Metallurgy

Approximately 50% of the run-of-mine (ROM) feed is low grade ore or waste, and consequently a beneficiation strategy is adopted where waste is removed on surface and the low grade feed is upgraded prior to being fed to CIP.  This results in increased gold content in feed to the CIP plant and reduced unit operating costs per ROM tonne.

The main planned change to currently employed plant is the addition of an Optical Ore Sorter which employs automated optical identification and pneumatic pulse sorting of processed ore.

The beneficiated gold ore, comprising screened fines, concentrate from the flotation plant and ore-sorter products are fed to the 216,000 tpa conventional CIP plant.

The original gold recovery plant comprised the 10,000 tpm CIL plant which has being upgraded to a CIP plant with a capacity of 18,000 tpm. The principle reason for the conversion to a CIP plant is to increase carbon loadings, thus reducing a bottleneck in the elution circuit.  The revised plant is now constrained, by mill and leach capacity, to 18,000 tpm until 2013 when additional CIP capacity will be added.

The net result of these concentrating processes is an upgrade of overall gold recovery from the originally designed 80% to a projected 86%.

Ore Beneficiation

Since the current overall plant capacity will be constrained by the CIP plant at 216,000 tpa feed tonnage, beneficiation stages of crushing, screening, ore-sorting and flotation will be implemented in order to maximise the grade into the CIP plant. This will be achieved using the following material flow methodology.

• Run of mine ore, containing higher grade fines due to preferential breakage of sulphides in the finer fractions, will be crushed to -75mm and screened at 25mm in the primary crushing and screening open circuit. An existing Extec mobile crusher and screen, capacity 90 to 110 tph, will be employed for this operation.
• The -25mm high grade material from the primary crushing and screening will be stockpiled separately for direct feed to the CIP.
• The +25mm material from the primary crushing and screening will then be fed into a circulating crushing circuit to be wet screened, into four size fractions; -75mm +25mm;  -25mm +4mm,  -4mm+1mm  and  -1mm. An existing double deck screen, of capacity 150 tph to accommodate a high circulating load, and an existing single deck screen, of capacity 70 tph will be employed for this operation.
• The +25-75mm size fraction will be fed to an optical ore sorter to separate the high grade reef material and waste.  The ore sorter will operate in batch/campaign mode to cater for two scenarios, i.e.

- sorted high grade material will be fed directly to the CIP when the plant is fed high grade stoped ore only, and

- sorted barren waste will be rejected when the plant is fed low grade development ore.

A new Commodes ore sorter unit, with a capacity of 40 tph of sorted product, will be employed in this operation.

- In the high grade sorting, rejects from the ore sorter will be crushed in an existing HSI crusher and recycled to wet screening.

- In low grade sorting, barren waste will be rejected from the plant and sold as aggregate in construction applications.

• The -25mm +4mm fraction will be crushed in an existing secondary VSI crusher to -4mm, and recycled to wet screening.
• The -4 mm; +1mm size fraction will be ground to -250 micron and fed to the flotation plant from which the upgraded concentrate will be fed to the CIP plant. An existing Ultimate flotation plant with a capacity of 40 to 50  tph will be used in this operation.
• Tailings from the flotation plant will be dewatered and stored in an existing tailings dam or deposited in open pits as part of rehabilitation or underground in allocated voids.
• The wet screened, -1 mm fines size fraction will be pumped directly to the process plant (for milling and CIP), this would also include all fine gold washed from the ore processed before.

All unit operations have been proven on an industrial scale using extremely hard oxide development ore, except for the Commodas optical ore sorting process which is currently been evaluated.

CIP Gold Recovery

The existing process plant will treat the products of the beneficiation process.

The CIP section of the process plant has a maximum capacity of 18,000 tonnes per month.  A conventional milling and classification, gravity gold concentrating, thickening, pre-leach, CIP, carbon strip, electrowinning and smelt processes are part of the process plant.

Milled products are fed to a gravity gold circuit, comprising a 20 inch Falcon concentrator, to recover liberated gold, and a Gemini shaking table, to produce a smeltable gold concentrate. The gold recovery obtained via this process is projected at a minimum of 30%.  Peer operations in the Witwatersrand area recover in excess of 50% of the gold via this method, and it is envisaged that as this is a recent installation, improvements on current projections are feasible.

The tailings from the concentrator and Gemini table are fed to the main CIP circuit for leaching.  A high rate thickener is used to thicken or dewater the slurry to a density of 45% to 55% solids, and the water from this thickener is re-used in the treatment plant.

The carbon absorption section is a conventional counter current circuit comprising 7 adsorption vessels with interstage screens to retain the loaded carbon.

Loaded carbon is recovered in a Zadra elution plant with a capacity of 700 kilogram of carbon per cycle.  The carbon stripping rate is one elution per day for six days per week.  The gold from the carbon is electro-won and smelted once a week.

Gold sludge and gravity gold are calcined at 700º C to remove organic impurities and smelted weekly in a diesel heated furnace.

All tailings from the cyanidation process are treated with peroxide to oxidise free cyanide. The dewatered tailing is deposited in an existing tailing storage facility, or a mined out open pit.  Underground deposition, whether cemented or not, is under review.  The evaluation is pending in 2011 to 2012 when the additional storage capacity is required.