Heat resistance and thermomechanical behaviour of ultralow and zero cement castables

Abstract

Knowledge of relative heat resistance as well as thermo-mechanical behaviour of refractory castables is very important for their use as linings in high temperature furnaces and refining vessels in the metallurgical, cement, and petrochemical industries. The present work aims at studying these properties for different types of refractory castable. Two classes of castable were prepared, namely ultralow and zero cement, containing either high alumina cement or hydratable alumina as bonding agent. For each class, two different castable systems were prepared, one containing an alumina-silica mixture in its matrix and the other containing magnesia-alumina. In all castables studied, calcined alumina was used as aggregate. The prepared castable samples were subjected to firing temperatures up to 1500°C. Relative heat resistance, bending strength before and after thermal cycling, hot modulus of rupture, and creep deformation were measured according to international standard specifications. It was concluded that a limited content of cement (ultralow cement castables) is beneficial with the magnesia-alumina mix in the matrix owing to the formation of calcium hexaluminate-magnesium aluminate-corundum (matrix advantage system) that results in excellent relative heat resistance as well as thermome-chanical properties. Zero cement castables on the other hand are recommended for use with the alumina-silica mixture, since the absence of cement improves the chances of mullite formation without glassy phases, thereby enhancing the properties of such refractory castables.     

Utilisation of South Libyan clay for preparation of mullite-alumina based refractory ceramics

Abstract

This work aims to benefit from the wide spreaded clay in south Libya with some alumina (Al₂O₃) additions to prepare and characterise mullite-alumina containing refractory ceramics. The starting materials used in this investigation are local raw clay and chemical aluminium oxide (Al₂O₃). Based on a previous geological survey carried out by the industrial research centre (Tripoli), four different clay samples were selected from different locations of the south of Libya. Representative samples of the four selected clays were investigated using X-ray diffraction (XRD) and differential thermal analysis. The four clay samples were used separately together with aluminium oxide to prepare six different batches containing different proportions of alumina from 0 up to 60% with an increament of 10%. The mullite containing bodies were investigated through their mineralogical compositions using XRD and scanning electron microscopy (SEM) attached with energy dispersive X-ray unit (EDX). The sintering, mechanical and refractory properties of the prepared mullite containing bodies were tested according to the international standard specifications. It is concluded that the densification, mechanical and refractory properties are improved as the alumina content increases on the expense of clay. Mixes containing 60% clay : 40% alumina and 50% clay : 50% alumina respectively, especially those prepared from Agar clay are characterised with good sintering, high mechanical and high refractory properties.     

Synthesis of high-performance mullite ceramics based on associated rare-earth kaolin

The objective of this work was to prepare high-purity, high-strength mullite ceramics from low-cost, associated rare-earth kaolin (AREK). A reaction sintering process using calcined AREK and γ-Al₂O₃ powders was used to synthesize high-performance mullite ceramics. Mineralogical, morphological, and chemical characteristics of AREK were given. The effects of associated REEs in kaolin and sintering temperature on the microstructural evolution, phase transformation, and physical properties of mullite were studied. The results showed that the mullite contents were 98.8%, the maximum aspect ratio was 8.22 μm, the relative density was 93.04%, and the micro-Vickers hardness and flexural strength were 10.63 GPa and 184.24 MPa, after sintering at 1500°C for 4 h. For comparison, calcined without rare-earth kaolin was also employed as a raw material to synthesize mullite ceramics, and the mullite content prepared by sintering the two kaolin clays at 1320–1480°C for 4 h was quite similar. However, mullite prepared using AREK forms secondary mullite in the temperature range of 1480–1500°C with a significantly higher mullite content, and therefore, the advantages of preparing mullite based on AREK as the raw material are high purity, low mullitization temperature, and high strength.     

Low Temperature Synthesis of High Alumina Cements by Novel Co‐Melt Precursors and Their Implementation as Castables with Some Micro Fine Additives

In the present investigations nano size high alumina cements (HAC) were prepared by very effective co‐melt precursor sintering technique from their metal nitrate precursors. The prime cementing phases observed were CA, CA₂, and C₁₂A₇. The addition of nano structured cements in refractory castables has improved the thermo‐chemical‐mechanical properties to a significant extent. Each batch of low cement castables (LCC) was prepared from calcined Chinese bauxite, HAC, and superfine additives. The effect of HAC in bauxite castable with the additives similar to Silicon Carbide, reactive alumina, and micro‐fine silica on the sinterability and properties of these castables was investigated. Physical properties such as apparent porosity and bulk density, mechanical properties such as hot modulus of rupture (HMOR), cold and hot modulus of rupture (CMOR), and cold crushing strength (CCS) of hydrated and sintered castables were studied. The sintered castables were also characterized for their solid phase compositions and microstructure using X‐ray diffraction (XRD) and FE‐SEM, respectively. In the castables new phases such as mullite, α‐alumina were formed at the expense of bauxite and silica. Solid solution of mullite formed at high temperature acts as a bonding phase and is accounted for high HMOR, CMOR, and CCS values. These excellent properties of such castables may enable their uses in various applications such as refractory lining for fabrication of steel, aluminium, copper, glass, cement, chemicals, and ceramics.     

Synthesis of mullite aggregates from fly ash: effect on thermomechanical behaviour of low cement castables

Abstract

Refractory aggregates were synthesised from beneficiated fly ash by reaction sintering with calcined alumina at 1600°C, and 83% mullite yield was achieved. The aggregates had low porosity, low thermal expansion and good refractoriness. To study compatibility in castable refractories, the aggregates were used in high alumina cement based low cement castables and their thermomechanical behaviour was studied. Microstructural characterisation revealed that the emergence of new bond phases such as mullite and calcium hexa-aluminate had a beneficial effect on the hot modulus of rupture and creep resistance of castables. An attempt was made to establish a structure–property relationship.     

Sintering of magnesia refractories through the formation of periclase–forsterite–spinel phases

Effect of calcined kaolin additions on sinterability of dead-burned magnesia was studied through additions of Egyptian kaolin. Five batches containing powders of 100/0, 90/10, 80/20, 70/30 and 60/40 magnesia/kaolin, respectively, were mixed, dried and fired up to 1550 °C for 4 h. Their physical (bulk density and apparent porosity), mechanical (cold crushing strength) as well as refractory (thermal shock resistance and refractoriness under load) properties were tested according to the International Standard Specifications. The phase compositions of the fired batches were investigated using X-ray diffraction as well as scanning electron microscope to illustrate the effect of the different developed phases on the above properties compared with the main magnesia source itself. Generally, samples containing 10–20 wt.% kaolin are characterized by their high sintering, mechanical and refractory properties, so they are recommended for use as a lining for different parts of steel furnaces as well as heating zone of cement rotary kilns. Samples containing 30–40 wt.% kaolin with lower refractory properties can be used in the heat exchangers or as kiln furniture in ceramic industries.     

Effect of MgO addition and sintering parameters on mullite formation through reaction sintering kaolin and alumina

Abstract

In the present work, the influence of MgO addition and sintering parameters on the formation and densification of mullite was investigated. The morphology of powders and the microstructure of the sintered samples were characterised by means of a scanning electron microscope. X-ray diffraction was used to characterise phases formed in sintered samples. The density of sintered samples was measured using a densimeter and quantified according to the Archimedes principle. MgO was added at 1, 2, 3, 4, 5 and 6 wt-% to kaolin and alumina and the powders were ball milled for 5 h then uniaxially compacted at 75 MPa and finally sintered at 1500, 1550, 1600 and 1650°C for 2, 4, 6 and 8 h. It was found that addition of MgO not only affected mullite formation but also promoted grain growth. For samples containing 0, 1 and 2 wt-%MgO only mullite was formed. While, in addition to mullite, Al₂O₃ was present in sample containing 3 wt-%MgO. At higher MgO content (4, 5 and 6 wt-%), three phases, i.e. mullite, Al₂O₃ and spinel, were formed. Addition of 1 wt-%MgO increased the density of all samples for all sintering times and higher densities corresponded to higher sintering temperatures. At higher MgO content, higher temperatures led to lower densities and lower temperatures led to higher densities for almost all sintering times.     

高岭土加入量对合成轻质微孔莫来石性能的影响

以高岭土、工业氧化铝、硅石粉为起始原料,采用湿法共磨,注浆成型,利用高岭石热分解过程中形成一次莫来石及高活性的无定形SiO2,合成高纯轻质微孔莫来石.研究了高岭土加入量及煅烧温度对轻质莫来石体积密度与莫来石生成量的影响.结果表明,随配比中高岭土加入量的增加,莫来石的体积密度有下降的趋势,且有利于莫来石相含量的提高.高的煅烧温度虽能增加莫来石相的生成量,但不利于莫来石合成料的轻质化,适宜的煅烧温度为1450℃.     

焦宝石基喷涂料的研制

以焦宝石、叶蜡石、硅微粉和铝酸钙水泥为原料制备了焦宝石基喷涂料,对比分析了焦宝石基喷涂料与焦宝石-铝矾土基喷涂料的物理性能。结果表明:焦宝石基喷涂料与焦宝石-铝矾土基喷涂料的性能相当,但是该喷涂料成本较低,可部分替代焦宝石-铝矾土基喷涂料修补高炉的衬里,使用效果良好。     

High alumina castables reinforced with SiC

Abstract

To improve the sintering, mechanical and refractory properties of high alumina castables, different contents of SiC (up to 8 wt.-%) were added at the expense of high alumina cement. Cold crushing strength of the green samples was measured and hydration behaviour was studied using differential thermal analysis (DTA). After firing at 1550°C for 3 h, the compositions of the fired samples were investigated using X-ray diffraction. Sintering parameters (bulk density and apparent porosity) and mechanical (cold crushing strength) as well as refractory (thermal shock resistance, permanent linear change, load bearing capacity) properties were tested according to standard specifications. It was concluded that increasing content of added SiC results in an improvement in sintering, mechanical and refractory properties but adversely affects green strength. However, castables containing 6 wt.-% SiC show a reasonable compromise between acceptable green strength and improved sintering, mechanical and refractory properties. The improved properties of the fired samples are related to the formation of SiC-mullite system in the matrix.     

High-toughness silicate ceramic

A silicate ceramic that is similar to porcelain and exhibits a maximum toughness of 4.6 MPa m^1/2 was obtained by tape casting from kaolin and 3 vol% of alumina fibers. Improved toughness and strength are achieved with the organized micro-composite microstructure that results from preferential orientation during the shaping of kaolinite particles and fibers in-plane of layers. During sintering, typical nucleation and growth processes of mullite produce specific microstructural characteristics, such as bulk zones, oriented fibers and large interfacial zones between the fibers and the bulk. Toughening is attributed to the decreased crack energy in the bulk ceramic, in which a dense and organized network of short mullite occurs, and in interfacial zones containing a superimposed network of large mullite. The silicate ceramic that is reinforced by only 3 vol% of the alumina fibers is strong (95 MPa) and tough (4.6 MPa m^1/2); although these properties are often mutually exclusive.     

In situ mullite foam fabrication using microwave energy

In situ mullite ceramic foams were fabricated using polymeric sponge replication method from ceramic slurry containing alpha alumina and kaolin mixtures. Ceramic preforms were processed using microwave energy and conventional heating. The sintered foam samples were characterized by SEM and XRD observations, density measurements and compression tests in order to observe the effect of two different sintering techniques on the structure and properties. It was found that the microwave processing was completed in a shorter burning out and sintering cycle and produced structures having higher mullite transformation ratio and fine grains.     

In situ synthesis and thermal shock resistance of mullite used for thermal storage ceramics in solar thermal power generation

Abstract

Mullite ceramic, as one of high performance thermal storage ceramics for solar thermal power generation systems, was in situ fabricated via semidry pressing and pressureless sintering in the air. Andalusite (57–68 wt-%) and calcined bauxite (24–29 wt-%) were used as the raw materials, with kaolin and a tiny of boric acid being added to promote the densification and improve the mechanical properties. The best physical properties and thermal shock resistance were obtained on an optimum A3 sample sintered at 1600°C for 3 h, i.e. a bending strength of 120·44 MPa and 30 cycles thermal shock cycling without cracking (wind cooling from 1000°C to room temperature) with a loss of bending strength of 8·7%.     

Effect of zirconia content on mechanical and thermal properties of mullite–zirconia composite

Abstract

Mullite–zirconia composites were prepared by adding various zirconia contents in the mullite ranging from 0 to 30 wt-% and sintering at 1400–1600°C for 2 h. The phase composition examined by X-ray diffraction showed that mullite was the major phase combined with developed t-ZrO₂ and m-ZrO₂ phase as a function of zirconia content, especially at 1600°C, wherein m-ZrO₂ predominated. Density increased when the zirconia content and sintering temperature were increased ranging from 2·2 to 3·53 g cm^?3. The morphology of mullite grain showed elongated grains, whereas dispersed zirconia showed equiaxed and intergranular grains. Flexural strength was continuously improved by adding zirconia during the sintering temperature ranging from 1400 to 1500°C, whereas flexural strength was initially improved up to 5 wt-% of zirconia addition and deteriorated with more than 5 wt-% of zirconia content during sintering between 1550 and 1600°C. The maximum strength, 190 MPa, was obtained when sintering mullite with 30 wt-% of zirconia content at 1500°C. The degradation of strength at high sintering temperature may be a result from more occurrence of m-ZrO₂ phase. Thermal expansion of sintered specimens indicated linear change and hysteresis loop change. The hysteresis loop obtained with increased zirconia content resulted in the t–m phase transformation. Martensitic start temperature M_s was determined to be 530°C for 15 wt-% zirconia sintered at 1500°C, implying that the t–m phase transformation occurred.     

Phase evolution and hydraulic properties of low cement castables matrixes

Abstract

To identify the hydration products of aggregate-free, low cement castables (LCC), cement matrixes were examined. Two series of cementing batches based on 33·3 and 50·0 wt-% high alumina cement (HAC) were processed by adding ultra-fine calcined alumina/fumed silica mixtures (FA/FS) with weight ratios of 1·0, 1·5, 2·3, 4·0, and 9·0 to each series. The hydrated batches were investigated for their hydraulic properties. Batches showing the highest cold crushing strength with minimum water of consistency (WOC) and reasonable setting time (ST) were selected and characterised with respect to phase composition, microstructure, and microchemistry, before and after firing up to 1400°C, by XRD, DTA, and SEM techniques. Cementing batches containing ≈33 wt-% HAC, and ≈67 wt-% FA/FS mixtures with ratios of 4·0–9·0 show optimum particle packing and hydration conditions with the least amount of WOC. This results in increases in cold crushing strength (CCS) of cementing batches up to 58 MPa after hydration for 3 d. The hydrated batches are composed mainly of unreacted α-Al₂O₃ particles bonded by CAH₁₀, AH₃, and C₂ASH₈ phases. On firing up to 1400°C, the hydrated phases are transformed into anhydrous CA₂ and CA₆ patches enclosing limited amounts of CAS₂ and/or C₂AS phases. Such batches are suitable for application as cementing matrixes for high alumina low cement castables. The low HAC content with high FA/FS ratio in the presence of more fine alumina in the matrixes of such castables leads to significant improvement in their hot mechanical properties.     

Influence of Zn2+ ion addition on properties of aluminous electrical porcelain

Abstract

ZnO was added as a dopant, in concentrations of 2, 3, and 4 wt-%, to alumina based porcelain batches, in order to study its effect on the fired materials characteristics. An additional composition containing 4 wt-%ZnO and 1 wt-%LiF was also investigated. The results showed that the addition of 4 wt-%ZnO decreased the maturing temperature and improved the electrical properties. It also promoted the development of a mullite phase from the glassy phase. On the other hand, the combination of ZnO and LiF had a negative effect on the electrical properties and on the mullite content. The Li⁺ ions caused the dissolution of mullite crystals and the crystallisation of new phases.     

Role of B2O3 in formation of mullite from kaolinite and α-Al2O3 mixtures

Abstract

The microstructure, phase constitution, and physical properties of mullite bodies prepared from α-Al₂O₃- kaolin mixtures with added B₂O₃ were investigated. Densification was found to be enhanced with small additions of B₂O₃. The results indicate that 0.5 wt-% B₂O₃ increases the content and growth rate of the mullite. It was found to be the optimum addition with respect to densification and resulting properties.     

粉煤灰为基制备耐火材料的研究

以粉煤灰为主要原材料,煤矸石和工业氧化铝为辅料,通过控制物料比例制备了不同Al2O3含量在不同烧结温度下的耐火材料.对所制备材料进行结构性能测试,并借助X射线衍射和扫描电镜对材料进行微观分析.实验结果表明:不同的烧结温度及氧化铝含量对耐火制品的致密性、力学性能、耐火度和晶型状态都有影响.制品在1350 ℃、Al2O3的含量为45%时,其性能指标为:体积密度2.47 g/cm3、常温耐压41 MPa、耐火度1450 ℃、晶型状态以莫来石相和刚玉相为主.该制品的力学性能和使用性能较好,可为进一步利用粉煤灰制备高附加值材料奠定基础.     

原位反应烧成莫来石-堇青石复合材料及其特性(英文)

通过原位反应烧成方法制备了不同莫来石、堇青石含量比的复合材料。首先,根据堇青石和莫来石的理论化学组成,分别将高岭土、工业氧化铝和滑石混合,配制出堇青石生料粉(C粉)和莫来石生料粉(M粉)。然后,再将C粉和M粉按不同的设计比例混合,烧结制备出一系列莫来石-堇青石复合材料。添加V2O5添加剂,考察其对烧结复合材料的影响。并通过X射线衍射,微观结构观察,热力学性质检验,比较了不同莫来石、堇青石含量比的复合材料的特性。制备的莫来石-堇青石复合材料所用的烧结温度可降低至1380℃,且复合材料中堇青石和莫来石晶体发育良好,复合材料抗热震性好。     

脱硅高岭土制备莫来石材料的研究

莫来石是一种高级耐火材料,具有非常广泛的用途。现研究用脱硅高岭土为原料生产莫来石材料,以解决目前莫来石生产中存在的产品质量与生产成本不能兼顾的问题。高岭土经脱硅处理后,主要成分为没有活性的Al2O3和部分没有被脱去的SiO2,其铝硅氧化物摩尔比已超过莫来石。在脱硅高岭土中加入部分未煅烧的生高岭土粉,用来调节莫来石的成分,并作为莫来石成型的粘结剂,然后经成型、煅烧,制成莫来石制品。结果表明：这种方法生产莫来石,不用加入工业氧化铝,有望降低生产成本及提高莫来石的纯度。