Effects of grinding media(ceramic ball and cast iron ball) on grinding products and flotation performance of chalcopyrite

Selective flotation has been considered the most promising strategy for treating chalcopyrite. Grinding, an es-sential procedure before flotation, has extraordinary significance in chalcopyrite flotation. In this study, the differences between the properties of the products obtained by grinding with cast iron ball (CIB, > 3.5% C) and ceramic ball (CB, > 90% Al2O3) media were investigated using various techniques: atomic absorption spectro-metry, scanning electron microscopy combined with energy dispersive spectrometry, X-ray photoelectron spectroscopy, and contact angle and zeta potential measurements. The performance of chalcopyrite flotation was used as the final factor in determining the characteristics of both the grinding media. The results indicate that the Fe3+ concentration and the pH of the pulp obtained by grinding with CB medium were lower, while the dis-solved oxygen content was obviously higher, than those of the pulp obtained by grinding with CIB medium. The Cu2+ concentrations were basically the same under the two grinding media systems. Furthermore, grinding with CB medium yielded products with relatively more even and smoother surface than grinding with CIB medium did. Fewer oxygen-containing floccules, i.e., FeOOH and Cu(OH)2, had formed and scattered on the chalcopyrite surface during grinding with CB medium, and this is mainly attributable to the limited local cell reaction of chalcopyrite alone. Additionally, it was found that better hydrophobicity and a lower iso-electric point of the chalcopyrite surface were achieved by grinding with CB medium. Consequently, grinding with CB medium exhibited better performance in terms of chalcopyrite recovery, which was approximately 16% higher than that obtained by grinding with CIB medium.

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 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 tem-perature, and weighed to calculate chalcopyrite recovery.





Characteristics of the grinding products

Several techniques were used to analyze the characteristics of the grinding products obtained using different grinding media systems. Three kinds of samples – pulp, solid and filtrate – were required for testing. The pulp could be obtained directly from grinding. The filtrate and filter cake were produced during pulp filtration, which was carried out in a laboratory-scale vacuum filter. The filter cake was further

Particle size distribution analyses
The particle size distributions of the grinding solid samples were
measured by the screen sizing method. A series of screens with mesh sizes of 25, 34, 43, 74, and 350 µm were used in the tests. Dry-screening was used to treat the samples screened with a mesh size of 350 µm, while wet-screening was used to treat the samples screened with mesh sizes of 25–74 µm.

Pulp chemical property analyses
The pH and DO content of the pulp samples were detected directly
using a pH meter (PHS-3C) and a portable DO meter (Multi 3510IDS), respectively. The concentrations of Fe3+ and Cu2+ in the filtrate




Surface morphology and species analyses
The surface morphologies of the solid samples were characterized
using a scanning electron microscope (SEM, Model No. SSX-550, Daojin Company, Japan) combined with an energy dispersive spectrometer (EDS, Model No. Inca, Oxford Company, Britain). The maximum mag-nification was 7000 times under an acceleration voltage of 15 kV. The species and their relative contents on the surface of the solid samples were examined by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Company, Britain). The XPS analyses were carried out using an Al KαX-ray source with operating voltage and current of 15 kV and 10 mA, respectively. The degree of vacuum in the analyzer chamber was 5 × 10−9 mbar during analysis. The analysis spot size was 700 × 300 µm, with analysis depth in the approximate range 2–5 µm.

Contact angle measurements
The solid samples were first pressed into a smooth slice. A contact
angle device, XG-CAMB, was then employed to measure the contact angle of the samples using the free sessile drop method. In this method, a stable water drop, 3–4 mm in diameter, is slowly transferred to the slice surface using a micro-syringe to measure the contact angle. Each sample was measured three times at different locations before the ad-dition of sodium butyl xanthate and the average value was obtained with an accuracy of ± 2°.

results of the zeta potential analysis were obtained from a computer processor.

Effects of grinding media on particle size distributions

The chalcopyrite samples were ground to a size of 90% passing 74 µm using different grinding media, and the resultant particle size distributions are shown in Fig. 3. The particle size distribution of the product produced by grinding with CB medium was close to that of the product produced by grinding with CIB medium. The particle sizes of both grinding products were mainly concentrated below 25 µm. In ad-dition, a small amount of coarse grains (> 350 µm) were observed in the products obtained from the two grinding media systems. This is due to a blind angle at the bottom of the vertical mill, which prevents a small amount of each sample from being ground by grinding media. However, minor differences still existed between the products particle size distributions obtained by grinding with CB and CIB media. The fine grain content of the product was higher in the case of grinding with CIB medium than with CB medium. This may be attributed to the high density and rough surface of the CIB medium, which increased the frictional force between the material and medium, leading to higher grinding efficiency and much finer particles.

Effects of grinding media on pulp chemical properties
The chemical properties of the pulp, i.e., ion (Fe3+, Cu2+) con-centration, pH, and DO content, had significant effects on flotation. The results obtained using different grinding media are shown in Fig. 4.
The Fe3+ concentration and the pH value of the pulp produced by grinding with CIB medium were obviously higher, while the DO content was significantly lower, than those of the pulp obtained by grinding with CB medium. Besides, the Cu2+ concentrations were basically the same under the two grinding media systems. The grinding of chalco-pyrite with CIB medium formed a complex system entailing not only local cell reactions within the CIB medium or within the chalcopyrite itself but also galvanic couple action between the two. The electro-chemical models are shown in Fig. 5 (Adam et al., 1986; Forssberg and Subrahmanyam, 1993). As illustrated in Fig. 5, the following redox reactions take place at the surfaces of the CIB medium and chalcopyrite (Healy, 1984).

Anodic oxidation: Fe ⇌Fe2+ + 2e (1)

CuFeS2 ⇌CuS + Fe2+ + S0 + 2e (2)

1/2O2 + 2Fe2+ ⇌2Fe3+ + O2− (3)

Fe3+ + 3H2O ⇌Fe(OH)3 + 3H+ (4)

1 2

2Fe3+ + O2−+ 4OH−→2FeOOH + H2O (5)

CuS + 2H2O ⇌Cu(OH)2 + 2H+ + S0 + 2e (6)

S0 + 4H2O ⇌SO42−+ 8H+ + 6e (7)
Cathodic reduction: O2 + 2H2O + 4e ⇌4OH− (8)

However, in the process of grinding with CB medium, only the local cell reaction of chalcopyrite took place (Fig. 5b). Therefore, compared to those occurring in grinding with CB medium, the redox reactions entailed in grinding with CIB medium were more intense, which caused more Fe3+ (Eq. (3)) to enter the pulp. Meanwhile, a large amount of iron was abraded and dissolved into the pulp (Eqs. (1) and (3)), ren-dering the Fe3+ concentration of the pulp much higher in the case of grinding with CIB medium than with CB medium. Consumption of DO rose corresponding to increased generation of OH− in the cathodic reaction (Eq. (8)). In addition, the further oxidation of Fe2+ to Fe3+ also consumed some DO (Eq. (3)). This led to much lower DO content in, and higher pH of, the pulp obtained from grinding with CIB medium.

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Post time: Feb-17-2020
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