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FLOTATION COLLECTOR PREPARATION AND EVALUATION OF OIL SHALE.

1. Introduction

With the exhaustion of petroleum resources and the growing energy demand due to the world economic progress, the development of substitute resources has become urgent. China's resource of oil is insufficient and its production is far from satisfying the social and economic needs of

the country. In 2016, China imported about 376 million tons of crude oil, the degree of external dependence making 65.5%. Oil strategic security issues have been given more and more prominence these years. China's overall oil shale reserves are estimated at 4.76 x [10.sup.10] tons and could significantly increase petroleum supply [1].

Pyrolysis is the main method to transform oil shale into shale oil. The oil shale pyrolysis process involves complex physical and chemical reactions. Lots of treatment processes and methods for oil shale pyrolysis have been carried out in order to enhance the yield of oil and improve its quality [2]. By this, parameters such as composition of source material, pyrolysis temperature, heating rate, residence time and particle size are taken into account [3, 4]. Pyrolysis under vacuum conditions has been shown to improve both the yield and quality of oil unlike pyrolysis under atmospheric pressure. Vacuum pressures accelerate the transport of pyrolysis products by affording a faster escape of primary oil from the reaction zone, therefore reducing the occurrence of secondary cracking reactions [5].

Oil shale, as a humic sludge substance, has not only some basic properties of coal but also unique properties of its own. Oil shale is characterized by a high content of ash and a low amount of organic matter compared to coal [6]. Generally, the amount of organic matter in oil shale is less than 35%, while that in coal is more than 75%. The H-to-C atomic ratio of oil shale is higher than that of coal. The amount of minerals is high in oil shale, which after the thermal decomposition go over to semicoke. As a result, the ash content of semicoke becomes higher and its calorific value lower than oil shale's, which reduces the economic value of semicoke and restrains its use [7]. Therefore, it is beneficial to reduce the ash content in oil shale by using the flotation technique [8].

Flotation is an easy and effective way to upgrade minerals [9]. The technique has been widely used in various fields, such as maceral separation, mineral separation, oil-water separation, paper pulp deinking, waste water treatment, etc. Utilization of oil shale in China will benefit from reducing its ash content via flotation [10]. One of the flotation research focuses is on selecting suitable reagents to improve oil shale flotation performance. However, up to now, only a few articles have touched upon oil shale flotation. Altun et al. [11] used the froth flotation technique to upgrade Turkish low-quality oil shale by employing amine-type collectors. It was found that ash could be reduced from 69.88% to 53.10% with a 58.64% combustible recovery, using an 800 g/ton Armoflote17 (AkzoChemical, Netherlands) at a neutral pulp pH. Altun et al. [12] further attempted to enhance the performance of flotation cleaning of oil shales by use of ultrasound. The results showed that the ultrasonic treatment increased the extent of ash rejection. After the ultrasonic treatment, the ash content of Himmetoglu oil shale decreased from 34.76% to 11.82% with an 82.66% combustible recovery, and that of Beypazari oil shale decreased from 69.88% to 34.76% with a 64.78% combustible recovery.

In this work, the flotation collector was synthesized by mechanical stirring and ultrasonic treatment. The optimal collector design and preparation conditions were found out. The study of the mechanism of interaction between the collector and mineral is useful to understand the structure and performance of the synthetic collector, so as to optimize and adjust the flotation conditions to achieve a higher degree of separation by flotation. In the current paper, the performance of synthetic collector A was studied by measuring the amount of adsorption by infrared spectroscopy. A single collector refers to kerosene or oleic acid, the synthetic collectors refer to the reagents synthesized by two or more reagents.

2. Experiment

2.1. Materials and instruments

The oil shale sample used in this paper was collected from Qingchuan mining district in Sichuan Province, China. The flotation experiments were carried out in a conventional mechanical flotation cell. The details about the equipment used (all China) are given in Table 1.

2.2. Ultimate and proximate analyses

The results of ultimate analysis of the oil shale sample showed that the contents of nitrogen, carbon and protium were 0.86%, 58.62% and 5.59%, respectively, indicating that the organic matter in it was abundant. The ratio of the number of carbon atoms to hydrogen atoms was 9:10, and was close to that of aromatic hydrocarbons, indicating that oil shale contained a lot of substantial benzene rings [13].

The results of proximate analysis of the sample demonstrated that its volatile content was 51.18% and that of ash 29.82%. Generally, the high volatile and low ash contents imply a high concentration of oil, which means that oil shale has a high utilizing value.

From Table 2 it can be seen that the ash content of the sample slightly increased with decreasing particle size. The increase was particularly significant in case of -75 [micro]m fractions. In case of -45 [micro]m fractions, the product yield was 18.40 %. Particles with a size of -250 to +75 [micro]m accounted for 55.28% of the total mass of the sample.

2.3. Experimental methods

Using kerosene and oleic acid as raw materials, synthetic collectors were prepared by mechanical stirring and ultrasonic treatment. During the experiment, the proportion between the two components and the instrument power were the two critical parameters. After the stability of the synthetic collector was measured, the optimal collector was used in flotation experiments.

The quantitative analysis methods included phase-resolved ultraviolet (UV) spectrophotometry and two-phase titration method [14, 15]. In this study, phase-resolved UV spectrophotometry was used to find the residual concentration of the flotation collector after adsorption by the mineral, and then the amount of the collector adsorbed on the mineral surface was calculated by the following formula [16]:

A=[([[rho].sub.0]-[[rho].sub.X])v/m], (1)

where A is the amount of adsorption, mg/g; [[rho].sub.0] is the initial concentration of solution, mg/L; [[rho].sub.x] is the concentration of solution after adsorption, mg/L; V is the volume of solution, L; m is the mass of solid, g.

3. Results and discussion

3.1. Synthesis of the collector

It was observed that when the ratio of kerosene to oleic acid was 1:1 and the agitation speed 1800 rpm, the synthetic collector was the most stable.

A comparison of the results presented in Tables 3 and 4 showed that the higher the proportion of kerosene in the solution, the higher its stability. When the kerosene-to-oleic acid ratio was 1.5:1, no sediments existed in the three kinds of solutions. When the respective ratio was 1:1.5, the sediments were present in a maximal amount, and kerosene and oleic acid were almost completely separated. So, at a 1.5:1 ratio of kerosene to oleic acid and the instrument power of 10 W, the synthetic collector was the most stable.

For dispersion measurements, better-performing stable reagents, labeled as A and B, were used. The kerosene:oleic acid ratio of reagent A was 1:1 and the agitation speed 1800 rpm, while reagent B had the kerosene:oleic acid ratio of 1.5:1 and the instrument power of 10 W.

The synthetic collectors were separately added to 200 mL of water, then stirred mechanically for 30 min and thereafter ultrasonically for 30 min. Finally, a Malvern Zetasizer Nano ZS90 particle-size zeta-potential analyzer (UK) was used to measure the particle size of the synthetic collector in the solution to evaluate its dispersion. The particle size of synthetic collector A was minimal, only 295 nm, while that of synthetic collector B was 433.2 nm. At the same time, the dispersion of collector A was the best, in terms of non-polar agents.

3.2. Flotation results

Octanol was used as the frother, with a dosage of 200 g/t. The collector dosages were 400, 500 and 600 g/t. The aeration rate was 0.12 [m.sup.3]/h and the oil shale ore mass 100 g. The flotation results are presented in Table 5.

Table 5 reveals that the yield of the concentrate was the highest when the collector dosage was 600 g/t, and the lowest when the respective dosage was 400 g/t. The ash content of the concentrate was the highest at the collector dosage of 600 g/t and the lowest at 500 g/t. The recovery rates at collector dosages of 500 and 600 g/t were nearly equivalent. These results demonstrated that the collector dosage of 500 g/t was the most optimal.

Next, kerosene and oleic acid were used as collectors and octanol as the frother. The collector dosage was 500 g/t and that of the frother 200 g/t. The aeration rate was 0.12 [m.sup.3]/h and the oil shale ore mass 100 g. The results are given in Table 6.

From Tables 5 and 6 it can be seen that the flotation concentrate ash contents of the single collector were mostly higher than those of the synthetic collector, and the flotation concentrate yields were lower than those of the synthetic collector. These results gave evidence of that the flotation effect of the single collector was poor.

In the next experiment, kerosene and oleic acid were used as collectors and octanol as the frother. The frother dosage was 200 g/t and the collector dosages were 400, 500 and 600 g/t. Kerosene and oleic acid were added at a 1:1 ratio. The aeration rate was 0.12 [m.sup.3]/h and the oil shale ore mass 100 g. The results are given in Table 7.

A comparison of the data given in Tables 5 and 7 shows that after addition of kerosene and oleic acid as collectors, their flotation concentrate ash contents were higher and concentrate yields lower than those of the synthetic collector.

So, the flotation performance of synthetic collectors was better.

3.3. Collector adsorption measurement

The experimental adsorption quantity data are shown in Figure 1.

It can be seen from Figure 1 that the quantity of adsorption of the collector on the oil shale surface increased with increasing collector dosage, and reached a relatively stable value at a collector dosage of 500 g/t.

3.4. Infrared spectroscopic analysis

The sample solution was prepared using a certain amount of oil shale and 500 g/t of the collector. Some water was added to obtain the solution concentration of 60 g/L. The samples were obtained by stirring the solution for 30 min, filtering through a funnel, rinsing with distilled water, and, finally, drying [17]. The Fourier transform infrared (FTIR) spectra were obtained between 4000 and 400 [cm.sup.-1].

The FTIR spectra of raw oil shale, kerosene and kerosene-infiltrated oil shale in Figure 2 illustrate the interaction between them.

The peaks in the range 4000-2800 [cm.sup.-1] in Figure 2a correspond to the stretching vibrations of OH, NH and N[H.sub.2] groups of phenol and alcohol. The broad band at 3480 [cm.sup.-1] shows the presence of dissociative N[H.sub.2] and hydroxyl OH groups. The double adsorption band at 2920 [cm.sup.-1] is due to the presence of C[H.sub.2]. The peak at 2340 [cm.sup.-1] is attributable to carboxylic acid. The adsorption band at 1440 [cm.sup.-1] indicates the unsaturated =C-H group. In addition, the adsorption band at 1030 [cm.sup.-1] denotes that an aliphatic C-H group exists in this adsorption band.

The double adsorption bands at 3000 [cm.sup.-1] and 2920 [cm.sup.-1] in Figure 2b are due to the presence of C[H.sub.2], indicating the carbon-chain or cycloalkane structure of kerosene. The bands at 1460 [cm.sup.-1] and 1380 [cm.sup.-1] are due to the deformation vibration of alkane or the stretching vibration of olefin. The adsorption band at 1640 [cm.sup.-1] signifies the stretching vibration of unsaturated olefin.

In Figure 2c, there are several new significant absorption peaks on the FTIR spectrum of the oil shale infiltrated by kerosene, in addition to the corresponding bands of minerals. For example, the adsorption band at 3520 [cm.sup.-1] is due to the consociation of hydroxyl and amino.

In Figure 3, the FTIR spectra of raw oil shale, oleic acid and oleic acid-infiltrated oil shale illustrate the interaction between them.

For Figure 3a, see the description above pertaining to Figure 2a. In Figure 3b, the broad band at 3630 [cm.sup.-1] belongs to the stretching vibration of the OH group. The absorption peaks at 3030 [cm.sup.-1], 2920 [cm.sup.-1] and 2850 [cm.sup.-1] are attributable to the deformation vibration of C[H.sub.3] and C[H.sub.2] groups. The peak at 1710 [cm.sup.-1] shows the stretching vibration of the C=O group. The strong band at 1280 [cm.sup.-1] is indicative of the presence of the stretching vibration of the C-O group and the deformation vibration of the O-H group, which conform to the respective vibrations of the alkyl and carboxylic functional groups of oleic acid.

From Figure 3c it can be seen that there have appeared several new absorption peaks on the FTIR spectrum of oil shale infiltrated by oleic acid, in addition to the corresponding bands of minerals. The peaks at about 1400 [cm.sup.-1] and 2300-2380 [cm.sup.-1] are attributable to chlorite, talc, epidote, hornblende, etc. The FTIR spectra in the figure include not only the absorption peaks of raw oil shale but also those of oil shale infiltrated by oleic acid. For example, the adsorption band at 1740 [cm.sup.-1], which is assignable to the stretching vibration of the C=O group, implies that oil shale absorbs a small amount of oleic acid.

In Figure 4, the FTIR spectra of raw oil shale, synthetic collector A and oil shale infiltrated by synthetic collector A illustrate the interaction between them.

For Figure 4a, see the description above relating to Figure 2a. The adsorption bands at 3030 [cm.sup.-1], 2920 [cm.sup.-1] and 2820 [cm.sup.-1] in Figure 4b refer to the deformation vibration of C[H.sub.3] and C[H.sub.2] groups. The peak at 1710 [cm.sup.-1] is assignable to the stretching vibration of the C=O group. Compared with the peaks of kerosene (Fig. 2b) and oleic acid (Fig. 3b), the peak of synthetic collector A at 1460 [cm.sup.-1] becomes narrower and smaller, suggesting that kerosene and oleic acid reacted with each other.

Figure 4c reveals that the adsorption peak of oil shale has shifted. For example, its deformation vibration peak has moved from 3480 [cm.sup.-1] and 2340 [cm.sup.-1] to 3470 [cm.sup.-1] and 2360 [cm.sup.-1], respectively. It means that a weak hydrogen bond whose intensity is higher than electrostatic attraction appeared between synthetic collector A and oil shale Therefore, the collecting capacity of synthetic collector A was higher even though the surface of oil shale was positively charged.

4. Conclusions

Oil shale represents the second largest solid fossil fuel deposit in the world. Due to the growing demand for alternative energy sources, improvement of oil shale quality has become a research hotspot lately. In this work, froth flotation technology was used to treat oil shale. The conclusions drawn are the following:

1. Analysis showed that treatment enhanced the utilization value of oil shale.

2. The synthetic collector exhibited the best performance when the ratio of kerosene to oleic acid was 1:1 and agitation speed 1800 rpm.

3. The flotation effect of synthetic collectors was shown to be the highest. Collector adsorption measurements indicated that with collector dosage increasing to 500 g/t, the adsorption quantity reached a relatively stable value.

4. Infrared spectroscopy measurements of raw oil shale, kerosene, oleic acid and synthetic collector A were made and the interactions between were analyzed. Synthetic collector A resulted from the reaction between kerosene and oleic acid. A weak hydrogen bond whose intensity was higher than electrostatic attraction appeared between synthetic collector and oil shale. The collecting capacity of synthetic collector A was higher than that of oil shale.

Acknowledgments

The authors acknowledge the support from the National Natural Science Foundation of China (Grant No. 51304157), the University Key Teacher by Henan Polytechnic University (Grant No. 2017XQG-12), and Henan Key Laboratory of Coal Green Conversion.

REFERENCES

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[7.] Li, J. H., Cao, Z. B. Composition and comprehensive utilization of oil shale. Liaoning Chem. Ind., 2007, 36(6), 110-112 (in Chinese).

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[9.] Gupta, N. Evaluation of graphite depressants in a poly-metallic sulfide flotation circuit. Int. J. Min. Sci. Technol., 2017, 27(2), 285-292.

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[11.] Altun, N. E., Hicyilmaz, C., Hwang, J. Y., Bagci, A. S. Evaluation of a Turkish low quality oil shale by flotation as a clean energy source: Material characterization and determination of flotation behavior. Fuel Process. Technol., 2006, 87(9), 783-791.

[12.] Altun, N. E., Hwang, J. Y., Hicyilmaz, C. Enhancement of flotation performance of oil shale cleaning by ultrasonic treatment. Int. J. Miner. Process., 2009, 91(1-2), 1-13.

[13.] Xue, Q. H., Li, S. Y., Wang, H. Y., Zheng, D. W., Fang, C. H. Utilization of Daqing oil shale and its pyrolysis products. Sci. Technol. Chem. Ind., 2009, 17(3), 54-56 (in Chinese).

[14.] Hu, Y. H., Cao, X. F., Jiang, Y. R., Li, H. P., Du, P. Synthesis of N-decyl-1, 3-diaminopropanes and their structure properties for flotation of aluminosilicate minerals. Conserv. Util. Miner. Resour., 2002, 22(6), 33-37 (in Chinese).

[15.] Zhu, Y. M. Mineral flotation testing techniques: Measurement and application of reagents adsorption capacity on mineral surface. Non-Ferr. Min. Metall. 1988, 4(2), 10-14 (in Chinese).

[16.] Yao, T. Y., Yao, F. Y., Li, J. S. Adsorption and adsorption enthalpy of cationic surfactant on different sand stone surfaces. Oil Drill. Prod. Technol. 2008, 30(2), 82-85 (in Chinese).

[17.] Zang, H. C., Zang, L. X., Zhang, H., Wang, J. F., Yang, H. L., Jiang, W., Liu, D. M., Wang, F., Hu, T. Research progress on application of near-infrared spectroscopy in pharmaceuticals. Journal of Pharmaceutical Research 2014, 33, 125-128.

Presented by E. Reinsalu and X. Han

Received October 1, 2017

LIJUN LIU (a), GAN CHENG (b,c*), WEI YU (a), CHAO YANG (a)

(a) School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, PR China

(b) College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454000, PR China

(c) Henan Key Laboratory of Coal Green Conversion, Henan Polytechnic University, Jiaozuo 454000, PR China

(*) Corresponding author: email chenggan464@126.com
Table 1. Equipment details

Equipment                  Producer

Flotation machine          Jilin Prospecting Machinery Factory
Vacuum filter              Wuhan Rock Crush & Grand
                           Equipment Manufacture Co., Ltd
Thermostatic drying oven   Tianjin Taisite Instrument Co., Ltd
Muffle furnace             Test Center of China Coal Research
                           Institute
Double Beam UV Visible     Beijing Purkinje General Instrument
Spectrophotometer          Co., Ltd
Ultrasonic cell disruptor  Ningbo Scientz Biotechnology Co.,
                           Ltd

Equipment                  Model

Flotation machine          XFDIV-1.5L
Vacuum filter              RK/ZL-o260/o200

Thermostatic drying oven   101-3AB
Muffle furnace             GW 300C

Double Beam UV Visible     TU-1900
Spectrophotometer
Ultrasonic cell disruptor  JY 92-IIN

Table 2. Particle size distribution analysis of the oil shale sample

Particle size,  Product yield,   Ash,   Cumulative    oversize, %
[micro]m              %           %    Product yield     Ash

 500                 1.54       25.03       1.54        25.03
-500 to +250        16.32       26.61      17.86        26.47
-250 to +125        32.65       26.55      50.51        26.52
-125 to +75         22.63       28.55      73.14        27.15
 -75 to +45          8.47       31.82      81.6         27.63
 -45                18.4        37.56     100           29.46
Total              100          29.46      --           --

Note: "--" represents no data.

Table 3. Collector preparation by mechanical stirring

Kerosene:oleic  Stirring time,  Stirring speed,
 acid ratio           h               rpm

                                     1100
  1:1                 1              1800
                                     2500
                                     1100
1.5:1                 1              1800
                                     2500
                                     1100
  1:1.5               1              1800
                                     2500

Kerosene:oleic  Observations after 3-day standing
 acid ratio

                Stratification
  1:1           Foam layer, no stratification
                Stratification
                Stratification
1.5:1           Slight stratification
                Slight stratification
                Obvious stratification
  1:1.5         Foam layer, obvious stratification
                Foam layer, obvious stratification

Table 4. Collector preparation by ultrasonic cell crusher

Kerosene:oleic  Processing time,  Power,  Observations after 3-day
 acid ratio            h            W           standing

                                    10      Existence of sediments
  1:1                  1            20      Abundance of sediments
                                    30      Existence of sediments
                                    10      Absence of sediments
1.5:1                  1            20      Absence of sediments
                                    30      Absence of sediments
                                    10      Abundance of sediments
  1:1.5                1            20      Abundance of sediments
                                    30      Abundance of sediments

Table 5. Flotation performance of synthetic collector A

Collector dosage,  Product      Product yield,   Ash,  Recovery,
       g/t                            %           %        %

                   Concentrate      96.41       24.52    98.80
       600         Tailing           3.59       75.45     1.20
                   Concentrate      95.94       23.18    98.74
       500         Tailing           4.06       76.85     1.26
                   Concentrate      95.27       23.64    98.49
       400         Tailing           4.73       76.36     1.51

Table 6. Flotation performance of the single collector

Collector   Product      Product yield,  Ash,   Recovery,
                               %           %        %

            Concentrate      87.46       24.41    94.27
Kerosene    Tailing          10.93       77.95     3.44
            Concentrate      68.90       25.31    86.73
Oleic acid  Tailing          31.10       74.69    13.27

Table 7. Flotation results after addition of kerosene and oleic acid

Collector dosage,  Product      Product yield,   Ash,  Recovery,
g/t                                   %           %        %

                   Concentrate      88.77       24.99    95.83
400                Tailing          11.23       74.21     4.17
                   Concentrate      91.18       25.97    96.78
                   Tailing           8.82       74.56     3.22
                   Concentrate      91.23       26.62    96.73
                   Tailing           8.77       74.22     3.27
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Author:Liu, Lijun; Cheng, Gan; Yu, Wei; Yang, Chao
Publication:Oil Shale
Article Type:Report
Geographic Code:9CHIN
Date:Sep 1, 2018
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