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The effect of ultra-fine coal on the flotation behavior of silica in subbituminous coal reverse flotation
Accepted Manuscript The effect of ultra-fine coal on the flotation behavior of silica in subbituminous coal reverse flotation
Yonggai Li, Jiawei Li, Peng Chen, Jianzhong Chen, Lijuan Shen, Xiangnan Zhu, Gan Cheng PII: DOI: Reference:
S0032-5910(18)30855-6 doi:10.1016/j.powtec.2018.10.014 PTEC 13788
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
27 July 2018 6 October 2018 10 October 2018
Please cite this article as: Yonggai Li, Jiawei Li, Peng Chen, Jianzhong Chen, Lijuan Shen, Xiangnan Zhu, Gan Cheng , The effect of ultra-fine coal on the flotation behavior of silica in subbituminous coal reverse flotation. Ptec (2018), doi:10.1016/j.powtec.2018.10.014
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ACCEPTED MANUSCRIPT The effect of ultra-fine coal on the flotation behavior of silica in subbituminous coal reverse flotation Yonggai Li a, *, Jiawei Li a, Peng Chen a, Jianzhong Chen Xiangnan Zhu b, Gan Cheng
a,
*, Lijuan Shen a,
a, c
a Key Laboratory of Coal Processing and Efficient Utilization (M inistry of Education), School of Chemical
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Engineering and Technology, China University of M ining and Technology, Xuzhou, Jiangsu 221116, China
Qingdao, Shandong 266590, China
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b College of Chemical and Environmental Engineering, Shandong University of Science and Technology,
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c College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China
Abstract: In this study, X-ray Photoelectron Spectroscopy (XPS) was used to analyze the
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elements on the surface of subbituminous coal, and reverse flotation was introduced to
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float subbituminous coal. The effect of ultra-fine coal on the flotation behavior of silica in coal reverse flotation was discussed by comparing the flotation performance with two
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different artificial mixed flotation feedings, with ultra-fine coal (UFC, -45μm) or without
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ultra- fine coal. The comparison results showed that the existence of UFC reduced the flotation rate and the recovery of silica, as well as the separation efficiency of
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subbituminous coal reverse flotation. Compared to the flotation performance of feeding
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with UFC, the concentrate ash content of feeding without UFC was lower (9.18%), and the separation efficiency was higher (68.69%). Total Organic Carbon Analyzer was used
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to test the adsorption quantity of collector by coal and silica to explain the mechanism. Conclusions were obtained that ultra- fine coal particles could adsorb more collector due to their larger specific surface area, leading to less collector left in the pulp to float silica particles. Selective flocculation reverse flotation method could increase the separation efficiency of coal reverse flotation with appropriate dosage flocculant.
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ACCEPTED MANUSCRIPT Key words: ultra-fine coal (UFC); coal reverse flotation; flotation rate; separation efficiency; selective flocculation
* Corresponding authors.
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E-mail address: [email protected] (Y. Li), [email protected] m (J. Chen).
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1. Introduction
Flotation is a common method to separate fine minerals. Coal has natural hydrophobicity.
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Conventional flotation could separate coal and gangue minerals efficiently by using kerosene or diesel as collector to increase the hydrophobicity of coal. The hydrophobic
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coal particles are scraped out as froth product while hydrophilic gangue minerals stay in
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the pulp. However, some low rank coals, such as subbituminous coal, have poor
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hydrophobicity because of high water content and a lot of oxygen-containing functional groups on the surface, which makes it difficult to be separated using conventional
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flotation [1-3]. Reverse flotation used in this research could take advantage of the poor surface hydrophobicity of low rank coal and achieve the objective of separation, turning
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the disadvantage to advantage [4]. During the process of reverse flotation, gangue
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minerals are scrapped out as froth product while coal particles stay in the pulp [4]. The flotation of fine or ultra- fine mineral is always a challenge in the area of mineral processing [5-9]. In many cases, there exists some bad effect caused by fine particles’ physical characteristics, such as low collision probability between fine particles and bubbles, slow flotation rate, hydraulic entrainment or mechanical entrapment, etc., as well as some problems resulted from the chemical characteristics of fine particles, such as large solubility, the adsorption and transformation of dissolved components on the
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ACCEPTED MANUSCRIPT surface of minerals, slime coating, agglomeration among fine particles and the nonselective adsorption of fine particles on reagents, etc [10]. Ni et al. [11] investigated the effect of slimes on the flotation recovery and kinetics of coal particles using flotation mixtures including slime particles and coal particles within 0.5–0.25mm, 0.25–0.125mm
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and 0.125–0.074mm size ranges, respectively and found that both the flotation recovery
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and rate of coal particles decreased with the increase in the mass propor tion of slime
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particles under the mixed flotation conditions, especially coarse coal particles. Ding [10] did some research about the flotation of feldspar and silica with different particle size and
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found that fine feldspar has weak inhibiting effect on coarse feldspar, but it could activate
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silica obviously. Selective flocculation is a very common method in fine mineral separation. Honaker et al. [12] has conducted research about the effect of pH on the
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selective flocculation of -10μm coal, and found that the coal’s selective cohesion
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happened during pH 8.5-9.5. Cai et al. [13] used selective flocculation to make low rank coal (ash content < 3%) and the combustible recovery could reach 90%. Coal reverse
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flotation is a concept opposite of conventional direct flotation. During the process of coal
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reverse flotation, minerals float to the froth product while coals are depressed and remain in the pulp [2]. Some research has been done about reverse flotation, such as the effect of
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kinds and dosage of collector or depressant, the pH of pulp, the conditioning time, the particle size, etc. on the performance of reverse flotation [14-22]. However, there are few papers about the effect of ultra- fine coal (UFC, -45μm) in coal reverse flotation. This research will focus on the effect of UFC particles on the flotation behavior of silica and separation efficiency of coal reverse flotation using two different feedings (feeding with
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ACCEPTED MANUSCRIPT UFC or feeding without UFC). The related mechanism was explained and selective flocculation reverse flotation was introduced. 2. Experiment materials and methods 2.1 Experiment materials
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The coal sample in this research is subbituminous coal (Peabody Energy, US). Coal
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sample was crushed to minus 200μm. The approximate analysis of subbituminous coal is: moisture 27.72%, ash content 8.81%, volatile component 46.62%, fixed carbon 16.85%,
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and sulphur content 0.72%. The particle size composition of subbituminous coal is shown in Table 1. It is can be seen that the ash content of the subbituminous coal is 8.81% which
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is very low. With the decrease of particle size, the ash content of coal increases. The main
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fraction is - 45μm with ash content 11.16%. The silica (ASTM quartz c-778, ebay) used in this research is close to pure minerals (98.8% purity). Particle size composition of
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silica is, -1.52μm 10%, -7.03μm 50%, and the mean particle size is 10.42μm.
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Subbituminous coal with different size composition (-200μm or -200+45μm) and silica was mixed with a ratio of 7:3 as artificial flotation feedings. This two different feedings
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are called feeding with UFC and feeding without UFC, respectively. Feeding without
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UFC was obtained by sieving out the UFC using lab wet screening method. The oversize product was dried in low temperature in the drying oven. 2.2 Chemical reagents The collector for silica in subbituminous coal reverse flotation is compound cationic ammonium Lilaflot D817M (AKZO NOBEL). Dextrin (Fisher Scientific) is used as depressant for coal. The collector and depressant used in the test are 1% solution prepared with deionized water. Methyl Isobutyl Carbinol (MIBC, Aladdin Bio
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ACCEPTED MANUSCRIPT Technology) was used as frother. The flocculant is non- ionic (Changshu Yiqing Environmental Protection Technology Co., Ltd.). The relative molecular weight of the flocculant is 10-12×10^6 and the electric density is lower than 5. Hexametaphosphate (chemically pure) was used as dispersant. The solution concentration of flocculant and
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dispersant used in the test are 0.05% and 1%, respectively. Tap water was used during the
2.3 Experimental methods
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2.3.1 X-ray Photoelectron Spectroscopy (XPS) study
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whole experimental process.
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X-ray Photoelectron Spectrometer (XPS, Thermo, Escalab 250X1) was used to analyze the elements. The data processing (peak fitting) was performed with XPS Peakfit
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software. The binding energies were corrected by setting the C1s hydrocarbon
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(−CH2−CH2− bonds) peak at 284.9 eV.
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2.3.2 reverse flotation
Flotation cell used in the reverse flotation test is lab-used 0.5L single flotation cell
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(RK/FD II). The pulp was in natural pH, and the inflating volume was open to the maximum. The flotation feeding was made up of 20g coal and 8.5g silica, and the pulp
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density was 57g/l. The conditioning speed was 1850rpm during the whole process. The slurry was initially conditioned for 3 min to ensure complete dispersion o f the samples. The pH of the pulp before the addition of dextrin and amine ranged from 6.83 to 6.97. Then 2kg/t depressant and 20ppm frother were added with 3min and 1min conditioning, respectively. At last the air valve was opened as soon as 1kg/t collector was added. The froth product was scrapped for 5 min. The products at different time points (20s, 40s, 80s,
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ACCEPTED MANUSCRIPT 120s, 200s) were also obtained. The concentrates (sink products) and tailings (froth products) were collected, dried, weighed and tested to calculate silica recovery rate and reverse flotation separation efficiency. The equations are as follows: 𝑅𝑆 (%) = 𝑀𝑇 ∗ 𝐴 𝑇 /[𝑀𝐹 ∗ 𝐴𝐹 ] ∗ 100
(1) (2)
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𝜂(%) = 100 ∗ 𝑀𝐶 ∗ (𝐴𝐹 − 𝐴𝐶 )/[𝑀𝐹 ∗ (100 − 𝐴𝐹 ) ∗ 𝐴𝐹 ∗ 100
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Among which,
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RS is the silica recovery in tailing, %; 𝜂 is separation efficiency, %;
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Mc, MT and MF are the weight of the concentrate, tailing and feeding, respectively, g; Ac, AT and AF are the ash content of the concentrate, tailing and feeding by weight,
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respectively, %. 2.3.3 Selective flocculation-reverse flotation
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Selective flocculation-reverse flotation was also conducted in the 0.5L flotation cell. The
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feeding is artificial mixture with 20g subbituminous coal and 8.5g fine silica. Pour the
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feeding pulp into flotation cell after being evenly stirred, and add water to 0.5L. The flotation steps are as follows: the pulp was conditioned with a speed of 1890r/min for
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3min initially, then 1kg/t sodium hexametaphosphate and 1kg/t dextrin were each
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conditioned in the slurry for 3min. Next the rotate speed was decreased to 1000r/min. Then moderate flocculating and 10ppm MIBC were each conditioned for 5min and 1min. At last 1kg/t Lilaflot D817M was added as collector when inflating, and the scrapping time was 5 min. 2.3.4 Adsorption test The total organic carbon analyzer was used to test the content of element nitrogen in the solution before and after the adsorption of minerals to calculate the adsorption quantity of
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ACCEPTED MANUSCRIPT collector amine by silica. Dilute the 1% Lilaflot D817M solution to 10-400mg/l. 20ml dilution was put into the test tube to measure the content of total nitrogen. The linear relationship between the concentration of Lilaflot D817M and the content of total nitrogen was obtained and the curve could be taken as the standard curve.
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The adsorption steps are as follows: firstly, put 1g subbituminous coal or silica into one
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100ml beaker and add deionized water to 80ml. After sufficient stirring, a certain amount
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of 1% Lilaflot D817M was put in the beaker and the volume was set to 100ml. Then put the beaker on the magnetic stirrer for 5min to make the adsorption sufficient. When
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adsorption equilibrium achieved, pour the pulp solution into the pressure filter with low
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speed qualitative filter paper to get the filtrate. At last, take 20ml filtrate into the test tube to measure the total nitrogen content using TOC instrument. Compare the result with the
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standard curve to get the adsorbing capacity by calculating the difference.
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3. Result and discussion
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3.1 Result of XPS
According to some papers, C1s peak of the combination form is C-C/C-H, C-O, C=O and
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O-C=O under 284.9eV, 286.14eV, 287.22eV, 288.75eV, respectively [23]. The main
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hydrophobic functional groups are C-C and C-H. The main hydrophilic functional groups are C-O, C=O, or O-C=O [24]. XPS Peak 4.1 software was used to split and simulate C1s peak. C1s peaks and the fitting curves of subbituminous coal is illustrated in Fig. 1. The area under different peaks was calculated to obtain the relative amount of C1s with different bond forms. The result is shown in Table 2. According to Fig. 1 and Table 2, the content of alkyl groups on subbituminous coal surface is 74.49%, and the content of all kinds of the oxygen-containing functional group is 25.5%. Xia has analyzed the surface
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ACCEPTED MANUSCRIPT properties of fresh anthracite using XPS, founding that the content of alkyl groups on fresh anthracite surface is 88.78%, and the content of oxygen-containing functional groups is 11.22% [25]. It can be seen that the content of oxygen-containing functional groups of subbituminous coal sample in this study is much higher than that of fresh
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anthracite, which is much easier to float. Combining the previous experiment results, the
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recovery of subbituminous coal using traditional flotation was very low, but better
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separation result could be obtained using reverse flotation method. Therefore reverse
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flotation was used to separate subbituminous coal next [2-4]. 3.2 The result of reverse flotation
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Fig. 2 shows the result of flotation rate of silica in subbituminous coal reverse flotation.
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According to Fig. 2, with the increase of time, the recovery of silica in tailing increases,
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but the increasing rate diminishes gradually. The largest recovery rate is obtained at about 120s. When it is at 200s, the silica recovery of the feeding without UFC is 82.76% while
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that of feeding with UFC is only 73.02%. The silica recovery rate of feeding without UFC is always higher than that of feeding with UFC when it is at the same time point. As
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the UFC has been removed from feeding without UFC, therefore the only difference
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between these two feedings is whether the UFC exists. According to the experiment result in Fig. 2, the existence of UFC has negative effect on the recovery of silica. This might be because of the much more adsorption of collector by UFC leading to the decrease of ammonium salt concentration in the pulp. According to the first-order kinetics rate model [26, 27]： 𝜀 = 𝜀∞ ∗ [1 − 𝑒𝑥𝑝(−𝑘𝑡)]
(3)
Among which: ε is the silica recovery in tailing, %; ε∞ is the silica recovery, %; k is the
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ACCEPTED MANUSCRIPT flotation rate constant. The calculation result of silica flotation rate constant is illustrated in Fig. 3 and Table 3. The flotation rate constant of feeding without UFC is 0.017, which is 0.006 higher than that of feeding with UFC. It means that the existence of UFC could deteriorate the flotation rate of silica in subbituminous coal reverse flotation.
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The separation efficiency of subbituminous coal reverse flotation is shown in Fig. 4. With
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the increase of time, the separation efficiency of subbituminous coal reverse flotation also
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increases, which reaches the biggest at 120s. During the whole separation process, the separation efficiency of feeding with UFC is higher than that of feeding without UFC.
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When it is at 200s, the separation efficiency of feeding without UFC reaches 68.69%,
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which is 3.53% higher than that of feeding with UFC (65.16%). It means that the existence of UFC reduces the separation efficiency of subbituminous coal reverse
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flotation.
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Table 4 shows the comparison result of clean coal quality with different flotation feedings. The clean coal recovery of feeding without UFC is 5.73% lower than that of
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feeding with UFC. But the clean coal ash content of feeding with UFC is 5.13% lower
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than that of feeding without UFC, which are 9.18% and 14.31%, respectively. For feeding with UFC, the combustible recovery in concentrate is 91.26%, which is 5.33%
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higher than that of feeding without UFC. This might be because coal particles could also adsorb cationic ammonium through electrostatic adsorption [2]. As the specific surface area of UFC is relatively larger, the total specific surface area of feeding with UFC is larger than that of feeding without UFC. Therefore, under the same concentration of collector, the coal particles of feeding with UFC could adsorb much more cationic ammonium, leading to the decrease of ammonium concentration in the pulp and reducing
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ACCEPTED MANUSCRIPT the recovery of silica relatively [2, 3]. At the same time, under the same concentration of ammonium, coal particles in the feeding without UFC are much coarser, which are much more easily lost in froth product and decreases the combustible recovery of concentrate. Next this phenomenon was explained by comparing the cationic ammonium adsorption
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quantity by coal and silica, respectively.
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3.3 Results of adsorption test
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During the adsorption process, subbituminous coal sample is the flotation feeding with
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UFC (the whole grade). The particle size of silica is the same with that in flotation feeding. The isoelectric point of silica is at about pH 2, and the surface of subbituminous
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coal is of electronegativity at neutral pH [2, 28, 29]. Therefore, during the process of
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subbituminous coal reverse flotation, a part of subbituminous coal particles could also adsorb cationic ammonium and be lost in tailings when cationic ammonium is used as
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collector.
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According to the adsorption result shown in Fig. 5, with the increase of the concentration of Lilaflot D817M, the adsorption quantity of subbituminous coal on ammonium salt
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increases linearly. The adsorption equilibrium has not been reached during the tested
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collector range. Within the experiment range, the adsorption of silica is always lower than that of subbituminous coal. When the concentration of Lilaflot D817M is 180mg/L, the adsorption of silica reaches equilibrium, and the adsorption quantity is 8.71mg/g. During the process of subbituminous coal reverse flotation, the flotation froth appears gray, which means more silica particles than subbituminous coal particles are floated. It indicates that there is no connection between the surface change degree of subbituminous coal and the adsorption quantity. Although the adsorption of silica on ammonium salt is
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ACCEPTED MANUSCRIPT very low, the surface becomes strong hydrophobicity. Subbituminous coal adsorbs a lot of ammonium salt, but the hydrophobicity is still very low. This might also because that the subbituminous coal surfaces are heterogeneous, so both the hydrophobic and the polar groups of the collector could orient toward the solution, which induced a lower
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hydrophobicity relative to the homogenous silica surfaces. So the subbituminous coal is
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very difficult to mineralize, which would stay in the bottom of the flotation cell and
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become clean coal products [22, 23]. As a result, the existence of ultra- fine subbituminous coal consumes lots of ammonium salt and has a negative effect on silica
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flotation. Therefore, the research next focuses on increasing the particle size of
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subbituminous coal, using selective flocculation method, to improve the recovery of silica and the separation efficiency of subbituminous coal reverse flotation.
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Fig. 6 shows the contact angle change of subbituminous coal under different dosage
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Lilaflot D817M. When the concentration of Lilaflot D817M increases from 1kg/t to 4kg/t, the contact angle increases from about 60o to 90o . It indicates that the hydrophobicity on
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Lilaflot D817M.
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the surface of subbituminous coal enhances graduately with the increase of the dosage of
In the previous study, we know that the hydrophilcity of silica is very strong and the
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water drop spreads completely on silica surface within 1s. After adsorbing Lilaflot D817M, the surface wettability of silica changes greatly. The result is shown in Fig. 7. When the concentration of collector is 1kg/t, the contact angle on silica surface reaches 120°. With the increase of collector concentration, the contact angle of silica keeps at about 130°. It indicates that the wettability on silica surface has been affected much more than subbituminous coal by the same concentration of Lilaflot D817M.
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ACCEPTED MANUSCRIPT 3.4 Results of selective flocculation reverse flotation test Table 5 shows the effect of PAM dosage on the selective flocculation-reverse flotation of subbituminous coal. It can be seen from Table 5 that the ash content of concentrate is the lowest (12.50%) when the dosage of flocculant is 50g/t. At this time, the ash content of
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tailing is 80.99%, and the combustible recovery is 90.69%. The separation efficiency at
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50g/t flocculant is also the highest comparing to the other two dosages. Therefore, the
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dosage of flocculant affects the selective flocculation-reverse flotation of subbituminous
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coal greatly. Therefore, appropriate flocculant dosage has a beneficial effect on reverse flotation of subbituminous coal.
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The comparison result between selective flocculation-reverse flotation test result and
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reverse flotation result of feeding with UFC (dextrin 1kg/t, collector 1kg/t) is shown in Table 6. According to Table 6, 12.50% ash content of concentrate could be obtained
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under the same flotation conditions using selective flocculation-reverse flotation with
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50g/t PAM. The ash content decreased by 1.6% and the separation efficiency increased from 58.39% to 66.07% comparing with reverse flotation results. Therefore, under
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certain conditions, selective flocculation-reverse flotation could improve low rank coal
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separation efficiency.
Measure 1g subbituminous coal (-74μm) or silica (-56μm) and put the sample into 100ml beaker. After stiring thoroughly, add 400g/t PAM to 100ml. Put the beaker on the surface of the maganetic stirrer and condition 5min. Take 1ml floc and put it on the glass slide. Observe the floc morphology using polarizing microscope. Fig. 8 shows the floc morphology of subbituminous coal. According to Fig. 8(a), subbituminous coal particles without any reagents disperse in the pulp very well. Though
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ACCEPTED MANUSCRIPT there are some flocs, the size of flocs is very small. It can be seen from Fig. 8(b) that after adsorbing PAM, large flocs form and the flocs are very tightening. It indicates that PAM has strong flocculation effect on subbituminous coal and it could increase the surface particle size of subbituminous coal.
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Fig. 9 shows the morphology of silica before and after adsorbing PAM. It can be seen
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that silica disperses very evenly. It is because that the surface of silica has strong polarity.
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The electronegative surface leads the repulsive force between particles very strong. It can be known that the size of subbituminous coal flocs is much larger than that of silica under
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the same PAM dosage by comparing Fig. 8 and Fig. 9. It means that the PAM has better
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flocculation effect on subbituminous coal than silica. Though the flocs of silica is smaller, it reaches 0.1mm when the dosage of PAM is 400g/t. The size will exceed the optimal
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particle size of silica for coal reverse flotation and deteriorates the separation efficiency
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of coal reverse flotation. Therefore, the concentration of PAM should be restricted within
4. Conclusions
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appropriate range in coal reverse flotation.
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(1) It is found that there are a lot of oxygen-functional groups on the surface of
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subbituminous coal through XPS test, which would affect its hydrophobicity. (2) The adsorption quantity of subbituminous coal on ammonium salt is much higher than that of silica. Subbituminous coal could adsorb more ammonium salt because of larger specific surface area, which leads to the decrease of the ammonium salt concentration in the pulp and less silica recovery. (3) The silica recovery, silica flotation rate constant, and separation efficiency of the feeding without UFC is 82.76%, 0.017, 68.69%, respectively, while that of feeding with
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ACCEPTED MANUSCRIPT UFC is only 73.02%, 0.011, 65.16%, respectively. The existence of ultra- fine subbituminous coal decreases the recovery of silica, silica flotation rate and separation efficiency of reverse flotation. (4) Selective flocculation method could flocculate subbituminous coal particles and
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change the surface particle size, which would decrease the collector adsorption quantity
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in subbituminous coal reverse flotation. Appropriate flocculant dosage could increase the
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separation efficiency of reverse flotation. Acknowledgments
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The work was supported by “the Fundamental Research Funds for the Central
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Universities (2018QNA20)” of China University of Mining and Technology. We also want to thank the support of A Priority Academic Program Development of Jiangsu
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Higher Education Institutions.
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Table 1 The particle size distribution of subbituminous coal
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Particle size (μm) 150-200 120-150 74-120 45-74 <45 Total
Yield (%) 15.71 12.92 15.3 11.73 44.34 100.00
Ash (%) 6.42 6.96 7.05 7.35 11.16 8.81
Table 2 The relative contents of C element with different binding forms on the surface of subbituminous coal Combining form Content (%)
C-C/C-H 74.49
C-O 16.59
C=O 5.18
O-C=O 3.73
Table 3 The effect of UFC on silica flotation rate in reverse flotation Feeding
k
R2
With UFC Without UFC
0.011
0.990
0.017
0.964
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Table 4 The comparison results of concentrate quality with different flotation feedings
Productivity (%)
Feeding with UFC 68.62
Ash Content (%)
14.31
9.18
Combustible Recovery (%)
91.26
85.93
Feeding without UFC 62.89
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Concentrate Quality
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Table 5 The effect of PAM dosage on the selective flocculation-reverse flotation of subbituminous coal Concentrate ash,%
Tailing ash,%
Combustible recovery,%
Separation efficiency,%
400
23.00
65.00
84.52
37.76
80
21.00
76.36
90.61
46.36
50
12.50
80.99
90.69
66.07
20
17.03
80.69
91.28
57.33
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PAM dosage,g/t
Combustible Recovery, %
Without PAM
94.31
With PAM
90.69
Concentrate ash, %
Tailing ash, %
Separation efficiency, %
14.17
83
58.39
12.50
80.99
66.07
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Conditions
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Table 6 The effect of PAM on reverse flotation
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[13] R.Q. Honaker, R.H. Yoon, G.H. Luttrell. Ultrafine coal cleaning using selective hydrophobic coagulation, Coal Prep. 25 (2) (2005) 81-97.
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[14] Y. Li, J. Chen, L. Shen. Beneficiation of coal-silica mixture using reverse flotation, Energ. Sources Part A 39(1) (2017) 103-109. [15] M.llyas, J. Akhtar, N. Sheikh, S. Munir, Reverse flotation of cut-of-grade of Lakhra coal, Energ. Sources Part A 39(20) (2019) 1999–2005. [16] K. Ding, J.S. Laskowski. Coal reverse flotation. Part I: Separation of a mixture of subbituminous coal and gangue minerals, Miner. Eng. 19 (1) (2006) 72-78. [17] K. Ding, J.S. Laskowski. Coal reverse flotation. Part II: Clean ing of a subbituminous coal, Miner. Eng. 19 (1) (2006) 79-86.
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ACCEPTED MANUSCRIPT [18] H. Zhang, J. Liu, Y. Cao, et al. Effects of particle size on lignite reverse flotation kinetics in the presence of sodium chloride, Powder Technol. 246 (2013) 658-663. [19] L. Shen，H. Wang. Properties of fatty acid/dodecylamine mixtures and their application in steam coal reverse flotation, Physicochem. Problem. Miner. Process. 52(1) (2016) 303-316. [20] M. Pawlik, J.S. Laskowski. Coal reverse flotation. Part I. Adsorption of dodecyltrimethyl
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ammonium bromide and humic acids onto coal and silica, Coal Prep. 23(3) (2003) 91-112. [21] M. Pawlik, J.S. Laskowski. Coal Reverse Flotation. Part II. Batch Flotation Tests, Coal Prep.
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23(3) (2003) 113-127.
[22] J. Solaris, A.D. Araujo, J. Laskowski. The effect of carboxymethyl cellulose on the flotation and
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surface properties of graphite, Coal Prep. 3(1) (1986)15-31.
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[23] W. Xia, J. Yang. Effect of pre-wetting time on oxidized coal flotation, Powder Technol. 250(12) (2013) 63-66.
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[24] W. Xia, Y. Peng, C. Ren, et al. Changes in the flotation kinetics of bituminous coal before and after natural weathering processes, Physicochem. Problem. Miner. Process. 51(2) (2015) 401−410.
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[25] W. Xia, J. Yang. Changes in surface properties of anthracite coal before and afterinside/outside
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weathering processes, Appl. Surf. Sci. 313 (2014) 320-324. [26] W. Zhang, R. Honaker, Y. Li, et al. The importance of mechanical scrubbing in magnetite-
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concentrate reverse-flotation, Miner. Eng. 69 (2014) 133–136. [27] W. Zhang, R. Honaker. Studies on carbon flotation from fly ash, Fuel Process Technol. 139
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(2015) 236–241.
[28] M. Sarikaya M, G. Özbayoǧlu. Electrokinetics of oxidized coal, Fuel Process Technol. 24 (1990) 459–466.
[29] M. Sarikaya, G. Özbayoğlu. Flotation characteristics of oxidized coal, Fuel 74(2) (1995) 291−294.
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Fig. 1. C1s peaks and the fitting curves of subbituminous coal
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Silica Recovery in Tailing ( % )
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Fig. 3. The calculated result of silica flotation rate in reverse flotation of subbituminous coal
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Adsorption quantity (mg/g )
Silica 40
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Fig. 5. The adsorption rule of silica and subbituminous coal in Lilaflot D817M solution with different concentration
Fig. 6. The contact angle change of subbituminous coal under different dosage Lilaflot D817M
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Fig. 7. The contact angle change of silica under different dosage Lilaflot D817M
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Fig. 8. The effect of PAM on the floc forms of subbituminous coal（a, Subbituminous coal; b, subbituminous coal floc with PAM）
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Fig. 9. The effect of PAM on the floc form of silica （a, silica b, silica floc with PAM）
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The calculated result of silica flotation rate in reverse flotation of subbituminous coal
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Reverse flotation was introduced to clean subbituminous coal.
The existence of ultra-fine coal decreases the flotation rate of silica.
Selective flocculation method was used to improve reverse flotation performance.
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Yonggai Li, Jiawei Li, Peng Chen, Jianzhong Chen, Lijuan Shen, Xiangnan Zhu, Gan Cheng PII: DOI: Reference:
S0032-5910(18)30855-6 doi:10.1016/j.powtec.2018.10.014 PTEC 13788
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
27 July 2018 6 October 2018 10 October 2018
Please cite this article as: Yonggai Li, Jiawei Li, Peng Chen, Jianzhong Chen, Lijuan Shen, Xiangnan Zhu, Gan Cheng , The effect of ultra-fine coal on the flotation behavior of silica in subbituminous coal reverse flotation. Ptec (2018), doi:10.1016/j.powtec.2018.10.014
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ACCEPTED MANUSCRIPT The effect of ultra-fine coal on the flotation behavior of silica in subbituminous coal reverse flotation Yonggai Li a, *, Jiawei Li a, Peng Chen a, Jianzhong Chen Xiangnan Zhu b, Gan Cheng
a,
*, Lijuan Shen a,
a, c
a Key Laboratory of Coal Processing and Efficient Utilization (M inistry of Education), School of Chemical
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Engineering and Technology, China University of M ining and Technology, Xuzhou, Jiangsu 221116, China
Qingdao, Shandong 266590, China
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b College of Chemical and Environmental Engineering, Shandong University of Science and Technology,
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c College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China
Abstract: In this study, X-ray Photoelectron Spectroscopy (XPS) was used to analyze the
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elements on the surface of subbituminous coal, and reverse flotation was introduced to
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float subbituminous coal. The effect of ultra-fine coal on the flotation behavior of silica in coal reverse flotation was discussed by comparing the flotation performance with two
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different artificial mixed flotation feedings, with ultra-fine coal (UFC, -45μm) or without
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ultra- fine coal. The comparison results showed that the existence of UFC reduced the flotation rate and the recovery of silica, as well as the separation efficiency of
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subbituminous coal reverse flotation. Compared to the flotation performance of feeding
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with UFC, the concentrate ash content of feeding without UFC was lower (9.18%), and the separation efficiency was higher (68.69%). Total Organic Carbon Analyzer was used
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to test the adsorption quantity of collector by coal and silica to explain the mechanism. Conclusions were obtained that ultra- fine coal particles could adsorb more collector due to their larger specific surface area, leading to less collector left in the pulp to float silica particles. Selective flocculation reverse flotation method could increase the separation efficiency of coal reverse flotation with appropriate dosage flocculant.
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ACCEPTED MANUSCRIPT Key words: ultra-fine coal (UFC); coal reverse flotation; flotation rate; separation efficiency; selective flocculation
* Corresponding authors.
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E-mail address: [email protected] (Y. Li), [email protected] m (J. Chen).
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1. Introduction
Flotation is a common method to separate fine minerals. Coal has natural hydrophobicity.
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Conventional flotation could separate coal and gangue minerals efficiently by using kerosene or diesel as collector to increase the hydrophobicity of coal. The hydrophobic
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coal particles are scraped out as froth product while hydrophilic gangue minerals stay in
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the pulp. However, some low rank coals, such as subbituminous coal, have poor
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hydrophobicity because of high water content and a lot of oxygen-containing functional groups on the surface, which makes it difficult to be separated using conventional
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flotation [1-3]. Reverse flotation used in this research could take advantage of the poor surface hydrophobicity of low rank coal and achieve the objective of separation, turning
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the disadvantage to advantage [4]. During the process of reverse flotation, gangue
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minerals are scrapped out as froth product while coal particles stay in the pulp [4]. The flotation of fine or ultra- fine mineral is always a challenge in the area of mineral processing [5-9]. In many cases, there exists some bad effect caused by fine particles’ physical characteristics, such as low collision probability between fine particles and bubbles, slow flotation rate, hydraulic entrainment or mechanical entrapment, etc., as well as some problems resulted from the chemical characteristics of fine particles, such as large solubility, the adsorption and transformation of dissolved components on the
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ACCEPTED MANUSCRIPT surface of minerals, slime coating, agglomeration among fine particles and the nonselective adsorption of fine particles on reagents, etc [10]. Ni et al. [11] investigated the effect of slimes on the flotation recovery and kinetics of coal particles using flotation mixtures including slime particles and coal particles within 0.5–0.25mm, 0.25–0.125mm
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and 0.125–0.074mm size ranges, respectively and found that both the flotation recovery
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and rate of coal particles decreased with the increase in the mass propor tion of slime
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particles under the mixed flotation conditions, especially coarse coal particles. Ding [10] did some research about the flotation of feldspar and silica with different particle size and
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found that fine feldspar has weak inhibiting effect on coarse feldspar, but it could activate
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silica obviously. Selective flocculation is a very common method in fine mineral separation. Honaker et al. [12] has conducted research about the effect of pH on the
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selective flocculation of -10μm coal, and found that the coal’s selective cohesion
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happened during pH 8.5-9.5. Cai et al. [13] used selective flocculation to make low rank coal (ash content < 3%) and the combustible recovery could reach 90%. Coal reverse
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flotation is a concept opposite of conventional direct flotation. During the process of coal
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reverse flotation, minerals float to the froth product while coals are depressed and remain in the pulp [2]. Some research has been done about reverse flotation, such as the effect of
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kinds and dosage of collector or depressant, the pH of pulp, the conditioning time, the particle size, etc. on the performance of reverse flotation [14-22]. However, there are few papers about the effect of ultra- fine coal (UFC, -45μm) in coal reverse flotation. This research will focus on the effect of UFC particles on the flotation behavior of silica and separation efficiency of coal reverse flotation using two different feedings (feeding with
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ACCEPTED MANUSCRIPT UFC or feeding without UFC). The related mechanism was explained and selective flocculation reverse flotation was introduced. 2. Experiment materials and methods 2.1 Experiment materials
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The coal sample in this research is subbituminous coal (Peabody Energy, US). Coal
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sample was crushed to minus 200μm. The approximate analysis of subbituminous coal is: moisture 27.72%, ash content 8.81%, volatile component 46.62%, fixed carbon 16.85%,
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and sulphur content 0.72%. The particle size composition of subbituminous coal is shown in Table 1. It is can be seen that the ash content of the subbituminous coal is 8.81% which
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is very low. With the decrease of particle size, the ash content of coal increases. The main
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fraction is - 45μm with ash content 11.16%. The silica (ASTM quartz c-778, ebay) used in this research is close to pure minerals (98.8% purity). Particle size composition of
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silica is, -1.52μm 10%, -7.03μm 50%, and the mean particle size is 10.42μm.
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Subbituminous coal with different size composition (-200μm or -200+45μm) and silica was mixed with a ratio of 7:3 as artificial flotation feedings. This two different feedings
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are called feeding with UFC and feeding without UFC, respectively. Feeding without
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UFC was obtained by sieving out the UFC using lab wet screening method. The oversize product was dried in low temperature in the drying oven. 2.2 Chemical reagents The collector for silica in subbituminous coal reverse flotation is compound cationic ammonium Lilaflot D817M (AKZO NOBEL). Dextrin (Fisher Scientific) is used as depressant for coal. The collector and depressant used in the test are 1% solution prepared with deionized water. Methyl Isobutyl Carbinol (MIBC, Aladdin Bio
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ACCEPTED MANUSCRIPT Technology) was used as frother. The flocculant is non- ionic (Changshu Yiqing Environmental Protection Technology Co., Ltd.). The relative molecular weight of the flocculant is 10-12×10^6 and the electric density is lower than 5. Hexametaphosphate (chemically pure) was used as dispersant. The solution concentration of flocculant and
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dispersant used in the test are 0.05% and 1%, respectively. Tap water was used during the
2.3 Experimental methods
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2.3.1 X-ray Photoelectron Spectroscopy (XPS) study
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whole experimental process.
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X-ray Photoelectron Spectrometer (XPS, Thermo, Escalab 250X1) was used to analyze the elements. The data processing (peak fitting) was performed with XPS Peakfit
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software. The binding energies were corrected by setting the C1s hydrocarbon
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(−CH2−CH2− bonds) peak at 284.9 eV.
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2.3.2 reverse flotation
Flotation cell used in the reverse flotation test is lab-used 0.5L single flotation cell
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(RK/FD II). The pulp was in natural pH, and the inflating volume was open to the maximum. The flotation feeding was made up of 20g coal and 8.5g silica, and the pulp
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density was 57g/l. The conditioning speed was 1850rpm during the whole process. The slurry was initially conditioned for 3 min to ensure complete dispersion o f the samples. The pH of the pulp before the addition of dextrin and amine ranged from 6.83 to 6.97. Then 2kg/t depressant and 20ppm frother were added with 3min and 1min conditioning, respectively. At last the air valve was opened as soon as 1kg/t collector was added. The froth product was scrapped for 5 min. The products at different time points (20s, 40s, 80s,
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(1) (2)
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𝜂(%) = 100 ∗ 𝑀𝐶 ∗ (𝐴𝐹 − 𝐴𝐶 )/[𝑀𝐹 ∗ (100 − 𝐴𝐹 ) ∗ 𝐴𝐹 ∗ 100
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Among which,
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RS is the silica recovery in tailing, %; 𝜂 is separation efficiency, %;
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Mc, MT and MF are the weight of the concentrate, tailing and feeding, respectively, g; Ac, AT and AF are the ash content of the concentrate, tailing and feeding by weight,
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respectively, %. 2.3.3 Selective flocculation-reverse flotation
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Selective flocculation-reverse flotation was also conducted in the 0.5L flotation cell. The
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feeding is artificial mixture with 20g subbituminous coal and 8.5g fine silica. Pour the
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feeding pulp into flotation cell after being evenly stirred, and add water to 0.5L. The flotation steps are as follows: the pulp was conditioned with a speed of 1890r/min for
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3min initially, then 1kg/t sodium hexametaphosphate and 1kg/t dextrin were each
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conditioned in the slurry for 3min. Next the rotate speed was decreased to 1000r/min. Then moderate flocculating and 10ppm MIBC were each conditioned for 5min and 1min. At last 1kg/t Lilaflot D817M was added as collector when inflating, and the scrapping time was 5 min. 2.3.4 Adsorption test The total organic carbon analyzer was used to test the content of element nitrogen in the solution before and after the adsorption of minerals to calculate the adsorption quantity of
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ACCEPTED MANUSCRIPT collector amine by silica. Dilute the 1% Lilaflot D817M solution to 10-400mg/l. 20ml dilution was put into the test tube to measure the content of total nitrogen. The linear relationship between the concentration of Lilaflot D817M and the content of total nitrogen was obtained and the curve could be taken as the standard curve.
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The adsorption steps are as follows: firstly, put 1g subbituminous coal or silica into one
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100ml beaker and add deionized water to 80ml. After sufficient stirring, a certain amount
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of 1% Lilaflot D817M was put in the beaker and the volume was set to 100ml. Then put the beaker on the magnetic stirrer for 5min to make the adsorption sufficient. When
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adsorption equilibrium achieved, pour the pulp solution into the pressure filter with low
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speed qualitative filter paper to get the filtrate. At last, take 20ml filtrate into the test tube to measure the total nitrogen content using TOC instrument. Compare the result with the
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standard curve to get the adsorbing capacity by calculating the difference.
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3. Result and discussion
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3.1 Result of XPS
According to some papers, C1s peak of the combination form is C-C/C-H, C-O, C=O and
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O-C=O under 284.9eV, 286.14eV, 287.22eV, 288.75eV, respectively [23]. The main
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hydrophobic functional groups are C-C and C-H. The main hydrophilic functional groups are C-O, C=O, or O-C=O [24]. XPS Peak 4.1 software was used to split and simulate C1s peak. C1s peaks and the fitting curves of subbituminous coal is illustrated in Fig. 1. The area under different peaks was calculated to obtain the relative amount of C1s with different bond forms. The result is shown in Table 2. According to Fig. 1 and Table 2, the content of alkyl groups on subbituminous coal surface is 74.49%, and the content of all kinds of the oxygen-containing functional group is 25.5%. Xia has analyzed the surface
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ACCEPTED MANUSCRIPT properties of fresh anthracite using XPS, founding that the content of alkyl groups on fresh anthracite surface is 88.78%, and the content of oxygen-containing functional groups is 11.22% [25]. It can be seen that the content of oxygen-containing functional groups of subbituminous coal sample in this study is much higher than that of fresh
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anthracite, which is much easier to float. Combining the previous experiment results, the
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recovery of subbituminous coal using traditional flotation was very low, but better
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separation result could be obtained using reverse flotation method. Therefore reverse
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flotation was used to separate subbituminous coal next [2-4]. 3.2 The result of reverse flotation
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Fig. 2 shows the result of flotation rate of silica in subbituminous coal reverse flotation.
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According to Fig. 2, with the increase of time, the recovery of silica in tailing increases,
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but the increasing rate diminishes gradually. The largest recovery rate is obtained at about 120s. When it is at 200s, the silica recovery of the feeding without UFC is 82.76% while
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that of feeding with UFC is only 73.02%. The silica recovery rate of feeding without UFC is always higher than that of feeding with UFC when it is at the same time point. As
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the UFC has been removed from feeding without UFC, therefore the only difference
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between these two feedings is whether the UFC exists. According to the experiment result in Fig. 2, the existence of UFC has negative effect on the recovery of silica. This might be because of the much more adsorption of collector by UFC leading to the decrease of ammonium salt concentration in the pulp. According to the first-order kinetics rate model [26, 27]： 𝜀 = 𝜀∞ ∗ [1 − 𝑒𝑥𝑝(−𝑘𝑡)]
(3)
Among which: ε is the silica recovery in tailing, %; ε∞ is the silica recovery, %; k is the
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ACCEPTED MANUSCRIPT flotation rate constant. The calculation result of silica flotation rate constant is illustrated in Fig. 3 and Table 3. The flotation rate constant of feeding without UFC is 0.017, which is 0.006 higher than that of feeding with UFC. It means that the existence of UFC could deteriorate the flotation rate of silica in subbituminous coal reverse flotation.
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The separation efficiency of subbituminous coal reverse flotation is shown in Fig. 4. With
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the increase of time, the separation efficiency of subbituminous coal reverse flotation also
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increases, which reaches the biggest at 120s. During the whole separation process, the separation efficiency of feeding with UFC is higher than that of feeding without UFC.
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When it is at 200s, the separation efficiency of feeding without UFC reaches 68.69%,
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which is 3.53% higher than that of feeding with UFC (65.16%). It means that the existence of UFC reduces the separation efficiency of subbituminous coal reverse
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flotation.
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Table 4 shows the comparison result of clean coal quality with different flotation feedings. The clean coal recovery of feeding without UFC is 5.73% lower than that of
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feeding with UFC. But the clean coal ash content of feeding with UFC is 5.13% lower
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than that of feeding without UFC, which are 9.18% and 14.31%, respectively. For feeding with UFC, the combustible recovery in concentrate is 91.26%, which is 5.33%
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higher than that of feeding without UFC. This might be because coal particles could also adsorb cationic ammonium through electrostatic adsorption [2]. As the specific surface area of UFC is relatively larger, the total specific surface area of feeding with UFC is larger than that of feeding without UFC. Therefore, under the same concentration of collector, the coal particles of feeding with UFC could adsorb much more cationic ammonium, leading to the decrease of ammonium concentration in the pulp and reducing
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ACCEPTED MANUSCRIPT the recovery of silica relatively [2, 3]. At the same time, under the same concentration of ammonium, coal particles in the feeding without UFC are much coarser, which are much more easily lost in froth product and decreases the combustible recovery of concentrate. Next this phenomenon was explained by comparing the cationic ammonium adsorption
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quantity by coal and silica, respectively.
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3.3 Results of adsorption test
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During the adsorption process, subbituminous coal sample is the flotation feeding with
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UFC (the whole grade). The particle size of silica is the same with that in flotation feeding. The isoelectric point of silica is at about pH 2, and the surface of subbituminous
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coal is of electronegativity at neutral pH [2, 28, 29]. Therefore, during the process of
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subbituminous coal reverse flotation, a part of subbituminous coal particles could also adsorb cationic ammonium and be lost in tailings when cationic ammonium is used as
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collector.
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According to the adsorption result shown in Fig. 5, with the increase of the concentration of Lilaflot D817M, the adsorption quantity of subbituminous coal on ammonium salt
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increases linearly. The adsorption equilibrium has not been reached during the tested
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collector range. Within the experiment range, the adsorption of silica is always lower than that of subbituminous coal. When the concentration of Lilaflot D817M is 180mg/L, the adsorption of silica reaches equilibrium, and the adsorption quantity is 8.71mg/g. During the process of subbituminous coal reverse flotation, the flotation froth appears gray, which means more silica particles than subbituminous coal particles are floated. It indicates that there is no connection between the surface change degree of subbituminous coal and the adsorption quantity. Although the adsorption of silica on ammonium salt is
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hydrophobicity relative to the homogenous silica surfaces. So the subbituminous coal is
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very difficult to mineralize, which would stay in the bottom of the flotation cell and
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become clean coal products [22, 23]. As a result, the existence of ultra- fine subbituminous coal consumes lots of ammonium salt and has a negative effect on silica
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flotation. Therefore, the research next focuses on increasing the particle size of
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subbituminous coal, using selective flocculation method, to improve the recovery of silica and the separation efficiency of subbituminous coal reverse flotation.
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Fig. 6 shows the contact angle change of subbituminous coal under different dosage
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Lilaflot D817M. When the concentration of Lilaflot D817M increases from 1kg/t to 4kg/t, the contact angle increases from about 60o to 90o . It indicates that the hydrophobicity on
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Lilaflot D817M.
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the surface of subbituminous coal enhances graduately with the increase of the dosage of
In the previous study, we know that the hydrophilcity of silica is very strong and the
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water drop spreads completely on silica surface within 1s. After adsorbing Lilaflot D817M, the surface wettability of silica changes greatly. The result is shown in Fig. 7. When the concentration of collector is 1kg/t, the contact angle on silica surface reaches 120°. With the increase of collector concentration, the contact angle of silica keeps at about 130°. It indicates that the wettability on silica surface has been affected much more than subbituminous coal by the same concentration of Lilaflot D817M.
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tailing is 80.99%, and the combustible recovery is 90.69%. The separation efficiency at
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50g/t flocculant is also the highest comparing to the other two dosages. Therefore, the
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dosage of flocculant affects the selective flocculation-reverse flotation of subbituminous
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coal greatly. Therefore, appropriate flocculant dosage has a beneficial effect on reverse flotation of subbituminous coal.
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The comparison result between selective flocculation-reverse flotation test result and
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reverse flotation result of feeding with UFC (dextrin 1kg/t, collector 1kg/t) is shown in Table 6. According to Table 6, 12.50% ash content of concentrate could be obtained
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under the same flotation conditions using selective flocculation-reverse flotation with
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50g/t PAM. The ash content decreased by 1.6% and the separation efficiency increased from 58.39% to 66.07% comparing with reverse flotation results. Therefore, under
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certain conditions, selective flocculation-reverse flotation could improve low rank coal
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separation efficiency.
Measure 1g subbituminous coal (-74μm) or silica (-56μm) and put the sample into 100ml beaker. After stiring thoroughly, add 400g/t PAM to 100ml. Put the beaker on the surface of the maganetic stirrer and condition 5min. Take 1ml floc and put it on the glass slide. Observe the floc morphology using polarizing microscope. Fig. 8 shows the floc morphology of subbituminous coal. According to Fig. 8(a), subbituminous coal particles without any reagents disperse in the pulp very well. Though
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ACCEPTED MANUSCRIPT there are some flocs, the size of flocs is very small. It can be seen from Fig. 8(b) that after adsorbing PAM, large flocs form and the flocs are very tightening. It indicates that PAM has strong flocculation effect on subbituminous coal and it could increase the surface particle size of subbituminous coal.
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Fig. 9 shows the morphology of silica before and after adsorbing PAM. It can be seen
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that silica disperses very evenly. It is because that the surface of silica has strong polarity.
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The electronegative surface leads the repulsive force between particles very strong. It can be known that the size of subbituminous coal flocs is much larger than that of silica under
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the same PAM dosage by comparing Fig. 8 and Fig. 9. It means that the PAM has better
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flocculation effect on subbituminous coal than silica. Though the flocs of silica is smaller, it reaches 0.1mm when the dosage of PAM is 400g/t. The size will exceed the optimal
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particle size of silica for coal reverse flotation and deteriorates the separation efficiency
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of coal reverse flotation. Therefore, the concentration of PAM should be restricted within
4. Conclusions
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appropriate range in coal reverse flotation.
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(1) It is found that there are a lot of oxygen-functional groups on the surface of
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subbituminous coal through XPS test, which would affect its hydrophobicity. (2) The adsorption quantity of subbituminous coal on ammonium salt is much higher than that of silica. Subbituminous coal could adsorb more ammonium salt because of larger specific surface area, which leads to the decrease of the ammonium salt concentration in the pulp and less silica recovery. (3) The silica recovery, silica flotation rate constant, and separation efficiency of the feeding without UFC is 82.76%, 0.017, 68.69%, respectively, while that of feeding with
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change the surface particle size, which would decrease the collector adsorption quantity
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in subbituminous coal reverse flotation. Appropriate flocculant dosage could increase the
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separation efficiency of reverse flotation. Acknowledgments
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The work was supported by “the Fundamental Research Funds for the Central
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Universities (2018QNA20)” of China University of Mining and Technology. We also want to thank the support of A Priority Academic Program Development of Jiangsu
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Higher Education Institutions.
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Table 1 The particle size distribution of subbituminous coal
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Particle size (μm) 150-200 120-150 74-120 45-74 <45 Total
Yield (%) 15.71 12.92 15.3 11.73 44.34 100.00
Ash (%) 6.42 6.96 7.05 7.35 11.16 8.81
Table 2 The relative contents of C element with different binding forms on the surface of subbituminous coal Combining form Content (%)
C-C/C-H 74.49
C-O 16.59
C=O 5.18
O-C=O 3.73
Table 3 The effect of UFC on silica flotation rate in reverse flotation Feeding
k
R2
With UFC Without UFC
0.011
0.990
0.017
0.964
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Table 4 The comparison results of concentrate quality with different flotation feedings
Productivity (%)
Feeding with UFC 68.62
Ash Content (%)
14.31
9.18
Combustible Recovery (%)
91.26
85.93
Feeding without UFC 62.89
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Concentrate Quality
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Table 5 The effect of PAM dosage on the selective flocculation-reverse flotation of subbituminous coal Concentrate ash,%
Tailing ash,%
Combustible recovery,%
Separation efficiency,%
400
23.00
65.00
84.52
37.76
80
21.00
76.36
90.61
46.36
50
12.50
80.99
90.69
66.07
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17.03
80.69
91.28
57.33
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PAM dosage,g/t
Combustible Recovery, %
Without PAM
94.31
With PAM
90.69
Concentrate ash, %
Tailing ash, %
Separation efficiency, %
14.17
83
58.39
12.50
80.99
66.07
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Conditions
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Table 6 The effect of PAM on reverse flotation
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Fig. 1. C1s peaks and the fitting curves of subbituminous coal
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With UFC Without UFC
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Silica Recovery in Tailing ( % )
100
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100 Time (s)
150
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Fig. 2. The test results of silica flotation rate in reverse flotation of subbituminous coal
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50
100
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-1.0
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Ln(1-R) (% )
-0.5
150
With UFC
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y = -0.017x - 0.0922 R² = 0.9647
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Without UFC
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Fig. 3. The calculated result of silica flotation rate in reverse flotation of subbituminous coal
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Separation Efficiency (% )
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0 0
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100 Time (s)
150
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Fig. 4. The separation efficiency of reverse flotation of subbituminous coal
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Subbituminous coal
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Adsorption quantity (mg/g )
Silica 40
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10
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100 200 300 400 Concentrate of Lilaflot D817M (mg/l)
500
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Fig. 5. The adsorption rule of silica and subbituminous coal in Lilaflot D817M solution with different concentration
Fig. 6. The contact angle change of subbituminous coal under different dosage Lilaflot D817M
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Fig. 7. The contact angle change of silica under different dosage Lilaflot D817M
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Fig. 8. The effect of PAM on the floc forms of subbituminous coal（a, Subbituminous coal; b, subbituminous coal floc with PAM）
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Fig. 9. The effect of PAM on the floc form of silica （a, silica b, silica floc with PAM）
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100
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y = -0.0111x + 0.0263 R² = 0.9906
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With UFC
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y = -0.017x - 0.0922 R² = 0.9647
Without UFC
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Ln(1-R) (% )
-0.5
150
-2.0
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The calculated result of silica flotation rate in reverse flotation of subbituminous coal
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Reverse flotation was introduced to clean subbituminous coal.
The existence of ultra-fine coal decreases the flotation rate of silica.
Selective flocculation method was used to improve reverse flotation performance.
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