Fuel 142 (2015) 235–242
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Understanding different roles of lignosulfonate in dispersing clay minerals in coal flotation using deionised water and saline water Di Liu, Yongjun Peng ⇑ School of Chemical Engineering, The University of Queensland, St Lucia, 4072 QLD, Australia
h i g h l i g h t s A small lignosulfonate addition mitigated clay surface coatings in deionised water. Electrostatic repulsion between coal and clay was enhanced in deionised water. In saline water, electrostatic repulsion was minimised. Steric repulsion was not sufficient to mitigate clay coatings at a low addition. A high lignosulfonate addition depressed coal flotation regardless of water quality.
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Article history: Received 23 August 2014 Received in revised form 26 October 2014 Accepted 30 October 2014 Available online 18 November 2014 Keywords: Coal flotation Clay mineral Lignosulfonate Dispersion Saline water
a b s t r a c t This study aims to understand the fundamental mechanism underpinning the different behaviour of ionic dispersants in fresh water and saline water observed in coal and mineral flotation and many other disciplines dealing with particles. Lignosulfonate D748 (LS), an anionic dispersant was used in this study to mitigate the clay slime coating in coal flotation using deionised water, and saline water of high ionic strength. In deionised water, the addition of a small amount of lignosulfonate enhanced coal flotation in the presence of clay minerals, but this beneficial effect was not observed in saline water. The addition of a large amount of lignosulfonate depressed coal flotation in both deionised water and saline water. The underlying mechanism was investigated by a range of techniques including flotation of pure coal, adsorption tests, and atomic force measurements (AFM). It was found that the enhanced electrostatic repulsion induced by a small amount of lignosulfonate in deionised water was responsible for the mitigation of clay coatings on the coal surface, resulting in improved coal flotation. In saline water, a low amount of lignosulfonate could not disperse clay minerals from the coal surface due to insufficient steric repulsion, and flotation improvement was therefore unable to achieve. The depression of coal flotation at a high lignosulfonate concentration in deionised water and saline water was attributed to the high adsorption on the coal surface, rendering it strongly hydrophilic regardless of the removal of clay coatings. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The Australian coal industry is facing the challenge of processing clayey coals as a result of the depletion of high quality coal deposits. Clay particles, usually less than 2 lm, may coat the coal surface, make it hydrophilic, prevent the adsorption of collectors and therefore depress coal flotation [1,2]. Oats et al. proposed that clay coatings occurred mainly as a result of strong van der Waals attraction, whilst the double-layer interaction played a secondary role [3]. In fact, clay minerals, such as kaolinite and bentonite,
⇑ Corresponding author. Tel.: +61 7 3365 7156; fax: +61 7 3365 4199. E-mail address:
[email protected] (Y. Peng). http://dx.doi.org/10.1016/j.fuel.2014.10.082 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.
are phyllosilicates which contain a continuous tetrahedral and octahedral layer. They usually have a complicated surface chemistry because of heterogeneity of charged edges and faces. The basal surface carries a permanent pH-independent negative charge, while the edge surface possesses a pH-dependent charge [4]. Although zeta-potential measurements suggest that at flotation conditions of pH 8, both coal and clay particles are negatively charged and hence the double-layer force between them is repulsive, calculation results based on colloid stability theory have suggested that van der Waals attraction is strong enough to overcome double-layer repulsion [3]. Recently, Wang et al. [5] detected the presence of clay coatings on coal surfaces by Cyro-SEM (scanning electron microscopy) and ascribed the low combustible recovery to clay coatings in coal flotation.
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To mitigate the adverse effect of clay minerals on mineral flotation, dispersants have been introduced to modify the colloidal interactions by creating electrostatic and/or steric repulsion. Seaman et al. [6] and Wei et al. [7] demonstrated that lignosulfonate improved copper and gold recovery by dispersing clay minerals in flotation using fresh water. Recently, we also found that a small amount of lignosulfonate improved coal flotation in the presence of clay minerals in deionised water [8]. However, the mechanism responsible for mitigating clay coatings on mineral surfaces by lignosulfonate has not been studied. On the other hand, it is interesting to find that lignosulfonate displayed a different behaviour in coal flotation in the presence of clay minerals when saline water was used. As demonstrated by Liu and Peng, lignosulfonate depressed coal flotation in saline water in comparison to its beneficial effect in deionised water [8]. In fact, a number of studies have revealed a similar phenomenon that anionic polymeric dispersants behave differently in flotation using fresh water and saline water. For instance, Peng and Seaman [9] examined the dispersion effect of carboxymethyl cellulose (CMC) in pentlandite flotation in the presence of serpentine minerals, and found that the effective use of CMC to mitigate the coating of serpentine minerals on pentlandite surfaces in the laboratory failed to be transferred into the plant. They attributed this disparity to water of different ionic compositions used in the laboratory and the plant. In the laboratory, fresh water was used while in the plant, saline water was applied. Wellham et al. [10] also demonstrated that CMC required an uneconomically high dosage to achieve satisfactory nickel flotation by dispersing serpentine slime particles in high ionic water. According to Vincent [11], the dispersion behaviour of polymers is strongly linked to their adsorption on colloidal particles, which is ionic strength dependent. Besides, polymer conformation can also be affected by salts [12], which may result in a structure change of adsorbed polymer layers formed on mineral surfaces, leading to a different dispersion and stabilisation mechanism. Studies on lignosulfonate conformation indicates that lignosulfonate takes a microgel model in solutions, with free charges on the surface and chargefree aromatic groups built up as the interior [13]. Yan et al. [14] pointed out that sulfonic groups and phenolic groups were mainly distributed on lignosulfonate surface, and carboxyl groups existed in the core. Meanwhile, lignosulfonate molecules are easy to aggregate in solutions [15,16]. The association among lignosulfonate molecules happens as a result of hydrogen bonding between carboxylic acid and phenolic hydroxyl groups [17]. The conformation of lignosulfonate used in this study was also investigated in the previous work by dynamic light scattering measurements [8]. It was found that at a low concentration, few lignosulfonate molecules aggregated in deionised water at weak alkaline conditions. With an increase in ionic strength, more and bigger lignosulfonate aggregates were formed in saline water at a same concentration. Apparently, saline water compressed the diffuse double layer around lignosulfonate molecules and facilitated randomly branched molecules to fold and associate with each other to form more and larger spherical aggregates. The change of lignosulfonate conformation in saline water may contribute to the different behaviour of lignosulfonate in coal flotation, which is the subject of this study. In this study, the dispersion effect of lignosulfonate on clay minerals was investigated in coal flotation and correlated with interfacial studies to establish the underlying dispersion mechanisms in deionised water and saline water. This study will help provide a new insight into understanding the role of anionic polymeric dispersants in dispersing clay minerals in the flotation process.
2. Materials and methods 2.1. Raw materials and reagents A problematic coal sample was obtained from a mine in the Bowen Basin in the state of Queensland, Australia. This is typically a low quality coal sample which is high in ash due to it being in a thinner section of the seam. The combustible feed grade of the coal is 82.9%. X-ray diffraction (XRD) analysis of the coal showed 3.9% kaolinite, 2.8% muscovite and 3.4% smectite as the main clay minerals [5]. In the laboratory, the coal sample was screened to 150 lm for flotation following the plant procedure. In the previous study, the slime coating of clay minerals on coal surfaces was detected by Cryo-SEM and was considered to be responsible for the low combustible recovery in the flotation [5]. In addition, a pure coal sample was obtained by hand-picking the coarse coal sample supplied from the same mine site. XRD analysis suggests that the purity of the coal sample is about 95%, containing no clay minerals. This pure coal sample was also crushed and sieved to below 150 lm for flotation tests and adsorption measurements to determine how lignosulfonate affected the surface property of coal and therefore coal flotation behaviour in the absence of clay minerals. Kaolinite was selected to represent the main clay mineral for the adsorption tests. It was purchased from Sibelco, Australia Limited Company, South Australia. Quantitative XRD analysis shows that this kaolinite sample contains 85 wt.% kaolinite with 11 wt.% muscovite and 4 wt.% quartz [18]. Other chemical reagents such as different salts (analytical grade) were purchased from Sigma– Aldrich. Deionised water and process water with a high salt content from the mine site were used in flotation tests. The composition of process water assayed by inductively coupled plasma mass spectrometer (ICP-MS) was listed in Table 1. The major ions are 2 Na+, Mg2+, Ca2+, SO2 4 , Cl , and CO3 . The pH and conductivity of the water are 8.5, and 9.32 mS/cm, respectively. Since traces of impurities such as residue collector and frother were present in the process water and may interfere with fundamental understanding for the flotation of the pure coal sample, adsorption measurements and AFM analyses, synthetic saline water with the same salt compositions of the process water by adding salts in deionised water was used. MIBC (Methyl Isobutyl Carbinol), 100% laboratory grade, and fresh Caltex diesel were used as frother and collector, respectively. Commercial lignosulfonate D748 was supplied by Borregaard– Lignotech USA and used in this study. The chemical properties of the lignosulfonate was well characterized by Ma and Pawlik [19]. It was reported that the average molecular weight of the lignosulfonate is 45 kDa and the proportion of Na, total sulphur, sulfonate sulphur, HPLC sugars, and Carboxylic groups in lignosulfonate is 7.0%, 6.5%, 6.2%, 1.0%, and 3.1% respectively. Fresh stock solutions of lignosulfonate were prepared at a concentration of 1 g/L daily. 2.2. Flotation tests After screening, the coal slurry was transferred to a 2.5 dm3 JK batch flotation cell and then conditioned with or without lignosulfonate, and collector (160 mL/t) for 7 min at 900 rpm of agitation. The flotation was then conducted following the addition of frother (110 mL/t). The solid percentage in the flotation cell was about 5% as used in the plant. In flotation, four concentrates were collected after a cumulative time of 1, 2.5, 5, and 10 min. Flotation was operated at an air flow rate of 3.0 dm3/min. Flotation froth was scraped every 15 s. The chosen deionised water, process water, or synthetic saline water was utilized in all stages of screening and flotation.
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D. Liu, Y. Peng / Fuel 142 (2015) 235–242 Table 1 The composition of the process water from mine site. Sample (mg/L)
Ca2+
K+
Mg2+
Na+
PO3 4
SO2 4
Cl
Carbonate
TSS
Recycled water
182
21
218
1037
0.1
3948
864
260
14
When saline water was used, the pH in flotation was constant, about 8.5, due to the buffer effect of the water. When deionised water was used, sodium hydroxide solutions were used to maintain pH at 8.5 during flotation. The flotation products were combusted at 815° for 2.5 h to obtain the ash and coal content [3]. 2.3. Adsorption isotherm tests Pure coal sample and kaolinite were used for the determination of adsorption isotherms. One gram sample was firstly conditioned for 15 min in 50 ml deionised water or synthetic saline water in the presence of different dosages of lignosulfonate. After conditioning, all solutions were centrifuged in order to separate the solid particles and determine the residue polymer concentration. It was noticed that centrifugation alone did not give a clear supernatant at the top of the centrifuge tubes and some fine particles were still present in solutions. Therefore, Millex filters with a pore size of 0.45 lm were used to filter any remaining particle from the solution and hence a clear supernatant solution was obtained. The concentration of lignosulfonate remaining in the supernatant was then determined using a Varian Cary 50 UV–vis spectrometer against a calibration curve. The absorbance of filtered and unfiltered standard lignosulfonate solutions were tested by UV–vis spectroscopy and it was found that there was no absorbance difference. To maintain a consistent experimental condition, standard lignosulfonate solutions used for calibration were also filtered through 0.45 lm filter unit. 2.4. Force analysis by AFM (Atomic Force Microscopy) Interaction force measurements were performed using a MFP3D Atomic Force Microscope (Asylum Research, Santa Barbara, CA) at room temperature. Since it is difficult to pick up the right sized spherical clay particle to perform surface force measurements, hydrophilic silicon nitride probe which is negatively charged at pH 8.5 [20], was selected to mimic clay particles which are also negatively charged at pH 8.5 [21]. The selected AFM cantilever, with a spring constant 0.27 N/m, was used. Besides, Graphite, (HOPG) SPI-1 Grade (10 mm 10 mm 2 mm), bought from SPI Supplies/Structure Probe, Inc. represents the hydrophobic coal particle. Both graphite and coal are negatively charged at pH 8.5 [21,22]. It was found that attraction forces existed between the probe and graphite at close contact, which could well serve the purpose of simulating the coating of clay minerals on the coal surface in the absence of lignosulfonate in flotation. Using the fluid cell, data was collected in the presence of lignosulfonate in either deionised or synthetic saline water at pH 8.5. 3. Results and discussion 3.1. Flotation of the problematic coal The effect of lignosulfonate on the flotation of the problematic coal was studied. Results would suggest whether lignosulfonate could be used to solve the industry problem. Fig. 1 shows the combustible and mineral matter (or ash) recovery in the presence of different lignosulfonate dosages using deionised water. For each dosage, three repeat tests were conducted and the average value
was calculated and used for plotting the curve. The experimental error for the same flotation tests was below 3%. As can be seen from Fig. 1(a), the addition of 25, 50, and 100 g/t lignosulfonate significantly increased combustible recovery. At the end of 10 min flotation, the combustible recovery was increased from 45% to 59%, 63%, and 54%, respectively. Fig. 1(b) shows the corresponding ash recovery increased slightly from 6.4% to 11.2%, 11.9% and 10.3%, with a similar product grade. As a result, the beneficial effect of lignosulfonate on coal flotation in the presence of clay minerals was evident in deionised water. It seems that a small amount of lignosulfonate mitigated the negative effect of clay minerals and therefore improved the coal flotation. 50 g/t lignosulfonate showed the most effective effect, which is in good agreement with observations by Seaman et al. [6] and Wei et al. [7] who used lignosulfonate to mitigate the negative effect of clay minerals in the flotation of copper and gold ores. Fig. 1(a) also shows that lignosulfonate may depress coal flotation at a greater dosage. As can be seen, the combustible recovery was reduced to 26% and 13% in the presence of 200 g/t and 500 g/t lignosulfonate, respectively. The problematic coal was then floated in process water. Results are shown in Fig. 2. Without the addition of lignosulfonate, the combustible recovery was much higher in saline water than that in deionised water. For example, in deionised water, at the end of 10 min flotation, 45% combustible recovery was achieved, whilst the combustible recovery was 74% in process water. This is consistent with the observations by other researchers indicating that saline water facilitated coal flotation as a result of the increased bubble-particle attachment efficiency [23–26]. Fig. 2 also shows that ash recovery and water recover were higher in process water than those in deionised water. It is known that entrainment is a strong function of water recovery [27]. Therefore, the increased water recovery should be responsible for the higher ash recovery in process water. Besides, Wang and Peng [28] identified that the recovery of ash in fine coal flotation was due to entrainment and entrapment and the use of saline water enhanced both entrainment and entrapment. The enhanced entrainment in saline water was due to the increased froth stability accompanied by higher water recovery, while the enhanced entrapment was due to the increased coal aggregation. Despite the increased ash recovery, the beneficial effect of process water is still evident because the quality of flotation concentrates in the end of flotation remained similar in process water and deionised water. However, when lignosulfonate was added in the flotation in process water, its beneficial effect was not observed. As shown in Fig. 2, the addition of lignosulfonate decreased both combustible recovery and ash recovery. The higher the lignosulfonate dosage, the lower the combustible recovery and ash recovery. It seems that lignosulfonate was adsorbed on both coal and mineral matter particle surfaces, rendered their surfaces hydrophilic and hence depressed their flotation in process water. In general, a small amount of lignosulfonate improved the flotation of the problematic coal containing clay minerals in deionised water but not in process water. It seems that a small amount of lignosulfonate was able to mitigate clay slime coatings from coal surface in deionised water but was not in process water. Meanwhile, a great amount of lignosulfonate depressed coal flotation in both deionised and process water. It seems that a great amount of lignosulfonate made coal surfaces hydrophilic regardless of the water
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Fig. 1. Combustible recovery (a) and ash recovery (b) from the flotation of the problematic coal as a function of water recovery in the presence and absence of lignosulfonate in deionised water.
Fig. 2. Combustible recovery (a) and ash recovery (b) from the flotation of the problematic coal as a function of water recovery in the presence and absence of lignosulfonate in process water.
quality used. The mechanism underpinning the different behaviour of lignosulfonate in coal flotation in deionised water and saline water was investigated below by using simpler mineral systems with well-defined conditions. 3.2. Flotation of the pure coal The flotation of the pure coal was conducted in both deionised and synthetic saline water to understand how lignosulfonate affected coal flotation in the absence of clay minerals as a result of the alteration of coal surface properties. Fig. 3 shows the combustible recovery as a function of water recovery in the absence and presence of lignosulfonate in deionised water. In the absence of lignosulfonate, the combustible recovery was high, about 86% indicating a good floatability of coal in the absence of clay minerals. However, the addition of lignosulfonate depressed coal
Fig. 3. Combustible recovery from the flotation of the pure coal as a function of water recovery in the presence and absence of lignosulfonate in deionised water.
Fig. 4. Combustible recovery from the flotation of the pure coal as a function of water recovery in the presence and absence of lignosulfonate in synthetic saline water.
flotation. As can be seen, the higher the lignosulfonate dosage, the lower the combustible recovery. 25 g/t, 50 g/t and 100 g/t lignosulfonate decreased the combustible recovery slightly from 82% to 78%, 75%, and 69%, respectively, but 200 g/t and 500 g/t lignosulfonate decreased the combustible recovery significantly to 29% and 12%, respectively. Obviously, lignosulfonate can modify the coal surface property and then depress coal flotation on its own right. However, the beneficial effect of a small amount of lignosulfonate in the flotation of the problematic coal in deionised water was indeed observed. This is probably because a small amount of lignosulfonate mitigated the negative effect of clay minerals outweighing its own adverse effect on coal flotation. On the contrary, a high amount of lignosulfonate depressed coal flotation regardless of the presence of clay minerals.
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A similar trend was found in the flotation of the pure coal with lignosulfonate using synthetic saline water. As shown in Fig. 4, 25 g/t, 50 g/t and 100 g/t lignosulfonate decreased the combustible recovery slightly from 91% to 88%, 84% and 77%, respectively in saline water, but 200 g/t and 500 g/t lignosulfonate significantly decreased the combustible recovery to 42% and 12%, respectively. 3.3. Adsorption isotherm tests The adsorption isotherms of lignosulfonate on kaolinite and coal were determined to explain whether the different behaviour of lignosulfonate in the flotation of the problematic coal using deionised water and process water was related to surface adsorption. Results are shown in Fig. 5. The particle size of the substrates is different. Therefore, it may not be possible to directly compare the adsorbed amount of lignosulfonate onto different substrates. However, it is possible to compare the adsorbed density of lignosulfonate onto the same substrate in deionised water and saline water. Fig. 5 shows that lignosulfonate displayed the lowest affinity toward kaolinite in deionised water. While there was a slight increase in adsorption density when lignosulfonate concentration was increased to 50 mg/L, the adsorption density levelled off at about 0.3 mg/g when lignosulfonate concentration was further increased to about 400 mg/L. In contrast, there is a significant increase in adsorption density of lignosulfonate on kaolinite surface in saline water. A steep increase in adsorption density can be found when lignosulfonate concentration was increased to 100 mg/L, followed by a slower growth rate. The maximum amount of the adsorbed lignosulfonate reached 2.8 mg/g at 500 mg/L lignosulfonate in saline water. As a result, the adsorption density of lignosulfonate on kaolinite increases with increasing the ionic strength, suggesting that electrostatic forces largely control the adsorption process [29–32]. The low adsorption affinity in deionised water may arise from the electrostatic repulsion between kaolinite and anionic lignosulfonate. In the presence of salts, however, charges are screened, and hence electrostatic repulsion between kaolinite and lignosulfonate is reduced. At the same time, the attractive forces, such as van der Waals force, hydrogen bonding attraction and metal cation-p interactions start to play a dominant role and consequently result in an increase of adsorption density on the kaolinite surface [33]. The growth of the adsorption density in saline water may contribute to the reduced ash recovery in problematic coal flotation in process water. Fig. 5 also shows that lignosulfonate displays a similar trend of adsorption density on the coal surface in deionised water and synthetic saline water. The adsorption density increased gradually with an increase in lignosulfonate concentration and levelled off when the concentration reached 350 mg/L. Compared with
Fig. 5. Adsorption isotherms of lignosulfonate on kaolinite and pure coal in deionised water and synthetic saline water at pH 8.5.
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deionised water, saline water promoted a little higher adsorption amount at the same lignosulfonate concentration as a result of the minimised electrostatic repulsion between coal and lignosulfonate. The maximum adsorption amount of lignosulfonate on coal surface was 2.8 and 1.9 mg/g at 500 mg/L lignosulfonate concentration, in saline water and deionised water, respectively. The adsorption of lignosulfonate on coal particles may mainly arise from the hydrophobic bonding between the polymer aromatic skeleton and coal surface, which is primarily independent of the ionic strength. In fact, studies on adsorption mechanism of polymeric dispersants, such as dextrin, polystyrene sulfonate and hydroxypropyl cellulose, on coal surfaces also suggested that hydrophobic interactions dominated the adsorption process [34,35]. The high adsorption density of lignosulfonate may render coal surface more hydrophilic at a high lignosulfonate concentration resulting in the depressed coal flotation as indicated in the flotation section. In this study, the adsorption isotherms were further fitted to the Langmuir and Freundlich models. The Langmuir equation can be written in a linear form, i.e. [33]: KF
1 1 1 1 ¼ þ q qmax K L C qmax where q is the adsorbed amount (mg/g), qmax is the maximum amount adsorbed (mg/g), KL is the equilibrium constant (L/mg), and C is the concentration of lignosulfonate in solution (mg/L). On the other hand, the Freundlich equation is an empirical nonlinear equation for a non-ideal and heterogeneous adsorption process and expressed linearly as:
1 log10 q ¼ log10 K F þ log10 C n where KF is the Freundlich constant and 1/n is a heterogeneity factor. Based on the above equations, the fittings were calculated and listed in Table 2. As can be seen, the correlation coefficients of R2 with the fitted Langmuir equation are all greater than those with Freundlich equation. Apparently, the adsorption of lignosulfonate on coal and kaolinite surfaces preferably adapted to monolayer adsorption in deionised water and saline water. Although lignosulfonate molecules and aggregates coexist in both solutions, single molecules are in the majority [8] and would probably take the priority of adsorbing on both coal and kaolinite particles. It is worth mentioning that when polymer concentration is lower than 5 mg/L which equals to about 100 g/t lignosulfonate in the previous flotation tests, lignosulfonate adsorption amount on either coal or kaolinite is actually quite low and adsorption isotherms are far from reaching equilibrium. At such a low lignosulfonate concentration, adsorption sites on coal and kaolinite surfaces cannot be fully occupied, and lignosulfonate could not generate enough steric layer on both coal and clay particles. Under this circumstance, steric repulsions could not be initiated to separate clay particles from coal surfaces. The insufficient steric repulsion combined with the minimised electrostatic repulsion in saline water may lead to a depression of problematic coal flotation in process water at a small amount of lignosulfonate. On the other hand, the beneficial effect by a small amount of lignosulfonate was observed in the flotation of problematic coal using deionised water, and this may arise from the enhanced electrostatic repulsion between kaolinite and coal particles in the presence of lignosulfonate. Although steric repulsion is limited at a small dosage of lignosulfonate, the adsorption of the negatively charged lignosulfonate would enhance the electrostatic repulsive force between kaolinite and coal particles, resulting in dispersed clay particles from coal surfaces and consequently an improved flotation of the problematic coal in deionised water. The particle interaction forces in the
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Table 2 Fitting results of the adsorption of lignosulfonate on kaolinite and pure coal surfaces in deionised or synthetic saline water. Mineral
Water
Langmuir equation
Freundlich equation
Fitting equations
R
2
Fitting equations
R2
Kaolinite
Deionised water Saline water
1/q = 136.06(1/c) + 1.6085 1/q = 22.038(1/c) + 0.2805
0.961 0.998
log10(q) = 0.443(log10c) 1.4677 log10(q) = 0.5459(log10c) 0.8851
0.811 0.903
Pure coal
Deionised water Saline water
1/q = 21.334(1/c) + 0.616 1/q = 16.042(1/c) + 0.3911
0.989 0.995
log10(q) = 0.4818(log10c) 0.9326 log10(q) = 0.4745(log10c) 0.7587
0.973 0.935
absence and presence of lignosulfonate were then measured by AFM. 3.4. Interaction force measured by AFM The interaction between kaolinite and coal in deionised water and saline water in the absence and presence of lignosulfonate was studied. AFM enables the direct measurement of interaction forces experienced between a tip and graphite, and may be used to demonstrate the effect of lignosulfonate on modifying the clay-coal particle interaction forces. Initially, the interaction forces between the silicon nitride tip and graphite were analysed without the addition of lignosulfonate. In Fig. 6, data for the normalized interaction force on approach between the AFM tip and graphite in deionised water and synthetic saline water at pH 8.5 are shown. In deionised water, upon approach, there was a little repulsive force between two surfaces that started from a separation distance of about 10 nm to 3 nm. This is probably because both the tip and graphite particle are negatively charged at this pH [20,23], resulting in an electrostatic repulsion. With the tip further approaching, repulsion disappeared and instead, stronger attraction was observed between the tip and graphite. In contrast, the repulsive force existed in deionised water was not observed in saline water as a result of minimised electrostatic repulsion in the presence of salts. This is consistent with the reports in literature [36]. There was an overall attraction between the tip and graphite within the separation distance of 30 nm in saline water. At close contact, saline water promoted a similar attractive interaction as deionised water. It is concluded that in the absence of lignosulfonate, attractive forces were present between the tip and graphite surface in both deionised and saline water. According to Oats et al. [3], at a short separation distance, the van der Waals attraction is dominant so that the double-layer repulsion is overcome, leading to clay coatings on coal surfaces. In Fig. 7, force-distance profile curves are plotted on approach in the presence of 1 mg/L lignosulfonate in both deionised and saline water. In the presence of 1 mg/L lignosulfonate which equals to 25 g/t lignosulfonate in flotation tests, it can be found that attraction between the AFM tip and graphite was no longer observed in
Fig. 6. Approach force curves between a silicon nitride tip and graphite substrate in deionised water and synthetic saline water at pH 8.5.
Fig. 7. Approach force curves between a silicon nitride tip and graphite substrate at pH 8.5 in the presence of 1 mg/L lignosulfonate using both deionised and synthetic saline water.
deionised water. Instead, a repulsive force arose within a short separation distance of 3 nm. Although there was a slight decrease in the repulsion force with the tip further approaching, the addition of 1 mg/L lignosulfonate successfully produced repulsions between the tip and graphite surfaces, resulting in a repulsive barrier to their approach. Since the adsorption density with the corresponding lignosulfonate is low in deionised water, this repulsive force should be generated by the enhanced electrostatic repulsion as discussed in the adsorption measurement section. As a result, the attractive van der Waals force at the close contact was overcome and the initial easy approach of tip to graphite surface was disabled in deionised water in the presence of lignosulfonate. In saline water, however, a strong attractive force at a separation of 2.5 nm was still observed, and there was a slight decrease in the overall attraction in the presence of lignosulfonate. Obviously, a small amount of lignosulfonate failed to generate enough repulsive forces to offset van der Waals forces in saline water. This result is consistent with the depressed coal flotation in the presence of a small amount of lignosulfonate in process water. As discussed earlier, the adsorbed lignosulfonate could not generate strong electrostatic repulsion between coal and kaolinite particles because of the screened electrical double layer in the presence of salts. Herb and Ross [37] also pointed out that the saline water with high ionic strength could suppress the range of electrostatic repulsion, and although electrostatic repulsion still existed and took part in the interactions, the main cause of stability came from the steric hindrance of the adsorbed lignosulfonate dispersant. However, as found in the adsorption isotherms section, a small amount of lignosulfonate displayed a low adsorption density on both coal and clay surfaces, and therefore could not produce sufficient steric repulsion to separate clay and coal particles. On the other hand, the adsorbed lignosulfonate on the clay coated coal surface may make it more hydrophilic and therefore decrease the flotation of the problematic coal in process water. When lignosulfonate concentration was further increased to as high as 15 mg/L which equals to higher than 200 g/t lignosulfonate in flotation tests, attraction forces between the AFM tip and
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Acknowledgements The authors are thankful for the financial support of the Australian Coal Industry’s Research Program (ACARP) as well as the discussions and suggestion from technical advisers at Xstrata and BHP Billiton Mitsubishi Alliance (BMA). Besides, the help from Dr. Elena Taran at Australian National Fabrication Facility (ANFF) in the University of Queensland on AFM measurement and analysis is greatly appreciated. References Fig. 8. Approach force curves between a silicon nitride tip and graphite substrate at pH 8.5 in the presence of 15 mg/L lignosulfonate using both deionised and synthetic saline water.
graphite cannot be observed any more as shown in Fig. 8. Instead, strong repulsive forces were produced in both deionised and saline water. As can be seen, repulsive forces produced in deionised water were stronger than that in synthetic saline water. While the repulsive interaction was observed to start at a separation distance of 11 nm in deionised water, the repulsive interaction happened only within a separation distance of 5 nm in saline water. As shown earlier, the adsorption amount of lignosulfonate on coal and clay surfaces increased with increasing the lignosulfonate concentration (lower than 15 mg/L). As a result, the strong repulsion produced in deionised water was caused by the increased electrostatic and some steric repulsion. The weaker repulsive forces generated by 15 mg/L lignosulfonate in saline water are mainly due to the increased steric repulsion. Indeed, a higher amount of lignosulfonate is required in saline water than in deionised water to achieve satisfactory separation of clay particles from coal surfaces with the minimised electrostatic repulsion. Pawlik, et al. [38] showed that at higher ionic strength, the initial CMC concentration at which the re-stabilisation of serpentine minerals began was much higher. On the other hand, although a large amount of lignosulfonate could generate strong repulsion to mitigate clay coatings in both deionised and saline water, the coal surface was made strongly hydrophilic resulting in the depression in flotation no matter whether clay coatings were removed from the surface. 4. Conclusions Lignosulfonate improved the flotation of the problematic coal containing clay minerals in deionised water at a low concentration but depressed the flotation in saline water. The different roles of lignosulfonate were explored by conducting flotation of pure coal in the absence of clay minerals, adsorption measurements and AFM tests. It was identified that different dispersion mechanisms of lignosulfonate applied in water of different ionic strength, leading to various flotation results. In deionised water, electrostatic repulsion was enhanced by a small amount of lignosulfonate between coal and clay minerals, which mitigated clay slime coatings from coal surfaces and improved coal flotation. With increasing the lignosulfonate concentration, more lignosulfonate was adsorbed on coal surfaces and rendered them hydrophilic resulting in the depression of coal flotation. In saline water, a low amount of lignosulfonate could not generate enough steric repulsion to separate clay particles from coal surfaces. Meanwhile, the adsorbed lignosulfonate made coal surfaces more hydrophilic, exacerbating the depression of coal flotation. Although a large amount of lignosulfonate may produce strong repulsion between coal and clay surfaces, the reduced coal surface hydrophobicity depressed coal flotation even when clay coatings were removed from coal surfaces.
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