Abstract
Preliminary work conducted at the Eland Platinum Mine indicated that the majority of the ultrafine Platinum Group Metals (PGMs) lost to final tails (more than 40% of the PGMs in the final tail) were in the fine/ultrafine region (-25 μm size fraction). This project was initiated to investigate the floatability of fine/ultrafine PGMs (-25 μm size fraction) using the column cells and a high shear FloatForce flotation mechanism to improve the recovery of fine-to-ultrafine PGMs lost to tailings.
Dual pilot scale columns (150 mm) were used solely for this purpose. The circuit showed an improvement in recovery of fine PGMs, with an overall recovery of about 83%. Thereafter, production scale (400 mm column and 800 mm turbo column cell) test work was initiated for both Tailings Storage Facility (TSF) material and Run of Mine (RoM) ore. This demonstrated a significant improvement in recovery of the fine/ultra-fine PGMs. The production column showed an increase in recovery of the ultra-fines of about 80%, where 60% of the concentrate recovered was -10 μm. The production scale dual columns improved the recovery of fine particles within – 25 μm size fraction by around 6% points from about 36% to about 42%. Thereafter, the FloatForce flotation mechanism was introduced. A cell-by-cell recovery was conducted for secondary rougher 3 (no FloatForce flotation mechanism installed), rougher 4 (FloatForce flotation mechanism installed) and rougher 5 (no FloatForce flotation mechanism installed). The introduction of the FloatForce flotation mechanism showed fine/ultra-fine PGMs recovery benefits, producing a recovery of approximately 80% for –25 μm, compared to secondary rougher 3 and secondary rougher 4, which all achieved a recovery of approximately 70%.
A dual column circuit was recommended at plant scale to improve fine/ultrafine PGMs recovery. A roll-out of an additional float force flotation mechanism along the remaining conventional secondary rougher high-grade float cells was also recommended to synergise the benefit of improved fine/ultra-fine PGMs recovery achieved with the two technologies. The plant performance with dual column installed and FloatForce flotation mechanism on secondary rougher 4 was compared to a period where none
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of the aforementioned was installed. The overall plant recovery increased by about 3% (relative), from about 81% to about 84 %.
This was also observed in the assay by size results, where the PGM losses in the -25 μm size fraction were reduced by 5.4% points, from 41.5% to 36.1%.
The synergy effect between the columns and FloatForce flotation mechanism also improved the recovery of coarse particles. This was attributed to the high-intensity agitation of the float force.
The introduction of FloatForce flotation mechanism showed a significant improvement in the recovery for size fractions of -25 and -38 μm. This mechanism was installed in a secondary rougher cell 4 and quantified against a cell upstream and downstream of it; no float force mechanism was installed. A recovery of 79% was observed in secondary rougher cell 4 and 70% in the upstream and downstream cells. This was attributed to the high-intensity agitation from the float force mechanism, suspending particles and improving particle-bubble interaction of finer particles. Additionally, the introduction of FloatForce flotation mechanism not only improved the recovery of fine to ultra-fine recovery, but it also improved the recovery of coarse PGM particles (53 μm and 75 μm). This infers that the turbulence force of this mechanism is not selective for only ultra-fine particles, but all particles in general.
Pilot-scale columns were initially used to identify a stream for potential recovery benefits. This stream was identified as the secondary rougher high-grade concentrate stream, which produced an initial recovery of 83%. The column showed improvement when fed with secondary high-grade rougher concentrate. This stream contains appreciable percentages of fine PGM at a of grind 80% passing 75 %. A recovery of 80% and 83 % was obtained with TSF feed and RoM when the column was fed with this stream, compared to other streams. This was attributed to the longer residence time of fine PGM particles and counter-current flow between feed and air, improving the interaction between particles and bubbles.
Production scale columns were introduced with the aim of increasing the column feed flow rate.Results showed an improvement in PGMs upgrade ratio at a lower mass pull (18 at a mass pull of 2.5%) with TSF ore. With RoM feed, the highest upgrade ratio obtained was about 15 at a mass pull of 1.66%.
The upgrade ratio decreased with an increase in mass pull and recovery. This was expected, as increasing mass pull results in grade dilution due to gangue mineral recovery.
The production scale column circuit showed a low % Cr2O3 in concentrate for both TSF and RoM feed at a mass pull below 5%. This was due to the improved froth drainage in the column cells. Additionally, lower mass pull results in lower entrainment of gangue. An increase in circuit recovery of 11.7% was observed with the introduction of the production scale column circuit with TSF (52.83% to 64 %). A recovery of 69% was achieved with Kukama ROM. The size-by-size assay showed a significant improvement in recovery of the -25 μm PGM (over 70%) in both TSF and Kukama. As previously mentioned, this was attributed to longer residence time and improved column cell hydrodynamics. Additionally, betasizer was used to analyse the particle size distribution of the -25 μm of the recovered final concentrate in the dual production scale column, primary final concentrate and secondary final concentrate. 61% of the -25 μm was ultra-fine (-10 μm) in the dual production scale column final concentrate and Pfico (45.12%) and Sfico (6.25%). This was attributed to the longer residence time in the column and proper circulation of feed within the column flotation cell. The simulation of the dual production scale columns on large-scale capacity showed a potential increase of the overall plant recovery by 2.83%, from 80.78% to 83.53%. This was in correlation with the assay by size result, where the PGM losses in the -25 μm size fraction were reduced by 5.4%, from 41.5% to 36.1%.
The improvements demonstrated in this study not only enhance ultra-fine PGM recovery efficiency but also align with South Africa’s critical minerals strategy by unlocking value from historical tailings. This has the potential to reduce the need for new mining, lower the cost per recovered ounce, and deliver significant environmental, social and governance (ESG) benefits through reduced waste and extended resource life.