Cleaning of the moist fine grained oil shale in the compound dry separator and the diffusion of the external moisture of the oil shale

Fuel 262 (2020) 116522

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Cleaning of the moist fine grained oil shale in the compound dry separator and the diffusion of the external moisture of the oil shale

T

Xiaodong Yua, , Haibin Lib, Zhenfu Luoc ⁎

a

College of Mining Engineering, North China University of Science and Technology, 063210 Tangshan, China Tsinghua Innovation Center in Dongguan, 523808 Dongguan, China c School of Chemical Engineering & Technology, China University of Mining and Technology, 221008 Xuzhou, China b

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Moist fine grained oil shale Compound dry separation External moisture diffusion Dehydrated Particles migration

Oil shale is a crucial alternative energy resource. The external moisture content of the fine grained oil shale is high and the grade is decreased. The physical separation can effectively remove inorganic impurities and the external moisture to enrich the concentrate. In this study, the external moisture diffusion process and its effects on the diffusion behavior of the moist fine-grained oil shale in the compound dry separator were studied. The effects of the operating parameters (vibration amplitude, vibration frequency and airflow) on the external moisture migration process were studied and the optimal parameters were obtained. The results illustrate the external moisture content decreases along the X-axis in the upper layer, but shows fluctuation in the central of the bed, and they are concentrated in the central area of the bed and loosely distributed in the side wall. The external moisture content also decreases gradually in the Y axis direction. The decrements of the external moisture content in the lower layer is greater than that in the upper bed as the particles in the lower bed is firstly affected by the airflow. With the increasing of the airflow, vibration amplitude and frequency, the standard deviation σmf of the external moisture content decreases firstly and then increases. When the vibration frequency

⁎

Corresponding author. E-mail address: [email protected] (X. Yu).

https://doi.org/10.1016/j.fuel.2019.116522 Received 10 August 2019; Received in revised form 9 October 2019; Accepted 26 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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(f), vibration amplitude (A) and airflow (Uv) are 54 Hz, 3.2 mm and 2.45 m/s, the removal rate of the external moisture is optimal. Under this condition, the probable error E value is 0.10 g/cm3 with the optimal dehydration and separation results.

1. Introduction

accuracy, and it would consume large quantities of water. Also, the treatment for the waste water is relatively complex, which is not favorable to the separation of oil shale. The hydrochloric acid and hydrofluoric acid needed in the pickling method are high in cost and the pickling method cannot reach the scale of commercial application, which limits its development. Bio-leaching technology is still being studied and far from industrialization. As a result, the utilization of the illustrated cleaning technologies for oil shale is limited, and the highefficiency cleaning of oil shale cannot be achieved. However, the dry sorting technology can solve the above problems to a certain extent [18]. At present, the dry sorting techniques mainly include compound dry cleaning, vibration air dense medium fluidized bed cleaning and air dense medium fluidized bed cleaning. The above cleaning techniques mainly focuses on the separation of the oil shale with a low external moisture content (< 5 wt%) in the previous studies, while the studies of the dry cleaning for the oil shale with higher external moisture content (> 7 wt%) is still blank [19]. The oil shale is mainly composed of kerogen, moisture and minerals. The kerogen is an organic substance that is insoluble in common organic solvents. Its structure is mainly composed of carbon and hydrogen. If the moisture content in oil shale is low, the kerogen content and the contents of carbon and hydrogen atoms would be higher. The higher of the oil content, the higher of the oil shale grade would be. Conversely, if the moisture content and mineral element content are higher, the kerogen content is relatively low, hydrocarbon ratio is low and the oil content is small, and the oil shale grade is poor. At present, researches have been done on oil shale dewatering in the world, mainly focusing on fluidized bed drying and dehydration process, vibration reentry drying process, microwave drying process, airflow drying process, etc. Through the analysis of the oil shale drying process, the higher of the oil shale moisture content, the larger the external moisture and the adsorbed moisture content in the oil shale pores would be, and the energy required for the drying process is greater. The higher of the required medium temperature and the greater of the fragmentation rate of oil shale particles would be, which is extremely disadvantageous for the post-dry distillation of shale oil and combustion oil shale power generation and directly determines the drying process of the economy. The higher of the moisture content will need greater energy for drying, which seriously affects the efficient utilization of oil shale and reduce

The utilization of fossil energy has brought the development of the world economy and human society into an unprecedented period [1]. At present, the fossil energy resources that can be utilized are mainly coal, oil and natural gas [2]. According to the report of the International Energy Agency (IEA), the world coal consumption will continue to increase until 2040 [3–5]. As a result, coal resources will be exhausted in the near future. According to the report from the US Geological Bureau, there are about 3 × 1013 barrels of traditional oil in the world that can be exploited and utilized [6–8]. However, the consumption of oil continues to increase and the exploitation of the remaining oil rises steadily, which leads to the rapid depletion of petroleum resources [9–11]. The natural gas resources that can be mined in the world is about 1.77 × 1015 m3, which can be used for 80 years according to the current consumption [12,13]. Therefore, with the intensification of energy consumption, energy security issues have become the most critical factor for the development of human society. Oil shale is a special sedimentary rock with concentrated organic matter and it has become the optimal choice for alternative energy resources. It consists of organic and inorganic minerals. The inorganic mineral content is generally about 70 wt%. The reserves of oil shale are huge and it distributed in many countries of the world. The oil shale resources in the United States, China and Russia are relatively large, accounting for about 90% of the world's oil shale resources [14,15]. In the process of exploitation, fine grained oil shale with a large amount of inorganic mineral impurities will be produced and the dust will be sprayed during the mining process, which would produce moist fine grained oil shale containing impurities [16]. The fine grained oil shale is of great value for its high oil content. However, due to the inorganic mineral impurities and high external moisture content, its grade is reduced and its utilization is limited. Therefore, it is necessary to beneficiate this part of the oil shale to remove inorganic mineral impurities and reduce the external moisture content and enrich the concentrate [17]. At present, most cleaning methods for oil shale are based on the sorting principle of coal. The main methods are wet and dry cleaning, acid pickling and bioleaching technology. In view of the high density of oil shale, wet cleaning is difficult to achieve the separation density and

Fig. 1. Diagram of oil shale compound dry separation system. 2

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utilization value. Therefore, it is of great significance to investigate the beneficiation of the moist fine grained oil shale with higher external moisture (> 7 wt%) by the compound dry separation. The external moisture content is the critical factor affecting the compound dry separation of the moist fine grained oil shale particles [20]. The variation of the external moisture content on the bed during the cleaning process is directly connected with the cleaning accuracy [21]. Therefore, the studies of the migration regularities of the water on the bed are crucial for investigating the separation performance of the oil shale and optimizing the operating parameters of the separator. In view of this, a compound dry separator is used to beneficiate the 6–0 mm moist fine grained oil shale and explore the migration of the external moisture affected by various parameters. The main reason for the decrements of the external moisture of the oil shale in the cleaning process is analyzed, which could provide new technical and theoretical support for the dry cleaning process of the moist fine grained oil shale.

Fig. 3. Sampling coordinate system in different areas of the bed.

The bed structure of the compound dry separator is irregular. Therefore, the migration trajectories of the oil shale particles in different sorting areas on the bed surface are different, which leads to the difference of the external moisture distribution in variable areas. In order to study the migration trajectory of the particles in different areas of the bed surface and the distribution of the external moisture, the sampling coordinate system at different areas on the bed surface is shown in Fig. 3. The bed surface is divided into three beneficiation areas of I, II and III along the X-axis, from 0 to 450 mm, from 450 to 650 mm and from 650 to 1000 mm, respectively. In the area I, 9 segments are equally distributed along the X-axis, and 10 segments are equally divided in the Y-axis. In the region II, the bed is equally divided into 4 and 7 segments along the X-axis the Y-axis, respectively. In the III region, the bed are divided into 7 and 6 segments in the X-axis and Yaxis, separately. The intersections for the X-axis and the Y-axis in each region are the sampling points.

2. Experimental 2.1. Experimental setup and procedure The compound dry cleaning system is shown in Fig. 1. The oil shale is firstly sieved by a 6 mm screen. The particles larger than 6 mm are crushed. The crushed oil shale is mixed with the particles less than 6 mm. Then they are delivered to the surge bunker and fed to the compound dry separator. Oil shale particles are loosely stratified by the synergistic effects of vibration and airflow in the separator bed. The light oil shale particles in the upper layer are discharged from the discharging side over the baffle. The heavy particles at the bottom of the layer are re-separated in the next cycle until being discharged in the gangue end of the bed. The airflow required for the sorting process is provided by the Roots blower. The dust particles generated during the cleaning process are separated by a cyclone and a bag filter and are collected, and the gas is expelled into the atmosphere. In this paper, the spatial migration of moist fine grain oil shale particles in the bed and the distribution of external moisture of the oil shale during the migration are studied. The sampling points during the experiments are determined. The spatial sampling coordinate system shown in Fig. 2(a) is established according to the cross section of the compound dry separation bed. The direction from the feeding end to the tailing end (transverse direction of the cross-section) is the X-axis direction and the bed is divided into 10 sections at equal intervals in the X-axis. The direction from the back-board to the discharge side (longitudinal direction of the cross section) is the Y-axis, and the bed is divided into 5 sections at equal intervals along the Y-axis. The intersection areas of the X-axis and the Y-axis are the sampling points in the bed. The direction of the height on the bed surface from the bottom to the top layer (the direction perpendicular to the X-axis and the Y-axis) is the Z-axis direction, and the bed layer is divided into two parts, the upper layer and the lower layer, along the Z-axis direction. Fig. 2 (b) is a schematic diagram of the layering of the oil shale particles on the bed.

2.2. Material properties This paper mainly studies the migration trajectory of the wet oil shale particles and the spatial distribution of the external moisture during the separation in the compound dry separator. The physical properties of the oil shale affect the migration process of particles, which in turn impacts the distribution of the external moisture and leads to the various beneficiation precision. The external moisture content of oil shale is directly influenced by its surface structure and the distribution of elements on the surface. The physical properties of the oil shale mainly depends on its micro-structure. Therefore, in order to study the physical properties of the oil shale particles, the microstructure of the oil shale particles is analyzed. In this paper, the X-ray Energy Dispersive Spectrometer (EDS) is used to analyze the elements in the oil shale, as shown in Fig. 4. In Fig. 4, the concentrations of C and O elements are relatively high and uniform, while the concentrations of elements Si and Al elements are lower than that of C and O elements. The distribution and concentrations of the elements Fe, S, P, Ti are obviously uneven and extremely low, which illustrates that the elements of the oil shale are

Z(mm) Upper layer Lower layer Upper layer

O

100

200

300

400

500

600

700

800

900

X(mm) 1000

Z

100

Back board end

X

Lower layer

Tailings end

200

300

Y

400

500 Y(mm)

(a)

(b)

Fig. 2. Spatial sampling coordinate system of the compound dry cleaning bed and corresponding material layering diagram. 3

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Fig. 4. Elemental distribution and X-ray wave analysis of oil shale.

mainly composed of C, O, Al, Si et al., and also contain some elements such as Ca, Mg, Fe, S, P, Ti. The organic matter in the oil shale is mainly composed of C and O elements. The concentration of C and O elements determines the oil content of the oil shale. The inorganic minerals are mainly composed of elements such as Al, Si, Ca, and Mg. Fig. 5 shows the 3D-X-ray-tomography of the oil shale. The XOY, XOZ and YOZ surfaces of the oil shale are imaged separately. The inorganic minerals and organic matters in the oil shale shown from the cross-section are associated together. The elements of inorganic minerals are mainly present in the pores of the oil shale, and the development of porosity is directly related to its density. The contents of inorganic minerals are high when the density of oil shale is high and the porosity is not developed relatively. Otherwise, the contents of organic matter are enriched as the oil shale density is relatively low and the porosity is developed. Therefore, the density of oil shale depends mainly on the type of elements and the content of organic matters and inorganic minerals. In view of this, the oil shale ore should be fully dissociated to effectively separate inorganic minerals from organic matters to recover organic matter and improve the grade of the oil shale. The external moisture contents and oil contents distribution of different grades of oil shale are shown in Fig. 6. The analysis of the sinkfloat tests for the oil shale ore is shown in the Table 1, and the corresponding washability curves are shown in the Fig. 7.

Fig. 6. Schematic diagrams of oil content and external moisture content distribution of the oil shale with different particle size.

mainly evaluated by the uniformity and stability of the distribution of the external moisture contents in the bed. The standard deviation σmf of the external moisture content is used to interpret the deviation of the measured external moisture content from the average moisture content.

2.3. Evaluation index The migration of the oil shale particles in the autogenous medium bed and the distribution of the external moisture content in the bed are

mf

=

1 n

Y

n

(mfi

mfa) 2

(1)

i=1

Z

Z

Z

X

O

O

Y

X O Fig. 5. 3D-X-ray-tomography of oil shale. 4

X

O

Y

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Table 1 Sink-float test result for moist fine grained oil shale of 6–0 mm. Density

Yield

Oil content

External moisture

Cumulative floats

Cumulative sinks

δp ± 0.1

(g/cm )

(%)

(%)

(%)

Yield (%)

Oil content (%)

Yield (%)

Oil content (%)

Density (g/cm3)

Yield (%)

< 1.8 1.8–1.9 1.9–2.0 2.0–2.1 2.1–2.2 2.2–2.3 2.3–2.4 2.4–2.5 > 2.5 Total

18.32 5.65 8.46 4.21 5.45 2.12 4.13 5.13 46.53 100.00

17.12 10.43 7.67 5.32 5.01 3.12 2.01 1.02 0.45 5.28

19.51 17.83 15.46 13.72 11.91 10.64 9.82 9.13 8.42 12.1

18.32 23.97 32.43 36.64 42.09 44.21 48.34 53.47 100.00

17.12 15.54 13.49 12.55 11.57 11.17 10.39 9.49 5.28

100.00 81.68 76.03 67.57 63.36 57.91 55.79 51.66 46.53

5.28 2.63 2.05 1.34 1.08 0.71 0.62 0.51 0.45

1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5

23.97 14.11 12.67 9.66 7.57 6.25 9.26 46.53

3

migration trajectory and sorting characteristics of the oil shale particles in the separator. The air flow(Uv), vibration amplitude(A) and frequency(f) are 2.55 m/s, 4.2 mm and 39hz, respectively. At first, the vibration and airflow are started simultaneously. After a certain time for the stratification and separation of the oil shale particles, the vibration and airflow are shut off at the same time and the free moisture contents of the oil shale particles at different measurement points shown in Fig. 2 are analyzed. The spatial distribution of the free moisture content of the oil shale shows the water migration trajectory, as shown in Fig. 8. Fig. 8 shows the spatial migration and diffusion trajectory of the free moisture of the oil shale in the bed. It can be seen that when the separation time T is 5 s, the particles diffuse along the bed to the intersection area of the X-axis of 310 mm and the Y-axis of 235 mm. There is no obvious difference for the change of the oil shale free moisture content in the upper and lower bed. At this time, the particles in the intersection area of the X-axis of 50–145 mm and the Y-axis of 35–120 mm have higher free moisture content, while the content of the free moisture at the side wall is relatively low. When the sorting time is short, the particles on the bed have strong adhesion due to the presence of the water on the surface of the particles, and the gap among particles is small and the particles are densely piled up and accumulated at the diffusion region on the bed. The material is gradually migrated under the effects of the backing plate and the feeding thrust. The particles at the side wall of the bed are affected by the thrust of the backing plate, which is larger than the collision friction among the particles. The particles’ dispersion at the side wall is relatively uniform and the particle are loosely dispersed. Most particles are concentrated in the middle of the diffusion zone and densely distributed with small particle gap. The vibration exciter is installed on the back plate and drives the bed to perform synchronous harmonic vibration and transfers the vibration energy to the bed. The particles at the bottom layer obtain the vibration energy firstly and the activity is enhanced. Since the particles have a certain amount of external moisture, the particles are relatively dense under the action of the liquid bridge force, although the underlying particles first obtain the vibration energy. However, as a result of the adhesion among the particles, the particles with higher activity at the bottom layer are more resistant in the migration process and the energy dissipation is higher, which results in less energy obtained by the upper layer particles, with less activity and disorder of the particle migration trajectory. At this time, the air flows through the air distribution plate. Due to the adhesion among the particles, air is blocked by the gap of the particles and thus cannot effectively acts on the particles. As the sorting time is short, the moist oil shale particles do not form extensive migration. Also, for the liquid bridge force the particles are densely arranged in certain area close to the feedings end, and the free moisture contents on the upper and lower bed show the same trend. The particle gap at the side wall area is larger and the looseness is increased. Then the air flow can effectively act on the particles to evaporate the surface water and the free moisture content is reduced.

Fig. 7. Washability curves of the moist fine grain oil shale samples.

n is the number of measurement points. mfi is the measured value of the external moisture content of the test point, %. mfa is the average external moisture content, %. The probable error Ep (Eq. (2)) was used to evaluate the oil shale separation efficiency in the compound dry separator. The lower the value, the better the sorting effect will be.

Ep =

1 ( 2

75

25)

(2)

where δ75 is the density when the partition coefficient is 75%, g/cm3; δ25 is the density when the partition coefficient is 25%, g/cm3. 3. Results and discussion 3.1. Spatial migration of the external moisture of the moist fine grained oil shale particles The compound dry cleaning relies on the synergy effects of the vibration and gas flow to fully fluidize the dense medium in the sorting bed to form a gas-solid autogenous medium fluidized bed with certain densities. The stratification and separation of the oil shale particles by density could be realized in the bed. The free moisture content of the oil shale affects the fluidization performance of the bed and directly determines the beneficiation precision of the compound dry separator. Therefore, it is critical to study the variations of the free moisture contents of the oil shale in different areas of the bed to analyze the 5

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(Upper layer)

(Upper layer)

(Upper layer)

(Upper layer)

T=5s

T=10s

T=15s

T=20s 6

(Lower layer)

(Lower layer)

(Lower layer)

(Lower layer) (caption on next page)

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Fig. 8. Oil shale external moisture space diffusion process.

As the sorting time is extended, under the effects of the vibration and the airflow the migration of the particles is significant. As the transport distance in the Y-axis decreases gradually, the concentrate can be quickly and effectively discharged, and the back-mixing phenomenon of the bed particles is gradually reduced or even disappeared. When the sorting time T reaches to 20 s, the particle separation process is completed. The separation of particles is a dynamic layering migration process. After separation, although the stratification and separation process of the particles are completed, few particles in the local small area around the bed are still subject to collision by the edge wall and the inter-particles interaction. The distribution of external moisture shows little fluctuation. The distribution of the external moisture content in the upper and lower layers of the bed is significant, and the distribution of the external moisture content at the upper and lower beds is significant, and the external moisture contents along the X-axis and Y-axis are both reduced, and the decrements of the external moisture contents at the lower layer is larger. It ensures that the concentrate products with lower external moisture content can be effectively discharged.

Fig. 9. Effect of vibration on the external moisture distribution of the oil shale particles.

3.2. Effect of vibration on the external moisture distribution of the oil shale particles in the bed

The particles in the middle area of the bed are densely packed and the looseness is poor, and the free moisture contents of the particles are basically unchanged. With the extension of the sorting time, the bed particles are subjected to the vibration of the bed and the continuous action of the gas flow, so the activity is enhanced and the free moisture contents decrease and the adhesion effect becomes weaker. Then the migration and separation occur. The particle gap becomes larger, and the bed particle distribution gradually becomes uniform. At the same time, the variations of the free moisture contents at the upper and lower beds are significantly different. When the sorting time T is 15 s, the free moisture contents of the particles decrease gradually along the X-axis and the Yaxis. However, the free moisture contents at the upper layer decrease slowly and is disordered distributed at the intersection areas of the 450–610 mm in the X-axis direction and 0–50 mm in the Y-axis. The free moisture contents at the lower layer are significantly reduced and relatively high with a disordered distribution in the intersection of 200–300 mm in the X-axis and 0–50 mm in the Y-axis and also at the intersection of 450–600 mm along the X-axis and 0–75 mm in the Yaxis. It is indicated that with the extension of the sorting time, the migration distance of the particles on the bed increases and the particles in the repeated sorting continue to be vaporized by the air flow. The evaporation rate of the external moisture is accelerated, and the bridge force is weakened, and the adhesion among particles is weakened and the arrangement is relatively loose, the looseness of the particle gap is intensified and the resistance of the airflow through the particle bed is reduced, so the airflow can effectively act on the surface of the particle to evaporate and reduce the external moisture. As the airflow acts firstly on the lower layer particles and continues to pass through the particle to the upper layer bed and looses the surface particles, the external moisture content at the lower layer is reduced by the evaporation of the gas stream, and the evaporated water is mixed into the gas stream and increases the humidity of the gas stream. Under the effects of the airflow with certain humidity, the moisture content at the upper layer decreases slower. At the same time, with the increasing of the separation time, the migration distance of the particles is increased, and for the trapezoidal structure of the bed the transport distance along the Y-axis in the X-axis direction is gradually reduced, which accelerates the discharge of the concentrates. The particles which can not be effectively discharged is blended with the particles having a lower external moisture content, which leads to back-mixing phenomenon and the disordered distribution of the external moisture.

In the compound dry separator, the bed surface is subjected to simple harmonic motion under the effects of the excitation motor on the back plate. The vibration energy is transferred to the particles in the bed, and affects the movement of the particles. The tests were carried out when the airflow Uv is 2.35 m/s. The external moisture contents of the oil shale in the I, II and III regions are shown in the Fig. 3. The average external moisture contents of the oil shale and the standard deviation of the external moisture contents were used to investigate the effects of the vibration amplitude and frequency on the free moisture distribution. The results are shown in Fig. 9. Fig. 9 shows the effects of the vibration on the distribution of external moisture contents on the bed. It can be seen from the Fig. 9 that the σmf values in different areas of the bed increase firstly and then decrease with the increasing of the vibration amplitude and frequency. However, there is significant difference between the increasing of the σmf-f curve and σmf-A curve in each area. For the σmf-A curve in the area I the σmf values increase faster with the increasing of the vibration frequency, while in the σmf-A curve the increasing of the σmf is relatively flat. In the area III, the changing of the standard deviations of the σmf-f curve and the σmf-A curve are tend to be flat with the increasing of the vibration frequency and amplitude. The standard deviation of the free moisture in the area II increases with the vibration amplitude and frequency, and the variation range is varied from the values of area I and area III. This shows that when the vibration amplitude and frequency are small, the particles can not obtain sufficient vibration energy, and the particle activity is poor with small particle gap, and the particles with different density and external moisture content do not have apparent migration and they are densely distributed. The area I is close to the feed end, and the oil shale enters this area first. As the particle gap is small and the internal resistance is large, the particle migration range is restricted. The particles are disordered distributed and most of them accumulated in the bed surface. The vibration energy obtained by the particles is increased with the increasing of the vibration amplitude and frequency. As a result, the particle activity is intensified and the migration speed of the particles increases gradually and the particle gap becomes larger. The evaporation rate of the external moisture is determined by the size and distribution uniformity of the particle gap. The more uniform of the particle distribution and the larger of the particle gap are, the faster of the external moisture evaporation rate would be. When the vibration frequency and amplitude are 54 Hz and 3.2 mm, 7

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respectively, the σmf reaches the maximum value, which indicates that the particle activity is in the optimal state and the inter-particle resistance is the smallest and the particle migration is obvious and the particle distribution is relatively uniform and also the gas flow can effectively act on the upper layer particles. As a result, the water evaporation rate reaches its maximum. With the further increasing of the vibration amplitude and frequency, the vibration energy on the particles is too large and the particle activity is over enhanced, which leads to the transformation of the the particles from the original regular motion to the random motion. The particles with higher external moisture are mixed with the particles with lower external moisture, which leads to the back mixing phenomenon. The particle distribution is disordered, and the σmf value decreases gradually. When the vibration frequency and amplitude increase to 59 Hz and 4.2 mm, the σmf value reaches the minimum. After the particle of the area II dehydration effects by the air flow from the area I, the particle gap is larger and the internal friction resistance decreases, and the migration trajectory is relatively regular. The external moisture distribution is more uniform compared with the external moisture distribution in the area I. Therefore, the σmf-f curve and the σmf-A curve are relatively flat compared with curves in area I. After the effects of the air flow from the area I and area II on the free moisture of the particles in the area III, the external moisture content is relatively low, and the particle distribution is relatively uniform, and the difference in external moisture content among the particles is further decreased. In this region, the particles are mainly gangue product with high density. The gangue product has a relatively low external moisture content. The dewater rate of the particles in this region is not significant with the increasing of the vibration amplitude and frequency. Therefore, the variable tendency of the σmf-f curve and the σmf-A curve has no significant difference.

velocity increases to 2.45 m/s, the σmf value reaches the maximum. In this state, the external moisture distribution is the most significant and the free moisture content reaches the minimum value with the optimal dewater effect. As the airflow increases further, the particles are excessively affected, and some particles with low density on the surface are blended with the particles in the other regions, which leads to serious back-mixing phenomenon. Some particles in the lower layer with higher free moisture are thrown onto the surface of the bed under the effects of the airflow, which leads to obvious mismatch phenomenon. The external moisture distribution is disordered and it is not obvious in different regions. The σmf values decreases gradually. When the airflow increases to 2.98 m/s, the σmf value reaches the minimum. The back-mixing phenomenon of the bed is serious, and the external moisture distribution is irregular. Since the area I is close to the feed end, the oil shale with higher free moisture first reaches this area and the gas flow firstly affects the particles in this area. Therefore, the free moisture of the particles on the bed surface is reduced and the concentrate is discharged from the baffle plate. The particles with higher external moisture continue to enter the next area and are further dehydrated under the effects of the gas stream. Particles in this area are most sensitive to the airflow and most external moisture is removed. After the effects of airflow in the area I, the particles with higher external moisture enter the area II for repeated separation. The free moisture of the particles is relatively low and its distribution is comparatively uniform. The sensitivity of the airflow is reduced and the water removal rate is decreased. After continuous cleaning in the area I and area II, the oil shale particles reach to the area III. The oil shale in the area III are mainly composed of high density particles. As for the effects of airflow in the first two areas, the free moisture content of the high density particles is relatively low and evenly distributed and has no obvious difference. Also, the external moisture of the particle is the least sensitive to the airflow, and the σmf values is the most stable.

3.3. Effects of the airflow on the external moisture migration

3.4. Beneficiation for the moist oil shale

The air in the compound dry separator enters the separation bed through the air distribution chamber, and is used to remove the external moisture of the oil shale and sufficiently loosen the material bed. The air flow has a great effects on the migration of the external moisture. The tests were carried out when the vibration amplitude and frequency are 3.2 mm and 54 Hz, respectively. The sampling points in the I region, II region and III region are shown in Fig. 3. The external moisture contents of the oil shale in the different areas were measured. The corresponding average external moisture content and the standard deviation of the external moisture content of the oil shale were calculated to explore the effects of the air flow on the external moisture distribution. The results are shown in Fig. 10. It can be seen from Fig. 10 that the standard deviation values of the external moisture content increases firstly and then decreases with the increasing of gas velocity, but they show different characteristics in the area I, II, III. The value of σmf in the area I shows the steepest tendency with the increasing of gas velocity and it is the most sensitive to the gas velocity. The value of σmf in the III area is relatively flat with the gas velocity, and the sensitivity to the air flow is the worst. The variation of σmf values is between the values in the area I and area III. When the gas velocity is small, the particles are closely packed in the bed, and the inter-particle space is small, so the gas flow is difficult to pass through the particle gap and the external moisture distribution is not significantly different. At this time, the σmf value is small. As the gas velocity increases, the migration velocity of the particle increases gradually and the looseness increases, and the airflow can smoothly pass through the particle gap to the bed surface. Then the external moisture contents of the particles on the bed surface are reduced. The external moisture distribution is uneven, and the free moisture contents in the upper layer and the lower layer are obviously different. The particles in the upper layer have lower moisture than the free moisture content in the bottom layer. The σmf value increases gradually. When the gas

The migration of the external moisture has been investigated and the optimal operating parameters have been obtained. The compound dry cleaning apparatus mainly relies on the synergy effects of the vibration and airflow to realize full fluidization of the autogenous medium bed and to achieve efficient separation of the oil shale. The external moisture content of the oil shale has great influences on the fluidization quality of the autogenous medium bed. Therefore, the distribution of the external moisture content and the oil content of the moist oil shale particles during the cleaning process have been studied, which are critical for improving the separation of the oil shale and optimizing the operating parameters of the separator.

Fig. 10. Effect of airflow on the external moisture distribution of the moist oil shale particles. 8

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3.4.1. Distribution of the oil shale during the beneficiation The separation of the oil shale in the compound dry separator is the process of dynamic migration of particles. The external moisture content directly affects the distribution of oil content and the cleaning effects. The tests were carried out when the vibration amplitude (A), vibration frequency (f), air flow (Uv) and separation time (T) are 3.2 mm, 54 Hz, 2.45 m/s, 18 s separately. The oil shale are sampled as shown in Fig. 2. The external moisture contents and oil contents of the samples are measured and analyzed, and the distribution of the external moisture contents and the oil contents can be seen in Fig. 11. It can be seen from Fig. 11 that the separation of the moist oil shale particles by the compound dry separator is a dynamic enrichment process, and the distribution for the oil contents and external moisture contents of the oil shale are different for the upper layer bed and the lower layer bed. In the initial stage of separation, as the moist oil shale particles have certain content of external moisture, the stratification of moist oil shale particles is relatively insignificant and the liquid bridge force among particles causes adhesion of the particles. The frictional resistance is increased, and the particle arrangement is relatively tight, and the particle gap is narrow. However, with the sorting process progresses, the particles are subjected to airflow in different areas of the bed, the external moisture of the particles is decreased and the bridge force is weakened, and the particle activity is increased. Also, the particle gap becomes larger, and the layering effect of the particles with different densities is improved under the effects of vibration. The external moisture contents in the upper layer bed decrease gradually from the feeding end to the tailing end, and decreases along the back plate to the discharge end, and the free moisture is low at the side wall and is relatively high in the middle of the bed. The oil contents also decreases gradually from the feeding end to the tailing end, but increase from the back plate to the discharge side. This phenomenon occurs for the combined effects of the force on the particles and the air flow. The particles at the side wall firstly obtain the vibration energy provided by the vibration motor at the back plate, and the activity of the particles is

enhanced, so the particle collision frequency increases and the energy is transmitted. As a result of the frictional resistance among the particles the energy is gradually dissipated during the transmission. Therefore, the particles at the side wall obtain greater vibration energy with stronger activity and larger particle gap and obvious airflow effect. The dissipation of the external moisture is apparent, and the content of the external moisture is low. On the contrary, the particles in middle region obtain less vibration energy with lower activity, and smaller particle gap, and insignificant airflow effect. Also, the dissipation of the external moisture is smaller, with higher external moisture content. The cleaning of the oil shale on the bed is mainly according to density. As the particles at the side wall obtain vibration energy, the activity of the particles is enhanced. Because of buoyancy effect of the high-density particle in the bed, the low density particles are in the upper layer and they are firstly collected at the discharge side. The particles with high densities and low oil contents enter the next cycle for cleaning and move gradually toward the tailings end. As a result, the regularities of the gradual increasing of oil contents from the back plate to the discharge side (Y-axis positive direction) and decreasing of oil contents from the feedings end to the tailings end (X-axis positive) are formed. The distribution of the external moisture content in the lower layer is consistent with that in the upper layer, but the external moisture content in the lower layer is less than that of the upper layer. As the particles in the lower layer contact firstly with the airflow and are significantly affected by the gas flow. The dissipation and contents of the external moisture are significant and low, separately. During the cleaning, the particle in the lower layer with smaller density and higher oil content are subjected to larger force, and preferentially move to the upper layer through particle gap, while the particles with heavier densities are deposited on the bottom layer and move gradually towards the back plate under the effect of the friction by the bed surface. The larger density particles at the back plate in the bottom layer continue to enter the next separation tank for repeated cleaning and gradually reach to the tailings end due to the thrust of the back plate. The

Fig. 11. Distribution of the oil content and external moisture of the moist oil shale on the bed. 9

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movement formed a distribution regularities that the oil contents decrease along the backing plate to the discharge side and from the feedings end to the tailings end. 3.4.2. Separation results The migration of the external moisture of the moist fine grained oil shale particles in the compound dry separator were systematically investigated, and the separation tests of 6–0 mm fine grain oil shale was carried out by the separator when A, f, Uv and T were 3.2 mm, 54 Hz, 2.45 m/s, 18 s, respectively. The external moisture content of the raw ore is 12.1% and the oil content is 5.28% and the actual sorting density is 2.38 g/cm3. The oil content, the external moisture content and the yield of the concentrate are 10.23%, 3.46% and 49.23%. Also, the oil content, the external moisture content and the yield of the tailing are 0.48%, 1.13% and 50.77%. The partition rates are shown in the Table 2. The corresponding partition curve can be seen in the Fig. 12. The product quality comparison is shown in the Fig. 13. It can be seen from Fig. 13 that the external moisture is significantly reduced after the cleaning by the compound dry separator. The external moisture contents of the raw ore of the oil shale, the concentrates and the tailings are 12.1%, 3.46% and 1.13%, respectively, which indicates that the external moisture of the moist oil shale particles is effectively reduced during the sorting process in the separator. During the beneficiation the bed layer is loosely layered and the gap among particles becomes larger under the effects of vibration and the airflow, so the external moisture of the particles can be effectively removed. As the concentrate sorting area is close to the feedings end and the effective transport distance in the Y-axis is much shorter than that in the X-axis on the bed, the effective separation time of the particles in the concentrate area is short. The tailings have a long effective transport distance along the X-axis, and are sorted repeatedly with longer sorting time. The airflow rates are different at various bed areas. As the densities of the particles are lower and bed layer are thicker and more compact at the concentrates end, the airflow rate is larger. Otherwise, the particles at the tailings end have been separated repeatedly and the external moisture contents are lower, and the airflow rates are relatively slow. In addition, the oil shale particles with lower particle size have lower density and higher oil content, while the particles with relatively larger particle size are mainly concentrated on tailings with larger density and lower oil content. The oil shale particles with lower density and higher oil content are firstly discharged from the discharge side to form a concentrate product. The tailings with relatively large particle size and relatively low oil content is discharged from the tailings end to form tailings. Therefore, the concentrate has a relatively small particle size and a large specific surface area and high adhesion water content. The tailings have small specific surface area and low adhesion water content. Secondly, from the analysis of the physical properties of concentrates and tailings, oil shale is a porous material with well developed pores in the structure. Also, oil shale is also a kind of crack-like material, which can adsorb more external water. The developed pores are the main space for free water to be adsorbed. The

Fig. 12. The partition curve of 6–0 mm moist fine oil shale, separated by a compound dry separator.

Fig. 13. The chart of the moist fine grained oil shale product quality comparison.

formation of developed pores in oil shale is mainly caused by residual pores of organic matter, matrix pores and secondary pores. The types of cracks mainly include diagenetic structural joints, diagenetic erosion joints, and structural dissolution joints. The oil shale has a high degree of metamorphism, high organic matter content, low density, high oil content, relatively loose structural layer, more primitive pores, more developed porosity, and more types of cracks. More free water is adsorbed with high water content. Therefore, the oil shale concentrate is developed due to its pore structure, with many cracks and high water content. Because of its low organic matter content, high content of

Table 2 Partition coefficient results of 6–0 mm moist oil shale for compound dry cleaning. Density (g/cm3)

Average density (g/cm3)

Feedstock sink-float result (%)

Tailings sink-float result (%)

Concentrate sink-float result (%)

Calculated feedstock sinkfloat result (%)

Partition coefficient (%)

−1.9 1.9–2.0 2.0–2.1 2.1–2.2 2.2–2.3 2.3–2.4 2.4–2.5 +2.5 Total

1.75 1.95 2.05 2.15 2.25 2.35 2.45 2.6

23.97 8.46 4.21 5.45 2.12 4.13 5.13 46.53

0.97 0.65 0.47 1.54 1.10 3.01 7.98 84.28 100.00

47.69 14.46 6.09 10.12 3.68 4.94 2.46 10.56 100.00

23.97 7.45 3.24 5.76 2.37 3.96 5.26 47.99 100.00

2.04 4.43 7.41 13.54 22.63 38.64 77.00 89.16

0.49 0.33 0.24 0.78 0.56 1.53 4.05 42.79 50.77

10

23.48 7.12 3.00 4.98 1.81 2.43 1.21 5.20 49.23

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inorganic minerals and compact structure, the tailings have single pore composition. The organic pores and matrix pores are very poor and the pores in the structure are relatively small and the types and quantities of cracks are small. The amount of free water adsorbed is low, which results in a low water content in the tailings. As a result, due to the differences in sorting time and airflow rates, the dewatering rates of the concentrates are lower than that of the tailings. The efficient dewatering of the particles has been achieved under the cooperative effects of the airflow and the sorting time.

Declaration of Competing Interest

4. Conclusions

Acknowledgement

In this paper, the spatial migration of the external moisture of the 0–6 mm fine grained moist oil shale in a compound dry separator was studied. The effects of the operating parameters such as the airflow rates, vibration amplitude and frequency on the external moisture migration have been systematically analyzed. The oil shale was sorted under certain operating parameters (A = 3.2 mm, f = 54 Hz, Uv = 2.45 m/s, T = 18 s) to investigate the removal of the external moisture during the sorting process. The main conclusions are as follows:

The financial support by The National Natural Science Foundation of China (51774283).

The results indicate that the compound dry separator can be used to remove the external moisture of the oil shale effectively with favorable separation.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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(1) The moist fine grade oil shale particles were analyzed X-ray Energy Dispersive Spectrometer (EDS). The concentrations and distribution of elements C and O are relatively high and uniform. The concentrations of elements Si and Al elements are lower. The concentration of elements Fe, S, P, Ti are extremely low, which indicates that C, O, Al, Si are the main elements of the oil shale. 3D Xray-tomography analysis illustrates that the oil content depends on the contents of elements C, O. (2) The spatial migration of the external moisture was analyzed. The results illustrate that in the X axis direction the particles are affected by the thrust of the back plate and the friction of the side wall during the migration process, so the particles are concentrated in the central area of the bed and loosely distributed in the side wall. In the Y axis direction the external moisture content decreases gradually. The content of the external moisture of the particles in the bottom layer is lower than that in the upper layer due to the effects of the airflow. (3) The effects of the airflow, vibration amplitude and frequency on the distribution of the external moisture content were analyzed, and the standard deviation of the external moisture content was used for evaluation. With the increasing of the airflow, vibration amplitude and frequency, the standard deviation σmf decreases firstly and then increases. When the f, A and Uv are 54 Hz, 3.2 mm, 2.45 m/s, respectively, the σmf reaches the maximum value. The fluctuation of the external moisture content is the most obvious, and the external moisture removal rate is in the optimal state. (4) The compound dry separator was used to beneficiate the 0–6 mm fine grained moist oil shale. After the separation, the yield, the oil content and the external moisture content of the concentrate are 10.23%, 3.46% and 49.23%, separately. The yield, the oil content and the external moisture content of the tailings are 0.48%, 1.13% and 50.77%, respectively and the probable error Ep is 0.1 g/cm3.

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