The PGMs in the MG ore mined by Tharisa Minerals occur in the silicates and are not associated with the chromite, thus enabling the process to extract chrome before PGMs without sacrificing PGM recovery.
This lowers the chrome content in the PGM circuit and results in much lower chrome content in the PGM concentrate compared to typical UG2 operations. Base metal content in the MGs is also significantly lower than Merensky and UG2 ores, resulting in a low matte fall during smelting, reducing base metal refining requirements.
Tharisa Minerals’ two independent processing plants are designed specifically to treat the MG Chromitite Layers of the Bushveld Complex. The smaller 1.2 Mtpa Genesis Plant, with the 100 ktpm chrome circuit, was commissioned in August 2011 with the PGM circuit being commissioned in December 2011. The larger 3.6 Mtpa Voyager Plant was commissioned in December 2012. Both plants operate at above nameplate capacity and milled collectively 4.8 Mt. The plants have a similar process flow that includes crushing and grinding, primary removal of chrome concentrate by spirals, followed by PGM flotation from the chrome tails and a second spiral recovery of chrome from the PGM tails.
Operating in parallel, the independent plants provide processing flexibility and production stability by allowing one plant to be shut down without hampering the production of the other. The modular design of the processing circuits allows sections of the plant to be stopped without affecting the rest of the operation (i.e. a crushing circuit can be stopped independently of the comminution, spiral and flotation circuits).
Using off the shelf technology, the processing circuits are uniquely engineered to deliver both PGM and chrome concentrates. This innovative approach to production has made Tharisa a world-class PGM and chrome co-producer.
The PGM rougher feed grade was marginally lower for the year at 1.47 g/t, while the Cr2 O3 ROM feed grade was virtually unchanged at 18.1% for the year.
Tharisa targets recoveries of 80.0% for PGMs and 65.0% for chrome. In FY2019 PGM recoveries were 82.1% while chrome recoveries were 62.0%.
During the year, the Group produced PGM concentrates containing 139.7 koz of contained PGMs (5PGE + Au) and chrome concentrates of 1.29 Mt with 312.1 kt being speciality grade concentrates. Third-party chrome concentrate produced was 241.1 kt.
Speciality chrome recovery circuits are integrated into the feed circuit of the Genesis Plant, known as the Challenger Plant. The Challenger Plant, which is owned by fellow subsidiary Arxo Metals, was commissioned in July 2013 and produces chemical and foundry grade chrome concentrates.
Production of speciality grade concentrates accounted for 24.2% of Tharisa’s chrome production in FY2019, and will be maintained at current levels to ensure that it maintains a strategic market share.