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

Transcription

1 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

2 Contents Why did we chose this theme? ( Meaning and Purpose ) Outline of method using halogen-containing organic system Rough experiment of dissolution/deposition of gold Application for stream sediment and ore Summary

3 Man-made pollution 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 people. View of high-grade Hg sediments at the river (Murao, 2003).

4 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 environments. It takes too much time to extract gold (several tens hour in usual)

5 Purpose 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 gold. In order to find some criteria in these phenomenon, we have here examined dissolution/deposition experiments with HOS and ascorbic acid. Tincture of iodine (HOS)

6 Outline of method using halogen-containing organic system (HOS) Dissolution 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 ] - complex. 2Au + 3I 3-2[AuI 4 ] - +I -

7 Deposition 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.

8 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.

9 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 equilibrium. 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 gold.

10 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.

11 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.

12 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.

13 Flow chart of experiments Samples Pulverized auriferous quartz vein Sieved stream sediments (particles under 0.063mm in diameter) HOS is composed of I 2, 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.

14 Sample:solvent=5g:100cc

15 Sample: solvent=5g:10cc Deposition of Au

16 Summary 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 sediments) 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.