Synthesis of kalsilite from microcline powder by an alkali-hydrothermal process

International Journal of Minerals, Metallurgy and MaterialsVolume 21, Number 8, August 2014, Page 826DOI: 10.1007/s12613-01 4-0977-7Synthesis of kalsilite from microcline powder by an alkali-hydrothermalprocessShuang qing Sul?2, Hong-wen Ma", Jing Yang", Pan Zhang2), and Zheng Luo2)1) National Laboratory of Mineral Materials, China University of Geosciences, Beiing 10083, China2) Blue Sky Technology Corporation, Beijing 100083, China(Received: 20 December 2013; revised: 26 February 2014; accepted: 27 February 2014)Abstract: The properties of aluminosilicate kalsilite have atracted the interest of researchers in chemical synthesis, ceramic industry, biofu-els, etc. In this study, kalilite was hydrothermally synthesized from microcline powder in a KOH solution. The microcline powder, rich inpotassium, aluminum, and silicon, was collected from Mountain Changling in Northwestem China. The effects of temperature, time, andKOH concentration on the decomposition of microcline were investigated. The kalsilite and intermediate products were characterized bymeans of wet chemistry analysis, X-ray Diffraction (XRD), infrared spectrormetry (R), 29Si magic angle spinning nuclear magnetic reso-nance (2"si MAS NMR), 27Al MAS NMR, and scanning elctron microscope (SEM). With increasing temperature, the microcline powdertransforms into a metastable KAISiO4 polymorph before transforming further into pure kalsilite. A mixture of both kalsilite and metastableKAISiO4 polymorph is obtained when the hydrothermal reaction is carried out within 2 h; but after 2 h, kalsilite is the predominant product.The concentration of KOH, which needs to be larger than 4.3 M, is an important parameter influencing the synthesis of kalsilie.Keywords: kalsilte; microcline; potassium hydroxide; hydrothermal synthesisminosilicate kalsilite is finding potential utilization in sev-1. Introductioneral applications.The main techniques for synthesizing kalsilite includeKalsilite (a polymorphic form of KAISiO4) is a feld-cation exchange from nepheline [7], solid-state sintering re-spathoid that exists mainly in K-ich silica undersaturatedaction from zeolite, kaolinite, or other silicate compoundsvolcanic rocks, and is usually accompanied by olivine, meli-[8- 10], sol-gel methods using ethyl silicate or a SiO2 sol aslite, clinopyroxene, phlogopite, nepheline, and leucite [1].a Si source and aluminum salt as an Al source [11-12], andKalsilite is macroporous, contains strongly basic potassiumhydrothermal methods from muscovite or kaolinite [13- 15].active sites, and is weakly soluble in methanol and oils. It isHowever, cation exchange and solid state reactions generateused as a catalyst additive in ammonia synthesis and hydro-mixtures of kalsilite polymorphs, while kalsilite synthesizedgen production from steam reforming and as a heterogene-by sol- gel methods often has poor crystal structures withous catalyst for transesterification of soybean oil withigher costs. Among these methods, hydrothermal synthesismethanol to biodiesel [2]. Kalsilite, with its high thermalis economical and produces the pure material with a fineexpansion coefficient, can also be prepared as a thermal ce-particle size at low temperature. However, when muscoviteramic for bonding to metals [3]. Nanokalsilite shows excel-is used as the starting materials, the hydrothermal methodlent and highly improved activity toward oxidation of car-needs a high pressure (100 MPa), a relatively high tempera-bon in diesel soot [4]. Kalsilite is also a precursor of leucite,ture (approximately 6009C), and a long reaction time (15 d)an important component of porcelain-fused-to-metal and[13]. Recently, Becerro e1 al. [14-15] showed that the kal-ceramic restoration systems [5]. Recently, kalsilite has beensilite can be synthesized at 300°C in 12 h when kaolinite re-shown to be a source for potassium sulfate [6]. Hence, alu.places muscovite as the raw material. The reaction time isCorresponding author: Hong-wen Ma E-mail: mahw @ cugb.edu.cn◎University of Science and Technology Bejjing and Springer- Verlag Berlin Heidelberg 2014中国煤化工包SpringerMHCNM HGS.Q. Su et al, Synthesis of kalsilite from microcline powder by an alkali-hydrothermal process827still relatively long for a large-scale industrial production.at 40 kV and 10 mA with nickel filtered Cu Ka radiationMicrocline is a K rich alkali feldspar and the principalcovering 20 between 10° and 65° in 0.02° steps and a scan-mineral phase of insoluble potash ore [16]. In a previousning speed of 8%/min. Fourier transform infrared spectros-paper, we showed that kalsilite was successfully synthesizedcopy (FTIR) of samples in KBr pellet were recorded with ahydrothermally from a mixture of microcline powder andNICOLET 750 spectrometer. 29si magic angle spinning nu-KOH solution [17]. In the present study, we examine the in-clear magnetic resonance (MAS NMR) spectroscopy meas-fluence of temperature, time, and alkali liquor concentrationurements were carried out in a Bruker Avance II 400 Mon the synthesis of kalsilite from microcline.spectrometer with a pulse length of 4.0 us, recycle delaytime of 3 s, and resonance frequency of 79.3 MHz. The2. Experimentalchemical shift was reported in 10 from tetramethylsilane.27Al MAS NMR measurements were measured at 104.0The microcline powder used in this work was preparedMHz with a pulse length of 3.0 μus and a reeycle delay timefrom a syenite, which was collected from Mountain Chan-of 1 s, using an aqueous solution of Al(NO3)3 (1.0 M) as thegling, Luonan County of Shaanxi Province, China. Its main reference solution. The morphology of each sample waschemical composition was as follows: SiO2 65.00wt%,observed by means of scanning electron microscopy (SEM,Al2O3 17.26wt%, Fe2O3 1.20wt%, MgO 0.84wt%, CaOLEO-1450, JSM-6301F).0.61wt%, Na2O 1.09wt%, and K2O 13.92wt%. The mainphase of syenite was microcline with unit cell parameters, a3. Results and discussion= 0.85665 nm, b= 1.29734 nm, c =0.71985 nm, a= 89°19',β= 115*53', and y = 90*48' (Fig. 1). Analytical grade KOHThe reaction temperature, KOH concentration, andused to prepare alkali liquor was supplied by Sinopharm leaching time are the major factors that influence the syn-Chemical Reagent Beijing Co., Ltd.thesis of kalsilite from microcline. Kalsilite is synthesizedunder various conditions to determine the optimum condi-tions. The influence of these parameters on the synthesis is30000息discussed below.3.1. Reaction temperature多2000The XRD patterns of products obtained after hydrother-mal treatment of microcline powder in 8.5 M KOH solution三10000at various temperatures are shown in Fig. 2. A sample of theproduct synthesized at 180°C presents only the diffractionpeaks of microcline, indicating that the crystal structure of1060microcline is not destroyed at 180°C within 3 h. However,201(9)after heating at 200°C, two peaks at 20 = 28.0° and 28.8, inaddition to the diffraction peaks of microcline, are observed.Fig. 1. XRD pattern of the microcline powder sample.The two peaks belong to the metastable orthorhombic KAI-Synthesis of kalsilite: the microcline ore was milled toiO4 polymorph (space group Ccmm, PDF 31-0965). Thesuch a grain size that >95% particles passed through apotassium feldspar diffaction peaks gradually become200-mesh sieve. A slurry of the milled microcline powderweaker with each subsequent increase in temperature, and(150 g) in KOH solution (540 mL) was transferred into adisappear completely when the synthesis is carried out atnickel hydrothermal stiring reactor (200 r/min) and heated240°C, suggesting that the lttice structure of microcline isat 180- -280°C for 0.5- -3.0 h. The pressurein the reactor wasully broken. All other peaks in the XRD spectrum of theapproximately equal to that of the vapor pressure of water atdecomposition product obtained after heating microcline forthe corresponding reaction temperature. After cooling to3 h at 240°C, except one, can be ascribed to the metastableroom temperature, the product was filtered and washed re-orthorhombic KAISiO4 polymorph, designated KAS-1. Thepeatedly using distilled water and dried at 105°C in air fordistinct peak at 20 = 22.2° is attributed to kalsilite (space8h.group P63, PDF 11-0579). The diffraction peak of metasta-The X-ray diffraction (XRD) patterns of samples wereble KAISiO4 \中国煤化Inperature, andcollected using a D/Max 2500 X -ray diffractometer operatedwhen the re:MHCNM HG”C, metastable .828Int. J. Miner. Metall. Mater., Vol. 21, No. 8, Aug. 2014KAISiO4 transforms completely to kalsilite, designatedlar (Table 1) and compare well with the theoretical compo-KAS-2. The unit cell parameters of kalsilite obtained atsition of KAISiO4 Chemical analysis of KAS-2 gives280°C from microcline are a = 0.51703 nm and c = 0.87162he chemical formula of pure kalsilite crystal asnm (normalized using CELL SR soffware [18]), which areKosiNao.cCosMg.zA.3.o39Si 003O04.slightly higher than those recorded by Becerro and Man-Table 1. Chemical composition of the metastable KAISiO4tovani [14] (a = 0.51662 nm and c = 0.87097 nm) in theirpolymorph (KAS-1) and kalsilite (KAS-2)wt%synthesis of kalsilite from kaolinite.Sample SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2OKK= KAISiO(11-0579)1KAS-1 37.80 29.60 1.88 0.40.84 0.38 27.54KK_。KKK280CKAS-2 37.89 29.91 1.97 0.68 1 23 0. 332: 7 56The SEM images of KAS-1 and KAS-2 are shown in Fig.3. KAS-1 is in the form of agglomerates, consisting of fineS = KAISiO, (31-0965)240°Cirregular particles, with the particle diameter of approxi-mately 300- 400 nm. Meanwhile, KAS-2 forms hexagonal220°Cplatelets with clear edges, and the length of each hexagonal2009Cedge is approximately 1 um. When compared to the struc-ture of metastable KAISiO4 polymorph, SEM results sug-F= KAISi,O,180gest that kalsilite possesses a more regular crystal morphol-2(507ogy and a larger particle size.201(0)To facilitate the interpretation of the XRD results, theFig. 2. XRD patterns of hydrothermal reaction products pre-products KAS-1 and KAS-2 were also analyzed by FTIRpared by heating microcline power in KOH solution (8.5 M)(Fig.4). Both KAS-1 and KAS-2 exhibited absorptionfor 3 h at different temperatures. F = microcline, S = metasta-bands at approximately 987 cm，689 cm，and 460 cm- ,ble KAISiO4 polymorph, and K = Kalsilite.which were assigned to asymmetric stretching vibration ofThe chemical composition of KAS-1 and KAS-2 is simi-the Si(AI)- O framework, symmetric stretching vibration of(a)()1 umFig. 3. SEM micrographs of KAS-1 (a) and KAS-2 (b). .100(a(b。60 t689.0|屋4020987.14000 3500 3000 2500 2000 1500 1000 5004000 3500 3000 2500 2000 1500 1000 50Wave number / cmr中国煤化工Fig. 4. FTIR spectra of KAS-1 (a) and KAS-YHCNMH Gs.Q. Su et al, Synthesis of kalsilite from microcline powder by an alkali-hydrothermal process829the Si(Al)- 0 framework, and bending vibration of thephous materials shows a main peak at 59.4 x 10° and aSi(A1)- 0 framework, respectively [19]. It is worth notingpronounced shoulder at 66.3 X 10, the result of Al in thethat KAS-1 displays an additional absorption band at 610.6unused amorphous materials. However, the resonance signalcmi . A similar observation has been made by Becerro et al.in the spectrum of KAS-1 showed a higher full-width at half[15], who obtained metastable ABW-type KAISiO4 duringmaximum (FWHM) value as compared with that of KAS-2,hydrothermal treatment of kaolinite. Compared to those inindicating the higher level of heterogeneity around Al inKAS-1, the bands in the FTIR spectrum of KAS-2 arKAS-1.sharper and the spectrum splitting is more distinct, indicat-Fig. 6. shows the solid 2Si MAS NMR spectra of KAS-1ing that Al is more ordered in the tetrahedron in KAS-2 thanand KAS-2. KAS-1 exhibits a resonance signal at- -85.3 xin KAS-1.10-6 (Fig.6(a)), which is in the range typical for the Q*(4AI)The solid 27Al MAS NMR spectra of KAS-1 and KAS-2environment; the high FWHM value of this signal suggestsare shown in Fig. 5. The spectrum of KAS-1 shows a uniquenon-equivalent Si sites in the asymmetric unit. KAS-2resonance centered at 61.0 X 10", which is typical of a tet-shows a unique and narrow resonance at- -88.3 X 10- ,rahedral Al. Meanwhile, the single, symmetric signal atwhich corresponds to the unique Si type of environment in59.6x 106 in the spectrum of KAS-2 is assigned to a tetra-the kalsilite structure, Q*(4AI). The presence of a single andhedrally coordinated Al, consistent with the Q*(4Si) ensharp 29Si MAS NMR resonance of KAS-2 is consistentvironment in a fully ordered kalsilite structure. Mozgawa etwith the ideal 1:1 Si/Al molar ratio and Al avoidance princi-al. [20] have shown that the 27Al MAS NMR spectrumple, indicating order in the arrangement of both Si and Alof kalsilite phase obtained on devitrification of the amor-atoms in pure kalsilite.(a61.0x 10-(b)59.6x 101401201008060402040120100806040200Chemical shift/ 10-*Chemical shift/ 10-6Fig. 5.27AI MAS NMR spectra of KAS-1 (a) and KAS-2 (b).a)-85.3x 10<|(b-88.3.x 10-0 -20-40-60-80-100-120-1400 -20 -40 -60 -80 -100 -120 -140Chemical shift/ 10*Fig.6. 29Si MAS NMR spectra of KAS-1 (a) and KAS-2 (b). .Although the signals assigned to tetrahedrally coordi-KAISiO4 polymorph and kalsilite phase, the FWHM valuesnated Al and Si are in the range expected for the Q(4Si) andof the signals in中国煤化Iectra are lowerQ*(4AI) environments, respectively, in both metastablefor KAS-2 (TabYHCN M H al order around830Int. J. Miner. Metall. Mater., Vol. 21, No. 8, Aug. 2014the Si and Al nuclei in the kalsilite phase is high, which wasfurther reduction in the concentration of KOH to 4.0 Min good agreement with the FTIR results. Both the 2'Al andcontains several diffraction peaks corresponding to micro-29Si resonances observed in this study are much broader thancline, indicating that the potassium feldspar is not com-those observed by Becerro et al [151., which are 240 Hz forpletely decomposed. This result implies that kalsilite can be27AI signal and 122 Hz for 29Si signal. The broader reso-synthesized from microcline powder with stoichiometricnances are very likely due to the high incorporation of Fe inamounts of KOH whereas, as reported previously, its synthe-the synthesized kalsilite from the starting microcline.sis from kaolinite requires an amount of KOH that is in ex-Table 2. FWHM values of signals in 29si and 27AI MAS NMRcess of that described by the stoichiometry of the reaction [15].spectra of KAS-1 and KAS-23.3. Leaching timeSample27A1 MAS NMR29si MAS NMRFig. 8 shows the XRD patterns of products obtained afterKAS-1801.6493.8hydrothermal treatment of microcline powder in 4.3 MKAS-2410.4182.4KOH after leaching for different periods of time at 280°C.3.2. Concentration of KOHThe XRD pattern of the product obtained after 0.5 h ofleaching exhibits microcline, metastable KAISiO4 poly-The decomposition reaction of microcline in KOH solu-morph, and kalsilite reflections, indicating that the shorttion can be represented by the reaction: .leaching time does not break down the crystal lattice ofKAISi;Og + 4K0H→KAISiO4 + 2K2SiO3 + 2H2O.microcline. After 1.0 h of treatment, the reflections ofThe reaction indicates that the decomposition of 1 molmicrocline disappear and the crystallized product is mainlymicrocline requires 4 mol KOH to obtain 1 mol kalsilite andkalsilite; only low-intensity reflections of metastable2 mol potassium silicate. Initially, kalsilite in this study isKAISiO4 polymorph are observed in the XRD pattern.synthesized using excess of KOH, i.e., a two-fold higherWhen the hydrothermal reaction is allowed to continue foramount of KOH than the required (150 g of microcline in2.0 h, metastable KAISiO4 polymorph transforms com-540 mL of 8.5 M KOH solution). Results for the synthesispletely into kalsilite. Extending the reaction time furtherof kalsilite using other concentrations of KOH are shown indoes not change diffraction peaks observed in the product,Fig. 7.demonstrating that the hydrothermal transformation ofmicrocline to kalsilite requires just 2.0 h, a time that is muchshorter than that required for the synthesis of kalsilite fromkaolinite (12 h at 300°C) [15].K炎KkK KK.02(30406070201(%)Fig. 7. XRD patterns of hydrothermal reaction products ob-- _0.5htained on heating microcline in various concentrations of KOH205Csolutions at 280°C for 3 h. F = microcline and K = kalsilite.20/()Fig. 8. XRD patterns of hydrothermal reaction products ob-As seen from Fig. 7, for all concentrations of KOH≥4.3tained on heating microcline in KOH solution (4.3 M) at 280°CM, there is no marked difference in the XRD patterns of thefor 0.5, 1.0, and 2.0 h. F = microcline, S = metastable KAISiO4products. All of the peaks in these spectra can be attributedpolymorph, and K = Kalsilite.to kalsilite, indicating that the microcline powder is com-pletely decomposed. Moreover, at 4.3 M, the KOH solution4. Conclusionsprovides the theoretical amount required by the stoichiome-中国煤化工try of the reaction. Hydrothermal product obtained afterPure kalsilit.MYHCN M H G hyrohemnalS.Q. Su et al, Synthesis of kalsilite from microcline powder by an alkali-hydrothermal process3Itreatment of microcline powder in KOH solution at 280°C.A metastable KAISiO4 polymorph is obtained when the re-[8] R. Dimtrjevic and V. Dondur, Synthesis and characteriza-tion of KAISiO4 polymorphs on the SiO2 KAIO2 join, J. 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