Hishn Hu,Ming Li,b,Lulu Li,Xiuxing To,*

a School of Chemical Engineering &Technology,China University of Mining and Technology,Xuzhou 221116,China

b National Engineering Research Center of Coal Preparation and Purification,China University of Mining and Technology,Xuzhou 221116,China

Keywords:Low rank coal Induction time Modification Flotation Bubble-particle attachment

ABSTRACTBased on the problems caused by many oxygen-containing functional groups and poor floatability on the surface of low rank coal,the characteristics of low rank coal were analyzed systematically by means of scanning electron microscopy (SEM),X-ray diffraction (XRD)and X-Ray photoelectron spectroscopy(XPS)techniques.The bubble-particle induction time was used to determine the characterization of the bubble-particle attachment,and the bubble-particle attachment of low rank coal modified by soaking the coal samples in an acid or alkaline solution was analyzed.The floatability of the modified coal surface was verified by flotation tests.The results show that the particle size of 0.125-0.074 mm of the coal sample exhibited better bubble-particle attachment characteristics.The small bubble,the larger approach velocity of bubble and the larger bubble deformation were more helpful to enhance the bubbleparticle attachment.For an acid solution,the smaller the pH was and the longer the soaking time was,the better the floatability of the coal sample and the higher the combustible material recovery were.The combustible material recovery of low rank coal was increased to 78.79% by soaking the sample in an acid solution of pH=0 for 180 min.On the contrary there was a best concentration for the alkaline solution.

1.Introduction

Low rank coal is abundant in China,such as lignite,long flame coal,non-caking coal and weak coal,which account for approximately 40% of the proven coal resources [1,2].With the improvement in the mechanization of coal mining,the deterioration of geological conditions of coal seams and the wide application of the heavy medium cyclone,the proportion of fine low rank coal has increased sharply [3].But the ash of low rank coal also shows a deteriorating trend.Floatation is one of the most economical and effective methods employed to treat fine coal [4-8].However,the surface of low rank coal contains a large number of polar oxygencontaining functional groups,and has poor hydrophobicity,so it is difficult for traditional hydrocarbon oil collectors to adsorb efficiently on the coal surface[9-12].Furthermore,the porosity of this kind of coal surface is well-developed and the pore permeability effect further increases the collector consumption [13].For conventional low rank coal flotation processes,more than 4 kg of collector,such as kerosene and diesel,is consumed per ton of dry coal[14].Coal surface modification is a technique used to optimize and change the properties of coal using physical and chemical methods.The surface modification technologies of coal include electromagnetic surface modification,ultrasonic modification,microwave modification,electrochemical modification,medicament modification,and mechanical modification[15-18].It has been shown that the implementation of the surface modification technology can promote the flotation effect of low rank coal to some extent.To reveal the difficult floating mechanism of low rank coal,the surface properties were analyzed,and the effect of the attachment characteristics of bubble on low rank coal were investigated by means of an induction time test device.The characteristics of the bubbleparticle attachment and flotation effect were studied by soaking the coal samples in an acid or alkaline solution,which provided some guidance for the practical low rank coal flotation.

2.Experimental

2.1.Materials

The long flame coal sample was from Zhuanlongwan coal mine in the Inner Mongolia Autonomous Region.A roll crusher was used to crush the coal sample to a size less than 1 mm,after which it was crushed again using the small shredding machine FS-100.After crushing,the coal samples with particle size smaller than 0.5 mm were sifted,filtered,and then kept in a drying oven at 70°C for 6 h for analysis and testing.The proximate and ultimate analyses of coal samples were carried out by means of a 5EMAG6700 automatic industrial analyzer and a Vario Macro Cube element analysis device,separately.The results are shown in Table 1.

The particle size composition of the coal sample is one of the crucial factors used to determine the flotation effect of low rank coal.Coal samples of sizes 0.5-0.25 mm,0.25-0.125 mm,0.125-0.074 mm,0.074-0.045 mm and -0.045 mm were obtained using the wet screening method.The coal samples were then filtered and dried in a drying oven at 70°C for 6 h,and the corresponding ash content was measured.According to the results shown in Table 2,the ash content of coal samples increases gradually with the decrease in the particle size of coal.In addition,the yield of the coal sample with a particle size smaller than 0.045 mm was 31.09%,and the ash content was the highest,which indicated that the coal contained many fine particles with a higher ash content.During the flotation process,this portion of fine particles with high ash content easily entered the foam layer through mechanical entrainment,resulting in a certain degree of pollution in the products of clean coal [19].

Diesel and methyl isobutyl methanol (MIBC)were used as collector and frother,separately.Hydrochloric acid (HCl)and sodium hydroxide (NaOH)were used to adjust the surface properties of coal samples.All reagents were purchased from Sinopharm Chemical Reagent Co.,Ltd.

2.2.Methods

2.2.1.Properties of coal samples

Scanning electron microscopy (SEM)was used to study the morphology and adhesion of impure minerals present on the surface of low rank coal.The Quanta250 scanning electron microscope was used to analyze the apparent morphology of low rank coal[20,21].

Depending on different crystals that cause the diffraction of the incident X-ray and produce diffraction patterns,an X-ray diffraction (XRD)analyzer can accurately assess the composition of the coal[22].The instrument used in this test was D8 ADVANCE,developed and produced by the Bruker Company of Germany.The coal samples were ground manually to a fine particle level(-325 mesh)to meet the requirements of instrument testing.Following this,the information of gangue minerals contained in the coal samples was obtained by analyzing the diffraction patterns using the Jade 6.5 software.There was also a certain degree of surface energy on the surface of these gangue minerals.The interference of gangue minerals during the flotation process of low rank coal was extremely complicated.

The X-Ray photoelectron spectroscopy (XPS)technique was used to excite the inner electrons (or valence electrons)of atoms or molecules in a sample using radiation,and the photoelectrons generated by the excitation were measured.The measured results were mapped into the XPS photoelectron spectra.In view of the significant differences in the electron binding energies of different elements,the elements in the sample can be clearly identified according to the position of the absorption peak in the transverse coordinate of the photoelectron spectroscopy.The instrument used in this experiment was Escalab 250Xi type electron energy spectrometer.With the help of this instrument,the surface of the coal sample was scanned for a wide spectrum,then for a narrow sweep C1s,and subsequently,the peak was fitted [23].

The contact angle is a measurement of the wetting degree of a solid surface.The hydrophobicity of the mineral surface was characterized by the contact angle,and the floatability could also be shown intuitively [24-26].The optical contact angle measuring instrument DSA100 and the YP-2 Compressor used in this paper are manufactured by the KRUSS Company of Germany and Shanghai Mountain Science instrument Co.,Ltd,respectively.The coal samples were made into smooth substrates under high pressure using YP-2 press,and the contact angle was measured using DSA100.

2.2.2.Induction time measurements

The coal particles were flattened at the bottom of a glass cell,and then the solution was injected into the cell.The liquid level was about 1 cm above the coal particles bed.With the help of a syringe,an air bubble was produced at the end of the capillary tube.Under the action of the driver,the capillary moved down with the bubble.The downward distance was set to ensure that the bubble was properly pressed to the surface of coal particles,and after the pulse ended,the capillary and bubble were moved to their initial position.A particle attaches to a bubble when the pulse time of the driver is longer than the bubble-particle induction time [27-29].By changing the pulse time and observing the attachment of the particles and the bubble,the minimum pulse time of the bubble-particle is defined as the induction time.Each group wasrepeated 50 times,and the average value was taken as the final induction time.The principle of the induction time apparatus is shown in Fig.1.

Table 1 Proximate and ultimate analyses of coal samples.

Table 2 Size distribution of Zhuanlongwan long flame coal.

2.2.3.Flotation tests

The flotation collector and frother used in this experiment were diesel and methyl isobutyl methanol (MIBC),respectively.The dosages of collector and frother were 10 kg/t and 300 g/t,respectively.XFD-63,a single flotation cell,was used with a volume of 0.5 L.The solid content in the form of pulp was set to 60 g/L.A mixture of 30 g of coal and 0.5 L of water was pretreated for 2 min and then the collector was added.After 2 min,the frother was added into the pulp for 1 min.The total flotation time was 5 min.

3.Results and discussion

3.1.Physical and chemical characteristics of coal samples

The surface morphology of the coal sample is shown in Fig.2.The surface was very rough and rich in silver grayish fragments,resulting from clay minerals attached to its surface.These microscopic characteristics lead to higher surface energy and lower hydrophobicity of the coal sample.

The results of the XRD of coal samples are shown in Fig.3.The main impure minerals present in coal samples were kaolinite,quartz,pearlite and calcite.Kaolinite and pearlite were typical clay minerals.Because of their special mineral layered structure and interlayer properties,they were easily muddied when they interacted with water,which resulted in a poor flotation effect and large flotation agent consumption.The surface hydrophilicity of quartz minerals was stronger than that of coal,which resulted in a strong hydrophilicity of incompletely dissociated coal-quartz,and the surface properties were different from those of coal.Moreover,these gangue minerals,especially the ultrafine grained gangue minerals,easily entered the flotation concentrate through a mechanical entrainment and fine mud cover to improve the ash content and reduce the quality of clean coal.

Fig.1.Testing principle diagram of induction time.

Fig.2.SEM results of low rank coal.

Fig.3.X-ray diffraction pattern of low rank coal.

The XPS scanning spectra of coal samples are shown in Fig.4,and the relative contents of the elements on the surface of coal samples are shown in Table 3.The relative contents of elements O and C in the coal sample were 30.65% and 58.03%,respectively.The hydrophilic oxygen functional groups on the surface of the coal samples improved the hydrophilicity of the samples.These hydrophilic groups easily formed hydrogen bonds with water molecules,which was not conducive to the interaction between coal surfaces and bubbles.Simultaneously,ordinary oil collectors were not effective in collecting low rank coal [30].

With the help of XPS Peak software,the peak fitting of C1s was carried out,and the fitting result is shown in Fig.5.The C1s data were calibrated based on 284.8 eV.At the binding energies of values 284.8 eV,286.3 eV,287.5 eV,and 288.80 eV,the carbon bonded functional groups corresponded to C-C or C-H,C-O-C or C-OH,C=O,and O-C-O,respectively.The relative contents of different functional groups were calculated according to the peak area,and the results are shown in Table 4.C-O was the main oxygencontaining group present on the surface of the coal sample.In addition,groups C-O and O=C-O were also present.The hydrophilic oxygen functional groups present on the surface of low rank coal increased the hydrophilic sites of the coal sample,and thus the hydrophilicity of low rank coal.Simultaneously,the oxygen functional groups easily combined with water molecules to form hydrogen bonds,and formed a stable hydration film on the surface of coal samples.

Table 3 XPS wide sweep results of coal samples.

Fig.5.C1s narrow sweep diagram of coal samples.

Table 4 Relative content of groups on coal surface.

The contact angle measurement is shown in Fig.6.The contact angle of coal sample was 32.2°,showing good surface hydrophilicity.The result of the contact angle was consistent with the results of XPS and SEM.

3.2.Bubble-particle attachment analysis

The bubble-particle attachment is a complex process,which is influenced by several factors such as particle size (d),bubble size(D),bubble approaching velocity (V)and bubble deformation (Δ h=H-h),as shown in Fig.7.The effects of particle size,bubble approach velocity,and bubble deformation on the induction time were discussed and analyzed to further clarify the bubbleparticle attachment characteristics.

3.2.1.Effect of bubble size (D)on induction time

Fig.6.The contact angle of low rank coal.

Fig.7.Process parameters of particle bubble adhesion.

Bubble size is a crucial factor affecting the flotation effect of low rank coal,considerably influencing the recovery rate and quality of clean coal.The bubble size was calculated by pixel transformation.The bubble diameters were set as 1.0 mm,1.2 mm,1.4 mm,1.6 mm,1.8 mm,and 2 mm.The bubble approaching velocity was set as 4.33 cm/s.The bubble return velocity was 3.46 cm/s.The bubble downward distance was 397.75 μm,h was 242.63 μm,and the bubble deformation (Δh)was 155.12 μm.Fig.8 shows the test results of the coal sample-bubble induction time for different bubble sizes.

It can be observed from Fig.8 that,with the increase in bubble size,the induction time increased gradually,showing that the good attachment of the small bubble-particle,and good mineralization performance of the bubble.When the bubble size was increased to 1.6 mm,the induction time of the coal sample increased rapidly,and the bubble became unstable,which could adversely affect the attachment between particles and bubbles,and impair the process of bubble mineralization.In addition,when the bubble size was the same,the minimum induction time of the coal sample was 0.125-0.074 mm particle size.When the ash contents of three sizes were similar,the gravity of the large particle was large,and it was difficult for bubbles to attach well to large particles during the process of floatation,thus affecting the yield of flotation concentrate coal.However,for the particle size of 0.125-0.074 mm,the hydrophobic site on the particle surface interacted with the bubble surface,and the coal particles did not detach easily during the process of mineralization and bubble rising.

Fig.8.Effect of different bubble sizes of coal sample-bubble on induction time.

3.2.2.Effect of bubble approach velocity (V)on induction time

The bubble downward distance (H)was 354 μm.The bubble falling times were 17 ms,15 ms,12 ms,10 ms,8 ms and 5 ms,and the bubble approach velocities were 2.08 cm/s,2.36 cm/s,2.95 cm/s,3.54 cm/s,4.43 cm/s,and 7.08 cm/s,separately.The values of bubble diameter,return velocity,downward distance(H),h,and bubble deformation (Δh)were 1.0 mm,3.46 cm/s,397.75 μm,242.63 μm,and 155.12 μm,respectively.Fig.9 shows the measurement results of the bubble-coal sample induction time at different bubble approaching velocities.The induction time decreased with the increase in the bubble approaching velocities.This was because under the same bubble downward distance (H),the greater the bubble approaching velocity,the greater the corresponding acceleration in the process of bubble moving downward,and the greater the force on the liquid film between the bubble and the particle.Therefore,it was more helpful to accelerate the discharge of liquid between particles and bubbles,thus accelerating the rupture of the liquid film and reducing the bubble-particle induction time.

3.2.3.Effect of bubble deformation (Δh)on induction time

The effect of bubble deformation (Δh)on induction time is shown in Fig.10.Bubble deformation is a key factor affecting the flotation effect of low rank coal,which has a profound influence on the recovery rate and quality of clean coal.The values of bubble diameter (D),approach velocity (V),and return velocity were 1.0 mm,4.33 cm/s,and 3.46 cm/s,respectively.The bubble deformation values (Δh)were 23.87 μm,43.75 μm,55.69 μm,75.57 μm,95.46 μm,115.35 μm,135.24 μm and 155.12 μm.With the increase in the value of bubble deformation,the induction time between particles and bubbles decreased gradually.Apparently,the larger the bubble deformation,the larger the contact range between particles and bubbles,the easier the formation of the three-phase wetting periphery,and the easier the hydrophobic particles attached to the bubbles.

3.2.4.Measurement results of induction time after modification of low rank coal

The treatment of the coal sample with an acid or alkaline solution will also have a certain influence on the coal quality.To further analyze the bubble-particle attachment characteristics after treatment with an acid or alkaline solution,different concentrations of hydrochloric acid solution and sodium hydroxide solution were used to soak coal samples.The soaked coal pulps were then heated using a water bath box.The heating temperature of the water bath was controlled at 70°C.

Fig.9.Effect of different bubble approach velocities on bubble-coal induction time.

Fig.10.Effect of bubble deformation on induction time.

To investigate the effect of the soaking time of the coal sample in an acid or alkaline solution on the bubble-particle induction time,the hydrochloric acid solution of pH=1 was selected,and the soaking time of the coal sample was set at 30 min,60 min,90 min,120 min,180 min and 420 min.Next,the coal samples were dried in an oven at 70°C for 6 h,and then the coal samples with particle size ranging between 0.125 mm and 0.074 mm were sifted.The variations in the bubble-particle induction time at different soaking times are shown in Fig.11.With the increase in the soaking time of the coal sample,the induction time of the coal sample first decreased and then changed gradually.The results above showed that the induction time of the particle size ranging from 0.125 to 0.074 mm without soaking in hydrochloric acid solution was 65 ms.However,after soaking in the acid solution(pH=1),the induction time reduced.Hence,soaking of the coal sample in hydrochloric acid can promote the bubble-particle attachment.When the soaking time was 420 min,the induction time reduced to 26 ms.The ash content of the above coal sample after acid treatment was measured,and was found to be lower than that of the raw coal ash,as shown in Table 5.In other words,soaking coal samples in hydrochloric acid solution dissolved acidsoluble minerals such as carbonate on coal surface,thereby,reducing the ash content and improving the hydrophobicity of coal samples,and reducing the bubble-particle induction time.

Fig.11.Effect of different soaking times on induction time.

Table 5 The ash content after acid treatment of coal samples.

Table 6 Results of the flotation test of Zhuanlongwan long flame coal.

3.3.Flotation results after pH modification

To study the effect of pH modification on flotation from a macroscopic point of view,the flotation test of coal samples soaked in an acid or alkaline solution for 180 min was carried out.The results of the flotation test are shown in Table 6.

Because the coal sample was soaked in the solutions of different pH values,the ash content of the floating coal sample was not consistent.To evaluate the flotation index better,the combustible material recovery was taken as the evaluation index of the flotation effect.It was worth noting that in the acidic solution(pH=0-7),the lower the pH,the lower the ash content of flotation raw coal.Meanwhile,the recovery of the combustible material decreased with the increase in pH,which was related to the removal of a few minerals by hydrochloric acid.On the contrary,in the alkaline solution (pH=7-14),with the increase in pH,the ash content of the flotation raw coal first decreased and then increased,and the recovery of the combustible material showed the opposite trend.When the concentration of the alkaline solution was less,it reacted with aluminum silicate minerals present on the surface of coal,causing a few hydrophilic minerals to dissolve.However,when the alkaline solution concentration was more,a new generation of minerals adhered on the surface of the coal and hence increased the ash content.Consequently,the floatability of coal samples was improved to a certain extent and the combustible material recovery improved by treating the coal samples with an appropriate concentration of an acid or alkaline solution.Therefore,when selecting an acid or alkaline solution to modify the surface of low rank coal,it is necessary to consider the properties of the solution and the characterization of the coal sample to improve the flotation effect of low rank coal to the greatest extent.

4.Conclusions

(1)Several impure minerals,such as quartz and kaolinite,with good hydrophilicity were present in low rank coal.These hydrophilic minerals were easily dispersed in water to form colloidal systems.The hydrophilicity of the coal sample was improved.It could be seen from the particle size distribution that the yield of particle size-0.045 mm was 56.41%and the ash content was high.The coal entrainment or fine mud cover could be formed easily in flotation.

(2)The surface of low rank coal was hydrophilic,and the contact angle was approximately 30°.Many hydrophilic oxygencontaining functional groups on the surface of low rank coal were the main reasons for the hydrophilicity.The results of the XPS showed that the oxygen-containing functional groups on the surface were composed of C-OH and C-O-C.The contents of C=O and O=C-O were less.These oxygencontaining functional groups had polarity,enabling them to easily form hydrogen bonds with water molecules and causing the water molecules to permutate on the surface to form hydration films,thus,inhibiting the adsorption of oil collectors used in conventional flotation on the surface of low rank coal.

(3)The best attachment effect of bubble-particle was observed for particle size 0.125-0.074 mm of the coal sample.Compared to large bubbles,small bubbles were more beneficial in increasing the bubble-particle attachment.In addition,the effects of the bubble approaching velocity and bubble deformation on bubble-particle collision,attachment,detachment,and other complex processes were also need to be considered.

(4)Treating coal samples with the appropriate concentration of an acid or alkaline solution removed a few coal minerals to a certain extent,thereby,increasing the floatability of coal samples and the combustible material recovery.However,when the solution was too alkali,new minerals formed on the surface of coal,thus worsening the bubble-particle attachment.

Acknowledgments

This work was financially supported by the National Key R&D Program of China(No.2018YFC0604702)and the National Natural Science Foundation of China (No.51774284).