Cordierite obtained from compositions containing kaolin waste, talc and magnesium oxide
Introduction
Cordierite is an aluminum magnesium silicate that can be represented in a ternary system, containing magnesium oxide (MgO), aluminum oxide (Al2O3) and silicon oxide (SiO2) in the proportions 2:2:5, respectively, with density of 2.53 g/cm3 and melting temperature of 1470 °C [1], [2]. This mineral has a low thermal expansion coefficient (α = (1−3) × 10−6 °C−1), high thermal shock resistance, low dielectric constant, high resistivity (ρ > 1012 Ω cm), high chemical and thermal stability, high refractoriness and high mechanical strength [3], [4], [5], [6], [7], [8], [9].
Cordierite-based materials are used in the production of components for the support of catalysts, filters, membranes, integrated circuit boards, furnace refractories and thermal insulators [10], [11], [12].
There are several conventional methods for the synthesis of cordierite, such as solid state sintering, sol-gel processes, the Pechini method and the glass crystallization technique [2], [13]. Among these methods, solid state sintering has the great advantage of being a low cost technique [11], [12], [14].
Raw materials commonly used in the solid state synthesis of cordierite are talc, kaolinite, sepiolite, ash, diatomite, feldspar, attapulgite, zeolite, pearlite, alumina and quartz [3], [5], [9], [14], [15], [16], [17], [18], [19]. Some researchers are investigating formulations containing wastes to obtain cordierite, including ferrochrome, magnesite and glass slag [8], [20], [21].
Kaolin is a raw material used in many industrial applications. Kaolin processing produces two types of wastes. One is composed of coarse particles (quartz in the form of sand), which is commonly referred to as coarse tailings. The second type is produced in the second phase of beneficiation to separate the fine fraction of the ore, purifying the kaolin and generating a residue called fine tailings. Most of the tailings thus generated are usually discarded on slag heaps and in floodplains, causing environmental impacts, and hence, posing risks to human health [22]. These wastes have been subject to strict inspections by environmental protection agencies, incurring costs for companies. On the other hand, these wastes can potentially be reused or incorporated as raw material in ceramic pastes.
In recent years, concern has increasingly focused on wastes generated by mining industry beneficiation around the world. Given the industrialization potential of kaolin in Brazil and the enormous amounts of wastes resulting therefrom, there is an ongoing need for studies aimed at the rational use of these tailings. Several studies have been developed in this area, including those [7], [23], [24], [25], [26]. Their studies indicate that these tailings can be used as precursor raw materials such as cordierite and mullite, which can be added to formulations of ceramic pastes to obtain refractories.
Using this approach, the purpose of this work was to obtain cordierite via the solid state sintering method from compositions containing kaolin waste, magnesium oxide and talc.
Section snippets
Materials and methods
The raw materials used in the development of this work were tailings from kaolin processing, called fine kaolin waste (FKW) and coarse kaolin waste (CKW), magnesium oxide (MO) and talc (T).
Prior to the characterization tests, the raw materials were sifted through an ABNT 0.075 mm sieve (No. 200) [27]. They were then subjected to chemical analysis by energy dispersive X-ray fluorescence spectroscopy (EDX) (Shimadzu EDX-720); X-ray diffraction (XRD) (Shimadzu Lab XRD-6000 X-ray diffractometer)
Results and discussion
Table 3 describes the chemical compositions of the formulations.
As can be seen in Table 3, composition M1 contained 38.0% SiO2, 31.3% Al2O3 and 17.1% MgO+K2O+Fe2O3+CaO, composition M2 presented 40.2% SiO2, 34.6 Al2O3 and 16.0% MgO+K2O+Fe2O3+CaO, while composition M3 contained 52.3% SiO2, 26.0% Al2O3 and 11.8% MgO+K2O+Fe2O3+CaO. Representations were made of the three compositions in the ternary equilibrium diagram (Fig. 1) of the SiO2-Al2O3-MgO system, the oxides percentage was calculated
Conclusions
After studying the compositions containing kaolin waste, magnesium oxide and talc aiming to obtain cordierite, it can be concluded that the increase in sintering temperature and in the average particle size influence the formation of cordierite. Cordierite nucleation started at 1250 °C and intensified at 1350 °C. The cordierite crystals showed a hexagonal tube-like morphology in the compositions containing kaolin and magnesium oxide wastes and kaolin and talc wastes. Compositions containing
Acknowledgements
The authors thanks to CNPq (303395/2016-8 and 308912/2016-0) and CAPES for financial support and PNPD/CAPES scholarship to Ester Pires de Almeida.
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