition. Partly because the
pickling baths has a significant value with respect to the
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content o remaining free acid, solved iron and a corresponding
amount of negative ions.
The hydrochloric solution can be regenerated through a
roasting process within an oil-burning oven. Thereby the solved
iron forms iron oxide and hydrogen chlorine. The latter is
absorbed while the pickling acid is recovered. One disadvantage
with this process is that a certain amount of hydrogen chlorine
is lost because also the remaining free acid goes into the oven.
It is also possible to regenerate the pickling solution
through electrolysis. The iron in the solution will be deposited
at the cathode of the electrolytic cell. The electrolyte (the
pickling solution) must form an almost neutral solution of
ferrous chloride in order to enable the electrolysis. Since the
pickling solution, however, contains a comparatively large
proportion of remaining acid, there will mainly be development of
hydrogen gas at the cathode, i.e. the deposition of metal will be
small in relation to used electric energy.
The solution can be neutralized by evaporation and
crystallization of iron chloride and separation of remaining
surplus acid, which can be returned to the pickling process.
Because of the solubility of iron ~hloride, the
evaporation of the bath must be complete to enable
crys$allization. Furthermore, the subsequent electrolysis will
consume a lot of electric energy, since a great deal of the
chloride is used up by the development of gas, while oxidation of
ferrous chloride to ferric chloride takes place at the anode.
The gradual increase of Fe III changes the polarlty leading to
the forming of basic salts, while the electrolytic efficiency
declines steeply.
From this, an electrolytic cell for recovPry of
pickling acid must have a ion-exchange membranes as a partltion
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be-tween anode and cathode, so that the acid ~ormed at the anode
will not prevent the deposition of metal at -the cathode.
An usefule pickling solution may comprise between 100
200 g free hydrochloric acid per liter solution wi~h between 0-80
g Fe2+ and usually attacks the base metal furiously. When
objects of iron or steel with scales, normally comprising Fe203
FeO-oxides, are pickled, pittings occur. This happens because
the surface of the base metal usually is rather small compared to
the oxide surface, i.e. the cathode, whereln the oxide has the
high electric potential in relation to the iron within sald
solution and is cathodic in relation to the base metal, which
becomes the dissolving electrode, because of its anodic
potential. Therefore, the scales are not dissolved in the acid,
but rather "explodes" away from its surface, because the acid
penetrates below the scale and lifts it away. The consequence is
severe pittings on the base metal, because of the anodic current
density (corrosive current~ is very high. Whils the proportion
of acid in the solution declines, the difference of potential
also declines and thereby the pickling effect of the solution.
The Fe304- oxide is deposited as a sludge on the bottom of the
pickling bath. The proportion of iron rises during conventional
pickling, while the proportion of acid declines. ~t the
beginning of the process the difference of potential between the
iron oxides is at least 1000 mV. The base metal Fe then acts as
anode, meaning that the iron oxides from FeO~ Fe2+. The
surface of the metal is therefore pitted when it is exposed by
fractures and pores in the oxide coating. It is normal to use an
inhibitor or restrainer in the pickling bath to reduce the
pitting damage on the base metal.
The present invention provides a method for pickling
objects of iron and steel, whlch enables a continuous
regenerating of the pickling solution, at a high level of
electrolytic efficiency, wherein the pickling can be performed in
a closed process having as residual products pickled ob;ects of
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iron and steel and pure electrolytic iron corresponding to the
amount of dissolved iron.
The method according to the present invention is
characterized in that the objects are transported into a pickling
solution having a temperature above 25C and con~aining ferrous
chloride and a low proportion of free hydrochloride acid, and
that said solution is continuously regenerated in a circulation
system with electrolytic cells, which are connected to a source
of direct-current, in order to deposite iron electrolytically and
recover free acid.
Preferably, the pickling fluid contains at least 300 g
FeC12 per liter and no more than 50 g of frPe acid reckoned as
HCl per liter.
Preferably the electrolytic cells are electrically
connected in series with the source of direct-current and in
parallel with the flow of pickling solution in said circulatlon
system.
Preferably, the proportion of ferrous chloride sinks no
more than 10 g per liter solution during its passage through any
of the electrolytic cells, and the current intensity at each of
their respective cathode is especially between 0.2-10 A per dm2
of cathodic surface.
A solution for pickling ob~ects of iron or steel,
before a subsequent surface treatment or mechanical processing,
comprising: FeC12, Fe2 and HCl, preferably contains the following
proportions: FeC12 between 250 and 450 g/l, Fe2 between 110 and
200 g/l and HCl between 5 and 50 g/l.
Since the pickling is carried through at a high
proportion of lron and therefore at a low proportion of free acid
and also at rised temperature, the oxides are dissolved
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efficiently, without any pitting of the base metal. This very
favourabl2 pickling effect, in spite of the high proportion of
iron in the solution, is a result of the ability of chloride to
form large complexes. That is, a large proportion of the iron in
the pickliny solution is bound in anion form as:
~FeC14 ) ~
This complex ion is in balance with other ions in the solution
tanks to the rised temperature, which results in an increased
proportion of free hydrogen ions.
Since the ionization energy is comparatively low in a
solution of ferrous chloride with a low proportion of free acid
it is possible to electrolytically deposit the iron with a high
electrolytic efficiency, and cathodically within a simple
electrolytic cell, without any partitioning ion-exchanging
membrane between anode and cathode.
The invention will now be described in further detail,
with reference to the accompanylng drawing, which represents a
schematic elevation of a pickling plant for using the method
according to the invention with continuous regenerating of the
pickling solution in a closed circuit.
The figure shows a pilot plant for pickling of cast
iron and hot rolled objects before hot zinc coating. A tank
containing pickling solution is generally denoted 10. Ob~ects
represented by the arrow 11 are transported down into the
solution after a preliminary degreasing and rinsing. Pickled
obejcts are represented by the arrow 12.
The pickling solution is continuously pumped, by means
of a pump 13 r from the tank 10, into five electrolytic cells 14,
connected in parallel with the flow ~rom the pump 13, and returns
to the tank 10 by means of a return pump 15. The electrolytic
cells are electrically connected in series to the positlve and
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negative terminals 16 and 17, respectively, of a direct-current
source. Each electrolytic cell 14 conventionally comprises one
anode 18 and one cathode 19.
These conditions prevail:
In the Picklinq solution
Volume: 10 m3
Flow of objects: 2 tons/h
Dissolved iron: 10 kg/h
Theoretical amount of acid
used per hour ~ 15 kg HCl at 100 % ~50 kg acid at 30 %
Concentrations: FeC12 340 g/l
Fe 150 g/l
free Hcl 20 g/l
Temperature: 40C
Among others, the following chemical reactions take place in the
20 solution: FeO~ Fe2+, Fe3+ . ~Fe2
In the reqeneratinq section
Dimensions: 1500xlOOOx1200 mm
Electrode system: 5 cells with blpolar electrodes
anode: graphite
cathode: stainless, acid resistant
Voltage drop per cell: 2.5 V
Current density: 2 A/dm2
Static current changer 16 V, 1000 A
Amperage per cell: 900 A
Total quantity of current: 4500 Ah
Theorectical amount of dissolved Fe: 4680 g
Real amount of dissolved Fe: 4300 g
Estimated electrolytic efficiency: 91 %
Amount of recovered acid: 22 kg at 30
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The following reactions take place in the electrolytic cells:
Cathode: Fe2~ ~ FeO, Anode: Fe2+--~3 Fe3+
The cathode plates 19 are made of 0.1 mm thin
stainless, acid resistant sheet. The cathodes are changed
regularily in the cells 14. By bending the sheets, the
electrolytic iron can be removed from said sheets.
The pickling solution circulates through the cell
lo system during the electrolysis at a rate of about 30 liter per
minute. This implies that the difference in concentration of
iron in the solution leaving vs. entering the tank is between
2.0-2.5 g/l. In the present cass, when the dissolving rate or
iron the pickling solution is 10 kg/h, 40% of the acid is
recovered.
In order to get a full recovery of acid, a cell system
with a total current of 10000 A is therefore required. This
means that the static current changer must be rated at, e.g. 16 v
and 200 A or 24 V and 1000 - 1500 A. The latter alternative i5
preferable, since the cost of a static current exchanger malnly
depends on current capacity and not on output or voltage.
The above described method radically alters the
properties of the pickling solution. The iron oxides Fe304
becomes the dissolving electrode and the base metal is not
attacked by pitting. The sludge or iron oxides is completely
dissolved leavlng no remains at the bottom of the tank. At the
same time the ob~ects are cleaned with no trace of dirt film.
The surrounding pickling solution is strongly reducing at the
conditions according to the invention.
The very high electrolytic efficiency during the
process according to the invention results from the relation
between the overpotential of the hydrogen towards the cathode and
the dissolving potential of the iron. The overpotential of
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hydrogen is preferably as high as possible, since the
electrolytic efficiency declines with a rising proportlon of free
acid in the solution.
The regenerating cells can be built as a modular
system, which can be adpated to most existing pickling plants,
while changing from a conventional process to the pickling
process according to the invention.
0 The above mentioned dimensions, rates and values can
obviously be varied within the scope of the following claims.
The method according to the invention can be used in combination
with conventional pickling methods. An actiyator can be added to
the above described pickling solution, comprising a surface
active agent with a high affinity to the base me-tal surface, to
facilitate the penetration of the pickling fluid beneath the
scales. Further, the electrolytic current between terminals 16,
17 can be pulsed at low frequency, givlng a lower electrolytic
polarization and therefore a higher electrolytic efficiency.
The pickling effect of the solution according to the
invention can be even more improved by addition of any of the
salts: magnesium chloride, calcium chloride or aluminium
chloride. This increases the overpotential at any given
proportion of free acid and therefore the electrolytic
efficiency. An additive of 50 g/l MgC12 reduces tensile stress
at the iron deposited at the cathode, so that a ductile film of
iron is formed. The iron can be plated to form any desired
thickness. The growth of about 1 m/min. at a current density of
5 A/dm2 and 90% electrolytic eEficiency. This addition of
magnesium chloride also improves the pickling process.
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