Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
:. ~.04~55
: This invention relates to an apparatus for the high
current density electrotreating (pickling, cleaning, etc.) of
metal strip and is more particularly related to improvement in
electrode positioning and construction.
Virtually all production of continuous lengths of metal
strip results in undesirable resiclues remaining on the surface of
~ the strip. Such residues may, for example, be oxides which are
; generally removed by pickling in acid, or lubricants which are
r
- rcmoved by cleaning in alkaline detergents. When the end uses of
the strip require a particularly clean surface (eg. when the metal
- is to be coated as in tin-plate) the art often resorts to
electrolytic processes which, in general, provide for more
` effective residue removal. Presently, the electrolytic treatment
of continuously moving metal strip is accomplished by passing the
., .
- r
. ' '` ~
~`
. .
.";'
`; ' '
',;
: . -
'~
~ .
... .
`. ~
. .
. ,.j ' ', .
.. ,~
~. ~
. ! . .
.'.. ' .
,... ~k
t. j
~i','''~
.,, 1 .
., .
'`"~: '
:-', . ': , , ' . ' ; .: ' ' . ' ' .'' ' : ' . ~
104G~SS
:i:
- strip between electrodes having a generally rectangular cross-
- section. Current transfer to the strip is achieved by either of
two basic means:
,:
. (1) use of conductor rolls, etc. for making direct
electrical contact with the strip, or
(2) transfer of current flow from an electrode of one
polarity, through the electrolyte to the strip and again through
the electrolyte to an electrode of opposite polarity. Since the
: ;
danger of sparking at contact points between the strip and rolls
; 10 increases as the current density employed increases, the latter
; method, which is sometimes referred to as bi-polar electrolyzing -
has been found especially beneficial (see, for example, U.S.
Patent 3,338,809) for electrolytic treatments employing very high
. current densities. Although not directed to the use of very high
.. . .
;~ densities, U.S. Patent 2,165,326, shows an electrotreating cell
'i which may be employed for effecting bi-polar electrolyzing.
However, if such a cell is employed for the electrolytic treat-
ment of metal strip at very high current densities, i.e. in excess
o about 500 Amps/ft2. and, ~ore desirably within the range 2000-
~ . .,
10,000 Amps/ft2, it is found that current efficiency is markedly
` decreased as a result of large losses within the electrolyte
itself. Additionally, it has been found, even when employing
~;; bi-polar electrolyzing, that such very high current densities
.~.
- nevertheless often give rise to arcing and other problems
associated with stray currents, such as overheated bearings.
,;..::
Therefore this invention provides an apparatus useful
for the bi-polar electrolyzing of metal strip and which is
- capable of achieving significantly enhanced current efficiencies.
The apparatus utilizes particular electrode
configurations and connections for minimizing the deleterious
:;
`i` effects of stray currents.
~:,
~ -2-
:::
;''','.''; -'i,~ .
,, ~
:: .
.... . ..
. .: . .
~- - .
~ ~09~55
Accordingly, there is provided by means of this invention
an apparatus for the electrolytic treatment of metal strip within
an electrolyte bath, which includes means for guiding the strip
~ .
in a substantially horizontal p:Lane, through the bath. There is
provided a number, n, of electrode pair configurations, n being
at least 2. Each of the electrodes is elongate and extends
` transversely of the strip. The electrodes forming each of the
said pairs are
i) in an electrotreating position,
ii) on opposite sides of the metal strip,
~:; iii) offset from each other so that the distance
.. .
~ between their respective vertical cross-
. . . , , ~
. sectional axes is greater than the sum of
- their effective cross-sectional widths/2, and
iv) the portion of that elongate electrode
facing the top surface of the strip exhibits
,-:~. ................................................................. - .
;~ a convex cross-section.
: . .
A d-c source of EMF supplies a current density within
; the range of 500 to 10,000 Amps/ft2 to the strip and the source
-~ 20 is connected so as to make the electrodes in each pair
, -;
approximately the same potential and of a polarity opposite
to that of the pair configurations adjacent thereto. Separating
each of the electrode pair configurations are insulative baffles
positioned for the unencumbered passage of the strip, whereby
the major portion of the current passing between oppositely
charged electrodes is caused to flow through the strip.
The above and other advantages of the present invention will
be more apparent from the following description when taken in
conjunction with the appended claims and drawing, in which:
,
,;. .,
;~ '
".,~ ~ .
;~ -3-
.:
~,",~
. . . ~,
1~ 5
; The figure is a diagrammatic representation of a
longitudinal cross-section of the apparatus, illustrating the
`~ essential features of this invention.
Referring to the Figure, the apparatus consists of a
tank 2, containing an electrolyte (not shown) suitable for the
~; contemplated electrolytic treatment. The metal strip 3, is
; passed in a substantially horizontal plane, through non-
conductive bumper rolls 4a, into the tank and ~etween electrode
~` pair configurations 5a - 5b, 6a - 6b, and 7a - 7b, respectively,
The electrodes are oriented in a conventional electrotreating
.:
~ position; that is, their longitudinal axes (i) lie in a segment
:..:
of a plane which is parallel to the plane segment formed by
., .
` ~ the strip face and (ii) are generally perpendicular to the
-~ direction of strip travel. On exiting the tank, the strip is
again passed through bumper rolls 4b, placed somewhat closer
to the strip than the electrodes themselves. Such bumper
., .
~ rolls, which also act as guide rolls, serve to prevent
'~,'. '.: , !
undesirable contact of the strip with the electrodes. Their use
is especially desirable in commercial prac*ice, where a high rate
.; i~.
of strip travel (increased vibration) and poor strip shape
i (waviness) would both combine to make such undesirable contact a
virtual certainty. Plastic pipe bumpers 8 may also be placed
within the tank to provide additional insurance against such
undesirable contact. In addition to the use of bumpers,
-` ~ however, it is a
. .
'''.'.~
, . .
:.
'`';
,:,.,~
.:
,..
-., ;
; ....
; -3a-
, ~ ,. . .
,........ . ..
,;:
: :i
~ .::
1044~45S
feature of this invention to preFerably employ an odd number _, of
electrode pair configurations. Three such pair configurations are
illustrated, but it should be understood that 5, 7, etc. such
configurations may be employed, as well. In using such an odd
number of electrode pair configurations, it has been fou~d that the
deleterious effects of stray currents are to a substantial extent
- eliminated, thereby further reducing the tendency to arcing and/or
strip burns, attendant in the use of very high current densities.
In employing an odd number of electrode pair configurations, it is
desirable that the cross-sectional area of the positively charged
-` electrodes be approximately equal to that of the negatively charged
electrodes. This factor is illustrated in the figure, wherein the
- negatively charged electrodes 6a - 6b, are of a greater cross-
section and are therefore each capable of concucting a greater
: :
amount of current than any of the individual positively charged
electrodes.
Three features for improving current efficiency are also
." ,
shown in the figure:
- (a) In contrast with electrodes conventionally employed
20 in treating strip, it is essential that the portion of the
electrodes facing the top surface of the strip exhibit a convex
shape and preferably be of a generally semi-circ~lar shape. For
ease of constructi~on, it will generally be most practical to produce
cylindrical electrodes, i.e. electrodes with a circular cross-
,.
section, as shown in the figure. It should he understood, however,
~ that the portion of the electrodes not facing the strip, ie., the
;~ top portions of electrodes 5a, 6a and 7a may, for example, be flat.
Similarly, the shape of the bottom electrodes is not critical and
. .
.
;~ - 4 -
,,. :
~.
'': ~ ~ '' ' ~ ' ' , . ' ' :
., ~ . '' ~ ;
104f~S5
conventional electrodes of rectarlyular cross-section may be employed.
In actua:L practice a large number of electrodes will be produced at
one time. In order that the electrodes may be interchangeable it
will generally be preferable to employ bottom electrodes 5b, 6b, 7b
which are similarly of a generally cylindrical shape.
The enhanced current efficiency, resulting from the use
of top electrodes of circular cross-section, is showm in Table I.
Ferrous metal strip having an oxide coating thereon, was electro-
pickled in 20~ H2SO4 solution maintained at a temperature of 125E.
- 10 The distance from both the top and bottom electrodes, to the strip,
was 1.5 inches, for all runs. The strip was passed at the same
; speed in all cases to provide an electrolyzing period of three
seconds. The data is an average of four samples, whereby the
reported current efficiencies were determined by measuring the
amount of scale removed (pic~led) from the strip surface. The
;- beneficial effect of round electrodes is readily seen; this effect
tending to increase with an increase in current density. For the
data of this Table 1, the electrodes were offset from each other
as shown in the figure.
~ Table I
High Current Density Pickling Efficiency
- Electrodes of rectangular vs. circular cross-section
., ,
- Pickling ~fficiency-~
Current 2 Rectangular Circular Efficiency
Density-Amps/ft Electrode Electrode Increase-~
6,666 52 59 13.5
8,333 64 77 20.3
10,000 77 93 20.8
. .
(b) It is seen, with respect to any electrode pair
configuration; for example 5a-5b, that contrary to conventional
.:.
_ 5 _
,,; .
: ,
, ., . . , , . .
'.- ,'.: . ~: ',
::. ., . -
.;:~ . .~ . :
:: . , ;
` 1~46~S
placements, top electrode 5a does not overlie electrode 5b. It has
been found that additional enhancement of efficiency may be achieved
if the electrodes forming a pair configuxation do not so overlie one
`- another. Stated another way, if we imagine a vertical axis drawn
through both the top and bottom electrodes, then the distance
...... .
between these two axes should be greater than the sum of the cross-
;.
sectional widths of both eelctrodes, divided by 2. In the specific
case where round electrodes are employed, then the distance between
- the vertical axes should be greater than: the sum of the diameters
of both electrodes divided by 2 (i.e. greater than the sum of the
- two radii). The beneficial effect of such electrode offset is shown
: ~ .
; in Table II. Unless otherwise stated, electrolyzing conditions were
the same as above. However, utilizing the knowledge gleaned from
the data of Table I, only round electrodes were evaluated in the
- 15 runs below.
:
Table II
High Current Density Picklinq Efficiency
; ~ -
~- Overlying Electrodes vs. Offset Electrodes
Pickliny Efficiency-%
~ 20 Current OverlyingOffset Efficiency
.`- Density-Amps/ft2 ElectrodesElectrodesIncrease-%
.: ;.
5,000 31 35 12.9
6,666 46 58 26.1
8,333 66 78 18.2
~ (~) Separating the respective electrode pair configura-
-~` tions are insulative baffles 9 for directing the current through
~ 25
i; the strip. These baffles, which may, for example, be made of
~ polypropylene, are positioned a short distance Erom the strip sur-
; face, but nevertheless sufficient ~o that the passage of the strip
therethrough is not encumbered. Since a significant portion of the
,.... . .
r. ' ~ 6 -
~, . .
.~. .",. ,
. i,.~ , .
~, ` ' '~ ' ' : -
:"`~'': :. . ' , :
.... . . . . .
~: .. . . . . . .
... ",. ~ .
~04G~S5
current would tend to travel in a straight line between the
electrodes (eg. a line between 5a and 6a) it may be seen that the
baffles serve to block that route, and force that otherwifie lost
current through the strip. It is therefore desirable that the
.,
;~,J'.' 5 baffles be sufficiently close to the strip 50 as to cause a
substantial portion of the current which would norm~lly travel in
such straight lines, to be diverted therefrom and travel through
the strip tsee the dashed lines of thè Figure).
~,
In addition to the above noted features for improving
current efficiency, it is nevertheless desirable that the effects
; of concentration polarization be reduced by employing any of the
.
well known techniques for effecting stirring of the solution? For
example, simple mechanical or propellor-type stirrers may be emplo-
yed. Particularly good results have been achieved through the use
~ 15 of flow headers 10 (driven by a circulation pump not shown) which
- force a bw pressure jet of electrolyte toward the strip face at a
small angle, e.g. 30. Even when current densities within the
range 2000 - 10,000 Amps/ft2 are employed, it has been found that
stirring of the electrolyte need not be particularly turbulent,
. . .
especially if the flow headers are strategically positioned, as
shown, near those portions of the strip surfaces undergoing maximum
: -
electrochemical activity.
s .
;.
.~ .
.'.~
. ~
.. ..
. . .
,~ :
, .
., .
:
., , ' '~,
:.
. . . . . .
. ~ .:
.. . . . .
:~ . . . .
:, . . : : . : .
, . , : : , .. .
