Mineral processing engineers Mineral Engineering Technical Services Pty Ltd (METS) are undertaking the testwork.
The results show a flowsheet that would incorporate flotation followed by Pressure Oxidation (POX) as a processing option that would achieve high recoveries of gold.
The Aphrodite Deposit is located 65 kilometres north of Kalgoorlie, Western Australia on which a Pre Feasibility Study (NYSE:PFS) is currently been conducted.
Key results of the testwork:
- Significantly, up to 96.8% gold recoveries were achieved from initial Pressure Oxidation (POX) testwork on sulphide concentrate.
- Further flotation work returns high gold recoveries with excellent very low mass pull to concentrate.
- Optimisation testwork is in progress to improve POX and flotation gold recoveries even further.
- Development of process flowsheets is at an advanced stage
A detailed metallurgical testwork program is currently being conducted on composite samples prepared from diamond core drilled in 2012.
These samples lie within, or adjacent to, resource blocks identified from the Scoping Study as potentially mineable from an open pit.
The testwork program is investigating both the Alpha and Phi lode material and is an extension of the initial program conducted for the Company's Scoping Study completed in February 2012.
Previous work done by Goldfields
A significant amount of testwork (including POX) was also completed by Goldfields Exploration between 1988 and 2001 (then owners of the nearby Paddington Gold Mine) on drill samples from the high grade, lower half of the Aphrodite Deposit which is currently planned for underground development (viz. below 190m to at least 440m depth).
Data from these earlier programs has been incorporated into the metallurgical database.
Background of Aphrodite ore
The primary (sulphide) resources at Aphrodite are refractory in nature requiring a process stage to be introduced to facilitate liberation and extraction of the gold.
As part of Aphrodite's Scoping Study METS completed a formal assessment of possible processes for the treatment of high grade sulphide concentrate - produced by separating (after crushing and grinding) the gold bearing sulphide minerals from 'barren' minerals using flotation technology.
These processing options were;
- Off-site concentrate treatment
- Pressure oxidation
- Bacterial oxidation and
Both concentrate sale and off-site treatment require the production of a high grade flotation concentrate on-site before transportation to a secondary processing facility.
However, these options do not produce gold doré on site - production of doré on site is considered more favourable as it attracts lower government royalty and provides Aphrodite with control over all stages of processing.
All other options investigated oxidise the gold bearing sulphides so that gold can be recovered as the final product of the process.
To determine the most appropriate processing option for the Aphrodite Deposit METS developed a number of ranking criteria to be assessed for each option. Criteria investigated included gold recovery, financial risk, process risk, environmental risk as well as OHS issues and capital and operating costs.
The outcome from the study was that pressure oxidation (POX) was the most preferred option going forward for processing of the gold bearing concentrate.
Pressure oxidation has been used extensively around the world for the past 30 years and continues to be the favoured option for a number of refractory gold plants currently in the design and construction stages including those for Barrick and Newcrest's deposits.
Pressure Oxidisation (POX) Tests
Current pressure oxidisation (POX) testing has investigated the effect of temperature, pressure and grind size on a bulk flotation concentrate prepared from Alpha Lode material.
The concentrate averaged 47.3 g/t gold and was prepared from a composite of core samples averaging 1.49g/t gold.
Sub-samples of the concentrate were subjected to POX for 1 hour resulting in rapid oxidation during this very short period.
Critically, the results to date confirm the fast oxidation and strong gold recoveries which could be achieved using POX technology.
Additional testing is being implemented on the Alpha lode flotation concentrate to optimise these findings, with both an extended residence time (2 hours compared to the 1 hour currently tested) and other grind size being investigated.
Flotation Variability Tests
Testing of the Phi lode material is also undergoing investigation, with the main focus being on the sulphide material.
An initial flotation variability testwork program on Phi lode sulphide material has been completed.
This testwork analysed sub-samples of the material at two different grind sizes (75 and 45 micron) at various reagent dosage schemes.
Because of the excellent mass pull no cleaner flotation testing has been undertaken at this stage.
Further optimising of reagents will be carried in order to gain further improvement in gold recoveries and mass pull of 2.9%.
Previous studies on Alpha lode material by Aphrodite Gold resulted in a 12.4% mass pull in the rougher concentrate.
Cleaner flotation testwork reduced the mass of the concentrate to 5.85% and achieved an overall recovery of gold of 93.5%. Bulk flotation tests produced an excellent mass pull of 2.9%.
In addition to the testwork being conducted for the PFS study, the engineering side of the project is also advancing.
Due to the sequential nature of the study, several deliverables have been developed as an outline to be completed once all testwork data is available.
These deliverables include capital and operating costs, mechanical equipment list and process description.
Process flowsheets have been developed and are currently being reviewed. The front end of the plant is being simulated using the updated testwork data with the objective of reducing capital costs and producing a more efficient circuit.
The metwork results achieved by METS is a stepping stone toward development of the Aphrodite deposit. The
high gold recoveries achieved from using initial Pressure Oxidation (POX) testwork on sulphide concentrate should silence some of the naysayers. We estimate this should provide a 10% filip to the current share price of $0.051.
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