# Investigation of Inner Lining Loss and Correlation with Steel Structure Quality in Rotary Kilns
## 2 Materials and Methods  
The measured object is a rotary lime kiln in the paper industry. The length of the kiln is 97 m, and the steel frame is 1.82 m in diameter. The kiln is about 3 m above the ground and is supported by four pedestals. There are two pedestals at each end of the furnace and two that are evenly spread along its length. On the two substations in the middle, there are motors that can be used to turn the furnace. The scheme is shown in Figure 2.  
The burner itself is made of steel and has a lining inside. Most of the steel frame of the furnace is 22 mm thick, and the riding ring has reinforcements that are 30-50 mm thick. The inner cover is split into several parts depending on the process being carried out: the burning zone, the intermediate zone, the heating zone, and the feeding zone (Figure 3). Most of the wear or loss occurs in the burning zone, where the high temperatures for lime firing break down the brick walls. Therefore, the inner lining becomes thicker in places where there is more wear than in other places. At the same time, the covering from the burning zone to the heating zone is made of two different materials to ensure adequate insulation. Methods used for monitoring rotary kilns
* Total station measurement: measuring the size of the kiln with an electronic surveying tool. * Laser scanning: using laser scanners to take point cloud data of the kiln's surface and then processing the data to make a 3D model of the kiln. * Terrestrial photogrammetry: A 3D model of the kiln is made from a collection of high-resolution photos taken from different angles. * Infrared thermography: thermal imaging cameras are used to find areas of wear or damage on the surface of the oven where the temperature changes. * Ultrasonic testing: the application of ultrasound sensors to measure the thickness of the kiln's shell and find places where it is getting thinner or eroding. * Vibration analysis: using sensors to watch the kiln's vibration patterns and find any strange moves or vibrations that could hint at a possible problem. A Leica RTC360 laser scanner (Leica Geosystems AG, Heinrich-Wild-Strasse, 9435 Heerbrugg, Switzerland) was used to measure the whole rotating kiln, as shown in Figure 4, The Leica RTC360 is a highly accurate laser scanning system used to capture the 3D measurements of objects, including rotary kilns. With a 3D point accuracy of up to \(\pm\)2 mm, Ref. [40], it provides precise and reliable measurement results. The system's advanced laser scanning technology and intuitive software enable efficient data acquisition and analysis. It offers multiple scanning modes and incorporates positioning technology for accurate data registration. During the measurement, 86 laser scan positions were created. There were 19 spots created inside the rotary kiln, 31 positions created outside, and 36 positions constructed to connect the outside and interior of the rotary kiln. The outer shell had a scan resolution of 6 mm/10 m, and the inner shell had a resolution of 12 mm/10 m (Figure 5). Figure 3: Detail of composition. Workflow
* Scanning rotary kiln exterior. * Scanning rotary kiln interior with spherical targets. * Scanning lime firing area to connect interior and exterior. * Registration of scans in Leica Cyclone Register360. * Clean scan from unnecessities. * Export separately interior and exterior scans. * Estimating axis of rotary kiln by fitting cylinder on exterior scans. * Modelling perfect state of inner lining. * Estimating lining loss by comparing model of perfect state with interior scans. The steel frame on the outside and the brick walls on the inside were both measured. The connection between the interior and exterior was determined through the entrance of the kiln in the lime firing area. Two accessible spaces were used in the measurement to sufficiently register the interior.