Beach Sand Sillimanite Research Articles

Thermo-mechanical properties of mullite—zirconia composites derived from reaction sintering of zircon and sillimanite beach sand: Effect of CaO

Mullite—zirconia composites containing 20% zirconia (mass fraction) were prepared by reaction sintering route utilizing Indian coastal zircon flour and sillimanite beach sand. 4%–12% of CaO (mole fraction) with respect to zirconia was used as additive. The effect of additive on densification, microstructure as well as various mechanical and thermo-mechanical properties was studied. Incorporation of CaO reduced the densification temperature of the composites to 1550 °C compared to 1600 °C (for CaO free samples). CaO formed small amount of liquid phase (calcium aluminosilicate), which facilitated sintering. Average grain size of the composites decreased up to 4% CaO addition, afterwards grain size increased with further addition of CaO. Samples with 4% CaO exhibited ∼225 MPa of flexural strength, ∼6 MP·m1/2 of fracture toughness and significant improvement in thermal shock resistance. CaO stabilized the tetragonal zirconia phase and thus improved the mechanical properties.

Synthesis and characterization of mullite–zirconia composites by reaction sintering of zircon flour and sillimanite beach sand

Mullite–zirconia composites containing 10–30 wt% zirconia were prepared by reaction sintering of zircon flour, sillimanite beach sand and calcined alumina. Raw materials were attrition milled, shaped into pellets and bars and sintered in the temperature range of 1450–1600∘C with 2 h soaking at peak temperature. Sintered products were analysed in terms of various physical, mechanical and thermo-mechanical properties. The analyses of phases developed and microstructural analyses were carried out by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. It was observed that the addition of ZrO2 up to 20 wt% significantly improves flexural strength and fracture toughness. The transformation of t → m zirconia was found to be the dominant mechanism for enhancement in mechanical properties. ZrO2 occupies both the intergranular as well as intragranular positions. However, intragranular zirconias are much smaller compared to intergranular zirconias.

Enhancement of thermal shock resistance of reaction sintered mullite–zirconia composites in the presence of lanthanum oxide

Mullite–zirconia composites containing 20wt.% zirconia were prepared by reaction sintering of zircon flour, sillimanite beach sand and calcined alumina. 0 to 8mol% of La2O3 with respect to zirconia was used as sintering aid. The effect of additive on the various physical, microstructures, mechanical and thermo-mechanical properties was studied. Quantitative phase analysis shows the change in tetragonal zirconia content with incorporation of lanthanum oxide. La2O3 addition has significantly improved the thermal shock resistance of the samples. Samples without additive retained only 20% of initial flexural strength after 5cycles, whereas samples containing 5mol% La2O3 retained almost 78% of its initial flexural strength even after 15 thermal shock cycles.

Optical and Micro-Raman Characterization of Fused Mullite Prepared from Beach Sand Sillimanite

Plasma-fused mullite compounds were prepared from beach sand sillimanite with varying sillimanite and alumina ratio (60:40 = MU1, 65:35 = MU2, and 75:25 = MU3) within five minutes of thermal plasma treatment. The X-ray diffraction (XRD) and micro-Raman studies indicate the presence of corundum phase in MU1 and MU2 compounds, whereas no signature of corundum phase is evident in MU3. This observation confirms that sillimanite and alumina taken in the ratio 75:25 (MU3) are exactly in the appropriate proportion for the complete conversion of mullite. The bulk density is higher in fused MU3 as compared to other two compositions. The optical band gap measured using UV-Visible spectrophotometer for this composition is found to be 3.37 eV.

Influence of raw material type and of the overall chemical composition on phase formation and sintered microstructure of mullite aggregates

Dense mullite aggregates with varied (47–70%) alumina contents have been prepared by a conventional dry-powder pressing technique followed by heat treatments at temperatures in the range of 1450–1725 °C. Different types of clays, beach sand sillimanite (BSS) and a high purity aluminium hydroxide were used as starting materials. Mullites derived from BSS consisted of equi-axed grains whereas those obtained from clay containing precursor mixtures exhibited elongated grains. The bulk density (BD), apparent porosity (AP) and water absorption (WA) capacity of sintered mullites were found to be strongly influenced by the pre-mullitization step of the precursors and in a less extent by the type of raw material, its hydration degree and the impurity contents of Fe 2O 3, CaO and Na 2O. Mullite aggregates obtained from the three different types of aluminosilicate raw materials (i.e., ball clay, china clay and beach sand sillimanite) through a double-stage heat treatment process exhibited better sintered properties in terms of bulk density, apparent porosity, water absorption capacity and higher mullite contents in comparison to those obtained following a single-stage firing process.

Formation and Densification Behavior of Mullite Aggregates from Beach Sand Sillimanite

Dense mullite aggregates with 60% and 70% Al2O3 have been prepared from precursor mixtures consisting of beach sand sillimanite and a high‐purity aluminum hydroxide following conventional single‐ and double‐stage firing processes. The bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity of sintered mullite aggregates were found to be strongly influenced by the premullitization step of this precursor mixture. Mullite aggregates formed in a double‐stage firing process exhibited higher BD and mullite content and lower AP and WA capacity in comparison with those obtained by the single‐stage firing process. The values of coefficient of thermal expansion of sintered mullite aggregates are close to those found in the literature reports for high‐purity stoichiometric mullite.

Comment on “Thermo-mechanical behaviour of low cement castables derived from mullite aggregates synthesised from beach sand sillimanite”

Comment on “Thermo-mechanical behaviour of low cement castables derived from mullite aggregates synthesised from beach sand sillimanite”

Properties of zirconia–mullite composites prepared from beach sand sillimanite

Zirconia–mullite composites were prepared from beach sand sillimanite in the presence of magnesia as an additive. The raw materials were attrition milled. Samples were isostatically pressed and sintered in the temperature range of 1500–1600 °C with 2 h of soaking. The resulting composites were characterized in terms of densification, thermal expansion, martensite start ( M s) temperature. The presence of magnesia changes the value of thermal expansion coefficient. The mechanical and thermomechanical properties, namely flexural strength and thermal spalling of the sintered composites, were characterized and effect of magnesia on it is discussed. The microstructural characterization of the sintered samples through scanning electron microscopy has shown that mullite and zirconia are the two major phases in the microstructure.

Effect of sillimanite beach sand composition on mullitization and properties of Al2O3-SiO2 system

Mullite was developed by reaction sintering of sillimanite beach sand and calcined alumina. Two varieties of sillimanite beach sand viz. S and Z having different compositions were selected. Synthesis and properties of mullite were very much dependent on the sillimanite beach sand composition. Presence of higher amount of impurities in the Z-variety of sillimanite sand favours the densification by liquid phase formation. Presence of zircon in Z-variety increases the hardness and fracture toughness. Alumina addition improves the mechanical/thermomechanical properties of the samples. Mullite retains the usual orthorhombic habit of sillimanite. Rounded to sub rounded zirconia dispersed within the mullite matrix of the sample ZA is noticed.

Effect of magnesia additions on the properties of zirconia–mullite composites derived from sillimanite beach sand

Zirconia–mullite composites with 4–8 mol% magnesium oxide have been prepared from Indian coastal sillimanite beach sand, zirconia and alumina. The X-ray diffraction study reveals all raw materials are crystalline in nature. In zirconia, monoclinic is the major phase detected. Alumina is having α-Al 2 O 3 as main constituent. X-R-D of magnesia shows periclase is major phase. Milling of the mixtures of all raw materials has been carried out in an attritor mill for 9 h. After drying and pressing the samples were sintered between 1500 °C and 1600 °C with 2 h of soaking. The physical properties of the sintered materials namely bulk density and linear shrinkage have been evaluated. The infra-red spectrums of sillimanite sand and sintered samples have been described. The thermo-mechanical and mechanical properties namely hot modulus of rupture, hardness and fracture toughness of sintered materials have also been evaluated. Microstructural characterization of the sintered samples through scanning electron microscope have shown mullite and zirconia are the two main phases.

Thermo-mechanical behaviour of low cement castables derived from mullite aggregates synthesised from beach sand sillimanite

Mullite aggregates were synthesised from beach sand sillimanite, a by-product generated during rare earth extraction, by reaction sintering with calcined alumina. The aggregates were characterised in terms of chemical composition, mineral phases, microstructure and important thermo-mechanical properties critical for refractory applications. To evaluate its suitability in castable refractories, low cement castables were prepared using the aggregates with commercial high alumina cement. The physical and thermo-mechanical properties such as bulk density, apparent porosity, cold crushing strength, hot modulus of rupture and compressive creep was studied in relation to temperature. The castables exhibited good modulus of rupture and excellent creep resistance at 1400 °C. Attempts were made to correlate these properties with corresponding microstructure and phases.

Preparation and properties of Y 2O 3 containing zirconia–mullite composites derived from sillimanite beach sand

Mullite–zirconia composites with 2–6 mol% yttrium oxide have been prepared from Indian coastal sillimanite beach sand, zirconia and alumina. All raw materials have been attrition milled and then isostatically pressed followed by sintering in the range of 1500–1600 °C with a 2-h soaking period. Then the sintered materials have been characterised in terms of bulk density, linear shrinkage, hot modulus of rupture, X-ray diffraction, infra red spectroscopy and microstructural features. The effect of yttrium oxide with change of sintering temperature for different properties have been discussed.

Synthesis and thermo-mechanical properties of mullite–alumina composite derived from sillimanite beach sand: effect of ZrO 2

A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO 2 (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO 2 on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO 2 slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO 2. ZrO 2 containing samples always show better resistance to thermal shock than the ZrO 2 free samples. Scanning electron micrography shows that ZrO 2 occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering.

Studies on Cordierite-Mullite Body—Part II: Effect of Sillimanite Sand in a Cordierite Composition

Beach sand sillimanite was substituted for china clay and fireclay grog in a standard cordierite body mix containing 60% china clay, 20% fireclay grog (IS-6 grade) and 20% talc. The specimens were fired at different temperatures ranged from 1200 to 1340°C with 2 h dwelling time at respective peak temperatures. The addition of 40 wt% sillimanite in the standard composition was found to improve the mechanical properties and to reduce the coefficient of thermal expansion from 25.7 × 10−7 to 20.43 × 10−7 at 500°C and 25.20 × 10−7 to 21.34 × 10−7 at 1000 C. The reduction in the coefficient of thermal expansion value of the body mix containing 40 wt% sillimanite was due to the presence of cordierite and mullite and/or sillimanite phases in the composition. The final body also showed enhanced fired strength from 147.67 kg.cm−2 to 241.90 kg.cm−2 after firing at 1340 C. The thermal shock resistance of the developed body mix containing 40 wt% sillimanite. measured in terms of percent loss of MOE after repeated thermal shock at gradually enhanced temperatures in water quenching, was found to be more than two times higher than that of the standard cordierite composition.

Thermal shock behaviour of high alumina aggregates derived from sillimanite beach sand with and without Fe 2O 3 doping

In this investigation, three types of high alumina aggregates namely sillimanite, mullite and alumina rich aggregates were developed from sillimanite beach sand and calcined Al 2O 3 by reaction sintering route. The various phases present in the sintered compacts were identified by X-ray diffraction study in conjunction with infrared study. Thermal shock behaviour of the aggregates with and without Fe 2O 3 doping were measured by the room temperature flexural strength retention on ambient air quenching from 1000°C against the number of cycles. The results obtained are presented and discussed in this paper. It was found that all Fe 2O 3 containing samples show a strength enhancement on quenching by means of tempering. Attempts have been made to correlate this parameter with the microstructure and phase assembly of the product. ©

Effect of chemical composition on sintering and properties of Al 2O 3–SiO 2 system derived from sillimanite beach sand

Reaction sintering of beach sand sillimanite and alumina is an innovative as well as inexpensive method of mullite formation. Beach sand sillimanite, a by-product generated during the separation of rare earth compounds, and calcined alumina were used as starting materials and were mixed in appropriate proportion and sintered in compacted form at 1500–1575°C. The study reveals that the use of submicron size powder enhances the sintering process. In this investigation, effect of chemical composition on sintering and mechanical/thermomechanical properties has been studied. It was found that the densification process is dependent on the Al 2O 3/SiO 2 ratio. Silica rich composition achieves highest density at 1525°C whereas the alumina rich composition requires 1550°C. Flexural strength measured at room temperature and at 1200°C initially de-creases with alumina content, with a minima at 71–74% Al 2O 3 and then increases again. Microstructure of the sintered is also dependent on the Al 2O 3/SiO 2 ratio of the batch. Mullite made from silica rich composition is needle shape in nature whereas in alumina rich composition they are non-acicular in nature.

Synthesis and mechanical properties of mullite from beach sand sillimanite: effect of TiO 2

Reaction sintering of beach sand sillimanite and calcined alumina is an innovative as well as cost effective method of mullite formation. In the present investigation, beach sand sillimanite and calcined alumina in microfine state were used as starting materials to develop high alumina aggregates for refractories applications using titania additive (0–6 wt%). The raw materials and additive (TiO 2) were mixed in definite proportion and sintered in compacted form at 1400–1550°C with 2 h soaking. It was determined that, alumina/silica ratio of the batch controls densification, flexural strength and microstructural development of the aggregates. Aggregates with almost zero porosity were developed at a sintering temperature of 1500°C with the help of titania additive. Gradual addition of TiO 2 up to 4 wt% promotes the densification process, however, TiO 2 affects the high temperature flexural strength of the aggregates. Aggregates developed from sillimanite sand at 1500°C indicates that the mullites formed are not ideal 3:2 type, but are 1:1 type. Mullite formed in the silica rich composition is long and elongated in nature. At the densification temperature of 1500°C up to 3·06 wt% TiO 2 enters into the mullite structure as solid solution.

Synthesis and Mechanical Properties of Mullite Developed from Beach Sand Sillimanite: Effect of Fe203

Mullite aggregate was developed from sillimanite beach sand and calcined alumina by the reaction sintering route using 1–4 wt% Fe2O3, as additive. Effect of Fe2O3, on the densification behaviour, microstructure and thermomechanical properties of the aggregate was discussed. It was found that Fe2O3 favours densification process by liquid phase formation, thereby deteriorating the thermomechanical properties particularly at higher sintering temperature. Mullite formed without Fe2O3 additive at 1600°C was mostly equiaxed in nature whereas it was needle shaped with few occasional equiaxed mullite grains within the matrix when formed in presence of Fe2O3 at 1500°C. Up to 4.08 wt% Fe2O3 enters into the mullite structure by solid solution formation at 1500°C.

Isothermal Sintering of Size-Graded Sillimanite

Beach sand sillimanite was subjected to suitable powder treatments and the sintering behaviour of the resultant narrow size-graded sillimanite powders as well as mixtures of these in various propor...

Slip-Casting Characteristics of Kerala Beach Sand Sillimanite

Ball-milled Kerala beach sand sillimanite could be conveniently slip-cast in an alkaline liquid medium to form useful refractory shapes like thermoelement supports, thermocouple sheaths and melting crucibles. The results of chemical analysis, particle size distribution of the ball-mill ground material, the pH-relative viscosity relationship using some deflocculants and a Mariott tube viscometer and the fired properties of well-dispersed, slip-cast sillimanite samples have been presented. It was observed that dense slip-cast sillimanite refractory wares could be obtained by a suitable treatment of the material.