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Optimisation of an Industrial Scale Ball Mill Using an Online Pulp and Ball

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Optimisation of an Industrial Scale Ball Mill Using an Online Pulp and Ball OPTIMISATION OF AN INDUSTRIALTown SCALE BALL MILL USING AN ONLINE PULP AND BALL LOADCape SENSOR of Thesis submitted in fulfilment of the degree of Master of Science University Pratish Keshav KSHPRA001 _______________ May 2013 Page | 1 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgementTown of the source. The thesis is to be used for private study or non- commercial research purposes only. Cape Published by the University ofof Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University ABSTRACT The secondary milling circuit at Waterval UG2 Concentrator had undergone a circuit change with the commissioning of the IsaMill, a horizontally stirred mill, in parallel with the secondary ball mill. The operation treats the PGM bearing UG2 ore type and produces a final concentrate enriched with PGM’s. The concept was to treat the finer silicate rich fraction in the IsaMill and the coarser chromite rich fraction through the ball mill. This circuit is typical of a UG2 plant in which maximum silicate with minimal chromite breakage is targeted. As a result of the circuit change an opportunity for optimisation around the industrial scale ball mill was considered for this study. Of concern in this study were new operating conditions for the mill in the changed circuit at which improved performance could be obtained. Another objective was to investigate if a difference in breakage response for the silicate and chromite fractions could be identified for different operating conditions in the ball mill. Town The secondary mill at Waterval UG2 Concentrator was already fitted with an online ball and pulp load sensor, the Sensomag. The information obtained Capefrom the sensor is in the form of shoulder and toe positions for the ball and pulp filling in the ofrotating ball mill. The mill was surveyed at various ball filling and mill % solids conditions and information from the online sensor was used to understand the mill performance, particularly with regards to mill load behaviour. Hence a final objective was to demonstrate that the information obtained from the online sensor could be related to mill operating conditions. The sensor output was envisioned to eventually form part of the mill control philosophy. Samples were taken of the mill feed and discharge streams at the different operating conditions andUniversity analysed for grind as well as PGM and Cr2O3 content. The majority of the PGM’s in the UG2 ore are in the silicates and thus the PGM distribution results would indicate the amount of breakage in the silicate fraction. Cr2O3 is used as an indicator of the chromite content in UG2 ore. In order to identify optimum mill performance the results were analysed using different measures which include general grind, particle and species distributions, reduction ratios, sieve efficiencies and specific energy. By comparing the results the differences and limitations of certain techniques were identified. It was found that the mill performance varied at different operating conditions. The optimum ball filling was found to be around 30%, which is similar to site operational target. The optimal % solids for this mill however seems to be higher than what the mill is typically operated at. No peak in % solids for mill performance was obtained. Scope exists to determine how far from the investigated maximum of 75% solids (by mass) does the optimum in-mill density lie for this mill. Thus new optimum conditions in terms of % solids do exist for the mill in the modified circuit. Results also showed that the size reduction of the silicates increased with an increase in mill % solids and ball filling degree. For chromite, the mill % solids did not appear to have any effect at low ball fillings, but a slight shift was observed at the higher ball fillings tested. The trend suggests that the size reduction of both silicates and chromite increased with an increase in ball filling, albeit at different rates. Finally, the test work has demonstrated that the online sensor Townoutputs can be related to mill performance. Differences in shoulder and toe positions for the ball and pulp loads were distinct between operating conditions. Improved grind performance was observed at conditions that resulted in lower free pulp angles. Thus the sensor could be usedCape as a control tool to identify and maintain optimum mill operational conditions. The Sensomag should be incorporated into a mill controller that looks at more than just mill ball filling. Conditionsof that result in optimum mill efficiency can be identified and the mill may be controlled using the sensor data. It is recommended that the mill continue to be run at 30% ball filling and at higher mill % solids than the maximum reached in this work. University DISCLAIMER I know the meaning of plagiarism and declare that all the work in the document, save for that which is properly acknowledged, is my own. Date:____________________________08/05/2013_____________________________________ Signature:___________________________________________________________________ ACKNOWLEDGEMENTS Special thanks have to be given to the many individuals and organisations that made this study possible. Firstly to my supervisor for your continued support and guidance over the great distance that typically makes a part time study like this that much more difficult.Town Thanks to Anglo American Platinum for supporting and allowing me to do this study and to publish this information. A great thank you needs to go out to all the operational staff and management at Waterval UG2 Concentrator for facilitating and assisting with the Capetest work required in this study. To all the various stakeholders involved in the test work - ofthank you for your contributions and support. These include, but are not limited to, Magotteaux, UCT, the University of the Witwatersrand and Anglo American Research. Finally, a most significant word of thanks must go out to my dear wife. None of the words in this thesis would have materialised without her motivation, support and love. Thank you for always believing in me and pushingUniversity me to understand that nothing is impossible or beyond our reach. Table of Contents 1 INTRODUCTION ................................................................................................. 1 1.1 Background .......................................................................................................................... 1 1.2 Objectives ............................................................................................................................. 2 1.3 Hypotheses ........................................................................................................................... 3 1.4 Layout ................................................................................................................................... 4 2 LITERATURE REVIEW ...................................................................................... 5 2.1 Preface .................................................................................................................................. 5 2.2 Mill load behaviour ............................................................................................................. 5 2.2.1 Charge shape with shoulder and toe position ................................................................ 5 2.2.2 Mill speed ....................................................................................................................... 6 2.2.3 Ball filling ......................................................................................................................Town 9 2.2.4 Ball size ........................................................................................................................ 10 2.2.5 Slurry percentage solids............................................................................................... 11 2.2.6 Slurry hold-up and residence time in a millCape ................................................................ 14 2.2.7 Liner wear and design.................................................................................................. 16 2.3 Mill load measurement methods usingof sensors and signals .......................................... 17 2.4 Techniques for analysing mill efficiency and performance ........................................... 23 2.5 Description of the UG2 ore type and its treatment in grinding circuits ....................... 26 2.6 Summary of chapter .......................................................................................................... 31 3 METHODOLOGY .............................................................................................. 32 3.1 Preface ................................................................................................................................University 32 3.2 Process description ............................................................................................................ 32 3.2.1 Current process ............................................................................................................ 32 3.2.2 Circuit before IsaMill introduction .............................................................................. 33 3.2.3 Process equipment ....................................................................................................... 34 3.2.4 Control equipment.......................................................................................................
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