Crocodile River Mine — 2022 Technical Report

The Zandfontein processing plant at Crocodile River Mine has been recommissioned with modified MF2 circuits to treat current underground ore and historical tailings, operating at actual throughputs below original design while achieving recoveries lower than the metallurgical test work indicates is possible.

The Zandfontein beneficiation plant at the Crocodile River Mine has been redesigned to process both underground ore and material reclaimed from the Tailings Storage Facility (TSF). The processing route incorporates crushing, milling, and flotation stages, with specific modifications made to the plant to accommodate Circuit B and Circuit D for treating WHIMS chrome plant tailings. Operational data from late 2020 to early 2022 shows the plant is operating with some recommissioning challenges that are affecting recovery rates. Design criteria suggest what the plant should achieve in terms of throughput and grinding performance, while plant data shows the actual operating envelope differs. Metallurgical test work indicates achievable PGM recoveries are above 40%, but the operating plant is currently performing below 15% recovery, indicating there is upside potential for optimisation.

Critical Data

Overview

The Zandfontein processing plant is designed to treat underground ore and TSF remining material through a circuit that includes crushing, milling, and flotation. The plant utilises two identical MF2 (Mill-Float-Mill-Float) circuits, designated A and B, with modifications made during recommissioning to accommodate Circuit B and Circuit D for treating WHIMS chrome plant tailings. The four ball mills at the plant each have 1,550 kW installed power, with design throughput of 136 tph per mill. The plant was operating at reduced throughput relative to design during the reporting period, with Circuit B operating at 100-120 tph against a design capacity of 164 tph, and Circuit D operating at 90-100 tph against a design capacity of 70 tph. Plant performance has been marked by instability, which has contributed to lower recoveries than indicated by the metallurgical test work.

Key Process Stages

The crushing circuit uses a jaw crusher followed by secondary and tertiary cone crushers to reduce underground ore to a suitable feed size for milling. The mill discharge passes through classification cyclones, with the oversize material returned to the mill feed conveyor and the undersize passing 0.63 mm pumped to primary flotation.

The milling circuit design is based on an open-circuit ball mill configuration with classification cyclones. The mill discharge from the primary mills is combined with the cyclone overflow and pumped to the secondary flotation stage. Typically, the secondary rougher feed PSD is 75-78% passing 75 µm. The design criteria for the four ball mills specify a P80 target of 75-80% passing 75 µm for mills #2 and #4, while mills #1 and #3 are screened at 630 µm. The classifier oversize material is returned to the mill feed conveyor and the undersize material passing 0.63 mm is pumped to primary flotation.

The flotation plant comprises two identical MF2 circuits, A and B, with rougher, cleaner, and re-cleaner banks. Reagent addition includes a depressant, collector, copper sulphate, and Dow 200 frother at 15 g/t. The primary rougher tailings are combined with the high grade cleaner tails and pumped to the secondary mill circuit for further grinding and PGM liberation. The concentrate resulting from this process is final high-grade concentrate with PGM grades ranging from 300-500 g/t 6E and chromite content of 1.5-1.7%. The high grade and low grade concentrates are combined in one slurry thickener to produce a slurry with a relative density of 1.5-1.55 and a combined PGM grade of 180-200 g/t 6E. The high-grade recleaner tail stream is recycled to the cleaner head. Rougher tails from this plant are cycloned, thickened, and pumped to the tailings dam for final deposition.

For the modified circuits, the milling circuit design is based on an open-circuit ball mill configuration with classification cyclones. Final flotation concentrate from Circuit B is thickened and transferred to dedicated concentrate storage tanks. Re-cleaner concentrate from Circuit D is pumped to a dedicated existing concentrate thickener for dewatering. The re-commissioned pressure filter, or a rented filter unit, is used to batch process both concentrates from Circuit-B and Circuit-D flotation processes. Concentrate material is re-claimed from the covered storage bunker by a front-end loader to feed into trucks for final product dispatch.

Metallurgical test work was completed by Maelgwyn South Africa, carrying out laboratory flotation tests to determine the best grind. The tests were done as open circuit multi-cleaner tests on material with PSDs of 80% passing of 106 µm, 75 µm, and 53 µm, followed by a locked cycle flotation test at best grind. From these results, the 6E recovery was 43.68% at a final concentrate grade of 83.3 g/t 6E. The 4E recovery was 48.12% at a final concentrate grade of 71.67 g/t 4E at a head grade of 2.9 g/t 6E.

Additional Interesting Data and Summary

The plant was operating with a total feed tonnage of 887,818 tonnes between December 2020 and February 2022, with a feed grade of 1.48 g/t. Final tailings amounted to 744,562 tonnes at a grade of 1.35 g/t, producing 3,654 tonnes of concentrate at a final grade of 45.7 g/t 4E. The design criteria for the four ball mills at the Zandfontein plant show that each mill has 1,550 kW installed power, with absorbed power ranging from 750 kW to 1,200 kW depending on the mill. The mills operate at 75% critical speed, except for Mill #4 which operates at less than 60%. The P80 target for mills #2 and #4 is 75-80% passing 75 µm, while mills #1 and #3 are screened at 630 µm.

The plant achieves lower recoveries when operating below 15%, primarily due to instability. The test work indicates that steady state production is required for the flotation circuit to be efficient, and lower recoveries are achieved when instability is present. The achievable recoveries from the metallurgical test work were above 40%, whereas the plant is operating below 15%, indicating there is upside potential in the recovery optimisations. A summary of the overall performance of the plant is detailed in the operations data, with the recovery of 11% used in the financial model. The total installed power for the processing plant is estimated as 4,288 kW, which includes power for the CRP tails guard cyclone feed, guard cyclones, secondary ball mills, and various reagent make-up circuits including copper sulphate, depressant, and flocculant make-up circuits.

Key Processes

• Crushing: Jaw crusher followed by secondary and tertiary cone crushers
• Milling: Open-circuit ball mill configuration with classification cyclones; four ball mills with 1,550 kW installed power
• Flotation: Two identical MF2 (Mill-Float-Mill-Float) circuits with rougher, cleaner, and re-cleaner banks
• Regrinding: Primary rougher tailings combined with high grade cleaner tails for secondary milling
• Concentrate handling: Thickening in slurry thickener, pressure filtration, and storage in covered bunker
• Reagent addition: Depressant, collector, copper sulphate, and Dow 200 frother at 15 g/t

Source: Crocodile River Mine , 2022 Technical Report, 2022.

Editorial Note: The source presents contradictory data on mill classification. The table text states the classifier oversize returns to the mill feed conveyor with undersize passing 0.63 mm to primary flotation, but the design criteria table shows mills #2 and #4 have a P80 target of 75-80% passing 75 µm with cyclone d50 of 75 µm, while mills #1 and #3 are screened at 630 µm. The plant capacity figures also appear contradictory, with Circuit D reported as having a design capacity of 70 tph but operating at 90-100 tph, which exceeds design. Additionally, the absorbed power for Mill #4 is given as 750 kW against 1,550 kW installed, which may require verification against the source.

Project website: Crocodile River Mine, 2022 Technical Report