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Hatch uses CFX for Grinding Mill Analysis
Posted Mon February 23, 2004 @06:41PM
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Application by Ross Haywood,
Non-Ferrous Metals Technology,
Hatch Australia

Hatch, a leading global engineering organization, has a reputation for the successful scale-up of process technology and the implementation of innovative solutions to technical challenges. To fulfill this reputation, Hatch’s Non-Ferrous Metals Technology Group uses advanced analytical tools, in particular CFD, for design evaluation and optimization, scale-up analysis and problem solving, where heat transfer, fluid flow, combustion and mass transfer are critical issues.

Recently we confronted the problem of scale-up analysis of a multiphase grinding mill – essentially a multiphase turbomachine! The mill works by vigorously “stirring” the feed material (pulp) and a coarse solid grinding media using a series of concentric disks rotating at high speed. At the discharge, a stationary separator is used to allow the pulp to flow out of the mill while retaining the grinding media. The essential features of the problem, from a modeling perspective were: complex geometry; multiple frames of reference (rotating disks and a stationary outlet), and a multiphase flow. As an added complication, the flow is strongly swirling with tangential velocities nearly three orders of magnitude greater than the mean axial velocity through the mill; this seemed like a perfect fit (or worthy challenge) for the new capabilities of CFX-5.


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The physical geometry and computational mesh for a typical segment of the mill were generated using CFX-Build, the native geometry/mesh pre-processor within CFX-5. The mesh was generated in three sections with general grid interfaces (GGIs) between the first two sections to accommodate a pitch change and between the intermediate and final section to model the change from a rotating reference frame to a stationary frame. A “frozen rotor” treatment was used to model the interface at the GGIs.

As the interest in this work was of a general nature, the total grid was modest, using only 130,000 tetrahedral volumes (30,000 nodes) in the three grid sections. The standard Eulerian multiphase model for liquid-solid systems was employed representing the pulp as the continuous phase and the grinding media as a dispersed second phase (solid). The solutions were computed on a single-CPU PC and required typically 500 iterations to achieve convergence to a tolerance of 10-4 in mass and momentum residuals. Actual clock time was approximately 8 hours per solution.

The CFD analysis has provided considerable insight into the internal workings of the mill. It has been possible to begin to understand the nature of media distribution, secondary flows, and anticipate qualitatively the wear characteristics of operating and pilot-scale mills. The CFD model has shown considerable promise in aiding the understanding of mill behavior, in design for scale-up, and also generally in improving operation.

Schematic diagram
Schematic diagram of a grinding mill.

 

Grinding Mill Installation
Grinding Mill Installation. Photo courtesy of MIM Process Technology.

 

grinding media concentration
Contour plot of the grinding media volume fraction in an axial slice, showing that the grinding media is forced to the outside periphery of the mill.

 

secondary flows
Vector plot of the grinding media phase secondary flows in an axial slice.

 

grinding media volume fraction
Contour plot of the grinding media volume fraction in a rotor passage in "blade to blade" view, approx 50% span. Note the high concentration on the pressure side of the "blade". (Rotor spinning counter-clockwise)

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