GB2229735A

GB2229735A - Electrolytic recovery of precious metals from electromechanical contacts

Info

Publication number: GB2229735A
Application number: GB9004445A
Authority: GB
Inventors: David Eric Bostock
Original assignee: BOSTOCK JILL
Current assignee: BOSTOCK JILL
Priority date: 1989-03-17
Filing date: 1990-02-28
Publication date: 1990-10-03
Anticipated expiration: 2010-02-28
Also published as: GB9004445D0; GB8906206D0; GB2229735B

Abstract

A method of recovering precious metal contacts from the tips of contact arms of base metal comprising providing an electrolytic system consisting of an anode, a cathode, an electrolyte and an electrical power source, electrically connecting the contacts to form the anode, placing the anode and the cathode in the electrolyte, and connecting the electrical power source to apply a potential difference across the anode and cathode, whereby metal is electrolytically removed from the arms to release the contacts.

Description

TITLE: RECOVERY OF PRECIOUS METALS FROM ELECTRO MECHANICAL CONTACTS DESCRIPTION This invention is concerned with the recovery of precious metals from electro-mechanical contacts and particularly, but not exclusively, from contacts in scrapped telephone exchange switch-gear (e.g. of the Strowger and crossbar types).

It has been a normal practice for many years to provide switch-gear with contacts comprising arms or springs of a base metal (usually a copper alloy, such as cupro-nickel or phosphor bronze) having bonded to their tips electrical contacts of a precious metal (which term includes, in the present specification, silver, gold, platinum, palladium, niobium and alloys thereof).

With the advent of solid state electronics, an increasing quantity of electro-mechanical switch-gear equipment is becoming redundant and it is important to recover metals from it. At present, precious metal contacts from such equipment are recovered by methods which involve cutting them from their arms or springs or granulating the relay sets which carry them. These methods are time consuming and result in the need for an expensive second process to recover the precious metals from the resulting mixture of base metals and precious metal. This task is made more difficult and expensive by the fact that the precious metal forms only a small part of the mixture, e.g. of the order of 1% by weight in the case of the granulating system and 10% by weight in the case where the contacts are cut from the arms.

It is an object of the invention to provide a method for the recovery of precious metals from electro-mechanical contacts which does not require the use of expensive equipment, which causes little or no noise or environmental pollution, and which enables the precious metals to be recovered in relatively pure form at low cost.

According to the invention, a method of recovering precious metal contacts from the tips of contact arms of base metal comprising providing an electrolytic system consisting of an anode, a cathode, an electrolyte and an electrical power source, electrically connecting the contacts to form the anode, placing the anode and the cathode in the electrolyte, and connecting the electrical power source to apply a potential difference across the anode and cathode, whereby metal is electrolytically removed from the arms to release the contacts.

Preferably the method comprises dipping the tips of the contact arms in the electrolyte only to a depth sufficient to immerse the contacts, so that the minimum of base metal is removed.

The method may comprise the step of recovering the released contacts from the electrolyte and washing the contacts.

Advantageously the method comprises electrically connecting the contact arms in circuit using the existing electrical connections of the contact arms. If desired the method may comprise applying electrical current to the electrolytic system at a level above that which would cause the metal electrolytically removed from the arms to become plated on the cathode, whereby the metal so removed is precipitated in powder form.

The following is a more detailed description of how the present invention may be carried out in practice.

Precious metal contacts in telephone exchange relays or switch-gear, which contacts are composed of silver, gold, platinum, palladium and niobium and their alloys, are bonded to the tips of metallic spring arms made from cupro-nickel, phosphor bronze or similar base metal alloys, which also act as electrical conductors. The spring arms are arranged in banks and are often electrically connected to removable electrical connectors by which the contacts are connected to other parts of the telephone exchange equipment. Alternatively, where no removable electrical connectors are provided, the contacts are directly connected to other parts of the equipment by means of multicore cables.

In accordance with the present invention, in one embodiment, relay or switch-gear banks are removed intact from supporting racks and inverted into a vat of an electrolyte, so that the tips of the spring contact arms are dipped into the electrolyte to a depth which just covers their precious metal contacts. The bank is then made the anode of the electrolytic bath, the cathode being one or more thin sheets of a conducting material such as copper, and direct current is passed between the bank and the cathode.

Some switchgear banks are provided with detachable electrical connections which may be connected to the D.C.

supply by means of an appropriate sliding connector from its equipment rack. Alternatively switchgear banks where the contacts are permanently wired and which terminate in cable can be connected via that cable, following the stripping of insulation therefrom, e.g. by heat.

The electrolyte used can be any suitable conducting fluid but for good separation a solution of 2 to 3% aqueous sulphuric acid by volume at ambient temperature has proved to be both cheap and efficient. Using this electrolyte, with a voltage of 4 to 24v D.C. and cathodes of copper sheet the base metal contact arms are dissolved preferentially so that the contacts become detached and fall to the bottom of the vat. They can then be recovered periodically and washed free of base metal powder and electrolyte. In practice care is taken to dip as little of the arms into the electrolyte as is possible so that the need to dissolve base metal is minimised.

Surprisingly it has been found that separation tends to occur at the junction between the precious and base metals. The purity of such contacts has been found to be of the order of 80 to 90% by weight. Therefore subsequent refining is relatively easy. The remainder of the switchgear bank is then available for copper refining in the usual way.

The base metals initially pass into solution but, as the bath matures, they are deposited on the cathodes, either as a coherent mass or as powder, depending upon the current density employed. The current density is in turn dependent on the voltage and electrolyte concentration, neither of which need to be closely controlled. However, although the process is finished much sooner with high current densities, it may result in foaming at the cathodes due to hydrogen evolution.

Preferably the current density is such that the base metals form a powder which tends to fall to the bottom of the electrolytic bath. It has been found that the powder forms a conductive layer which acts as an extension of the cathodes and thereby increases the efficiency of the process.

Since the contact arms are electrically insulated from their supports in the switchgear bank, once they have dissolved to the level of the surface of the electrolyte the current automatically ceases to flow, so that the process is self limiting and does not require any supervision. With a D.C. voltage of 12v and a sulphuric acid concentration of 3% the contacts will usually have separated in about 7 hours.

Claims

1. A method of recovering precious metal contacts from the tips of contact arms of base metal comprising providing an electrolytic system consisting of an anode, a cathode, an electrolyte and an electrical power source, electrically connecting the contacts to form the anode, placing the anode and the cathode in the electrolyte, and connecting the electrical power source to apply a potential difference across the anode and cathode, whereby metal is electrolytically removed from the arms to release the contacts.

2. A method according to claim 1, comprising dipping the tips of the contact arms in the electrolyte only to a depth sufficient to immerse the contacts.

3. A method according to claim 1 or claim 2, comprising the step of recovering the released contacts from the electrolyte and washing the contacts.

4. A method according to any preceding claim, comprising electrically connecting the contact arms in circuit using the existing electrical connections of the contact arms.

5. A method according to any preceding claim, comprising applying electrical current to the electrolytic system at a level above that which would cause the metal electrolytically removed from the arms to become plated on the cathode, whereby the metal so removed is precipitated in powder form.

6. A method according to claim 5, wherein the arrangement is such that the precipitated metal powder forms a layer which becomes an extension of the cathode.

7. A method according to any preceding claim comprising using a solution of 2 to 3% aqueous sulphuric acid by volume at ambient temperature as the electrolyte and applying a D.C. voltage in the range 4 to 24v.

8. A method according to claim 1, substantially as hereinbefore described.