J. Cent. South Univ. (2018) 25: 1306-1316

DOI: https://doi.org/10.1007/s11771-018-3827-3

Surface property variations in flotation performance of calcite particles under different grinding patterns

XU Peng-yun(许鹏云)^{1, 3}, LI Jing(李晶)², HU Cong(胡聪)⁴, CHEN Zhou(陈洲)⁵,

YE Hong-qi(叶红齐)¹, YUAN Zhong-quan(袁中权)³, CAI Wen-ju(蔡文举)³

1. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Basic Teaching Department, Nanchang Institute of Science and Technology, Nanchang 330108, China;

3. Hefei Cement Research and Design Institute, Hefei 230051, China;

4. Northwest Mining and Geology Group Co., Ltd., for Nonferrous Metals, Xi’an 710054, China;

5. Sino-steel Maanshan Institute of Mining Research Co., Ltd., Maanshan 243000, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: Based on the working principles of particle bed comminution, particles produced by high-pressure grinding rolls (HPGR) have surface properties different from particles produced by other grinding patterns, which exert great influence on mineral flotation. Flotation performances of calcite particles under different grinding patterns involving the use of HPGR, a jaw crusher, a dry ball mill, a wet ball mill, and a wet rod mill were studied using single mineral flotation tests. The surface properties of the particles under different grinding patterns were characterized to determine the flotation performance variation in terms of specific surface area, particle size distribution, AFM, XPS, and zeta potential. The results show that particles ground by HPGR exhibited improved flotation performance within the lower range of grinding fineness in both NaOL and dodecyl amine flotation systems compared to the particles prepared using other grinding patterns. Specific surface area, particle size distribution, surface roughness, Fe(Ⅲ) contamination, binding energy, and zeta potential are greatly influenced by grinding patterns, which is the main cause of the flotation performance variation.

Key words: calcite; surface property; grinding patterns; high-pressure grinding rolls; flotation performance

Cite this article as: XU Peng-yun, LI Jing, HU Cong, CHEN Zhou, YE Hong-qi, YUAN Zhong-quan, CAI Wen-ju. Surface property variations in flotation performance of calcite particles under different grinding patterns [J]. Journal of Central South University, 2018, 25(6): 1306–1316. DOI: https://doi.org/10.1007/s11771-018-3827-3.

1 Introduction

From the working principle of particle bed comminution [1], high-pressure grinding rolls (HPGR) exhibit advantages such as low energy consumption, high productivity, and easy maintenance which are not possible with other grinding patterns [2, 3]. Because of these advantages, HPGR has attracted intense scrutiny and interest from both domestic and foreign researchers since it had been invented by Schonert in the 1980 s [4]. It is now widespread in cement, mineral processing, coal mining, construction aggregate, metallurgy, sintering, and pellet formation [5–7]. As a new type of grinding equipment, HPGR is gradually becoming an important component of grinding systems for mineral processing. Recent developments have indicated that HPGR will undoubtedly replace traditional crushers and mills. From a mining perspective, HPGR has played key roles in decreasing the cost of mineral extraction, creating many benefits [8]. Especially in current economic conditions, HPGR as new energy-saving and cost-reducing equipment will increasingly benefit relevant personnel and has become a focal point of practice and research.

Recently research regarding HPGR has focused on the following four aspects: particle size distribution and theoretically modeling the particles ground by HPGR [9, 10]; energy efficiency in the grinding process of HPGR [2]; the formation mechanism of micro cracks in the particles ground by HPGR [6]; and the influence of HPGR on the subsequent separation procedures such as flotation and leaching [6, 11–13]. For mineral separation and processing, the fourth aspect is the most important. KODALIA et al [6] reported that products from HPGR crushing have more particle damage and higher copper recoveries when compared with products of the same size class from jaw crushing, as determined by X-ray computed tomography analysis and laboratory column leaching experiments. Through column leaching experiments of low grade gold ore ground by a conventional jaw crusher and HPGR, TANG et al [11] showed that HPGR played a significant role in strengthening the penetration effect of heap leaching which can improve the leaching rate of gold ore and decrease leaching solution consumption. Meanwhile, using Mo-Cu ore from Bangpu (Tibet, China), HOU et al [12] performed grinding-flotation tests of the products ground by HPGR and jaw crusher. The floatation effect of the products ground by HPGR was better than those ground by jaw crusher. Thus, it can be concluded that HPGR has played a positive role in separation procedures such as flotation and leaching. However, some studies have shown the opposite, that HPGR does not improve flotation or leaching compared to a conventional jaw crusher or ball mill. For example, OLIVEIRA et al [13] reported that there was no evidence that acid leaching of Cu from low-grade Brazilian ore was more efficient in the products crushed by HPGR compared to those crushed by a jaw crusher.

The discrepancy between these two sets of studies showed that the influence of HPGR on the separation and the mechanism of this influence remain unclear. In fact, the influence of HPGR on the separation and its related mechanism are closely related to the particle surface properties which include specific surface area [14], particle size distribution [15], surface roughness [16], and Zeta potential [17]. Therefore, to understand the influence of HPGR on subsequent separation and its mechanism, the surface properties of particles under different grinding patterns must be further investigated.

Based on the studies described above, flotation performances of particles under different grinding patterns including the use of HPGR, a jaw crusher, a dry ball mill, a wet ball mill, and a wet rod mill were investigated using single mineral flotation tests. The surface properties of the particles were characterized and compared to understand the flotation mechanism of the particles under different grinding patterns through evaluation of specific surface area, particle size distribution, surface roughness, XPS, and zeta potential.

2 Materials and methods

2.1 Mineral samples and reagents

Pure mineral samples of calcite were obtained from the Rongan Mine in Guangxi Province, China. X-ray diffraction analysis was performed to characterize the chemical and mineral compositions (Figure 1), and chemical analysis was performed to determine the purity of the minerals. The XRD results showed that the calcite sample was quite pure, and chemical analysis showed that the calcite sample was 99.21% pure. The preparation of mineral samples used for the flotation tests and their surface properties are shown in Figure 2. During sample preparation loss, contamination and any other scenario which may change the identity of the samples were prevented.

In the flotation tests, the anionic collector, NaOL, and cationic collector, dodecyl amine, were chemically pure and obtained from Tianjin Fuchen chemical reagent factory. HCl (AR) and NaOH (AR) were used to adjust the pH of the flotation system and tap water was used for all flotation tests.

Figure 1 XRD diffraction pattern of ore sample

Figure 2 Flow chart of sample preparation methods

2.2 Flotation tests

Cationic flotation tests were performed in a lab-used single flotation cell of XDF_IV0.5 which was fabricated in a prospecting machinery factory in Jilin province. The volume of the flotation cell was 500 mL and the impeller speed was 1992 r/min. First, 50 g samples prepared with different grinding patterns were mixed with 450 mL of tap water in the flotation cell for 1 min. Then, NaOH was added to adjust the pH to approximately 9.5 and the pulp was conditioned for 1 min. Next, the cationic collector dodecyl amine was added to the pulp to a final concentration of 2 mg/L. The flotation time was fixed for 4 min at room temperature (22 °C). Finally, the concentrate and tailings were weighed separately after filtration and drying, and the recovery was calculated based on the dry weight and grade. Anionic flotation tests were performed under the same conditions except for pH and collector type. In the anionic flotation system, the pulp pH was maintained at approximately 7.0 by addition of low concentration of HCl and NaOH solutions. The anionic collector NaOL was added to the pulp to a final concentration of 5 mg/L.

2.3 Characterization of surface properties

2.3.1 Particle size and particle size distribution

A Cilas-1180 laser particle analyzer was used to measure and calculate the frequency distribution of the particle size. The mineral samples were air-dried and 3 g of the sample particles were dispersed by addition to anhydrous ethanol. Finally, the sample solutions were prepared for 10 min by ultrasonic dispersion and transferred to the laser particle size analyzer for measurement.

2.3.2 Specific surface area (SSA)

A DBT-127 breathable specific surface area analyzer was used to measure the specific surface area of the sample particles based on national standard of GB/T 8074-2008 (China).

2.3.3 Atomic force microscopy (AFM) and surface roughness

A multimode-8 atomic force microscope from Bruker was used to observe the three-dimensional morphology of the sample particles. Sample particles were positioned on the test bench in air and the probe was operated in tapping mode. The schematic for surface roughness measurements is shown in Figure 3. A more detailed description of AFM operation to measure surface roughness can be found in Refs. [18–20].

Figure 3 Schematic plot of surface roughness measurements

2.3.4 X-ray photoelectron spectroscopy (XPS)

A 250-Xi X-ray photoelectron spectroscope from Thermo Fisher Scientific was used to measure the binding energy and atomic concentration on the surface of the samples.

2.3.5 Zeta potential

A suspension containing 0.1 wt% sample particles (ground to 5 μm in an agate mortar) was prepared in 30 mL of a KCl (1×10^-3mol/L) solution and conditioned by magnetic stirring for 3 min. After allowing the solution to settle for 10 min, the supernatant of the dilute fine particle suspension was removed for zeta potential characterization. A ZetaPALS-90plus potentiometric analyzer from Brookhaven Company (US) was used for the zeta potential measurements. The conductivity and pH of the suspension were adjusted using low concentration of HCl and NaOH solutions continuously during the measurements and the environmental temperature was maintained at 22°C. The average of three readings was taken for each measurement.

3 Results and discussion

3.1 Flotation performance of particles with different grinding patterns

Anionic and cationic flotation tests were performed on the particles with different grinding patterns. The flotation performance of particles is shown in Figure 4.

Figure 4(a) shows that particles ground by HPGR exhibit better flotation performance than the other grinding patterns when the grinding fineness was less than 90%, exceeding 0.074 mm in the NaOL flotation system. Conversely, flotation performance of the particles ground by jaw crusher is equal to or better than that of particles ground by HPGR when the grinding fineness was larger than 90% exceeding 0.074 mm. Besides these two grinding patterns, flotation performance of the particles ground by dry ball, wet ball, and wet rod mills was reduced sequentially. Figure 4(b) shows that the particles ground by HPGR exhibit better flotation performance than those obtained with the other grinding patterns at grinding fineness less than 65% passing 0.074 mm in the dodecyl amine flotation system. When grinding fineness was further improved, the particles ground by wet ball and wet rod mills showed superior flotation performance over the particles ground by HPGR, while those ground by jaw crusher were on par with the particles ground by HPGR. Compared with the NaOL flotation system, particles ground by wet ball and wet rod mills exhibited better flotation performances in the dodecyl amine flotation system.

Figure 4 Flotation performances of particles under different grinding patterns:

The summary of Figures 4(a) and (b) indicates that the particles ground by HPGR exhibit superior flotation performance within the smaller range of grinding fineness in both NaOL and dodecyl amine flotation systems compared to the other grinding patterns. When grinding fineness is larger than a critical value, the superior properties of the particles ground by HPGR disappear. Specifically, the critical value of grinding fineness in the NaOL flotation system is 90% passing 0.074 mm and in the dodecyl amine flotation system it is 65% passing 0.074 mm. Differences in flotation performances of the particles under different grinding patterns can be determined from surface properties which are significantly affected by grinding pattern.

3.2 Effects of grinding patterns on specific surface area

During flotation, the adsorption of the collector on the surfaces of mineral particles plays a vital role [21]. Thus, studies regarding the specific surface area of particles with different grinding patterns are important for understanding the flotation performances described in Section 3.1. The effects of the grinding patterns on specific surface area are shown in Figure 5.

Figure 5 Specific surface area of particles with different grinding patterns (a—Advantage zone; b—Transition zone; g—Disadvantage zone)

Figure 5 shows that the particles ground by HPGR have a higher specific surface area than those with other grinding patterns when the grinding fineness is less than 75% passing 0.074 mm. Unfortunately, the higher specific surface area of the particles ground by HPGR was unsustainable. In particular, when the grinding fineness ranges between 75% and 90% passing 0.074 mm, the specific surface area of the particles ground by HPGR is only slightly larger than that of particles ground by wet ball mill, approximately equal to those ground by jaw crusher and wet rod mill, and less than that of the particles ground by dry ball mill. When the grinding fineness rises above 95% passing 0.074 mm, the particles ground by HPGR do not have any advantage in specific surface area. To distinguish among the three different phenomena regarding the specific surface area of particles ground by HPGR, the concepts of the “advantage zone”, “transition zone”, and “disadvantage zone” are introduced in Figure 5.

Previous studies [2, 6, 11] indicated that particles ground by HPGR with a larger size exhibited more micro cracks than particles ground by HPGR with a smaller size. Another study [22] demonstrated that the specific surface area of the particles is determined by particle size and morphology as well as pores, cracks, and defects in the particles. Thus, three conclusions can be drawn: 1) when the grinding particles are in the advantage zone, small specific surface areas of the particles typically are observed for all grinding patterns. A large number of micro cracks exist in the particles ground by HPGR at this grinding fineness, which improves the specific surface area and increases the risk of collector adsorption on the surface of the particles [23, 24]. This is likely why the particles ground by HPGR exhibit improved flotation performance within the smaller range of grinding fineness in both NaOL and dodecyl amine flotation systems compared to other grinding patterns; 2) when the grinding fineness is in the disadvantage zone, large specific surface areas of particles are typical for all grinding patterns and the first factor becomes the crucial factor resulting in large specific surface area. Although the grinding fineness remains constant, differences exist in the particle size distribution. Dry and wet ball milling widened the particle size distribution through the working principle of “point to point” (as shown in Figure 6), which improves the specific surface area of the particles. This explains why superiority of the particles ground by HPGR disappears when grinding fineness is larger than the critical value.

Figure 6 Particle size distribution of particles under different grinding patterns at same grinding fineness

3.3 Effects of grinding patterns on surface roughness

Although the origin of the improve floatation by HPGR was explained clearly in Section 3.2, the importance of surface roughness in flotation performances should not be neglected [16]. Generally, high surface roughness influences two major aspects: 1) high surface roughness can improve the specific surface area of particles, which increases the risk of collector adsorption; and 2) high surface roughness exposes more crystal sites to the “tip point”, which improves the surface energy of the particles and enhances the adsorption capacity towards the collector on the crystal sites [25, 26]. Consequently, AFM was used to observe the three-dimensional morphology of the sample particles and surface roughness was calculated from the three-dimensional morphology data. Three- dimensional surface images of sample particles with different grinding patterns are shown in Figure 7. The corresponding surface roughness values of the sample particles with different grinding patterns are shown in Figure 8.

Figure 7 Three-dimensional surface images of particles with different grinding patterns:

Figure 7 shows that the surfaces of particles ground by HPGR and jaw crusher are the roughest and those ground by dry ball mill were rather smooth. In contrast to the previous grinding patterns, the surfaces of particles ground by wet ball and wet rod mills can be classified as “medium” in terms of roughness. The surface roughness values of particles with different grinding patterns presented in Figure 8 illustrate this trend.

Figure 8 Surface roughness values of particles with different grinding patterns:

Grinding can be considered as a process of constant breaking of the old surface and building of a new surface [27, 28]. Thus, the grinding rate has a significant influence on the surface roughness. HPGR and jaw crusher grind the mineral particles rapidly through a powerful applied force [29], while dry ball, wet ball, and wet rod mills grind the mineral particles less forcefully through weak gravity from the medium. Therefore, the difference between applied forces of the different grinding patterns resulted in the variable surface roughness. Surface roughness can be used as a supplementary explanation for the flotation performances described in Section 3.1, especially for the particles ground by HPGR in the NaOL flotation system.

3.4 Surface energy spectra analyses

During grinding, new surfaces cannot be isolated from the grinding environment. Thus, significant differences exist in the new surfaces of the particles with different grinding patterns [30]. These significant differences include two aspects:1) binding energy of the surface; and 2) contamination on the surface. Previous studies [25, 31, 32] have indicated that binding energy and contamination on the surface have significant effects on the flotation performance of particles. Therefore, XPS was used to measure the binding energy and contaminant concentration on the surface of the particles. The XPS spectra of particles with different grinding patterns are shown in Figure 9. Atom fraction and binding energy of the surface elements are listed in Tables 1 and 2, respectively.

Figure 9 X-ray photoelectron spectroscopy (XPS) of particles with different grinding patterns:

Table 1 Atom fraction of surface elements

Table 2 Binding energy of surface elements

Figure 9 shows that Fe(III) contamination was found on the surfaces of particles ground by wet ball and wet rod mills, but was not observed in the dry ball mill, HPGR, and jaw crusher particles. Table 1 further clarifies the amount of Fe(III) by atom fraction of surface elements. Combined with the flotation performances shown in Figure 4, it is clear that Fe(III) contamination has different effects in different flotation systems. As the pH value in the NaOL flotation system is neutral (approximately 7.0), Fe(III) can stably exist in the pulp and shows strong competitive adsorption with Ca(II). According to the theory of electronegativity, Fe(III) is more readily adsorbed, which suppresses the flotation performances of the particles ground by dry ball, wet ball, and wet rod mills. However, Fe(III) cannot exist in the pulp of the dodecyl amine flotation system as the pH is approximately 9.5. Thus, an important conclusion was reached that competitive adsorption did not occur in the dodecyl amine flotation system and flotation performance was mainly influenced by the specific surface area of the particles. Furthermore, this conclusion explains why the particles ground by HPGR exhibit better flotation performances than those obtained using the other grinding patterns at grinding fineness of less than 65% passing 0.074 mm, while the particles ground by wet ball and wet rod mills exhibit superior flotation performance over HPGR when the grinding fineness is higher than 65% passing 0.074 mm in the dodecyl amine flotation system.

Table 2 shows that the binding energy of Ca(II) on the surface of the particles ground by HPGR and jaw crusher is higher than that on the surface of the particles ground by dry ball, wet ball, and wet rod mills. Considering the sites of the collector adsorption are the Ca(II) on the surface of calcite [33], the particles ground by HPGR and jaw crusher promote flotation performance.

3.5 Effects of grinding patterns on zeta potential

The zeta potential of mineral particles can dramatically influence the adsorption of collectors on their surfaces, which has already been shown in Refs. [34, 35]. Different grinding environments will inevitably lead to different zeta potentials. In particular, Fe(III) accumulated with extended grinding time seriously affects the zeta potential of mineral particles [36]. The zeta potentials of particles with different grinding patterns are shown in Figure 10. The upper right corner of Figure 10 shows the details regarding the potential of zero charge (PZC). It should be noted that the calcite particles cannot stably exist in acidic solution. Thus, the pH range of the tests was from 5.8 to 14.

Figure 10 Zeta potential of particles with different grinding patterns as a function of pH

It can be seen from the Figure 10 that the zeta potentials of the particles ground by HPGR and jaw crusher are lower than those ground by wet ball and wet rod mills at the same pH and the zeta potentials of the particles ground by dry ball mill lie somewhere in between. All particles have positive charges and the surface charges of particles with different grinding patterns are basically the same when the pH is approximately 7 in the NaOL flotation system. For electrostatic adherence between particles and anionic collector, the positive charge is beneficial. Thus, it can be considered that the zeta potential of the calcite particle is nearly independent on grinding pattern in the NaOL flotation system. When the pH value is approximately 9.5 in the dodecyl amine flotation system, all particles have negative charges. This is better for electrostatic adherence between the particles and cationic collector. However, the charge values of the particles with different grinding patterns differ significantly. Thus, it can be considered that the zeta potential of the calcite particle is appreciably affected by grinding patterns in the dodecyl amine flotation system. In addition, it is clear from the PZC values that the adsorbed Fe(III) changes the structure of the double-electric layer and shifted the PZC in the positive direction.

4 Conclusions

1) The results of the flotation tests show that particles ground by HPGR exhibit superior flotation performance within the smaller range of grinding fineness in both NaOL and dodecyl amine flotation systems compared to other grinding patterns. When the grinding fineness is larger than a critical value, the particles ground by HPGR show equivalent or worse performance than the other particles. Specifically, the critical value of grinding fineness in the NaOL flotation system is 90% passing 0.074 mm while in the dodecyl amine flotation system it is 65% passing 0.074 mm.

2) When the grinding fineness is in advantage zone, a large number of micro cracks exist in the particles ground by HPGR, which improves the specific surface area and increases the risk of collector adsorption on the particle surface. This explains why the particles ground by HPGR exhibit superior flotation performance within the smaller range of grinding fineness in both NaOL and dodecyl amine flotation systems compared with the other grinding patterns. When the grinding fineness is in disadvantage zone, dry and wet ball mills widen the particle size distribution by the working principle of “point to point”, which improves specific surface area. This explains why the particles ground by HPGR exhibit equivalent or worse flotation performance than the other methods when grinding fineness is larger than a critical value.

3) Surface roughness was characterized by AFM and represents a supplementary explanation for the flotation performances of the particles with different grinding patterns, especially for the particles ground by HPGR in the NaOL flotation system. Surface energy spectra analyses showed that Fe(III) contamination occurred on the surfaces of the particles ground by wet ball and wet rod mills, while it was absent in the particles ground by dry ball mill, HPGR, and jaw crusher. The binding energy of Ca(II) on the surface of particles ground by HPGR and jaw crusher was higher than that of Ca(II) on the surface of particles ground by dry ball, wet ball and wet rod mills.

4) The zeta potential of the particles ground by HPGR and jaw crusher are lower than that of particles ground by wet ball and wet rod mills at the same pH, and the zeta potential of the particles ground by dry ball mill is somewhere in between. The adsorbed Fe(III) changes the structure of the double-electric layer and shifted the PZC in the positive direction.

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(Edited by FANG Jing-hua)

中文导读

不同磨矿方式下方解石浮选行为差异的颗粒表面特性

摘要：基于料层粉碎工作方式，高压辊磨制备的矿物颗粒具有区别于其他磨矿方式制备颗粒的表面特性，对矿物浮选会产生较大的影响。本文研究了高压辊磨、颚式破碎、干式球磨、湿式球磨及湿式棒磨下方解石纯矿物颗粒的浮选行为，并通过比表面积、粒径分布、原子力学显微镜、表面电子能谱及动电位等手段表征了不同磨矿方式制备颗粒的表面特性以揭示其浮选机理。结果表明：当磨矿细度较低时，相比于其他磨矿方式高压辊磨制备的方解石颗粒在油酸钠和十二胺体系均能够获得更好的浮选指标；由磨矿方式导致的矿物颗粒比表面积、粒度分布、表面粗糙度、Fe³⁺沾染物、键合能及Zeta电位差异是其浮选行为差异化的主要原因。

关键词：方解石；表面特性；磨矿方式；高压辊磨；浮选行为

Foundation item: Project(2013EG132088) supported by Special Program for Research Institutes of the Ministry of Science and Technology, China; Project(12010402c187) supported by Key Science and Technology Program of Anhui Province, China; Project(GJKJ-14-89) supported by Science and Technology Program of Nanchang Institute of Science and Technology, China

Received date: 2017-01-04; Accepted date: 2017-03-06

Corresponding author: XU Peng-yun, PhD, Engineer; Tel: +86–15256046331; E-mail: xupengyun01@163.com; ORCID: 0000-0002- 1110-1566