Decision support models for the maintenance and design of mill liners

Abstract: Mining companies use heavy-duty equipment that work round the clock in highly abrasive environments. Autogenous (AG) mills used in the mining industry and in ore dressing plants are examples of major bottlenecks in the context of downtime and exert an influence on production economics. The rubber liners inside the mill are critical components for mill shell protection and ore grinding. The replacement and inspection of mill liners are major factors regarding mill stoppages and lead to production losses. The wear readings of mill liners are critical when making replacement decisions, and periodic inspections need to be carried out to obtain wear readings. Wear measurement devices are important in terms of measurement accuracy. The appropriate selection of wear measurement devices can lead to a significant reduction in the overall costs, including the device costs and the production loss costs incurred during inspections of the wear of mill liners. An approach has been developed for selection of the optimum wear measurement method, considering the industry specifications. Based on an analysis performed in a case study, a cost-effective wear measurement method has been suggested, taking the industry requirements into consideration. The wear of mill liners affects the production performance of the mining mill. Hence, the replacement decision for mill liners affects not only the lining cost and the production loss cost, but also the mill revenue due to variation in the metal output. Therefore, the production performance has to be considered when developing decision models for making maintenance replacement policies for mill liners. Variation in the ore properties also affects the liner wear and ore value. Both these parameters have an effect on the cost and revenue parameters. Therefore, consideration of the ore properties in the life cycle profit (LCP) formulation will provide more effective maintenance decisions. A maintenance decision support model has been developed to meet the specific requirements of process,purchase and maintenance departments. The lifetime of different parts (components) of mill liners also varies due to the wear of the parts, for technical reasons concerning the grinding process and the different wear zones inside the mill. The mill needs to stop on a number of occasions for the replacement of parts of mill liners. These stoppages are also one of the major causes of mill downtime and corresponding production losses, which can be minimized by a cost-effective maintenance schedule for the optimum grouping of mill liners for combined replacement. The maintenance schedule depends on the lifetime of the mill liners. The life cycle cost (LCC) can be reduced further through optimum improvement of the lifetime of different parts of the mill liners. A decision support model has been developed to determine the optimum grouping of parts of mill liners for replacement and the optimum improvement in the lifetime of parts of mill liners. The proposed LCC model is also useful for the design departments ofliner manufacturers for optimizing the material and dimensions of mill liners, in order to achieve a proposed improved lifetime. A significant improvement in the annual profit was observed when the maintenance policies suggested in the thesis replaced the existing maintenance policies. The research presented in this thesis has used a systematic evaluation approach and pair-wise comparison methods for selection of the optimum wear measurement method for mill liners. Time sampling, correlation studies and simulation methods have been used for LCP optimization, in order to determine the optimum replacement interval for major replacements of mill liners. An approach based on an exhaustive enumeration search has been used for LCC optimization for determining the optimum grouping and the optimum improvement in the lifetime of parts of mill liners. The usefulness of LCC and LCP analyses for mill liners in mining companies is illustrated in the thesis and decision models for effective maintenance planning are demonstrated. The results of the present research are promising for the possibility of making a significant reduction in production losses and the LCC after optimizing the maintenance activities and the lifetime of the mill liners studied in this specific case study. The results of the present thesis are related to specific industrial requirements, and are expected to enhance the capability of making cost-effective replacement decisions. Keywords: Mining industry; Maintenance decisions; Mill liners; Life Cycle Cost (LCC); Life Cycle Profit (LCP); Mill liner design; Economic model; Production Economics; Optimization; Replacement decision; Wear measurement device; Combined replacement; Life improvement