In December 2015 the Board agreed that it would be prudent to commence certain low-cost but time-consuming studies and investigations while the pre-drilling environmental studies and associated documentation were being completed.
These activities include the following, some of which are explained in more detail in the next section including interim results:
- Bulk Sampling;
- Metallurgical Laboratory Testwork;
- Preparation and analysis of project specific Assay Reference Material;
- X-Ray Ore Sorting and Optical Ore Sorting Trials;
- Bench Scale Magnetic Separation Testwork;
- QEMSCAN mineralogical studies of mineralised and unmineralised silexite; and
Bulk Sampling – a nominally three tonne bulk sample of mineralised silexite was manually collected from the historic Mt Everard pit walls. Silexite is the quartz – topaz rich host rock of the Torrington tungsten (ferberite) mineralisation.
The hand sampling highlighted the presence and predominance of banded/fine-grained disseminated ferberite mineralisation. The banded distribution had not been previously reported. Previous mining had concentrated on coarse high-grade mineralisation along joints, in veins and vughs and processed only minor amounts of the fine-grained style of mineralisation which is expected to be the dominant source of mineralisation.
Approximately 1,100kg of the crushed material was split off and placed in four 200 litre sealable steel drums and delivered by Townes on a pallet to freight agents in Brisbane for shipment to the selected metallurgical engineering group.
The remaining material is stored in sealed 200 litre steel drums at the Townes plant site in Tenterfield for future use if required.
Metallurgical Laboratory Testwork – An exhaustive selection process for the most appropriate metallurgical engineering group and laboratory to conduct the testwork was conducted over a 3-month period before deciding on Appropriate Processing Technologies (APT) in conjunction with their associates Peacocke and Simpson (Johannesburg and Harare in Zimbabwe) to undertake the metallurgical testwork. APT was selected as only they offered a vertical integrated operation given their ability to also build modular processing plants. The sample was then airfreighted to the mineral processing facility in Zimbabwe in early January 2016.
The initial / sighter metallurgical testwork on approximately 200kg of material commenced early January and was completed in early March. This testwork highlighted some issues to be resolved prior to setting the parameters for the processing the remaining ~850kg of sample material. During May and June further small-scale testwork programmes have been conducted with roughly 700kg of sample now remaining.
It was resolved that the initial crushing to minus 3mm was unsuccessful in liberating much of the tungsten mineralisation. Similarly, recoveries at finer grinding to minus 1mm were not much better and therefore that jigs will serve no useful purpose in the gravity concentration circuit. The metallurgical testwork carried out on the additional small batches of the bulk sample during May and June resulted in a successful processing route comprising wet impact crushing to minus 1mm and the sequential screening-off of the minus 300 micron fraction for spiral gravity concentration followed by (Wilfley) tabling and then magnetic separation to produce a clean tungsten concentrate (ASX 6 September 2016).
Topaz has an absolute hardness twice that of quartz. Given the ore contains 15 to 20% topaz and given its abrasiveness, much emphasis is being placed on not overgrinding or milling as the added cost may outweigh the benefit of higher recoveries of the tungsten. Physical strength and work indexes (crushing and milling) will be carried out during the final testwork phase, or on PQ (large diameter) diamond core in future.
Metallurgical testwork on additional small batches of the bulk sample prior to processing the now remaining +700kg of the material has been carried out. This testwork resulted in a successful processing route being envisaged comprising wet impact crushing to minus 1mm and the sequential screening-off of the minus 300 micron fraction for spiral gravity concentration followed by (Wilfley) tabling.
This process results in average recoveries of +80% of the contained tungsten, which is acceptable and may be improved upon during further refinement and proof of concept trials.
The difficult part of the testwork has been to ensure high (+80%) recovery of the finely disseminated tungsten into a small mass of concentrate with a grade of 62% WO3 or better with the minimal amount of process steps and without the use of chemicals (e.g. by flotation). This has now been achieved, but still needs optimization and confirmation on larger samples and diamond core.
Bench-Scale Magnetic Separation Testwork on the concentrate product – The tungsten mineralisation at Torrington is predominantly the iron-rich member of the Wolframite (FeWO4) series called Ferberite which is paramagnetic due to its iron content.
The minus 300 micron concentrate (and also the 212, 106 and minus 106 micron fractions of it) produced from the gravity testwork above was processed using a high-powered setting on electromagnetic Carpco magnetic separation equipment. This gave excellent results with a minimum recovery of 98% of the available tungsten into a reduced concentrate mass and importantly the concentrate grade was also increased from about 48% WO3 to an average of 69% WO3 (see table below).
|Top Tung +300µ SP Mag Conc
|Top Tung -300µ SP Mag Conc
|Spiral and tabling concentrate
|Top Tung Table Mag Conc -300+212µ
|Top Tung Table Mag Conc -212+106µ
|Top Tung Table Mag Conc -106µ
|Average WO3 concentration
The successful magnetic separation testwork has shown the potential to possibly by-pass the use of tables in the gravity recovery process by going straight from the spirals to a magnetic separation process. This will be further investigated as it could vastly simplify the processing plant.
Assay Reference Material – The two 25kg visually higher (~0.5%) and lower grade (~0.3% WO3) samples were collected from the bulk sample material for the production of site-specific standards for use as reference material. The samples were processed and homogenised by ALS Global in Brisbane and 10 splits of each assayed. These samples have also been distributed to a number of accredited Australian and offshore laboratories for analysis.
The standards will be used as reference material when submitting samples for assay and for the calibration of the Company’s desktop XRF unit which will be used for routine assaying of prepared samples when drilling commences.
X-Ray Ore Sorting Trials – - Ore sorting testwork carried out by TOMRA Sorting Solutions - Mining (TOMRA) at their test facility in Sydney on hand selected Torrington tungsten mineralised and unmineralised silexite utilising their X-Ray Transmission (XRT) system has been successfully completed.
Summary of Results: The head grade of the silexite* sample feed was 0.78% WO3 and following sorting through the plant:
- The mineralised product represents 56% of the sample feed and assayed 1.38% WO3; and,
- The unmineralised (waste) product represents 44% of the sample and assayed 0.029% WO3.
What this means is that approximately half of the material going through the sorter is rejected as waste, thus significantly reducing the mass to be processed while also increasing the grade of the mineralised fraction.
The finely disseminated tungsten mineralisation was very clearly detected by the TOMRA XRT system. From this small-scale sorting testwork, it appears that almost all the mineralised samples would be classified as product and the unmineralised samples as waste in a commercial-scale sorting plant.
To verify that significant mass reductions and high recoveries can be achieved from a full scale XRT sorter, a testwork programme will be carried out in 2017 using drill core that is more representative of the mineralised silexite zone and the grade of the deposit.
Full details of the testwork are given in a release to the ASX on 18 October 2016 (View full report here)
*Silexite is the quartz-topaz host rock to the tungsten mineralisation at Torrington.
A commercial-scale TOMRA XRT Ore Sorter in Sydney where the test samples were processed.
Optical Ore Sorting Trials – Mogensen has had some early success with optical ore sorting testwork on feed rock sizes of 10 to 30mm in that it has been possible to separate finely disseminated tungsten (ferberite) mineralised and unmineralised silexite. This visual success is yet to be quantified, but if proven correct this will reduce the amount of material to be processed and increase its W grade, i.e. it will result in an improvement in the project’s financial viability.
The Company has also hand-sorted some 10 to 30mm samples of both visually mineralised and unmineralised silexite for additional optical ore-sorting trials at another company interested in conducting research into separating this material. This company also manufactures ore sorting equipment utilizing X-Rays, optical and laser technology.
Interim Results – Conceptually the recent metallurgical testwork findings may result in a smaller and vastly simplified process flowsheet for a commercial tungsten recovery plant at Torrington as follows: -
- Crush and screen the mined rock to 30mm with the 10 to 30mm fraction being optically sorted. The mineralised (smaller) mass will be returned to the minus 10mm fraction for wet impact (or similar) crushing to minus 1mm, with either all this material or two separate screened fractions of minus 1mm and minus 300 micron being concentrated through spirals and then magnetically separated without the requirement of tabling or other gravity or metallurgical processing.
- Optimising this entire process will be the main focus of research on the remaining 700 kg sample in the near-term.
- Torrington is historically renowned for the occurrence of large masses of ferberite and coarse grained mineralisation in veins and bungs as referred by historical small-scale miners. The processing route as described above will also recover any such coarse mineralisation encountered during mining.
QEMSCAN Mineralogical Studies – This study has shed light on many aspects of the ore and unmineralised silexite including liberation sizes and composition of minor impurities in both the tungsten and topaz. Its findings will be invaluable going forward.
A copy of the QEMSCAN study can be viewed by following this link.