CL-220499 Received: November 24, 2022 | Accepted: January 9, 2023 | Web Released: February 25, 2023

Separation and Purification of Montmorillonite from Raw Bentonite Using a Simple

Hydrothermal Treatment
Ema Yoshikawa,* Shingo Yokoyama, and Yasutaka Watanabe
Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko, Chiba 270-1194, Japan

E-mail: yoshikawa3829@criepi.denken.or.jp

Herein, a simple hydrothermal treatment method is proposed stones are dispersed in water, and particles below a certain
to separate high-purity montmorillonite from natural clay stone, particle size, such as 2 ¯m, are collected by sedimentation
bentonite, which contains various associated minerals. Specifi- methods, including centrifugation.11­15 However, in this method,
cally, a suspension of bentonite is placed in a closed vessel and fine, associated minerals cannot be removed. Therefore, further
chemical treatment can be performed to remove fine, associated

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heated to temperatures above approximately 100 °C. High-purity
montmorillonite is collected by recovering the gel fabricated by minerals.14,16­19
heating. Evidently, the purity and collectability of montmor- Natural smectite clays can be divided into Na-type and
illonite is higher with higher temperatures and longer periods of Ca-type smectite, with different swelling properties. In the case
treatment. of Na-type smectite with large swelling properties,20 although
these methods can be used to separate a high-purity sample, they
Keywords: Montmorillonite | Bentonite | Purification contain numerous processing steps and are time-consuming.
As is well-known, Ca-type smectite, which has lower swelling
Smectite is a clay mineral found in natural clay stones such properties compared with Na-type, has poor dispersibility21,22
as bentonite, which has properties such as swelling capacity, ion and is extremely difficult to separate from the associated
exchange capacity, and organic affinity. In particular, montmo- minerals. Therefore, a decisive step preceding separation is the
rillonite contained in bentonite is a kind of smectite. Smectite, preparation of a stable dispersion of the smectite by replacing
including montmorillonite, has been used to develop various the divalent exchangeable cations with sodium ions.23,24 If
functional materials, such as clay-polymer nanocomposites,1­3 smectite of any ionic type can be purified by a simple method,
clay-organic hybrid materials,1,4,5 and clay-metal complex hybrid the options of natural smectite available for materials develop-
materials.6,7 Both natural and synthetic smectite have been ment will be considerably expanded. Herein, we investigated a
utilized for industrial applications, depending on the purpose.8 relatively simple method for separating high-purity smectite
Synthetic smectite has a simple chemical composition and with from Ca-type bentonite, which is relatively difficult to purify.25
small impurities, whereas natural smectite is found in clay stones, Kunibond (KB: Kunimine Industries. Co. Ltd, Japan) was
such as bentonite, and contains large amounts of associated used as the sample of Ca-type bentonite to separate high-purity
minerals, such as quartz and feldspar.9 Depending on its origin, montmorillonite. Figure 1 shows the flow of separation of high-
natural smectite has a great variety of properties, such as crystal purity montmorillonite from raw bentonite by hydrothermal
shape, crystal aspect ratio, chemical composition, layer charge, treatment. The key point in this method is the separation of the
exchangeable cation composition, and water dispersibility.10 sample into several layers of varying purity by heating the
Therefore, if natural smectite can be used for the aforementioned suspension. First, to create bentonite suspension, KB was added
material development, the most suitable host material can be to deionized water and ultrasonically dispersed. The suspension
selected from among smectites with different properties. was made at a liquid/solid weight ratio of 30 or 50. To
To use natural smectite for materials development, high- investigate the reasonable conditions for the purification, the
purity smectite must be separated from natural clay stones. bentonite suspension was hydrothermally treated under varying
Therefore, associated minerals other than smectite must be temperatures and heating periods (Table 1). The bentonite
removed. Generally, to increase the purity of smectite, clay suspension was poured into a closed vessel, such as an autoclave,

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Figure 1. Schematic of separation of high-purity smectite from natural clay by hydrothermal treatment.

132 | Chem. Lett. 2023, 52, 132–135 | doi:10.1246/cl.220499 © 2023 The Chemical Society of Japan
 Table 1. Test conditions D 㻞㻜㻜㻜㻜
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Ca-type bentonite 㻯㼞㻦㻌㼏㼞㼕㼟㼠㼛㼎㼍㼘㼕㼠㼑
Sample 㻲㻦㻌㼒㼑㼘㼐㼟㼜㼍㼞
(Kunibond) 㻝㻡㻜㻜㻜

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㻽㻦㻌㼝㼡㼍㼞㼠㼦
Liquid Deionized water 㻼㻦㻌㼜㼥㼞㼕㼠㼑

Liquid/solid ratio 30*, 50 㻝㻜㻜㻜㻜

Heating temperature 90, 110, 200 °C
Heating period 3 or 7 d 㻡㻜㻜㻜 㻹 㻯㼞
㻽 㻯㼞
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and heated at 90, 110, and 200 °C for each period (3 and 7 d) in a 㼻㻞䃗㻔㻯㼡㻷D㻕
thermostatic oven. Subsequently, the vessel was removed from

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the oven and air-cooled to approximately 23 °C by leaving it for E
6 to 12 h. Finally, after opening the vessel, the liquid covering
the solid sample was removed with a syringe, and the gels were
picked up at the top by using a spoon. The collected gels were
dried under a vacuum and crushed to analyze their characteristics.
For the sample with no gels, the top 1 cm of the suspension
was collected and dried. The samples were labeled as KB
“temperature” °C - “heating days” D. Note that, the results of
the samples with liquid/solid ratio of 30 are omitted because
high-purity montmorillonite was not separated after the treat-
ment. To compare the purification results with the conventional
separation methods, i.e., sedimentation method, KB was dis-
persed in deionized water, and small fractions with particle sizes
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of >0.2 ¯m (KB-0.2) and 0.2­2 ¯m (KB-2) were separated using
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centrifuges.
For mineral identification, the X-ray powder diffraction
Figure 2. (a) XRD pattern of random sample and (b) SEM
(XRD; RIGAKU, SmartLab) patterns of raw bentonite and image of raw bentonite (KB: Kunibond).
treated samples were obtained using randomly oriented and
oriented samples, respectively.26 The vacuum-dried samples Heating period
were observed and analyzed chemically using field emission Temp.
3 days 7 days
scanning electron microscopy with energy dispersive X-ray
spectroscopy (FESEM-EDS; JEOL JSM-7001M). To evaluate
the purity of montmorillonite, the amount of methylene blue
(MB) absorbed on the sample was measured based on the 200 °C
Japanese Industrial Standard (JIS Z 2451). A particle size
analyzer (Shimadzu SALD-2300) was used to determine the KB200 °C-3D KB200 °C-7D
particle size distribution. Before measurements, the samples
were dispersed in deionized water and sonicated for 20 min.
The mineral compositions of raw KB were montmorillonite, 110 °C
cristobalite, quartz, feldspar, and pyrite (Figure 2a). From the Gel
Forming
SEM image and EDS analysis (Figure 2b), flake-shape particles as KB110 °C-3D KB110 °C-7D
montmorillonite27 were observed. Furthermore, several aggrega-
tions of a few hundred nm composed of fine associated minerals, No
primarily composed of Si, were observed in raw KB. From XRD gels
pattern (Figure 2a), the fine associated minerals should be 90 °C
cristobalite or quartz which are primarily composed Si.28
Figure 3 shows the photos of the samples after hydro- KB90 °C-3D KB90 °C-7D
thermal treatment. Several white, clear gels were observed at
the top of both of KB200 °C samples, whereas no gels were Figure 3. Gel formation at each temperature and heating
observed in either of the KB90 °C samples. Although the gels period.
were not observed in KB110 °C-3D, the gels were observed in
KB110 °C-7D. These results suggest that gel formation occurred below the clear-colored gel, was muddy-colored and exhibited a
over 110 °C, and it was affected by the heating period. The stronger gel structure. At the bottom, large dark-colored particles
sample, which formed a gel, was separated into several layers were sedimented and formed a solid layer. Conversely, in
with different properties, as shown in Figure 1. White, clear- samples without gel, such as those heated at 90 °C, no structure
colored gels were on the top of the solid phase. The middle part, was present.

Chem. Lett. 2023, 52, 132–135 | doi:10.1246/cl.220499 © 2023 The Chemical Society of Japan | 133
 㻭㼙㼛㼡㼚㼠㻌㼛㼒㻌㻹㻮㻌㼍㼐㼟㼛㼎㼑㼐㻌㻔㼙㼙㼛㼘㻛㻝㻜㻜㼓㻕
㻔㼍㻕 㻹 㻹㻦㻌㼙㼛㼚㼠㼙㼛㼞㼕㼘㼘㼛㼚㼕㼠㼑
㻔㼎㻕 㻵㼚㼠㼑㼚㼟㼕㼠㼥 㻔㼍㻕㽢㻝㻜㻜 㻝㻢㻜
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㻝㻞㻜

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㻝㻜㻜
㻵㼚㼠㼑㼚㼟㼕㼠㼥㻌㻔㼍㻚㼡㻚㻕

㻹 㻹 㻷㻮㻥㻜䉝㻙㻟㻰
㻷㻮㻥㻜䉝㻙㻣㻰
㻤㻜
㻷㻮㻝㻝㻜䉝㻙㻟㻰 㻢㻜
㻷㻮㻝㻝㻜䉝㻙㻣㻰 㻠㻜
㻷㻮㻞㻜㻜䉝㻙㻟㻰 㻞㻜
㻷㻮㻞㻜㻜䉝㻙㻣㻰 㻜
㻷㻮㻙㻞 㼞㼍㼣㻌㻷㻮 㻷㻮㻙㻜㻚㻞 㻞㻥 㻷㻮㻞㻜㻜䉝㻙㻣㻰
㻷㻮㻙㻜㻚㻞
Figure 6. Adsorption of methylene blue onto raw KB and
㻡 㻝㻜 㻝㻡 㻞㻜 㻞㻡 㻟㻜 㻟㻡 㻠㻜 㻠㻡 㻞㻜 㻞㻞 㻞㻠 treated samples.

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㻿㼑㼐㼕㼙㼑㼚㼠㼍㼠㼕㼛㼚 㻷㻮㻙㻜㻚㻞
Figure 4. XRD pattern of treated, oriented samples. 㻷㻮㻞㻜㻜䉝㻙㻣㻰

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㻜
㻜㻚㻜㻝 㻜㻚㻝 㻝 㻝㻜 㻝㻜㻜 㻝㻜㻜㻜
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Figure 7. Distribution of particle size after different separating

treatment.
㻯㼛㼘㼘㼑㼏㼠㼕㼛㼚㻌㼞㼍㼠㼑㻌㼛㼒㻌㼠㼞㼑㼍㼠㼑㼐㻌㼟㼍㼙㼜㼘㼑㻌㻔㻑㻕

㻝㻡
Figure 5. SEM image of hydrothermal treated sample 㻿㼑㼐㼕㼙㼑㼚㼠㼍㼠㼕㼛㼚
㻴㼥㼐㼞㼛㼠㼔㼑㼞㼙㼍㼘㻌㼠㼞㼑㼍㼠㼙㼑㼚㼠
(KB200 °C-7D).

Figure 4 shows XRD patterns of oriented samples that were 㻝㻜

hydrothermally treated and other purified samples that were
treated via the sedimentation method. In all samples, no peaks
reflecting quartz, feldsher, and pyrite were observed, except for 㻡
montmorillonite and cristobalite. Furthermore, no peaks of the
new product formed by hydrothermal treatment were identified.
However, the peaks of cristobalite were still present in almost 㻜
㻷㻮㻙㻜㻚㻞 㻷㻮㻥㻜䉝㻙㻣㻰 㻷㻮㻝㻝㻜䉝㻙㻣㻰 㻷㻮㻞㻜㻜䉝㻙㻣㻰
all samples. In hydrothermally treated samples, cristobalite
peaks were smaller at higher treatment temperatures and longer
Figure 8. Collection rate for each treated sample.
heating periods. In samples obtained by the sedimentation
method, weak peaks of cristobalite were observed in KB-2,
whereas they were negligibly observed in KB-0.2. In a sample montmorillonite, identical to KB-0.2. Figure 7 shows a different
that was hydrothermally treated, the highest removal effect particle size distribution for each purification method. The broad
of cristobalite was recognized for KB200 °C-7D, which was particle size distribution indicated an asymmetrical shape for
approximately identical to KB-0.2. Figure 5 shows an SEM KB200 °C-7D whereas KB-0.2 was identified as the narrow
image of KB200 °C-7D. Almost all particles in the image were symmetrical distribution. These observations indicate that the
flake-shaped montmorillonite, and the associated minerals, such gel collected by hydrothermal treatment was composed of high-
as in Figure 2, were rarely observed. purity montmorillonite with a wide particle size distribution.
Evidently from Figure 6, approximately 150 mmol/100 g of Figure 8 shows the ratio of the dry mass of the gel collected
methylene blue was adsorbed by KB200 °C-7D; this amount after hydrothermal treatment for 7 d to the dry mass of the KB
greater than the 122 mmol/100 g for raw KB. By contrast, the added to the suspension. The collection rate of the fraction <0.2
value was identical to montmorillonite, with particle sizes of less ¯m obtained by the sedimentation method were compared with
than 0.2 ¯m, which was collected from KB using the sedimen- that. In hydrothermal treatment, the higher the heating temper-
tation method after exchangeable calcium ions replace with ature, the greater the amount of gel collected. The collecting
sodium ions.29 This suggests that KB200 °C-7D had high-purity amount of KB200 °C-7D was double that of KB-0.2, and both

134 | Chem. Lett. 2023, 52, 132–135 | doi:10.1246/cl.220499 © 2023 The Chemical Society of Japan
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10.1246/cl.220499. 25 E. Yoshikawa, Y. Watanabe and S. Yokoyama, Japan
Platform for Patent information, Application number
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Chem. Lett. 2023, 52, 132–135 | doi:10.1246/cl.220499 © 2023 The Chemical Society of Japan | 135