Effects of grinding media on grinding products and flotation performance of chalcopyrite

1. Introduction
Grinding is extensively used in mineral processing to reduce particle size. It is well known that grinding is a complex physicochemical process with significant effects on the subsequent flotation of sulfide minerals. The various physical and chemical changes in sulfide minerals influenced by grinding are mainly reflected in aspects such as particle size composition, chemical properties of the pulp, surface properties, and crystal structure (Zhu et al., 2015; Xu et al., 2016), as well as in other issues caused by fine grinding.
During grinding of sulfide minerals, a mechanical action is established between the sulfide minerals and the grinding medium 。The combined action of the impact force and grinding force of the grinding medium is mainly used to provide appropriate particle size and liberate the valuable mineral for flotation. Varying degrees of elastic-plastic deformation thus take place on the surface and subsurface of the sulfide minerals, which affects their surface semiconductor properties and electrochemical behaviors .

Many studies have reported that changes in the surface properties and roughness of sulfide minerals during grinding are mainly related to the grinding medium and grinding methods, and these properties play an important role in the wettability and flotation behavior of the minerals . A smoother surface results in better hydrophobicity and floatability (Wang and Xie, 1990). A similar observation was made by Forssberg et al. (1988). Hu et al. (2002) and Gu and Zhong (2008) indicating that galena and pyrite surfaces were relatively flat under the action of non-mechanical forces but grew rough under the action of mechanical forces with the appearance of dark substances with different optical properties on their surfaces, which proves that these surfaces exhibited higher reactivity following

mechanical force action. As reported by Song et al. (2007), grinding with ceramic medium is much more beneficial to subsequent pyrite flotation than grinding with iron medium. The improvement in hydrophilicity and low flotation recovery in the case of pyrite ground with iron medium are mainly due to high surface roughness and serious corrosion.
In the grinding of sulfide minerals, there also exists a series of electrochemical actions, i.e., local cell actions among the minerals or the grinding media themselves, and galvanic couple actions between the minerals and grinding media (Huang et al., 2006; Mu et al., 2018). Redox reactions thus take place on the surfaces of the minerals and grinding medium, leading to significant changes in the pulp and surface properties of the grinding products. Several studies have shown that grinding with different media introduces vast variations in the electrochemical reactions and resultant products on the surface of sulfide
minerals, thereby resulting in significantly different flotation behaviors. In the process of grinding with non-ferrous media, the formation of metal-deficient sulfur-rich surfaces enhances the hydrophobicity of the sulfide minerals and promotes the adsorption of collectors on mineral surfaces. When steel balls were used as the grinding medium, the iron hydroxides produced by oxidation covered the sulfide mineral surface, thereby enhancing its hydrophilicity and deteriorating its flotation behavior.
Tayebi-Khorami et al. (2018) and Xiong et al. (2018) agreed that moderate oxidation on the sulfide minerals surface is favorable to its flotation recovery. This might be due to the fact that moderate oxidation could contribute to the formation of a metal-deficient sulfur-rich surface or sulfur on the minerals surface Although the effects of grinding media on chalcopyrite grinding and flotation performance have been investigated by several researchers
, the literature lacks more systematic studies, especially pertaining to particle size composition, and pulp and surface properties. In the present study, a detailed discussion on the effects of two types of
grinding media, cast iron balls (CIB) and ceramic balls (CB), on the characteristics of chalcopyrite grinding products, i.e., particle size distribution, ion (Fe3+, Cu2+) concentration, pulp pH, dissolved oxygen
(DO) content, surface morphology and species, contact angle, as well as zeta potential, is presented. Furthermore, the results obtained in the grinding experiments were validated through chalcopyrite flotation tests. The aim of the present study is to enhance further theoretical insights into the effects of the grinding environment on the grinding and flotation of chalcopyrite, thus providing a practical reference for the beneficiation of copper sulfide ores.
2. Experimental
2.1. Materials, grinding media and reagents
2.1.1. Materials
Chalcopyrite samples were obtained from the Dexing Yinshan mine in the Jiangxi province of China. Samples of high purity were selected and crushed using a jaw crusher and roll crusher, and then screened to obtain particle size fractions of −1 + 0.5 mm. Gravity separation was used to remove impurities from the samples. To prevent oxidation in air, the samples were placed in closed wide-mouth amber packets and stored in a cool and dry place. The results of the chemical composition analysis of these samples are given in Table 1. The samples had a  copper content of 32.56%. The chemical formula of chalcopyrite is CuFeS2. In theory, the copper content of pure chalcopyrite mineral is 34.78%, and hence the purity of the chalcopyrite samples was as high as 93.62%. The main impurity component is SiO2, with a content of 4.51%. The X-ray diffraction (XRD) analysis of the samples (Fig. 1) also

indicated that the chalcopyrite samples were of high purity with quartz being the main impurity.
2.1.2. Grinding media
Two types of grinding media – CIB and CB – were used in this study. The spherical diameters of both grinding media were 3 mm. The CIB medium was supplied by Jiangxi Sanxin New Materials Co.,Ltd.
The average densities of the CIB and CB media were 7.38 g/cm3 and 3.57 g/ cm3, respectively.
2.1.3. Reagents
Sodium butyl xanthate (C5H9OS2Na, AR grade) and 2# oil (ROH, RAlkyl group, AR grade) were used in this investigation as collector and frother, respectively. During the flotation, the pH was adjusted by the addition of HCl (AR grade) and NaOH (AR grade) solution. All reagents used in this study were obtained from Tieling Beneficiation Reagent
Co., Ltd., Liaoning Province, China. Deionized water was used throughout the experiments.
2.2. Grinding and flotation
The grinding tests were carried out in a vertical mill (JM-2, Changsha, China) at a constant rotational speed of 450 rpm. To better verify the effects of the grinding media on grinding products and chalcopyrite flotation behavior, the mill barrel was made of corundum with a volume of 200 mL, and the top of the stirring rod was covered with a nylon material. In each grinding test, 30 g of chalcopyrite sample combined with 30 mL deionized water was added to the vertical mill, and ground with a medium-charge ratio of 35% for 6 min with CIB and 10 min with CB to render 90 wt% of the sample less than 74 µm in diameter. The pulp was then transferred to a beaker and the volume was increased to 360 mL by the addition of deionized water. Finally, the pulp was divided into 6 equal portions for analyses of particle size distribution, ion (Fe3+, Cu2+) concentration, pulp pH, DO content,
surface morphology and species, contact angle, and zeta potential, and for flotation tests. The procedure is shown in Fig. 2. The flotation tests were carried out in a XFG flotation machine (Jilin, China) at a spindle speed of 1992 rpm. The portion of the pulp (60 mL) for evaluation by flotation was transferred to a 100 mL flotation cell, and deionized water  Fig. 3. Particle size distributions of products obtained using different types of grinding media.was added to attain a certain level for flotation. The process of flotation can be seen in Fig. 2. The flotation products were dried at low temperature, and weighed to calculate chalcopyrite recovery.

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Post time: Dec-13-2019
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