*2.3. Crushing and Grinding Equipment*

Physical treatments were carried out on a lab-scale, in order to try to reproduce the typical comminution forces and stresses that characterize comminution circuits [1] and control the gradual size reduction of the sample. Using the available equipment, the selected comminution and grinding route takes into account the necessity to provide an input material dimension, for each machine, suitable for feeding hoppers dimensions. Output materials dimensional characteristics have been set up in order to allow the material to fit in the subsequent machine's hopper. In addition, the equipment arrangement selected for this kind of ore samples was set up taking into account the possibility of the material to be subsequently tested for processing and separation purposes.

Figure 1 shows the comminution equipment arrangement that was selected in order to test the response in terms of the mineral distribution of the material. A preliminary crushing and grinding test on representative samples of the collected materials was performed. OM observations carried out on the preliminary test outputs showed that, generally, 85% of the grains was free below the 0.425 mm size. Consequently, each machine output was screened at 0.425 mm. At the end of the process, all passing 0.425-mm materials were collected as a composite product.

**Figure 1.** Lab-scale crushing and grinding equipment scheme.

A Hazemag impact crusher was used as the first crushing stage. This machine has a four-lug bolt rotor, powered by an electrical motor. The running speed was 500 rpm, and the gap width between the bolts and impact elements was set at 10 mm. The feeding hopper has 120 × 120 mm aperture, limiting the maximum input material dimension to 50 mm.

A Magutt-10 jaw crusher was used for the second crushing step. It is powered by a 0.75-kW electric engine at 250 rpm. The selected closed-side setting (CSS) was 5 mm, with an open-side setting (OSS) of 15 mm. Feeding hopper is 120 × 60 mm, limiting input material dimension to d <40 mm.

A S.I.M.A. Milano disk mill was used as the first grinding stage. The disk gap selected was 2 mm. Feeding hopper dimensions allow an input material dimension below 5 mm.

A lab-scale rod mill was used as the final stage of grinding. It is equipped with a 193-mm-diameter, 267-mm-long drum, containing nine steel rods with a diameter of 8 mm inside, powered by a 0.4-kW electrical engine. The drum was filled with 1 kg of material at a 30% v/v filling ratio. The grinding time was 4 min at 50 rpm rotation speed.

#### *2.4. Sieve Analysis*

Sizing of the material was performed using different combinations of sieves in dry and wet conditions, in order to know the size distribution of the equipment output materials. Dry sieving was selected for the general output materials resulting from each crushing and comminution machine. A portion of the materials was collected and screened by means of nested screens in decreasing sizes, from the top screen to 0.125–0.075 mm, and shaken for 3 min in the lab screen shaker.

Wet sieving was selected for the grain size distribution definition of the composite comminution product passing 0.425 mm. The sieve sequence consisted of 0.425, 0.355, 0.300, 0.25, 0.212, 0.180, 0.125, 0.090, and 0.063 mm. The materials were oven-dried before XRPD analysis.

#### **3. Results**

#### *3.1. Optical Microscopy*

The observation of thin sections, shown in Figure 2, was carried out with OM in parallel transmitted light (PL) and cross-polarized transmitted light (XPL), with objectives between 4× and 10×. There was an abundant presence of sphalerite minerals surrounded by a calcite matrix. Light-opaque mineralization, such as galena and its altered compounds, were found. It was possible to spot some opaque veins filled with black organic material.

Sphalerite grains were massively present in the samples, showing a yellow-brownish and black coloration transparent in PL and opaque in XPL. Calcite appeared as a white and striated matrix, transparent both in PL and XPL. Opaque minerals, like galena and cerussite, were sparse and usually occurred in the surroundings of sphalerite grains.

Coarser sphalerite grain sizes, ranging in average between 0.400 and 0.425 mm, were defined using digital measurements from the OM images.

#### *3.2. Scanning Electron Microscopy*

In order to have elemental information related to the mineralization nature, SEM punctual analyses were performed. The investigation targeted those areas difficult to characterize by OM and the intergranular filled voids present. Figure 3 represents the SEM analysis on uncovered rock slices, which was performed in order to confirm the OM outcomes and the total number of mineral phases present in the sample before XRPD analyses.

SEM images and Energy Dispersive Spectroscopy (EDS) spectra, shown in Figure 3, confirmed a widespread presence of sphalerite [25] in the calcite [26] matrix and highlighted the presence of Zn and Pb alteration compounds, mainly cerussite [27], anglesite [28], and smithsonite [29]. The presence of organic matter in the microfractures was detected [30].

#### *3.3. Crushing and Grinding*

In the arranged comminution and grinding scheme, samples entered with maximum size of 50 mm and they were reduced to millimetric and sub-millimetric grains. According to first equipment feeding hopper specifications, a preliminary manual size reduction has been performed with a 5-kg hammer. The purpose of this test was to bring most of the material below 0.425 mm, limiting the size fractions that could negatively affect an eventual flotation separation [31–34]. The process can be divided into two stages:

The particle size distribution of the broken products from each comminution was analyzed. Granulometric curves for the output materials are shown in Figure 4.

**Figure 2.** Optical microscopy (OM) thin section images in parallel transmitted light (PL) and cross-polarized transmitted light (XPL) (Sphalerite, Sph; Calcite, Cal; and Galena, Gal): (**a**) thin section n.1, PL and (**b**) XPL and (**c**) thin section n.3, PL and (**d**) XPL.

**Figure 3.** *Cont.*

**Figure 3.** SEM images and spectra of thin sections: (**a**) sphalerite (Sph, blue dot), cerussite (Cer, green dot), organic matter (Org, orange dot), (**b**) sphalerite (Sph), and smithsonite (Sm, blue dot).

**Figure 4.** Output product granulometric curves for each equipment.

A mass balance, assuming negligible losses, for 5 kg of materials resulting from crushing and grinding is plotted in Figure 5. The impact crusher, selected as the first stage of crushing, resulted in a dimension reduction with a contained fine production <0.425 mm of 16.2 wt %. The jaw crusher, chosen for the second crushing stage, gave a lower production of fines. In fact, only 6.9 wt % of <0.425 mm was obtained. At the end of the crushing stage, the overall fraction having dimensions below a 0.425 mm-size was 22 wt % of the input materials, while the oversized materials sent to the grinding stages were 78 wt %.

**Figure 5.** Mass balance resulting from the crushing and grinding stages.

For a further size reduction, a disk mill was selected as the first stage of grinding. It resulted in materials characterized by *d*50 > 1.4 mm, with the fraction of the fines (*d* < 0.425 mm) being 12.3 wt % The output material was homogeneously sized. Rod milling was selected for the final grinding stage. It was selected aiming at a drastic reduction of the material below the size of 0.425 mm. The results showed how four min of grinding time produced 56.8 wt % of undersized material. At the end of the secondary crushing test, material having *d* < 0.425 mm was 62.1 wt % of the feed material obtained by primary crushing.

Oversized material, accounting for about 30 wt % of the total output, was not reintroduced in any grinding stage. All the materials passing 0.42–5 mm screens were collected as a composite sample and further analyzed in terms of the granulometric distribution and XRPD.