Temperature influence on structural changes of foundry bentonites

Journal of Molecular Structure 1004 (2011) 102–108

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Temperature influence on structural changes of foundry bentonites ⇑ _ Mariusz Holtzer, Artur Bobrowski, Sylwia Zymankowska-Kumon AGH University of Science and Technology, Faculty of Foundry Engineering, 30-059 Cracow, Reymonta 23 Str., Poland

a r t i c l e

i n f o

Article history: Received 11 April 2011 Received in revised form 19 July 2011 Accepted 20 July 2011 Available online 10 August 2011 Keywords: Bentonite Foundry FTIR Thermal analysis Determination of montmorillonite Structure of montmorillonite

a b s t r a c t The results of investigations of three calcium bentonites, activated by sodium carbonate, applied in the foundry industry as binding material for moulding sands, subjected to the influence of high temperatures – are presented in the paper. Investigations were performed by the thermal analysis (TG) method, the infrared spectroscopy (FTIR) method and the modern Cu(II)–TET complex method (used for the determination of the montmorillonite content in bentonite samples). The occurrence of the dehydration process and two-stage dehydroxylation process was confirmed only for bentonite no. 2. This probably indicates that cis- and trans-isomers are present in the octahedric bentonite structure. Tests were performed at temperatures: 500, 550, 700, 900, 1000, 1100, 1200 °C. Ó 2011 Published by Elsevier B.V.

1. Introduction Bentonite is one of the main binding materials used in a metal foundry practice for making moulds. In addition, bentonites are applied in other fields such as: drilling, food industry, crude oil processing. Moulding sands with bentonite constitute app. 70–80% of sands, in which castings from ferrous alloys are carried out. One of the reasons that bentonite is used as a binding material is the possibility of its multiple use. One of the most important parameters of foundry bentonites is their thermal resistance and strength at elevated temperatures [1–4]. A casting mould made of moulding sands with bentonite is poured with liquid metal alloy of a temperature approaching 1550 °C. The main component, which decides on the binding properties and thereby on the bentonite thermal resistance, is montmorillonite. The determination of the bentonite deactivation degree under the influence of high temperatures is very important in foundry practice, since it decides on the necessary rebounding of sands being in circulation. In practice, only a small part of sands in moulds – being in a direct contact with the casting – is endangered by the temperature causing the complete bentonite deactivation. Montmorillonite belongs to the mineral group called smectites. These are laminar hydroxyaluminosilicates of aluminium, iron and magnesium, which are marked by packets 2:1 (two tetrahedrite silica-oxygen layers, and the octahedrite metal–oxygen layer between them) [3,5]. Exchangeable cations: Na+, K+, Ca2+, NH4+ and others, as well as water particles are between the packets. A dis⇑ Corresponding author. _ E-mail address: [email protected] (S. Zymankowska-Kumon). 0022-2860/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.molstruc.2011.07.040

tance between packets in the montmorillonite structure depends on the exchangeable cation and increases along with a water saturation. Water stored in interpacket spaces causes montmorillonite swelling. Due to this, bentonites obtain colloidal and mechanical properties, allowing to make casting moulds. The bentonite ability to cations exchanges is being used for a sodium activation of calcium bentonites for foundry needs. A calcium bentonite activation is performed by means of sodium carbonate according to the following reaction:

Na2 CO3 þ Ca2þ ! 2Naþ þ CaCO3 The effect of this reaction is a significant increase of water absorption and an improvement of mechanical properties by the so-called activated bentonite. Calcium carbonate presence is characteristic for the activated bentonite. Under an influence of a temperature montmorillonite undergoes various transformations, which are the reasons of the loss of its binding properties. Thus, the following processes can occur [6]:  dehydration,  dehydroxylation,  decomposition (transformation of a crystal structure into an amorphous form),  recrystallization (once more formation of a crystal structure). The temperature range of these transformations depends on the chemical composition of montmorillonite. E.g. at a higher iron content they dehydrate at lower temperatures. The first two processes are of an endothermic character and correspond to a mass loss,

M. Holtzer et al. / Journal of Molecular Structure 1004 (2011) 102–108

while the third process, being also endothermic does not cause a mass loss. A recrystallization process is an exothermic one. In the case of the activated bentonite an effect related to the CaCO3 decomposition can occur. During dehydration adsorbed water from outer surface and interlayer water from inner surface and hydration shells of interlayer cations desorbs in the endothermic reaction between room temperature and 300 °C [7]. The dioctahedral character of the montmorillonites leeds to vacancies in the octahedral sheets. Only two of the three octahedral positions are occupied. The vacancies are of cis- or trans-vacant character. A cis- or trans-position is defined by the position of hydroxyl groups (Fig. 1) [8]. At the trans-octahedron the two hydroxyl groups are positioned at opposite corners. The cis-position is defined by two hydroxyl groups at one edge. Drits et al. [9] showed that the dehydroxylation behaviour of aluminous dioctahedral 2:1 layer silicates is related to the distribution of the metal ions and vacancies in the octahedral sheet. They demonstrated that the dehydroxylation peak temperature obtain by thermal analysis can be used to characterize the structure of the octahedral sheet. Cis-vacant (cv) minerals dehydroxylate at about 650–700 °C and trans-vacant (tv) varieties dehydroxylate at about 500–550 °C. The boundary between cis-vacant and trans-vacant was found at about 600 °C [10]. On the bases of data concerning the thermal analysis, collected for many years, it was found that the hydroxyl group is the most often liberated from montmorillonite at a temperature of app. 650–700 °C [11]. Therefore montmorillonites, which undergo dehydroxylation at a temperature of 700 °C are called ‘‘normal’’ or ‘‘ideal’’ [12–14], while montmorillonites undergoing dehydroxylation at a temperature of 500 °C are called ‘‘abnormal’’. Some of them exhibit a double peak: the first with the maximum at about 550 °C and the second one usually at 700 °C. Different authors postulate different probable causes of this two-stage dehydroxylation [15]:  substitution in the tetrahedral and octahedral layers (the role of iron) [16],  defects in the montmorillonite structure [17],  differences in the mineral particle sizes [18]. According to Stoch [19] the montmorillonite originating from the weathering of volcanic ashes give a dehydroxylation peak at 700 °C, while those formed by weathering of other minerals (primarily micas) display an additional endothermic peak at 560– 600 °C. During dehydroxylation structural water from two hydroxyl groups per formula unit (half unit cell) migrates out of the mineral

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structure according to the reaction: 2(OH) ? H2O + Or (r, residual). This process is influenced by three different structural features [20]:  the nature of interlayer cation changes dehydroxylation temperature up to about 60 °C,  the nature of octahedral cations and their bonding strength affect the endothermic peak position; the dehydroxylation temperature increases as Fe–OH < Al-OH < Mg–OH [5];  trans- and cis-isomers fraction in a mineral structure. Decomposition and recrystallization reveal varying intensities and sometimes only one of the two reactions can be observed. Decomposition and recrystallization take place between 850 and 950 °C [8]. Grim and Bradley [21] describe the third endothermic effect (with a maximum usually at 900 °C), which is related to the breakdown of the crystal structure of the montmorillonite anhydride and to the formation of an amorphous product. Very often this third endothermic effect is not seen. However, reasons for its absence are still not clear. In contrast with previous endothermic effects, the third endothermic effect is not associated with a change in mass of the samples. Fajnor and Jesenak found first exothermic peak at about 1050 °C in the DTA curves [15]. This is caused by crystallization of the high temperature phases (from amorphized product), probably spinel [21]. The process depends on the chemical composition of the sample, including the type of exchangeable cations. The exothermic reaction is closely related to the iron content. Samples with low iron content show the exothermic reaction above 1000 °C, and hence it is not recorded unless the temperature is carried above 1000 °C [22]. Structural changes occurring in montmorillonite – under an influence of a temperature – trigger also changes in the spectrum obtained by the infrared spectrometry method. The thermal analysis and the FTIR spectroscopy were applied for the determination of bentonite structural changes. In order to assess how these structural changes influence binding properties of montmorillonite, its content in bentonites was additionally determined.

2. Materials applied in investigations Investigations were performed for three bentonites used in foundry practice, originated from the main European producers:  German bentonite, no. 1,  German bentonite, no. 2,  Polish bentonite, no. 3. The basic properties of these bentonites are given in Table 1. The chemical composition for all bentonites are collected in Table 2.

3. Investigation methods

Fig. 1. Cis and trans-isomers in the octahedric structure of bentonite.

The thermal analysis of bentonites was carried out by means of the derivatograph of the F. Paulik, J. Paulik, L. Erdey type, at a heating rate of 10 °C/min, in the air atmosphere. FTIR studies were carried out using Excalibur spectrometer equipped with a standard DTGS detector. All standards and technological samples were prepared using KBr technique. The spectra were collected with 4 cm1 resolution in transmission mode and were taken in the frequency region from 4000 cm1 to 400 cm1.

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Table 1 Characteristics of the investigated bentonites. Bentonite

pH

Conductance of Moisture content Loss on electrolytes (lS/cm) (110 °C) (%) ignition (%)

Bentonite no. 1 10.42 255 Bentonite no. 2 10.39 858 Bentonite no. 3 10.41 260

9.6 11.0 8.5

16.20 17.66 13.88

Table 2 Chemical composition of the investigated bentonites. Bentonite no. 1

Bentonite no. 2

Bentonite no. 3

18.02 3.00 0.002 3.60 2.32 3.49 0.11 0.31 0.068 48.75 0.024 0.33

16.00 1.63 0.010 2.11 0.80 4.73 0.075 2.65 0.034 56.65 0.018 0.11

% mass Al2O3 CaO Cr2O3 Fe2O3 K2O MgO MnO Na2O P2O5 SiO2 SO3 TiO2

16.23 3.19 0.006 6.80 0.52 3.18 0.036 2.63 0.15 49.85 0.23 0.70

The FTIR spectra were recorded for bentonite samples hold at temperatures: 500, 700, 900, 1000, 1100 and 1200 °C for 1 h. For the determination of the montmorillonite content in bentonites the spectrophtometric Cu(II)–triethylentetraamine (Cu–TET) complex method, developed by the authors [23,24] was applied. Bentonite samples were heated, respectively, to temperatures: 500, 600 and 700 °C and hold at them for a period of 0.5; 1 and 2 h to determine the optimal heating time (with a heating rate: 20 °C/min). Samples were cooled together with the furnace and then kept in the desiccator until testing. The German VDG Ref. Bentonite (Bavarian) containing 75% of montmorillonite was used as a reference at the montmorillonite content determinations. For technological researches such as condensation zone and compression strength moulding sands with bentonite (100% of quartz sand, 8% of bentonite, water up to compactibility of 45%) were prepared. Samples were made of moulding sands heated to temperatures: 300, 550 and 700 °C and hold at them for 1 h. 4. The obtained results and their discussion 4.1. Thermal analysis of bentonite The TG curves are presented in Fig. 2, while the DTG and DTA curves, for bentonite no. 2, in Fig. 3. Temperatures of dehydration and dehydroxylation together with the corresponding mass loss of bentonite samples are given in Table 3. The endothermic dehydration stage (at a temperature range: 155–169 °C) and endothermic dehydroxylation stages (for bentonite no. 2 – the first one at temperatures: 519 °C and the second at: 672 °C) for each sample were recorded. The highest dehydroxylation temperature was demonstrated by bentonite no. 1. The dehydration process was accompanied by the water loss in amounts from 5.74% for bentonite no. 3 to 7.38% for bentonite no. 2. In the dehydroxylation process bentonite no. 3 demonstrated also the smallest total mass loss, being equal to 5.94%. 4.2. Infrared spectrometric analysis (FTIR) Two wavelength ranges: 400–1800 cm1 and 3000–4000 cm1, characteristic for bentonites, can be singled out in the spectra obtained by the FTIR method.

Fig. 2. TG derivatographic curve for bentonite no. 2.

Fig. 3. DTG and DTA derivatographic curves for bentonite no. 2.

Table 3 Bentonite characteristics obtained from the DTA/DTG/TG analysis. Dehydroxylation Mass loss caused by temperature dehydroxylation (°C) (%)

Bentonite

Dehydration Mass loss temperature caused by dehydration (°C) (%)

I

II

I

II

Bentonite no. 1 Bentonite no. 2 Bentonite no. 3

155

6.54

–

697

–

7.07

169

7.38

519

672

4.01

2.9

156

5.74

–

693

–

5.94

The FTIR spectra in wavelength range 400–1800 cm1 obtained for bentonite no. 1 are presented in Fig. 4, for bentonite no. 2 in Fig. 5, while for bentonite no. 3 in Fig. 6. The identified characteristic peaks for all bentonites are collected in Table 4. In the spectrum of bentonite, which was heated to 550 °C, at the wave number 3620 cm1, the band which corresponds to stretching vibrations of the Al–OH and Si–OH bonds occurs. The broad band having maximum app. at 3450 cm1 corresponds to stretching vibrations of –OH from water contained within interpacket spaces (Ist stage of dehydroxylation), whereas deformative bending vibrations at 1640 cm1 – correspond to the –OH group in absorbed water. The band at 1135 cm1 indicates presence of the Si– O vibrations behind the plane. The strongest band at app. 1040 cm1 is attributed to the Si–O stretching vibrations in the plane, and the band at 529 cm1 is related to bending vibrations

M. Holtzer et al. / Journal of Molecular Structure 1004 (2011) 102–108

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Fig. 4. IR spectrum of bentonite no. 1 in the wave numbers range: 1800–400 cm1 before (a) and after heating to the following temperatures: (b) 500 °C, (c) 550 °C, (d) 700 °C, (e) 900 °C, (f) 1000 °C, (g) 1100 °C, (h) 1200 °C.

Fig. 5. IR spectrum of bentonite no. 2 in the wave numbers range: 1800–400 cm1 before (a) and after heating to the following temperatures: (b) 500 °C, (c) 550 °C, (d) 700 °C, (e) 900 °C, (f) 1000 °C, (g) 1100 °C, (h) 1200 °C.

Si–O, for Si–O–Al and Si–O–Si. The bands at 915 cm1, 875 cm1 and 844 cm1 originate from bending vibrations of Al–Al–OH, Al– Fe–OH and Al–Mg–OH. The band at the wave number 797 cm1 is characteristic for microcrystalline SiO2 – free silica and corresponds to symmetric stretching vibrations: O–Si–O. An appearance of absorption bands at: 640–680 cm1, indicates the presence of the Si–O–Si bonds in the tested sample [25]. An

absorption in the vicinity of the wave number 620 cm1 can indicate the presence of the Si–O–Al bond [26]. Bentonite hold at temperatures 500–550 °C is losing the adsorbed water both from the surface (dehydration) and from the interpacket spaces (Ist stage of dehydroxylation), which causes either a decreased intensity or a complete disappearance of bands at wave numbers of app. 3450 cm1 and 1640 cm1. Such changes

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Fig. 6. IR spectrum of bentonite no. 3 in the wave numbers range: 1800–400 cm1 before (a) and after heating to the following temperatures: (b) 500 °C, (c) 550 °C, (d) 700 °C, (e) 900 °C, (f) 1000 °C, (g) 1100 °C, (h) 1200 °C.

are visible in each investigated bentonite. Vibrations originated from OH group, at the wave number app. 3630 cm1 are still visible. A further temperature increase causes numerous changes in bentonites. At a temperature of 700 °C the band at a wave number of 3630 cm1 (OH group) decays. This is related to the total (IInd stage) dehydroxylation of bentonite, i.e. the interpacket water loss. The activated bentonite exhibits a broad band in the range of wave numbers: 1460–1480 cm1, which are related to the carbonate ions (CO3 62) presence. The Si–O band at the wave number 1114 cm1 is shifted to 1140 cm1 with a simultaneous intensity increase. At a temperature of 700 °C the band characteristic for carbonate ions decays. The band 915 cm1 originated from the Al–Al–OH bond – is not seen any more in the spectrum of bentonite heated to this temperature. At a temperature of 500 °C the band in the vicinity of 843 cm1 (Al–Mg–OH) disappears also. Instead the band at the wave number 738 cm1 appears, which can be related to the polymorphic change of quartz occurring at a temperature of 573 °C (low-temperature b-quartz turns into hightemperature a-quartz). It can be assumed that shifting of the band from the wave number 520 cm1 to 573 cm1 at a temperature of 700 °C is related to dehydroxylation. Investigations performed for bentonites indicated that changes in absorption bands at 830, 750 and 530 cm1, are attributed to the Si–O–Al bonds. These changes are related to disturbances of the silicate structure and seems to be characteristic for certain minerals [27]. As it results from Figs. 4–6, each temperature increase results in changing of bandwidth of the main Si–O band at the wave number app. 1040 cm1. At a temperature of 900 °C structural changes occur in bentonite samples. The intensity maximum of the band 1040 cm1 corresponding to the Si–O vibrations shifts towards a higher wave number (1095 cm1) and its character indicates that it is more complex. These changes can indicate that the successive polymorphic quartz transformation occurs, (since at a temperature above 870 °C a high-temperature quartz turns into trydymite), but also can point out the degradation of the bentonite structure (montmo-

rillonite) of laminar silicate into simple silicate and then into an amorphous form. At the wave number of 680 cm1 a new band appears and a further temperature increase from 900 to 1200 °C increases its intensity (probably a new crystal structure is being formed). 4.3. Montmorillonite content analysis Structural changes occurring in montmorillonite under the influence of the temperature caused losing of bentonite binding properties. The obtained results of the montmorillonite content in investigated bentonites are presented in Figs. 7–9. Measurements performed after various times of holding bentonite samples at the given temperature indicated, that after 1 h none further changes in the montmorillonite content occurred. After holding at a temperature of 500 °C the montmorillonite content in all investigated bentonites was not decreasing. Differences between bentonites occurred after heating them to a temperature of 600 °C. The largest decrease of the montmorillonite content demonstrated bentonite no. 2 (above 50%), while the most thermal resistant was bentonite no. 1. After 1 h of holding at a temperature of 700 °C the montmorillonite content practically reached zero in all bentonites, and they were losing their binding properties. However, after shorter time (0.5 h) of holding at this temperature bentonites no. 1 and 3 still contained significant amounts of montmorillonite, 55% and 32% respectively. Table 5 contains the results of some technological properties of moulding sand with bentonites. 5. Conclusions Performed broad investigations of the temperature influence on properties and structure of bentonites used in foundry practice as moulding sands binding agents confirmed the occurrence of the endothermic dehydration process (at temperatures: 155–169 °C), as well as the two-stage endothermic dehydroxylation process for bentonite no. 2 at temperatures: 519 °C and 672 °C. Moreover,

Table 4 Listing of bands in the temperature range: 20–1200 °C for individual bentonites. 1 – German bentonite no. 1; 2 – German bentonite no. 2; 3 – Polish bentonite no. 3. Temp. (°C)

Listing of bands in the temperature range: 20–1200 °C for individual bentonites 20 °C

Wavenumber (cm1)

500 °C

550 °C

2

3

1

2

3

1

2

3

3632

3630

3633

3632

3630

3633

3632

3630

3633

3455 1642 1461

3460 1643 1485

3450 1642 1461

3455 1642 1461

3460 1643 1485

3450 1642 1461

3455 1642 1461

3460 1643 1485

3450 1642 1461

1135

1114

1035 914

1040 915

848 794

843 795

1039 914 876

1035 914

790

794

1040 915

795

1039 914 876

1035 914

790

794

1040 915

1

900 °C 2

3

1

1000 °C 2

3

1

1100 °C 2

3

1

1200 °C 2

3

1

2

3 –OH

1642

1643

1642

1135

1135

1140

1033

1035

1040

794 730

793 730

H–OH –H–OH

1095

1090

1086

1095

1090

1086

1095

1090

1086

1095

1090

1086

786

787

787

786

787

787

786

787

787

786

787

787

675

681

686

675

681

686

675

681

686

675

681

686

565

572

570

565

572

570

565

572

570

565

572

570

459

466

462

466

466

462

466

466

462

466

466

462

876 795

792 730

642

521

623

623

521

521

521

466

623

623

521

521 466

521

623

623

521

521 466

565

572

573

481

478

481

CO2 3 Si–O Si–O Si–O Al–O–H AlFeOH Al–Mg–OH Si–O Si–O Si–O–Si Si–O–Si Si–O–Al Unknown Si–O–Al Unknown O–Si–O

700 °C

107

550 °C

4.06 24.42 47.98

Fig. 7. Montmorillonite content in bentonite no. 1 in dependence of the temperature and time of holding.

300 °C

87.03 60.56 130.02

Fig. 8. Montmorillonite content in bentonite no. 2 in dependence of the temperature and time of holding.

Base

95.68 76.43 152.81

Compression strength (kPa)

72.19 82.1 116.61

Fig. 9. Montmorillonite content in bentonite no. 3 in dependence of the temperature and time of holding.

Condensation zone Pmax (kPa); T = 315 °C, t = 31 s

1.72 1.81 2.16

Table 5 Technological properties of moulding sand with bentonite.

Bentonite no. 1 Bentonite no. 2 Bentonite no. 3

the highest dehydroxylation temperature exhibited bentonite no. 1. These processes were reflected both in the thermal analysis and the FTIR analysis results (spectrum changes). An occurrence of two stages of dehydroxylation can attest to the presence of the octahedral sheet cis-vacant and trans-vacant varieties in the structure [10].

M. Holtzer et al. / Journal of Molecular Structure 1004 (2011) 102–108

1

1039 914 876 840 790

Bond

700 °C

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M. Holtzer et al. / Journal of Molecular Structure 1004 (2011) 102–108

Due to the limited temperature range (to 900 °C) of the performed thermal analysis, the identification in the DTA curves of the endothermic peak appearing at app. 900 °C – related to the transformation of the crystal montmorillonite structure into the amorphous one – was not successful. Structural investigations were confirmed by examinations of binding properties (determination of the montmorillonite content and technological properties of moulding sands) of bentonites subjected to a high temperature influence. After heating to a temperature of 500 °C the montmorillonite content practically did not change. It was only when bentonites were heated to 600 °C a gradual decrease of the montmorillonite content was observed (the most pronounced for bentonite no. 2). Probably this was related to the first stage of montmorillonite dehydroxylation. After holding bentonite samples at a temperature of 700 °C for 1 h the montmorillonite content was practically zero and bentonite was without any binding properties. This corresponds to the second stage of dehydroxylation. The increase in temperature of holding sands at the range of 700 °C clearly resulted in the drop of compression strength. The highest drop of strength was shown by the sand with bentonite 2 (from 82.1 kPa in room temperature to 24.42 kPa in temperature 700 °C). In order to be able to explain fully structural transformations occurring in bentonites under the influence of heating to temperatures above 900 °C an additional X-ray radiographic analysis will be necessary.

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Acknowledgement [27]

The study was performed within the Research Project National Science Centre No. N N507320440 (2011–2012).

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