Effect of Calcium Ion on Coal Flotation in the Presence of Kaolinite Clay

Jan 8, 2016 - The settlement tests and AFM analysis show that the electrostatic repulsive force is fully suppressed by excessive Ca2+ .... and atomic ...
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Effect of Calcium Ion on Coal Flotation in the Presence of Kaolinite Clay Yaowen Xing,† Xiahui Gui,*,‡ and Yijun Cao‡ †

School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China Chinese National Engineering Research Center of Coal Preparation and Purification, Xuzhou 221116, Jiangsu, China

‡

ABSTRACT: The addition of electrolytes considerably increases the recovery of mineral matter in coal flotation. In this paper, the effect of calcium ion on coal flotation in the presence of kaolinite clay was investigated through microflotation experiments on a mixed coal−kaolinite system. The interaction behavior and mechanism between coal particles and kaolinite were indicated by a rotational viscometer, scanning electron microscope (SEM), settlement tests, and atomic force microscope (AFM). The microflotation results reveal that the combustible recovery increased dramatically as the concentration of calcium increased, and then reached a maximum of 85.36% at 5 mmol/L calcium concentration, after which the recovery declined. The ash content of the concentrate always increased with increasing calcium concentration. The rheology measurements and SEM analysis show that the heterocoagulation between coal and kaolinite does not occur when the Ca2+ concentration is lower than 5 mmol/L. A kaolinite coating on the coal surface is confirmed in the Ca2+ concentration range of 5−8 mmol/L, leading to a sharp increase in the concentrate ash content. The settlement tests and AFM analysis show that the electrostatic repulsive force is fully suppressed by excessive Ca2+ addition and that the attractive force dominates the kaolinite−particle interaction. These results can give a more detailed description of the flotation behavior of kaolinite in inorganic electrolyte solution and provide guidelines for the industrial application of saline water coal flotation. and the operation recovery of the fine clay particles were found to decrease. However, unlike entrainment, the mechanical coating behavior is mainly determined by the particles’ interaction force. A new method, the zeta potential distribution measurements of coal and clay suspensions, was proposed to study the interactions of clay with coal particles.5,12,13 The degradation of coal flotation performance was mainly attributed to the addition of montmorillonite, and not kaolinite. Bakhtiari et al. investigated the effect of caustic on the interaction of bitumen with various types of model clays in an oil sand extraction process.14 They found that the type of clays plays an important role in the interaction between clay and bitumen. Further, Honaker et al. calculated the interaction energies for a coal−montmorillonite/kaolinite system using the extended DLVO theory.15 Their results showed that the energy between coal and montmorillonite was negative and attractive, whereas the converse was true for kaolinite. However, Otaset al. claimed that the clay coating was governed by the van der Waals attraction and that the double-layer interaction played a secondary role.1 Thus, the best way to increase the flotation recovery in the presence of clays was the removal of these fine minerals by using a hydrocyclone in the coal plant. Recently, coal flotation using saline water has attracted considerable interest because of the scarcity of fresh water and the stringent regulations on the quality of discharged water.16−19 The coal flotation performance could also be intensified by using inorganic electrolyte. Three mechanisms for explaining the flotation results have been proposed, namely,

1. INTRODUCTION Microfine minerals, mainly clays (kaolinite, illite, and montmorillonite), have a detrimental effect on coal flotation because they form mechanical coatings on the coal surface and result in water entrainment. The coating makes the coal particles hydrophilic and prevents bubbles or collectors from adhering to the coal particles, leading to low flotation recovery and high ash content of the concentrate.1,2 Fine mineral particles are also easily dragged by the Prato boundary between air bubbles and thus degrade the froth product quality in flotation; this phenomenon is commonly referred to as water entrainment.3,4 In recent years, with the mechanization of coal mining operations to maximize coal production and utilization, raw coal contains high proportions of mineral matter, which pose a severe challenge to fine coal flotation.5 It is, therefore, important to develop an appropriate flotation separation process that can optimize the flotation efficiency, similar to the chemical process of multiphase reactions.6−8 Several studies have been conducted to explore the effect of fine clays on coal flotation over the last 60 years. Wang et al. comprehensively reviewed the mechanisms, contributing factors, and modeling of entrainment in flotation.9 They found that entrainment is generally strongly dependent on water recovery, which is considered to be the medium that transfers the mineral particles into the concentrate during the flotation process.9 Akdemir and Sonmez investigated the effect of particle size, impeller speed, and pulp density on the coal flotation and clay entrainment.10 Particle size was found to be the most important parameter affecting the recovery and entrainment behavior of coal and clay. Gui et al. also studied the dynamic characteristics of fine clay particles in coal flotation.11 With the progress of flotation, the flotation rate © XXXX American Chemical Society

Received: October 20, 2015 Revised: January 5, 2016

A

DOI: 10.1021/acs.energyfuels.5b02474 Energy Fuels XXXX, XXX, XXX−XXX

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M high-speed centrifuge, with a centrifuge speed of 3000 rpm, was used for the float-and-sink separation. The specification of the centrifuge tube was 4 × 250 mL; that is, four centrifuge tubes with volumes of 250 mL each were arranged symmetrically in the centrifuge. The centrifugal liquid was an organic solution prepared by mixing carbon tetrachloride, benzene, and tribromethane. This procedure has also been described in another study.22 Table 1 lists the

destabilization of hydration layers surrounding the particles, inhibition of bubble coalescence, and compression of the electrical double layer around the particles.17 However, most attention has been focused on the increased flotation recovery while the quality of the final product has been neglected. It should be noted that the use of saline water considerably increased the recovery of mineral matter (mainly kaolinite, illite, and montmorillonite) in coal flotation. As a result, the use of saline water further deteriorated the flotation selectivity.3,20,21Therefore, in addition to the corrosive effect of inorganic ions on the pipe line, the poor flotation selectivity is a main factor restricting the industrial application of saline water flotation. The influence mechanism of inorganic salt ions on the flotation behavior of clay particles must be studied further. The aim of the present study is to investigate the effect of calcium ion on coal flotation in the presence of kaolinite clay through microflotation experiments of a mixed coal−kaolinite system. The interaction behavior and mechanism between coal particles and kaolinite were analyzed by a rotational viscometer (RV), scanning electron microscope (SEM), settlement tests, and atomic force microscope (AFM).The results of this research are expected to provide a more detailed description of the flotation behavior of kaolinite in inorganic electrolyte solution and guidance for the industrial application of saline water flotation.

Table 1. Proximate Analysis of the Pure Coal Samples project

Mad, %

Aad, %

Vad, %

FCad, %

yield

2.07

4.10

38.85

54.98

results of the proximate analysis of the pure coal samples. The elemental compositions of the pure coal samples were analyzed by using the Elementar (vario MACRO cube, Germany), and the results are listed in Table 2.The size distribution of the pure coal samples and

Table 2. Element Compositions of the Pure Coal Samples element yield,%

S

C

H

N

O

0.50

78.83

4.425

1.47

14.78

kaolinite was tested using the Microtrac S3500 laser particle size analyzer, and the results are shown in Figure 2. The mean particle size

2. EXPERIMENTAL SECTION 2.1. Materials. Coal samples were collected from a coal preparation plant in Shandong Province, China. The compounds of the coal samples were determined via X-ray diffraction, and the results are shown in Figure 1.The main mineral matter in the raw coal is clay,

Figure 2. X-ray diffraction patterns of raw coal. of the sample was 45.14 μm. Analytical grade kaolinite powders with 99% purity were collected from Yongcheng City, Henan Province. The size distribution of kaolinite, also shown in Figure 2, was found to be much finer than that of coal. The mean particle size of kaolinite was only 3.22 μm. The enriched size fraction was 2−5 μm, and more than 80% of the particles were smaller than 10 μm. 2.2. Method. 2.2.1. Microflotation Tests. A laboratory plexiglass flotation cell (XFG) with an effective volume of about 100 mL was used. Pure coal and kaolinite were mixed at a mass ratio of 4:1. For each test, 8 g of kaolinite−coal samples and reagent grade calcium chloride was added into a 100 mL beaker with 50 mL of deionized water (DI, Academic Milli-Q system). The suspension was then dispersed by magnetic stirring for 24 h. The well-dispersed pulp and the rest of the 50 mL of deionized water were poured into the flotation cell for the microflotation tests at an impeller speed of 1800 rpm. After 2 min of the prewetting process, a common diesel collector and an octanol frother were added into the pulp in a stepwise manner. The dosages of the diesel and octanol were 0.8 and 0.3 kg/t, respectively.

Figure 1. X-ray diffraction patterns of raw coal. primarily kaolinite, followed by lesser amounts of quartz, pyrite, calcite, and dolomite. Kaolinite is easily broken into ultrafine powder in the flotation pulp, which decreases the flotation selectivity. To explore the effect of calcium on the coal flotation in the presence of kaolinite more precisely and provide a full insight into this phenomenon, the flotation samples were prepared by mixing pure coal and analytical grade kaolinite powders with 99% purity. First, lump coal samples were crushed using a laboratory hammermill to give particles smaller than 0.5 mm. Then, a wet screening test was carried out to collect −0.125 mm fine fractions. Finally, pure fine coal was obtained via a float-andsink experiment by using a 1350 kg/m3 density heavy fluid. A GL-21 B

DOI: 10.1021/acs.energyfuels.5b02474 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Experimental setup and schematic representation. The collector and frother conditioning periods were 2 and 1 min. At the end of the conditioning period, bubbles induced by an air pump were passed through the pulp, forming froth on the upper pulp surface. The flotation process was maintained for another 4 min. The froth products and tailings were dried and weighed for ash analysis. 2.2.2. Rheology Measurements. The rheological behavior of the flotation pulp was determined by using the HAAKE rotational viscometer 550 (Thermo Fisher Scientific, America). Rheology refers to the deformation and flow behavior of the fluid under sheer stress. It is well-accepted that the flotation performance is closely related to rheological behavior.23 The particle−particle or bubble−particle interaction could be identified by the rheological behavior. Rheology measurements were, therefore, carried out to probe the effect of calcium concentration on the interaction behavior between kaolinite and coal particles. A 10 mL aliquot of the coal−kaolinite suspension was taken for the rheology measurements from the upper level of the flotation cell directly before the aeration process. Diesel and octanol were added normally. The shear stresses were recorded under different rotation speeds from 0 to 750 rpm, corresponding to 0−800 s−1 shear rate. The viscosity was obtained by the plot of the shear stress against the shear rate. Each experiment was carried out at 7.0 pH and 25 °C and repeated thrice. The average value was presented for further analysis. 2.2.3. SEM Measurements. An SEM (FEI Quanta TM 250, America) equipped with an energy-dispersive X-ray spectrometer (EDS) was used to directly observe the kaolinite coating on the coal surface. A fraction of the flotation concentrate froth was collected and transferred carefully to a glass dish and naturally dried in order to avoid the damage to the surface structure. Images and elemental composition of the concentrate were obtained after it was coated with a layer of gold. It should be noted that EDS is only used for qualitative analysis to indentify whether the sedimentary particles on the coal surface are kaolinite. 2.2.4. Settlement Tests. The sedimentation experiments for the kaolinite suspension were conducted at different calcium concentrations in a 500 mL glass measuring cylinder. A 15 g portion of kaolinite was weighed and then poured into the measuring cylinder. The required inorganic salt solution with different calcium concentrations (0, 3, 5, 8, 12, 16, and 20 mmol/L) was added to a total volume of 500 mL. The cylinders were inverted for several times to ensure uniform mixing. The photos were taken for analysis after 24 h settlement. 2.2.5. AFM Measurements. The interaction force measurement between the coal substrate and kaolinite particle was analyzed under different calcium concentrations by AFM. The coal surface substrate was prepared as follows. First, the coal was subjected to cutting with a diamond saw. Then, rough grinding, fine grinding, lapping finish, and polishing were performed using a metallographic polishing machine (ZMP-2000). For the kaolinite particles used to prepare a colloid probe, a particle with diameter of approximately 50 μm was collected using a micromanipulator under an optical microscope and glued to

the apex of a triangular cantilever using epoxy resin. This procedure has been described in detail elsewhere.24All samples were then thoroughly rinsed with deionized water, alcohol, acetic acid, and deionized water sequentially. Finally, the samples were dried by blowing pure nitrogen gas over the surface in order to avoid oxidation. A Bioscope Catalyst AFM (Bruker, Germany) in contact mode was used for the force measurements. The experimental setup and schematic representation are presented in Figure 3. The measurement principle is described briefly as follows.25 The triangular cantilever itself or the sample surface is mounted on a piezocrystal, which allows the position of the probe to be moved with respect to the surface. Deflection of the cantilever is monitored by the change in the path of a laser light beam deflected from the upper side of the cantilever end by a photodetector. As the tip is brought into contact with the sample surface by the movement of the piezocrystal, its deflection is monitored. This deflection can then be used to calculate the interaction force between the probe and the sample. The long-range force was recorded during the approaching stage, while the pull-off force was recorded in the retraction stage. The pull-off force is generally defined by the adhesion force. All experiments were carried out at room temperature, 25 °C. However, it must be noted that, owing to the irregular geometry and surface heterogeneity of a coal or clay surface, the measurement results cannot be explained quantitatively with the standard DLVO (Derjaguin−Landau− Verwey−Overbeek) theory of colloid stability. Consequently, the force curves are only qualitatively compared, and we restrict our discussion to qualitative speculations. This procedure has previously been successfully used to study the coal particle−bubble interaction and the presence of the nanobubbles and/or slime removal on ZnS surfaces after methanol treatment.16,26

3. RESULTS AND DISCUSSION 3.1. Microflotation Tests. The effect of calcium concentration on coal flotation performance is presented in Figure 4. The combustible recovery increased dramatically as the concentration of calcium increased, and the recovery reached a maximum of 85.36% at 5 mmol/L calcium concentration, after which the recovery declined. From the perspective of flotation kinetics, the ultimate flotation recovery is determined by flotation rate constant k and flotation time t. In the present study, the flotation time was kept constant and k was the unique variable. It is well-established that the first-order flotation rate constant k can be expressed as27 k=

1 S bP 4

(1)

where P is the collection efficiency, Sb is the bubble surface area flux. Sb is related to the superficial gas velocity Jg by the relationship28 C

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Figure 4. Effect of calcium concentration on coal flotation.

Sb = 6

Figure 5. Effect of calcium concentration on the rheology of coal− kaolinite suspension.

Jg Db

(2)

where Db is the diameter of the bubble, Jg is defined as the volumetric air rate divided by the cross-sectional area of the flotation cell. The collection efficiency is the consequence of three subprocesses, namely, collision, attachment, and detachment in the overall collection process. The attachment energy barrier between bubble and particle decreases at high calcium concentration due to the compression of the electric double layer on the particle surface. Meanwhile, bubble coalescence in salt solutions is inhibited.17 Smaller bubbles lead to a high bubble surface area flux, froth stability, and collision probability, which, in turn, increase the flotation recovery. The subsequent decline in the recovery may be due to the excessive kaolinite coating on the coal surface seriously. Considering the hydrophilic nature of kaolinite particles, it should be expected that the hydrophobicity of coal surfaces may decrease. A low contact angle increased the wetting film stability and energy barrier, leading to a low flotation recovery. The ash content of the concentrate always increased with the calcium concentration. However, it should be noted that the ash content curve can be divided into three parts roughly based on the slope differences. When the Ca2+ concentration was lower than 5 mmol/L, the ash content increased slightly with Ca 2+ concentration (AB dotted line, as shown in Figure 4). When the Ca2+ concentration was within 5−8 mmol/L, the ash content increased considerably with a high slope (BC dotted line). Finally, a nearly flat region CD was achieved. It should be noted that the changes in the ash content were determined by the flotation behavior of kaolinite. As a result, the different variation tendencies of ash content at different Ca 2+ concentration ranges indicate that the conditions of kaolinite flotation behavior and coal−kaolinite interaction are always disparate. 3.2. Rheology Measurements. To identify the interaction between coal particles and kaolinite, the rheology measurements of the coal−kaolinite pulp were conducted. The effect of calcium concentration on the rheology of the coal−kaolinite suspension is shown in Figure 5. It is well-accepted that mineral pulp often exhibits the characteristics of Newtonian, Bingham, pseudo-plastic, or plastic fluid. For a Newtonian fluid, the slope of the shear stress−shear rate curve is constant and shows zero

yield stress; i.e., the viscosity is constant. For non-Newtonian fluids, the apparent viscosity always changes with an increase in the shear rate. As shown in Figure 5, the pulp shows Newtonian behavior at low calcium concentrations and pseudo-plastic behavior when the concentration increased to 5 mmol/L. The apparent viscosity decreased as the shear rate increased, which is referred to as shear-thinning behavior. In general, shearthinning behavior is the result of decomposition of the particle network structures under high shear in the pulp. At the quasistatic condition, aggregates may occur because of the attractive interparticle interaction force. These clusters were destroyed if a critical destructive hydrodynamic force was applied. The rheological behavior at 5 mmol/L calcium concentration indicates that heterocoagulation of kaolinite−coal or kaolinite−kaolinite/coal−coal aggregates existed in the pulp. With a further increase in the calcium concentration, only a small shear rate was needed to collapse the aggregates, which indicates that a large amount of clusters formed when the calcium concentration was higher than 8 mmol/L. However, the composition of the aggregates could not be identified here. 3.3. SEM Measurements. To confirm the composition of the aggregates in the flotation pulp, SEM measurements combined with EDS were conducted. SEM images and EDS analysis results of flotation concentrates at calcium concentrations of 2 and 8 mmol/L are shown in Figure 6. No obvious aggregate was observed at 2 mmol/L calcium concentration. This is consistent with the rheology analysis results because of the Newtonian behavior at low calcium concentrations. In contrast, a large number of fine particles were found on the coal surface (aggregates) at 8 mol/L calcium concentration, which is also consistent with the rheology analysis. The EDS analysis shows that O, Si, and Al were the major elements in the fine particles. As a result, the aggregates can be identified as the kaolinite coating. Along with the microflotation results, it can be said that the kaolinite coating makes the coal surface hydrophilic, leading to a slight decrease in the combustible recovery. It has been reported that the precipitation of hydrophilic calcium hydroxide can also reduce the surface hydrophobicity.29 However, the fact that calcium was not detected via EDS analysis indicates that calcium hydroxide precipitation did not occur under neutral conditions. It can also D

DOI: 10.1021/acs.energyfuels.5b02474 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 6. SEM pictures and EDS analysis of flotation concentrates: (a) 2 mmol/L calcium concentration; (b) 8 mmol/L calcium concentration.

Figure 7. Photographs of kaolinite suspensions at different calcium concentrations after 24 h settlement. The calcium concentration from left to right is as follows: 0, 3, 5, 8, 12, 16, and 20 mmol/L, respectively.

sedimentation experiments on kaolinite suspensions were conducted only in the present study. As shown in Figure 7, the suspension remained turbid at 0 mmol/L Ca2+ concentration. It indicates that the interaction force between kaolinite particles is repulsive and the suspension is highly stable. As the Ca2+ concentration increased, the supernatant became more and more clear. The electrostatic force between kaolinite particles is screened by Ca2+ addition. On the basis of this observation, it is speculated that heterocoagulation between coal and kaolinite will also occur at high Ca2+ concentration condition because of the increasing of zeta potential on the coal surface. 3.5. AFM Measurements. The effect of calcium concentration on the interaction force between coal and kaolinite is shown in Figure 8. More information could be obtained to deepen our understanding on the coal−kaolinite heterocoagulation by measuring the interaction force directly using AFM. Here, the force curves are only qualitatively compared, and we

be concluded that the slight increase in the ash content at low Ca2+ concentration may be attributed to water entrainment due to the foaming effect of Ca2+ and not the heterocoagulation of coal with kaolinite. The sharp increase in the ash content within 5−8 mmol/L Ca2+ concentration is due to the kaolinite coating on the coal surface. When the Ca2+ concentration was 8 mmol/ L, kaolinite adsorbed on the coal surface particles excessively. There was no residual site for the sedimentation of kaolinite. This also can be confirmed by the rheology analysis because the minimum shear rate for the decomposition of coal−kaolinite aggregates did not decrease further. A further increase in Ca2+ could not increase the concentrate ash content. 3.4. Settlement Tests Analysis. The photographs of kaolinite suspensions at different calcium concentrations after 24 h settlement are presented in Figure 7. The calcium concentration from left to right is as follows: 0, 3, 5, 8, 12, 16, and 20 mmol/L, respectively. Because it is difficult to verify a clear supernatant for an ultrafine kaolinite−coal suspension, the E

DOI: 10.1021/acs.energyfuels.5b02474 Energy Fuels XXXX, XXX, XXX−XXX

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4. CONCLUSIONS The effect of calcium ion on coal flotation in the presence of kaolinite clay was investigated through microflotation experiments of a mixed coal−kaolinite system. The interaction behavior and mechanism between the coal particles and kaolinite were investigated by a rotational viscometer (RV), scanning electron microscope (SEM), settlement tests, and atomic force microscope (AFM). The microflotation results show that the combustible recovery increased dramatically as the concentration of calcium increased, and the recovery reached a maximum of 85.36% at 5 mmol/L calcium concentration, after which the recovery declined. The ash content of the concentrate always increased with increasing calcium concentration. When the Ca2+ concentration was lower than 5 mmol/L, the ash content increased slightly with an increase in the Ca2+ concentration. When the Ca2+ concentration was within 5−8 mmol/L, the ash content increased considerably with a high slope. The rheology measurements and SEM analysis showed that the heterocoagulation between coal and kaolinite did not occur at low Ca2+ concentrations and that the addition of kaolinite to the coal flotation system should not be a serious concern. The sharp increase in the ash content within 5−8 mmol/L Ca2+ concentration is due to the kaolinite coating on the coal surface. AFM measurements were also demonstrated to be a powerful tool for qualitative study of clay coatings in coal flotation systems. The results of this research can give a more detailed description of the flotation behavior of kaolinite in inorganic electrolyte solution and provide guidelines for the industrial application of saline water flotation.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-13775989229. Fax: +86 0516 83884289. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 8. Effect of calcium concentration on the interaction force between coal and kaolinite: (a) approach curves; (b) retraction curves.

ACKNOWLEDGMENTS This research was supported by the National Nature Science Foundation of China (51574236) and the Fundamental Research Funds for Central Universities (2015XKMS095) for which the authors express their appreciation.

restrict our discussion of the interaction mechanism of the kaolinite coating to qualitative speculations. As shown in Figure 8, the interaction forces between coal and kaolinite changed from weak repulsive to strong attractive and increased monotonously with an increase in the Ca2+ concentration from 0 to 10 mmol/L, so did the jump-in distance and the corresponding adhesion force. When the Ca2+ concentration was 0 mmol/L, no jump-in was observed and the repulsive force corresponded to that in the case of no kaolinite coating. A sudden jump-in was found when Ca2+increased up to 3 mmol/ L. The electrostatic repulsive force was fully suppressed by excessive Ca2+ addition, and the attractive force dominated the kaolinite particle interaction. It should be noted that the Ca2+ concentration for the kaolinite coating in the AFM experiment (3 mmol/L) is lower than that in the microflotation tests and rheology measurements (5 mmol/L). This is probably due to the large amount of particle numbers and lower Ca 2+ adsorption density in the practical flotation suspension compared with that of the AFM fluid cell. AFM measurements were also demonstrated to be a powerful tool for qualitative study of clay coating in coal flotation systems.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b02474 Energy Fuels XXXX, XXX, XXX−XXX