A trial of extractingA trial of extracting goldgold from stream from stream sediment and High Au/Agsediment and High Au/Ag ore ore

usingusing halogenhalogen--containingcontaining organic systemorganic system

H. Murakami and Y. NakaoInstitute for Geo-Resources and Environment, AIST

H. MurakamiH. Murakami andand Y. Y. NakaoNakaoInstitute for GeoInstitute for Geo--Resources and Environment,Resources and Environment, AISTAIST

Geological Survey of Japan

Contents Contents Why did we chose this theme?( Meaning and Purpose )

Outline of method using halogen-containing organicsystemRough experiment of dissolution/deposition of goldApplication for stream sediment and ore Summary

ManMan--mademade pollutionpollution• gold-mercury amalgamation

Even in more recent times, mercury has been employed to extract gold inthe small-scale mining. (e.g.Philippine, Brazil). Procedure fromcreating amalgams with mercury through heating to drive off mercury,results in a large number ofPOISONINGs that attack to local people.

View of high-grade Hg sediments at the river (Murao, 2003).

AgainstAgainst cyanide leachingcyanide leaching• In case of extracting gold by using

cyanide, cyanide-solution is quitedangerous for worker to treat.

• Waste of cyanide-leaching process,especially aerosol from leachingpads has potential to damage theenvironments.

• It takes too much time to extract gold

(several tens hour in usual)

Tailing of waste from Cyanide-leaching

PurposePurpose• 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), well-known as “tincture of iodine” can serve as good solventsfor noble metal such as gold. Tincture of iodine is easy tohandle and poisoning-free as can be used for sterilizer.

• Gold can be precipitate by adding ascorbic acid, well-known as “Vitamin-C” into such a solvent containinggold.

• In order to find some criteria in these phenomenon, we have here examined dissolution/deposition experimentswith HOS and ascorbic acid.

Tincture of iodine(HOS)

Outline of methodusing halogen-containing organic system

(HOS)Dissolution

In our experiments, HOS is composed of I2, NaI and ethanol. A triiodide ion, I3- is isolated from this system and active for thedissolution of metal. A dark brown color of the system reflectｓpresence of I3-.

I2 + I－ (in NaI) ⇔ I3－

The I3- ion is effective oxidant toward noble metal, and in the presence of I-, reacts with gold to form the very stable [AuI4]-complex.

２Au + ３I3－ ⇔ 2[AuI４]－ + I－

DepositionAfter gold is dissolved, ascorbic acid is added to reduce I2.

I2 ⇒ 2I－This reduction results in deficiency of I3－, HOS begins tobehave as “poor-solvent” for gold. A dark brown color of ＨＯＳis lost reflecting depletion of I3-. Gold can be precipitated.

Hydrogen peroxide liquid (H2O2) can be oxidized the above solution.2 I− → I2

The system turns into original HOS which can dissolve gold.

RoughRough experimentexperiment of of dissolution/deposition of golddissolution/deposition of gold

In order to estimate dissolution rate of gold in HOS and how muchascorbic acid needs to be precipitated gold, model experiment has beencarried out.

1. A gold wire (ca. 1m, 0.2mm in diameter) was added into HOS(40ml) of I2 (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. DissolutionThis 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 equilibrium.

If gold was saturated in 0.1g at room temperature, It can be estimated 0.1mol of gold can dissolve in HOS with I3

- of 1mol.

The increase temperature caused a rapid growth of dissolution of gold.

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 I3

- ion was lost. Deposition of gold starts at high water content with making fine-grained particles.

Before applicationBefore application

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 I3

- 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.

Rough experiments here indicates that;

Application forApplication for stream sediment and orestream sediment and oreIn 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 experimentsFlow chart of experimentsSamples

• Pulverized auriferous quartz vein

• Sieved stream sediments (particles under 0.063mm in diameter）

• HOS is composed of I2, NaI and ethanol.

• 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:100ccSample:solvent=5g:100cc

Sample: solvent=5g:10ccSample: solvent=5g:10cc

Deposition of Au

SummarySummaryIt 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 than1000ppb of gold.

Gold in natural materials should be concentrated at the time ofsampling and preparation stage. (e.g. pan concentrate of stream sediments)

After the recycling HOS by H2O2, 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.