## 4 Conclusions  
In this study, the technology of DE-XRT combined with variable gas-ejection realized an in-line automatic sorting process for multicategory scrap of bulk non-ferrous metals in one measurement. The main findings and innovation are as follows:  
1. By selecting the shading of low-energy as the main characteristic parameter, combined with the coordinates of detector line scan direction and comparing the mean distance between the R_value of each effective point within the material shadows and \(S_{L}\)-\(x\)-\(R\) R_value calibration surface to identify non-ferrous metals, it can eliminate the influence of material thickness and the fan-shaped effect of the X-ray source, and thus improve material identification accuracy.
2. By means of threshold judgment of the low-energy shading, it can select unidentifiable overexposed and underexposed materials for secondary sorting.
3. By means of threshold judgment of low-energy shading, it can improve identification accuracy by screening effective points within the material shadows, which can be supported by laboratory results, including 85% identification accuracy by count basis, 100% identification accuracy of heavy and light metals.
4. Taking full advantage of additional morphological features from DE-XRT, it uses \(S_{L}\)-\(x\)-\(t\) calibration surface thickness to calculate material mass and center-of-gravity coordinates, with the former error less than 5% and the latter error less than 4%. Meanwhile, the application of the variable gas-ejection can realize the measurement and identification of in-line automatic separation for multicategory materials at one time, and thus improve sorting efficiency.  
The method in this paper can be applied to sort bulk solid waste with fixed shape and deformation resistance, as a result of its features of high accuracy, high efficiency, low operation cost, dry-type sorting and zero secondary pollution. In the future, identification accuracy and classification of solid waste sorting will be improved by a combination of DE-XRT and other sensors, which is worthy of further research. Therefore, this technology has fair potential and competitive power in industrial applications.  
Conceptualization, T.X. and W.Y.; methodology and software, T.X.; validation, T.X. and X.X.; formal analysis and investigation, T.X. and X.X.; resources, T.X.; data curation, T.X.;writing--original draft preparation, T.X.; writing--review and editing, T.X.; visualization, T.X.; supervision, X.X.; project administration and funding acquisition, W.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Jiangsu Provincial Key Research & Development Program of China, grant number BE2020779 and BE2018722; the Jiangsu Provincial Science and Technology Support Program of China, grant number BE2013715; and the Jiangsu Provincial Special Fund for Transformation of Scientific and Technological Achievements of China, grant number BA2014067.  
Not applicable.  
Data Availability Statement: Data form this study can be made available upon request to the corresponding author after executing appropriate data sharing agreement.  
The authors declare no conflict of interest.