Experimental investigation into the kinetics of Falcon UF concentration: Implications for fluid dynamic-based modelling

Abstract

Centrifugal gravity separators, such as the Falcon UltraFine (UF) concentrator, are the most common gravity concentration techniques used for fine particles processing. Hence, understanding the kinetics and separation mechanisms at play within these separators is of paramount interest. Recent research yielded a predictive physical model for the Falcon UF which however does not explain some results obtained with industrial ores. The Falcon UF kinetics have been investigated through the processing of fine-grained ores from the Altenberg tin deposit (Germany), the Tabuaço tungsten deposit (Portugal), a synthetic iron ore as well as results from previous studies on kaolin residues. Results have shown an evolution of Falcon UF performance with time/feed mass in contradiction with the stationary separation hypothesis on which the physical model was based. In terms of Falcon UF separation timing, four phases can be distinguished. First, upon initial feeding of the bowl, particles are trapped or rejected depending on their settling velocity. It yields a relatively ineffective selection according to density so that only ultrafine particles are ejected from the bowl, resulting in the quick growth of the concentrate bed. When the bed reaches a critical size, recovery and enrichment continue to increase through selective resuspension phenomenon that favours the concentration of dense particles and the ejection of larger particles. This way, the bed builds up while the content of concentrate bed surface evolves until resuspension balances the stream of dense material reaching the bed and recovery drops. The evolution of partition curves over time confirmed the low recovery of ultrafine particles during the whole operation but also showed a decrease of coarse particles recovery with time. It suggests that the second separation mechanism is less sensible to particle size compared to the first one and that size even has a negative impact on recovery. Furthermore, erosion figures in furrows are observed in the concentrate bed which may play locally an active role in the separation. These observations suggest that two separation mechanisms are at play. Firstly, differential particles settling within the flowing film which is already accounted for in the existing physical model. Secondly, resuspension of particles from the concentrate bed by the action of a lift force acting preferentially on coarse particles deposited at the surface of the bed and resulting in the rejection of coarser and lower-density particles. The addition of a lift force component to the existing model is discussed and a resuspension criterion is proposed as a guidance of the physics involved in this second separation mechanism. Future developments will require a dynamic model which would need to integrate the evolution of the concentrate bed content over time.

Application of Falcon Centrifuge as a Cleaner Alternative for Complex Tungsten Ore Processing

“…Enhanced gravity separators such as Falcon concentrators, which use additional centrifugal force, have been specifically developed to process fine to ultrafine particles, down to 3 µm [22][23][24]. Falcon concentrators can be used successfully for recycling processes [25,26] as well as for mine tailings reprocessing [27,28] that are key approaches to improve environmental outcomes [29]. Developed in the 1980s by Steve McAlister [30], such separators consist in a bowl capable of spinning at high "G" force, enabling the density separation of fine particles [22,24].…”

“…The bowl can be fluidized to avoid compaction and to remove light particles from the concentration zone (Falcon SB), while the Falcon UltraFine (UF) bowl, specifically designed to recover very fine particles, is not fluidized. Rotary speed, pulp density, feed flowrate, and fluidization pressure (for the Falcon SB only) are the main operating parameters involved in the separation [27,28,[31][32][33][34]. Falcon concentrators are operated in semi-batch mode but, at industrial scale, continuous Falcon concentrators (Falcon C) can process continuously up to 100 t/h of ore with high feed pulp density (up to 45 wt %) [24].…”

“…Recently, authors demonstrated that Falcon concentrators, as other centrifugal separators, allow to eliminate efficiently the fine particles (−10 µm), also called slimes [28]. Desliming, which is recommended for the froth flotation stage since fine particles (−10 µm) are known to disturb the process, is usually performed using hydrocyclones.…”

“…In addition, a by-pass was placed at the output of the feed tank to allow operating in closed circuit during feed preparation, for pulp density adjustment and flowrate control. Feed dry weight was set at 500 g for both Falcon UF and Falcon SB, based on preliminary saturation tests which allowed to set the maximum feed weight to work with to avoid saturation of the bowl [28].…”

“…This result can be attributed to the presence of phyllosilicates in light silicates and dense silicates groups, since their characteristic shape can result in stronger entrainment phenomenon [41][42][43]. Moreover, it can also be linked to the erosion mechanisms in the bowl that are undoubtedly stronger for higher pulp flowrates and might affect more significantly the light minerals [28]. Overall, for the gangue mineral recoveries, the pulp density has a higher effect in absolute value than the rotary speed, indicating a strong control of the settling mechanisms on the separation.…”