U.S. patent application number 10/584068 was filed with the patent office on 2007-10-04 for metal strip electroplating.
This patent application is currently assigned to CORUS STAAL BV. Invention is credited to Daniel Adriaan De Vreugd, Kokkie Schnetz, Jacques Hubert Olga Joseph Wijenberg, Eric Bob Wijnbeek.
Application Number | 20070227632 10/584068 |
Document ID | / |
Family ID | 34717202 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070227632 |
Kind Code |
A1 |
Schnetz; Kokkie ; et
al. |
October 4, 2007 |
Metal Strip Electroplating
Abstract
Process for high speed metal strip electrotinning wherein the
strip is plated by anodically dissolving tin anodes facing the
strip into an electroplating solution, and depositing said
anodically dissolved tin on at least part of the strip acting as
cathode, wherein that tin is supplied to the electroplating
solution in the form of pellets held in an anode basket.
Inventors: |
Schnetz; Kokkie; (Haarlem,
NL) ; De Vreugd; Daniel Adriaan; (Beverwijk, NL)
; Wijnbeek; Eric Bob; (Heiloo, NL) ; Wijenberg;
Jacques Hubert Olga Joseph; (Amsterdam, NL) |
Correspondence
Address: |
STEVENS DAVIS MILLER & MOSHER, LLP
1615 L STREET, NW
SUITE 850
WASHINGTON
DC
20036
US
|
Assignee: |
CORUS STAAL BV
IJMUIDEN
NL
|
Family ID: |
34717202 |
Appl. No.: |
10/584068 |
Filed: |
December 23, 2004 |
PCT Filed: |
December 23, 2004 |
PCT NO: |
PCT/EP04/14894 |
371 Date: |
March 28, 2007 |
Current U.S.
Class: |
148/518 |
Current CPC
Class: |
C25D 7/0614 20130101;
C25D 17/10 20130101 |
Class at
Publication: |
148/518 |
International
Class: |
C25D 1/04 20060101
C25D001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2003 |
EP |
03078988.7 |
Claims
1. Process for high speed metal strip electrotinning wherein the
strip is plated by anodically dissolving tin anodes facing the
strip into an electroplating solution, and depositing said
anodically dissolved tin on at least part of the strip acting as
cathode, characterised in that tin is supplied to the
electroplating solution in the form of pellets held in an anode
basket.
2. Process according to claim 1 wherein part of the tin anodes is
masked out using adjustable masking means that are controlled and
guided dependent on strip width and/or tin coating thickness
distribution.
3. Process according to claim 1 or 2, characterised in that the
masking means comprise a shutter or blind
4. Process according to any one of the preceding claims,
characterised in that the pellets are electrically contacted via a
current collector made of a material with a low electrical
resistance allowing for good electrical contact with the tin
pellets and being electrochemically inert in the electrolyte.
5. Process according to claim 4, characterised in that the anode
basket is so designed that it is the current collector.
6. Process according to any one of claims 1-5, characterised in
that an automated supply system is provided to add tin pellets to
the anode basket.
Description
[0001] The invention relates to a process for high-speed metal
strip electrotinning wherein the strip is plated by anodically
dissolving tin anodes facing the strip into an electroplating
solution.
[0002] Such a process is known from practice and is described in
detail e.g. in the handbook "The Making, Shaping and Treating of
Steel", 10th ed., pp. 1146-1153, where a description of a typical
commercial tinplating process called FERROSTAN is given which
description is considered to be incorporated herein by
reference.
[0003] As known, see also FIG. 36-5 of said handbook, in the said
known process the anode bars are to be replaced and the anode bar
positions adjusted regularly, which is labour intensive because of
the weight of the anode bars of typically 50 kg, potentially
hazardous in view of fumes, strong acids and high electrical
currents and deteriorates the uniform tin coating thickness over
the strip width.
[0004] When the anode bars are spent to an agreed minimum
thickness, they are removed from the plating section and recycled
in a remelting process for new cast anodes.
[0005] Since optimal placement of the anodes is important for
stable and uniform plating, the anode positions must be adjusted
regularly.
[0006] It is an objective to minimize relatively unhealthy, heavy
and uncomfortable work on parts of and above or near plating units
used in electrolytic tinplating processes.
[0007] Furthermore, it is an objective to provide a highly stable
electroplating process that can be adequately controlled,
minimizing disturbances caused by the supply, (lack of) adjustment
and removal of anode parts.
[0008] At least some of these and other objectives and further
advantages are achieved in a process according to aspects of the
invention as claimed in claims 1 et seq.
[0009] The term "facing the strip" in this connection is intended
to indicate that at least part of the anodic tin "is visible" from
at least part of the strip.
[0010] In a process according to the invention the problem of
having to adjust the anode positions to minimise tin edges when the
strip path and/or the strip width changes may be avoided.
Adjustments can e.g. be suitably made by controlled masking out
part of the anode. In this context masking out is held to mean
positioning an object between anode and cathode so as to impede
plating "in the shadow of the object" if the anode is seen as a
light source.
[0011] In view of the fact that the anode substance, viz. tin is
supplied in pellet form and fed to baskets, tin bars as described
above are no longer used and so there is no need to adjust them
anymore. The need to supply heavy anode bars is eliminated. Instead
anode substance is supplied in the form of easily handled anode
pellets. The invention also avoids removal of spent anode material
since the pellets may be completely consumed.
[0012] It is remarked that for the purpose of this application the
term pellets shall mean rounds, ovoids, briquets, granules and the
like.
[0013] In a preferred embodiment part of the anode is masked out
according to claim 2. Preferably the masking means have the
features of claim 3. Surprisingly by simply masking e.g. edge
portions of the anode by using a mechanical device that acts as a
regulable shutter or blind it turns out to be possible to easily
and optimally control tinplating also at the edge portions of the
strip.
[0014] In an embodiment the pellets are electrically contacted via
a current collector made of a material with a low electrical
resistance allowing for good electrical contact with the tin
pellets and being electrochemically inert in the electrolyte.
Suitable materials for the current collector include Ti and Zr.
[0015] In an aspect an automated supply system is provided to add
tin pellets to the anode basket.
[0016] The invention will now be elucidated using examples in the
form of a description of aspects of the conventional process as a
comparative example and aspects of the invention.
IN THE DRAWINGS
[0017] FIG. 1 shows a cross section of a conventional tinning cell
and various elements used in such a cell;
[0018] FIG. 2 shows an example of a screen shot of process control
apparatus displaying coating thicknesses at different positions
over the strip width in a conventional tinning line;
[0019] FIG. 3 shows a top view of an anode bridge forming part of a
conventional tinning cell;
[0020] FIG. 4 schematically indicates the movement of the anode
bars along the anode bridge in a conventional tinning process;
[0021] FIG. 5 schematically indicates removing or adding anode bars
in a conventional tinning process;
[0022] FIG. 6 schematically indicates placement and appearance of
an anode basket for use in the process according to the
invention;
[0023] FIG. 7 schematically indicates an anode basket for use in
the process according to the invention in more detail;
[0024] FIG. 8 a graph generally indicating i/i.sub.avg as a
function of D ES;
[0025] FIG. 9 schematically indicates a shutter placed as a mask in
front of an anode basket for use in a process according to the
invention.
COMPARATIVE EXAMPLE
Sacrificial Anode System
[0026] A typical soluble anode system is illustrated in FIG. 1. In
FIG. 1 tin is supplied by tin anode 1 which has an anode gap 2 and
an anode notch 3. Each of a series of tin anodes 1 is supported by
an anode bridge 4 at a top portion near its anode notch 3 and at a
bottom portion in anode box 5. Isolated plate 6 separates two
tinning sections in one plating cell. Electrical power is supplied
to the strip via conductor roll 7. Near the bottom of the plating
cell the strip is guided by sink roll 8. Also hold-down roll 9 is
shown. Anode bridge 4 comprises an insulated parking space 10 for a
fresh tin anode 1. The tin anodes 1 are connected to the anode
bridge 4 via contact strip 14.
[0027] Three different procedures can be distinguished during
operation of the soluble anode system.
[0028] Procedure 1--Anode Spacing
[0029] During tinplating the anodes have to be properly positioned
to obtain a uniform tin coating thickness over the strip width. In
FIG. 2 an example is given of values of the tin coating thickness
over the strip width in a situation in which the anodes were not
properly positioned.
[0030] To prevent the situation described above, the anodes have to
be positioned as can be seen in FIG. 3, which gives a top view of
anode bridge.
[0031] Depending on the width of the strip 11, tin coating
thickness and line speed, the optimal anode positions are given by
parameters A-G. In one specific example the optimal parameters are
given for a line speed of 400 m min.sup.-1, a strip width of 732 mm
and a tin coating thickness of 2.8 gm.sup.-2 on each side of the
strip.
[0032] A=95 mm (at height anode bridge) and 85 mm (at height anode
box)
[0033] B=60 mm (at height anode bridge) and 50 mm (at height anode
box)
[0034] C=13 mm
[0035] D=14 mm (anodes positioned at equidistance)
[0036] E=76 mm (fixed anode width); 8 anodes in total
[0037] F=50 mm
[0038] G=15 mm
[0039] Using these settings a uniform tin coating thickness over
the strip width can be realised. Parameter C is of special
importance as this position results in the well-known phenomenon
"tin edge" also known as "dog-bone" effect.
[0040] Furthermore the anode is closer to the strip at the bottom
to compensate for olimic losses in the anode and strip, which would
otherwise cause unwanted differences in current density over the
height of the strip. Therefore parameter A and B are smaller at the
bottom of the anode than at the top.
[0041] In the soluble anode system, anode spacing is a regularly
recurring operation after replacement of spent anodes (see
procedure 2), after a change of strip width, and after a change to
differential coating (see procedure 3). Anodes are manually spaced
by placing an insulated hook into the anode gap.
[0042] At least three important disadvantages of the soluble anode
system can be identified in connection with anode spacing. A first
disadvantage is the occurrence of variations of tin coating
thickness over the strip width, e.g. in the form of tin edges; the
outer anodes may be positioned too close to the strip edge
(parameter C), or the anodes may be a non-equidistanced (parameter
D), or not evenly consumed over the length of the strip caused by
improper anode positioning. A second disadvantage is the labour
intensiveness of adjustment, and a third disadvantage is that
adjustment is hazardous in view of exposure to electrolyte, fumes
and the presence of electrically charged installation parts.
[0043] Procedure 2--Replacing Spent Anodes
[0044] The thickness of the worn anodes is regularly checked with a
thickness gauge. When the anode thickness in the optimal anode
arrangement previously described (see procedure 1) becomes less
than 15 mm, the anode is detached from the anode bridge and placed
on the nearest insulated parking space, see FIG. 4 where the arrows
indicate how the anodes "move" along the anode bridge. On the other
side a new anode is placed on the insulated parking space and
transferred to the anode bridge. After each replacement, anodes
need to be repositioned again (see procedure 1). In FIG. 4 a fresh
tin anode is designated with N and a worn one with W.
[0045] During tinplating the anodes dissolve which results in a
changing anode to strip distance. This causes a non-homogeneous tin
coating thickness distribution over the strip width. In practice
this is compensated by placing the anode bridge and the strip at a
small angle (see procedure 1, parameters A and B).
[0046] The disadvantages of the soluble anode system due to anode
replacement are mainly related to anode spacing (see procedure 1).
An additional disadvantage is that the anodes are not constantly
positioned according to the optimal anode arrangement during anode
replacement. This causes variations in the tin coating thickness
over the strip width.
[0047] Procedure 3--Changing to Another Strip Width or to
Differential Coating
[0048] After changing strip width, parameter C in FIG. 3 no longer
has the optimal value. Furthermore after changing to differential
coating, i.e. a lower coating weight on one side of the strip, tin
edge build-up becomes more severe on the low coating weight side.
In practice both situations are compensated by removing (or adding)
and/or repositioning the anodes on the anode bridge.
[0049] In this connection reference is made to FIG. 5 indicating
removing or adding anodes after changing to another strip width or
to differential coatings.
[0050] If the strip width changes e.g. from 732 mm to 580 mm in the
previously described optimal anode arrangement (see procedure 1)
two anodes have to be detached from the anode bridge (see FIG. 5).
After removal of the anodes, the remaining anodes need to be
repositioned again (see procedure 1).
[0051] If a differential coating is applied of 2.8/5.6 gm.sup.-2 in
the previously described optimal anode arrangement (see procedure
1) one anode has to be added on the anode bridge facing the high
coating weight side of the strip. After adding, the anodes need to
be repositioned again (see procedure 1). At more extreme coating
weight differences the outermost anodes also have to be shifted
more inwards (parameter C in FIG. 3) with respect to the strip
edge.
DISADVANTAGES PRIOR ART AND ADVANTAGES INVENTION
[0052] The disadvantages of the soluble anode system due to
changing to another strip width or to differential coating are
mainly related to anode spacing (see procedure 1). An additional
disadvantage is that the anodes are not positioned according to the
optimal anode arrangement (see procedure 1) during removal or
adding of anodes. This causes variations in the tin coating
thickness over the strip width.
[0053] To overcome the disadvantages of soluble anodes (SA)
mentioned in the comparative example, dimension stable anodes (DSA)
are sometimes used. This system is less labour intensive and
results in less variations of tin coating thickness over the strip
width. The main disadvantage of DSA is that an external dissolution
reactor is required to replenish tin to the electrolyte.
[0054] According to the invention the advantages of an SA and a DSA
system are now combined into a system, which is totally new for
high-speed strip electrotinning, the new system hereinafter
referred to as a DSSA (dimension stable soluble anode) system.
[0055] According to the method of the invention more uniform tin
coatings can be applied, even where it is less labour intensive,
involves less hazards and is lower in costs. The tin stock can be
lower and compared to the DSA system no separate dissolution
reactor is needed. Also less personnel is needed for anode
handling. Also, by using as the anode tin in the form of pellets
held in an anode basket according to the invention, the cell
voltage can be lowered. Probably this is due to the increase of
anodic surface. It will be clear that this also opens up routes to
increased production speeds and thus potentially higher yield for
the electrotinning production line in question.
[0056] The invention will now be described in more detail by
describing an example according to the invention.
EXAMPLE ACCORDING TO THE INVENTION
[0057] In the example according to the invention the plating
installation parts and the process fluids and parameters were
conventional except where mentioned.
[0058] According to an aspect of the invention instead of
individual tin bars, reference being made to FIGS. 1 and 6, anode
baskets 12 were mounted on the anode bar 4 via contact strip 14.
The contact strips 14, made of copper in the experiments according
to this example, may be coated on their surface contacting the
anode basket 12 with a noble metal like Au or Pt. In the embodiment
of the invention the contact strips 14 were coated with Pt, which
worked well.
[0059] The anode baskets 12 in FIG. 6 were filled with tin pellets
(2-20 mm preferably between 5-9 mm in diameter). In order to
replenish anodic substance, tin pellets are supplied regularly,
which can be done while the plating line is fully operational. The
anode baskets 12, in the experiments according to this example made
of titanium, are designed and positioned in such a way that the
anode is closer to the strip at the bottom to compensate for holmic
losses in the anode and strip, which would otherwise cause unwanted
differences in current density over the height of the strip. For
part of the production according to this example, the anode basket
was covered with an anode bag to prevent small tin fines entering
the electrolyte. Under normal operating conditions the anode bags
may need replacement 1-2 times a year. On the other hand, it turned
out that for another part of the production according to this
example where no anode bag was used, this did not pose a problem of
small tin fines entering the electrolyte.
[0060] By providing the DSSA system with an edge mask 13, see FIG.
7, even the build-up of tin (dogbone effect) can be reduced. The
construction of these edge masks and the system to move them are
designed in such a way that they can be operated from a safe
distance from the plating line excluding labour intensive and
possibly dangerous work.
[0061] In a cathode/anode geometry where the strip width is 1020 mm
and the anode width exactly overlaps the strip at also 1020 mm,
when the strip width is subsequently changed from 1020 to 940 mm, a
normalised current density defined as i.sub.avg, wherein i stands
for the local current density and i.sub.avg for the average current
density (e.g. in A/m.sup.2), and therefore the amount of tin
build-up at the edge of the strip reaches an unacceptable level,
see upper curve in FIG. 8.
[0062] In FIG. 8 the horizontal axis shows D ES representing the
distance in mm from the edge of the strip, the lower curve shows
the relation i/i.sub.avg versus D ES for a strip and anode width of
1020 mm, and the upper curve shows i/i.sub.avg after the strip
width has changed to 940 leaving the anode configuration configured
for a strip width of 1020 mm.
[0063] To overcome this problem of tin build-up at the edge of a
smaller width strip, a shutter is placed as a mask in front of the
anode basket. In FIG. 9 a schematic representation of this
situation is given. In FIG. 9 the vertical axis (the Y-axis)
represents a plane through the centre of the strip perpendicular to
the surface of the strip. Y=0 represents a cross section of the
face of the strip, and Y=50 represents a cross section of the face
of the anode and the values on the Y-axis represent the distance
from the cathode abbreviated as D AC. The horizontal axis (the
X-axis) represents the distance from the centre of the strip, D CS.
The grey area at X=(450;700) and Y=(10;15) represents a cross
section of the shutter indicated by M.
[0064] If in FIG. 9 the placement of the shutter is varied from
X=470 mm (corresponding to 0 mm overlap with a strip having a width
of 940 mm) to 440, 425 and 410 mm (corresponding to an overlap with
the strip of 30, 45 and 60 mm respectively) the current density at
the edge of the strip is reduced, see FIG. 10. In FIG. 10 the upper
curve corresponds to an overlap of 0 mm, the next lower curve to 30
mm, the next lower curve to 45 mm and the lower curve to 60 mm.
[0065] In practice, an optimum tin layer thickness distribution may
be found at an overlap of mask and anode of about 45 mm.
[0066] It will be clear that the invention involves a great leap
forward whereby the features and operation of existing
electrotinning lines can be greatly improved by providing a method
that can be easily controlled, is less labour intensive, eliminates
risks and reduces waste (regeneration) flows.
* * * * *