Variations in the pore structure of clays treated by a decomposition-hydration-reducomposition treatment

Surface Technology, 13 (1981) 189- 195

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V A R I A T I O N S IN THE PORE STRUCTURE OF CLAYS T R E A T E D BY A DECOMPOSITION--HYDRATION-REDECOMPOSITION TREATMENT

N. SH. PETRO, W. E. MOURAD and B. S. GIRGIS Surface Chemistry Laboratory, National Research Centre, Dokki, Cairo (Egypt) (Received October 28, 1980)

Summary Six local mineral deposits (serpentine, kaolin, Aswan clay, ball clay, bentonite and diatomite) were subjected to a d e c o m p o s i t i o n - h y d r a t i o n redecomposition treatment. The initial calcination of each solid was performed at a high enough t em pe r at ur e sufficient t hat it decomposed completely as shown by differential thermal analysis curves. The final decomposition in vacuum of the magnesium and aluminium silicates resulted in products which had a higher adsorption capacity and were essentially microporous in nature. This was ascribed to r e h y d r o x y l a t i o n of the dehydrated silicate layers to an extent which depends on the degree of shrinkage. The pore volume of bentonite decreased, probably because of the collapse of the decomposed silicate sheets and their inability to rehydroxylate. The calcium oxide in the decomposed diatomite hydrated and produced an active product after a second decomposition at 400 ~C.

1. I n t r o d u c t i o n The activation of clays is frequently carried out in order to obtain useful industrial products, e.g. gas and liquid adsorbents, catalysts and catalyst carriers. Various procedures are used including thermal and/or acid t r e a t m e n t [1 - 6]. The s t r u c t u r e and t ext ur e of the products are determined by the conditions under which the calcination is performed [7, 8]. The presence or absence of air during the thermal t r e a t m e n t affects the rate and mechanism of gas loss as well as the r e h y d r a t i o n behaviour of the products [9]. A technique in which the raw clay is decomposed at a moderately high t e m p e r a t u r e and then r e h y d r a t e d and redecomposed at a lower t e m p e r a t u r e has recently been developed. The t r e a t m e n t of talc-magnesite [10] and serpentine-magnesite [11] in this m a n n e r has given products of astonishingly high surface area t hat were characterized by microporosity. It has 0376-4883/81/0000-0000/$02.50

© Elsevier Sequoia/Printed in The Netherlands

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been suggested that this type of activation is a characteristic property of the magnesium silicate minerals investigated and/or is due to the presence of a decomposable magnesium salt which is involved in the dehydration and decomposition [lo, 111. Therefore we tested this activation method on a number of mineral deposits with different structures. The variations in the structure and texture were determined using differential thermal analysis (DTA) and the adsorption of nitrogen gas.

2. Experimental

details

2.1. Materials Five mineral deposits were used: kaolin, Aswan clay, ball clay, bentonite and diatomite. This set of clays was obtained from the Fayoum Depression and the Aswan region. We also used a purified serpentine deposit prepared by leaching a raw serpentine deposit obtained from the Barramyia district with 5% HCl. The mixture was shaken continually for 24 h, washed and finally dried at 110 “C. The DTA of the product showed that the decomposition effect due to magnesium carbonate had completely disappeared. 2.2. Techniques The chemical compositions of the clay and serpentine specimens were determined using atomic absorption spectroscopy. The results are given in Table 1 which also includes the weight loss on ignition at 1000 ‘C. TABLE Chemical

1 analysis

Sample

of the raw materials SO2 cd”)

403 w

Fez03 cx0)

CaO 0”)

MgO FM

Weight loss on ignition (%)

Raw serpentine Acid-treated serpentine Kaolin Aswan clay Ball clay Bentonite Diatomite

32.28 41.90

3.63 2.60

5.13 1.58

1.71 0.30

36.72 39.40

19.69 14.50

44.40 51.10 54.52 49.10 33.10

39.90 25.78 27.60 18.89 5.01

1.51 6.61 3.50 4.71 2.90

1.38 1.81 1.06 3.30 27.59

1.20 2.00 1.81 7.20 1.10

11.31 10.90 15.02 15.00 28.50

The DTA of the samples was performed using an apparatus with an automatic recorder. The rate of heating was 10 “C min I. Each of the six materials under test was calcined at the temperature at which its principal decomposition reaction occurred. Thus kaolin, ball clay, Aswan clay and bentonite were calcined for 5 h at 650 “C, diatomite at 900 “C and serpentine at 750 “C. A portion of each product was hydrated by immersion in excess

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water at room temperature, left for 3 h and then dried to constant weight at 110 °C. Redecomposition of the hydr at ed products was performed by heating in vacuum at 400 °C for 2 h. The nitrogen adsorption isotherms were measured at 77 K and used in the evaluation of the surface area, the total void volume and the mean pore radius. The surface area SBET (m E g-1) was calculated using the B r u n a u e r E m m e t t - T e l l e r (BET) method [12], the total void volume Vp (ml g-1) was evaluated from the amount of nitrogen adsorbed at the sat urat i on vapour pressure and the mean pore radius ~ (A) was estimated from the relation = 2Vp/SsET.

3. R e s u l t s

3.1. Chemical analysis The data in Table 1 indicate that the magnesium silicate clay serpentine (Mg3Si2Os(OH)4) was contaminated by the presence of extra magnesia which was confirmed to be magnesium carbonate [11]. After the acid t r e a t m e n t an almost stoichiometric magnesia c o n t e n t was attained (see Table 1). The aluminosilicate clays (kaolin, Aswan clay and ball clay) were essentially composed of kaolinite (AlzSiEOs(OH)3). However, the Aswan clay and the ball clay were contaminated by quartz, as can be seen from the exceptionally high silica content. The bentonite was composed essentially of montmorillonite (Nao.7(A13.aMgo.~)SisO20(OH)4.nH20) but nevertheless a large number of divalent and tr iv a l ent cation exchanges had t aken place, as was evident from the high iron, calcium and'magnesium contents. The diatomite had been shown earlier to be composed mainly of silica diatom particles embedded in a calcium car b onat e matrix. This deposit has been chemically described as composed of calcite and a low percentage of aluminosilicate clays in addition to boehmite and gibbsite [13]. 3.2. Differential thermal analysis The DTA curves of the raw materials are shown in Fig. 1. The two endothermic peaks in the curve for pure serpentine correspond to the loss of the physically adsorbed water at 110 °C and to the loss of the water of crystallization at 680 °C [14 - 17]. The exothermic effect at 800 °C is usually ascribed to the formation of forsterite (Mg2SiO 4) [11, 13, 16 - 19]. The three s truct ur a l l y similar clays (kaolin, Aswan clay and ball clay) exhibit two prominent thermal effects (Fig. 1). T h e endothermic reaction at 580 °C is associated with de hydr oxyl at i on of the kaolin component and leads to the formation of metakaolinite (A12Si207) and the exothermic reaction at 940- 980 °C is ascribed to the formation of aluminium silicate or spinel (A14SiaO12). Bentonite is characterized by a large dehydration peak at 140 °C and a thermal d eh y d r o xyl a t i on effect at 560 °C. There is also a small exothermic

192

s h o u l d e r n e a r 910 °C w h i c h denotes spinel f o r m a t i o n [19]. D i a t o m i t e loses its adsorbed w a t e r at 110 - 160 °C and the calcite shows a d e l a y e d d e c o m p o s i t i o n at the r a t h e r h i g h t e m p e r a t u r e of 880 °C. A n e x o t h e r m i c s h o u l d e r at a b o u t 900 °C is u s u a l l y ascribed to a r e a c t i o n b e t w e e n the c a l c i u m oxide and the free silica to form c a l c i u m silicate [13, 18]. The small e n d o t h e r m i c effect at 540 °C and the peak at 150 °C are a s s o c i a t e d with d e h y d r a t i o n of the low p e r c e n t a g e of m o n t m o r i l l o n i t e or k a o l i n i t e c o n t a m i n a t i n g the deposit [20].

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Temperature ( ' C )

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Fig. 1. The DTA curves of the raw materials investigated.

3.3. Nitrogen adsorption The v a r i a t i o n s in the pore s t r u c t u r e p a r a m e t e r s t h a t a c c o m p a n y the d e c o m p o s i t i o n - h y d r a t i o n - r e d e c o m p o s i t i o n process are given in Table 2. The c a l c i n a t i o n of s e r p e n t i n e at 750 °C p r o d u c e s small decreases in its m e a s u r e d s u r f a c e area and pore volume. This could be due to s i n t e r i n g of the solid at this t e m p e r a t u r e . R e d e c o m p o s i t i o n after h y d r a t i o n proved to be an effective w a y of c r e a t i n g new pores, leading to m a r k e d increases in the surface a r e a (more t h a n fivefold) and the t o t a l p o r o s i t y (more t h a n twofold). However, the

193

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194

p r o d u c t is m i c r o p o r o u s in c h a r a c t e r , as is evident from the v a l u e of 20 A e s t i m a t e d for its m e a n pore radius. The p a r e n t k a o l i n i t e clays (kaolin, Aswan clay and ball clay) h a v e small surface areas (6 - 17 m 2 g 1) and small t o t a l pore volumes (0.015 - 0.035 ml g-1). A small d e c r e a s e in surface a r e a is found after decomposition at 650 °C and is p r o b a b l y due to the s i m u l t a n e o u s d e h y d r o x y l a t i o n of k a o l i n i t e and f o r m a t i o n of m e t a k a o l i n i t e . This t r a n s f o r m a t i o n is also a c c o m p a n i e d by slight decreases in b o t h the t o t a l pore v o l u m e and the m e a n pore radii. The p r o d u c t s of the d e c o m p o s i t i o n - h y d r a t i o n - r e d e c o m p o s i t i o n t r e a t m e n t showed twofold to fivefold increases in b o t h the surface area and the total porosity c o m p a r e d with the u n t r e a t e d deposits. These products also had v e r y small m e a n pore radii (22- 26 A). In general, w h e n k a o l i n i t e decomposes, the w a t e r loss o c c u r s m o r e or less u n i f o r m l y from all unit cells with a decrease in the n u m b e r of o x y g e n atoms per u n i t cell. This may lead to a partial collapse of the silicate layers, t h e r e b y explaining the small decrease in the t e x t u r a l p a r a m e t e r s [21]. The p a r e n t b e n t o n i t e is c h a r a c t e r i z e d by a high a d s o r p t i o n area and a large pore v o l u m e c o m p a r e d with the o t h e r raw deposits. Direct decomposition at 650 °C with the f o r m a t i o n of m o n t m o r i l l o n i t e d e h y d r o x y l a t e results in the loss of the available a d s o r p t i o n area and pore volume (a d e c r e a s e of a b o u t fivefold) but the m e a n pore dimensions are u n c h a n g e d . R e d e c o m p o s i t i o n of the h y d r a t e d p r o d u c t in v a c u u m produces a small i n c r e a s e in the a d s o r p t i o n a r e a of the decomposed b e n t o n i t e and the p r o d u c t has a m i c r o p o r o u s texture. The d i a t o m i t e has a small surface area and a large pore volume and h e n c e is of m a c r o p o r o u s c h a r a c t e r . T h e r m a l t r e a t m e n t at 900 C is accomp a n i e d by a s u b s t a n t i a l increase in the surface area available. This may be due to the f o r m a t i o n of active calcium oxide from the calcium c a r b o n a t e which is almost u n a f f e c t e d by h y d r a t i o n and s u b s e q u e n t decomposition in v a c u u m at 400 C . 4. D i s c u s s i o n

The a c t i v a t i o n of s e r p e n t i n e by the d e c o m p o s i t i o n h y d r a t i o n r e d e c o m p o s i t i o n t e c h n i q u e appears to be r e l a t e d to the s t r u c t u r e of the m a g n e s i u m silicate and not to the p r e s e n c e of excess m a g n e s i a since the specimen i n v e s t i g a t e d was free of this material. Thus the p r o d u c t i o n of such an active solid could be because the silicate l a y e r s t r u c t u r e expands and thus is stable (i.e. it does not collapse) at a t e m p e r a t u r e of 400 °C u n d e r vacuum. A s u b s t a n t i a l p a r t of this n e w l y c r e a t e d v o l u m e will be enclosed b e t w e e n layers s e p a r a t e d by only a few molecules; this will cause large increases in the surface a r e a and the m i c r o p o r e volume. A n o t h e r possible e x p l a n a t i o n of the activation, which has been r e f e r r e d to earlier [11], is. the f o r m a t i o n of a f o r s t e r i t e h y d r a t e t h a t m a y d e h y d r a t e at a b o u t 400 °C. The k a o l i n i t e group of clays p r o b a b l y decomposed with a limited degree of s h r i n k a g e such t h a t partial r e h y d r o x y l a t i o n took place. Redecomposition

195

of the h y d r a t e d p r o d u c t s will t h e n lead to an a p p a r e n t i n c r e a s e in the a d s o r p t i o n capacity. However, this a c t i v a t i o n would be d e p e n d e n t on the s t r u c t u r e of the r a w m a t e r i a l as well as on the degree of its c o n t a m i n a t i o n with o t h e r oxides. The a p p a r e n t d e a c t i v a t i o n of b e n t o n i t e m a y be b e c a u s e r e h y d r a t i o n of d e h y d r o x y l a t e d bentonite, m o n t m o r i l l o n i t e etc. is only possible if the t e m p e r a t u r e of the initial h e a t i n g is lower t h a n the s h r i n k a g e t e m p e r a t u r e [22]. The t e m p e r a t u r e of 650 °C used in this w o r k m a y h a v e been too high and h e n c e s h r i n k a g e of the s t r u c t u r e w o u l d h a v e o c c u r r e d and p r e v e n t e d the r e h y d r o x y l a t i o n and s u b s e q u e n t e x p a n s i o n in the l a y e r s t r u c t u r e . In addition, the presence of large a m o u n t s of sodium, m a g n e s i u m and c a l c i u m in the b e n t o n i t e m a y h a v e facilitated the collapse of the silicate layers. Finally, the d i a t o m i t e m u s t h a v e been a c t i v a t e d by a different mechanism since it is n o t c o m p o s e d of silicate l a y e r s t r u c t u r e s . The calcite in w h i c h the silica diatoms are embedded u n d e r g o e s delayed d e c o m p o s i t i o n at a high t e m p e r a t u r e ; the c a l c i u m oxide m i c r o p a r t i c l e s formed are p r e v e n t e d from c o m i n g into c o n t a c t with e a c h o t h e r a n d t h e r e f o r e do n o t grow into l a r g e r crystals. These crystallites h y d r a t e easily and decompose to smaller c r y s t a l l i t e s b e c a u s e of the release of w a t e r molecules.

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