**Gabriele Baldassarre \*, Oliviero Baietto and Paola Marini**

Department of Environmental, Land and Infrastructure Engineering (DIATI), Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129 Turin, Italy; oliviero.baietto@polito.it (O.B.); paola.marini@polito.it (P.M.)

**\*** Correspondence: gabriele\_baldassarre@polito.it; Tel.: +39-011-090-7614

Received: 21 July 2020; Accepted: 8 October 2020; Published: 9 October 2020

**Abstract:** Every mining operation is followed by a beneficiation process aimed at delivering quality materials to the transformation industry. Mainly, in order to separate valuable minerals from gangue in mineral processing, the crushing and grinding of extracted ore are crucial operations for the following separation steps. Comminution is the most energy-consuming operation in mining, and the quality of the results is strictly related to the characteristic of the material under treatment, the type of equipment used in comminution, and the circuit design adopted. A preliminary study was performed in order to understand the crushing behavior under different comminution forces of a high-grade mixed Zn-Pb sulfide ore sample, collected in a Mississippi-Valley Type (MVT) deposit, and the distribution of the target minerals among the products of the process. Ore samples were examined and characterized through thin section observation and SEM analyses for the determination of grain size and texture features, while X-ray powder diffraction (XRPD) quantitative analyses were performed for the definition of target mineral concentrations of comminuted product samples. The selected crushing and grinding circuit comprised lab-scale equipment. For each stage of the process, products below the estimated free-grain size threshold were collected, and particle size analyses were carried out. Comminution products were divided into size distribution classes suitable for further separation operations, and XRPD analyses showed a mineral-grade distribution varying with the dimensions of the products. Characterization of the ore material after crushing and grinding force applications in terms of the distribution of target minerals among different-sized classes was achieved. The important trends highlighted should be considered for further investigation related to an efficient separation.

**Keywords:** comminution; mineral processing; mixed sulfides; sphalerite; galena

#### **1. Introduction**

The mining industry is characterized by essential operations for a proper transformation process of raw materials into final products. Generally, the exploitation activity in a mine is followed by a beneficiation process designed to increase the quality of the valuable minerals naturally present in the excavated material. The very early stage of the transformation is represented by the comminution and grinding of extracted ore aimed at obtaining different changes in terms of the dimension and shape of the material that will be further treated [1–3]. Progressively, separation processes are set up in order to concentrate and separate target minerals from gangue.

Comminution is a highly energy-consuming operation, and it is characterized by very low efficiency [1,4–6] and the quality of the final product being strictly related to the physical characteristics of the material under treatment [7,8].

The principal objective of comminution is to liberate valued minerals, which is not an easy and predictable outcome. However, it is important to achieve this, because it affects the efficiency and control of the entire downstream process, especially when the ore has to be delivered to a flotation process [9–14]. The shape, dimension, and distribution of minerals also play a strategic role for the achievement of an effective beneficiation process [15,16].

In this study, a representative sample of a specific mineralized area of the Gorno Mining District was comminuted with lab-scale equipment, and the size distribution related to the selected equipment was studied.

Determination of the mineral composition and distribution of the concentration of target minerals among the different-sized classes of the crushing products was the key focus of this work.

The importance of having a clear comprehension of the redistribution of target minerals in mid-processing products could lead to future effective separation designs. Benefits that could be achieved can enhance a reduction in end-process waste, as well as the control of water use, reagents, and machinery utilization during the beneficiation stage.
