[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.
[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 are 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] Part of the anode is masked out according to claim 1 using adjustable masking means
that are controlled and guided dependent on strip width and/or tin coating thickness
distribution. Preferably the masking means have the features of claim 2. 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.
[0017] In the drawings
Fig. 1 shows a cross section of a conventional tinning cell and various elements used
in such a cell;
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;
Fig. 3 shows a top view of an anode bridge forming part of a conventional tinning
cell;
Fig. 4 schematically indicates the movement of the anode bars along the anode bridge
in a conventional tinning process;
Fig. 5 schematically indicates removing or adding anode bars in a conventional tinning
process;
Fig. 6 schematically indicates placement and appearance of an anode basket for use
in the process according to the invention;
Fig. 7 schematically indicates an anode basket for use in the process according to
the invention in more detail;
Fig. 8 a graph generally indicating i/iavg as a function of D ES;
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
[0018] 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.
[0019] Three different procedures can be distinguished during operation of the soluble anode
system.
Procedure 1 - Anode spacing
[0020] 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.
[0021] 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.
[0022] 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
-1, a strip width of 732 mm and a tin coating thickness of 2.8 g m
-2 on each side of the strip.
- A = 95 mm (at height anode bridge) and 85 mm (at height anode box)
- B = 60 mm (at height anode bridge) and 50 mm (at height anode box)
- C=13 mm
- D = 14 mm (anodes positioned at equidistance)
- E = 76 mm (fixed anode width); 8 anodes in total
- F = 50 mm
- G = mm
[0023] 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.
[0024] Furthermore the anode is closer to the strip at the bottom to compensate for ohmic
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.
[0025] 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.
[0026] 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.
Procedure 2 - Replacing spent anodes
[0027] 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.
[0028] 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).
[0029] 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.
Procedure 3 - Changing to another strip width or to differential coating
[0030] 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.
[0031] In this connection reference is made to Fig. 5 indicating removing or adding anodes
after changing to another strip width or to differential coatings.
[0032] 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).
[0033] If a differential coating is applied of 2.8 / 5.6 g m
-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
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] The invention will now be described in more detail by describing an example according
to the invention.
EXAMPLE ACCORDING TO THE INVENTION
[0039] In the example according to the invention the plating installation parts and the
process fluids and parameters were conventional except where mentioned.
[0040] According to an aspect of the invention instead of individual tin bars, reference
being made to Fig.s 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.
[0041] 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.
[0042] 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.
[0043] 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
avg, wherein i stands for the local current density and i
avg for the average current density (e.g. in A/m
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.
[0044] 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
avg versus D ES for a strip and anode width of 1020 mm, and the upper curve shows i/i
avg after the strip width has changed to 940 leaving the anode configuration configured
for a strip width of 1020 mm.
[0045] 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.
[0046] 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.
[0047] In practice, an optimum tin layer thickness distribution may be found at an overlap
of mask and anode of about 45 mm.
[0048] 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.
1. Verfahren zum Hochgeschwindigkeitselektroverzinnen eines Metallbands bzw. -streifens,
bei dem der Streifen durch anodisches Auflösen von dem Streifen zugewandten Zinnanoden
in einer Elektroplattierungslösung plattiert bzw. beschichtet wird, und Ablagern des
anodisch gelösten Zinns auf wenigstens einem Teil des Streifens, der als Kathode wirkt,
wobei Zinn der Elektroplattierungslösung in Form von Pellets, die in einem Korb gehalten
werden, zugeführt wird, dadurch gekennzeichnet, dass ein Teil der Zinnanoden unter Verwendung von Maskierungsmitteln maskiert bzw. verdeckt
ist, die, abhängig von der Streifenbreite und/oder der Zinnbeschichtungsdickenverteilung,
gesteuert und geführt werden.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass die Maskierungsmittel einen Verschluss oder Blende umfassen.
3. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, dass die Pellets elektrisch über einen Stromabnehmer kontaktiert sind, der aus einem Material
mit einem niedrigen elektrischen Widerstand hergestellt ist, was einen guten elektrischen
Kontakt mit den Zinnpellets zulässt, und der elektrochemisch inert in dem Elektrolyt
ist.
4. Verfahren nach Anspruch 3, dadurch gekennzeichnet, dass der Anodenkorb so gestaltet ist, dass er der Stromabnehmer ist.
5. Verfahren nach einem der Ansprüche 1-4, dadurch gekennzeichnet, dass ein automatisiertes Versorgungssystem vorgesehen ist, um Zinnpellets dem Anodenkorb
hinzuzufügen.