Electrolytic process for the simultaneous deposition of gold and replenishment of elemental iodine

An improved process for recovering gold from gold-containing materials by iodide/iodine leaching is provided wherein the pregnant lixiviant containing solubilized gold and iodine is treated in an electrolytic cell to reduce gold in solution to elemental gold for recovery and reduce iodine to iodide at the cathode, so as to prevent iodine from interfering with subsequent gold recovery processes, and at the same time reoxidize iodide present at the anode to elemental iodine to regenerate the leach solution to the desired iodide:iodine weight ratio, e.g. about 2:1 to about 10:1. Gold is precipitated in the cathode compartment, and if desired, the cathode effluent may be treated for further removal of traces of gold before being passed to the anode compartment. A method for preventing iron contamination of the cathode is also provided comprising buffering the lixiviant solution to a pH of about 5.

TECHNICAL FIELD 
This invention lies in the field of hydrometallurgy. In particular, it 
involves the simultaneous electrolytic recovery of gold and restoration of 
elemental iodine in spent lixiviant solutions used, for example, in in 
situ mining, heap-leach mining, or agitated-leach recovery processes. 
BACKGROUND OF THE INVENTION 
Iodine/iodide leaching processes for the recovery of gold from 
gold-containing materials are well known to the art, and are described for 
example, in Applicant's U.S. patent application Ser. No. 598,706, to be 
issued as U.S. Pat. No. 4,557,759, incorporated herein by reference, and 
in U.S. Pat. Nos. 2,304,823 and 3,957,505. 
When iodine and iodide attack an auriferous ore containing pyrite or other 
reducing materials, gold is solubilized as an iodide complex, either 
AuI.sub.2.sup.- or AuI.sub.4.sup.-, and pyrite or other reducing materials 
are oxidized by elemental iodine, forming iodide as one of the products. 
For the mining process to be economical, the lixiviant must be recycled; 
thus, any iodine which has been reduced must be restored to the solution 
in the oxidized (elemental) form, and the gold must be removed. 
This combination of requirements, restoration of iodine and removal of 
gold, is difficult to accomplish inexpensively either simultaneously or 
sequentially. The difficulty arises from the fact that elemental iodine 
and the iodo-gold complex behave similarly in the presence of reducing 
agents, ion-exchange resins, and activated carbon. Thus, cementation of 
gold onto iron or zinc results in the solubilization of large amounts of 
these metals because iodine as well as gold effects their oxidation. 
Adsorption of gold onto an anion resin or activated carbon is inhibited by 
simultaneous adsorption of iodine onto available sites, and elemental 
iodine is quantitatively removed from the solution in the process. 
The object of the present invention is to provide an economic process for 
recovering gold from an iodine-containing lixiviant in a one-step process 
in which gold and iodine are electrolytically reduced simultaneously with 
the oxidation of iodide. Iodine is produced at a rate sufficient to 
provide for a recycle leaching of gold. A further object of the invention 
is to prevent iron fouling of the electrolysis.

SUMMARY OF THE INVENTION 
An improved process for recovering gold from goldcontaining materials 
including auriferous ores by iodide/iodine leaching is provided wherein 
the pregnant lixiviant containing dissolved gold and iodine is treated in 
an electrolytic cell to reduce gold in solution to elemental gold for 
recovery and simultaneously reduce iodine to iodide at the cathode, so as 
to prevent iodine from interfering with subsequent gold recovery 
processes. Concomitantly, iodide present at the anode is oxidized to 
elemental iodine to regenerate the leach solution to the desired 
iodide:iodine weight ratio, e.g. about 2:1 to about 10:1. Gold is 
precipitated in the cathode compartment, and if desired, the cathode 
effluent may be treated for further removal of traces of gold before being 
passed to the anode compartment. A method for preventing iron 
contamination of the cathode is also provided comprising buffering the 
lixiviant solution to a pH of about 5. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process described herein involves the simultaneous electrolytic 
reduction of iodine and reduction and precipitation of gold at a cathode, 
preferably a stainless steel cathode, with concomitant reoxidation of 
iodide to iodine at the anode, preferably a carbon anode, of the same 
cell. In a modification of the process, the cathode effluent is subjected 
to a process for removal of gold, such as by being passed through a bed of 
activated carbon or anion exchange resin to remove traces of gold before 
passing into the anode compartment. All iodine in the solution entering 
the ore zone is reduced to iodide either in the ore zone or in the cathode 
compartment of the electrolytic cell, and is then completely replenished 
by oxidation in the anode compartment of the cell. Since no iodine is 
present in the cathode effluent, no iodine is available for adsorption 
onto the carbon bed or anion exchange resin in the modified process. 
The current in the cell is adjusted to generate the desired concentration 
of elemental iodine in the anode effluent. The total combined iodine and 
iodide concentration depends on the characteristics of the material being 
leached (3 gpl is a typical total iodide-iodine concentration for use in 
in situ mining). The iodide to iodine ratio in the regenerated lixiviant 
is also a function of the qualities of the feed material, however when too 
low an iodide to iodine ratio, i.e., too much iodine, is produced, the 
rate of solubility of iodine at the anode will not keep pace with iodine 
generation and cause iodine to crystallize on the anode and interfere with 
the process. If the iodide to iodine ratio is too high, i.e., too little 
iodine, the iodine may be exhausted before the gold leaching is completed, 
causing gold to redeposit in the feed material. Preferably an iodide ratio 
of at least about 2:1 is desirable, and ratios up to about 10:1 are 
generally useful. For solutions containing depleted iodine concentrations 
after interaction with reducing materials in the ore zone, the quantity of 
oxidized iodine in the cathode compartment will be insufficient to carry 
the total current used to regenerate the iodine in the anode compartment. 
The remainder of the current must then be carried by some other cathode 
half-reaction; usually this additional half-reaction will be the reduction 
of water to hydrogen gas (in addition to the small contribution from the 
reduction of gold complex to elemental gold). The cell voltage necessary 
to be applied to effect the gold-recovery/iodine regeneration is then the 
difference in half-cell potentials between the iodide/iodine oxidation and 
the reduction of water at the cathode pH, plus any cathode overvoltage for 
the water reduction plus the ohmic drop in the cell. Algebraically, this 
may be expressed as: 
EQU E.sub.app =E.sub.I -.sub./I.sbsb.2 +E.sub.H.sbsb.2.sub.O/H.sbsb.2 
+IR.sub.cell +E.sub.cath,over 
The power requirement for effecting this process is the product of the 
current times the applied voltage: 
EQU P=IE.sub.app 
It is thought that the chemical reactions taking place in the process are 
described by the following equations: 
In the ore zone: 
EQU 8H.sub.2 O+7I.sub.3.sup.- +FeS.sub.2 .fwdarw.Fe.sup.2+ +2SO.sub.4.sup.2- 
+21I.sup.- +16H.sup.+ 
EQU 2Au+I.sup.- +I.sub.3.sup.- .fwdarw.2AuI.sub.2.sup.- 
In the cathode compartment: 
EQU I.sub.3.sup.- +2e.sup.- .fwdarw.3I.sup.- 
EQU AuI.sub.2.sup.- +e.sup.- .fwdarw.Au+2I.sup.- 
EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- 
In the anode compartment: 
EQU 3I.sup.- .fwdarw.I.sub.3.sup.- +2e.sup.- 
When lixiviant solution first contacts a pyrite ore zone, it is common for 
extensive interaction between the iodine and the pyrite to take place 
rapidly because of the presence in the ore of finely divided pyrite 
particles. For unbuffered lixiviant solutions, the pH can drop rapidly to 
about three or less and a considerable quantity of iron can be 
solubilized. The high concentrations of iron thus produced will then be 
transported to the cathode where undesirable electrode reactions or 
chemical precipitations can occur. To prevent this untoward situation, the 
lixiviant solution may be buffered with a non-reducing buffer, preferably 
sodium acetate/acetic acid, to a pH sufficient to prevent iron 
dissolution, preferably at least about 5; this keeps the iron 
concentration to a low enough level that it interferes very little with 
the operation of the electrolysis cell. Buffering the lixiviant also 
retards dissolution of the stainless steel cathode, the degradation of the 
cathode being a hundred times faster at pH 3 than at pH 5. 
FIG. 1, depicting a preferred embodiment of this invention, shows original 
or regenerated lixiviant containing an optimum iodide:iodine ratio at 
reservoir R.sub.1 being pumped into an ore column, from which pregnant 
lixiviant containing dissolved gold and reduced iodine, i.e. a higher 
iodide:iodine ratio than present in the original or regenerated lixiviant, 
flows into reservoir R.sub.2. The pregnant lixiviant is conducted to the 
stainless steel mesh cathode of an electrolytic cell equipped with a 
conventional power supply where gold is reduced to elemental gold and 
precipitated onto the cathode, and substantially all the iodine present in 
the lixiviant is reduced to iodide. The cell is shown with a Nafion (Du 
Pont Co.) membrane separating the anode and cathode compartments. This 
membrane is a sheet of cation exchange material which is impervious to 
water and anions but permits passage of cations from the anode compartment 
to the cathode compartment. This arrangement prevents mixing of anolyte 
and catholyte while still permitting maintenance of electro-neutrality in 
the cell compartments as the electrode reactions take place. The 
goldbarren cathode effluent containing virtually no iodine is collected at 
reservoir R.sub.3 from whence it may be conducted in the modified path to 
an activated carbon bed for removal of traces of gold, and then be 
conducted to the graphite rod anode compartment of the electrolytic cell. 
Alternatively, the gold- and iodine-barren lixiviant may be conducted from 
R.sub.3 via the primary path directly to the anode compartment. In the 
anode compartment, iodide is oxidized to iodine in an amount sufficient to 
provide a regenerated lixiviant for reuse, and the regenerated lixiviant 
is conducted to R.sub.1 and recycled. 
The following examples are illustrative of the invention and not intended 
to be limiting. 
EXAMPLE 1 
In a typical laboratory-scale embodiment of this process, unbuffered 
solution containing 9 grams/liter ionic iodide and 1 gram/liter elemental 
iodine was pumped from a reservoir at 75 ml./min. into a glass column 23 
cm. wide by 30 cm. long containing approximately 15 kg. of pyritic gold 
ore. Effluent from the column was depleted somewhat in iodine and 
contained gold ranging in concentration 10 to 0.1 mg./liter. This effluent 
solution was pumped into the cathode compartment of the electrolysis cell 
and reduced at 1.0 amp of current using an applied voltage of 2.0 volts. 
No gold or elemental iodine was detected in the cathode effluent. The 
cathode effluent was then pumped into the anode compartment of the same 
electrolysis cell wherein iodide was oxidized to elemental iodine at such 
a rate that the anode effluent was replenished to 1 gram/liter iodine. A 
cell current efficiency of 95 percent was achieved. 
EXAMPLE 2 
In a typical laboratory-scale embodiment of the electrolysis process, a 
current of 1.00 amp was required to replenish 1.00 gram iodine/liter 
flowing through the cell at 75 ml./min., and the applied voltage was 
measured to be 2.0 volts. That current reflects a 95 percent current 
efficiency for the anode half-reaction. Thus the power consumption rate 
(current times voltage) was 2.0 watts. Simultaneously in the cell, gold 
was being reduced at the cathode in the amount of 2 mg./liter at the same 
flow rate of 75 ml./min. At this rate, in one hour's time 9 mg. of gold 
metal was deposited at the cathode. The energy consumption was then 2 
watt-hours per 9 mg. Au or 0.22 KWH/gm. Au. At 10, per KWH this computes 
to 2.2.cent./gm. Au. Since a gram of gold is currently valued at about 
$10, this cost is insignificant.