2010 – Precious Metals Conference, Falmouth UK
Merrill Crowe preceded Carbon In Pulp (CIP) technology and is now only used for
very high grade gold projects or where the silver to gold ratio exceeds ten to one. CIP
technology is generally more efficient and cost effective for lower grade ores whereas
Merrill Crowe is superior for high grade silver ores.
Gravity processes are not as efficient for high silver ores as for gold. Metallic silver or
electrum can be recovered at modest recoveries whereas acanthite (Ag2S) exhibits
poor gravity recovery even with centrifugal concentrators.
Leaching of high silver ores requires significantly higher cyanide (>1500 ppm) and
oxygen levels and extended leach times (>72 hours). Usually the recovery is lower
than for gold.
The adsorption capacity needs to be higher for high grade silver ores with a larger
inventory of carbon and more carbon movement. Carbon activity and solution loss
become more critical with a general requirement for more adsorption stages in order
to achieve equilibrium.
Silver elutes first off the carbon and at a lower temperature. These conditions are at
odds with the gold elution. Generally, operations optimise to maximise gold recovery
from the carbon. The silver cyanide complex (Ag(CN)2) is less stable than gold
cyanide (Au(CN)2) and breaks down at the lower temperature and forms metallic
silver which remains on the carbon. This has a negative impact on carbon activity and
hence solution loss.
Where high silver values are present during electrowinning, the cathodes can be
washed with high pressure water and the electrowon gold and silver comes off very