Hydrothermal synthesis of nano-kaolinite from K-feldspar

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Hydrothermal synthesis of nano-kaolinite from K-feldspar ⁎

Jiangyan Yuana,b, Jing Yanga, Hongwen Maa, , Shuangqing Suc, Qianqian Changa, ⁎ Sridhar Komarnenib, a Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, PR China b Department of Ecosystem Science and Management and Materials Research Institute, 204 Energy and the Environment Laboratory, The Pennsylvania State University, University Park, PA 16802, USA c Blue Sky Technology Corporation, Beijing, 100083, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Nano-kaolinite K-feldspar Hydrothermal synthesis Chemical weathering Kaolinite of paper-coating grade

Development of sustainable routes for the synthesis of kaolinite in nano-scale (nano-kaolinite) is very significant for producing high quality kaolinite of paper-coating grade in kaolin industry. Duplicating chemical weathering processes in nature, two routes were developed and compared for the synthesis of nano-kaolinite from K-feldspar. Kaolinite of uniform plate-like morphology with thickness of around 14 nm was obtained in this study. Both synthesis routes may lead to the comprehensive utilization of K-feldspar for the synthesis of pure kaolinite for not only high quality paper-coatings but also medical and other uses.

1. Introduction Materials circulation on the surface of the Earth largely depends on chemical weathering processes [27]. Natural kaolinite is the product of long-term chemical weathering of aluminosilicate rocks, the formation of which is closely correlated with the types of primary minerals, fluid concentration and washout time [7,12]. Initially, the synthesis of kaolinite was just aimed at studying the genesis and stability conditions of clay minerals owing to geological interest [15]. K-feldspar is one of the least reactive minerals and its conversion to kaolinite takes a long time by chemical weathering process [7]. The kaolinization reactions of feldspar play a significant role in deformation processes and this reaction needs the aid of acidic fluids. Understanding the weathering of minerals under natural environmental conditions can provide clues for the accelerated experimental synthesis of relevant mineral phases for industrial applications by a facile method. Excellent chemical and thermal stability of kaolinite [19] makes it widely useful in ceramic technology [1], advanced inorganic glass, cosmetics [32], coatings [24] and binder [3]. Kaolinite occurs widely throughout the world and it can be mined for commercial applications from more than 60 locations in many countries such as the United States, Britain, Brazil and China. It was reported that global kaolin reserves reach to the extent of 22.2 billion tons, but kaolin of high-quality or paper coating grade is in short supply [3]. China has abundant and widely distributed kaolin resources of different purities including

abundant ordinary kaolinitic soils of low quality with lot of impurities. But, more or less pure kaolin is distributed in Suzhou, Maoming, Longyan and Datong areas with better quality, the latter can be used to prepare catalysts for crude oil cracking, paper coatings, advanced ceramics and as raw materials for other applications [37]. However, kaolinite of high-quality for special applications of paper-coating grade, medicinal use etc. is extremely rare and has become an urgent necessity in kaolin industry. Therefore, how to produce cost-effective and highquality kaolinite has become an important issue. At the present time, several complex processing techniques, such as mechanical grinding, classification, stripping, intercalation and other treatments were used to achieve high quality kaolinite [4]. However, high energy consumption, low yield, high cost and minor impurities make the ultra-fine production of kaolinite impractical even for premium products. Chemical synthesis method may be the best route to obtain much purer kaolinite of nano-scale. Here, using the natural chemical weathering processes of feldspar to kaolinite as a model, we used K-feldspar for suppling SiO2 and Al2O3 components for the synthesis of kaolinite by hydrothermal method. This process of using natural K-feldspar not only can solve the problem of expensive cost of inorganic potassium salts and organic compounds, but also lead to synthesis of purer kaolinite. Moreover, feldspars are one of the most abundant minerals in the Earth's crust [6], which constitute 60% of both the continental and the oceanic crusts of the Earth [33]. Adequate conversion of K2O, SiO2 and Al2O3 in K-feldspar into sustainable

⁎ Corresponding authors at: Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, PR China. E-mail addresses: [email protected] (H. Ma), [email protected] (S. Komarneni).

https://doi.org/10.1016/j.ceramint.2018.05.227 Received 5 May 2018; Received in revised form 21 May 2018; Accepted 26 May 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Yuan, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.05.227

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Table 1 Chemical compositions of K-feldspar (wt/%). Sample

SiO2

TiO2

Al2O3

TFe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

LOI

Total

XS-16

64.84

0.29

17.95

0.83

0.010

0.65

0.36

0.63

14.15

0.082

0.47

100.24

* TFe2O3 = Total iron oxides as FeO and Fe2O3.

2KAlSiO4 + H2 SO4 + (n − 1) H2 O → 2K2SO4 + Al2O3⋅2SiO2⋅nH2 O

resources will play a significant role in modern ceramic industry. Herein, we designed two different routes, mimicking chemical weathering processes in nature, to synthesize kaolinite from K-feldspar aiming to develop and find a cost-effective, most useful and sustainable method to synthesize nano-kaolinite.

(2)

Al2O3⋅2SiO2⋅nH2 O + HCl → Al2 [Si2 O5] (OH) 4 + (n − 2) H2 O + HCl (3) The second synthesis route (named M-2): The kalsilite obtained in the pretreatment stage (Eqs. 1, 4) was reacted with HNO3 solution (Eq. (5)) by hydrothermal method directly to obtain kaolinite. After hydrothermal reaction, autoclaves were cooled quickly with cold water. The solids were separated from the solution by filtration, washed with deionized water, dried and then characterized. However, the obtained solution by filtration was used to prepare potassium nitrate. The second method developed here uses the following steps:

2. Materials and methods 2.1. Materials The bulk K-feldspar sample (XS-16) used in this study was collected from Xiyuanxia village in Rongcheng county of Shandong province, China. K-feldspar powder was obtained by crushing, grinding, ballmilling and passing through a 200-mesh sieve with most particles smaller than 74 µm in diameter and its chemical composition is shown in Table 1. Major chemical components are SiO2 (64.84%), Al2O3 (17.95%) with a K2O content of 14.15%. Potassium hydroxide (85%, analytical reagent grade) was supplied by Beijing Modern Eastern Fine Chemical Co., Ltd. HNO3 solution (analytical purity, 65~68%) was supplied by Beijing chemical works and CH3COOH solution (≥ 99.5%) by Sinopharm chemical reagent Co., Ltd. Hydrochloric acid (HCl) and sulfuric acid (H2SO4) were supplied by Beijing chemical factory. Deionized water was produced in the local laboratory.

K-feldspar (KAlSi3O8) + 4KOH → Kalsilite (KAlSiO4) (S) ↓ +2K2SiO3 + 2H2O (4) 2KAlSiO4 + 2HNO3 + H2O → Kaolinite Al2[Si2O5](OH)4 + 2KNO3 (5)

3.2. Characterization The chemical composition of K-feldspar sample and kalsilite were determined by wet chemical analysis. Powder X-ray diffraction patterns of raw materials and as-prepared samples were recorded by a SmartLab (Rigaku) X-ray diffractometer with Cu Kα radiation (40 kV/40 mA). Fourier-transform infrared (FTIR) spectra of samples were collected by a Perkin Elmer 2000 in the 4000–400 cm−1 region using potassium bromide as the diluent and binder. The morphologies of as-prepared samples were examined by Sirion 200 scanning electron microscope (SEM) under the analytical conditions of EHT = 5.00 kV and Signal A = SE. The thermal decomposition of product sample was studied by differential scanning calorimetric and thermal gravimetric analysis (DSC-TGA) using an SDT Q600 V20.9 Build 20 instrument in air atmosphere at a heating rate of 10 °C/min.

3. Experimental methods 3.1. Pretreatment of K-feldspar Firstly, K-feldspar powder was decomposed in KOH solution in a high pressure hydrothermal autoclave at 280 °C for 2 h [34]. The alkali hydrothermal treatment of K-feldspar was aimed at dissolving the stable structure of microcline and getting kalsilite [22,36]. Table 2 gives the chemical composition analyses of kalsilites, which were synthesized in different batches. Next, two routes to synthesize kaolinite from kalsilite were given and compared. The first synthesis route (named M-1) was previously developed [20] but also used here for comparison: The kalsilite obtained in the pretreatment stage (Eq. (1)) was dissolved in H2SO4 solution to obtain aluminosilicate gel (AS) (Eq. (2)) with higher chemical reactivity and the filtrate was collected by filtering for further use. The filtrate can be used to prepare K2SO4 by evaporation [35,21]. The obtained aluminosilicate, AS was treated hydrothermally with HCl solution (pH=2) (Eq. (3)) to obtain kaolinite [20]. The different steps are given as follows:

4. Results and discussion As a highly stable and poorly reactive mineral, K-feldspar must be pretreated firstly according to Eq. (1) before being used as a precursor for the synthesis of kaolinite. In the pretreatment stage, K-feldspar was treated using KOH solution in order to transform K-feldspar, microcline into kalsilite (KAlSiO4) completely, where 2/3 SiO2 of microcline was released into solution and existed in the form of [SiO3]2-, which can be precipitated by adding lime milk to obtain CaSiO3·nH2O and KOH solution for recycling [22]. Here, synthesis of kaolinite (named method M-2) was accomplished by reactions based on Eqs. (4) and (5), which show that all K+ can be collected by the evaporation of water as KNO3, which is an important potassium resource for some crops sensitive to

K − feldspar (KAlSi3O8) + 4KOH → Kalsilite (KAlSiO4 ) (S ) ↓ + 2K2 SiO3 (1)

+ 2H2 O Table 2 chemical composition of kalsilite in different batches. Sample

SiO2

TiO2

Al2O3

TFe2O3a

MnO

MgO

CaO

Na2O

K2O

P2O5

LOI

Total

gl-1 sj-1 Avg

37.82 37.68 37.75

0.10 0.05 0.08

28.74 30.13 29.44

2.32 0.87 1.60

0.16 0.01 0.09

0.61 0.13 0.37

0.38 0.79 0.59

0.60 0.57 0.59

27.18 28.13 27.66

0.03 0.08 0.06

1.63 0.93 1.28

99.57 99.37 99.47

a

TFe2O3 = Total iron oxide as FeO and Fe2O3. Avg represents the average values of above data. 2

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the products synthesized by M-1 method were reported previously by Ma et al. [20]. Here, HIs of both methods were compared and analyzed as shown in Fig. 3a. Combining the results in Fig. 2 and Fig. 3a, it can be seen that kaolinite did not form in the initial 12 h at 250 °C using the M2 method. However, the HI of kaolinite increased from 0 to 0.62 when the reaction time was extended from 12 to 18 h and thereafter, the Hi increased slowly with the continued increase of reaction time. Here, the kaolinite crystallinity was divided into four categories (levels) [17] ≥ 1.3); ordered (1.3 according to their HI: highly ordered (HI > HI ≥ 1.1); disordered (1.1 > HI ≥ 0.8); highly disordered (HI < 0.8). It is obvious that all the products of M-2 method at 250 °C crystallized from 12 to 72 h existed in the disordered categories. The HI of M-1 method rose to 1.39 (maximum value) during the initial 24 h at 250 °C, indicating that aluminosilicate precursor was more reactive than KAlSiO4. The HI using M-2 method at 270 °C jumped from 0.53 to 1.19 during the initial 10 h rapidly but remained relatively stable after 10 h, indicating that kaolinite was crystallized quickly during the initial stage of method M-2 at 270 °C. We can see that the temperature is obviously a significant influencing factor on the crystallization of kaolinite. Although the HI of M-1 method had a rapid increase to the maximum value of 1.39 by 24 h of crystallization, the formation of alunite, KAl3(SO4)2(OH)6 could not be avoided as shown in Fig. 3b. In contrast, the HI of M-2 method at 270 °C is lower than that of M-1 method at 24 h but the final product of the former is purer than the latter although a trace amount of muscovite also formed in KAO M-2 (Fig. 3b). However, this trace impurity will not affect the properties of kaolinite. The M-2 method is simpler and more efficient than M-1 because the M-2 method just needs one step from KAlSiO4 obtained in the pretreatment stage. From here on, we named the samples whose conditions were marked in Fig. 3a by M-1 and M-2 method at 270 °C and as KAO M-1 and KAO M-2, respectively. Fig. 3b shows the XRD results of KAO M-1 and KAO M-2 prepared at 270 °C for 10 h, where obvious alunite phase can be seen in KAO M-1, which will affect the purity of kaolinite and hence will adversely affect the uses of kaolinite. The d001 spacing is the important parameter to identify and certify kaolinites. In general, kaolinites have d001 spacings in the range of 7.1–7.4 Å [25]. The d001 spacings of KAO M-1 and KAO M-2 synthesized here (Fig. 3b) were 7.20 and 7.19 Å, respectively. Fig. 4 shows the SEM images of KAO M-1 and KAO M-2 synthesized by the M-1 and M-2 methods, respectively. Fig. 4a is the image of KAO M-1, which shows uniform sheet shape with the thickness of 35–45 nm. Compared with the KAO M-1 prepared by M-1 method, the shape of KAO M-2 sample synthesized by M-2 method appears to be thinner sheets with thickness of around 14 nm and smooth surfaces (Fig. 4b). Higher magnification of the KAO M-2 shown as inset in Fig. 4b reveals spherical-type aggregation, which may suggest self-assembly of nanoparticles by oriented attachment to form single crystals of nano-kaolinite. This type of nano-scale kaolinite is particularly suitable for the paper coating applications without involving the stripping stage, which is normally practiced with natural kaolinites. Kaolinite generally shows three types of morphologies such as spheres, laths and plates based on the temperature of formation and the ratio of Si/Al in solution during its formation [9]. Platy kaolinite is generally formed from undersaturated solutions having a low concentration of Si and Al according to the thermodynamic considerations. Here, the ratio of Si/Al was constant because all the Si and Al components were supplied by K-feldspar in a ratio of 3:1 and therefore, the two methods used here were controlling the morphologies of kaolinite similarly. Fig. 5 shows the FTIR spectra of kaolinites synthesized by the two methods. There are three peaks in the 3620–3700 cm−1 range corresponding to –OH stretching, which relies on the degree of ordering and crystallinity of kaolinite. Based on the intensity and separation of the peaks in this region, the degree of crystallization of KAO M-1 appears to be lower than that of KAO M-2. In the Si‒O‒Si (including Si‒O‒Al) region, sample KAO M-1 synthesized by M-1 method showed overlapping peaks at 1114, 1035 and 1007 cm−1 which can be assigned to

Fig. 1. XRD patterns of products synthesized using M-2 method with various ratios (r) of r = n(HNO3)/n(KAlSiO4) at 250 °C (∇- Kaolinite, • - Boehmite, ♦Kalsilite).

chloride. Fig. 1 presents the XRD patterns of kalsilite and product samples obtained at various r = n(HNO3)/n(KAlSiO4) using M-2 method. All of the diffraction peaks can be indexed to the hexagonal phase of kalsilite (JCPDS file 11-0579), triclinic phase of kaolinite (JCPDS file 14-0164) and orthorhombic phase of boehmite (JCPDS file 21-1307). As shown in Fig. 1, boehmite was generated besides kaolinite when the ratio of n (HNO3)/n(KAlSiO4) was 0.95, which is in accordance with the results of Bentabol [2], who showed that kaolinite formation was always accompanied with minor boehmite. Satokawa [31] proposed that kaolinite was the crystalline product, which was templated on the sheet structure of the boehmite. When the ratio of n(HNO3)/n(KAlSiO4) was increased from 0.95 to 1.15, the relative intensity of boehmite phase weakened gradually. The reason for this is that the dissolved amount of silica increased as the concentration of H+ was increased, which then reacted with boehmite and led to the formation of kaolinite steadily [10]. When the ratio of n(HNO3)/n(KAlSiO4) was increased from 0.95 to 1.05, the Hinckley Index (HI) of synthesized kaolinite increased from 0.85 to 1.01 (Fig. 1). This index is the ratio of the sum of the heights of the reflections (1 1 0) and (11 1) measured from the inter-peak background to the height of (1 1 0) from the general background [28]. The higher the HI, the greater is the crystallinity of kaolinite. When the ratio of n(HNO3)/n(KAlSiO4) was increased further, the HI of kaolinite remained relatively stable. Therefore, the ratio of n(HNO3)/n(KAlSiO4) of 1.05 was chosen as the optimal concentration for further studies. Kaolinite was synthesized using two steps, i.e., M-1 method as described in our previous study [20]: 1) kalsilite was leached by H2SO4 solution to yield aluminosilicate gel (AS) as precursor and 2) then the AS was reacted with HCl solution to obtain kaolinite as shown in Eqs. (2) and (3). All the K2O component of kalsilite in both M-1 and M-2 methods could be recovered in the forms of K2SO4 and KNO3, respectively and thus, the comprehensive utilization of K-feldspar could be achieved. Fig. 2 shows the XRD patterns of products synthesized at 250 and 270 °C with different crystallization times by the M-2 method while 3

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Fig. 2. XRD patterns of products synthesized by M-2 method at 250 (a) and 270 °C (b) for different times. (∇- Kaolinite, • - Boehmite).

Fig. 3. Hinckley indices of kaolinite products as a function of time and temperature by M-1 and M-2 methods (a) and XRD patterns (b) of kaolinite products from M-1 ). method (KAO M-1) and M-2 method (KAO M-2) prepared at 270 °C for 10 h (♣- alunite,

Fig. 6 shows the Tg and DSC curves of KAO M-1 and KAO M-2. The small weight losses of 1.31% and 1.40% of KAO M-1 and KAO M-2 at around 200 °C are attributed to the removal of physisorbed water from the surfaces of kaolinites. Kaolinite has an endothermic peak in the 400–500 °C owing to the dehydroxylation of structural OH groups of kaolinite [31] depending on its crystallinity and we can see from Fig. 6 that the endothermic peaks are located at 494 and 477 °C for KAO M-1 and KAO M-2, respectively. In the range of 400–550 °C, the weight losses were 13.35% and 9.38% for kaolinites prepared by M-1 and M-2 methods, respectively, which is consistent with the dehydroxylation of structural OH [13]. The higher weight loss (13.35%) in the temperature range of 400–500 °C and the weight loss of 3.05% in the temperature range of 747–796 °C of KAO M-1 is attributed to the dehydration and the release of SO3, respectively, during the decomposition process of alunite, KAl2(SO4)2(OH)2 impurity (Fig. 3b). Table 3 lists the raw materials and synthesis conditions of kaolinite

in-plane Si‒O vibrations. These peaks are weaker than those of the KAO M-2 sample prepared by M-2 method which showed separated peaks at 1107, 1035 and 1006 cm−1. The absorption band at 914 cm−1 is related to the Al‒OH. The peak shoulder at 940 cm−1 of KAO M-1 can be assigned to Al‒OH band, which occurs as a small inflection on the OHbending vibrations. The characteristic peaks at 915 cm−1 correspond with the inner-hydroxyl groups of Al‒OH and those present in KAO M-1 and KAO M-2 are sharp, suggesting both the kaolinites had better crystallinity as has been shown by XRD results given above. The two peaks at 794 and 754 cm−1 are related to Si‒O‒Al vibration and particularly represent characteristic absorption modes of kaolinite. Sharp stretching bands of Si‒O at 698 cm−1 also can show the differences in crystallinity of the two kaolinites synthesized by the two different methods. The bands at 539, 471, and 430 cm−1 could be attributed to the in-plane Al‒O‒Si and Si‒O‒Si bending vibrations and their degree of sharpness corresponds with the degree of ordering of kaolinite. 4

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Fig. 4. SEM images of kaolinite synthesized using two methods: (a) KAO M-1 by M-1 method, (b) KAO M-2 by M-2 method and see the TEM image of KAO M-2 at higher magnification in the inset.

expected to lower the cost of raw materials for synthesis and the cost of kaolinite in turn. In addition, most of the previous syntheses in Table 3 needed a longer time to synthesize kaolinite, which will lead to the reduction of efficiency and increased cost. Therefore, here we designed two ways to synthesize nano-kaolinite using K-feldspar as cost-effective precursor. Fig. 7 shows the synthesis process of KAO M-1 and KAO M-2. We can see from Fig. 7 that both methods involve the decomposition of K-feldspar for about two hours. It took 4 h to dissolve kalsilite subsequently into highly reactive AS, followed by crystallization of kaolinite in 24 h by hydrothermal treatment for the synthesis of KAO M-1. However, alunite existed as an impurity in the final product (Fig. 3b). In order to avoid alunite, we designed the second method (M-2) to synthesize KAO M-2, which just needs one step after the decomposition of K-feldspar to obtain kaolinite by hydrothermal treatment. Compared with method M-1, method M-2 is simpler and better because shorter crystallization time was needed (Fig. 3a). Both methods developed here can lead to the comprehensive utilization of K-feldspar and the extra SiO2 and K2O can be transformed to CaSiO3·nH2O and KNO3 as by-products, both these are significant to ceramic industrial and agricultural necessities. More importantly, the thickness and diameter of as-prepared kaolinites using both the methods are in the nano-scale, which are expected to be useful for paper coating and in making advanced ceramic materials.

Fig. 5. FTIR spectra of KAO M-1 and KAO M-2.

in previous reports by others. All of these researchers used chemical reagents as precursors for kaolinite synthesis, which will lead to higher cost for such synthetic kaolinites. However, using naturally occurring Kfeldspar to supply silica and alumina as was done in this work is

Fig. 6. Tg (a) and DSC (b) curves of KAO M-1 and KAO M-2. 5

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Table 3 Summary of raw sources and reaction conditions for the synthesis of kaolinite in previous studies. Raw sources

Reaction time

Temperature

References

aluminum hydroxide, silicic acid, HCl solution silica gel, amorphous Al(OH)3·xH2O, HCl solution Natural biotite, AlCl3 Na metasilicate (SiO2·Na2O·5H2O), AlCl3·6H2O and FeCl3·6H2O Na2CO3, Al2(SO4)3, Na2SiO3, oxalic acid, H2SO4 solution Sodium metasilicate (SiO2·Na2O·5H2O), [Al(NO3)3·9H2O], NaOH solution tetraethyl orthosilicate aluminum tri-isopropoxide, KOH solution tetraethyl orthosilicate, AI(OH)3, HCl solution silica-sol and alumina-sol Sodium metasilicate SiO2·Na2O·5H2O, (Al(NO3)3·9H2O and Fe(NO3)3·9H2O

1–3 weeks 36 h 72 h 60 days 6 days 21 days 6–60 days 3–10 days 1–144 h 7–36 days

170–250 °C 250 °C 200 °C 225 °C 175 °C 200–240 °C 150–250 °C 220 °C 220 °C 200 °C

[29] [30] [5] [16] [18] [8] [14] [31] [23] [8]

Fig. 7. Graphical representation of the synthesis process of kaolinite.

referring to the chemical weathering processes of K-feldspar, nanokaolinite was obtained by controlling the environmental medium conditions of reaction process. K-feldspar was placed in alkaline solution environment, and then decomposed into Al(OH)4-, Si(OH)4 and K+ to form kalsilite. Compared with K-feldspar, kalsilite was easily dissolved into Al(OH)4-, Si(OH)4 and K+ in the acid environment. So, prior to the acid treatment, alkaline solution was used here to treat feldspar in order to accelerate the transformation rate from feldspar to kaolinite (Fig. 8). Therefore, mimicking the millions of years of geological weathering processes in the laboratory scale took only several hours (Fig. 8) and hence, the current results have a significant meaning for the synthesis of kaolinite in nano-scale by the hydrothermal method.

4.1. Synthesis mechanism In nature, kaolinite was formed mainly during the chemical weathering process and low temperature hydrothermal alteration process of aluminosilicate rocks in the acidic fluid. Kaolinite obtained by the chemical weathering of feldspar can be expressed in the following equation: 4KAlSi3O8 (Feldspar) + H2O + 2CO2 → Al4[Si4O10](OH)8 (Kaolinite) + 8SiO2 + 2K2CO3 Feldspar was extremely stable due to crystallizing from the pegmatitic stage of magmatism in both intrusive and extrusive igneous rocks as well as many types of metamorphic rocks. Fig. 8 shows the schematic diagram of chemical weathering (A) and the experimental synthesis process of kaolinite from feldspar. When K-feldspar was exposed in water and acidic fluid, the charge-balancing cations (K+) were released by ion exchange [26], the dissolved species might be retained by adsorption/desorption at the surface of mineral particle and T-O-T linkages generated by hydrolytic degradation [11]. New solid was precipitated by the assembly of species generated in the dissolution of K-feldspar. After nucleation, growth, and recrystallization, a secondary mineral (kaolinite) was formed [33]. However, it would take a long time (millions of years) to form natural kaolinite from feldspar. Here,

5. Conclusions In summary, two methods (M-1 and M-2) were developed and compared to synthesize kaolinite from K-feldspar powder. The kaolinite by M-1 method showed uniform plate-like morphology with the thickness of 35–45 nm. Kaolinite samples synthesized by M-2 method resulted in plate-like sheets with thickness of around 14 nm. The nanoscale kaolinite obtained by M-2 method will help in producing high quality paper-coating grade of kaolinite for industry.

Fig. 8. Schematic diagram of the reaction mechanism of kaolinite in natural chemical weathering process (A) and experimental synthesis (B). 6

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Acknowledgements

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