Improved relationships for discharge in SAG/AG mills

Mitchell, Daniel (2015). Improved relationships for discharge in SAG/AG mills MPhil Thesis, Sustainable Minerals Institute, The University of Queensland. doi:10.14264/uql.2015.941

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Author Mitchell, Daniel
Thesis Title Improved relationships for discharge in SAG/AG mills
School, Centre or Institute Sustainable Minerals Institute
Institution The University of Queensland
DOI 10.14264/uql.2015.941
Publication date 2015-10-09
Thesis type MPhil Thesis
Supervisor Nirmal Weerasekara
Malcolm Powell
Total pages 208
Language eng
Subjects 091404 Mineral Processing/Beneficiation
0914 Resources Engineering and Extractive Metallurgy
Formatted abstract
Comminution is an essential component of mineral processing operations and its proper operation and design is vital to ensure that the plant operates efficiently. In recent years, there has been a shift of emphasis on ball mill optimization to semi-autogenous grinding (SAG) mills. In order to optimize this section, the skills to accurately model and understand its underlying mechanisms are required.

The literature exposed a clear lack of previous research on the influence and performance of grate/pulp lifter design in AG/SAG mill discharge. The current understanding from literature lacks knowledge of mill content size distribution and its effect on discharge through the grate at various aperture sizes and radial positions. It also indicates that in order to advance the understanding in a meaningful way, a more mechanistic approach must be taken to modelling these principles.

There is currently no accepted pilot test procedure for measuring discharge and product size distributions in AG/SAG mills. The existing models and understanding of grate discharge and classification are known to be flawed and have been identified as one of the key limitations in AG/SAG modelling.

The aim of this thesis is to improve on the current understanding of discharge in the SAG/AG model. It focuses on the present limitations of the JK AG/SAG model in predicting accurate grate and pebble port discharge. The thesis objective was to design a pilot scale test that can accurately measure and explore discharge mechanisms. The results of doing this can lead to the replacement of non-mechanistic equations with either comprehensive mechanistic modelling or an improvement of the current empirical relationships.

An in-depth pilot scale test methodology has been developed and an extensive experimental program undertaken on a 1.8 m diameter batch mill adapted to provide accurate discharge measurements. Controlled discharge tests with associated full content measurement allowed accurate measurement of the grate discharge function only, without complications of unknown pulp lifter effects, grinding rates and transport through the mill. Successful initial predictions for flow through the grate and its classification have been achieved and analysed. Finally the extensive material gained by conducting this study can be utilised in improvement of current and development of new discharge models.

The results of the extensive pilot scale test program show similarities to previous literature however the studies have uncovered unique and previously unrefined relationships for SAG/AG discharge. The overall discharge results indicate that it is controlled by the ore filling in the mill and not related to total mill filling. Discharge through an individual opening on the grate is driven by the factors of ore filling, radial position and the size of the aperture.

The results indicate that 1.18 mm is approximately the size below which particles experience no classification (i.e. if presented to the aperture they will pass through). At this point discharge is at a maximum in relation to mill contents and is much larger than previously estimated in the literature.

The classification at the grate for both slurry apertures and pebble ports has substantially extended previous research studies. The results show clear trends in relation to the radial position, aperture size and particle size.

The thesis has provided improvements to the previous AG/SAG mill discharge understanding in terms of discharge rates and grate classification in relation to mill load and operating
Keyword AG/SAG milling
SAG mill
Discharge
Comminution
Aperture
Pebble port
GR
Classification

Document type: Thesis
Collections: UQ Theses (RHD) - Official
UQ Theses (RHD) - Open Access
 
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Created: Wed, 30 Sep 2015, 23:13:15 EST by Daniel Mitchell on behalf of Scholarly Communication and Digitisation Service