A trial of extracting gold from stream
sediment and High Au/Ag ore
H. Murakami and Y. Nakao
Institute for Geo-Resources and Environment, AIST
Geological Survey of Japan
Why did we chose this theme?
( Meaning and Purpose )
Outline of method using halogen-containing organic
Rough experiment of dissolution/deposition of gold
Application for stream sediment and ore
• gold-mercury amalgamation
Even in more recent times, mercury
has been employed to extract gold in
the small-scale mining. (e.g.
Philippine, Brazil). Procedure from
creating amalgams with mercury
through heating to drive off mercury,
results in a large number of
POISONINGs that attack to local
View of high-grade Hg sediments at the river (Murao, 2003).
Against cyanide leaching
Tailing of waste from Cyanide-leaching
• In case of extracting gold by using
cyanide, cyanide-solution is quite
dangerous for worker to treat.
• Waste of cyanide-leaching process,
especially aerosol from leaching
pads has potential to damage the
• It takes too much time to extract
(several tens hour in usual)
• If we can establish another harmless technique to extract
gold, that will be more effective for the above situations.
• Halogen-containing organic solvent systems (HOS), wellknown
as “tincture of iodine” can serve as good solvents
for noble metal such as gold. Tincture of iodine is easy to
handle and poisoning-free as can be used for sterilizer.
• Gold can be precipitate by adding ascorbic acid, wellknown
as “Vitamin-C” into such a solvent containing
• In order to find some criteria in these phenomenon, we
have here examined dissolution/deposition experiments
with HOS and ascorbic acid.
Tincture of iodine
Outline of method
using halogen-containing organic system
In our experiments, HOS is composed of I 2 , NaI and ethanol. A
triiodide ion, I 3 - is isolated from this system and active for the
dissolution of metal. A dark brown color of the system reflects
presence of I 3 - .
I 2 +I - (in NaI) ⇔ I 3
The I 3- ion is effective oxidant toward noble metal, and in the
presence of I - , reacts with gold to form the very stable [AuI 4 ] -
2Au + 3I 3- ⇔ 2[AuI 4 ] - +I -
After gold is dissolved, ascorbic acid is added to reduce I 2 .
I 2 ⇒ 2I -
This reduction results in deficiency of I 3 - , HOS begins to
behave as “poor-solvent” for gold. A dark brown color of HOS
is lost reflecting depletion of I 3 - . Gold can be precipitated.
Hydrogen peroxide liquid (H2O2) can be oxidized the above solution.
2 I − → I 2
The system turns into original HOS which can dissolve gold.
Rough experiment of
dissolution/deposition of gold
In order to estimate dissolution rate of gold in HOS and how much
ascorbic acid needs to be precipitated gold, model experiment has been
1. A gold wire (ca. 1m, 0.2mm in diameter) was added into HOS
(40ml) of I 2 (6mmol), NaI (6mmol) and ethanol (29.1g) with
stirring at room temperature.
2. After a hour, 10 ml HOS containing gold was separated, and used
for deposition experiments using ascorbic acid and water.
This graph shows dissolution of
gold into HOS with time. In first
a hour, there is immediate sharp
rise in dissolution of gold. After a
hour, dissolution of gold slightly
increase. It is likely to achieve
If gold was saturated in 0.1g at
room temperature, It can be
estimated 0.1mol of gold can
dissolve in HOS with I 3-
The increase temperature caused
a rapid growth of dissolution of
In order to check a condition for
deposition of gold from HOS, water
and ascorbic acid were added step by
step. Intervals of each step are around
This graph shows variation of
concentration for water and ascorbic
acid in HOS with 0.175% of gold.
At 1st and 2nd steps, gold
precipitation did not occur, however
ion was lost. Deposition of gold
starts at high water content with
making fine-grained particles.
Rough experiments here indicates that;
1. HOS can be used to extract gold.
2. Dissolution rate of gold reaches to the maximum in the first
3. It can be estimated 0.1mol of gold can dissolve in HOS
with I 3- of 1mol. It means that gold can be dissolved up
until around 0.1% order of gold content in the system.
4. By using ascorbic acid, deposition of gold is likely to be
controlled by water content. Because ascorbic acid can be
dissolve only in system including water.
Application for stream sediment and ore
In Japan, there are a lot of placer gold
regions. Of these, Kitakami mountain area
in northern part of Japan represents
mesothermal Au deposits which is
characterized by production of native gold.
Its Au/Ag ratio is generally ranges from 800
Samples of stream sediment and ore were
taken from Kitakami region.
Flow chart of experiments
• Pulverized auriferous quartz vein
• Sieved stream sediments (particles
under 0.063mm in diameter)
• HOS is composed of I 2
, NaI and
• Solutions after dissolution of sample
and addition of ascorbic acid were
analyzed by ICP-MS.
• Sample dissolved by 100cc of HOS
were carried out for 2 cycles, that by
10cc for 1 cycle.
• It is necessary to consider volume ratio of sample / solvent in case
of extracting gold from natural sample (showing low gold grade).
Expected concentration of gold in HOS should be more than
1000ppb of gold.
• Gold in natural materials should be concentrated at the time of
sampling and preparation stage. (e.g. pan concentrate of stream
• After the recycling HOS by H 2 O 2 , it seems that Mn and Fe
occasionally prevent gold from extracting into HOS.
• Using ascorbic acid to precipitate gold is disadvantage to recycling
system due to its requirement water for dissolution into HOS.