# Investigation of Inner Lining Loss and Correlation with Steel Structure Quality in Rotary Kilns
## 4 Discussion  
This article provides an evaluation of two methods, namely laser scanning and drilling, to estimate the loss of lining in rotary kilns. Laser scanning offers several advantages as a non-destructive technique. It enables comprehensive data acquisition, capturing the detailed geometry of the kiln's inner surface, and facilitating precise measurements. Laser scanning is time-efficient, requires minimal setup, and eliminates the need for physical contact with the kiln, reducing safety risks. However, it has limitations, including high costs for equipment and data processing, limited penetration depth, and susceptibility to environmental conditions such as high temperatures and pollution.  
However, drilling provides direct measurements of the thickness of the lining, making it a reliable method of estimating loss. It is a cost-effective approach that utilises basic equipment and techniques. However, drilling is a destructive method that may cause structural damage and provide limited samples, potentially underestimating the extent of the lining loss.The choice between laser scanning and drilling depends on various factors, such as project requirements, precision needed, the magnitude of lining loss, costs, schedule constraints, and kiln condition. A thorough analysis of these factors can determine the optimal method for estimating the loss of insulation in a rotary kiln (Table 1).  
Furthermore, this study investigated the relationship between inner lining loss and the design of the kiln steel frame. The findings revealed a positive correlation, indicating that higher ovality values were associated with greater lining loss. This highlights the importance of considering structural changes when assessing the integrity of the kiln's inner layer. The correlation coefficients ranging from 0.38 to 0.57 indicate a moderate to moderately strong positive correlation between the ovality and lining loss in the studied rotary kiln. Although these coefficients may not be exceptionally high, they signify a significant relationship between the variables (Table 2).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Subject** & **Drilling** & **TLS** \\ \hline Measurement time & 2 h & 5 h \\ Processing time & 0 h & 3 h \\ Cover area & 1\% & 95\% \\
3D visualisation & no & yes \\ Exact periodic comparison & no & yes \\ Gap detection & no & yes \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison—drilling versus terrestrial laser scanning.  
Figure 17: Imperfect RANSAC cylinder detection (**left**), manual alignment of ideal cylinder (**middle**), and distribution of the values of riding ring loss (**right**).  
This study also discovered that the loss of brick lining is less than the value reported from the wells. The resulting median loss was taken from 6 metres to 66 m of the stope due to inadequate lime brick lining cleaning, which occupied most of the brick lining area in the section that began at 66 m of the stope. At the same time, it was no longer anticipated that the brick lining throughout this length would show significant wear. Due to the high frequency of lime in the brick lining, the marquee could not be viewed much further from its 74 m vantage point (Figure 7).  
The method of estimation of lining loss in rotary kiln with laser scanning is simple to apply on any rotary kiln. The main factors to use laser scanning for the analysis of the rotary kiln are the visibility of the rotary kiln itself. If the rotary kiln is surrounded by other technology and a large part of it is not visible, the following analysis steps cannot be performed. A follow-up condition is sufficient space to connect scans inside and outside the rotary kiln. Smaller and narrower interconnection spaces will impair the quality of point cloud from laser scans. A similar condition of visibility applies inside the rotary kiln. Material must be visible for the analysis of loss of internal lining. If the masonry is contaminated with lime or other material, the scanner is unable to measure the condition of the internal lining.