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  • ISSN: 2333-6633
    Chem Eng Process Tech 1(3): 1019.
    Submitted: 04 December 2013; Accepted: 27 December 2013; Published: 29 December 2013
    Research Article
    Simultaneous Recovery of Boron and Other Major Elements from a Coal Fly Ash by pH Control
    Kyoko Oishi*, Yugo Maehata, and Masashige Hashino
    Department of Civil Engineering, Graduate School of Engineering, Kyushu University, Japan
    *Corresponding author: Kyoko Oishi, Department of Civil Engineering, Graduate School of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan; Tel/Fax: +81 92 802 3423; Email: ohishi@civil.kyushu-u.ac.jp
    Abstract
    Coal fly ashes, which contain various hazardous elements, may be disposed of or reused. The existence of the hazardous elements in coal fly ashes can pose environmental problems. Therefore, it is desirable to remove these elements before reusing coal fly ash. In this study, the simultaneous recovery of boron and other major elements from a coal fly ash was investigated. The amounts of boron and other major elements in solution were measured after leaching using pure water and HCl of several different concentrations with leach times of 6 h at a liquid/solid ratio of 10. Pure water leached boron and calcium. Dilute HCl leached aluminum, boron, calcium, iron, and magnesium. The amount of all elements almost reached a maximum when the final pH of leachate was approximately 1. The boron and other elements were co-precipitated when the pH of these leachates was adjusted to 12. Aluminum, magnesium, and iron were removed completely by the precipitation while boron and calcium were removed at the ratios of 30-40% and 60-75%, respectively. Residual boron was removed using glucomannan semi-gel at a pH of approximately 12. Approximately 70% of the boron that leached from the coal fly ash was finally removed by co-precipitation and adsorption with glucomannan semi-gel.
    Keywords: Coal fly ash; Boron; Recovery; Leachate
    Introduction
    Coal fly ash is a waste material produced at coal-fired plants. Nowadays, great amounts of the ash are required to be recycled. Coal fly ashes contain various hazardous elements in traces but the large amounts of the ash have a great effect on ecosystem. The hazardous elements may cause environmental problems by leaching into environmental waters when the ashes are disposed or reused. Boron, arsenic, and molybdenum contained in coal ashes are easily leached [1,2]. Boron is chemically classified as a semi-metal; however, it is considered a rare metal as it has great value as a functional material. On the other hand, boron may be toxic to animals, plants and humans [3-7]. Therefore, these fly ashes are pre-treated to prevent contamination of various hazardous elements into water environments before reusing the ashes. Several leaching methods are proposed [8]. Deionized water has been used in the leaching of coal fly ash. Recently acid solution such as dilute HCL and H2SO4 are used as leaching solvents to obtain a large fraction of elements from fly ashes [1,8-14]. These acidic solutions leached not only boron but also other elements, such as calcium, iron, magnesium, and aluminum. It is important for the reuse of coal fly ashes to determine the amount of each element that can be leached from the coal ash and to recover selectively these elements from the leachates. Accordingly, a technics for effective recovery of metals containing boron is required. It is also desired to be a simple and low-cost.
    Authors reported that dissolved boron was recovered selectively using glucomannan semi-gel at pH ≥ 11 [15]. The higher pH condition was one of important factors for effective boron removal.
    The effects of the pH control on leaching amounts of elements from a coal fly ash and on effective recovery of boron and other major elements from the leachate were investigated in this study.
    Materials and methods
    Materials
    A coal fly ash was obtained from the Kyushu Electric Power Co., Inc., Fukuoka, Japan. PROPOL ISLB® was produced from konjac glucomannan at Shimizu Chemical Co., Hiroshima, Japan. This is a “semi-gel”, which is gelled only at the surface when placed in water and is insoluble.
    Determination of boron and other elements
    Boron concentrations were measured by the azometin H method [16]. Other elemental concentrations were measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Perkin-Elmer Optima 5300 DV) after pretreatment with 0.1 N HNO3 at the Center of Advanced Instrumental Analysis, Kyushu University, Fukuoka, Japan.
    Flow for recovery of boron and other elements from coal fly ash
    A summary of the overall fractionation scheme for the elements released from a coal fly ash is illustrated in (Figure 1).
    Leaching of boron and other elements from coal fly ash
    Leachates were produced by shaking the 50 g of coal fly ash and solvent in a liquid to solid ratio (L/S) of 10 for 6 h at room temperature. Pure water and dilute HCl were used as leaching solvents. The dilute HCl was prepared at 0.1, 0.25, 0.5, and 1.0 N concentrations.
    Analysis of elements in leachates
    The leachates obtained in the above section were centrifuged at 800 xg for 10 min. The supernatants were filtered through a membrane of 0.1 μm pore size. The pH of the filtrate was measured and then adjusted to the same pH (12) as the leachate using pure water by using 1N NaOH. One hour later, the leachates were again centrifuged at 800 xg for 10 min. The supernatant and precipitate were obtained. The concentration of elements in the supernatants was measured after treatment with 0.5% HNO3. The precipitates were dissolved using 0.5% HNO3 and the elemental concentrations measured.
    Adsorption of dissolved boron with glucomannan semi-gel
    A total of 15 g of the semi-gel powder was added to 300 mL of the supernatant obtained in the above section (Figure 1). The mixture was incubated at 20°C for 4 h on an orbital shaker at 120 rpm. After incubation, the mixture was centrifuged at 800 xg for 10 min. The boron concentrations in the supernatant were determined. Boron adsorbed by the semi-gel was calculated by comparing the concentration of the final to the initial concentrations. All experiments were conducted at least in triplicate.
    Results and Discussion
    Effect of pH on leaching of elements from coal fly ash
    The major elements and their amounts in the leachates from coal fly ash by pure water or dilute HCl of various concentrations are shown in Table 1. Boron and calcium were slightly soluble in the leachates when using pure water, but other elements were not detected. Dilute HCl tended to increase the amount of the elements leached in the following order: Ca > Al > Fe > Mg > B. In the leachates produced using 0.25 N HCl, these elements approached a maximum value. The leachate produced in pure water reached a pH of approximately 12. Leachates at concentrations of 0.1, 0.25, 0.5 and 1.0 N HCl reached a final pH of 3.5, 1.1, 0.6 and 0.2, respectively. The amount of elements leached increased with decreasing final pH of the leachate and almost reached a maximum when pH of leachate was approximately 1 (Table 1).
    Table 1 Chemical composition of leachates from coal fly ash using pure water and dilute HCl of several concentrations and mass balances of elements in the supernatants and the precipitate after adjusting the leachates to pH=12.

    Leaching   soluvent   [-]

    elements  [-]

    final pH  [-]

    leachate [mg]*

    surpernatant [mg]

    precipitation [mg]

    recovery    [%]

    Al

    ND

    -

    -

    -

    B

    1.7

    -

    -

    -

    pure

    Ca

    11.8

    178

    -

    -

    -

    water

    Fe

    ND

    -

    -

    -

    Mg

    ND

    -

    -

    -

    Al

    48

    ND

    53

    110

    B

    7.6

    5

    2.4

    98

    HCl

    Ca

    3.5

    957

    688

    208

    94

    0.1N

    Fe

    ND

    ND

    ND

    -

    Mg

    49

    ND

    74

    152

    Al

    210

    0.2

    198

    95

    B

    8.8

    4.6

    3.8

    96

    HCl

    Ca

    1.1

    1146

    382

    666

    92

    0.25N

    Fe

    132

    ND

    139

    105

    Mg

    87

    ND

    104

    120

    Al

    209

    ND

    209

    100

    B

    8.2

    5.5

    3

    104

    HCl

    Ca

    0.6

    1176

    166

    902

    91

    0.5N

    Fe

    98

    ND

    135

    137

    Mg

    21

    ND

    86

    408

    Al

    244

    ND

    222

    91

    B

    7.8

    6.2

    3.4

    122

    HCl

    Ca

    0.2

    1196

    113

    893

    84

    1.0N

    Fe

    125

    ND

    145

    116

    Mg

    28

    ND

    84

    297

    *amounts leachated from 50g of a coal fly ash

    Table 1 Chemical composition of leachates from coal fly ash using pure water and dilute HCl of several concentrations and mass balances of elements in the supernatants and the precipitate after adjusting the leachates to pH=12.

    ×
    Boron exists in two phases in coal fly ash: the leachable and mineralogical bound phase. It has been shown that a large fraction of boron is leached from coal fly ash under low pH conditions [1, 8-12,17]. The pH has an important role in the distribution between the fly ash and liquid phase [18]. Most of the boron is likely to be distributed at the ash surface. Therefore, boron in coal fly ash is removed efficiently using dilute HCl. It is reported that the boron level in fly ash was as high as 1900 ppm, and approximately 50% was leachable into water [11]. Many types of coal from various countries are used in coal fired power plants. Thus, the properties of coals and their ashes are quite varied; however, an essential factor for effective leaching of the elements from coal fly ashes is control of the final pH of leaching solution.
    Recovery of elements from leachates by co-precipitation with pH control
    According to the fractionation scheme illustrated in (Figure 1), the major elements in leachates produced using pure water and dilute HCl were fractionated. The mass balance of these elements was calculated from the sum of their amounts in the supernatant and precipitate after adjustment to a pH of 12 (Table 1). Total recovery ratios of all of these elements except for magnesium ranged from 80 to 120%. The recovery ratios are able to be evaluated with the high reliability. Except for boron and calcium, almost all of elements in these leachates were recovered as a precipitation. Generally, dissolved metals form metal hydroxides and are precipitated when adjusted to a high pH. Metal-rich solutions such as the leachate from coal fly ash cause the precipitation of metal hydroxides under higher pH conditions. On the other hand, aluminum and boron are dissolved as Al(OH)4- and B(OH)4- at a pH greater than 10, respectively. Therefore, they should not precipitate in theory. However, they were concentrated in the precipitates (Table 1). Aluminum was not precipitated with boron at a pH of 12 when only boron and aluminum co-exist [15]. Table 1 shows that aluminum and boron co-precipitated with calcium, iron and magnesium hydroxides when leachates by dilute HCl were adjusted to a pH of 12. Dissolved metals incorporate boron into their hydroxides under higher pH condition [1, 21]. Boron is precipitated mainly in the form of magnesium borate that is converted to the boracite mineral [Mg3Cl (B7O13)] [22,23]. The formation of positive borate-ion pairs may cause coordinative bonds with negatively charged magnesium hydroxide [18]. Therefore magnesium hydroxide results in the co-precipitation of boron and its removal from solution [18]. Boron is also formed both CaB(OH)4+ and B(OH)4- in the presence of calcium [24]. The former will be precipitated by ion-pair formation and the latter will remain dissolved.
    Figure 1 Flow sheet for elements removal from a coal fly ash.

    Figure 1 Flow sheet for elements removal from a coal fly ash.

    ×
    The magnesium was not detected in the supernatant, but done in the precipitation with 1.2 to 4-fold excess of the leachate amount (Table 1). A possible explanation for this result is that a calcium matrix effects on ICP-AES analysis [25-27]. The calcium/magnesium ratio is more than 10-fold (Table 1). The presence of more than a 10-fold excess of calcium to magnesium interferes in magnesium analysis with ICP-AES. The solubility of Mg(OH)2 is very low in comparison with Ca(OH)2. Therefore, the mass balance of magnesium may lack in accuracy, but magnesium must have been concentrated in the precipitation.
    Generally, leachates from coal fly ashes have been disposed after dilution or controlling to neutral pH value. However, boron and other elements in the leachates were recovered as precipitation under pH≥12 condition (Table 1). The combination of the leaching of elements from coal fly ash by dilute HCl and controlling of the leachates to pH≥12 is useful for the effective removal of boron and other major elements from coal fly ashes. The pH control of the leachates is one of most important processes in the simultaneous recovery of boron and other elements from coal fly ash leachates.
    Recovery of boron from leachates using glucomannan semi-gel
    The major elements were almost all recovered except for boron and calcium by precipitation when the leachates were increased to a pH of 12 (Table 1). In a previous paper, authors reported that glucomannan semi-gel had a high capacity for B(OH)4- removal at the same pH condition as above [15]. Therefore, this semi-gel was used to recover boron from the supernatant after precipitation process at pH=12 under the same pH conditions. Results are summarized in (Figure 2). About 40% of the dissolved boron was removed by adsorption to the semi-gel. The diol groups in the semi-gel form complexes with B(OH)4- under high pH conditions [15]. The dissolved boron forms B(OH) 4- and CaB(OH) 4+ in the presence of calcium at high pH conditions. Therefore boron species not removed by the semi-gel will be mainly the form of CaB(OH) 4+.
    Figure 2 Boron removal by precipitation and adsorption using glucomannan semi-gel at pH=12 from leachates of a coal fly ash by pure water and dilute HCl of several concentrations.

    Figure 2 Boron removal by precipitation and adsorption using glucomannan semi-gel at pH=12 from leachates of a coal fly ash by pure water and dilute HCl of several concentrations.

    ×
    Conclusion
    Leaching from coal fly ashes by dilute HCl, controlling of the leachates up to a pH of 12, and adsorption using glucomannan semi-gel under the same pH conditions were performed successively for the effective removal of boron and other major elements. The amount of boron and major elements leached approached a maximum value when the final pH of the leachates was approximately 1. The major elements in the leachates were aluminum, boron, calcium, iron, and magnesium. These elements were precipitated by the formation of metal hydroxides when the pH of the leachates was increased to approximately 12. Dissolved aluminum and boron were co-precipitated by ion-pair formation with calcium and magnesium under their pH condition.
    The combination of leaching with dilute HCl, controlling of the leachates up to a pH of approximately 12 and adsorption of the dissolved boron by glucomannan semi-gel led to the removal of approximately 70% of the boron and almost all of the major metals. The methodology may provide a new technique for the effective removal of boron and major elements from coal fly ash.
    References
    1. Izquierdo M, Querol X. Leaching behavior of elements from coal combustion fly ash: An overview. Inter J coal Geol. 2012; 94: 54-66.
    2. Jones DR. In environmental aspects of trace elements in coal. Kuwer Academic Publ Dordrecht. 1995; 221-55.
    3. Hatcher JT, Bower CA. Equilibrium and dynamics of boron adsorption by soils. Soil Sci 1958; 85:319-23.
    4. Weir RJ Jr, Fisher RS. Toxicologic studies on borax and boric acid. Toxicol Appl Pharmacol. 1972; 23: 351-364.
    5. Landolph JR. Cytotoxicity and negligible genotoxicity of borax and borax ores to cultured mammalian cells. Am J Ind Med. 1985; 7: 31-43.
    6. Moseman RF. Chemical disposition of boron in animals and humans. Environ Health Perspect. 1994; 102 Suppl 7: 113-117.
    7. Wester RC, Hui X, Hartway T, Maibach HI, Bell K, Schell MJ, et al. In vivo percutaneous absorption of boric acid, borax, and disodium octaborate tetrahydrate in humans compared to in vitro absorption in human skin from infinite and finite doses. Toxicol Sci. 1998; 45: 42-51.
    8. Kim A G, Hesbach P. Comparison of fly ash leaching methods. Fuel 2009; 88 : 926-37.
    9. Hollis JF, Keren R, Gal M. Boron release and sorption by fly ash as affected by pH and particle size. J Environ Qual. 1988; 17: 181-184.
    10. Nathan Y, Doverckek M, Pelly I. Mimran, U. Characterization of coal fly ash from Israel. Fuel 1999; 78: 205-213.
    11. Cox J A, Lundquist G L, Przyjazny A, Schrmulbach CD. Leaching of boron from coal fly ash. Environ Sci Technol. 1978; 12: 722-723.
    12. James WD, Graham CC, Glascock MD, Hanna ASG. Water-leachable boron from coal ashes. Environ Sci Technol. 1982; 16: 195-197.
    13. Iwashita A, Sakaguch Y, Nakajima T, Takanashi H, Ohki A, Kambara S, et al. Leaching characteristics of boron and selenium for various coal fly ashes. Fuel. 2005; 84: 479-485.
    14. Kukier U, Ishak CF, Sumner ME, Miller WP. Composition and element solubility of magnetic and non-magnetic fly ash fractions. Environ Pollut. 2003; 123: 255-266.
    15. Oishi K, Maehata Y. Removal properties of dissolved boron by glucomannan gel. Chemosphere. 2013; 91: 302-306.
    16. Kluczka J, Trojanowska J, Zolotajkin M, Ciba J, Turek M, Dydo P. Boron removal from wastewater using adsorbents. Environ Technol. 2007; 28: 105-113.
    17. Tsuboi I, Kunugita E, Komasawa I. Recovery and purification of boron from coal fly ash. J Chem Eng Japan 1990; 23: 480-485.
    18. Polat H, Vengosh A, Pankratov I, Polat A. A new methodology for removal of boron from water by coal and fly ash. Desalination 2004; 164: 173-188.
    19. Sims JR, Bingham FT. Retention of boron by layer silicates, sesquioxides, and soil materials: II. Sesquioxides. Soil Sci Soc Am Proc. 1968; 32: 346-369.
    20. Keren, R. Gast, R G. pH-dependent boron adsorption by montmorillonite hydroxy-aluminum complexes. Soil Sci. Am. J. 1983; 47: 1116-1121.
    21. Hobbs MY, Reardon EJ. Effect of pH on boron coprecipitation by calcite: Futher evidence for nonequilibrium partitioning of trace elements. Geochemica et Cosmochimica Acta. 1999; 63: 1013-1021
    22. Valyashko MG. Genesis and exploration of borate deposits related to marine salt deposits. Internat Geol Rev. 1970; 12: 711-719.
    23. Aksenova TD, Borisenkov VI, Dorofeyeva VA. Stability of natural magnesium borates in marine brine at various stages of halogenesis. Geochem Internat 1989; 26: 31-39.
    24. Mattigod S, Frampton J, Lim C. Effect of ion-pair formation on boron adsorption by kaolinite. Clays Clay Miner 1985; 33: 433-437.
    25. Danzaki Y, Takada K, Wagatsuma K, Oku M. Accurate and rapid estimation on spectral interferences in routine analysis by ICP-AES: use of mutual interference coefficients. Fresen J Anal Chem. 1998; 361: 410-418.
    26. Brenner IB, Marchand AL, Daraed C, Chauvert L. Compensation of Ca and Na interference effects in axially and radially viewed inductively coupled plasmas. Microchem J 1999; 3: 344-355.
    27. Villiers SM, Greaves M, Elderfield H. An intensity ratio calibration method for the accurate determination of Mg/Ca and Sr/Ca of marine carbonates by ICP-AES. Geochem Geophys Geosyst. 2002; 3 : DOI: 10.1029/2001GC000169.
    Cite this article: Oishi K, Maehata Y, Hashino M (2013) Simultaneous Recovery of Boron and Other Major Elements from a Coal Fly Ash by pH Control. Chem Eng Process Tech 1(3): 1019.
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