Gold recovery system

A procedure is described for recovering gold from rinse water. The recovery procedure involves electrochemical removal of the gold from the rinse water and is particularly advantageous for removing gold in low concentrations from rinse water.

TECHNICAL FIELD 
The invention involves recovery of gold from aqueous solutions. The process 
is particularly useful for recovering gold from rinse water used in 
various processes involving gold such as gold plating processes. 
BACKGROUND OF THE INVENTION 
Gold plating processes have many industrial applications including 
production of jewelry and other decorative articles, production of optical 
devices and production of electronic circuits and components. There are 
several advantages to the use of gold. First of all, it does not form a 
surface insulating film such as an oxide film. For this reason, gold has a 
high surface luster which is quite attractive when used in jewelry 
articles. For the same reason, the optical reflection properties of gold 
are attractive, which makes its use in optical devices highly desirable. 
Again, for the same reason, its use in electric circuits and components is 
highly desirable because surface contact to gold usually has low 
electrical resistance. 
Gold also has the advantage of being chemically inert. This is due to the 
fact that no surface insulating layer is formed on gold. The use of gold 
often increases the lifetime and reliability of devices and articles since 
gold is not affected by many chemicals and adverse conditions of 
temperature and humidity. A particular case in point is the production of 
integrated electronic circuits. Here quite thin and narrow conducting 
paths are required. Many metals (i.e., copper) might be satisfactory as 
far as electrical conductivity is concerned, but they rapidly degrade with 
time. Gold has the advantage of being inert, and also has quite high 
electrical conductivity. 
Because of its excellent electrical contact properties, gold is often used 
in electrical connectors, switches and relays. By the addition of small 
amounts of various elements (for example, arsenic, cobalt, nickel), gold 
can be made quite hard and resistant to abrasion. For the above reasons, 
and because of the extensive growth of the integrated circuit industry, 
the industrial use of gold has increased tremendously in recent years. 
This fact, together with the high and increasing cost of gold has made it 
highly desirable to use gold in as efficient a manner as possible. In 
particular, it has become economically desirable to ensure that gold not 
actually plating onto a surface is recovered and not lost. 
In many processes involving gold, such as gold plating processes, extensive 
use is made of rinse water to remove various reagents and prevent 
contamination of solutions used in subsequent steps in the process. Such 
rinsing operations are useful in all types of processes involving gold 
including, electroplating, electroless plating (displacement and 
autocatalytic plating), etc. Where gold plating solution is removed by the 
rinse water, significant amounts of gold are lost which adds to the cost 
of the plating process. It is highly desirable from an economic point of 
view to recover this gold. Also, from an ecological point of view, removal 
of gold from rinse water is desirable. 
SUMMARY OF THE INVENTION 
The invention is an electrochemical process for recovering gold from dilute 
aqueous solution using an electrolytic recovery cell. The unique 
parameters of the recovery process permits recovery of gold from highly 
dilute solutions, often less than 500 parts per million gold by weight. 
The recovery process is most advantageously used in conjunction with a 
continuous gold electroplating process where the gold is recovered from 
rinse water used to remove electroplating bath solution from plated parts. 
The structure and composition of the cathode is of special significance. A 
perforated structure is preferred so as to permit more rapid flow of the 
dilute aqueous solution past the cathode. Although a variety of metals may 
be used for the cathode, (copper, nickel, etc.) it is preferred that a 
metallic material be used which permits easy removal of gold without 
affecting the cathode material. Suitable metallic materials are titanium, 
tantalum, etc., with titanium preferred because of availability and 
relatively low cost. Alloys of titanium are useful provided they remain 
unaffected by procedures for removing gold but generally essentially pure 
titanium (greater than 98 weight percent titanium) is most preferred. A 
special design is used in the electrolytic recovery cell to ensure 
complete removal of gold from the rinse water even when the gold is 
present only in small concentrations. Typically the gold recovery removes 
gold down to a concentration of one part per million by weight. Such a 
recovery system prevents loss of gold washed off of plated parts and is 
economically highly advantageous because of the high cost of gold. 
Optionally, the gold may be reintroduced into a gold electroplating bath 
by exposing the plated gold on the titanium cathode to a cyanide 
electroplating bath.

DETAILED DESCRIPTION 
The invention is a process for recovering gold from dilute aqueous solution 
using an electrolytic recovery cell. The dilute aqueous solution is 
typically the rinse water in a gold electroplating process and typically 
has gold concentration in the range from 500 to one part per million by 
weight. More often, the concentration range is from 100 or even 10 to one 
parts per million. Higher concentrations are not usually encountered 
because the solution would no longer be an effective rinsing agent. 
Indeed, much lower concentrations are usually preferred so that the 
solution removed most of the gold from the part being rinsed. The lower 
concentration is approximately the limiting concentration that can be 
easily removed from the solution. A typical gold plating process where 
gold recovery is useful is described in U.S. Pat. No. 4,153,523 issued to 
D. E. Koontz and D. R. Turner on May 8, 1979 and U.S. Pat. No. 4,230,538 
issued to D. R. Turner on Oct. 28, 1980. 
In many applications involving gold, rinse water is recirculated and the 
gold (as well as other constituents) are allowed to accumulate. Before 
disposal, the gold is removed in accordance with the invention. In other 
processes, the gold is continuously removed and in some situations, the 
rinse water is disposed of continuously. 
Although the gold recovery process is highly useful with a large variety of 
gold recovery processes, it is particularly convenient when used in 
conjunction with a gold strip line plating process like that described in 
the above two references. Such processes are often used to electroplate 
gold on high-volume items such as connector pins. In such a process, a 
strip travels down a succession of electrochemical processing containers 
(at least two cells but usually more) including cleaning, electrochemical 
polishing, electroplating, etc. Included in the strip line process are one 
or more gold electroplating sections, often one devoted to gold flash 
plating and another to hard gold electroplating. After the gold plating 
processes, the strip line is rinsed to remove residual gold plating 
solution. The rinse water is often recirculated and gold removed either 
periodically (as before disposal of the rinse water) or on a continuous 
basis. 
In broad terms, the gold removal process involves flowing the dilute 
aqueous solution through a recovery cell made up of container (to hold the 
aqueous solution), anode and cathode. The physical arrangement of the 
anode and cathode are not critical and often depend on convenience. For 
example, the same container might serve another purpose such as filtering 
of the dilute aqueous solution or analysis of the dilute aqueous solution. 
Certain design criteria increase the efficiency of the cell in removing the 
dissolved gold. First of all, high flow rates are preferred because it 
reduces the depletion layer thickness and increases the rate at which gold 
is removed from the dilute aqueous solution. It also increases the volume 
of solution exposed to the cathode per unit time and thereby increases the 
rate of removing gold from the dilute aqueous solution. For this reason, 
it is preferred that the flow rate be greater than three cm/sec, more 
preferred greater than 10 or even 20 cm/sec. Turbulent flow is also 
preferred so as to permit the more efficient removal of gold. Generally, 
it is preferred that the cathode (and often the anode) be of the 
flow-through type (sometimes called expanded metal construction) in which 
there are open holes or spaces in the electrode. Such a construction 
increases the velocity of flow and turbulence of the flow. 
Large surface area for the cathode and anode are also preferred. This 
ensures more rapid and complete removal of the gold from the solution. 
Often, the cathode and anode are in the form of parallel, close-spaced 
surfaces such as parallel planes or concentric cylinders. The electrode 
potential is generally from 3 to 4 volts and the current is monitored to 
determine progress in the gold removal process. As gold is removed, the 
current (for a constant voltage) reduces and essentially complete removal 
of the gold is indicated by a constant current at constant voltage. 
A large variety of materials can be used for anode and cathode. For the 
anode, traditional anode material usually used in electroplating 
operations may be used such as platinum, platinized titanium, etc. A 
particularly convenient anode structure is titanium coated with a mixture 
of iridium oxide and tantalum oxide. Typical compositions range from 10 to 
90 mole percent iridium oxide, remainder tantalum oxide. 
The cathode can be made of a variety of metallic materials. A particularly 
convenient material is titanium because of chemical stability and the ease 
with which gold electroplates on this material. 
The use of titanium is advantageous when the gold recovery process includes 
redissolving the gold into the electroplating bath. The gold can be put 
back into the electroplating bath in a variety of ways. Particularly 
convenient is exposing the cathode structure with electroplated gold to 
the electroplating bath. Cyanide solution containing oxygen dissolves the 
gold attached to the cathode and converts it to the gold species 
(generally monovalent gold cyanide ion) used in the electroplating 
process. Titanium cathodes are advantageously used because they are inert 
chemically in such a process. Indeed, it is advantageously used in other 
redissolution processes such as dissolving the gold in Aqua Regia, 
electrochemical dissolution, etc. 
A unique and highly convenient apparatus 10 for carrying out the recovery 
process is set forth in the FIGURE. This particular apparatus combines 
both the gold recovery function with a solution filtering function. The 
apparatus comprises a container 11 with entrance port 12 and exit port 13. 
Inside the container there is an anode 14 and cathode 15. These electrodes 
are in the form of perforated metallic cylinders with the cathode cylinder 
fitting inside the anode cylinder. Both cathode and anode have screw-type 
structures which permit electrical connection with the outside of the 
container. These structures are labeled 16 for the anode and 17 for the 
cathode. The container is fitted with a cover 18 with pressure screw 19 to 
ensure against leaks and proper alignment of cathode and anode. The 
incoming solution makes its way through the cathode and anode structure 
and into the filter assembly 20. After the solution goes through the 
filter assembly it goes into a manifold 21 and then through the exit port 
13. Proper observation of the electrode current at constant voltage 
permits an estimate of the dilute solution composition.