Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process and device for cleaning galvanic baths to plate metals
Nature of the invention
The invention concerns a process and a device for cleaning galvanic baths to
plate
metals, in particular alkaline zinc-nickel alloy baths, using ion exchangers
in order to
prolong the lifetime of electrolytes and remove any undesirable decomposition
products.
Background of the invention
Zinc-nickel coatings are used in all applications that require high quality
surface
protection when subject to corrosion. The conventional field of application is
the
automobile manufacture for components that are used in the engine bay, on
braking
systems and in the landing gear bay. For this reason, alkaline zinc-nickel
electrolytes
have been used more recently as published in US 4,889,602, which for example
have the following electrolyte composition:
Table 1: Electrolyte deposit of a zinc-nickel electrolyte
Zinc oxide ZnO 11.3 g/l
Nickel sulphate hexahydrate NiSO4*6H20 4.1 g/l
Sodium hydroxide NaOH 120 g/l
Polyethyleneamine (complexing agent) eg. (C2H5N)n 5.1 g/l
The amines in the electrolyte act as complexing agents for the nickel ions.
Complexing agents are constituents of numerous galvanic and chemical processes
which are used in the separation of metals. The zinc-nickel electrolyte is
usually
driven by insoluble nickel anodes. The zinc content is kept constant by adding
a
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suitable zinc ion source and the nickel content is kept constant by adding a
source of
nickel ions. The, colour of the zinc-nickel electrolyte however changes from
blue-
purple to brown after a certain time of operation.
After a certain time of operation, nitrites (so-called organically bonded
cyanide which
can contain nitriles as well as isonitriles) and cyanide ions are formed in
the zinc-
nickel electrolytes through anodic oxidation from the amine-containing
complexing
agents. The problem of cyanide pollution requires the continuous replacement
of the
electrolytes and a special waste water treatment which in turn significantly
affects the
operating costs of the electrolyte. After several days, or weeks, there is a
noticeable
increase in the discolouration and a separation into two phases. The top phase
is
dark brown. This phase causes considerable problems when the work pieces are
coated, for instance the uneven distribution of the coating thickness or
blistering. The
continuous removal or skimming of this second brown phase is therefore
absolutely
essential. This operation requires a considerable amount of time and money.
The
formation of the second phase is traced back to the concept that the amines in
an
alkaline solution on the nickel anodes are transformed to nitriles
(organically bonded
cyanides). This however means that because of the decomposition of the amines,
new complexing agents have to keep being added which in turn increases the
process costs.
Several processes are described in the prior art to reduce the concentration
of
cyanides.
The activated carbon cleaning process is a common process that is used in
electroplating to remove organic impurities in nickel electrolytes. The
quantities of
activated carbon used are determined in preliminary tests. The quantities most
frequently used for activated carbon cleaning are 2 - 5 g/l. The activated
carbon is
added at a temperature of between 50 - 60 C. Once added, the electrolyte is
stirred
intensively. After approximately half an hour, the absorbable substances are
absorbed by the activated carbon and are filtered out. The disadvantage of
this
process, however, is that all organic constituents are thereby removed from
the
electrolytes. For zinc-nickel electrolytes this would mean that not only the
decomposition products, but also all other organic constituents such as for
example
brighteners and complexing agents, are removed.
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The publication EP 1 344 850 Al features a device to reduce the build-up of
cyanide
by separating the anode from the alkaline electrolyte using an ion exchanger
membrane. This separation prevents a reaction of the amines on the nickel
anodes
and therefore also any undesirable side-reactions. The occurring side-
reactions,
problems of disposal, formation of a second phase and the adverse impact on
the
quality of the plated zinc-nickel layer, are thereby also avoided. It is
therefore no
longer necessary to replace the bath and spend lots of time and money on
skimming
the second phase which has formed. The zinc-nickel electrolyte acts as a
catholyte.
The medium in the anode compartment which is separated using the
aforementioned
ion exchanger membrane, is known as the anolyte whereby in this case either
sulphuric acid or phosphoric acid can be used. The disadvantage of this
process is
the use of a costly and high-maintenance ion exchanger membrane, which can
also
not be used for all common metallization baths.
The publication EP 0 601 504 131 describes the cleaning of galvanic baths for
the
separation of metals using polymer absorber resins. Similar to the activated
carbon
treatment, the disadvantage is that not only the decomposition products, but
also all
other organic constituents such as for example brighteners and complexing
agents,
are removed.
Description of the drawings:
Figure 1: Ion exchanger regeneration unit
Figure 2: Hull cell set-up
Figure 3: Procedure and regeneration effect using a combination of an ion
exchanger and the freezing out of sodium carbonate
Figure 4: Comparison of the layer thickness distribution of different zinc-
nickel
electrolytes
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Description of the invention
The aim of this invention is to selectively remove the cyanide and nitriles
that have
formed during the metallization process, from the electrolytes. Surprisingly,
by using
ion exchange resins which are able to bind cyanide ions, it was possible to
remove
not only the cyanide ions but also the nitriles from the bath. The use of ion
exchange
resins for this specific purpose is unknown in prior art.
In alkaline zinc-nickel electrolytes with amine-containing complexing agents
(eg.
polyethyleneamine), a nitrile compound is formed during the operation. The
disadvantage of the decomposition product is that as the lifetime of the
electrolyte is
extended or as the decomposition product increases, an oily and waxy second
phase
is formed. The formation of the decomposition product is responsible for the
loss of
expensive complexing agents and the formation of highly toxic cyanide. From
the
amine-containing complexing agents, nitriles (R-CN, this always includes
isonitriles,
R-NC) are formed, initially in the oxidative reaction at the anode, which then
react
further to form cyanide ions (CN-).
These problems lead to reduced efficiency and loss of quality of the plated
layer.
Here the efficiency is the percentage part of the total current introduced to
plate a
defined amount of metal. To counteract the reduced efficiency, the current
density is
usually increased, which however in turn accelerates the decomposition rate of
the
complexing agent to the nitrile (R-CN) and cyanide. Tests have shown that the
second phase contains large quantities of cyanide, metal and sodium carbonate
(Na2CO3). It can therefore be assumed that these decomposition products are
influenced by the nitrile or that they exist together as the concentration
continues to
increase and form a second phase. From a procedural point of view, it is
difficult to
separate the second phase since the liquid in the bath is constantly moving.
Furthermore this also means a constant loss of complexed metal ions and other
precious additives which are also in this phase. It is therefore an object of
the present
invention to selectively remove the cyanide and organically bonded cyanide
(nitrile)
from the electrolyte.
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The reduction in organically bonded cyanide would be noticeable in the change
in
content of the total organic carbon found in the process solution. This could,
however, also mean that other vital organic constituents, such as brighteners
or
organic complexing agents are lost in the electrolyte. If it was the case that
brighteners are removed from the electrolyte, this would considerably affect
the
optical quality of the plated layer. A reduction in the content of cyanide and
nitrite
compounds would subsequently increase the efficiency.
According to the present invention, the cyanide and organically bonded cyanide
is to
be removed using an ion exchange resin.
Ion exchange resins are used to remove toxic substances or interfering anions
or
cations from waste water. The advantage of this process is that it does not
require a
precipitation or chemical destruction since the interfering substances can be
removed
from the waste water without being changed. Ion exchange resins are high-
molecular
organic substances. The rigid and insoluble frame has easily interchangeable
counterions on it. These are easily movable and interchangeable counterions,
usually
hydrogen ions or hydroxyl ions. The regeneration of galvanic process baths is
therefore a suitable process to extend the lifetime of electrolytes by
removing
interfering cations or anions. The batch operation is a process for the ion
exchange.
The ion exchanger resins come into contact with the electrolyte solution in a
receptacle. The process is finished as soon as there is an exchange
equilibrium
between the counterions from the exchanger and similarly charged ions from the
electrolyte solution. If additional ions have to be removed from the
electrolyte using
the ion exchanger resins, then new resins have to be added. The resins are
filtered
out once the equilibrium is established.
The column process is the process most commonly used in the laboratory. Here,
the
ion exchanger resin is packed into a column. All necessary operations are then
performed in the pack which has been created. Two different work techniques
are
distinguished, namely working with a decreasing and increasing liquid layer.
With the
decreasing liquid layer, the electrolyte flows through the column from the top
down
and with the increasing liquid layer from the bottom up. Filling the column is
a
straightforward operation. The resin in its current form is first of all
transferred to a
beaker containing distilled water'to swell the resins. This operation is
necessary to
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prevent the column from shattering and to avoid the column from being to
densely
packed as the resins.swell. Two hours is usually sufficient for the resins. to
swell. The
resin is then sludged in the column whilst making sure that the resin which is
already
layered, is covered with water at all times. This is necessary in order to
prevent any
effects from air bubbles. Any excess water is constantly removed from the
column.
Finally, once the resin has been filled, a piece of cotton wadding is placed
across the
top of the pack. The following sub-processes should be carried out during the
operating cycle of a ion exchanger column:
1. Load (ion exchange)
2. Wash exchanger pack
3. Regenerate
4. Wash exchanger pack
Washing between operations is necessary to remove any residues of reagents in
the
ion exchanger column. During the regeneration process, the exchanger pack is
transformed to its original state (non-loaded state). If the ion that was
exchanged
during the ion exchange is to be recovered again, it is removed by the ion
exchanger
by eluting with a suitable liquid. According to the invention, the process
solution flows
through the ion exchanger resins, whereby the cyanides are taken up on the
anchor
groups through interactions and the hydroxide anions are released on the
electrolytes. Surprisingly, nitrile compounds can also be removed in this way.
Each ion exchanger resin that is capable of binding cyanide ions, can be used
within
the framework of the present invention. Suitable ion exchange resins to bind
cyanide
ions are for example described in Ludwig Hartinger: Handbuch der Abwasser- and
Recyclingtechnik, 2nd ed. 1991 on pages 352 - 361, which is incorporated
herein by
reference. According to paragraph 5.2.3.3.4 and Table 5-1 anions like cyanide
can be
exchanged utilizing strongly alkaline anion exchange resins. Such resins
comprise
resins made from polyacrylamide possessing quarternary ammonium groups. Such
resin material is commercially available and for example described in Table 13
(page
89) of: Robert Kunin, Ion Exchange Resins, reprint 1985, which is herein
incorporated by reference. Quarternary strong base resins suitable comprise
Amberlite IRA-400 (Rohm & Haas Co.), Amberlite IRA-401 (Rohm & Haas Co.),
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Amberlite IRA-410 (Rohm & Haas Co.), Dowex 1 (Nalcite SBR) (Dow Chemical Co.),
Dowex 2 (Nalcite SAR) (Dow Chemical Co.).
All such resins are also capable of binding nitriles.
By way of example, tests were carried out using the ion exchanger resins
Lewatit
MonoPlus M600 and MonoPlus M500 produced by Lanxess Deutschland GmbH.
These resins are extremely alkaline anion exchangers which as a functional
group,
have quaternary amines. The matrix is a cross-linked polystyrene. The bulk
density is
680 g/l, the effective grain size is 0.62 mm.
Figure 1 shows the column process with an increased liquid layer according to
one
embodiment of the present invention. In the bottom part of the column (4) is a
glass,
ceramic or plastic frit, or a spray register or spray pole or sieve (6)
through which the
process solution can flow evenly through the ion exchanger resin (5). The ion
exchanger resin (5) is embedded in the column. At the top end of the column,
there is
a glass, ceramic or plastic frit or a sieve (3). This is to prevent the resins
from moving
upwards and to ensure that only the process solution gets through. In the
collection
receptacle (1) which is used for the galvanic bath for the separation of
metals, is the
contaminated process solution which is conveyed through the column using a
hose
pump (2). Once the process solution has passed through the column, it is
collected in
a receptacle (7) which can be identical to receptacle (1). The device used for
the
metallization process comprises, as shown in figures 1 and 3, a receptacle (1)
to take
a zinc or zinc alloy bath, a connected pump system (2), which is connected to
the ion
exchanger device (4) to take the zinc or zinc alloy bath, which contains ion
exchanger
resin (5) and a collection device (7) for the zinc or zinc alloy bath passing
through the
ion exchanger resin (5), which can be identical to receptacle (1).
The ion exchanger resin (5) in the ion exchanger device (4) can be on a spray
register, spray pole or sieve.
The receptacle (1) is generally equivalent to the galvanic zinc or zinc alloy
bath and
consists of at least an anode, a cathode (the substrate to be coated) and a
voltage
source.
In addition, there can also be - as shown in Figure 3 - a freezing device (8)
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between the receptacle (1) and the ion exchanger device (4) to cool the
solution and
separate a sodium carbonate solid. The freezing device (8) includes a cooling
unit (9)
to cool the solution to a temperature that is preferably below 10 C, more
preferably
between 2 - 5 C and an outlet (10) to separate the crystallised sodium
carbonate.
There can also be a receptacle (11) between the freezing device (8) and the
ion
exchanger (4) to take the zinc or zinc alloy bath that has been cleaned from
carbonates.
It was not possible to regenerate the resins Lewatit MonoPlus M600 and M500
using
a sodium hydroxide solution. A stronger anion is needed to exchange the bonded
cyanide anions. According to the invention, the proposal is to transform the
ion
exchanger resins back to their chloride form. Strong acids such as for example
hydrochloric acid (HCI) cannot be used as this would immediately form toxic
hydrogen cyanide. During the regeneration test of the resins, sodium chloride
was
used to separate the cyanide from the resins and transform the resins back to
the
chloride form. The regeneration solution with sodium chloride was moved into
the
very alkaline range (pH value > 10) with a 0.5 % by weight sodium hydroxide,
since
cyanides can quickly decompose below this pH value and form toxic hydrogen
cyanide. The regeneration tests were examined using three different
concentrations
of sodium chloride (6, 12 and 18 % by weight, Tables 2 - 7). The regeneration
operation was realised at a linear speed of 5 m/h. One litre of sodium
chloride
solution was used for the regeneration and conveyed through the ion exchanger
pack. Four portions of sample fractions having a volume of 250 ml each were
taken
and the content of different electrolyte parameters was analysed, compared and
assessed. Data from the analysis was used to calculate the amount of cyanide
which
had bonded to the resin and was able to be eluted through the regeneration
process.
Referring to Table 2: a total volume of 1 1 of regeneration solution with 6 %
in weight
of NaCl was used to elute the cyanide (including organic nitrile) from the
column
containing Lewatit MonoPlus M600. Sample 1 is an analysis of the first 250 ml
of
regeneration solution used to elute the cyanide from the column, Sample 2 the
second portion of 250 ml, Sample 3 the third portion of 250 ml and Sample 4
the
fourth portion of 250 ml, giving 1 1 of total eluent. The amount of total
cyanide in 1 1
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eluent is 1.525 mg. The same was performed for the other regeneration cycles
according to Tables 3 - 7.
The results show that it is beneficial to use solutions with a high chloride
ion content
to regenerate the columns.
Table 2: Lewatit MonoPlus M600 - Determination of the eluted quantity of
cyanide,
regeneration with 6 % in weight of NaCl solution
Lewatit Cyanide Quantity of cyanide in
MonoPlus M600 concentration 250 ml sample volume
Regeneration [mg/I] [mg/sample volume]
Sample 1 1.8 0.450
Sample 2 1.5 0.375
Sample 3 1.5 0.375
Sample 4 1.3 0.325
From 100 ml resin, eluted quantity of cyanide -> 1.525
Table 3: Lewatit MonoPlus M500 - Determination of the eluted quantity of
cyanide,
regeneration with 6 % in weight of NaCl solution
Lewatit Cyanide Quantity of cyanide in
MonoPlus M500 concentration 250 ml sample volume
Regeneration [mg/I] [mg/sample volume]
Sample 1 3.6 0.900
Sample 2 3.7 0.925
Sample 3 2.9 0.725
Sample 4 2.2 0.550
From 100 ml resin, eluted quantity of cyanide -> 3.100
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Table 4: Lewatit MonoPlus M600 - Determination of the eluted quantity of
cyanide,
regeneration with 12 % in. weight of NaCI solution
Lewatit Cyanide Quantity of cyanide
MonoPlus M600 concentration in 250 ml sample
Regeneration volume
[mg/I] [mg/sample volume]
Sample 1 7 1.750
Sample 2 6.5 1.625
Sample 3 6.5 1.625
Sample 4 6.5 1.625
From 100 ml resin, eluted quantity of cyanide -> 6.626
Table 5: Lewatit MonoPlus M500 - Determination of the eluted quantity of
cyanide,
regeneration with 12 % in weight of NaCl solution
Lewatit Cyanide Quantity of cyanide in
MonoPlus M500 concentration 250 ml sample
Regeneration volume
[mg/I] [mg/sample volume]
Sample 1 18.0 4.500
Sample 2 8.5 2.125
Sample 3 8.5 2.125
Sample 4 6.2 1.550
From 100 ml resin, eluted quantity of cyanide -> 10.300
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Table 6: Lewatit MonoPlus M600 - Determination of the eluted quantity of
cyanide,
regeneration with 18 % in weight of NaCI solution
Lewatit Cyanide Quantity of cyanide
MonoPlus M600 concentration in 250 ml sample
Regeneration volume
[mg/I] [mg/sample volume]
Sample 1 35 8.75
Sample 2 41 10.25
Sample 3 44 11.00
Sample 4 45 11.25
From 100 ml resin, eluted quantity of cyanide -> 41.25
Table 7: Lewatit MonoPlus M500 - Determination of the eluted quantity of
cyanide,
regeneration with 18 % in weight of NaCl solution
Lewatit Cyanide Quantity of cyanide
MonoPlus M 500 concentration in 250 ml sample
Regeneration volume
[mg/I] [mg/sample volume]
Sample 1 17 4.25
Sample 2 18 4.50
Sample 3 17 4.25
Sample 4 17 4.25
From 100 ml resin, eluted quantity of cyanide -> 17.25
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Tests where the temperature was increased (flow temperature in the beaker 55
C,
average temperature in the ion exchanger column 35 C) have shown that
achieving
the correct temperature significantly reduced the regeneration medium
requirement.
The solution used had a sodium chloride ion concentration of 18 % in weight.
Table 7b): Lewatit MonoPlus M600 - Determination of the eluted quantity of
cyanide,
regeneration with 18 % in weight of NaCI solution
Volume Temperature Concentration Elution capacity Volume
flow rate (NaCl + NaOH) regeneration
solution
[BV/h] [ C] [%] [mg cyanide / per litre [BV]
regeneration solution]
33 RT 18+5 42 238
2.5 RT 18+5 119 84
2.5 55 C first runnings, 18+5 310 33
35 C in the column
2.5 55 C first runnings, 18+5 312 32
35 C in the column
2.5 55 C first runnings, 18+5 286 35
35 C in the column
[BV/h] = bed volume per hour
* [BV] = bed volume
The aged electrolyte which is to be regenerated, should if possible be as
close as
possible to the original state (new batch). New batches of alkaline zinc-
nickel
electrolytes usually have an efficiency of 70 % for a current density of 1
A/dm2. In
electroplating, in order to assess the regeneration effect, the Hull cell test
can be
used and there is the option to determine the efficiency of the electrolyte
using
Faraday's law. Based on the layer thickness distribution of the electrolyte,
it is
possible to assess how good the regeneration effect is using an ion exchange
resin.
The Hull cell is used to determine the effects of the bath parameters (eg.
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temperature, pH value, electrolyte composition, lack of or surplus of
additives,
cleanliness, impurities from foreign metals) on the property of the plated.
layer
depending on the current density.
Since in a Hull cell the cathode is diagonal to the anode (see Figure 2),
there is a
distribution of current densities on the cathode. This makes it possible to
examine the
effect of the current density in a single experiment. Understandably the
current
density is higher at the edge nearest the anode than at the edge away from the
anode (Figure 2). The quality of coated surfaces, ie. the composition,
thickness,
evenness and other properties, therefore primarily depend on the composition
of the
electrolyte and the plating conditions. The key quality factors are the
composition of
the electrolyte and the current parameters which must be monitored to assure a
high
quality coating. The composition of the electrolyte plays a significant role
in this
instance. Each individual additive in the electrolyte influences the
properties of the
electrolyte and the plated layer. In order to obtain the desired layer
quality, the
concentration of the electrolyte constituents must be within certain limits.
The majority
of electrolytes contain, in addition to the inorganic constituents, additional
organic-
type additives. These organic constituents are designed to influence the
properties of
the layer that is to be plated. This includes for instance brightening,
levelling,
hardness, ductility and throwing power ability. The Hull cell test was carried
out to
examine the appearance of the plated layer and the zinc-nickel composition.
Tests
were carried out with the Hull cell on a new, on an aged and on an electrolyte
that
had been regenerated using ion exchange resins. This test is designed to give
an
indication as to how effective it is to come close to the original state (new
batch). The
Hull cell can be used to establish how losses during the ion exchange process
affect
the plating rate. The additives however only work effectively if they are used
in a
certain concentration and composition.
Qualitatively, by visually assessing the brightness of the coated plates, it
can be said
that the reduction in the TOC (Total Organic Carbon) content, as shown in
Tables 8
and 9, is due to the reduction of the nitrite concentration and that of the
amine-
containing complexing agents.
The ion exchange process can preferably be carried out in conjunction with the
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freezing out of sodium carbonate to further increase the efficiency of the
process and
match the plating performance of a non-aged electrolyte. The electrolyte
solution can
be conveyed through a cooling device either before or after treatment in the
ion
exchange resin column (see Figure 3). During cooling, a sodium carbonate phase
which can be separated, is formed. The old electrolyte is preferably treated
in the
freezer unit first and then in the ion exchange resin unit.
Operational examples
Tests were carried out on an electrolyte to plate zinc-nickel alloys in
accordance with
Table 1.
For this test, 100 ml of resin was swelled in fully desalinated water for two
hours and
then sludged into the column. Prior to the loading process, the Lewatit
MonoPlus
M600 was regenerated using a 2 % by weight sodium hydroxide solution to
transform
the resins to the OH" form. For the Lewatit MonoPlus M500, this was done using
a
3 % by weight sodium hydroxide solution. The loading process is realised
according
to the values indicated by the manufacturer. In practice, it is customary to
indicate the
loading process in bed volume per hour (BV/h). This value in turn refers to
the
embedded quantity of resin which is embedded in the column. Loading generally
takes place at 10 BV/h. Based on our quantity of resin used (100 ml), the
volume flow
rate is 1000 ml/h. This represents a rate of 1.51 m/h and is within the value
range
specified by the manufacturer. Before the tests were carried out, a reference
sample
was taken from the zinc-nickel electrolyte which was to be regenerated (Sample
0 in
the tables corresponds to an aged electrolyte). In the preliminary test which
was
carried out to examine the selectivity of the ion exchanger resins, 1000 ml of
alkaline
zinc-nickel electrolytes were conveyed through the ion exchanger column, where
250 ml of sample fractions were taken every fifteen minutes (Samples 1 - 4 in
Tables
8 and 9).
The content of the different constituents was then examined in the sample
fractions
and compared with one another. The metal content, sodium hydroxide content,
sodium carbonate content, sodium sulphate content, content of the complexing
agents, TOC content and the total cyanide content of the samples was examined.
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Tables 8 and 9 show the test results.
Table 8: Test results of the loading process for Lewatit MonoPlus M600 (aged
electrolyte)
Sample 0 Sample 1 Sample 2 Sample 3 Sample 4
Lewatit MonoPlus M600 Time Time Time Time Time
[min] [min] [min] [min] [min]
0 0-15 15-30 30-45 45-60
Zinc Zn [g/1] 12.4 11.9 12.0 12.1 12.0
Nickel Ni [g/1] 1.5 1.5 1.5 1.5 1.5
Sodium NaOH [g/I] 94.5 78.9 94.7 95.5 94.1
hydroxide
Sodium Na2CO3 [9/1] 56.6 58.8 59.2 57.8 58.3
carbonate
Sodium sulphate Na2SO4 [g/1] 4.50 4.73 4.77 4.51 4.70
Complexing - [MI/1] 140 132 135 138 135
agent
Total cyanide CN- [mg/I] 92.0 5.2 5.1 5.2 4.8
TOC - [g/1] 45.8 40.0 44.2 44.0 44.0
Table 9: Test results of the loading process for Lewatit MonoPlus M500
Sample 0 Sample 1 Sample 2 Sample 3 Sample 4
Lewatit MonoPlus M500 Time Time Time Time Time
[min] [min] [min] [min] [min]
0 0-15 15 - 30 30 - 45 45 - 60
Zinc Zn [g/I1 11.5 7.7 11.4 11.7 11.5
Nickel Ni [g/I] 1.4 1.4 1.4 1.5 1.4
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Sodium NaOH [g/1] 147 140 148 146 149
hydroxide
Sodium Na2CO3 [9/1] 72.1 60.4 71.6 71.2 72.8
carbonate
Sodium sulphate Na2SO4 [g/1] 7.69 5.48 7.59 7.52 8.19
Complexing - [mI/I] 137 97 128 132 131
agent
Total cyanide CN" [mg/I] 75.0 17.0 7.5 7.6 7.7
TOC - [g/1] 42.9 25.9 40.8 41.6 42.2
The values in Tables 8 and 9 show that the metal content concentrations are
virtually
constant and do not change significantly. The nickel concentration remains
unchanged and does not fluctuate. The sodium hydroxide concentration slightly
drops at first. The reason for this is that the resins could not be fully
transformed to
the OH" form during the regeneration process. The resins therefore were still
able to
absorb the hydroxide ions. The sodium hydroxide concentration, however, takes
on
the same order of magnitude again as that of Sample 0 and remains virtually
the
same. The content of sodium carbonate and sodium sulphate remain virtually
unchanged.
There is a clear reduction in cyanide and a lower yet significant reduction in
the TOC.
The test shows that the resin Lewatit MonoPlus M600 retains the interfering
cyanide
from the process solution. The test also shows that the resin's absorption
capacity
has by no means been reached and that the cyanide content dropped even after
60 minutes. In comparison to the Lewatit MonoPlus M500 it is clear that the
cyanide
concentration is initially accompanied by a reduction in the concentration of
zinc,
sodium hydroxide, sodium carbonate, sodium sulphate and the complexing agent
in
the first fraction (Sample 1). The nickel concentration remains virtually
constant
throughout the whole test period.
The Hull cell tests were carried out to examine the appearance of the plated
layer
and the zinc-nickel composition. Tests were carried out with the Hull cell on
a new,
on an aged and on an electrolyte that had been regenerated using ion
exchangers.
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This test is designed to give an indication as to how important it is to come
close to
the original state (non aged electrolyte). The Hull cell can be used to
establish how
losses during the ion exchange process effect the plating rate.
The Hull cell was filled with 250 ml of electrolyte as per Table 1. A nickel
anode was
used as the anode. Once the Hull cell plate had been cleaned, a 1-ampere
current
was applied. The coating time was fifteen minutes.
The low current density range (see Fig. 2) shows an even and bright plating
result.
The electrolyte which was treated using the ion exchanger resin Lewatit
MonoPlus
M600, revealed an even and bright surface across the whole current density
spectrum. The assessment of the surface should be classed as bright. It can
therefore be confirmed that the ion exchange process which is used to remove
cyanide from the alkaline zinc-nickel electrolyte, significantly improves the
appearance of the plated layer. More importantly however is the finding that
the
appearance was not in any way worsened, which indicated that no organic
additives,
which are responsible for the appearance of the depsit, were removed from the
plating bath by the ion exchange process.
This also leads to the conclusion that the reduction of the TOC content is due
to the
reduction of the organic complexing agent and the organically bonded cyanide
(nitrile). The same result was obtained using an electrolyte which was treated
with
the Lewatit MonoPlus M500 ion exchanger resin.
The high and low current density ranges shown in Figure 2 act as measuring
points
for determining a layer thickness and the alloy composition of the zinc-nickel
layer.
After the coating process in the Hull cell, the layer thicknesses were
measured using
an X-ray fluorescence measurement device at the two measuring points A (high
current density range) and B (low current density range). Five measurements
were
taken at each measuring point. In electroplating, the X-ray fluorescence
analysis is a
standard method used for a quick and non-destructive determination of layer
thicknesses. By using this measurement method, it was possible to ascertain
the
layer thickness and the amount of nickel and zinc. Based on the layer
thickness
distribution, it was then possible to draw a conclusion concerning the effect
of the ion
exchange process on the electrolyte parameters. The base or reference value
which
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is to be obtained using the regeneration process, is the layer thickness
distribution of
the newly included electrolyte [Table 10].
A comparison of the layer thickness distribution for a new and an aged
electrolyte
[Table 11] also shows how quickly the efficiency level and thereby also the
separation rate of the electrolyte drops as the lifetime increases. In order
to retain the
same matrix relating to the batch, it is necessary to replenish the quantities
of metal
ions for the old electrolyte as per Table 11 as well as those lost through the
ion
exchange process. The initial concentration (Sample 0) is needed for this.
Table 10: Composition of the layer - new electrolyte
Layer thickness Measuring point A Measuring point B
distribution of the new Layer Nickel Zinc Layer Nickel Zinc part
electrolyte thickness part part thickness part
[pm] [%] [%] [pm] [%] [%]
1 5.15 13.9 86.1 1.76 13.7 86.3
2 5.22 13.8 86.2 1.77 14.2 85.8
3 5.14 14.5 85.5 1.78 13.9 86.1
4 5.10 14.2 85.8 1.87 14.5 85.5
5.18 13.9 86.1 1.80 13.7 86.3
Mean value 5.16 14.1 85.9 1.80 14.0 86.0
Table 11: Composition of the layer - aged electrolyte
Layer thickness Measuring point A Measuring point B
distribution of the aged Layer Nickel Zinc Layer Nickel Zinc part
electrolyte thickness part part thickness part
[pm] [%] [%] [pm] [%] [%]
1 3.13 14.2 84.7 1.27 15.4 84.6
2 3.14 14.5 85.5 1.25 15.0 85.0
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3 3.13 15.1 84.9 1.26 14.2 85.8
4 3.16 14.5 85.5 1.26 14.5 85.5
3.13 14.3 85.7 1.25 14.2 85.8
Mean value 3.14 14.5 85.3 1.26 14.7 85.3
After the regeneration and replenishment of the aged electrolyte [Tables 12
and 13],
the Hull cell test shows that the plated layer thickness at measuring points A
and B is
considerably higher and is closer to the non aged electrolyte, in comparison
to the
aged electrolyte [Table 11]. The result also shows that the nickel and zinc
composition has not changed in the layer. It can therefore be said that
removing the
cyanide and organically bonded cyanide accelerates the separation rate of the
alkaline zinc-nickel electrolyte and that the bath quality is significantly
increased in
comparison to the aged plating bath by using an ion exchanger system.
Table 12: Layer thickness at the measuring point / electrolyte regenerated
with
Lewatit MonoPlus M600 and missing quantities supplemented
Layer thickness Measuring point A Measuring point B
distribution Layer Nickel Zinc Layer Nickel Zinc part
Electrolyte regenerated thickness part part thickness part
and supplemented [pm] [%] [%] [pm] [%] [%]
Lewatit MonoPlus M600
1 3.60 14.0 86.0 1.30 13.5 86.5
2 3.59 14.8 85.2 1.33 14.0 86.0
3 3.66 14.9 85.1 1.39 14.5 85.5
4 3.65 14.7 85.3 1.38 14.0 86.0
5 3.63 13.6 86.4 1.39 14.3 85.7
Mean value 3.63 14.4 85.6 1.36 14.1 85.9
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Table 13: Layer thickness at the measuring point / electrolyte regenerated
with
Lewatit MonoPlus M500 and missing quantities supplemented
Layer thickness Measuring point A Measuring point B
distribution Layer Nickel Zinc Layer Nickel Zinc part
Electrolyte regenerated thickness part part thickness part
and & supplemented [pm] [%] [%] [pm] [%] [%]
Lewatit MonoPlus M600
1 3.75 14.7 85.3 1.41 14.3 85.7
2 3.69 14.5 85.5 1.35 14.3 85.7
3 3.69 14.5 85.5 1.39 14.9 85.1
4 3.70 14.3 85.7 1.38 14.5 85.5
3.71 14.4 85.6 1.40 14.4 85.6
Mean value 3.71 14.5 85.5 1.39 14.5 85.5
The efficiency of the electrolyte can be increased further by freezing out the
sodium
carbonate. A comparison of the layer thicknesses in the aged electrolyte with
the
lower concentration of sodium carbonate [Table 14] after the freezing out
process
and the aged electrolyte with the higher concentration of sodium carbonate
[Table 11]
where there was no freezing out, shows that the decrease in the sodium
carbonate
concentration at the least affects the separation rate. There was no evidence
that the
metal composition was affected. An examination of the efficiency of the
electrolyte
once the sodium carbonate had been frozen out revealed a 7 % increase in the
efficiency of the electrolyte. A regeneration of the zinc-nickel electrolyte
by freezing
out the sodium carbonate and removing the cyanide and nitrite using the ion
exchanger is particularly advantageous.
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Table 14: Layer thickness distribution - electrolyte - sodium carbonate frozen
out.
Layer thickness Measuring point A Measuring point B
distribution Layer Nickel Zinc Layer Nickel Zinc part
Electrolyte thickness part part thickness part
Na2CO3 removed by [pm] [%] [%] [pm] [%] [%]
means of freezing out
1 3.22 13.8 86.2 1.23 13.8 86.2
2 3.22 14.1 85.9 1.25 14.6 85.4
3 3.22 13.8 86.2 1.24 14.6 85.4
4 3.25 14.0 86.0 1.25 13.7 86.3
3.22 14.5 85.5 1.24 14.9 85.1
Mean value 3.23 14.0 86.0 1.24 14.3 85.7
Using macro throwing power measurments, it was also possible to assess how
effectively the ion exchange process can bring the aged electrolyte to the
original
state (non aged electrolyte). 250 ml of electrolyte were again filled into the
Hull cell as
per Table 1. The Hull cell plate was galvanised for 15 minutes. In order to be
able to
assess the throwing power and the regeneration effect as a result of the ion
exchanger and the freezing out process, the throwing power abilities of
various
electrolytes were assessed. To do so, the Hull cell plate which was to be
coated and
was 30 mm from the lower edge of the plate, was measured at intervals of a
centimetre. The measuring points were indicated by crosses on the coated
plate. The
measurement was taken using the X-ray fluorescence measurement process. From
the layer thickness distribution of the plates it was possible to determine
the effects of
the regeneration process on the electrolyte. The measured layer thicknesses
were
applied across the length of the plate [Figure 4]. It shows that after 15
minutes of
metallization, across the whole length of the Hull cell plate, the
electrolytes which
were regenerated with Lewatit MonoPlusM500 and Lewatit MonoPlusM600 created a
layer thickness which was significantly higher than could be achieved using
the aged
bath.
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