Seventh International Latin American Conference on Powder Technology, November 08-10, Atibaia, SP, Brazil

Mullite Evidence For Rapid Firing From Bentonite Clay of Paraíba

JOSILEIDO Gomes1,a, ALYSSON Israel Oliveira Rocha2,b, ROMUALDO Rodrigues Menezes3,c, GELMIRES de Araújo Neves 4,d , LISIANE Navarro de Lima Santana 4,e, MARIA Isabel Brasileiro1,f

1Universidade Federal de Campina Grande, UFCG/DEP – PB, Departamento de Engenharia de Processos - Aprígio Veloso, 820 – CEP 58109-970 – Campina Grande, PB, Brazil 2,4Unidade Acadêmica de Engenharia de Materiais, Universidade Federal de Campina Grande, Aprígio Veloso, 820, Campina Grande-CEP:58.109-970, PB, Brazil 3Universidade Federal da Paraíba, Campus de João Pessoa, Cidade Universitária - João Pessoa - PB - Brazil - CEP - 58059-900

[email protected], [email protected], [email protected],[email protected], [email protected], [email protected]

Keywords: Mullite, bentonite, microwaves heating.

Abstract. Bentonite clays are aluminium–silicates that when heated turn into mullite. The sintering of mullite obtained from these mineral clays by quick microwaves heating comes up as an alternative process for mullite powders synthesis. The use of quick heating on ceramics nano-powders synthesis is a recent technology that is being successfully used on synthesis with microwaves and synthesis process by combustion. The quick microwaves heating enables adding heat quickly and equally, accelerating the nucleation kinetics and the development of the mullite stage. Thus, the purpose of this work is to analyze the effect of the microwaves heating process variables, analyzing the influence of the applied power and of the heating rate on the mullite powders obtaining from bentonite clays. The clays have been favored and submitted to the following characterizations: chemical granulometric and mineralogically. Subsequently, the clays have been delamined aiming disagglomeration and separation of the thinner fractions and submitted to granulometric and mineralogical characterization. The synthesis has been realized on a domestic microwaves oven. The obtained powders have been characterized by X-ray diffraction. The results showed that the applied power variation and the sintering time are fundamental on the obtaining of mullite powders.

Introduction

The natural mullite occurs very rarely because of their conditions of formation, high temperature and low pressure. This mineral has good mechanical strength and the fracture toughness, low thermal expansion, which results in an excellent resistance to thermal shock, low thermal and electrical conductivity, excellent resistance to creep, the infrared transmittance and resistance to oxidation at high temperature [1]. In recent decades, many researchers search synthesize it, mainly through the mixing of SiO2 and Al2O3, or extract it [2] from minerals containing Al2O3 and SiO2 in his compositions . According to Bernard [3], a variety of methods of synthesis have been proposed for the preparation of mullite, most of them based on heating mixtures of alumina and silica in stoichiometric proportion, with the only difference being the source for these particular precursor oxides. According Quatib [4], besides the high cost, the techniques used to produce synthesis usually powder with heterogeneous morphology in the case of mullite with shapes of particles that do not favor the densification during sintering and with large amount of agglomerates and / or aggregates.

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1108 Seventh International Latin American Conference on Powder Technology, November 08-10, Atibaia, SP, Brazil

The bentonite is a material which presents dimensions of nano particles, in contrast to kaolinite, which may put it in as a potential precursor for the production of nano-powder of mullite. Previous experience [5,6] demonstrated the great potential this clay for the production of mullite, despite the high content of iron oxide. In nature this structure contains primary particles formed by 5 to 10 lamellae that remain together by interlayer ions. These primary particles form large aggregates visible of 0.1 to 10 μm [7]. According LeBaron [8], the sheet clays can be delamined crystallites in tens of nanometers thick (tactóides) or even individual lamellae with thickness of about 1 nm. The rapid sintering is a technique in which the rapid heating followed by sintering at high temperature in short time and may result in products of smaller size of grain, compared to conventional sintering. The use of microwaves as a method of processing at high temperatures has many benefits [9]. As the heating occurs by interaction with the electromagnetic field, is a volumetric heating of the material, which allows to work with high heat and reduce the processing cycles [10]. Sales [11] and Sacks [12] showed that the use of rapid heating in conventional oven allow to obtain crystals of mullite. While other studies [13], involving rapid in microwave synthesis of composite alumina / SiC, indicated that, apparently, the faster the heating the agglomeration of smaller particles and lower the size of the particles obtained. Within this context, this work aimed to the synthesis of nano powders of mullite and submicrometrics using bentonite clay as precursor material and rapid cycles of heating in microwave in order to produce powders with characteristics that achieve high density in process of sintering.

Materials and Method

Materials. For the development of this work were used two samples of natural bentonite clays called clay White (A) and ash clay (B) from the deposit located in Cubati - Paraíba.

Method. Samples of bentonite clay was dried in oven at 60 º C, then ground in mill type climb and sieved through 200 mesh (opening 74μm) in vibrating table. After treatment, the clay was the following characterization: chemical, mineralogical and grain. Later there was the process of delamination through a process of desaglomeration the wet. Was used 87g of bentonite to 315ml of deionized water. The mill was in desaglomeration in alumina, the rotation of 30rpm was used for 4 and 8 hours. After of the desaglomeration the solutions was prepared with concentrations of 15g/l then poured into test tubes with a capacity of one liter. After 24 hours was the first collection and then were the size analysis of the fraction collected. The volume collected was dried in oven at 60ºC, after drying the material was ground in mortar and passed in the ABNT 200 mesh. The material disagglomerated was then dried and subjected to synthesis in domestic microwave oven, model NN-GD587SRU PANASONIC brand. The times of synthesis were 10, 15, 20 and 25 minutes using high power. After microwave heating in the powder were analyzed by X-ray diffraction.

Results and Discussion

The Table 1 shows the chemical composition of semi-quantitative samples of bentonite studied. Table 1 – Chemical composition of the samples (%wt) other Samples LOIa SiO Al O Fe O MgO Na O K O CaO Total 2 2 3 2 3 2 2 oxides A 12,04 46,30 24,96 3,74 0,00 0,00 0,66 0,43 11,84 100 B 10,52 54,64 25,51 3,50 3,48 0,76 0,62 0,60 0,34 100 aLOI= loss of ignition

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1109 Seventh International Latin American Conference on Powder Technology, November 08-10, Atibaia, SP, Brazil

Observe that the content of Fe2O3 in the samples was approximately 4,0% and is within the range observed in the literature for bentonite clays South American [14,15]. The levels of alkali and alkaline earth oxides (MgO, CaO and K2O) are also observed inside the bentonite in South America. The XRD patterns of the samples of bentonite A and B delamined are processed and presented in Fig. 1 and 2. There is, qualitatively, that the samples A and B show levels of clay mineral smectite. However, there is also the presence of kaolinite and quartz in these samples.

E- Esmectita C C-Caulinita C E- Esmectita E Q-Quartzo E C-Caulinita C Q-Quartzo Q Q C E C CC E E E C Q C Q Q Q M8 - 24P Q C M8 - 24P C E E Q E C C Q CC C E Q E Q C Q Q Q Q M4 - 24P M4 - 24P E C C E E CC C C Q Q Q Q Q Amostra inicial Amostra inicial

0 102030405060 0 102030405060 2θ 2θ

Fig. 1 – XRD patterns of sample (A) Figura 2 - XRD patterns of sample (B)

The Fig.3 an 4 shows the results of granulometric analysis of sample A and B, respectively, in fractions below 0.10, 0.20, 0.75 and 2.0 μm, after disagglomeration for 4 and 8 hours, followed by settling for 24 hours in test tubes.

BENTONITE CAMPOS NOVOS CUBATI A - M8 - 24P – Desaglomeration for 8 hours and 24 hours of 250,00 sedimentation in test tubes. 200,00 A – M8- Desaglomeration for 8 150,00 hours. 100,00 A – M4 - 24P – Desaglomeration 50,00 for 8 hours e 24 hours of sedimentation in test tubes. 0,00 D.Average 0,10 µm 0,20 µm 0,75 µm 1,00 µm 2,00 µm A – M8 – Desaglomeration for 4 hours A - M8 24P 1,48 0,00 0,81 15,01 30,41 77,54 A - INITIAL – Initial sample A - M8 3,33 1,30 4,87 7,25 10,45 30,85 A - M4 24P 1,62 0,11 1,12 15,01 26,74 69,90 A - M4 4,56 0,34 0,98 4,09 7,03 22,63 A - INITIAL 7,66 0,39 1,06 2,80 4,77 16,73

Fig. 3 – Granulometric analysis the sample A Analyzing at the Fig.3 it was observed that there was a significant reduction in the diameter of the clay after disagglomeration for 4 and 8 hours and after decanting of the 24 hours. After the desaglomeration wet for 4 and 8 hours, the average diameter of particles of the sample was 7.66 μm pass to 4.56 μm and 3.33 μm respectively, when the sample was subjected to decantation in measuring these values seemed to is more significant, because the average diameter initially was 7.66 μm pass to 1.62μm and 1.48μm respectively. For the cumulative fraction below of 2.0 μm, the sample had accumulated fraction of 16.73 % after milling for 4 hours this value pass to 22.63%, the same clay after the process of decanting for 24 hours in measuring the fraction accumulated was more significant going to 69.90 %. When the sample was subjected to 8 hours of milling, the fraction of particles with

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this diameter increased to 35.85 % after the desaglomeration wet, followed by decanting into beaker for 24 hours, the stage had collected 77.54 % of particles less than 2.0 µm, this shows that there was a reduction with respect to the original size. The desaglomeration wet proved to be very efficient in reducing the average diameter of particles in the fraction below 2.0 µm, however, to fraction below 0.20 µm which corresponds to 200nm, the data showed that there is a reduction significant after 4 and 8 hours of desaglomeration. According Oliveira [16], fine powders between 1 and 0.001µm, have strong tendency to cluster, staying together by attractions of origin electrostatic, which are pronounced in liquid medium, directly affecting the state of dispersion.

BENTONITE CAMPOS NOVOS CUBATI (GRAY) B-M8-24P–Disagglomeration for 8 hours and 24 hours of sedimentation 200,00 in test tubes. B–M8-Disagglomeration for 8 hours. 150,00 B–M4-24P-Disagglomeration for 8 hours and 24 hours of sedimentation 100,00 in test tubes. B–M8- Disagglomeration for 4 hours. 50,00 B-INITIAL– Initial Sample.

0,00 D. average 0,10 µm 0,20 µm 0,75 µm 1,00 µm 2,00 µm B - M8 24P 1,85 5,16 13,51 27,05 27,05 50,89 B - M8 5,61 0,00 0,17 1,34 2,79 10,94 B - M4 24P 1,57 3,54 9,25 30,28 34,14 65,43 B - M4 5,87 0,41 1,14 3,42 5,35 16,43 B - INITIAL 7,75 0,15 0,44 1,94 3,55 12,86

Fig.4 - Granulometric analysis the sample B

Analyzing the Fig.4, the results showed that there was a reduction in the average diameter of particles, as well as an increase in accumulated fractions below 0.10 μm, 0.20 μm, 0.75 μm and 2.0 μm. The average diameter of particles was 7.75 μm went to 5.87 and 5.61 μm. This reduction was observed for milling after 4 and 8 hours, and after decantation in their respective test tubes for 24 hours, where such a reduction became even greater with values of 1.57 and 1.85 μm respectively. For the cumulative fraction below 2.0 μm, the reduction was significant, the sample initially had accumulated fraction of 12.86% after disagglomeration for 4 hours, and the cumulative fraction went to 16.43% and 65.43% after sedimentation sample tube. When the sample was subjected to 8 hours disagglomeration and 24 hours settling, the accumulated fraction was 50.99%. However, for the sample disagglomerated for 8 hours and decanted, was obtained a higher value for accumulated fractions below 0.20 μm corresponding to nano-fraction of the sample, the cumulative fraction from 0.44% went to 13.51%. The sample A showed the lowest average diameter for all the processes, this behavior is probably due to quartz and kaolinite in the sample, Fig.1 and Table 1, which may have settled, favoring phase separation. Fig.5 and 6 present the XRD patterns of sample A subjected to the processes of delamination and heating in the microwave for 10, 15, 20, and 25 minutes using high power. Analyzing at Fig.5 and 6, the samples studied had virtually the same mineralogical behavior (appearance of new phases) when subjected to heat. Identified characteristic peaks of the following phases: aluminum calcium silicate, silica, iron hydroxide, magnesium oxide and iron, aluminum potassium silicate and sodium, silicon oxide and mullite, showed that during

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the microwave synthesis, the samples heated at a time of lower synthesis, no showed peaks of mullite formation. Second Herculano[2], several variables can influence the process of mullitization including: particle size, sintering atmosphere, temperature, time and heating rate. The intensive mullitization occurs in a temperature range of 1600 to 1700°C, temperatures these would probably not have been achieved when the samples were 10, 15 and 20 synthesis in the microwave. For the time of 25 minutes, observed yet the formation of mullite more intense in all samples, probably during this time was more reaction between the SiO2 and Al2O3 forming mullite crystals.

SI -Silicato de aluminio e cálcio SI -Silicato de aluminio e cálcio F - Fosfato de ferro AMOSTRA (A) Q - Quartzo AMOSTRA (A) M4 -24P F - Fosfato de ferro M - Mulita Q - Quartzo Q C- Caulinita Q O- Óxido de titânio Fe - Hidróxido de ferro M - Mulita O - Óxido de magnésio e ferro Fe - Hidróxido de ferro S - Aluminio silicato de potássio e sódio O - Óxido de magnésio e ferro SiO - Óxido de silício M 2 M S - Aluminio silicato de potássio e sódio M M M TI - Óxido de titânio SiO - Óxido de silício M Fe M 2 MQM MMM M M 25 minutos M M M M M M M M Q Fe M M M 25 minutos SiO Q Fe 2 Fe SiO M FeS O 20 minutos 2 FeQ S O 20 minutos SiO Q Q 2 O FeQ S Fe S O Q O TI 15 minutos Fe 15 minutos SiO 2Fe Q SiO SI F S S TI C 2 FQ 10 minutos SI S TI Q 10 minutos 0 102030405060 0 102030405060 2θ 2θ

Fig.5 - XRD patterns of sample A after Fig.6 - XRD patterns of sample (A M4- synthesis in microwave 24P) after synthesis in microwave

The Fig.7 and 8 shows the XRD patterns of sample B subjected to the processes and conditions as the previous sample was submitted. Pattern was quite similar to the sample A, the peaks of mullite appeared after heating for 25 minutes, which may be related to higher crystallization of this phase. In both samples can be observed the formation of silicon oxide and quartz, this probably occurred due to excess silica present in all samples (Table 1).

SI -Silicato de aluminio e cálcio AMOSTRA (B) F - Fosfato de ferro AMOSTRA (B) M4-24P Q - Quartzo SI -Silicato de aluminio e cálcio Q M - Mulita Q Fe - Hidróxido de ferro F - Fosfato de ferro Q - Quartzo O - Óxido de magnésio e ferro M - Mulita TI - Óxido de titânio Fe - Hidróxido de ferro S - Aluminio silicato de potássio e sódio O - Óxido de magnésio e ferro SiO - Óxido de silício 2 TI - Óxido de titânio M S - Aluminio silicato de potássio e sódio M M SiO - Óxido de silício M MM M M 2 M Fe M M M 25 minutos M M M M M Fe M MM Q M M M 25 minutos Q SiO 2 S S O 20 minutos SiO2 FeQ S O 20 minutos Q SiO SiO SI 2 F S S 2Fe Q S TI 15 minutos O TI 15 minutos Q SI Q Q Q FQ S SI S TI 10 minutos SI F S TI 10 minutos 0 102030405060 0 102030405060 2θ 2θ

Fig.7 - XRD patterns of sample B after Fig.8 - XRD patterns of sample (B M4- synthesis in microwave 24P) after synthesis in microwave

Conclusion

Were studied two samples of bentonite in the city of Cubatí- Paraíba, aiming to obtain mullite by rapid sintering in the microwave. After carrying out preliminary experiments can be reached the following conclusions: all the samples showed levels of iron oxide and oxides of alkali and alkaline earth (MgO, CaO and K2O) in the bentonite observed for South American;

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the samples was fraction accumulated less than 2.0µm between 12.86 and 16.73%. The sample did not present sodium oxide in its composition showed a higher fraction accumulated to almost all the fractions evaluated. The process of delamination note that the longer milling favored disagglomeration. Through the XRD patterns of the synthesized samples can be observed that the formation of mullite was observed when the residence time in the microwave oven was 25 minutes.

Acknowledgement. The authors gratefully acknowledge financial support provided by Capes and CNPq, process number 479674/2007-8 and 307068/2007-2.

References

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