The selection and design of mill liners

Dramatic shortcomings of mill liner designs, especially of large SAG mills, such as rapid failure and even mill shell damage arising from impacting of the charge directly on the liner, and unsuitable spacing of lifter bars yielding unfavourable compromises between lifter bar height and liner life have highlighted the significance of correct mill liner selection. Liners protect the mill shell from wear and transfer energy to the grinding charge, and a careful balance is required to optimise these conflicting requirements. This review serves to highlight these problems and how they can be tackled in a logical and often inexpensive manner by considering charge trajectories and liner spacing criteria, in conjunction with liner wear monitoring. An overview is given of the principal types and materials of construction of mill liners. Examples of good and bad liner design are given, followed by a rigorous approach to liner design based on the best technology available, combined with experience and logical engineering thinking. Methods of monitoring the progressive wear of liners, and relating this to the performance of the mill are presented. The value of wear monitoring in ongoing liner optimisation and cost saving, through balancing the lives of the lifters and shell plates, and providing reliable comparative data for testing different liner materials and designs, is explained. Wear testing techniques and their drawbacks and limitations are discussed, along with new tests under development. The contribution of advanced computation techniques, such as the Discrete Element Method (DEM), to predicting the wear profiles of liners, and integrating this information into optimising the overall performance of the mill from a production and cost perspective, are considered in some detail. This takes into account the change of the charge trajectories, energy transfer, and milling efficiency, as the mill liner wears and the profile changes. It is hoped that this review will better arm mill operators to select suitable mill liners, with a view to decreasing production costs while maintaining mill performance near optimal levels. INTRODUCTION Poor liner design has a detrimental affect on milling performance and on liner life, (Powell 1991b). This results in a loss of revenue and increased operational costs. Reduced milling efficiency can result in excess power usage and decreased recovery of the valuable minerals. Excess liner wear results in exorbitant liner materials costs, and excessive downtime which reduces mill availability and impacts on plant throughput. For a plant with a number of mills this also entails the employment of extra mill relining staff and the risks and costs associated with frequent relining. Optimised liner design can be used to strike the best economic balance between liner life and mill grinding performance, thus enhancing the profitability of a mining operation. Protection of the mill shell from the aggressive impacting and abrasive environment inside a mill is well known as the primary purpose of mill liners. Generally the liners fell under the Maintenance and Engineering Department, where the objective was to utilise a liner that lasted as long as possible, or was as cheap as possible, or of course preferably both. Liners were treated merely as a cost overhead, and a cause of downtime, and the maintenance approach has been to reduce the cost, while remaining within acceptable downtime constraints. Cost saving lead to the development of profile liners and lifter bars, as these dramatically increase the life of the liner. The downtime constraints and high stresses in large SAG mills helped to drive the development of greatly improved liner materials. However, this cost engineering approach ignored the mill performance and overlooked the other key function of mill liners. The second primary function of a liner is to transfer rotary motion of the mill to the grinding media and charge. After all, it is the interface between the mill and the grinding charge. Although work on the grinding action in mills was published 100 years ago (White 1905 and Davis 1919), the first publication on the influence of liner design on the charge motion only appeared in 70 years later (McIvor 1983). With the advent in the eighties of larger and larger SAG mills running in single stream circuits, it became apparent to the operating staff that the liner was having a significant influence on mill performance. This had been hidden previously by the regular changing of liners over a number of mills in the older plants that had many mills in parallel. In fact this is generally still the case in the multi-stream plants, where mill liner design and selection is only tackled on a cost consumables basis. However, the gains to be had through good liner design and selection are just as great as on the large SAG mills. This paper looks at recognising problems in liner design and selection in existing operations, and then at liner selection for new applications.