J. Cent. South Univ. (2019) 26: 75-87

DOI: https://doi.org/10.1007/s11771-019-3983-0

Effect of magnetic seeding agglomeration on flotation of fine minerals

YUE Tao(岳涛), WU Xi-qing(伍喜庆), DAI Liang(戴亮)

School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Magnetic seeding agglomeration (MSA), i.e., adding magnetic seeds and a low intensity pre-magnetization for fine agglomeration, was applied to the flotation of coal, pyrite and hematite ore slimes. Size analysis and flotation tests highlight that the MSA improved flotation recovery and kinetics of pyrite ore while causing some loss in selectivity, and in the presences of the polyacrylamide for coal and starch for hematite the agglomeration flotation was further strengthened due to the synergetic effect between the flocculants and magnetic seeds. Magnetism analyses and calculation confirmed the adsorption of magnetic seeds onto minerals, resulting in a decreased threshold magnetic field intensity for the MSA to happen. Then atomic force microscope (AFM) study found that there exists a long range force between magnetic seeds and minerals, which facilitates the adsorption of magnetic seeds on minerals. FTIR shows both the polyacrylamide and starch adsorbed onto minerals and magnetic seeds, thus acting as the bridging media between minerals and magnetic seeds, intensifying the agglomeration in flotation. Surface characterization of the MSA was understood by SEM imaging, and models of the MSA were proposed.

Key words: magnetic seeds; magnetic seeding agglomeration (MSA); magnetic seeding flotation (MSF); fine agglomeration; fine flotation; flocculation

Cite this article as: YUE Tao, WU Xi-qing, DAI Liang. Effect of magnetic seeding agglomeration on flotation of fine minerals [J]. Journal of Central South University, 2019, 26(1): 75–87. DOI: https://doi.org/10.1007/s11771-019-3983-0.

1 Introduction

Flotation is a famous technique in mineral processing. However, the conventional flotation was limited to its properly-sized minerals range of 5–300 μm [1]. The main reason behind this limitation lies in that fine particles present huge specific surface areas, so they need relatively high dosage of reagents (especially collectors and depressants) and exhibit poor flotation performances while intermediate or properly-sized particles produce high flotation kinetics and need low consumption of collectors. Sampling of most industrial flotation plants tabulated the similar pattern [2]. Not surprisingly, this leads to a universal notion that ‘fines do not float’, and plant operators are careful to avoid overgrinding or ‘sliming’ of feed. On the other hand, though flotation has been extensively applied in the recovery of fine minerals known as slimes, traditional flotation methods generally turn out their poor flotation efficiency that leads to serious challenges to mineral processing economically. In some cases, with the limitation for good separation many fine minerals have to be discarded as tailings prior to proper flotation.

To improve the fine flotation performance, a few of flotation methods were investigated. They are based on particle aggregation, namely, by enlarging the apparent particle size, such as the carrier flotation, shear flocculation, selective flocculation, and oil agglomeration [3, 4]. Carrier flotation [5, 6] employs the readily-floatable and coarse mineral particles as carriers to carry fine particles with less floatability, and the resultant coated particles (aggregates of carrier-fine particle) are then floated. This technique has been tested for ores [6, 7] and coals [8]. Apart from the mineral processing field, flotation of fine ink using talc minerals as carriers was also studied [9]. In comparison to the traditional flocculation, selective flocculation-flotation is based on the aggregates or “flocs” formed by the target mineral particles, and aggregates are then floated, and it has been studied for several decades [10–12]. This technique requires high selective flocculation for adsorption, which produces huge limitation due to the high selectivity [13]. Shear flocculation flotation has been reported as a testing flotation method needing high turbulence. Studies [14–16] on shear flocculation confirmed that this method is limited in industry due to its deficient theoretic-understanding and huge energy for agitation. Oil agglomeration is an old method so-called ‘liquid phase agglomeration’ which makes use of the oil’s hydrophobicity and its bridging adsorption onto particles [17, 18]. Recently one technique called as ‘temperature responsive flotation’ has been published [19–22]. This method uses special polymers as flocculants for flotation, for example, poly (N-isopropyl acrylamide) (PNIPAM) [19, 21], which acts as the flocculent and collector for fine particles around the temperature of ‘lower critical solution temperature’ (LCST), enlarging the effective size of fine minerals and facilitating particle–bubble collision and attachment [23].

Take coal slimes as an example, it is well known that the commonly used method for recovering coal from coal slimes is flotation rather than gravity separation although in many cases, the flotation efficiency of coal slimes is technically and economically poor due to the presence of ultrafine particles and resultant huge consumptions of chemical reagents [24, 25]. As a matter of fact, the optimum sizes of coal particles for flotation are in the range of –125 – +74 μm [26], but the coal slimes are far finer than this granularity range. To solve the problem, studies have tested some new methods above remarked, such as carrier flotation [8], selective flocculation-flotation [27], oil agglomeration flotation [18], and all these methods have a certain positive effect on the flotation performance of coal slimes.

Fundamentally, these techniques as mentioned above are aimed to enlarge the effective granularity of fine minerals to an appropriate size (so-called apparent size) through aggregation, improving bubble–particle collision and attachment or more mineralized froth phase [28–30]. But most of them have not found practical applications yet, so still remain “promising” processes due to their varied limiting factors and wait for further improvement. The author [31, 32] introduced an original technique, named ‘magnetic seeding flotation’ (MSF), of especially recovering fine minerals by magnetic seeding agglomeration. In comparison with conventional flotation and its derivative techniques, the method differs mainly in that magnetic seeds and pre-magnetization are brought in the conditioning of the slurry prior to aeration of flotation. In this study, effect of magnetic seeding agglomeration (MSA) on the flotation of coal along with the pyrite and hematite ore slimes was investigated, and then corresponding mechanisms were explored.

2 Materials and methods

2.1 Materials

The fine coal (66.80% less than 38 μm) used for flotation tests was obtained from a coal mine in Shanxi province, China, and contained 44.20% C and 28.40% ash respectively. The pure coal (containing 1.82% ash) with an average size of 16.41 μm was obtained from the fine coal slimes via gravity separation, being mostly used for the mechanism analysis.

The sample of pyrite ore slimes derived from the overflow of secondary hydro cyclone after grinding, 73.95% <23 μm, containing 6.21% S, was taken from a Cu-S concentrator in Jiangxi province, China. The pure pyrite (average size of 18.04 μm) with purity of 96.43% obtained from the above pyrite slimes was mainly used for measurements.

The hematite ore (96.70% less than 15 μm) used for flotation tests was obtained from an iron ore mine in Gansu province, China, and contained 43.73% Fe (Total) and 24.11% SiO₂ respectively. The pure hematite (containing 98.02% hematite) with an average size of 17.11 μm was collected from the above slimes by high-intensity magnetic separation and gravity separation, being mostly used for MSA analysis.

Magnetic seeds were produced through the air-oxidation technique [33]. The seeds were synthesized in the deionized water and then moderate Fe(II) salts and NH₃·H₂O were added to the system at 85 °C while stirring. About 35 min later, the solution of magnetic seeds was synthesized with an average size of 100–200 nm (as shown in Figure 15). It is worth noting that magnetic seeds with a high specific magnetic susceptibility should be dispersed fully by an ultrasonic cleaner prior to use due to their self-agglomeration.

Polyacrylamide (PAM) for coal flotation belongs to the anionic kind with a 3 million molecular weight. In the reverse flotation of hematite ore, a 0.02 g/mL caustic starch solution prepared with a ratio of 4 parts starch to 1 part sodium hydroxide by boiling the mixture for about 20 min was used as the depressant, and then a DDA solution of 0.05 g/mL with a 1:1 mole ratio of dodecyl amine to acetic acid was prepared as the collector.

2.2 Methods

2.2.1 Laser granularity analysis for agglomerates

The suspension with a 5% concentration of the fine minerals, conditioned with the magnetic seeds and the polyacrylamide/starch, was conducted by the pre-magnetizer [34] for several minutes, and then was transferred for size analysis through the analyzer Mastersizer2000. It was worth noting that the speed of the analyzer should be less than 1000 r/min due to the unsteady or fragile agglomeration, and the ultrasonic of the analyzer should be turned off too.

2.2.2 Magnetic seeding flotation (MSF) tests

Each test sample of certain ore slimes was dispersed in the above stirring pre-magnetizer [34] (now its volume is 0.5 L) for 2 min, and stirring for 5 min after adding moderate magnetic seeds. Following the magnetic seeds, other reagents were added into the pulp and conditioned for 4 min. Then the pulp was introduced to the flotation cell for flotation.

2.2.3 Vibrating sample magnetometer (VSM) analysis for fine minerals

Magnetisms of fine minerals in the presence of magnetic seeds were obtained by the magnetometer Lake Shore 7404. The fine mineral coated with a desired amount of magnetic seeds was conducted while stirring in the pre-magnetizer for 5 min. Then, the suspension was introduced into the magnetic separation tube (150 mT) to discard those unabsorbed magnetic seeds, and the following tailings was finally used for VSM analysis [35].

2.2.4 Atomic force microscope (AFM) study on interaction forces

The interaction forces between the magnetic seeds and the mineral substrate were measured by a multimode SPM atomic force microscope (AFM). The substrate surface of hematite or quartz with high purity was produced by slicing, lapping, polishing, and then using deionized water, alcohol and deionized water to wash it sequentially, and at last drying the surface by nitrogen. Glass fiber with a 10 mm-diameter magnetic seeding aggregate was manipulated to stick to the tip of the probe by epoxy resin. Probe with the seed and substrate surface (hematite or quartz) were fixed to the corresponding positions of the AFM. Start button was pushed to make the probe move to, contact with and remove from the substrate surface. The corresponding picture and data were recorded by the computer automatically.

2.2.5 Fourier transform infrared spectroscopic (FTIR) characteristics

To study the adsorption characteristics of the coal/hematite in the presence of the PAM/starch, the FTIR analysis of the samples was measured via the transmission method using an IRAffinity-1 spectrometer (Shimadzu Corporation, Kyoto, Japan). The 1% PAM/starch solution (i.e., 10 g/L) was prepared firstly, and then the pure mineral (<2 mm) and magnetic seeds conditioned with the PAM or caustic starch solution in the thermostatic shaker for 20 min at 25 °C were prepared, respectively. Then, a small part of the suspension was collected for centrifugation and the precipitate was dried at 55 °C using a vacuum drying oven. Finally, the FTIR studies were carried out.

2.2.6 Scanning electron microscope (SEM) imaging

Samples of 2 g pure hematite in the size range of 10–20 μm were prepared firstly by the elutriation method, and then dispersed in 50 cm³ deionized water for 2 min. Then reagents (caustic starch, magnetic seeds) were added to the suspension, and suspensions of hematite, hematite with 200 mg/L caustic starch, and hematite mixed with 200 mg/L caustic starch plus 180 mg/L magnetic seeds under the 50 mT were prepared. After conditioning, the dried samples with gold powder were prepared for SEM imaging by the Model JSM-6360LV (JEOL Corporation, Japan).

3 Size analysis in magnetic seeding agglomeration and flotation tests

3.1 Size analysis in magnetic seeding agglomeration

Table 1 shows the size changes of fine minerals in the presences of magnetic seeds and organic flocculants, such as the fine coal, pyrite and hematite. As shown in Table 1 for coal particles, the average size of the coal increased up to 21.73 μm after the addition of 11 mg/L PAM plus 48 mg/L magnetic seeds, while in the blank and in the case of the 11 mg/L PAM or 48 mg/L magnetic seeds only the average sizes were 16.41 μm, 20.25 μm and 18.24 μm, respectively. It is a surprise that the ultrafine sizes of d(0.5) were increased by 8.84% (from 8.60 to 9.36 μm) and 7.95% (10.06 to 10.86 μm), revealing that the ultrafine particles (less than 11 μm) in the suspension were greatly agglomerated in the presence of magnetic seeds. As a strong flocculent, the PAM flocculates fines well [36], and in comparison to the PAM, it can be demonstrated the PAM plus the magnetic seeds agglomerated strongly with the fine coal, and certainly the PAM plus the magnetic seeds is a better flocculent than the PAM alone here.

The influence of the increased dosage of magnetic seeds on the particle size of pyrite is summarized in Table 1. It is shown that the apparent size of pyrite increased with the increased concentration of magnetic seeds, and then the size of d(0.5) was improved by 7.62% (from 11.86 to 11.02 μm), or in other words the magnetic seeds facilitated the agglomeration of the fine pyrite.

Table 1 also presents the influence of magnetic seeds on the apparent particle size of hematite particles. It can be seen that the apparent size of hematite raised up to 20.40 μm after adding 800 mg/L caustic starch plus 120 mg/L magnetic seeds while in the blank or adding the starch their average particle sizes were 17.11 μm, and 19.72 μm, respectively. In addition, the ultrafine size of d(0.1) was increased by 22.84% (from 5.56 to 6.83 μm) in the presence of magnetic seeds, i.e., the ultrafine particles (most vulnerable faction to loss by the entrainment in flotation) in the suspension were greatly agglomerated. The fine hematite particles were agglomerated well by adding moderate starch, however, agglomerated better while adding moderate magnetic seeds and pre-magnetization.

Thus, the addition of magnetic seeds into the slurry of fine particles results in enlarging the fine particle size (apparent size), and this phenomenon is defined as magnetic seeding agglomeration (MSA); in the cases of presences of traditional organic flocculants, such as PAM and starch, the interactions between the magnetic seeds and flocculants are able to further promote the MSA.

3.2 Effect of magnetic seeding agglomeration (MSA) on flotation

3.2.1 Flotation of coal slimes

Coal flotation tests in the presence of magnetic seeds and pre-magnetization were conducted, as presented in Figure 1. The recovery of coal increases with the increasing concentration of the collector, but in the case of presence of the magnetic seeds, the recovery increases even more sharply than the blank. These indicate that adding magnetic seeds is able to improve the flotation recovery of coal slimes, i.e., the presence of magnetic seeds in the flotation can reduce the collector consumption at the same recovery. In addition, the magnetic seeds range of 100–200 nm displaying finer than coal particles, cannot acted as carriers in the flotation. The positive effect on the flotation recovery, therefore, can be attributed to the magnetic seeding agglomeration (MSA) by producing a positive apparent size for coal flotation. The flotation selectivity, however, decreases in the presence of magnetic seeds probably due to the presence of agglomeration between coal and ash particles.

Table 1 Effect of magnetic seeds and pre-magnetization on particle size of fine minerals

Figure 1 Effect of magnetic seeds on flotation using mixed collector (containing kerosene and certain ester)

It was demonstrated that the PAM plus magnetic seeds were agglomerate strongly with the fine coal in Table 1, therefore, the collaborative effect between them on flotation can be predicted. It was also demonstrated in Figure 2 that there is a synergistic effect between magnetic seeds and the PAM on the coal flotation, and the optimal ratio of magnetic seeds to PAM is 7:3. This synergistic effect exhibits a strong recovery of the coal, probably due to the aid of the bridging adsorption of the PAM between particles (as confirmed in Figure 11), but also produces a high ash grade.

3.2.2 Flotation of pyrite ore slimes

As shown in Figure 3, the flotation recovery of S (mainly belongs to the pyrite mineral) increases with the increased dosage of magnetic seeds, and this agrees with the increased agglomerate size of the MSA in Table 1. However, the grade of S in the concentrate slightly decreases with increased magnetic seeds dosage, implying that the magnetic seeds also adsorb onto the gangue particles, forming futile agglomeration and causing loss in selectivity of the flotation process. Figure 4 shows the effect of magnetic seeds on the flotation kinetics of pyrite ore. It can be inferred that the addition of magnetic seeds is able to speed up the flotation rate, and the increases of both the rate constant k and the maximum cumulative recovery ε_∞ suggest that the particles (aggregate of fine particles and magnetic seeds) in this system are more suitable for flotation in terms of change of granularity rather than hydrophobicity, probably being due to the MSA of fines on the flotation.

Figure 2 Synergistic effect of PAM and magnetic seeds on coal flotation (100 mg/L dosage of collector (containing kerosene and certain ester), 80 mg/L total concentration of PAM and magnetic seeds)

Figure 3 Effect of magnetic seeds on flotation of pyrite ore using 100 g/t butyl xanthate (flotation time t=210 s)

Figure 4 Effect of magnetic seeds on flotation kinetics of pyrite ore slimes using 100 g/t butyl xanthate

3.2.3 Flotation of hematite ore slimes

Figure 5 shows the effect of the magnetic seeds dosage on the reverse cationic flotation. It can be inferred that the flotation depression ability was improved with the addition of the magnetic seeds. The iron recovery increases with the increase of the magnetic seeds with an optimum recovery of 56.79% by using 180 g/t magnetic seeds, but beyond the dosage of 180 g/t magnetic seeds, the recovery decreases with further increase of magnetic seeds dosage. The fact that the depressing effect on the flotation was strengthened by the addition of magnetic seeds due to the agglomeration of the starch with particles in the suspension, surprisingly consistent with the agglomeration of hematite fines in Table 1.

Figure 5 Effect of magnetic seeds on reverse cationic flotation performance of hematite ore slimes at natural pH under pre-magnetization of about 50 mT (collector: DDA 400g/t; depressant: caustic starch 1500 g/t)

3.2.4 Effect of pre-magnetization field intensity on flotation of ore slimes

It was demonstrated that a low pre- magnetization facilitated the MSA of the coal, pyrite and hematite to happen. In view of this, effects of pre-magnetization field intensity on the flotation in the presence of magnetic seeds were investigated, as shown in Figure 6. Figure 6 revealed that the increased pre-magnetization intensity is beneficial to the flotation recovery with an optimum intensity range of 50–100 mT, but beyond that intensity the recovery decreases with further increase of magnetic intensity. These can be attributed to the particular self-agglomeration of the added magnetic seeds themselves under the higher magnetic field, therefore, a moderate pre- magnetization for the flotation is critical here.

Figure 6 Effect of pre-magnetization field intensity on flotation of ore slimes (magnetic seeds: coal 175 g/t, pyrite 200 g/t, hematite 180 g/t; pre-magnetization time: 5 min)

4 Mechanism of formation of MSA

It is known that that the total potential energy of a fine suspension plays an important role on the stability. Also based on the expanded DLVO theory to a fine suspension [37, 38], the magnetic interaction V_M is an essential part for the total energy V_T and the volume magnetic susceptibility x of particles plays an important role for the magnetic attraction in the external magnetic field B_o (the magnetic induction of pre-magnetization mentioned in this study). While the particles were placed in the applied magnetic field, the calculation of the magnetic energy was given below [38]:

                      (1)

where R_o is the radius of particles; x is the volumetric magnetic susceptibility of particles; B_o is the magnetic induction; μ_o represents the permeability of vacuum and d is the distance between particles.

So, the fine suspension would be instable due to the pre-magnetization B_o and the increased magnetic susceptibility x. To probe the MSA, analysis and related calculations were given as follows.

4.1 Threshold magnetic field intensity B_A for fines to agglomerate

4.1.1 VSM analysis of fine minerals

The hysteresis loops of coal, pyrite, and hematite minerals coated with the magnetic seeds are given in Figures 7–9, respectively.

It was presented in Figure 7, the saturation magnetic induction of the pure coal with a small magnetism of 0.064 A·m²/kg increases to 0.090 A·m²/kg after adding moderate magnetic seeds. Surprisingly, after adding moderate PAM and magnetic seeds, the saturation magnetic induction increases up to 0.245 A·m²/kg which might be owned to the associated effect of the PAM and magnetic seeds. So, the improvement of the coal magnetism produces a good effect on the magnetic interactions between coal particles due to the Eq.(1).

Figure 7 Effect of magnetic seeds on magnetism of coal (1–Coal; 2–Coal 34 mg/L magnetic seeds; 3–Coal 11 mg/L PAM and 34 mg/L magnetic seeds)

Figure 8 Hysteresis loops of pyrite (1–Pyrite; 2–Pyrite 40 mg/L magnetic seeds)

Figure 9 Magnetic hysteresis loops of hematite (1–Hematite; 2–Hematite 10 g/t magnetic seeds; 3–Hematite 100 g/t magnetic seeds; 4–Hematite 1000 g/t magnetic seeds)

Figure 8 shows the VSM figure of pyrite minerals. After the conditioning with magnetic seeds the pyrite minerals exhibit a strong saturation magnetic induction of 0.253 A·m²/kg, while in the blank it is only 0.067 A·m²/kg. Like the coal or pyrite, the hematite also produces an increased saturation magnetic induction by the addition of magnetic seeds as given in Figure 9.

In addition, DAI [39] calculated the adsorption density of magnetic seeds onto the coal particles via the indirect models based on Figure 7, and the calculation demonstrated that the PAM improved the coverage of magnetic seeds onto the coal surface, and this will be further studied by the FTIR below.

4.1.2 Threshold magnetic field intensity of fine minerals to agglomerate

Based on Eq. (1), it was suggested that the external magnetic field B_o is a decisive factor for fine particles to agglomerate. For fine particles to agglomerate, WATSON [40] proposed a method to analyze the threshold magnetic field intensity B_A for fine particles to agglomerate below:

                        (2)

where K is the Boltzmann constant (1.3806505× 10^–23 J/K); T is the temperature in Kelvin; the value of the magnetic permeability of vacuum μ_o is 4π×10^–7 N/A²; R_o is the radius of mineral particle, and x is the volume magnetic susceptibility of the target mineral. The related result was shown in Table 2.

Table 2 Threshold magnetic field intensity for fine minerals to agglomerate

Table 2 indicates that the value B_A for coal particles to agglomerate decreases strongly after the addition of magnetic seeds, sharply declines by the addition of the PAM and magnetic seeds. It can also be found in Table 2 that B_A for pyrite and hematite coated with magnetic seeds decreases sharply. So, in the MSA tests, the actual magnetic susceptibility of fine minerals increases by the addition of magnetic seeds, consequently decreases the threshold magnetic field intensity and facilitates the magnetic seeding agglomeration readily in a low external magnetic field of the pre-magnetizer.

4.2 Interaction force between particles (AFM)

The AFM was used to investigate the interaction forces between solid surfaces, and the interaction forces between a magnetic seeding aggregate and substrate surfaces (hematite and quartz) were obtained in the dry atmosphere and recorded in Figure 10.

As shown in Figure 10, the attractive force can still be measured even the separation distance is more than 200 nm and the maximum value of the attractive force is up to 10 nN. So, the force can be characterized as a long range force due to high magnetic energy in the presence of magnetic seed.

Figure 10 Measured forces as functions of separation distance between a 10 mm-diameter magnetic seeding aggregate and a flat substrate surface in dry atmosphere (1–Quartz substrate; 2–Hematite substrate)

This long range force would greatly strengthen the adsorption of magnetic seeds onto fine minerals and increase the magnetic susceptibility of fines, and then help the agglomeration of fine particles to happen under the condition that the particles was placed in the external magnetic field [38].

4.3 FTIR spectra of fine minerals conditioned with PAM or starch

4.3.1 FTIR spectra study of coal, magnetic seeds, and particles in presence of PAM

As shown in Figure 11, after the interaction of the coal with polyacrylamide, the adsorption of C—N in 1116 cm^–1, characteristic of the stretching and vibrating of the bond, increases for the coal-PAM, and there appears a new adsorption of N—H in 3349 cm^–1. It can be also observed from the spectrum of magnetic seeds-PAM, the adsorption of C—N in 1124 cm^–1 appears, and also there is an adsorption in 3342 cm^–1, characteristic of NH₂. So, the polyacrylamide chemisorbs on the surfaces of both magnetic seeds and coal, and acts as the bridging media between the two types of particles, resulting in an intensified coverage of the seeds onto the coal surface and positive effect on the MSA.

4.3.2 FTIR spectra study of hematite, hematite- starch and magnetic seeds-starch

It can be observed in Figure 12, in the spectrum of hematite–starch, the new peak around 1031 cm^–1 is the C—OH bending and C—O stretching vibration, and the peak around 1070 cm^–1 is the C—H bending vibration, and then the small bands of COO^- asymmetric plus symmetric stretching are found around 1625 and 1404 cm^–1 [41], and the band near 3419 cm^–1 is the stretching vibration of O—H group. It can be also found in the spectrum of magnetic seeds-starch, the new adsorptions around 1634 and 1400 cm^–1 are the COO^- asymmetric and symmetric stretching peaks [41], demonstrating the chemisorption of caustic starch onto magnetic seeds, and the C—O stretching vibration around 1028 cm^–1 and the band of 1073cm^–1 regarded as the C—H bending vibration are observed. Other peak around 3423 cm^–1 is the O—H stretching vibration. Like the adsorption onto hematite, the starch was also absorbed onto the magnetic seeds through the hydrogen bonding adsorption and chemisorption.

Figure 11 FTIR spectra of magnetic seeds and coal treated with PAM

Figure 12 FTIR spectra of magnetic seeds and hematite conditioned with the starch

Therefore, the starch was adsorbed onto the magnetic seeds and hematite, and also acted as a bridging media between the two types of particles. Like the PAM, the organic flocculent starch also leads to an intensified coverage of the magnetic seeds onto hematite particles and positive action in the agglomeration via the pre-magnetization. It can be predicted that the presence of magnetic seeds strengthens the adsorption of starch on the hematite, which might produce a good depressing ability.

4.3.3 Model of magnetic seeding agglomeration (MSA)

The FTIR analysis suggest that the adsorption of the PAM or starch onto fine minerals and magnetic seeds acts as a bridging media for the agglomeration and the proposed model of the magnetic seeding agglomeration for coal and hematite can be illustrated in Figure 13. Furthermore, a proposed model of the agglomeration for pyrite fines was given in Figure 14.

4.4 Surface characterization (SEM imaging)

In order to further understand the behavior of the magnetic seeding agglomeration (MSA), SEM images are given in Figure 15. Figure 15(a) shows the incompact characteristic of fine hematite conditioned with magnetic seeds (no pre- magnetization), and in Figure 15(b) the agglomerates of most magnetic seeds adsorb onto hematite although a relatively small number of single magnetic seeds are scattered on hematite surface. In contrast to Figure 15(a), Figure 15(c) presents more compact adsorption morphology between particles including the hematite and magnetic seeds in the presence of pre-magnetization, suggesting the presence of the MSA. The morphologies of starch and its adsorption onto hematite are given in Figure 15(d), indicating a good flocculating effect of the starch on hematite. In the presence of magnetic seeds and pre- magnetization, the flocculating behavior in Figures 15(e) and (f), however, presents more compact and stronger agglomerates than that in Figures 15(c) and (d). In addition, Figure 15(f) also shows that the starch absorbed onto the hematite prior to the magnetic seeds and the agglomerates of magnetic seeds are found outside the adsorption layer of the starch which acts as the bridging media between hematite and magnetic seeds. These highlight that the co-agglomeration of the hematite and starch in the presence of magnetic seeds and pre-magnetization happens here, resulting in strong agglomerates of particles.

Figure 13 Proposed model of MSA for fine coal or hematite particles in presence of organic flocculants

Figure 14 Proposed model of MSA of fine pyrite particles without organic flocculants

Figure 15 SEM images of hematite conditioned with magnetic seeds (a, b), hematite conditioned with magnetic seeds in presence of 50 mT pre-magnetization (c), ematite coated with caustic starch (d), hematite treated with magnetic seeds and caustic starch under the 50 mT pre-magnetization (e, f) (MS: 180 mg/L magnetic seeds (100–200 nm), H: hematite (10–20 μm), S: 200 mg/L caustic starch)

5 Conclusions

1) The introduction of moderate magnetic seeds and pre-magnetization facilitated the agglomeration and flotation of fine minerals. The combined use of magnetic seeds and the polyacrylamide or starch under a low pre-magnetization particularly helped the agglomeration and flotation of coal or hematite slimes. Magnetic seeding agglomeration resulted in properly-sized agglomerates for fine flotation, but also causing certain loss in selectivity of the flotation due to the adsorption of magnetic seeds onto the gangue particles. In addition, a moderate pre-magnetization intensity was also critical to the fine flotation, and a high magnetic intensity decreased the positive effect of MSA due to the particular self-agglomeration of magnetic seeds themselves.

2) The mechanism of the magnetic seeding agglomeration mainly results from the magnetic interactions between the magnetic seeds and fine minerals under the pre-magnetization. The magnetic interaction is characteristic of the long range force. This long range magnetic force greatly enhanced the adsorption of magnetic seeds onto fine minerals and increased the magnetic susceptibility of fines, and then helped the agglomeration of fine particles in the presence of a low external magnetic field.

3) Organic flocculants, such as polyacrylamide (PAM) and starch, chemisorb on the surfaces of magnetic seeds as well as fine minerals, and act as the bridging media between magnetic seeds and minerals (including the coal and hematite). This bridge promoted the agglomeration of fine minerals, and then enlarged their apparent particle size right for flotation.

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(Edited by HE Yun-bin)

中文导读

磁种团聚对细粒矿物浮选的影响

摘要：磁种团聚（MSA），即通过向浮选矿浆中引入磁种和低强度预磁场，使细粒矿物发生团聚，然后将该团聚应用到煤泥、黄铁矿及赤铁矿细泥浮选中。通过粒度分析和浮选试验发现，磁种团聚提高了细粒浮选的回收率和黄铁矿矿泥的浮选速率，但对浮选选择性造成了一定损失；在聚丙烯酰胺和淀粉分别存在条件下，煤泥和赤铁矿细泥的团聚浮选皆得到了增强，磁种与絮凝剂之间存在着协同作用。机理分析和计算证实了磁种在矿物表面发生了吸附，该吸附降低了细粒磁种团聚所需要的临界磁场强度。原子力测试表明磁种与矿物颗粒间存在着一种长程作用力，该作用力有利于磁种在矿物颗粒上的吸附。红外分析表明，聚丙烯酰胺和淀粉在矿物和磁种表面皆发生了吸附，起到了桥联作用，强化了矿物粒子间的团聚。探讨了磁种团聚的表面特征（SEM）和吸附模型。

关键词：磁种；磁种团聚（MSA）；磁种浮选；细粒团聚；细粒浮选；絮凝

Foundation item: Project(51274256) supported by the National Natural Science Foundation of China

Received date: 2017-11-10; Accepted date: 2018-05-17

Corresponding author: WU Xi-qing, PhD, Professor; Tel: +86-13036795285; E-mail: xiqingwu@hotmail.com; ORCID: 0000-0002- 0368-1513