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Synthesis of Hydroxysodalite from Paper Sludge Ash
Using NaOH-LiOH Mixtures
Takaaki Wajima
To cite this version:
Synthesis of Hydroxysodalite from Paper Sludge Ash Using NaOH-LiOH
Mixtures
67Takaaki Wajima1
1 – Department of Urban Environment Systems, Chiba University, 1-33, Yayoi-cho, Inagek-ku, Chiba 263-8522, Japan DOI 10.2412/mmse.40.5.622 provided by Seo4U.link
Keywords: hydroxysodalite, katoite, gehlenite, paper sludge ash, NaOH-LiOH mixture, synthesis reaction.
ABSTRACT. Hydroxysodalite zeolite was synthesized at 90 oC from paper sludge ash, which is industrial wastes in
paper manufacturing, using NaOH-LiOH mixed solution. Paper sludge ash was discharged from paper making plant as industrial wastes, and the amount is increasing annually. The new utilization of paper sludge ash is desired. Hydroxysodalite can be used to remove HCl gas at high temperature, and there are papers for hydroxysodalite synthesis from various ashes, for example, coal fly ash. In my previous study, hydroxysodalite can be synthesized from paper sludge ash. However, little information can be available on the synthesis of hydroxysodalite from paper sludge ash. Therefore, we attempted to examine the synthesis of hydroxysodalite from paper sludge ash using NaOH-LiOH mixtures. Hydroxysodalite [Na6Al6Si6O24‧8H2O] was obtained in the mixed solution with Li / (Li + Na) ratios smaller than 0.25,
while katoite [Ca3Al2(SiO4)(OH)8] was formed in the mixed solutions with the other molar ratios, due to the dissolution
of gehlenite [Ca2Al2SiO7]. The observed concentrations of Si and Al in the solution during the reaction explain the
synthesis of reaction products, which depends on alkali species.
Introduction. Zeolites are a group of more than 40 crystalline hydrated aluminosilicate minerals with
structures based on three-dimensional network of aluminum and silicon tetrahedra linked by sharing of oxygen atoms. Due to their unique pore structures and ion-exchange properties, zeolites can be used not only as cation exchangers and adsorbents but also molecular sieves and catalysts [1].Paper sludge is generated as industrial waste during the manufacture of recycled paper products, and the amounts are increasing annually. The sludge consists of organic fibers, inorganic clay-sized materials, and about 60% water, and is incinerated to produce paper sludge ash (PSA) by burning out the organic materials, thereby reducing the volume of waste. Although a small portion of the ash has been used as cemented fillers, lightweight aggregates in the construction industry and other minor applications [2, 3], most is dumped in landfills. The large daily output of PSA and the limited landfill capacity causes social and environmental problems, and new techniques of ash utilization for further recycling are desired. In our previous study, hydroxysodalite [Na6Al6Si6O24‧8H2O] was synthesized from PSA
in NaOH solution at low temperature (< 100 oC) [4]. Hydroxysodalite is one of high aluminum containing crystalline tectoaluminosilicates, and used to remove HCl gas at high temperature for applying to waste incinerator [5]. It is one way to use PSA for recycling. However, little information can be available on the conversion of PSA into hydroxysodalite. To our knowledge, no previous effort has been made to determine the dependence of alkali species on alkali synthesis of zeolite from PSA. In the present study, hydroxysodalite was synthesized from industrial waste, PSA, using NaOH-LiOH mixtures.
Experimental. Raw PSA was obtained from one of the major paper manufacturers in Japan. The
chemical composition of the ash, determined by scanning electron microscopy (SEM) (Hitachi, S-2600H) equipped with energy dispersive spectrometry (EDS) (Horiba, EX-200) [4], is shown in Table 1. It is noted that Li content is analysed by an inductively coupled plasma method (ICP, SPS4000, SEIKO) after dissolving the sample in aqua regia, because EDS cannot detect Li content. The ash
consists mainly of SiO2 (43.0 %), Al2O3 (23.9 %) and CaO (22.9 %) in the form of amorphous matter
and the minerals gehlenite (Ca2Al2SiO7) and anorthite (CaAl2Si2O8), determined by X-ray diffraction
(XRD) (Rigaku, Rint-2200U/PC-LH), as shown in Fig. 1. The remaining components are essentially lower-concentration impurities, such as Na2O, MgO, Fe2O3 and TiO2.
Fig. 1. Powder X-ray diffraction patterns of PSA.
Table 1. Chemical composition of PSA and the products.
Sample Reaction solution
Chemical composition [wt.%]
SiO2 Al2O3 CaO Na2O MgO Fe2O3 TiO2 Li
PSA 43.0 23.9 22.9 0.3 7.3 0.8 1.8 0.0 The product 4M NaOH 38.1 19.4 26.3 6.4 7.1 0.9 1.7 0.0 3M NaOH + 1M LiOH 38.8 18.9 27.7 5.3 6.0 1.1 1.6 0.5 2M NaOH + 2M LiOH 46.5 21.0 20.4 1.1 7.0 0.5 1.2 2.4 1M NaOH + 3M LiOH 42.1 20.6 26.0 0.5 5.7 0.7 1.3 3.2 4M LiOH 31.9 27.4 26.5 0.1 7.5 0.9 1.5 4.2
Paper-sludge ash was converted to zeolites and other minerals by reaction with alkaline solutions. In order to investigate the effect of cation in alkali solution on zeolite synthesis, two-component alkali solutions of NaOH/LiOH was used as alkali sources. Total alkali concentration in the solution was 4 mol/L. The amounts of Na+ and Li+ were changed under constant of OH-. The alkali reaction using the above alkali solutions was carried out as follows. In each reaction experiment, 100 grams of ash were added to 1 L of alkali solution in a 1 L Erlenmeyer flask (made of poly methyl pentene) with a dimroth condenser, and the mixture (slurry) was continuously stirred at 90 °C. Five mL aliquots of each slurry were removed at varying time intervals to monitor the reaction process over a period of 24 hours. The aliquots were filtered, the solid residue was washed with purified water (using a Millipore Milli-Q Labo system) and dried for 12 hours at 60 °C in a drying oven. The solid residue was then analyzed by XRD to determine the minerals present. The intensity of the major XRD peaks for mineralogical phases: gehlenite (2 1 1), hydroxysodalite (1 1 0), and katoite (4 2 0), were used to determine changes in the mineralogical phases. The chemical composition of the product was analyzed by the same method as the ash. The filtrates were analyzed by ICP to determine the concentration of Si and Al in the alkali solution during the reaction. The amounts of Na+, Ca2+ and
Li+ released from the product in ammonium solution were examined as the fixation of Na+, Ca2+ and Li+ in the product structure for investigating synthesis mechanism. 0.1g of the solid residue was treated with 10 mL of 1 M-ammonium acetate solution, separate from the solution by centrifugation, and added again into fresh 1 M-ammonium acetate solution. This process was repeated three times for 20 min per exchange. The concentration of Na+, Ca2+ and Li+ in the ammonium acetate solution was analyzed by ICP to determine the amount of released Na+, Ca2+ and Li+ from the product. Results and discussion. PSA was reacted with NaOH-LiOH mixed solutions at 90 oC for 24 h. XRD patterns of the reaction products in each NaOH-LiOH mixed solution after 24 h reaction are shown in Fig. 2. In the original ash, two mineral phases, gehlenite and anorthite, existed (Fig. 1). In the product, peaks of hydroxysodalite were confirmed as product phases, and those of anorthite diminished and gehlenite remained in the solids using NaOH-LiOH with low Li/(Li + K) ratios of 0 or 0.25, while calcium hydrate minerals, such as hydrocalumite [Ca2Al(OH)7•3H2O], katoite
(Ca3Al2(SiO4)(OH)8) and portlandite (Ca(OH)2) were confirmed in the product, and both anorthite
and gehlenite decreased using NaOH-LiOH with high Li/(Li + K) ratios more than 0.5.
Fig. 2. Powder X-ray diffraction patterns of the product derived from PSA with NaOH-LiOH mixtures; (a) 4 M NaOH, (b) 3 M NaOH + 1 M LiOH, (c) 2 M NaOH + 2 M LiOH, (d) 1 M NaOH + 3 M LiOH, and (e) 4 M LiOH.
Table 1 shows the chemical compositions of PSA and the products synthesized in different alkali solutions. The Na content in the product increases with increasing the Na fraction of reaction solution, while the main contents, SiO2, Al2O3 and CaO, are almost same. The Li content in the product also
increased with increasing Li content in the solution.
The intensities of the major mineralogical phases in the product, and the amounts of Na+, Ca2+ and Li+ released from the product after 24 h reaction are shown in Fig. 3. With increasing the Li/(Li +Na)
ratio in the mixed solution, the intensities of gehlenite in the product at Li/(Li + Na) ratios = 0 and 0.25 were almost constant, and then gradually decreased to zero above Li/(Li + Na) = 0.5. The intensities of hydroxysodalite in the products at Li/(Li + Na) ratios = 0 was higher than that at other ratios and decreases, while those of katoite in the products at Li/(Li + Na) ratios = 0 was lower than at other ratios and increases, with increasing Li/(Li + Na) ratios of the mixed solution. It is considered that dissolution of gehlenite causes the formation of calcium hydrate minerals.
The amount of Na+ released from the product was almost constant at 100 cmol/kg at Li/ (Li + Na) = 0 and gradually decreased to zero above Li/(Li + Na), which correlated with intensity of hydroxysodalite in the product due to the cation exchange property of hydroxysodalite zeolite. The amount of Ca2+ released gradually decreased to zero with increasing Li/(Li +Na) ratio in the mixed solution, which correlated with the intensity of katoite due to the fixation of calcium in the structure of calcium hydrate product. It is noted that the amount of Li+ released from the product lineally
increased with increasing Li content in the solution, which means that Li+ is not fixed in the structure of the product to remove easily.
Fig. 3. Intensities of hydroxysodalite, katoite and gehlenite in the product and amounts of Na+,
Ca2+and Li+ released from the product.
The reaction process was monitored by measuring the concentrations of Al and Si in the solutions and analyzing the properties of the solid product for Na+ and Ca2+ release during each 24 h experiment (Fig. 4). Although Ca is also a major elemental constituent of the ash, its concentration in solution is not a reliable indicator of the bulk system chemistry, because Ca is incorporated into insoluble solid phases in alkaline solutions [6]. The concentration of Al in solution always exceeded that of Si, even though the Si concentration exceeded that of Al in the starting ash. In the case of Li/(Na + Li) = 0 and 0.25 (Fig. 4 (a), (b)), the concentrations of Si and Al increased initially after introduction of PSA, then both Si and Al rapidly decrease after 1 - 2 h, and the Al content gradually increase and the Si content continued to rapidly decrease to approximately 10 mM after 2 h. The changes shown in Fig. 4 (b) is bigger than those shown in Fig. 4 (a). The Na+ release from the solid product increased and became almost constant after 4 h to 100 cmol/g and 65 cmol/g in Fig. 4 (a) and Fig. 4 (b), respectively, meaning that Al increase in the solution was caused by hydroxysodalite crystallization and Li addition did not promote zeolite synthesis. On the other hand, the Ca2+ release increased rapidly in the initial stage and gradually increased after 4 h to 300 cmol/kg and 350 cmol/kg in Fig. 4 (a) and (b), respectively, meaning that amorphous calcium aluminium silicate hydrate gel (CASH gel) with releasable Ca was formed in these solutions. In the case of Li/(Na + Li) ratios higher than 0.5 (Fig. 4 (c)-(e)), the concentrations of Si and Al initially increased after introduction of PSA and the Si and Al concentrations decreased. The rates of decrease for Si and Al were faster with increasing Li content in the solution and the amounts of Si and Al dissolved decreased. The amount of Na+ released from the solid product decreased because of the decrease in hydroxysodalite zeolite crystal content. The
0 100 200 300 400 0 200 400 600 800 0 0.25 0.5 0.75 1
Hydroxysodalite Katoite Gehlenite
Na+ Ca2+ Li+ R el ea sed a m o u n ts o f N a + , C a 2 + , a n d L i + [ cm o l/ k g ]
Ratio of Li+/total cation
Ca2+ release rapidly increased to 300 cmol/kg and then decreased, which was correlated with the Al content in the solution. The rate of decrease in Ca2+ release was also higher with increasing Li content in the solution. It is considered that with increasing the Li/(Na + Li) ratio of the solution dissolution of gehlenite is promoted to supply lager Ca and Al to the solution and the formation of calcium silicate hydrate minerals is promoted.
Fig. 4. Concentrations of Si and Al in the solution during the reaction, and the Na+ and Ca2+ release properties of the solid product after synthesis in each mixed solution; (a) 4 M NaOH, (b) 3 M NaOH + 1 M LiOH, (c) 2 M NaOH + 2 M LiOH, (d) 1 M NaOH + 3 M LiOH, and (e) 4 M LiOH.
Summary. Hydroxysodalite was synthesized from PSA in NaOH-LiOH solutions at 90 °C.
Hydroxysodalite, katoite, hydrocalumite and portlandite were synthesized in the products. Hydroxysodalite was synthesized at Li/(Li + Na) ratios lower than 0.25, while calcium hydrate minerals, such as katoite, hydrocalumite and portlandite, were synthesized using other ratios. Increasing the Li content gradually decreased Na+ and Ca2+ release from the product, increasing Li+
release from the product. The concentrations of Si and Al in the solution observed during the reaction explain the synthesis of products. The release of Na+ from the product depends on formation of hydroxysodalite zeolite crystals, while Ca2+ release depends on formation of CASH gel and calcium minerals, such as katoite, hydrocalumite and portlandite.
References
[1] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, 1978.
[2] M. Singh, M. Garg, Cementitious binder from fly ash and other industrial wastes, Cem. Concr. Res., Vol. 29, 1999, 309-314. DOI: 10.2138/am.2007.2251
[4] T. Wajima, H. Ishimoto, K. Kuzawa, K. Ito, O. Tamada, M.E. Gunter, J.F. Rakovan, Material conversion from paper-sludge ash in NaOH, KOH, and LiOH solutions, Am. Mineral., Vol. 92, 2007, 1105-1111. DOI: 10.2138/am.2007.2251
[5] T. Wajima, Kinetics of the removal of hydrogen chloride gas using hydroxysodalite at high temperatures, Int. J. Chem. Eng. Appl., Vol. 7, 2016, 235-238. DOI: 10.18178/ijcea.2016.7.4.580 [6] J. Kragten, Atlas of Metal-ligand Equilibria in Aqueous Solution, Ellis Horwood Limited, 1978.
