Structural analysis and condition monitoring of grinding mills : a case study
Abstract: Grinding mills are large rotating cylindrical steel vessels used to grind ore and minerals into finer particles. The mills are important parts of the mineral enrichment process and the grinding is the last step of the comminution process, where the particle size is reduced by a combination of abrasion and impact.The rotation of the mill under loaded conditions can result in fatigue cracks. Fatigue cracks and associated failures have been identified as a major problem in mineral processing plants. The cracks lead to unpredicted and unplanned production stoppages for inspections and for repair and replacement of the cracked mill parts. This leads to increasing costs due to production loss, additional man-hours and spare parts.The purpose of the research presented in this licentiate thesis was to calculate the structural strains, stresses, displacements, etc. in grinding mills in operation, to prevent overloading, to calculate crack propagation speeds and critical crack lengths, and to develop new improved mills that would withstand the current loading. This research has also aimed to propose, develop and test methods for the detection and monitoring of fatigue cracks in mills during operation, in order to facilitate optimal maintenance decision-making based on current crack sizes.The performed research is a case study of the secondary pebble mills of LKAB, a mining company in northern Sweden. The mills are situated inside dressing plants KA1 and KA2 in Kiruna. To achieve the goals, a number of crack detection and monitoring methods were investigated and evaluated as to their ability to find and monitor fatigue cracks on the running mills. Measurements with wireless strain measurement equipment, infrared thermography and crack propagation sensors were performed on the mills in operation.A finite element model of a mill was developed to calculate the strains and stresses in the mill at any position in the mill and for any loading condition. A variety of spatial discretizations, boundary conditions, material properties and loading alternatives were considered to simulate the behaviour of the real mill in the best possible way. To calculate the loading on the mills in operation, a mathematical model and computer software were developed to calculate the charge configuration, as well as the loading and the magnitude and distribution of the forces acting on the mill in operation. Using the finite element model and the computer software, the global displacement field of the entire mill structure was calculated using quasi-static loading for different inputs of the charge and process parameters.To verify the finite element results, the measured strain ranges for one complete rotation of the mill were compared with the corresponding calculated ones. The numerical results were also verified with logged process data, such as bearing reaction forces. One conclusion, based on the comparisons, is that the developed finite element model and the developed software tools can be considered useful for engineering applications.The developed software tools, together with the finite element model, make it possible to calculate the global displacement field of the entire mill structure for any situation. This is achieved by inputting the desired process data and charge parameters into the software, calculating the loads and force distributions, exporting them to the finite element model, and running the simulation. From the global displacement field, strains, stresses, reaction forces, displacements, etc. can be calculated with standard routines for any position in the mill.The performed research work gives a deeper understanding of the field of structural analysis and load calculation of grinding mills in operation. The complexity of modelling the behaviour of mills in operation is high. Consequently, it is difficult to obtain accurate estimations of crack propagation speeds and critical crack sizes based on the calculated stresses.It has been found that strain measurements, with strain gauges attached to the mill mandrel, can be used to detect and monitor larger circumferential cracks near the flanges in the mill in operation, since the measured strain ranges increase with the crack size. It has further been found that infrared thermography can be used as a method to indicate cracks without stopping the mill, as the increased thermal gradient around the cracks can be detected by a special type of thermal instrument.Crack propagation sensors have proven to be ideal for high-precision online monitoring of the crack propagation of smaller cracks at the corners of the manholes in the mill. Finally, it has been found that strain measurement is a useful method not only to verify finite element results and to detect and monitor cracks, but also to prevent overloading of the mill and to estimate charge features such as the filling level, the charge shape and the position of the charge circumferentially inside the mill during operation.
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