Coal Preparation » Gravity Separation
As part of a longer term thrust, this project has developed a framework for the development of a fundamentally based mathematical description of dense medium cyclones (DMCs). The dynamics of multiphase flow through dense medium cyclones have been derived from first principles utilising Computational Fluid Dynamics (CFD) model. Discrete element modelling (DEM) methodology has then been applied to incorporate the dynamics of concentrated particle flow.
As the methodology and concepts used in this project were new to ACARP, specific project objectives were set by the ACARP Project Monitors to allow some possible outcomes from the approach to be demonstrated. The approach was able to realistically model:
- Surging of both medium and solid particles
- The impact of changing a tangential to an involute feed configuration.
The model frameworks developed in this project were validated by successfully predicting the:
- tangential and radial flow velocity profiles in a hydrocyclone with no solids present
- magnetite slurry density profiles within a dense medium cyclone
- partitioning of coal particles on a dense medium cyclone.
Once validated, the CFD-DEM model developed in this project allowed the beginnings of a full understanding of how a dense medium cyclone operates. For example, it was used to understand:
- the pressure distributions inside an operating dense medium cyclone
- the tangential, axial and radial velocity distributions inside an operating dense medium cyclone.
- the formation of a helical twisted cylinder-type air core that was asymmetrical, continually in motion through which air passed from spigot to vortex finder outlets. The vortex did not coincide with the geometrical axis of the DMC and had a curved surface rather than being straight.
- the presence of a high density ring that was identified to be around the air core through which low density material could not pass. This was identified as a possible cause of low relative density (RD) material erroneously passing to underflow.
- internal flow collisions around the vortex finder that resulted in short circuiting of barely separated coal to overflow as well energy losses which diminish inherent separating efficiency.
- that less short circuiting to vortex finder should occur in DMCs with involute entries. This seemed to be due to a significant reduction in internal DMC flow collisions. Involute entry units were predicted to give sharper separations at much reduced offsets.
- the “surging” phenomenon in DMC operation reproduced by CFD and DEM.
- the dominant force for separation is predicted to be that due to the pressure gradient.
Significant additional work is required to extract high value from this project, beginning with validation of the DEM modelling, followed by modelling of existing/proposed dense medium cyclone designs and possible design of higher efficiency dense medium cyclone units. The latter aspect could involve, for example, minimising those factors that cause by-passing to overflow and underflow.
This is a work in progress.