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The effects of rheology and stability of magnetite dense media on the performance of dense medium cyclones He, Ying Bin

Abstract

The main objective of this thesis is to investigate the effect of the stability and non-Newtonian rheological properties of magnetite dense medium on the performance of dense medium cyclone (DMC). In addition, the DMC separation mechanism, the rheology of magnetite dense medium, and the hydrodynamics of particle movement in non- Newtonian fluids are also studied. The tests, carried out on a 150 mm pilot scale DMC loop with density tracers as the cyclone feed, reveal that the 0/U flowrate ratio is the fundamental operating parameter directly related to the DMC performance, other operating parameters influence the DMC performance through, or partially through the OAJ flowrate ratio. The medium rheology and stability exert opposite influences on DMC performance. In addition, the separation of fine feed particles (2.0 mm) is more strongly affected by medium stability. As a consequence, contradicting results are observed depending on which of these factors is predominant. A DMC separation mechanism based on a modified equilibrium orbital hypothesis by taking into account the medium density gradient and medium inward radial flow was proposed. Based on this hypothesis, a theoretical model is derived to predict the DMC separation density and cutpoint shift behaviour: The main objective of this thesis is to investigate the effect of the stability and non-Newtonian rheological properties of magnetite dense medium on the performance of dense medium cyclone (DMC). In addition, the DMC separation mechanism, the rheology of magnetite dense medium, and the hydrodynamics of particle movement in non- Newtonian fluids are also studied. The tests, carried out on a 150 mm pilot scale DMC loop with density tracers as the cyclone feed, reveal that the 0/U flowrate ratio is the fundamental operating parameter directly related to the DMC performance, other operating parameters influence the DMC performance through, or partially through the OAJ flowrate ratio. The medium rheology and stability exert opposite influences on DMC performance. In addition, the separation of fine feed particles (2.0 mm) is more strongly affected by medium stability. As a consequence, contradicting results are observed depending on which of these factors is predominant. A DMC separation mechanism based on a modified equilibrium orbital hypothesis by taking into account the medium density gradient and medium inward radial flow was proposed. Based on this hypothesis, a theoretical model is derived to predict the DMC separation density and cutpoint shift behaviour: [formula] where the first term represents the influence of medium stability and the second term reflects the influence of medium rheology. The rheological studies reveal that the non-Newtonian magnetite suspensions are best described by the Casson equation. The Casson yield stress is the principal rheological parameter and responds to the changing medium properties in a well defined pattern. On the other hand, the Casson viscosity is a subordinate rheological parameter which can be treated as a constant for coarse (commercial) and intermediate magnetite suspensions, or can be ignored for very fine magnetite suspensions. To interpret the influences of the non-Newtonian medium rheology on DMC performance, a theoretical framework of particle movement in non-Newtonian fluids is established. A shear rate equation bridging the hydrodynamic and rheology theories is derived: [formula] It is further derived that the general form of Reynolds number applicable to any rheological fluid types is: [formula] For magnetite suspensions conforming to the Casson equation, the modified Reynolds number is: [formula] Thus the influences of the viscosity and yield stress on DMC performance can be interpreted via the general relationship between drag coefficient and the modified Reynolds number: [formula] The yield stress not only determines the threshold drag on particles before particle-to-fluid relative movement is achieved, more importantly, it also determines the viscous drag during the particle-to-fluid relative movement. Since the Casson yield stress value is much greater than the corresponding Casson viscosity value, the DMC separation of fine particles is mainly determined by the Casson yield stress. This postulation is verified by the experimental evidence obtained from the DMC separation tests and the rheological measurements.

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