Advanced Powder Technology 27 (2016) 2121–2127

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Advanced Powder Technology

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Original Research Paper

Microwave-assisted and conventional hydrothermal synthesis ofpotassium merlinoite from K-feldspar

http://dx.doi.org/10.1016/j.apt.2016.07.0250921-8831/� 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

⇑ Corresponding author at: Beijing Key Laboratory of Materials Utilization ofNonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,China University of Geosciences, Beijing 100083, China.

E-mail address: mahw@cugb.edu.cn (H. Ma).

Changjiang Liu a,b, Jing Yang a,b, Hongwen Ma a,b,⇑, Pan Zhang a,c

aBeijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, China University of Geosciences, Beijing100083, Chinab School of Materials Science and Technology, China University of Geosciences, Beijing 100083, ChinacBlue Sky Technology Corporation, Beijing 100083, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 March 2016Received in revised form 4 July 2016Accepted 30 July 2016Available online 10 August 2016

Keywords:Potassium merlinoiteK-feldsparEco-fertilizerMicrowave-assisted synthesis

Potassium merlinoite was successfully synthesized from K-feldspar powder by microwave-assisted aswell as conventional hydrothermal methods. Effects of aging, H2O/Al2O3 molar ratio, crystallization tem-perature, crystallization time, and mixed heating strategy on potassium merlinoite formation were sys-temically studied. It was observed that aging was not a necessary factor to the formation of merlinoite,and merlinoite was the unique crystalline product at a wide range of H2O/Al2O3 molar ratio, crystalliza-tion temperature and time. The reaction rate of microwave-assisted hydrothermal synthesis was 2–3times as much as conventional method. The acceleration effect of microwave was proved to work onlyat the initial period of irradiation, and a mixed heating method was considered as an appropriate strategyto prepare merlinoite. A simple and economical chemical process has been proposed to convert the nat-ural K-feldspar powder into merlinoite-type eco-fertilizer in this paper.� 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

Zeolite materials, with specific adsorption and ion exchangeproperties, are able to maintain water in soil, improve the avail-ability of nutrient elements, and remediate the heavy metal-polluted farmland if applied in agriculture [1,2]. The agriculturaluse of natural Na-zeolites (such as clinoptilolite and stilbite) hasbeen reported in literatures [3,4]. Some nutrient-loaded (N- andK-) zeolites have also been used as slow-release fertilizers toimprove the low efficiency situation of traditional chemical solublefertilizers [5]. Nutrients-loaded zeolites, with both soil remediationand fertilizer effects, are excellent eco-fertilizers in modernagriculture.

Potassiummerlinoite, which is also called zeolite W in some lit-eratures [6], together with other two types of K-zeolites, zeolite Land zeolite T, is a common product in K2O-Al2O3-SiO2-H2Ohydrothermal system [7]. Merlinoite synthesized from coal flyash has been proved to be an efficient slow-release K-fertilizer

for plant growth [8]. It has also been studied as a molecular sievefor the extraction of potassium from seawater [9,10].

Conventional zeolite synthesis method requires aluminosilicategels prepared from synthetic chemical agents. Besides, the utiliza-tion of natural rocks and minerals, industrial and agriculturalwastes, including kaolin [11], coal fly ash [12], and rice husk ash[13,14] etc., as starting materials for zeolites has attracted muchof attention. However, those raw materials only provide the alu-minum and/or silicon sources for zeolites. Expensive alkalinechemical agents, NaOH and KOH, are still required. Finding alterna-tive cheap raw materials with all the necessary components forzeolite is of great significance for zeolite preparation.

As a widely abundant rock-forming mineral, K-feldspar (KAlSi3-O8) is an important insoluble potassium source [15]. Recently,extraction of potassium from K-feldspar to meet the agriculturalpotash needs has become an international topic [16,17]. Conver-sion of K-feldspar into slow-release K-fertilizer (merlinoite), cannot only supply potassium element for plants, but also take fulladvantage of the aluminum and silicon components in the insol-uble mineral, which satisfies the principle of green chemistry [18].

K-feldspar has a stable tectosilicate framework. Directhydrothermal treatment on K-feldspar with KOH solution resultedin the formation of kalsilite (KAlSiO4) phase, instead of K-zeolites[19]. In order to obtain high reactive amorphous intermediates,

2122 C. Liu et al. / Advanced Powder Technology 27 (2016) 2121–2127

pre-activation on rawmaterials is a necessary step, such as calcina-tion of kaolinite before zeolitization, which requires a thermal acti-vation temperature of 500–900 �C [20]. Besides, alkaline fusion isanother activation method, needing a relative low temperature of400–600 �C [21].

In addition to conventional hydrothermal (CH) synthesis,microwave-assisted hydrothermal (MH) heating can alwaysincrease reaction kinetics by several orders of magnitudes, whichhas been considered as an effective method for the preparationof zeolites. Though lots of papers reported the microwave-assisted hydrothermal synthesis of zeolite [22], the effects ofmicrowave working strategies on zeolite formation are seldomreported. The microwave-assisted hydrothermal synthesis ofpotassium merlinoite from K-feldspar has not been reported, aswell.

The aim of present work is to systematically study the forma-tion of potassium merlinoite from K-feldspar by microwave heat-ing. As a comparison, conventional hydrothermal synthesis undersame experimental conditions are investigated. To explore theeffects of heating strategies on merlinoite formation, the mixedmethods of CH and MH heating are studied.

2. Experimental

2.1. Materials

The raw K-feldspar powder (particle size <74 lm, Sample ID:SK15-2) was obtained from crushed and grinded syenite, a kindof potassic rock collected from Shangdu county of Inner Mongoliaautonomous region, China. Its chemical composition determinedby wet chemical analysis is listed in Table 1. Based on the principleof material balance [23], considering the three feldspar end-members (KAlSi3O8, NaAlSi3O8, CaAl2Si2O8) and other commonminerals in igneous rocks (micas, apatite, magnetite, etc.), mineralcomposition of the powder is (wt%): K-feldspar 91.35, quartz 6.91,and minor other minerals (mica, apatite, magnetite, etc.) 1.74.Potassium hydroxide (G.R. 85% KOH, Beijing Chemical Works)and deionized water were used in the following experiments.

2.2. Methods

5 g K-feldspar powder was firstly mixed with potassiumhydroxide at mass ratio of 1:1.2 in a nickel crucible, and then fusedat 400 �C for 90 min. The mixtures after fusion were dissolved intodifferent volume of deionized water, 30, 40, 50, and 60 mL, toobtain aluminosilicate gels. The gels were heated immediately, orafter 15 h of aging at room temperature.

MH synthesis was conducted in 100 mL Teflon-reactors placedin a microwave digestion oven (HG08Z-4, HuaGangTong Technol-ogy Co., Ltd). A microwave generator (Panasonic 2M261-M32)was equipped on the oven to generate microwave irradiation witha working frequency of 2.45 GHz. Temperature-rising period wasset for 4 min, with 0–100% of 1000W output power, and thetemperature-holding period was operated at 0–100% of 600 W out-put power. CH synthesis under the same conditions was conductedin a Teflon-lined stainless steel autoclave. To examine the effects ofheating strategy on potassium merlinoite synthesis, mixed heatingmethods between microwave field and conventional heating ovenat different heating period were carried out (see part 3.4, Fig. 5).

Table 1Chemical composition of the K-feldspar powder (SK15-2) (wB%).

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO

66.27 0.03 17.50 0.04 0.24 0.01 0.23

After experiments, the products were filtered, washed repeatedly,and dried at 105 �C for 12 h.

2.3. Characterization

X-ray diffraction (XRD) analysis was done by a X-ray diffrac-tometer (SmartLab, Rigaku) using Cu Ka radiation in steps of0.02� over 2h range of 3–50� (scanning speed: 0.10 s/step). Relativecrystallinity (RC) was calculated by comparing of the diffractionintensities of three strong peaks (I(111), I(114), I(311)) with themaximum-intensity-sample among all the synthesized products.Morphology was observed by a scanning electron microscopy(JSM-IT300, JEOL). CECs of the synthesized potassium merlinoitesamples were determined by measuring the exchanged NH4

+ in2 M KCl solution from NH4

+-saturated samples, prepared by equili-brating with 2 M ammonium chloride solution. NH4

+ concentrationwas determined using colorimetric method.

3. Results and discussion

3.1. Effects of aging

All the diffraction peaks in Fig. 1 confirm the crystallization ofpotassium merlinoite phase (PDF card No. 30-0902). The RC ofdirect-crystallized samples (Fig. 1b, 30.32% for 2 h, 100% for 4 h)show much higher than the aged samples (Fig. 1a, 15.51% for 2 h,64.28% for 4 h), even when increase crystallization time from 2 hto 4 h. For MH method, the difference becomes more obvious.Poorly crystalline products were obtained after aging (Fig. 1c),whereas, high X-ray intensity merlinoite pattern was identifiedafter 45 min of microwave heating in the direct-crystallized sam-ple (Fig. 1d).

It is well known that aging before zeolite crystallization is anessential step for the nucleation of zeolite from the starting gels.For example, the synthesis of zeolite A can be finished in 1 minafter prolonged aging using MH heating [24]. A ten-hour aging per-iod was suitable for merlinoite formation in other report [9]. How-ever, what was found in this work illustrates that aging has aretardation effect on potassium merlinoite zeolitization. Directlyhydrothermal synthesis is superior to long time aging in this study.This phenomenon may be caused by the rawmaterial that is differ-ent from traditional aluminosilicate gels. The K-feldspar afterfusion may provide some sub-units that were beneficial to directlyzeolitization, but easy to be destroyed by prolonged aging time.

As a consequence, we conclude that the synthesis process ofpotassium merlinoite from potassium aluminosilicate proceduresusing K-feldspar as starting material can be simplified by neglect-ing the unnecessary aging period.

3.2. Effects of H2O/Al2O3 molar ratio

According to the report of Ref. [6], potassiummerlinoite has a K:Al:Si ratio of 1:1:(1.78–2.37). Theoretically, K-feldspar (K:Al:Si = 1:1:3) can provide sufficient and necessary potassium, alu-minum, and silicon sources for merlinoite. The raw K-feldsparpowder has a M2O/Al2O3 ratio of 1.05, where M2O belongs to thealkali metal oxides (K2O with minor Na2O) and trace amount ofdivalent metal oxides (CaO, MgO). Therefore, no extra potassium,aluminum, and silicon sources are needed in the synthetic process.

CaO Na2O K2O P2O5 LOI SUM

0.70 0.89 13.83 0.04 0.44 100.23

Fig. 1. XRD patterns of potassium merlinoite synthesized by CH for 2 h and 4 h after (a) 15 h, and (b) 0 h of aging, and by MH for 30 min and 45 min after (c) 15 h, and (d) 0 hof aging. M-Merlinoite.

C. Liu et al. / Advanced Powder Technology 27 (2016) 2121–2127 2123

The excess potassium hydroxide in pre-activation stage can bereused by further causticization. The only varied factor that mightaffect merlinoite crystallization was H2O/Al2O3 ratio. The gel com-position was 6.4SiO2:1.0Al2O3:6.3M2O:xH2O in this study, where xstands for H2O/Al2O3 molar ratio values, varying as 197.3, 261.2,325.2, 389.2, depending on the amount of water introduced intothe system after fusion.

Fig. 2 shows the XRD patterns and RC of potassium merlinoitesynthesized by CH and MC methods from different H2O/Al2O3

molar ratios. It was found that potassium merlinoite was the onlycrystalline phase with different RC in various H2O/Al2O3 molarratios by both CH and MH methods. The optimal gel compositionof (5–10)SiO2:Al2O3:(0.05–0.31)Na2O:(0.7–7.7)K2O:(27–720)H2Ofor merlinoite (zeolite W) crystallization at temperature range of100–150 �C for 24–240 h was summarized in reference literature[7]. The synthesis conditions in this study are in the range of abovecomposition, but beyond the scope of the given time and temper-

Fig. 2. XRD patterns of the synthesized merlinoite samples obtained from different H2OMerlinoite.

atures limits, indicating there is a more broad crystallization zoneof potassium merlinoite in the K2O-Al2O3-SiO2-H2O hydrothermalsystem.

It can be observed that H2O/Al2O3 molar ratio of 325.2 wasmore appropriate for the synthesis of highly crystalline potassiummerlinoite by both CH and MH heating methods.

3.3. Effects of synthesis temperature and time

Zeolites are thermodynamically metastable materials, and theirformation are generally controlled by kinetics factors such as syn-thesis temperature and time. In Na2O-Al2O3-SiO2-H2O system,three different zeolites, Y-type, P-type and analcime can beacquired from the same gel composition by only changing the tem-perature and time during synthesis processes [25]. Therefore, tem-perature and time are important factors that may affect theformation of potassium merlinoite. In this work, crystallization

/SiO2 molar ratio by (a) CH for 4 h, and (b) MH for 1 h at temperature of 180 �C. M-

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from 140 to 180 �C, and heating time of 0–6 h for CH, 0–90 minfor MH, were investigated. Systematically experimental results

Fig. 3. Relations of relative crystallinity (RC) vs. crystallization time for CH and MHmethods under different temperatures.

Fig. 4. Morphology of the products after 6 h of CH heating, a

analyzed by XRD (patterns not shown) identify the unique crys-talline phase of merlinoite under above temperature and timeranges, which proves that merlinoite is thermodynamically stablewithin the above conditions. Therefore, kinetic study of merlinoiteformation under CH and MH methods is necessary.

Fig. 3 plots the RC values vs. crystallization time under differenttemperatures. Generally, the crystallization of zeolite from startinggels mainly includes three steps: an induction period, an acceler-ated crystal growth period, and deceleration of the growing pro-cess [26]. For MH method, at 180 �C, the induction period can beneglected, and accelerated crystal growth was lasted from 0 to45 min. As a comparison, it took approximate 1 h for inductionby CH heating at 180 �C, after which merlinoite phase began togenerate. The fast crystallization period was from 1 h to 4 h, andthe best crystalline merlinoite was obtained after 4 h by CH heat-ing. It was found that decreasing temperature to 160 �C and140 �C sharply reduced the RC of merlinoite products for both CHand MH methods. At 140 �C, merlinoite began to crystallize after30 min using MH synthesis, or 3 h using CH heating. Even pro-longed synthesis only resulted in the RC of 7.25% for CH and14.45% for MH methods. As a result, MH for 60–90 min at 180 �Cis suitable for merlinoite synthesis from the K-feldspar.

nd 90 min of MH heating under different temperatures.

Fig. 6. RC and CEC values of potassium merlinoite samples synthesized fromdifferent heating strategies.

C. Liu et al. / Advanced Powder Technology 27 (2016) 2121–2127 2125

It is observed obviously that merlinoite crystallization wasenhanced by MH method. The rapid crystallization of zeolites byMH method has been compared with CH in other reports. The suit-able time that is needed for zeolite synthesis depends largely onthe types of targeting zeolites. As a summary, the reaction rateincreased by 3–4 times for zeolite Y at 100 �C [27], about 4 timesfor zeolite T at 120 �C [28], and over 20 times for zeolite X at90 �C [29]. This work demonstrates that the formation rate of mer-linoite by MH method is 2–3 times faster than by CH method, sim-ilar to the formation of zeolite A synthesized from metakaolinite at70–80 �C [20].

SEM images of the products obtained by CH and MH methodsunder different temperatures is shown in Fig. 4. At 140 �C, irregularaggregates probably belonging to the X-ray amorphous productswere observed for both CH- and MH- samples. Merlinoite obtainedfrom CH heating exhibits as dumbbell-like pillars at length ofapproximate 10 lm. The shape of both ends is in the form of singlepillar similar to that of cauliflower. The MH-sample obtained at160 �C shows slender stick-like morphology, with length of about5 lm. When temperature increased to 180 �C, small clusters withcolumn-like morphology are observed. It is a common results thatzeolite morphology is affected by microwave irradiation [30,31].The merlinoite grains synthesized using MH method were relativesmall and narrow compared with those using CHmethod. This maybe attributed to the homogeneous heating by microwave irradia-tion inside the reaction mixture. An ‘‘effective temperature [32]”at the reaction interface was more promotive to the nucleation ofmerlinoite, and resulted in the small but uniformed-size crystalgrains. In CH method, as heat was from outside, the temperaturedistribution was heterogeneous in the mixture, by which thenucleation and crystal growth was affected. Consequently, grainmorphology became different.

3.4. Effects of heating strategy

The reaction mixtures were microwave-heated for an interval oftime, then transferred to a conventional heating oven; or firstlyconventional-heated for 45 min before MH heating (see Fig. 5).For all experiments total heating time was maintained for 90 min.

The CECs and RC of the synthesized potassium merlinoite sam-ples are shown in Fig. 6. CECs have a slight change within 54.84–57.23 mgNH4

+�g�1, along with a variation of RC at different heatingstrategies. This is probably caused by the special material. Somesub-units or structures may existed in the fused K-feldspar withsimilar ion-exchange properties to merlinoite. Thus, even the sam-ple with poor RC (68.18%) had a CEC close to the maximum value.

It was found that 4 min of MH heating during temperature-rising period followed by 90 min of CH heating for the next90 min caused the RC of 68.18% (CM-A). In contrast, RC of the sam-ple synthesized by CH method for 90 min is below 20% (Fig. 3,

Fig. 5. Graphical representation of CH

curve of CH 180 �C). This finding particularly indicates that evenshort-time microwave irradiation can greatly promote zeolite for-mation. However, the strategies of CM-B (MH for 15 min then CHfor 75 min, RC = 83.90%), and CM-C (MH for 30 min then CH for60 min, RC = 86.90%) illustrated in Fig. 5 had almost the same effectas direct MH heating for 90 min (89.73%), which means prelongedirradiation time after 15 min has unconspicuous effect on zeolitiza-tion. In the case of strategy CM-D, 45 min of MH heating followedby 45 min of CH heating even generated a poor RC of 68.56%,equaling to that of 45 min-MH product (RC = 68.61%), indicatingthat CH at the following 45 min had no effect on crystallization.

From above results, two aspects findings are summarized: (i)the reaction rate can be accelerated only at the initial period ofmicrowave heating, (ii) the effect of CH heating after microwaveirradiating for 45 min was insignificant. Similar consequence wasalso reported by Inada et al. in their synthesis of NaP zeolite fromcoal fly ash. Their results showed that the yield of NaP increased atearly stage, decreased at middle stage, and was not influenced bymicrowave heating at last stage [33]. However, in this work thebest crystalline merlinoite (RC = 99.25%) can be obtained by45 min irradiation after 45 min CH heating (CM-E). It turned outthat pre-heating by CH method followed by MH heating was moreappropriate for merlinoite formation. It is deduced that CH pre-heating stage in the induction period led to a supersaturation inthe gel. The followed MH heating could promote nucleation ofmerlinoite from the supersaturated gel. Thus, highly crystallinemerlinoite was synthesized via strategy CM-E. Additionally, boththe CH and MH time was shortened to 45min by using the mixed

and MH mixed heating methods.

Table 2Summary of raw sources, methods and conditions for the synthesis of potassium merlinoite (zeolite W).

Silicon, Aluminum, and Potassium sources Synthesis method Temperature/�C Time/h Reference

Boehmite, Silica solution, Potassium hydroxide CH 140 96 [34]Coal fly ash, Amorphous SiO2 or quartz, Potassium hydroxide CH 180–250 1–2 [35]Tetramethyl orthosilicate, Aluminum tri-sec-butoxide, Potassium hydroxide CH 165 48 [36]Ludox, Alumnium hydroxide, Potassium hydroxide/Potassium nitrate CH 150 240 [6]Silica acid, Aluminum hydroxide, Potassium hydroxide CH 165 72 [37]Water glass, Sodium aluminate, Potassium hydroxide CH 150 24 [10]

170 22 [9]Coal fly ash, Potassium hydroxide CH 150–200 8–24 [8]Diatomite, Aluminum hydroxide, Potassium hydroxide CH 150 20–120 [38]TEOS, Aluminate solution, Potassium hydroxide CH 170 48 [39]Silica sol, Sodium aluminate, Potassium hydroxide CH 165 72 [40]Ludox, Aluminum hydroxide, Potassium hydroxide CH 165 72 [41]

MH 4K-feldspar CH + MH 180 1.5 This work

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heating method, which also reduces energy cost. Despite thatmicrowave working mechanism is complex and need furtherinvestigation, its acceleration effect on the formation of merlinoitewas obvious.

Table 2 lists a series of raw sources, synthesis methods and con-ditions of potassiummerlinoite in other literatures. For most of theresearches, pure chemical agents were the major sources for mer-linoite. Though coal fly ash and diatomite were introduced in somereports, the additional potassic agent, KOH, was still required. It isunreasonable to convert the expensive potassic agents intoK-fertilizers! In contrary, all the necessary sources for merlinoitewere from the natural material of K-feldspar powder in this work,which could largely reduce the raw material costs. It is obviousthat CH heating was the dominated synthesis method, and littleresearch reported MH method for merlinoite. The mixed heatingstrategy by the combination of CH and MH methods has hithertonot been reported. This strategy shortened the synthesis timewithin 90 min, which is a considerable way to reduce the energycost for merlinoite synthesis.

In conclusion, this work has proposed a simple and economicalchemical process to convert the natural K-feldspar powder intomerlinoite-type eco-fertilizer, which shed new light on the com-prehensive utilization of potassium, aluminum, and siliconcomponents.

4. Conclusion

Potassium merlinoite was synthesized by both microwave-assisted (MH) and conventional hydrothermal (CH) methods fromraw K-feldspar after pre-activation by fusion with potassiumhydroxide as additive.

It was found that aging was not a necessary step, even had aretardation effect on merlinoite crystallization. Merlinoite wasthe unique thermodynamically favored product at temperaturerange of 140–180 �C, and H2O/Al2O3 molar ratio of 197.3–389.2,within 90 min for MH and 6 h for CH. The optimum conditionsfor the synthesis of high crystallinity potassium merlinoite isH2O/Al2O3 of 325.2, temperature of 180 �C, time of 60–90 min forMH, and 6 h for CH methods. MH method can enhance the reactionrate by 2–3 times of CH method. The slender and uniformed stick-like morphology is affected by microwave irradiation.

Microwave has been proved to promote zeolitization at the ini-tial period of irradiation. The mixed heating strategy with CH pre-heating followed by MH has been considered appropriate forpotassium merlinoite synthesis.

This work provide a new sight on the process of K-feldspar byconverting it into merlinoite-type eco-fertilizer, with the compre-hensive utilization of natural insoluble potassium sources.

Acknowledgements

This work was supported by the funds from FundamentalResearch Funds for the Central Universities (2652015371) andChina Geological Survey Project (12120113087700).

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