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Recovering unburned carbon from gasification fly ash using saline water
Waste Management 98 (2019) 29–36
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Recovering unburned carbon from gasification fly ash using saline water Rui Zhang a,b, Fangyu Guo a,b, Yangchao Xia a,b, Jinlong Tan c, Yaowen Xing a,⇑, Xiahui Gui a,d,⇑ a
Chinese National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China c School of Chemistry and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China d Henan Province Industrial Technology Research Institute of Resources and Materials, Zhengzhou University, Zhengzhou 450001, Henan, China b
a r t i c l e
i n f o
Article history: Received 29 May 2019 Revised 1 August 2019 Accepted 9 August 2019 Available online 14 August 2019 Keywords: Gasification fly ash Saline water Recovery Foam stability Surface tension
a b s t r a c t Gasification fly ash is one of the wastes generated by coal gasifiers, and the unburned carbon therein seriously restricts the resource utilization of gasification fly ash. Flotation is one of the best ways to recover unburned carbon from it; however, surface pores of gasification fly ash are developed and contain several hollow hydrophilic glass beads, which makes it difficult for conventional flotation to recover unburned carbon effectively and the dosage of the flotation reagent is too high. In this study, different concentrations of saline water (NaCl, MgCl2, and AlCl3) are configured to the flotation solution, and their effect on the recovery of unburned carbon of gasification fly ash is investigated. Furthermore, the gasification fly ash treated with saline water is chosen to study the basic properties by the measurement of Zeta potential, surface tension, and flotation foam behavior. The experimental results show that with an increase in the valence state of the inorganic salt cation, the unburned carbon recovery efficiency of the gasification fly ash is significantly improved. When the concentration of Al3+ reaches 0.4 mol/L and the dosage of frother is 7.5 kg/t, the unburned carbon removal rate of the tailings reaches 95% or more. Saline water reduces the surface tension of the flotation system and weakening bubble decay; in the solution of Al3+, the flotation foam size is the smallest, followed by the solution of Mg2+, Na+. Furthermore, the saline water effectively reduces the Zeta potential of the particle surface and improves the floatability of the solid particles. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Gasification fly ash is one of the wastes produced by the gasification furnace in coal chemical enterprises (such as coal-to-oil and coal-to-gas), and its annual emissions can reach several hundred thousand tons or more (Xu et al., 2009; Hower et al., 2017). As gasification fly ash exhibits the characteristics of high ash content, fine grain size, and low direct utilization rate (Huggins et al., 2016), most of them are piled up in the fly ash storage yard, thus occupying considerable space (Yang et al., 2019a). Besides, the fly ash contains some hazardous metals; thus, during the long-term stacking process, the environment is polluted as the fly ash gradually penetrates the soil and groundwater (Huang et al., 2003). At present, the gasification fly ash treatment is mainly used for the admixture of concrete (Walker and Wheelock, 2006), and some is used as a bulk building material (such as cement) (Ding et al., 2016). However, the carbon in the gasification fly ash, which is not completely ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Xing), [email protected] (X. Gui). https://doi.org/10.1016/j.wasman.2019.08.014 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
burned by combustion, seriously affects the use of gasification fly ash as building materials or admixtures (Xu et al., 2017). A large amount of gasification fly ash is difficult to use, and there is still a great deal of it disposed in ash pond or landfill (Tian et al., 2019). In addition, unburned carbon can be used as a substitute for rubber carbon black or as an adsorbent material (such as activated carbon) (Zhang and Honaker, 2015). The loss on ignition of (LOI) gasification fly ash is about 20–30%. It was found that a relatively high content of unburnt carbon which hinders their utilization as additives in cement and concrete. (Wu et al., 2007). And the direct use of gasification fly ash is not only difficult to meet the building materials use standards (Lee et al., 2010), but also causes significant wastage of unburned carbon resources (Xu et al., 2009; Zhu et al., 2019). Therefore, how to effectively recover unburned carbon from gasification fly ash is key to the secondary utilization of gasification fly ash solid waste resources. Foam flotation is a method for achieving mineral separation by utilizing the difference in the hydrophobicity of the surface of component minerals (Xia et al., 2019a, 2019b). Many studies have shown that the use of froth flotation is one of the most effective and economical methods for removing unburned carbon from
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different types of fly ash (Demir et al., 2008; Mccarthy et al., 2013). Altun et al. (2009) used a flotation column to remove unburned carbon from fly ash. By optimizing the bubble generator device, the recovery rate of unburned carbon was over 98% and the tailing loss was <1%. Xu et al (2017) compared the effects of a traditional flotation machine and a static micro-bubble flotation column on the decarbonization and separation performance of fly ash. Under the optimum flotation conditions, the recovery of unburned carbon is 6.5% higher than that of a flotation machine. The removal of unburned carbon from fly ash by foam flotation is a mature process and the separation effect is remarkable (Yang et al., 2019b). However, the problem of high-flotation-reagent consumption has always been a difficult problem for laboratory researchers and field workers. As gasification fly ash is obtained by spray sedimentation of the fly ash in the high-temperature gasification process of coal, and in the process of gasification, in addition to the melting of various mineral components, reactions also occured between mineral components (Bai et al., 2008). Taking calcium as an example, the main components are anorthite, silica and other minerals at 1300 °C. At 1400 °C, the crystalline materials include anorthite, quartz, mullite and so on. At 1500 °C, the main mineral in ash is Ca3(Si3O9), Ca2(SiO4), Ca(Al2O3)2 anorthite and some amorphous materials (Li et al., 2006). So the hydrophobicity of gasification fly ash surface is very poor. Besides, the physical and chemical properties of the surface of unburned carbon particles have been changed significantly and the surface oxidation degree has become serious; flotation reagents are not easy to spread on its surface. For the removal of unburned carbon in gasification fly ash by flotation, the phenomenon that the consumption of the foaming reagent too high is particularly obvious. In addition, gasification fly ash contains several hollow glass microspheres, which are easily broken during the flotation process, thereby exposing a more specific surface area. This is another reason for the low efficiency and high reagent consumption of conventional flotation foaming reagents (Kiani et al., 2015). In response to this, researchers have actively explored the best conditions for recovering unburned carbon from the fly ash (Bournival et al., 2012; Kurniawan et al., 2011; Castillo et al., 2011). For a flotation reagent, (Walker and Wheelock, 2006) used nonylphenol, hexadecane, fuel oil, and methyl isobutyl phenol (MIBC) as a foaming reagent to conduct a laboratory study on a power-plant fly ash in a twostage flotation cell. The results showed that the recovery rate of unburned carbon in the concentrate reached over 95% by using the two-stage flotation method. In addition, the use of a collector consisting of nonylphenol and either hexadecane or fuel oil, in combination with MIBC, allows for a better separation of unburned carbon from fly ash under conditions of smaller collectors. Drzymala et al. (2005) used 4-dodecylphenol and hexadecane as collectors to achieve the separation of unburned carbon in fly ash on the laboratory flotation machine, based on the two-stage flotation test. The carbon content of the selected fly ash could be reduced from the initial 25–6.0% or lower. In addition, many scholars have recently investigated the influence of saline water on flotation, and argued that the flotation of hydrophobic coals can be improved using inorganic ions (Laskowski and Castro, 2015). On the one hand, some researchers have proposed that after the inorganic salt ions are added to the solution, the electric double layer on the surface of the mineral particles is compressed and the absolute electric potential of the particles decreases (Arnold and Aplan, 1986). On the other hand, the inorganic salt ions can promote the instability and rupture of the hydration film around the coal particles, thereby enhancing the hydrophobicity of the particles and increasing the probability of contact and collision of the particles with the bubbles (Liu et al., 2013). Based on this, Wang et al. (2013, 2014) studied the behavior of minerals in brine flotation fine coal. The experiment examined the entrainment of
minerals during flotation through foam image analysis. Saline water increases the recovery of minerals as well as that of combustibles. Zhang, 2015 studied the flotation enhancement mechanism of natural hydrophobic coal particles in different cation (Na+, Ca2+, Al3+) salt solutions. In the test, the flotation performances of AlCl3 and NaCl solutions showed the highest and lowest improvement effects, respectively. In addition, Chang et al. (2018) studied the effects of water salinity on the flotation of oxidized coals, and found that the compression of the electric double layer in saline water could increase the flotation efficiency by reducing the electrostatic repulsion between the particle and bubble, which could also increase the flotation efficiency. The surface hydrophobicity of gasification fly ash is poor, and saline water plays an important role in mineral flotation. However, little research has been conducted on the recovery of unburned carbon from gasification fly ash using saline water. In this study, the effect of saline water with different concentrations and frother with different dosages on the recovery of unburned carbon from gasification fly ash is studied, and the work’s aims to use the special properties of saline water to change the behavior of bubbles to reduce the consumption of chemicals while achieving the purpose of secondary recycling of gasification fly ash resources.
2. Materials and methods 2.1. Test samples and chemicals The gasification fly ash was taken from a chemical plant of Shenhua Coal Industry Group Ningxia, China. Gasification reaction was carried out by GSP coal gasification technologies (the temperature is 1450–1650 °C). In the gasification process, some unreacted carbon and fine slag will be brought out by syngas, collected by spray and dehydrated to produce a large amount of gasification fly ash (Wang, 2017). The original ash was dried and sieved through a 0.5 mm standard sieve, and the 0.5 mm portion under the sieve was taken as the material for this test. Table 1 shows the proximate and ultimate analyses of the test gasification fly ash measured, and the content of carbon and hydrogen is determined by fully burning the sample at a temperature of 850 °C. However, the method for determining the oxygen content is not yet mature, and the practical oxygen content is generally calculated by difference. Table 1 shows that the air-dried ash of the ash sample is 73.70%, the moisture (Mad) is 1.74%, volatile matter (V) is only 3.08%, and in the elemental analysis, the carbon content is 95.12%, and the oxygen element content is 0.99%. The industrial and elemental analyses of the sample showed that the ash content was high after burning at high temperature, and the residual carbon content in organic matter was higher, while the oxygen and hydrogen elements were fewer after combustion. However, the content of fixed carbon was still 21.48%, indicating that some of the carbon in the ash sample was not fully burned and this part was not realized. The flotation reagent used in the experiment was purchased from Hunan Xinghui Washing Chemical Technology Development Co., Ltd.; W501 was used as the collector and W502 was used as the frother. The flotation reagent’s chemical compositions were characterized using FTIR (Nicolet IS5, Thermo Fisher, USA) and the results are shown in Fig. 1(b). The water used in the test was deionized water, with pH = 7; the saline water comprised NaCl, MgCl2, and AlCl3, since of this is three representative inorganic salts. Through the small sieve screening and muffle furnace burning method, the content of each fraction of gasification fly ash and the loss on ignition were measured.
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R. Zhang et al. / Waste Management 98 (2019) 29–36 Table 1 Proximate and ultimate analyses of the gasification fly ash. Proximate analysis (wt%, ad)
Ultimate analysis (wt%, daf)
M
A
V
FC*
C
H
N
S
O*
1.74
73.70
3.08
21.48
95.12
1.59
0.48
1.82
0.99
ad-air dry basis; daf-ash free basis; *-by difference.
Fig. 1. LOI distribution of gasification fly ash (a) and FTIR spectra of flotation reagent (b).
2.2. XRD analysis The composition of inorganic minerals in gasification fly ash was studied by the German Brook D8 ADVANCE-type X-ray diffraction (XRD). The light source under test was X-ray-tube copper target radiation, the tube pressure was 40 kV, the current was 40 mA, the goniometer accuracy was 0.0001°, and the scanning range was 0–140°. 2.3. SEM/EDS analysis The morphology of gasification fly ash was analyzed by FEI Quanta TM250 scanning electron microscopy (SEM) and energydispersive spectroscopy (EDS). The surface morphology of the sample was observed by high-energy electron beam scanning and some element in the gasification fly ash was positioned. In the EDS photos, different colors represent different elements. Appropriate amounts of the sample were adhered to the surface of the conductive tape before the test began, and placed on the sample holder. The ear wash ball was used to blow off the particles that stuck to the conductive tape. The sample was then subjected to a gold spray treatment prior to testing, to prevent charge accumulation during the scan. 2.4. Flotation test The test used an XFD-type flotation machine with volume of 1 L, and the slurry concentration was maintained at 100 g/L. Before the test, the weighed inorganic salt was dissolved in a flotation cell, and the concentration of the saline water was 0.1, 0.2, and 0.4 mol/L, respectively (the concentration of NaCl was 0.2, 0.4, and 0.8, respectively). After pulp conditioning (stirring at an impeller speed of 1800 r/min for 2 min), the collector (W501) and frother (W502) were added for contact times of 120 and 30 s, respectively. The size of the inflation valve switch was adjusted, the inflation amount was controlled within 0.2 m3/h, and foaming was started. After flotation for 7 min, the concentrate and tailings were
separately filtered, dried, and weighed, and the ash was tested according to the national standard to calculate the yield and ash. 2.5. Surface tension test The surface tension of the synergistic system of the saline water and frother was measured using a K 100 surface tension instrument. First, 0.4 mol/L of saline water was prepared with deionized water as the base solution, because this concentration is the best flotation test concentration, and then, different types and concentrations of the frother were prepared with different base solutions. The surface tension test was carried out at 20 ± 0.5 °C. The repeatability of the test was verified three times for each test and the average value of the test results was taken as the final test value. 2.6. Dynamic foam stability test An inorganic salt solution having a concentration of 0.4 mol/L was prepared in a flotation cell having a volume of 1 L, and 100 g of the ash sample was added to the flotation cell to maintain the entire slurry concentration at 100 g/L. After the slurry preparation was completed, the flotation machine was turned on, impeller speed was adjusted to 1800 r/min, and the slurry was continuously adjusted for 2 min. Then, a certain amount of frother (2.5 kg/t) was added, and the air intake was maintained at 0.2 m3/h. When the inflation valve was opened, the foaming behavior was recorded every 2 s until the foam reached the upper edge of the flotation cell. 2.7. Zeta potential test The ash sample was further ground to less than 74 lm in a mortar; 1 g of the ground ash sample was weighed each time with a precise electronic balance and placed in a 100 mL beaker. Then, salt ions were sequentially added in accordance with the concentration of the flotation solution. It was then stirred with a magnetic stirrer for 30 min to allow the ash sample to diffuse sufficiently. Finally,
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the sample to be tested was allowed to stand for 12 h. During the test, a certain amount of the suspension was sucked into the electrophoresis tank by a syringe. According to the measuring operation procedure of the electrophoresis apparatus, the gray motor potential at the natural pH (pH = 7) was measured, and each sample was measured three times, and finally, the average value was taken.
mullite in the gasification fly ash is mainly encapsulated in the glass beads by kaolinite in raw coal after high-temperature combustion, and the quartz and illite are mainly contained in the raw materials. In the XRD pattern of the sample, there are also characteristic peaks composed of amorphous minerals, which are mainly composed of vitreous bodies in the gasified ash and unburned carbon.
3. Results and discussion
3.3. SEM/EDS test results
3.1. The characterization of sample and reagent
The SEM-EDS photos of gasification fly ash are shown in Fig. 3 (a), where the main elements of the regular spheroids located in the original ash I region are Si and Al. The main element of the irregular multi-space body located in region II is C. The analysis of gasification fly ash indicates that the regular sphere in region Ⅰ is a glass bead formed by high-temperature combustion of gasification fly ash, and this part of the substance is the main component of ash. The irregular hollow body in region Ⅱ is the unburned carbon particles, which are not fully burned in gasification fly ash. The effective separation of glass beads and unburned carbon particles in gasification ash will be the key link in the utilization of the gasification ash resource. The results of SEM analysis of gasification fly ash, concentrate, and tailings are shown in Fig. 3(b). As seen in Fig. 3(b), (1), there is not only a part of unburned carbon in the raw ash but also several glass beads with crystalline or amorphous phase. In the raw ash, the unburned carbon surface is rough and porous, and the developed pores are the main cause of the high consumption of flotation drugs. Fig. 3(b), (2) shows an image of the flotation concentrate of the gasified ash, which indicates that some unburned carbon is embedded with glass beads in the flotation concentrate; such a special co-existence state will cause the ash content of the concentrate to exceed the standard in the flotation process. Fig. 3 (b), (3) shows a photograph of the appearance of gasification fly ash flotation tailings. Most of the tailings are glass beads, whose sizes vary. Many glass beads of different shapes will bond at a high temperature (red circle), and the main components of this glass bead are quartz and alumina (Asokan et al., 2005). After separating the carbon and ash of gasification fly ash, this part of the tail ash can be used as a substitute for traditional building materials (such as cement and hollow bricks) (Golewski, 2017) or chemical products (such as polymerized aluminum and alumina) (Sujjavanich et al., 2017).
The test results of the gasification fly ash particle size and collector are shown in Fig. 1(a), (b) respectively. The Fig. 1(a) shows that the main particle size of gasification fly ash is concentrated at 0.045 and 0.125–0.074 mm, wherein the LOI of the 0.125–0.074 mm grain is the highest and that of the 0.045 mm grain is relatively low. Therefore, the high ash content in the gasification fly ash is concentrated in the fine particle size and the flotation of the 0.125–0.074 mm fine grain material is key to the removal of unburned carbon from the gasification fly ash. The Fig. 1(b) shows the infrared spectra of flotation reagent. It can be seen the absorption peak around 3453 cm1 is attributed to hydroxyl (AOH) with stretching vibration. The absorption peak around 2928 cm1 and 2867 cm1 is attributed to aliphatic CAH with asymmetrically stretching vibration (ACHvas) and aliphatic CAH with symmetrically stretching vibration (ACHvs), respectively (Li et al., 2015). The peak at 1712 cm1 are assigned to carboxyl (@COOH). In the FTIR spectra of W501, the peak around 1600 cm1 is attributed to C@C bonds. The peak around 1455 cm1 and 1375 cm1 are attributed to bending vibration of methylene (ACH2) and methyl(ACH3), respectively (Akyıldırım et al., 2017). The peak at 1084 cm1 and 1081 cm1 originate from CAO stretching vibration (alcohols and ethers, respectively). The oxygen-containing groups, such as AOH, ACOOH, AC@O, and ACAOA, are the main polar compositions of flotation reagent (Wang et al., 2018) the characteristic peaks of these functional groups have been marked with red dotted lines in the figure. 3.2. XRD test results The results of gasification fly ash XRD test analysis are shown in Fig. 2, where the main crystalline minerals in the sample are quartz and illite, as well as a small amount of mullite and gypsum. The
3.4. Flotation test results In the gasification fly ash flotation test, LOI and removal rate of unburned carbon (RUC) in the tailings are usually evaluated to determine the degree of perfection of the sorting equipment and process (Xu et al., 2018). The RUC of the tail ash in the flotation test is equal to the carbon recovery rate of the concentrate (Xu et al., 2017).
LOI ¼ 100 Ay
ð1Þ
where Ay represents raw ash.
RUC ¼
Fig. 2. X-ray diffraction patterns of gasification fly ash.
cj Lj Ly
100%
ð2Þ
where cj represents the concentrate yield, %; Lj represents the loss on ignition of the concentrate, %; and Ly represents the loss on ignition of raw ash, %. The respective experiments investigate the improvement effects of three inorganic salts of NaCl, MgCl2, and AlCl3 on the separation of gasification fly ash flotation under different amounts of the foaming reagent. The test results are shown in Fig. 4.
R. Zhang et al. / Waste Management 98 (2019) 29–36
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Fig. 3. SEM-EDS photos of gasification fly ash (a) and SEM results for raw materials, concentrates, and tailings (b).
Fig. 4 shows that as the concentration of inorganic salt cations increases, the carbon removal rate of the flotation tailings also increases and the loss on ignition of tailings decreases. The results of the flotation test indicate that the use of saline water as the aqueous solution of flotation can effectively achieve unburned carbon recovery of the gasified fly ash. Moreover, under the condition of the same inorganic salt ion concentration, the higher the valence state of the inorganic salt cation, the smaller is the amount of frother reagent required to achieve the optimum flotation carbon-ash separation test effect. Note that due to the small amount of charge carried by Na+, a small amount of Na+ is insufficient to affect the flotation test results (Lessard and Zieminski, 1971). Therefore, the gradient of Na+ concentration is set to be high the addition of inorganic salt ions can effectively reduce the degree of merger between bubbles, and the higher the valence state of the added inorganic salt cations, the more obvious is the blocking effect (Henry and Craig, 2008). In the solution environment, there is a plateau channel between the bubble and the bubble for the liquid to flow. The addition of the inorganic salt cation can reduce the flow rate of the liquid in the plateau channel, thereby reducing the discharge rate between the bubbles and further preventing the occurrence of the bubble-tobuking phenomenon (Bhakta and Ruckenstein, 1997). Meanwhile, after the inorganic salt ions are added to the solution, the electric double layer on the surface of the ore particles is compressed, the absolute value of the zeta potential of the particles is reduced, and the repulsive force between the particles and the bubbles is reduced (Paulson and Pugh, 1996). 3.5. Surface tension analysis The microcosmic phenomenon of foam decay is the rupture of a liquid film, which is affected by the seepage process. The driving
force DPS is a net result of the suction pressure in the adjacent plateau border channels and the disjoining pressure (P) in the films, and is given by (Bhakta and Ruckenstein, 1997)
DP ¼ DP S
Y
¼
2c Y R
ð3Þ
where DPS is the pressure difference between the liquid film and the plateau channel, also known as the capillary pressure difference; R is the radius of the bubble; and c is the surface tension of the liquid Q phase. is the disjoining pressure, which refers to the interaction between the gas–liquid interface on both sides of the liquid film. The pressure is expressed as the mutual exclusion of the two gas– liquid interfaces and is an opposite force to the capillary pressure. Both of them maintain the force balance of the liquid film. The foam decay process in the two-phase system is carried out under the action of gravity and DP. The change in the properties of the liquid phase system will affect the size of DP, and then the speed of the decay process. Fig. 5 shows that the surface tension of the frother–saline system is lower than that of the frother–deionized water system, that is, saline water is helpful in reducing the surface tension of the system as well as the driving force of the entire liquid membrane drainage. Thus, the purpose of preventing bubble annexation is achieved. In addition, Fig. 5(a)–(d) shows a schematic of film drainage by the capillary pressure. 3.6. Dynamic foam stability test results The results of bubble behavior in the flotation are shown in Fig. 6. After the inflation valve is opened, the bubbles are recorded every 2 s, and after 6 s, the bubble reaches the upper edge of the flotation tank and the recording is ended. The figure shows that
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Fig. 4. Flotation effect of saline water with different concentration.
Fig. 5. Surface tension of frother and saline water.
at the same time, the size of the bubbles in the Al3+ condition environment is the smallest, followed by Mg2+ and Na+. However, with time, the growth rate of the bubbles under different inorganic salt ion conditions has an opposite trend. In the Na+ environment, the
Fig. 6. Study on bubble behavior in flotation.
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relatively small and the foam stability is the strongest. In addition, by SEM analysis of gasified fly ash concentrate, the results show that the high concentration of concentrate ash in the flotation test is mainly due to the existence of carbon-ash continuous organisms. (3) The results of the Zeta potential test of gasification fly ash particles under different inorganic salt cations and concentrations are consistent with those of the flotation test. The smaller the absolute value of the Zeta potential, the more favorable is the improvement in the flotation effect. In the AlCl3 solution, the salt ion concentration required for the vaporized ash particles to reach the zero point is much smaller than that in MgCl2 and NaCl.
Declaration of Competing Interest None. Fig. 7. Effect of salt concentration on Zeta potential.
Acknowledgement bubble size grows the fastest, the thickness of the foam layer is the largest, and the foam is easy to break, followed by Mg2+ and Al3+. The difference in the bubble behavior in different inorganic salt ion solutions indicates that the bubble annexation degree is also different. The degree of merger of bubbles in the Al3+ solution environment is the smallest, and the stability of the bubbles is relatively strong. The addition of inorganic salts compresses the electric double layer of the bubble, increasing the electrostatic repulsion between the bubbles, thereby preventing the mutual agglomeration between the bubbles and reducing their size. 3.7. Zeta potential test results The variation in the Zeta potential of gasification fly ash particles with salt ion type and concentration is shown in Fig. 7. As seen in the figure, under natural pH conditions (pH = 7), as the salt ion concentration increases, the Zeta potential of the vaporized ash particles also increases. In flotation test, reducing the absolute value of Zeta potential is helpful in reducing the electrostatic repulsive force between particles and improving their floatability (Xia et al., 2019c; Yu et al., 2012). In the Al3+ solution, the salt ion concentration at the zero point is much smaller than that at the zero point of the Mg2+ and Na+ solutions, that is, the optimum salt ion concentration in the Al3+ solution is much smaller than that of the other two ions. The law is related to the electron radius and charge difference of salt ions, and the variation in Zeta potential of gasification fly ash particles with different salt ion species is consistent with the flotation results. 4. Conclusion (1) The results of particle size analysis of gasification fly ash show that the particle size of 0.045 mm and 0.125– 0.074 mm is dominant in the original ash, and all this part is the key to the removal of unburned carbon from gasification fly ash. (2) The flotation test results show that the difference concentration of inorganic salt cations improves the recovery efficiency of unburned carbon in gasification fly ash flotation. Al3+ and Na+ show the highest and lowest improvement, respectively. Salt ions reduce the surface tension of the system and weaken the bubble decay process. By studying the bubble behavior in different salt ion solution environments, it is also found that the bubble size of Al3+ solution is
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Recovering unburned carbon from gasification fly ash using saline water Rui Zhang a,b, Fangyu Guo a,b, Yangchao Xia a,b, Jinlong Tan c, Yaowen Xing a,⇑, Xiahui Gui a,d,⇑ a
Chinese National Engineering Research Center of Coal Preparation and Purification, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China c School of Chemistry and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China d Henan Province Industrial Technology Research Institute of Resources and Materials, Zhengzhou University, Zhengzhou 450001, Henan, China b
a r t i c l e
i n f o
Article history: Received 29 May 2019 Revised 1 August 2019 Accepted 9 August 2019 Available online 14 August 2019 Keywords: Gasification fly ash Saline water Recovery Foam stability Surface tension
a b s t r a c t Gasification fly ash is one of the wastes generated by coal gasifiers, and the unburned carbon therein seriously restricts the resource utilization of gasification fly ash. Flotation is one of the best ways to recover unburned carbon from it; however, surface pores of gasification fly ash are developed and contain several hollow hydrophilic glass beads, which makes it difficult for conventional flotation to recover unburned carbon effectively and the dosage of the flotation reagent is too high. In this study, different concentrations of saline water (NaCl, MgCl2, and AlCl3) are configured to the flotation solution, and their effect on the recovery of unburned carbon of gasification fly ash is investigated. Furthermore, the gasification fly ash treated with saline water is chosen to study the basic properties by the measurement of Zeta potential, surface tension, and flotation foam behavior. The experimental results show that with an increase in the valence state of the inorganic salt cation, the unburned carbon recovery efficiency of the gasification fly ash is significantly improved. When the concentration of Al3+ reaches 0.4 mol/L and the dosage of frother is 7.5 kg/t, the unburned carbon removal rate of the tailings reaches 95% or more. Saline water reduces the surface tension of the flotation system and weakening bubble decay; in the solution of Al3+, the flotation foam size is the smallest, followed by the solution of Mg2+, Na+. Furthermore, the saline water effectively reduces the Zeta potential of the particle surface and improves the floatability of the solid particles. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Gasification fly ash is one of the wastes produced by the gasification furnace in coal chemical enterprises (such as coal-to-oil and coal-to-gas), and its annual emissions can reach several hundred thousand tons or more (Xu et al., 2009; Hower et al., 2017). As gasification fly ash exhibits the characteristics of high ash content, fine grain size, and low direct utilization rate (Huggins et al., 2016), most of them are piled up in the fly ash storage yard, thus occupying considerable space (Yang et al., 2019a). Besides, the fly ash contains some hazardous metals; thus, during the long-term stacking process, the environment is polluted as the fly ash gradually penetrates the soil and groundwater (Huang et al., 2003). At present, the gasification fly ash treatment is mainly used for the admixture of concrete (Walker and Wheelock, 2006), and some is used as a bulk building material (such as cement) (Ding et al., 2016). However, the carbon in the gasification fly ash, which is not completely ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Xing), [email protected] (X. Gui). https://doi.org/10.1016/j.wasman.2019.08.014 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
burned by combustion, seriously affects the use of gasification fly ash as building materials or admixtures (Xu et al., 2017). A large amount of gasification fly ash is difficult to use, and there is still a great deal of it disposed in ash pond or landfill (Tian et al., 2019). In addition, unburned carbon can be used as a substitute for rubber carbon black or as an adsorbent material (such as activated carbon) (Zhang and Honaker, 2015). The loss on ignition of (LOI) gasification fly ash is about 20–30%. It was found that a relatively high content of unburnt carbon which hinders their utilization as additives in cement and concrete. (Wu et al., 2007). And the direct use of gasification fly ash is not only difficult to meet the building materials use standards (Lee et al., 2010), but also causes significant wastage of unburned carbon resources (Xu et al., 2009; Zhu et al., 2019). Therefore, how to effectively recover unburned carbon from gasification fly ash is key to the secondary utilization of gasification fly ash solid waste resources. Foam flotation is a method for achieving mineral separation by utilizing the difference in the hydrophobicity of the surface of component minerals (Xia et al., 2019a, 2019b). Many studies have shown that the use of froth flotation is one of the most effective and economical methods for removing unburned carbon from
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different types of fly ash (Demir et al., 2008; Mccarthy et al., 2013). Altun et al. (2009) used a flotation column to remove unburned carbon from fly ash. By optimizing the bubble generator device, the recovery rate of unburned carbon was over 98% and the tailing loss was <1%. Xu et al (2017) compared the effects of a traditional flotation machine and a static micro-bubble flotation column on the decarbonization and separation performance of fly ash. Under the optimum flotation conditions, the recovery of unburned carbon is 6.5% higher than that of a flotation machine. The removal of unburned carbon from fly ash by foam flotation is a mature process and the separation effect is remarkable (Yang et al., 2019b). However, the problem of high-flotation-reagent consumption has always been a difficult problem for laboratory researchers and field workers. As gasification fly ash is obtained by spray sedimentation of the fly ash in the high-temperature gasification process of coal, and in the process of gasification, in addition to the melting of various mineral components, reactions also occured between mineral components (Bai et al., 2008). Taking calcium as an example, the main components are anorthite, silica and other minerals at 1300 °C. At 1400 °C, the crystalline materials include anorthite, quartz, mullite and so on. At 1500 °C, the main mineral in ash is Ca3(Si3O9), Ca2(SiO4), Ca(Al2O3)2 anorthite and some amorphous materials (Li et al., 2006). So the hydrophobicity of gasification fly ash surface is very poor. Besides, the physical and chemical properties of the surface of unburned carbon particles have been changed significantly and the surface oxidation degree has become serious; flotation reagents are not easy to spread on its surface. For the removal of unburned carbon in gasification fly ash by flotation, the phenomenon that the consumption of the foaming reagent too high is particularly obvious. In addition, gasification fly ash contains several hollow glass microspheres, which are easily broken during the flotation process, thereby exposing a more specific surface area. This is another reason for the low efficiency and high reagent consumption of conventional flotation foaming reagents (Kiani et al., 2015). In response to this, researchers have actively explored the best conditions for recovering unburned carbon from the fly ash (Bournival et al., 2012; Kurniawan et al., 2011; Castillo et al., 2011). For a flotation reagent, (Walker and Wheelock, 2006) used nonylphenol, hexadecane, fuel oil, and methyl isobutyl phenol (MIBC) as a foaming reagent to conduct a laboratory study on a power-plant fly ash in a twostage flotation cell. The results showed that the recovery rate of unburned carbon in the concentrate reached over 95% by using the two-stage flotation method. In addition, the use of a collector consisting of nonylphenol and either hexadecane or fuel oil, in combination with MIBC, allows for a better separation of unburned carbon from fly ash under conditions of smaller collectors. Drzymala et al. (2005) used 4-dodecylphenol and hexadecane as collectors to achieve the separation of unburned carbon in fly ash on the laboratory flotation machine, based on the two-stage flotation test. The carbon content of the selected fly ash could be reduced from the initial 25–6.0% or lower. In addition, many scholars have recently investigated the influence of saline water on flotation, and argued that the flotation of hydrophobic coals can be improved using inorganic ions (Laskowski and Castro, 2015). On the one hand, some researchers have proposed that after the inorganic salt ions are added to the solution, the electric double layer on the surface of the mineral particles is compressed and the absolute electric potential of the particles decreases (Arnold and Aplan, 1986). On the other hand, the inorganic salt ions can promote the instability and rupture of the hydration film around the coal particles, thereby enhancing the hydrophobicity of the particles and increasing the probability of contact and collision of the particles with the bubbles (Liu et al., 2013). Based on this, Wang et al. (2013, 2014) studied the behavior of minerals in brine flotation fine coal. The experiment examined the entrainment of
minerals during flotation through foam image analysis. Saline water increases the recovery of minerals as well as that of combustibles. Zhang, 2015 studied the flotation enhancement mechanism of natural hydrophobic coal particles in different cation (Na+, Ca2+, Al3+) salt solutions. In the test, the flotation performances of AlCl3 and NaCl solutions showed the highest and lowest improvement effects, respectively. In addition, Chang et al. (2018) studied the effects of water salinity on the flotation of oxidized coals, and found that the compression of the electric double layer in saline water could increase the flotation efficiency by reducing the electrostatic repulsion between the particle and bubble, which could also increase the flotation efficiency. The surface hydrophobicity of gasification fly ash is poor, and saline water plays an important role in mineral flotation. However, little research has been conducted on the recovery of unburned carbon from gasification fly ash using saline water. In this study, the effect of saline water with different concentrations and frother with different dosages on the recovery of unburned carbon from gasification fly ash is studied, and the work’s aims to use the special properties of saline water to change the behavior of bubbles to reduce the consumption of chemicals while achieving the purpose of secondary recycling of gasification fly ash resources.
2. Materials and methods 2.1. Test samples and chemicals The gasification fly ash was taken from a chemical plant of Shenhua Coal Industry Group Ningxia, China. Gasification reaction was carried out by GSP coal gasification technologies (the temperature is 1450–1650 °C). In the gasification process, some unreacted carbon and fine slag will be brought out by syngas, collected by spray and dehydrated to produce a large amount of gasification fly ash (Wang, 2017). The original ash was dried and sieved through a 0.5 mm standard sieve, and the 0.5 mm portion under the sieve was taken as the material for this test. Table 1 shows the proximate and ultimate analyses of the test gasification fly ash measured, and the content of carbon and hydrogen is determined by fully burning the sample at a temperature of 850 °C. However, the method for determining the oxygen content is not yet mature, and the practical oxygen content is generally calculated by difference. Table 1 shows that the air-dried ash of the ash sample is 73.70%, the moisture (Mad) is 1.74%, volatile matter (V) is only 3.08%, and in the elemental analysis, the carbon content is 95.12%, and the oxygen element content is 0.99%. The industrial and elemental analyses of the sample showed that the ash content was high after burning at high temperature, and the residual carbon content in organic matter was higher, while the oxygen and hydrogen elements were fewer after combustion. However, the content of fixed carbon was still 21.48%, indicating that some of the carbon in the ash sample was not fully burned and this part was not realized. The flotation reagent used in the experiment was purchased from Hunan Xinghui Washing Chemical Technology Development Co., Ltd.; W501 was used as the collector and W502 was used as the frother. The flotation reagent’s chemical compositions were characterized using FTIR (Nicolet IS5, Thermo Fisher, USA) and the results are shown in Fig. 1(b). The water used in the test was deionized water, with pH = 7; the saline water comprised NaCl, MgCl2, and AlCl3, since of this is three representative inorganic salts. Through the small sieve screening and muffle furnace burning method, the content of each fraction of gasification fly ash and the loss on ignition were measured.
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R. Zhang et al. / Waste Management 98 (2019) 29–36 Table 1 Proximate and ultimate analyses of the gasification fly ash. Proximate analysis (wt%, ad)
Ultimate analysis (wt%, daf)
M
A
V
FC*
C
H
N
S
O*
1.74
73.70
3.08
21.48
95.12
1.59
0.48
1.82
0.99
ad-air dry basis; daf-ash free basis; *-by difference.
Fig. 1. LOI distribution of gasification fly ash (a) and FTIR spectra of flotation reagent (b).
2.2. XRD analysis The composition of inorganic minerals in gasification fly ash was studied by the German Brook D8 ADVANCE-type X-ray diffraction (XRD). The light source under test was X-ray-tube copper target radiation, the tube pressure was 40 kV, the current was 40 mA, the goniometer accuracy was 0.0001°, and the scanning range was 0–140°. 2.3. SEM/EDS analysis The morphology of gasification fly ash was analyzed by FEI Quanta TM250 scanning electron microscopy (SEM) and energydispersive spectroscopy (EDS). The surface morphology of the sample was observed by high-energy electron beam scanning and some element in the gasification fly ash was positioned. In the EDS photos, different colors represent different elements. Appropriate amounts of the sample were adhered to the surface of the conductive tape before the test began, and placed on the sample holder. The ear wash ball was used to blow off the particles that stuck to the conductive tape. The sample was then subjected to a gold spray treatment prior to testing, to prevent charge accumulation during the scan. 2.4. Flotation test The test used an XFD-type flotation machine with volume of 1 L, and the slurry concentration was maintained at 100 g/L. Before the test, the weighed inorganic salt was dissolved in a flotation cell, and the concentration of the saline water was 0.1, 0.2, and 0.4 mol/L, respectively (the concentration of NaCl was 0.2, 0.4, and 0.8, respectively). After pulp conditioning (stirring at an impeller speed of 1800 r/min for 2 min), the collector (W501) and frother (W502) were added for contact times of 120 and 30 s, respectively. The size of the inflation valve switch was adjusted, the inflation amount was controlled within 0.2 m3/h, and foaming was started. After flotation for 7 min, the concentrate and tailings were
separately filtered, dried, and weighed, and the ash was tested according to the national standard to calculate the yield and ash. 2.5. Surface tension test The surface tension of the synergistic system of the saline water and frother was measured using a K 100 surface tension instrument. First, 0.4 mol/L of saline water was prepared with deionized water as the base solution, because this concentration is the best flotation test concentration, and then, different types and concentrations of the frother were prepared with different base solutions. The surface tension test was carried out at 20 ± 0.5 °C. The repeatability of the test was verified three times for each test and the average value of the test results was taken as the final test value. 2.6. Dynamic foam stability test An inorganic salt solution having a concentration of 0.4 mol/L was prepared in a flotation cell having a volume of 1 L, and 100 g of the ash sample was added to the flotation cell to maintain the entire slurry concentration at 100 g/L. After the slurry preparation was completed, the flotation machine was turned on, impeller speed was adjusted to 1800 r/min, and the slurry was continuously adjusted for 2 min. Then, a certain amount of frother (2.5 kg/t) was added, and the air intake was maintained at 0.2 m3/h. When the inflation valve was opened, the foaming behavior was recorded every 2 s until the foam reached the upper edge of the flotation cell. 2.7. Zeta potential test The ash sample was further ground to less than 74 lm in a mortar; 1 g of the ground ash sample was weighed each time with a precise electronic balance and placed in a 100 mL beaker. Then, salt ions were sequentially added in accordance with the concentration of the flotation solution. It was then stirred with a magnetic stirrer for 30 min to allow the ash sample to diffuse sufficiently. Finally,
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the sample to be tested was allowed to stand for 12 h. During the test, a certain amount of the suspension was sucked into the electrophoresis tank by a syringe. According to the measuring operation procedure of the electrophoresis apparatus, the gray motor potential at the natural pH (pH = 7) was measured, and each sample was measured three times, and finally, the average value was taken.
mullite in the gasification fly ash is mainly encapsulated in the glass beads by kaolinite in raw coal after high-temperature combustion, and the quartz and illite are mainly contained in the raw materials. In the XRD pattern of the sample, there are also characteristic peaks composed of amorphous minerals, which are mainly composed of vitreous bodies in the gasified ash and unburned carbon.
3. Results and discussion
3.3. SEM/EDS test results
3.1. The characterization of sample and reagent
The SEM-EDS photos of gasification fly ash are shown in Fig. 3 (a), where the main elements of the regular spheroids located in the original ash I region are Si and Al. The main element of the irregular multi-space body located in region II is C. The analysis of gasification fly ash indicates that the regular sphere in region Ⅰ is a glass bead formed by high-temperature combustion of gasification fly ash, and this part of the substance is the main component of ash. The irregular hollow body in region Ⅱ is the unburned carbon particles, which are not fully burned in gasification fly ash. The effective separation of glass beads and unburned carbon particles in gasification ash will be the key link in the utilization of the gasification ash resource. The results of SEM analysis of gasification fly ash, concentrate, and tailings are shown in Fig. 3(b). As seen in Fig. 3(b), (1), there is not only a part of unburned carbon in the raw ash but also several glass beads with crystalline or amorphous phase. In the raw ash, the unburned carbon surface is rough and porous, and the developed pores are the main cause of the high consumption of flotation drugs. Fig. 3(b), (2) shows an image of the flotation concentrate of the gasified ash, which indicates that some unburned carbon is embedded with glass beads in the flotation concentrate; such a special co-existence state will cause the ash content of the concentrate to exceed the standard in the flotation process. Fig. 3 (b), (3) shows a photograph of the appearance of gasification fly ash flotation tailings. Most of the tailings are glass beads, whose sizes vary. Many glass beads of different shapes will bond at a high temperature (red circle), and the main components of this glass bead are quartz and alumina (Asokan et al., 2005). After separating the carbon and ash of gasification fly ash, this part of the tail ash can be used as a substitute for traditional building materials (such as cement and hollow bricks) (Golewski, 2017) or chemical products (such as polymerized aluminum and alumina) (Sujjavanich et al., 2017).
The test results of the gasification fly ash particle size and collector are shown in Fig. 1(a), (b) respectively. The Fig. 1(a) shows that the main particle size of gasification fly ash is concentrated at 0.045 and 0.125–0.074 mm, wherein the LOI of the 0.125–0.074 mm grain is the highest and that of the 0.045 mm grain is relatively low. Therefore, the high ash content in the gasification fly ash is concentrated in the fine particle size and the flotation of the 0.125–0.074 mm fine grain material is key to the removal of unburned carbon from the gasification fly ash. The Fig. 1(b) shows the infrared spectra of flotation reagent. It can be seen the absorption peak around 3453 cm1 is attributed to hydroxyl (AOH) with stretching vibration. The absorption peak around 2928 cm1 and 2867 cm1 is attributed to aliphatic CAH with asymmetrically stretching vibration (ACHvas) and aliphatic CAH with symmetrically stretching vibration (ACHvs), respectively (Li et al., 2015). The peak at 1712 cm1 are assigned to carboxyl (@COOH). In the FTIR spectra of W501, the peak around 1600 cm1 is attributed to C@C bonds. The peak around 1455 cm1 and 1375 cm1 are attributed to bending vibration of methylene (ACH2) and methyl(ACH3), respectively (Akyıldırım et al., 2017). The peak at 1084 cm1 and 1081 cm1 originate from CAO stretching vibration (alcohols and ethers, respectively). The oxygen-containing groups, such as AOH, ACOOH, AC@O, and ACAOA, are the main polar compositions of flotation reagent (Wang et al., 2018) the characteristic peaks of these functional groups have been marked with red dotted lines in the figure. 3.2. XRD test results The results of gasification fly ash XRD test analysis are shown in Fig. 2, where the main crystalline minerals in the sample are quartz and illite, as well as a small amount of mullite and gypsum. The
3.4. Flotation test results In the gasification fly ash flotation test, LOI and removal rate of unburned carbon (RUC) in the tailings are usually evaluated to determine the degree of perfection of the sorting equipment and process (Xu et al., 2018). The RUC of the tail ash in the flotation test is equal to the carbon recovery rate of the concentrate (Xu et al., 2017).
LOI ¼ 100 Ay
ð1Þ
where Ay represents raw ash.
RUC ¼
Fig. 2. X-ray diffraction patterns of gasification fly ash.
cj Lj Ly
100%
ð2Þ
where cj represents the concentrate yield, %; Lj represents the loss on ignition of the concentrate, %; and Ly represents the loss on ignition of raw ash, %. The respective experiments investigate the improvement effects of three inorganic salts of NaCl, MgCl2, and AlCl3 on the separation of gasification fly ash flotation under different amounts of the foaming reagent. The test results are shown in Fig. 4.
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Fig. 3. SEM-EDS photos of gasification fly ash (a) and SEM results for raw materials, concentrates, and tailings (b).
Fig. 4 shows that as the concentration of inorganic salt cations increases, the carbon removal rate of the flotation tailings also increases and the loss on ignition of tailings decreases. The results of the flotation test indicate that the use of saline water as the aqueous solution of flotation can effectively achieve unburned carbon recovery of the gasified fly ash. Moreover, under the condition of the same inorganic salt ion concentration, the higher the valence state of the inorganic salt cation, the smaller is the amount of frother reagent required to achieve the optimum flotation carbon-ash separation test effect. Note that due to the small amount of charge carried by Na+, a small amount of Na+ is insufficient to affect the flotation test results (Lessard and Zieminski, 1971). Therefore, the gradient of Na+ concentration is set to be high the addition of inorganic salt ions can effectively reduce the degree of merger between bubbles, and the higher the valence state of the added inorganic salt cations, the more obvious is the blocking effect (Henry and Craig, 2008). In the solution environment, there is a plateau channel between the bubble and the bubble for the liquid to flow. The addition of the inorganic salt cation can reduce the flow rate of the liquid in the plateau channel, thereby reducing the discharge rate between the bubbles and further preventing the occurrence of the bubble-tobuking phenomenon (Bhakta and Ruckenstein, 1997). Meanwhile, after the inorganic salt ions are added to the solution, the electric double layer on the surface of the ore particles is compressed, the absolute value of the zeta potential of the particles is reduced, and the repulsive force between the particles and the bubbles is reduced (Paulson and Pugh, 1996). 3.5. Surface tension analysis The microcosmic phenomenon of foam decay is the rupture of a liquid film, which is affected by the seepage process. The driving
force DPS is a net result of the suction pressure in the adjacent plateau border channels and the disjoining pressure (P) in the films, and is given by (Bhakta and Ruckenstein, 1997)
DP ¼ DP S
Y
¼
2c Y R
ð3Þ
where DPS is the pressure difference between the liquid film and the plateau channel, also known as the capillary pressure difference; R is the radius of the bubble; and c is the surface tension of the liquid Q phase. is the disjoining pressure, which refers to the interaction between the gas–liquid interface on both sides of the liquid film. The pressure is expressed as the mutual exclusion of the two gas– liquid interfaces and is an opposite force to the capillary pressure. Both of them maintain the force balance of the liquid film. The foam decay process in the two-phase system is carried out under the action of gravity and DP. The change in the properties of the liquid phase system will affect the size of DP, and then the speed of the decay process. Fig. 5 shows that the surface tension of the frother–saline system is lower than that of the frother–deionized water system, that is, saline water is helpful in reducing the surface tension of the system as well as the driving force of the entire liquid membrane drainage. Thus, the purpose of preventing bubble annexation is achieved. In addition, Fig. 5(a)–(d) shows a schematic of film drainage by the capillary pressure. 3.6. Dynamic foam stability test results The results of bubble behavior in the flotation are shown in Fig. 6. After the inflation valve is opened, the bubbles are recorded every 2 s, and after 6 s, the bubble reaches the upper edge of the flotation tank and the recording is ended. The figure shows that
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Fig. 4. Flotation effect of saline water with different concentration.
Fig. 5. Surface tension of frother and saline water.
at the same time, the size of the bubbles in the Al3+ condition environment is the smallest, followed by Mg2+ and Na+. However, with time, the growth rate of the bubbles under different inorganic salt ion conditions has an opposite trend. In the Na+ environment, the
Fig. 6. Study on bubble behavior in flotation.
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relatively small and the foam stability is the strongest. In addition, by SEM analysis of gasified fly ash concentrate, the results show that the high concentration of concentrate ash in the flotation test is mainly due to the existence of carbon-ash continuous organisms. (3) The results of the Zeta potential test of gasification fly ash particles under different inorganic salt cations and concentrations are consistent with those of the flotation test. The smaller the absolute value of the Zeta potential, the more favorable is the improvement in the flotation effect. In the AlCl3 solution, the salt ion concentration required for the vaporized ash particles to reach the zero point is much smaller than that in MgCl2 and NaCl.
Declaration of Competing Interest None. Fig. 7. Effect of salt concentration on Zeta potential.
Acknowledgement bubble size grows the fastest, the thickness of the foam layer is the largest, and the foam is easy to break, followed by Mg2+ and Al3+. The difference in the bubble behavior in different inorganic salt ion solutions indicates that the bubble annexation degree is also different. The degree of merger of bubbles in the Al3+ solution environment is the smallest, and the stability of the bubbles is relatively strong. The addition of inorganic salts compresses the electric double layer of the bubble, increasing the electrostatic repulsion between the bubbles, thereby preventing the mutual agglomeration between the bubbles and reducing their size. 3.7. Zeta potential test results The variation in the Zeta potential of gasification fly ash particles with salt ion type and concentration is shown in Fig. 7. As seen in the figure, under natural pH conditions (pH = 7), as the salt ion concentration increases, the Zeta potential of the vaporized ash particles also increases. In flotation test, reducing the absolute value of Zeta potential is helpful in reducing the electrostatic repulsive force between particles and improving their floatability (Xia et al., 2019c; Yu et al., 2012). In the Al3+ solution, the salt ion concentration at the zero point is much smaller than that at the zero point of the Mg2+ and Na+ solutions, that is, the optimum salt ion concentration in the Al3+ solution is much smaller than that of the other two ions. The law is related to the electron radius and charge difference of salt ions, and the variation in Zeta potential of gasification fly ash particles with different salt ion species is consistent with the flotation results. 4. Conclusion (1) The results of particle size analysis of gasification fly ash show that the particle size of 0.045 mm and 0.125– 0.074 mm is dominant in the original ash, and all this part is the key to the removal of unburned carbon from gasification fly ash. (2) The flotation test results show that the difference concentration of inorganic salt cations improves the recovery efficiency of unburned carbon in gasification fly ash flotation. Al3+ and Na+ show the highest and lowest improvement, respectively. Salt ions reduce the surface tension of the system and weaken the bubble decay process. By studying the bubble behavior in different salt ion solution environments, it is also found that the bubble size of Al3+ solution is
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