2011 – ALTA 2011, Perth WA


The classic method of recovery of gold and silver from cyanide solution was Merrill Crowe
until the introduction of Carbon In Pulp (CIP) in the 1970’s.
The CIP process indicated savings in CAPEX. This meant that lower grade ores could be
economically treated than otherwise would have been the case. Soluble losses from CIP
plants are lower than Merrill Crowe and the process is less vulnerable to impurities such as
sulphides, arsenates in the leach liquor. The CIP process is more robust and forgiving than
Merrill Crowe.
Until recently for very high grade feeds Merrill Crowe was far superior to CIP and a clear
choice. However, the trends toward the application of EMEW cells, ion exchange resins and
hybrid plants are changing the way we might look at Merrill Crowe in the future for high
grade gold and more particularly high silver feeds.
The problem with direct electrowinning is that cell efficiency drops with solution tenor. The
high grade gravity ILR pregnant solution may be able to be treated separately with direct
electrowinning and save on zinc dust. Similarly the CCD overflow could be electrowon to
save zinc. Solutions above 1,000 ppm silver for metallic silver to plate out and around 300
ppm for powder to be deposited on the cathode are required. The EMEW powder cell design
incorporates 200 mm dia tube that acts as the cathode and has a stainless steel anode fitted.
Solution is pumped through at high velocity and the powder is drained to a filter using a cycle
timed PLC.
The use of resins and IXEW (ion exchange followed by electowinning) hybrid circuits and
Resin In Pulp (RIP) are also being considered, but as a retrofitted design based on piloting in
an existing plant in order to minimise process risk.

Read the paper here