Oily bubble flotation technology combining modeling and optimization of parameters for enhancement of flotation of low-flame coal

Accepted Manuscript Oily bubble flotation technology combining modeling and optimization of parameters for enhancement of flotation of lowflame coal

Songjiang Chen, Lulu Li, Jinzhou Qu, Quanzhou Liu, Longfei Tang, Xiuxiang Tao, Huidong Fan PII: DOI: Reference:

S0032-5910(18)30334-6 doi:10.1016/j.powtec.2018.04.053 PTEC 13354

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

1 September 2017 17 April 2018 19 April 2018

Please cite this article as: Songjiang Chen, Lulu Li, Jinzhou Qu, Quanzhou Liu, Longfei Tang, Xiuxiang Tao, Huidong Fan , Oily bubble flotation technology combining modeling and optimization of parameters for enhancement of flotation of low-flame coal. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Ptec(2017), doi:10.1016/j.powtec.2018.04.053

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Oily

bubble

flotation

technology

combining

modeling

and

optimization of parameters for enhancement of flotation of low-flame coal Songjiang Chen a, Lulu Li a, Jinzhou Qu b, Quanzhou Liu a, Longfei Tang a, Xiuxiang Tao a,*,

PT

Huidong Fan a

RI

a Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), School of Chemical Engineering and

SC

Technology, China University of Mining and Technology, Xuzhou 221116, P.R. China.

b School of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, P.R. China

NU

Abstract: There are abundant low-medium rank coal resources in China that provide an important

MA

guarantee for China’s economic and social development as well as security of energy supply. However, it is difficult to achieve the economic recovery of low-medium rank coals by

D

conventional flotation owing to its high natural surface hydrophilicity. In this study, the

PT E

characteristics of low-medium rank coal samples were comprehensively investigated based on their size/density distributions, XPS, BET, and SEM analyses. An improved oily bubble flotation

CE

combining modeling and optimization of technological parameters was employed to enhance the

AC

flotation of low-medium rank coal samples. The results indicated that the studied samples were rich in oxygenated functional groups, pores, and cracks that existed on the surfaces of the coal samples, thereby making it difficult for them to float. As a result, the collector dosage was maintained at a high level ranging from 10–50 kg/t in the conventional flotation of the coal sample. However, the oily bubble flotation technology showed much better flotation efficiency in the case of low-medium rank coal, achieving a higher combustible matter recovery at a much smaller *Corresponding author.

E-mail addresses: [email protected] (X. Tao), [email protected] (S. Chen )

1

ACCEPTED MANUSCRIPT collector dosage. Additionally, various parameters, such as the collector dosage, impeller speed, flotation pulp concentration, and air flow rate, had a profound effect on the flotation performance in the oily bubble flotation of the coal samples. An optimization of the technological parameters was conducted regarding the flotation of oily bubbles of low-medium rank coals, and a quadratic

PT

function model for the combustible matter recovery was established. Finally, an optimization

RI

scheme was established attaining a combustible matter recovery of 86.32% for oily bubble

SC

flotation of coal samples, where the frother dosage, collector dosage, impeller speed, pulp concentration, and air flow rate were 0.4 kg/t, 2.32 kg/t, 2198.53 rev/min, 52.47 g/L, and 0.23

NU

m3/h, respectively. As a consequence of the optimization scheme, the collector dosage was

MA

decreased by 95.36%.

Keywords: Low-medium rank coal; oily bubble flotation; response-surface methodology;

D

Box–Behnken experimental design; optimization; quadratic model

PT E

1. Introduction

China is rich in coal resources that account for 94% of China's energy reserves. Low-medium

CE

rank coals, such as lignite, long-flame coal, noncaking coal, and weakly-caking coal, account for

AC

45.68% of existing coal reserves in China. Coal has been the major primary energy source and has provided an important guarantee for China’s rapid economic and social development over the past decades. However, the combustion of low-quality coals results in a considerable environmental pollution. Additionally, with the deterioration of geological conditions of coal resources, and the continuous improvement of the mechanization of mining, the amount of fine coal has increased significantly. Furthermore, it is usually difficult to achieve the economic recovery of low-medium rank coals by conventional flotation because of their strong hydrophilicity caused by rich 2

ACCEPTED MANUSCRIPT oxygenated functional groups on their surfaces [1–6]. Additionally, the common oily collector dosage is usually as high as 30–50 kg/t in the flotation of low-medium rank coals [7–10]. Therefore, owing to these technical and economic limitations, the industrialization of flotation of low-medium rank coal slime has not been realized thus far. The slime of low-medium rank coal is

PT

usually recycled with dehydration instead of flotation. If this fraction of high-ash coal slime was

RI

mixed with clean coal based on gravity separation as the final product, it would lead to the

SC

degradation of the quality of the final concentrated product, and the reduction of the economic efficiency of enterprises. Furthermore, it would also elicit serious environmental pollution issues

NU

owing to the direct combustion of the high-ash coal slime [11]. Consequently, the flotation of

MA

low-medium rank coal is an important technical issue that needs to be resolved urgently. In order to enhance the flotation performance of low rank/oxidized coal, pretreatments are

D

usually used to improve the floatability of coal samples, or the properties of bubbles. This process

PT E

mainly involves grinding/dry-grinding with collector [4, 6, 12–15], thermal, and microwave heating, for removal of water from the coal structure [3, 16–20], or hydrophilic functional groups

CE

[3, 21, 22], ultrasound for preconditioning [23, 24], or distribution of flotation reagents [24, 25],

AC

and particle surface modification [9, 10, 26–29]. Reinforcing prior arguments, there are many research studies regarding the enhancement of flotation of coals that are difficult to float using high-efficiency flotation equipment and other methods [30–34]. In the flotation system, fine particles exhibit large variability in their physical and chemical surface properties, and respond poorly to the conventional flotation processes. Use of microbubbles with diameters under 100 µm increases the collision efficiency between fine particles and bubbles, but the recovery of water and gangue minerals also increases [35–37]. In comparison to air bubbles of the same size, the inertia 3

ACCEPTED MANUSCRIPT of oil droplets is larger. This attribute can increase the collision efficiency, and hence enhance the flotation performance of fine particles. In addition, the oil droplets spread and adhere on the particles more readily, which minimizes the water recovery [38, 39]. However, this method requires a tremendously large amount of oil that is not economical. To reduce the operating costs,

PT

the use of oil-coated bubbles was recently proposed as a carrier in the flotation of coals [40, 41].

RI

Inspired by the demonstrated success of the use of oil-coated bubbles in flotation, an

SC

oily-bubble flotation method was proposed, whereby a thin layer of oil or other regents is coated on the air bubble surface [37]. Fig. 1 shows a microflotation cell for reactive oily bubble flotation.

NU

The bottom chamber of the flotation cell was fully filled with collector-containing kerosene. The

MA

compressed air was aerated within the oil phase, and then through a fritted glass disk into the pulp to generate oily bubbles. At the same time, a magnetic stirrer bar was used on the fritted disk to

D

magnetically agitate the pulp and break the air streams into bubbles. As a result, the generated air

PT E

bubbles were coated with a thin oil film as they entered the flotation pulp. The oily-bubbles carried ore mineral particles, floated to the top of the flotation cell, and were collected by the

CE

froth-collecting lauder. The feasibility of the reactive oily bubble technology that was used to

AC

attain a selective flotation was confirmed in the single mineral flotation paradigm [37]. This method was also proven to be feasible by other research studies, and exhibited a better flotation performance than conventional air bubble flotation [42–44]. However, it should be noted that the flexible adjustment of the air flow rate is restricted. In this method, an increased air flow rate corresponds to a large oil or collector dosage, which is unfavorable for controlling the reagent dosages. In addition, excessive air flow rates may cause a large amount of oil to flow into the pulp, and hence result in the failure of the oily bubble flotation tests. 4

ACCEPTED MANUSCRIPT To address these technical problems, an improved apparatus for oily bubble flotation was proposed by Xia et al. [45], and the experimental apparatus is presented in Fig. 2. As shown in Fig. 2, the dodecane collector was first heated to approximately 215 °C to form oil steam in a flask with three necks, one of which was connected to the air inlet valve of the flotation machine. The

PT

dodecane steam with air was induced to the flotation cell through the inlet valve because of the

RI

negative pressure generated by the increased rotational speed of the agitation impeller. In this

SC

process, the dodecane steam was rapidly condensed into droplets in the flotation pulp. Once the air was injected into the flotation cell, it was sheared into a large volume of fine air bubbles

NU

containing diesel oil drops owing to the high-speed of the rotating impeller. Correspondingly, the

MA

dodecane droplets could be spread and coated on the surface of the air bubbles owing to the molecular or drop motions. As a result, the oily bubbles were formed, namely bubbles covered by

D

a thin oily collector film. Using this apparatus, a much higher combustible matter recovery and a

PT E

higher flotation efficiency index were attained. Moreover, the oily bubble flotation yielded a better selectivity for coal particles compared to conventional flotation [7, 45].

CE

In order to improve the security and adjustability, an advanced experimental device for oily

AC

bubble flotation was assembled by our team, as presented in Fig. 3. This oily bubble flotation system is mainly consisted of a 1.5 L XFD flotation machine, an air-compressed atomizer (an air compressor and an atomizer), a gas flowmeter, a heating tube, and a temperature control box. Unlike the oily bubbly flotation apparatus introduced by Xia et al. [45], the oily collector was held in an atomizer instead of a heating flask. The collector was first dispersed into fine oil droplets, and was then heated to form oil steam in the heating tube, which was preheated to 280 °C. This separated the oily collector from the heating device, thereby improving the security in the flotation 5

ACCEPTED MANUSCRIPT process. Additionally, the collector dosage may be adjusted flexibly through the air flow rate injected into the atomizer. The purpose of this study is to achieve the economic recovery of the low-medium rank coal. In the present work, the novel oily bubble flotation technology was employed to enhance the flotation of low-medium rank coal.

PT

2. Experimental materials and methods

RI

2.1. Materials

SC

The coal slime sample—a long-flame coal used for this study—was provided by the Daliuta Mine in the Shendong mine area, which is located at the junction of the Shanxi, Shaanxi, and

NU

Mongolia provinces. Table 1 shows the results of proximate and ultimate analyses of the coal

MA

samples, where Mad is the moisture content, Aad is the ash content, Vdaf is the volatile matter content, and FCdaf is the fixed carbon content. Furthermore, Cdaf, Hdaf, Odaf, Ndaf, and Sdaf,

D

respectively represent the content of carbon, hydrogen, oxygen, nitrogen, and sulfur elements. As

PT E

seen in Table 1, the ash content of the coal sample is 33.98%, which indicates that it is high-ash coal slime. The oxygen content (Odaf) is 24.55% in the coal organic matrix, and it seems that there

CE

are abundant oxygenated functional groups on the surface of the coal samples. Fig. 4 presents the

AC

results of XRD analyses for the coal sample. The primary mineral matter in the raw coal sample was found to be quartz, followed by kaolinite and muscovite, with fewer amounts of pyrite, clinochlore, and amesite. Quartz particles are naturally hydrophilic, and they are hard to attach to bubbles in the flotation pulp. Thus, they are easily discharged from flotation cells as tailings. However, kaolinite, clinochlore, and amesite minerals, have similar characteristics in that they break into ultrafine particles in the pulp. Therefore, they are easy to coat the coal surface, resulting in mechanical entrainment of minerals in the clean coal. The characteristics of coal samples will 6

ACCEPTED MANUSCRIPT be discussed in detail in Section 3.1. 2.2. SEM measurement The analysis of surface morphology for coal particles was conducted with a scanning electron microscope (SEM, Quanta 250). The magnification was fixed at 200, 5000, and 6000. For a

PT

typical measurement, a layer of coal particles adhered to conductive tape on the sample holder. A

RI

bulb syringe was used to blow off the particles that failed to adhere on the conductive tape to

SC

avoid damaging the vacuum system. Afterwards, coal samples were sputtered with a layer of gold powder for the subsequent scanning tests. The detailed operating parameters for SEM

NU

measurements were as follows: HV was 25.00 kV, the focal spot was 5.0 nm, and the working

MA

distance (WD) was kept constant at approximately 17.8 mm. 2.3. XPS measurement

D

An X-ray photoelectron spectroscopy analyzer (ESCALAB 250Xi) was employed to analyze

PT E

the surface elements and chemical functional groups. The instrument was equipped with an Al Kα (1486.6 eV) source. Coal samples used for the measurements were first grounded until all the

CE

powder could pass through a 0.074 mm sieve. Subsequently, coal grounded samples that weighed

AC

approximately 0.2 g, were pressed into pellets using a 10 MPa pressure for the measurement. In order to ensure the reliability of the results, all analyses were performed in an ultrahigh vacuum (5×10−8 Pa) environment at a constant temperature of 25 °C. A wide scanning test model with step energy of 1 eV was used to identify the relative content of carbon, oxygen, silicon, and aluminum, on the coal surface, while the narrow scanning model with step energy of 0.05 eV was used to determine the relative contents of carbon forms. The binding energy of raw data of measurements should be calibrated by setting the C1s peak at 284.6 eV, while all subsequent peak fitting was 7

ACCEPTED MANUSCRIPT performed with the software XPSpeak4.1. 2.4. BET measurement In this study, an automatic surface area/pore analyzer (BEL Company, Japan) was used to analyze the structural parameters of the pores of the coal samples, such as the pore volume,

PT

specific surface area, pore size distribution, and average pore diameter. For a typical measurement,

RI

approximately 0.6 g coal particles with full or −0.045 mm size fractions were carefully transferred

SC

to the sample tube, cleaned, and dried in advance. It should be noted that in order to obtain reliable test results, the weighing accuracy of the coal sample used for the measurement ought to be as

NU

high as 0.0001 g. Additionally, for all measurements, the degassing processing of coal samples

MA

was conducted under a vacuum condition at 60 °C for 12 h. After the degassing pretreatment, the N2 gas adsorption/desorption process was carried out at the liquid nitrogen temperature (−196 °C)

D

to measure the pore structure. According to the nitrogen adsorption/desorption isotherms at

PT E

−196 °C, the total pore volume, specific surface area, and average pore diameter, were analyzed. The detailed description for this measurement method can be found elsewhere [46–48].

CE

2.5. Attachment time measurement

AC

An induction timer instrument (2015EZ) was employed to measure the attachment time of the coal particles onto air/oily bubbles, as shown in Fig. 5. The apparatus consists of a 40 W speaker used as a power driver, an integrated control box, including a digital-analog converter, a CCD camera system with a macro lens, a microdisplacement sensor, a longitudinal displacement motor, a manual controller, an illumination device, a sample table with a heating device, and a device generating bubbles. The power driver is the core component, and is connected to an amplifier interfaced with a computer to control the movement of the bubble via a capillary tube. The 8

ACCEPTED MANUSCRIPT microdisplacement sensor is used to detect the displacement of bubbles during the measurement. Fig. 6 presents the measurement process of the attachment time. In this figure, H0 denotes the displacement of the power driver or the bubble holder, h0 is the initial gap between the bubble and the coal particle bed, db is the diameter of the bubble, and ua and ur are the velocities of the

PT

bubbles approaching towards and retracting from the particle bed. For a typical measurement,

RI

approximately 1.0 g of coal particles with sizes within the range of 0.074–0.125 mm were

SC

transferred to a sample cell filled up with water to prepare a flat coal particle bed with an even thickness. The bottom of the capillary glass tube was then immersed in the water in the sample

NU

cell through the manual controller, and an air/oily bubble with a controllable size was generated at

MA

the end of the capillary tube using a microsyringe. However, it should be noted that for an oily bubble generation, a small volume of diesel oil should be sucked in the capillary tube before

D

immersed in the water. During the measurement, the air/oily bubble was driven towards the

PT E

mineral particle bed and established contact with it for a given period of time before retracting away from the mineral particle bed. The attachment of coal particles onto the air/oily bubbles were

CE

observed on the monitor if the preset contact time was sufficiently large. Thus, if the preset contact

AC

period was inadequate, no particle-bubble attachment occurred. The minimum time required for attachment at every contact was considered as the attachment time, while the average value of ten repeated measurements was considered as the final result. What is more important is that all the parameters in the measurements were fixed, unless otherwise specified. The oily bubble size was set up at 1.30 mm, H0 at 353.39 µm, h0 at 271.65 µm, and ua and ur at 2.36 cm/s. The detailed description on the attachment time measurements can be found in prior published work [49, 50].

9

ACCEPTED MANUSCRIPT 2.6. Flotation tests 2.6.1. Conventional flotation tests The conventional flotation tests of coal samples under 0.5 mm were conducted in a 1.5 L XFD flotation cell. Firstly, 90 g of coal samples and 1.5 L of water were transferred into the flotation

PT

cell. The prewetting process was then performed for 120 s at an impeller speed of 2000 rev/min.

RI

Afterwards, the diesel oil used as a collector was added into the pulp with a microsyringe, and the

SC

conditioning process was carried out for 120 s. Lastly, methyl isobutyl carbinol (MIBC) that was used as a foaming agent, was added to the pulp, and another conditioning process was performed

NU

for 60 s. The collector dosages were 4, 10, 20, 30, and 50 kg/t. Therefore, according to the

MA

exploratory experiments listed earlier, the dosage of the foaming agent (MIBC) was kept constant at 0.4 kg/t (optimal dosage), and the aeration rate was 0.2 m3/h. After opening the inlet valve and

D

the scraper, the froth concentrates were collected for 300 s. The clean coal and tailings were

PT E

filtered and vacuum dried at 60 °C, weighed, and collected for ash determination. The combustible matter recovery (EC) and flotation efficiency index (εc) were used to analyze the flotation results,

AC

CE

which can be calculated using Eqs. (1) and (2), respectively.

EC (%) =

MC (100 - AC )  100 MF (100 - AF )

(1)

εC (%) =

MC (AF - AC )  100 MFAF (100 - AF )

(2)

where MC is the weight of the concentrate (%), AC is the ash content of the concentrate (%), MF is the weight of the feed (%), and AF is the ash content of the feed (%). 2.6.2. Oily bubble flotation tests The oily bubble flotation tests were performed with the oily bubble flotation system, as shown 10

ACCEPTED MANUSCRIPT in Fig. 3, whereas Fig. 7 presents the specific flotation procedures. Unlike the conventional flotation, the suspension of coal samples and water was preceded by a prewetting process over 240 s. After the prewetting process, the MIBC frother was added into the flotation pulp. The pulp was then conditioned for 60 s. Meanwhile, the collector was first held in an atomizer and dispersed

PT

into fine oil drops by an air compressed atomizer. After this step, these fine oil drops were

RI

vaporized by the heating tube that was preheated to 280 °C. The diesel oil drops mixed with air

SC

were sucked into the flotation cell after opening the inlet valve, and the flotation was conducted for 300 s to collect either the froth products or clean coal.

NU

3. Results and discussions

MA

3.1. Characteristics of low-medium rank coal slime 3.1.1. Size distribution of coal slime samples

D

The coal sample was screened to six size fractions using a wet screening test with a set of

PT E

standard sieves. The test results, i.e., particle sizes and ash content distributions, are listed in Table 2. It was found that the dominant size fraction of −0.045 mm accounted for more than 50%, and

CE

the ash content was as high as 45.07%, which indicated the existence of sliming of highly ground

AC

ash, fine-gangue minerals. According to flotation kinetics, the collision probability of ultrafine particles with bubbles is considerably low because these particles with small inertia readily follow the liquid streamlines around the rising bubbles instead of establishing contact with the bubbles in the flotation pulp, thereby resulting in low-mineralization efficiency [30, 37]. In addition, these ultrafine particles with highly ground ash are susceptible to adhere into cracks and pores on the surface of coarse particles in the flotation pulp. As a result, it is difficult to obtain the effective separation of ultrafine coal particles from ultrafine slime. During the flotation process, the 11

ACCEPTED MANUSCRIPT high-ash slime is readily transferred to the froth product by mechanical entrainment, thereby polluting the clean coal. More importantly, the yield of coal particles with size fractions in the range of 0.074−0.125 mm was 17.33%, which is the optimal size range for fine-coal flotation. 3.1.2. Density distribution of coal slime samples

PT

The settlement separation of particles with different densities in mixed organic heavy liquids was

RI

performed with a high-speed centrifuge. Centrifugal liquids at different densities, namely, 1.3, 1.4,

SC

1.5, 1.6, and 1.8 g/cm3, were prepared by mixing benzene, carbon tetrachloride, and tribromethane. The experiments were carried out sequentially from low to high density. For every density

NU

separation experiment, after centrifugation for 10 min, the float was collected, filtered, and

MA

vacuum dried, for density and ash content distribution analyses. Table 3 shows the detailed data of density and ash content distributions of the coal samples.

D

As seen in Table 3, the cumulative yield of the low-density fraction (less than 1.5 g/cm3) is

PT E

32.76%, while the ash content of cumulative floating objects is 7.99%, which is the main source of low-ash clean coal. The content of the intermediate density fraction (1.5−1.8 g/cm3) is 28.17%,

CE

and its ash content is 25.16%, which suggests that good liberation or complete liberation were not

AC

attained, i.e., the valuable coal was closely associated with the useless gangue minerals. In addition, the yield of the high-density fraction (more than 1.8 g/cm3)—one of the major components of tailings in flotation—is 38.67%, and its ash content is 64.58%. According to the density and ash content distributions analyses, a yield of 60.93% was obtained for clean coal, while the ash content was 15.93%, at a separation density of 1.8 g/cm3. 3.1.3. XPS analysis for surface chemical properties of coal-slime samples The surface chemical properties of the coal samples were identified by XPS analyses. Table 4 12

ACCEPTED MANUSCRIPT presents the broad scanning results obtained for the surface of coal samples for primary elements, such as carbon, oxygen, silicon, and aluminum. The peaks at binding energies of 284.66, 531.95, 399.75, 102.81, and 74.57 eV, belong to C1s, O1s, Si2p, and Al2p, respectively. The concentration of oxygen was found to be as high as 45.04% on the coal surface, and the O/C concentration ratio

PT

reached 1.20, which may indicate poor coal sample surface hydrophobicity. Additionally, the total

RI

atomic content of silicon and aluminum, which resulted in hydrophilic sites on the coal surface,

SC

was 17.99%.

Narrow scanning for carbon and peak-fitting analysis for C1s were performed to identify the

NU

chemical form and content of organic oxygen. The fitted results of C1s using the software

MA

XPSPEAK 4.12 are shown in Fig. 8. The peaks of C1s at binding energies of 284.6 eV, 285.82 eV, 287.30 eV, and 289.20 eV, are respectively assigned to the groups of C-H/C-C, C-O-C/C-OH

D

(alcohol, phenol, or ether), C=O (carbonyl), and O=C-O (carboxyl). The relative content of

PT E

different functional groups was calculated according to their peak areas, and the results are listed in Table 5. The results indicated that the content of the oxygenated functional or hydrophilic

CE

groups was 34.36%. Additionally, it was found that the organic oxygen on the coal surface mainly

AC

exists in the form of alcohol, phenol, ether, and carbonyl groups, and some in the form of carboxyl groups. It is well known that the oxygenated functional groups, i.e., hydrophilic groups, on the coal’s surface increased the hydrophilic sites on the coal’s surface, making the coal hydrophilic. Furthermore, oxygen atoms are negatively charged and form hydrogen bonds with water molecules, which results in a stable hydration film on the coal’s surface. As a result, the hydrophilicity of coal increases further, and the energy barrier for the process of attachment of coal particles onto bubbles also increases. Therefore, it can be envisaged that it is difficult for 13

ACCEPTED MANUSCRIPT these coal particles to complete the mineralization process during flotation and achieve effective separation. 3.1.4. SEM and BET analyses for surface morphology of coal slime samples The representative SEM images for the surface morphology of coal samples with full and

PT

−0.045 mm size fractions are shown in Fig. 9. It is observed in Fig. 9c that the particle size

RI

distribution of the coal sample less than 0.045 mm was uniform, which agreed with its size

SC

distribution analyses. The surface morphology of both coal samples was found to be clearly rugged with a large amount of pores and cracks, as seen in Fig. 9b and 9d. Additionally, the coarse

NU

coal particle surface and its pores were found to absorb many ultrafine particles, as shown in Fig.

MA

9b. To further understand the pore structure on the coal’s surface, specific surface area measurements were performed, and the detailed results, including pore volume, specific surface

D

area, pore size distributions, and average pore diameter, are presented in Table 6 and Fig. 10. As

PT E

shown in Table 6, the surface area of raw coal is 6.114 m2/g and the pore volume is 0.014 cm3/g. Coal that had a size fraction smaller than 0.045 mm, was found to have larger pore volume,

CE

surface area, and average pore diameter, compared to raw coal. Additionally, it should be noted

AC

that coal samples with a size fraction of −0.045 mm have an increased mesopore (2–50 nm) than coal samples with full size fraction, as seen in Fig. 10. It is well known that the pores and cracks on the coal surface will be filled up with water when they establish contact with water in the flotation pulp. As a result, the coal surface will be wrapped by a hydration film, rendering coal hydrophilic. It is considered that the hydration film on the surface of coal that is less than 0.045 mm may be thicker and more stable because it contains an increased number of pores and cracks on its surface. Furthermore, this behavior may reduce the hydrophobic sites of the coal’s surface 14

ACCEPTED MANUSCRIPT leading to a decreased hydrophobicity. On the other hand, more pores and an increased surface area imply that an increased collector quantity is required to increase the hydrophobicity of the coal’s surface in flotation. In summary, it is concluded that the ultrafine particles that account for more than 50% of clean

PT

coal would have a negative influence on its efficient recovery because of its low-collision

RI

probability and floatability. In addition, there were abundant oxygenated functional groups on the

SC

coal’s surface that render the coal hydrophilic. Furthermore, the hydration film on the coal’s surface caused by its pores and cracks makes the attachment of coal particles onto air bubbles

NU

difficult to achieve the mineralization process in conventional flotation paradigms. More

MA

importantly, it is envisaged that it will be difficult to attain the economic recovery of coal samples owing to high collector consumption resulting from the large surface area and developed pores.

D

Consequently, the coal sample exhibits low floatability. Correspondingly, it will be difficult to

PT E

achieve a satisfactory separation efficiency using the conventional flotation method. 3.2. Flotation results and optimization of technological parameters for oily bubble flotation of

CE

low-medium rank coal

AC

3.2.1. Conventional flotation results Conventional flotation was carried out to evaluate the floatability of coal slime samples. Fig. 11 illustrates the conventional flotation results, and the flotation performance was evaluated based on the calculated recovery of combustible matter, the flotation efficiency index, and the ash content of concentrate. The results indicated that the oily collector dosage in conventional flotation was maintained at a high level ranging from 10–50 kg/t. As the dosage of the collector increased, a sudden increase was documented in the combustible matter recovery and flotation efficiency index. 15

ACCEPTED MANUSCRIPT The combustible matter recovery and flotation efficiency index only reached levels of 42.21% and 30.61%, respectively, when the collector consumption was as high as 10 kg/t, which indicated the poor floatability of coal samples. Elicited results were consistent with the analyzed results obtained from XPS, SEM, and BET. When the dosage of the collector reached to 50 kg/t, the

PT

concentrate had an ash content of 8.75%, a combustible matter recovery of 88.18%, and a flotation

RI

efficiency index of 68.46%, which indicates good selectivity and strong collecting power for the

SC

coal samples. This behavior may be attributed to the separation effect that results from oil agglomeration. However, it should be noted that the collector dosage is too high to be accepted by

NU

the coal preparation plant. Therefore, it is concluded that the conventional flotation of the

MA

low-medium rank coal owing to its natural hydrophilicity and developed surface pores and cracks, encountered high collector dosage and low recovery problems. Therefore, increased attention must

D

be paid to the economic recovery of low-medium rank coals.

PT E

3.2.2. Preliminary exploration on test conditions of oily bubble flotation of low-medium rank coal To obtain satisfactory yields or combustible matter recovery, high oily collector consumption

CE

within the range of 20–50 kg/t is usually required in conventional flotation of low-medium rank

AC

coals. To achieve such a task, a novel, oily bubble flotation technology was proposed to enhance the flotation of low-medium rank coal with a smaller collector dosage. This technology used oily bubbles instead of air bubbles as a carrier, thereby improving considerably the collecting power of the carrier for coal particles. In this section, the oily bubble flotation experiments at different collector dosages, pulp concentration, impeller speed, and air flow rate, were carried out in conjunction with relevant data analyses to determine the effective parameters and their levels in oily bubble flotation of low-medium rank coal. Moreover, the frother dosage was kept constant at 16

ACCEPTED MANUSCRIPT 0.4 kg/t in all oily bubble flotation tests for consistency with the conditions used in the conventional flotation. 3.2.2.1. Effect of collector dosages on oily bubble flotation of coal samples Fig. 12 presents the oily bubble flotation results at different collector dosages (1, 2, 4, 6, and

PT

8 kg/t). The flotation pulp concentration was 60 g/L, the impeller speed was 2000 rev/min, and the

RI

aeration rate was 0.2 m3/h. It was found that the recovery of combustible matter and flotation

SC

efficiency index markedly increased, while the ash content declined, with noted increases in collector consumption. A concentrate with an ash content of 9.32%, a combustible matter recovery

NU

of 94.42%, and a flotation efficiency index of 71.47%, were obtained when the dosage of the

MA

collector reached 4.0 kg/t. These outcomes were superior to those obtained from the conventional flotation of coal samples at the collector dosage of 50 kg/t, as seen in Figs. 11 and 12. This

D

behavior indicated that oily bubble flotation exhibited good selectivity and strong collecting power

PT E

for coal samples, and that the oily bubble flotation could significantly reduce the collector’s consumption. However, it should be noted that the curves of combustible matter recovery and

CE

flotation efficiency index tend to level when the dosage is higher than 4 kg/t. In other words, the

AC

recovery could not be considerably improved by further increasing the collector dosage. Therefore, the collector dosage was fixed at 4 kg/t in the subsequent oily bubble flotation tests. In the conventional flotation, the oily collectors, such as diesel and kerosene, are directly added into the flotation pulp or water phase, as shown in Fig. 13a. However, the oily collector is insoluble. It is therefore difficult to adequately disperse it in water because of the thermodynamic instability due to the increased interfacial area. Even though the dispersion of the oily collector in the flotation pulp can be enhanced by the energy input generated from mechanical agitation, the 17

ACCEPTED MANUSCRIPT collector is still dispersed in the pulp in the form of fine oil droplets. In this case, the attachment of coal particles and air bubbles occurs after the adsorption of collector on the coal particle surface and air-water interface, as shown in Fig. 13a. As a result, the attachment process is delayed owing to the lack of direct contact between coal particles and air bubbles. In comparison to the

PT

conventional flotation, the oily collector was spread on the surface of the air bubbles in the form

RI

of an oil film in the oily bubble flotation, as shown in Fig. 13b, thereby considerably enhancing

SC

the dispersion of the oily collector in the pulp. Consequently, the collecting power was enhanced because of the higher concentration of collector molecules localized on the air-water interface.

NU

Moreover, the direct contact between coal particles and air bubbles was attained. In other words,

MA

the coal particles are captured by air bubbles when they adhere onto oily bubbles. As a result, the expended work for adhesion during the attachment process is decreased, thus enhancing the

D

attachment efficiency. Furthermore, the collector consumption in flotation was reduced because of

PT E

a greater degree of dispersion of diesel oil, which reduced the collector molecules in the flotation pulp, and hence avoided the unexpected mineralization and capture for gangue mineral particles,

CE

thereby enhancing the selectivity of the bubbles.

AC

For a successful flotation, the mineralization process, i.e., the attachment of the hydrophobic particles onto the air bubbles should be completed after the collision process in the flotation pulp. The captured particles are then floated and transferred into the froth zone. As a result, a specific separation between the coal particles and gangue minerals is achieved to which the attachment process between particles and bubbles is crucial. Particle-bubble attachment involves a series of events, as seen in Fig. 14. When particles closely approach bubbles after collision, the hydration film, or wetting film, forms between them. If attractive surface forces are large enough, the 18

ACCEPTED MANUSCRIPT hydration film becomes unstable and thins to a critical thickness. In the second stage, as the distance between particles and bubbles further declines, the film ruptures to form a three-phase contact line with a critical radius. Finally, the expansion of the three-phase contact line occurs to form a wetting perimeter required for a stable attachment. The time required for completing the

PT

attachment processes is called the attachment time [54]. To ensure the attachment between coal

RI

particles and bubbles, the attachment time must be less than the contact time. Therefore, the

SC

attachment time is an effective and sensitive parameter for the evaluation of the attachment efficiency. Fig. 15 presents the results of attachment between coal particles and (a) air, or (b) oily

NU

bubbles. It was found that the attachment time of particles onto air bubbles was 315.1 ms, while

MA

that of particles onto oily bubbles was considerably decreased to 51.7 ms. These findings suggested that upon use of oily bubbles, the attachment efficiency was significantly enhanced,

D

which agreed with the flotation results shown in Figs. 11 and 12. This may be attributed to a

PT E

stronger, long range, molecular force of attraction between the oil and coal interface compared to that between air and coal. Given that this force arose from hydrophobic surfaces, it was considered

CE

deterministic for the wetting film’s lifetime and for the expansion rate of the three-phase contact

AC

line [53–56]. More importantly, it was observed that oily bubbles picked up more particles than air bubbles, as shown in Fig. 15, which meant that there existed a stronger tendency for collecting oily bubbles than air bubbles. 3.2.2.2. Effect of flotation pulp concentration, impeller speed, and aeration rate, on the oily bubble flotation of low-medium rank coal The collector dosage was kept constant at 4 kg/t, and the optimum flotation pulp concentration, impeller speed, and aeration rate for oily-bubble flotation of long-flame coal were successively 19

ACCEPTED MANUSCRIPT determined. The floatation results are illustrated in Fig. 16. As shown in Fig. 16a, it was found that the best flotation performance was attained at the pulp concentration of 60 g/L. Above this concentration, the flotation performances exhibited decreases in the combustible matter recovery and in the flotation efficiency index. The ash content showed a gradual increase as the flotation

PT

pulp concentration increased, which suggested that the mechanical entrainment of highly ground

RI

ash fine slime intensified with the increase of pulp concentration, resulting in the increase of ash

SC

content in the cleaned coal. Additionally, increased pulp concentration indicates that a high content of fine slime in the flotation pulp, and a fine slime with greater specific surface area, will

NU

lead to ineffective adsorption of a large collector amount. This behavior led to a decrease in the

MA

combustible matter recovery at high concentrations. Consequently, an efficient flotation is necessary to keep the pulp concentration within a reasonable range, and the optimal pulp

D

concentration for oily bubble flotation was 60 g/L.

PT E

Fig. 16b presents the effect of the impeller’s speed on the flotation performance. The flotation performance increased initially and then decreased with increases in the impeller’s speed.

CE

However, the ash content showed a completely opposite variation trend with impeller speed

AC

increases. It is well known that low-intensity turbulence for the pulp caused by low impeller speeds is detrimental to the collisions and adhesions of oily bubbles and particles, especially for microfine particles, resulting in a poor flotation performance. Even if the collision between microfine particles and bubbles is attained due to the low inertia of microfine particles, a considerable energy input is still required to rupture the hydration film between them to form a proper three-phase contact line for a successful attachment [57, 58]. Additionally, inadequate dispersion between coal particles and gangue mineral particles, or particle aggregation resulting 20

ACCEPTED MANUSCRIPT from low impeller speeds, led to a low selectivity in flotation, increasing the ash content of concentrates. However, impeller speeds that are too high are also detrimental to flotation, which increases the probability of detachment of coal particles from oily bubbles, thereby leading to a decline in the combustible matter recovery. Moreover, a low selectivity was obtained owing to the

PT

entrainment of gangue mineral particles caused by the increased intensity of turbulence. This also

RI

led to an increase of the ash content of concentrates. As a result, it is necessary for an efficient

SC

flotation to keep the impeller speed within a reasonable range, and the best flotation performance was attained at an impeller speed of approximately 2200 rev/min.

NU

A similar effect of air flow rate on flotation performance to that of impeller speed was observed

MA

in Fig. 16c. The best flotation performance was obtained at the air flow rate of 0.20 m3/h. When the air flow rate was insufficient, fine particles due to their small gravity were preferentially

D

floated, and transferred to the froth products. However, coarse particles with low ash failed to

PT E

attach themselves to bubbles and were left in the pulp, which led to a low combustible matter recovery and relatively high ash content. When the air flow rate becomes excessive (more than

CE

0.25 m3/h), it may cause merging of the bubbles, an environmental disorder of the flotation pulp,

AC

and instability in the froth zone, which is also unfavorable to mineralization in flotation. Correspondingly, the flotation performance exhibited a decline at air flow rates higher than 0.20 m3/h.

Summarizing the above, it can be preliminarily concluded that when the collector and foaming agents were kept constant at 4 kg/t and 0.4 kg/t, respectively, the optimal technological parameters of oily bubble flotation for low-medium rank coal were as follows: flotation pulp concentration of 60 g/L, impeller speed of 2200 rev/min, and air flow rate of 0.2 m3/h. 21

ACCEPTED MANUSCRIPT 3.2.3. Optimization of technological parameters for oily bubble flotation of low-medium rank coal 3.2.3.1. Box–Behnken experimental design The oily bubble flotation results presented above indicated that various factors, including collector dosage, flotation pulp concentration, impeller speed, and air flow rate, had significant

PT

effects on the flotation of coal samples. In this section, the response-surface methodology, namely

RI

a statistical design method, was employed to determine the optimum parameter combination of

SC

oily bubble flotation by simultaneous optimization of the multiple response variables. Additionally, the software Design-Expert 8.0.5 was used for the experimental design and for the analyses of the

NU

flotation test results, in order to construct mathematical models of combustible matter recovery

MA

[59, 60]. While the Box–Behnken design method—derived from three-level factorial designs—is a commonly used response-surface methodology that is provided by the software Design-Expert

D

8.0.5, and depends on the Box–Behnken design matrix [61]. Use of the Box–Behnken method,

PT E

and in accordance to the above oily bubble flotation results, the four-factor and three-level experimental designs for collector dosage, flotation pulp concentration, impeller speed, and air

CE

flow rate, are shown in Table 7. Twenty-nine oily bubble flotation experiments were performed to

AC

reveal the effects of these four parameters on the flotation performance. Therefore, the detailed parameter combination for the experimental design, the random order adopted for the execution of experiments, and the flotation results, are presented in Table 8. As seen from Table 8, the highest combustible matter recovery of 96.37% was obtained at the 24th run, while the lowest combustible matter recovery was obtained at the 21th run. 3.2.3.2. Establishment of model for the combustible matter recovery and variance analysis For the data fitting of the experimental results, Design-Expert 8.0.5 provides four function 22

ACCEPTED MANUSCRIPT models. The fitted results of these function models are listed in Table 9. This indicates that the quadratic model showed increased adjusted and predicted regression coefficients (0.9830 and 0.9666), which indicates that the model could explain 96.66% of changes in the combustible matter recovery using the four parameters. In other words, the model was considerably powerful.

PT

Therefore, the quadratic model was used for data fitting of the experimental results. The generated

RI

results of variance analyses for the combustible matter recovery are listed in Table 10. The P value

SC

of the quadratic model is far less than 0.01, while the P value for lack of fit (error P value) is much higher than 0.05. These findings indicate that this model exhibits a high correlation for

NU

combustible matter recovery, i.e., this model is statistically significant. After the quadratic,

MA

multiple regression fitting, is applied on the experimental results, the regression model of combustible matter recovery can be expressed as

D

Y=95.27+1.70×A+3.09×B−2.21×C+2.39×D−0.34×A×B−0.10×A×C+0.020×A×D−0.13×B×C−0.1

PT E

6×B×D−0.15×C×D−3.09×A2−3.24×B2−2.90×C^2−4.21×D2 where Y is the combustible matter recovery, A is the impeller speed, B is the collector dosage, C is

CE

the flotation pulp concentration, and D is the air flow rate.

AC

The function model for the combustible matter recovery can be used to calculate and predict the combustible matter recovery of the flotation of oily bubbles at different collector dosages, pulp concentrations, impeller speeds, and air flow rates. Meanwhile, the same model was used to map the response surfaces and contour maps, and optimize the scheme of oily bubble flotation, as described in the Sections 3.2.3.3 and 3.2.3.4.

23

ACCEPTED MANUSCRIPT 3.2.3.3. Analysis of interaction effects of variables on the combustible matter recovery using response-surface methodology To understand the interaction between variables and their effects on the response values, 3D graphs and contour maps were portrayed according to the quadratic, multiple function model for

PT

combustible matter recovery. These are presented in Fig. 17. The color and height of the points on

RI

response surfaces (3D graphs) represent the magnitude of the response values, i.e., the higher the

SC

heights of the points are on the response surfaces, and the closer their colors are to red, the larger the response values are. The contour line is the projection of points on the response surface.

NU

Additionally, the closer the shapes of the contour lines are to circles, the weaker the interactions

MA

between the two factors are. On the contrary, the closer the shapes of contour lines are to ellipses, the stronger the synergistic effects of the factors are on the response values. When the response

D

values have extremums in the tested range, the highest point of the response surfaces will be

PT E

exactly at the center point of the smallest ellipses in the contour maps. When the flotation pulp concentration and air flow rate were set as the central values (level 0

CE

values in Table 7), the interaction effect of the impeller speed and collector dosage on combustible

AC

matter recovery can be represented as shown in Fig. 17 (a and b). It was found that to increase combustible matter recovery, higher collector dosage and higher impeller speed was required—up to 2175 rev/min—but it was reversed for higher impeller speeds. In contrast, at low collector dosages, no obvious change was found in combustible matter recovery with increasing impeller speeds. Consequently, it seemed that higher collector dosages elicited higher recovery, and there was an optimum combination of impeller speed and collector dosage that led to a maximum combustible matter recovery. As shown in Fig. 17 (c and d), a contour line that was approximately 24

ACCEPTED MANUSCRIPT elliptical was observed, which reflects a considerable interaction between impeller speed and aeration rate. The contour lines are densely distributed along the axial direction of the air flow rate, and are sparsely distributed along the axial direction of the impeller speed. This suggests that the effect of air flow rate on combustible matter recovery was superior to that of the impeller speed.

PT

More importantly, the combustible material recovery was found to reach maximum value at higher

RI

air flow rates and impeller speeds, while at low air flow rates the degrees of variations of

SC

combustible matter recovery, and the increase of impeller speed, were small. Fig. 17 (e and f) shows that an increase in the collector dosage caused significant increases and subsequent

NU

decreases in combustible matter recovery at low-pulp concentrations, and led to minor variations

MA

at high-pulp concentration. Similarly, it can be found from Fig. 17 (g and h) that as the collector dosage increases, the combustible matter recovery increases dramatically to the maximum value at

D

medium-to-high air flow rates, while the variation it exhibits is small at low air flow rates. In other

the maximum value.

PT E

words, when the air flow rate and the collector dosages were large, the response value tended to

CE

3.2.3.4. Optimization of oily bubble flotation scheme

AC

According to the results of the experimental design and the quadratic model of combustible matter recovery, the optimization schemes used for maximum combustible matter recovery, as calculated by Design-Expert 8.0.5, are shown in Table 11, when the response value (combustible matter recovery) was set as its maximum of 96.37%. Four factors were set within the ranges in Table 7. The results indicate that the expectation of each scheme is 100%, as seen in Table 11, which indicates that the fitting of the data is highly reliable. Theoretically, the combustible matter recovery reached 96.53% at the minimum collector dosage of 4.35 kg/t (scheme 2). However, the 25

ACCEPTED MANUSCRIPT present study focused on realizing the economic recovery of low-medium rank coal. Therefore, the optimization schemes for reduction of collector consumption were obtained when the collector dosage was set as the minimum value of 2 kg/t (Table 13). Other parameters were set within the ranges, as shown in Table 12. It was found that the expectation of Scheme 1 was highest, and the

PT

collector dosage reached the minimum value of 2.32 kg/t. Meanwhile, the theoretical combustible

RI

matter recovery was as high as 91.50%. Compared to the optimization scheme for maximum

SC

combustible matter recovery, even though the combustible matter recovery led to a slight decrease, the collector dosage decreased by 47.67%. Consequently, the optimal technological parameters of

NU

oily bubble flotation for low-medium rank coal were determined as follows: the collector dosage

MA

was 2.32 kg/t, the flotation pulp concentration was 52.47 g/t, the impeller speed was 2198.53 rev/min, and the air flow rate was 0.23 m3/h.

D

To confirm the reliability of the optimization schemes for maximum combustible matter

PT E

recovery and collector consumption reduction, corresponding oily bubble flotation tests were performed. Both types of flotation experiments were repeated three times, and their average value

CE

was calculated and considered as the final outcomes. The maximum combustible matter recovery

AC

was found to be 95.12% when the calculated parameters of Scheme 2 in Table 11 were used in the practical flotation, which corresponded to the high forecasting accuracy of 98.54%. For the practical flotation using the parameters of Scheme 1 in Table 13, the combustible matter recovery was 86.32%, and exhibited a slight deviation compared with the predicted value. This behavior may be attributed to its relatively low expectation. It is concluded that the two optimization schemes, and the function model for combustible matter recovery based on the Box–Behnken design, were highly reliable. A maximum combustible matter recovery of 95.12% could be 26

ACCEPTED MANUSCRIPT attained when the frother dosage, collector dosage, impeller speed, pulp concentration, and air flow rate, were respectively set at 0.4 kg/t, 4.35 kg/t, 2152.50 rev/min, 56.00 g/L, and 0.22 m3/h. Upon use of the optimization scheme for collector consumption reduction, a combustible matter recovery of 86.32% was obtained at a collector dosage of 2.32 kg/t, which was comparable to the

PT

conventional flotation performance at collector dosages of 50 kg/t. In other words, at comparable

RI

combustible matter recovery, the collector dosage was reduced by 95.36% using this optimization

SC

scheme of oily bubble flotation. 4. Conclusions

NU

This study aimed to achieve the effective recovery of long-flame coal slime in the Daliuta coal

MA

preparation plant. The characteristics of low-medium rank coal samples were comprehensively investigated, which included their size and density distributions, surface chemical properties, and

D

surface morphology (i.e., structural parameters and morphology of pores). The results suggested

PT E

that ultrafine (−0.045 mm) particles accounted for more than 50% of the coal sample, and their ash content was 45.07%. In order to obtain the effective recovery of coal samples, increased attention

CE

should be paid on this size fraction. XPS analyses indicated that there are many oxygenated

AC

functional groups on the coal surface, thereby rendering the coal hydrophilic. Additionally, according to the SEM and BET analyses, it was found that there were developed pores and cracks on the surfaces of the coal, which may further increase the hydrophilicity of coal samples by coating the coal with a thick hydration film. As a result, the collector dosage needed to obtain a satisfactory yield in the conventional flotation was as high as approximately 30 kg/t. In order to attain economic recovery of the coal samples, the oily bubble flotation technology was employed. The results indicated that the concentrate yielded an ash content of 9.32%, increased combustible 27

ACCEPTED MANUSCRIPT matter recovery of 94.42%, and a high-flotation efficiency index of 71.47%, when the dosage of the collector reached 4.0 kg/t. These outcomes were superior to those obtained based on the conventional flotation of coal samples at a collector dosage of 50 kg/t. In order to determine the optimal technological parameter combination for oily bubble flotation of the coal samples, the

PT

built-in response-surface methodology in Design-Expert 8.0.5 was used for the experimental

RI

design and relevant analyses of the flotation results. It was found that the parameters had a

SC

profound effect on the flotation performance in the oily bubble flotation tests, which involved collector dosage, impeller speed, flotation pulp concentration, and air flow rate. Based on the

NU

fitting of the flotation results, a quadratic function model for the combustible matter recovery was

MA

established using Design-Expert 8.0.5. This model was used to calculate and predict the combustible matter recovery at different conditions, without the conduct of practical flotation

D

experiments. Furthermore, according to the results of the experimental design and the quadratic

PT E

model of combustible matter recovery, the optimization scheme for oily bubble flotation of the coal samples was obtained using Design-Expert 8.0.5. Using the optimization scheme for collector

CE

consumption reduction, a combustible matter recovery of 86.32% was obtained at a collector

AC

dosage of 2.32 kg/t that was comparable to the conventional flotation performance at a collector dosage of 50 kg/t.

Acknowledgements

This study was supported by the Fundamental Research Funds for the Central Universities (Grant No. 2018BSCXA10) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (Project Name: Study on interactions and the mechanisms of mineralization between low rank coal particles and air/oily bubbles, which is under the charge of Songjiang Chen during 2018 28

ACCEPTED MANUSCRIPT to 2019). References [1]

K.B. Quast, D.J. Readett, The surface chemistry of low rank coals, Advances in Colloids and Interface

Science 27 (1987) 169-187.

S. Dey, G.M. Paul, S. Pani, Flotation behavior of weathered coal in mechanical and column flotation cell,

PT

[2]

M. Cinar, Floatability and desulfurization of a low-rank (Turkish) coal by low-temperature heat treatment,

Fuel Processing Technology 90 (2009) 1300-1304.

Hussin A.M. Ahmed, J. Drzymala, Upgrading difficult-to-float coal using microemulsion, Minerals &

[5]

MA

Metallurgical Processing 29 (2012) 88–96.

NU

[4]

SC

[3]

RI

Powder Technology 246 (2013) 689-694.

T. Uslu, E. Sahinoglu, and M. Yavuz, Desulphurization and deashing of oxidized fine coal by Knelson

S. Chen, L. Tang L, X. Tao, L. Chen, Z. Yang, L. Li, Effect of oxidation processing on the surface properties

PT E

[6]

D

concentrator, Fuel Processing Technology 101 (2012) 94–100.

and floatability of Meizhiyou long-flame coal, Fuel 210 (2017) 177-186.

T. Li, Study on flotation of Yi-tai low-rank coal slime, China University of Mining and Technology 2014.

[8]

J. Qu, Research on reactive oily bubble flotation behavior of low rank coal and its flotation technique, China

AC

CE

[7]

University of Mining and Technology 2015.

[9]

S. Dey, Enhancement in hydrophobicity of low rank coal by surfactants-a critical overview, Fuel Processing

Technology 94 (2012) 151-158. [10] Y. Cebeci, The investigation of the floatability improvement of Yozgat Ayrıdam lignite using various

collectors, Fuel 81 (2002) 281-289.

[11] W. Xia, G. Xie, Y. Peng, Recent advances in beneficiation for low rank coals, Powder Technology 277 (2015)

29

ACCEPTED MANUSCRIPT

206-221.

[12] W. Xia, J. Yang, B. Zhu, Flotation of oxidized coal dry-ground with collector, Powder Technology 228 (2012)

324–326.

[13] W. Xia, C. Zhou, and Y. Peng, Enhancing flotation cleaning of intruded coal dry-ground with heavy oil,

PT

Journal of Cleaner Production 161 (2017) 591-597.

RI

[14] Jovica M. Sokolovic, R.D. Stanojlovic, Z.S. Markovic, Activation of oxidized surface of anthracite waste

SC

coal by attrition, Physicochemical Problems of Mineral Processing 48 (2012) 5-18.

[15] H.A. Temel, A.K. Majumder, Investigation of the effectiveness of desliming and flotation in cleaning

NU

Malatya-Arguvan lignite, Energy Sources 38 (2016) 1048-1054.

MA

[16] E. Lester, S. Kingman, The effect of microwave pre-heating on five different coals, Fuel 83 (2004)

1941-1947.

D

[17] G. Oezbayoglu, T. Depci, N. Ataman, Effect of microwave radiation on coal flotation, Energy Sources Part

PT E

A-Recovery Utilization And Environmental Effects 31 (6) (2009) 492–499.

[18] B.K. Sahoo, S. De, B.C. Meikap, Improvement of grinding characteristics of Indian coal by microwave

CE

pre-treatment, Fuel Processing Technology 92 (2011) 1920–1928.

AC

[19] B.C. Meikap, N.K. Purohit, V. Mahadevan, Effect of microwave pretreatment of coal for improvement of

rheological characteristics of coal–water slurries, Journal of Colloid and Interface Science 281 (2005)

225–235.

[20] E. Lester, S. Kingman, Effect of microwave heating on the physical and petrographic characteristics of a U.K.

coal, Energy & Fuels 18 (1) (2004) 140–147.

[21] W. Xia, J. Yang, C. Liang, A short review of improvement in flotation of low rank/oxidized coals by

pretreatments, Powder Technology 237 (2013) 1-8.

30

ACCEPTED MANUSCRIPT

[22] W. Xia, J. Yang, and C. Liang, Effect of microwave pretreatment on oxidized coal flotation, Powder

Technology 233 (2013) 186-189.

[23] S.G. Ozkan, Effects of simultaneous ultrasonic treatment on flotation of hard coal slimes, Fuel 93 (2012)

576-580.

PT

[24] S.G. Ozkan, and H. Z. Kuyumcu, Investigation of mechanism of ultrasound on coal flotation, International

RI

Journal of Mineral Processing 81 (2006) 201-203.

SC

[25] S.G. Ozkan, Beneficiation of magnesite slimes with ultrasonic treatment, Minerals Engineering 15 (2002)

99–101.

NU

[26] W. Xia, C. Ni, and G. Xie, Effective Flotation of Lignite Using a Mixture of Dodecane and 4-Dodecylphenol

MA

(DDP) as a Collector, International Journal of Coal Preparation & Utilization 36 (2016) 262-271.

[27] R. Yang, L. Shen, and H. Wang, Research on emulsified tyre pyrolysis oil as a coal flotation collector."

D

Physicochemical Problems of Mineral Processing 53 (2017) 279-287.

PT E

[28] X. Gui, Y. Xing, T. Wang, et al, Intensification mechanism of oxidized coal flotation by using oxygen-containing collector α-furanacrylic acid, Powder Technology 305 (2017) 109-116.

CE

[29] B. Wen, W. Xia, and J.M. Sokolovic, Recent advances in effective collectors for enhancing the flotation of

AC

low rank/oxidized coals, Powder Technology 319 (2017) 1-11.

[30] Y. Xing, X. Gui, Y. Cao, et al, Clean low-rank-coal purification technique combining cyclonic-static

microbubble flotation column with collector emulsification, Journal of Cleaner Production 153 (2017)

657-672.

[31] O.H. Han, M.K. Kim, B.G. Kim, et al, Fine coal beneficiation by column flotation, Fuel Processing

Technology 126 (2014) 49-59.

[32] D. Tao, S. Yu, X. Zhou, et al, Picobubble column flotation of fine coal, International Journal of Coal

31

ACCEPTED MANUSCRIPT

Preparation & Utilization 28 (2008) 1-14.

[33] D. Tao, B. Li, S. Johnson, et al, A flotation study of refuse pond coal slurry, Fuel Processing Technology 76

(2002) 201-210.

[34] H. Zhang, J. Liu, Y. Wang, et al, Cyclonic-static micro-bubble flotation column, Minerals Engineering 45

PT

(2013) 1-3.

RI

[35] R.H. Yoon, Microbubble flotation, Minerals Engineering 6 (1993) 619-630.

SC

[36] J. wang, L. Wang, J. Hanotu, et al, Improving the performance of coal flotation using oscillatory air supply,

Fuel Processing Technology 165 (2017) 131-137.

NU

[37] J. Liu, T. Mak, Z. Zhou, et al, Fundamental study of reactive oily-bubble flotation, Minerals Engineering 15

MA

(2002) 667-676.

[38] E. Kusaka, Y. Kamata, Y. Fukunaka, et al, The role of hydrolyzed metal cations in the liquid-liquid extraction

D

of ultrafine silica with dodecyl sulfate, Minerals Engineering 10 (1997) 155-162.

PT E

[39] E. Kusaka, Y. Nakahiro, and T. Wakamatsu, The role of zeta potentials of oil droplets and quartz particles

during collectorless liquid-liquid extraction, International Journal of Mineral Processing 41 (1994) 257-269.

CE

[40] M. Shen, T.D. Wheelock, Coal Agglomeration with Microbubbles, Coal Preparation 21 (2000) 277-298.

AC

[41] M. Shen, T.D. Wheelock, Development and scale-up of a gas-promoted oil agglomeration process for coal

beneficiation, Miner. Met. Process. 18 (2001) 87-94.

[42] F. Zhou, L. Wang, Z. Xu, et al, Reactive oily bubble technology for flotation of apatite, dolomite and quartz,

International Journal of Mineral Processing 134 (2015) 74-81.

[43] F. Zhou, L. Wang, Z. Xu, et al, Application of reactive oily bubbles to bastnaesite flotation, Minerals

Engineering 64 (2014) 139-145.

[44] F. Zhou, L. Wang, Z. Xu, et al, Interaction of reactive oily bubble in flotation of bastnaesite, Journal of Rare

32

ACCEPTED MANUSCRIPT

Earths 32 (2014) 772-778.

[45] W. Xia, J. Yang, Experimental design of oily bubbles in oxidized coal flotation, Gospodarka Surowcami

Mineralnymi-Mineral Resources Management 29 (2013) 129-136.

[46] J. Tang, L. Feng, Y. Li, et al, Fractal and pore structure analysis of Shengli lignite during drying process,

PT

Powder Technology 303 (2016) 251-259.

RI

[47] J. Liu, J. Zhu, J. Cheng, et al, Pore structure and fractal analysis of Ximeng lignite under microwave

SC

irradiation, Fuel 146 (2015) 41-50.

[48] X. Meng, M. Gao, R. Chu, G. Wu, Q. Fang, Multiple linear equation of pore structure and coal-oxygen

NU

diffusion on low temperature oxidation process of lignite, Chinese Journal of Chemical Engineering 24

MA

(2016) 818-823.

[49] S. Chen, L. Tang L, X. Tao, H. He, L. Chen, Z. Yang, Enhancing flotation performance of low rank coal by

D

improving its hydrophobicity and the property of oily bubbles using 2-ethylhexanol, International Journal of

PT E

Mineral Processing 167 (2017) 61−67.

[50] S. Chen, Z. Yang, L. Chen, X. Xiu, L. Tang, H. He, Wetting thermodynamics of low rank coal and

CE

attachment in flotation, Fuel 207 (2017) 214-225.

AC

[51] A.V. Nguyen, J. Ralston, H.J. Schulze, On modelling of bubble-particle attachment probability in flotation.

Int J Miner Process 53 (1998) 225–49.

[52] B. Albijanic, O. Ozdemir, A.V. Nguyen, et al, A review of induction and attachment times of wetting thin

films between air bubbles and particles and its relevance in the separation of particles by flotation, Advances

in Colloid and Interface Science 159 (2010) 1-21.

[53] L. Pan, and R.H. Yoon, Hydrophobic forces in the wetting films of water formed on xanthate-coated gold

surfaces, Faraday Discussions 146 (2010) 325-340.

33

ACCEPTED MANUSCRIPT

[54] S. Tchaliovska, P. Herder, R. Pugh, et al, Studies of the contact interaction between an air bubble and a mica

surface submerged in dodecylammonium chloride solution, Langmuir 6 (1990) 1535-1543.

[55] N.V. Churaev, Aqueous wetting films in contact with a solid phase, Advances in Colloid & Interface Science

114-145 (2005) 3-7.

PT

[56] J. Wang, Z. Li, R.H. Yoon, et al, Surface forces in thin liquid films of n-alcohols and of water-ethanol

RI

mixtures confined between hydrophobic surfaces, J Colloid Interface Sci 379 (2012) 114-120.

SC

[57] J. Ralston, W. Kent, G. Newcombe, Polymer-stabilized emulsions and fine-particle recovery, I. The

calcite-quartz system, Int. J. Miner. Process. 3 (1984) 167–186.

MA

Int. J. Miner. Process. 14 (1985) 217–232.

NU

[58] W. Kent, J. Ralston, Polyer-stabilized emulsions and fineparticle recovery, II. The chalcopyrite-quartz system,

[59] S. Koca, D.O. Aksoy, A. Cabuk, et al, Evaluation of combined lignite cleaning processes, flotation and

D

microbial treatment, and its modelling by Box Behnken methodology, Fuel 192 (2017) 178-186.

PT E

[60] A. Dashti, E. Nasab, Optimization of the performance of the hydrodynamic parameters on the flotation

performance of coarse coal particles using design expert (DX8) software, Fuel 107 (2012) 593–600.

CE

[61] S. Ferreira, R. Bruns, H. Ferreira, et al, Box-Behnken design: an alternative for the optimization of analytical

AC

methods, Anal Chim Acta 597 (2007) 179–86.

34

ACCEPTED MANUSCRIPT Fig. 1. Microflotation cell for reactive oily bubble flotation [37] Fig. 2. Experimental apparatus for oily bubble flotation [45] Fig. 3. Experimental device of flotation of oily bubbles Fig. 4. XRD spectrum of a coal sample Fig. 5. Image of induction timer instrument [49, 50]. Fig. 6. A schematic depicting the measurement of the attachment time of the coal particles [6, 49]. Fig. 7. Oily bubble flotation test procedures Fig. 8. Fitted spectra of the coal sample for C1s Fig. 9. SEM images of coal sample with full (a and b) and (c and d) a size fraction of −0.045mm

Fig. 12. Effect of collector dosages on oily bubble flotation of coal sample Fig. 13. Conventional flotation (a) and oily bubble flotation (b) [37]

RI

Fig. 11. Effect of collector dosages on conventional flotation of coal samples

PT

Fig. 10. Pore size distribution of coal samples with full and −0.045 mm size fractions

SC

Fig. 14. Attachment between a particle and a bubble. Stage (1): the thinning of the hydration film. Stage (2): the rupture of the hydration film and the formation of a three-phase contact line. Stage (3): the expansion of the three-phase contact line [51, 52].

NU

Fig. 15. Attachment between coal particles and (a) air, or (b) oily bubbles

Fig. 16. Oily bubble flotation curves at different (a) flotation pulp concentrations, (b) impeller speed, and (c) aeration rate

MA

Fig. 17. (a, c, e and g) Response surfaces and (b, d, f and h) contour maps of interaction effects of parameters on

AC

CE

PT E

D

the combustible matter recovery

35

ACCEPTED MANUSCRIPT

Table 1 Proximate and ultimate analyses of coal slime samples Proximate analysis, %

Ultimate analysis, %

Mad

Aad

Vdaf

FCdaf

Cdaf

Hdaf

Odaf

Ndaf

Sdaf

7.68

33.98

36.68

63.32

68.38

3.90

24.55

1.14

2.03

Table 2 Size distribution of coal samples Oversize Yield, %

Undersize

Ash, % Yield, %

Ash, %

18.31

19.84

18.31

19.84

0.25–0.125

9.19

18.25

27.50

19.31

0.125–0.074

10.53

21.58

38.03

19.94

0.074–0.045

6.80

21.84

44.83

−0.045

55.17

45.07

100.00

Total

100.00

33.93

Ash, %

100.00

33.93

81.69

37.09

72.50

39.48

20.23

61.97

42.52

33.93

55.17

45.07

SC

RI

0.50–0.25

Yield, %

PT

Size, mm

NU

Table 3 Density distribution of coal samples

Cumulative distribution of sediments

Ash, %

Yield, %

Ash, %

5.46

100.00

34.94

5.93

93.31

37.05

Cumulative distribution of floating

Yield, %

MA

Density, g/cm3 Yield, % Ash, % objects

6.69

5.46

6.69

1.30–1.40

9.57

6.26

16.27

1.40–1.50

16.50

10.01

32.76

7.99

83.73

40.57

1.50–1.60

9.19

15.58

41.95

9.65

67.24

48.07

1.60–1.80

18.98

29.80

60.93

15.93

58.05

53.21

+1.80

39.07

64.58

100.00

34.94

39.07

64.58

Total

100.00

34.94

CE

PT E

D

−1.30

Table 4 Survey scan of coal samples Peak binding energy

FWHM, eV

Area (P), CPS×eV

Atomic content, %

O1s

531.95

3.22

1308744.97

45.04

284.66

3.06

429026.31

37.47

102.81

2.96

113330.53

10.20

74.57

2.88

50928.25

7.29

C1s Si2p Al2p

AC

Element

36

ACCEPTED MANUSCRIPT

Table 5 Fitting data of coal sample for C1s Range of

Peak binding

Area (P),

Relative

CPS×eV

content, %

FWHM, eV fitting,eV

Chemical group

energy,eV 284.60

1.29

17293.36

65.64

C-C/C-H

285.82

1.87

6243.70

23.70

C-O-C/C-OH

287.30

4.00

2142.75

8.13

C=O

289.20

2.60

666.41

2.53

O=C-O

PT

281.12–291.0

RI

Table 6 Analyses of pores and specific surface areas of coal samples with full and −0.045 mm size fractions Surface area, m2/g

Pore volume, ×10−2 cm3/g

Raw coal

6.114

1.14

Coal less than 0.045 mm

8.835

2.45

SC

Samples

Average pore diameter, nm 7.470 11.127

NU

Table 7 Level of factors investigated using the Box–Behnken experimental design Level Factors

A

Impeller speed (rev/min)

B

Collector dosage (kg/t)

−1

0

1

1800

2100

2400

2

4

6

40

60

80

0.1

0.2

0.3

MA

Number

D

Flotation pulp concentration C

CE

Air flow rate (m3/h)

AC

D

PT E

(g/L)

37

ACCEPTED MANUSCRIPT

Table 8 Box–Behnken experimental design and results

Standard order

Run

Pulp

Impeller speed Collector (rev/min)

dosage (kg/t)

concentration (g/L)

Aeration rate Combustible matter (m3/h)

recovery (%)

3

1800

6

60

0.2

89.94

2

1

1800

2

60

0.2

83.19

3

28

2100

4

60

0.2

94.58

4

17

1800

4

40

0.2

5

10

2400

4

60

0.1

6

13

2100

2

40

0.2

7

19

1800

4

80

8

11

1800

4

60

9

23

2100

2

60

10

2

2400

2

60

11

7

2100

4

12

8

2100

4

13

14

2100

6

14

29

2100

4

15

4

2400

6

16

20

2400

4

17

15

2100

18

16

2100

19

6

20

RI

PT

1

SC

0.2

90.13 87.19 88.11 85.77 89.06

0.3

87.46

0.2

88.04

40

0.3

92.52

80

0.3

87.67

40

0.2

94.63

60

0.2

95.22

60

0.2

93.42

80

0.2

88.49

2

80

0.2

84.24

6

80

0.2

90.25

2100

4

80

0.1

83.51

12

2400

4

60

0.3

92.18

21

21

2100

2

60

0.1

82.21

22

26

2100

4

60

0.2

95.16

23

22

2100

6

60

0.1

88.75

24

25

2100

4

60

0.2

96.37

27

2100

4

60

0.2

95.02

5

2100

4

40

0.1

87.76

24

2100

6

60

0.3

93.35

28

9

1800

4

60

0.1

84.15

29

18

2400

4

40

0.2

93.26

26 27

MA

D

PT E

CE

AC

25

NU

0.3

38

ACCEPTED MANUSCRIPT

Table 9 Model’s summary statistics Sum-of-residual

Models

Standard deviation

R2

Adjusted R2

Predicted R2

Linear model

2.88

0.5802

0.5102

0.5045

235.76

2FI model

3.32

0.5818

0.3495

0.3497

309.41

Quadratic model

0.54

0.9915

0.9830

0.9666

15.89

0.69

0.994

0.9722

0.6708

156.63

model

PT

Cubic curve

squares

Source

Sum-of-squares

Model

Degrees-of-fre Mean-squared value

471.74

14

33.7

A

34.48

1

B

114.64

1

C

58.43

1

D

68.5

1

AB

0.47

AC

0.042

AD

0.16

BC

0.065

F-value

SC

edom

RI

Table 10 Variance analyses of regression model used in this study

Prod>F < 0.0001

34.48

119.37

< 0.0001

114.64

396.91

< 0.0001

58.43

202.31

< 0.0001

68.5

237.16

< 0.0001

1

0.47

1.62

0.0223

1

0.042

0.15

0.0139

1

0.16

0.54

0.0451

1

0.065

0.23

0.0237

0.11

1

0.11

0.37

0.0359

0.09

1

0.09

0.31

0.0315

61.77

1

61.77

213.85

< 0.0001

68.08

1

68.08

235.7

< 0.0001

54.39

1

54.39

188.33

< 0.0001

115.08

1

115.08

398.44

< 0.0001

Residual

4.04

14

0.29

Lack of fit

2.28

10

0.23

Pure error

1.76

4

0.44

Corrected total

475.79

28

CD A2

C2

AC

D2

D

PT E

BD

B2

MA

NU

(significant)

CE

116.67

P-value

0.52

0.8186 (not significant)

Note: P value Pr>F less than 0.05 represents a significant effect; P value Pr>F less than 0.01 represents a highly

significant effect.

39

ACCEPTED MANUSCRIPT

Table 11 Optimization scheme of the maximum combustible material recovery Impeller speed

Collector dosage

(rev/min)

(kg/t)

(g/L)

(m3/h)

recovery (%)

1

2209.33

4.71

47.11

0.21

96.53

100

2

2152.50

4.35

56.00

0.22

96.53

100

3

2156.21

5.27

49.91

0.26

96.45

100

4

2152.96

4.93

51.39

0.22

96.91

100

5

2169.09

4.91

48.79

0.22

6

2119.57

4.69

52.43

0.25

7

2224.97

5.09

43.56

0.23

96.38

100

8

2144.49

4.41

47.92

0.25

96.38

100

9

2114.42

4.58

49.44

0.22

96.68

100

10

2150.38

4.66

43.86

0.23

96.41

100

Expectation (%)

PT

Pulp concentration Air flow rate Combustible matter

96.90

100

96.67

100

RI

SC

Schemes

NU

Table 12 Parameter setting of model Setting

Ranges

A: impeller speed

Within the range

1800–2400 (rev/min)

Minimum value

2–6 (kg/t)

Within the range

40–80 (g/L)

Within the range

0.1–0.3 (m3/h)

Within the range

82.21–96.37 (%)

MA

Types

B: collector dosage C: pulp concentration

D

D: air flow rate

PT E

Y: combustible matter recovery

Table 13 Optimization scheme of the maximum combustible material recovery at minimum collector dosage Impeller speed Collector dosage Pulp concentration (rev/min)

(kg/t)

CE

Schemes

Air flow rate

Combustible matter

Expectation

(g/L)

(m3/h)

recovery (%)

(%)

2198.53

2.32

52.47

0.23

91.50

81.4

2

2190.87

3.24

52.31

0.23

94.69

80.4

3

2196.54

2.52

52.42

0.23

92.32

79.9

2196.5

2.58

52.4

0.23

92.55

79.7

2192.55

3.05

52.3

0.23

94.14

79.6

2194.87

2.79

52.38

0.23

93.29

79.3

7

2194.59

2.79

52.36

0.23

93.29

79.3

8

2194.74

2.79

52.37

0.23

93.30

79.3

4 5 6

AC

1

40

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

Highlights  The coal samples were rich in oxygenated functional groups, pores, and cracks.  The collector dosage was maintained at a high level in the conventional flotation.  Oily bubble flotation was used to enhance the flotation performance of coal samples.  Design-Expert 8.0.5 was used for the technological parameters of optimization.  As a result of the optimization scheme, the collector dosage was decreased by 95.36%.

41

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17