GB2293390A - Simultaneous etchant regeneration and metal deposition by electrodialysis - Google Patents

Abstract

A solution of spent etchant eg. ferric chloride etchant for etching nickel or copper chloride for etching brass is electrolysed as the analyte in a cell divided by a semipermeable membrane so that simultaneously the etchant is regenerated and metal eg. iron-nickel alloy or zinc containing minimum Cu respectively is deposited directly on the cathode, using a cathode current density of under 3000 A/m<2>. The catholyte preferably comprises a Gp 1A halide eg. NaCl, a catalyst for reduction of metal cations eg. boric acid and a reducing agent for the deposition of the cations eg. sodium hypophosphite and has an independently adjusted pH eg. of 1 - 5.

Description

ETCHANT REGENERATION This invention relates to etchant regeneration, that is, to a method by which spent etchant (which has been used e.g. for patternwise dissolution of selected areas from a metal foil), apart from being rendered less environmentally harmful, is regenerated, i.e. oxidised back to a reusable form and/or ions of the said metal are removed to a sufficient extent for them to cease inhibiting or upsetting the etching action. Such regeneration is necessary from time to time in order to maintain a consistent etch rate and product quality in the etching bath, the alternative being to use fresh etchant and throw away the spent etchant, with all the attendant costs and environmental problems. However, in requiring simultaneous reduction of the said ions and oxidation of the etchant, such regeneration will not be straightforward.The problem is a large-scale one, with an estimated 103 tonnes of nickel being dissolved into etchant per year.

For reasons set forth in "Nickel Etching Economics", Allen and White, The Journal, Fall 1992 pp 4-11, electrodialysis is considered to be a more promising approach to disposal/regeneration than is solvent extraction, ion exchange or cementation.

The problem is not confined to nickel. For example, brass which has been etched (photochemically machined) with ferric chloride gives rise to a most inconvenient cocktail of cations, and the etchant must similarly be regenerated (or otherwise replaced) from time to time.

The invention also addresses the related problem that it may be desirable to recover metals from the cations in different proportions than in the spent etchant composition.

According to the present invention, there is provided a method of etchant regeneration, comprising electrodialysing the etchant as the anolyte in a cell divided by a semipermeable membrane, the catholyte being at an independently adjusted pH, e.g. from 1 to 5 and at a current density on the cathode of under 3000 A/m2, the catholyte preferably comprising 0.1 M to 6M Gp IA halide, and 0.1 M to 2M catalyst, adsorbable on the cathode, for reduction of the cations arising from the metal which has been etched.

There is also provided a method of electroplating a metal or an alloy, comprising electrodialysing a solution of mixed cations as the anolyte in a cell divided by a semipermeable membrane, the catholyte being at an independently adjusted pH.

e.g. from 1 to 5, and at a current density on the cathode of under 3000 Alum2, the catholyte preferably comprising 0.1M to 6M Gp IA halide, and 0.1M to 2M catalyst, adsorbable on the cathode, for reduction of the cations arising from metal which has been etched, whereby the metal or an alloy thereof is deposited on the cathode. The catalyst may be a multi-effect substance such as boric acid.

Preferably the catholyte also contains a reducing agent for deposition (as a metal! of the said cations, e.g. sodium hypophosphite, e.g. up to IM, such as 0.005-O.SM. The catholyte may also contain a stabiliser for the metals which respectively form the etchant (thus Fe where the etchant is FeC13 solution) and the metal being etched (for example Ni) and may be the stabiliser known at the priority date by the trade mark Attotech Iron-Nickel Stabiliser C, present in a concentration of preferably 2-100 g/l, such as 5-50 g/l. The stabiliser may be or comprise a reductant, oxidant, complexant or chelating agent, such as tartaric acid, e.g. 90% tartaric acid and 10% other components.

The current density of the electrodialysis is preferably from 50 to 2500 A/m-.

preferably under 1000, e.g. under 500, e.g. under 350, such as 300 A/m2 or less, for example under 200 or 150, on the cathode, which may be any convenient inert material.

Electrodialysis conveniently proceeds at from room temperature to 1 00 C, for example 400C to 70"C.

Anolyte may be continuously cycled from an etching bath into the cell and back to the etching bath. The etching bath may be connected to a number of cells in parallel for operating flexibility and to match regeneration rate to demand.

In the above-mentioned case of etching Ni with FeC13, the result of the regeneratior method according to the invention may be the plating of Ni and Fe together onto the cathode. This can be technically useful in its own right, for example in the manufacture of magnetic recording heads. Because of the complexities of etching brass (Cu/Zn) with FeC13 and regenerating the etchant, brass may instead be etched using cupric chloride CuC12. This gives rise to Cu(II), Cu(I) and Zn(II) ions. The ideal would be to oxidise the Cu(I) back to Cu(II) and to recover Zn with a minimum of Cu .

Discussing the invention in a non-limitative way, considering the case of FeC13 etchant used on nickel foil and being regenerated in such a way as simultaneously to produce Fe-Ni alloy for magnetic recording heads, the electroplating of Fe and Ni is an anomalous case, wherein the Fe is usually deposited preferentially (even if Ni is present in excess) although Fe is nominally more electronegative than Ni. To achieve a reasonable proportion of Ni in the final alloy, such as 35 to 60 times the Fe (e.g. one-tenth to one-fifth the Fe) the cations to be deposited should be present in the proportions Ni:Fe or Fe:Ni = 35-60:1. The actual proportions in spent etchant are typically Ni:Fe = 1:23.

This conflict can be resolved by treating the spent etchant as an anolyte separated from the catholyte (from which the desired alloy is to be plated) by a semi-permeable ion-exchange membrane. By keeping Fe+tt substantially out of the catholyte, it is also prevented from etching nickel freshly deposited on the cathode. Also, the membrane allows the pH of the catholyte to be kept different from the anolyte and within an appropriate range, e.g. 2 to 4 especially 3 to 33/4 (too low, and H2 gas evolves (wasting current); too high, and electroless precipitation occurs). Fe++ in the anolyte may migrate through the membrane to the cathode and be deposited as Fe" or may, despite the electrostatic repulsion, strike the anode and be regenerated to Fe+++. When this no longer occurs. the counter ion, for example Cl-, is oxidised to Cl2 .

The composition of the alloy deposited on the cathode depends on two types of variable, the bath composition and the process parameters.

The bath composition can only be varied, since the spent etchant must be considered as invariable b! using a semipermeable membrane to separate the spent etchant (in the anode compartment) from a catholyte of one's choice. The catholyte composition is considered later. The anolyte may however be acidified by e.g. HC1, which prevents hydrolysis of Fe and deposition of iron hydroxides in the membranes.

The process parameters include current density, not as useful a variable in practice as in theory, temperature, and agitation of one or both electrode compartments.

The catholyte preferably contains an indifferent carrier salt for conductivity, e.g. NaCI, a multieffect additive such as H3B03, and an alkali such as NaOH to adjust the initial pH to a suitable value, e.g. 3l/2 + V2. In summary,

ANOLYTE CATHOLYTE Comments ELECTROLYTE Spent Ferric Sodium Chloride + anions common to Chloride Etchant Additives both (Fe2+,Fe3+,Ni2+ (Na+,H+,OH-,Cl-) THEORETICAL Fe2± Fe3+ + e Ni2+ + 2e' - Ni Ratio of Ni::Fe REACTION (oxidation) Fe2+ + 2e' - Fe plated out depends (reduction) on electrodialysis parameters OPERATIONAL PARAMETERS 1) Current density RANGE 10-70 mA/cm2, preferably under 30 mA/cm2 for high nickel deposit 2) pH < 1 3.5+0.5 3) Temperature 50 C 50 C 4) Agitation None Used in some expts.

5) Redox < 640 mV N/A possibility of clan Potential - 610 mV generation if > 640 mV TABLE 1 - Electrodialysis parameters for removal of nickel Duration : 260 minutes* Mass of alloy deposited on cathode : 2.1 g/l of catholyte Agitation: none Current consumption: 10.4 Ah/l of catholyte *After this time, the etchant was sufficiently regenerated for re-use. The catholxte still contained depositable cations.

The invention will now be described by way of example with reference to the accompanying drawings, which is a schematic diagram of an electrodialysis cell for carrying out the method.

A hot plate 1 with spacers heats a water jacket 2 supporting a glass electrodialysis cell 3, which is divided by a semipermeable membrane 4 into anode and cathode compartments 5 and 6 of capacity 3/4 litre each. The anode compartment 5 is equipped with a thermometer 7 and a platinum combination redox probe 8, as well as a graphite anode 9 parallel to the membrane 4 and insulated on the back and sides. The cathode compartment 6 is equippped with a pH probe 10 and a temperature probe 11 as well as a stainless steel cathode 12 parallel to the membrane 4 and insulated on the back and sides.

The surface of the cathode 12 is agitated by a stream of bubbles from a gas supply 13. The cell can be controlled according to either anode/cathode voltage or anode/cathode current.

The membrane 4 was Nafion 450 by du Pont or CMX I by Tokuyama Soda.

The anolyte is spent 8.6% 45" BaumC ferric chloride etchant used for etching pure nickel A solution of identical composition can be synthesised from 0.324 M ferrous chloride 0.162 M nickel chloride 3.44 M ferric chloride 1% w/w hydrochloric acid Distilled water to make up to specific gravity 1.42.

The cathol!rte is synthesised from 0.8 M sodium chloride 0.4 M boric acid 8-31 g/l Attotech Iron-Nickel Stabiliser C 10-100 mM sodium hypophosphite NaH2PO2 sodium hydroxide to bring initial pH of catholyte to 3.5.

Specific gravity is 1.05.

Operational parameters: 1) Current density ranged from 10-75 mA/cm2 2) Temperature (anolyte and catholyte) maintained at 50i5 C 3) pH of catholyte 3.5i0.5 (i.e. initially set as close to 3.5 as posisble, would then increase, decrease or remain constant according to other variables ideally within the given range) 4) Electrode separation was 2-3 cm 5) Air agitation (in the catholyte) was used in a few experiments.

Electrolysis is continued until the mass of deposit on the cathode is 1.58 g.

In two experiments the following results were obtained: EXPERIMENT 1: Nickel yield = 12.05% Catholyte composition: 0.8M NaCI, 0.4M H3BO3, iron-nickel stabiliser 15 girl.

100 mMNaH2PO2 (8.8 girl), initial pH = 3.3 using NaOH Operational parameters: average voltage = 3.3 V, current density = 30.21 mA/crrr.

temp= 50 C Final pH of catholyte was 3.9.

EXPERIMENT 2: Nickel yield = 18.4% Catholyte composition: 0.8M NaCI, 0.4M H3BO3, iron-nickel stabiliser 30 g1.

100 mMNaH2PO2 (8.8 g/l), initial pH = 3.3 using NaOH Operational parameters: average voltage = 2.21V, current density = 12.3 mA/cm-.

temp = 530C Final pH of catholyte was 3.23.

From these experiments, it may be seen that etchant regeneration and nickel recovery can be achieved simultaneouslv, with nickel forming 12-18% of Fe-Ni alloy deposited on the cathode, i.e. a higher proportion of nickel in the deposit than in the catholyte.

Claims (15)

1. A method of etchant regeneration, comprising electrodialysing the etchant as the anolyte in a cell divided by a semipermeable membrane, the catholyte being at an independently adjusted pH, e.g. from 1 to 5, and at a current density on the cathode of under 3000 A/m2, the catholyte preferably comprising 0.1M to 6M Gp IA halide, and 0.1M to 2M catalyst, adsorbable on the cathode, for reduction of the cations arising from the metal which has been etched.

2. A method of electroplating a metal or an alloy, comprising electrodialysing a solution of mixed cations as the anolyte in a cell divided by a semipermeable membrane, the catholyte being at an independently adjusted pH, e.g. from 1 to 5, and at a current density on the cathode of under 3000 A/m2, the catholyte preferably comprising 0.1M to 6M Gp IA halide, and 0.1M to 2M catalyst, adsorbable on the cathode, for reduction of the cations arising from metal which has been etched, whereby the metal or an alloy thereof is deposited on the cathode.

3. A method according to Claim 2, wherein the said solution comprises used etchant.

4. A method according to Claim 1 or 3, wherein the etchant contains Fe l i | ions.

5. A method according to Claim 4, wherein the etchant has been used to etch nickel.

and wherein proportionately more Ni than Fe is preferably deposited on the cathode.

6. A method according to any preceding claim, wherein the catholyte further comprises a reducing agent for deposition as a metal of the said cations.

7. A method according to Claim 6, wherein said reducing agent is a hypophosphite.

8. A method according to any preceding claim, wherein the catholyte contains stabilisers for the electroplatable cations of the anolyte.

9. A method according to Claim 8, wherein the stabilisers are reductants. oxidants.

complexants and/or chelating agents.

10. A method according to any preceding claim, wherein the catholyte is agitated.

11. A method according to any one Claims 1 to 9, wherein the catholyte is not agitated.

12. A method according to any preceding claim, wherein the current density does not exceed 300 A/m2 on the cathode.

13. A method according to any preceding claim, when performed at from 40"C to 70or.

14. A method according to any preceding claim, wherein the pH of the catholyte is within the range 2-4.

15. A method according to any preceding claim, wherein the anolyte is continuously cycled from an etching bath into the cell and back to the etching bath.