Introduction
Since the completion of the Pre-Feasibility Study (PFS) in mid-2018, COB has been advancing the metallurgical program for the Broken Hill (Thackaringa) Cobalt Project. As previously announced on 26 February 2019, COB has been treating 45 t (wet basis) of Reverse Circulation (RC) drill chips through a pilot-scale concentrate circuit at ALS Metallurgy, Burnie Tasmania. The Company is pleased to provide this update to the market with the finalised results.
The RC chips were collected at regular 1 m intervals and stored in individual bags during drilling. Using geological logging and a nominal 500 ppm cobalt cut-off, contiguous mineralised sections down each drill hole were selected for the concentrate testwork program. Criteria for sample selection, allowed up to 8 m of continuous dilution (<500 ppm), resulting in a wide range of cobalt grades being included for testing. In total, there were 1588 individual samples (each representing a 1 m interval of drill hole), covering a range of 31 ppm to 3600 ppm cobalt.
Concentrate flowsheet
As part of the 2017 Scoping Study and 2018 PFS, COB developed a simple circuit for upgrading the ore into a pyrite-concentrate prior to refining. The key driver for the circuit selection, was to take advantage of the large (coarse) grained pyrite, and thereby minimise crushing and milling costs.
After extensive mineralogical characterisation, and bench-scale evaluation of different concentrate methods, a combined gravity-flotation circuit was selected. The ore was crushed/milled to 1 mm topsize, and passed across gravity spirals, to produce a gravity concentrate. The gravity rejects were size-classified, and the fine fraction (nominally <125 um) sent to a flotation circuit for scavenging the remaining cobalt-pyrite.
In the current pilot-scale program, the circuit has now been shown to be robust, with successful concentration of variable ore grades. Furthermore, the combination of two techniques – gravity and flotation, provides an effective method for treating ore samples which vary in particle size distribution. These are two common hurdles for reliable concentrator operations.
Key results
- The overall recovery of cobalt was 90%, in-line with the PFS testwork results.
- Cobalt grades in the gravity and flotation concentrates were remarkably similar, ranging from 4444 ppm to 5075 ppm. This confirmed that the cobalt content in the host pyrite mineral was consistent across the range of ore grade samples. Variations are likely due to differences in liberation of pyrite grains from the gangue feldspar/silicates.
- Tailings rejects typically graded 100–180 ppm Co, which was close to the analysis detection limit of 100 ppm Co (XRF method was used). Sub-samples have been sent for characterisation of acid-forming properties (Bureau Veritas Adelaide), and other sub-samples are now being evaluated for incorporation into overall mine waste rock and process plant tailings management studies (ATC Williams).
- Optimisation of the classification step, and the possible inclusion of a regrind unit operation, may lead to improved cobalt recoveries. This will be considered in future testwork programs.
Implications
COB is encouraged by the positive results achieved, when upscaling the quantity of material by 50x to the pilot trial. The successful use of commercial-sized spirals bodes well for commercial implementation of the circuit.
The simplicity of the circuit equipment, and the robustness to account for low-average-high grade ore and variable particle size distributions, provides a strong foundation for the process plant operations.
The ability to upgrade the ore by concentration, while retaining ~90% of the cobalt and rejecting ~80% of the feed ore to tails, significantly reduces the capital and operating costs for a refinery.
Next steps
COB is currently preparing detailed plans for continued pilot testing of the flowsheet. This will include the following steps:
- During 2H 2019, the 7–8 t of concentrate produced from the pilot-trial, will be progressed through a pilot-scale furnace operated by ANSAC in Bunbury.
- Plans are being prepared for a second concentrate trial, utilising the remaining 40–45 t of mineralised material stored at Broken Hill. This will focus on seeking improved recoveries via process optimisations.
- Engineering design work will now commence to update the PFS costings for the concentrate circuit.
- Geo-metallurgical studies will now commence, with a focus on linking in-ground ore characteristics to extractive mining techniques (blasting, load and haul), and crushing/milling requirements.