Objectives of the 2017 Savannah North metallurgical program are to:
- Identify variations in mineralogy or flotation performance requiring separate metallurgical domaining;
- Demonstrate that Savannah North mineralisation performs in a similar manner to the historically treated Savannah ore under typical Savannah plant conditions; and
- Identify alternative processing regimes (i.e. reagent schemes) that may lead to improved recoveries, reduced reagent costs and/or increased concentrate grades.
QEMSCANTM mineralogical testwork has confirmed that the Savannah North mineralisation is similar to the Savannah ore, i.e. pyrrhotite-dominant sulphides with sub-ordinate pentlandite and chalcopyrite. The mineralogical testwork shows that approximately 85% of the elemental nickel occurs as pentlandite, with around 7% of the nickel occuring as non-sulphide. The remainder of the nickel is assumed to be present in pyrrhotite. Approximately 60% to 75% of the pentlandite occurs in a liberated form, with the less liberated pentlandite mainly locked with pyrrhotite (occurring as fine inclusions, thin or thick flames). The P80 of pentlandite varies between 59µm and 78µm across the sample set. Chalcopyrite has a P80 of about 80µm and about 75 % is classified as ‘liberated’; the less liberated chalcopyrite is mainly locked with pyrrhotite and less frequently with pentlandite. Cobalt occurs within the pendlandite, with no discrete cobalt minerals observed in the QEMSCANTM study.
Comminution testwork returned Bond Work Indices ranging from 9.5 to 12.5 (“medium”). SAG mill comminution (SMC) testing showed the Savannah North mineralisation to be “very soft” to “soft”. The softer comminution parameters will provide some scope to either maintain mill throughput while producing a finer grind P80 or alternatively maintain the existing grind P80 at a slightly higher mill throughput.
Flotation testwork is on-going. Testwork conducted to date is indicating that both the Savannah North Upper and Lower Zones are expected to perform within the range of typical Savannah ore performance, and that bulk concentrate grades containing 9% to 11% nickel are achievable.
The current focus of the metallurgical program is to identify alternative processing regimes that may lead to improved recoveries and/or concentrate grades, without modifying plant hardware. The key to improving Ni recovery and grade is the selective depression of pyrrhotite. A range of different reagents and dosages, along with variation of pH levels, are currently under trial.
As the improvement phase of the metallurgical program remains on-going, the grade-recovery relationships based on actual Savannah plant performance derived from processing records for the twelve-year operating period to 2016, were used for the optimisation work. These records demonstrate a consistent relationship between nickel recovery, head grade and concentrate grade. Historical grade-recovery data for the targeted concentrate grade of 9% Ni is shown in Figure 3.
Figure 3 – Historical Savannah plant nickel recovery versus head grade for a concentrate grade of 9% Ni
The Savannah FS Optimisation assumes no change to the existing Savannah processing plant configuration or capacity. The nominal throughput capacity of the Savannah plant is approximately 1.0Mtpa. Between February and May 2016, prior to going into care and maintenance, the Savannah plant was operating between 120tph and 140tph, averaging approximately 130tph (85,000t per month).
For the optimisation work, plant throughput is capped at approximately 80,000t per month (~120tph). Life-of-mine mill throughput averages 0.9Mtpa, up from 0.8Mtpa in the Feasibility Study. Due to changes in mining productivity, the milling profile is smoother over life-of-mine, ranging from 830ktpa in the first year of operation, up to 950ktpa, in the third year. A three-month ramp-up to full production is assumed in the optimised plan.
Life-of-mine nickel head grade for the optimised plan averages 1.42% Ni, with quarterly averages varying from 1.1% Ni to 1.6% Ni. Lower grades are processed in the first year of production, associated with the remnant Ore Reserves at Savannah. The annual milling profile is shown in Figure 4.
Figure 4 – Savannah Optimisation – Annual production and grade profile
For the Savannah FS Optimisation, the Company is targeting production of a bulk Ni-Cu-Co concentrate with a concentrate grade of 9% Ni. Processing recoveries at the target concentrate grade over life of mine are expected to average 87% Ni, 96% Cu and 90% Co, based on historic Savannah plant performance.
Metal in concentrate production is forecast to average 11,000t Ni, 5,800t Cu and 760t Co per year with 93,800t Ni, 49,100t Cu and 6,500t Co in concentrate produced over life of mine. Annual metal in concentrate production is shown in Figure 5.
Figure 5 – Annual nickel processing recovery and metal in concentrate production
Savannah Processing Plant
The existing processing plant and accommodation camp are sufficient for the proposed future operations under the optimised plan.
All infrastructure required for a re-start of operations remains on site and is being maintained by the care and maintenance crews. Major fixed plant and mobile equipment is either ready to be recommissioned or can be at short notice, with refurbishments costed in the financial model. Underground decline and pumping systems have been maintained and operated so that minimal rehabilitation will be required to recommence underground operations.
The major infrastructure works required on a re-start of operations for the optimised case are unchanged from the Feasibility Study, and include: Savannah North decline access development, Savannah North Fresh Air Rise and tailings storage facility wall lift.
The Savannah FS Optimisation includes provision for solar power. A proposal for solar power has been provided by our existing third-party power provider, utilising a 4MW system generating approximately 10% of the life-of-mine power requirement. Modelling assumes the Company will own the solar power infrastructure. Annual savings on power costs of around $2M are estimated to be achievable, compared with 100% diesel power.