Mecanochemistry_enstatite_spinel_cordierite

7/28/2019 Mecanochemistry_enstatite_spinel_cordierite

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THE EFFECT OF MECHANOCHEMICAL ACTIVATION IN THE

MgOAl2O3SiO2 SYSTEM REACTIVITY

C. A. dAzevedo1,2

, F. M. S. Garrido2, M. E. Medeiros

2 *

1 Instituto de Pesquisas da Marinha - Rua Ipir no. 2, Ilha do Governador, 21931-090, Rio deJaneiro, Brazil

2Departamento de Qumica Inorgnica, Instituto de Qumica, Universidade Federal do Rio de Janeiro,

Ilha do Fundo, 21945-970, Rio de Janeiro, Brazil

ABSTRACT

Samples on the MgOAl2O3SiO2 ternary system, constituted by 28.5mol % of MgO, 28.5mol % of

Al2O3 and 43mol % of SiO2, were activated in a roll mill and calcined at different temperatures. The

influence of grinding time, the ratio between powder mass and the grinding element mass, the SiO2

precursor and the activation medium, in the system reactivity were studied. The analysis of the

infrared spectra and of the X-ray powder diffraction patterns indicates the formation, in some

samples, of Mg(OH)2 at room temperature, of forsterite (MgSi2O5) and enstatite (MgSiO3) at 1223K

and of spinel (MgAl2O4) between 1223K and 1523K. The presence of cordierite (Mg2Al2Si5O18) was

observed at 1523K, a reaction pathway for its formation was proposed.

KEYWORDS: mechanochemistry, enstatite, spinel, cordierite.

__________________________________

*Corresponding author: E-mail address: [email protected].

INTRODUCTION

The energy transfer during the impact in a grinding process and its effect over the reactivity has been

the subject of several studies, usually using high-energy mills with speeds higher than

500 rpm. Besides the increase in the homogeneity and the variation of the particle size, the intensity

of the locally applied force over a determined powder volume, imprisoned at the contact point during

the impact, determines the powder mechanochemical activation. In this process is common the

formation of amorphous phases, creation of point defects or deformations in the crystalline network.

In some special situations, mechanochemical reactions between the mixture components can occur

during the grinding process [1-7].


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Cordierite based ceramics on the ternary system MgOAl2O3SiO2 present great economic

importance. Cordierite composition can vary between the limits expressed by the formulas

2MgO.2Al2O3.5SiO2 and MgO.Al2O3.3SiO2. In these ceramics secondary crystalline phases, such as

corundum, mullite, spinel, forsterite, enstatite and cristobalite are presents very often [7-11]. As a

consequence, the properties of these ceramics are strongly dependent on the composition, the

presence of additives and manufacturing techniques. Recently, it was discussed the importance of

spinel in the cordierite formation process [7,9-11]. Some authors proposed that cordierite would be

formed from the reaction between spinel and a silica precursor phase [7,9]. However, other authors

suggest that the utilization of additives would allow the direct synthesis of cordierite, either by the

reaction between MgO, Al2O3 and SiO2 [10] or by the reaction between enstatite and alumina [11].

In this work, the mechanochemical activation of the MgO-Al2O3-SiO2 ternary system was studied.

The objective was to produce ceramics based on the cordierite where spinel is present as the main

secondary phase, trying to elucidate the influence of the spinel phase in the cordierite formation

process.

MATERIALS AND METHODS

Merck silicic acid (H4SiO4) (dehydrated by treatment with Merck concentrated perchloric acid) [12]

and calcined at 1173K for 4h was used as precursor of silica (SiO2) for the cases of type samples.

Another silica precursor was the Merck silica gel HF254 (Type 60) used as it was removed from the

bottle, for type samples. The magnesium oxide precursor (MgO, P.A. Merck) was submitted to a

thermal treatment at 823K for 4h, forming periclase [13]. The aluminum oxide precursor (Al2O3,

Ridel-of-Han) was calcined at 1473K (4h) and used in the -corundum form (-Al2O3) [12, 14].

On the grinding process, 10g to 4,5g of the oxide mixture, with equal composition 28.5mol % of

MgO, 28.5mol% of Al2O3 and 43mol% of SiO2, were placed in a Nalgene bottle of 250ml, together

with the grinding elements. This composition, with an excess of MgO and Al 2O3, was used to

evaluate the influence of the spinel phase in the cordierite formation process. The grinding was

performed in a US STONEWARE mill of roll at a 100rpm speed. Zirconia cylinders (NETSCH) with

1cm of diameter and 1cm of length were used as the grinding elements in the place of zirconia

spheres, to increase the shock area [7]. Absolute ethyl alcohol P.A. (Grupo Qumica) and distilled and

deionized water (MILLI-Q Water System equipment - Millipore) were used as activation medium.

Table 1 presents the conditions used for grinding each one of the samples.

The samples were heated for 2 hours at temperatures of 393K, 1223K, 1423K, 1523K or 1623K. The

heating process was performed in two stages: 1) Starting from ambient temperature in steps of

6K/min until to reach 423K, remaining there for 1h; 2) Starting from 423K in steps of 10K/min until


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to reach the temperature of calcination and remaining there for 2h. A Thermolyne F46240CM electric

oven was used for all the thermal treatments.

The infrared spectra were obtained with a resolution of 4cm-1

in a FTIR Nicollet Magna 760

Spectrophotometer, using the KBr disc technique. The crystalline phases were determined by X-ray

diffraction (XRD) using a Rigaku Miniflex Diffractometer with Ni-filtered Cu Kradiation, with a

scanning speed of 20/min.

Table 1. Grinding Conditions

SiO2 Precursor Activation Medium Grinding Time

(h)

M/C*

Ratio

Sample

Dehydrated Silicic

Acid

Absolute Ethyl Alcohol

2 1/20 D0

18 1/20, 1/40 D1, D248 1/20, 1/40,

1/80

D3, D4,

D5

Silica Gel

Absolute Ethyl Alcohol

2 1/20 aD0

48 1/40, 1/80 aD4, aD5

Mixture of Absolute Ethyl Alcohol

and Water

2 1/20 bD0

48 1/40, 1/80 bD4, bD5

Water

2 1/20 cD0

48 1/40, 1/80 cD4, cD5

M/C* = powder mass / zirconia cylinders mass

RESULTS AND DISCUSSION

Dehydrated and calcined silicic acid as SiO2 precursor

Table 2 shows the results of DRX and FTIR for the samples D0 to D5 heated to different

temperatures. The XRD results for the samples heated at 394 K and for the samples calcinated at

1223 K (Table 2 and Figure 1) show a progressive decrease in the relative intensity of the peak at

2 = 42.900. This can be associated with a progressive amorphization of the periclase (MgO) [13],

that takes place as a consequence of the increase on the grinding time and of the reduction in the M/C

ratio.

For the samples heated at 1223K (Table 2 and Figure 1), the predominant crystalline phase observed

was -Al2O3 [14]. In addition, different amounts of periclase (MgO) were also observed. For the

sample D0 the amount of periclase was greater. For all the samples there is also the formation of a

small amount of Mg and Al spinel (MgAl2O4), as indicated by the presence of DRX peaks in 2 =

36.800

and 44.800

[15]. However, with exception for the sample D0, bands were observed in IR

spectra (at 952cm-1

and 894cm-1

, region of Si-O stretching [16, 17]) as well peaks in the DRX (in 2


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= 22.900, 32.30

0, 35.70

0and 39.80

0[18]) that indicate the formation of a small amount of forsterite

(Mg2SiO4). In addition, for samples D3, D4 and D5, enstatite (MgSiO3) appears as a minor

crystalline phase, this fact being confirmed by IR bands at 1013cm-1

and 860cm-1

, and by DRX peaks

in 2 = 28.130

and 31.400

[19, 20]. The enstatite occurs in a higher amount in the sample D5, this

fact points out the importance of the degree of mechanochemical activation on the enstatite

formation.

Table 2. Observed phases on samples using dehydrated silicic acid as SiO2 precursor.

Temperature Sample Observed Phases

393K D0 to D5 -Al2O3, SiO2, MgO

1223K

D0 -Al2O3, SiO2, MgO, MgAl2O4

D1, D2 -Al2O3, SiO2, MgO, MgAl2O4, Mg2SiO4

D3, D4, D5 -Al2O3, SiO2, MgO, MgAl2O4, Mg2SiO4, MgSiO3

1523K

D0, D1, D2 -Al2O3, -SiO2, MgAl2O4

D3, D4,D5 -Al2O3, -SiO2, MgAl2O4, Mg2Al4Si5O18

1623K D0 to D5 -Al2O3, -SiO2, MgAl2O4, Mg2Al4Si5O18

SiO2 = amorphous silica (observed by IR), -SiO2 =-cristobalite

F ig 1 . X - ra y p o w d er d if fra c ti on p a tt er n s o f s a m p le s A ) D 0 , B ) D 3 an d C ) D 5

ca lc ined a t 1223K .c o ru n du m p e ric la se fo rs te rite e n sta tite s pin el

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

A

2

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 01 0 0 0

1 2 0 0

B

1 0 2 0 3 0 4 0 5 0 6 0

02 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

C

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

A

2

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 01 0 0 0

1 2 0 0

B

1 0 2 0 3 0 4 0 5 0 6 0

02 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

C

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

A

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 01 0 0 0

1 2 0 0

B

1 0 2 0 3 0 4 0 5 0 6 0

02 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

A

1 0 2 0 3 0 4 0 5 0 6 00

2 0 04 0 06 0 08 0 01 0 0 0

1 2 0 0

B

1 0 2 0 3 0 4 0 5 0 6 0

02 0 04 0 06 0 08 0 0

1 0 0 01 2 0 0

C


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Table 2 and Figures 2 and 3 show that spinel (MgAl2O4) [15,21] and -cristobalite [22-24] are the

predominant phases in the samples calcined at 1523K, with exception of the sample D0 where a

great amount of-Al2O3 is still observed. However, a small amount of cordierite (Mg2Al4Si5O18) was

formed in the samples D3, D4 and D5, as indicated by IR bands at 1179cm-1 (asymmetrical

stretching of tetrahedral Si-O), 960cm-1

(stretching of Al-O bonding) and 770cm-1

(symmetrical

stretching of Si-O bonding), and by a DRX peak in 2 = 28.490 [17, 25-27]. As can be seen in Figures

2 and 3 the amount of cordierite increases with the increasing in the mecanochemical activation, and

therefore it is maximum for the sample D5. This corresponds to the case where the amount of

enstatite is the highest for the samples heated at 1223K. This fact suggests a correspondence between

the presence of enstatite, on the samples heated at 1223K, and the cordierite formation at 1523K.

In all samples calcined at 1623K the formation of cordierite (Mg2Al4Si5O18) ocurred. As the grinding

time increased and the M/C rate decrease, the amount of cordierite increased with a simultaneous

diminishing of cristobalite (-SiO2). Therefore, the amount of cordierite formed on sample D0 is the

lower one according to data obtained from DRX and FTIR. These results indicated that calcined and

dehydrated silicic acid is not an adequate SiO2 precursor for the synthesis of cordierite at 1523K, and

hence another precursor was studied.

Fig 2. X-ray podwer diffraction patterns of samplesA) D0, B) D3 and C) D5 calcined at 1523K.

-corundum cristobalite spinel cordierite

Fig 3. IR spectra of samples A) D0, B) D3 and C) D5calcined at 1523K.

-corundum cristobalite spinel cordierite

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0.000.020.040.060.080.100.120.140.16

cm-1

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Silica gel as SiO2 precursor

On this stage a variation of the activation medium was also introduced, as well a different SiO2

precursor. The options used were: absolute ethyl alcohol (samples aD), a mixture of distilled and

deionized water 10% v/v in absolute ethyl alcohol (samples bD), or distilled and deionized pure

water (samples cD). The results of DRX and FTIR are resumed on Table 3 for the samples aD0

tocD5, heated at different temperatures.

Table 3. Observed Phases on samples using silica-gel as SiO2 precursor.

Temperature Sample Observed Phases Temperature Sample Observed Phases

393K

aD0, aD4

aD5

SiO2, -Al2O3,

MgO

1423K

aD4, bD4,cD4, aD5,

bD5, cD5

-SiO2, -Al2O3,

MgSiO3, MgAl2O4

bD0, cD0,bD4, bD5

SiO2, -Al2O3,MgO, Mg(OH)2

cD4 SiO2, -Al2O3,

Mg(OH)2

cD5 SiO2, -Al2O3,

Mg(OH)2 *

1223K

aD0, bD0 SiO2, -Al2O3,

MgO, Mg2SiO4

1523K

a

D0,bD0,

cD0

-SiO2, -Al2O3,

MgAl2O4,

Mg2Al4Si5O18

cD0 SiO2, -Al2O3,

MgO, Mg2SiO4,

MgSiO3

aD4, bD4,

cD4, aD5

-SiO2, -Al2O3,

MgSiO3, MgAl2O4,

Mg2Al4Si5O18

aD4,aD5

SiO2, -Al2O3,

Mg2SiO4, MgSiO3

bD4, cD4,

bD5, cD5

SiO2, -Al2O3,

MgSiO3

bD5 -SiO2, MgAl2O4,

Mg2Al4Si5O18

cD5 MgAl2O4,

Mg2Al4Si5O18

SiO2 = amorphous silica, * = amorphous Mg(OH)2 (observed by IR), -SiO2 = -cristobalite

For all samples that suffered activation in the presence of water (bD and cD), the presence of an IR

band at 3700cm-1

was observed and attributed to the O-H stretching of Mg(OH)2 [28]; meanwhile, a

DRX peak in 2 = 18,590

[29] was observed for some samples (Figure 4 and Figure 5). These data

indicated the formation of the magnesium hydroxide - Mg(OH)2 - during the grinding process (Table

3). On samples cD4 and cD5, as it is showed on Figure 4, all MgO reacted since the DRX peak at

2 = 42,900

was not observed [13]. For the sample cD5 (Figure 4F) the DRX peak at 2 = 18,590

[29] was not observed, what indicated that in this sample the Mg(OH) 2 produced is amorphous or

reacted with the SiO2 , forming an amourphous magnesium silicate as observed by MacKenzie for the

MgO-SiO2 system[29].


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From Figure 6 and Table 3 we can conclude that there was no spinel formation for the samples

calcined at 1223K, as it was observed for the D samples calcined at same temperature. However,

when compared with D5 sample (Figure 1) these D samples contain a higher amount of enstatite.

The analysis of X-ray diffraction patterns and IR spectra also indicated that the amount of enstatite

formed in the sample aD5 was greater than that formed in the sample aD4, and that the amount of

forsterite was greater in aD4 than in aD5. In addition, the absence of DRX peaks at 2 = 22.900,

32.300, 35.70

0and 39.80

0demonstrated that there was no formation of forsterite [19] in those samples

that suffered activation in presence of water (bD4, bD5, cD4 and cD5). Therefore, it can be

affirmed that a decrease in the M/C ratio and an increase in the amount of water in the activation

medium result in an increase in the amount of enstatite formed, as well a inhibition of the forsterite

formation.

Fig6. X-ray podwer diffraction patterns of samples A) aD4, B) bD4, C) cD4, D)aD5, E) bD5,

F) cD5 calcined at 1223K.

-corundum enstatite forsterite

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3800 3700 3600 3500 3400 3300 3200

-0,02-0,010,000,010,020,030,040,050,060,070,080,090,100,110,120,130,140,15

CM-1

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B

C

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-0.03-0.02-0.010.000.010.020.030.040.05

CM-1

D

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-0,02-0,010,000,010,020,030,040,050,060,070,080,090,100,110,120,130,140,15

CM-1

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CM-1

D

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0.0

E

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0.00

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F

Fig 4. X-ray podwer diffraction patterns of samplesA) aD4, B) bD4, C) cD4, D)aD5, E) bD5, F) cD5dried at 393K .

-corundum periclase magnesium hidroxide

Fig 5. IR spectra of samples A) aD4, B) bD4,

C) cD4, D)aD5, E) bD5, F) cD5 dried at 393K .

magnesium hidroxide


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The above results suggest that the utilization of water as activation medium allowed a higher

degree of mechanochemical activation of silica gel and MgO, resulting in increasing of reaction

between the silica gel and the magnesium precursor at 1223K. The final result is the formation of

enstatite and the inhibition of spinel formation, since all the magnesium precursor probably reacted

with silica gel.

As can be seen in the Figures 7 and 8 (X-ray diffraction patterns and IR spectra), when these samples

are calcined at 1523K some DRX peaks appear at 2 = 10.500

and 28.490

and IR bands are observed

at 1179cm-1

, 960cm-1

and 770cm-1

, confirming the formation of the cordierite [17,25-27]. In addition

to an increasing in the intensity of these peaks and bands, a gradual reduction of intensity of

cristobalite IR band at 1090cm-1

[22, 23] is observed which indicates that the silica consumption is

associated with the formation of cordierite. As the degree of mechanochemical activation and the

amount of water in the activation medium are increased we can also observe a gradual

disappearance of the DRX peaks related to enstatite and -Al2O3. Therefore, the formation of the

cordierite is also associated with the consumption of these phases. There is also the formation of a

MgAl2O4 phase in all the samples, since DRX peaks in 2 = 36.800, 44.80

0, 59.40

0[15] and an IR

band at 690cm-1

[21] were found. This phase is in a larger amount in the sample aD5.

Fig 7. X-ray podwer diffraction patterns of samples

A) aD0, B) aD5, C) bD5 and D)cD5 calcined at1523K .

-corundum cristobalite enstatite spinelcordierite

Fig 8. IR spectra of samples A) aD0, B) aD5, C)

bD5 and D)cD5 calcined at 1523K .

-corundum cristobalite enstatite spinelcordierite

1400 1200 1000 800 600-1

A

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C

D

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Figures 9 and 10 present X-ray diffraction patterns and IR spectra that illustrate the evolution of the

formation of phases with the temperature on sample cD5. For the sample, ground and dried at 393K,

the X-ray diffraction pattern (Figure 9A) indicates that -Al2O3 [14] is the only crystalline phase

presents. IR spectrum (Figure 10A) indicates the presence of -Al2O3 by bands at 640cm-1

and

590cm-1

, and of amorphous SiO2, due to the presence of a characteristic band around 1110cm-1

. The

amorphous magnesium hydroxide - Mg(OH)2, is indicated by the presence of IR band around

3700cm-1

(Figure 5F). There are also a series of shoulders that in principle can be associated to other

amorphous phase, as for example the amorphous magnesium silicate observed by MacKenzie for the

MgO-SiO2 system [29]. When heated at 1223K, an expressive amount of enstatite is formed (Figure

9B). An IR band related to the amorphous SiO2 is observed around 1110cm-1

, in addition to the -

Al2O3 referring bands (Figure 10B). For the sample calcined at 1423K, the DRX peaks and IR bands

found are an indication of the presence of -cristobalite [22-24], -Al2O3 [14,16], enstatite [19,20]

and a small amount of MgAl2O4 [15,21]. For the sample heated at 1523K, the almost disappearance

of the IR band at 1090cm-1

is observed [22, 23], what indicates a pronounced consumption of SiO2

(Figure 10D). The main peaks observed in the X-ray diffraction pattern of this sample, are those

referring to cordierite [27], the main phase, and to the MgAl2O4 [21], the secondary phase. For the

sample calcined at 1623K, there are no significant alterations in the X-ray diffraction patterns and in

the IR spectrum with respect to the sample heated at 1523K. Therefore, this sample is composed

basically by the cordierite, the main phase, and by the secondary phase MgAl2O4.

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1 4 00 12 00 10 0 0 8 0 0 6 0 0

0 .4

0 .6A

c m-1

1 4 00 12 00 10 0 0 8 0 0 6 0 0

0. 240. 260. 280. 300. 32

B

1 4 00 12 00 10 0 0 8 0 0 6 0 0

0. 14

0. 16

C

1 4 00 12 00 10 0 0 8 0 0 6 0 0

0. 00

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1 4 00 12 00 10 0 0 8 0 0 6 0 0

0. 1350. 1400. 1450. 1500. 1550. 1600. 165

E

1 4 00 12 00 10 0 0 8 0 0 6 0 0

0 .4

0 .6A

c m-1

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0. 240. 260. 280. 300. 32

B

1 4 00 12 00 10 0 0 8 0 0 6 0 0

0. 14

0. 16

C

1 4 00 12 00 10 0 0 8 0 0 6 0 0

0. 00

D

1 4 00 12 00 10 0 0 8 0 0 6 0 0

0. 1350. 1400. 1450. 1500. 1550. 1600. 165

E

Fig10. IR spectra of sample cD5 A)grounded and

dried at 393K, B) 1223K, C) 1423K, D) 1523K andE) 1623K.

-corundum SiO2 spinel enstatite cordierite

Fig9. X-ray powder diffraction patterns of sample cD5

A) grounded and dried at 393K, B) 1223K, C) 1423K, D)1523K and E) 1623K.

-corundum SiO2 spinel enstatite cordierite


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From these results it is evident that the formation of enstatite at 1223K has a major influence on the

temperature and reaction pathway of cordierite formation. It is also clear that the degree of

mechanochemical activation and the amount of water in the activation medium are the most

important factors to the formation of enstatite. Similar results were reported in the literature when the

synthesis of cordierite was made in the presence of additives such as fluorite [11] or when the

mecanochemical activation was made in an attrition mill at 4500rpm and clay minerals were used as

starting materials [30]. However, in these papers [11,30] it was not clear the importance of enstatite in

the cordierite formation process.

With respect to the influence of spinel phase on the cordierite formation process, some authors

proposed that cordierite would be formed from the reaction between spinel and a silica precursor at

around 1600K [9]. Our results show that for the samples with high degree of mechanochemical

activation the spinel phase do not participate on the cordierite formation process.

CONCLUSION

From the results obtained by DRX and IR spectroscopy, we concluded that:

i. Conditions of grinding with t = 18h or M/C higher than 1/40 do not present good results respecting

to the formation of cordierite (Mg2Al4Si5O18);

ii. For the objective of cordierite synthesis, silica gel as SiO2 precursor works better than dehydrated

and calcined silicic acid.

iii. The formation of Mg(OH)2 during the mechanochemical activation in the samples where water

was used as the activation medium provides the formation of enstatite (MgSiO3);

iv. In the samples with high degree of mechanochemical activation, the formation of the cordierite at

1523K can be describe by the following reaction:

2MgSiO3 + 2Al2O3 + 3SiO2 Mg2Al4Si5O18

v. The mechanochemical activation, using water as activation medium, allows a great amount of

cordierite to be formed at 1523K.

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Mecanochemistry_enstatite_spinel_cordierite