Mullitization process of andalusite concentrates – Role of natural inclusions

doi:10.1016/j.ceramint.2013.10.036

Ceramics International

Volume 40, Issue 4, May 2014, Pages 5129-5136

https://doi.org/10.1016/j.ceramint.2013.10.036 Get rights and content

Abstract

In the article the results of investigations into the mullitization process of two andalusite concentrates are presented. XRD, XRF and ICP investigations revealed nearly identical phase and chemical composition of both concentrates. However, the andalusites considerably differ in the rate of the mullitization process. XPS and LM investigations revealed that the factors responsible for mullitization include not only the content of mineral inclusions and grain size distribution but also impurities dispersion and the content of naturally occurring carbon in andalusite grains.

Introduction

Andalusite Al₂SiO₅ crystallizes in an orthorhombic system in Pnnm space group. It occurs in a form of euhedral, pseudotetragonal columnar crystals, characterized by good fissility, parallel to plane (110) and slightly poorer along plane (100) [1], [2]. Theoretically, its chemical composition contains 62.93% of Al₂O₃ and 37.07% of SiO₂, but in nature it occurs with various impurities, for example carbon-rich andalusite. When carbon is specifically located in a form of a cross visible in the crystal along (001) plane, it is known as chiastolite [3].

The biggest deposits of mined andalusite are metamorphogenic (primary) deposits, related to plutonism. Another extensively exploited type are supergenic (secondary), elluvial and alluvial deposits. The above mentioned types of deposits may differ in their age of mineralization. The term “mineralization” is understood as a process of andalusite formation from rocks which are rich in aluminum and poor in silica, referred to as metapellits, in the conditions of contact metamorphism of igneous intrusions [4].

The interest in andalusite is related to the fact that it is a useful raw material to be applied in the ceramic industry, including high-temperature applications (refractory materials). The usefulness of the mineral in question results mainly from its properties such as low thermal conductivity (the lowest in the group of silimanite minerals) [5], a possibility of using andalusite in a raw state for ceramic mixes, both in fine and coarse fractions, as well as its relatively low price [6], [7], [8]. From the point of view of refractory applications, an extremely important phenomenon is thermally activated transformation of andalusite into mullite. Mullite Al₆Si₂O₁₃ is a phase characterized by high melting point and good thermo-mechanical properties, resistance to chemical corrosion and thermal shocks [9]. As a formality, it should be added that the mullitization process is accompanied by a release of amorphous phase rich in SiO₂ [10]. It is assumed that the process of andalusite mullitization is a diffusion assisted phase transformation with silica dissolution [6], [11]. The andalusite mullitization process was investigated by many authors and in various experiments a high number of results were obtained for the process temperature (1100–1480 °C) [12], [13], [14], [15], [16]. In general, on the basis of literature review, it can be stated that transition temperature and kinetics depend on the grain size distribution and the degree of raw andalusite material purification [15], [17], [18]. Undoubtedly, the type and content of impurities that accompany andalusite are important. Impurities can be related to the genesis of the deposit [19].

It is worth emphasizing that in the analysis of andalusite transition into mullite the attention was focused mainly on the total content of impurities related to mineral inclusions, while the manner of their distribution was ignored. Neither discussed the role of carbon naturally occurring in andalusite grains as a factor which might influence the mullitization process. In this article we would like to discuss both above mentioned issues. In order to do that, we analyzed 2 andalusite concentrates differing considerably in their susceptibility to mullitization but having a very similar chemical composition as well as grain size distribution.

In search of factors that are evidently different for both above quoted andalusites, several experimental techniques were applied: optical light microscopy LM, X-ray fluorescence spectrometry XRF, X-ray diffraction XRD, plasma emission spectrometry ICP and X-ray photoelectron spectroscopy XPS.

The aim of this article is to enrich the discussion on factors which may play a significant role in the process of andalusite phase transformation into mullite, which is of great practical importance.

Section snippets

Investigated andalusite concentrates

Commercial concentrates of andalusite from two separate deposits, differing in their genesis and mineralization age, were used as a research material: the elluvial deposit from the Republic of South Africa, hereinafter referred to as andalusite A, and the autochthonous deposit from France, hereinafter referred to as andalusite B. Admittedly, andalusite A comes from a secondary, elluvial deposit but its primary mineralization is related to igneous granite intrusion of Bushveld complex, which

Characteristics of the raw materials

As mentioned in the introduction, andalusites A and B considerably differ in their susceptibility to mullitization. The results of degree of conversion α_B determined at 1350 °C, 1400 °C and 1450 °C are presented in a form of α_B=f(t) dependence in Fig. 1. Transformation of andalusite B is evidently slower – lower values α_B are reached for the same periods of transformation compared to andalusite A. Kinetic measurements of the mullitization process were carried out for powdered samples with grain

Summary

In the article the results concerning two types of andalusite – andalusite A, originating from the secondary (elluvial) deposit, and andalusite B, from the primary (autochthonous) deposit – have been presented. XRD, XRF and ICP investigations revealed a very similar phase and chemical composition of both samples. However, the andalusites considerably differed in the rate of the mullitization process. A possible explanation of this problem was provided by investigations conducted by XPS and LM.

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Effects of andalusite and kyanite addition on the microstructure, thermo-mechanical properties and corrosion resistance of ultralow-cement bauxite-corundum castables
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Ultralow-cement bauxite-corundum refractory castables are increasingly attracting attention because of the advantages in performance and cost. To improve its volume stability and high-temperature performance, andalusite and kyanite were incorporated into this type of refractory, respectively, and a comparative investigation of their effects on microstructure, thermo-mechanical properties, and slag corrosion resistance was carried out in this work. The expansion effects induced by the mullitization of these additives not only suppressed the sintering shrinkage but also made the matrix structure more compact and uniform, mainly owing to the generation of many new and strong mullite bonds. This improvement resulted in a significant increase in the refractoriness under load and creep resistance. This effect was far more pronounced for the andalusite-added castables, for which the RUL increased by 62 °C and the creep rate at the stationary stage decreased by 26%. Besides, adverse effects on microstructure and properties appeared when adding a higher amount of kyanite due to the excessive expansion. Moreover, the corrosion mechanisms of these refractory castables by refining slag were discussed, and the active role of andalusite addition in the corrosion resistance was also proposed.
Fracture behavior and microstructure of Al<inf>2</inf>O<inf>3</inf>–SiO<inf>2</inf> castables containing andalusite
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Citation Excerpt :
The in situ mullite formed by the decomposition of andalusite is called primary mullite. Complete mullitization of andalusite results in about 83 wt% mullite with a capillary network filled with 17 wt% silica rich glass (as shown in Fig. 1) [5–12], accompanied by a volume expansion of about 5%. Primary mullite is arranged in an orientation with its c axis parallel to c axis of the initial andalusite[13], as shown in Fig. 2.
To evaluate the andalusite role on the fracture behavior of Al₂O₃–SiO₂ castables and reveal the improvement mechanism of flexibility, Al₂O₃–SiO₂ castables with various andalusite additions (0, 10, 20, 30, 40, and 50 wt%) were prepared adopting homogenized bauxite, andalusite, calcium aluminate cement, microsilica and ultrafine α-Al₂O₃ powder as raw materials. Wedge splitting test was used to determine fracture behavior of Al₂O₃–SiO₂ castables. The improvement mechanism of toughness was discussed according to the analysis of microstructure. The results show that incorporation of andalusite is an effective way to change fracture behavior and improve the flexibility of the Al₂O₃–SiO₂ castables. The force-displacement curve of the Al₂O₃–SiO₂ castables without andalusite addition is featured with a linear fracture behavior, while the force-displacement curves of the Al₂O₃–SiO₂ castables containing andalusite are characterized by a nonlinear fracture behavior. The flexibility parameter (G_F/σ_NT ratio) rises markedly from 35.3 μm of the castable without andalusite to 197.8 μm of the castables with 40% of andalusite addition, which increases more than twice. Microcracks toughening plays one major role in the improvement of the flexibility of Al₂O₃–SiO₂ castables containing andalusite. The formation of microcracks can be attributed to mismatch of coefficient of thermal expansion between andalusite particles and matrix, caused by anisotropic behavior of andalusite aggregates and mullitized andalusite aggregates. The interlocking microstructure of in situ formed mullite grain in the matrix is the second reason for the toughness improvement of Al₂O₃–SiO₂ castables containing andalusite.
Andalusite transformation and properties of andalusite-bearing refractories fired in different atmospheres
2019, Ceramics International
Citation Excerpt :
Andalusite-containing refractories have been widely used due to their advantageous properties, such as volume stability, thermal shock resistance, and creep resistance, which are related to the mullitization of andalusite at high temperatures [1–8]. The mullitization of andalusite fired in an air atmosphere has been extensively investigated in terms of different parameters, such as firing temperature, particle size, and impurities [9–14]. It has been reported that the volumetric stability of andalusite-containing refractories results from volumetric expansion during andalusite transformation, which counterbalances the sintering shrinkage of refractories at high temperatures [15,16].
We found in our research that andalusite aggregate fired in a reducing atmosphere exhibits a lower mullitization rate than that fired in an air atmosphere. For investigating the effect of atmosphere on the transformation of andalusite and the properties of andalusite-containing refractories, andalusite powder (≤0.074 mm) and refractories containing andalusite aggregate (3–1 mm) were fired in air and carbon embedding, respectively. The phases and microstructure of the andalusite fired in both atmospheres were characterized by X-ray diffraction and scanning electron microscopy, respectively. The correlations of the properties of the andalusite-bearing refractories with the firing atmospheres were investigated in terms of volume stability, mechanical strength, and thermal shock resistance. The difference in the properties of the refractories was discussed with respect to the varied transformation rates of andalusite, and in terms of the different viscosities of the silica-rich glass caused by the different atmospheres.
Enhanced alkali vapor attack resistance of bauxite-SiC refractories for the working lining of cement rotary kilns via incorporation of andalusite
2018, Ceramics International
Bauxite-SiC refractories as the working lining of cement rotary kiln are subjected to severe alkali attack due to alternative fuel combustion. Herein, the present work aims to enhance their alkali attack resistance via incorporation of different types of andalusite (aggregates and powder). Results show that addition of andalusite reduces penetration of alkali vapor by decreasing their pore size. In particular, the presence of andalusite aggregate traps alkali vapor inside its pore network, retarding the corrosion of refractory matrix. By comparison, the presence of andalusite powder contributes to formation of liquid corrosion products to block the pores, which prevent alkali vapor from further penetration. Incorporation of andalusite into BSRs is straightforward and sought-after, providing an effective approach for high-performance refractory materials with excellent alkali attack resistance.
Densification behavior of mullite-Al<inf>2</inf>TiO<inf>5</inf> composites by reaction sintering of natural andalusite and TiO<inf>2</inf>
2018, Ceramics International
Citation Excerpt :
Furthermore, the phases evolution of the samples with 8 wt% TiO2 addition can also be illustrated by the TiO2-Al2O3-SiO2 ternary equilibrium phase diagram [26]. As shown in Fig. 4, the theoretical chemical composition of andalusite (62.93 wt% of Al2O3 and 37.07 wt% of SiO2 [27]) should be located at Point F. With 8 wt% TiO2 addition, the composition of the mixture would be located at Point L, which is in the AT+M+S phase region. Therefore, AT, mullite and SiO2 should be the only phases present in the samples with addition of 8 wt% TiO2 when equilibrium has reached in the calcination process.
In this work, mullite-Al₂TiO₅ composites were fabricated by natural andalusite with TiO₂ as an additive. The densification characteristic, phase composition and mullitization process of andalusite with TiO₂ addition was investigated by the Archimedes’ principle, dilatometry, X-ray diffraction and scanning electron microscopy (SEM-EDS) techniques. The results showed that the incorporation of TiO₂ not only enhanced the thermal stability of in-situ Al₂TiO₅ in the silica liquid yielded from the mullitization of andalusite, but also accelerated andalusite decomposition and retarded mullite formation, which facilitated the sintering and densification of mullite-Al₂TiO₅ composites.
Preparation and thermal shock resistance of corundum-mullite composite ceramics from andalusite
2017, Ceramics International
Citation Excerpt :
The transformation of andalusite to mullite is extensive (about 88%), along with a negligible volume change [9–12]. In addition, mullitized andalusite grains exhibit specific microstructural characteristics, which are beneficial to improve thermal shock resistance in comparison with monocrystalline mullite [13–17]. During the temperature from 1100 °C to 1600 °C, andalusite is converted into mullite and silica rich glass [18–22] (as show in equation 1, 2).
Corundum-mullite composite ceramics have high hardness, small plastic deformation and other excellent performances at high temperature. Corundum-mullite composite ceramics were fabricated from andalusite and α-Al₂O₃ by in-situ synthesis technology. Effects of mullite/corundum ratio and sintering temperatures on the water absorption, apparent porosity, bulk density, bending strength, thermal shock resistance, phase composition and microstructure of the sample were investigated. Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily, which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock (1100 °C-room temperature, air cooling). Formula A4 (andalusite: 37.31 wt%, α-Al₂O₃: 62.69 wt%, TiO₂ in addition: 1 wt%, mullite: corundum=6:4 in wt%) achieved the optimum properties when sintering at 1650 °C, which were listed as follows: water absorption of 0.15%, apparent porosity of 0.42%, and bulk density of 3.21 g⋅cm⁻³, bending strength of 117.32 MPa. The phase composition of the sintered samples before and after thermal shock tests were mullite and corundum constantly. The fracture modes of the crystals were transgranular and intergranular fractures, which could endow the samples with high thermal shock resistance.

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Mullitization process of andalusite concentrates – Role of natural inclusions

Abstract

Introduction

Section snippets

Investigated andalusite concentrates

Characteristics of the raw materials

Summary

J. Eur. Ceram. Soc.

Ceram. Int.

Ceram. Int.

J. Catal.

Appl. Surf. Sci.

Thin Solid Films

Surf. Sci.

J. Europ. Ceram. Soc.