Biografias

A novel thiosulfate leach process for thetreatment of carbonaceous gold ores

• Placer’s R&T department initiated research to find a viable alternative
to cyanide in early 1999
• Thiosulfatewas selected as the most promising alternative to cyanide
• Unlike cyanide, thiosulfate is only weakly preg-robbed by native
organic carbon
• Research focused on treating carbonaceous preg-robbinggold ores
from Nevada and elsewhere using thiosulfate
• Three processes have been developed from this research:
– Pressure thiosulfate leaching – copper and ammonia free
– HATS – Heap ammoniumthiosulfate leaching
– MATS – Mill ammonium thiosulfate leaching
• This presentation will focus on the MATS process

Leaching:
– 2Au + 4S2O32- + 1/2O2 + H2O  2Au(S2O3)23- + 2OH-
• Thiosulfatedegradation during leaching:
– 4S2O32- + O2 + 2H2O  2S4O62- + 4OH-
– 3S2O32- + 2O2 + H2O  2S3O62- + 2OH-
• Precipitation:
– 2Au(S2O3)23- + HS- + OH- 2Au + 4S2O32- + 2H2O + S°
– S 3O6 2- +HS- + OH- 2S2 O3 2- +H2 O
– S4O62- + HS- + OH- 2S2O32- + H2O + S°
• Sludge Upgrading:
– S° + SO32-  S2O32-
* Pipeline carbonaceous ore from the Cortez Joint Venture
• XRDanalysis indicates that the primary composition of the ore is
quartz, illite, calcite & dolomite
• 82% gold recovery obtainable with thiosulfate from diagnostic leaching

• Close to theoreticalgold recovery is obtained with thiosulfate
• Cyanide gold recovery is less than 5%
• 15 g/L ammonium thiosulfate, pH 9, 2 reactor volumes/hour aeration
Continuously stirred reactors in series used• Results from 30 minute residence time shown below, but 15 minutes is sufficient to precipitate gold
Au S2O3
2- S3O6
2- S4O6
2-
/L /L /L /L
• Ammonium sulfide added to maintain a potential of –90mV vs. SHE in the
mg/g/g/g/Initial 1.02 4.38 4.83 1.45
Final <0.01 6.14 4.64 0.08
circuit (0.5 g of ammonium sulfide per liter of solution treated)
• Under some conditions (lower pH,...