Note: Descriptions are shown in the official language in which they were submitted.
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Precious Metal Recovery
Specification:
The invention relates to a method and a device for the plating of work pieces
with a fluid containing precious metals. The invention is especially
applicable in
processes for producing electrical circuit carriers.
To electroplate work pieces, surfaces thereof must first be treated in such a
manner that they are made electrically conductive if the surfaces of the work
pieces are non-conductive. For this purpose, the work pieces are immersed
into a solution containing ionic, ionogenic or colloidal palladium. Ionic
palladium
may more specifically be present in the form of a salt, such as palladium
chloride for example, which is generally dissolved in a hydrochloric acidic
solution. lonogenic palladium is present as a complex, an aminopyridine
complex for example. Colloidal palladium may contain diverse protective
colloids, e.g., a protective colloid formed from tin(II) chloride or
consisting of an
organic polymer. The palladium nuclei which are thereupon adsorbed on the
surfaces of the work pieces serve for example as activators to initiate an
electroless metal deposition that causes an electrically conductive layer to
form
on the surfaces so that the surfaces are then ready to be metal-plated with
any
metal. This method is utilized for producing printed circuit boards and other
circuit carriers as well as metal-plated parts in the sanitary, automotive and
furniture industry for example and more specifically for chromium-plating
plastic
parts.
The palladium containing solution may also be used for forming an electrically
conductive layer. In this direct electroplating method, further metal is
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electrolytically deposited after palladium treatment without a metal layer
being
previously formed with an electroless metal coating method.
In treating the work pieces having electrically non-conductive surfaces, part
of
the palladium containing solution still adheres to the work pieces when the
previously immersed work pieces are emersed from the solution. The adhering
solution is usually rinsed with water.
With known activating methods using colloidal palladium for example, a
solution is used that usually contains 50 - 400 mg/I of palladium. In treating
plastic parts with a geometrical surface of one square meter, about 5 - 10 mg
of
palladium are typically adsorbed. This quantity is necessary to activate the
plastic surface. When the work pieces to be treated leave the corresponding
processing station, about 0.2 I activating solution per square meter is
carried
over from the bath and is still left on the surfaces. Therefore, approximately
10
- 50 mg of palladium get lost to the bath because the adhering solution is
entrained out of the processing bath, then rinsed and transferred to waste
water treatment.
In direct electroplating electrically non-conductive surfaces without
electroless
metal-plating palladium containing solutions are also utilized. In this case,
a
higher concentration of palladium, of e.g., 400 mg/I, in the solution is
needed.
In carrying out the known direct metal-plating methods, the palladium
entrained
from the processing solution amounts to about 50 mg/m2. By taking appropriate
measures such as the previous adsorption of polyelectrolyte compounds to the
non-conductive surfaces, the adsorption of the palladium particles may be
increased from a relatively low value to about 50 mg/m2 of the surface of the
work pieces. Even though, about 60 - 70% of the palladium utilized in the
solution gets lost by being entrained. 40 - 30% only can actually be used for
metal-plating the surfaces of the work pieces.
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It has been known for example to recover palladium from processing solutions.
U.S. Patent No. 4,078,918 e.g., describes a recovery process for reclaiming
e.g., palladium from various materials that contain dissolved or non-dissolved
palladium. The materials are at first treated with an oxidizing agent to
destroy
possible organic components and are then treated with ammonium hydroxide in
order to form amine complexes. The thus obtained palladium complexes are
next reduced with ascorbic acid so that palladium deposits from the processing
solution as a metal and may be filtered.
Furthermore, in "Reclamation of palladium from colloidal seeder solutions" in
Chemical Abstracts, 1990: 462908 HCAPLUS there is described a method of
reclaiming palladium from solutions of colloidal PdlSnCl2 as a pretreatment
prior to electroless metal-plating in which the solution is air-gassed for 24
hours
so that palladium is caused to flocculate. The deposit is separated, dried and
further processed.
In "Recovery of palladium and tin dichloride from waste solutions of colloidal
palladium in tin dichloride" in Chemical Abstracts, 1985:580341 ~HCAPLUS
there is described a method of precipitating palladium by the addition of
metallic tin at 90°C.
U.S. Patent No. 4,435,258 discloses another method for recovering palladium
from spent baths that are utilized for activating electrically non-conductive
surfaces for the subsequent electroless metal-plating process. The activating
solutions are reprocessed by causing the colloidal palladium to oxidize into
the
solution by adding an oxidizing agent such as hydrogen peroxide for example,
by subsequently heating the solution to destroy the residual hydrogen peroxide
and by thereafter electrolytically depositing palladium from these solutions
onto
a cathode.
In "Recovery of the colloidal palladium content of exhausted activating
solutions
used for the current-free metal coating of resin surfaces" in Chemical
Abstracts,
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1976:481575 HCAPLUS finally there is described a method of obtaining
palladium from Pd/SnCl2 in which palladium is precipitated by the addition of
concentrated nitric acid and is filtered.
DE 100 24 239 C1 describes a method of electroplating work pieces with a
palladium colloid solution by contacting the work pieces with the colloid
solution
according to which palladium is recovered after the colloid solution was used,
by separating palladium colloid particles from the colloid solution by means
of a
membrane filter. Materials made from ceramics for example may be used for
filtration. The pore exclusion size of the membranes amounts to 200 to 10,000
Dalton. It is stated therein that the palladium particles pass through the
membrane filter when the pore exclusion size used is in excess of 10,000
Dalton.
The prior art methods for electroplating work pieces with a palladium colloid
solution are complicated and expensive.
The basic problem the present invention faces is to avoid the drawbacks of the
known methods and to find a method for plating work pieces with a fluid
containing at least one precious metal that may be carried out at low cost.
Small quantities of additional chemicals only should be necessary to carry out
the method. Furthermore, the method should involve little expense of energy
and time and should more specifically require low maintenance.
This problem is overcome by the method according to claim 1 and by the
device according to claim 15. Preferred embodiments of the invention are
indicated in the subordinate claims.
The method in accordance with the invention serves to plate work pieces with a
fluid, said fluid containing at least one precious metal, the method
comprising
contacting the work pieces with the fluid. For the purpose of recovering the
precious metal from the fluid, said fluid is filtered after plating of the
work
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pieces through at least one ceramic membrane filter to separate the precious
metal from the fluid, the ceramic membrane filter having an exclusion pore
size
in excess of 10,000 Dalton. Due to the filtering the precious metal is
separated
from the fluid.
5
By plating, any treatment with fluids is meant that is directed to alter the
surface
of the work pieces, the fluid having to contain precious metals. Not included
therein though are methods of coating work pieces with polymer coatings, more
specifically enamelling methods.
The work pieces to be plated include metallic work pieces, non-metallic work
pieces and work pieces consisting of both metallic and non-metallic materials.
The work pieces may have all conceivable forms and be intended for all
conceivable utilizations. Preferred pieces are semi-finished products for
producing circuit carriers, more specifically for producing printed circuit
boards
and hybrid circuit carriers such as multichip modules for example.
The precious metals that can be separated from corresponding fluids are all
the
elements of Group I and VIII of the Periodic Table of Elements, i.e., more
specifically Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au. The invention preferably
relates to a method for treating work pieces by plating with a palladium
containing fluid.
More specifically, the fluid may be a solution. This is more specifically the
case
when the precious metal is present in ionic or ionogenic form. By ionic form
of
the precious metal, salts of the precious metal dissolved in water or in
another
solvent that promotes the dissociation of said salts is more specifically
meant.
By ionogenic form of the precious metal, precious metal complexes are meant,
more specifically complexes of the precious metal ions with organic complex
ligands. The complexes may be uncharged or be present in the form of ions.
The fluid may be present in the form of a colloid, more specifically of a
colloid
of the elemental precious metal.
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The precious metal containing fluid may be both a processing fluid for
treating
the work pieces or a rinsing fluid. By processing fluid a fluid is meant that
serves to alter the surface properties of the work pieces, e.g., a coating
fluid,
including an activating fluid, a cleansing fluid, an etching fluid or the
like. By
contrast, a rinsing fluid only serves, after treatment of a work piece with
the
processing fluid, to rinse off the processing fluid still adhering on the
surface of
the work piece.
After use of the fluid for plating the work pieces, the precious metal is
filtered in
the at least one ceramic membrane filter. This means that the fluid is at
first
utilized for plating and is only filtered afterwards in the membrane filter
for the
purpose of recovering the precious metal that it contains. The fluid can for
example be contacted with a work piece by spraying, jetting, flooding or
blasting, the fluid dripping off the work piece collected and the collected
fluid be
conducted to the membrane filter immediately thereafter. The collected fluid
may also be first retained in a reservoir from where it is delivered back to
the
work piece, though. In this case, the fluid may be either conducted to the
membrane filter after having been collected for a predetermined period of time
(intermittent method), or part of the fluid may be tapped continuously from
the
reservoir and be transferred to the membrane filter (continuous method). To
achieve a stationary filling condition in the reservoir in this case, new
processing fluid is permanently introduced into the reservoir in a quantity
per
unit of time that equals the quantity of the fluid permeating the membrane
filter
per unit of time. The work pieces may also be contacted with processing fluid
contained in a treatment container by immersing them thereinto. In this case,
the processing fluid is, after use, either conducted to the membrane filter
after
having been collected for a predetermined period of time (intermittent
method),
or part of the fluid in the reservoir may be tapped continuously from the
treatment container and be transferred to the membrane filter (continuous
method).
The method of the invention permits to achieve in a simple manner and with
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little expense of chemicals, energy and time as well as with little
maintenance,
a far-reaching separation of precious metal from the exhausted processing
solutions under continuous operation. It more specifically permits to
regenerate
the exhausted processing solutions after the fraction containing the palladium
has been separated so that the entire palladium may be recirculated into the
process.
Over the method described in Chemical Abstracts, 1990:462908 HCAPLUS for
reclaiming palladium from colloidal Pd/SnCl2, the present method has the
advantage that the fractions are completely separated whereas with the
precipitation method described in Chemical Abstracts a non-negligible part of
the palladium is oxidized to form the bivalent, soluble stage thereof so that
it
cannot be completely separated from the solution by filtration. Accordingly,
this
part of the palladium cannot be recovered and will be lost.
Another advantage of the method of the invention over the method described in
Chemical Abstracts, 1985:580341 HCAPLUS is that there is no need for
considerable expense of additional chemicals like metallic tin and of energy
and time as they are required for the known method for the purpose of heating
the colloid solution.
The method in accordance with the invention also has substantial advantages
over the method described in U.S. Patent No. 4,435,258, namely that palladium
may be removed almost entirely from the solutions, whereas, by the method
according to U.S. Patent No. 4,435,258, only an extremely low current
efficiency may be achieved, especially when the palladium concentration
is low, which occurs after a long period of electrolysis. Therefore, it is
either
very complicated or not possible at all to completely remove palladium with
this
prior art method.
In contrast to the method described in Chemical Abstracts, 1976:481575
HCAPLUS, the method and the device in accordance with the present invention
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are more specifically suited for continuous operation. Furthermore, the method
presented in this publication imperatively needs additional chemicals.
Surprisingly, and as contrasted with the properties of membrane filters having
a
pore exclusion size clearly in excess of 10,000 Dalton as they are indicated
in
DE 100 24 239 C1 and according to which the palladium particles of colloidal
palladium colloid solutions permeate the filter, the separating properties of
ceramic filters having a pore exclusion size of e.g., 20,000 Dalton proved
excellent with regard to colloidal palladium. In this connection, reference is
made to the tests No. 1 and 2 in Example 1.
The method and the device in accordance to the present invention have the
following advantages over known methods and devices:
a. Precious metal, more specifically palladium, may be recovered from ionic,
ionogenic and colloidal solutions with but one device. It is not necessary to
use
several matched devices. As a result thereof, the solutions may be mixed and
collected prior to being regenerated. The same also applies to the processing
and rinsing fluids: processing fluids with a high concentration of precious
metal
can be mixed with rinsing fluids containing precious metal in a very low
concentration and then be processed together.
b. Ceramic membrane filters that are largely resistant to chemicals and to the
effects of temperature may be utilized since separation of precious metal is
also successful with the larger pores of ceramic membrane filters. Maintenance
is low as a result thereof as the filters do not need cleaning very often.
Ceramic
membrane filters also have a long durable life. Moreover, precious metal does
not adsorb to the membrane material.
c. The fluid to be treated can be reprocessed with a very simple method. It is
e.g., not necessary to work in a protective atmosphere to prevent colloidal
particles from dissolving in the fluid.
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Colloidal activators on the basis of palladium comprise palladium particles
that
are surrounded by a protective coating (protective colloid). Tests using high
resolution transmission electron microscopy (HTEM) and atomic force
microscopy (AFM) showed that the palladium particles have a diameter of at
least 2.5 nm. The mean particle diameter amounts to approximately 4 nm,
which corresponds to the gaussian distribution of particles. In testing a
rinsing
fluid that was obtained after treatment of work pieces with the colloidal
activator, a wide particle size distribution that showed particles with a
maximum
size of about 18 nm as well as smaller particles (from 2 to 18 nm) was
determined.
In practical utilization, colloid solutions are acidic, often highly
hydrochloric
acidic, and contain chloride ions as well as possibly tin in the oxidation
stages
(II) and (IV) or organic, polymeric stabilizers like gelatine or polyvinyl
pyrrolidone and reducing agents. Except for the polymers, which are utilized
in
small quantities, all the other substances contained therein are ionic. It is
presumed that these ionic constituents are much smaller than the palladium
particles.
Surprisingly, palladium particles may be removed very selectively and
completely from these colloid solutions by means of appropriate membrane
filters comprising different porosities, although, in the case of the tin
containing
colloidal solutions, tin, which is simultaneously present, is confiained in a
high
concentration (usually more than 70 times the palladium concentration) and
although the tin compounds are known to form colloidal solutions that are
difficult to filter.
For ultrafiltration, diverse types of membranes made of various materials have
been tested. The tests showed that what matters in selecting the membrane
filter is more specifically that it be sufficiently stable to the fluid that
contains the
precious metal and that may contain 15 percent by weight of hydrochloric acid
for example.
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To separate the palladium colloid particles, ceramic membrane filters may be
utilized that have an exclusion pore size of from about 15,000 Dalton to about
25,000 Dalton, more specifically an exclusion pore size of from about 17,500
Dalton to about 22,500 Dalton and most preferably of approximately 20,000
5 Dalton.
A preferably utilized ceramic membrane filter is made of a ceramic material
containing aluminum oxide, more specifically a-AI203, titanium dioxide and
possibly zirconium dioxide. In principle, other filter materials may also be
10 utilized. As a rule, the filter material is deposited onto a highly porous
supporting body that provides the filter with the required mechanical
stability.
This supporting body may consist of a-AI2~3 or of SiC (silicon carbide) for
example.
The filter may be configured in the form of a disc or as a tube. In the first
case,
a flow is directed onto the disc, approximately normal to the surface thereof,
said flow being deviated in radial direction. A pressure difference is built
up
between the two surfaces of the disc so that permeate may pervade the disc. If
the filter has the shape of a tube, the fluid is conveyed through the tube in
axial
direction, a pressure difference being built up between the inner space and
the
outer space of the tube. As a result thereof, permeate can pervade the wall of
the tube e.g., from the interior volume of the tube to the space external of
the
tube. This second method is called dynamic filtration. In this case, the
precious
metals are retained within the inner space of the tube, whereas the fluid,
which
has been largely freed from precious metal, permeates through the wall of the
tube from the inner volume of the tube to the space external of the tube.
Some fluids may be filtered directly without any further pretreatment. In this
case, very good results are obtained with the ceramic membrane filters.
In some cases, the fluids to be reprocessed are chemically pretreated first.
After having been used for plating and prior to being filtered through the
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membrane filter, the fluid is mixed for this purpose with chemical substances
that are suited to alter the at least one precious metal in such a manner that
it
is almost completely retained during filtration. It is presumed that, by
adding
these chemical substances, the particle size of the precious metal is altered
in
such a manner that the particles that contain precious metal cannot pass
through the pores of the membrane filter. For this purpose it should be
sufficient to adjust the average particle size to a value in excess of about
10 nm
when the particle size fits the Gaussian distribution. In this case, a
membrane
filter with an exclusion pore size in excess of 10,000 Dalton would already
retain almost the entire quantity of precious metal in the concentrate. Larger
particles may be set accordingly by adding these chemical substances when
membrane filters with a greater exclusion pore size are being used.
If palladium is present in the solution in ionic and/or ionogenic form, the
fluid
may be mixed with chemical substances selected from the group comprised of
reducing agents, sulfur compounds, selenium compounds and tellurium
compounds. The chemical substances for pretreatment are most preferably
selected from the group comprised of boron hydrides, amine boranes,
hypophosphites, inorganic sulfides and organic thio compounds, more
specifically the alkali and ammonium salts of dimethyl dithiocarbamate, of
sulfides, of boron hydrides such as tetrahydroboranate for example, and of
hypophosphites. The organic thio compounds considered are more specifically
organic compounds in which sulfur is bonded to one or to two atoms of carbon
to form a single or a double bond therewith i.e., thioles, sulfides,
disulfides and
polysulfides, thioamides and thioaldehydes for example.
If palladium is present in the fluid in colloidal form, pH adjusting agents
are
used as chemical substances. The fluid is mixed with the pH adjusting agents
in such a manner that solution pH ranges from 3 to 12.
In both cases, a solution is obtained that is very well suited for separating
the
precious metal, in this case for separating palladium.
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The following advantages derive from this improvement of the present
invention:
a, the pretreatment is very simple. It suffices to mix the fluids containing
the
precious metal with the required substances or with the pH adjusting agent
respectively.
b. The expense of additional chemicals is very low. To process 200 I of
rinsing
water from the treatment with an aqueous solution of an organic palladium
complex (7 mg/l of Pd), only 7.5 ml of a solution containing 467 g! of sodium
dimethyl dithiocarbamate are needed. If rinsing waters originating from the
treatment with a palladium colloid (organic protective colloid, 25 mg// Pd)
are to
be processed, mere 0.5 I of an aqueous solution of 432 g// of NaOH will
suffice.
It could be inferred from the observations and tests that led to the present
invention that it is possible to recover precious metals from rinsing fluids
and/or
processing fluids by means of membrane filters. For this purpose
a. the work pieces are contacted with a palladium containing processing
fluid,
b. then, the processing fluid still adhering to the surfaces of the work
pieces is removed with rinsing fluid, and
c. the processing fluid and/or the rinsing fluid are passed (preferably
under pressure) through the at least one ceramic membrane filter for
filtration thereof, the fluid being passed through the at least one
membrane filter being a permeate fluid and the fluid that has not passed
through the at least one membrane filter being a concentrate fluid.
After treatment with a palladium containing fluid, the work pieces, which are
preferably made of an electrically non-conductive material, are rinsed in a
suitable device with a rinsing fluid by immersing them into it, by flooding or
preferably by spraying said rinsing fluid onto said work pieces in order to
keep
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the volume of rinsing solution as small as possible. The rinsing fluid is next
conducted through a ceramic membrane filter by means of a pressure pump,
said filter retaining the palladium particles and allowing the rinsing water
to
permeate. Said permeate may then be transferred to waste water treatment.
Prior to being conducted through the membrane filter, the processing fluid
and/or the rinsing fluid may be mixed with the chemical substances such as for
example the reducing agents, sulfur compounds, selenium compounds,
tellurium compounds or the pH adjusting agents.
In a particularly preferred embodiment of the invention, only the rinsing
fluid, or
a rinsing fluid containing preferably up to 5 percent by volume of processing
fluid, is conducted through the membrane filter (preferably under pressure).
The work pieces are contacted with fresh rinsing solution, a predetermined
quantity of fresh rinsing solution per unit of time being permanently
available.
The quantity of the permeate fluid formed per unit of time may be more
specifically adjusted to approximately equal the quantity of the rinsing fluid
that
is contacted with the work pieces per unit of time. As a result thereof, a
stationary condition is achieved in the processing plant: in that the amount
of
fresh rinsing fluid delivered to the work pieces is exactly the same as the
amount of permeate fluid drained from the plant, the result obtained is a
constant flow of substances. This, of course, only applies if the amount of
added chemical substances is negligible and if no further influencing
variables
affect the process. For, in practice, evaporation of rinsing fluid could play
a
major part.
Retained palladium, which is present as a concentrate in the form of a
homogeneous dispersion of metal or of a metal compound, e.g., in the form of
a PdS dispersion, may be recycled. The retained palladium may e.g., be
dissolved, converted to palladium chloride and be utilized to synthesize a new
palladium containing processing fluid or for any other application. The
palladium containing concentrate solution may also be concentrated to near
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dryness in a filter press. For this purpose, the concentrate fluid coming from
the
membrane filter is directed into a container in which palladium containing
slurry
that has formed during concentration deposits, the slurry suspension being
directed to the filter press. The palladium containing filter cake obtained by
means of the filter press may be utilized as a base substance for producing
pure palladium and palladium compounds.
The device in accordance with the invention for plating work pieces with a
fluid
containing at least one precious metal is typically provided with means for
contacting the work pieces with the fluid as well as with holding means for
the
work pieces.
The means for contacting the fluid with the work pieces are e.g., nozzles by
means of which processing or rinsing fluid is sprayed, jetted, flooded or
discharged onto the surfaces of the work pieces. This arrangement is used for
example when the fluid is to reach the surface at a high flow velocity or when
the quantity of the fluid needed is to be minimized. In another embodiment of
the invention, the contacting means are treatment containers in which the
processing fluid is disposed and into which the work pieces are immersed.
The holding means for the work pieces may also be embodied in very diverse
forms: the work pieces may for example be retained in a conventional way by
means of cramps, clamps, tongs or screw fastenings. Furthermore, the work
pieces may also simply be held, transported and conducted in a horizontal
position on rolls, wheels or cylinders or they may be clamped therein between.
Aside from the features mentioned, the device also comprises a facility for
separating the at least one precious metal from the fluid. This facility
comprises
at least one ceramic membrane having an exclusion pore size in excess of
10,000 Dalton. Further the facility comprises at least one pump for delivering
the fluid to the at least one membrane and fluid conduits for conducting the
fluid from the means for contacting the work pieces with the fluid to the at
least
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one ceramic membrane. By a pump, any pump which is not motor operated or
simply delivery of the fluid by gravity is also meant.
In accordance with the explanations given herein above, the facility for
5 separating the precious metals from the fluid is furthermore provided with a
mixing facility. In this mixing facility, fluid coming from the means for
contacting
the work pieces with the fluid can be mixed with chemical substances. For this
purpose, any conventional mixing facility known in the chemical reaction
technique, such as e.g., stirring facilities and mixing zones in flow
reactors, may
10 be utilized.
Furthermore, the facility for separating the precious metals from the fluid
may
also be provided with a multiphase separating unit in which slurry may deposit
which is produced during separation from the fluid and which comes from the
15 facility for separating the precious metal from the fluid. A multiphase
separating
unit of this type is formed by a sedimentation tank for example in which
virtually
no fluid convection is taking place. Said slurry suspension may then be
directed
into a filter press in order to largely purify and dry the slurry, which
mainly
contains precious metal.
The invention will be understood better upon reading the description of the
Figs. More specifically,
Fig. 1: is a perspective, schematic view of a ceramic membrane filter;
Fig. 2: is a schematic representation of a device for plating work pieces
in accordance with the invention.
Fig. 1 illustrates a ceramic membrane filter in the form of a tube 1. The tube
is
made of a highly porous ceramic material that serves as support 3 and that is,
in the present case, of aluminum oxide. The support 3 is provided, on its
inner
side, with another ceramic layer of an oxide that serves as a membrane filter
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layer 2. Said membrane filter layer 2 in turn consists of two layers (not
specifically shown), i.e., a first microfiltration layer made of a-AI203 and
of a
second ultrafiltration layer made of Zr02 and Ti02, Ti02 having the finest
pore
size so that filtration is also possible with an exclusion pore size of e.g.,
20,000
Dalton. The membrane filter layer 2 has an exclusion pore size of about 20,000
Dalton. Accordingly, the mean pore size amounts to approximately 20 nm.
The tube has an inside diameter of about 6 mm. The tube is about 1000 mm in
length. The flow passes therethrough under pressure in the direction of flow
referred to be reference numeral 4. The pressure difference between the
entrance and the exit of the tube ranges from 1.5 to 3 bar.
In order to collect the permeate passing through the internal wall of the
tube,
the ceramic tube is positioned concentrically within another tube.
Fig. 2 comprises two of the filter tubes 1 represented in Fig. 1 in the lower
part
of the figure, the filter tubes 1 being part of ceramic tubes with several
bores of
the type shown in Fig. 1. For this purpose 19 axial bores are for example.
drilled
in a ceramic tube consisting of a highly porous ceramic material, said axial
bores being paralleled.
In the upper part of Fig. 2, the processing stations of a processing plant for
printed circuit boards is partially shown. The printed circuit boards are
successively conveyed through the difFerent processing stations in the
processing direction R. A typical example of such a method is described, inter
alia, in WO 93/17153 A1.
After having already performed the pretreatment steps, the printed circuit
boards (which are not shown herein) are immersed, in activating station A-Pd,
into an activating bath containing palladium in colloidal form. For this
purpose,
the fluid is contained in an immersion bath tank.
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Then, the printed circuit boards are conveyed through three successive rinsing
stations S" S2 and S3. There, the activating fluid that adheres to the
surfaces of
the printed circuit boards is successively rinsed off. The different rinsing
stations S" S2 and S3 are provided with spray nozzles to serve this purpose.
The rinsing stations S~, SZ and S3 are configured as open top containers that
are provided with nozzles arranged on the walls of the long sides thereof. In
order to rinse off the adhering activating fluid, rinsing fluid is sprayed
onto the
surfaces of the printed circuit boards as the printed circuit boards are
lowered
into and/or are raised out of the stations S" SZ and S3_ The rinsing fluid is
collected at the bottom of a container in the respective one of the rinsing
stations S~, S2 and S3. Fresh rinsing fluid is dispensed to the rinsing
station S3
at an average flow rate of 200 I/h, from there it is conducted in a direction
counter to the processing direction R of the printed circuit boards to the
rinsing
station S2 arranged upstream thereof from where it is brought into the rinsing
station S,, the flow rate remaining the same. Each rinsing station S,, S2 and
S3
is also allocated a collecting tank (not shown) in which the respective
rinsing
fluid is collected. The collected rinsing fluid is drained at a flow rate of
200 I/h
from the collecting tank of rinsing station S~ toward further processing.
After the surfaces of the printed circuit boards have been freed from adhering
activating fluid by rinsing, they are subjected to posttreatment. Such
processing
fluids are for example solutions of sulphinic acids. In the posttreatment
station
S, the printed circuit boards are immersed for treatment into these solutions
which are contained in the treatment containers.
Then, adhering posttreatment solution is rinsed off again in the further
rinsing
stations S4, S5 and S6. Again, the rinsing fluid is sprayed from nozzles
arranged
in the stations S4, S5 and Ss onto the surfaces of the printed circuit boards.
The
collected rinsing fluid is directed to collecting tanks (not shown) from where
it is
conducted successively back, in a direction counter to the processing
direction
of the printed circuit boards R, to the rinsing stations S5 and S4 which are
arranged upstream thereof. The rinsing fluid is drained from rinsing station
S4
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toward subsequent waste water treatment.
The printed circuit boards are next immersed into an etch solution contained
in
a container in etch station C-Pd. There, palladium adsorbed to the copper
surfaces is removed from activation by etching slightly the copper surfaces.
In
this case as well, the printed circuit boards are immersed into the etch
solution.
After that, adhering processing fluid is again rinsed off the surfaces of the
printed circuit boards. For this purpose, the printed circuit boards are
conveyed
to the rinsing stations S7, S$ and S9. Etch solution adhering to the surfaces
of
the printed circuit boards is removed by means of rinsing fluid that is
sprayed
from nozzles onto the surfaces. For this purpose, fresh rinsing fluid is
conducted into rinsing station S9 at a flow rate of 200 I/h and the rinsing
fluid
gathering in this rinsing station is collected in collecting tanks (not
shown).
Again, the collected rinsing fluid is conducted in a direction counter to the
processing direction of the printed circuit boards R, from rinsing station S9
to
rinsing station S$ and from there into rinsing station S~. From rinsing
station S,
the palladium enriched rinsing fluid is conducted to a regenerating
arrangement
at a flow rate of 200 I/h.
The afore mentioned way of treating the printed circuit boards is but one
possible alternative. The printed circuit boards may also be processed in a so
called horizontal plant. The boards are hereby conducted through the various
stations in a horizontal direction of transport and in a horizontal or
vertical
orientation. In the diverse stations, the fluids may delivered to the surfaces
by
nozzles.
The rinsing fluid originating from rinsing station S4 contains virtually no
precious
metal and can be dispensed to the conventional waste water processing
system. By contrast, the rinsing fluid originating from the rinsing stations
S, and
S, contains palladium and is regenerated in the inventive way:
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In the first place, the various rinsing waters are collected in buffer tanks
11.1
and 11.2, respectively. Rinsing fluid drained from the buffer tanks 11.1 and
11.2
at a flow rate of 200 I/h is next conducted into the conduits 13.1 and 13.2,
respectively, by means of pumps 12.1 and 12.2, respectively, and is delivered
to a common conduit 13.3. To adjust the pH, the combined rinsing fluids are -
if
necessary - mixed with a pH adjusting agent, in the present case with NaOH.
For this purpose, NaOH solution is added from a reservoir 14 to the combined
rinsing fluids. An electric control circuit (not shown) serves to control the
dosage
of the NaOH solution. Said control circuit comprises a pH probe 15, a pH
measuring electrode for example, for controlling a dosing pump (not shown) for
the NaOH solution. In case the pH of the rinsing fluid is near 7, the pH needs
not be adjusted to the precise value of 7.
If, instead of a palladium colloid fluid, an ionic or ionogenic palladium
solution is
utilized, solutions of other suitable chemical substances are added to the
flow
of fluid in place of a pH adjusting agent in order to make certain the
palladium
containing fluid is filtrable.
The rinsing fluid, the pH of which is now adjusted to a value of about 7, is
then
directed by means of another pump 12.3 through a conduit 13.4 into a
collecting tank 16.
A lower fill level sensor 17.1 and an upper fill level sensor 17.2 are
provided in
collecting tank 16. If the fluid level is higher than the upper fill level
sensor 17.2,
fluid is directed through conduit 13.5 from container 16 to pump 18. If, by
contrast, the fill level of the collecting tank 16 is below the lower fill
level sensor
17.1, the rinsing fluid is not pumped oufi of the collecting tank 16.
By means of pump 18, the fluid is conducted, under a pressure ranging from
1.5 to 3 bar, through two membrane filter tubes 1 connected in series. The
permeate fluid pervading the walls of the tube is drained toward further waste
water treatment A. The concentrate fluid remaining in the filter tube is
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WO 03/029518 PCT/EP02/09125
recirculated via the closed circular conduit 13.6 so that the fluid is
permanently
and increasingly concentrated with regard to palladium. Via the branching
13.7,
part of the concentrated rinsing fluid is permanently circulated back to the
collecting tank 16 from where it is directed to the membrane filters by way of
5 pump 18 so that palladium gradually enriches in this fluid.
In collecting tank 16, palladium containing slurry, which results from
concentration, deposits in a multiphase separating zone. Said slurry
suspension
may be drained into another container 19.
Fluid coming directly from the activating station A-Pd can also be discharged
directly for regeneration and be directed toward ultrafiltration. For this
purpose,
said fluid may be either transferred by hand into a collecting tank 20 using
the
path referred to by reference numeral M or small quantities thereof may be
conducted to the bufFer tank 11.1 by means of a pump 12.4. Fluid that has
been removed and transferred by hand to collecting tank 20 may then be
dispensed to collecting tank 16 by way of another pump 12.5 for example.
The slurry suspension contained within the container 16 in the multiphase
separating unit is directed to a filter press 21 for further separation of
palladium.
The filter press 21 is shown in dashed lines in Fig. 2. It contains filter
material
having a pore size of about 50 pm. The pressure in the press amounts to about
4 bar. Excess fluid can either be circulated back to the collecting tank 16 by
way of the additional conduit 22 or be dispensed to waste water treatment A.
The following examples will serve to explain the invention:
Example 1:
To perform a test, printed circuit boards were treated with a colloidal,
acidic
activating fluid that contained 400 mg/I of colloidal palladium, a protective
colloid in the form of a polymer and a reducing agent in the form of sodium
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21
hypophosphite. The mean particle diameter of the palladium colloid particles
was about 4 nm.
After rinsing, the printed circuit boards were treated with an posttreatment
solution containing an organic sulphinic acid, were then rinsed again and
finally
treated in an etch solution containing 300 g/1 of sodium persulfate. The
amounts of palladium thereby removed from the copper surfaces were
dispensed to the etch solution and, via the etch solution adhering to the
surfaces of the prinfied circuit boards, to the subsequent rinsing fluid.
The rinsing fluids obtained in the rinsing stations S~ to S3 and S7 to S9 (see
Fig.
2) under the afore mentioned conditions were dispensed to the regeneration
arrangement described at a flow rate of 200 I/h respectively. The fluids were
separated at a filter membrane made of ceramic (a-AI203 as a support material
with two ultrafiltration layers of ~r02 and Ti02 applied thereon, Ti02 being
provided with the finest pore size and effecting a filtration with a pore
exclusion
size of approximately 20,000 Dalton; the Ti02 layer was applied by a Sol Gel
method). The concentration of palladium in the rinsing fluids as well as the
pH
of these fluids are indicated in Table 1 (tests No. 1 and 2).
The pH of the fluids originating from rinsing stations S, to S3 and S, to S9
were
not adjusted with pH adjusting agents.
During ultrafiltration, the concentrate fluid was conducted through the
ceramic
membrane filter at a flow rate of 2,800 I/h. The permeate flow rate obtained
was
of 40 to 45 I/h.
After ultrafiltration, a permeate fluid and a concentrate fluid were obtained.
The
concentrations of palladium in the permeate and in the concentrate according
to the tests No. 1 and 2 are also indicated in Table 1.
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22
Example 2:
In another test, a mixture of rinsing fluids from the colloidal activating
fluid and
from the etch solution was prepared at a volume ratio of 1 : 1 (test No. 3).
The
same ceramic membrane filter was used as in Example 1. The initial palladium
concentration in the combined rinsing fluids and the pH of the mixture are
indicated in Table 1. To adjust the pH of the combined rinsing fluids to 7, a
NaOH solution was added to the rinsing fluid.
The permeate solution obtained after having carried out ultrafiltration had a
palladium concentration of < 0.5 mg/I. The palladium concentration in the
concentrate was > 1 g/1 (see Table 1 ).
Example 3:
In another test No. 4, the same ceramic membrane filter as in Example 1 was
used. Colloidal activating fluid was added at a volume ratio of 1 : 100 to the
mixture of rinsing fluids obtained according to Example 2. The palladium
concentration in this fluid was equal to 15.0 mg/I. The pH of this fluid was
adjusted to 7 by means of NaOH solution. The palladium concentrations in the
permeate and in the concentrate after ultrafiltration was performed are
indicated in Table 1.
Example 4:
In another test No. 5, the same ceramic membrane filter as in Example 1 was
used. In this test, the solution of an ionogenic activator was used in place
of a
colloidal activating solution. The activator contained an organic palladium
complex (Neoganth° Activator, Atotech Deutschland GmbH, Germany), the
palladium concentration in this solution was 250 mg/I.
The printed circuit boards activated with this solution. were again treated in
a
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23
rinsing cascade of three rinsing stations S~, SZ and S3, the direction of flow
of
the rinsing water corresponding to that shown in Fig. 2. The palladium
concentration in the rinsing water originating from rinsing station S~ was
about
1.5 mg/I. For adjusting the ultrafiltrability of the rinsing water, an aqueous
solution of 467 g/1 of sodium dimethyl dithiocarbamate was added to the
rinsing
water. The palladium concentrations obtained in the permeate and in the
concentrate as a result of ultrafiltration of this solution are indicated in
Table 1
(Test No. 5).
Example 5:
In another test No. 6, the same ceramic membrane filter as in Example 1 was
used. In this test, the rinsing water obtained according to Example 4 was
mixed
at a volume ratio of 100 : 1 with the solution of the activating bath. An
aqueous
solution of 10 g/1 of sodium sulfide was added to the mixture. The initial
palladium concentration was 8.0 mg/l. The palladium concentrations in the
filtrate and in the concentrate after ultrafiltration are indicated in Table
1.
The tests described herein above yielded concentrate fluids that had a
considerable amount of slurry. After the slurry had settled, the concentrate
was
dispensed to a filter press. The palladium concentration in the enriched
concentrate amounted to 2 to 5 g/1. The filter cake obtained during
compression
had a palladium concentration of 2 to 15 percent by weight.
Example 6:
In another test No. 7, the same ceramic membrane filter as in Example 1 was
used. A mixture according to Example 5 was added at a volume ratio of 2 : 1 to
the mixture of rinsing fluids obtained according to Example 2.
The palladium concentration in this fluid was 4.2 mg/I. The pH was adjusted to
7 by means of NaOH solution. Furthermore, an aqueous solution of 467 g/1 of
CA 02460015 2004-03-08
WO 03/029518 PCT/EP02/09125
24
sodium dimethyl dithiocarbamate was added to the fluid. The palladium
concentrations in the permeate and in the concentrate after ultrafiltration
had
been performed are shown in Table 1.
It is understood that the examples and embodiments described herein are for
illustrative purpose only and that various modifications and changes in light
thereof as well as combinations of features described in this application will
be
suggested to persons skilled in the art and are to be included within the
spirit
and purview of the described invention and within the scope of the appended
claims. All publications, patents and patent applications cited herein are
hereby
incorporated by reference.
CA 02460015 2004-03-08
WO 03/029518 PCT/EP02/09125
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CA 02460015 2004-03-08
WO 03/029518 PCT/EP02/09125
26
Listing of numerals:
1 ceramic membrane filter
2 ceramic filter layer
3 highly porous ceramic supporting tube
4 direction of flow
processing plant for printed circuit
boards
10 11.1, 11.2 buffer tanks
12.1 -12.5 pumps
13.1 -13.7 conduits
14 reservoir
pH probe
15 16 collecting tank
17.1 lower fill level sensor
17.2 upper fill level sensor
18 pump
19 container
20 collecting tank
21 filter press
22 conduit
A-Pd activating station
posttreatment station
S, - S9 rinsing stations
C-Pd etch station
N1 removal by hand
waste water treatment
processing direction of the printed
circuit boards
