*3.5. X-Ray Powder Di*ff*raction Analyses*

XRPD analyses, performed on each dimension class obtained by wet sieving from comminution composite output products < 0.425 mm, distinguished five different mineral phases: sphalerite, galena, calcite, anglesite, and cerussite, as reported in Figure 7a–i. Sphalerite's main peaks were observed at 2θ angles of 28,57◦, 33.11◦, 47.57◦, 56.39◦, 76.77◦, and 88.52◦ [35]. Calcite's representative peaks were observed at 2θ angles of 29.44◦ and 48.58◦ [36]. Galena's peaks were observed at 2θ angles of 26◦, 30.09◦, 43.06◦, and 50.98◦ [37]. Cerussite's and Anglesite's characteristic peaks were observed at 2θ angles of 24.80◦, 25.49◦, and 43.48◦ and 20.84◦, 23.36◦, and 26.75◦ respectively [38,39].

Qualitatively, each class showed a constant presence in terms of the mineral phases. Reference spectra [24,35–39] were compared with the experimental ones. These outcomes generally confirmed what was previously observed by OM and SEM.

Quantitative XRPD analyses obtained by Rietveld [21–23] refinement showed residual errors Rwp ranging from 5.50% to 6.49% and goodness-of-fit in the range from 2.06 and 2.41, as shown in Table 1.

**Table 1.** Observed residual errors (Rwp) and goodness-of-fit (S) from the quantitative X-ray powder diffraction (XRPD) analysis.


**Figure 7.** *Cont.*

**Figure 7.** *Cont.*

**Figure 7.** *Cont.*

**Figure 7.** X-ray powder diffraction (XRPD) spectra of different comminution product-sieved classes: (**a**) 0.425–0.350 mm; (**b**) 0.350–0.300 mm; (**c**) 0.300–0.250 mm; (**d**) 0.250–0.212 mm; (**e**) 0.212–0.180 mm; (**f**) 0.180-0.125 mm; (**g**) 0.125-0.090 mm; (**h**) 0.090-0.063 mm; (**i**) <0.063 mm.

According to some authors, a Rietveld refinement showing low *R*wp and *S* values can be considered as reliable, with errors in the weight fraction estimation between 0.5 wt % and 1.5 wt % [40,41]. The quantitative results are shown in Table 2. On average, sphalerite was the most abundant phase in the samples, ranging 50–70 wt %, calcite, defined as the gangue mineral, attained 17–27 wt %, while galena ranged 4–9 wt % The other phases could be considered as minor. The accuracy of the quantitative analyses was not assessed in detail.


**Table 2.** Quantitative XRPD results for each particle size.

(1) Estimation errors must be considered in the range ± 0.5–1.5%, according to [40,41].

Some of the mineral phases present in the samples showed a different concentration among different granulometric classes. The clearest evidence was the ones linked with galena and Pb-related compounds. Their total concentration was around 9.5 wt % in particle sizes 0.425–0.350 mm and reached 21.6 wt % in particle sizes <0.063 mm, constantly increasing their presence with the decrease of their material dimensions. Galena was the most abundant regarding Pb-related compounds and showed a worthwhile concentration for further recovery.

Sphalerite was the most abundant phase in each size, but its presence decreased from 0.180 mm, where it peaked with 70.4 wt %, toward finer classes. The lowest concentration of ZnS was 62.6 wt %, found in the <0.063-mm class. The calcite phase had its lowest presence in class 0.212–0.180 mm with 17.3 wt %, while its highest concentration of 26.9 wt % was measured in the <0.063-mm class.

In general, high values of valuable minerals were found in the different classes of comminution products, underling the necessity of further separation in order to obtain high-quality concentrates from the ore. In the choice of recovery and separation methods of Pb-related minerals—galena, in particular—the increase of their concentrations in the finest classes should be taken into consideration.

#### **4. Discussion**

Ore samples were collected in the Pian Bracca extension area in the Gorno Mining District under an exploration operation by Alta Zinc Ltd. In order to have an overview of the characteristics of the sampled ores uncovered, polished thin sections from rock slices were realized and observed. Petrographic observations were necessary for the determination of the shape, composition, and dimensions of grains present in the mineralized mixed sulfide ores collected. The results showed typical characteristics related to MVT deposits [19,42], with relatively high contents of sphalerite minerals embedded in a calcite matrix. Galena was sparsely present in very small crystals, difficult to be distinguished only by OM.

SEM characterization brought additional information, especially concerning alteration products and the filling of the ore micro-fractures. The presence of cerussite, anglesite, and organic matter was detected, confirming the presence of Pb-related minerals and traces of organic matter, arguably linked with the formational geological environment of the site [19]. The observation made on the dimensional characteristics resulted in estimated sizes of valuable mineral grains below the 0.400 mm and 0.450 mm thresholds. On these parameters, a lab-scale crushing and grinding circuit was arranged, aiming to obtain products <0.425 mm. Ground materials passing the 0.425-mm screening were collected as unique products, accounting for 70 wt % of the initial input quantity. The other 30 wt % of the initial input materials needed to be reground and were not considered for further analysis (Figure 5). Passing 0.425 mm, the material grain size distribution was studied: material <0.063 mm accounted for 27 wt % (Table 1).

XRPD quantitative analyses were realized on oven-dried samples resulting from the wet sieving of <0.425-mm composite product samples. The results highlighted the important presence of sphalerite and galena in dimension classes ranging between the 0.250-mm and 0.063-mm classes, as shown in Figure 8a,d. Target Pb and Zn phase concentrations were observed as fluctuating, varying with the reduction in the dimensions of the products. This phenomenon could be framed as a selective comminution behavior of the ore [43], but further data on mineral liberation grades and comminution efficiency should be collected in order to better define the parameters related to this specific possibility.

Sphalerite was the most abundant mineral phase in the composite product samples <0.425 mm obtained by crushing and grinding; its concentration peaked at class 0.180 mm with 70.4 wt %, as plotted in Figure 8c, assuming a decreasing trend toward finer-grained sizes, whereas galena and Pb-related compound concentrations, plotted in Figure 8a, showed an increasing trend towards finer-grained sizes, peaking at class <0.063 mm with a total concentration of 21.6 wt %

The sum of the ZnS and Pb-related compounds', corresponding to the valuable mineral phases present in the ore, peak concentration was reached in the 0.212–0.180-mm class with 82.7 wt %, while their lowest occurrence of 73.1 wt % was in the <0.063-mm particle sizes, as shown in Figure 8d. Their trend is mainly dominated by sphalerite concentration oscillations.

The calcite concentration, plotted in Figure 8b, trend appeared as V-shaped, encountering a descending behavior until the 0.212–0.180-mm class, strictly related to the total valuable phase ZnS + Pb-tot peak.

Moreover, according to the quantitative analyses, important information was obtained for the evaluation of the most abundant mineral phases present in the material and their collocation among the grain size classes of the comminuted samples.

**Figure 8.** Mineral phase weight fraction variations among the dimension classes in comminution products below 0.425 mm: (**a**) Pb-related compounds (Pb-tot), (**b**) calcite (CaCO3), (**c**) sphalerite (ZnS), and (**d**) valuable phases assumed as a sum of the sphalerite and Pb-related compounds (ZnS+Pb-tot).

#### **5. Conclusions**

The purpose of this study was the characterization of ore materials after crushing and grinding force applications in terms of the distribution of target minerals among different-sized classes. Petrographic observations were carried out on polished thin sections by optical microscopy and scanning electron microscopy. Crushing and grinding equipment was used in order to reproduce the comminution forces typically present in a comminution plant, and the output material samples were characterized by means of XRPD quantitative analysis. Important trends in the valuable mineral phase fluctuations among different-sized classes were highlighted from results.

Further detailed studies should be taken into consideration. Comminution configurations should be properly assessed in terms of grindability studies on materials, specific energy consumptions, and industrial-relevant comminution flowsheet set-ups. Mineral liberation studies should also be implemented in order to have a clearer understanding of the comminution behavior of this material under different conditions.

Generally, applicable separation processes for this kind of ore, such as froth flotation or gravity separation methods, should be taken into account [1,44,45]. Concerning the presence of Pb-related compounds in very fine classes, a nonconventional flotation [46,47] should be considered. In general, the amount of valuable minerals present in the sampled area is relevant and worthwhile for industrial purposes.

**Author Contributions:** Conceptualization, G.B. and P.M.; methodology, G.B. and P.M.; investigation, G.B. and O.B.; resources, P.M.; data curation, G.B.; writing—original draft preparation, G.B.; writing—review and editing, G.B., O.B., and P.M.; and supervision, P.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors are thankful to Alta Zinc Ltd. and Energia Minerals (Italia) s.r.l. for their availability in welcoming access, interest, recovery, and the sharing of relevant information related to their industrial activities.

**Conflicts of Interest:** The authors declare no conflict of interest.
