This year’s production outlook for Cyprus-headquartered integrated resources company Tharisa Minerals’ flagship chromite and platinum-group- metals (PGMs) expansion project, near Marikana on the western limb of the Bushveld Igneous Complex, in the North West, is “promising” and the company remains on track to attain steady-state production by 2016, says CEO Phoevos Pouroulis.

This is partly owing to the development of a new 300 000 t/m stand- alone processing plant at the project site, constructed and commissioned by minerals processing and project management company MDM.

The Tharisa mine was developed following a staged approach, with the new processing plant constructed in March 2011 alongside the existing 100 000 t/m processing plant during the first phase of the project, thereby providing Tharisa with a 400 000 t/m run-of-mine (RoM) capacity.

Tharisa’s concentrator processes the middle group (MG) chromitite layers of the Bushveld Complex and is the first local concentrator to produce a chromite concentrate and a PGM concentrate simultaneously. This highlights the uniqueness of Tharisa’s processing plant as the MG chromitite layers have traditionally only been processed for chromitite recovery.

Plant Progression
MDM was first contracted in 2011 to undertake the front-end engineering and design of Tharisa’s processing plant, along with various phases of laboratory testwork to confirm process design assumptions and all phases of the commissioning process.

“By December 2012, the plant was running at full throughput design capacity of 300 000 t/m and meeting full recovery design capacity,” says MDM executive director George Bennett.

He explains that the process route followed in the concentrator is a three-stage crushing of the RoM to a product of 80% passing 25 mm.

“The crushed product is fed into two ball mills that are operated in closed circuit with vibrating screens to produce a 0.6 mm primary grind. The original design of the plant was for two semiauto- genous grinding mills, 6.4 m in diameter, 4.12 m in length and each with 3 350 kW installed power,” he states.

Bennett points out that, the competence of the rock was not fully understood when the mills were ordered.

“After the final set of testwork had been completed, the ore was proven to be harder than expected and two additional crushing stages were added before milling. “The mills were also changed to ball mills to ensure the throughput and required grind size,” he says.

Bennett adds that the primary chromite circuit comprises a complex spiral plant with rougher, scavenger, cleaner, recleaner and middlings spiral banks to produce a final chromite concentrate with a chromium oxide grade of between 41% and 44%.

“The tailings from the primary chromite removal stage are milled in a secondary milling stage with an overflow ball mill of 6.4 m in diameter and 6.4 m in length. Tharisa uses 5 500 kW of installed power to produce a final grind size of about 80%, passing 75 µ,” Bennett explains.

He notes that, after the milling process, the material is treated in a PGM recovery flotation plant with a rougher circuit and a multiple-stage cleaner circuit.

“The cleaner circuit is configured as a high-grade and low-grade flotation circuit to produce a final PGM concentrate . . . which is thickened and filtered before it is sent to a PGM smelter for toll treatment.”

The tailings from the flotation circuit are conveyed to a second complex spiral circuit with similar configuration to the primary spiral circuit, which produces a secondary chromite concentrate with a chromium oxide grade of between 41% and 42%.

The total chromite yield for the plant is between 29% and 42% and, depending on the chromite feed grade, the plant produces between 116 000 t/m and 168 000 t/m of chromite concentrate.

According to Tharisa, this makes the plant the single biggest chromite concentrate producer in South Africa.

The company adds that the mine’s total contained chrome resources of about 800-million tons make it the largest single chrome mineral resource deposit in the world.

The “favourable” geological and metallurgical characteristics of the Tharisa deposit are expected to support about 17 years of large-scale, openpit mining, giving the company the option of expanding the mine underground.

Bennett enthuses that MDM successfully completed the Tharisa concentrate project in a record time of 13 months and within budget.

“Since commissioning, the project has reached full production in a short time, which MDM believes is testament to the company’s work ethic to design and build cost- effective, functional and efficient plants,” he says.

Company Outlook
Speaking at the company’s inaugural interim results presentation for the six months ended March 31, 2014, Pouroulis stated that the economic fundamentals of PGMs and chrome, supported by strong global demand and underpinned by supply constraints, would remain sound for the foreseeable future, thereby providing a platform for more favourable commodity prices.

He added that Tharisa would continue to pursue its operational improvement strategy through the implementation of capital projects, like Phase 1 of the concentrator plant project.