Activation-deactivation mechanisms in spodumene samples

Solid State Ionics 42 (1990) 1-6 North-Holland

ACTIVATION-DEACTIVATION MECHANISMS IN SPODUMENE SAMPLES H.J. G A S A L L A ~ and E. P E R E I R A Centro de Tecnologia de Recursos Minerales y Cerftmica, (CETMIC)(CIC), Provincia de Buenos Aires, Casilla de Correo 49, 1897 Manuel B. Gonnet, Argentina

Received 18 August 1989; accepted or publication 20 January 1990

Thermal transformation phases of mechanically treated a-spodumene (constitutional oxide composition LizO, A1203and SiO2 is 1: 1: 4) and, the residue of mechanically treated ct-spodumene leached with sulfuric acid ( l : 1: 10) are studied and compared with thermal transformation of vitreous spodumene (composition 1: l : 4) and with a vitreous phase of spodumene together with SiOz (composition l : l : l 0). All samples show 7-spodumene formation before the formation of the [3-form. The mechanism of activation-deactivation in those samples is explained. DTA and XRD techniques were employed.

1. Introduction

The control o f reactivity o f solids has been studied in the last few years both by fractomechanics [ 1,2 ] and by m e c h a n o c h e m i s t r y [ 3 ]. The p r i m a r y p h e n o m e n o n o f application o f mechanical stress leads to the solid activation a n d generates a metastable ( a m o r p h o u s ) state, whereas the deactivation corresponds to the secondary effect o f application o f that mechanical stress, that is, release o f the solid stress. This deactivation is achieved either by acid or thermal t r e a t m e n t [4]. The degree o f crystalline ordering i n d u c e d by application o f mechanical stress ( a m o r p h o u s state) is a d e t e r m i n i n g factor in solid reactivity control. A vitrified material shows only short-range ordering [ 5 ], and so does an a m o r p h o u s material o b t a i n e d by application o f mechanical stress [4]. Both are therm o d y n a m i c a l l y metastable although the former is energetically m o r e stable than the latter. It is obvious that the a c t i v a t i o n - d e a c t i v a t i o n m e c h a n i s m s involved are not the same in both cases. The purpose o f the present work is to d e t e r m i n e the differences a n d similarities o f those m e c h a n i s m s in samples o f mechanically treated s p o d u m e n e and in Present address: Laboratorio de Tecnologia Quimica, Facultad de Ciencias Exactas, 9 de julio 1449, 3400 Corrientes, Argentina. Elsevier Science Publishers B.V. (North-Holland)

vitrified spodumene. Research on that subject was carried out by Berger et al. [ 6 - 1 0 ] , who c o m p a r e d spodumene samples mechanically activated in air and water m e d i a and their thermal deactivations. The new evidence contributed by the present work is in agreement with the results o b t a i n e d by Berger.

2. Experimental

The s p o d u m e n e used is a pyroxene belonging to the monoclinic crystallographic system, whose oxide c o m p o s i t i o n is Li20"AI203.4SiO2 [11]. Sample M~ corresponds to mechanical treatment o f the s p o d u m e n e that has a 1 : 1 : 4 oxide composition, milled for 20 min at 20 s intervals in a H E R Z O G H S M 100 oscillating mill [ 11,12]. Sample M 2 is M l treated with H2SO4 3 N at 170°C for 100 min, which removes the Li20 and A1203 generating a SiO2-rich material having an oxide ratio o f 1:1:10. Sample V~ corresponds to a glass-like s p o d u m e n e having a 1 : 1 : 4 c o m p o s i t i o n o b t a i n e d by melting s p o d u m e n e at 1400°C for 30 min and further quenching at r o o m temperature. Sample V2 is a glass-like s p o d u m e n e having a 1 : 1 : 1 0 c o m p o s i t i o n o b t a i n e d by melting spodumene a n d SiO2 at 1400°C for 30 min and further quenching at r o o m temperature.

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H.J. Gasalla, E. Pereira ~Activation-deactivation in spodumene samples

DTA was performed on all samples in a NETZSCH equipment using 150 mg samples, Gt-AI203 as standard and a 10°C/min heating rate. X-ray diffractograms were obtained in a PHILIPS PW 1140/00 difractometer with Cu Kot radiation ( 2 = 1.542 A) at 40 kV and 20 mA. A thermal treatment sequence was performed for 1 g samples of M~, M2, V~ and V2, for one hour between 800 and 1050°C at 50°C intervals. Quantitative XRD analyses were performed by measuring the peak area corresponding to planes (100) or (200) and (112) for 7-spodumene and to planes (102) and (211 ) for ~-spodumene of samples M~ and V~ and planes (112) and (211 ) of samples M2 and V>

3. Discussion

Fig. 1 shows DTA obtained for each sample and for the original a-spodumene. DTA traces on mechanical activation of the original ct-spodumene (sample M~) shows an exothermal peak at 820°C due to formation of y-spodumene and an endothermal peak at 1000°C due to formation of [3-spodumene [ 8-12 ]. XRD diffractograms (fig. 2) obtained from thermal treatment of 1 g of M t for an hour show 7-spo-

V÷y y K~ M2 A

M1 i l c~

"V 700 I

800 I

900 l

1000 I

1100 I

1200~:(/

Fig. 1. DTA of orignal c~-spodumene and of sample Ml, M2, V I and V2.

dumene formation at 950°C and 13-spodumene formation at 1100 ° C. Formation of 7-spodumene, which does not occur by thermal treatment of natural a-spodumene, shows deactivation of the material that had previously been mechanically activated. This aspect was dealt with in earlier reports [ 6-12 ]. Sample Me shows endothermal peaks at 960°C and 1030°C (see fig. 1 ). The endothermal peak is absent due to deactivation of the amorphous material, which is eliminated by the acid treatment [ 13 ]. However, fig. 3 shows diffractograms obtained at 950°C, which show 7-spodumene formation indicating that there still are residual stresses in the material that favour stabilization of that crystalline structure. The material still showing activation due to application of mechanical stress is deactivated by 7-spodumene stabilization and shifts the DTA peak to 960°C, also modifying its enthalpy. This explains the fact that 7spodumene is generated as a direct consequence of the degree of crystalline ordering (higher degree of ordering due to elimination of the most amorphisized material by acid treatment) and of the type of structural metastability (higher stability). Sample V~ (vitreous spodumene) shows a lower degree of crystalline ordering than samples M~ and ME. The DTA presents two exothermal peaks: one at 890°C and another at 920°C. The XRD of V~ (fig. 4) evidences formation of 7-spodumene and then, of [3-spodumene. It is apparent that the mechanism of this transformation differs to a great extent from that of M~ and M2. Since the vitreous state is energetically more stable than that generated by mechanical treatment, 7-spodumene formation involves thermal activation of the glass and further deactivation to stabilize the 7-form. This implies a direct mechanism of stress generation and a rapid deactivation, so that the DTA peak for 7-spodumene formation is intermediate between those of M~ and M2 and the enthalpic content is particular to that transformation. The fact that 13-spodumene formation occurs at lower temperatures and near the 7-spodumene peak, and, that its enthalpic variation is exothermal (contrait to those of M~ and M2) suggests a displacive transformation from 7~fl. DTA for sample V2 does not allow an interpretation of the type already offered. However, there is also 7-spodumene formation (fig. 5).

H.J. Gasalla, E. Pereira ~Activation-deactivation in spodumene samples

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H.J. Gasalla, E. Pereira ~Activation-deactivation in spodumene samples

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Fig. 5. XRD of Vz, V2 at 950°C and V 2 at 1100°C. Analysis of sample V~ also shows the importance of the degree of crystalline ordering in the activation-deactivation mechanism and of the type of structural metastability. Figs 2-4 and 5 show only those XRD diffractograms in which y- and ILspodumene are more in evidence. Because the experimental conditions of those diffractograms are so different from the ones used in DTA (heating rate, sample weight), the temperature of higher formation of y-spodumene (9 50 ° C) is not

coincident with that from DTA. However, the purpose of those figures is to show that, in the temperature treatment sequence, y-spodumene is generated first and ILspodumene, afterwards. Table 1 lists y- and ~-spodumene unit cell lattice parameters obtained from each of the samples treated. It also gives unit cell volumes. Unit cell volumes both for y- and [~-spodumene of M1 and V~ show a certain similarity but are somewhat higher for M]. Conversely, unit cell volumes

H.J. Gasalla. E. Pereira ~Activation-deactivation in spodumene samples

5

Table 1 Unit cell lattice parameters of'/- and I]-spodumenefrom samples M~, M2, Vn and V2. Spodumene

Crystallographic system

a 7

monoclinic hexagonal

~l

tetragonal

Sample

Lattice parameters

M~ M2 V~ V2

a= 9.52 a= 5.30 a=5.20 a=5.24 a=5.21

M~ M2 V~ V2

a = 7.56 a=7.49 a=7.56 a=7.49

b= 8.32

Volume c= 5.25 c= 5.41 c=5.45 c=5.52 c--5.43

387.3 394.8 382.9 393.8 382.9

c= 9.23 c=9.23 c=9.20 c=9.23

527.5 517.8 525.8 517.8

Planes determined: 7-spodumene (100) or (200) and ( 112); I~-spodumene(102) and (211 ) for Mz and V~, (112) and (211 ) for M2 and V2. for y- and 15-spodumene of M2 and V2, in which there is an increase of SiO2, are similar or the same, but they are quite lower than those of M~ a n d V1. The fact that 7- a n d 13-spodumenes of M I show higher cell volumes than those of V~ evidences that M t m a i n t a i n s crystalline defects to a higher degree than when in the vitreous state and, therefore, in spite of the fact that V1 has a lower degree of ordering, the ordering is more "compact". O n the other hand, the 7-and 13-spodumenes of M2 and V2, having lower cell volumes, show that the excess SiO2 plays an i m p o r t a n t role in the polymorphic transformation [14]. The temperature sequence in 7-spodumene formation, namely M~-V~-M2, is similar to that of unit cell volumes (394.8-393.8-382.9). The same conclusion has been reached for 13-spodumene (527.5-525.8-517.8). To a certain extent, this research allows us to correlate the p h e n o m e n o n of degree of crystalline ordering and structural metastability with that of the solid activation-deactivation.

4. Conclusions Crystalline 7- and 13-spodumenes obtained by thermal treatment of an a - s p o d u m e n e having a structurally disordered state show differences in their lattice parameters, mainly as regards unit cell volume. These differences have been attributed to the previous history of the samples, which generated the disordered state (mechanical treatment, mechanical

treatment a n d further acid treatment, and vitrification ). Mechanical treatment of et-spodumene causes its activation. Its deactivation by either thermal or acid treatment and further thermal treatment stabilizes 7spodumene. The acting stress relaxation mechanism is indirect and depends on the degree of crystalline ordering and on the type of structural metastability. Melting of ct-spodumene generates a vitreous material that, through thermal treatment, produces both activation and deactivation by 7-spodumene stabilization. The acting stress relaxation mechanism is direct and also depends on the degree of ordering and on the type of structural metastability.

References [ 1] B.R. Lawn, J. Am. Ceram. Soc. 66 (1983) 83. [2] A.G. Evans and K.J. Faber, J. Am. Ceram. Soc. 67 (1984) 255. [ 3 ] A.P. Chupakhim,A.A.Sidel'nikovand V.V. Boldyrev,React. Solids 4 (1987) 163. [4] V.V. Boldyrev,Termochim. Acta 110 (1987) 303. [ 5 ] A.R. West, Solid state chemistryand its applications (Wiley, New York, 1984). [6]A.S. Berger, L.T. Menzheres, N.P. Kotsupalo, T.I. Samsonova, M.I. Tatarintseva, I.A. Poroshina and V.V. Boldyrev, lzv. Otd. Akad. Nauk, SSSR 1 ( 1981 ) 67. [7]A.S. Berger, L.T. Menzheres, N.P. Kotsupalo, T.I. Samsonova, M.I. Tatarintseva, I.A. Poroshima and V.V. Boldyrev, lzv. Otd. Akad. Nauk, SSSR 1 ( 1981 ) 74. [8]A.S. Berger, V.V. Boldyrev, H.P. Kotsupalo and L.T. Menzheres, Dokl. Akad. Nauk, SSSR 257 (4) ( 1981 ) 888.

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H.J. Gasalla, E. Pereira ~Activation-deactivation in spodumene samples

[9] A.S. Berger, L.T. Menzheres, N.P. Kotsupalo and V.V. Boldyrev, Izv. Sib. Otd. Akad. Nauk, SSSR 5 (1983 ) 9 I. [10] A.S. Berger, L.T. Menzheres, N.P. Kotsupalo and V.V. Boldyrev, 10th Intern. Symp. React. of Solids (Dijon, 1984) p. 331. [ 11 ] H.J. Gasalla, Estudio de efectos secundarios por aplicaci6n de esfuerzos mec~inicos en variedades polim6rficas de espodumeno, Tesis (La Plata, Argentina, 1988 ).

[ 12] H.J. Gasalla, E.F. Aglietti, J.M. Porto Lopez and E. Pereira, Mat. Chem. Phys. 17 (1987) 379. [ 13 ] H.J. Gasalla and E. Pereira, Latinoamerican Appl. Res., to be published. [ 14] Ch.T. Li, Z. Kristollogr. 132 (1970) 118.