Appendices 5-13 - Nautilus Cares - Nautilus Minerals

seawater was very slow. Lead still showed substantial rates of dissolution which
reflects the rapid oxidation of galena, reported to initiate in minutes (Kim et al, 1994).
The Phase 2 experiment showed that there appears to be a temperature effect on
dissolution of metals with a concomitant decrease in pH, increase in Ca dissolution
and increase in sulfur as sulfate. While it has been reported that metal sulfides are
less stable at higher temperatures (Tsang and Parry, 2004) the magnitude of increase
measured in these experiments may be higher than expected due to air oxidation of
the ore that may have occurred between the two experiments.
Acknowledging that these experiments may represent a worst case scenario, as a
result of air oxidation of the ore, the results for the quality of elutriate water show that
at least a 300 fold dilution will be required at 6 o C and 600 fold dilution if the slurry
will be between 12 and 24 o C, to produce a discharge water quality that would be
considered to have minimal if any environmental impact. A more conservative
dilution of 1000 fold would reduce metal concentrations to below the
ANZECC/ARMCANZ (2000) guideline values for 99% protection level for most
metals even at 24 o C, except for Co. This dilution would also reduce Ca and sulfate
levels and increase pH and alkalinity to background levels. This assumes that the
slurry will be processed within 180 minutes as there is no data for dissolution at 12 or
24 o C beyond 180 minutes.
The required dilution could be readily achieved by initially mixing the processed
seawater 1:10 with background seawater and pumping that back into the ocean at
depth though a diffuser. A diffuser should be able to achieve a further 100 fold
dilution within meters of the diffuser depending on currents.
These conclusions are based on the two sets of experiments that addressed the
variables of temperature and time, but did not address turbulent mixing directly.
However all experiments included a 5 minute shaking period at the beginning which
may represent the mixing that will occur when the slurry is pumped to the surface.
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