Multotec Spiral improves chrome recovery from UG2 tails

Multotec senior applications engineer, Sithembiso Mayisela, outlines an ongoing success story from a chrome recovery plant in the North West province of South Africa for the recovery of chrome from UG2 tailings from the platinum concentrator upstream.

The chrome recovery plant receives UG2 tails from a MF2 platinum concentrator and recovers chromite before the material is sent to the tailings dam facility.

The chromite head grade received by the plant ranges from 16- 22% Cr. The plant had been struggling to reach the required plant performance targets while using previous spirals, which had reached the end of their life. The previously installed spirals were from a different supplier.

The plant invited Multotec to quote on replacement spirals for the plant in 2022, as the current spirals had reached the end of their lifespan. The initial request was for a quote on replacement spirals to match the existing footprint, but during plant inspection, it became apparent that the flowsheet had to be optimised before quoting on a full spiral solution.

The client had already initiated the process of modifying the flow sheet, and Multotec assisted the client with spiral selection through test work. Although Multotec can rely on a wealth of experience to guide the choice of spiral selection, test work is always advisable to confirm selections. Test work is typically done by the Multotec Technology Department using similar spirals identified for installation, which makes the test work reliable.

After test work, the results suggested that the HX5 spirals be used on the rougher. The HX5 spiral, which has a pitch angle of 19°, was found to achieve the best recovery and yields on the rougher stage. At 1.0 m wide, the HX5 spiral is able to accommodate a feed rate of 4-7 tons per start, with a nominal feed rate of 5.0 tons/h. The spiral can be assembled in single, double, or triple start, and a triple start assembly can process a nominal feed rate of 15 t/h. Smaller diameter SC21 spirals were recommended for the cleaning stages after test work. The SC21 spiral, which has a pitch angle of 21°, was found to achieve the best upgrade ratios for delivering the required 41% Cr2O3 grade.

The HX5 spiral was also the spiral of choice for the scavenger application, as suggested by the test work.

The SC21 can process 1.0 to 2,3.0 t per start. Like the HX5, it can be assembled in single, double, and triple start, and a triple start assembly can process a feed of up to 6.9 t/h. The material received by the client contains a significant amount of ultra-fines, more than 10% less than 38 µm, so desliming was recommended before feeding the tailings into the spiral separation plant. Ultra-fines increase the viscosity of the slurry, which then reduces the efficiency of the spiral.

An eight-way Multotec classification cyclone cluster of 250 mm cyclones at 10 degrees with one barrel (VV250-10-1) was selected for the application. Multotec VV-type cyclones, which are polyurethane cyclones, have the advantage of being lightweight when compared to their steel counterparts and offer similar wear rates. The use of a crane during maintenance is not required, as these cyclones can be easily taken apart and lifted by hand. For the scavenger/dewatering application, a six-way cluster with similar cyclones was selected.

The client placed orders in August 2022 for cyclones and spirals, which included ancillary equipment such as distributors, launders, and piping. All the items were delivered to the client within four months of order placement and installed over December 2022 and January 2023. The plant started running in mid-January 2023 when mining resumed operations after the Christmas break.

By the end of January 2023, the plant was producing on-spec concentrate of 41% Cr2O3. The first full month of running the plant achieved a 6% increase on the previous plant yield. The increase in yield saw a 40-50% increase in the production output of the plant.

The client was able to achieve a project payback period of three months, with the project including the rebuild and modification of the plant. The project was completed safely, with no injuries experienced, which was a testament to the good project management from the client.

Jafta Mashele, the Plant Manager of the K3 Chrome Recovery Plant at Sibanye-Stillwater, expressed his satisfaction with the service provided by Multotec’s R&D division, for their flexibility in conducting test work to meet the client’s needs and their willingness to adapt to specific requests. He stated, “I appreciate Multotec's ability to offer flexible payment terms which suited and helped us manage cash flows effectively.”

Mashele further commended Multotec, saying, “Multotec employs Application Engineers with extensive processing experience and in-depth product knowledge, which enables us to address technical inquiries promptly, both remotely and on site.”

The plant is currently conducting test work with Multotec to further explore the recovery of chrome from ultra-fine material, with Mashele consistently emphasising the importance of data-driven decisions. This work is being carried out using the new UX7 ultra-fine heavy minerals spirals, that is specifically designed to enhance the recovery of ultra-fine material.

Credit: MechChem Africa