A trial of extracting gold from stream sediment and High Au/Ag ore ...


A trial of extracting gold from stream sediment and High Au/Ag ore ...

A trial of extracting gold from stream

sediment and High Au/Ag ore

using halogen-containing

organic system

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

carried out.

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.

1. Dissolution

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-

of 1mol.

The increase temperature caused

a rapid growth of dissolution of


2. Deposition

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

20 minutes.

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

I 3-

ion was lost. Deposition of gold

starts at high water content with

making fine-grained particles.

Before application

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

one hour.

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

to 950.

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.


Sample: solvent=5g:10cc


of Au


• 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.

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