# Hyperspectral Imaging Sorting of Refurbishment Plasterboard Waste
## 3 Results
### Validation of the HSI-Based System for Refurbishment Plasterboard Waste Sorting  
Small particles in batch 2 of the refurbishment plasterboard waste were removed with a screen with an aperture of 10 mm. These particles were removed because the laboratory-scale HSI classification process setup was operated manually and the classification of particles <10 mm in size would be a very lengthy process. The sieved material with particles \(\geq\)10 mm in size represented 90.73 wt% of the refurbishment plasterboard waste, and thus, small particles <10 mm in size accounted for the remaining 9.27 wt%. Refurbishment plasterboard waste particles \(\geq\)10 mm in size were used as input of the HSI-based classification process, whereas particles <10 mm in size were discarded. The input material was placed in the sliding scanning table and analysed by the HSI-based classification system (Figure 2b). The amounts and percentages of the input and output streams and their components were quantified (Table 3). The percentage of paper in the product was 6.1 wt%, which can mostly be removed during subsequent crushing and sieving stages of the physical recycling process. The percentage of contaminants in the product was very small (0.64 wt%). The plasterboard recovery yield of the HSI-based classification system calculated using Equation (4) was 98.53 wt%, which implies that only 1.47 wt% of plasterboard was lost in the rejects. However, 9.27 wt% of the collected refurbishment plasterboard waste had sizes <10 mm, which were discarded as rejects. Therefore, the actual plasterboard recovery yield of the HSI sorting process will depend on whether particles \begin{table}
\begin{tabular}{l c c} \hline
**Sample** & **Plasterboard (wt\%)** & **Contaminants (wt\%)** \\ \hline Refurbishment plasterboard & 94.81 & 5.19 \\ waste (batch 1) & & \\ \hline \end{tabular}
\end{table} 
Table 2: Composition of the refurbishment plasterboard waste used for HSI system training. <10 mm in size present in the refurbishment plasterboard waste are discarded as rejects or not. Figure 8 shows the contaminants in outputs 1 and 2, the unclassified material found in output 3, and the refurbishment plasterboard waste with particles <10 mm that was not processed. The contaminants in outputs 1 and 2 were mainly wood and mortar. There was also loose paper in these outputs that did not come from the plasterboard. The unclassified material of output 3 was mostly constituted of rubber fragments. The unprocessed material with particles <10 mm in size was mostly constituted of gypsum powder and plasterboard fragments. The chemical purity of gypsum from the discarded refurbishment plasterboard waste fraction <10 mm in size and from the sorted plasterboard in outputs 1 and 2 was determined. Figure 9 shows that the chemical purity of gypsum obtained from unprocessed refurbishment plasterboard waste <10 mm in size and gypsum obtained from the plasterboard in output 1 (product) was about 95.8 wt%. Therefore, the unprocessed refurbishment plasterboard waste could be incorporated into the sorted refurbishment plasterboard waste in output 1 to increase the plasterboard recovery yield without compromising gypsum quality. Figure 8: Contaminants in output 1 (a), contaminants in output 2 (b), unclassified material of output 3 (c), and unprocessed refurbishment plasterboard waste particles <10 mm in size (d). \begin{table}
\begin{tabular}{l c c c c} \hline
**Stream** & **Weight (g)** & **Weight (\%)** & **Component** & **Weight (g)** & **Weight (\%)** \\ \hline Input & & & & \\ (\(\geq\)10 mm fragments) & 8249 & 100 & & 7561 & 91.66 \\  & & & Contaminants & 688 & 8.34 \\ \hline Output 1 & & & & \\ (product) & 7498 & 90.90 & & 7450 & 99.36 \\  & & & Contaminants & 48 & 0.64 \\ \hline Output 2 & & & & \\ (rejects) & 700 & 8.49 & & 111 & 15.86 \\  & & & Contaminants & 589 & 84.14 \\ \hline Output 3 & & & & \\ (unclassified material) & 51 & 0.61 & & Not & Not \\  & & & & \\ \end{tabular}
\end{table}