CA1220161A - Metal recovery from spent electroless and immersion plating solutions - Google Patents

Abstract

Abstract of The Disclosure A new process is disclosed which enables an effi-cient and effective recovery of dissolved metals from a spent electroless or immersion plating solution and without the use of a permeable membrane. This is accomplished, by pro-viding a bath of the spent solution in which an insoluble metal anode and a cathode of an aluminum shape are introduced, by adjusting the solution within a selected range of pH of about 2 to 12, by applying electric current to the electrodes only in an amount sufficient to restrain or inhibit chemical dissolution of the cathode by galvanic action, and impor-tantly, by maintaining the selected pH throughout the process.
A directly usable pure metal deposit in the form of a film or slab is deposited on the aluminum cathode that has an adhering but peelable relation thereon. The aluminum cathode may be in the form of any suitable shape, such as a sheet, rod, plate, or wire. In a conventional process, the metal salt in solution is electrodeposited on a cathode made of the same metal that is to be extracted. However, such a process has been found to be unsatisfactory in that a coarse, powdery deposit in the nature of a metal oxide or compound results and at a very low efficiency. The deposit has to be further processed to obtain the desired pure metal. The process herein involved provides a rate of metal deposition that is considerably in excess of the 100% that can be achieved with electrolysis governed by Faraday's law, and is not based on electroplating deposition.

Description

~22~

Electroless plating is a growing segment of metal finishing application in the electroplating and electronics in-dustries. The electroless plating processes employ a solution in which the salts of the metal to be plated are dissolved, usually in the form of an inorganic metal salt and a metal che-late, complexed by an organic-type chelating agent. The tight-ness of the chelated bond regulates the rate of deposition which otherwise is governed by the presence of a reducing com-pound in the solution. The deposition is autocatalytic on an initially sensitized metal, plastic, or glass surface. Elec-troless plating, also known as autocatalytic deposition, results in a fine-grained metal deposit very similar in appearance and function to an electroplate. The donor electrons are not de-rived from an external power source, but are furnished by the reducing chemical compound incorporated into the chemical solu-tion make-up. Electroless plating, a growing and expanding in-dustrial application, is most often used as a starting deposit for subsequent electroplating in the process of plating on plastics, printed circuit boards, electroformed dies and other metal objects, plating of internal or hard-to-reach, intricate metal surfaces, and the like.
Some electroless plating processes are also known as "immersion plating" systems. These systems are not based on the electron donor being a reducing compound, but depend on an elec-trochemical exchange reaction. The metal to be deposited is dissolved in an acidic solution, the acid component of which is also a good solvent of the basis metal on which the deposit is to occur. The basis metal of the object to be plated is re-placed by chemical displacement. The electromotive potential of the metal to be deposited has to be more negative than the electrochemical potential of the basis metal, which is slowly 6~L

dissolving in the given environment and is replaced by the re-duced metal taken out of the solution.
As the metal is deposited, the metal content of the solution is depleted. Metal salt additions replenish the metal losses and combine with the chelating compound from which the metal has been removed by the deposition. In time, after sev-eral chemical replenishment additions, the inorganic salts from the metal salt additions and the decomposed reducing chemicals accumulate. This slow accumulation of the chemical content of the solution reduces the deposition rate and may affect the crystal structure of the deposit. Therefore, the solution must be either regenerated or discarded. Regeneration of the spent chemical solution is highly complex and most of-ten is considered uneconomical.
Discarded plating solutions and waste rinse waters, because of their metal content, require waste treatment so that the residual me~als discharged in the effluent will be at a min-imal concentration. Chelated metal compounds are not respon-sive to the conventional waste treatment technology o neutral-ization or high pH treatment, precipitation and settling. Formany years now intensive research on a widespread international scale has been conducted to overcome these difficulties in metal finishing waste treatment. Additionally, an economical loss is created by the disposal of electroless plating solutions since the conventional metal recovery techniques utilized for the far less frequently disposed electroplating baths, such as crystal-lization, precipitation, and recovery of a pure metal hydroxide sludge, ion exchange, or electrolytic removal of the metal be-fore final discharge, are not as applicable to the electroless . 3~0 solutions.
Our efforts to find an economic and simple solution for the treatment problems discussed above have centered on the recovery of the metals in an easily resuable form from the rela-~2~6~

~ively concentrated discarded process solutions. The reasons for this are, first, because most of the metal discharged from a plant would be due to the periodic disposal of the spent pro-cess solution, and secondly because it is far more economical to treat a more concentrated stream inlow volume than a high volume of a dilute waste (rinse water). The process can be made applicable for dilute rinse waters with a modified rinsing process. As will be well appreciated by those familiar with the art, a rinsing system can always be so arranged that the flow volume is reduced and the concentration increased. Ex-amples of such approaches would be counterflow rinsing or a stagnant rinse following the process. This slow-flowing or stagnant rinse accumulates the major portion of the dragout and is periodically discarded. Another example would be discarding the stagnant rinse each time the process solution is changed and recovering the metals from both at the same time. A simple system such as the metal recovery process described hereinafter could solve both the metal recovery and waste treatment needs of the industry.
In our investigations, we have found that convention-al electroplating approaches to strip out the metal content of an electroless solution yield a coarse, powdery deposit at very low efficiency, even if the solution has been modified by the addition of various chemicals, pH changes, etc.
Electrochemical means to recovery have been tried subsequently with some success. The process is based on the addition of zinc or aluminum powder, both base metals with an anodic polarity, to displace nickel, copper, or tin from the solution. This approach yielded a powdery deposit with rapid dissolution to the metal powder added. The use of metal powder is based on the need to increase the surface area which, in combination with rapid stirring, attempts to establish maximum contact between the metal in solution and the powder metal that is displacing it by dissolving in the solution. As an example, the reaction for nickel removal could be written as follows:

2 Al + 3NiC12 ~ 3Ni + 2AlC13 To an electroless nickel plating solution that has contained 400 mg/l Ni we have added 550 mg/ of Al powder (the stoichiometric equivalen~ would have been ~22.7 mg). This so-lution was vigorously stirred at 160F for four hours but only 100 mg of Ni was removed, while the aluminum was completely dis-solved. Somewha~ better results could be achieved with elec-troless copper and tin solutions, but the aluminum consumption was high and the recovered powder metal was not of good value.
The deposlt had a high percentage of oxide lncluded, especlal-ly with the copper, and the recovery of the by-product would have to be effected through a smelter or reEiner.
Most surprisingly, we have found that by using alumi-num shapes such as sheets, rods or wire instead of powder, fast recovery and good quality metal can be gained by an electro-chemical displacement method.
Considering the possibility that we might be able to counter the electrochemical charges developed between a base metal in a solution or more noble metal ions, we have applied a cathodic potential from an external source, such as would be utilized for electroplating, to reduce the chemical dissolution of the aluminum The applied current had to be just sufficient to provide cathodic protection to the aluminum, which now has become a cathode sheet on which the metal deposition could pro-ceed. This modification has been highly successful and has led to the optlmization of this process The results of our re-search have shown that by applying an electrolytic potentlal to the spent electroless platlng solution using aluminum shapes as cathodes, the process has been further improved. For the anode i2~61 electrode, depending on the particular process requirements, insoluble anodes such as steel, platinized titanium, carbon or graphite, etc., have been found suitable. The electrodeposition is very rapid; the cathode plate is uniform, fine grained, of high purity, and can be built up to a sufficient thickness (~/4"

- 3/8") so that the deposit can be periodically removed and re-used in an electroplating process as anode material, or sold as a valuable by-product. The deposit is easily separated from the aluminum cathode; the copper is peelable; the nickel, in view of the high internal stresses, is brittle and will spall off at the boundary layer between the nickel and the aluminum.
The dissolution of the aluminum is significantly in-hibited by the cathodic potential. The pH of solution should also be controlled (pH 2-12) so that the dissolution of the alu-minum is minimized.
Each of the electroless solutions marketed is somewhat different. The variability is due to the particular organic chelating agent and reducing chemical compound utilized, the re-commended pH for the process, and the like. This arises not only due to each supplier's particular formulation, but also be-cause for each metal a different chelate and reducer is appro-priate. The utilitarian use o~ the deposit, the desired ductil-ity, tensile strength, or hardness; luster or conductivity, etc., are the guiding aims in the development and will control the formulation of the process.
There is an optimum pH for each process. During depo-sition, the pH drops due to the removal of the metal and freeing of the combining organic or inorganic acid ion. It is therefore desirable to control the pH during the operation of the recovery process. The examples cited below will be indicative of the pH
ranges found most useful.

3~22~

In the following examples, we have provided typical examples of the process as adapted to particular spent electro-less or immersion plating solutions. We show the rate of metal removal as a function of time and also as related to the applied cathodic current efficiencJT, which most often is considerably above the calculated maximum (100%) cathode current efficiency.
This anomaly is believed to be due to the autocatalytic metal deposition in view oE the residual reducing chemicals in the electroless plating solutions.
The Eollowing Examples are for the purposes of illus-tration only and are not to be considered as limiting on the present invention.

In this example, spent electroless nickel plating so-lution, at an initial pH of 6.1, is treated. The solution pH is raised with 50% caustic soda solution to pH 8.0-8.5. An alumi-num sheet of 2" x 1-1/2" at a temperature of 140F is immersed into 250 ml of this solution, while agitating it by a motorized stirrer. The nickel is removed by a non-electrolytic, immersion process.
Time hr. Ni (~1)Ni removed (g) AlS
0 5.6 --1 4.1 0.37 2 0.7 0.86 3 0.5 0.05 30.7 mg/l The deposit was powdery, non-adherent, and a greyish-blac~ suspension in the solution.
In these examples, the S sign over the particular metal indicates the soluble metal concentration in the solution.

In this example, spent electrcless copper plating so-lution, at an initial pH 12.4, was treated. The solution pH is 6~L

reduced with 50% sulfuric acid solution to ph 11.5-12. An alu-minum sheet of 2-1/4" x 2-3/4" is immersed into 200 ml of this solution at room temperature, while agitating it by a mechanical stirrer. The copper is removed from the solution by non-electrolytic, immersion process.
Time, hr. Cu (g/l) Cu removed (g) AlS

o 5.21 --0.5 0.0885 1.024 1.91 g/l 1.0 0.00015 0.018 2.92 g/l This deposit was powdery, non-adherent, and consisted of a metallic copper with cuprous oxide mixture.

In this example, spent electroless nickel plating so-lution, at an initial pH 6.3, was treated. The solution pH is raised with 50% caustic soda solution to pH 8.0-8.5. The pH is maintained during electrolysis by caustic soda additions. The initial soluble nickel (NiS) concentration is 5.23 g/l. The electrolysis conditions employed are as follows:
Solution volume - 2.5 1; temperature - 140F;
pH - 8.0-8.5; mechanical agitation by propeller mixer.
Mild steel anode, sheet aluminum cathode.

Time CathodeCumulative Cath.
hour Ni (~/1) Efficiency Efficiency AlS

0 5.23 __ __ 1 4.11 1534% 1534%
2 2.85 1726~ 1630%
3 2.25 812% 1358%

4 1.29 1324% 1349%
6 0.70 399% 1033%
7 0.40 416% 944%
8 0.35 73% 836%
9 0.29 27.4mg/1 After the 9-hour electrolysis, the cathode deposit was inspected and was found to be adherent, bright, metallic, suit-able for build-up of additional metal thickness.
The solution on analysis showed a build-up of AlS = 27.4 mg/l and FeS = 14.0 mgll.

12~ 61 Example 4 In this example, spent electroless nickel plating solution at an initial pH of 6.3 was treated. The pH of 6.2 is adjusted to pH 8.0-8.5; nickel content 5.6g/1. The elec-trolysis conditions employed are the same asin Example 3, but the cathode sheet used is one previously employed in the depletion of several other batches of spent solutions, such as of the type of Example 1.
Time CathodeCumulative Cath. S
Hour NiS (g/l)EfficiencyEfficiency Al 0 5.6 1.167 3.522 2009% 2009%
2 2.442 1479% 1789%
3 1.879 644% 1407%
4 0.895 1119% 1335%
0.543 40Z% 1148%
6 0.330 8 0.277 30.2% 767%
12 0.272 1.4% 512% 45.3mg/1 The solution showed a build-up of Al = 45.3 mg/l., Fe = 51.0 mg/l. The deposit was adherent, bright and metallic.
Example 5 In this example, spent electroless nickel plating solutions at a pH of 6.3 was treated. The electrolysis con-ditions employed are the same as in Example 3 but using a coiled aluminum wire cathode at a cathode current density of 4.5 A/sf.
Time CathodeCumulative Cath. S
Hour NiS ~g/l)EfficiencyEfficiency Al 0 6.20 1-3/4 1.70 762% 762%
3 0.764 224% 538%
4 0.453 98% 428% llOmg/l The deposit obtained is metallic, adherent, suit-able for continuous build-up.
Example 6 In this example, spent electroless copper plating solution, at an initial pH of 10, was treated. The pH was not further controlled and during the 6-hour electrolysis it dropped to a pH of 7.9. The electrolysis conditions are as :~2;~

Solution volume 500 ml; temperature - room;
mechanical agitation.
Platinum pLated titanium anode, sheet aluminum cathode.
DC power 2~5V~ 0~3~0~4 A/sf cathode area.
Time CathodeCumulative Cathode Hour CuS (g/l) EfEiciency Efficiency 0 5~1 1 4 ~ 2 113% 113%
2 3~0 154% 134%
3 1.6 159% 142%
4-1/2 57 mg/l 120% 135%
2 ~ 77 mg/l 2 ~4% 121%
6 2 ~ 29 gm/l The aluminum concentration at the end of 6 hours is 106 mg/l.
The deposit obtained is metallic, but with cuprous oxide inclusions and non-adherent copper oxide powder in the solution.

In this example, spent electroless copper plating so-lution, at an initial pH of 11~8~ is treated. The pH is main-tained at 11.0-11.5 during electrolysis.
The electrolysis conditions are as follows:
Solution volume 500 ml; room temperature;
mechanical agitation; aluminum cathode, mild steel anode.
DC power, 2 ~ OV ~
Time S Cathode Cumulative Cath. S
30 Hour Cu (g/l) Efficiency Efficiency Al 0 2~99 1 1 ~ 63 819% 819%
2 1.21 252% 536%
0.175 208% 339%
6 0 ~ 064 67% 294%
7 O ~ OOS9 35~/~ 256% 1800 mg/l Aluminum concentration at the end of 7 hours = 1.8 g/l .

--9 ~

~.Z~6~
The deposit obtained is bright, metallic and suitable for cathode plate buiLd-up 1/4" - 3/8" thick.

In this example, spent electroless copper plating solution is treated.
The electrolysis conditions employed are the same as in Example 7.
Time Cathode Cumulative Cath. S
Hour CuS (g/l) Efficiency Efficiency Al 0 2.050 1 0.676 827% 827%
2 0.322 213% 520%
4 0.242 24% 272%
4.67 0.0351 186% 260%

5.17 0.0024 41% 239% 840 mg/l Aluminum concentration after 5.17 hours of electrol-ysis was 840 mg/l. The deposit is the same in character as that obtained in Example 7.

In this example, spent immersion tin plating solu-tion is treated. This solution solidifies on cooling to room temperature and is in the form of a gelatinous semi-solid ~hat has to be heated to loo& to re-establish the liquid consis-tency. The solution is highly acidic, containing free hydro-chloric acid. Added caustic soda adjusted the pH to 2Ø The tin content is 13 g/l.
During immersion plating or electrolysis the solu-tion must be cooled as heat is generated by aluminum dissolu-tion and the electrolysis. The temperature was maintained in the range of 80-95C.
Aluminum shapes in the form of rods are immersed in the spent tinning solution for immersion plating. In approxi-mately 45 seconds most of the tin is removed as a coagulated precipitate in the cell. The tin content of the sample has been reduced from 13 g/l to 310 mg/l, while the dissolved alu-~.~2~3~6~

minum has increased by 12.6 g/l. the deposit is not adherent, but consists of easily removable aggregate of metallic tin crystals.
Example 10 In this example, spent immersion tin plating solution is heated and partially neutralized to pH 2 as in Example 9.
This solubion~ contained 11 g/l tin. Subsequently the solution was electrolyzed to remove the tin metal content.
Electrolysis conditions employed are: temperature 90-95C; plantiniæed titanium anodes and aluminum wire cathode;
mechanical agitation, D.C. power is 2V, 3.5-7 A/sf.
Several tests were conducted using these conditions, and it is found that the tin removal rate is approximately 15.1 g/A hr. F'or each gram of tin removed, approximately lg of Al is dissolved in the solution. The residual tin remaining in the solution is in the range o~ 100-350 mg/l. The tin deposit can be built up to a thickness of 3/8" and it is an adherent, but peelable pure metallic deposit.
The sign in these examples indicates soluble metal in solution.
From the previous discussion, it is apparent that an important or key feature of the present invention involves the use of a pH within a range of about 2 to 12 and the main-tenance of such a selected pH throughout the process. This assures the recovery of the dissolved metal content in the waste solution, such as the copper, nickel, or tin, as a pure metal film or slab in an inherent relation on the aluminum shape which is employed as the cathode. this will be apparent from a comparison of the results obtained in EXAMPLES 3, 4, 5, 7, 8, and 10 with the adverse results obtained in EXAMPLES
1, 2, 6, and 9.

Claims (9)

THE EMBODIMENTS OF THE INVENTION CLAIMED ARE AS FOLLOWS:

1. A process for recovering metal values in an adherent pure metallic slab form from a spent electroless or immersion plating solution containing soluble metal to be recovered that is more noble than aluminum and without requiring the use of a permeable membrane which comprises: providing a bath of the spent solution with a pair of electrodes comprising an insoluble metal anode and an aluminum shape to be solely employed as a cathode in a spaced-apart relation in the bath, adjusting the solu-tion to a selected pH within a range of about 2 to 12 and substantially maintaining the selected pH throughout the process, while applying electric current from an external direct current power source to the electrodes to energize them in an amount sufficient to provide cathode protection, to assure employment of the aluminum shape as a cathode and to inhibit its chemical dissolution, and thereby removing the soluble more noble plating metal from the solution and forming it as a slab-like peelable pure metal in an adhering relation on the surface of the aluminum shape.

2. A process as defined in claim 1 wherein the insoluble anode is of mild steel.

3. A process as defined in claim 1 wherein direct electric current of at least 0.2A/s.f. is applied to the electrodes.

4. A process as defined in claim 1 wherein the solution is agitated.

5. A process as defined in claim 1 wherein, the insoluble anode is of a material in the nature of mild steel, platinized titanium, carbon or graphite, and the cathode is in the form of aluminum sheet, rod or wire.

6. A process as defined in claim 1 wherein the operation is continued in the defined manner until a slab-like peelable, uniform, fine grained, highly pure metal deposit is formed on the aluminum cathode of a thickness of about 1/4 to 3/8 of an inch.

7. A process for recovery of metal values in pure metallic plate-like form from a spent electroless or immersion plating solution containing a chemical compound for reducing metal more noble than aluminum, comprising providing an insoluble anode and a solid aluminum shape in the spent solution as spaced-apart electrodes therefor, agitating the solution, selecting a pH
within a range of about 2 to 12, and maintaining the selected pH
in the solution during the processing while applying electric current to the electrodes in an amount to inhibit galvanic action on and protect the aluminum cathode from dissolution, while building-up plating metal from the solution in the form of a pure metal coating adhering in peelable form on the surface of the aluminum cathode shape.

8. A process as defined in claim 7 wherein electro-chemical displacement is employed to initiate a metallic deposit of the more noble metal displaced from the solution on a cathode shape of the aluminum base metal which is cathodically protected to build up a substantial metallic deposit by the use of direct current supplied from an external power source.

9. A process as defined in claim 7 wherein an electric current of at least 0.2A/s.f. is applied.