Metso mill lining solutions – a total cost approach
Mill lining design
When selecting suitable mill lining designs, there are certain application-related factors that must always be considered. The type of mill, the mill size, the feed size, ore type, mill speed (rpm), pH-value, temperature etc. are some of the more important selection criteria. In many cases the grinding circuit design (open or closed, pebble crusher etc), discharge capacity and of course whether the mill is operated in single or dual rotation also need to be considered.
SAG and AG mills typically treat coarse, angular feeds and hence normally exhibit relatively high wear rates. Linings for these mills must be quite highly profiled to lift the charge enough to achieve the required impact, and also to maintain an adequate profile throughout their service life. Ball mill liners are normally less profiled as compared to SAG and AG mill liners, and primary ball mills are also generally of overflow type.
Method of installation and available installation equipment should always be considered when designing a lining. The overall plant layout, as well as the maximum size and allowable weight of the individual lining components must be taken into account to ensure quick and safe installations.
Metso always tries to meet the customer objectives and to design for their specific application. There is no such thing as a generic liner or a typical lining wear life, because applications, customer objectives and maintenance schedules vary widely from case to case. A lining of high and uniform quality that is properly sized and balanced in design ensures that maintenance is minimized, mill availability is maximized,
and that maintenance intervals and necessary stoppages are easily predicted.
Metso's range of mill liners for different mill types
Metso’s product range covers almost all mill sizes and types irrespective of brand name. There are however a few unusual applications that need to be considered on a case to case basis. Mills with very high temperatures, extreme pH-values or generally chemically hostile environments can in some cases require special liners or liner materials.
Metso has a range of different types of lining materials (rubber, metallic alloys) that perform differently in different applications. Some are formulated for optimum resistance to impact, cutting and tearing in primary applications, while other compounds have been tailored for maximum abrasion resistance in fine grinding applications, while still others are designed to resist high temperatures.
Metso has recently developed and delivered a new liner concept for big AG mills called the Megaliner™. The Megaliner ™ is an entirely custom-made lining system which integrates multiple rows of lifters and shell plates into one large piece, covering up to 4 square meters per liner element. These liners use a minimum number of attachment points, and the liner is bolted from the outside of the mill. This is an entirely new concept that not only substantially shortens the liner installation and change out time, but also significantly reduces the risk of injury, since the liner handler operator is the only person who needs to be inside the mill while stripping worn liners and installing new ones.
Grinding efficiency is influenced by the lining design. A mill has a certain size and available power, and this means that any mill has a theoretical maximum grinding capacity. Since the lining forms the final link of power transmission to the tumbling load, it must be profiled in such a way to achieve the correct balance between impact and attrition grinding. On a broad scale, the design of a lining will also affect media and energy consumption, while on a finer scale close attention to fit and tolerances allows for quick liner change out, as well as providing superior protection to the mill itself.
Metso's mill liner systems are designed to maximize service life as well as mill utilization. Computer simulations are conducted based on operating parameters and wear profile histories. 2D-DEM modeling is routinely used for general design selection, from which different lining design alternatives can be compared and media trajectories calculated. 2D-DEM modeling provides reliable comparative values for power draw and media to liner impact.
To further optimize and fine-tune lining designs 3D-DEM capability is used. This tool allows a possibility to predict the performance of a lining over its entire service life, and to calculate trajectories, full energy spectra, media to liner impact and power draw with a very high degree of accuracy. These services are generally available for all mills with Metso liners regardless of brand name, but when a customer purchases
genuine Metso equipment, many of these services can be rolled into a package. Metso also offer process audit services, up to and
including full 'mine to mill optimization.
Mill liner inspections and maintenance planning
When Metso ventures into new applications, best effort is made to conduct regular and frequent inspections to determine specific wear patterns, from which Metso not only can estimate upcoming liner replacement, but also identify if any design improvements are possible. With opportunity and adequate notification, three ormore inspections are often conducted over the life of an initial liner set. For well-documented mills with stable operating conditions, one or two inspections between liner change-outs are generally sufficient to
permit accurate forecasting.
Based on several decades of experience with all types of mill applications, Metso's engineers and technicians know when a lining has reached a critical thickness or can predict whether the remaining lining profile will survive until the next planned service stop. Wear rates tend to increase when the lining profile becomes too low. Once a lining loses its 'grip' on the load, the solids and slurry begin to slide over the worn lining surfaces, causing increased abrasion and poor grinding performance. Linings also need a certain minimum thickness to withstand media impacts, although this varies widely from case to case. If worn too thin, a lining will begin to spontaneously fail, so it is critical that replacement takes place before these limiting thicknesses are reached.
Availability of parts and maintenance schedules must also be considered. The cost of lost production due to unnecessary downtime can often be many times higher than the cost of the lining. It is therefore often economically favorable to replace larger portions of the lining according to schedule, even if some parts are not completely worn out, rather than to replace liners piece by piece or to postpone maintenance and be forced to make unplanned production stoppages.
Mill lining service concept
Metso can provide installation supervision, or in many cases execute the complete installation or change-out using Metso's own expert installation crews. Expertise at drilling or re-drilling mills as required, to optimize the lining configuration, can also be provided. Mill lining installation supervisors can train the customer's own maintenance crew or a local contractor to do maintenance and lining inspections. This
training is often conducted in conjunction with a typical installation at site, but Metso also often arranges on or off-site seminars for managers, supervisors, and operators on a more technical and theoretical level.
Long-term service contracts seem to be increasing in popularity within the mineral processing sector. Contracts are generally set up for a minimum of two years, then often extended or open-ended at customer request. All contracts are individually packaged for each specific customer, based on application and client preferences. Scopes range from on-demand inventory, to consignment stocking arrangements, to
complete mill lining service packages on a cost per ton basis, whereby Metso takes care of supply, inventory management, liner inspections, maintenance planning (together with the customer) and installations including provision of the complete installation crew. It is perhaps worthy to note that some of these service contracts have been in place for more than 25 years.
Metso can also offer full-blown Life Cycle Service (LCS) contracts covering several different products (pumps, belting, crushers, mills etc.) which include not only wear parts, but also capital equipment spares, training, support, service and maintenance.