**4. Results**

#### *4.1. Spectral Measurement from Chalcopyrite*

The first step was to calculate the average spectra obtained in each test, which are depicted in Figure 3. The average calibrated spectrum of chalcopyrite combustion for each size is shown in Figure 3. It is observed that the behavior of the spectral irradiance was different for each size of chalcopyrite analyzed. It is possible to observe that for the finer sizes, the intensity of the spectrum is greater; this is because finer particles are completely oxidized, releasing more energy. A cloud of fine particles represents a greater surface area compared to a cloud of "coarse" particles, which also has a higher oxygen consumption which in turn translates into greater heat generation. For their part, the larger particles take time to burn in the reaction zone, resulting in a lower intensity emission spectrum. Even so, this behavior is not possible to observe in the smallest sample size (CpyF), which may be related to an agglomeration problem. The smallest size tends to form clumps in the cloud combustion, increasing its particle size and thus behaving as coarse particles.

**Figure 3.** Average emission spectrum of chalcopyrite combustion at different sizes.

It is not possible to observe emission peaks in the chalcopyrite spectra compared to those reported by Toro et al. [16] in their combustion experiments with pyrite. This is likely because chalcopyrite in an oxidizing environment releases less energy than pyrite under the same conditions, which results in a lower temperature combustion flame, making it more difficult to identify discontinuities. Some of the reactions that occurred during the oxidation of chalcopyrite and pyrite are shown in Table 2, a value of ΔH < 0 indicates that the reaction is exothermic (releases thermal energy), while a value of ΔH > 0 indicates a reaction is endothermic (absorbs thermal energy).



1 Values are calculated with HSC Chemistry®.

In this work, multivariate techniques were implemented to detect weak spectral emission profiles. The application of PCA and the analysis of loadings of the combustion spectra of chalcopyrite, allowed us to find peaks at 606 and 616 nm, which are associated with copper oxides [19,20]. These peaks were observed slightly from the loading of the PC3 of the CpyD sample, being more visible in the CpyF sample (Figure 4d–f). Moreover, the peaks associated with Na and K emissions were observed, around 589 and 767 nm, respectively.

**Figure 4.** *Cont*.

**Figure 4.** PCA analysis on the combustion spectrum of chalcopyrite of different sizes. (**a**) CpyA, (**b**) CpyB, (**c**) CpyC, (**d**) CpyD, (**e**) CpyE, (**f**) CpyF.

#### *4.2. Spectral Measurement from Copper Concentrates*

In Figure 5, the average spectral signals of the combustion of copper concentrates are represented. It can be noted that the signal associated with the sample Conc. A presents greater intensity of irradiance compared to the spectral signals of the samples of chalcopyrite and Conc. B. In addition, the spectral lines of emission of Na and K are presented with greater intensity.

**Figure 5.** Average emission spectrum from the combustion of copper concentrates.

Through exploratory analysis with PCA to each set of spectral data of the concentrates and the subsequent graphic representation of the first three loadings (Figure 6a,b), it is revealed that the emission spectral lines of Conc. A are more intense than those of Conc. B and, in addition, there are certain characteristics at ~606 and ~616 nm that are associated with copper oxides, while close to the potassium emission, two spectral emissions appear at 779.1 and 793.9 nm, the same ones that appear in the pyrite combustion spectral signals reported by Toro et al. [16].

**Figure 6.** PCA loadings of the (**a**) Conc. A and (**b**) Conc. B average emission spectrum.
