GB1574989A

GB1574989A - Metal plating process

Info

Publication number: GB1574989A
Application number: GB1816479A
Authority: GB
Original assignee: Pitt Metals and Chemicals Inc
Current assignee: Pitt Metals and Chemicals Inc
Priority date: 1976-08-20
Filing date: 1977-08-15
Publication date: 1980-09-17
Also published as: JPS5326237A; GB1574990A; GB1574988A; CA1133418A

Description

PATENT SPECIFICATION

( 11) ( 21) Application No 18164/79 ( 22) Filed 15 Aug 1977 o ( 62) Divided Out of No 1 574 988 Ci ( 31) Convention Application No.

716 225 ( 32) Filed 20 Aug 1976 in ( 33) United States of America (US) ( 44) Complete Specification published 17 Sept 1980 ( 51) INT CL 3 C 25 D 21/22 3/30 ( 52) Index at acceptance C 7 B 102 106 120 265 267 552 737 KC ( 72) Inventor RICHARD E HORN ( 54) METAL PLATING PROCESS ( 71) We, PITT METALS AND CHEMICALS, INC, a corporation organised and existing under the laws of the Commonwealth of Pennsylvania, U S A, having its principal place of business at 4314 Main Street, Pittsburgh, Pennsylvania 15224, United States of America do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to electroplating processes, e g tin plating processes, in which rinse water used to rinse the plated articles is treated to recover valuable plating metal, e g tin therefrom.

Alkali metal stannate tin plating, using potassium or sodium stannate is a commercial method of plating tin to base metal substrates Conventionally this process has been carried out using soluble tin anodes.

The use of tin anodes causes a substantial number of control problems If the anode current density is too low, the tin dissolves in the form of stannite or stannous tin which causes rough, poorly adhering plate.

If the anode current density is too high, an insoluble film forms on the surface of the anodes and the tin does not dissolve These problems require accurate control of the anode current density which will vary as the soluble tin anodes are consumed This in turn limits the range of current density usable in alkali metal stannate.

An alternative to using soluble anodes is to use insoluble anodes As there is no dissolution of the anode material, the allowable current densities are much greater and more flexible However, in both cases substantial losses of tin may occur through drag out from the plating bath and accumulation in the rinse water used to rinse the plated articles These losses can result in an otherwise economic process becoming uneconomic and there is therefore a need for an economic recovery process for the plating metal from the rinse water and in accordance with the present invention this is achieved by electrodialysis 50 of the rinse water and recycle of the recovered plating metal to the plating bath.

Broadly speaking, the process of the present invention comprises a metal plating process, which comprises electrodepositing 55 a metal on a workpiece from an aqueous metal plating bath containing, in solution, a compound containing said metal as an ionic complex and an alkali metal, removing the plated workpiece from the bath, rinsing 60 the plated workpiece with water, and treating the rinse water to recover excess plating metal therefrom, wherein said treatment comprises electrodialysing the rinse water in an electrodialysis cell, said cell having an 65 anode compartment containing an aqueous anolyte solution, a cathode compartment containing an aqueous catholyte solution, and a neutral compartment positioned between said anode and cathode 70 compartments and into which is fed said rinse water, said neutral compartment being separated from said anode compartment by a permselective membrane permeable to the metal ions to be recovered, 75 and from said cathode compartment by a cation permselective membrane, thereby to accumulate said metal ions in said anolyte.

The invention will be further described with reference to the accompanying draw 80 ing in which the single figure is a flow diagram illustrating a metal plating tank, a rinse water tank and a three compartment electrodialytic cell as may be used in the practice of this invention The circulation 85 of the rinse water to the centre compartment and the recovery of the plating metal therefrom, is illustrated diagrammatically.

Referring to the drawing there is diagrammatically illustrated a plating tank 90 1 574 989 1 574 989 and a rinse tank The plating tank holds the aqueous plating bath, e g aqueous potassium stannate solution and the electrochemical plating process takes place within the plating tank The material plated is then transferred to a rinse tank where the material is thoroughly rinsed with water.

Where elongated strips are plated the metal strips pass through the plating tank and then into the rinse tank A substantial amount of the aqueous tin-plating bath is transferred from the plating tank to the rinse tank by the rapidly moving metal strips.

This loss of the plating bath is commonly referred to as a 'drag out' of the plating bath.

Although the plating quality with the high drag out may be acceptable, there is an extremely high loss of valuable potassium stannate in the drag out and the possibility of corrosion problems and safety problems from the high caustic levels in the rinse water The rinse water could be concentrated by conventional methods to recover a concentrated solution of tin compounds and the alkali metal hydroxide.

This, however, is expensive and would then result in excessive alkali metal hydroxide in the plating bath of the concentrated solution were returned to the plating tank.

As illustrated in the drawing the plating tank is generally designated by the numeral and the rinse tank by the numeral 102.

The transfer of the plating bath to the rinse tank is indicated diagrammatically by the line 104 and designated drag out The process for recovering the tin units from the rinse water includes an electrodialytic cell 106 that has a centre compartment 108, an anode compartment 110 and a cathode compartment 112 In the latter two there are suspended respectively an anode 114 and a cathode 116.

A conduit 120 is arranged to convey rinse water containing potassium stannate and potassium hydroxide from the tank 102 to the electrodialytic cell centre compartment 108 Water is introduced into the cathode compartment 112 and the tinplating bath which includes both the potassium stannate and potassium hydroxide in solution is withdrawn from the plating tank 100 and introduced into the cell anode compartment 110 through conduit 124.

When a current is impressed across the anode and cathode of the cell 106 the stannate ions in the rinse water in the centre compartment 108 migrate through the anion permselective membrane or neutral membrane 126 into the anode compartment 110.

In the anode compartment 110 the stannate ions react with the alkali metal hydroxide in the plating bath introduced into the anode compartment 110 through conduit 124 and form potassium stannate The plating bath enriched in potassium stannate and reduced in potassium hydroxide is withdrawn from the anode compartment through outlet conduit 130 and returned to the plating bath in the plating tank 100 70 The rinse Water is withdrawn from the centre compartment 108 through conduit 132 and the water introduced into the cathode compartment 112 is withdrawn through conduit 134 The rinse water and 75 the water from cathode compartment 112 contain potassium hydroxide and may be neutralised for re-use or may be discarded.

The potassium ions that migrate from the cell centre compartment 108 to the cathode 80 compartment form an alkali hydroxide solution in the cathode compartment 112.

In another embodiment illustrated in the drawing by dotted lines the water introduced into the cathode compartment 112 is 85 withdrawn therefrom through conduit 136 as an alkali hydroxide solution and introduced into the anode compartment 110.

This eliminates recycling the plating bath and the alkali metal hydroxide in the 90 solution introduced through conduit 136 from the cathode compartment 112 reacts with the stannate ion in the anode compartment 110 to form potassium stannate.

The potassium stannate is withdrawn 95 through conduit 138 and introduced as a part of the plating bath.

With the above process it is apparent that it is now possible to recover the tin units in the rinse water by the migration 100 of the stannate ions from the rinse water in the electrodialytic cell centre compartment 108 to the anode compartment 110.

A typical electroplating bath usable in the above process comprises an aqueous 105 potassium stannate solution containing g/l of potassium stannate and 23, g/l excess potassium hydroxide Suitable cation permselective membranes usable in the electrodialysis cell are manufactured by 110 the Ionac Chemical Company and designated 'MC-3470 '.

Although the embodiment illustrated in the drawing discloses a three compartment cell it should be understood that cells 115 having a greater number of compartments may be employed as long as there is a neutral' compartment between the anode and cathode compartments.

In conclusion, attention is drawn to our 120 copending application No 7918165 (Serial No 1574990).

Claims

WHAT WE CLAIM IS:-

1 A metal plating process, which comprises electrodepositing a metal on a work 125 piece from an aqueous metal plating bath containing, in solution, a compound containing said metal as an ionic complex and an alkali metal removing the plated workpiece from the bath, rinsing the plated 130 1 574989 workpiece with water, and treating the rinse water to recover excess plating metal therefrom, wherein said treatment comprises electrodialysing the rinse water in an electrodialysis cell, said cell having an anode compartment containing an aqueous anolyte solution, a cathode compartment containing an aqueous catholyte solution and a neutral compartment positioned between said anode and cathode compartments and into which is fed said rinse water, said neutral compartment being separated from said anode compartment by a permselective membrane permeable to the metal ions to be recovered, and from said cathode compartment by a cation perselective membrane, thereby to accumulate said metal ions in said anolyte.

2 A process according to claim 1, wherein the anolyte solution comprises a portion of the metal plating bath withdrawn therefrom and wherein, after accumulation of the metal ions therein, the anolyte is recycled to the metal plating bath.

3 A process according to claim 1, wherein the catholyte is water and wherein the alkali metal hydroxide solution, which accumulates in said cathode compartment as the result of migration of alkali metal ions from the rinse water into the catholyte 30 through said cation permselective membrane, is passed to said anode compartment as said anolyte thereby to form in said anode compartment, with the metal ions accumulating therein, an aqueous alkali 35 metal plating solution which is recycled to the plating bath.

4 A process according to any one of the preceding claims being a tin plating procss and wherein the aqueous plating bath com 40 prises an aqueous solution of an alkali metal stannate.

A process according to claim 1, substantially as hereinbefore described with reference to the accompanying drawing 45 For the Applicants, D YOUNG & CO, Chartered Patent Agents, 9 & 10 Staple Inn, London, WC 1 V 7RD.

Printed for Her Majesty's Stationery Office by The Tweeddale Press Ltd, Berwick-upon-Tweed, 1980.

Published at the Patent Office, 25 Southampton Buildings, London, Wic 2 A l AY, from which copies may be obtained.